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Casting is the basic process technology of mechanical manufacturing. Due to various reasons, there will be various defects in the production process, especially the cylinders of diesel engines and gasoline engines in the automobile industry. Such complex thin-walled parts have higher scrap rate. In addition, the cast iron parts in various mechanical equipment are extremely vulnerable during use. So far, there is no theory or method to completely solve the above problems at home and abroad.
1.2 International News
Internationally, as the cold welding of cast iron (referring to the preheating before welding), the pure nickel cast iron electrode invented in the United States in the 1940s is still used. Under certain conditions, spray welding of nickel-based powder can be used. 308 wire produced in the United States (similar to the present invention). Compared with the present invention, the fusion zone HB ≤ 300 (≤ 240 in the present invention), and continuous welding cannot be performed at the time of use (because slag inclusion and cracking are liable to occur).
1.3 Domestic News
In the 1980s, the State Planning Commission promoted the "low carbon steel electrode" researched by Chen Zhongsheng (professor-level senior engineer) of Beijing 211 Factory during the period of "Seven. Five" and "Seven. Eight". It proved that the technology could not be effective. To solve the fundamental problem of welding of cast iron parts; in the 1990s, Chongqing University studied spray welding, which was recommended by the Sichuan Provincial Planning Commission, and has certain applications in diesel engine manufacturers, but the following problems exist in the application process:
1 welding temperature is not easy to control. Before the welding of the technology, the gas to be welded is heated by the gas welding flame. The heating temperature is determined by manual visual inspection. The temperature is low and the welding is not strong. If the temperature is too high, the welding joint will be too hard;
2 Preparation work before welding is more cumbersome, technically required to clean the place to be welded before welding;
3 There are special requirements for operation during the welding repair process, and improper operation may cause the loss of the welding fluid;
4 The requirements for the area and thickness of the welding plane are high, and it is only suitable for the working surface with large and shallow welding area;
5 The welding staff has a bad working environment, high temperature during summer operation, and long preheating time for workpiece operation in winter.
In summary, although spray welding can achieve machining and thermal cracking under certain external conditions, there are many shortcomings and drawbacks in this technology, and there are still hot cracks in the cast iron welded parts in actual operation engineering. (The probability of hot cracking is determined to be 80% or more), which limits it in large-scale application.
1.4 Several aspects of the invention patents solved in theory and process:
(1) White mouth problem, no heat treatment is required after welding.
At present, the international and domestic welding repair of cast iron parts has not solved the problem of white mouth. The thickness of the 308 welding wire (currently the most advanced) invented in the United States in the most ideal case can reach 0.1MM, and the white mouth tends to be obvious. The special welding wire and CO2 protection semi-automatic cold welding technology adopted by the invention. The current density is large, the cooling rate after welding is large, and the hardening layer of the base metal and the welding zone of the welding wire is intermittent from the metallographic white layer. Generally, it is <0.07MM, which has little effect on the cutting tool before cutting. Heat treatment is required.
(2) Solving the problem of no heat before welding and no cracking.
Welding repair is a local rapid heating of the cast iron parts, so temperature unevenness is inevitable. When the thermal stress is greater than the tensile strength of the welded iron, the thermal crack will inevitably occur. The invention uses the lowest yield limit "200MPA when designing the chemical composition of the welding wire, and can be as high as 400-500MPA, so that the stress is generated during the welding repair process, and the weld seam is deformed and the stress is lowered when the temperature exceeds 200MPA, thereby being most effective. Prevent the occurrence of hot cracks.
A new type of welding wire for solving the problem of fusion welding of white cast iron
1.1 Basic situation