Analysis on Mechanism and Control Measures of Cracks in High Manganese Steel Castings

DataTime:[2022-9-15]

1. Influence of chemical composition and tissue

At the beginning of the invention, the chemical composition of high manganese steel was C: 1.0% - 1.4%, Mn: 10% - 14%. In domestic and foreign experiments and researches, Cr, Ni, V and other alloy components were added in an attempt to improve the yield strength and hardness, while some added Ti, Al and others to promote grain refinement. This is required for our track shoes, front and rear guide wheels and other castings. However, no matter how the alloy composition changes, it must be controlled within a certain range to play a good role. These effects are mainly aimed at improving the properties of high manganese steel, and are not obvious for crack control. The ideal effect can be obtained by studying the influence of each element of standard composition high manganese steel on crack. Carbon and manganese are the main components of high manganese steel. Normally, the structure of high manganese steel is single phase austenite. When the carbon content is low, martensite and 8 phases (dense hexagonal) are easily formed. When the carbon content is high. Carbide precipitation can not be avoided under the condition of water toughening treatment, which will also reduce the properties of steel. Higher carbon content can increase the yield point, but reduce the elongation. The steel with low carbon content and high manganese content has high toughness, and the carbon content of high manganese steel should be controlled at the middle and lower limits as far as possible under the premise of ensuring the performance. Manganese content shall be controlled at the upper middle limit. It is better to keep it in a narrow range to prevent carbide precipitation and crack caused by reduction of elongation and toughness. The silicon content is generally below 1.0%, which has no significant effect on the mechanical properties of manganese steel. When the silicon content exceeds 1.0% and gradually increases, the strength and elongation will decline sharply. Silicon content in production is relatively low. The sulfur content in manganese steel is low, because most of the sulfur and manganese sulfide are easily combined to form manganese sulfide into the slag. A small amount of sulfide remains in the steel as non-metallic inclusion, which has little influence on the properties of the steel and can be ignored. When the phosphorus content is above 0.06%, the extensibility decreases sharply, which is easy to cause hot cracks in castings. Phosphorus content shall be controlled during production. Chemical composition and its content are closely related to mechanical properties. It is sometimes difficult to manufacture within the range of components that meet the performance requirements, so its balance should be considered during production. This will avoid cracks due to differences in composition.

2. Casting process

Casting technology is the key to ensure the quality of high manganese steel castings. The process of this steel is mainly designed to avoid crack formation. The main factors affecting the crack are: casting structure, process parameters, gating and riser settings, core structure, pouring temperature, etc. The structural form of high manganese steel castings shall minimize the thickness difference of connecting walls as much as possible under the premise of ensuring use. Avoid cracks caused by shrinkage stress due to large temperature difference. Although the wall thickness difference of some parts is reasonable, the design thickness value is too large, carbide precipitates more when the cooling speed is slow, and cracks will also appear due to the influence of shrinkage stress. When the design structure cannot be changed, the process planning should consider adding some elements that can refine the grain during molten steel smelting to increase the flexibility of the core. Other structural problems shall be determined as the case may be, such as increasing rounded corners, adding transition walls, lacing, etc. The shrinkage of high manganese steel is higher than that of plain carbon steel and alloy steel. The cast blanks are in a brittle state before the water toughening treatment. Any factor that affects the shrinkage will cause cracks in the shrinkage stage of the castings after pouring. The more obvious is the box casting, which forms an inner cavity from the sand core. The sand core has good flexibility, and cracks are very easy to occur in the high stress area. The solution is to add sawdust, foam, etc. into the core to ensure that the sand core has a proper yield. The box belt, riser and flash burr all have conditions to prevent shrinkage. It is also an effective way to solve the yielding problem by properly increasing the sand intake, loosening the layer and reducing the core gap. The position and size of the gating and riser also have an impact on the generation of cracks. The setting position shall not affect the shrinkage of the casting. The gating system shall be distributed evenly as far as possible. The steel water flow shall be stable to reduce the temperature difference of various parts of the casting. The riser shall be of appropriate size. The thermal conductivity of high manganese steel is very poor. If the size of the riser is too large, the excessive temperature difference between the riser and the casting in the late cooling period of the casting will inevitably produce high stress, which will often lead to cracks in the connection between the riser and the casting. During process design, the riser modulus shall be controlled within 1.2-1.3 times of the casting modulus. The liquidus temperature of high manganese steel with standard composition is 1400 ℃, the solidus temperature is 1350 ℃, the solidification temperature range is relatively narrow, and the reaction to pouring temperature is very sensitive. If the pouring temperature is too high, cracks will easily occur, and if the temperature is low, cold shuts and insufficient pouring will easily occur. The ideal pouring temperature is 1450-1470 ℃. The lower limit shall be taken for thick wall parts, and the upper limit shall be taken for thin wall parts. During pouring, the pouring shall be completed at one time without stopping the flow.

