THE DEVELOPMENT OF SOFT FERRITIC STAINLESS-STEELS FOR COINING

The limiting criterion in the use of stainless steel for certain formi ng operations such as coining is its hardness, which determines parame ters such as required striking pressure, wear on dies, and integrity o f pattern detail. Attention is given here to the development of ultra- soft ferritic stainless steels for coinage. In the first instance, the influence of carbon content on the striking properties of a series of commercial and experimental 17%Cr alloys was evaluated. It was found that a lower carbon content permitted the striking of relatively highl y profiled and intricate patterns, provided that the hardness did not exceed 125 HV. Hardnesses as low as 115 HV were achieved in the fully annealed condition, but these alloys required high striking pressures. In a further study, three grades of experimental 14%Cr ferritic stain less steels were produced with a view to optimizing the ductility. Whe n the residual impurities were minimized, hardness values of about 100 HV and lower were achieved. Micro-alloying additions allowed the heat -treatment response and grain size to be controlled. Excellent tensile properties, with elongations of up to 42 per cent, were achieved in t he fully annealed condition, and highly profiled patterns were produce d in coining trials.