THE DELTA-TRANSITION TO GAMMA-TRANSITION IN FE-CR-NI ALLOYS DURING LASER TREATMENT

Laser remelting experiments have been performed with Fe-18%Cr-Ni alloy s to evaluate the effects of composition (Cr/Ni ratios between 1.30 an d 1.83) and growth velocity (0.9 to 50 mm/s) on the microstructure sel ection during solidification between primary delta-ferrite and primary gamma-austenite. The experimental results are compared with theoretic al predictions from a dendrite growth model. The experimental transiti ons from ferrite to austenite are in good agreement with theory for bo th an increase in Ni content and increase in growth rate. The transiti ons from austenite to ferrite are found to occur at lower Ni contents than predicted. It is thought that the delta-gamma transition shows no nucleation barrier and is controlled by dendrite growth kinetics and the gamma-delta transition is controlled by nucleation of ferrite.