What are the technological characteristics of cast iron welding?

Instructions for using cast iron welding electrodes: Due to the high carbon content, uneven microstructure, and low plasticity of cast iron, it is a material with poor weldability. During welding, defects such as white cast, cracks, and pores are likely to occur; therefore, welding cast iron requires a high level of skill from the welder. Cast iron welding can generally be divided into two methods: preheated welding and cold welding. To ensure good welding results, it is recommended to adopt the following welding measures for reference and selection when welding castings made of different materials or repairing defects. EZC-type gray cast iron welding electrode: an EZC-type cast iron welding electrode with a steel core and highly graphitized flux, suitable for both AC and DC power. By adding an appropriate amount of graphitizing elements to the flux of steel-core cast iron welding electrodes, the weld can turn into gray cast iron when cooled slowly; however, if the cooling rate is fast, white cast iron is formed, which is difficult to machine. The cooling rate has a significant impact on machinability and weld microstructure; therefore, the welding procedure differs from that of ordinary cold welding electrodes. These electrodes require continuous welding followed by heat retention to allow the weld to cool slowly. The microstructure, properties, and color of the welds in gray cast iron are generally similar to those of the base material; however, due to their poor plasticity, they are unable to relieve welding stresses, resulting in weak resistance to thermal stress cracks. For defects in the areas of small, thin-walled parts where stiffness is low, welding can be carried out without preheating; however, in most cases, preheating to around 400°C is necessary before welding, followed by slow cooling. This approach helps to prevent cracks and case hardening. EZNi-type pure nickel cast iron welding rod: The EZNi type is a cast iron welding rod with a pure nickel core and a highly graphitized flux coating; it can be used with both AC and DC power and enables welding in all positions. During welding, the workpiece does not need to be preheated. It is a type of cold-welding electrode for cast iron that offers good comprehensive properties such as crack resistance, machinability, workability, and mechanical strength; it is widely used for patch welding thin cast iron parts and their surfaces. EZNiFe-type nickel-iron cast iron welding rod: The EZNiFe type is a cast iron welding rod with a nickel-iron core and a strongly graphitized flux coating; it can be used with both AC and DC power and enables welding in all positions. During welding, the workpiece does not require preheating; it features high strength, good plasticity, excellent crack resistance, and strong fusion with the base material. It can be used for patch welding on important gray cast irons and ductile iron.

Cast iron has a high carbon content and many impurities; it also features low plasticity, poor weldability, and sensitivity to cooling rates. After welding repairs, it is prone to the formation of white cast 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 is preheated as a whole or in parts 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 martensitic structures 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 greater 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 scales 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°–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 rod 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), or the weld area can be covered with asbestos cloth (sheet) or charcoal ash to enable the formation of a uniform microstructure in the weld while preventing 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 carried out 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 distributed evenly 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 made 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, following 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 bead 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 give its surface a pitted appearance, 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 advisable 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-Reduced Stress Welding Method: This method does not involve pre-heating the workpiece; instead, it heats the \"heat-reduced stress 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 criteria are as follows: 1) They should be areas that prevent the weld from expanding. When this area is heated and cooled, the weld has the possibility to undergo free thermal expansion and contraction. 　　2) It should be a section with little 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 due to its deformation. 　　When selecting a welding method, the following principles should be considered: ① Choose different welding methods based on factors such as machinability, color, and strength. The hot welding method of shielded metal arc welding is most suitable for castings that require high quality and good machinability, while the cold welding method of shielded metal arc welding is appropriate for machined surfaces and large castings that are difficult to preheat. 　　② Different welding methods are selected based on factors such as the volume, shape, thickness of the welded parts, and the operating conditions. For small and medium-sized thin-walled parts (such as cylinders), gas welding, cold welding, or hot welding can all be used; for larger parts, gas welding and hot welding should be employed.

The two introductions were very comprehensive. I’ve learned it.*