Influence of Tool Rotational Speed and Post-Weld Heat Treatments on Friction Stir Welded Reduced Activation Ferritic-Martensitic Steel

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2017-08, Vol.48 (8), p.3702-3720
• Main Authors: Manugula, Vijaya L., Rajulapati, Koteswararao V., Reddy, G. Madhusudhan, Mythili, R., Bhanu Sankara Rao, K.
• Format: Article
• Language: English
• Subjects: Activation; Air cooling; Austenite; Base metal; Characterization and Evaluation of Materials; Chemistry and Materials Science; Coalescing; Ferritic stainless steel; Ferritic stainless steels; Friction; Friction stir welding; Grain boundaries; Hardness; Heat affected zone; Heat treating; Heat treatment; Impact strength; Martensitic stainless steel; Martensitic stainless steels; Materials Science; Mechanical properties; Metallic Materials; Metallurgy; Microstructure; Nanotechnology; Precipitates; Structural Materials; Surfaces and Interfaces; Tempered martensite; Tempering; Thin Films; Tool steels; Toughness
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-017-4143-5

The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M 23 C 6 on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe 3 C precipitates were found at both the rotational speeds in SZ. The Fe 3 C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M 23 C 6 on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed.