Microstructure and mechanical properties of stainless maraging steel laser weldments after ageing at different temperatures

The microstructure and mechanical properties of Fe-Cr-Ni-Mo-Ti stainless maraging steel laser weldments have been investigated after post-weld ageing treatments in a temperature range from 420 degrees C to 580 degrees C. Metallographic characterization of laser weldment revealed three zones, i.e. fusion zone, heat-affected zone and unaffected parent material zone. Martensite was the predominant phase in the three zones when the ageing temperature was lower than 500 degrees C whereas a large amount of reverted austenite occurred in three zones on ageing above 500 degrees C, leading to hardness decrease accordingly. Hardness distribution across the welds demonstrated that after all ageing treatments, the hardness in heat-affected zone was greatly increased to above 420 HV from 320 HV in as-weld condition, and hardness in three regions varied with different trends with increasing ageing temperature. The different ageing behavior in fusion zone and parent material zone has been discussed by comparison of dissolution of Ti(C, N)(x) inclusion within them during welding thermal cycle. The tensile properties were found satisfactory in weld subjected to ageing at 460 degrees C; the yield strength was increased by 14.5% as compared with as-received aged parent material. A graph was established for determination of fracture location and post-weld ageing efficiency using measured hardness values in both heat-affected zone and parent material zone.