How to select welding method of high carbon steel?
High carbon steel is mainly used for machine parts that require high hardness and wear resistance, such as rotating shafts, large gears and couplings. In order to save steel and simplify the processing process, these machine parts are often combined with welded structures.
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. Welding materials: High carbon steel welding generally does not require the strength of the joint and the base metal. In arc welding, low hydrogen type welding wire with strong desulphurization ability, low diffusible hydrogen content of deposited metal and good toughness is generally used. When the strength of the weld metal and the base metal is required, the low hydrogen type welding wire 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 welding wire with a 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 welding wire can be selected to obtain austenitic structure with good plasticity and strong crack resistance.
2. Groove: 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 the groove and grooves within 20mm on both sides of the groove. Clean the rust, etc.
3. Preheating: The preheating temperature is controlled at 250℃～350℃.
4. Interlayer treatment: In multi-layer multi-pass welding, the first pass uses small-diameter welding wire and low-current welding. Use the wire to oscillate laterally so that the entire base metal heat affected zone is heated for a short period of time for preheating and thermal insulation.
5. Post-weld heat treatment: Immediately after welding, put the workpiece into the heating furnace, and keep it at 650 ℃ for stress relief annealing.
Due to the high hardening tendency of high carbon steel, hot and cold cracks are prone to occur during welding.
1. Prevention 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.
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 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 alkalinity of the welding wire or flux to reduce the impurity content in the weld and improve the degree of segregation.
2. Prevention 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 appropriate assembly and welding sequence to reduce the restraint stress of welded joints and improve the stress state of weldments.
4) 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.
5) Dehydrogenation treatment should be carried out immediately before welding so that hydrogen can fully escape from the welded joint.
6) Stress relief annealing treatment should be carried out immediately after welding to promote the outward diffusion of hydrogen in the weld.