What are the key points to pay attention to when welding high carbon steel pipes

High carbon steel refers to carbon steel with w(C) higher than 0.6%. It has a greater tendency to harden than medium carbon steel and forms high carbon martensite, which is more sensitive to the formation of cold cracks. At the same time, the martensite structure formed in the heat-affected zone of welding is hard and brittle, which greatly reduces the plasticity and toughness of the joint. Therefore, the weldability of high carbon steel pipes is quite poor, and special welding processes must be adopted to ensure the performance of the joints. Therefore, it is rarely used in welded structures. High carbon steel pipes are mainly used for machine parts that require high hardness and wear resistance, such as shafts, large gears, and couplings. In order to save steel and simplify the processing technology, these machine parts are often combined with welded structures. In heavy machinery manufacturing, welding problems of high carbon steel parts will also be encountered. When formulating the welding process of high carbon steel weldments, various welding defects that may occur should be fully analyzed, and corresponding welding process measures should be taken.

1 Weldability of high carbon steel pipe

1.1 Welding method: High carbon steel pipe is mainly used for structures with high hardness and high wear resistance, so the main welding methods are arc welding, brazing, and submerged arc welding.

1.2 Welding materials: High carbon steel pipe welding generally does not require the joint to have the same strength as the parent material. Low-hydrogen type welding rods with strong desulfurization ability, low diffusible hydrogen content in the deposited metal, and good toughness are generally used for arc welding. When the weld metal is required to have the same strength as the parent material, a low-hydrogen type welding rod of the corresponding grade should be selected; when the weld metal is not required to have the same strength as the parent material, a low-hydrogen type welding rod with a strength level lower than the parent material should be selected. Remember not to select a welding rod with a higher strength level than the parent material. If the parent material is not allowed to be preheated during welding, in order to prevent cold cracks in the heat-affected zone, austenitic stainless steel welding rods can be used to obtain an austenitic structure with good plasticity and strong crack resistance.

1.3 Bevel preparation: In order to limit the mass fraction of carbon in the weld metal, the fusion ratio should be reduced. Therefore, U-shaped or V-shaped bevels are generally used during welding, and attention should be paid to cleaning the oil, rust, etc. within 20mm on the bevel and both sides of the bevel.

1.4 Preheating: When welding with structural steel electrodes, preheating must be done before welding, and the preheating temperature should be controlled at 250℃～350℃.

1.5 Interlayer treatment: When welding multiple layers and multiple passes, the first pass uses a small diameter electrode and a small current. Generally, the workpiece is placed in a semi-vertical welding position or the electrode is swung horizontally so that the entire heat-affected zone of the parent material is heated in a short time to obtain the preheating and insulation effect.

1.6 Post-weld heat treatment: Immediately after welding, the workpiece is placed in a heating furnace and kept warm at 650℃ for stress relief annealing.

2. Welding defects and prevention measures of high carbon steel pipes

Due to the great tendency of high carbon steel pipes to harden, hot cracks and cold cracks are prone to occur during welding.

2.1 Preventive measures for hot cracks:

1) Control the chemical composition of the weld, strictly control the content of sulfur and phosphorus, and appropriately increase the manganese content to improve the weld structure and reduce segregation.

2) Control the cross-sectional shape of the weld, and the width-to-depth ratio should be slightly larger to avoid segregation in the center of the weld.

3) For welds with high rigidity, appropriate welding parameters, welding sequence, and direction should be selected.

4) Take preheating and slow cooling measures when necessary to prevent the occurrence of hot cracks.

5) Increase the basicity of the welding rod or flux to reduce the impurity content in the weld and improve the degree of segregation.

2.2 Preventive measures for cold cracks:

1) Preheating before welding and slow cooling after welding can not only reduce the hardness and brittleness of the heat-affected zone but also accelerate the diffusion of hydrogen in the weld.

2) Select appropriate welding measures.

3) Use appropriate assembly and welding sequences to reduce the restraint stress of the weld joint and improve the stress state of the weld.

4) Choose appropriate welding materials. Dry the welding rod and flux before welding and use them as needed.

5) Before welding, carefully remove water, rust, and other dirt on the surface of the base metal around the groove to reduce the content of diffused hydrogen in the weld.

6) Dehydrogenation should be carried out immediately before welding to allow hydrogen to fully escape from the weld joint.

7) After welding, stress-relieving annealing should be carried out immediately to promote the diffusion of hydrogen in the weld.

3. Summary

Due to its high carbon content, high hardenability, and poor weldability, high-carbon martensitic structure and welding cracks are easily generated during welding. Therefore, when welding high-carbon steel pipes, it is necessary to reasonably select the welding process and take corresponding measures in time to reduce the occurrence of welding cracks and improve the performance of the welded joints.