Improvement in elevated temperature strength, hydrogen attack resistivity and stress relief cracking susceptibility of Cr-Mo steels

Aiming at development of advanced Cr-Mo steels with mechanical properties and resistivity to embrittlements superior to those of commercially available heavy section 2 1/4Cr-1Mo steels, the effects of alloying elements and impurity atoms on strength, toughness, resistance to hydrogen attack and susceptibility to stress relief cracking in low Si 2 1/4Cr-1Mo steels were investigated. The results obtained are as follows, 1. Nb and V additions increase room temperature and elevated temperature strengths and creep rupture strength remarkably. 2. Lowering effective S improves the susceptibility to stress relief cracking and hydrogen attack which are related to the pre-existing microcavities nucleated on MnS. 3. Reducing N improves the susceptibility to hydrogen attack and increases the creep rupture strength. 4. Improvements of strength and hydrogen attack susceptibility by V and Nb additions are related to the lowering carbon activity and stabilization of alloy carbides by solution of alloying elements to carbides. (orig.)