Do alloy characteristics impact formability?
04 August 2020
Deep-drawing stainless steel and nickel alloys means balancing many opposing factors. Adjusting hardness values to suit process requirements aids the decision-making process.
Stampers, understandably, want ductile materials that easily cold form. But specifiers of high-performance alloys usually don’t consider formability—they want strong, hard and tough parts. The ideal is a material that bends easily during forming but doesn’t bend at all once it’s a part. That’s a tall order. But with a little help from two under-appreciated factors – work hardening and heat treatment – stainless steel can approach that ideal.
The 300 series of austenitic stainless steels can only be hardened by cold working – heat treating is not an option. Because cold working takes place within the plastic range between yield strength and tensile strength, a look at a table of properties might suggest that Type 301 would be a good candidate for stamped parts because its range is comparatively wide. This grade can handle much pulling and stretching but has a tendency to work-harden quickly. For that reason, 301 is not recommended for the deep drawing process.
Type 305 exhibits a much narrower range between yield and tensile strengths but is the preferred grade for deep-drawn applications. About 90% of stainless deep-drawn parts are produced from this grade. Because of its relatively high nickel content, work hardening increases very slowly during the forming process. It can be drawn over a series of does without becoming extremely hard or brittle, and extensive drawing usually is possible before annealing is required. The good initial elongation of 305 falls off rapidly, however, so it is not suitable for operations that induce severe stretching.
Type 302 is the mid-range choice. Its mechanical properties and forming behavior fall somewhere between that of 301 and 305, so it offers benefits and shortcomings of both.
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Courtesy of Ulbrich Stainless Steels and Special Metals Inc.