The Study On Microstructure Transformation And Properties Of Forged Steel Grinding Ball

With the change in the process of metal mines to large semi-autogenous(SAG)mills,the demand for large-size grinding balls has increased,and their comprehensive performance has become more and more valued to mining customers.It is of great significance for the enterprise to study the structure transformation of steel and develop large-size grinding balls suitable for the working condition of SAG.In this paper,the critical cooling rate of martensitic transformation of 10 kinds of test steels is measured by Formaster-II automatic transformation dilatometer,and then the mathematical expression between the critical cooling rate of martensitic transformation and chemical composition is established.The calculation equation of critical cooling rate and the coefficient of effect of C,Si and Cr are obtained by multiple linear regression.The MATLAB software is used to simulate the quenching temperature field,which provides a useful tool for the quenching process and microstructure analysis of large-size grinding balls.The numerical simulation results of 6C?140 mm grinding ball in 37 ? water show that it takes 128.8 s for the surface temperature to be pre-cooled from 950 ? to 800 ?.After quenching in water,it takes210 s for the center to reduce to 300 ?,and its maximum return temperature of the surface is 157.1 ? after leaving water.Combined with the simulation results of the quenching process,the optimal process is determined as follows: the surface is air-cooled to 800 ?? 820 ?,then quenched with water for 210 s,and then cooled in air.After a while,the return temperature of the surface reached the maximum temperature of 152 ?,which is similar to the simulated data.The volume hardness reaches 58.5 HRC,the content of retained austenite is less,the primary austenite grain size(PAGS)of the center is 6.9,the impact energy at 1/2R is more than 24 J,the depth of decarburization layer is approximately 182 ?m,and the crushing resistance is excellent.In order to reduce the quenching stress,the secondary water-cooling quenching process of 6C?140 mm grinding ball is developed,namely W90A30W120 A and W120A30W90 A.The volume hardness reaches above 56 HRC,and the internal stress is small.The microstructure is mainly composed of martensite and a small amount of bainite from the surface to 1/2R.The microstructure is a mixture of martensite,bainite and a small amount of pearlite from 1/4R to the center.The ?120 mm grinding balls of high-volume hardness soft center are prepared using 1C steel.The residual quenching temperature is 780 ?? 800 ?,water cooling170 s,and the maximum return temperature is 140 ?.The volume hardness of grinding ball is 58.5 HRC,the hardness of the center region is lower than 53 HRC,the impact energy of 1/2R and the center is greater than 28 J,and the number of single ball drops reached 4,000 without cracking.