CHINA FOUNDRY

136

Vol.9 No.2

High Cr white cast iron/carbon steel bimetal liner by lost foam casting with liquid-liquid composite process

Male, born in 1979, Ph.D candidate and Lecturer. Research interests: wear-resistant materials, lost foam casting, CAD/CAM/CAE. E-mail: 18971600996@189.cnReceived: 2011-09-04; Accepted: 2012-03-01

*Xiao Xiaofeng

*Xiao Xiaofeng 1, 2, Ye Shengping 1, Yin Weixin 3, Zhou Xiaoguang 1, and Xue Qiong 4

(1. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan

430074, China; 2. School of Mechanical Engineering and Automation, Wuhan Textile University, Wuhan 430073, China; 3. Wuhan ZhiKe

Abrasive-Resistant Material S&T Development Co., Ltd, Wuhan 430073, China; 4. College of Science, Wuhan University of Technology,

Wuhan 430079, China)

The liners in wet ball mill for mineral processing industry encounter impact, wear and corrosion from the grinding

balls and the grinding medium during the grinding process, resulting in various liner failures under the wear modes including impact, abrasive and/or corrosive wear [1]. The service life of liners made from traditional materials, such as Hadfield steel and alloyed steels, is rather limited in these conditions. High-Cr white cast iron (HCWCI), one of the most commercial hard-facing alloys, has also been selected as ball mill liner’s material. The presence of a high volume fraction of M7C3-type hard carbides is the primary reason for the excellent corrosive and abrasive wear resistance. However, the same carbides are also responsible for the observed brittleness of the material, often limiting its use to non-repetitive impact applications. Therefore, fracture toughness is an important property of liners made from HCWCI [2].

Since a large amount of carbides is required in the microstructure to maximize the wear resistance, some researchers have sought to improve the toughness through modifications to the carbide structures [3,4]. But no matter how

Abstract: Liners in wet ball mill for mineral processing industry must bear abrasive wear and corrosive wear, and consequently, the service life of the liner made from traditional materials, such as Hadfield steel and alloyed steels, is typically less than ten months. Bimetal liner, made from high Cr white cast iron and carbon steel, has been successfully developed by using liquid-liquid composite lost foam casting process. The microstructure and interface of the composite were analyzed using optical microscope, SEM, EDX and XRD. Micrographs indicate that the boundary of bimetal combination regions is staggered like dogtooth, two liquid metals are not mixed, and the interface presents excellent metallurgical bonding state. After heat treatment, the composite liner specimens have shown excellent properties, including hardness > 61 HRC, fracture toughness αk >16.5 J·cm-2 and bending strength >1,600 MPa. Wear comparison was made between the bimetal composite liner and alloyed steel liner in an industrial hematite ball mill of WISCO, and the results of eight-month test in wet grinding environment have proved that the service life of the bimetal composite liner is three times as long as that of the alloyed steel liner.

Key words: bimetal liner; liquid-liquid composite process; lost foam casting; high Cr white cast ironCLC numbers: TG249.5/252 Document code: A Article ID: 1672-6421(2012)02-136-07

uniform is the carbide distribution and how fine is the carbide size, the fracture toughness of singular HCWCI does not seem to meet the requirement of impact wear condition in ball mill. To overcome the problem, some researchers have paid so much attention to the composites possessing superior properties including high hardness, high corrosive wear resistance and reasonable fracture toughness [5-7].

To solve these problems above mentioned, we conducted the present study to produce a bimetal liner from HCWCI and carbon steel composite based on lost foam casting (LFC) with liquid-liquid composite process. LFC is a type of evaporative pattern casting process that is similar to investment casting except that foam is used for the pattern instead of wax. Plenty of carbon generates and gathers around the cavity when foam pattern is melted by molten metal. Enrichment of carbon is disadvantageous for cast steel parts, but it is very suitable to produce wear resistant parts because less metal matrix will be oxidized and more hard carbides will be produced and compounded. Moreover, LFC is also dimensionally accurate, maintains an excellent surface finish, requires no draft, and has no parting lines formed by flash, so metal parts made by LFC could avoid subsequent processing or decrease machining allowance. The purpose of present study is to explore a new method for producing a composite liner with high wear resistance, precise dimension and low consumption.

