Thermographic and Microstructural Analysis of Martensitic Stainless Steel ASTM A743 CA6NM Arising Out of Layered GMA Welding Process Using AWS 410 NiMo Welding Wire

摘要

The martensitic stainless steel ASTM A743 CA6NM is typically used in the production of hydroelectric turbines due to its known high resistance to cavitation induced surface damages. Despite the fact the material presents a high resistance to cavitation, depending on the loading condition to which the turbine runner is subjected and on its geometry, fatigue cracks can develop, thus requiring repair by means of removing material around the crack, up to its complete elimination, and by depositing weld metal in the cavity followed by a grinding process, in order to recover the original runner geometry. Such a repair process is normally done on site, which means that it is not possible to carry out the post weld heat treatment necessary to bring the newly deposited weld metal and the base metal to the same microstructure encountered in the runner when it comes out of manufacture. In order to study the effect that a layered welding has on the base material and on the weld metal, this work aims at studying the microstructural changes that occur in the CA6NM stainless steel welded in multiple layers with a AWS 410 NiMo welding wire. In order to attain such an objective, several 410 NiMo weld beads were deposited in successive layers on the border of a 5mm thick sheet while the resulting temperature fields were monitored by a thermographic camera. After the welding process, the samples cut from the welded sheet were examined in the perpendicular direction of deposition and their resulting microstructures where analyzed and correlated with the temperature history recorded during the welding process. Hardness tests were also carried out.