Investigation of the effect of heat treatment on microstructure and mechanical properties of ST37 steel pipes in potable water transmission lines joined by submerged arc welding

In this study, 4 mm thick St37 steel materials used in drinking water transmission lines were joined by submerged arc welding at current values of 400 and 450 A, and heat treatment was applied to impart different hardness values to the joined welded connections. The effects of current values and hardening heat treatment on the tensile strength, hardness and microstructure of steel materials were analyzed. According to the tensile test results, the stress values of the welded connections joined with current values of 400 and 450 A were obtained as 458 MPa and 401 MPa, respectively. These values were higher than the stress values of the un-welded sample, which was 390 MPa. With the hardening heat treatment, the stress values of the samples increased, and the strain values decreased. The stress values of the samples with maximum hardness were obtained as 1110 MPa for the un-welded samples, 1057 MPa for the samples joined with a current value of 400 A, and 1001 MPa for the samples joined with a current value of 450 A. According to the hardness test results, the hardness value increased from the base material to the weld metal. The hardness of the base material was measured between 140~147 HV. In the weld metal where high hardness values were obtained, the hardness values measured for 400 A current value were between 197-205 HV, while the hardness values measured for 450 A current value were between 191-195 HV. While higher hardness values were measured on the outer surfaces of the materials directly exposed to carbon through the hardening heat treatment, the hardness values partially decreased towards the center of the material. The difference between the hardness values measured on the outer and inner surfaces of the steel samples with high hardness was greater than the difference in the outer and inner surfaces of the steel samples with low hardness. However, the hardness values of the base material, heat affected zone (HAZ) and weld metal were measured to be close to each other. According to the microstructure results, St37 steel consisted of an equiaxed ferrite phase, and a typical columnar structure was formed in the weld metal with post-welding solidification. Although some grain coarsening was observed in the HAZ between the base material and the weld metal, there was no significant difference in the microstructures of the base material, HAZ and weld metal. A martensitic microstructure was obtained on the surfaces exposed to carbon through the hardening heat treatment, and the ferrite phase increased in the structure as it moved towards the center. This situation revealed that the current values of 400 and 450 A are preferable values for joining St37 steel.