Fatigue crack propagation in 2.25 Cr-1.0Mo steel weldments in air and hydrogen

Abstract

Fatigue crack growth (FCG) behaviour of 2.25Cr–1.0Mo steel plate and welds tested in air and gaseous hydrogen was investigated. Microstructural observations indicated that both the weld metal (WM) and heat-affected zone (HAZ) were composed of mainly bainitic microstructure. A ferrite matrix with uniformly dispersed carbides was found in the annealed base metal. The change in microstructures of WM and HAZ between the as-welded and tempered welds was limited. The as-welded WM had the lowest impact toughness among the specimens, however, the impact toughness was improved after postweld heat treatment at 700°C/1 h. Regardless of the testing environments, the as-welded WM exhibited a higher resistance to crack growth than the steel plate as the crack propagated transversely to the welding direction, especially in the low ΔK range. However, higher fatigue crack growth rates in the as-welded HAZ than those of the steel plate were found. As the crack grew along the fine-grained heat-affected zone in the as-welded condition, significant retardation of crack growth as compared with that of the steel plate was obtained when tested in air or gaseous hydrogen. In various regions of the weld, the FCG behaviour of tempered welds was similar to that of the steel plate in air. The results also indicated that the tempered bainitic microstructure present in WM or HAZ was more resistant to unstable crack growth than the annealed steel plate tested in gaseous hydrogen. Environmentally induced acceleration of FCG in various regions of the weld was usually exhibited a change of fracture features.