Welding knowledge of medium carbon steel

Medium carbon steel refers to the carbon content of 0.25% ~ 0.60% of the carbon steel, including high quality carbon structure in 30, 35, 45, 50, 55, and cast carbon steel in ZG230-450, ZG270-500, ZG310-570, ZG340-640 and other steel. Because the carbon content of medium carbon steel is higher than that of low carbon steel, its weldability is worse than that of low carbon steel. When the mass fraction of carbon is close to 0.30% and the content of manganese is not high, the weldability is good, but with the increase of the content of carbon, the weldability gradually becomes worse. When the mass fraction of carbon reaches about 0.50%, the weldability becomes obviously worse. First, welding medium carbon steel easy to appear problems and technological measures The problems that tend to occur when welding medium carbon steel are as follows: 1, cold crack problem Due to the high carbon content of steel, it is easy to produce hard and brittle martensite structure in heat-affected zone during welding, so it is easy to produce cold crack. If the welding material is not selected properly or the welding process is not formulated properly, the welding seam is easy to produce cold cracks. 2, hot crack problem When welding, the base metal with higher carbon content will also bring carbon into the weld after melting, so that the content of carbon in welding increases. Carbon can aggravate the effect of hot crack caused by sulfur and phosphorus in metal, so it is easy to produce hot crack when welding medium carbon steel. Especially when the content of sulfur and phosphorus in the base metal or welding material is not strictly controlled, it is more likely to produce hot cracks. In addition, the high content of carbon in steel also increases the tendency to produce CO pores in welding joints. Process measures Due to the cold crack, hot crack and other defects in the welding of medium carbon steel, it is necessary to take some special technological measures to weld it normally. 1. Welding method All kinds of arc welding methods can be used for welding medium carbon steel. Since the medium carbon steel is mostly used in the manufacture of machine parts rather than large welding structures, electrode arc welding is the most widely used method in production. 2. Welding materials In order to prevent cold and hot cracks in the welded joint, low hydrogen welding electrode is usually used in arc welding. Low hydrogen electrode not only makes the weld hydrogen content is low, but also has a certain desulfurization, phosphorus effect, can improve the plasticity and toughness of the weld. When the carbon content in the steel is low and the joint restraint is not large, you can also use ilmenite type or titano-calcium type welding rod welding, but must take strict process measures to match, such as to minimize the fusion ratio, strict preheating of welding parts and control the temperature between layers. If the welding can not be preheated, you can also use chromium-nickel austenitic stainless steel welding electrode, For example, E308L-16 (A102), E308L-15 (A107), E309-16 (A302), E309-15 (A307), E310-16 (A402), E310-15 (A407), etc. 3, preheating temperature and inter-layer temperature Preheating before welding is the most effective technological measure to prevent cracking in welding medium carbon steel. Preheating can not only reduce the cooling rate of the joint, thus preventing martensite formation, but also reduce welding stress and accelerate hydrogen diffusion escape. In most cases it is necessary to preheat and maintain interlayer temperatures. The choice of preheating temperature and interlayer temperature depends on the carbon equivalent of the steel, the thickness of the base metal, the rigidity of the structure, the type of electrode, etc. The preheating temperature can be determined by the weldability test or by the empirical formula T0=550 (C-0.12) +0.4δ, where T0 represents the preheating temperature (℃), C represents the mass fraction of carbon in the base metal being welded (%), and δ represents the thickness of the steel plate (mm). 4, groove type It is better to open the U or V groove of the weldment, so as to reduce the proportion of base metal into the weld. If the casting defects are repaired, the grooves dug out by the shovel should be smooth to reduce the amount of base metal melted into the weld. 5. Welding parameters Reverse polarity DC power supply should be used for welding. Multi-layer welding, because the base metal dissolved into the first layer of the weld proportion (mass fraction) up to about 30%, so it is appropriate to use small diameter electrode, small current, slow welding speed welding. 