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Introduction

Steels are undoubtedly the most successful group of engineering materials. Theyenjoy widespread use throughout most industries and can attain an unrivalled rangeof affordable mechanical properties. The current world production of steel is about780 million tonnes per annum, and is predicted to rise to about 930 million tonnes bythe year 2000.

The Profile concentrates upon the mass-market engineering steels, and indicates thegeneral effects of adding various amounts of carbon and other elements to iron.Steels contain carbon in amounts ranging from very small (about 0.005 wt% in ultra-low carbon, vacuum degassed steels), to a maximum of 2.00 wt% in the highestcarbon tool steels. Iron containing over 2.00 wt% carbon is classified as cast ironand will be dealt with in a future Profile. Carbon profoundly alters the microstructureand properties in steel. Generally, carbon content is kept low in steels that requirehigh ductility, high toughness and good weldability, but is maintained at higher levelswhen high strength, high hardness, fatigue resistance and wear resistance arerequired.

The performance of a steel component can be as highly dependent upon its thermaltreatment as its alloying additions. This review refers to thermal treatment and coldworking effects but does not attempt to explain the theories in great detail. Theinterested reader is referred to the section Useful Texts .

Structure of Steel

The component elements in steel can be categorised in terms of their crystalstructures. At least a basic knowledge of the practical implications of these crystalarrangements is essential to understand the performance of steel in service. Thestructures are dependent upon the concentrations of each element, the fashion inwhich the steel is cooled from furnace temperatures, and the amount of cold workperformed on the steel.

Crystal Structures

Ferrite, α, is the crystal arrangement for pure iron. This form exists as part of thestructure in most steels and can usefully absorb carbides of iron and other metals bydiffusion in the solid state. Ferrite takes a body centred cubic (bcc) form and is softand ductile.

CORROSION

Austenite, γ, is a solid solution, that is, the component elements are arranged as if insolution (it also exists as an allotrope of pure iron). All steel exists in this form atsufficiently high temperatures (see Figure 1). Some alloy steels stabilise thissingular phase and it is present even at room temperatures. The crystalarrangement is face centred cubic (fcc) and, like ferrite, it is soft and ductile.

Cementite is iron carbide (Fe3C). When carbon atoms can no longer beaccommodated in solution in ferrrite and austenite (due to an increase in carboncontent or reduction in temperature), cementite forms, as it can accommodate morecarbon in its crystal structure. Like other carbides, it is hard and brittle.

Pearlite is a phase mixture consisting of alternating platelets of ferrite and cementite(α+ Fe3C) which grows by conversion from austenite. A steel containing 0.77 wt%carbon can consist solely of pearlite if cooled sufficiently slowly from austenite (seeFigure 1).

Under the microscope it can have an iridescent mother-of-pearl appearance, hencethe name.

Fig.1: Part of the equilibrium phase diagram for the Fe-C system

Martensite is commonly found in steel that has been rapidly cooled ( quenched )from austenite (see ‘Thermal Treatment’ below). It is a particularly hard, brittlearrangement. Essentially it forms because any carbon in solid solution in austeniteat high temperatures does not have enough time to be incorporated into cementitewhen cooled rapidly. The austenite crystals undergo a transformation involving theshearing of atom planes over each other. Martensite does not appear on the phasediagram (Figure 1) as it is not an equilibrium phase. The strain energy involved inthe martensitic reaction is enormous and a large undercooling is necessary. In lowand medium carbon alloys, the martensite tends to form in lath-shaped crystals thatare generally too fine to resolve in the light microscope. In high carbon steels, plate

martensite forms. For certain steels, the rapid cooling necessary to produce amartensitic structure (e.g. water or brine baths) introduces large surface tensilestresses and may cause quench cracking. However, when medium carbon steelsare alloyed with elements such as nickel, chromium and molybdenum, thedevelopment of equilibrium phases is suppressed and martensite can be formed withless drastic cooling, such as oil quenching.

If the steel is cooled such that the formation of pearlite by the short range diffusion ofiron atoms is not possible, bainite can be produced. The bainite that forms attemperatures just below those at which pearlite forms is termed upper bainite (seeFigure 2). At lower temperatures, lower bainite forms. Both lower and upper bainiteconsist of aggregates of platelets or laths of ferrite, separated by regions of residualphases consisting of untransformed austenite or of phases such as martensite orcementite.

