High-carbon steel is a type of steel with a carbon content (C) higher than 0.6%. It exhibits strong hardenability, forming high-carbon martensite and being more sensitive to the generation of cold cracks. During the welding process, the martensitic structure formed in the heat-affected zone increases the hardness of the joint and exacerbates its brittleness, leading to a significant decrease in ductility and toughness of the joint. Therefore, the weldability of high-carbon steel is quite poor, requiring special welding processes to ensure the performance of the joint. Consequently, high-carbon steel is generally not widely used in structural welding.
However, high-carbon steel is still widely used in some machine components such as shafts, large gears, and couplings, among others. These components are usually fabricated using welded structures to save steel and simplify manufacturing processes. Welding issues with high-carbon steel components are also encountered in heavy machinery manufacturing. Therefore, when determining the welding process for high-carbon steel, it is necessary to comprehensively analyze various possible welding defects and take corresponding process measures.
I. Weldability of High-Carbon Steel
1）Welding Methods
High-carbon steel is mainly used in structures requiring high hardness and wear resistance, so the main welding methods include shielded metal arc welding, brazing, and submerged arc welding.
2）Welding Materials
High-carbon steel welding generally does not require the weld metal to have the same strength as the base metal. For shielded metal arc welding, low hydrogen electrodes with strong desulfurization ability, low diffusible hydrogen content in the deposited metal, and good toughness are usually selected. When the weld metal needs to have the same strength as the base metal, electrodes of the corresponding grade of low hydrogen type should be selected; while when the weld metal does not need to have the same strength as the base metal, electrodes of a strength grade lower than that of the base metal should be selected. If preheating of the base metal is not allowed, austenitic stainless steel electrodes can be selected to avoid the formation of cold cracks in the heat-affected zone and obtain an austenitic structure with good ductility and crack resistance.
3）Preparation of Groove
To limit the carbon content in the weld metal, the melting ratio needs to be reduced. Therefore, U-shaped or V-shaped grooves are usually used for welding, and attention should be paid to cleaning the groove and the surrounding area within 20mm of oil stains, rust, and other impurities.
4）Preheating
When using structural steel electrodes for welding, preheating is necessary before welding, and the preheating temperature should be controlled between 250°C and 350°C.
5）Interlayer Treatment
In multi-layer multi-pass welding, the first pass should be welded with small-diameter electrodes and low current. The workpiece is usually placed in a semi-standing position or the electrode is swung laterally to ensure that the entire heat-affected zone of the base metal can be heated in a short time, thereby achieving preheating and heat preservation effects.
6）Post-Weld Heat Treatment
The workpiece should be immediately placed in a heating furnace after welding, and held at 650°C for annealing to eliminate stress and tempering. Due to the large hardenability of high-carbon steel, it is prone to hot cracks and cold cracks during welding.
II. Welding Defects and Prevention Measures of High-Carbon Steel
1）Prevention Measures for Hot Cracks:
Control the chemical composition of the weld seam, strictly control the content of sulfur and phosphorus, and appropriately increase the manganese content to improve the weld metal structure and reduce segregation.
Control the cross-sectional shape of the weld seam, with a slightly larger width-to-depth ratio to avoid segregation at the center of the weld.
For large rigid weldments, appropriate welding parameters, welding sequences, and directions should be selected.
Preheating and slow cooling measures should be taken if necessary to prevent the occurrence of hot cracks.
Increase the alkalinity of the electrodes or flux to reduce the impurity content in the weld metal and improve the degree of segregation.
2）Prevention Measures for Cold Cracks:
Preheating before welding and slow cooling after welding to reduce the hardness and brittleness of the heat-affected zone, and accelerate the outward diffusion of hydrogen in the weld.
Choose appropriate welding measures.
Use appropriate assembly and welding sequences to reduce the restraint stress of the welded joint and improve the stress state of the weldment.
Choose appropriate welding materials, preheat electrodes and fluxes before welding, and use them immediately after taking them out.
Carefully clean the surface of the base metal around the groove to reduce the hydrogen content in the weld.
Immediately carry out dehydrogenation treatment before welding to allow hydrogen in the welded joint to escape fully.
Immediately carry out stress-relief annealing after welding to promote the outward diffusion of hydrogen in the weld.
III. Conclusion
Due to the high carbon content and large hardenability, high-carbon steel has poor weldability, easily forming high-carbon martensitic structure and welding cracks. Therefore, when welding high-carbon steel, it is necessary to choose welding processes reasonably and take corresponding measures in a timely manner to reduce the occurrence of welding cracks and improve the performance of welded joints.
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