EVALUATION OF HOT CRACKING SUSCEPTIBILITY OF SOME AUSTENITIC STAINLESS STEELS AND A NICKEL-BASE ALLOY

摘要

For the Prototype Fast Breeder Reactor (PFBR), a modified version of 316L stainless steel, designated as 316L(N), has been chosen as the major structural material. In order to reduce the risk of sensitisation, the carbon content has been reduced to less than 0.03 wt-%, and to compensate for the loss in strength due to the reduced carbon content the nitrogen content has been specified to be about 0.08 wt-%. For fuel clad and wrapper applications, a radiation-resistant variation of 316 stainless steel containing titanium about 6 times the carbon content, named Alloy D9, has been chosen. Weld metal and heat-affected zone (HAZ) cracking of austenitic stainless steels Alloy D9 and 316L(N) were investigated. Specifically, the role of titanium in Alloy D9 and nitrogen in 316L(N), along with the impurity elements, were studied. In Alloy D9, cracking increased with Ti/C ratio, but a significant contribution to cracking came from the nitrogen of about 200 ppm picked up during welding even when using high purity argon shielding gas. Titanium to carbon (Ti/C) ratio of about 4 was found to show least susceptibility to solidification as well as HAZ cracking. In modified 316 weld metals with 3-7 FN, nitrogen in the range 0.06-0.12% had no detrimental effect on weldability. Weldability of Inconel 718 base material was also investigated. From hot cracking considerations, ENiCrFe-3 consumable was found more suitable to weld Inconel 718 than consumable of matching composition. Weldability was tested in various geometrical configurations such as T-, butt- and rod-to-strip in similar as well as dissimilar combination with 9Cr-1 Mo steel using ENiCrFe-3 consumable. The studies showed the need for careful joint preparation and use of techniques to enhance weld penetration for minimising defects. This paper discusses the weldability problems associated with these austenitic stainless steels chosen for use in the construction of PFBR. Various criteria in use for weldability evaluation as per codes in relation to the present data on stainless steels and nickel-base alloys are also discussed. The importance of hot cracking evaluations in determining the fabrication weldability of these austenitic stainless steels is also discussed in detail.