چكيده به لاتين
In order to investigate the influence of the mixture of the shielding gas and welding frequency on the weld metal in welding of stainless steels, a 4 mm thick sheet has been welded autogenously using pulsed current gtaw methode under the pulse frequencies of 40, 80, 120, 160 and 200 Hz and the nitrogen composition of 0, 0.5, 1, 2 and 10 mollar precent in the Argon shielding gas. The welded samples were cut and metallographic procedures were carried out for every section. Then, the images of microstructure of weld metal were produced using optical (OM) and scanning electron microscopy (SEM). Furthermore, penetration depths of weld metal have been observed and calculated using a stereography micorscope. Moreover, microhardness, ferritoscopy, nitrogen content analysis (LECO) tests were carried out as well as tensile test from the longitudinal sections of the weld metal after which, the fractography images were provided on the cross sections of fracture. By evaluating the results, it was found that the volume of the melt in the weld zone increased with increasing the pulse current frequency and the depth to width ratio was also raised. The hardness and strength of the weld metal began to slump due to the coarser microstructure the higher frequencies had caused. Also, increasing the nitrogen composition in the sheilding gas, caused the nitrogen content of the weld metal bacame more noticible at the higher frequencies. Admixture of the nitrogen into the sheilding gas has not only affected the heat input and changed the melt volume and the depth to width ratio, but the microstructure also got coarser. By dissolving nitrogen in the weld pool, the ferrite content in the weld zone tended to reduce and the morphology of the residual ferrite turned in to fully skeletal from the primary lathy-skeletal-like mixture. Investigation of mechanical properties in the presence of the nitrogen in the shielding gas has proved that hardness of weld metal increased in all frequencies and reached the peak at the composition of 5% nitrogen. Results showed that in the zones close to the surface of the weld pool, hardness had higher values and as going toward the deeper zones, hardness gradually decreased. The yielding strength of the weld zone increased with the composition of nitrogen but the fracture strength did not seem to experience any remarkable changes. Fractography results showed that the tendency of brittle fracture had raised in the higher nitrogen compositions and lower pulsed current frequencies.
