Influence of Shielding Gas Composition and Pulsed Current Frequency on the Microstructure of Austenitic Stainless Steel Welded by Pulsed Current GTAW

摘要

In the current research, an attempt was made to study the effect of shielding gas composition and operational parameters on the microstructure of austenitic stainless steel. Fulfilling this purpose, a 4 mm austenitic stainless steel sheet was provided. Pulsed current gas tungsten arc welding was carried out using nitrogen gas with the volume percent of zero, 0.5, 1, 2, 5, and 10 in addition to argon as the shielding gas and under pulsed current with frequencies of 40, 80, 120, 160, and 200 Hz. After welding, samples were cut, and the metallographic study was done on weld metal by optical microscope and scanning electron microscope. Furthermore, a ferrite-scope test was performed on the weld metal, and the results were evaluated. Mechanical properties were investigated, and fracture surface was studied. Results showed that increasing the frequency of pulsed current leads to a decrease in ferrite amount in the microstructure, and the area fraction of ferrite decreased to 23% by increasing the frequency. Moreover, it was proven that the addition of nitrogen to the shielding gas resulted in an ascending change in the heat input in the weld pool. Also, with the increase in welding metal nitrogen, ferrite at frequencies of 40 and 200 Hz decreased to 56% and 62%, respectively. In addition, the morphology of the remaining ferrite transformed from mixed lacy-vermicular to completely vermicular. The hardness of the weld metal increased to 66% and 37% at the frequencies of 40 and 200 Hz, respectively. Similarly, the yield strength increased to 11% and 10% at similar frequencies. [GRAPHICS] .