Microstructure and Mechanical Properties of 21-6-9 Stainless Steel Electron Beam Welds
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ELECTRON beam (EB) welds in annealed 21-6-9 stainless steel (SS) sheet were made and characterized to determine the weld fusion zone (FZ) and base metal mechanical properties for use in structural design calculations. Alloy 21-6-9 SS, also known as Nitronic 40, was developed as an improved austenitic stainless steel over traditional 300 series stainless steel alloys. This alloy contains nominally 21 pct Cr, 6 pct Ni, 9 pct Mn (compositions in wt pct) and has improved corrosion resistance due to its higher Cr content of 21 pct. In addition, 21-6-9 SS uses N rather than C as a strengthener, which reduces the tendency for corrosion sensitization that can occur during heat treating and welding. Nitrogen can be added up to 0.35 pct, which improves the room temperature yield strength of 21-6-9 SS over 300 series stainless steel alloys, while maintaining good toughness and high ductility (>40 pct elongation to failure). Manganese is added to 21-6-9 SS to increase austenite stability, particularly at cryogenic temperatures, while reducing the Ni content from 8 to 6 pct as compared to 300 series stainless steels. 21-6-9 SS can be further strengthened by cold work and can be machined, forged, and welded using the same methods as 300-series stainless steels. A review report on characterization of 21-6-9 SS base metal can be found in Reference 1, and its yield stress over a wide range of strain rates and temperatures can be found in Kassner and Breithaup.[2]
Although the base metal properties of 21-6-9 SS are well documented, the properties of the welds are not as well understood. Measuring the mechanical properties of weld joints is complicated by the fact that microstructure and property gradients are formed in the weld FZ and heat-affected zone (HAZ) due to the localized nature of welding heat sources. Oftentimes, the welds are not large enough to allow the extraction of all-weld metal tensile bars from the FZ, and under these circumstances cross-weld tensile bars are sometimes used. The cross-weld tensile samples give an overall measure of the joint performance and fail in the weakest portion of the weld joint region, which may be the base metal, HAZ or FZ depending on the material and the welding conditions. In order to measure the mechanical properties of the weld FZ itself, hardness measurements are often performed and sometimes correlated with stress–strain behavior using microhardness and nanoindentation methods.[3,4] This study characterizes EB welds tested in a crossweld geometry with different sample sizes and configurations, and the results are compared to a nanoindentation method for predicting stress–strain behavior. The results are modeled using the Steinberg–Guinan and Johnson–Cook models[5,6] to create mechanical property data for 21-6-9 SS welds over a wide range of strains under quasi-static conditions at room temperature.
II. JOHN W. ELMER, G. FRED ELLSWORTH, JEFFREY N. FLORANDO, ILYA V. GOLOSKER, and RUPALEE P. MULAY are with the Lawrence Livermore National Laboratory, Livermore, CA, 94550. Contact
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