Advances in SPecail Steels : Super alloy ni and ti alloys

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Super AlloysContent:1) Application2) Introduction3) Specific Characteristics of Super Alloy4) Production of Super AlloysIntroduction to Nickel and its alloys5) Composition of Ni- Base Super Alloy:6) Composition–microstructure relationships in nickel

alloys7) Heat Treatment of Ni-AlloysIntroduction to Ti Alloys8) Ti –Alloys Composition, Properties and Uses9) Heat Treatment of Ti- Alloys

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Application of Super alloy

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Boeing 777 aircraft engine• When significant resistance to loading under static, fatigue and creep conditions is required, the nickel-base superalloys have emerged as the materials of choice for high temperature applications.

• Particularly true when operating temperatures are beyond about 800 ◦C.

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• single-crystal superalloys are being used in increasing quantities in the gas turbine engine.

• creep rupture lives of the single-crystal superalloys about 250 h at 850 ◦C/500MPa

• a typical first-generation alloy such as SRR99, to about 2500 h • the third-generation alloy RR3000. Under more demanding conditions,

for example, 1050 ◦C/150MPa, rupture life has improved fourfold from 250 h to 1000 h.

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• Fan - The fan is the first component in a turbofan. The large spinning fan sucks in large quantities of air. Most blades of the fan are made of titanium.

• Compressor - The compressor is made up of fans with many blades and attached to a shaft. The compressor squeezes the air that enters it into progressively smaller areas, resulting in an increase in the air pressure.

• Combustor - In the combustor the air is mixed with fuel and then ignited. There are as many as 20 nozzles to spray fuel into the airstream. The mixture of air and fuel catches fire. This provides a high temperature, high-energy airflow. The fuel burns with the oxygen in the compressed air, producing hot expanding gases. The inside of the combustor is often made of ceramic materials to provide a heat-resistant chamber. The heat can reach 2700°.

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• Turbine - The high-energy airflow coming out of the combustor goes into the turbine, causing the turbine blades to rotate. The turbines are linked by a shaft to turn the blades in the compressor and to spin the intake fan at the front. This rotation takes some energy from the high-energy flow that is used to drive the fan and the compressor. The gases produced in the combustion chamber move through the turbine and spin its blades. The turbines of the jet spin around thousands of times. They are fixed on shafts which have several sets of ball-bearing in between them.

• Nozzle - The nozzle is the exhaust duct of the engine. This is the engine part which actually produces the thrust for the plane. The energy depleted airflow that passed the turbine, in addition to the colder air that bypassed the engine core, produces a force when exiting the nozzle that acts to propel the engine, and therefore the airplane, forward.

• The combination of the hot air and cold air are expelled and produce an exhaust, which causes a forward thrust. The nozzle may be preceded by a mixer, which combines the high temperature air coming from the engine core with the lower temperature air that was bypassed in the fan. The mixer helps to make the engine quieter.

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Introduction(After World war II)

• The term "super alloy" used to describe a group of alloys

developed for use in

turbo superchargers and aircraft turbine engines that required high

performance at elevated temperatures.

• The range of applications for which super alloys are used has

expanded to many other areas and now includes aircraft and land-

based gas turbines, rocket engines, chemical, and petroleum

plants.

• These alloys have an ability to retain most of their strength even

after long exposure times above 650°C

• “Super-alloys are unique high temperature materials used in gas turbine engines, which display excellent resistance to mechanical and chemical degradation”

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Super-alloys are based on Group VIIIB elements and usually consist of various combinations of Fe, Ni, Co, and Cr, as well as lesser amounts of W, Mo, Ta, Nb, Ti, and Al. But in general.The three major classes of Super-alloys are considered:

1. Nickel-based2. Iron-based 3. Cobalt-based alloys.

• Nickel-based superalloys are used in load-bearing structures to the highest homologous temperature of any common alloy system (Tm = 0.9, or 90% of their melting point).

