Discussion on the Mechanism of Cracks in High Manganese Steel Castings

The causes of cracks in high manganese steel castings in actual production and the influence of various factors in the solidification process of castings are analyzed. Preventive measures are proposed from structural design, chemical composition control, casting process and heat treatment process.

High manganese steel accounts for a certain proportion of wear-resistant materials. However, in actual production, waste products are often caused by crack defects, which brings great economic losses to manufacturers. Therefore, for high manganese steel castings subjected to high impact load, it is an urgent problem for high manganese steel manufacturers to analyze the causes of cracks and take preventive measures.

Causes of cracks

The cracks in high manganese steel castings are mainly hot cracks. Because the free linear shrinkage value (2.4% – 3.0%) of high manganese steel during casting is much larger than that of carbon steel, this is the main factor causing hot cracking of high manganese steel. Secondly, the thermal conductivity of high manganese steel is low, which is 1/4-1/6 of that of carbon steel. Therefore, the temperature difference between various parts of the casting during heating and cooling is large, causing considerable thermal stress, which is another important reason for hot cracking. If the accumulation of shrinkage stress is considered again, the damage to castings will be even greater.

Influence of various factors on crack formation during casting solidification

During the crystallization of high manganese steel castings, coarse grains and columnar crystals are easy to form. Brittle carbides on the as cast grain boundary make the castings brittle and reduce the strength of high manganese steel castings, which is the internal condition for the formation of cracks in castings. When the solidified shell is formed, the first crack occurs in the stress concentration of the hot spot, and the crack expands inwards with the thickening of the solidified shell. Sometimes the crack is filled with liquid steel. If it cannot be filled with liquid steel, it will form a crack after solidification, or it may crack for the second time after being filled with liquid steel. The cracks usually occur at the uneven wall thickness of the casting, the transition fillet connecting the two walls and the local slow solidification. Anatomical tests on castings have proved that internal cracks originate from shrinkage cavity and porosity inside castings, so internal cracks are always accompanied by shrinkage cavity and porosity. At the periphery of shrinkage cavity and porosity, the solidification is mostly dendrite, and the opening created by interdendritic solidification cannot be filled with liquid metal to form cracks. The production practice also proves that the micro cracks on the surface of the castings after machining are related to the surface defects of the castings. Under the action of cutting force, microcracks occur around the blowholes and slag holes on the casting surface due to stress concentration. Some internal cracks are formed due to improper process and operation during casting sand falling, which cause them to expand to the surface.

Factors causing cracks during casting use

The practice has proved that the cracks of high manganese steel castings in use are mainly caused by the existence of casting defects inside the castings. Under the impact load, cracks will sprout around the shrinkage cavity and porosity, and then develop into external cracks after fatigue growth. On the other hand, carbides are precipitated on the grain boundary due to improper temperature and water entry time during casting heat treatment. Or the phosphorus content in the steel is high, and the phosphorus eutectic on the grain boundary weakens the strength and toughness of the grain boundary. On the one hand, these places can initiate cracks under high impact load, and at the same time, the fatigue growth of cracks will be accelerated here.

Preventive measures for casting cracks

In view of the main links in the production of high manganese steel castings, measures should be taken from the following aspects to prevent cracks.

Structural design of castings

The structural problems such as too large wall thickness difference, improper wall thickness transition and too small fillet transition of castings are easy to produce cracks. Therefore, the casting design should be closely combined with the casting process to avoid unreasonable casting design. For example, you can change the “+” section to a “T” section.

Casting process design (including various process factors and gating system design)

The most important factor in the casting process is the flexibility of the mold and sand mold, followed by the unreasonable design of the sand box. For example, the box reinforcement can prevent shrinkage and produce cracks. Therefore, the box reinforcement must be a certain distance from the casting and riser.
Due to improper design of the gating system, multiple ingates introduced dispersedly often crack at the joint with the ingate due to hindering the shrinkage of the casting. It should be specially pointed out that at the entrance of the ingate of the casting, the local temperature is high and the final solidification occurs. Because there is not enough feeding, the shrinkage stress causes the casting to crack, so generally a riser feeding should be set at the ingate.

Setting of riser and chill of high manganese steel castings

The riser of high manganese steel castings shall be set on the principle of not using ordinary top riser, because cracks are easily caused when the riser is cut with acetylene flame. Therefore, it is better to use side risers and easy cutting risers, which are usually knocked off with a hammer. The riser is set for the casting to feed the hot spot, so that the casting will not produce shrinkage cavity and porosity, which is an effective measure to prevent internal cracks. However, the riser is set to produce contact hot spot, and other process measures should be properly coordinated with it. If cold iron is used reasonably, internal crack can be prevented and external crack will not occur.
The cold iron can adjust the solidification speed of each part of the casting, which can make the defects of the casting move. The feeding range of the riser can be expanded by matching with the riser, but the cold iron is not used properly. For example, when the cold iron with bending deformation is used, cracks are often caused due to the uneven solidification speed of the casting within the inappropriate length range of the cold iron. Large interval between cold irons can also cause cracks. High manganese steel castings are very sensitive to this, so special attention should be paid to process design.

Chemical composition and smelting process

In high manganese steel, carbon and phosphorus have the greatest influence on crack generation. The higher the carbon content is, the easier the casting is to crack.
The influence of reduction refining of molten steel on the crack of high manganese steel castings should also be paid attention to. In the smelting process of high manganese steel, the sum of FeO+MnO in the slag should be strictly controlled to be no more than 1.2%, because with the increase of the sum of FeO+MnO in the slag, the FeO+MnO in the molten steel must also increase, and the precipitation on the grain boundary after solidification will make the steel brittle.
Controlling pouring temperature and opening temperature is also an effective measure to prevent cracks in high manganese steel castings. With the increase of pouring temperature, the shrinkage stress of the casting increases, and more importantly, the coarse grains and serious columnar grains greatly weaken the strength of the steel. In addition, the high manganese steel castings shall not be boxed when they are hot and exposed to the air for sudden cooling, but shall be slowly cooled in the mold. For complex castings, the temperature shall be reduced to about 200 ℃ before boxed.

Heat treatment process

Whether the temperature difference between the furnace temperature and the casting is appropriate during charging is an important factor for crack generation. After the casting is put into the furnace, it shall be heated up after 1h-1.5h to make the casting slowly warm up. The heating rate at the low temperature stage (below 650 ℃) is the key to crack. Generally, the heating rate of more complex castings shall not exceed 50 ℃/h, otherwise the castings are easy to crack.

Conclusion

In conclusion, when producing high manganese steel castings, as long as the casting shape optimization design is fully considered and the correct and appropriate casting process is strictly implemented, the cracking can be controlled, and high-quality high manganese steel castings can also be produced.
Author: Xue Fulian