Study on the heat treatment process of high chrome cast iron (2)
Although high chromium cast iron has good hardenability, high chromium cast iron contains a large amount of chromium, making its thermal conductivity worse. When the cooling rate is high, the steep thermal gradient is easily produced in the casting. Then at different parts of the casting, the phase transition occurs at different times. Also, the material has a large expansion and contraction coefficient, which can easily lead to a high-stress zone in the casting. This will lead to casting deformation and even fracture. Besides, when the cooling rate is high, the AUSTENITE will be fully transformed into martensite. When the cooling rate is high, high chromium cast iron’s impact toughness worsens after quenching, and the hardness increases. So it’s very important to do the heat treatment.
Therefore, in the actual production, the general use of cooling strength is weak air cooling to avoid casting in order to prevent serious consequences.
Even if the high chromium cast iron is cooled in the air, there is still large internal stress, so tempering should be carried out as soon as possible. Martensite high chromium cast iron after quenching is usually tempered at 200 ~ 260 °C. Tempering in this temperature range can improve the shearing property of high chromium cast iron and improve the casting’s reliability under impact load. The results in Ref. 2 show that high chromium cast iron’s hardness after tempering in the temperature range of 200 °c-450 °C does not change much and still keeps high hardness. When the tempering temperature exceeds 550 °c, the hardness decreases sharply.
The more carbides, the higher the material’s hardness, and the main factor affecting the number of carbides is the carbon content. With the increase of carbon content, the carbide becomes more, the hardness increases, but the Brittleness increases. Under the condition of high-stress Abrasive wear, in order to obtain good impact toughness, the amount of carbide is generally controlled to be less than 30%.
The relationship between the size and distribution of carbides and their toughness and wear resistance is not well established. Reference 3 when comparing the effect of permanent mold casting with sand mold casting on the wear resistance of high chromium cast iron, it is found that the product of carbide in permanent mold casting is larger than that in sand mold due to the small size of carbide relative to sand mold, as the source and propagation path of cracks become more and more, it shows lower impact toughness and poor anti-abrasive wear performance. However, E43 reduced carbides by modification and granulated the carbides, but obtained a good combination of toughness and wear resistance.
The multi-element alloying can improve the structure, hardness, and toughness of high chromium cast iron. The alloy elements commonly used in high chromium cast iron are carbon, chromium, silicon, manganese, molybdenum, titanium, nitrogen, and so on
This is mainly because the Matrix of high chromium cast iron is embedded with carbide with high hardness, which determines that high chromium cast iron has excellent wear resistance under low-stress conditions. However, as-cast high chromium cast iron may not be able to obtain the required structure. To give full play to its wear resistance, it is often treated as martensite Matrix, which makes the Matrix itself more wear-resistant and can better support carbides; once the chemical composition and casting process are determined, the size distribution of the carbide is determined, and heat treatment becomes the decisive factor for the wear resistance and toughness of the material. The quenching temperature is 910 °C, 930 °C, 950°C, 970°C, 1000°C, 1050 °C, the quenching method is water quenching and air quenching.
The heat treatment and healing process of high chromium cast iron is actually a dynamic process of secondary carbide precipitation and dissolution, so the selection of quenching temperature and holding time is very important. At a certain temperature, the higher the quenching temperature is, the higher the hardenability is. With the increase of CR content in the alloy, the temperature range of secondary carbide precipitation moves towards high temperature, so the suitable quenching temperature will also change with CR content. The thicker the wall, the higher the quenching temperature should be. As the austenitizing of high chromium cast iron is accomplished by the diffusion of carbon and alloy elements, the diffusion rate determines the phase transformation time and the holding time. Insulation time can be generally based on wall thickness; choose for 2 ~ 4H, thick-walled parts and longer. After quenching, high chromium cast iron’s impact toughness becomes worse, and the hardness increases obviously. Therefore, in the actual production, the air cooling with weak cooling intensity is generally used to avoid cracks in the casting to prevent serious consequences.