Wear 250 (2001) 7175

Abrasion resistance of high Cr cast irons at an elevated temperature

Hua-Nan Liua, Michiru Sakamoto, Mikio Nomurab, Keisaku Ogica National Institute of Advanced Industrial Science and Technology, Shuku, Tosu, Saga 841-0052, Japan

b Isobe Iron Works Co. Ltd., Tokyo, Japanc Kyushu University, Kyushu, Japan

Abstract

A novel wear tester was employed to investigate the comparative abrasive stability of metallic materials at elevated temperatures. Thistester consists of a symmetric -shaped specimen holder that is rotated in a stainless steel tank filled with quartz sand, and an electricalfurnace surrounds this tank to heat the abrasive sand. The test material and its comparative, of a size of 20 mm 20 mm 5 mm, were setin the respective branches of the holder. During the atmospheric condition experiment, only one 20 mm 20 mm face of each specimenwas suffered with abrasion, while the other faces which were mounted in the holder were oxidized. Then the volume loss of the specimenscan be accurately calculated with the aid of additional oxidation experiments, and by choosing the same comparative material the abrasivestability of different materials can be compared. A newly developed high Cr cast iron 4.5C40Cr8Ni9Nb5Mo was employed as the testalloy, while the comparative was the widely used 25Cr cast iron (Fe25Cr2.9C0.5Ni0.5Mo). The results show that, at the temperatureof 925 K, the test alloy exhibited a superior abrasive stability to the 25Cr alloy; the average wear rate of the former was only about 36%of that of the latter. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Abrasion; High chromium cast iron; Elevated temperature

1. Introduction

High Cr cast irons are used in various applications wherestability in a abrasive environment is a principal require-ment, such as power plants, mineral industrial plants, steelmaking plants, etc. and their exceptional wear resistanceresults primarily from their high volume fractions of hardcarbides. It was generally considered that the primarycoarse carbides would cause a drastic drop of toughness,and the use of hypereutectic alloys is very limited. How-ever, the recent studies have revealed that the fracturetoughness of high Cr cast irons with a smaller amountof primary M7C3 type carbides is rather higher than thatof the hypoeutectic alloys [1], and the mechanical prop-erties of the hypereutectic high Cr cast irons can be im-proved by alloy design and structure control techniques[25]. As a study concerning the development of supe-rior abrasion resistant materials, we have made a series offundamental researches on phase diagrams, solidificationphenomenon, hardenability, and mechanical properties ofhypereutectic high Cr cast irons [6]. This paper reports

Corresponding author. Tel.:+81-942-81-3675; fax:+81-942-81-3698.E-mail address: michiru-sakamoto@aist.go.jp (M. Sakamoto).

the progress in abrasion tests of these alloys at elevatedtemperatures.

2. Experimental procedures

2.1. Materials tested

In the present study, the hypoeutectic 25Cr cast iron(Fe25Cr2.9C0.5Ni0.5Mo), widely used in high tem-perature applications, was employed as the standard sam-ple, and a newly developed hypereutectic high Cr castiron (4.5C40Cr8Ni9Nb5Mo) was employed as thetest material. Both specimens were cut directly from theas-fabricated 100 mm 100 mm 25 mm blocks cast insand molds. As a typical presentation of microstructuresshown in Fig. 1, the 25Cr cast iron mainly consists ofthe dendritic primary-phases leaving the eutectic phasesfilled in the arm spaces (Fig. 1a), while in the test alloythe greater primary carbides constitute the main part ofthe structure (Fig. 1b). The compressive strength (fracturestrength) of the test alloy was lower than that of the 25Crstandard alloy at room temperature, but was greater than thelatter at the temperatures over 700 K, as shown in Fig. 2.The hardness curves of both alloys plotted in Fig. 3 show

0043-1648/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S0043-1648(01)00665-2

72 H.-N. Liu et al. / Wear 250 (2001) 7175

Fig. 1. Microstructures of the specimens: (a) 25Cr alloy; (b) test alloy.

that the developed alloy exhibited greater hardness than the25Cr alloy at all test temperatures.

2.2. Abrasion test

A variety of laboratory testers had been used in the studyof the wear behavior of high Cr cast irons at normal at-mospheric temperature as summarized by Tabrett et al. [7],and the reported high temperature wear testers are usuallyin ring-on-disc or pin-on-disc types [8,9]. However, in somecases the high temperature light abrasion would be the pre-vailing type of the wear of the components. One example isthe wear of the boiler impeller of a powder plant, this im-peller acts as a mixer to insure that the coal powder and theheavy oil could be fully mixed with the air and then fullyburned, as shown in the schematic view of Fig. 4. The im-

Fig. 4. Schematic view of the boiler impeller of a power plant.

Fig. 2. Compressive strength of the specimens as a function of testtemperature.

Fig. 3. Hardness of the specimens as a function of test temperature.

peller works at the temperature range from 973 to 1073 Kand is suffered from the abrasion of coal powders. In or-der to study this kind of abrasion, a low stress wear testerwas developed in the present study. This tester consists ofa symmetric -shaped specimen holder that was rotated ina stainless steel tank filled with quartz sand. An electricalfurnace surrounds this tank to heat the abrasive sand, asshown in Figs. 5 and 6. The specimens, each of which was20 mm 20 mm 5 mm in size, were set in the branchesof the holder, leaving one 20 mm 20 mm face to be wornby the abrasive.