6. Post-welding heat treatment It is desirable that the weldment undergo stress relief heat treatment immediately after welding. Especially for large thickness weldments, large rigid structures and weldments working under dynamic or impact loads. The temperature of stress relief tempering is generally 600 ~ 650℃. If stress relief heat treatment cannot be carried out immediately after welding, post heating, that is, heating slightly above the preheat temperature, shall be carried out for a holding time of approximately 1 hour per 10mm thickness. Two, typical medium carbon steel welding process compilation (a) 35 steel and ZG270-500 cast carbon steel The mass fraction of 35 carbon steel is 0.32% ~ 0.39%, and that of ZG270-500 cast carbon steel is 0.31% ~ 0.40%. The carbon equivalent is about 0.45%, so the weldability of this type of steel is tolerable. However, during welding, the heat affected zone may still produce hard and brittle martensite structure and have a tendency to produce cracks, so certain technological measures should be taken when welding this kind of steel. 1. Selection of welding materials When welding electrode arc welding, when the welding seam and base metal are required to be equal in strength, E5016 (J506) or E5015 (J507) welding electrode can be selected; When the strength of weld and base metal is not required, welding rods such as E4316 (J426), E4315 (J427), E4303 (J422), E4310 (J423) can be used for welding. In submerged arc welding, HJ430 and HJ431 can be used as flux. Welding wire can choose H08MNA, H10MN2, etc. Electroslag welding, flux can choose HJ430, HJ431, HJ360; Welding wire can choose H10Mn2, H08Mn2Si, H08Mn2SiA, etc. 2, preheating temperature and inter-layer temperature When welding 35 steel and ZG270-500 cast steel, the preheating temperature and interlayer temperature of the welded parts are usually about 150℃. When the rigidity of the welded parts is relatively large, the preheating temperature and interlayer temperature should be raised to 200 ~ 250℃. The heating range of local preheating is 150 ~ 200mm on both sides of the groove. 3. Post-welding heat treatment For large thickness welds, large rigid structure parts and welds with dynamic or impact load, stress relief tempering should be carried out immediately after welding, tempering temperature is generally 600 ~ 650℃. For welds of general thickness, post heating may be used to allow diffused hydrogen to escape. The post-heating temperature is generally 200 ~ 350℃, and the holding time is 2 ~ 6h, depending on the thickness of the welding parts. (2) 45 steel and ZG310-570 cast carbon steel The carbon content of 45 steel is 0.42%-0.5%, and that of ZG310-570 cast steel is 0.41%-0.50%. The carbon equivalent is about 0.56%, the quenching tendency is relatively large, easy to produce cracks, and the weldability is relatively poor. 1. Selection of welding materials When welding electrode arc welding, try to choose low hydrogen electrode, when the welding seam and base metal strength, can choose E5516-G (J556), E5515-G (J557) electrode; When the strength of the weld and the base metal is not required, the welding rod E4316 (J426), E4315 (J427), E5016 (J506), E5015 (J507), E4303 (J422), E4301 (J423) can be used. For submerged arc welding, HJ350 or SJ101 can be used as flux and H08MnMoA as welding wire. 2. Selection of welding parameters For welding 45 steel and ZG310-570 cast carbon steel, a smaller welding current should be selected to reduce the fusion ratio of the weld and reduce the amount of carbon transition from the base metal to the weld. 3, preheating temperature and inter-layer temperature Welding this kind of steel, it is best to take the overall preheating before welding, preheating temperature is more than 200℃. For the T-joint, because the heat dissipation direction is more than that of the flat joint, it will lead to the accelerated cooling of the welded joint and increase the tendency to produce cold cracks. The preheating temperature should be appropriately increased. The preheating temperature is 250 ~ 400℃, depending on the thickness of the welding parts. The interlayer temperature should not be lower than preheating temperature. 4. Post-welding heat treatment The welding parts should be immediately after the stress relief tempering treatment, tempering temperature of 600 ~ 650℃.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods