Welding Technology for Medium and Heavy Plates in Steel Structure Manufacturing
I. Thick Plate Welding During the welding of thick and ultra-thick plates, a large amount of welding filler metal is deposited, the welding duration is long, and the total heat input is high. The components have high weld restraint and large residual welding stresses during welding, resulting in significant post-weld stresses and deformations. Hot cracks and cold cracks tend to occur during welding.
Before welding thick plates, the plate temperature is relatively low. At the start of welding, the arc temperature reaches 1250–1300°C. Under such rapid temperature changes, the temperature distribution in the thick plate is uneven, which easily causes hardening (martensite structure) in the heat-affected zone of the weld, making the weld metal brittle and increasing the tendency for cold cracking. To avoid such problems, preheating is mandatory before welding thick plates. In actual production and manufacturing, the welding process must be controlled to prevent welding cracks. 1. Tack Welding Tack welding is the most problematic area in thick plate construction. During tack welding of thick plates, the temperature at the tack weld is rapidly cooled by the surrounding “cooling medium”, causing excessive local stress concentration, cracking, and damage to the material. The solution is to increase the preheating temperature, lengthen the tack welds, and enlarge the fillet size during tack welding of thick plates. 2. **Multi-Layer and Multi-Pass Welding** An important technological principle for thick plate welding is multi-layer and multi-pass welding; wide weaving passes are strictly prohibited. Because the groove of thick plate welds is large, a single pass cannot fill the groove. Wide weaving leads to high restraint stress from the base metal on the weld, relatively low weld strength, and a high risk of weld cracking or delayed cracking. The advantages of multi-layer and multi-pass welding are: - The previous pass acts as preheating for the next pass; - The next pass acts as post-heat treatment for the previous pass. This effectively improves the stress distribution during welding and helps ensure welding quality. 3. **Intermediate Inspection During Welding** Unlike medium and thin plates, thick plate welding of one component can take several hours or even dozens of hours. Therefore, strengthening intermediate inspections during welding is particularly important for timely problem detection. Inspection shall be carried out concurrently with welding, not stopping construction. For example, carefully check for cracks during slag removal and address any defects immediately. 4. **Post-Weld Heat Treatment (PWHT)** Immediately after butt welding of thick plates, locally heat the weld and the base metal within 100–150 mm on both sides using infrared electric heating plates. Heat to 250–350°C, then cover with asbestos for insulation for 2–6 hours, followed by air cooling. This post-heat treatment allows diffusible hydrogen that entered the weld pool due to inadequate pre-weld cleaning or improper flux baking to escape rapidly, preventing hydrogen-induced cracking in the weld and heat-affected zone. Ultrasonic testing (UT) of thick steel welds shall be performed 48 hours or longer after welding. If the schedule permits, re-inspection can be conducted before the component leaves the factory to ensure qualification and prevent damage from delayed cracking.