Investigation On Correlation Between Microstructure And Property Of Friction Stir Welding Joint Of High Nitrogen Steel

In this study, we chose high nitrogen austenitic stainless steel (HNS) as researchobject and obtained a defect-free welded joint using friction stir welding (FSW). Based onthe theory of strength matching, we further investigated the effect of gradientcharacteristics on the mechanical property of FSW joint. In addition, for the severe plasticdeformed microstructures in nugget zone (NZ) of the joint, we put forward the concept ofeffective grain size and modified the Hall-Petch (H-P) relationship between the hardnessand grain size in NZ. Furthermore, the strengthening mechanism of low angle boundaries(LABs) was also discussed.The analysis of microstructures and properties showed as follows. The averagenitrogen-content of the FSW joint was very close to that of base material (BM), almost noloss. All the microstructures in the joint were austenitic phases. The NZ exhibited finegrains due to the dynamic recrystallization occurred during FSW. Compared with BM, theas-welded joint showed much higher yield strength (YS) and slightly higher ultimatetensile strength (UTS), respectively. However, the elongation of as-welded joint was onlyachieved78%of BM. After the post-welded heat treatment (PWHT), the YS of the jointdecreased to the level which is slightly higher than BM, but the UTS changed a little.Moreover, the elongation of the joint recovered to90%of BM.We found that significant microstructure and mechanical property gradients along thetransverse and thickness directions existed in the as-welded FSW joint of HNS. Thesegradients resulted in the combined overmatching feature with higher strength but lowerplasticity. Moreover, the PWHT was shown to effectively weaken these gradients anddecreased the matched factor of the as-welded joint. The reduction in the matched factorwas beneficial to the increment in elongation, but had little effects on the YS and UTS ofthe joint. The enhanced YS, UTS, and equivalent elongation to BM can be obtainedsimultaneously by decreasing both the matched factor and the proportion of NZ in thegauge length of the joints.In addition, by calculating the grain orientation spread and introducing the concept of effective grain size, we solved the problem that the classical H-P equation was unsuitablefor the linear fitting of the relationship between the hardness and grain size of severeplastic deformed microstructures. The H-P equation was modified as:Hv=H0+kH[(kRX+kD)/kRX]1/2d-1/2where the kRXand kDare recrystallized factor and deformed factor, respectively. Theeffective H-P slope Keffequals the term of kH[(kRX+kD)/kRX]1/2. Its physical meaningrevealed that the boundary strengthening increased accompanied with the increase indeformed degree. Similarly, the boundary strengthening decreased accompanied with theincrease in recrystallized degree. Furthermore, according to the results, we preferred thepile-up model to explain the strengthening mechanism of LABs. The LABs are alsoconsidered as the barriers to dislocation motion.