Abstract: The bolt of highpressure main stop valve of a power plant steam turbine was fractured. The physical and chemical test and the failure analysis for the fractured bolt, the material of which is 20Cr1Mo1VNbTiB, were carried out. The results show that during manufacturing process of the bolt of the main stop valve, the control of the heating processing was improper, and there was coarse grain in metallographic structure; in unit operation process, the intergranular crack occurred in coarse grain zone with high stress of the bolt, and the crack continuously extended. Finally, the leakage of the main stop valve and the bolt fracture failure were occurred. The measures to prevent the bolt failure caused by coarse grain were given.

Key words: high-pressure main stop valve, 20Cr1Mo1VNbTiB, macroscopic metallographic phase, coarse grain, fracture