Abstract
In this work, the effect of Cr3C2 on the microstructure and properties of 310 coatings by plasma transfer arc welding was studied. The microstructure, composition, hardness and wear resistance of the coatings were analyzed by scanning electron microscope, EDS, X-ray diffractometer, microhardness tester and friction wear tester. The results show that when Cr3C2 is less, the coating is composed of columnar crystal and cellular crystal, mainly including (Fe, Ni) Fe3Ni2FeCr0.29Ni0.16C0.06Cr2Fe14C and CrFe7C0.45. With the increase of Cr3C2, the structure becomes dendrites and new phases Cr7C3 and Cr23C6 appeared. However, the crystallinity of 5% is poor and there is no obvious grain structure. The hardness of the coatings increases first and then decreases with the content of Cr3C2. The hardness with the content of 3% is the highest. The friction coefficient corresponds to the hardness and the main wear mechanism is abrasive wear and oxidation wear.
Introduction
310(H12Cr26Ni21Si) is a kind of austenitic stainless steel. Owing to its high content of chromium and nickel, it has good comprehensive properties such as high-temperature oxidation resistance and high-temperature creep strength [1-3]. Especially when the alloy contains a certain amount of Si, C, B and other alloy elements, it also has good wear resistance, cold and hot fatigue resistance, air erosion resistance and high-temperature abrasion resistance [4]. The stainless steel is widely used in high-temperature boiler, machinery industry, aerospace, petrochemical and other fields. However, under some harsh working conditions, the simple alloy surfacing layer can no longer meet the working requirements. The PTA surfacing metal-based ceramic composite coating combines the high toughness of metal with the high melting point, high hardness and high wear resistance of ceramic materials, which can effectively improve the service life of parts [5-9]. Some researchers have studied the properties of WC-containing Fe-based alloy layer [10-13]. But the active oxidation of WC begins at 500°C in the air and the antioxidant capacity is weak. As a common ceramic hard additive phase, Cr3C2 can significantly improve the microstructure and wear properties of alloy coatings [14-16]. And it is a high-temperature wear-resistant hard phase with excellent performance, high thermal hardness and excellent high-temperature wear resistance [17-21]. However, there is very little research on overlaying welding by adding Cr3C2 to 310 stainless steel powders.
In this paper, five kinds of 310 powders with different Cr3C2 contents were surfacing on Q235 steel by manual PTA surfacing equipment. And the microstructure, chemical composition, hardness and wear resistance of the surfacing layer were analysed and studied in detail.
Materials and Methods
Coatings preparation
The base metal of the test is Q235 steel, which is widely used in industrial production. The surfacing material are 310 stainless steel powder supplied by Weiye Metal Powder Company and Cr3C2 powder supplied by Bocheng research and metallurgy Center. The chemical composition (wt-%) is shown in Table 1 and the SEM images of the two powders are shown in Figure 1. Cr3C2 with 1%, 2%, 3%, 4% and 5% were added to 310 powders, respectively, mixed evenly and dried at 80°C for 4 h. PTA-BX-400A high energy plasma arc powder surfacing equipment (Shanghai Benxi mechanical and Electrical Technology Co., Ltd, China) was used for this experiment. The transfer arc voltage is 50 V and the experimental currents are 200 A. The flow rate of ion gas is 300 L h −1 and that of argon protection gas is 200 L h −1.
SEM images of two powders: (a) 310 powder, (b) Cr3C2 powder. Composition of 310 powder (wt-%).
Cutting the surfacing sample into appropriate size by wire cutting. Sandpaper was used to polish the cross section, and diamond polishing paste was used to polish the cross section. After polishing, aqua regia was used for corrosion. Scanning electron microscope (SEM) (MIRA 3, LMH, TESCAN Brno, s.r.o.) was used to observe the microstructure of each coating and its energy spectrum analyser (EDS) (Aztec Energy ES, X-Max 20) was used to measure the elemental composition.
Phase composition characterization
The phase analysis of coatings was carried out by Japanese physical SmartLab X-ray diffractometer (XRD). The diffraction angle is between 5° and 90°. The test results were analysed with jade 6 software and compared with PDF standard card to determine all phases.
Microhardness test
The microhardness of all samples was tested by HXS-1000A microhardness tester. During the test, 5 points shall be taken from each area at the interval of 0.1 mm along the direction from the coatings to the base metal. A preload of 200 g was held for 8 s. The average value was taken after the test.
Friction and wear test
The friction coefficient of the coatings was measured by MS-T3001 friction and wear tester, and the surface was ground and polished before the experiment. The tests were performed with a ball-on-disc configuration on a normal temperature. Si3N4 ball with a diameter of 3 mm was used as the counter-grinding surface and the load was 500 g. The rotation speed was 100 rev min–1 and the friction time was 20 min.
