Nitride Precipitation during Isothermal Heating after Rapidly Cooled from High Temperature in Duplex Stainless Steel

Nitride precipitation during isothermal heating after rapidly cooled from the ferrite phase enriched temperature in duplex stainless steel was investigated to clarify the effect of nitrogen in steel as an alloying element and austenite phase growth. Nitride precipitation is frequently harmful for the properties such as toughness or corrosion resistance in duplex stainless steels. The heat affected zone (HAZ) in the vicinity of fusion boundary in weldments tends to have more remarkable nitride precipitation because of rapid cooling from the ferrite phase enriched temperature. During cooling process the growth of austenite phase occur resulting in the prevention of nitride precipitation.Employing 25%Cr duplex stainless steels containing various level of nitrogen of 0.1% to 0.3%, the measurement and observation of nitride precipitation were conducted in the specimens isothermally heated at 873 K to 1073 K after rapidly cooled from 1653 K. As results the low nitrogen containing steel had relatively much nitride precipitation than the higher nitrogen containing one because of the less increase in the fraction of austenite phase growth with larger nitrogen solubility. This result can be explained by the decease of oversaturation of nitrogen in ferrite phase by contribution of austenite phase growth during thermal cycle.