Residual stress in welding parts is internal stress caused by uneven localized high-temperature heating and cooling during the welding process. This stress can reduce structural stiffness, stability, and fatigue strength, and even cause brittle fracture. Heat treatment processes, through controlled heating, holding, and cooling, induce plastic deformation or phase transformation within the material, effectively eliminating or redistributing residual stress. This is a key method for improving the performance of welded parts.
Full-body high-temperature tempering is a key method for eliminating residual stress. This process heats the entire welded part to a temperature below the A1 phase transition point (e.g., 600-650°C for mild steel), holds the temperature for a specified period, and then slowly cools it. During heating, the material's yield strength decreases, and residual stress drives localized plastic deformation, leading to stress relaxation. The holding phase ensures uniform heat transfer, eliminating temperature gradients, and slow cooling prevents the generation of new stresses. This method can eliminate approximately 80% of residual stress and is particularly suitable for large structures or welded parts requiring high precision, such as pressure vessels and bridge components.
Localized high-temperature tempering targets welded parts that cannot be heated throughout. Heating specific areas using a gas flame, induction heating, or resistance wire, combined with insulation to reduce cooling rates, is performed. This method eliminates stress through localized plastic deformation and is suitable for applications with localized stress concentrations, such as thick plate butt welds and tube-to-tube connections. For example, when welding a boiler tube sheet to a tube bundle, localized tempering of the tube sheet edge can significantly reduce residual tensile stresses caused by excessive restraint.
Pre-weld preheating is an important adjunct to the heat treatment process. Pre-welding the weldment, either fully or locally, to a temperature of, for example, 150-300°C, reduces the temperature difference between the weld zone and the parent material, slowing the cooling rate and thus reducing thermal stress. Preheating also reduces hydrogen solubility, preventing hydrogen-induced cracking. For materials with a high hardening tendency (such as high-carbon steel and alloy steel), preheating prevents embrittlement caused by martensitic transformation, indirectly reducing residual stress.
Post-weld heat treatment and preheating form a closed-loop process. Immediately after welding, heat the weld and heat-affected zone to 200-350°C, hold for 2-4 hours, and then slowly cool. This process promotes hydrogen escape, reduces hydrogen-induced stress concentrations, and relaxes some residual stresses through plastic deformation.
Post-heat treatment is often combined with overall tempering, forming a complete process chain of "preheating - welding - post-heating - tempering" to further enhance stress relief.
Heat treatment parameters must be precisely controlled based on the material properties and weld structure. Excessive heating rates can increase temperature gradients and induce new stresses; excessive cooling rates can cause cracks due to phase transformation or thermal contraction. For example, austenitic stainless steel requires controlled cooling rates to avoid embrittlement caused by σ phase precipitation. The tempering temperature of quenched and tempered high-strength steel must be lower than the original tempering temperature to prevent strength loss. Furthermore, the heating method (e.g., furnace heating or flame heating) should be selected based on the workpiece size and shape to ensure temperature uniformity.
The synergistic application of heat treatment with other processes can enhance the overall effectiveness. For example, sequential control during welding, such as symmetrical weld placement and segmented de-welding, can reduce constrained stress. Post-weld mechanical vibration aging, through resonance, rearranges the material's internal grains, further relaxing stress. For complex structures, numerical simulation can be combined to optimize heat treatment parameters and achieve precise control of stress distribution.
Heat treatment processes, through the interaction of temperature and stress fields, provide a scientific solution for controlling residual stress in welded parts. From global tempering to localized treatment, from preheating before welding to post-heating after welding, process refinement and parameter precision are key to achieving optimal stress reduction.
With the advancement of intelligent temperature control technology and multi-physics coupled simulation, heat treatment processes will more efficiently serve high-end equipment manufacturing, ensuring the safety and durability of welded structures.
