January 28, 2003 -- Correlation of microstructure with hardness and wear resistance of VC/Steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

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Correlation of microstructure with hardness and wear resistance of VC/carbon steel surface-alloyed materials fabricated by high- energy electron-beam irradiation was investigated. The mixtures of VC powders and flux (50 pct MgO-50 pct CaO or CaF^sub 2^) were deposited on a plain carbon steel substrate, and subsequently irradiated using a high-energy electron beam. The surface-alloyed layers of 1.2 to 3 mm in thickness were homogeneously formed without defects, and contained a large amount (about 10 vol pct) of VC precipitates in the bainitic or martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface-alloyed layers improved hardness and wear resistance. Particularly in the surface- alloyed material fabricated with the lower input energy density, the wear resistance was greatly enhanced over the steel substrate because of the increased size and volume fraction of VC particles, although the thickness of the surface-alloyed layer decreased. Microstructural modifications including melting, solidification, precipitation, and phase transformation of the surface-alloyed layer were also predicted from a thermal transfer modeling and a Fe-V-C ternary phase diagram. The predicted results were found consistent with those data from actual electron-beam irradiation and microstructural analysis.

I. INTRODUCTION

THOUGH steels have been widely used as basic structural materials in all industries due to their excellent ductility as well as high strength, they are increasingly exposed to severe industrial working environments. Since such an exposure is mostly limited to their surface region, efforts to enhance their surface properties, or to strengthen or coat the surface, have been made, some of which have already been commercialized or are near commercialization.

Transition metals usually form carbides such as TiC and VC in steels, when combined with carbon, and these carbides can be used as reinforcing or coating materials because they have very high hardness and excellent wear resistance.[1] Particularly, steel- based surface-alloyed materials reinforced with VC can be applied to structures or parts requiring hardness, wear resistance, and high- temperature properties, and their microstructures can be modified by subsequent heat treatments.[2] Fabrication methods for steel-based surface-- alloyed materials reinforced with VC include coating methods using laser evaporation[3,4] and ion implantation,[5] surface alloying methods using laser beam,[6,7] and in-situ alloying methods.[8] However, the coated materials have poor bonding strength between coated layer and substrate, while the surface-alloyed ones using laser beam have shortcomings of thin surface-hardened layer and obvious overlapping phenomenon. The in-situ alloying methods require the existence of liquid phases, and the coated layer should be made prior to casting.

Recently, active research has been undertaken on surface-- alloyed materials or surface composites in which advantages of ceramics such as excellent resistance to heat, corrosion, and wear are fully taken by direct irradiation of high-energy beam such as pulsed-laser beam and electron beam. In particular, a new fabrication method under development is the use of an irradiating high-energy accelerated electron beam directed toward the substrate materials in air.19,10,11 When an electron beam with high energy (several MeV) is irradiated on the substrate surface, incident electrons having high kinetic energy collide with electrons inside the substrate, and transfer their energy to excitation energy and kinetic energy of secondary electrons.

Since the excited electrons subsequently thermalize and transfer their excitation energy to the lattice by electron-- phonon collisions, ceramics having high melting point can be easily melted by this thermal energy. [12,13] Upon electronbeam irradiation of the metal surface, where ceramic powders are evenly deposited, the material surface and ceramic powders are either partially or completely melted, and ceramic elements are dispersed into the metal. In the melted layer, carbides, borides, and nitrides are precipitated during solidification, thereby fabricating ceramic/ metal surface-- alloyed materials or surface composites. This high- energy electron-beam irradiation has several advantages: (1) a strong interface between the surface-alloyed layer and the substrate, (2) little influence on substrate properties because of short irradiation time, and (3) homogeneous heating and cooling.[9,10] Compared with the laser beam method, this method has two times higher thermal efficiency, produces thicker surface- alloyed layer, and enables continuous process in the air. [14]

In the present study, VC/steel surface-alloyed materials were fabricated by evenly depositing VC powders on a plain carbon steel substrate and irradiating high-energy electron beam. For the formation of the surface-alloyed layer through surface melting and alloying processes, an inorganic flux has to be used to protect melted VC powders and substrate from the air and to promote homogeneous melting.[15] By varying electron-beam irradiation conditions and the kind of flux, three different specimens were fabricated, and mechanisms for surface alloying and surface property enhancement were elucidated by investigating microstructure, hardness, and wear resistance and the correlation between them. Since an appropriate input energy is needed to optimize the microstructure of the surface-alloyed layer, the maximum heating temperature, cooling rate, and temperature distribution of the surface-alloyed layer were predicted by thermal modeling analysis.

