Influence of the nitriding time in the wear behaviour of an AISI H13 steel during a crankshaft forging process

Abstract

The influence of nitriding time in the wear behaviour of an AISI H13 hot work steel used on forging dies has been studied. AISI H13 steel was nitrided by a patented sursulf bath, varying nitriding time from 1 to 24 h. Optical microscopy and microhardness deep profile through the nitrided layer were performed for each nitriding time. Standard pin-on-disk wear test were conducted at temperature similar to that attained during forging process. Sliding distance was varied from 450 to 1500 m. It has been observed that friction coefficient does note change with nitriding time and wear rate varies as a function of the sliding distance due to the presence of different wear mechanisms. Thus, for short sliding distances wear rate depends on two mechanisms: plastic deformation and abrasive wear, whereas for large sliding distances the mechanisms controlling wear rate are abrasive and oxidative wear. Based upon laboratory results, performance tests on hot-forging tools were carried-out under industrial conditions. Crankshaft forging dies were sursulf nitrided during 7 and 13 h, for which the number of forged parts was 4065 and 10381, respectively. These results showed that increasing the nitriding time and thus the thickness of nitrided layer give rise an increase of the forging die durability.