An investigation of effects of welding residual stresses on creep crack growth for a low alloy butt weld

R5 is an assessment procedure for the high temperature response of structures and R5 Volume 7 specifies the creep crack growth assessment for low alloy welds. In power plants, almost all welds operating in creeping regime have been post weld heat treated (PWHT). However, low levels of welding residual stresses may still exist. It is known that the welding residual stresses in weldments can be difficult to estimate, as their magnitude and distribution depend on materials, geometry, weld procedure and heat treatment.For high temperature low alloy steel, CrMoV, butt welds, R5 Volume 7 suggests that a bending welding residual stress of 60 MPa should be assumed for a circumferential defect following a post weld heat treatment. For conservatism, the relaxation of this low level of the welding residual stress in a service component operating at high temperatures for a long period time has been discounted in R5. As the actual welding residual stresses can be difficult to estimate, it is necessary to carry out a detailed investigation for the effects of the welding residual stresses on the creep crack growth and a number of associated crack growth parameters.In this paper, for a typical low alloy steel butt weld in ½Cr½Mo¼V (‘½CMV’) pipework with 2¼Cr1Mo weld metal under a given set of internal pressure, temperature and system moment, a series of creep crack growth analyses have been conducted using the R5 Volume 7 assessment procedure, assuming the welding residual stresses of 10 MPa–100 MPa and three different crack initiation times. A postulated service-initiated internal circumferential defect (semi-elliptical surface-breaking surface defect) of 2.3 mm deep by 15 mm long has been used.The investigation results are presented by a series of graphs which show the effects of the welding residual stresses and different crack initiation times on the creep crack growth for the weld studied. After a detailed discussion, conclusions can be drawn.