II. EXPERIMENTAL

A. Alloying Powders and Substrate

Ceramic powders used for the fabrication of surface-- alloyed materials were VC powders with high hardness, excellent resistance to wear and heat, high thermal conductivity, and high melting point, which were obtained from CERAC, Inc. (Milwaukee, WI).[16] The MgO, CaO, and CaF2 powders were used as flux after drying them at 200 deg C for 1 hour, and their physical properties and sizes are shown in Table I. A plain carbon steel plate (thickness: 20 mm) was used as a substrate, and its chemical composition is Fe-0.15C-1.06Mn-0.44Si- 0.003S-0.01P-0.01Ni-0.02Cr-- 0.002Mo-0.04A1 (wt pct).

B. Fabrication of Surface-Alloyed Materials by High-- Energy Electron-Beam Irradiation

Process parameters that should be taken into consideration when fabricating surface-alloyed materials by electron-beam irradiation include material parameters of the substrate, content and kind of flux, input energy, specimen traveling speed, beam size, and so on. In this study, optimal process parameters were established by changing the flux and varying the input energy (or beam current). First, two VC/flux mixtures were fabricated, one by mixing VC powders with 20 wt pct of CaO-MgO flux powders (CaO:MgO = 1:1 (wt pct)), while the other with 50 wt pct of CaF^sub 2^ flux. The substrate surface was polished and rinsed with acetone, on which VC/ flux mixtures were evenly deposited at about 0.18 g/cm^sup 2^ density. After pressing the mixtures with a 600 N load, the high- energy electron beam was used to irradiate the surface. For convenience, the 20 wt pct CaO-MgO flux-mixed specimens fabricated under the irradiation conditions of beam current of 26 and 24 mA are referred to as "A1" and "A2," respectively, while the 50 wt pct CaF^sub 2^ flux-mixed specimen fabricated under beam current of 24 mA is referred to as "B." shows a schematic diagram for the fabrication process of surface-alloyed materials.

Table I.

A high-voltage electron accelerator (model: ELV-6) at the Budker Institute of Nuclear Physics (Novosibirsk, Russia) was used for irradiation.[17] This accelerator has energy ranges from 0.5 to 1.5 MeV and maximum power of 100 kW. In this study, other process parameters except beam current were fixed at the optimal conditions established by the Budker Institute of Nuclear Physics (electron energy: 1.4 MeV, specimen moving speed: 10 mm/s, scanning width: 50 mm, and beam diameter: 12 mm). Three specimens, indicated as A1, A2, and B, were produced with different flux/process history combination, which was optimized by the previous works,[10,11] as summarized in Table II. The energy that is input into the material surface per unit area, i.e., the input energy density (W), is calculated in terms of specimen moving speed (v), scanning width (1), beam voltage (V), and beam current (I) as follows:[14]

C. Microstructural Analysis and Wear Resistance Test

The surface-alloyed specimens were sectioned parallel to the irradiation direction, polished, and etched by a nital solution or a Murakami etchant (3 g K^sub 3^Fe(CN)^sub 6^ + 10 g NaOH + 100 mL H^sub 2^O)[19] in which carbides are selectively etched but not the matrix. Their microstructure was observed by an optical microscope, a scanning electron microscope (SEM), and a transmission electron microscope (TEM). Phases present in the surface-alloyed layer were analyzed by X-ray diffraction and energy dispersive spectroscopy (EDS), and their volume fractions were measured by an image analyzer. Hardness was measur\ed from the surface down to the substrate by a Vickers hardness tester under a 500-g load, and microhardness of the matrix was measured under a 50-g load.

Abrasive wear resistance tests were conducted by a dry sand/ rubber wheel abrasion wear test method, in accordance with ASTM G65- 85 specifications.[20] Wear test specimens were machined into a size of 25 X 75 X 15 mm. They were worn in contact with sands (average diameter: 0.2 mm) between rubber-lined wheels under a testing load of 20 kgf, and the weight loss and specific wear rate [(wear volume)/ (load X distance)] were evaluated as resistance to abrasive wear. Wear testing was performed at room temperature for 5 minutes without using a lubricant, and the total wear distance was 377 in. After the wear test, a worn surface and cross section of each specimen were observed by an SEM.

III. RESULTS

A. Microstructure

Figures 2(a) through (c) are low-magnification optical micrographs of the VC/steel surface-alloyed specimens. The surface- alloyed layer is formed when the powder mixture and part of the carbon steel substrate surface are melted and solidified, and carbides are precipitated in the layer. The specimens show evenly thick surface-alloyed layers without pores. Though there are some variations in the thickness of the layer depending on the location, it decreases in the order of the A1, A2, and B specimens.

