Common welding methods for cast iron parts

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Common welding methods for cast iron parts: Due to certain advantages of cast iron, it occupies a significant proportion in automotive manufacturing materials. Cast iron parts are mostly basic components with high machining precision and high costs, such as cylinder blocks, cylinder heads, and transmission housings. During manufacturing and use, cast iron parts often develop cracks, pores, damage, and other issues. According to statistics, under normal use of vehicles, when such components reach their wear limit, the change in their dimensions is only 0.08% to 0.40%, and the loss in weight is only 0.1% to 1.8%. It would be extremely wasteful to discard these components at that point. Therefore, it is highly necessary to study and utilize advanced repair experience to properly repair cast iron parts. Welding is a very effective method for repairing cast iron parts. Cast iron has a high carbon content and many impurities; it also features low plasticity, poor weldability, and sensitivity to cooling rates. After welding, it is prone to the formation of martensitic structure and cracks. To improve the quality of welding repairs on cast iron parts, the following methods can be adopted. 1. Hot welding method: Before welding, the workpiece as a whole or in certain areas is preheated to 600–700°C; during the repair welding process, the temperature remains above 400°C, and after welding it is cooled slowly to room temperature. The hot welding method can effectively reduce the temperature difference at the weld joint, thereby decreasing stress; it also improves the plasticity of the casting and prevents the formation of martensite structure and cracks. The common welding methods are gas welding and shielded metal arc welding. Gas welding commonly uses cast iron gas welding wires, such as HS401 or HS402, along with flux CJ201 to remove oxides. The gas welding preheating method is suitable for patch welding small and medium-sized thin-walled parts. For shielded metal arc welding, cast-iron core cast-iron electrodes Z248 or steel core cast-iron electrodes Z208 are used; this method is primarily employed for repairing cast-iron parts with a large thickness (greater than 10 mm). The welding equipment for hot welding mainly includes heating furnaces, welding torches, electric furnaces (oil furnaces or ground furnaces), etc. The welding process is as follows: 1) Preparation and preheating before welding: Remove oil and oxide scale around defects to expose the metallic luster of the base material; create a groove, with a depth generally equal to 2/3 of the thickness of the workpiece’s wall, and an angle of 70° to 120° ； Place the welded piece in the furnace and heat it slowly to 600–700°C (do not exceed 700°C). 2) Welding: Use a neutral flame or a weak carbonizing flame (make sure the molten iron does not flow to one side during welding); once the base metal has been fully melted, then add the weld metal ； When white spots appear in the molten pool, stop adding weld rod metal, add an appropriate amount of flux, use the weld rod to remove the impurities, and then continue welding ； To obtain a smooth weld, the weld after welding should be slightly above the surface of the cast iron part; the slag that overhangs the weld should be remelted. Once it has cooled to a semi-molten state, the excess portion can be leveled off using welding wire along the surface of the casting. 3) Post-weld cooling: It is generally necessary to cool slowly to room temperature within the furnace (usually taking more than 48 hours). Alternatively, asbestos cloth (sheet) or charcoal ash can be used for covering, so as to enable the weld to develop a uniform structure and prevent cracks from forming. 2. Cold welding method: In this method, the workpiece is not preheated before welding, or the preheating temperature does not exceed 300°C. Cast iron cold welding is commonly performed using shielded metal arc welding. Depending on the requirements of the cast iron parts, different cast iron welding electrodes can be selected. For repairing the unmachined surfaces of ordinary gray cast iron parts, Z100 electrode is used, while for repairing high-strength gray cast iron and ductile iron parts, Z116 or Z117 electrodes are employed. The welding equipment for cold welding is ordinary arc welding equipment, and the welding process is as follows: 1) Pre-welding preparation: Remove oil and impurities from the surface to be welded so that the metallic luster of the base material is exposed; if cracks are present, drill stop holes at each end of the cracks to prevent them from extending during welding ； A groove is cut along the crack, with its shape and size determined by the thickness of the area to be welded and the process requirements. For large castings, a certain number of screws can also be screwed into the welds to reinforce the joint. The diameter of the screws generally should not exceed 16 mm (if the wall thickness is less than 15 mm, the screw diameter should be less than or equal to 6 mm). The number of screws can be determined based on the cross-sectional area; that is, the total cross-sectional area of the screws should not exceed 25% of the cross-sectional area of the crack in the casting. Moreover, these screws should be evenly distributed on both sides of the crack. 2) Selection of welding repair specifications: The diameter of the welding rod is determined by the thickness of the area to be repaired; generally, it is advisable to use rods with a smaller diameter in order to reduce the heat input into the workpiece. While ensuring good fusion between the welding rod metal and the base material, the welding current should also be kept as low as possible to avoid excessive temperature in the workpiece, which could lead to stress ； The arc length is generally 0.5 to 1.1 times the diameter of the welding rod, in order to ensure stable combustion; if a direct current power supply is used, the workpiece is usually chosen as the negative pole to prevent it from overheating. 3) Requirements for the welding process: It is generally necessary to follow the principle of \"working from the inside out (first inside the holes, then on the outside of the machine body, and finally on its upper surface), using short weld segments, carrying out intermittent welding, and applying multiple layers of welds. After the first layer is welded, use a grinding wheel to remove some of the weld metal from the entire weld seam; only after confirming that there are no pores or cracks should the second layer be welded\" ； For each layer, start welding from both sides of the groove first and then the middle; follow the principle of \"low current and hammer-driven welding\". ① The entire weld seam should be divided into several small sections; welding must not be carried out continuously. The length of each section depends on the thickness of the welded parts, usually ranging from 10 to 50 mm. After each section is welded, it must be cooled to room temperature before proceeding with the next section. Each small weld patch should not extend across both sides of the groove, as this helps the unwelded areas to contract freely and prevents the arc from staying in those areas for too long. ② When the temperature of the metal after welding is around 800°C, the weld should be hammered to create a pitted surface, thereby relieving welding stresses and eliminating cracks and pores. Hammering should not be carried out when the temperature is below 300°C, to avoid the formation of cold-crack defects. ③ During welding, it is preferable to use a straight-line, small-circle motion for the welding rod; the weld should transition smoothly into the base material to facilitate the distribution of stresses in the weld. 3. Heat-based stress reduction welding: This method does not involve pre-heating the workpiece; instead, it heats the \"stress reduction zone\" of the workpiece before and during welding, so as to prevent interference with the contraction of the weld seam, thereby reducing internal stresses and avoiding crack formation. One or more locations in the heat-affected zone can be selected, and the selection principle is as follows: 1) It should be a location that hinders the expansion of the weld. When this area is heated and cooled, it allows the weld to undergo free thermal expansion and contraction. 2) It should be a section with limited connection to other parts and high strength. 3) Its own deformation should not have a significant impact on other parts; it should not cause damage to those parts as a result of its own deformation.