Ti-Containing Rapid Nitriding Steel (2nd Report)

We reported in the previous report that we obtained hard and thick hardened layers when Cr-Ti steels (Ti=2∼3%) were nitrized. But the hardness gradient at the boundary between the hardened and the unhardened parts was too large. Therefore, we intended to improve this defect. We prepared six samples (C:0.2∼0.4, Cr:0∼2.25, Ti:2.0∼2.6) and measured the hardness distribution and tested the heat resistance. The results obtained were as follows: (1) Primary nitriding was carried out at high temperature to increase the depth of nitrization, and the secondary nitriding at 650°C to obtain good results. By this double nitriding the surface hardness was not raised and a distinct peak of hardness appeared at the border point between the hardened and the unhardened layers. (2) Next, the primary nitriding was operated at 600∼675°C for the purpose of raising the surface hardness, and the secondary nitriding was done at a slightly higher temperature than that of primary nitriding (700∼750°C) lower the hardness gradient. By this method, we obtained high surface hardness (VHN:900), small hardness gradient and good surface conditions. The hardness was not lowered by reheating about at 600∼700°C.