Study of Reheat Cracking of Cr-Mo Steels (Report 10). Influence of Vanadium Carbide on Reheat Cracking of Cr-Mo Steels.

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.