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Specific characteristics of Super Alloy (718 –Ni Base)

1. Good ductility is evidenced at 649°C to 760°C.

2. Yield & tensile strength, creep, and rupture strength properties are

exceedingly high at temperatures up to 705°C.

3. The unique property of slow aging response permits heating and

cooling during annealing without the danger of cracking. Problems

associated with welding of age hard-enable alloys are eliminated with

Alloy 718.

4. Fracture toughness tests have been conducted on the alloy in forms

other than tubing at temperatures from –195°C to 538°C with

excellent values reported.

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Production of Super Alloy – (Summary)The following is a summary of processes for the manufacture of typical super

alloys:

1. Vacuum Induction Melting:

• Vacuum induction melting is used as the standard melting practice for the

preparation of superalloy stock.

• Raw metallic materials including scrap are charged into a refractory crucible

and the crucible is maintained under a vacuum during melting of the charge.

• Typically, more than 30 elements are refined or removed from the superalloy

melt during VIM processing. A final step involves the transferring of the liquid

metal from the crucible into a pouring system, and the casting of it into

molds under a partial pressure of argon.

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2. Investment Casting:• The investment casting or 'lost-wax' process is used for the

production of superalloy components of complex shape, e.g. turbine blading or nozzle guide vanes.

• A wax model of the casting is prepared by injecting molten wax into a metallic 'master' mold. These are arranged in clusters connected by wax replicas of runners and risers; this enables several blades to be produced in a single casting.

• Next, an investment shell is produced. Finally, the mold is baked to build up its strength. After preheating and degassing, the mold is ready to receive the molten superalloy, which is poured under vacuum.

• After solidification is complete, the investment shell is removed and the internal ceramic core leached out by chemical means, using a high pressure autoclave.

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3. Secondary Melting (Vacuum Arc Remelting / Electroslag Remelting )

• For many applications, a secondary melting process needs to be applied to

increase the chemical homogeneity of the superalloy material and to reduce the

level of inclusions. (Normally Super alloys double or tripled VAR)

• After VIM processing, it would be normal for the cast ingot to possess a

significant solidification pipe and extensive segregation. Removing the pipe

reduces productivity and the segregation can lead to cracking and fissuring

during subsequent thermal-mechanical working.

• Furthermore, VIM can leave non-metallic (ceramic) inclusions present in the

material which can be harmful for fatigue properties.Application of the

secondary melting practices can reduce the problems associated with these

effects. Vacuum arc remelting (VAR) or electroslag remelting (ESR) are used for

this purpose, sometimes in combination with each other.

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4. Powder Metallurgy:

• P/M is an expensive processing route but it is used for producing

heavily alloyed alloys with acceptable chemical homogeneity.

• The need for powder metallurgy (P/M) first arises for the production

of some high-integrity superalloy components such as turbine discs.

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5. Heat Treatment:• The term "heat treatment" when applied to superalloys may mean many

different processes, including1. stress relieve annealing,2. in process or full annealing, 3. solution treating, 4. Precipitation hardening.• In-process annealing may be used after welding to relieve stress or in

between severe forming operations. • Full annealing is used to obtain a fully recrystallized, soft and ductile

structure.• Solution treating is done to dissolve second phases so that the additional

solute is available for precipitation hardening.• Precipitation hardening (also called age hardening) is used to bring out

strengthening phases and to control carbides and the topologically close packed phases.

6. Applications of coatings also involve exposures to elevated temperatures.

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An Introduction to Nickel and its alloys• Nickel is the fifth most abundant element on earth. • The atomic number is 28, weight is 58.71• The crystal structure is face-centred cubic (FCC), from

ambient conditions to the melting point, 1455 ◦C, which represents an absolute limit for the temperature capability of the nickel-based superalloys.

• The density under ambient conditions is 8907 kg/m3. Thus, compared with other metals used for aerospace applications, for example, Ti (4508 kg/m3) and Al (2698 kg/m3), Ni is rather dense.

