Effect of Nitrogen on Evolution of Dislocation Substructure in 316LN SS during Creep

Abstract

Effect of nitrogen on the creep deformation behaviour and the dislocation substructure development and precipitation on creep exposure in 316LN austenitic stainless steel has been studied. Four heats of the steel with nitrogen in the range 0.07 - 0.22 wt. % were melted Creep tests on the steels were carried out at 923 K over a stress range of 140 - 225 MPa. Norton's law (έ = Aσn) relating steady state creep rate with applied stress was found to obey by the steels. The power law exponent ‘n’ was found to decrease from 13.6 to 5.5 with the increase in nitrogen content. Transmission electron microscopic studies were carried out on the creep tested steels to understand the evolution of dislocation substructure on creep exposure. Dislocations were observed to have rearranged in the form of subgrains in the material containing 0.07 wt. % nitrogen. The tendency to form subgrain decreased with the increase in nitrogen content. In the steel containing 0.22 wt. % nitrogen, there was no evidence of dislocation subgrain formation on creep exposure and the dislocations were found to be uniformly distributed in the matrix. Presence of intragranular nitride or carbonitride precipitates in the 316 LN steel containing 0.07 wt. % was not seen even after creep exposure for 9200 h. However, fine intragranular carbonitride precipitates were observed in the steel containing 0.14 and 0.22 wt. % nitrogen after creep exposure for 9700 h and 16000 h respectively. The precipitation and dislocation substructure formation in the steel having different nitrogen content on creep exposure have been correlated with the creep deformation mechanism.