Metallurgy Into Duct Ion

8/8/2019 Metallurgy Into Duct Ion

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USE OF MATERIALS COURSEINTRODUCTION TO METALLURGY LECTURE NOTES

INDEXPage

1. Introduction............... ................................................................................... 4

1.1 Metals ........ ................................................................................. 41.2 Alloys ......... ................................................................................. 71.3 Solid Solutions ............................................................................. 71.4 Phase Mixtures ............................................................................ 8

2. Equilibrium Diagrams ............................................................................... 10

2.1 Iron - Carbon Diagram ............................................................... 102.2 Slowly Cooled Structures .......................................................... 122.3 Quenched Structures................................................................. 132.4 Critical Temperatures ................................................................ 15

3. Hot and Cold Working............................................................................... 17

3.1 Stress Relief .............................................................................. 173.2 Recrystallisation ......................................................................... 173.3 Grain Growth ............................................................................. 18

4. Hardness and Strength............................................................................. 19

4.1 Influence of Grain Size .............................................................. 20

4.2 The effects of Structural Changes ............................................. 204.3 Hardenability .............................................................................. 21

5. Heat Treatment.......... ................................................................................ 24

5.1 Hardening Mechanisms ............................................................. 24

5.1.1 Solid Solution Hardening ......................................... 245.1.2 Precipitation Hardening ........................................... 25

5.1.3 Work Hardening ....................................................... 265.1.4 Strain Ageing ........................................................... 28


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5.2 Isothermal Transformations ....................................................... 29

5.2.1 TTT Curves .............................................................. 295.2.2 Continuous Cooling Transformation Curves ........... 30

5.2.3 Effects of Alloys on Transformations ....................... 30

5.3 Normalising ................................................................................ 315.4 Full Annealing ............................................................................ 315.5 Quenching and Tempering ........................................................ 345.6 Importance of Grain Size ........................................................... 355.7 Grain Size Determination .......................................................... 37

6. Alloying...................... ................................................................................. 39

6.1 Alloying Elements in Steel ......................................................... 39

6.1.1 Carbide Formers ...................................................... 406.1.2 Crystal Growth ......................................................... 416.1.3 Corrosion Resistance .............................................. 426.1.4 Strength ................................................................... 42

6.2 Effects of Alloying on Steel ........................................................ 42

6.2.1 Carbon ..................................................................... 426.2.2 Manganese .............................................................. 426.2.3 Chromium ................................................................ 436.2.4 Nickel ....................................................................... 436.2.5 Molybdenum ............................................................ 446.2.6 Niobium .................................................................... 446.2.7 Vanadium ................................................................. 456.2.8 Aluminium ................................................................ 456.2.9 Sulphur ..................................................................... 456.2.10 Phosphorus .............................................................. 466.2.11 Silicon....................................................................... 47

6.3 Alloy Addition in Steel - Summary ............................................. 47

7. Microstructures in Steel Arc Welds......................................................... 49

7.1 Weld Metal ................................................................................. 497.2 Heat Affected Zone ................................................................... 517.3 Welding Heating Cycle .............................................................. 527.4 Carbon Equivalent ..................................................................... 53


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8. Structure of Ingots.... ................................................................................ 54

8.1 Pipe........... ................................................................................. 548.2 Segregation ............................................................................... 558.3 Steel Types ................................................................................ 56

8.3.1 Rimming Steel.......................................................... 568.3.2 Semi-killed Steel ...................................................... 578.3.3 Killed Steel ............................................................... 57


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1. INTRODUCTION

1.1 Metals

Metals are chemical elements or alloys, which in general exhibit the following

properties:

- Good conductivity of heat and electricity

- Relatively hard, yet possess properties of malleability and ductility

- Solid at normal temperatures - except Mercury

- High densities

- Low specific heats

- Reflect light and are generally white in colour -except Gold and Copper

- Difficult to penetrate with X-rays

- Magnetic to some degree but only Iron, Nickel and Cobalt are ferromagnetic

However, it is the properties of strength, ductility and toughness which dictatetheir principal uses.

All metals have a crystalline structure of which there are three common types:

Body-centred cubicFace-centred cubicHexagonal close-packed

Body-centred cubic Face-centred cubic Hexagonal close packed

- Iron (ferrite)Vanadium

TungstenChromium

- Iron (austenite)Copper

SilverGold

AluminiumLeadNickel

Platinum

MagnesiumTitanium

ZincCadmium

The most economical packing arrangements for the metal atoms are thehexagonal close packed and face centred cubic, followed the body centredcubic structure, which occupies a larger volume (Fig 1).


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Fig 1 The three Principal Types of Structure in which metallic elementscrystallise

Fig 2 Volume changes in Iron with Temperature


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Fig 3 The Early Stages in the Growth of a Metallic Dendrite

The importance of different packing densities is illustrated by the fact that themetal iron is allotropic, that is it adopts different crystal structures according toits temperature. Up to a temperature of 910C it h as a body-centred cubicform, from 910C to 1400C it is face centred cubic , and above 1400C itreverts to body centred cubic until it melts at about 1535C. Heating themetal will not only produce an increase in volume by thermal expansion, butalso, volume changes will be caused by alterations in crystal structure.

The diagram (Fig 2) demonstrates the volume effects of different atomicpacking arrangements resulting from the allotropic changes from bodycentred cubic structure to face centred cubic structure and back to bodycentred cubic structure.

When a metal alloy solidifies, each crystal begins to form independently froma nucleus or centre of crystallisation. The nucleus will be a simple unit ofappropriate crystal lattice, and from this the crystal will grow. It develops bythe addition of atoms according to the lattice pattern and rapidly begins toassume visible proportions in what is called a dendrite. (Dendritic-tree-like).

A metallic crystal grows in this way because heat is dissipated more quicklyfrom a point, leading to the formation of a rather elongated skeleton (Fig 3).

The dendrite arms continue to grow and meet neighbouring dendrites whichwill be orientated differently owing to their independent formation; that is, theirlattices will meet at odd angles. Hence the independent formation of eachcrystal leads to the irregular overall shape of crystals and to the characteristicgrain structure of metals when in the as cast condition.


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1.2 Alloys

An alloy is essentially a mixture of two or more elements, the principalcomponent being a metallic element (the 'parent metal' or 'solvent'), so that

the resultant mixture exhibits metallic properties. A wide variety ofmechanical and physical properties may be obtained by alloying, so thatalloys, rather than pure metals, are of the greatest importance forengineering.

If the constituent metallic atoms are chemically similar to one another, theywill crystallise as a single set of crystals, since all the atoms will behave as ifthey belonged to the same species. A single-phase solid solution is thensaid to form, and its microstructure is often indistinguishable from that of apure metal.

However, there may be a tendency for the elements to crystallise separatelyto form distinct and different crystals joined at mutual grain boundaries. Sucha structure is an example of a phase mixture, which can usually bedistinguished from a single-phase solid by metallographic examination.

Note that this could include the formation of an intermetallic compound.These compounds are in themselves of little practical value, since they tendto be hard and brittle, but they can be important as constituents of alloysystems.

1.3 Solid Solutions

Initially when a solid solution is formed the crystal structure is the same asthat of the parent metal - the atoms of the solute or alloying element aredistributed throughout each crystal, and a range of composition is possible.The solution may be formed in two ways:

(a) In substitutional solid solutions the atoms share a single commonarray of atomic sites (Fig 4a).

In some systems the parent metal will dissolve any proportion of the

solute and retain its original crystal structure. However, in many casesthere is a limited solubility and in order to accommodate a largerproportion of the added alloying element a change in the initial crystalstructure becomes necessary to form a different solid solution, that is,another phase. In this way two solid solutions may exist together overa range of composition.

(b) In interstitial solid solutions the atoms of the solute element aresmall enough to fit into the spaces between the parent metal atoms, asillustrated (Fig 4b).


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(a) substitutional

(b) interstitial

Fig 4 The formation of solid solutions

Because of the atom size limitation, interstitial solid solutions are lesscommon than substitutional solutions, although Carbon atoms can dissolvein iron crystals in this way in steel. Similarly Nitrogen can dissolve in steel

and this is the basis of the Nitriding surface hardening process. The verysmall atoms of Hydrogen will dissolve interstitially in ferrous alloys, usuallyproducing brittleness.

