The effect of the precipitates type on the thermal fatigue properties of 18% Cr ferritic stainless steel weld HAZ

The microstructural changes and precipitation behavior during thermal fatigue testing of the 18% Cr ferritic stainless steel weld heat affected zone (HAZ) were investigated. The simulation of the weld HAZ and thermal fatigue testing were carried out using a metal thermal cycle simulator (MTCS) under a complete constraint force in the static jig. The thermal fatigue properties of the weld HAZ deteriorated during cyclic heating and cooling in the temperature range of 200-900 degrees C due to the decrease of Nb content in the solid solution, coarsening of MC type precipitates, the Laves phase, and M6C precipitates with grain coarsening and softening of the matrix. Primary and secondary fatigue cracks were initiated from the notch of the specimen after around 60% of the total number of cycles due to the coarsened particles and grain boundary oxidation. As the number of fatigue cycles increased, a transgranular crack propagated towards the center of the specimen and finally the fracture was complete as it merged with the spontaneously formed and coarsened cavities in the specimen during the thermal cycling. (C) 2012 Elsevier B.V. All rights reserved.The authors wish to thank the POSCO Technical Research Laboratory for supporting this research