Importance Of Preheating Before Welding And Heat Treatment After Welding Of Lined Ball Valve

Preheating and post-weld heat treatment are crucial for ensuring weld quality in Lined Ball Valve. Preheating is essential for welding critical components, alloy steels, and thick parts. The main benefits of preheating are as follows:
1. Preheating slows the cooling rate after welding, facilitating the escape of diffusible hydrogen from the weld metal in fully welded ball valves and preventing hydrogen-induced cracking. It also reduces the degree of hardening in the weld and heat-affected zone, improving the crack resistance of the welded joint.
2. Preheating reduces weld stress. Uniform localized or global preheating reduces the temperature difference (also known as the temperature gradient) between the welded parts in the weld area. This reduces weld stress and weld strain rate, helping to prevent weld cracking.
3. Preheating reduces the constraint of the weld structure, especially in fillet joints. As the preheat temperature increases, the crack incidence decreases. The selection of preheating temperature and interpass temperature depends not only on the chemical composition of the steel and welding rod, but also on the rigidity of the welded structure, the welding method, and the ambient temperature. These factors should be comprehensively considered and determined. Furthermore, the uniformity of the preheating temperature across the thickness of the steel plate and within the weld area plays a significant role in reducing weld stress. The width of the local preheating zone should be determined based on the constraints of the workpiece being welded. Generally, it should be three times the wall thickness around the weld area and should not be less than 150-200 mm. Uneven preheating will not only fail to reduce weld stress, but may actually increase it.

Post-weld heat treatment of fully welded ball valves has three objectives: dehydrogenation, weld stress relief, and improvement of weld microstructure and overall performance.
Post-weld dehydrogenation treatment of fully welded ball valves refers to a low-temperature heat treatment performed after welding, before the weld cools below 100°C. The general standard is heating to 200-350°C and holding for 2-6 hours. The primary function of post-weld dehydrogenation treatment is to accelerate the escape of hydrogen from the weld and heat-affected zone, making it extremely effective in preventing weld cracks when welding low-alloy steel.
During the welding process, due to uneven heating and cooling, as well as inherent or external component constraints, weld stresses are always generated in the component after welding. The presence of weld stresses in the component reduces the actual load-bearing capacity of the weld joint, causing plastic deformation and, in severe cases, even component failure.
Stress relief heat treatment reduces the yield strength of the welded workpiece at high temperatures, thereby relaxing weld stresses. Two common methods are: One is overall high-temperature tempering, in which the entire weld is placed in a furnace, slowly heated to a certain temperature, then held at that temperature for a period of time, and finally cooled in air or in the furnace. This method can eliminate 80%-90% of weld stresses. The other method is localized high-temperature tempering, in which only the weld and the surrounding area are heated, followed by slow cooling. This reduces the peak weld stress, smoothes the stress distribution, and achieves the partial stress relief. After welding, some fully welded ball valves made of alloy steel may develop a hardened structure in the weld joint, deteriorating the material's mechanical properties. Furthermore, this hardened structure can damage the joint under the effects of welding stress and hydrogen. Heat treatment improves the metallographic structure of the joint, enhancing its plasticity and toughness, and thus improving the overall mechanical properties of the welded ball valve joint.