1.4 Phase Mixtures

A phase, present in an alloy as a separate entity, can be pure metal, a solidsolution or an intermetallic compound. Any mixtures of two or more of thesecan occur. In binary systems, that is those of two elements, generally notmore than two phases can exist together.


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2. EQUILIBRIUM DIAGRAMS

Thermal equilibrium (also known as Phase or Constitutional) diagrams areof great importance in metallurgy for with their aid it is possible to determineexactly the structure of a particular alloy at any given temperature, provided

the alloy has been allowed to reach a state of equilibrium. Thus the phasespresent, their quantities and the chemical composition of each phase can beshown with precision. The diagrams are constructed principally by thermalanalysis but also with microscopic studies, the examination of volumechanges, X-ray diffraction and other techniques.

Equilibrium can be considered as a state of balance ultimately arrived at bythe components at the temperature of the system concerned. However, insome cases such a state would take a very long time to be reached while inothers it may never be reached at the temperature in question. For example,if an alloy is rapidly cooled by quenching to room temperature, chemical and

physical changes may be suppressed such that they will never take placeunless the alloy is reheated to allow them to occur. Very slow cooling mustthen follow.

2.1 Iron-Carbon Equilibrium Diagram (Fig 5)

Steel may be defined as an alloy of Iron and Carbon (up to about 1.7%C).Here it may be helpful to recall the allotropic nature of iron and that up to

910C it has a body centred cubic crystalline form known as alpha Iron,

from 910C-1400C a face centred cubic structure, g amma Iron, reverting to

body centred cubic delta . Iron above that temperature. These terms aremodified in steel to Ferrite, Austenite and Ferrite. Other phases in theequilibrium structure are Cementite the inter-metallic compound Fe3C, and,Pearlite a phase mixture known as a Eutectoid consisting in this case ofalternate layers of Cementite and Ferrite. Pearlite contains about 0.83%C.

Ferrite and Ferrite, the body centred cubic structures dissolve only verysmall amounts of carbon: less than 0.01% at room temperature. The facecentred cubic Austenite however, is capable of dissolving up to nearly 2%Cat 1150C although this structure will change on re aching the Lower CriticalTemperature 723C below which the Eutectoid reactio n will be complete. (Fig

6)


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Fig 5 The Iron-Carbon phase diagram


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Fig 6 Part of the Iron-Carbon Thermal-equilibrium diagram

Note that the Lower Critical Temperature 723C bel ow which all Austenitehas been converted to Ferrite and Cementite is commonly known as the A1temperature. The temperature above which the structure will be whollyAustenite, the Upper Critical Temperature, is known as the A3 temperature.

Also the temperature above which the steel reverts to a wholly body centredcubic ferrite is known as the A4 temperature.

2.2 Slowly Cooled Structures

The most important reaction in steel is the decomposition of austenite oncooling. Consider the slow cooling of a steel of 0.83%C content (i.e. of theeutectoid composition) (Fig 7a); at 723C the structure will transform to aneutectoid mixture consisting of alternate lamellae or plates of ferrite andcementite.

A steel of higher carbon content (known as a 'hyper-eutectoid' steel) (Fig7b), 1.20%C, will remain austenitic down to the temperature around, say,870C at which the solvus line is crossed, so that Fe3C will start to beprecipitated at the austenite grain boundaries.

Continued cooling and precipitation of cementite Fe3C will reduce the carboncontent of the austenite until it reaches that of the eutectoid 0.83%C. Whenthe temperature falls to below 723C, this residual austenite will transform topearlite, and the final microstructure will be cementite plus pearlite.


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A lower carbon steel (i.e. a 'hypoeutectoid steel) of 0.4%C (Fig 7c) will beginto transform when the temperature falls below the solvus line by theprecipitation of ferrite at the austenite grain boundaries.

Continued cooling and precipitation of ferrite will increase the carbon content

of the austenite until it reaches that of the eutectoid 0.83%C. At 723C thisremaining austenite will transform to pearlite resulting in a final structure offerrite plus pearlite.

2.3 Quenched Structures

The previous microstructures form in plain carbon steels which have beenmoderately slowly cooled (e.g. by cooling in air) from temperatures within theaustenitic phase field, say from 50C above the low er boundary line CED.This is called a 'normalising' heat treatment, but medium and high-carbonsteels are very commonly subjected to more complex treatments in order fully

to exploit their properties. These treatments involve, first, heating the alloyinto the austenite phase field, as before, but then quenching it in water orbrine which suppresses diffusion and thus the formation of ferrite andcementite. Under these conditions the austenite transforms by a process notinvolving diffusion into a metastable distorted form of body-centred ironknown as 'Martensite' (Fig 7d). This process is extremely rapid and thetransformation may be completed in a few microseconds.

Fig 7a Structure of Eutectoid Steel (1000X) (Pearlite)

Fig 7b Structure of Hypereutectoid Steel (1000X)


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Fig 7c Structure of Hypoeutectoid Steel (1000X)

Fig 7d Martensite (2000X)

All the carbon originally dissolved in the Austenite at high temperatureremains after quenching in interstitial solution in the Martensite crystals. Thishas the effect of distorting the lattice from cubic to tetragonal symmetry. Thislattice distortion by the dissolved carbon has the effect of hardening thestructure and the resulting steels will not only be hard but brittle, for which

there is little practical application. A second heat treatment called temperingis therefore required and this will reduce hardness and brittleness. IfMartensitic steel is reheated to the temperature range 200-600C (below theAustenite forming temperature) it rapidly decomposes to form body centredcubic ferrite and particles of Cementite.

This structure is on an extremely fine scale, the size of the carbide particlesbeing dependent on the time and temperature of the treatment. The higherthe temperature and the longer the time, the softer and less brittle theproduct.


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2.4 Critical Temperatures

There has been reference to the A1 lower critical temperature, the A3 uppercritical temperature and the A4 temperature above which austenite reverts to

ferrite. Examination of the Iron-Carbon equilibrium diagram shows that the

latter change will not occur when the carbon content approaches andexceeds 0.5%C. In such cases liquid steel will solidify directly to austenite.

Concern may be felt regarding the non-appearance of an A2 temperature andalso the absence of a beta phase in the equilibrium diagram. Originally thedesignation A2 was given to the temperature 769C, the Curie poin t, at whichiron ceases to be magnetic, a fact that was expected to be accompanied by aphase change. This proved not to be the case and the A2 has no structuralsignificance.

It cannot be emphasised too strongly that the structural changes in the

diagram and the temperatures at which they occur refer to conditions ofequilibrium. In practice it is found that, on heating, it is necessary to exceedthe equilibrium temperatures to achieve the expected structural changes.Such temperatures would normally be about 50C abov e the A1 and A3 andcan be determined with reasonable accuracy for particular conditions ofheating. Here the lower critical point is termed the Ac1 and the upper criticalpoint the Ac3 temperature. The suffix C has been derived from the Frenchchauffage meaning heating. In a similar way it may be expected that oncooling it is necessary to reach temperatures lower than those of theequilibrium diagram to obtain the appropriate structural changes. Thus, for

example, in hot rolling operations, which are accompanied by continuouscooling, temperatures of 100-150C below equilibriu m become necessary forthe expected structural changes. In such cases AR1 and AR3 temperaturesare referred to, the suffix R again being French, derived fromRefroidissement, cooling.

The Effect of Heating Rates on Lower and Upper Critical TemperaturesSteel AISI 1045 (0.45%C)

Equilibrium Temperature HEATING RATES C/SEC FROM 700-1000C

3 30 300 1400

Ac1 723 780 790 800 840

Ac3 770 820 830 860 935


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3. HOT AND COLD WORKING

In a process such as rolling, the crystals making up a metal are deformed inthe direction of rolling. If the temperature of the operation is such that thedeformation is accompanied by simultaneous recrystallisation the process is

Hot Working. Here relatively minor ductility changes can be expected andthese can be attributed largely to improved crystal structures and betterdispersion of non-metallic inclusions. On the other hand if there is norecrystallisation and the crystals become progressively more deformed onworking then the process is Cold Working. In this case major propertychanges arise with a loss of ductility and marked increases in hardness andstrength.