Thermal Treatment

Conditioning of steel by thermal or heat treatment relies on the different mechanicalproperties which are exhibited by the structures above. Figure 1 illustrates theequilibrium structures present at different temperatures with changing carbon contentfor the iron-carbon system. Figure 2 demonstrates the effect of chill rate upon finalstructure, and is called a time-temperature-transformation or TTT diagram.Essentially, when cooling from the melt or high temperature phases, there is anincubation period below the equilibrium melting point or transformation temperature(7230C in the case of the steel shown) before the transformation occurs. Thisundercooling provides the driving force for the transformation. During a furnace cool(i.e. slow cooling rate) the austenite will start to transform to ferrite and cementiteafter sufficient undercooling, resulting in a microstructure of coarse pearlite. With ahigh cooling rate, such as experienced with a water quench, it is possible to miss thenose of the TTT curve altogether. Martensite is produced, starting at about 2200Cfor the composition shown. The finish temperature of the martensite reaction forcertain alloys can be below room temperature, so that at room temperature someunstable austenite is present.

Fig.2: The TTT diagram for AISI 1080 steel(0.79%C, 0.76%Mn) austenitised at 9000C

The design of steels and cooling conditions to produce required amounts ofmartensite has its own branch of technology, hardenability . In plain carbon steels,the nose of the TTT curve occurs at very short times, hence fast cooling rates arerequired to produce martensite. In thin sections of steel, a rapid quench can producedistortion and cracking. In thick plain carbon steels, it is not possible to produce anall martensitic structure. All common alloying elements shift the nose of the TTTdiagram to longer times, thus allowing the development of martensite in thicksections at slower cooling rates. Thermal treatment of steels will be dealt with in afuture Profile.

Work Hardening

Resistance to continuing plastic flow as a metal is worked is termed workhardening . When work is performed below hot-working temperatures (i.e. belowabout 0.5Tm, where Tm is the melting point), and the crystal structure is forced todeform to accommodate the strain, microscopic shearing (or slip) occurs alongdefinite crystalline planes. Discontinuities in the crystal structure, present in allmetals and known as dislocations, increase in density during plastic flow and thosemoving on intersecting slip planes tangle and pile up. This means that an ever-increasing shear stress is required for deformation, increasing the yield stress.Eventually the stress required to move dislocations is high enough for a crack to

initiate and subsequently propagate, and the material breaks. Fig.3 demonstratesthe effect of work hardening during a tensile test.

Fig.3: Loading and unloading cycles in a tensile testdemonstrating work hardening

Most steels with appreciable alloy content possess a complex crystal structureresulting in numerous potential slip planes and intersection points, consequentlymost engineering steels are highly susceptible to work hardening.

Work hardening improves tensile strength, yield strength and hardness at theexpense of reduced ductility (see Table 1). These effects can only be removed byannealing or normalising.

Table 1: The effect of heat treatment and condition on theproperties of plain carbon steels

Plain Carbon Steels

Carbon steels are supplied in the as-rolled, normalised, or hardened and temperedcondition, with the best properties developed by hardening and tempering. Theeffect of carbon content on the tensile strength, elongation to failure and hardness ofannealed plain carbon steel is shown in Fig.4.

Fig.4: The effect of carbon content and heat treatment on thetypical properties of plain carbon steel

Very low carbon content (up to 0.05%C)These steels are ductile and have properties similar to iron itself, they cannot bemodified by heat treatment. They are cheap, but engineering applications arerestricted to non critical components and general panelling and fabrication work.

Low carbon content (0.05% to 0.2%C) e.g. 080M15, 150M19, 220M07, AISI 1006,AISI 1009, AISI 1020Such steel cannot be effectively heat treated, consequently there are usually noproblems associated with heat affected zones in welding. Batches which are free oftramp elements such as chromium are ductile with good forming properties, as littlework hardening is exhibited. However, chromium as low as 0.1% and vanadium andmolybdenum contents as low as 0.05% can have a dramatic effect on hardenability.

Surface properties can be enhanced by carburising and then heat treating thecarbon-rich surface. High ductility results in poor machinability, although thesesteels can be machined if high spindle speeds are employed. More commonlysulphur and lead are added to form free-machining inclusions. Low quality steelswith high quantities of sulphur and phosphorus will have better machinability thangood quality steels which are clean and free from oxides and slag inclusions.

