Impact of laser beam welding on mechanical behaviour of 2.25Cr–1Mo (P22) steel

Abstract

The use of welding processes in the manufacturing and repair of structures intended for the energy industry plays a key role in the guarantee of a continuous supply of fossil fuels, which is the basic condition for ensuring energy security. A square butt joint of 10 mm thick plate of 2.25Cr–1Mo (P22) steel was fabricated by autogenous laser beam welding process and then post-weld heat treatment (PWHT) for two sets of process parameters (760 °C for 90 min–T1 and 730 °C for 180 min–T2). The joints were subjected to detailed metallographic investigations and hardness measurements, static tensile tests and impact toughness tests. The joint in as-welded state is characterized by significant heterogeneity of structural morphology and mechanical properties, while PWHT significantly improves uniformity in microstructure and mechanical properties along weldments. PWHT in particular causes tempering of the bainite and promotes the formation of carbide precipitates (M7C3, M23C6, and Mo2C) of globular and spherical shape along boundaries and bainite blocks. Laser welding caused a drastic increase in the weld metal hardness (388 HV) and coarse prior austenite grain bainite region (390 HV) compared to the hardness of base metal (BM) (below 200 HV). Both of the applied PWHT variants reduced the hardness of these areas of welded joints to the BM level. During the tensile tests, all the joints were fractured in BM. In the case of AW joints the tensile strength was higher than BM and it was 625 ± 10 MPa and after both PWHT, tensile strength was decreased by about 100 MPa. Due to over tempering of bainite in BM the elongation of the joints was several percentage points lower (27 ± 3 for AW joints) than the value for BM (35% ± 2), but slightly higher in the case of joints after PWHT (31 and 33 ± 1). Impact energy of the weld metal (259 ± 8 J) was lower than P22 base metal (320 ± 8 J), but PWHT causes a significant increase in impact energy of the weld metal (291 ± 6 J and 306 ± 10 J for T1 and T2, respectively). The analysis of these results shows that the proposed laser welding procedure with a high temperature gradient and PWHT enables the fabrication of P22 steel welded joints that meet the quality criteria.