Analysis of the Causes of Cracking and Failure of Q235B Welded Pipe

Analysis of the Causes of Cracking and Failure of Q235B Welded Pipe

Q235B is a carbon structural steel specified in the Chinese national standard GB/T700. It is widely used in welded pipe production because of its low carbon content and good welding performance. Although Q235B steel has shown excellent performance in many industrial fields, in some cases, HFW welded pipes (high-frequency welded pipes) produced using this steel may crack at the welded joints. Recently, a steel plant was found to have cracks at the welded joints when producing Q235B steel pipes, and the cracks were obviously perpendicular to the welds. Through a series of chemical composition, metallographic structure, non-metallic inclusions, scanning electron microscopy and energy spectrum analysis, the causes of cracking and failure of Q235B steel pipes were analyzed in detail. This article will comprehensively discuss this incident, analyze the causes of cracking of welded pipes, and propose corresponding improvement measures.

1. Macroscopic manifestation of welded pipe cracking
In the actual production process, the cracking phenomenon of welded pipes is manifested as obvious transverse cracks at the weld. The crack direction is perpendicular to the weld. The crack runs through the entire weld area and even extends to the parent material. Specifically, the diameter of the welded pipe is 273mm and the wall thickness of the steel plate is 7.5mm. The crack length is about 45mm, and the depth of the crack has reached the thickness of the steel plate, showing a more serious failure of the welded joint.

2. Inspection and analysis
2.1 Chemical composition analysis
In order to find out the cause of the cracking of the welded pipe, the chemical composition analysis was first carried out. The chemical composition of the failed pipe was fully inspected by direct reading spectrometer. The results showed that the main components of Q235B steel meet the requirements of national standards, with a low carbon content, and belong to low-carbon steel. The contents of the main elements specified in the standard, such as carbon, silicon, manganese, sulfur and phosphorus, are all within the allowable range, and no obvious excess phenomenon was found.

However, although the chemical composition meets the standard requirements, some trace elements may be affected by local heating during welding, resulting in local changes in composition. For example, the temperature increase in the welding area may cause the volatilization or redistribution of elements, which in turn affects the quality of the welded joint. Therefore, although the chemical composition itself is not a problem, in the actual welding process, the stress concentration may be caused by the inhomogeneity of the composition or local overheating, resulting in cracking.

2.2 Non-metallic inclusion inspection
Non-metallic inclusions are usually impurities that are not completely removed during the steel smelting process, which may have an adverse effect on the quality of the welded joint. The inclusion analysis of the failed welded pipe using a metallographic microscope showed that there was a certain amount of non-metallic inclusions in the metallographic structure of the weld area, which mainly included oxides and sulfides. Especially at the welded joint, the presence of non-metallic inclusions may lead to local stress concentration, thus becoming the source of crack initiation.

The effect of non-metallic inclusions on the welded joint is usually manifested as a reduction in the toughness of the weld, resulting in easy crack propagation in the weld area. This phenomenon is clearly reflected in the cracking of the Q235B steel pipe welded joint. The experimental results also found that these inclusions may not be completely removed during the welding process, resulting in brittle areas at the welded joint.

2.3 Metallographic analysis
In order to further understand the microstructure of the welded joint, metallographic analysis was performed. The analysis results show that there is a certain degree of structural coarsening in the heat affected zone (HAZ) of the welded joint, and the coexistence structure of pearlite and ferrite appears in some areas. The formation of pearlite is a phase transformation phenomenon caused by local overheating during welding, while ferrite shows the brittle characteristics of metal. In the heat affected zone, due to the temperature fluctuation during welding, the microstructure of some areas may undergo uneven transformation, forming a relatively fragile metal structure, which is prone to cracking under external force or thermal stress.

In addition, at the junction of the parent material and the weld, the weld metallographic structure is relatively fine, but there are still certain microcracks. Although these cracks are short in length, they may expand rapidly due to stress concentration in a high temperature and high pressure welding environment, resulting in the final cracking of the welded pipe.

2.4 Scanning electron microscopy and energy spectrum analysis
In order to further study the morphology of the crack and its propagation mechanism, scanning electron microscopy (SEM) observation and energy spectrum analysis were carried out. Scanning electron microscopy showed that the crack edge at the welded joint showed typical brittle fracture characteristics, and the crack surface showed obvious flash texture, which indicated that the crack propagation was mainly caused by brittle fracture.

Energy spectrum analysis further revealed that the crack surface contained a high concentration of oxygen, indicating that the formation of the crack may be related to the oxidation reaction during the welding process. Under high temperature environment, the metal surface of the welded joint may react with oxygen in the air to form an oxide film, which will reduce the toughness of the metal itself and increase the possibility of brittle fracture. Combined with the observation results of scanning electron microscopy, it can be inferred that the occurrence and propagation of cracks are closely related to local oxidation reactions and stress concentration.

3. Summary of failure causes
Through multiple tests and analyses of cracking failure of Q235B welded pipes, combined with the macroscopic and microscopic manifestations of cracking of welded pipes, the following conclusions are drawn:

Stress concentration during welding: During welding, due to the uneven temperature gradient, the heat affected zone (HAZ) in the welded joint area may produce excessive stress, leading to the initiation of local brittle cracks.

Influence of non-metallic inclusions: Non-metallic inclusions in steel are not completely removed during welding, resulting in a decrease in the toughness of the welded joint and becoming the source of crack propagation.

Welding temperature is too high: Local overheating during welding causes coarsening of the metal structure, which in turn affects the toughness and strength of the welded joint and increases the risk of cracks.

Influence of oxidation reaction: The oxide film on the surface of the welded joint may reduce the toughness of the joint area, resulting in brittle fracture.

4. Improvement measures
Based on the above analysis results, the following improvement measures are proposed:

Optimize welding process: Control the temperature during welding to avoid local overheating, ensure that the microstructure of the heat affected zone is uniform and fine, and improve the toughness of the welded joint.

Improve steel quality: Strengthen the control of non-metallic inclusions during steel smelting, reduce the presence of inclusions, especially in the welding area, to avoid their impact on welding quality.

Surface treatment of welded joints: Pre-treat the joints before welding to remove possible oxides and impurities, reduce the occurrence of oxidation reactions, and avoid reducing the quality of welded joints due to oxidation.

Strengthen post-welding heat treatment: Through post-welding heat treatment, improve the metallographic structure of the welded joint, reduce the formation of brittle phases, and improve the comprehensive mechanical properties of the welded joint.

Through these measures, the cracking problem of Q235B welded pipes in the production process can be effectively reduced, ensuring the quality and safety of welded pipe products.


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