What are Austenitic Stainless Steels and Why They Cannot Undergo Heat Treatment?

04 April 2019

Copious quantities of nickel and chromium bolster a base ferrous structure. That’s what gives Austenitic Stainless Steel its face-centred microcrystalline identity. Infused with a little carbon, the alloy family gains its mainstream mechanical properties. So, what is that austenitic edge? The nickel-based flavouring is clear enough, but we want to know more about its practical features and applications, plus the reason such a popular alloy group can’t undergo heat treatment.

Questioning the Face-Centred Differences

Austenitic steels are highly formable, weld-amenable and non-magnetic. They’re popular, there’s no denying that point. In fact, 300 series stainless steel is part of this family, and that’s the most popular type of steel on the market today. Members of that series are polished and tough, and they’re found in high and low-temperature applications. In the kitchen or on a high-end automobile, 3xx alloys function equally well. Their crystal matrices, formed as face-centred cubes, cram toughness and ductility into a compact package. Having said all that, there’s that heat treatment problem to solve.

Austenitics Refuse Heat Treatment

Tough and malleable, there’s just one problem that prevents the alloy from being an all-around winner: austenitic stainless steels cannot experience conventional heat treatment procedures. The high quantities of nickel and chromium do gift the alloy with that signature look. The metal is polished and corrosion-resistant. Without sufficient amounts of carbon in the amalgamated blend, however, it’s a hard, almost impossible form of stainless steel to heat treat. While martensite forms without too much energy expenditure in other types of steel, that metallic state and its quench-facilitated austenitic structure cannot develop without a requisite quantity of alloying carbon.

Cold Worked and Annealed

Because of the above issues, austenitic steels are work hardened. They’re exposed to bending operations and drawing work, which causes the metal’s microcrystalline structure to deform. The result is material stress. Now, with that deformation energy locked inside the alloy, heat treatment technology can finally make its contribution. Instead of a hardening process, however, it’s an annealing service that heats the stainless steel workpiece until the work hardened stress is neutralized. Again, heat treatment hardening is not applicable here, but an annealing routine will help when cold work stress threatens the structural integrity of an austenitic stainless steel product run.

Carbon-rich steels are a heat treatment engineers bread-and-butter. Opting to break away from that microcrystalline paradigm, austenitic stainless steels pack their cubic ferrous matrices with nickel and chromium, although there is a small amount of carbon inside that crystal framework. It’s this nickel high base that gives the alloy its corrosion resistant character, but that feature comes at a cost. The cost is exacted in the form of a heat treatment-less fabrication process.

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