Why Do Cracks Occur During Seamless Pipe Welding?

As an important industrial material, seamless pipe often needs to be welded during processing and production. However, seamless pipe welding often leads to cracks of varying degrees, which seriously affects product quality and safety performance. So, what are the causes of cracks in seamless pipe welding?

Causes of cracks

1. Grain boundary precipitates

During welding, the precipitates at the grain boundaries will melt and aggregate at high temperatures, forming a certain cooling stress, causing cracks in the pipe.

2. Poor welds

During welding, the thermal expansion coefficients and linear expansion coefficients between different materials are different, which can easily cause deformation when the weld cools and cause cracks in the pipe.

3. Alloy elements

During the manufacturing and production process of seamless pipes, copper, manganese and other alloy elements are easy to gather at the grain boundaries during welding, forming brittle phases, causing intergranular cracks.

4. Improper welding operation

If the welding temperature is uneven, the welding parameters are inappropriate, the welding speed is too fast, etc., it will have an adverse effect on the welding quality, thus causing cracks.

Remedial methods

1. For cracks caused by grain boundary precipitates

First, local heat treatment is required, and the crack area is annealed (the temperature is controlled at 600~800℃) to eliminate the aggregation of grain boundary precipitates and relieve residual stress. Secondly, re-cladding welding, use low-hydrogen welding rods (such as E7018) to grind and clean the cracks, re-weld and control the interlayer temperature below 150℃. Finally, add grain boundary stabilizers, add trace amounts of titanium (Ti) or boron (B) to the repair welding material to inhibit the formation of brittle phases at grain boundaries.

2. For cracks caused by poor welds

First, crack assessment is performed, and the crack depth and direction are determined by penetration testing (PT) or ultrasonic testing (UT). Then repair the V-shaped groove, grind a 60°~70° groove along the crack direction, and completely remove the oxide layer and impurities. Use segmented welding method, small current, multi-pass welding (each weld width ≤3mm), and hammer stress relief after each welding. Finally, for important pressure-bearing pipelines, reinforcement rings or casing pipe can be added after repair.

3. For intergranular cracks caused by alloy element segregation

First, it is necessary to adjust the material composition. When repairing, use nickel-based welding materials (such as ERNiCrMo-3) to dilute the copper and manganese segregation in the parent material. After repairing, dehydrogenation treatment (250~350℃ for 2 hours) is immediately carried out to prevent hydrogen-induced crack expansion. Finally, laser cladding repair is carried out. Laser cladding technology is used for microcracks to quickly remelt the surface layer and refine the grains.

4. For cracks caused by improper welding operation

First, parameter optimization verification is carried out, and parameters are re-established through welding process qualification (WPS/PQR) (such as reducing the current by 10%~15%). Secondly, TIG remelting repair is carried out. For the areas where thermal cracks have occurred, TIG welding without wire filling is used for remelting to eliminate weld impurities. Finally, a stop hole (diameter 3~5mm) is drilled at the end of the crack to prevent further expansion.

Measures to prevent cracks

1. Optimize welding process

For different seamless pipe welding materials and thickness parameters, reasonable matching is carried out, and appropriate welding parameters such as welding temperature and welding speed are selected to reduce residual stress after welding, thereby avoiding cracks.

2. Fine welding operation

Strictly control the quality of welding operation, such as selecting appropriate welding technology and tools, ensuring the neatness and tightness of welding joints, shortening the welding process, and reducing the inertial effect of residual internal stress.

3. Adding appropriate amounts of special elements such as rare earth elements

In the production process of seamless pipes, special elements and compounds are added to form a certain crystal degradation effect to reduce the generation of cracks.

4. Increase pipe preheating treatment

Preheat the seamless pipe before welding to make the temperature and temperature distribution of the material uniform, avoid residual stress after welding, and thus reduce the generation of cracks.

5. Choose the right processing method

For example, adopt the forging and hot manufacturing method (FV) or the thermal expansion method (TD) to reduce the intergranular stress and deformation during the processing, thereby reducing the occurrence of cracks.

Summary

In summary, the causes of cracks in seamless pipe welding are relatively complex, but they can be effectively prevented by optimizing the welding process, fine welding operations, adding special elements, etc. Only when these preventive measures are effectively implemented can the welding quality and the safety performance of the pipe be guaranteed.