At present, large and medium-sized cement plants, steel and non-ferrous metal concentrators and thermal power plants built in the early stage at home and abroad often use high chromium cast iron grinding balls. Among them, martensitic high chromium cast iron grinding ball occupies a large proportion. The most important reason is that the wear rate and crushing rate of martensitic high chromium cast iron mill ball are very low, the working life is long, the proportion of balls in the warehouse is stable, the number of times of supplementing and replacing the mill ball and the clearance time are reduced, so as to improve the productivity of the mill. Cement plants in industrialized countries mostly use large ball mills (barrel diameter more than 5m), and the ball consumption rate of high-chromium cast iron grinding balls is generally not more than 30g/t cement. The crushing mill is hardly seen in the silo.
The manufacture of high-quality martensitic high-chromium cast iron grinding balls requires mastering the following three points:
(1) Strictly control the eutectic carbide volume fraction of high chromium cast iron. Eutectic carbide volume fraction not only affects whether the mill fails in advance (broken or out of round), but also affects the cracking tendency of the mill during heat treatment.
In order to determine the appropriate amount of carbide in cast iron grinding balls, we have carried out abrasion resistance tests on several ball mills with different chromium content and chrome-carbon ratio on the diameter of 2.2~3.5m, and then calculated the volume fraction of the corresponding carbide. The results show that when the volume fraction of eutectic carbide of martensitic high chromium cast iron is 22%~23% and the macroscopic hardness is HRC56~60, the wear resistance and crushing resistance of the grinding ball maintain a good level. The average ball consumption of coarse grinding process is 24.5~29.2g/t cement raw material, and the ball breaking rate is 0.09%~0.12%, which is satisfactory. When the volume fraction of carbide exceeds 25%, the ball consumption decreases slightly, but the crushing rate increases. When the same composition is used, the performance of the martensitic high chromium cast iron mill ball in the fine grinding chamber is better than that in the sand casting mill ball after the same heat treatment, and there is no significant difference in the coarse grinding chamber.
(2) Ensure that the material has sufficient hardenability. The hardenability of high chromium cast iron is closely related to chromium content and chromium carbon ratio. These two factors determine the hardenability and quenching hardness of high chromium cast iron, the types and properties of eutectic carbides, the carbon content of martensite and the incidence of quenching micro-cracking. Considering that the high chromium cast iron grinding ball is a mass consumable, its production cost should not be too high. The chromium content of martensitic high-chromium cast iron grinding ball is generally 12% to 15%, and the chrome-carbon ratio should not be less than 6.
The results show that the minimum chrome-carbon ratio of 100mm thick high chromium cast iron without molybdenum is 10, which is cooled in the air after the destabilization treatment. Below this value, it is difficult to have sufficient hardened layer. If the mill contains 12% to 15% chromium, the carbon content should be 1.2% to 1.7%. Carbide is only 6.2% to 15.1%. Such a small amount of carbide is difficult to play the role of anti-wear skeleton. Moreover, this component is far from the eutectic point, and the casting performance is poor. In order to improve this situation, it is only appropriate to reduce the chrome-carbon ratio, or to add elements that can improve hardenability - molybdenum and copper. Adding molybdenum will increase production costs, but the effect is reliable.
It is suitable to keep the macroscopic hardness of martensitic high chromium cast iron mill ball at HRC55~57, and high hardness often increases the crushing rate of the mill ball. Unless the hardness of the grinding material is particularly high, the hardness of the grinding ball does not need to be further increased.
(3) Use the correct heat treatment process. Reasonable heat treatment will make the grinding ball obtain enough hardness and toughness, residual internal stress and residual austenite volume are controlled at a relatively low level.
It is necessary to control the residual austenite volume during heat treatment of high chromium cast iron grinding ball. The residual austenite volume has a significant effect on the durability of the mill. The residual austenite content is generally more than 30% in the mills with more severe spalling. It is considered that residual austenite is helpful to improve the toughness of the material and the actual hardening property of martensitic high chromium cast iron. The volume fraction of residual austenite in martensitic high chromium cast iron should be controlled in the range of 10%~20%.
Manganese has the effect of stabilizing austenite and reducing Ms point. The content of manganese in martensitic high-chromium cast iron should not be too high, generally 0.5% to 0.8% is appropriate. The addition of excessive alloying elements is also an important reason for the significant increase of residual austenite in high chromium cast iron grinding balls. Under the premise of ensuring the working performance of the grinding ball, the amount of alloying elements added should be reduced as much as possible.
W (C) = 2.0% ~ 2.2%,
w(Cr)=12%~15%,
W (Si) {{0}}.6% ~ 1.0%, w (Mn)=0.6% ~ 0.8%,
w(Mo){{0}}.5%~1.0%(whether to add molybdenum according to the ball diameter and hardness requirements),
w(P)<0.07%,
w(S)<0.05%.