A cold-worked metal is in a state of considerable mechanical stress,resulting from elastic strains internally balanced. These elastic strains arelargely due to inhomogeneous deformation having taken place during cold-

working. If the metal is heated to a sufficiently high temperature the strainswill be removed; at the same time the tensile strength and hardness of themetal will fall to approximately their original values and the capacity for cold-work return. This form of heat-treatment is known as annealing, and isemployed when the metal is required for use in a soft but tough state or,alternatively, when it is to undergo further cold deformation. Annealing takesplace in three stages as follows:

3.1 Stage 1 - Stress Relief

This occurs at relatively low temperatures at which dislocations are able tomove to equilibrium positions in the crystal lattice (see Heat treatment sectionfor an explanation of dislocations). Such small movements can reduceinternal mechanical stress without, however, producing any visible alterationin the distorted shape of the cold-worked crystals. Moreover, hardness andtensile strength will remain at the high value produced by cold-work and mayeven increase.

3.2 Stage II - Recrystallisation

As mentioned previously, a low-temperature anneal to relieve internal stress

may sometimes be used, but generally annealing involves a definite andobservable alteration in the crystal structure of the metal. If the annealingtemperature is increased a point is reached when new crystals begin to growfrom nuclei produced in the deformed metal. These nuclei are formed atpoints of high energy, such as crystal boundaries. The crystals so formed areat first small, but grow gradually until they absorb the entire distorted structureproduced originally by cold-work. The new crystals are equi-axed in form,that is, they do not show any directional elongation, as did the distorted cold-worked crystals which they replace.

This phenomenon is known as recrystallisation, and it is a method

employed, in conjunction with cold-work, of course, to produce a fine-grainedstructure in non-ferrous metals and alloys. Only in some cases - notably in


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steels and aluminium bronze, where certain structural changes take place inthe solid state - is it possible to refine the grain size by heat-treatment alone.

3.3 Stage III - Grain Growth

The temperature at which recrystallisation will take place is called therecrystallisation temperature and if the annealing temperature is above this,the crystals will continue to grow until the structure is relatively coarsegrained. The amount of grain growth is governed to a large extent by theannealing temperature, the duration of annealing and the degree of previouscold-work. A high annealing temperature or a long annealing time willencourage grain growth. Heavy deformation will lead to a small grain size,light deformation will give rise to a larger grain size on annealing since therewill be fewer nuclei for crystal growth.


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4. HARDNESS AND STRENGTH

Although hardness is a property difficult to describe precisely, tests havebeen devised which are quick and easy to apply. Such tests are non-destructive, unlike tensile and impact determinations, and they are therefore

attractive means of assessing the mechanical properties of metals. Inpractice, routine hardness tests on manufactured parts will be applied overlimited property ranges and reliable correlations between hardness, ultimatetensile strength, yield strength, elongation and reduction of area can beestablished. As such they will provide excellent guides to the acceptability ofthe items concerned (and possibly their chemical compositions), the suitabilityof the manufacturing processes and heat treatment to which they have beensubjected.

Increases in hardness values (which results from an increase in C content)will usually be associated with increases in ultimate tensile and yield strength,

but there will be reductions in ductility (elongation) expressed as reduction ofarea and elongation (Fig 8). Note that an increase in C content will also beaccompanied by a reduction in impact (Charpy) toughness.

Fig 8 Diagram showing the Relationship between Carbon Content,Mechanical Properties, Microstructure and Uses of Plain Carbon Steelsin the Normalised Condition


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4.1 Influence of Grain Size on Hardness

Usually in metals a smaller grain size will lead to some increase in hardnessand tensile strength. This strengthening effect is due not only to the complexintersecting slip process in the various grains but also to the grain boundaries

which are themselves obstacles to the movement of slip planes whichtherefore cannot propagate freely from grain to grain.

In structural steels, processing for optimum properties is aimed at producingvery small sized relatively soft grains. In this way Yield Strength is enhancedand the Impact properties markedly improved especially at low temperatures.

4.2 The Effects of Structural Changes on the Hardness of Steels

It must be emphasised that in steels the most profound changes inmechanical properties occur as a result of changes in microstructure. These

are illustrated in the table below relating to the different structures obtained ina plain carbon eutectoid steel (0.83%) which has been subjected toisothermal transformations. (See later section).

Structure/Hardness

STRUCTUREHARDNESS

BHNHARDNESSROCKWELL

C

TEMPERATURE OFTRANSFORMATION

C

Coarsepearlite

170293

531

720660

Fine pearlite 388 41 580

Featherybainite (upper)

401415

4244

500400

Acicularbainite (lower)

555578

5658

280230

Bainite &Martensite

601 60 175

Martensite 682 66 RT


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4.3 Hardenability

Hardenability is a measure of the depth a steel will harden on quenching.

The uniform rapid cooling of a heavy steel section is impossible even with

drastic quenching. Such a section will not harden completely to its corewhereas a thin section would be wholly martensitic. This difficulty canhowever be overcome by the addition to the steel of alloying elements whichwill in general increase the time available to begin and completetransformation. Thus a martensitic structure becomes possible with the lowercooling rates found in heavier sections. This is one of the most importantfunctions of alloying and to ensure the correct application of such steels, fromboth the technical and economic aspects, some measure of hardenabilitybecomes necessary. To determine this, tests have been devised to estimatethe maximum diameter at which the required structures can be produced byquenching. If this diameter is exceeded, hardening at the core will be

incomplete resulting in non-uniform properties.

The Jominy end-quench test (Fig 9) is widely used in evaluating thehardenability of steel. Here a standard test piece is heated to its austeniticregion, dropped into a frame and quenched, at one end only, by a measured jet of water at 25C. Thus different rates of cool ing are obtained along thelength of the bar and the resulting hardness values can be determined. Thedrawings illustrate the test and the graphs show typical hardness values fromthe tests of three steels of differing compositions (Fig 10). It will be noted thatthe depth of hardening increases markedly with growing alloy content eventhough the proportion of carbon at 0.45% is the same in each case. Usingthe Jominy test results as a basis it is possible, for a particular steel, tocalculate a maximum diameter at which uniform properties can be obtained.Such a measure is known as the ideal diameter or ruling section. The idealdiameter here represents the section at which a structure of 50% martensiteis achieved.


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(A) The standard form of test piece used

(B) Diagrammatic representation of the apparatus used in the test

Fig 9 The Jominy End-Quench Test


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Fig 10 The Depth of Hardening of Three Different Steels as indicated bythe Jominy Test


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5. HEAT TREATMENT

5.1 Hardening Mechanisms

It has been calculated that the theoretical strengths of pure metals should be

much greater than those observed. These differences have beenconvincingly explained by dislocation theory.

This theory has been likened to a situation where one carpet lying on top ofanother is very difficult to move by pulling at one end. However a ruck in theupper carpet will move across it very easily. This ruck represents thedislocation moving between slip planes in a metal giving rise to plasticdeformation, which otherwise could not occur. Calculations confirm that thestress required to make the dislocation lines move is in good agreement withthe measured yield stress so that such faults can account for the weakness ofmetals. The dislocation lines finish only when they reach a metal surface or

grain boundary. If dislocation motion is impeded, for example by interactionwith other dislocations, there will usually be an increase in hardness andstrength. It should be emphasised that dislocation theory is much morecomplex than this grossly over-simplified version (Fig 11).

Fig 11 The Movement of a Dislocation During Slip

5.1.1 Solid Solution Hardening

The most common reason for alloying is to increase the yield strength of ametal. This requires the movement of dislocations to be impeded by makingalterations to the structure on an atomic scale. The effectiveness of foreignatoms as barriers to the movement of dislocations depends first upon the sizedifference between the solute atoms and those of the parent metal andsecond upon the proportion of foreign atoms present in the crystal. Elasticstrain fields will be set up around the misfitting atoms making dislocationmotion more difficult.

Interstitial solute atoms such as carbon and nitrogen in body centred cubiccrystals provide an important example of solute hardening alloys. Thus


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carbon or nitrogen dissolved in iron produces a local strain which causes avery steep rise in yield strength with increasing solute. If only a very smallquantity of such a solute is present one effect of this high local strain is thatthe solute element will tend to migrate to the space provided along thedislocations present rather than being uniformly distributed in solution in each

crystal. This has the effect of pinning (holding) the dislocations in place, aphenomenon which is revealed by the presence of a Yield Point when plasticflow sets in during a tensile test.

