Welding on C67 steel grade sheet: Influence of the parameters and post welding heat treatment

In batch operation, most industries require engineers to maintain low hardness on the welded parts, particularly for low carbon steel. This article focuses on tungsten inert gas (TIG) welding performed on 0.90 mm of a C67 grade sheet by varying different welding parameters such as current, velocity, and temperature. Samples were collected from the tool side, mid-wall, and operator side for metallographic and micro-hardness examinations considering various parameters. Without post welding heat treatment (PWHT), the welded parts were quenched at room temperature, while with PWHT the welded parts were kept at 710 ℃ for 99 s after welding, and subsequently, the samples were cooled slowly by air at ambient temperature. An increase in hardness was registered in either the fusion zone or melted zone, with decrements in the heat affected zone (HAZ) for both procedures. When the the welding was performed without PWHT, a martensitic and bainitic microstructure was noticed in the melting zone (MZ) and HAZ, respectively. In contrast, a bainitic microstructure was observed in either the melting or heat affected zone in the welding with PWHT. Metallographic images revealed crack propagation when welding was performed without PWHT. A larger HAZ was noted in the welding sample with PWHT, and hardness was also relatively lower compared the samples without PWHT. There was no significant difference in hardness among the samples taken from tool side, mid-wall, and operator side for both procedures. Finally, the lowest microhardness (265 HV) was found in the MZ when the welding was carried out with PWHT employing a 90 A current and 10 mm/s velocity.