Welding of Medium and Thick Plates

Medium-thick plates (typically referring to steel plates with a thickness of 6mm to 100mm) are widely used in heavy-load structures.  The core challenges lie in controlling welding stress deformation and preventing cracks (especially cold cracks and lamellar tearing), while ensuring full thickness penetration and internal quality.

Key process measures include:

1.  Strict preparation: Precisely machining the groove (such as V-type, X-type), thoroughly cleaning the welding area, and strictly preheating (usually 100-250°C) to slow down cooling, reduce hardening, and drive out hydrogen to prevent cracking.

2.  Multi-layer and multi-pass welding: Breaking down thick welds into multiple layers and passes, with low heat input for each pass, preheating the subsequent layer with the previous one, and tempering the previous layer with the subsequent one to improve microstructure performance and disperse stress.  Strict control of interlayer temperature is crucial.

3.  Welding methods: Submerged arc welding (efficient and stable) is the main method, while stick arc welding (flexible) and gas shielded welding (high efficiency) are also commonly used.  For ultra-thick plates, electroslag welding can be chosen for vertical welding.

4.  Back gouging: After single-sided welding, it is often necessary to gouge the back side to remove defects before re-welding to ensure root quality.

5.  Post-heating and heat treatment: Immediate post-heating after welding to maintain temperature and drive out hydrogen;  high-demand structures require post-weld heat treatment to eliminate stress and improve performance.

6.  Deformation control: Managing deformation through reasonable assembly, reverse deformation, and optimizing the welding sequence.

Quality control: Relying on non-destructive testing (primarily ultrasonic testing) to ensure there are no defects internally and on the surface, supplemented by destructive testing to verify performance.

In summary: Welding of medium-thick plates is a systematic engineering process, and success depends on the strict execution of core processes such as preheating, multi-layer welding, gouging, and interlayer temperature control, as well as the qualifications of the welders and compliance with standards and specifications.