137

Research & DevelopmentMay 2012

percent of carbide M7C3, carbon and chromium, respectively. Approximate 34wt.% of M7C3 carbide was designed to meet the requirement of service condition in ball mill. Tang et al [9] thought that the C content of 3wt.% was the optimized value for achieving high wear resistance, so the C content in present study was designed as 3wt.%; and the Cr content can be computed out by Eq.(1). Finally, the designed chemical composition of HCWCI is shown in Table 1, and the chemical composition of carbon steel is also given in the table.

1 Experimental details

1.1 Experimental materialsTable 1 shows the designed chemical composition of HCWCI which was quantized by F. Maratray’s expression [8] as follows:

(1)

In Eq.(1) wt(M7C3), wt(C) and wt(Cr) represent weight

Fig. 2: Schematic representation of LFC process (a) step 1: sand fill and compaction; (b) step 2: HCWCI pouring and (c) step 3: carbon steel pouring

Table 1: Chemical compositions of HCWCI and carbon steel (wt.%)

1.2 Process of liquid-liquid composite lost foam casting

First, the pattern was made using polystyrene foam. Due to simple main profile of liner, it could be cut from a solid block of foam by using a hot-wire foam cutter. Runner system, risers and other patches were hot glued to the pattern. The whole foam pattern is shown in Fig. 1.

Fig. 1: Foam pattern of composite liner

Next, the foam pattern was coated with ceramic investment, also known as refractory coating, via dipping and brushing. This coating creates a barrier between the smooth foam surface and the coarse sand surface. Secondly controlling permeability was conducted, which allows gas created by the vaporized foam pattern to escape through the coating and into the sand. Controlling permeability is a crucial step to avoid sand erosion, so molten metal does not penetrate or cause sand erosion during pouring.

After the coating dries, the foam pattern was placed into a flask and backed up with un-bonded quartz sand. The sand was then compacted using a vibrating table, and a vacuum system was also used to increase sand compaction. Once compacted, the mold, shown in Fig. 2(a), was ready to be poured. The pouring process in liquid-liquid composite casting is very critical and uneasily controlled as compared to that in conventional casting. As shown in Fig. 2(b), the present approach is to finish pouring molten HCWCI quantitatively; then, the molten carbon steel is poured into cavity timely as shown in Fig. 2(c).

Molten high Cr white cast iron

Vacuum

(b)

Vacuum

(a) Runner system Flask

Carbon steel

HCWCI

Foam

Quartz sand

wt(M7C3) = 12.33 · wt(C) + 0.55 · wt(Cr) -15.2

Molten carbon steel

Vacuum

Molten high Cr white cast ironRunner system Flask

(c)

Vacuum Vacuum

(a) (b)

C Si Mn S P Cr W Mo Ni HCWCI 2.9-3.1 <0.8 0.8-1.1 <0.04 <0.04 21-23 0.4-0.6 0.4-0.6 0.4-0.6Carbon steel 0.18-0.25 0.6-0.9 0.9-1.2 <0.035 <0.035

CHINA FOUNDRY

138

Vol.9 No.2

Through the above process, the castings o f t he compos i t e l i n e r s h a v e b e e n successfully made, as shown in Fig. 3(a) and Fig. 3(b). The rough dimension of the liner is 90 mm × 400 mm × 900 mm .

Fig. 4: Schematic representation of movement of grinding balls in a ball mill

Fig. 3: Bimetal composite liner castings: (a) for Ф3.6 m ball mill and (b) for Ф5 m ball mill

1.3 Heat treatmentThe composite liners were heat-treated at 1,223 K for 120 min to destabilize austenite and then air cooled to room temperature. The purpose of the heat treatment was to obtain martensite matrix in order to enhance wear resistance of the composite.

1.4 Experimental summaryBimetal composite liners with precisely controlled chemical compositions have been prepared. The specimens, cut from the composite liner by E. D. M. Wire, were polished to a 1 μm diamond finish and chemically etched in a freshly prepared solution which contains 50 mL FeCl3, 20 mL HCl and 20 mL ethanol. This solution attacks preferentially the matrix leaving

carbides relatively unaffected, which provides good contrast between carbides and matrix [10]. Microstructure of the HCWCI sample was characterized using optical microscope, XRD and SEM.

