Due to some of the advantages of cast iron, it accounts for a large proportion of automotive materials. Most of the cast iron parts are basic parts with high processing precision and high price, such as cylinder block, cylinder head and transmission housing. Cracks, pores, damage, etc. often occur during the manufacture and use of cast iron parts. According to statistics, under normal use conditions, when such parts reach the wear limit, the dimensional change is only 0.08% to 0.40%, and the quality loss is only 0.1% to 1.8%. It is undoubtedly very wasteful to scrap the parts at this time. Therefore, it is necessary to research and utilize advanced repair experience to properly repair cast iron parts. Welding is a very effective way to repair cast iron parts.
Cast iron has high carbon content, many impurities, and has low plasticity, poor weldability, sensitivity to cooling rate, etc. It is easy to appear white joint structure and crack after welding. In order to improve the quality of the weld repair of cast iron parts, the following methods can be used.
1. Hot welding
Before the welding, the workpiece is preheated to 600~700 °C in whole or in part, not less than 400 °C during the repair welding process, and slowly cooled to room temperature after welding. The hot welding method can effectively reduce the temperature difference of the welded joint, thereby reducing the stress, and at the same time, improving the plasticity of the casting and preventing the white mouth structure and crack.
Commonly used welding methods are gas welding and electrode arc welding. Gas welding commonly used cast iron gas welding wire, such as HS401 or HS402, with flux CJ201 to remove oxides. The gas welding preheating method is suitable for repairing small and medium-sized thin-walled parts. The electrode arc welding uses cast iron core cast iron welding rod Z248 or steel core cast iron welding rod Z208. This method is mainly used for repairing cast iron parts with large thickness (greater than 10mm).
The welding equipment of the heat welding method mainly includes a heating furnace, a welding torch, an electric furnace (oil furnace or a ground furnace), and the welding process is as follows:
1) Preparation and preheating before welding: remove the oil stain and scale around the defect, revealing the metallic luster of the base: the groove opening, the depth of the groove is generally 2/3 of the wall thickness of the weldment, and the angle is 70° to 120°; The weldment is slowly heated to 600-700 ° C (not more than 700 ° C).
2) Welding: use neutral flame or weak carbonized flame (do not let molten iron flow to one side during welding), after the base metal is melted, melt into the electrode metal; when it appears white bright spots in the molten pool, stop filling Into the electrode metal, add the appropriate amount of flux, remove the debris with the electrode and then continue welding; in order to obtain a flat weld, the weld after welding should be slightly higher than the surface of the cast iron, and will overflow the slag outside the weld Remelting, when the temperature is lowered to the semi-molten state, the upper portion of the casting is scraped with the wire.
3) Post-weld cooling: Generally, it should be slowly cooled to room temperature with the furnace (usually more than 48h). It can also be covered with asbestos cloth (plate) or charcoal to make the weld be uniform and prevent cracks.
2. Cold welding
This method does not preheat the workpiece before welding, or the preheating temperature does not exceed 300 °C. Commonly used electrode arc welding for cast iron cold welding. According to the requirements of cast iron workpieces, different cast iron welding rods can be used. For example, the Z100 welding rod is used for the non-machined surface of the gray cast iron parts. The Zll6 or Z117 welding rods are used for the repair of high-strength gray cast iron and ductile iron parts.
The welding equipment of the cold welding method is an ordinary arc welding equipment, and the welding process is as follows:
1) Preparation before welding: remove the oil stains and impurities on the surface of the soldering surface to expose the metal luster of the substrate: if there is a crack, drill a crack stop at each end of the crack to avoid crack propagation during welding; The port, its type and size are determined by the thickness of the weld and the process requirements. In the case of large castings, a certain number of screws can be screwed into the weld to strengthen the joint. The screw diameter is generally not more than 16mm (if the wall thickness is less than 15mm, the screw diameter should be less than or equal to 6mm), the number of screws can be calculated according to the cross-sectional area, that is, the total cross-sectional area of ​​the screw is not more than 25% of the crack cross-sectional area of ​​the casting, and these The 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 welding repairing part. Generally, a small-diameter welding rod should be used to reduce the heat of the input welding part: in the case of ensuring the welding of the welding rod metal and the base body, the welding current is also Should be used as small as possible to avoid excessive stress on the weldment to produce stress; the arc length is generally 0.5 to 1.1 times the diameter of the electrode to ensure stable combustion: if DC power is used, the weldment is generally selected as the negative electrode. The weldment is heated and the temperature is too high.
3) Operational process requirements: Generally, it should be followed by “inside and outside (first hole, rear body, and then upper plane), short section, intermittent, dispersive welding, multi-layer multi-channel (after the first layer is welded) Grinding some welding meat on the whole weld with a grinding wheel, checking that there is no porosity, crack and then welding the second layer; each layer is first welded from the sides of the groove, after welding the middle), small current, hammer welding The principle of sewing.
1 The whole weld is divided into several small sections, which cannot be continuously welded. The length of each section depends on the thickness of the weldment, generally 10~50mm: after each section is welded, it should be cooled to room temperature and then welded to the next section: each small weld Do not cross the wave to the sides of the groove, which will help the unwelded part to shrink freely and prevent the arc from staying on the sides of the groove for too long.
2 When the metal temperature after welding is about 800 °C, the weld should be hammered to make the surface pockmark, to loosen the welding stress, remove cracks and pores: when the temperature is lower than 300 °C, it can not be hammered again, so as to avoid cold brittleness. crack.
3 It is better to use a straight line and small circle method in the welding. The weld should be smoothly transitioned with the base metal to facilitate the weld stress direction.
3. Heating minus welding
This method does not heat the weldment beforehand, but heats the "heating reduction zone" of the weldment before and during the welding, so that it does not hinder the shrinkage of the weld, thereby reducing the internal stress and avoiding cracks. The heating reduction zone can be selected in one or more places. The selection principle is:
1) It should be the part that hinders the expansion of the weld. When the portion is heated and cooled, the weld has the possibility of obtaining free thermal expansion and cold shrinkage.
2) It should be a part with less contact and other strengths.
3) The deformation of itself should not have much influence on other parts, and it will not damage other parts due to its deformation.
Pay attention to the following principles when choosing a welding method:
1 Choose different welding methods for different machinability, color, strength, etc. The electrode arc welding hot welding method is most suitable for castings with high quality and good machinability. The electrode arc welding cold welding method is suitable for machined surfaces and large castings that are not convenient for preheating.
2 Select different welding methods for different weldment volume, shape, thickness and conditions of use. For small and medium-sized thin-walled parts (such as cylinders), gas welding, cold welding, and heat welding can be used. For larger parts, gas welding heat welding should be used.
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