H.-N. Liu et al. / Wear 250 (2001) 7175 73

Fig. 5. Schematic view of high temperature abrasion tester: (a) vertical section of the tester; (b) cross-section of the specimen holder.

Fig. 6. Photo of the specimen holder.

The test temperature was 923 K, the rotation speed of thespecimen holder was 200 rpm (the specimens speed wasabout 1.36 m/s), the total weight of the abrasive sand (withmean diameter of 1 mm) filled in the stainless tank was5.3 kg, and the test time for each measurement of the massloss was 20 h. The final results of abrasion test were givenby the volume loss of the specimens as a function of testtime.

3. Results and discussions

3.1. Calculation of the wear rates

The volume loss of the specimens within a certain testtime is

W = Swvwt kSovot (1)whereW, S, v, , and t are the mass loss, surface area,wear or oxidation speed, density, and the test time, respec-tively, k the coefficient (weight gain per oxidized volume),and w and o denote the wear and oxidation process, re-spectively.

The wear rate is

vw = W + kSovotSwt

(2)

Then the comparative wear rate is

vw

vw= (W + kSovot)/Sw

(W + kSovo)/Sw(3)

where () denotes the standard specimen.By this sequence, an accurate calculation can be realized

with the aid of additional oxidation experiments. However,for most of the high Cr cast irons, the weight gain resultedfrom oxidation increase, which occurred rapidly and thengradually approached to a constant [10]. Then, the sequence3 can be simplified as

vw

vw= W/Sw

W /Sw(4)

In present studies, both specimens were in the same size,i.e. Sw = Sw, therefore,vw

vw= W/

W /(5)

Since the relative wear rate(vw/vw) calculated by Eqs. (4)and (5) are time-independent, the wear data obtained fromseparate experiments can be compared.

3.2. Abrasion results and the wear behavior

The additional oxidation experiments performed at 923 Krevealed that, for both alloys, the weight gain resulted fromoxidation were negligible compared with the weight lossby the abrasion, therefore, the influence of oxidation wasignored in the calculations of present studies.

The wear rates of both alloys have an initial transientperiod followed by a steady period, as shown in Fig. 7. Theaverage wear rate of the test alloy was only 36% of that of the

74 H.-N. Liu et al. / Wear 250 (2001) 7175

Fig. 7. Volume loss of the specimens as a function of test time.

25Cr standard alloy, indicating excellent high temperatureabrasive resistant of the former.

The worn surface of the 25Cr alloy, as shown in Fig. 8,was obviously structure-dependent. The primary-phasethat was softer at the test temperature was worn rapidly,leaving the eutectic phase carbide that was harder and wearresistant forming the hills of the worn surface. The microgrooving and deformation were considered to be the domi-nant mechanisms of the wear of the primary-phase, whilethe brittle fracture process was considered to be the processof the wear of eutectic carbides, as a highly amplified SEMimage shows in Fig. 9. The oxidation exerted no obviousinfluence on the wear mechanism of this alloy.

Compared with the 25Cr standard alloy, the worn mor-phology of the test alloy was rather smooth, as shown in aSEM image in Fig. 10, indicating that both primary carbidesand eutectic phases were resistant to the quartz abrasive atthe test temperature. The brittle fracture process was alsoinferred as the dominant wear mechanism of this specimen.The oxidation-resulted expansion of the Nb content carbideswas observed in the worn surface of this specimen (Fig. 10),indicating that the oxidation process had some aggravatingeffect to the wear process of this alloy.

Fig. 8. Worn morphology of 25Cr alloy after abrasion at 923 K.

Fig. 9. An SEM images of worn surface of 25Cr alloy after abrasion at923 K.

Fig. 10. An SEM images of worn surface of the test alloy after abrasionat 923 K.

4. Summary

A novel wear tester was developed to investigate the com-parative abrasive stability of metallic materials at elevatedtemperatures. This tester consists of a symmetric -shapedspecimen holder that is rotated in a stainless steel tank filledwith quartz sand heated by an electric furnace. The specimenand its comparative were set in the branches of the holder.During the experiment performed in atmospheric conditions,only one face of the specimen was suffered with abrasion,while the others were oxidized. Then the volume loss of thespecimens can be accurately calculated with the aid of ad-ditional oxidation experiments. By choosing the same com-parative the abrasive stability of different materials can becompared.

A newly developed hypereutectic high Cr cast iron wasemployed as the test alloy, and the comparative was thewidely used hypoeutectic 25Cr cast alloy. The results showthat, at a temperature of 923 K, the test alloy exhibited asuperior abrasive stability to the 25Cr alloy, where the av-erage wear rate of the former was only about 36% of thatof the latter. Through the analysis of the worn surface, theabrasion mechanism of the developed alloy was inferred asa micro grooving and brittle fracture process.

H.-N. Liu et al. / Wear 250 (2001) 7175 75

Acknowledgements

This work is part of a project on the superior high-temperature wear resistant composite materials financed bythe New Energy and Industrial Technology DevelopmentOrganization of Japan (NEDO). The authors would liketo acknowledge this support as well as the permission topublish the results.

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