Development Of High Performance And Large Diameter Forging Grinding Ball

Large ball mill has been used widely to solve the problem of low productivity of mineral resources and high production cost which caused by decline of mineral quality. The large ball mill needs to match with large diameter of grinding ball. Therefore, it is great significance to prepare high-performance large diameter of grinding ball. In this thesis, cracking problem in the process of preparation of 65 Mn forging ball was investigated by optical microscopy, SEM, EDS, Rockwell hardness and impact toughness. The cause of crack in the 65 Mn forging ball was discussed and the prioritization scheme was put forward to improve its mechanical properties. A new type of forging ball — R2 forging ball was studied, because of thinner hardening layer of 65 Mn forging ball. Small quantities were prepared to probe the cause of crack. Austenitic transformation temperature, quenching medium, quenching temperature on the material organization and performance aspects of R2 forging ball has been studied to optimizing the heat treatment quenching process.The results showed that the failure mode of 65 Mn forging ball was the crack which is 15 mm from forging ball surface and extend in the circumferential direction. Holes and inclusions were the main reason gave rise to crack which is convenient to crack germination; ductility and toughness reduced due to rough widmanstatten structure was appeared in subsurface region, its bad for crack. Internal stress produced when quenching greater than fracture toughness. Optimization of preparation process: high-quality raw material(?90mm) — cutting — heating to 1050?— forging — transferring(900- 920?) — quenching(800- 820?), quenching liquid concentration 2.5% — tempering(220?- 4h).After the process optimization, there was no widmanstatten structure exist in subsurface region, hardness changed steady, surface hardness was enhanced about 3 HRC, and the depth of hardening layer increased 2 mm approximately.To exploit a new ball forging material — R2 steel, small quantities manufacture R2 forging ball, hardness was less than 4 HRC between maximum and minimum, ?125mm was through-hardening, volume hardness ranging from 56 to 58 HRC, impact toughness was from 17 to 20J/cm2. The failure modes of R2 forging ball was crack generated from the surface and ended near the core. Coarsening of martensite(7- martensite) was the major reason, reduced material strength; Inappropriate quenching caused a great stress exceeded fracture strength. Oil quenching can avoid crack; quenched at 920? obtained fine austenite grains with no coarsening of martensite, quenching hardness was 62.9 HRC, impact toughness was 26.3J/cm2 after tempering at 220?. Optimization of preparation process: original material(?90mm) — cutting — heating to 1050? — forging — transferring and air cooling — heating to 920? and oil quenching — tempering(220?- 4h). Compared with small quantities manufacture forging ball, after the process optimization, R2 forging ball had no obvious crack with good morphology, martensite coarsening phenomenon is improved. Impact toughness was 26.04J/cm2, which were improvements of 30%, the whole hardness was about 59 HRC, volume hardness was about 58.5 HRC.