Comprehensive mechanical properties of specimens, including hardness, toughness and bending strength, have been studied. The dimensions of specimens for toughness test and bending strength test were 10 mm × 10 mm × 55 mm and 20 mm × 30 mm × 170 mm, respectively. Moreover, wear resistance was compared between liners made from bimetal composite and alloyed steel in industrial hematite ball mill of Wuhan Iron and Steel (Group) Corporation (WISCO) according to the schematic diagram in Fig. 4.

2 Results and discussion

2.1 Microstructures of HCWCI and carbon steel

The different specimens were obtained respectively from the HCWCI layer and the carbon steel layer of the composite liner, as shown in Fig. 3, and their chemical compositions are shown in Table 1. It can be seen from Fig. 5(a) and Fig. 5(b) that the typical as-cast microstructure of HCWCI layer is hypoeutectic, consisting of eutectic carbides in a mainly austenitic matrix, while the typical as-cast microstructure of carbon steel layer is hypoeutectoid, being composed of ferrite and pearlite.

Carbides and mat r ix of the HCWCI layer and the microstructure of the carbon steel layer were investigated by means of SEM-EDS. Relevant elements shown in Fig. 6 have been detected, and their weight percentages are given quantitively in Table 2. The results prove that the above microstuctural analysis is reasonable and accurate.

After the destabilization heat treatment, the secondary carbide precipitation promoted the change of the microstructure from an austenitic matrix (Fig. 5a) to a mainly martensitic one (Fig. 7a) with some retained austenite. In addition, the newly formed matrix was reinforced with secondary carbides of the type M7C3 (Fig. 7b). In studies on high-Cr white cast irons with

HCWCI layer

(a)

Carbon steel layer

(b)

HCWCI layer

Carbon steel layer

Grinding ball

Mineral particles

Composite liner

Alloy steel liner

139

Research & DevelopmentMay 2012

Fig. 5: Optical photograph of composite microstructure in as cast: (a) HCWCI layer and (b) carbon steel layer

Fig. 6: SEM-EDS analysis: (a) carbide of HCWCI layer, (b) matrix of HCWCI layer and (c) carbon steel layer

Fig. 7: Microstructures of HCWCI layer after heat treatment: (a) optical photograph and (b) SEM photograph

Table 2: EDS analysis of composite

Point in Fig. 6(a) Point in Fig. 6(b) Point in Fig. 6(c)

wt.% at.% wt.% at.% wt.% at.%

C 10.20 33.50 8.96 31.02 7.54 26.37

O - - - - 2.07 5.42

Si - - 0.46 0.68 0.27 0.40

Cr 57.60 43.70 15.32 12.25 0.70 0.57

Mn - - - - 1.32 1.01

Fe 31.44 22.21 75.26 56.05 88.09 66.22

similar chromium contents and destabilization temperatures of 1,223 K, other authors also reported this kind of secondary carbides [11, 12].

Moreover, the microstructure of the composite has been characterized by XRD. Two X-ray patterns for the as-cast and the as-heat treated irons are shown in Fig. 8. For the as-cast condition, only the austenite and the eutectic carbide peaks were detected; while for the heat treated iron, the intensity of the peaks of austenite diminished and that the peaks of martensite increased. This has proved the microstructural change observed from micrographs in Fig. 5(a) and Fig. 7.

(a) (b) (c)

50 μm

(a) (b)

200 μm50 μm

(a) (b)

Carbide

Austenite

Ferrite

Pearlite

Martensite

Carbide

Martensite

Carbide

Element

CHINA FOUNDRY

140

Vol.9 No.2

Fig. 8: XRD traces indicating phases present in HCWCI microstructure: (a) as cast and (b) after heat treatment

Fig. 9: Microstructure of composite interface after heat treatment: (a) optical photograph and (b) SEM photograph

2.2 Composite interfaceThe specimens with composite interface shown in Fig. 9 were obtained from composite liner shown in Fig. 3(a), which was produced using liquid-liquid composite LFC. According to the micrograph shown in Fig. 9(a), the boundary of bimetal combination regions was staggered like dogtooth, and the two liquid metals were not mixed. It can also be noticed from Fig. 9(b) that a transition region between HCWCI and carbon steel of about 0.1 mm in thickness has been formed, and the interface presents excellent metallurgical bonding state.

