Key Words: Grinding Ball; High Carbon Chromium GCR 15; hardness; wear resistance

Introduction
As a kind of grinding body, grinding ball requires high hardness, the higher the hardness, the better the wear resistance. From the carbon content of the grinding ball: With the increase of carbon content of the grinding ball, the higher the hardness of the grinding ball, the greater the wear resistance, when the carbon content is about 1 %, the wear resistance reaches the maximum. Based on this principle, Changshu Lonte wear-resistant Ball Co., Ltd. (hereinafter referred to as “Lone wear-resistant ball”) using GCR15 steel to make the grinding ball. However, because of the high carbon content in GCR15 steel and the big size of grinding ball, the crack of water quenching will increase significantly. According to the characteristics of GCR15 steel, new technology must be developed to satisfy the production of grinding ball of GCR15 steel.

1. Classification of grinding balls
From the grinding ball mode of production, there are casting grinding ball, rolling grinding ball, forging grinding ball. Judging from the grinding ball’s texture, the grinding balls in China are divided into high chromium multi-alloy cast iron grinding balls, medium chromium and medium chromium multi-alloy cast iron grinding balls, low alloy cast iron grinding balls, Bainite martensite multi-phase ductile iron grinding balls, bainite steel balls, medium-high carbon low alloy steel balls, etc. . Grinding balls of different materials have their own advantages and disadvantages.

High Chromium cast iron grinding ball has excellent wear resistance because of its high content of chromium, discontinuous carbide distributed on the martensite body, high hardness, and certain toughness. In order to improve its hardenability, MO, CU and other alloy elements are added properly. For Dry grinding operations, in the cement, electric power industry widely used. However, due to the principle of electrochemical corrosion, its application in wet grinding is limited. In addition, the high chromium grinding ball consumes a lot of chromium resources and also limits its large-scale use.

All kinds of low alloy cast iron grinding ball, due to less chromium content, low price. Mold casting, low-temperature tempering to eliminate stress. Used for dry wear resistance is better than ordinary steel ball, but used for mine grinding its crushing.

The air-cooled bainite grinding ball has high impact wear resistance and is suitable for wet grinding because of the good combination of lower bainite and martensite duplex structure. But the metallurgical quality control must be strict, otherwise, the qualified BAINITIC steel cannot be produced.

High Carbon low alloy steel ball, less alloy element content, after forging and rolling, quenching and tempering treatment can obtain a combination of strength and toughness, hardness and wear resistance significantly improved, in wet grinding operations, especially in Non-ferrous metal widely used.

2. Develop content and goals

2. 1 Product Specifications
(1) nominal diameter of grinding ball (M M M) : 60,65,75;
(2) the theoretical diameter of Grinding Ball (mm) : 62. 4,67. 6,78. Zero.

2.2 Quality goals for grinding balls
(1) grinding ball surface and core hardness (HRC): 60 ~ 63;
(2) The microstructure of the grinding ball: fine needle martensite with a small amount of retained austenite.

3  PROCESS DESIGN
3. 3. 1 heating of RAW MATERIALS
The heating of raw materials is a very important process, especially for GCr15 steel, GCr15 steel is an over-eutectoid steel, it is very sensitive to heating, it should be thoroughly heated, and the defects of serious oxidation and over-heating of the material surface should be prevented And heating along the length direction of the material should be uniform. In addition, the distribution of a small amount of unmelted carbide in the grinding ball structure is required to increase the wear resistance of the grinding ball. In order to achieve the above requirements, the company designed a walking-beam furnace, the use of natural gas furnace reductive atmosphere to achieve less oxidation decarbonization and to ensure the uniform heating material. The temperature of the furnace is controlled by Pid, and the temperature difference fluctuates little. It provides the organization prepared for the later grinding ball quenching.

3. 3. 2 Grinding Ball Rolling
Grinding Ball Rolling is a mature rolling process of Longte wear-resistant Ball Company, which is the process of rolling round steel into grinding balls. The process mainly controls the surface quality of the grinding ball and ensures that the qualified grinding ball products are rolled.

3. 3. 3 COOLING AFTER ROLLING
After ball rolling, because the ball temperature is still very high, it must be cooled to the quenching temperature. Therefore, we designed a conveyor belt, the speed of the conveyor belt operation can be stepless speed, to achieve a significant change in climate on the ball temperature of the smaller impact. The carrying capacity of the belt is sufficient to ensure that the rolled balls are dispersed in the effective range and uniformly cooled.