This group represents the bulk of the market for general purpose steel, finding usagein car bodies, ships and domestic appliances. Stainless steels and aluminium alloyscompete with these steels in certain areas.

Medium carbon content (0.2% to 0.5%C) e.g. 070M20, 080M40, 216M44, AISI1023, AISI 1030, AISI 1046.Heat treatment and work hardening are now effective methods for modifyingmechanical properties. Hardenability increases in proportion to carbon content.Welders must now take note of the hardening effects in the heat affected zone andtake precautions against excessive energy input, as increased hardenability resultsin an increased likelihood of brittle structures forming. All common alloying elementsincrease the hardenability and hence a carbon equivalent scale has been devisedas an approximate guide to weldability (see below).

In the normalised condition, machinability is improved compared with low carbonsteels due to their lower ductility and it can be further enhanced with the addition ofsulphur or lead if special free-machining properties are required. Ductility andimpact resistance is, however, reduced.

The corrosion resistance of these steels is similar to low carbon steel, although smalladditions of copper can lead to significant improvements when weatheringperformance is important. Most steels in this category contain some silicon andmanganese, added as de-oxidising and de-sulphurising elements duringmanufacture (see below). While the quantities present are not considered to effectmechanical properties, an indication of the quality of the steel is given by thephosphorus and sulphur content, where the lower the content, the higher the quality.

This category represents medium strength steels which are still cheap and commandmass market. They are general purpose but can be specified for use in stressedapplications such as gears, pylons and pipelines.

Medium-high carbon content (0.5% to 0.8%C) e.g. 070M55, 080M50, AISI 1055,AISI 1070These steels are highly susceptible to thermal treatments and work hardening. Theyeasily flame harden and can be treated and worked to yield high tensile strengthsprovided that low ductility can be tolerated. For example, spring wire in this categorycan have an ultimate tensile strength (UTS) >2GPa. Clearly, welders must take careto prevent heat affected zone (HAZ) cracking with these steels, and specialist adviceshould always be obtained. The carbon equivalent can be used to evaluate potentialwelding problems.

Although high strengths and hardnesses are attainable, impact strengths are poor.These steels are not normally used in stressed applications subjected to shock.They are used where hardness is valued, such as for blades, springs, collars, etc.

High carbon content (>0.8%C) e.g. 050A86, 080A86, AISI 1086, BS 1407Cold working is not possible with any of these steels, as they fracture at very lowelongation. They are highly sensitive to thermal treatments. Machinability is good,although their hardness requires machining in the normalised condition. Welding isnot recommended and these steels must not be subjected to impact loading.

These steels can have UTSs greater than 1 GPa, and care needs to be taken toavoid hydrogen embrittlement following electroplating. Advice should be sought from

the plating-shop. As with the medium-high plain carbon steels, steel with >0.8%C isused for components requiring high hardness such as cutting tools, blades, etc.

Alloy Steels

By the addition of alloying elements such as chromium, nickel, molybdenum andvanadium it is possible to increase the hardenability of a carbon steel, along withother properties, such as corrosion resistance and fatigue strength.

The general trend of improved response to heat treatment and cold working inproportion to carbon content, described for plain carbon steel, applies equally to alloysteels. However, final properties are sensitive to the alloying additions as well.

The number of alloy steels available makes the choice of a steel for any givenapplication difficult, and in most cases there will be a number of steels that wouldmeet the requirements.

Chromium (Cr) can improve general high temperature properties, and alsocorrosion and oxidation resistance. It forms carbides with the available carbon inpreference to iron which aids carburisation. It also slows down metallurgicalreactions, thus increasing hardenability. Chromium results in larger grainedstructures which can cause problems as a result of the associated poorermechanical performance. (See below for high chromium content steels)

Nickel (Ni) lowers critical heat treatment temperatures and generally allows foreasier conditioning. Nickel strengthens and toughens steel by dissolution into theferritic matrix. It is particularly valued in low temperature service where impactstrengths can be maintained at sub-zero temperatures.

Vanadium (V) is a very good carbide former, although it is also useful as adeoxidiser. Vanadium carbides are particularly fine and evenly distributed, and theyprovide the best grain refining properties, which generally improves mechanicalproperties. Vanadium carbide is very hard and has a stabilising effect on othercarbides (notably chromium carbide) which might otherwise precipitate, causinggrain growth and brittleness during heat treatment. Vanadium forms nitrides andconsequently is often present in nitriding steels.

