Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

Sheet material specimens from 18Ni (250-grade) maraging steel were gas tungsten-arc welded using two different filler wires: one matching the base material in composition and the other with higher cobalt but lower molybdenum and titanium contents. Welding was carried out both in constant-current and pulsed modes. Post-weld aging was performed at three different temperatures, viz., 425, 480 and 520°C. Metallographic characterization revealed pronounced segregation, presumably of Ti and Mo, along interdendritic and intercellular boundaries in the weld metal produced with filler of matching composition. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Segregation and austenite reversion were not noticed when Ti and Mo contents were reduced in the filler wire, except at the highest aging temperature used. The use of pulsed welding resulted in some grain refinement, but the effect was not considerable.