Substitutional solute addition is the commonest way of solution-hardening ametal. Brass (copper-zinc) and bronze (copper-tin) are two familiar materialswhich make use of this hardening effect.

5.1.2 Precipitation Hardening

Thermal treatment can be used to control the size and distribution of second

phase particles in any alloy which undergoes a phase transformation in thesolid state. Alloy systems which have a phase diagram showing a decreasingsolid solubility limit with decreasing temperature are particularly appropriatefor such treatments and particles of the second phase can often be made toprecipitate in a very finely dispersed form.

In Figure 12, an alloy C exists as a single phase solid solution () at hightemperatures but on slow cooling it becomes supersaturated with respect to

the second phase, which therefore separates out.

The distribution of the phase may be controlled as follows.

The alloy is first solution heat-treated at the high temperature and then rapidlycooled by quenching into water or other cooling fluid. Solid-state diffusion is

suppressed in this way, so that the phase cannot separate and the alloyexists at the low temperature in an unstable supersaturated state. If thetemperature is now increased, so that diffusion can take place at ameasurable rate, the second phase will nucleate and grow.

In alloys of relatively low melting-point, there will be an appreciable diffusionrate of solute atoms at room temperature, so that over a sufficient length of

time, the second phase will precipitate out. This effect is known as 'ageing',but in most alloys the temperature has to be raised in order to causeprecipitation to occur and the material is said to be 'artificially aged'. Therate of growth of the precipitate is controlled by the rate of atomic diffusion, sothat the precipitation increases with increasing ageing temperature. The sizeof the precipitate becomes coarser as the ageing temperature is increased,as shown schematically in Fig 12.

If, at any temperature, the time of heat treatment is very prolonged,coagulation or coarsening of the particles occurs; the small ones tend toredissolve and the large ones to grow at their expense.


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Precipitation hardening is the term now used to describe ageing. Just asheating will accelerate precipitation hardening, refrigeration will impede theprocess.

Fig 12 Variation of precipitate size with ageing temperature


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5.1.3 Work Hardening

Owing to the interaction of dislocations within the grains, plastic deformationwill progressively harden a metal. The dislocations will multiply with strainand their density will continually rise with increasing deformation and thus the

number of interactions per unit volume will rise as the strain continues. Whena polycrystalline metal is stressed, each grain is deformed into a shape that isdictated by the deformation of its neighbours which requires the operation ofseveral slip systems. The following graphs given in Figs 13, 14 and 15 showthe effects of cold work on mechanical properties.

Ultimate tensile strength can be increased by up to 1040 N/mm2

by reducingthe cross-section by cold working by 90%. A reduction in cross-section by10% will reduce percentage elongation from about 30% to 4%. Thereafterthere will be little decrease to about 2%.

Fig 13 Stress-strain curves for metal subsequent to varying degrees ofprior cold-working. It will be observed that cold working brings about anincrease in tensile and yield strengths and a decrease in elongationprior to fracture.


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Fig 14 Effect of cold deformation on the strength and hardness of metal. It willbe seen that the spread between the yield strength and tensile strength curvesbecomes less with greater amounts of deformation and a consequentreduction in ductility.

Fig 15 Stress-strain curves for metal subsequent to varying degrees of priorcold-working. It will be observed that cold working brings about an increase intensile and yield strengths and a decrease in elongation prior to fracture.


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Gain in UTS (N/mm2)

% Reduction in Cross-Section 0.20%C 0.80%C

30

5080

140

280450

200

350700

5.1.4 Strain Ageing

Strain ageing is observed when low carbon steel has been subjected to asmall amount of deformation and then allowed to age for a period. Thisresults in an increase in hardness and strength with reduced ductility and adrastic reduction of toughness as measured by impact tests. Even less than1% cold reduction can produce such ageing which can be attributed largely to

the movement of nitrogen atoms in the steel to sites which allow dislocationsto be pinned in place. The maximum effect is with about 15% reduction.Such ageing proceeds very slowly at room temperatures and may takeseveral months to reach a peak. Temperature increases will however rapidlyincrease the rate of hardening and at 300C this wi ll reach the maximumvalue in a few seconds.

5.2 Isothermal Transformations

So far consideration has been given to very slow cooling, when conditions willapproach equilibrium, or very fast cooling to ambient temperatures.Variations in cooling rates between these extremes are possible and theresults can be shown in isothermal transformation curves, also known as Scurves, or time-temperature-transformation (TTT) curves. These areconstructed by taking a number of specimens of the steel in question, heatingthem into the austenitic range and then quenching them in baths of differenttemperatures. At predetermined time intervals individual specimens aretaken from their baths and quenched in water. They are then examinedmicroscopically to evaluate the extent of the transformation that has occurred.

5.2.1 TTT Curves for Steel

Figure 16 shows TTT Curves for a eutectoid plain carbon steel i.e. 0.83%C.First to be noted is the horizontal broken line at 723C which is thetemperature above which a eutectoid steel will be wholly stable austenite.Below this line the austenite is unstable and the two C shaped curves indicatethe times necessary for the transformation of this austenite to ferrite pluscementite to begin and go to completion following quenching to a particulartemperature.

The horizontal lines at the foot of the TTT diagram, strictly speaking, are notpart of the TTT curves but represent the temperatures at which the formation

of martensite will begin MS, and end MF, during the cooling of unstableaustenite through this range. Unlike the formation of pearlite and bainite the


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transformation of unstable austenite to martensite is not dependent on timeas it occurs almost instantly. The degree to which this change takes place isdetermined simply by the temperature to which the steel is cooled. Once atransformation has gone to completion a steel will usually be water quenchedwith no further change in its constitution.

Fig 16 Time - Temperature - Transformation (TTT) Curves for a PlainCarbon Steel of Eutectoid Composition

5.2.2 Continuous Cooling Transformation Curves

The TTT curves indicate structures which are produced by transformationswhich take place at a fixed temperature and specify a given incubationperiod before the transformation begins. It is important to realise that there

can be no direct connection between such isothermal transformations andthose which take place under continuous cooling at a constant rate from723C to room temperature. Therefore it is not pos sible to super-imposecurves which represent continuous cooling onto a TTT diagram. However,modified TTT curves which are related to continuous cooling can beproduced. They are of a similar shape to the TTT curves but are displaced tothe right.


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5.2.3 The Effects of Alloy Additions on Isothermal Transformations

With the exception of the metal cobalt, all alloy additions to steel will increaseboth the incubation period and the time necessary for transformations atspecific temperatures. In effect this will increase hardenability. These

changes are illustrated in the attached series of TTT diagrams (Fig 17). Itshould be noted that alterations in composition will be accompanied byvariations in the temperatures A1, A3 and A4 and also those of MS and MF.Attention is drawn to the fact that the diagrams are based on a logarithmicscale so that even apparently minor movements of the TTT curves to the rightwill result in major increases in incubation and transformation periods. Thesewill be reflected in the continuous cooling transformation diagramsappropriate to these alloys.

5.3 Normalising

The process refers only to steel and consists of heating the metal to atemperature about 50C above its Upper Critical Poi nt (Ac3 temperature) oraustenitising temperature and having given adequate time for full conversionto Austenite allowing it to cool in still air. On heating, the change from bodycentred to face centred cubic structure will result in small austenite crystalswhose sizes will determine those of the body centred cubic crystals formed oncooling. The process is therefore important in producing a refined grain size.If too high a temperature is employed, grain growth of the austenite will occurwhich will be reflected in the final cooled structure.

5.4 Full Annealing

This process is also applicable only to steel and consists of heating thematerial to the appropriate normalising temperature and then cooling it veryslowly (Fig 18), usually in the furnace. The resulting structure, as withnormalised steel, will consist of ferrite or cementite with grains of pearlite butfull annealed steel will be expected to have a somewhat larger grain size.More important is the effect of slow cooling on the structure of pearlite andthis will become more coarsely lamellar with decreasing cooling rate whichmay even lead to very coarse lamellae balling up into coalesced particles ofcementite in a groundmass of ferrite. Steel subjected to full annealing

treatment will have lower tensile strength, impact strength and yield point buthigher elongation values than those of normalised steel.


