Laser-welded forged steel pistons can meet the needs of the new era of heavy truck engines. 42CrMo4 and 38MnVS6 are widely used as piston materials due to the good mechanical properties. This study investigates the influence of preheating on microstructure and mechanical properties of 42CrMo4/38MnVS6 laser welding joint. The experimental results show preheating increases the laser absorption capacity of the metal, which can lead to an increase in weld width. The microstructure of weld is the high-hardness and poor toughness twin martensite without preheating. As the temperature of preheating increases, the twin martensite in the weld begins to transform into lath martensite and regenerates ferrite and bainite. As the preheating temperature increases, the plane fracture toughness (K1C) of the weld increases and then decreases, reaching the highest value of 2322.94 MPa·mm −1/2 at 150 • C. Compared with no preheating conditions, the tensile strength of the welded joint after preheating is improved. The fracture mode of welded joints changes from brittle fracture to ductile fracture. When the preheating temperature is 100-200 • C, the tensile strength of the welded joint reaches 1018.1-1032.5 MPa; when the preheating temperature is 250 • C-300 • C, the tensile strength decreases. the heat input is in the range of 275 J/mm 0.5 ·s 0.5 to 435 J/mm 0.5 ·s 0.5 , and there was no defect in the weld [12]. Kuryntsev et al. studied the effect of heat treatment on laser welded joints of 0.349C-1Cr-1Si steel. The results showed that the laser welded joint had the characteristics of high hardness and low elongation. After heat treatment, the elongation and ductility of the welded joint are improved [13]. Laser welding has a high energy density, and the base metal is not easy to deform after welding. It can optimize the piston structure and is conducive to the full combustion of fuel. Therefore, the development of laser welding forged steel piston has become one of the development directions of piston structure [14][15][16][17]. Preheating before welding can increase the laser absorption rate of the metal and reduce the cooling rate of the weld, adjust the stress distribution of the welded joint, avoid the generation of hardened structure in the weld, and is one of the welding methods to effectively improve the mechanical properties of the welded joint [18,19].At present, there are more studies on laser welding of aluminum alloy and low carbon steel with preheating, but less on medium carbon steel [20][21][22]. In this paper, the effect of preheating on the weld structure of 42CrMo4/38MnVS6 laser welding is studied, and the mechanical properties of welded joints under different preheating temperature are compared and analyzed, which provides the basis of experimental and theoretical for the application of laser welding technology in a forged steel piston.
Materials and Methods
Materials