3. 3. 4 Water Quenching
When the ball is cooled to the temperature range set for quenching, the ball will be quenched automatically in order of priority. Under the rotation of the water-quenched Drum, the ball is driven to rotate in the water on one hand and move forward on the other, so that all parts of the ball are cooled evenly. On the other hand, in order to equalize the water temperature, a water stirring device is added in the pool to make the water flow continuously to realize the uniformity of the water temperature, which is beneficial to the grinding ball quenching. By adjusting the rotating speed of the spiral drum to change the time of the ball in the water, the temperature of the ball out of the water can be controlled. Quenching water temperature control by the thermal resistance of the water temperature measurement, if the water temperature is high, the intake valve automatically opens; if the water temperature is low, the intake valve automatically closed. The quenching water can also be preheated for climatic reasons.

3. 3.5 CONTROL OF EFFLUENT TEMPERATURE
The quenching degree of the ball is controlled by controlling the temperature of the water out of the ball, and the high-temperature energy of the ball is diffused to the surface continuously, which makes the temperature of the ball’s center tend to be the same as that of the ball’s surface. After quenching, the internal energy of the grinding ball has not been released completely, which is reflected in the temperature, that is, the surface temperature of the grinding ball increased. This will ensure that the core of the ball can also be hardened while preventing the ball from cracking. The control of the ball’s outlet temperature can be realized by adjusting the spiral drum in the water. The spiral drum rotates fast, the ball’s outlet velocity is also fast, the surface temperature of the ball after outlet is high; the spiral drum rotates slowly, and the ball’s outlet velocity is also slow The surface temperature of the ball rises slowly after it emerges from the water.

3. 3. 6 COOLING CONTROL AFTER WATER DISCHARGE
After the ball mill water temperature is divided into two parts to control. The first is that after the ball is discharged, the temperature rises back to the highest temperature, and the martensite is cooled naturally for a period of time to stabilize the formed martensite and has the effect of a certain self-tempering The transformation of austenite to martensite still exists in the forced cooling process.

3.3. 7 control of tempering temperature and tempering time
The tempering furnace uses the tail gas of the walking beam furnace to realize the tempering and reheating of the grinding balls, thus saving energy and protecting the environment. Because the tempering temperature of the ball is not high, the ball to be cooled to a certain temperature can enter the tempering furnace, the ball flat in the furnace, with the net belt movement. The temperature of each zone is automatically controlled by the ramp plate of the flue and the air duct of each zone, so the heating is uniform and the tempering effect is good. When the core temperature of the ball reaches the maximum temperature of the ball quenching, the heat preservation can be out of the furnace for a certain time to cool. Ensure full tempering, eliminate quenching stress, not to reduce the hardness too much.

4. Effect Analysis

Taking the production of D75mm grinding balls as an example, the 75mm round steel produced by the Nanjing Iron and Steel is organized and produced according to the above technological route, in order to investigate the hardness of the grinding balls and the crushing resistance of the grinding balls.

4.1 CHEMICAL COMPOSITION OF MATERIALS

The chemical composition of the material is shown in Table 1.

4. 2 Grinding Ball Hardness Test
The finished balls 1 # , 2 # were sampled by wire cutting. The cutting surface passed through the center of the ball, and the thickness of the sample was 10 mm. 5 ~ 1 mm. Rockwell hardness tester is used to test the hardness from the surface of the ball to the center of the ball every 5 mm or so. The results of the hardness test are shown in Table 2.

Table 2 hardness test results / HRC

It can be seen from table 2 that the hardenability of the grinding ball is good, and the hardness of the grinding ball can meet the design requirements. The microstructure is fine-needle martensite with a little retained austenite and a little troostite.

4. 3 DROP TEST
The 10 grinding balls produced above were used in the drop ball test at 6M height. The balls were dropped 2400 times without breaking or spalling, indicating that the grinding balls have a strong anti-breaking ability.

5 CONCLUSION
(1) GCR15 grinding balls produced by this process have good hardenability, high hardness, and strong impact resistance, and are widely used in Non-ferrous metal mining industry

(2) the GCR 15 grinding balls produced by this process extend the carbon content to1 %, the hardness and wear resistance of rolling mill ball is further improved, and the competitive product ability of rolling mill ball is enhanced.

(3) the GCR 15 steel grinding ball produced by this process overcomes the defects of the traditional GCR15 steel grinding ball produced by residual heat quenching and opens up a new way for the production of large size GCr15 steel grinding ball.