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Composition of Ni- Base Super Alloy:• The compositions of most important nickel-based superalloys is given

in the table. • One can see that the number of alloying elements is often greater

than ten and consequently, if judged in this way, the superalloys are amongst the most complex of materials engineered by man.

• Nickel 50.0-55.0%• Chromium 17.0-21.0%• Niobium + Tantalum

4.75-5.50%• Molybdenum 2.80-3.30%• Aluminum 0.20-0.80%• Titanium 0.65-1.15%• Carbon 0.08% max

• Silicon 0.350% max• Manganese 0.350% max• Sulfur 0.015% max• Copper 0.300% max• Phosphorus 0.015% max• Cobalt 1.00% max.• Iron Balance

Alloy 718

• Certain superalloys, such as IN718 and IN706, contain significant proportions of iron, and should be referred to as nickel–iron superalloys.

• the behaviour of each alloying element and its influence on the phase stability depends strongly upon its position within the periodic table as shown in fig 1.

• Fig.1: Categories of elements important to the constitution of the nickel-based superalloys, and their relative positions in the periodic table.

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Composition–microstructure relationships in nickel alloys

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• A first class of elements includes nickel, cobalt, iron, chromium, ruthenium, molybdenum, rhenium and tungsten; these prefer to partition to the austenitic γ and thereby stabilize it. These elements have atomic radii not very different from that of nickel.

• A second group of elements, aluminum, titanium, niobium and tantalum, have greater atomic radii and these promote the formation of ordered phases such as the compound Ni3(Al, Ta, Ti), known as γ’

• Boron, carbon and zirconium constitute a third class that tend to segregate to the grain boundaries of the γ phase, on account of their atomic sizes, which are very different from that of nickel. Carbide and boride phases can also be promoted. Chromium, molybdenum, tungsten, niobium, tantalum and titanium are particularly strong carbide formers; chromium and molybdenum promote the formation of borides.

Composition–microstructure relationships in nickel alloys

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• The microstructure of a typical superalloy consists therefore of different phases, drawn from the following list.

(i) The Gamma phase, denoted γ . This exhibits the FCC structure, and in nearly all cases it forms a continuous, matrix phase in which the other phases reside. It contains significant concentrations of elements such as cobalt, chromium, molybdenum, ruthenium and rhenium, where these are present, since these prefer to reside in this phase.

(ii) The Gamma Prime precipitate, denoted γ’ .This forms as a precipitate phase, which is often coherent with the γ -matrix, and rich in elements such as aluminium, titanium and tantalum. In nickel–iron superalloys and those rich in niobium, a related ordered phase, γ’’ , is preferred instead of γ’

(iii) Carbides and borides. Carbon, often present at concentrations up to 0.2 wt%, combines with reactive elements such as titanium, tantalum and hafnium to form MC carbides. During processing or service, these can decompose to other species, such asM23C6 and M6C,which prefer to reside on the γ –grain boundaries, andwhich are rich in chromium, molybdenum and tungsten. Boron can combine with elements such as chromium or molybdenum to form borides which reside on the γ –grain boundaries.

Composition–microstructure relationships in nickel alloys

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Heat treatment of Ni-Alloys• Following are the treatments:1. Annealing:• Produce recrystallized grain structure.• To soften the work hardened Ni- alloy.• Normally done from 700 to 1200°C (Depending upon alloy)2. Stress Relieving • given to work hardened and non-age hardened Ni-alloys.• For reduce or remove residual stresses.• Normally done from 430 to 870°C.3. Stress Equalizing • Low temperature HT process.• Given to Ni-alloys to balance stresses in cold worked alloys without

affecting mechanical properties.

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4. Solution Treating and age hardenig• Some Ni-Alloys solution treatment and age hardening treatment are

summarized below.

