Hardening & Tempering Of Stainless Steel
The basics of this group of classic heat treatments is that the stainless steel components are heated to the appropriate hardening temperature of the alloy (usually between 800-900°C), held at temperature, then "quenched" (rapidly cooled) in a protective gas. This is followed by tempering (a soak at a lower temperature) which develops the final mechanical properties and relieves stresses. The processes are typically relevant for ferritic and martensitic stainless steels.
Heat treatment of ferritic stainless steel
Ferritic stainless steels are like their martensitic counterparts in terms of corrosion-resistivity, albeit with significantly decreased nickel and elevated levels of chromium. This decreases the cost of the alloy alongside the material’s proclivity towards stress corrosion cracking. Yet it still cannot be hardened by conventional thermal processing.
Process annealing is used to impart maximum ductility and corrosion resistance at the expense of hardness. This follows a similar process to martensitic annealing albeit with shorter recrystallization periods at lower temperatures of approximately 760 – 955°C.
Heat treatment of martensitic stainless steel
Martensitic stainless steels could be described as the most conventional alloy in terms of its hardenability properties. They typically display a proportional hardenability to carbon (C) content and are subsequently suited to standard austenitizing, quenching, and tempering. The alloy is heated to a point above its critical temperature long enough for recrystallization to occur. This causes a decrease in ductility in favor of hardness, up to a maximum point. It is then quenched and tempered to achieve the optimal intersection of workability, corrosion resistance, and hardness.
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