Molybdenum (Mo) like vanadium, usefully yields a fine grain structure withconsequent improvements in overall strength. This fine structure is a result of thestable, even distribution of molybdenum carbide. These carbides also serve tostabilise steels with nickel and chromium additions which can otherwise show temperbrittleness due to carbide precipitation. Molybdenum also enhances corrosionresistance in stainless steels.

Tungsten (W) forms carbides which are exceptionally hard. These carbides arebeneficial in a similar fashion to molybdenum carbides, although far greaterconcentrations are required. Tungsten is valued in steels requiring hardness withstability at high temperatures, for example, tool steels.

Boron (B) is able to improve hardenability in concentrations as low as 0.001%. Thisparticularly sensitive behaviour is only effective with low to medium carbon steels.Despite increasing hardenability, these steels are still easily welded and are oftenspecified where controlled hardenability in the weld is required. It is not known forsteel to use boron as the sole alloying element, but it is frequently found inconjunction with other elements such as vanadium, chromium and molybdenum, asit also increases their hardening effect.

Copper (Cu) generally enhances corrosion resistance, although if it is present as atramp element, possibly due to poorly separated scrap, it can be disastrous, causinggrain segregation during hot working.

Cobalt (Co) is never present alone, but always as an addition to alloy steels. It isnot a carbide former but dissolves in the ferrite matrix, like nickel and silicon.Additions of up to 30% cobalt to ferrous alloys have a significant effect on thematerials magnetic properties. Cobalt can not only strengthen the ferrite, but alsoappears to stabilise the carbides and maintains their properties to much highertemperatures.

Titanium (Ti) forms very stable carbides, combining with carbon in preference toiron and chromium. For the titanium to combine with all the carbon, a minimum ofeight times as much titanium as carbon is used, resulting in titanium stabilisedweldable austenitic steels.

Aluminium (Al) is a good deoxidiser, but alumina (aluminium oxide) is a brittlematerial which can be a damaging inclusion in steel. Aluminium can, however,increase the ability of the steel to nitride and has some grain refining properties.

Manganese (Mn) is a useful deoxidiser and desulphuriser as the oxides andsulphides are particularly ductile and harmless. It is found in almost every steel forthese reasons. In fact, because it is such a common addition, it is often omitted fromspecifications unless it is present in quantities >2%. Manganese lowers heattreatment temperatures and can give a wholly austenitic steel in concentrationsgreater than about 15%. These steels are non-magnetic. Manganese alsostrengthens steel and can yield high carbon steels that are tough and workable. Itshould be noted that manganese tends to increase the likelihood of quench cracking.

Silicon (Si) is a cheap and harmless deoxidiser found almost without exception insteels. It raises heat treatment temperatures and forms graphites which are usefulfor decarburising. At levels above about 0.5%, silicon can increase corrosionresistance and fatigue strength, although not to the same extent as other alloyingelements.

High Chromium Steels

Additions of chromium in excess of 12% gives rise to a stable surface film ofchromium oxide, the stability of the film increasing with increasing chromium content.

This oxide film confers corrosion resistance and is the basis on which the stainlesssteel family is built.

Low carbon steels containing 12-30% chromium are the ferritic stainless steels(e.g. 430, 409) which are not heat treatable. Increases in mechanical properties canonly be achieved by cold working. The corrosion resistance of this group issignificantly better than the high carbon-high chrome steels.

High carbon-high chrome steels are heat treatable as a consequence of the highercarbon content, and are known as martensitic stainless steels (e.g. 410, 416).They do, however, exhibit lower corrosion resistance due to chromium depletion ofthe oxide film. They exhibit good strength and oxidation resistance up to 7500C,although their creep strength above 6000C is poor.

Austenitic stainless steels (e.g. 302, 316) result from additions of nickel (usuallybetween 10-20%) to low carbon steels containing 18-25% chrome. These steelsexhibit superior corrosion resistance in a wide range of environments. Theproperties can only be modified by cold work. They are also significantly moreexpensive than the straight chromium grades. When mention is made of stainlesssteel it is generally these non-magnetic steels that are being referred to. While thethermal expansion of these steels is similar to that of copper, their thermalconductivity is less than that of alumina at room temperature.

