| Abstract Scope |
Introduction
17-7 PH is a semi-austenitic precipitation-hardening (PH) stainless steel. It offers good corrosion resistance, and can be easily fabricated in the austenitic condition before heat treatment to the high strength, PH condition. Applications for 17-7 PH stainless steel include heat exchangers, springs, and a range of components in the aerospace industry.
Welding of 17-7 PH can be performed in any heat treated condition; however, the austenitic condition is recommended, as welding results in fully austenitic microstructures in the heat affected zone (HAZ) and weld metal. Welding is followed by a three-step post weld heat treatment (PWHT) comprised of annealing, austenite conditioning, and aging. In some instances application of the recommended PWHT may not be feasible. Instead, the component is left in the as-welded condition or only the aging step may be applied.
The goal of this project was to understand the role of PWHT in determining the strength of the HAZ and weld metal, as compared to the base material. This was achieved through characterization of the microstructure, microhardness, and calculated strength for welds produced by electron beam welding (EBW) and gas tungsten arc welding (GTAW) and PWHT under various conditions.
Experimental Procedures
17-7 PH plate was received in the mill annealed, austenitic condition. Coupons were cut and heat treated to the martensitic condition through the application of the mill anneal (1066 °C, 90 min) and austenite conditioning (760 °C, 90 min) heat treatments. Bead-on-plate welds were produced on the coupons using EBW or GTAW. Each coupon was then cut into three sections, and a different heat treatment was applied to each section. The parameters of these heat treatments were: mill anneal (1066 °C, 90 min, argon quench), austenite conditioning (760 °C, 90 min, argon quench) and aging/precipitation hardening (566 °C, 90 min, air cool). The first section of each coupon was left in the as-welded condition (W), the second section was aged (W + A), and the third section was mill annealed, austenite conditioned, and aged (W + 3A). Microhardness maps and micrographs were collected from each coupon which characterized the base material, HAZ and weld metal. Yield strength and ultimate tensile strength of the welded material was calculated from the microhardness values [Pavlina and Van Tyne, 2008].
Results and Discussion
Average microhardness of the weld was compared to the microhardness of the parent material. Weld metal produced by EBW and GTAW, left in the as-welded condition, exhibited microhardness of 70% and 65% of the parent material, respectively. Aging after welding was not as effective in increasing the hardness of the weld metal and HAZ relative to the base material.
Though the weld metal did exhibit an increase in microhardness, the parent material
microhardness increased to a larger extent. This was attributed to precipitation of AlN in the base material resulting in full hardness, while the weld zone remained austenitic. Therefore the microhardness of the weld as a percentage of the parent material is lower, of about 64% and 56% for EBW and GMAW, respectively. Following the full three step PWHT (W+ 3A), the weld metal exhibited microhardness of 96% and 99% of the parent material for EBW and GMAW, respectively. In the W and W + A conditions, EBW exhibits slightly higher microhardness in the weld in comparison to GTAW. However, the W + 3A condition produced similar microhardness values for both EBW and GTAW, which is about equal to the parent material. The calculated values for YS and UTS followed similar trends as the microhardness results.
Conclusion
This study explored the effect of the PWHT and welding methodology on the strength of the HAZ, weld metal, and base material in 17-7 PH stainless steel. With regards to welding methodology, hardness was shown to increase slightly with EBW, as compared to GTAW. Overall, the PWHT was shown to have a greater influence on the hardness (and strength) of the weld region, as compared to welding methodology. The highest microhardness and strength in the weld metal was achieved through the application of the 3 step heat treatment. This resulted in a weld metal microhardness of 96% and 99% of the parent material for EBW and GMAW, respectively. In instances where the weld metal strength requirement may be less than that of the parent material components, 17-7 PH may be used in either the welded and aged or as-welded condition. |