Results and discussions
Coatings microstructure and phase composition
Figure 2 shows the SEM image of each component Cr3C2/310 coating. It can be seen from the figure that with the increase of Cr3C2 content, the microstructure of the coatings has changed significantly. When the addition amount is only 1% or 2%, the microstructure of the coating is mainly composed of coarse columnar dendrite and fish scale cellular crystal (circles in Figure 2(a)), and the difference between grain and grain boundary is not significant. The whole structure has obvious growth orientation along the cooling direction. When the content of Cr3C2 reaches 3% and 4%, the microstructure changes obviously. The whole coating becomes into dendrite having obvious difference between grain and grain boundary, and the grain size becomes smaller and finer. In addition, some primary dendrites were broken to form secondary dendrite arms (circle in Figure 2(c)) [22]. When the content of Cr3C2 continues to increase to 5%, macro cracks appear. The microstructure difference of the whole coating becomes smaller again in which the grain boundaries cannot be clearly distinguished. And there are many microscopic defects such as pores (circle in Figure 2(e)).
Microstructure of coatings after surfacing with different Cr3C2 content: (a) 1% (b) 2% (c) 3% (d) 4% (e) 5% (f) cracks in the 5% coating.
Figure 3 is the EDS line scanning image of the coating and base metal interface with 4% Cr3C2 content. It can be seen from the element content that there is no ‘steep cliff’ change of all elements at the interface. It shows that a metallurgical union may have taken place between the coatings and the base metal. The existence of O element may be due to certain oxidation at the interface during the corrosion process, while Cr and Ni elements have almost no concentration height in the base metal, only a slowly rising concentration gradient near the interface appears, indicating that there is no obvious element diffusion and the dilution rate of the base metal is very low.
EDS line scan image of the interface between coating and substrate with 4% content.
Figure 4 is the image of the coatings of 1% and 3% Cr3C2 content under 5000X electron microscope magnification. Table 2 shows the EDS test results of each point in Figure 4. It can be seen that there is a significant difference between the two grains in Figure 4. The grain boundary structure with 1% content is uniform and continuous, but the difference between grain and grain boundary contrast is small. Compared with that, the grain boundary with 3% content is composed of obvious lamellar and bar like structures. Combined with the analysis of element content in Table 2, there is a certain difference in the distribution of elements between the grain and the inter-crystalline. The content of Cr in the inter-crystalline (point B) is obviously higher than that in the grain (point A). And with the increase of Cr3C2 content, the content of Cr and C are significantly increased (point D).
Contrast images of different Cr3C2 content welding layers under high power electron microscope: (a) 1% (b) 3%. Results of EDS measurements of each point in the coatings (wt-%).
Because the difference of microstructure between 1% and 2%, 3% and 4% Cr3C2 is not obvious, the samples with 1%, 3% and 5% Cr3C2 content are selected for XRD analysis. Figure 5 shows the test results. It indicates that the positions of the diffraction peaks are basically the same, but the peak heights are different. For 1% and 5% of the coatings, the diffraction peak intensity near 43° is greater than that near 50°, while the result of 3% is opposite. In addition, the diffraction peak intensity of 5% is small, indicating that its crystallinity is very poor. From the results of phase analysis, when the content of Cr3C2 is only 1%, the coating is mainly composed of (Fe, Ni) solid solution, Fe3Ni2 phase, austenitic phase FeCr0.29Ni0.16C0.06, and carbide phase Cr2Fe14C, CrFe7C0.45. When the addition amount reaches 3%, new austenite phase Cr–Ni–Fe–C and Cr7C3, Cr23C6 carbide phases appear. Compared with 1%, 5% of the coatings has new Cr7C3 and Cr23C6 carbide phases.
XRD test results of different Cr3C2 content coatings.
The melting temperature of Cr3C2 is about 1890°C. However, some scholars [23,24] have shown that in the system with Ni, Cr3C2 will begin to melt and surface diffusion will take place at a temperature of about 700∼800°C. When the temperature reaches about 1000°C, a considerable part of Cr3C2 has been decomposed into Cr and C atoms. Therefore, in the process of surfacing, because of the extremely high temperature of plasma arc (the centre of the arc column is more than 10,000°C), all the Cr3C2 added will basically decompose. Generally speaking, there are three stable carbides of Cr, Cr3C2, Cr7C3 and Cr23C6. The standard Gibbs free energy equation for forming these carbides is as follows [25]:
From the above equation, it can be seen that under the same temperature (T), generally speaking, the Gibbs free energy to form Cr23C6 is the smallest so it has the highest tendency of formation from dissolved Cr and C. However, the XRD analysis results detected that both Cr7C3 and Cr23C6 exist. This is because the reaction is not only affected by thermodynamic factors, but also by kinetic factors. The above equation is based on pure Cr–C system, but the actual situation is more complex. Owing to the presence of other elements, such as Fe, Ni, etc., uphill diffusion will occur to the melted Cr3C2 region and the reaction between Cr and C will be affected [23,24,26]. In addition, due to the high cooling rate after plasma surfacing, the diffusion of elements cannot reach the ideal level. Cr7C3 and Cr23C6 are formed by decarburization of Cr3C2 [27]. If the cooling rate is too fast, C will not diffuse or react with other elements, which will hinder the formation of Cr23C6.
