Warm and Cold Heading of Stainless Steel

A series of warm and cold heading tests ranging from room temperature to 400 deg C have been conducted on 305 and copper bearing 302 stainless steel wires as well as 0.2 percent C steel wire using a fully instrumented double blow Waterbury-Farrel machine heading 100 blanks per minute. Total and frictional forces developed during the first blow using a variety of lubricants, together with the second blow loads have been recorded. The effectiveness of the different lubricants employed was examined. The optimum blank temperature for warm heading stainless steel is in the range 200 to 400 deg C where the punch loads are reduced about 25/30 percent, compared to cold heading. Within this temperature range the friction coefficient for the copper coated stainless steel possessing a stearate film on the surface remains relatively low (μ ∼ 0.01/0.02). Reduced punch loads are shown to effect an increase in punch life. For all temperatures the heading loads for the copper bearing 302 stainless steel were lower than for 305 stainless steel. Both these materials exhibit a weak dynamic strain aging effect compared with plain carbon steel. In fact the heading loads were higher between 300 deg and 400 deg C for the plain carbon steel. At room temperature components made from both copper bearing 302 stainless steel and 305 stainless steel possessed fewer shear cracks when employing the former material and this effect is attributed to a lower strain hardening rate.