Heat treatment process of the high chrome cast iron Grinding Media

Guo Zhi-hong, Wei Ying-hui, Wang Hong-wei

(Taiyuan University of Technology, materials science, Taiyuan, Shanxi)

ABSTRACT: A heat treatment process of high chromium cast iron grinding media with trace alloying elements Mo, V, NB was studied. The heat treatment processes of quenching at 980 °C, tempering at 400 °C and 600 °C was adopted. The microstructure of the quenched Matrix is quenched Martensite, tempered at 400 °C and tempered at 600 °C is tempered Sorbate.

The results of hardness analysis and wear resistance analysis show that the hardness of the sample treated by quenching is 65HRC and the wear amount is the smallest, the hardness decreases to 62.8 HRC after tempering at 400 °C and the wear amount increases by 18.2% compared with the quenched state, and the hardness decreases to 57.6 HRC after moderating at 600 °C The wear rate increased by 30.3% compared with the quenching state.

The optimum heat treatment process combined with the practical application of grinding media should be quenched at 980 °C and tempered at 400 °C.

Keywords: High Chromium Cast Iron; Grinding media; wear resistance; Heat Treatment

Introduction

Ball mill is widely used in cement, electric power, mineral processing, building materials, and other industries. As the grinding medium in the ball mill, the grinding ball must have both high wear resistance and good toughness. In recent years, with the rapid development of China’s industry, the consumption of grinding ball is very large. The method of improving the performance of the grinding ball and increasing its service life will produce great economic benefits. The wear resistance of the grinding ball is closely related to its heat treatment process. In this paper, the metallographic structure and properties of the high chromium alloy ball are analyzed through the experimental study on the composition design and heat treatment process A heat treatment process for improving the wear resistance of high chromium alloy balls was proposed.

1.Experimental materials and methods

1.1 OPTIMUM DESIGN OF COMPONENTS

Based on the requirement of Matrix and carbide type, the chemical composition of the grinding media was optimized in laboratory and factory.

(1) Carbon: C has a significant effect on the matrix structure and carbide of high chromium cast iron. C is the main element for the formation of eutectic carbide (C R, F e)7 c 3, which plays a vital role in wear resistance.

(2) CR: Cr is a basic alloy element which ensures excellent wear resistance and toughness of high chromium white cast iron. The content of CR determines the type of carbide. When the content of CR reaches a certain amount, the increase of the content is not obvious for the improvement of wear resistance, too little cannot form a high hardness carbide (CR, Fe)7 C3.

(3) MN: Mn is an effective element to form austenite, which can improve the hardenability of steel, but the content of MN cannot be too high in the grinding media.

(4) Si: Si is a non-carbide forming element, which can decrease the hardenability of steel and increase the eutectoid transformation temperature.

(5) Niobium: After adding Niobium into high chrome cast iron, the properties of high chromium cast iron can be improved by improving the matrix structure and carbide morphology. The improvement of wear resistance and impact toughness is the primary performance.

(6) Trace alloying element: In High Chromium cast iron, the concentration of chromium in the Matrix is not enough to restrain the transformation of pearlite shape because most of the added chromium goes into carbide. In order to improve the as-cast hardenability of the grinding ball, it is necessary to add Mo, Cu, V, and w to the grinding ball for micro-alloying, the content (mass fraction) of which is less than 1.0 %. The combination of Molybdenum and copper can improve the impact toughness of the alloy and prolong the service life of the grinding ball.

Based on the failure analysis of the wear-resistant ball in a steel plant in Shanxi Province, the key to improve the wear-resistant performance of the ball is how to make the hardness and toughness of the Matrix match well. The toughness of the material can be further improved through grain boundary purification, grain refinement, carbide content control, carbide morphology, and size improvement while obtaining high hardness. The effect of composition optimization on the microstructure and properties of high chromium white cast iron was studied by increasing the content of CR, Mo and adding Re. The chemical composition of the optimized wear-resistant ball is shown in table 1.

Table 1 chemical composition (mass fraction) of optimized high chromium White Cast Iron Grinding Media

Element Mn Si Cr Mo Ti Re Nb Fe
Contain 2.84 1.26 0.048 0.053 0.81 15.83 0.16 0.07 <0.02 0.04 0.02 0.01 0.26 Balance
  • cast iron balls

Fig. 1 Grinding media metal mold drawings

  • heat treatment figure

Heat treatment figure in figure 2.

The grinding media was quenched and tempered in a box-type resistance furnace at 980 °C, air-cooled, 400 °C and 600 °C, respectively. The heat treatment process curve is shown in figure 2.

The microstructure of high chromium white cast iron grinding ball as-cast and after heat treatment was observed by metallographic microscope.