At higher temperatures above the melting point of HCWCI when carbon steel was poured, both Cr and C elements were gradually dissolved into the molten steel. In consequence, an

over-saturated solution was closely around composite interface with temperature drop, which tends to make the precipitation on them through diffusion. According to the principles of crystallization kinetics, small and fine compound carbides shown in Fig. 9(b) can nucleate and crystallize in the transition region between the HCWCI layer and the carbon steel layer.

Porosities have also been identified in Fig. 9(b), which was caused by the following reasons: the fine grains reduce the sand permeability, which can not match up to the velocity of generated gases from the EPS pyrolisis [12]. So the high temperature and the velocity of carbon steel pouring, combined with lower escape velocity of gases, must induce heat concentration and generate porosities in the region.

Furthermore, the distributions of relevant elements around the composite interface were analyzed using SEM-EDX. According to the EDX analysis result shown in Fig. 10, the thicknesses of diffusion layers of Cr and C elements are about 0.1 mm and 0.2 mm, respectively.

2.3 Hardness, toughness and bending strengthComprehensive mechanical properties are shown in Table 3 and Table 4. It can be noticed that the fracture toughness of specimens has been improved significantly and can meet the requirement of the impact wear in ball mill applications.

Porosity

Carbon steel layer

HC

WC

I layer

Small carbide

50 μmCarbon steel layer

HCWCI layer

(b)(a)

30 40 50 60 70 80 90 100

800

700

600

500

400

300

200

100

0

As-cast

2 q (°)2 q (°)

30 40 50 60 70 80 90 100

(a) (b)500

400

300

200

100

0

AustensiteM7C3

Heat treated

(a) (b)

Austenite

141

Research & DevelopmentMay 2012

Cr

C

O

Si

V

Mn

Fe

Fig.10: SEM-EDX analysis of relevant elements in composite interface

Fig. 11: Wear resistant comparison between liners made from bimetal composite and alloyed steel in industrial hematite ball mill of WISCO: liners served and disassembled from ball mill for five months (a) and for eight months (b)

Table 3: Results of fracture toughness test

Table 4: Results of bending strength and hardness test

Specimen Sectional No. dimension

1 10 mm ×10 mm 18.8 2 10 mm ×10 mm 20.2 3 10 mm ×10 mm 16.9

Sectional Bending dimension strength (MPa)

1 20 mm × 30 mm 1,610 61.82 20 mm × 30 mm 1,745 62.33 20 mm × 30 mm 1,683 61.2

a k (J·cm-2)

SpecimenNo.

Hardness(HRC)

2.4 Corrosive and abrasive wear resistanceWear resistant comparison between liners made from bimetal composite and alloyed steel has been made in an industrial hematite ball mill at WISCO. The dimension standard of the ball mill is 3.6 m × 6 m, and its full productivity is 160 tons per hour.

As shown in Fig. 4, the composite liners and the alloyed steel liners were assembled and arranged separately by each other in the ball mill during the present research. After having served in the wet grinding environment for five months, it could be obviously noticed from Fig. 11(a) that the bimetal liners made from the composite are more wear resistant than the ones made from the alloyed steel. After eight months of

Table 5: Weight loss in wet grinding environment

1 170 133 37 2 170 138 32

Alloyed 1 185 82 103 steel 2 185 81 104

Wear resistant factor

3

1

service with full loads, all liners were disassembled from the ball mill. As shown in Fig. 11(b), the liners made from the alloyed steel were no longer serviceable because their minimum wall thickness was only about 10 mm. Their working surfaces have been seriously worn, deformed and fully oxidized. However, working surfaces of the composite liners still kept their original profile with uniform decrease in thickness, and silvery white proved that the composite liners have been oxidized only a little because of high chromium content.

