Quenching and tempering steel in low alloy high strength steel is one of the fastest developing and most dynamic steel types in recent decades. Due to its high strength, good plasticity and toughness, as well as uniformity and stability in welding, it occupies a mainstream market. More than half of the high strength steel standards worldwide are delivered in quenched and tempered condition.

Low alloy high strength steel can be divided into non-quenched and tempered steel and quenched and tempered steel. Generally, non-quenched and tempered steel refers to steel with a tensile strength below 600MPa at room temperature, while quenched and tempered steel is used for machinery, pressure vessels, and submarines; the yield strength reaches 880~1176MPa, mainly for medium carbon quenched and tempered steel, which is commonly used in products or components with very high strength requirements. Research has shown that the heat input during welding affects the toughness and microstructure of quenched and tempered HQ130 steel (yield strength 1313MPa). The test results indicate that as the welding heat input increases from 9.2kJ/cm to 26.4kJ/cm, the impact toughness of HQ130 steel HAZ gradually decreases. Therefore, it is widely used in fields such as construction, shipbuilding, high-pressure vessels, engineering machinery, and vehicles. Low alloy quenched and tempered high strength steel maintains a relatively stable microstructure under complex conditions such as repeated heating, deformation, and cooling, ensuring good final performance of the steel plate. Quenched and tempered high strength steel possesses both high strength and plasticity.

1. In the engineering machinery industry, a large amount of quenched and tempered steel is used. Low alloy high strength steel plates for engineering machinery are widely applied in hydraulic supports for coal mines, excavators, cranes, dump trucks, ball mills, port machinery, etc. They can be categorized based on performance requirements into strength type and hardness type; the latter is also known as high wear-resistant steel plates.

With the continuous improvement of comprehensive mechanization levels in domestic coal mining, the demand for high-end hydraulic supports is increasing. To meet the requirements for high strength and reliability of supports while using high-strength steel plates as much as possible. For higher grade high strength steels like Q690, quenched and tempered steel is generally selected. The new type of quenched and tempered high strength steel WH80Q (yield strength ≥685MPa) developed by Wugang has been applied to hydraulic support projects produced by Zhengzhou Coal Mining Machinery Group Co., Ltd. for Shenhua Group.

2. Pressure vessels and pipelines have seen significant improvements in strength, toughness, and pressure resistance in recent years in China; however, there is still a gap compared to advanced countries regarding weldability and corrosion resistance. Since pressure vessels are special equipment with potential hazards, the welding process of the steel plates must be carefully controlled.

Research on the welding process and joint performance of SHT900D quenched steel (yield strength 925MPa) used in high-end hydraulic supports by Zhengzhou Coal Mining Machinery Group Co., Ltd. has been conducted. The test results indicate that SHT900D steel has a strong hardening tendency in the heat-affected zone (HAZ) during welding processes that can achieve good comprehensive mechanical properties for welded joints meeting design and usage requirements.

Research on hydrogen-induced cracking during welding of HG980D steel (yield strength 945MPa) used in hydraulic supports indicates that when t8/3 ≤ 150s, martensitic structure appears in HG980D steel HAZ with increased hardening tendency. The results show that under different cooling times, the microstructure and microhardness results of HG980D steel HAZ vary; when the hydrogen content in the joint is low, only larger joint restraint stress will lead to hydrogen-induced cracking; however, when hydrogen content is high, even small restraint stress can cause hydrogen-induced cracking. Therefore, ultra-low hydrogen welding materials must be selected for welding HG980D steel.

Since the 1980s, China has developed HG series steels (such as HG70 steel, HG80 steel) and HQ series steels (HQ70 steel, HQ80 steel, HQ100 steel) to replace WEL-TEN steels imported from Japan. These steels are suitable for wear-resistant parts of engineering machinery such as lifting mechanism components, pressure vessels, excavator buckets, dump truck bodies etc., where joint toughness must be strictly controlled with welding heat input below 20kJ/cm.

The FZQ1380 type self-elevating tower crane uses quenched H80 high-strength steel pipes as main chord members for its boom. Each section of the boom is connected by pins made from quenched D80 material. The welds connecting H80/D80 steels serve as connection points for the boom under load concentration; thus their welding quality is critical during fabrication. Research indicates that for H80/D80 high-strength steels at 800MPa level using stick arc welding methods while adhering to 'equal strength principle', welding heat input should be controlled below 25kJ/cm with interpass temperature below 150℃ followed by post-weld heat treatment to meet design usage requirements.

The main arm of the QY100H truck crane developed by Puyuan Company uses DILLIMAX 965 (yield strength ≥960MPa) high-strength steel produced by French company Ugine & ALZ. Research indicates that using UNION X90 welding wire with an 80% Ar + 20% CO2 gas shielded welding method while strictly controlling cooling time t8/5 between 5~12s along with preheating temperature and diffusion hydrogen content within welds can ensure that mechanical properties of welded joints meet design usage requirements.

The requirements are very strict. In standards worldwide due to the special nature of pressure vessel steels, most require delivery of plates in quenched state.