To ensure the performance and quality of welded components, enhance the plasticity of the weld metal, eliminate or reduce residual stresses from welding, and prevent cracking, it is necessary to subject the components to heat treatment when required. Depending on the sequence of the heat treatment process and the welding process, the heat treatment of welding components is commonly divided into three types: preheat treatment, post-weld heat treatment, and post-welding heat treatment.
Preheating treatment
Preheating is the process of heating the assembled components as a whole or locally before welding. Its purpose is to slow down the temperature gradient and cooling rate during the heating of the welding joint, extending the cooling time from 800℃ to 500℃ for the joint, which facilitates the escape of hydrogen from the weld, reduces the formation of quench hardening, and prevents the occurrence of cold cracks. Additionally, preheating the components as a whole can also improve the uniformity of the temperature distribution in the weldment and reduce internal stresses.
The determination of preheating temperature can be calculated through empirical formulas or referenced from relevant experimental data. It is important to note that these calculation formulas or experimental data are established based on specific test conditions and the lowest preheating temperature that does not cause cracking. Therefore, special attention should be given to their applicability to avoid unnecessary losses. Excessive preheating temperature not only increases costs but also worsens the working conditions for welders. Therefore, under conditions that meet the process requirements, the lowest possible preheating temperature should be used. When interlayer heating is performed on multi-layer welded components, the interlayer temperature should not be lower than the preheating temperature to prevent the occurrence of delayed cracks; otherwise, the formation of delayed cracks cannot be effectively prevented.
2. Post-heat Treatment
Post-weld heat treatment involves immediately heating the weldment or welding area with a preheating device after welding stops, and maintaining the temperature for a certain period. The purpose is to allow hydrogen in the weld metal to fully escape, preventing the formation of cracks. In practical applications, post-weld heat treatment is particularly effective in preventing cracks in high-strength low-alloy steel with greater thickness.
Selecting an appropriate post-heat treatment temperature can reduce the preheat temperature by up to 50°C, and even eliminate the need for preheating. Lowering the preheat temperature benefits worker conditions, improves production efficiency, reduces energy consumption, and decreases the risk of defects such as heat cracks caused by excessive preheat temperatures.
Generally, the higher the tempering temperature in the post-heat treatment, the shorter the required holding time. To avoid cracking in the post-heat treatment of some common low-alloy high-strength steels in China (including tempered steels with tensile strength below 800 MPa), the temperature and time for post-heat treatment should be appropriately selected (refer to the test data in the welding manual).
Post-weld heat treatment
Post-weld heat treatment is the process of applying overall or local heat treatment to the welded components after the welding is completed. Its purpose is to improve the metal's structure and properties, reduce residual stresses from welding, soften the quenching structure in the heat-affected zone of welding, enhance the toughness of the weld joints, and improve the geometric stability of the structure. Depending on the specific objectives of the post-weld heat treatment, it can be categorized into stress-relieving heat treatment, welding annealing, normalizing, quenching, and low-temperature stress-relieving.
Post-weld heat treatment improves the toughness of low-carbon steel and other steels, but it may reduce the toughness of some high-strength steels. For quenched and tempered high-strength steel, if the heat treatment temperature exceeds the tempering temperature, the material will lose its original quenched and tempered effect, resulting in a decrease in both strength and toughness of the steel.
Additionally, when conducting overall heat treatment on welding components, it is crucial to ensure uniformity in heating temperatures and strictly control the heat treatment process. Otherwise, not only can residual stresses not be reduced, but they may also increase due to uneven temperatures.
Despite the complex heat treatment process for welding components, as long as the goal is clear, the methods are appropriate, and potential negative factors are prevented proactively, qualified products can always be manufactured.
