The white coloured metal nickel has good resistance to oxidation and corrosion with good workability and mechanical properties (just like mild steel). It (has high melting point) retains its strength at elevated temperatures and its ductility and toughness at low temperatures. Because of good electrical conductivity, the lead and terminals of many electronic applications are made of nickel. Nickel is commonly alloyed with copper, iron, chromium, silicon, molybdenum.
Depending on the alloy and properties required, nickel and nickel alloys can be given one, or more of following principal types of heat treatment:
1. Annealing:
It consists of heating to predetermined temperature, normally of 700° to 1200°C for a definite time, and slowly, or rapidly cooling (precipitation hardenable type alloys are cooled rapidly to get maximum softness). The aim of annealing is (also called soft annealing) to completely soften as a result of recrystallisation to use them as, it is or for further cold working. If annealing is done at higher temperatures and for longer periods, ‘dead soft’ alloy is obtained-then, it is called Dead-Soft annealing, but grain coarsening occurs here.
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2. Stress-Relieving:
In this heat treatment, the residual stresses of strain hardened state of non-age hardenable alloys is removed or reduced with the process of recrystallisation. The normal temperature range of heating for nickel and nickel alloys is 425°C to 875°C depending on the alloy composition and the amount of strain-hardening of the alloy.
3. Stress-Equalizing:
It is a low temperature heat treatment that is given to nickel alloys to balance the stresses of cold worked state without appreciable decrease in mechanical strength (of cold worked state). This is partial recovery stage of annealing in which no change in microstructure is seen with increased yield strength, slight increase in tensile strength and hardness and complete recovery of electrical conductivity.
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Though the temperature depends on the amount of cold work and the composition of the alloy, but is about 230°C to 315°C (275°C is very commonly used temperature).
This treatment is used for nickel alloys in cold deformed state of coil springs, wire, and flat-spring stampings.
4. Precipitation Hardening:
Adding Mg, Al, Si, Ti, etc. to nickel and its alloys increases response to age hardening to yield high strength and hardness after age hardening.
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(a) Solution Heat Treatment:
It is a high temperature heat treatment to dissolve age hardening constituents and carbides into solid solution. Some of the nickel alloys do not require solution treatment, but in some alloys, it is required to develop special properties. For example, Inconel X-750 is solutionised at 1150°C for 2-4 hours, and air cooled prior to double ageing treatment to improve creep and rupture strength. This results in good properties for high temperature springs and turbine blades of Inconel X-750.
(b) Age Hardening:
Table 14.16 gives ageing temperature and time of some of nickel alloys. Nickel alloys are soft when quenched from 790 to 1250°C and can be hardened by holding in the range 480 to 540°C and then air cooled. Nimonic alloys are solution treated at 1080°-1200°C for 10 hours and then aged at 700-850°C for 10-16 hrs. Here γ’ is the fine precipitating phase to cause increased hardness.
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Nimonics too are high temperature materials used for making rotor blades, turbine discs, rings, fasteners, nozzle vanes etc. of gas turbines operating at 650-850°C.
Chromium imparts high scale resistance, while Ti and Al increase heat resistance. After solutionising (1080-1120°) and cooling, alloy has supersaturated FCC structure having high ductility to allow deep drawing, bending and shaping and can be welded. Age at 700°C Intermetallic γ’-phase, Ni3(Ti, Al) is formed which has coherent interface.
Also are formed TiC and TiN which increase strength at high temperatures. Mo and W (if added) retard diffusion and raise recrystallisation temperature. Co increases heat resistance and processing ductility. B and Zr increase grain boundary strength by removing impurities like Bi, Sb. Sn, S, Pb as compounds.
