Creep behaviour of post-weld heat-treated 2.25Cr-1Mo ferritic steel base, weld metal and weldments

Abstract

The evaluation of the creep rupture behaviour of 2.25Cr-1Mo ferritic steel at 773 and 823 K over a stress range of 130-300 MPa has been carried out. The material conditions examined include base material, weld metal and weldments (comprising base, weld and heat-affected zones). Specimens for creep testing were taken from single-V-weld pads, fabricated by manual metal arc welding using basic coated 2.25Cr-1Mo electrodes, and were given a post-weld heat treatment (973 K for 1 h). Microstructure and hardness in the as-welded, post-weld heat-treated and creep-tested conditions were evaluated. The heat-affected zone consisted of coarse-grain bainite, fine-grain bainite and intercritical structure regions. Generally, the weld metal exhibited significantly higher creep rupture strength than the base material, while the composites showed inferior creep strength. Ductility was found to decrease with increasing rupture time for all the material conditions. The rupture elongation exhibited by the base material was significantly higher than that shown by weld metal and composite specimens. Composite specimens crept at a faster rate while weld metal deformed at a slower rate than base metal. The applied stress (σ) dependence of the secondary creep rate (ε_s was found to follow a power-law relationship of the form ε=Aσⁿ. At all test conditions, transgranular fracture was observed. Failure in the composite specimens occurred in the intercritical structure of the heat-affected zone. Interrupted creep tests at 823 K on composite specimens have revealed progressive localization of creep deformation in the intercritical structure prior to fracture. An attempt has been made to establish equations that can predict the weld joint creep properties on the basis of the base and weld metal properties.