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Fig 17 TTT-curves for alloy steels (after US Steel Corp)


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Fig 18

(A) Normalised(B) Fully Annealed(C) Water-quenched(D) Water-quenched and tempered


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5.5 Quenching and Tempering

If a plain carbon steel is sufficiently rapidly cooled from above its upper criticaltemperature, i.e. within the wholly austenitic area, martensite will be formed.This phase becomes harder and more brittle as the carbon content increases

up to the eutectoid value, 0.83%C. The hardness so attained will varysomewhat with the rate of cooling. The upper set of values shown in Fig 19are the result of rapid cooling while those below are of average rates.

Fig 19 The effect of carbon content on hardness of martensite

Occasionally such high martensitic hardnesses are useful for applicationsrequiring good wear resistance but more often they are the essential startingpoints for tempered structures. Thus a steel may be hardened by heating it to30-50 degrees C above its upper critical temperature and quenching it insome medium to produce the desired rate of cooling. In this condition, a toolsteel for example, will be hard and brittle and some treatment to relieveinternal stresses and to reduce brittleness is needed. Tempering will cause

martensite to transform and the higher the temperature the nearer will thestructure revert to the stable pearlitic type appropriate to that temperature.Tempering is always carried out below the lower critical level and when thedesired temperature is reached may be followed by an immediate waterquench, if the alloy is susceptible to temper embrittlement. Rate of coolingfrom the tempering temperature has little other effect.

Up to 200C tempering will provide only some stre ss relief but from 230-400C the martensite will change to form a new cons tituent consisting of avery finely dispersed granular mixture of ferrite and cementite. This phase ismuch tougher than martensite and somewhat softer and less brittle.

Toughness may be defined as the resistance to fracture by impact.Increasing tempering temperatures causes coalescence of the cementite


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particles until at about 400C they become visible at magnifications of about500 times. Subsequent increases of tempering temperature to above 550Cwill cause strength to fall away with no rise in ductility.

The importance of the fact that steels can be heat treated to give marked

changes in properties cannot be over-emphasised. Thus it is possible bycorrect treatment to produce a relatively soft and ductile material that can becold worked and machined to the desired shape and size and then to changeits properties, drastically increasing its hardness and strength, making it farmore suitable for its eventual service life. Compared with the pearlitic andbainitic structures of the same hardness, tempered martensites will havehigher yield, tensile and impact strengths compatible with a high level ofductility. For most purposes therefore they are the most desirable structures.

Tempering and Water Quench

0.40%C Steel As Quenched 200C 350C 450C 600C

Hardness HVTensile N/mm

2

Elongation %Charpy Impact (J)

6701850

511

62017001219

44014201414

37012001630

2208802475

5.6 The importance of grain size

As steel is heated through the critical range it is recrystallised with the

formation of fine grains (Fig 20).

Fig 20 Schematic representation of the effect of temperature and grain-

refining elements on grain size


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These grains tend to grow as the temperature is raised, but the growth ismore or less abrupt at a characteristic temperature - with the formation offirst some coarse and some fine grains and finally all coarse grains, asindicated above. Prolonged heating also tends to increase the grain size.

Each steel has a characteristic coarsening-temperature range and if thattemperature is below the maximum commonly used for hardening, the steel isclassed as coarse grained. If the steel is treated with effective amounts ofaluminium, niobium, vanadium or titanium, or a combination of theseelements, it does not coarsen at the usual hardening temperatures and isclassed as fine grained.

Coarse-grained steels (ASTM grain size 1-5) have some advantages in easiermachining, higher creep strength at elevated temperatures, greaterhardenability, and possibly less notch sensitivity in fatigue, but they have

lower notched-bar impact strength and the impact resistance falls off rapidlyat subnormal temperatures. Fine-grained steels (ASTM grain size 5-8 plus)have much better toughness, and virtually all heat-treating alloy steels used inmachine construction are treated during melting so that they will be finegrained. Steels that develop a mixture of coarse and fine grains tend to giveerratic properties, and are unlikely to be used.

The grain-coarsening temperature of fine-grained steels varies somewhat butcan be expected to be in the neighbourhood of 980C for aluminium-treatedsteel. This temperature may, however, vary widely with the composition andprior treatment. The coarsening temperature of high-carbon steels tends to

be relatively low but grain growth may be restrained by undissolved carbides.

5.7 Grain Size Determination

Grain size is commonly measured according to ASTM Standard Method E112, Determining The Average Grain Size. In materials having two or moreconstituents, the grain size usually refers to that of the matrix. Minorconstituent phases, inclusions, and additives are not normally considered.

It is important in using these methods to recognise that the measurement ofgrain size is not precise, but an estimate. A metal grain is a three-

dimensional shape of varying sizes. The grain cross section produced by arandom plane (surface of observation) is dependent upon where the planecuts each individual grain. Thus, no two fields of observation can be exactlyequal.


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Of the three methods listed in the standard, the comparison procedure is mostpopular since it takes the least time to carry out. This method involves viewinggrains in a microscope and comparing them at the same magnification, 75X or100X, to charts defined in ASTM E112, with two examples shown in Figure 21.The ASTM Grain Size Number corresponds to a certain number of grains/in

2

according to Table below:

ASTM No

0 1 2 3 4 5 6 7 8 9 10

Grains/in2

0.5 1 2 4 8 16 32 64 128 256 512

The relationship between the Grain Size Number and the number of grains/in2

is given by the expression:

Where:

N = ASTM Grain Size Number n = 2(N 1)

n = number of grains/in

2at the specified magnification

Fig.21 Examples of ASTME E 112 Comparison procedure grain sizecharts for No. 5 and 8

Thus they may be added to improve mechanical properties. They may beused to enhance resistance to corrosion or high temperature oxidation.Further, they may be present to develop special characteristics such as thoseof an electrical or magnetic nature, strength at high temperatures or for thesteel to remain austenitic at room temperatures.


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6. ALLOYING

6.1 Alloying Elements in Steel

The principal function of alloying elements in steel is to improve its properties.

Thus they may be added to improve mechanical properties. They may beused to enhance resistance to corrosion or high temperature oxidation.Further, they may be present to develop special characteristics such as thoseof an electrical or magnetic nature, strength at high temperatures or for thesteel to remain austenitic at room temperatures.

The alloying elements added may either simply dissolve in the ferrite or theymay combine with some of the carbon, forming carbides.

The principal effects which these alloying elements have on themicrostructure and properties of a steel include those on the allotropic

transformation temperatures.

Some elements, notably nickel, manganese, cobalt and copper, raise the A4temperature and lower the A3 temperature, as shown in Figure 22 (A). In thisway these elements, when added to a carbon steel, tend to stabilise austeniteand increase the range of temperature over which austenite can exist as astable phase. Other elements, the most important of which includechromium, tungsten, vanadium, molybdenum, aluminium and silicon, have

the reverse effect, in that they tend to stabilise ferrite by raising the A3temperature and lowering the A4, as indicated in Figure 22 (B). Such

elements restrict the field over which austenite may exist, and thus form whatis often called a loop.

The elements of the -stabilising group generally have a face-centred cubiclattice. Since this is the same as austenite, these elements will retard thetransformation of austenite to ferrite. At the same time these elements retardthe precipitation of carbides, and again this has the effect of stabilisingaustenite. The a-stabilising elements are usually those with a body-centredcubic lattice. These will dissolve more readily in ferrite than in austenite, andat the same time diminish the solubility of carbon in austenite. In this waythey stabilise ferrite. As shown in Figure B, progressive increase in one or

more of the -stabilising elements will cause a point to be reached beyondthe confines of the -loop, where the austenite cannot exist at anytemperature.


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Fig 22 Relative effects of the addition of an alloying element on the allotropictransformation temperature at A3 and A4

(A) Tending to stabilise (B) Tending to stabilise

6.1.1 Carbide or Graphite Formers (Fig 23)

Some alloying elements form very stable carbides when added to a plaincarbon steel. This generally has as hardening effect especially when the

carbides formed are harder than iron carbide. Such elements includechromium, tungsten, molybdenum, titanium and manganese. When one ormore of these elements is present, a structure containing complex carbides isoften formed.


