Low alloy high strenth steel is an alloy steel with less than 5% of the total amount of alloying elements in the steel. According to the provisions of GB/T1591-2008, low-alloy steel is divided into 8 grades: Q345, Q390, Q420, Q460, Q500, Q550, Q620, Q690; Due to the different quality, it is divided into A, B, C, D, and E grades. The main problems when welding low-alloy steel structural steels are the cold cracks of the welded joints and the hardening tendency of the welded heat-affected zone. Different welding methods have different effects on welded knots. The welding methods of low-alloy steel are: electrode arc welding, submerged arc welding, tungsten argon arc welding, molten pole argon arc welding, CO2 gas shielded welding, electroslag welding and brazing, etc., and the welding methods used should be determined according to the product structure, plate thickness, performance requirements, production conditions, etc. The welding material selection principle of low-alloy steel is generally treated according to the principle of equal strength, and the toughness, plasticity and crack resistance of the weld are comprehensively considered, as long as the strength of the weld metal is not lower than or not higher than the lower limit of the base metal.

When low-alloy steel is welded, it is usually preheated before welding, which is conducive to reducing the speed of the cold zone after welding, avoiding the appearance of hardened structure, reducing welding stress, and is an effective measure to prevent cracks; It also helps to improve the structure and performance of joints, so that the process measures commonly used in the welding of low-alloy high-strength structures, the steel preheating temperature depends on the composition of the steel, the thickness of the plate, the shape of the welded structure, the degree of restraint and the ambient temperature and other factors. Generally, the preheating temperature is 100~200°C, but it is not preheated for small parts and thin plates. After welding, the tempering temperature of electrode arc welding is 600~650 °C; Electroslag welding 900~980°C normalizing, 600~650°C tempering.

The determination of welding parameters, that is, the size of the heat input depends on the embrittlement and the tendency of the cold crack in the superheated zone, because the embrittlement and the tendency of the cold crack of various steels are different, so the input requirements for the welding heat are very different. When the carbon content of welding is low, there is no strict control over the heat input, but it is better to have a small heat input. When the welding carbon content is high, the heat input should be larger, because the strength grade of the low alloy high-strength steel is higher, the hardenability tendency is larger, the larger heat input should be selected, but it should not be too large, otherwise the embrittlement tendency of the coarse grain area will be increased. For example, there is no strict control of the welding line energy during the welding of 16MnR steel, because the embrittlement tendency of the welding heat-affected zone of this steel is small, but for the steel containing trace alloying elements such as V, Nb, Ti, etc., a smaller welding line energy should be selected. For low-alloy steels with high carbon-based alloying element content and high yield strength, such as 18MnMoNbR, due to the large hardening tendency of this steel, and the overheating tendency of its heat-affected zone, it is necessary to increase preheating before welding and timely heat treatment after welding while selecting small linear energy. In short, it is safer to use preheating and post-welding heat treatment when welding low-alloy steels, and to minimize heat input during welding.