The Effect of Cementite Size and Morphology on the Abrasive Wear Behavior of UHC Steel

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Niyazi Ozdemir

Furkan Sarsilmaz

e-mail: fsarsilmaz@firat.edu.tr

Nuri Orhan

Department of Metallurgy, Faculty of Technical Education, University of Fırat, 23119 Elazıg˘, Turkey

The Effect of Cementite Size and

Morphology on the Abrasive Wear

Behavior of UHC Steel

In this study, the abrasive wear behavior of a grain refined hypereutectoid carbon steel containing 1.2% C, 4% Al, 0.20% Mo, and 0.1% Ti was investigated experimentally. Thermal cycles were applied to all specimens about ten times to obtain a fine-grained structure and to gain more softness structures, such as spheroidized cementite for these steels. After every thermal cycle, the microstructures of specimens were examined by scanning electron microscopy to determine the transformation mechanism of structure. Microstructure analyses showed that size of cementite decreased as a function of heat treatment cycle. With the increase of heat treatment cycle, the grain size of cementite in all specimens started to decrease. As a consequence, these cementite islands transformed to spherical cementite having an average grain size of 5␮m. In addition, the wear test results indicated a correspondence between wear rate and thermal cycling. However, the hardness values decreased with increasing heat treatment cycle.

关DOI: 10.1115/1.2842238兴

Keywords: hypereutectoid steel, thermomechanical treatment, abrasive wear

1 Introduction

Ultrahigh carbon 共UHC兲 steels are comprised of a ferrite-carbide or austenite-ferrite-carbide dual phase structure关1兴. As a newly developed and advanced material, UHC steel has a broad potential field of applications as structural tool, mould materials, and so on 关2兴.These steels can provide a fine-grained microstructure by add-ing small amounts of V, Nb, Ti, Cr, B, Mo, and Al关1,2兴. However, it has been reported that a fine ferrite grain structure with a uni-form distribution of spheroidized cementite Fe3C, i.e., a共ferrite

+ cementite兲 microduplex structure, can be obtained in UHC steels through adequate thermal processing关2,3兴. Carbides and nitrides of these elements not only prevent austenite grain size growth during heat treatments even in subaustenite regions but also affect ferrite nucleation and produce a fine-grained microstructure. In addition, smaller austenite grains easily transform to pearlite. Martensite formation is rarely achieved in slowly cooling关3–6兴. When UHC steels are slowly cooled from the austenite region, the final microstructure consists of a widespread pearlite network in a ferrite matrix. This carbide network deteriorates mechanical prop-erties of steel 关5,6兴. For achieving very fine-grained structure, thermomechanical processes are required 关7,8兴. This comprises two steps: In the first step, the material is rapidly cooled from 1150° C to 730° C then hot and warm deformed. This process produces fine austenite grains and distributes carbides in subgrain boundaries and inside the austenite uniformly. The second step is to quench the material from A1 共730°C兲 temperature. This step

produces fine and spherical cementite distributed in the matrix of ferrite. If this step is repeated 8 to 12 times, ferrite will separate because of volume change, and consequently cementite in the structure will be spheroidized and distributed more homoge-neously关9兴.

UHC steels or hypereutectoid carbon steels are the most widely reported in the literature but the role of grain size and morphology effects of several thermomechanical processes on the wear behav-ior have not been studied in enough detail. UHC steels having a

microduplex structure might be applied to the construction indus-try in the near future: therefore, in this study the effect of the change of grain size on the wear behavior of fine-grained hyper-eutectoid steel was investigated by observing the grain size at every step of the thermomechanical processes.

2 Experimental Procedure

2.1 Materials. In this study, the given chemical composition

of UHC steel in Table 1 was used as a test material. UHC steel was manufactured by means of an induction casting technique with addition of different alloying elements共Ti, Mo, Mn, Ni, and Al兲. The specimens were melted and cast into bars about 60 mm in diameter and 400 mm in length. The eutectoid transformation point of the materials was determined by differential thermal analysis 共DTA兲. Then they were homogenized at 1100°C for 60 min and cooled to room temperature. Homogenized specimens were hot forged at共A3兲 830°C with a maximum 94% reduction in

area. Hot forged specimens were tempered for 30 min at 730° C in the two phase field, which is slightly over the A1 temperature

determined by DTA for this steel and then quenched into oil to produce a spherical cementite structure. This heat treatment cycle was repeated up to ten times to produce grain refinement.

