High carbon steel refers to carbon steel with w(C) higher than 0.6%, which 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 welding heat affected zone is hard and brittle, which leads to a great decrease in the plasticity and toughness of the joint. Therefore, the weldability of high carbon steel is quite poor, and special welding processes must be adopted to ensure the performance of the joint. . Therefore, in welded structures, it is generally rarely used. High carbon steel is mainly used for machine parts that require high hardness and wear resistance, such as shafts, large gears and couplings [1]. In order to save steel and simplify the processing process, these machine parts are often combined with welded structures. Welding problems of high carbon steel components are also encountered in heavy machinery manufacturing. When formulating the welding process of high carbon steel weldments, various welding defects that may occur should be comprehensively analyzed, and corresponding welding process measures should be taken.
1 Weldability of high carbon steel
1.1 Welding method
High carbon steel is mainly used for structures with high hardness and high wear resistance, so the main welding methods are electrode arc welding, brazing and submerged arc welding.
1.2 Welding materials
The welding of high carbon steel generally does not require the strength of the joint and the base metal. In electrode arc welding, low-hydrogen electrodes with strong desulfurization ability, low content of diffusible hydrogen in the deposited metal and good toughness are generally used. When the strength of the weld metal and the base metal is required, the low-hydrogen type electrode of the corresponding level should be selected; when the strength of the weld metal and the base metal is not required, the low-hydrogen type electrode with the strength level lower than that of the base metal should be selected. Welding electrodes with a higher strength class than the base metal cannot be selected. If the base metal is not allowed to be preheated during welding, in order to prevent cold cracks in the heat affected zone, austenitic stainless steel electrodes can be used to obtain austenitic structures with good plasticity and strong crack resistance.
1.3 Groove preparation
In order to limit the mass fraction of carbon in the weld metal, the fusion ratio should be reduced, so U-shaped or V-shaped grooves are generally used during welding, and attention should be paid to cleaning the groove and the oil stains and rust within 20mm on both sides of the groove.
1.4 Preheating
When using structural steel electrodes for welding, it must be preheated before welding, and the preheating temperature should be controlled at 250°C to 350°C.
1.5 Interlayer Processing
In multi-layer multi-pass welding, the first pass uses small-diameter electrodes and low-current welding. Generally, the workpiece is placed in semi-vertical welding or the welding rod is used to swing laterally, so that the entire heat-affected zone of the base metal is heated in a short time to obtain the effect of preheating and heat preservation.
1.6 Post-weld heat treatment
Immediately after welding, put the workpiece into the heating furnace, and keep it at 650℃ for stress relief annealing
2 Welding defects of high carbon steel and preventive measures
Due to the high hardening tendency of high carbon steel, hot and cold cracks are prone to occur during welding.
2.1 Preventive measures for thermal 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-depth ratio should be slightly larger to avoid segregation in the center of the weld.
3) For welding parts with high rigidity, appropriate welding parameters, appropriate welding sequence and direction should be selected.
4) If necessary, take preheating and slow cooling measures to prevent the occurrence of hot cracks.
5) Increase the basicity of the electrode 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 outward diffusion of hydrogen in the weld.
2) Select appropriate welding measures.
3) Adopt the appropriate assembly and welding sequence to reduce the restraint stress of the welded joint and improve the stress state of the weldment.
4) Choose suitable welding materials, dry the electrodes and fluxes before welding, and use them as needed.
5) Before welding, the water, rust and other contaminants on the surface of the basic metal around the groove should be carefully removed to reduce the content of diffusible hydrogen in the weld.
6) Dehydrogenation treatment should be carried out immediately before welding so that hydrogen can fully escape from the welded joint.
7) Stress relief annealing treatment should be carried out immediately after welding to promote the outward diffusion of hydrogen in the weld.
3 .Conclusion
Due to the high carbon content, high hardenability and poor weldability of high carbon steel, it is easy to produce high carbon martensitic structure during welding, and it is easy to produce welding cracks. Therefore, when welding high carbon steel, a reasonable welding process should be selected. And take corresponding measures in time to reduce the occurrence of welding cracks and improve the performance of welded joints.
