A new type of welding wire for solving the problem of fusion welding of white cast iron

1.1 Basic Situation Casting is a fundamental process in mechanical manufacturing, yet it often results in various defects due to multiple factors. Especially in the production of cylinder parts for diesel and gasoline engines in the automotive industry, which are complex and thin-walled components, the scrap rate tends to be relatively high. Moreover, cast iron parts used in various mechanical equipment are highly prone to damage during operation. To date, no theory or method has been developed globally to completely address these issues. 1.2 International Developments Internationally, cold welding of cast iron—defined as preheating before welding—has been practiced since the 1940s with the use of pure nickel cast iron electrodes developed in the United States. In some cases, nickel-based powder spray welding is also applied. The 308 wire from the U.S., similar to the present invention, is commonly used. However, compared to the current invention, the fusion zone hardness (HB) is limited to ≤ 300, whereas the present invention achieves ≤ 240. Additionally, continuous welding at the time of use was not possible due to issues like slag inclusion and cracking. 1.3 Domestic Progress In the 1980s, the State Planning Commission promoted the "low carbon steel electrode" developed by Professor Chen Zhongsheng from Beijing 211 Factory during the "Seven-Five" and "Seven-Eight" periods. However, this technology proved ineffective in fundamentally solving the welding challenges of cast iron parts. In the 1990s, Chongqing University researched spray welding, which was recommended by the Sichuan Provincial Planning Commission. Although it found some applications in diesel engine manufacturers, several limitations were identified during its implementation: 1. Temperature control during welding is difficult. Before welding, the workpiece is heated using gas welding flames, and the temperature is determined manually through visual inspection. This often leads to low temperatures, resulting in weak welds, or excessive heat, causing the weld joint to become too hard. 2. Pre-weld preparation is complicated, requiring thorough cleaning of the area to be welded. 3. The welding process demands precise operational skills; improper handling may lead to loss of the welding material. 4. There are strict requirements for the size and thickness of the welding surface, making it suitable only for large and shallow areas. 5. The working environment for welders is poor, with high temperatures in summer and long preheating times in winter. In summary, although spray welding can achieve machining and reduce thermal cracking under certain conditions, it still suffers from numerous shortcomings. In actual engineering practice, hot cracks still occur in cast iron welded parts, with a probability of over 80%, limiting its widespread application. 1.4 Theoretical and Process Innovations in the Invention (1) Solving the White Cast Iron Problem Without Post-Weld Heat Treatment Currently, both international and domestic methods for repairing cast iron parts have not effectively addressed the issue of white cast iron. Even the most advanced 308 welding wire from the U.S. can only reach a thickness of 0.1 mm, with a noticeable tendency toward white cast iron. The invention uses a special welding wire combined with CO₂ shielded semi-automatic cold welding technology. This approach allows for a high current density and rapid cooling after welding, resulting in an intermittent metallographic white layer that is generally less than 0.07 mm. This has minimal impact on cutting tools before machining, eliminating the need for heat treatment. (2) Eliminating Pre-Weld Heating and Preventing Cracking Welding repair involves localized rapid heating of cast iron parts, making temperature unevenness inevitable. When thermal stress exceeds the tensile strength of the cast iron, thermal cracks will occur. The invention addresses this by designing the welding wire's chemical composition with a minimum yield strength of 200 MPa, capable of reaching up to 400–500 MPa. During the welding process, stress is generated, and once the temperature exceeds 200 MPa, the weld seam deforms, reducing stress and effectively preventing hot cracks.

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