Iron comes in two basic forms: ferrit and austenit. Ferrit is magnetic, austenit isnīt. When you heat iron above a certain temperature it becomes austenit (when you make a nail red hot itīll lose itīs magnetic properties as long as it stays hot). Due to the alloy metals low carbon stainless steel stays austenitic at room temperature therefore itīs not magnetic. As a side effect it canīt be hardend by heat treatment.
Hope that helps.
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