Studies On The Hardening-Toughening Heat Treatment Process Of Wear-Resistant Cast Iron Grinding Balls

High chromium cast iron grinding balls have a large number of applications in metallurgical mining, building materials, cement and power industries because of their excellent wear resistance performance. After quenching and tempering heat treatment, the low toughness high chromium cast iron grinding balls appeared plenty of spalling and broken failure problems when used in the large ball mill. Thus, to improve the impact toughness and impact fatigue resistance of grinding balls, the influence of austempering heat treatment process and multi-stage austenitizing austempering heat treatment process on the matrix microstructures and carbides of the grinding balls were experimentally investigated. In this paper, the main results are as follows:1. The matrix microstructure of high chromium cast iron grinding balls is fine acicular martensite and with a lot of dispersively distributed block and rod secondary carbides after quenching-tempering process. The surface hardness of the grinding ball is about 60 HRC, the core hardness is about 56 HRC, and the impact toughness is about 4.1 J/cm2.Hardness is high but impact toughness is low.2. The high chromium cast iron grinding balls were austempered at 230℃、260℃ and 290℃ for 15 min to 50min. The experimental results show that with the increase of austempering temperature, the hardness and impact toughness of grinding balls all raised first and lowered later; with the extension of austempering time, the hardness of the grinding balls reduced and the impact toughness raised first and lowered later. The matrix of grinding balls is austenite-bainite microstructure with main fine acicular bainite when austempered at 260℃ for 40min, in addition, a certain number of round granular secondary carbides dispersively distributed in the matrix. The hardness and impact toughness of the grinding balls cooperated better, the surface hardness is about 58 HRC, the core hardness is about 55 HRC, and the impact toughness can be up to 7.2 J/cm2 that increased about 75% relative to the quenching-tempering process.3. With the reduce of secondary austenitizing temperature, the hardness of the high chromium cast iron grinding balls was on the decline when the second austenitizing temperature was between℃ 950 and 830℃. By multi-stage austenitizing of 980℃× 150min+920℃×30min, then austempered at 260℃ for 30min to 50min, the shape of angled eutectic carbides were improved, with a large number of round granular secondary carbides dispersively distributed in the matrix. The hardness and impact toughness have a certain extent increase, the surface hardness keep around 59 HRC, the core hardness remain at around 54.5 HRC, and the impact toughness in the range of 7.1 J/cm2 to 7.4 J/cm2.4. The falling ball fatigue test results show that high chromium cast iron grinding balls appeared a large number of spalling and even broken failure problems after quenching-tempering process, but spalling rarely appeared after austempering process or multi-stage austenitizing austempering process. Thus, the impact fatigue wear resistance is greatly superior to the grinding balls after quenching-tempering process. Accordingly, austempering heat treatment process is better for high chromium cast iron grinding balls when served in the large ball mill.5. The numerical simulation and experimental analysis results show that the boundary between grinding ball and flow gate was the last solidification area, the surface layer microstructure of this area is coarse, brittle and stress concentrated, results in the brittle phase cracked easily and formed crack under impact, eventually led to spalling failure occur in this region firstly.