Alloy Solution Treatment

Ageing

Nimonic 9015%Cr. 0.85%Co,2.5%Ti,1.5%Al,0.05%C

8-12 Hrs at 1080-1180°C, Air cooling

12-16 Hrs at 700-850°C, air cooled

Inconel X15%Cr, 18%Co,6.75%Fe,0.8%Al,2.5%Ti,0.7%Mn,0.04%C

2-4 Hrs at 1165°C, Air cooling

24Hrs, at 860°C air cooled followed by 20 hrs at 740°Cair cooled.

Hastelloy B1%Cr,2.5%Co,23-30%Mo,2.6%V,1.0%Si,1.0%Mn,0.05%C

1200°C air cooled 24 Hrs at 860°C, air cooled

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Example: 1. Solution treatment and age hardening:• Nimonic alloys are heated to 1080-1200°C for about

10hrsand subsequently aged at 700-850°C for 10-16 Hrs.2. Result:• A super saturated (gamma) γ-solution with FCC lattice

formed. Upon ageing, the supersaturated γ-solid solution decomposes and fine precipitates of (gamma prime) γ’-phase, i.e. Ni(Al) and etc. compounds are formed.

Other important features: (Brittleness in Ni Alloy)• On longer ageing at 850-900°C, the stable (Eta) η-phase

(Ni3Ti) is formed which is of Hexagonal type and may cause embrittlement to the alloy.

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Titanium Alloys

Content• Introduction• Ti –Alloys Composition, Properties and Uses• Heat Treatment of Ti- Alloys

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Introduction:• Ti (Alloys) gives variety of light weight strong materials with good

fatigue and corrosion resistance. • Ti alloys is used as substitute of AL alloy in aircraft structure in the

temperature range 200-500 °C.• Two allotropic form 1) Alpha (α) Ti = HCP up to 882 °C.2) Beta (β) Ti = BCC, stable above 882 °C.Effect of Alloying addition:• Al is α stabilizer, when Al is added the α to β transformation

temperature is raised.• Cr, Mo, V, Mn and Fe are β stabilizer, when these are added the α

to β transformation temperature is lowered.• The relative amount of α and β stabilizing elements in Ti-Alloys,

and the Heat Treatment determine whether its microstructure would be mainly single α or single β or mixture of α –β over the range of desired temperature.

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• On the basis of phases present, Ti alloy may be of three (3) basic types namely, α, β and. α-β

1. α- Alloys show excellent weldability, good strength at high and low temperature, and stability at moderate temp: for sufficient long duration.

2. α-β Alloys are two phase alloy at room temperature and are stronger than α- Alloys .

3. β-Alloys retain their structure at room temp: and can be age hardened to give high strength.

4. Other is near-α Alloy containing mainly α- stabilizing element + less than 2% β-Stabilizing element.

• Following Table shows composition, properties and uses of some heat treatable Ti-Alloys.

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Alloy Type Form UTS(MPa)

Yield(MPa)

Elongation Uses

α- Alloys Ti-5Al-2.5Sn

Sheets, bars & Forgings

800 760 10 Compressor Blades and Welded assemblies

Near α- AlloysTi-6Al-3Mo-1Zr-Si

Bars & Forgings 1200 6 Compressor Blades and Dies

α-β Alloys Ti-6Al-4V

Sheets, bars & Forgings

1200 1060 8 Pressure Vessels, air frame and engine part

α-β Alloys Ti-4Al-4Mo-4Sn-Si

Bars & Forgings 1400 1250 10 Air frame structural Forging

β-Alloys TI-13V-11Cr-3Al

Sheets, bars & Forgings

920 850 10 Fasteners, rivets, sheet metal part and tubing.

