PRECIPITATION OF ALPHA'-PHASE IN NI-CR-FE OVERLAY WELD ALLOY WITH DISPERSED NBC PARTICLES

Isothermal aging at 773 approximately 1023 K has been carried out to investigate precipitation in low Fe, high Cr, Ni-base overlay weld alloys which were formed on a mild steel by plasma powder welding process. In this work, effects of NbC addition on precipitation process were studied by hardness measurements, X-ray diffraction and metallographic methods. It was found that the NbC dispersed alloy was higher in age-hradenability than the NbC-free alloy in the as-welded condition and both alloys had a peak hardness followed by slowly softening during aging. In the NbC-free alloy, nodular precipitates appeared at grain boundaries and then continuous precipitation of alpha'-phase occurred within the matrix (gamma-phase). However, in the NbC dispersed alloy, any precipitaion has not occurred except for continuous precipitation of alpha' in the matrix. It was considered that such different precipitation phenomena have been caused by niobium atoms dissolved in the matrix. Age hardening observed is due to the continuous precipitation of alpha' particles and the softening is caused by their coarsening.