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1 How to prevent cracks in high-manganese steel castings 1. Structural design of castings – Structural issues such as excessive differences in wall thickness, improper transitions between wall thicknesses, and too small radii at the corners of the castings can all lead to the formation of cracks. Therefore, casting design should be closely integrated with the casting process to minimize unreasonable casting designs. For example, the “+” shaped cross-section can be changed to a “T” shaped cross-section, etc. 2. Foundry process design (including various process factors and the gating system): Among the various factors in the foundry process, the most important one is the shrinkage allowance of the mold, followed by an unreasonable design of the sand box. For example, contraction hindered by box ribs can cause cracks; therefore, there must be a certain distance between the box ribs and the casting as well as the riser. Due to an improper design of the gating system, the multiple runners that feed material in separately often crack at the points where they connect to the main runner, as this hinders the contraction of the casting. It should be specifically noted that at the inlet of the gating system in the casting, the local temperature is high and solidification occurs there first; due to a lack of sufficient feeding, shrinkage stresses cause the casting to crack. Therefore, risers are generally provided at the gating system location for feeding purposes. 3. Placement of risers and chillers in manganese steel castings: The principle for placing risers in manganese steel castings is to avoid using conventional top risers, as cutting these risers with an acetylene flame can easily cause cracks. Therefore, it is best to use side risers and easily removable risers, which are generally removed by hammering. Installing risers in castings to compensate for hot spots helps prevent shrinkage cavities and porosity in the castings, and it is an effective measure to avoid internal cracks. However, the presence of risers also creates contact with hot spots, so other processing measures need to be properly coordinated with them. By using the chill properly, it is possible to prevent both internal cracks and external cracks. Cold iron can regulate the solidification rate of different parts of the casting, allowing the location of defects in the casting to be shifted; when used in conjunction with risers, it can expand the range of filling provided by those risers. However, if the chill iron is used improperly, for example by using one that is bent or deformed, cracks can occur due to uneven solidification rates of the casting within an inappropriate range of chill iron length. Large gaps between cold irons can also cause cracks; high-manganese steel castings are particularly sensitive to this, so special attention should be paid to it during process design. 4. Chemical composition and melting process: In high-manganese steel, carbon and phosphorus have the greatest impact on crack formation. The higher the carbon content, the more prone the casting is to cracking. Attention should also be paid to the impact of reduction refining of molten steel on cracks in high-manganese steel castings. During the smelting of high-manganese steel, it is necessary to strictly control the sum of FeO+MnO in the slag to be no more than 1.2%, because as the sum of FeO+MnO in the slag increases, the level of FeO+MnO in the molten steel also rises; upon solidification, these substances precipitate at the grain boundaries, making the steel brittle. Controlling the casting temperature and the temperature upon opening the mold are also effective measures to prevent cracks in high-manganese steel castings. As the casting temperature increases, the shrinkage stress of the casting rises; more importantly, the grain size becomes larger and columnar grains become more pronounced, **weakening the strength of the steel. Furthermore, high-manganese steel castings should not be removed from the mold while still red-hot and exposed to air for rapid cooling; instead, they should cool slowly within the mold. For complex castings, it is advisable to remove them from the mold only when the temperature has dropped to around 200°C. 5. Heat treatment process: Whether the temperature difference between the furnace and the casting during furnace loading is appropriate is an important factor in the formation of cracks. After the castings are placed in the furnace, they need to be allowed to reach an even temperature over 1–1.5 hours before the temperature is increased, so as to allow the castings to warm up slowly. The rate of temperature increase during the low-temperature stage (below 650°C) is crucial in determining whether cracks will form. Generally, the heating rate for more complex castings should not exceed 50°C/h; otherwise, the castings are prone to cracking. 2 How is the heat treatment of high-manganese steel carried out? 1. Purpose of the solution treatment for wear-resistant high-manganese steel castings: To eliminate the carbides present within the grains and at the grain boundaries in the as-cast structure, thereby obtaining a single-phase austenitic structure; this improves the strength and toughness of high-manganese steel and expands its range of applications. To eliminate the carbides in its as-cast structure, the steel must be heated to above 1040°C and held at that temperature for an appropriate period of time, allowing the carbides to fully dissolve in the single-phase austenite; thereafter, rapid cooling is carried out to obtain an austenite solid solution structure. This solution heat treatment is also known as water toughening. The as-cast microstructure of water-toughened wear-resistant manganese steel contains a large amount of precipitated carbides, resulting in low toughness and susceptibility to fracture during use. 1 Temperature for water tempering: The temperature for water tempering depends on the composition of the manganese steel; it is usually between 1050 and 1100°C. For manganese steels with a high carbon content or high alloy content, the upper limit of this temperature range should be used, as in the case of ZGMn13 steel and GX120Mn17 steel. However, an excessively high water tempering temperature can cause severe decarburization of the casting surface and accelerate the growth of the grains in high-manganese steel, thereby affecting its performance. 2 Heating rate: Manganese steel has poorer thermal conductivity than ordinary carbon steel; therefore, manganese steel castings experience high stresses during heating and are prone to cracking, so the heating rate should be determined based on the thickness and shape of the casting. Generally, thin-walled, simple castings can be heated at a faster rate, while thick-walled castings should be heated slowly. To reduce deformation or cracking of castings during heating, a manufacturing process is commonly used in which the parts are first held at around 650°C to minimize the temperature difference between the inner and outer parts of thick-walled castings and to ensure uniform temperature within the furnace; after that, the temperature is raised rapidly to the water-toughening temperature. 3 Holding time: The holding time depends mainly on the wall thickness of the casting, in order to ensure that the carbides in the as-cast structure are completely dissolved and the austenite is homogenized. Generally, the holding time can be calculated as 1 hour for every 25 mm of casting wall thickness. 4 Cooling: The cooling process has a significant impact on the performance characteristics and microstructural state of castings. During water tempering, the temperature of the casting before it is placed in water must be above 950°C to prevent the re-precipitation of carbides. Car-type heat treatment furnaces are commonly used for the thermomechanical treatment of high-manganese steel. The quenching of castings when placed in water is commonly done by automatic tipping or with a crane basket. The former tends to cause deformation in large-sized and complexly shaped thin-walled parts, and it is also difficult to remove the castings from the water tank after quenching; the latter makes it easy to remove the castings after quenching, but it results in high consumption of lifting baskets. 2. As-cast residual heat treatment of wear-resistant manganese steel castings: To shorten the heat treatment cycle, the as-cast residual heat can be utilized for the water-toughening treatment of manganese steel. The process is as follows: The casting is removed from the mold at 1100–1180°C; after core removal and sand cleaning, the temperature of the casting is allowed to drop to 900–1000°C. It is then placed in a furnace heated to 1050–1080°C, where it remains for 3–5 hours before being cooled by water. This processing method simplifies the heat treatment process, but it presents certain difficulties in terms of production operations. 3. Precipitation strengthening heat treatment of wear-resistant manganese steel castings: The purpose of precipitation strengthening heat treatment for wear-resistant manganese steel is to, based on the addition of appropriate amounts of carbide-forming elements (such as molybdenum, tungsten, vanadium, titanium, niobium, and chromium), use heat treatment to produce a certain quantity and size of dispersed carbide second-phase particles within the manganese steel, thereby strengthening the austenite matrix and improving the wear resistance of the material. This heat treatment method is costly and the process is relatively complex.