Cr-Mo鋼の再熱割れにおよぼすバナジウム炭化物の影響

Cr-Mo鋼の再熱割れに関する研究(第10報)

抄録

The authors have confirmed in the previous paper that vanadium increased the reheat cracking sensitivity of Cr-Mo steels by the role of decreasing the stress relaxation. This role of vanadium may be related to the precipitation hardening induced by vanadium carbide. In this paper, the cracking sensitivity of Cr-Mo-V steels was discussed from the view point of the precipitation of various carbide phases. X-ray analysis was carried out on the carbide phases extracted electrolytically from the simulated-HAZ specimen tempered at 973 K. Following experimental results were obtained on the steels of the chemical composition ranges of 0 to 2%Cr, 0.28 to 0.8%Mo and 0 to 0.4%V. (1) The increase of cracking sensitivity brought by the addition of vanadium corresponds closely to the increse of hardness induced by the secondary-hardening. (2) The steels of lower molybdenum content are originally less sensitive to cracking. With the addition of vanadium, vanadium carbide precipitates and it converts the steels secondary-hardenable, and hence, cracking-sensitive. (3) Steels of higher molybdenum content are originally secondary-hardenable ; this is brought by molybdenum carbide. In the vanadium-bearing steel, vanadium carbide precipitates replacing molybdenum carbide. However, the original cracking sensitivity is varied little by vanadium, because those two carbides induce the secondary hardening of the same magnitude.