In-service behaviour of creep strength enhanced ferritic steels Grade 91 and Grade 92 – Part 2 weld issues

In Creep Strength Enhanced Ferritic steels control of both composition and heat treatment of the parent steel is necessary to avoid producing components which have properties below the minimum expected by applicable codes. The degree of tempering involved in manufacture will modify the material hardness. While under most conditions hardness is reduced by tempering, exceeding the AC1 temperature can lead to an increase in hardness. In this heat treatment the properties will be relatively poor even though the measured hardness may be apparently acceptable. Thus, care should be exercised in imposing an acceptance test of components based on simple hardness alone.Differences in parent material heat treatment and composition apparently have remarkably little influence on the creep life of the heat affect zone (HAZ). Thus, Type IV cracking in the fine grained or intercritically heat treated regions of the HAZ does not appear to directly depend on the strength of the base steel. This form of in-service damage is relatively difficult to detect using traditional methods of non-destructive testing. Moreover, since repeated heat treatment leads to over tempering and a degradation of properties, specific procedures for making and then lifing repair welds are required. The present paper summarizes examples of damage and discusses best option repairs.

► For many components damage in the weld heat affected zone will be the primary source of in-service problems.
► Repair approaches should consider the influence of heat flow on metallurgical transformations.
► Both development of residual stresses and the local properties of the constituent zones influence Type IV damage.
► Serviceability of components in the creep range must consider stress, temperature and applicable material properties.