PART 2 : HEAT TREATMENT

ALLOY SYSTEMS STEELS ALUMINUM ALLOYS TITANIUM ALLOYS NICKEL BASE SUPERALLOYS

STEELS Annealing Normalizing Stress Relieving Hardening and Tempering Solution Treatment and Aging

IMPORTANT EQUILIBRIUM PHASES IN STEELS Ferrite (α )…Body Centered Cubic (BCC) Cementite (Fe 3 C) … Orthorhombic Austenite ( γ )…Face Centered Cubic (FCC) δ Ferrite…Body Centered Cubic (BCC)

BCC CRYSTAL MODEL Packing Density- 68%

FCC CRYSTAL MODEL Packing Density- 74%

BASIS FOR HEAT TREATMENT: Fe-C PHASE DIAGRAM FEATURES SOLUBILITY OF C IN  Fe(BCC)-O.O2%  Fe(FCC)-2.11% EUTECTIC REACTION C, 4.3%C EUTECTOID REACTION C, 0.77%C

OCTAHEDRAL VOID SPACE IN BCC VOID SIZE IS nm SIZE OF C ATOM IS 0.07 nm OCTAHEDRAL VOID SPACE IN FCC VOID SIZE IS nm SIZE OF C ATOM IS 0.07 nm SIZE OF VOID SPACE

ISOTHERMAL TRANSFORMATION CURVE

1. Heat the steel piece to the specified austenitizing temperature and hold so that the entire job achieves the specified temperature 2. Quench fast enough so as to avoid the knee to prevent formation of the high temperature transformation products (pearlite, ferrite and cementite) HOW IS HARDENING DONE

EMERGENCE OF MARTENSITE FROM AUSTENITE

EFFECT OF CARBON HARDNESSHARDNESS WT% CARBON Effect of Carbon on a) Hardness b) Ms (a) (b)

Diameter Hardness,Rc Diameter Hardness,Rc HARDENESS DISTRIBUTION Hardness distribution in water quenched steels (a) SAE1045 and (b) SAE6140 (a) (b)

Depending upon requirement, appropriate tempering temperature is selected. A typical case is shown in the graph for 4340 STEEL. The tempering temperature depends upon the required strength and hardness after tempering. All these are tabulated and are available in ASTM literature for every steel. Temper, C (MPa) (1725) (1035) (1380) (690) Tempering Temperature Strength EFFECT OF TEMPERING

These are softening processes used for producing steel with high ductility and low hardness. Though annealing is used in a very broad sense it has a distinct cycle. Annealing involves heating the steel to elevated temperature, holding for a time dictated by section thickness and cooling in the furnace. The elevated temperature is in the range of c above a 3 for hypo eutectoid steels and c above a 1 (not a cm, to avoid precipitation of pro-eutectoid cementite along grain boundaries) for hyper eutectoid steels Normalising involves heating the hypo eutectoid and hyper eutectoid steels above a 1 and a cm, respectively holding and air cooling. ANNEALING AND NORMALISING TEMPERATURES

HEAT TREATMENT OF ALUMINIUM ALLOYS Solid solution strengthened alloys Soaked in Furnace followed by air cooling Precipitation hardened Solution treated and quenched ( quench delay < 15 seconds ) Aged natural ( room temperature ) or artificial ( higher temperature )

Al-Cu Phase DiagramAl-Zn Phase Diagram PRECIPITATION HARDENABLE ALUMINIUM ALLOY SYSTEMS

1) Solution Treatment- the alloy is heated above the solvus temperature and soaked there until a homogeneous solid solution (α) is produced. 2) Quenching is the second step where the solid α is rapidly cooled forming a supersaturated solid solution of α SS. 3) Aging is the third step where the supersaturated α, α SS, is heated below the solvus temperature to produce a finely dispersed precipitate(θ). The formation of a finely dispersed precipitate in the alloy is the objective of the precipitation-hardening. PRECIPITATION HARDENING PROCESS

STRENGTHENING PRECIPITATES IN DIFFERENT ALLOY SYSTEMS Al- Cu systems and Al Cu Li systems Al 2 Cu, Al 2 CuMg, Al 2 CuLi, Al 3 Li Al –Mg-Si systems Al 5 Cu 2 Mg 8 Si 6 Al-Zn-Mg, Al-Zn-Mg-Cu systems MgZn 2, Mg(ZnCuAl) 2

Al-Mg Phase Diagram Al-Mn Phase Diagram SOLID SOLUTION STRENGTHENING Solid solution strengthening is due to dissolved solute. The solute atmosphere interacts with moving dislocations impeding their motion.

WHY NO PRECIPITATES IN Al-Mg AND Al-Mn SYSTEMS?  Despite sloping solvus, Mg coming out of super saturated solution is extremely sluggish. Therefore strengthening is only by solid solution.  Very slow cooling such as furnace cooling from annealing temperature brings out Mg in blocky form as Al 3 Mg 2, reducing the strength.  In higher Mg containing Al alloys (>4wt%), these precipitates appear at grain boundary, reducing ductility and resistance to stress corrosion cracking.  Post annealing cold work accentuates this problem.  Mn in Al alloys is added below its high temperature solubility limit. Therefore no question of forming precipitates.  Both Mg and Mn increase work hardening rate. Therefore strengthening is done by cold working(Al and Mg alloys < 3wt% Mg).

OTHER IMPORTANT Al ALLOYS Al-Si Alloys Do not form any precipitates Weak solid solution strengthening Si improves fluidity Therefore used as sheets for brazing, Welding rods and castings. Al-Si-Mg alloys  Si and Mg in proper proportion produce AlMg 2 Si  precipitates. 2 Groups  1 st Group- (Mg +Si ) can be easily extruded and air cooled.  2 nd Group-(Mg +Si ) >1.4% develops high strength on aging after ST + Quenching. Cu also added to enhance mechanical properties.

HEAT TREATMENT OF TITANIUM ALLOYS Annealing Mill Annealing Normal Annealing Aging Treatment Solution treatment and quench Ageing at elevated temperature

HEAT TREATMENT

DEVELOPMENT OF MICROSTRUCTURES

HEAT TREATMENT OF Ni BASE SUPERALLOYS Solution Treatment Ageing Treatment

HOMOGENIZATION To make the composition uniform SOLUTION TREATMENT Heating to temperature above γ’ solvus and below incipient melting to take all the γ’ into solution, followed by quenching. Wrought alloys C Cast alloys C AGING 1.Primary aging at 925 C to precipitate coarse γ’ 2.secondary aging at 750 C to precipitate fine γ’ 3.Tertiary aging at 700 C to precipitate very fine γ’ and to form M 23 C 6 carbides. HEAT TREATMENT DETAILS