Abstract
An electrode with different C additives was developed. The microstructure of the hardfacing surface layer was observed by optical microscopy. The phase structure was determined by X-ray diffraction. The hardness and wear resistance of the hardfacing surface layer were measured respectively. The worn-out surface and three-dimensional morphology were observed by field emission scanning electron microscope equipped with energy dispersive X-ray spectrometry. The relation curve between mass fraction of M7C3 carbide and temperature was calculated according to thermodynamics software Thermo-Calc. The results show that, with the increase in C additive, the hardfacing surface layer changes from a hypoeutectic structure to a hypereutectic one. Meanwhile, the primary phase changes from austenite to carbide. The hardness and wear resistance of the hardfacing surface layer increase gradually with the increase in C additive, and when the C additive is 25 wt-, they are the largest. With the increase in C content, the precipitation temperature of M7C3 decreases from 1280 to 1260°C, while the maximum amount of M7C3 increases instead, which is from 0·154 to 0·313. The reason of the improvement for wear resistance of the hardfacing surface layer is that the carbide initiates and the amount of M7C3 increases.
Introduction
Fe–Cr–C alloy has been applied widely because of its superior wear resistance and high hardness.1–5 After being used for a period of time, the Fe–Cr–C alloy workpieces failed due to severe abrasion on their surface.6 The failed workpieces can be repaired by the hardfacing (harden face welding) method to restore their dimension and shape and to obtain a higher performance.7–9
In order to obtain more excellent performance of the Fe–Cr–C alloy workpieces after hardfacing, the composition of the hardfacing surface layer should be optimised, so as to improve its microstructure and property.3,10–16 Previous researches indicated that, in the Fe–Cr–C alloy, element Ti can refine the primary M7C3 carbide, so as to improve its wear resistance.3,11,12 Element W can increase tempering resistance and red hardness, which ensure workpieces with high hardness and wear resistance at high temperatures.13,14 Element V not only refines the matrix microstructure, to improve the toughness of the Fe–Cr–C alloy, but also combine with C to form VC, to increase the hardness and wear resistance of the hardfacing surface layer.15–17
However, in all kinds of elements, the influence of element C on the microstructure and property of the Fe–Cr–C alloy is the most significant. Depending on the C content, the Fe–Cr–C alloy is divided into hypoeutectic, eutectic and hypereutectic ones respectively.18 Especially, the C content in the hardfacing surface layer directly influences the amount, size and distribution of the primary phase, as well as the property of the Fe–Cr–C alloy.19
Therefore, four kinds of electrodes with different C additives were developed. Subsequently, the influence of element C on the microstructure and property of the Fe–Cr–C alloy hardfacing surface layer was investigated, and the corresponding mechanism was analysed, which can supply the theoretical foundation for optimising the microstructure of the Fe–Cr–C alloy and improving the property of the hardfacing surface layer.
Experimental
Experimental materials
The electrode for hardfacing the Fe–Cr–C alloy was manufactured. The core of the electrode was made of low carbon steel H08A. The outer coating was composed of ferrosilicon, ferrochrome, ferromanganese and graphite (C additive). In order to analyse the influence of C additive on the microstructure and property of the hardfacing surface layer, the mass fractions of the graphite added into outer coating were 0, 2, 4 and 6 wt- respectively.
Experimental methods
Base metals for the welding surface were prepared from low carbon steel plate Q235, and three layers were welded onto each specimen. The welding parameters are listed in Table 1. In order to analyse the influence of C additive on the microstructure and property of the hardfacing surface layer, the hardfacing surface layer was corroded with 4 nitric acid alcohol after being metallographically polished. Subsequently, the microstructure of the hardfacing surface layer was observed by an Axiovert 200 MAT optical microscope. The phase structure was determined by X-ray diffraction using a D/max-2500/PC diffractometer. The macrohardness and wear resistance of the hardfacing surface layer were measured using an HR-105A Rockwell hardness tester and dry sand rubber wheel abrasive tester respectively. Worn surface and fractography were characterised by a Hitachi S4800 field emission scanning electron microscope, and the composition of each phase was analysed by energy dispersive X-ray spectrometry. The relation curve between mass fraction of M7C3 carbide and temperature was calculated according to thermodynamics software Thermo-Calc.