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Fig 23 The condition in which alloying elements are present in steel

Other elements have a graphitising effect on the iron carbide; that is, theytend to make it unstable so that it breaks up, releasing free graphitic carbon.This effect is more evident if no carbide stabilisers are present. Elementswhich tend to cause graphitisation include silicon, nickel and aluminium.

Therefore, if it is necessary to add appreciable amounts of them to a steel, itcan be done only when the carbon content is extremely low. Alternatively, ifthe carbon content needs to be high, one or more of the elements of the firstgroup, namely the carbide stabilisers, must be added in order to counteractthe effects of the graphitising element.

6.1.2. Crystal Growth

The rate of crystal growth is accelerated, particularly at high temperatures, bythe presence of some alloy additions, notably chromium. Care must therefore

be taken that steels containing elements in this category are not overheatedor, indeed, kept for too long at an elevated temperature, or brittleness, whichis usually associated with a coarse grain size, will result.

Fortunately, grain growth is retarded by other elements, notably niobium andvanadium, whose presence thus produce a steel which is less sensitive to thetemperature conditions of heat-treatment.


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6.1.3 Corrosion Resistance

The corrosion-resistance of steels is substantially improved by the addition ofaluminium, silicon and chromium. These metals form thin but dense andadherent oxide films which protect the surface of the steel from further attack.

Of the elements mentioned chromium is the most useful when mechanicalproperties have to be considered. When nickel also is added in sufficientquantities, the austenitic structure is maintained at room temperature.

6.1.4 Strength

One of the main reasons for alloying is to effect improvements in themechanical properties of steel. These improvements are generally the resultof physical changes already referred to. For example, hardness is increasedby stabilising the carbides; strength is increased when alloying elementsdissolve in the ferrite; and toughness is improved by refinement of the grain

size.

6.2 The Effects of Alloying on Steel Strength and Hardenability

6.2.1 Carbon

It has been shown that increasing carbon content in plain carbon steels willlead to increases in hardness and, up to the eutectoid composition (0.83%C)increases the tensile and yield strengths. Corresponding to these changeswill be decreases in ductility, malleability and impact strength. Increasing the

carbon content of martensitic steels may be expected to show increases inhardness up to the eutectoid point. Tempered martensitic structures will showincreases in hardness and brittleness up to about 1.4%C. Hardenability inplain carbon steels is at a maximum at the eutectoid composition. Increasingor decreasing the carbon content from the eutectoid composition will lead tolower hardenability, i.e. it becomes increasingly more difficult to obtainmartensitic structures, and increasingly rapid quenching is required.

6.2.2 Manganese

Some manganese is present in nearly all steels. It is usually below 1% and it

is only when this amount is exceeded that it is regarded as a deliberatelyadded alloying element. Like nickel, manganese stabilises austenite butunlike nickel it also has the effect of stabilising the carbides, by itself formingMn3C. It has a considerable strengthening effect on ferrite and increaseshardenability.


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6.2.3 Chromium

Very small amounts of chromium when added to carbon steel will cause aconsiderable increase in hardness. At the same time strength is raised with

some loss of ductility which is not apparent with less than 1.0%Cr. Theincrease in hardness is mainly due to the fact that chromium is a carbidestabiliser forming hard carbides itself or double carbides with Fe3C.Chromium is a ferrite stabiliser and austenite may be eliminated entirely whenmore than 11%Cr is added to pure iron.

The main disadvantages in the use of chromium as an alloying element is itstendency to promote grain growth, with its attendant brittleness. Care musttherefore be taken to avoid overheating or holding for too long at the normalheat-treatment temperature.

Steels containing small amounts of chromium and up to 0.45% carbon areused for axle shafts, connecting-rods and gears; whilst those containing morethan 1.0% carbon are extremely hard and are useful for the manufacture ofball-bearings, drawing dies and parts for grinding machines.

Chromium is also added in larger amounts - up to 21% - and has apronounced effect in improving corrosion-resistance, due to the protectivelayer of oxide formed. This oxide layer is extremely thin, and these steelstake a very high polish. They contain little or no carbon and are thereforecompletely ferritic and non-hardening (except by cold-work).

Stainless steels which have only chromium as the main alloying element andhave C levels that exceed 0.1% are of the martensitic type, the structurebeing obtained by rapid cooling. If these steels are allowed to cool slowly,carbides will be precipitated, with consequent loss in corrosion-resistance(sensitisation).

6.2.4 Nickel

The addition of nickel to a plain carbon steel tends to stabilise the austenitephase.

For example, the addition of 25% nickel to pure iron renders it austenitic, andso non-magnetic, even after slow cooling to room temperature.

Nickel makes the carbides unstable and tends to cause them to decomposeto graphite. It is therefore inadvisable to add nickel by itself to high carbonsteels and most nickel steels are of the low carbon type. Generally the 3.5%nickel steels are the most widely employed, those with about 0.12%C beingused mainly for case hardening, while the 0.30%C types are used forstructural purposes and in engines for shafting and axles etc. Nickel also hasa grain refining effect enabling the alloys to be employed in case hardening

as grain growth will be limited during prolonged treatment at about 900C.


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Nickel was one of the first elements added to plain carbon steels, its mainadvantage being to increase tensile strengths without adversely affectingductility. Its effect of lowering critical temperatures for heat treatment is alsoan advantage in cost reduction.

Nickel in moderate amounts also increases hardenability allowing slowerquench rates than in plain carbon steels.

6.2.5 Molybdenum

Molybdenum is a strong carbide stabiliser and relatively small amounts willmarkedly enhance the properties of plain carbon steels. Usually it is added inquantities ranging from 0.12-0.65% depending on the intended application ofthe steel. More often it is used in combination with manganese, nickel and/orchromium to give a noticeable improvement in properties. Hardenability isincreased especially when used in conjunction with chromium. One of the

metal's advantages is that it will reduce the tendency to temper brittleness inlow nickel low chromium steels when heat treated at 250-400C. In generalNi-Cr-Mo alloy steels possess the best all-round combination of properties,especially when high tensile strength and good ductility are required in largecomponents. Such steels are relatively free from the mass effects of heattreatment, the transformation rates of the Ni-Cr steels being even furtherreduced by the presence of molybdenum which contributes considerably tohardenability. Its presence will also raise the high temperature strength andcreep resistance of high temperature alloys and it is added to stainless steelsin proportions of up to 3.0% to improve corrosion resistance especially inchlorides and acids.

The disadvantages of molybdenum steels are that they require highertempering temperatures to obtain properties comparable to plain carbonsteels and they need a longer holding period at quenching and normalisingtemperature to ensure complete solid solution of the molybdenum ironcarbide.

6.2.6 Niobium (US Columbium)

Plain low carbon steels for structural purposes benefit greatly from small

additions of niobium and quantities as low as 0.02-0.04% will increase YieldPoint by 62-110N/mm2

and Ultimate Tensile Strength 54-85N/mm2. This will

give a higher Yield to Ultimate Tensile Strength ratio than in plain carbonsteels. Above 0.04%Nb this strengthening effect falls away rapidly.

Niobium is a strong carbide former but its effect on low carbon steels isthought to be mainly by precipitation hardening. It has a marked grainrefining function sometimes reinforced by aluminium and/or vanadiumadditions. Such refined grain structures give high impact values.

Niobium as a very strong carbide stabiliser, is also added to some stainless

corrosion resisting steels (type 347) of the 18/8 chromium-nickel variety toprevent weld decay sensitivity.


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6.2.7 Vanadium

Plain vanadium steels are manufactured to a very limited extent, butchromium-vanadium steels containing up to 0.2% vanadium are widely usedfor small and medium sections. The mechanical properties resemble those of

nickel-chromium steels, but usually show an advantage in respect of the limitof proportionality and percentage reduction in area. Chromium-vanadiumsteels are also easier to forge, stamp and machine, but are more susceptibleto mass effects of heat-treatment than the corresponding nickel-chromiumsteels.

Vanadium has a strong carbide-forming tendency. It also stabilisesmartensite and low temperature tempered martensite on heat-treatment andincreases hardenability. Like nickel, it restrains grain growth of the austenite.One of the most important effects of vanadium is that it induces resistance tosoftening at high temperatures provided that the steel is first heat-treated to

absorb some of the vanadium carbide into solid solution. Consequentlyvanadium steels are used for hot-forging dies, extrusion dies, die-casting diesand other tools operating at elevated temperatures.