Figures 3(a) through (c) are optical micrographs of the Al specimen showing microstructural modifications according to the depth from the surface. The surface-alloyed materials are roughly divided into four regions: (1) The surface-alloyed (i.e., melted) layer, (2) the interfacial region, (3) the heat-affected zone (HAZ), and (4) the unaltered substrate. As shown in Figure 3(a), the matrix of the surface-- alloyed layer is composed of bainite, the solidification cell structure is visible, and carbides are relatively homogeneously dispersed throughout the layer. In the surface-- alloyed layer close to the interfacial region, dendritic growth is observed as in the interface of welded regions. Coarsegrained HAZ grown in a Widmanstatten pattern exists near the interfacial region (Figure 3(b)). Beneath this HAZ lies the fine- grained HAZ, in which decomposed pearlite is observed (Figure 3(c)). An SEM micrograph of the surface-alloyed layer of the A1 specimen is presented in Figure 4(a) and shows a bainitic structure where fine carbides are dispersed in the ferritic matrix. A TEM micrograph and a selected area diffraction pattern of fine cuboidal carbides are provided in Figure 4(b). As the diffraction pattern indicates a (001) plane of a face-centered cubic (fcc) structure, these carbides are identified to be VC. Cementite is also formed in a columnar shape, as indicated by arrows in Figure 4(b). Consequently, carbides present in the surface-alloyed layer of the A1 specimen are either VC or vanadium-containing cementites.

Optical micrographs of the surface-alloyed layers of the A2 and B specimens are shown in Figures 5(a) and (b). Solidification cells are clearly visible, and plate-type martensite is observed as a matrix of the surface-alloyed layer. Figure 6(a) is an SEM micrograph of the A2 specimen. Here, eutectic carbides are formed in solidification cell boundaries, and a few cuboidal carbides are observed. In Figure 6(a), the cuboidal carbide, eutectic carbide, and matrix are marked as "1", "2", and "3", respectively, and their EDS spectra are presented in Figure 6(b). The cuboidal carbide and eutectic carbide are identified to be VC particles. A large amount of Fe detection is attributed to the matrix near the carbides because the spatial resolution of the SEM is over 1 (mu)m. The EDS analyses data of the B specimen are about the same as those of the A2 specimen. Figures 7(a) and (b) provide a TEM micrograph and an EDS spectrum of fine carbides formed in the A2 specimen, according to which they are confirmed to be VC.

Table III summarizes the results of quantitative analysis of the three surface-alloyed specimens, listing the thickness of the surface-alloyed layer, size and volume fraction of VC carbides, solidification cell size, and carbon content in the matrix of the layer, which was measured by EDS. Actual carbon content in the matrix can be lower than the measured value since fine carbides are included in the matrix. The A1 specimen shows the thickest layer of 2.9 mm, followed by the A2 and B specimens. This indicates that the thickness of the surface-alloyed layer increases when beam current increases or MgO-CaO flux is used. The volume fraction and size of VC carbides in the A2 and B specimens are about the same, ranging from 10 to 11 pct and 1.4 to 1.5(mu)m, respectively. The solidification cell size of the B specimen is smaller than that of the A2 specimen. The VC carbides present in the A1 specimen are finer, sized about 0.3 (mu)m, and their volume fraction is lower than those of the A2 and B specimens. The carbon content in the matrix shows the highest in the B specimen, followed by the A2 and A1 specimens.

The VC carbide has a B1 (NaCl) structure, where C atoms occupy octahedral interstices of the V lattice (fcc structure) to form a new fcc structure. Also, they have a VC^sub 1-x^ type because vacancies are formed at interstitial sites due to nonstoichiometry. Since the lattice constant decreases with an increasing amount of vacancies, the relationship between CN ratio and lattice constant exists, as shown in Figure 8.[21] The C/V ratio of VC ranges from 0.59 to 0.92, and free carbon is present at the ratio above 0.88.[21,22] Based on the X-ray diffraction analysis data of the A2 and B specimens, lattice constants can be approximated, from which the C/V ratio can be roughly obtained.[23] Because the lattice constants of the A2 and B specimens lie in 4.14 to 4.16 Angstrom and the CN ratios lie in 0.77 to 0.85, the VC formed in the surface- alloyed layer exists in types of VC^sub 0.77^ to VC^sub 0.85^. This is because vacancies take the interstitial sites of C atoms as V and C atoms are not completely diffused during solidification.