The sample of 10mm* 10 mm* 8 mm in size at the center of the grinding ball was cut by weld and polished with 200 # to 1500 # metallographic sandpaper. The etching agent was 4% nitric alcohol and the etching time was 10S. The microstructure was observed and analyzed by MDS-type metalloscope.

Hardness is an important index of wear resistance, and the uniformity of ball hardness reflects the uniformity of wear. In this experiment, HR-150A Rockwell hardness tester was used, the load was 5kgs, the loading time was 5m, and the hardness was measured from the center of the specimen to the edge of the specimen. The hardness values of high chromium white cast iron after as-cast and heat treatment were measured and compared to study the effects of composition and heat treatment process on the hardness of the material.

Wear resistance is evaluated by the amount of wear under the same wear condition. Adopts ML-10 type tester, the Motor Speed is 90 r / min, the quartz sandpaper is 140 # , the load is 200g, 400g, and 800g respectively, the specimen size is dia 6 * 10mm, and the specimen is measured by the Electronic Analytical Balance (accuracy is 0.1 mg) after the specimen is worn by positive rotation and reverse rotation Wear capacity ΔM = mass before wear M 1-mass after wear M2.

2.Experimental Results and Analysis

2.1 microstructure

Fig. 3 is the microstructure of grinding media after different heat treatment processes. The as-cast structure shown in Fig. 3(a) is composed of Pearlite P + Carbide + Abnormal Ledeburite, with less ledeburite and more cementite, and the carbides are mostly net-like carbides. 3(B) shows the microstructure of the sample quenched at 980 °C and air-cooled to room temperature, and its Matrix is acicular martensite m + granular carbide + a small amount of retained austenite. 3(C) is quenched at 980 °C and air-cooled to room temperature and tempered at 400 °C.  The microstructure from air cooling to room temperature, the matrix structure is tempered tobolite T + Carbide + a small amount of retained AUSTENITE A, Fig. 4(d) is the microstructure from air cooling to room temperature after quenching at 980 °C, and then after high temperature tempering at 600 °C, Air Cooling to room temperature. The microstructure is tempered sorbite + carbide + a small amount of retained austenite. High-temperature tempering results in the decomposition of Martensite to form sorbite, and the ferrite and carbide are coarse.

  • alloyed steel balls

Fig. 3 Grinding media metal mold drawings

  • casting  grinding media

Fig. 4  Grinding media metal mold drawings

2.2 Hardness Analysis

Fig. 4 is the hardness contrast of grinding media under different heat treatment process. From Fig. 4, it can be seen that the hardness of grinding media is the highest after quenching at 980 °C, and the hardness reaches 65HRC. The results show that the microstructure of sample an at room temperature is metamorphosed, Ledeburite as Matrix and contains pearlite and carbide, and the hardness is not high; the microstructure of sample B is mainly quenched acicular martensite with high hardness; The Matrix is mainly tempered troostite, the hardness decreases, and sample D is tempered from high temperature to room temperature, wear-resisting, the martensite in the ball structure decomposes and forms tempered sorbite, which causes the hardness to decrease seriously.

2.3 Analysis of wear

Fig. 5 is a comparison of abrasive wear of grinding media under different heat treatment processes. As can be seen from Fig. 5, after quenching at 980 °C, the wear amount is the smallest, which is related to the high hardness of the quenched structure, but the internal stress is easy to exist in the quenched structure, and after quenching and tempering at 980 °c, the wear amount is larger than that of the wear resistant ball quenched at 980 °C The results show that the wear resistance of the ball after tempering at 400 °C is lower than that after tempering at 600°C, and the wear resistance of the ball after tempering at 400 °C is better than that after tempering at 600°C.

  • results of the heat treatment for grinding media

3. Experimental Results

In this experiment, the grinding media samples were heat-treated by different processes in a high-temperature heating furnace. The microstructure and wear resistance of the grinding media after heat-treatment were analyzed by means of the metallographic microscope, Rockwell hardness tester and wear tester And came to the following conclusions:

1) At room temperature after quenching at 980 °C, the Matrix structure is mainly quenched martensite, at which time the grinding media has high hardness and wear resistance, but the internal stress often exists in the quenched structure, which is easy to cause the crack and deformation of the grinding media, so it must be tempered to eliminate the internal stress and improve the toughness of the grinding media.

2) After tempering at 400 °C, the Matrix structure is mainly tempered tobolite at room temperature, and the grinding media has higher hardness and wear resistance, and after tempering at 600 °C, the Matrix structure is mainly tempered sorbite and contains a lot of coarse ferrite and carbide, which leads to the poor hardness and wear resistance of the grinding media. Therefore, the optimum heat treatment process for the alloy components studied is quenching at 980 °C and tempering at 400 °C.