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The weldability investigation on the nuclear grade austenitic stainless steels was conducted using Gleeble hot ductility and Varestraint hot cracking tests. The study provided an evaluation of the hot ductility response, and hot cracking susceptibility (HAZ liquation cracking and fusion zone solidification cracking) of modified nuclear grade and standard austenitic stainless steels. Extensive microstructural characterization using state-of-the-art analytical electron microscopy (TEM and STEM) as well as SEM (equipped with EDS) and OLM was performed to correlate material behavior with metallurgical characteristics. In addition, studies on the effect of Si, N, and rare earth elements on hot cracking susceptibility and the significance of the ductility dip phenomena was also performed. Furthermore, based on the metallurgical evaluation, possible mechanisms involved in solidification cracking and HAZ liquation cracking of the modified alloys are proposed. Finally, recommended optimized chemical specifications and requirements for nuclear grade stainless steels are also suggested. The hot ductility and weldability behavior of the modified 316NG and 347NG alloys were found to be generally superior to the standard AISI 304, 316 and 347 alloys. The hot ductility response and hot cracking susceptibility of the stainless steels was dependent upon major and minor element content (P, S, Si and Nb), ferrite content (potential), grain size and solidification mode (Creq/Nieq ratio). Among these, the primary solidification mode was the most important factor, such that primary ferritically solidified alloys exhibited a better behavior than alloys solidified in a primary (fully) austenitic mode when other variables were constant. The influence of the primary solidification mode was found to be more important for crack propagation than for initiation. The effect of impurity level (P+S) on the extent of cracking was found to be dependent on the Creq/Nieq ratio, such that when the ratio is greater than approximately 1.6, the P+S content can be as high as 0.06%. However, when the ratio is less than about 1.5, a P+S content as low as 0.02% can exert significant influence on solidification cracking. The relative amount of Nb to C and N/1/2x%Nb/30x%C+50x%N appears to be a predominant factor for hot cracking in type 347. When this ratio is less than approximately 0.1, P, S and Si play a significant role in solidification cracking. In order to predict material behavior more quantitatively and definitively, "new" parameters: critical strain temperature range (CSTR) and ductility recovery ratio (DRR) for Gleeble hot ductility testing and the cracked HAZ length (CHL) for the Varestraint base metal HAZ cracking, were developed and analyzed as a function of metallurgical variables and chemical composition during this study. The correlation among these parameters indicates that when the CSTR is less than 70 10 F and DRR is greater than 40 F, the material is least susceptible to HAZ liquation cracking (virtually zero CHL).

The ductility dip phenomenon related to fracture along the embrittled grain boundaries (by melting and resolidification due to impurity element segregation) at temperatures above the equichohesive temperature but below the dynamic recrystallization temperature. Grain boundary embrittlement may be further exacerbated by the precipitation of chromium rich carbides and thus an increased amount of liquid formation by exposure to a high peak temperature. The ductility dip phenomena may be characteristic of the testing method which involves significant deformation and dynamic recrystallization (under rapid strain rate conditions at high temperatures) and may not be likely to occur in an actual weld HAZ. The addition of nitrogen to high silicon (1.25%) 304 decreases the solidification cracking susceptibility when the primary solidification mode is maintained as ferritic. The decreased cracking susceptibility was found to be due to a decrease in the amount of low melting sulfides by the formation of high sulfur containing silicates. For a fully austenitic 316 stainless steel, the extent of solidification cracking decreased with an increase in the nitrogen content from 0.047% to 0.12%. The decrease in solidification cracking for a fully austenitic material containing nitrogen is due to a decrease in the solidification cell size and a narrower "critical strain temperature range (CSTR)".

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Published: 2006 Number of Pages: 276 File Size: 1 file , 14 MB