CONTROLLED DRAWING TO PRODUCE DESIRABLE HARDNESS AND MICROSTRUCTURAL GRADIENTS IN ALLOY-302 WIRE

The production of a macroscopically duplex microstructure in stainless steel alloy 302 wire, fine grains on the wire surface and coarse grai ns at the wire interior, was investigated by systematically varying th e drawing angle from 8 to 32, deg and the reduction from 1 to 15 pet. The measured hardness gradient was correlated to the microstructure af ter heat treating at 1000 degrees C for 0.5 hours. It was determined t hat the wire surface must exceed a hardness level of 207 KHN for recry stallization to a fine grain size, while the wire core must be hardene d to a level between 166 and 207 KHN for grain growth. The deformation zone geometry parameter (Delta) for wire drawing, which is convention ally employed to give a relative measure of the strain distribution in a wire workpiece as a function of die angle and reduction, was utiliz ed in the design of the experimental drawing schedules. The magnitude of measured hardness gradients and the corresponding calculated value of Delta were found to vary similarly with die angle but differently w ith reduction. At constant total reduction; multiple- and single-step drawing schedules produced equivalent hardness gradients, even though the calculated values for Delta indicated that the former would give a steeper gradient. Wires with-two : widely differing grain size gradie nts, coarse and fine vs. fine and coarse at the wire surface and cente r, were headed. The wire with fine grains on the surface had the highe r resistance to surface cracking.