Welding parameters
Results
Influence of C additive on microstructure of hardfacing surface layer
Influence of C additive on phase structure of hardfacing surface layer
Figure 1 shows X-ray diffraction of the hardfacing surface layer with different C additives. The hardfacing surface layer with 10 C additive consists of γ-Fe and M7C3 carbide. When the C additive is 15, the amount of γ-Fe decreases, while that of M7C3 carbide increases. Moreover, M23C6 and MC carbides appear in the hardfacing surface layer. With the increase in C additive, the amount of γ-Fe decreases continually, while that of M7C3 carbide increases. When the C additive is 25, the microstructure of the hardfacing surface layer is mainly M7C3 carbide, with a small amount of γ-Fe, MC and M23C6 carbides.
X-ray diffraction of hardfacing surface layer with different C additives
Influence of C additive on microstructure of hardfacing surface layer
Figure 2 illustrates the microstructure of the hardfacing surface layer with different C additives. From Fig. 2a, when the C additive is 10, the microstructure of the hardfacing surface layer consists of the trick dendritic primary austenite and the fine eutectic structure. When the C additive increases to 15, as shown in Fig. 2b, the dendritic primary austenite is refined and the zone of eutectic structure, which consists of eutectic austenite and eutectic carbide increases. When the C additive is 20, the primary austenite further refines and the dendritic austenite disappears, while the eutectic structure increases continually. When it is 25, the primary carbide initiates in the microstructure largely enclosed by the eutectic structure, which is shown in Fig. 2d.
Microstructure of hardfacing surface layer with different C additives
Influence of C additive on hardness and wear resistance of hardfacing surface layer
Influence of C additive on hardness of hardfacing surface layer
Figure 3 shows the hardness of the hardfacing surface layer with different C additives. When the C additive increases from 10 to 20 wt-, the hardness of the hardfacing surface layer increases, which is from 50 to 56 HRC. When the C additive is 25 wt-, the hardness of the hardfacing surface layers increases obviously, which is 63 HRC.
Hardness of hardfacing surface layer with different C additives
Influence of C additive on wear resistance of hardfacing surface layer
Figure 4 shows the weight loss of the specimens with different C additives. The weight loss of the hardfacing surface layer with 10 wt-C additive is the largest. With the increase in C additive, weight loss decreases gradually, and it is the lowest with 25 wt-C additive when the wear time is <225 min. However, when the wear time is >225 min, the weight loss is the lowest with 20 wt-C additive.
Weight loss of specimens with different C additives
The worn morphologies of the specimens with different C additives when the wear time is 250 min are shown in Fig. 5. From Fig. 5a, the wear scratches on the surface are both wide and deep when the C additive is 10 wt-. With the increase in C additive, the wear scratches on the surface are narrow and shallow gradually. When the C additive is 20 wt-, the wear scratches are the most smooth, which is shown in Fig. 5c.
Worn morphologies of specimens with different C additives
When the C additive is 25 wt-, roughness of the worn surface increases again. The reason is that, the size and density of the carbides increase significantly, so the wear resistance of the hardfacing surface layer with short wear time increases. However, with longer wear time, the carbides on the surface maybe broken, even parts of them take off from the substrate, so the wear resistance of the hardfacing surface layer decreases, which is shown in Fig. 5d.
Influence of C additive on porosity and cracking of hardfacing surface layer
The surface morphologies of the specimens with different C additives after being polished are shown in Fig. 6. As shown in Fig. 6, some pores, instead of cracks, can be found on the hardfacing surface layers with different C additives. However, with the increase in the C additive, the number and size of the pores are not changed significantly, which indicates that the C additive has no effect on the porosity and the cracking of the hardfacing surface layer by means of the appropriate welding method.