6.2.8 Aluminium

The presence of aluminium in plain carbon and low alloy steels will tend tostabilise ferrite but as it will normally be in very low concentrations (0.015%) which may reduce hardenabilitysomewhat although improving tensile and impact strengths. It is widely usedas a deoxidant in low carbon mild steels where ductility is the principal quality.In such steels the presence of small amounts of nitrogen introduced duringsteel making will cause strain ageing to the detriment of their properties. Asmall amount, up to about 0.08%, of residual aluminium by forming aluminiumnitride will suppress this phenomenon in cold rolled annealed temperedgrades. In offshore structures a Al to N ratio of at least 2:1 is sometimesspecified to overcome strain ageing during welding.

6.2.9 Sulphur

The element sulphur will tend to decrease hardenability somewhat, but as,with a few exceptions, considerable efforts are made to eliminate it from plaincarbon and alloy steels, it is usually present only in small amounts and itseffect in this respect may be neglected.

It is the most deleterious impurity commonly present in steel. If precautionsare not taken to render it harmless it will form the brittle sulphide, FeS. This issoluble in molten steel, but when solidification takes place the solid solubilityfalls to an equivalent of 0.03% sulphur. If the effects of extensive coring arealso taken into account amounts as low as 0.01% sulphur may causeprecipitation of the sulphide at the grain boundaries. In this way the austenite

grains will become virtually coated with brittle films of ferrous sulphide. Sincethis sulphide has a fairly low melting point, the steel will tend to crumble


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during hot-working. Being brittle at ordinary temperatures, ferrous sulphidewill also render steel unsuitable for cold-working processes, or, indeed, forsubsequent service of any type.

The sulphur is rendered harmless by the presence of manganese with which

it combines preferentially to form manganese sulphide MnS. This MnS isinsoluble in molten steel and some will be lost in the slag while the rest will bepresent as globules distributed throughout the steel and not associated withthe structure when solidification takes place. These globules are plastic at hotworking temperatures and the tendency of the steel to crumble is thusremoved while the globules will be rolled out as threads in subsequentoperations.

With modern steel making practices levels of


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6.2.11 Silicon

Silicon is widely used as a deoxidant in steelmaking and it is often present inquantities of 0.2-0.5%. Like nickel it encourages graphite formation and itmust therefore be kept low in high carbon steels. It dissolves in ferrite where

it has a substantial strengthening effect. Hardenability is only moderatelyaffected by the element. Silicon is included in some heat resisting steels inamounts up to 1.5% as it aids high temperature resistance to oxidation.

6.3 Alloying Additions in Steel - Summary

The effects of individual alloying elements are shown in tabular form below:

AlloyingElement

Effect onStrengthening

Ferrite

Effect inForming

Carbides

Effect onTransformation

Temperatures

Effect onHardenability

Manganese Strong Weak Lowers Strong

Silicon Strong None Raises Moderate

Phosphorus Strong None Lowers Moderate

Nickel Moderate None Lowers Moderate

Chromium Weak Moderate * Strong

Copper Moderate None Lowers Weak

Molybdenum Strong Strong Raises Strong

Vanadium Weak Strong Raises Mild

Tungsten Moderate Strong Raises Moderate

* Raises or lowers depending on carbon content

1. Elements which tend to form carbides - Cr, W, Ti, V, Nb, Mo, Mn.

2. Elements which tend to graphitise the carbide - Si, Co, Al, Ni. Elementsfrom category 1. should be present to avoid graphitisation when smalladditions of the above are exceeded.

3. Elements which tend to stabilise the Austenite - Mn, Ni, Co, Cu.

4. Elements which tend to stabilise the Ferrite - Cr, W, Mo, V, Si.

The Austenite phase may disappear when a closed loop is formed. If no

carbon is present in the alloy the loop will disappear when the followingelements are present in these approximate percentages.

Cr 12.8%, S 2.0%, W 6.0%, P 0.5%, Al 1.1%, Ti 0.75%, Mo 4.0%


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7. MICROSTRUCTURES IN STEEL ARC WELDS

7.1 Weld Metal

The large number of variables inherent in welding such as the welding process

itself, the final composition of the melt, welding speed and thermal cycle make itunrealistic even to attempt to predict with any precision the microstructures thatcan arise. Nevertheless, some general points can be made. In the weldingoperation itself a pool of molten metal is retained within the work piece. Thismay be likened to molten metal within an ingot mould, and on freezing, largecolumnar crystals will form and since the weld pool is small, and in the case ofa single arc weld, solidification rapid, they will usually occupy the completecross-section of weld metal. This metal has a characteristic cellular-dendriticstructure of cast metal consisting of the rather coarse columnar austeniticgrains which curve into the weld centre line and a fine cellular network withinthe grains. The transformation products resulting from the decomposition of

these grains is dependant mainly on the rate of cooling of the weld metal.

Although it has been pointed out that accurate predictions of weld metalstructure are not possible a schematic CCT (Continuous Cooling Time) (Fig 24)in which general changes arising with differing cooling rates and the effects ofheat input, alloying elements and slag inclusions are related in a qualitativeway. Thus C, N, Mn etc tend to move the diagram to the right to give more timefor transformations. If cooling curves are superimposed on such a diagram itwill be clear that rapid cooling rate curves will approach the vertical whereasslow cooling rate curves will approach the horizontal. All phases intersected bysuch curves will be present in the final structure.

It may be expected that the strength and hardness of welds would be generallyhigh as rapid cooling rates tend to promote lower temperatures oftransformation and there are usually large quantities of impurities and alloyingelements. On the other hand the impact properties and toughness of suchsteel welds can be impaired especially at the weld centre line which is the areaof maximum segregation where the columnar crystals meet. To this must beadded the deleterious effects of the coarse columnar structure with mixtures oftransformation products which may result in the precipitation of carbides whichon impact tend to rupture to produce brittle fractures of the pro-eutectoid ferrite.


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Fig 24 Continuous Cooling Time. Schematic CCT diagram for steel weldmetal, summarising the possible effect of microstructure and alloying onthe transformation products for a given weld cooling time

In a plain carbon steel of 0.15-0.20%C with slow cooling and small under-cooling below A3, a structure of blocky ferrite with pearlite will be formed. Amedium-slow cooling rate with larger undercooling below A3 will result in a formof ferrite known as Pro-eutectoid ferrite and on further cooling the production

of ferrite needles known as Widmanstatten side plates and the remainingaustenite at even lower temperatures transforming into pearlite and cementite.Another possible phase in such weld metals at undercooling near A1 isAcicular Ferrite the transformation to which is enhanced by the presence ofstrong carbide formers such as Mo or Cr. The presence of acicular ferrite givesimproved toughness to such welds.

The acicular ferrite is formed intragranularly resulting in randomly orientatedshort ferrite needles with a basket weave feature. This interlocking naturetogether with the fine grain size provides maximum resistance to crackpropagation by cleavage. Notch toughness increases with increasing volume

fraction of acicular ferrite in the weld metal. The formation of either grainboundary ferrite, ferrite side plates or upper bainite is detrimental to weld metaltoughness since these microstructures provide easy crack propagation paths.

Medium high cooling rates result in the transformation occurring below A1 with afast initial ferrite growth resulting in rapid carbide concentration at the austenite-ferrite interface causing the precipitation of cementite which is again followed byvery rapid ferrite growth with the same consequences. This is known as thePeriodic Pearlite reaction. A fast cooling rate results in the formation of thephase Upper Bainite, while a very fast cooling rate may produce LowerBainite or Lath martensite.


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7.2. Heat Affected Zone (HAZ)

To be satisfactory, dilution of a weld metal must occur by melting part of thebase metal which in the region of the melt will be exposed to similar hightemperatures. Subsequently, conditions of rapid cooling under conditions of

severe restraint will be imposed and as a result of this thermal cycle the originalmicrostructure and properties of the metal close to the weld will be changed.This region is normally referred to as the HAZ.

The HAZ can be conveniently divided into a series of sub-zones and illustratedin fig 25 are those of a 0.15%C transformable steel.