Figure 6 is a microhardness test result curve of five groups of samples, in which the dashed line represents the interface of the coatings and the base metal. The hardness difference of the base metal of all samples is small and the average hardness is about 160HV0.2. According to the XRD test results from Figure 5, the melted Cr and C elements have enough time to diffuse and combine with other elements to form new structures without producing new secondary carbides due to the small addition amount. When the content of Cr3C2 reaches 3% and 4%, the hardness of the coatings is obviously improved, and the average hardness is also increased to 323.4 HV0.2 and 298.4 HV0.2, respectively, with the highest reaching 376.7 HV0.2. This can be attributed to the formation of new secondary carbides in the process of surfacing with high Cr3C2 content. The existence of these carbides can significantly improve the hardness of the coatings due to the role of Orowan mechanism. On the other hand, because of the tiny and small carbides, which can act as the core of grain nucleation or as hard particles, pinning at the grain boundary to prevent grain growth during the cooling process, finer and more uniform crystal grains can be obtained, thereby improving the hardness of the coatings. As for the coatings with 5% content, because of the bad weld-ability, the molding state is poor and the crystallinity is low. There are many defects such as cracks and pores, which greatly affect the density of coatings, and then affect the hardness. The average hardness is only about 186.6 HV0.2.
Microhardness test results of coatings with different Cr3C2 content.
Figure 7 shows the friction coefficient curve of each sample. The average friction coefficient is 0.5921, 0.5162, 0.3416, 0.4152 and 0.6080, respectively, according to the content of Cr3C2 from small to large. This is consistent with the hardness test result, and the average friction coefficient of the highest hardness coatings is also the smallest. The friction curves of 3% and 4% Cr3C2 coatings are narrower and have less fluctuation when they are stable, which indicates that their microstructure is more uniform and more stable during friction. However, the curve of 5% is very wide and fluctuates greatly after stabilization, which is related to its poor crystallinity and non-compact structure.
Friction coefficient curve of different Cr3C2 content coatings.
Figure 8 is the SEM image of the wear track of each sample after friction, and Table 3 is the EDS test results of each point indicated in Figure 8. It can be seen from Figure 8(a,b) that during repeated friction, there are a large number of peeling pits and debris particles on the surface of the coatings with low Cr3C2 content. Combined with EDS results, it can be seen that the oxygen content of the darker colour area and the brighter debris particles is very high. Therefore, it is concluded that although the experiment is carried out at room temperature, there is still a serious oxidation reaction in the friction process, which will produce an oxide layer on the surface. However, under the action of load, the oxide layer will undergo strain hardening, so cracks (dotted circle area in figure 8(a)) will be generated under the pressure of the small balls, and eventually will break and fall off, forming peeling pits with bright edges. The oxide chips with high hardness formed by the falling pieces will become new abrasives and further wear the coatings. When the content of Cr3C2 increase to 3% and 4%, the width of wear track becomes smaller which means that the depth of Si3N4 ball pressed into the coatings becomes shallow because the hardness is significantly improved. Wear marks show a wavy shape, which is usually due to the plastic flow caused by a large plane shear force between the coating and the ball [28]. It can also be seen from Figure 8 that compared with the coatings with less Cr3C2 content, its peeling pit and oxide debris are significantly less. The finer and uniform structure and higher hardness make the wear effect of oxide debris weaken, and the oxide scale is more uniform and not easy to fall off. This is also the main reason why the friction coefficient is small and the curve is stable. However, due to the poor forming, low crystallinity and low hardness of the 5% Cr3C2 content coatings, the spalling and oxidation debris particles wear are obvious. Therefore, it can be concluded that the main wear mode of coatings is oxidation wear and abrasive wear caused by brittle fracture of oxide layer.
The surface morphology image of different Cr3C2 content coatings after wear: (a) 1% (b) 2% (c) 3% (d) 4% (e) 5% EDS test results of the coating surface after the wear test (wt-%).
When the content of Cr3C2 is less, the coatings are mainly composed of columnar crystal and cellular crystal which is mainly composed of (Fe, Ni) solid solution, Fe3Ni2, FeCr0.29Ni0.16C0.06 and carbide phases Cr2Fe14C and CrFe7C0.45. With the increase of Cr3C2 content, the structure becomes dendrite and new Cr7C3 and Cr23C6 carbide phases are produced. However, the crystallinity of 5% content is poor, the microstructure is unclear and there are many defects such as holes and cracks. The hardness of the coatings first increased and then decreased with the content of Cr3C2. The hardness of the coating with 3% content was the highest, and the average hardness reached 323.4 HV0.2. The next is the coating of 4% content, which is 298.4 HV0.2. The hardness of 1%, 2% and 5% coatings are similar and the values are 189.7 HV0.2, 188.8 HV0.2 and 186.6 HV0.2, respectively. The friction coefficient of the coatings corresponds to the hardness, and the average friction coefficient of the coatings with the highest hardness is only 0.3416. The coating with 4% content ranks second with an average friction coefficient of 0.4152 and that of coatings with 1%, 2% and 5% contents are 0.5921, 0.5162 and 0.6080, respectively. The main wear mechanism of coatings is abrasive wear and oxidation wear. The brittle fracture of oxide skin will produce peeling pit, and the oxide debris particles with high hardness will become new abrasives, which will further wear the coatings and cause material removal.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the author(s).