Surface morphologies of specimens with different C additives
Discussion
Primary phase of hardfacing surface layer
Figure 7 demonstrates the two-dimensional and three-dimensional morphologies of the specimens with different C additives. From Fig. 7a and e, when the C additive is 10 wt-, the primary austenite exists in the hardfacing surface layer largely besides a little eutectic structure. The morphology of the primary austenite in two-dimension is branch-like and that of three-dimension is embossment. Because of the primary austenite that occupies the surface with lower hardness, the wear resistance of the hardfacing surface layer is lower too. Therefore, the scratches are shown as wide and deep strips throughout the entire worn surface.
Two-dimensional and three-dimensional morphologies of specimens with different C additives
With the increase in C additive, from Fig. 7b, c, f and g, the amount of the primary austenite decreases, while that of the eutectic structure increases. The uniformity of the microstructure increases obviously, so wear resistance of the hardfacing surface layer improves largely and the wear scratches narrow and shallow.
From Fig. 7d and h, when the C additive is 25 wt-, a large amount of primary carbide initiates, which can be the excellent wear particle. The morphology of the primary carbide in two-dimension is hexagon and that in three-dimension is plane chunk. With high hardness of the primary carbide, the wear resistance of the hardfacing surface layer can be improved in short wear time. However, primary carbide in the hardfacing surface layer is lack of support because of the very few surrounding matrix. So, with longer wear time, the surface friction exceeds the binding force between primary carbide and matrix, so the primary carbide prone to peel off from the wear surface, which leads to the weight loss, increases suddenly.
M7C3 carbide of hardfacing surface layer
From Fig. 1, the eutectic and primary carbide are both M7C3. From Fig. 7, the amount of M7C3 carbide increases with the increase in C additive. Therefore, the wear resistance of the hardfacing surface layer is related with the M7C3 carbide.
The precipitation regular of the M7C3 carbide at different temperatures was calculated by Thermal-Calc software. The hardfacing surface layers with four C additives were taken, and their chemical compositions are listed in Table 2.
Chemical compositions of hardfacing surface layers/wt-
The precipitation regulars of M7C3 in the hardfacing surface layer with different C contents are shown in Fig. 8. When the C content increases from 0·685 to 2·74 wt-, the precipitation temperature decreases gradually, which are 1280, 1270, 1265 and 1260°C respectively. With the decrease in temperature, the mass fraction of M7C3 increases slowly firstly and then rapidly, and reach a steady finally. Moreover, with the increase in C content, its maximum amount of M7C3 decreases obviously. As the C content is 0·685 wt-, the maximum amount of M7C3 is 0·154 when the temperature is 800°C. When the C contents are 1·37, 2·065 and 2·74 wt-, the maximum amounts of M7C3 are 0·206, 0·259 and 0·313, when the temperatures are 792, 788 and 776°C respectively.
Precipitation regular of M7C3 carbide in hardfacing surface layer with different C contents
From the above analysis, it is found that with the increase in C content, the amount of M7C3 carbide increases. Combined with Fig. 7, when the C contents are 0·685, 1·37 and 2·065 wt-, which means the C additives are 10, 15 and 20 wt- respectively, the M7C3 carbide is a eutectic one and the microstructure shows homogeneous. So, with the increase in C additive, the amount of hard M7C3 carbide increases, and the wear resistance of the hardfacing surface layer increases.
When the C content is 2·74 wt-, the size and amount of the primary M7C3 are the largest, in short wear time, and the wear resistance of the hardfacing surface layer is the largest too. However, with long wear time, the M7C3 carbides often peel off from the surface, so the wear resistance of the hardfacing surface layer decreases again.
Conclusions
With the increase in C additive, the amount of primary austenite in the hardfacing surface layer decreases continually, while that in the eutectic structure increases gradually. When the C additive is 25 wt-, the primary carbide initiates in the hardfacing surface layer largely.
With the increase in C additive, the hardness and wear resistance of the hardfacing surface layer increase gradually, and when the C additive is 25 wt-, they are the largest.
With the increase in C content, the precipitation temperature of M7C3 decreases from 1280 to 1260°C, while the maximum amount of M7C3 increases instead, which is from 0·154 to 0·313.
With the increase in C additive, the primary phase changes from austenite to carbide and the amount of M7C3 increases, which results in the improvement of the wear resistance of the hardfacing surface layer.