Fig 25 Various regions of the HAZ in a single pass weld

Changes taking place in the Solid-liquid transition zone depend largely on the

composition of the metal and whether the austenite melts directly or istransformed to delta ferrite. In the latter case boundary segregation will resultin reduced grain growth and a layer of small crystals, but in the former there willbe no such changes. In the grain-coarsened HAZ pro-eutectoid ferritenetworks feature prominently for lower carbon equivalent x 2 grades but withhigher steels this will be reduced in favour of lower temperature transformationproducts such as Widmanstatten side plates with possibly some martensitemainly in the mid-grains. In the grain refined HAZ the structure tends to be finegrained ferrite-pearlite. The intercritical HAZ can yield a wide range of probablestructures, depending on the cooling rate, and may be pearlite, upper bainite,autotempered martensite or high carbon martensite. The subcritical HAZ

produces its most notable change in the degradation of pearlite to spheroidalparticles of cementite. The zone of unaffected base material at lowertemperatures may result in embrittlement of the structure caused by dynamicstrain ageing.

The production of high strength structural materials is mainly based ondeveloping a product with as small a grain size as possible but the severity ofthe weld thermal cycle is such that the structure is completely modified near theweld. In high energy processing such as submerged arc it is not uncommon forthe grain size to be increased tenfold or more in the zone closest to the fusionline. In steels of high Carbon equivalent (above 0.40) it is likely that the graingrowth zone will contain martensite. Thus the microstructure of the grain


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growth zone more than other zones in the HAZ will determine the properties ofthe weld.

The thermal cycle that is to say the heating time, the time at temperatures ordwell time, and the cooling time are important and must be considered together

with the presence of precipitates and their solubility at high temperatures.Moreover the previous thermal and mechanical history is also important. Thusfor example the original steel may have been in the cold rolled condition, it mayhave been annealed or normalised or subjected to other treatments. Withthese aspects in mind the sub-zones may be considered separately since eachone will have a different type of microstructure with appropriately differentproperties. Notwithstanding the complexities involved such structure can bepredicted with greater certainty then those arising in the weld metal.

7.3 Welding Heating Cycle

The importance of the welding heating cycle is not always fully recognised. Thetemperature increases can be 200-300C/sec and thus to transformationswill be above that of equilibrium and the phase will be substantially superheatedbefore the change occurs. The degree of superheating will be affected by thewelding process and the thermal cycle is such that in most metals some graingrowth will occur in the HAZ which not only affects strength and toughness butalso influences grain size in the weld metal. In most steels the presence ofcarbide or nitride particles tends to hinder grain growth and it has been shownthat such growth occurs predominantly at temperatures above the equilibrium-solubility limits of such particles. Also it has been observed that most grain

growth occurs during the heating part of the thermal cycle and in some steelsexperiments indicate that only about 20% of growth occurs in the cooling cycle.

In general some degree of grain growth control can be exerted either by limitingthe duration of the weld thermal cycle or by precipitate pinning, that is, mainlyby Carbides and Nitrides in steels. The best way to control grain size is to uselarge quantities of very small precipitates but this is not always easy to achieve.

The employment of multi-run welds will result in a refinement of microstructureand improvements in toughness with reductions in residual stress whencompared with a single run of the same cross-section.


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7.4 Carbon Equivalent (Ceq)

Weldable structural steels are normally capable of being welded without theneed for any pre-heat because the fectors which cause high hardness and thelikely presence of martensite would be absent in such welds. These

characteristics are usually expressed by reference to the Carbon Equivalentvalue. This is assessed in terms of how the alloying elements affect thetransformation characteristics including the martensite start temperature Ms ofthe steel. An empirical formula has been devised for Carbon Equivalent (CE)and adopted by the International Institute of Welding as follows:

A weldable structural steel will normally have a CE value not greater than0.40%. Other formulae such as PCM used in Japan are also available. It shouldbe realised that such formulae and the values calculated from them areprincipally guides and circumstances may arise when departures from them arejustified.

Essentially a Carbon Equivalent is a measure of the hardenability of a steel.


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8. STRUCTURE OF INGOTS

When molten metal is poured into a metal ingot mould, solidification will usuallybegin with the formation of small chill crystals at the mould-metal interface (Fig26). These are followed by long columnar crystals growing at right angles to

the mould surface. The size of these crystals will depend on the rate ofsolidification and they will be small if this is rapid, increasing in size with slowerrates. With very slow solidification, as with a sand mould for example, thecolumnar and chill crystals will not generally form but dendrites will develop intoirregular equi-axed grains. Where columnar crystals have formed, planes ofweakness can be expected at sharp corners and in extreme cases even at theingot centre.

Fig 26 Ingot Solidification

8.1 Pipe

As a layer of cooled metal forms around the ingot walls, contraction occursresulting in a fall of the level of liquid. Successive layers of solid form, eachaccompanied by a fall in the liquid level, the fall increasing as the volume ofliquid decreases. In this manner a central cavity is formed, known as PrimaryPipe. With the ingot mould designed narrow end up (Fig 27) a conicalvolume of metal still remains liquid after the top portion of the ingot is solid.Solidification of this metal will give rise to further cavities known as SecondaryPipe.


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Fig 27 Effects of Ingot Taper and Hot Top/Feeder Head

This can be prevented by using a mould wide end up when shrinkage will becounteracted by molten metal feeding down from the top. The primary pipe canalso be reduced by using a brick-lined top on the mould - called a hot top orfeeder head, in which the metal will remain molten for a prolonged period actingas a reservoir of molten metal, which will consequently feed the ingot.

8.2 Segregation (fig.28)

Molten steel contains soluble impurities - sulphur and phosphorus - and solublealloying elements together with insoluble impurities or slag particles insuspension. The first crystals to separate contain less impurity than the averagecomposition and those elements which lower the freezing point, such assulphur, phosphorus, carbon, silicon and manganese collect in the last portionsto solidify. This phenomenon is known as segregation and in the case of steelingots means that there is a concentration of sulphur, phosphorus and carbonin the centre and upper portions of the ingot. Associated with the pipe they are

largely removed when it is discarded.

Segregation also occurs as the microscopic scale. Impurities and solublealloying elements are rejected by the solidifying crystal with the result that analloy rich area surrounds each crystal. This can be clearly seen in carbon steelcastings, the original crystal lower in alloy content tends to be ferritic whilst themore highly alloyed crystal surround is pearlitic.


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Fig 28 Segregation in Steel

8.3 Steel Types

Steel, manufactured from impure pig-iron or scrap, is first purified to a greatextent by a process of oxidation which inevitably leaves the molten materialwith excessive oxygen which must be reduced to render it suitable for casting toan ingot. The steel type is defined by the degree of deoxidation.

8.3.1 Rimming Steel (Fig 29)

The structure of this type of ingot is characterised by an outer envelope or rimof solid, comparatively pure steel, with the inner core of less solid or pure

character. Such ingots are made from steel of an effervescing nature, i.e. steelin which deoxidation either in the ladle or in the mould has been intentionallylimited with the result that a free evolution of gas progressively takes placeduring freezing. The evolution of gas towards the centre of the ingot producesglobular blowholes which counteract cooling and solidification shrinkage andare subsequently welded together during rolling. The thickness of the rim andthe proportion of blowholes will both decrease with increasing carbon content.This type of steel improves the yield of ingot to finished product and will providerolled material of good surface finish especially important in applications for lowcarbon grades such as cold rolled sheets and strip. Rimming steel is notusually available with carbon contents exceeding 0.35% and is not acceptablefor critical structural applications.


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Fig 29 Rimming Steel

8.3.2 Balanced or Semi-killed steel

A balanced steel is a non-piping steel in which no observable gas evolutiontakes place but where sufficient gas is formed during solidification to balance oroffset normal shrinkage. Numerous blow-holes are produced so the centralshrinkage cavity or pipe is prevented or considerably reduced in extent.Balanced steels are only partially deoxidised and small additions of aluminiumare made to the moulds, if necessary, to reduce the oxygen content to thatamount which will give an ingot with a flat or slightly bulging top.

8.3.3 Killed steel

Killed steel is steel that has been completely deoxidised by additions ofmanganese, silicon and sometimes aluminium, before casting, so that there ispractically no evolution of gas during solidification and sound ingots areobtained. The shrinkage cavity or pipe is limited to upper portions of the ingotor in the feeder head with which such ingots are usually provided. Steel usedfor continuous casting is always fully killed.

Metallurgy Into Duct Ion

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