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YES. The 300 series stainless steel can be "hardened" BUT only by "work hardening." That is by cold working the material, either by cold rolling down to lighter and lighter gauges, or by "drawing" through a die or other size altering operation. "Annealing" stainless steel will REMOVE the work hardening effect. YES and NO. The 400 series have two different stainless steel structures. One is called "ferritic" (409, 430, 434. 439) which cannot be hardened by heat treatment. The other is called "martensitic" (403, 410, 416, 420, & 440 A,B,C) which CAN be hardened by heat treatment.