3. Heat treatment

High manganese steel can obtain its properties only under proper composition and correct heat treatment conditions. If it is not heat treated or the heat treatment conditions are incorrect, the steel will become brittle or even crack. The normal heat treatment specification is solution and high temperature water toughening treatment to obtain a single austenite. Production practice has proved that the solution temperature should be within the range of 1050-1090 ℃. It is generally believed that the thermal conductivity of high manganese steel is very poor, and the expansion rate is very low. During solution treatment, slow heating should be used, and the temperature should be kept at 600-700 ℃ to prevent cracking. The research results show that the retained austenite will strongly precipitate carbide at 500-700 ℃, which will reduce the plasticity of the steel, increase the internal stress caused by the specific volume difference between the new and old structures, and thus increase the cracking tendency. Therefore, the casting should pass this temperature range as soon as possible during heating.

4. Clean cutting

One of the characteristics of high manganese steel is its poor thermal conductivity, short heat preservation time after casting and fast cooling speed, which make the casting stress increase and easy to crack. To prevent cracks, cool the sand mold slowly as much as possible to extend the insulation time, and the box beating temperature should be controlled below 200 ℃. The practice proves that there is no crack when the box is opened at this temperature. During the cutting of high manganese steel casting riser, flash and burr, due to the poor thermal conductivity of the material, a large temperature difference is generated in the heat affected zone of the cutting part, and a high stress state is formed. Slight neglect of the notch will cause cracks. In order to control the temperature difference in the heat affected zone within the minimum range, the pouring, riser and burr cutting must be submerged in the pool to keep the water temperature below 40 ℃, which is not easy to cause cracks.

5. Repair welding

Repair welding is an effective method to eliminate the defects of casting products, and the possibility of cracks appearing in the repair welding stage is also great. In order to ensure the product quality, the following problems must be paid attention to:

① Preheating is not required before repair welding. Reheating at a lower temperature can cause carbide to precipitate along the grain boundary and crystal surface, promoting the re emergence of embrittlement and prone to cracks.
② It is impossible to obtain satisfactory results by welding repair on manganese steel without water toughening heat treatment.
③ Flame cutting or carbon arc air gouging shall not be used to remove defects, and air shovel and grinding wheel are preferred.
④ The composition of repair welding electrode shall be similar to that of base metal, and stainless steel electrode can also be used. No matter which electrode is used, the carbon content shall be low, and the carbide precipitation can be reduced during repair welding.
⑤ The parts to be repaired shall be smooth and clean without oil and rust, and the proper groove shall be made according to the requirements of welding repair process.
⑥ Repair welding shall be carried out in strict accordance with the process.
⑦ It is very important to avoid the thermal stress that may cause the risk of cracks during repair welding. As manganese steel has a high expansion coefficient and a low thermal conductivity, it will generate stress at the repair weld when cooling. Therefore, hammer weld the repair metal when cooling, so that the compressive stress and the tensile stress generated during cooling are offset.
⑧ Keep the parent metal heated to the minimum during repair welding, so as to minimize the adverse effects of reheating.

All the above methods have been adopted in production, and the crack defects in the weld repair area can be avoided by proper operation.

6. Conclusion

There are many causes of cracks in high manganese steel castings. Based on years of research and practice, the causes and preventive measures of cracks in high manganese steel castings are found out, which effectively guide production and control the cracks within the minimum range.

Source: China High Manganese Steel Castings Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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