Effect of grain boundary precipitation and δ-ferrite formation on surface defect of low nickel austenitic stainless steels

Abstract

The present study elaborately discussed the effect of grain boundary precipitation and delta (δ) ferrite formation on surface defects of low nickel austenitic stainless steels. Several specimens were collected from different locations (i.e. centre and side) of cast slabs and rolled plates with variable nitrogen and carbon contents. The alloy segregation, precipitation and δ-ferrite formations were analyzed using several characterization tools. It was observed that the segregation of alloying elements along the grain boundaries is solely responsible for variation in precipitation and δ-ferrite formation among the specimens. Type of precipitation is mainly composition dependant. Relatively higher nitrogen and low carbon containing specimens show Cr2N and/or Cr3C1.52N0.48 rich continuous precipitation along the grain boundaries. On the other hand, decrease in nitrogen content along with the increase in carbon content results in discontinuous or cellular type of precipitation. Continuous grain boundary precipitates were found more detrimental compare to discontinuous or cellular precipitations. Micro-cracks are probably generated at the stress concentrated areas of precipitation–matrix interfaces and propagate along the interfaces or grain boundaries towards the stress free surface. Again, additional driving forces towards the surface crack formation and propagation are facilitated by the δ-ferrite and austenite interfaces during hot rolling.