Precipitation-hardening stainless steels (e.g. 17-4 PH, PH 13-8 Mo) arechromium-nickel alloys containing precipitation hardening elements such as copper,aluminium or titanium. The alloys are of two general types: semi-austenitic, requiringa dual heat treatment to achieve final strength properties. The main advantage ofthese alloys is the low temperature heat treatment required to achieve final strength,which can be as high as 2 GPa, resulting in minimal scaling and distortion, thusenabling parts to be finished machined prior to final heat treatment.

It should be noted that chromium has a tendency to migrate to grain boundaries atelevated temperatures, where it forms chromium carbide. This is a series problem inthe heat affected zones of welds. This effect is known as weld decay and causesfailure due to corrosion along grain boundaries where there is a depletion ofchromium. For welding, a carbon content <0.03% is specified to avoid significantcarbide formation. Alternatively, the steel can be stablised with the addition oftitanium or niobium which form carbides in preference to chromium.

Although stainless steels are more corrosion resistant than other steels, they aresubject to specific corrosion mechanisms, such as weld decay. Advice must besought for particular applications.

Selected Special Steels

High strength-low alloy (HSLA) steels are a group of low-carbon steels that utilisesmall amounts of alloying elements to attain yield strengths in excess of 275 MPa inthe as-rolled or normalised conditions. These steels have better mechanicalproperties than as-rolled carbon steels, largely by virtue of grain refining and

precipitation hardening. Because the higher strength of HSLA steels can beobtained at lower carbon levels, the weldability of many HSLA steels is at leastcomparable to that of mild steel. They can allow more efficient designs withimproved performance, reductions in manufacturing costs and component weightreduction. Applications include oil and gas pipelines, automotive beams, offshorestructures and shipbuilding.

Maraging steels differ from conventional steels in that they are hardened by ametallurgical reaction that does not involve carbon. These steels are strengthenedby intermetallic compounds such as Ni3Ti and Ni3Mo that precipitate at about 5000C.These steels typically have very high nickel, cobalt and molybdenum contents whilecarbon is essentially an impurity and its concentration is kept as low as possible inorder to minimise the formation of titanium carbide, which can adversely affectmechanical properties. Ultra-high strengths may be obtained with these steels, andweldability is good. Toughness is superior to all low alloy carbon steels of similarstrength, particularly the low temperature toughness. Although they are expensive,they are easy to machine and heat treat, so that some economies result incomponent production.

The Editor thanks John Donlon of MIS Midland Region and Ian Marr of MDPAssociates for their help and advice in the preparation of the Profile.

Useful Addresses

Engineering Steels

While British Steel do not currently operate a Technical Advisory Service for generalenquiries, the product-based advisory services may be able to help, e.g.:

Strip Products Advisory ServiceTel: 01633 290022 x 4090

Plates Advisory ServiceTel: 01698 266233 x231

Structural Steel Advisory CentreTel: 01642 404242

UK StockholdersNational Association of Steel StockholdersGateway HouseHigh StreetBirmingham B4 7SYTel: 0121 632 5821Fax: 0121 643 6645

Stainless SteelStainless Steel Advisory Centrec/o The Institute of MaterialsThe Innovation Centre217 PortobelloSheffield S1 4DPTel: 0114 224 2240Fax: 0114 273 0444

Nickel Development InstituteEuropean Technical Information CentreThe HollowayAlvechurchBirmingham B48 7QBTel: 01527 584777Fax: 01527 585562

SpecificationsWorld Metal IndexSheffield Libraries & Information ServicesCentral LibrarySurrey StreetSheffield S1 1XZTel: 0114 273 4714Fax: 0114 275 7405

Useful Texts

Properties & Selection: Irons, Steels & High Performance Alloys’ASM Metals Handbook, Vol 1 (1990) ASM International

‘Materials Selector’N A Waterman & MF Ashby (1994) Chapman & Hall

‘Bainite in Steels’HKDH Bhadeshia (1992) The Institute of Materials

‘Metals Databook’Colin Robb (1988) The Institute of Materials

‘Metallic Materials Specification Handbook’Robert B Ross (1992) Chapman & Hall

‘Iron & Steel Specifications’1989 British Steel plc

‘Worldwide Guide to Equivalent Irons & Steels’1993 ASM International