Objective Compared with conventional welding repair methods, laser cladding, an advanced surface modification technology, uses nonequilibrium processing conditions, such as rapid heating and cooling, to fabricate similar alloy compositions on the surface of high-strength steel components. The coating can exhibit refined grains and high dislocation density to achieve high strength and ductility of the repair zone. Therefore, it is a potential for the laser repair of high-strength steel surfaces. Traditional welding methods are used to repair high-strength steel using multilayer and multipass repair welding. Multiple welding thermal cycles induce coarse grains in the heat-affected zone (HAZ), which can lead to significant embrittlement and poor impact toughness of the high-strength steel substrate. Similar to traditional welding, the multilayer and multipass thermal cycles in laser cladding can have multiple tempering effects on the substrate hardened zone, which can lead to grain coarsening and strength softening of the substrate in HAZ. For the multilayer and multipass laser repair of high-strength steel components, in addition to the effective control of the microstructure and performance of the cladding layer, the softening problem of HAZ in the high-strength substrate and the deterioration of mechanical properties (i.e., low strength and poor elongation) must be overcome. Therefore, in this study, the variation trends of the microstructure evolution and mechanical properties of HAZ in the 30CrMnSiNi2A substrate were shown to be beneficial in controlling the strength and ductility of the repaired high-strength steel parts.
Methods Multilayer laser cladding 30CrMnSiA powders were processed on thick 30CrMnSiNi2A steel plates with geometric sizes of 120 mm×60 mm×10 mm using the 8-kW semiconductor laser (Laserline LDF-8000-60). The laser cladding parameters were as follows: 2100-W laser power, 7.3-mm beam diameter, 9-mm/s laser scanning speed, 10-g/min powder feed rate, 10-L/min powder feed gas flow, 20-L/min coaxial shielding gas flow, and 0.3-mm single clad layer. The substrates were separately cladded using 1--8 layers, and the samples were retained in the air-cooling state. The microstructure was characterized using the Zeiss-AxioCam MRc5 optical microscope and TESCAN-LYRA3 scanning electron microscope. The microhardness was characterized using the Zwick/Roell ZHμ Vickers microhardness tester with 0.5-kgf load and 15-s holding time. To investigate the mechanical properties of HAZ subjected to different laser thermal cycles, the cladding coating was cut off, and the hardened layer and tempered zone of the substrate were retained. Tensile and impact samples were prepared using the Zwick/Roell Z100 tensile testing machine and 300 J impact testing machine.
Results Owing to rapid laser heating, the first cladding layer did not significantly decompose the residual austenite of the substrate for tempering; however, the following layers had an obvious effect on the residual austenite decomposition, which slightly decreased the sample ductility and impact toughness. With an increase in the number of cladding layers, the ductility of the specimen samples increased, and initial crack and crack growth occurred in the high-temperature tempering zone (HTTZ); however, when the cladding layers did not quench the substrate, initial crack and crack growth occurred in the incomplete quenching zone (IQZ) and HTTZ. Moreover, the uniform plastic deformation decreased, resulting in a significant decrease in the elongation.
Conclusions and Discussions Each cladding layer can repetitively quench the substrate during multiple cladding. However, the quenching depth gradually decreased with an increase in the number of cladding layers because the former deposited layers absorbed the heat. IQZ occurred when the thermal cycle could not heat the substrate above the austenitizing temperature and the cladding layer that did not quench the substrate began to produce a tempering effect with an increase in the number of cladding layers. Each cladding layer had the tempering effect on the 30CrMnSiNi2A substrate during the multilayer process. With the increasing number of cladding layers, the residual austenite among the substrate martensite lath bundles first decomposed, the carbides gradually precipitated, and the martensite laths coarsened, becoming wider and blocky until the lath martensite completely transformed into a sorbite microstructure. In terms of the mechanical properties of the substrate in HAZ, the tensile strength gradually decreased and the impact toughness gradually increased with the number of cladding layers. Owing to rapid laser heating, the first cladding layer will not decompose the tempered retained austenite (RA) in the substrate; however, the double cladding layers will significantly decompose RA. Because of the decrease in the ductile RA phases, the tensile elongation and impact toughness of the double cladding layers were slightly reduced.
