FeCrBSi composite coating prepared by gas tungsten arc welding

Wear 260 (2006) 25–29

Microstructure and wear properties of in situ TiC/FeCrBSi composite coating prepared by gas tungsten arc welding Wang Xinhong∗ , Zou Zengda, Song Sili, Qu Shiyao School of Material Science and Engineering, Shandong University, Jinan 250061, PR China Received 19 August 2004; received in revised form 25 November 2004; accepted 7 January 2005 Available online 5 February 2005

Abstract Using a gas tungsten arc welding (GTAW) process, in situ synthesis TiC particles reinforced Fe-based alloy composite coating has been produced by preplaced FeCrBSi alloy, graphite and ferrotitanium powders. The microstructure and wear properties of the composite coatings were studied by means of scanning electron microscopy (SEM), X-ray diffractometer (XRD) and wear test. The effects of thickness of the pre-placed powder layer on the microstructure, hardness and wear resistance of the composite coatings were also investigated. The results indicated that TiC particles were produced by direct metallurgical reaction between ferrotitanium and graphite during the GTAW process. TiC particles with sizes in the range of 3–5 ␮m were dispersed in the matrix. The volume fraction of TiC particles and microhardness gradually increased from the bottom to the top of the composite coatings. The TiC-reinforced composite coatings enhance the hardness and wear resistance. The highest wear resistance of the composite coating with a 1.2 mm layer was obtained. © 2005 Elsevier B.V. All rights reserved. Keywords: TiC particles; Wear properties; In situ synthesis; Gas tungsten arc

1. Introduction Ceramic particles of TiC have high hardness and thermal stability and can be used to reinforce Fe-based composites. These composites find extensive applications in tools, dies, and wear as well as high temperature oxidation resistance components. TiC particles reinforced iron-based composites are produced mainly by powder metallurgy routes involving the addition of TiC powders to iron alloy powders [1,2]. The advantage of powder metallurgy is that it has good surface quality and precision of products. However, in this process, homogeneous mixing of TiC and iron-based powders is difficult, and powder surfaces can be easily contaminated during mixing. In recent years, in-situ synthesis of TiC particles reinforced metal matrix composites, which were produced by using liquid Fe–Ti–C alloys, liquid phase sintering, self-sustaining high temperature synthesis and surface ∗

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0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.007

alloy using laser beam, has received much interest worldwide [3–9]. Their eminent advantage is that they eliminate interface incompatibility of matrices with reinforcements by creating more thermo-dynamically stable reinforcements based on their nucleation and growth from the parent matrix phase. Recently, in situ synthesis of TiC-reinforced metal matrix surface composite materials were reported. TiC particles were successfully synthesized in the coatings by laser melting [10–12]. It is desirable that the surface layer of components is reinforced by TiC particles to offer high wear resistance to them whilst they retain the high toughness and strength. However, problems always persist owing to differences in the laser beam absorption rates of different cladded powders [13]. Furthermore, comparing complex components using the GTAW method is expensive and difficult. Tungsten inert gas heat source has a potential to be used for surface modification [14,15]. However, a limited application of this process is updated in the literature. In the present study, a simple process is suggested to process TiC-reinforced Fe-based surface composites by evenly

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Table 1 Chemical composition (wt.%) of the specimen and powders for surface alloying Material

Chemical composition

1045 Steel Ferrotitanium Fe-based alloy

0.45 C, 0.25 Si, 0.66 Mn, balance Fe 41.5 Ti, 0.08 C, 0.035 P, 0.025 S, balance Fe 0.1–0.2 C, 8.0 Ni, 18 Cr, 2.5 B, 3.0 Si, 1.0 Mo, 1.0 Mn, 1.0 V, balance Fe

depositing Fe-based alloy powders, graphite and ferrotitanium powders on an AISI 1045 steel substrate and gas tungsten arc welding (GTAW) process.