2.2 Abrasive Wear Tests. The abrasive wear tests were

per-formed using a pin-on-disk type apparatus共see Fig. 1兲. Before the wear tests, each specimen was finished up to 1200 grade SiC abrasive paper 关10兴, making sure that the wear surface was in complete contact with the surface of the abrasive paper. The cross-sectional dimensions of pins were cylindrical in shape with pol-ished flat ends of 12 mm diameter. All the wear tests were per-formed against 80 grade SiC abrasive paper under loads of 10 N and 20 N关11兴. The arm that held the specimen was traversed at 0.02 m/min constant traverse speed in a radial direction such that the specimen followed a spiral path across the surface of the ro-tating disk. The minimum and maximum radii of spiral path were kept at 15 mm and 80 mm. The disk was rotated about 21 rpm, which corresponds to 20.1 m sliding distance for each test. Mass loss was measured by using an electronic balance with a reso-lution of 0.01 mg. Two specimens were tested under each load. For determination of the wear behavior of this steel, the worn

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF

TRIBOLOGY. Manuscript received September 8, 2006; final manuscript received December 12, 2007; published online March 13, 2008. Assoc. Editor: Thierry Blanchet.

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surfaces were examined by scanning electron microscope共SEM兲. The wear rate was calculated by the following formula:

Wa=

⌬G

dMS 共mm

3/Nm兲

where Wais the wear rate,⌬G the mass loss, M the applied load,

d the density of material, and S the sliding distance, respectively 关11–13兴.

3 Results and Discussion

3.1 Microstructure and Hardness. Figure 2 shows the

opti-cal micrograph of S1 hypereutectoid steel after casting. The figure clearly reveals the presence of pearlite along with a small quantity of ferrite network around the pearlite colonies near the grain boundary cementite. This could be attributed to the 1.2% carbon equivalent of the steel. In Figs. 3共a兲–3共e兲, the microstructures after thermal cycle of Specimens S2, S3, S4, S5, and S6 are given, respectively. Microstructural observation on these specimens shows the existence of lamellar cementite and spherical cementite in ferrite matrix. Dissolution rate of lamellar cementites increased with the increasing thermal cycles. As a consequence, cementite size decreased in all specimens gradually. Moreover, the space between lamellar islands was increased. Measured values of spherical cementite size by image analyses program are given as a function of heat treatment cycle共Table 2兲. It may be noted from Table 2 that the size and morphology of cementite changed with Table 1 The chemical compositions of the hypereutectoid steel

Chemical compositions wt %

C Si Mn P S Cr Ni Al Mo Ti

1.18 0.15 0.70 0.02 0.03 0.02 0.02 4.0 0.20 0.10

Fig. 1 Illustrations of pin-on-disk abrasive wear apparatus

Fig. 2 The microstructure of specimen prior to thermal cycle

Fig. 3 SEM micrographs of microstructure after thermal cycle: „a… second, „b… fourth, „c… sixth, „d… eight, and „e… tenth

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increasing heat treatment cycle.

Cementites in S2 sample can be shown as equaxial lamellas packed one after the other. The average cementite grain size is lower than 5␮m 共Fig. 3共b兲兲. After the fourth thermal cycle, de-composed cementites in the S3 sample can be easily seen and in relation both grain size and distance between grains are decreased. In Fig. 3共e兲, it can be clearly seen that lamellar cementites changed to spherical cementite in ferritic matrix, and by the end of the tenth thermal cycle, the average grain size of cementite reached minimum size. In the end of every thermal cycle, type of cementites and size variations was measured and given in Table 2. After the tenth thermal cycle, grain size was determined to be about 5␮m. When hypereutectoid steels are quenched from the A1temperature, this process produces fine and spherical cementite in a ferrite matrix. If this step is repeated 8 to 12 times, ferrite will be separated because of volume change, and consequently ce-mentite in the structure will be spheroidized and distributed more homogeneously关12兴.

Figure 4 shows the hardness measurements of samples plotted as a function of heat treatment cycle. With the increase of heat treatment cycle, the hardness values of all samples started to de-crease because of the formation of ductile phases in microstruc-tures.

3.2 Abrasive Wear Behavior. In order to effectively discuss

the wear behavior of the samples, it would be worth analyzing the phenomena of high-stress abrasion and the factors controlling the operating material removal mechanism关12兴. During the abrasion test, the specimens are subjected to a relative motion against the fixed abrasive particles under load. As a result, the specimen sur-faces experience normal as well as shear stresses. The normal stress helps to penetrate the abrasive into the specimen surfaces and the shear stress facilitates scratching action of the abrasive. Under the combined actions of normal and shear stresses, the abrasive removes material from the specimen surface by cutting and ploughing action.

