Some aspects of the thermal stability of 316L weld metal

This investigation has been concerned with some aspects of the thermal stability of the austenitic stainless steels and in particular that of 316L weld metal. Thermal stability might be considered as the ability of a material to maintain its properties after exposure to elevated temperatures. Poor thermal stability of the austenitic steels is generally linked to precipitation reactions and most often to formation of the brittle, intermetallic compound sigma phase. The thermal stability of welds is often found to be lower than that of the base metal. This effect is most often linked to the presence of ferrite in the weld deposit which greatly increases the rate of sigma formation. The presence of certain residual elements in the weld deposit is found to have major effects on the thermal stability.

This investigation was directed toward characterizing the effect of ferrite content upon the structural stability of 316L weld metal. Standard argon shielded 316L gas tungsten arc deposits contained about 5% ferrite. The addition of one volume percent nitrogen to the shielding atmosphere effectively prevented ferrite presence in the weld deposit. These two types of deposits when aged at 650°C had significantly different responses. The argon shielded-high ferrite deposit formed sigma phase, carbides, and probably austenite. The argon-nitrogen shielded deposit formed carbides with only trace amounts of sigma phase.

The effect of stress on structural stability was studied. An eight station stress-rupture laboratory was set up for stressed aging experiments. Results of this investigation indicated that stress had no effect upon the structural changes of the standard deposits but appeared to retard carbide formation in the argon-nitrogen shielded deposits.