B. Hardness

Vickers hardness was measured from the surface down to the substrate, and the results are shown in Figures 9(a) through (c). Overall bulk hardness and microhardness of matrix are also shown in Table IV. The bars associated with each hardness data point represent the standard deviation of each hardness measurement. All three specimens show a similar hardness distribution. The maximum hardness of the Al specimen is about 450 VHN, an increase of 3 times greater than that of the carbon steel substrate (150 VHN) (Figure 9(a)). The hardness of the surface-alloyed layer is maintained at 430 to 480 VHN, abruptly drops in the interfacial region, and reaches down to about 150 VHN in the HAZ and substrate. Though the A2 and B specimens show similar trends to the At specimen, their surface-alloyed layers show higher hardness than the At specimen, ranging from 630 to 660 VHN in the A2 specimen and 660 to 670 VHN in the B specimen (Figures 9(b) and (c)). This is because the volume fraction and size of VC formed in their surface-alloyed layers are higher than those of the At specimen. Microhardness of the martensitic matrix of the surface-- alloyed layers in the A2 and B specimens is higher than that of the bainitic matrix in the At specimen or the substrate (Table IV). This also contributes to the hardness enhancement.

C. Wear Resistance

Table IV also lists the wear loss and specific wear rate of the A2 and B specimens and the substrate. The two specimens have lower wear rate than the substrate. Particularly, the B specimen shows excellent wear resistance, with far lower wear rate than the substrate by as much as 2.5 times. These data indicate that wear resistance is enhanced as the size and volume fraction of carbides and the hardness in the surface-alloyed layer increase. In comparison with the A2 specimen, the B specimen has better wear resistance because the cell size of the surface-alloyed layer is smaller and the overall hardness of the layer and the matrix hardness are higher.

Figures 10(a) through (d) are SEM micrographs of the worn surface and cross-sectional area of the A2 and B specimens. The worn surface in both specimens shows a typical abrasive wear mode with scratches formed along the wear direction (Figures 10(a) and (b)). Because the B specimen has smaller solidification cells and more homogeneous microstructure than the A2 specimen, its surface shows mild scratches evenly distributed throughout the surface. Closer examination of the cross-sectional area reveals that eutectic carbides near the worn surface are cracked (arrow marked in Figures 10(c) and (d)). The A2 specimen has more cracked carbides than the B specimen, thereby showing rougher surface. In the B specimen, severe shear deformation occurs near the worn surface. Its worn surface is in good condition because the matrix and carbides are simultaneously worn due to the high hardness of the matrix (Figure 10(d)).

IV. DISCUSSION

A. Thermal Analysis of Electron-Beam Irradiation

V. CONCLUSIONS

The VC/steel surface-alloyed materials were fabricated using high- energy electron-beam irradiation, and their microstructure, hardness, and wear resistance were investigated to reach following conclusions.

1. When powder mixtures of VC and flux were deposited on the surface of a carbon steel substrate and irradiated by high-energy electron beam, 1.2- to 3-mm thick, defectfree surface-alloyed layers were formed as the thermal energy of the electron beam was well transferred into the VC/flux mixture.

2. The microstructure of the surface-alloyed materials was composed of a surface-alloyed layer, an interfacial region, a heat- affected zone, and the unaltered substrate. Fine VC particles were evenly distributed in the bainitic ma\trix of the surface-alloyed layer. When the input energy density of the electron beam was reduced, the thickness of the surface-alloyed layer decreased because the amount of melted substrate was accordingly reduced. However, the size and volume fraction of VC particles formed in the surface-alloyed layer were greatly increased, and solidification cells composed of eutectic VC particles were well developed.

3. The formation of VC particles as well as the hardened martensitic or bainitic matrix tripled the hardness of the surface- alloyed layer, while doubling the wear resistance, over the substrate. Hardness and wear resistance increased as the layer thickness decreased with increasing VC volume fraction.

4. The microstructural modification process of the surface-- alloyed layer upon electron-beam irradiation was predicted from the thermal transfer model and Fe-V-C ternary phase diagram. The predicted results were found consistent with those data from actual electron-beam irradiation test and microstructural analysis.

ACKNOWLEDGMENTS

The authors thank Professors Yangmo Koo and Nack J. Kim, POSTECH, and Mr. M. Golkovski and Dr. N. Kuksanov, Budker Institute of Nuclear Physics, for their helpful discussion on the fabrication of the surface-alloyed materials.

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KWANGJUN EUH and SUNGHAK LEE

KWANGJUN EUH, Senior Researcher, is with the Department of Materials Technology, Korea Institute of Machinery and Materials, Changwon, 641-010, Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea, is jointly appointed with the Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: shlee@postech.ac.kr