Figure 5 shows variation of wear rate for grain refined

hyper-eutectoid steel specimens at two different loads and constant slid-ing distance. It can be seen that S1 exhibited lower wear rate due to having lamellar pearlite microstructure. However, in S6, the transformation of lamellar pearlite to spherical cementite led to gradually increasing wear rate. The lamellar pearlitic structure shows less wear rate than the other structures 关13兴. This lower wear rate in Specimen S1 is directly related to the great amount of lamellar pearlite in the microstructure. Lamellar cementites pro-vide more effective work hardening than spherical cementites in the matrix of ferrite. It is well known from other studies that the rate of work hardening decreases in succession from pearlite to spheroidized cementite关13兴. It is suggested that the energy con-sumption of pearlitic structure may be greater than those of mar-tensitic structure and spheroidized carbide structure, due to the large plastic deformation of the ferrite matrix and the cementite lamina fractures in pearlitic structure. The energy consumption in the steel with spheroidized cementite is small because the defor-mation of the ferrite matrix can occur with little or no defordefor-mation of cementite 关13兴. The wear rate of Specimens S1 and S2 with lamellar structure are also small. Specimens S3, S4, S5, and S6 with spheroidized structure show a larger wear rate. This higher mass loss in these specimens is associated with higher sphe-roidized cementite content and cementite size in the matrix of ferrite.

The existence of spherical cementite in the microstructure in-duces higher deformability or superplastic properties关14兴. Abra-sive wear behavior, which is a function of deformation behavior of a material, can be considered to depend on a material’s micro-structure 关15兴. The highest mass loss occurred in Specimen S6, which has better deformation properties. Lamellar cementite af-fects the whole grain’s wear behavior. On the other hand, spheri-cal cementite affects only ferrites transforming into cementite spheroids.

From the results of the microstructural observation and wear test, it was seen that there was a relationship between both amount and size of spherical cementites on abrasive wear rate of the fine-Table 2 The changes of cementite size and morphology in ferrite matrix after thermal cycling

progress

Sample No. Thermal Cycling Progress Microsutructure Cementite size and formation

S1 After casting Pearlite+ ferrite+ cementite lamellar

S2 Second Cementite+ ferrite lamellar

S3 Fourth Cementite in matrix of ferrite ⬎10␮m

S4 Sixth Cementite in matrix of ferrite ⬃10m

S5 Eighth Cementite in matrix of ferrite ⬍10m

S6 Tenth Cementite in matrix of ferrite ⬍5␮m

Fig. 4 Hardness result of all samples as a function of heat treatment cycle

Fig. 5 Abrasive wear behaviors of specimens under applied load of 10– 20 N as a function of heat treatment cycle

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grained specimens. Among the grain refined specimens, the lowest wear rate has been seen in the specimens that have the most ho-mogeneously dispersed spherical cementites共see Fig. 3共e兲兲. How-ever, when a comparison was made in wear rate between a fine-grained specimen and a pearlitic specimen, it was seen that the wear rate of the fine-grained specimen was higher than those of pearlitic specimens. This was directly related to the different pearlitic structure with spheroidized cementites. Lamina-formed cementites in microstructure have shown less wear rate than spherical-formed cementites, because the ferrite phases between cementite laminas absorb the energy intensity.

Examinations of microstructure near the worn surface and wear debris are very useful to understand the wear behavior of the abrasive wear system. SEM micrographs of worn surface of speci-mens are presented in Figs. 6共a兲–6共f兲, respectively. From the SEM photos, it was seen that severe plastic deformation on the worn surfaces increased with thermal cycling. Severe wear produced higher amount of deformation and fragmentation in contacting surface, hence deep grooves occurred on the surface 共Figs. 6共c兲–6共e兲兲. These grooves in the S2 were about 120␮m deep, while the S6 grooves were about 160␮m deep.

According to the above results, increasing thermal cycling led to transformations in pearlite with decreasing cementite size and morphology in the matrix of ferrite. This also results in higher wear rate and decreased hardness with decreasing cementite size and morphology.

4 Conclusions

From the investigation of the abrasion wear behavior of a grain refined UHC steel, the following conclusions can be made:

1. A desired microstructure in the specimens was obtained by heat treatment cycle. The microstructure of specimens changed from pearlite to grain boundary cementite and spherical cementite in a matrix of ferrite with increased heat treatment cycles. The cementite size in the ferrite matrix was smaller with increasing heat treatment cycles.

2. The lowest hardness and highest wear rate were achieved in Specimen 6. The differences of wear rate for various micro-structures depend on their grain morphology with respect to level of thermal cycle.

3. The abrasive wear behavior of UHC steel was basically af-fected by grain size and morphology of cementite obtained from heat treatment cycling. Therefore, a correspondence always exists between the wear rate and thermal cycle.

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