The paper concerns the effect of annealing time and temperature on the properties of the nitride layer on X37CrMoV51 tool steel used in the extrusion aluminium industry. Samples made from X37CrMoV51 steel were hardened and tempered, and then nitrided at 530 °C. After nitriding, the samples were annealed in a furnace at 470 °C for 8 h, 12 h, 24 h, 30 h and 60 h, and additionally for 20 h at 270 °C. The samples were tested for structure, hardness and abrasion immediately after nitriding and again after annealing. It was found that annealing the nitrided samples leads to degradation of the nitride layer, accounting for the decrease of hardness. The annealing of the samples at 470 °C, over 12 h causes a decrease in mean hardness value from 1176 HV to 1103 HV, and annealing the samples over 30 h at this temperature leads to a decrease in hardness to 964 HV. The changes in nitrogen content in the white (compound) and diffusion layers and the resulting consequences of changes in phase composition and properties were evaluated. Annealing over 30 h at 470 °C caused the white layer to disappear and the average nitrogen content in the diffusion layer to decrease to the level of about 5-6 at%.
Keywords: extrusion dies; hardness; nitriding; structure.