Effects of Heat Treatment Process on Microstructure and Properties of High Chromium Grinding Ball

ABSTRACT: The heat treatment process of high chromium grinding ball produced by sand coated iron mould casting process was researched. Shrinkage and porosity defects have not been discovered after cutting along the diameter of grinding ball. The grinding ball in cast state has poor mechanical properties. Mechanical properties of grinding ball significantly increase after heat treatment After heat treatment of 950 °C * 2 h oil quenching and 350 °C *2 h tempering, grinding ball gets the best comprehensive properties. Compared with cast state, impact toughness and Rockwell hardness are increased by 55% and 26% respectively. After determination of metal matrix microhardness and microstructure observation, it can be judged that metal matrix in cast state is mainly austenite. During quenching, most of the austenite transforms into martensite and micro-hardness increases obviously*Ehning tempering, martensite transforms to tempering martensite or tempering troostite.
The quenching stress is relieved, impact toughness increases and microhardness decreases.

KEYWORDS: sand coated iron mould; microhardness; tempering martensite; tempering troostite

High Chromium wear-resistant cast iron is widely used because of its excellent wear resistance. High Chromium cast iron grinding balls are widely used in cement, mining, thermal power generation, etc. To improve the production and heat treatment process of grinding balls and actively explore the relevant laws in the production process has a good guiding role for the actual production. Meanwhile, the production process can be simplified, the product quality can be improved, and the production cost can be reduced.

1.CHEMICAL COMPOSITION 

High Chromium wear-resistant white cast iron contains more chromium elements, the cast will produce more high hardness of chromium carbon compounds. When the CR / C ratio in high chromium white cast iron is over 3.5, the main carbides are MTC3 and its micro-hardness reaches 1200 something 1800HVra. The relationship between the content of carbides and the content of elements in castings is shown in formula (1). MARTENSITE was selected to improve the wear resistance of castings.

                                  Formula (1)  Carbides (vol%%)12.33 XC + 0.55 xcr-15.2.

2.CASTING PROCESS SELECTION

The traditional production technology of wear-resistant grinding ball is sand mold casting and metal mold casting. The too slow cooling speed of sand casting can easily lead to shrinkage cavity and porosity, at the same time, the coarse grain, the impact toughness and the service life of grinding ball will be decreased sharply. Metal Mold casting has the high cooling rate to refine grains, but the too fast cooling rate will cause great casting stress. The sand-coating process with iron mold combines the advantages of sand mold and metal mold casting. This casting process cooling speed moderate, mold use many times will not appear mold cavity depression, the production of the high-chromium grinding ball smooth surface. Mold with the exhaust hole, exhaust effect is good, the production of high-chromium grinding ball stable performance. Is now widely used in the production of wear-resistant grinding balls.

3.HEAT TREATMENT STUDY

3.1 Pre-test Treatment

The as-cast grinding balls with the diameter of 110 mm were selected randomly from the grinding balls produced by the sand-coating process in the iron mould, and 16 standard impact samples with the dimension of 55mm * l0 mm * l0 mm were cut by linear cutting machine. No shrinkage or porosity was found in the sample taken from the middle of the grinding ball, as shown in Fig. 1. The chemical composition of the grinding ball is shown in table 1. The CR / C ratio is about 4.74, the content of CR is low, and the content of carbon is correspondingly reduced to increase the toughness of grinding ball.

grinding media

 Tab.1 Chemical composition of grinding ball (wt%)

C Cr Si Mn S P RE
2.421 11.468 0.636 0.656 0.046 0.045 0.04

3.2 Heat Treatment

All 16 specimens were small and of uniform shape. The heat treatment process adopts conventional austenitizing quenching and tempering (quenching and tempering) treatment, and the heat preservation time of quenching and tempering is set at 2H. 16 samples were numbered as 1 #, 2 #, 3 #, 4 #…. 16 # 1 # sample, without heat treatment (as cast) , 2 #-5 # sample was quenched at 900 °C and tempered at 250 °C, 350 °C, 450 °C and 550 °C respectively. Use Oil for quenching, tempering after heat preservation air cooling. The quenching temperature is 950 °C, oil quenching, and the tempering temperature is 250 °C, 350 °C, 450 °C, and 550 °C, respectively. 10 # sample heat treatment is 950 °C, air cooling + 350 °c tempering, 11 # is 950 °C, air cooling + 550 °c tempering, 12 # is 950 °C, oil quenching, no tempering. The quenching temperature is 1000 °C, oil quenching, tempering temperature is 250 °C, 350 °C, 450 °C and 550 °C respectively.

3.3 Test Results

NI300C impact testing machine was used to carry out impact toughness test on 16 samples. The Rockwell hardness is measured by the Rockwell hardness tester. The impact toughness and Rockwell hardness of the specimen are shown in table 2

Tab.2 Results of samples impact toughness and Rockwell hardness

NO. Impact value
/(J*cm^)
Rockwell hardness

(HRC)

NO. Impact value
/(J*cm^)
Rockwell hardness

(HRC)

1 3.3 47.0 9 6.4 48.9
2 4.1 58.1 10 5*2 57.8
3 4.3 57.2 11 4.3 49.2
4 4.5 56.1 12 4.0 63.0
5 5.1 47.5 13 4.4 61.2
6 4.9 60.1 14 4.7 60.9
7 5.1 59.2 15 4.7 60.2
8 5.1 58.0 16 6.2 44.1

3.4 analysis of Test Results

When the Matrix is austenitized, secondary carbides will be precipitated during the quenching process. The austenite is unstable, and the martensite transformation occurs during oil quenching. Most of the AUSTENITE is transformed into martensite. During tempering, the MARTENSITE is transformed into tempered martensite and the resulting martensite transformation is eliminated

Greater stress. Therefore, the impact toughness will be improved after tempering. From the 12 # sample, after 950 degrees: oil cold quenching, no tempering, casting hardness up to 63HRC.

