CREEP STRESS-CONCENTRATIONS IN MISMATCHED WELDMENTS - STARTING POINTSFOR EARLY CREEP DAMAGE AND CREEP CRACKING

The material creep properties of weldments, i.e. the minimum creep str ain rate, the creep rupture strength and the creep ductility, do in ge neral differ from those of the parent material. It is often assumed th at if the weld material creep strength is higher than that of the pare nt, the life of the weldment will also be higher. This is not always t rue. In fact, when subjected to a prescribed displacement, e.g. a circ umferential weldment in a pipe subjected to internal pressure, a creep hard weld develops a stress concentration which may result in a reduc ed life expectancy of the weldment system. Similarly, the properties o f the heat affected zone often differ from those of the parent and wel d material properties which may be one reason for early Type IV cracki ng. In addition, the weld preparation geometry, the width of the HAZ a nd other geometric parameters influence the stress distribution. The p resent paper summarizes some of the effects of material property varia tions in weldments and their impact on the creep life expectancy. It i s shown that the stress concentrations resulting from material propert y mis-matching may be high enough to produce premature creep damage in the weldment, or will cause creep cracking starting from initial defe cts.