2. Materials and experimental procedures A powder mixture Fe-based self-fluxing alloy FeCrBSi, Ferrotitanium alloy and crystalline graphite (99.5% purity) was used as the coating material. The substrate material with the dimension of 100 mm × 25 mm × 10 mm was AISI 1045 steel in a quenched and tempered condition. The surfaces of the samples were thoroughly cleaned, dried and finally rinsed by acetone. The main chemical composition of Fe-based alloy, ferrotitanium alloy and substrate steel are listed in Table 1. The average size of the ferrotitanium and graphite particles was less than 10 ␮m. The particles of Fe-based alloy powder have an average size of 20 ␮m and appear to be spherical. The ferrotitanium and graphite powders were combined in the desired molar ratio (TiFe:C = 1:1). The weight ratio ωFeTi+C /ωFeCrBSi was 6:4. In order to obtain homogeneous distribution, the combined powders attrition-milled for 1 h using agate ball mill with an agate container and balls operated at 300 rpm. The milled mixture with a thickness of 0.5–2.0 mm was preplaced on the surface of the substrate. Cladding was conducted using the GTAW process, which is presented in Fig. 1. A tungsten electrode of 2-mm diameter was used to produce a stable arc. The arc was controlled by the supply of current and voltage to the electrode. Table 2 lists the parameters of the cladding process used in this work. Pure

Table 2 GTAW process parameters Welding current (A) Welding voltage (V) Welding speed (mm/min) Electrode Arc gap (mm) Electrode polarity

150 15–17 55 W–2% thorium 2 DCSP

argon was used as a shielding gas, and the flow rate of the argon is 8 l/min. After cladding process, samples were cut from the alloyed specimens for microstructural examination and hardness measurement. The samples were prepared for metallographic examination by grinding on SiC wheels followed by polishing and etched with a solution of alcohol and 2% nitric acid. Conventional characterization techniques such as optical microscopy, scanning electron microscopy (SEM), electron microprobe micro-analysis (EPMA) and X-ray diffraction were employed for studying the microstructure and elemental analysis of the cladding coatings. The micro-image analyzer (Model: XQF-2000, China) with microprocessor was used to determine the TiC volume fraction and particles size in the cladding coatings. The block-on-ring wear testing was carried out without lubrication at room temperature using a friction and wear-testing machine (Model: MM-200, China). The test specimens were machined to block with size of 10 mm × 10 mm × 30 mm. The ring material of the wear couple was a hardmetal containing 92 wt.% WC and 8 wt.% Co. The wear conditions were 49–196 N normal load, 0.84 m s−1 sliding speed and 252 m sliding distance. The average width of the wear trail was measured with the help of a toolmicroscope, and the wear volume was calculated using the following formula [16,17]: 1/2

V = B{r 2 sin−1 (b/2r) − b/2(r 2 − b2 /4)

}

= Bb3 /12r (mm3 ) where, B is width of wear ring (mm); b, width of wear trail (mm) and r, out radius of wear ring (mm).

3. Results and discussion 3.1. Microstructure of coating

Fig. 1. Schematic representation of the weld cladding.

When the preplaced thickness of the mixture powder is less than 1.2 mm, the melt tracks were found to be free from gas porosity, inclusions and crack, as shown in Fig. 2(a). Melted tracks gave a smooth rippled surface topography. However, when the thickness of the preplaced thickness is beyond 2.0 mm, the formation of the tracks became poor, gas porosity and incomplete fusion can be found, as shown in Fig. 2(b).

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Fig. 2. Melted track produced on the surface of specimen with a preplaced thickness: (a) 1.5 mm and (b) 2.0 mm.

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Fig. 5. Concentration of Ti, Cr and Ni in the GTAW composite coating with a 1.2 mm preplaced coating.

TiC can be synthesized during GTAW process. In the ternary Fe–Ti–C system, the possible reaction and the companying free energy changes are as the following [18]: FeTi + C = TiC + Fe G◦

Fig. 3. XRD spectra of the GTAW surface composite coating.

The X-ray diffraction pattern of the surface composite coating is shown in Fig. 3. It shows that the phases of the composite coating is mainly TiC, ␣-Fe, ␥-Fe and intermetallic compounds. The compounds are mainly lamellar (Fe, Cr)7 C3 and FeB, forming interdendritically during the late stages of solidification. It clearly confirms that TiC particulates can be synthesized by direct reaction between ferrotitanium and graphite. Fig. 4 shows the EPMA face scanning of the elements in the top surface of the cladding coating. It also confirms that

