Development of high performance large diameter forging steel ball

In order to solve the problems of low production efficiency and high production cost caused by the decline of ore grade, the advantages of large-scale ball mill are gradually highlighted and widely used. It is necessary to use the large-diameter ball in the large-scale ball mill, so it is important to prepare high-performance large-diameter forging steel ball.

In this paper, the cracking problem of 65mn forging steel balls produced in a factory is discussed. By means of metallographic observation, SEM micromorphology observation, EDS energy spectrum analysis, Rockwell hardness test, and impact toughness test, the causes of crack formation of the 65Mn forged steel ball was discussed and the optimized scheme was put forward In order to prepare 65Mn large diameter forging ball with good mechanical properties. In view of the thin hardened layer of 65Mn forging ball, a new type of forging ball, R2 forging ball, was studied. The cause of crack formation of the trial-produced forging ball with a small number of cracks was discussed. The quenching process of R2 forging ball was optimized by studying R2 austenite transformation temperature, the choice of quenching medium and the effect of quenching temperature on the structure and properties of the material To prepare high-performance large diameter R2 forging ball.

The results show that the failure mode of 65Mn forging ball is that the crack occurs about 15mm from the surface layer of 65Mn forging ball, and the crack extends along the circumferential direction The coarse widmanstatten structure is formed in the subsurface area of the forging ball, which reduces the plasticity and toughness of the forging ball and intensifies the crack propagation.

The optimized preparation process was as follows: high-quality raw bar (90mm)-cut short bar — heating to 1050 °C — forging — transferring (900-920 °c) — quenching at 800-820 °C, quenching solution concentration 2.5% — tempering (220 °c-4h). After optimization, there is no widmanstatten structure on the sub-surface of 65mn forging ball, the change trend of hardness is stable, the surface hardness is increased about 3HRC and the depth of hardened layer is increased about 2mm. A new type of forging ball material, R2 steel, and R2 steel, was developed. The whole 125 mm forging ball is quenched, the bulk hardness is 56 ~ 58HRC, the impact toughness is 17 ~ 20J / CM2. The failure mode of R2 steel forging ball is that the surface cracks appear and the crack propagation stops near the center of the ball. The main failure causes are martensite coarsening (Grade 7 martensite), reducing the strength of the material, improper quenching process, resulting in greater internal stress, exceeding the fracture strength of the material.

The quenching hardness is 62.9 HRC and the impact toughness is 26.3 J / CM2 after tempering at 220 °C. The optimized preparation process was as follows: original bar stock (90mm) — CUT STUB — heating to 1150 °C — Forging — Transfer Air Cooling — heating to 920 °C — Oil quenching — tempering (220 °c-4h). Compared with the small batch trial-produced forging ball, R2 forging ball has no obvious crack, good forging ball structure, martensite structure coarsening has been improved, impact toughness is 26.04 j / CM2, increase about 30 %, the whole hardness is about 59HRC Volume hardness of about 58.5 HRC.