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 form 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, causing the plasticity and toughness of the joint to be greatly reduced. 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, it is generally rarely used in welded structures. High carbon steel is mainly used for machine parts that require high hardness and wear resistance, such as rotating shafts, large gears and couplings [1]. In order to save steel and simplify the processing technology, these machine parts are often combined with welded structures. In heavy machine manufacturing, welding problems of high carbon steel components are also encountered. When formulating the welding process for high carbon steel weldments, various possible welding defects should be comprehensively analyzed and corresponding welding process measures should be taken.
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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
High carbon steel welding generally does not require equal strength between the joint and the base metal. When arc welding, low-hydrogen electrodes with strong sulfur removal capabilities, low diffusible hydrogen content in the deposited metal, and good toughness are generally used. When the strength of the weld metal and the base metal is required to be equal, a low-hydrogen welding rod of the corresponding grade should be selected; when the strength of the weld metal and the base metal is not required, a low-hydrogen welding rod with a strength level lower than that of the base metal should be selected. Remember Welding rods with a higher strength level 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 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, 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, rust, etc. within 20mm on both sides of the groove.
1.4 Preheating
When welding with structural steel electrodes, it must be preheated before welding, and the preheating temperature is controlled between 250°C and 350°C.
1.5 Interlayer processing
When welding multiple layers and multiple passes, a small-diameter electrode and low current are used for the first pass. Generally, the workpiece is placed in a semi-vertical welding or the welding rod is used to swing laterally, so that the entire base metal heat-affected zone is heated in a short time to obtain preheating and heat preservation effects.
1.6 Post-weld heat treatment
Immediately after welding, the workpiece is placed in a heating furnace and kept at 650°C for stress relief annealing [3].
2 Welding defects of high carbon steel and preventive measures
Because high carbon steel has a strong tendency to harden, hot cracks 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 sulfur and phosphorus content, and appropriately increase the manganese content to improve the weld structure and reduce segregation.
2) Control the cross-sectional shape of the weld and make the width-to-depth ratio slightly larger to avoid segregation in the center of the weld.
3) For rigid weldments, 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 thermal cracks.
5) Increase the alkalinity 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[4]
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) Choose appropriate welding measures.
3) Adopt appropriate assembly and welding sequences to reduce the restraint stress of the welded joint and improve the stress state of the weldment.
4) Choose appropriate welding materials, dry the electrodes and flux before welding, and keep them ready for use.
5) Before welding, water, rust and other contaminants on the basic metal surface 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 to allow hydrogen to fully escape from the welded joint.
7) Stress-relieving annealing treatment should be performed 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 martensite structure and welding cracks during welding. Therefore, when welding high carbon steel, the welding process must be reasonably selected. And take corresponding measures in a timely manner to reduce the occurrence of welding cracks and improve the performance of welded joints.
Post time: May-27-2024