= −186 606 + 13.22T J mol−1

It can be seen that the Gibbs free energy for the formation of TiC is always negative. For Fe–Ti–C system, Yan et al. [19] reported that in situ TiC took place only in the heated to 1373 K. In the GTAW welding pool, the temperature is far higher than 1373 K, therefore, in situ synthesis TiC particles in the GTAW welding pool is possible. That is, thermodynamic considerations indicate that TiC is stable during the GTAW. The formation of TiC occurs through diffusion of carbon to titanium site, where it precipitates as carbide by chemical reaction. The concentration of Ti, Cr, and Ni in the surface layer of sample took on a gradient distribution as presented in Fig. 5, i.e. with the distance from the outside increasing, and the concentration of these elements decreased gradually into that of a master-alloy. Fig. 6 shows the representative microstructures of the surface coatings produced by GTAW. Although some areas show more TiC particles than others, the distribution of TiC particles is, in general, uniform in a matrix of low carbon martensite and retained austenite. In the surface of specimens, uniform TiC dispersions of 25.4–31.6% by volume fraction were achieved with particles size in the range 3–5 ␮m. All of the TiC particles are either rectangular or irregular in shape. This distribution of particles may be ascribed to the interaction between the particles and the advancing solid/liquid interface. Due to the rapid movement of the solid/liquid interface limits the in situ synthesis TiC particles pushing effect and brings about a uniform distribution. Additionally, with an increase in the local interface solidification speed, partial of TiC particles are pushed and captured by the solid/liquid interface. 3.2. Microhardness of the coating

Fig. 4. EPMA face scanning of elements in the cladding coating with 1.2 mm thickness of the preplaced coating: (a) EPMA morphology; (b) Ti; (c) C and (d) Cr.

The microhardness of TiC particles reinforced Fe-based surface composite coatings was measured along the depth

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Fig. 7. Microhardness of the composite coatings: (a) hardness via layer depth and (b) maximum hardness values of the composite coatings. Fig. 6. Morphology of the surface composite coating produced by GTAW process: (a) distribution of TiC particles and (b) SEM image after deep etching.

from the irradiated surface; all of the data were an average of three measurements. The typical microhardness profile of the coatings with a 1.2 mm thickness is shown in Fig. 7(a). The gradient microstructures led to a gradual hardness distribution of the coating. The microhardness of the coating gradually increased with increase of distance from bottom of the coatings. It is noted that there is no sudden transition from the coating to the substrate in the hardness, which indicates an absence of a sharp demarcation in materials properties across the interface. It can also be found that the average microhardness of coating gradually increased with increase of content of TiC, which was due in part to much more TiC particles that were formed during GTAW processing. In addition, the hardness of the coatings varied with the preplaced thickness of coating, as shown in Fig. 7(b). The hardness value increased with the increase of the thickness of the preplaced coating at thickness ranging from 0.6 mm to 1.2 mm, but decreased when the thickness was greater than 1.2 mm. A higher hardness value was obtained in the coating with a 1.2 mm thickness. It is attributed to the effect of the dilution of the substrate on the coatings. With the increase of the thickness of the coating, the effect of the dilution decreased. However, when the thickness of the coating was greater than 1.2 mm, micro-cracks, gas porosity and incomplete fusion were found in the coatings, bonding strength between the

coating and substrate decreased. As a result, the hardness of the coatings decreased. 3.3. Wear characterization of the coatings The wear volume results for the coatings are given in Fig. 8. It indicates that the composite coatings are more effective in improving wear resistance and give a much lower increase in wear volume with increasing normal load than that of AISI 1045 steel. This is mainly related to the hard carbides of TiC particles. Additionally, amongst the coatings and substrate, the lowest wear volume occurred for the cladding coating with a 1.2 mm thickness of the preplaced coating. It is directly attributed to the hardness of the cladding coating.

Fig. 8. Wear volume of the composite coatings and 1045 steel via normal loads.

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ite coating. TiC particles, which were synthesized from graphite and ferrotitanium, with a size in the range of 3–5 ␮m were mainly uniformly dispersed in the matrix. 2. The TiC particles enhance the hardness and wear resistance of the composite coatings. The highest wear resistance of the composite coating was obtained with a 1.2 mm layer. In situ TiC particles reinforced Fe-based composite coating appeared a mild wear with fine scratches.

Acknowledgements This research was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20020422032) and Youth Foundation of Shandong University.

References

Fig. 9. Wear scar of: (a) the composite coatings and (b) 1045 steel at normal load, 98 N.

The SEM micrograph of the worn scar for the coating with a 1.2 mm thickness and AISI 1045 steel are shown in Fig. 9. It can be seen that in situ TiC particles reinforced Fe-based composite coating appeared a mild wear with fine scratches, but 1045 steel showed a severe adhesive wear with scalelike feature. Due to the hard barriers effect of TiC particles, the scratching was interrupted or the path of scratching was converted. That is, in situ synthesis TiC particles reinforced the matrix and protected it from serious abrasion. As a result, the composite coatings possess a much higher resistance to plastic deformation and scratching than that of 1045 steel.

4. Conclusions 1. A new in situ method has been developed to produce TiC particles reinforced Fe-based alloy surface compos-

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