Improvement of High Temperature Strength of 1Cr-1Mo-1/4V Steel for Boiler Main Steam Pipe

Abstract

In order to develop a low alloy heat resistant steel which has a higher creep-rupture strength than that of 21/4Cr-1Mo steel for boiler main steam pipe, the effects of cooling rates at quenching (1400°C/h-300°C/h) and of thermal cycle on the high temperature strength of 1Cr-1Mo-1/4V steel have been investigated. The thermal cycle is designed so as to simulate the thermal condition encountered in the hot bending of the main steam pipe. The high temperature strength of the welded joint of the steel has also been investigated. It is found from the results that this steel is unsuitable for main steam pipe.
A trial, therefore, has been made on the improvement of the hardenability and the high temperature strength of 1Cr-1Mo-1/4V steel, and the effects of increase in the content of C, Cr and Mn, and the effects of Ni or B addition on the microstructure and the high temperature strength of this steel have been investigated. Experimental results show that 1Cr-1Mo-1/4V steel with 0.003wt%B has a favourable structure (all bainite) and high temperature strength even at the cooling rate as low as 300°C/h.