Research On The Laser-MAG Hybrid Welding Process Of HQ785T1 HSLA Steel

The current method which enable to avoid cold crack of the welding of high strength low alloy steel is preheated before welding process. In response to this situation, the laser-MAG hybrid welding process and joints property of low-alloy high strength steel were studied.This paper has systematically studied the effect of the main parameters on the weldings during the laser-MAG hybrid welding process.The crack sensitivity of HQ785T1 (28mm thick) low alloy high strength steel by laser-MAG hybrid welding under the room temperature without preheating has been in-depth studied.The mechanical properties and the physical and chemical properties of the hybrid welding joints have been studied.The methods of improve the welding efficiency while ensuring the mechanical properties of joints have also been researched.High quality welding joints of HQ785T1 (28mm thick) low alloy high strength steel were obtained without preheating as well as the welding efficiency was significantly increased.The results show that, the crack resistance of laser-MAG hybrid welding is better than MAG welding for HQ785T1(28mm thick) low alloy high strength steel.The experiments show that, use the HS-70 welding wire, crack-free welding joints of HQ785T1(28mm thick) low alloy high strength steel were obtained at room temperature without preheating. The low-intensity matching welding wire HS-70 is well matched with the base metal by the laser-MAG hybrid welding process.The stability of butt plates hybrid welding process and the appearance of joints are very well. The mechanical properties of stretching and bending(Lateral bending, facade bending and root bending) for hybrid welding joints have good performance,-40℃impact values are superior to conventional MAG welding.Micro structures of hybrid welding joints are better than MAG welding joints, the organizations of bainite and some martensite can improve mechanical performance of the joints. The micro-impact fracture mechanism of hybrid welding joints at-40℃is dimple fracture and quasi-cleavage fracture, the impact energy absorption is higher than MAG welding joints. The hardness of the hybrid welding joint center increases from the top to the bottom, the hardness of.the root of the joint is higher than the base materia, while the situation of the upper part of the welding joint is the opposite.Hardened and softened areas has been found in the HAZ.However, the width is narrow, about 0.5mm,the phenomenon of hardened and softened is not obvious. With the increase of welding heat input, the mechanical properties for hybrid welding joints decrease significantly. Cold cracks could be found in the root area if the heat input is too large. Therefore, the welding heat input should be controlled no more than 1.1KJ/mm in order to ensure the formation and mechanical properties of joints.As the dilution ratio of the root area of the weld is great and the conten of alloy elements is relatively high, the harden tendency of the roots is great.This situation has some influence on the root certain impact values.When selecting groove parameters as follows:one-side welding groove root size H =7mm, bevel angleα=30°, double-side welding groove root size H=4mm, bevel angleα=45°and the welding speed isl.2m/min, the appearances of joints are very well, and no defects in the joints. Stretching, bending, impact and other mechanical properties are better than the conventional MAG welding.The amounts of the filled metal of one-side hybrid welding groove and double-side hybrid welding groove are 28% and 57% of MAG welding respectively.The general welding efficiency ratio improved of hybrid welding is about 70% of the MAG welding.