The transfer behavior of nitrogen into the welding metal during gas tungsten arc welding process of 32Mn-7Cr-1Mo-0.3N steel was investigated. The effects of gas tungsten arc welding process variables, such as the volume fraction of nitrogen in shielding gas, arc holding time and arc current on the nitrogen content in the welding metal were also evaluated. The results show that the volume fraction of nitrogen in gas mixture plays a major role in controlling the nitrogen content in the welding metal. It seems that there exhibits a maximum nitrogen content (depending) on the arc current and arc holding time. The optimum volume fraction of nitrogen in shielding gas is 4% or so. The role of gas tungsten arc welding processing parameters in controlling the transfer of nitrogen is further (confirmed) by the experimental results of gas tungsten arc welding process with feeding metal.
SEM and Field emitting TEM-EDAX were used to investigate the fracture surface of series impact specimens and th e grain boundary chemistries of VIM (vacuum-induction-melted) Fe-38Mn austeni tic alloy before and after ESR (electroslag remelting,). The quantity and the si ze of inclusions were also examined. The results show that the VIM Fe-38Mn aust enitinic alloy water-quenched from 1 100 ℃ undergoes an obvious ductile-to-b rittle transition, and the impact work at ambient temperature is 242 J, the corr esponding fracture surface exhibits a dimple character. However, the impact work at 77 K of VIM alloy is only 25 J and the fracture mode is IGF (intergranular f racture). After ESR, the impact work at ambient temperature is 320 J and the fra cture surface exhibits a character of “volcano lava” (meaning excellent toughn ess); The impact work at 77 K is up to 300 J and the fracture mode is microvoid coalescence mixed with quasi-cleavage. The segregation of Mn is not found in al l specimens, but the segregation of S is observed, and the S segregation is decr eased after ESR. The examined results of inclusions show that ESR reduces the q uantity and improves the morphology of inclusions. From the above results it can be seen that the cryogenic IGF of VIM Fe-38Mn austenitic alloy is related to t he S segregation at grain boundary. After ESR the decrease in the quantity and s ize of inclusion results in the increase of the impact work at ambient temperatu re, while the restriction of IGF is related to the decrease in the total level, and hence in the grain boundary segregation of S.