Table: Ti-Alloys (heat treatable) : Composition, Properties and Uses

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Heat treatment of Ti-Alloy • Purpose of heat treatment of Ti-Alloys are: 1. To reduce residual stress developed fabrication operation,2. To get optimum combination of ductility, machinability, and dimensional and

structural stability,3. To obtained improved strength and specific Mechanical properties such as

fracture toughness, fatigue strength and creep resistance.• α and near- α Ti-alloys are subjected to stress relieving and annealing. High

strength can not be obtained in α and near- α Ti-alloys by HT.• The commercial β-alloys respond to HT. Ageing at elevated temperature after

solution treatment results in the decomposition of β-phase, and hence strengthening occurs.

• α – β alloy are two phase alloys and are most popular of the three types of Ti-alloys.

• The following three HT process are adopted for Ti-alloys.1. Stress Relieving2. Annealing3. Solution Treating and ageing.

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1. Stress Relieving• Ti and its alloys are stress relieved to minimized the undesirable residual

stress due to cold working, non-uniform hot forging and solidification.• Components can be cooled from stress relieving temperature either by air

cooling or slow cooling.• Following table shows Stress relieving temperature and time for Ti and its

alloys. Alloy Temperature Range

(°C)Time(hr)

Commercially Pure Ti 480-590 1/4 -4

α or Near α- AlloysTi-8Al-1Mo-1VTi-6Al-2Cb-1Ta-0.8Mo

590-700600-650

1/4 -41/4 -2

α-β Alloys Ti-6Al-4VTi-3Al-2.5VTi-8Mn

485-645540-650480-590

1-41/2 -21/4 -2

β-Alloys Ti-13V-11Cr-3AlTi-10V-2Fe-3Al

710-730680-700S

1/2 -1/4 1/2 -2

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2. Annealing• For = improves fracture toughness, ductility, dimensional and thermal

stability and creep resistance. • Different types of annealing is given to Ti-alloys. (i) mill annealing (ii) duplex

annealing (iii) triplex annealing (iv) recrystallization annealing and (v) beta annealing.

• Following table shows annealing temperature and time for Ti and its alloys. Alloy Temperature Range

(°C)Time(hr)

Commercially Pure Ti 650-760 1/10-2

α or Near α- AlloysTi-8Al-1Mo-1VTi-6Al-2Cb-1Ta-0.8Mo

785795-900

1-81-4

α-β Alloys Ti-6Al-4VTi-3Al-2.5V

710-790650-760

1-41/2-2

β-Alloys Ti-13V-11Cr-3AlTi-15V-3Al-3Cr-3Sn

710-790790-810

1/6 -1 1/12 -1/4

Cooling Medium

Air

Air or FurnaceAir

Air or FurnaceAir

Air or WaterAir

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3. Solution Treating and Ageing• α-β and β-alloys are solution treated and aged to obtain a wide

range of strength level….. How strength achieved ?• Β-phase is unstable at low temperature b/c it is high temp: phase.• Higher ratio of β is produced by heating an α-β alloys to the

solution treating temperature, upon quenching proportion of the phases are maintained.

• During subsequent ageing the decomposition of metastable β-phase takes place….this result improves the strength level.

Quenching conditions ?• α-βalloys are quenched in water or a 5% brine or caustic soda

solution.• β alloys are air quenched.

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Example: • Ti-6Al-4V (α-β alloy) is solution treated at 995-970°C for 1Hour followed

by water quench. This is aged b/w 480-590°C for 4-8 Hours.• This treatment gives maximum tensile strength properties to the

alloy.• This alloy contains α’ (Ti- Martensite) which has a needle like

structure. Brittleness in Ti-Alloy (Omega, ω-Phase)• When β-phase transforms to a metastable transition phase termed as

ω-Phase. (Due to more rapid quenching and fast reheating to ageing temperature above 430°C)

• This is generally observed in highly β- stabilized α-β alloys.• This phase introduces brittleness in Ti alloys.How ω-Phase is to be suppressed?1. Avoid rapid quenching and fast reheating to ageing temperature

above 430°C.2. Addition of Al, Mo and Sn in the alloys prevent the formation of ω-

Phase.

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