Metallurgy Interview Question:

How is solution annealing carried out? What type of furnace is required? Does one need to be heat in a Salt Bath?

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Solution annealing is carried out by heating up the alloy to a temperature in which typically only one phase is stable. This temperature depends on the alloy to be solution heat-treated; for precipitation hardenable Al-Cu base alloys, solution-annealing temperature is ~550C in which only alpha phase persists and for some precipitation hardenable steel grades like precipitation hardening martensitic stainless steel 17-4PH, this is something like 1050C where only gamma (austenite) phase exists.

Solution annealing is the second stage of a two-stage process; the second stage is precipitation hardening which is performed by heating the alloy to a temperature far below the solution annealing temperature, in which very small precipitates begin to form. This leads to enhancement of mechanical properties of the alloys and desired properties mainly high strength are reached only at the end of the precipitation hardening stage, whereas after solution annealing, material is rather soft. The precipitation hardening temperature for Al-Cu base alloys is something around 180C, while that of 17-4PH stainless steel is around 500C. Such materials are mainly used in aerospace applications where materials having high strength/weight ratios are required.
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