Microstructure Aspects of Creep Resistant Welded Joints

Abstract

A soft zone in the welded joint, negatively influencing the creep behaviour, was detected and localised in the intercritical zone of the heat affected zone (HAZ), limiting the long-time creep strength of welds. Energy-filtering transmission electron microscopy (EFTEM) has been used for imaging precipitates and grain boundary phases in the soft zone of the HAZ and in the weld deposit of advanced ferritic martensitic Cr steels. Due to different creep duration times at a temperature of 600°C, many different precipitates and grain boundary phases and a new type of Cr-V-Nb-N rich precipitation, the so-called modified Z phase emerge in the joint. The appearance of the Cr and V rich Z phase was followed by a decrease of the population of M23C6 and MX precipitates. According to these results a decrease of the creep strength in the soft zone of the HAZ beginning after about 10,000 h was found, which seems to be in connection with the observed microstructural changes. This effect must be taken into account in the design of welded components made from this type of materials. Basic investigations using mainly the Gleeble HAZ simulation technique corroborate a drop in the hardness (soft zone) and creep resistance that occurred in the inter critical zone of the HAZ, where the peak temperature reached a level of about 900–1,000°C (zone of α/γ-transformation). This and previous investigations have shown a drop in the creep resistance for the HAZ. Compared with the base material, reductions of the creep resistance in weld samples of about 20–25% have to be taken into account. At lower stress values, the fracture location shifts from the base material towards the softened intercritical zone of the HAZ (type IV cracking). The microstructure of the HAZ shows after stressed condition pores and micro-cracks in the softened intercritical zone (temperature between Ac1 and Ac3). A correlation between the creep resistance and the size distribution of particular particle populations could not be clearly proven. For the total evaluation of the microstructural development the reduction of the dislocation density and the recovery processes (formation and growth of subgrain boundaries) must be taken into account. The tests were made on cast G-X12 CrMoWVNbN 10.1.1 parent material and Cromocord 10M weld deposit