From Table 2, it can be seen that the hardness is only 3.3 j / CM2 with 47HRC impact toughness, and the Comprehensive Mechanical Properties are poor. After heat treatment, the hardness and impact toughness of the samples were improved remarkably. Fig. 2 shows the mechanical properties of the sample after cold quenching and tempering of oil at different temperatures. Fig. Shows that with the increase of tempering temperature, the impact toughness increases and the hardness decreases. At 550 deg: temper impact toughness increased significantly, but the hardness decreased sharply, even lower than the cast hardness. The reason may be that the carbide precipitates in the retained austenite coarsened and lost strengthening effect, the hardness of the retained austenite and martensite decreased at the same time and changed into tempered troostite and tempered sorbite. When tempered at 350 °c, the impact toughness does not change much compared with 450 °C, but the hardness should be high at 1 something 2HRC, while the impact toughness is low at 250 C °C. Therefore, the choice of 350 C tempering is ideal.

  • Microstructure and Properties of High Chromium Grinding Ball

Fig. 3 shows the Mechanical Properties of samples quenched with oil at different temperatures. 350 C tempering. As can be seen from FIG. , The comprehensive mechanical properties at 950 C quenching are better. 900 C quenched, due to the lower temperature, carbon, saw elements in austenite

The hardness of produced martensite decreases with the lower solubility in the matrix. 1,000 C quenching, due to the higher temperature, secondary carbide re-dissolve austenite, the AUSTENITE carbon content increases, improve the organization stability, quenching

The retained austenite content is higher, the martensite hardness is higher, but the impact toughness is lower.

The results show that the hardness is 59.2 HRC and the impact toughness is 5.1 j / cm2o. The hardness and impact hardness is increased by 26% and 55% respectively.

3.5 Organizational Analysis

Figure 4 shows the microstructure of the samples in different states. The samples were all corroded with 4% nitric acid alcohol. The white tissue is carbide which cannot be corroded and appears white. The composition of the grinding ball belongs to hypoeutectic, and the carbide will precipitate in the form of carbide + austenite eutectic structure at the final solidifying position of the Metal Matrix. The volume fraction of carbides can be calculated to be about 20.96 %.

Figure 4 shows that there is a black structure between the white carbides. The carbides are precipitated at the base grain boundary in eutectic form. The carbides are more dispersed. The CR / C ratio is 4.74, which indicates that the main type of carbides is M7C3. MARTENSITE transformation and secondary carbides occur in the Matrix during quenching treatment. Therefore, the Black Matrix in Fig. 4(b) and (c) has the diffuse white speckled tissue, whereas no such tissue exists in Fig. 4(a). The point-like substance can be judged to be secondary carbides.

  • Effects of Heat Treatment Process on Microstructure and Properties of High chrome Grinding Ball

The microstructure of Fig. 4(b) and (c) is not significantly different. The main difference between tempered and untempered tissues is the type of martensite. When tempering, martensite changes into tempered martensite precipitates micro-carbides, but the amount is very small, and the volume is difficult to distinguish. Mh micro-hardness tester was used to determine the micro-hardness of the Matrix of 1 #, 12 #, and 7 # samples.

The results are shown in Table 3.

Test No. Test value 1 Test value2 Test value 3 Test value 4 Test value 5 Test value 6 Average
1 386.1 381.6 403.3 421.9 413.5 373.1 395.8
12 719.8 695.8 742.5 713.5 722.9 745.8 723.4
7 552.9 598.2 593.4 591.0 582.4 579.4 582.9

The cooling speed of sand coating process is faster in the iron mold, and the Matrix structure is as-cast austenite, and a little martensite and other structures. According to the table, the micro-hardness of 1 # sample as-cast is only 395.8 HV. When the sample is not tempered after quenching, more martensite will be formed because of the smaller sample has better hardenability. The micro-hardness of the Matrix was 723.4 HV. It is proposed that the hardness of Martensite is 750 something 900HV [9]. The quenching and tempering state is due to the formation of tempered martensite and partly tempered torrent. Compared with the quenched Matrix, the hardness of the 7 # sample Matrix decreased obviously, only 582.9 HV. It shows that tempering plays an important role in the change of Martensite.

4. CONCLUSION

(1) the impact toughness is 3.3 j / CM2 and the hardness is 47HRC. After heat treatment, the impact toughness and hardness of the material increased obviously, and the impact toughness reached 5.1 j / CM2 and the hardness reached 59.3 HRC after 950 C oil quenching + 350 C tempering air cooling. The impact toughness and hardness increased by 55% and 26% respectively.

(2) according to the micro-hardness test and microstructure observation, the majority of the Matrix structure of grinding ball is austenite, and the micro-hardness is 395.8 HV. MARTENSITE transformation and precipitation of secondary carbides occurred during quenching. A large amount of martensite and a small amount of residual austenite were found. The micro-hardness of Matrix was raised to 723.4 HV. During tempering, the MARTENSITE is changed into tempered martensite and the micro-hardness of Matrix is reduced to 582.9 HV.

(The original article is written by KU GUANGQUAN, SU YONG, GONG SHENGWEI, XIE NAIXIONG, the article No.: 1001-3814(2016)12-0226-04. Show great respect to our researchers. )

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