Effect of nitrogen on solidification cracking resistance in ERNiCr-3 weld metal

Abstract

Additions of nitrogen were made to ERNiCr-3 (FM 82) weld metal to investigate the effect on weld metal microstructure and solidification cracking susceptibility. Weld samples were prepared with different argon-nitrogen shielding gas mixtures to produce nitrogen variations in the weld metal. The cast pin tear test (CPTT) was then used to evaluate solidification cracking susceptibility as a function of weld metal nitrogen content. Phase fraction and composition of constituents in the final solidification microstructure were characterized via optical and electron microscopy. Thermodynamic (Scheil) calculations were performed to determine the phase formation during solidification, associated solidification path, and solidification temperature range. Solidification cracking susceptibility was found to increase significantly with increasing nitrogen content The overall amount of second phase in the solidified microstructure increased when nitrogen was added to the weld metal. Small skeletal constituents in the interdendritic regions, primarily Nb-rich carbides (NbC). Were more frequently observed with increasing weld metal nitrogen content Larger cuboidal Ti-rich nitrides (TiN) and carbonitrides (Ti, Nb)(N, C) were found only when nitrogen was added to the weld metal. Their location in dendrite core regions indicates formation during an earlier stage of the solidification process. Scheil calculations confirmed the strong effect of nitrogen additions on the amount and sequence of phase formation during solidification of ERNi-Cr-3 weld metal. High nitrogen levels (> 100 ppm) facilitate primary nitride formation prior to the solidification of the gamma (y) dendrites, and increase the amount of phase constituents in the solidification microstructure. The pres-ence of nitrogen also shifts the start of the eutectic y/NbC formation at the end of solidification to lower temperatures, which results in an increase in the solidification temperature range. This occurs at much lower nitrogen levels (> 25 ppm) and correlates with the observed increase in solidification cracking susceptibility.