The weight losses of the composite liner and the alloyed steel liner are listed in Table 5. It can be proved that the service life of the bimetal composite liner is three times as long as that

Material type Liner number Original weight (kg) Present weight (kg) Weight loss (kg)

Composite

Alloy steel liners

Composite liners

Alloy steel liners

(b)

Composite liners

(a)

CHINA FOUNDRY

142

Vol.9 No.2

of the alloyed steel liner. Wear resistant difference between the composite liner and the alloyed steel liner results from their chemical composition and microstructure, which can be explained as follows: Microstructure of the alloyed steel after heat treatment is normally composed of singular martensite, while the HCWCI layer of the composite after heat treatment consists of eutectic carbides(M7C3) in a mostly martensitic matrix.

3 Conclusions(1) Bimetal composite liner made from HCWCI and carbon

steel has been successfully developed and produced by liquid-liquid composite LFC. The composite possesses superior properties, including high hardness, high corrosive wear resistance and reasonable fracture toughness.

(2) The obtained micrographs indicate that the boundary of bimetal combination regions is staggered like dogtooth, two liquid metals are not mixed, and the interface presents excellent metallurgical bonding state.

(3) After heat treatment, the composite liner specimens have shown excellent properties, including hardness > 61 HRC, fracture toughness a k >16.5 J·cm-2 and bending strength >1,600 MPa.

(4) Wear resistant comparison has been made between liners made from the bimetal composite and the alloyed steel in an industrial hematite ball mill at WISCO. The result shows that the service life of the bimetal composite liner is three times as long as that of the alloyed steel liner.

References[1] Prasad B K, Modi O P, Jha A K and Patwardhan A K. Effects

of Some Material and Experimental Variables on the Slurry Wear Characteristics of Zinc-Aluminum Alloys. J. Mater. Eng. Perform., 2001, 10: 75-80.

[2] Correa R, Bedolla-Jacuinde A, Zuno-Silva J, et al. Effect of boron on the sliding wear of directionally solidified high-chromium white irons. Wear, 2009, 267: 495-504.

[3] Ma N, Rao Q, Zhou Q. Effect of boron on the structures and properties of 28%Cr white cast iron. AFS Trans., 1990, 98: 775-781.

[4] Han Fusheng and Wang Chaochang. Modifying high Cr-Mn cast iron with boron and rare earth–Si alloy. Mater. Sci. Technol., 1989, 5: 918-924.

[5] Rong Shoufan, Guo Jiwei, Zhu Yongchang, et al. Study on BiMetal Liquid Composite Casting Lining Board. In: Proceesings of the 12th Conference on Chinese Wear-Resistant Materials, 2009: 85-91. (in Chinese)

[6] Li Yefei, Gao Yimin. Three-body abrasive wear behavior of CC/high-Cr WCI composite and its interfacial characteristics. Wear, 2010, 268: 511-518.

[7] Liu D, Li L Q, Li F Q, Chen Y B. WCP/Fe metal matrix composites produced by laser melt injection. Surface and Coatings Technology, 2008, 202 (9): 1771–1777.

[8] Maratray F, Poualion A. Austenite retention in high-chromium white irons. ASF Trans., 1982, 27–82: 795–804.

[9] Tang X H, Chung R, Li D Y, et al. Variations in microstructure of high chromium cast irons and resultant changes in resistance to wear, corrosion and corrosive wear. Wear, 2009, 267: 116-121.

[10] Bedolla-Jacuinde A, Correa R, Quezada J G, Maldonado C. Effect of titanium on the as-cast microstructure of a 16% chromium white iron. Materials Science and Engineering A, 2005, 398: 297–308.

[11] Powell G L F, Laird G, II. Structure, nucleation, growth and morphology of secondary carbides in high chromium and Cr-Ni white cast irons. J. Mater. Sci., 1992, 27: 29–35.

[12] Shivkumar S, Yao X, Makhlouf M. Polymer–melt interactions during casting formation in the lost foam process. Scripta Metall. Mater., 1995, 33: 39-46.

This work was financially supported by the National Natural Science Foundation of China under grant No. 50805109, and the Fundamental Research Funds for the Central Universities under grant No. 2011-1a-023.