1) The casting composition is uneven and segregation exists.
2) The high temperature holding time is too long, the grain grows, and the mechanical properties decrease.
3) The heat transfer of cast iron is slow. When the external temperature is quickly cooled to the ms point, the internal temperature is still high when the transformation of martensite begins. When the external transformation of martensite is completed, the internal transformation begins, and the external endurance is internal. The tensile stress caused by the expansion of the tissue transformation causes cracking when the tensile stress exceeds the tensile strength of the material.
1. Improvement of heating specifications
In order to minimize the temperature difference between the inside and outside of the casting, the alloying elements and carbon can be fully dissolved and diffused, the holding time of the casting at high temperature is reduced, and the oxidative decarburization degree and grain growth tendency of the casting are controlled to a minimum, and 200 ° C can be adopted. 400 ° C, 800 ° C section preheating heating method, according to the weight of the parts and the amount of furnace, at 980 ° C for 45 minutes, and then out of the furnace.
2. Improvement of cooling specifications
Supercooled austenite has different stability at different temperatures. In the most unstable region of austenite at 650 ° C ~ 400 ° C, it should be cooled at a critical cooling rate greater than quenching to prevent the transformation of austenite in the pearlite type, and the cooling rate in the interval above 650 ° C and below 400 ° C should be Slowly, reduce thermal stress and tissue stress due to temperature difference and martensite transformation to reduce deformation and prevent cracking. Therefore, after the parts are discharged at 980 ° C, they are pre-cooled to about 700 ° C in air, and then oil is introduced. In the oil, the oil is cooled to 400 ° C, and air-cooled to about 150 ° C and then tempered immediately. The tempering temperature was 320 ° C and the temperature was kept for 2 hours.https://www.wuxiorient.com/