关键词: 汽门螺栓, 20Cr1Mo1VNbTiB钢, 第二相, 裂纹

Abstract: Reasons of the cracking failure were analyzed for 20Cr1Mo1VNbTiB steel high pressure speed regulating valve bolt and medium pressure combination valve bolt in the steam turbine of a power plant. The results show that the chemical composition and hardness of the two bolt specimens are in accordance with the relevant standards. In the metallographic structure of the two bolt specimens, there are localized mixed grains and a large number of hard and brittle MC-type second phase particles, the latter of which can act as the crack nucleation sites, thus increasing the crack initiation probability, accelerating the crack propagation of the material, and increasing the crack failure tendency of the material. Such precipitated phases have high stability and are difficult to be eliminated by heat treatment, and are usually formed in the smelting process. Thus the smelting process control should be strengthened to avoid the occurrence of a large number of such hard and brittle MC phases.

Key words: valve bolts, 20Cr1Mo1VNbTiB steel, second phase, cracks