Secondary hardening in some low-chromium hot-work tool steels

Abstract: Low-chromium hot-work tool steels combine good heat conductivity with excellent tempering resistance. This property combination has made these steels ideal for die casting and extrusion. The overall aim of this work has been to investigate the precipitation during secondary hardening of these low-chromium steels using electron microscopy and atom probe. Modelling and experimental evaluation was also done in order to see if it is possible to add nitrogen to low-chromium steel and still get secondary hardening. The steels was analyzed as-hardened as well as tempered. With no nitrogen added the secondary precipitates were found to be MC and M2C with mixed compositions regarding vanadium, molybdenum and chromium. In the as-hardened state there was some auto-tempering, resulting in the precipitation of ?-carbide and M3C. When tempering at 625ºC for 2+2 hours the microstructure is peppered with small (< 5 nm) secondary precipitates and after slight overtempering there had been some coarsening. However, there was still large amounts of very small precipitates, which indicates that there had been further precipitation during the overtempering. With the nitrogen addition the modelling suggested that the M(C,N) phase would be difficult to dissolve much during austenitizing, but it also predicted that there would be a potential for secondary precipitation. The three trial alloys confirmed that the hardening response was lowered as the nitrogen level was increased; and all three alloys had clear secondary hardening during high temperature tempering. The present work thus shows that nitrogen addition could be possible to use as a mean to reach improved tempering resistance in low-chromium hot-work tool steels.