Laser welding on 10 mm thick grade 92 steel for USC applications: microstructure and mechanical properties

Abstract

High-power lasers have been shown to be more effective for welding plates with thicknesses of 10 mm or greater. In the present research, a heat-resistant P92 steel plate was welded using the laser beam welding process. The laser-welded joint underwent mechanical testing and metallographic characterization in both the as-welded condition and after post-weld heat treatment (760 °C for 2 h). The macrostructure analysis revealed that the welded joint had full penetration with negligible internal defects. The widths of the heat-affected zone (HAZ), the weld metal at the top, and the weld metal in the root region were 1.77 mm, 3.83 mm, and 3.12 mm, respectively. Inhomogeneity in both the microstructure and microhardness was observed along the welded joint. The coarse-grained structure with negligible precipitates in the coarse-grained HAZ resulted in a maximum hardness of 432 HV, while a minimum hardness of 225 HV was measured in the inter-critical HAZ, likely due to the formation of a complex microstructure. Another important observation in the fine-grained HAZ and inter-critical HAZ was the presence of two types of grain boundaries: one decorated with a high density of precipitates and the other free from precipitates. This contributed significantly to the heterogeneity in the microstructure. The weld metal exhibited a lath-elongated martensitic microstructure, which showed significant hardness variation due to the presence of soft ferrite patches. The hardness of the untempered martensite in the weld metal ranged from 385 to 403 HV, with an average of 398 ± 7 HV. In contrast, the hardness of the soft ferrite patches was measured in the same range of 234–349 HV. The ultimate tensile strength and percentage elongation were 1014 ± 11 MPa and 27 ± 3%, respectively, which are significantly close to those of the P92 base metal, as fracture occurred in the P92 base metal. The Charpy toughness measured higher than the recommended value of 47 Joules, confirming the suitability of the welded joint for USC boiler applications. The PWHT significantly reduced the inhomogeneity in microstructure and mechanical properties, though some variation remained. There was a notable decrease in hardness for the weld metal, coarse-grained HAZ, and fine-grained HAZ after PWHT, while the hardness of the delta ferrite patches and inter-critical HAZ remained relatively unaffected, leading to continued microstructural heterogeneity. The tempering of martensite due to PWHT resulted in a drop in ultimate tensile strength and an increase in percentage elongation, with failure still occurring in the P92 base metal in the PWHT condition. Additionally, Charpy toughness increased significantly after PWHT, confirming the applicability of the PWHT for welded joints of P92 steel before final application. A good correlation between microstructure and mechanical properties was established based on these findings.