Creep rupture strength of V-modified 2 1/4Cr-1Mo steel

Abstract The V-modified 2 1/4Cr–1Mo (Mod. 2 1/4Cr–1Mo) steel plate has been developed for the use in modern petro-chemical pressure vessels. Its resistance to hydrogen attack is improved owing to vanadium addition. As it also has the excellent creep rupture strength, it could be used in the fossil power plant. Here the creep rupture properties have been analyzed with the observation of microstructure, precipitates and sub-structure. Its creep rupture strength lies at the same level with the Mod. 9Cr–1Mo steel. Though it declines at higher temperatures and longer rupture times, the creep rupture elongation tends to increase in that condition, indicating the less susceptibility to brittle creep fracture. Before creep testing, many finely distributed precipitates exist in the matrix. They are as small as 20 nm and containing V, Mo and small amount of Cr. They are classified into two types; one is V rich V 4 C 3 and the other is presumably Mo 2 C that is rich in Mo content. These precipitates grow to the size of about 50 nm by creep test at 600 °C for about 3000 h. Along prior austenite grain boundary, lath boundary and sub-grain boundary, globular M 7 C 3 exists before creep test and it changes to M 2 C growing into matrix during creep test. The decline of creep rupture strength should be caused by the ripening of fine precipitates in the matrix and the coarsening of sub-grain; the latter is ascribed to the decreased coverage of boundaries with M 7 C 3 due to the precipitation sequence of M 7 C 3 to M 2 C. The coarsening of sub-grain size could accommodate the creep strain at grain boundary, leading to the increase of creep rupture elongation through suppressing the void formation.