2019 年 59 巻 7 号 p. 1354-1361
In this work, different ratios of molybdenum carbide (1, 3 and 5 mass% Mo2C) powders were added to Vanadis 4 extra alloy steel powders and then mixed by ball milling for 6 h. The composite powders underwent vacuum sintering at 1200, 1220, 1240, 1260 and 1280°C for 1 h, respectively. The results showed that the optimal sintering temperature for the addition of 5 mass% Mo2C powders was 1220°C. It also represented that the apparent porosity was 0.18%, and that a transverse rupture strength (TRS) value of 2281.3 MPa and a hardness value of 79.2 HRA were obtained, respectively. Additionally, the TRS value was obviously enhanced to 2437.6 and 2491.4 MPa by the addition of 5 mass% Mo2C powders after heat treatment and sub-zero plus heat treatments, respectively. Meanwhile, the hardness value also increased to 80.6 and 81.3 HRA, respectively, whereas the Mo-rich M6C carbides distributed in the grain boundaries, and V-rich MC carbides appeared in the grain and grain boundaries after sub-zero plus heat treatments. Significantly, a series of heat treatment processes is effective in improving the microstructure and strengthening the mechanical properties of the sintered Vanadis 4 extra composites.
In many industrial applications, tool steels are subjected to extremely high and variable loads. Among them, cold-work tool steels are the most important category, as they have a wide usage area in the industry because of their high hardness, toughness and wear resistance.1,2) Vanadis 4 Extra (also known as V4 Extra) alloy steel is a Cr–Mo–V alloyed steel which possess the good ductility, high wear resistance and compressive strength, and excellent tempering back resistance. In general, V4 Extra is a typical cold-work tool steel and especially suitable for applications where adhesive wear and/or chipping are the dominant failure mechanisms. It also very suitable for blanking and forming of advanced high strength steels by proper heat treatments.2,3)
In order to further improve the tool life, over the past 10 years, industrial tool steels for cutting or wear of materials have widely used metal-ceramic composite materials (MMCs), whereas, powder metallurgy (P/M) methods offer two very different types of materials with a view to achieving lightweight, high strength, hardness and wear resistance tool steels.4) Among them, MMCs have gained a considerable interest in the last decades. The driving force has been the fact that addition of ceramic reinforcement in the metallic matrix can improve specific strength, stiffness, wear, fatigue and creep properties compared to conventional engineering materials.5,6) The properties of MMCs are greatly influenced by the nature of reinforcement and its distribution in the metal matrix. Simultaneously, iron-based alloys or steels have been used as matrix materials for metal-matrix composites owing to the lower cost, adequate mechanical and wear properties.7,8)
Generally speaking, tool steels produced by P/M has extremely refined, homogenous microstructure such as homogenous distribution of primer alloying carbides came from production resulting as alloying process compared with their conventionally produced cast and wrought counterparts.6,7) Conventional P/M involves mixing the metal powders, compacting of the mixed powders into moulds and sintering of the compact powders under different atmospheres. However, sintered P/M parts usually over 5% porosity. Enhanced sintering techniques can be applied to obtain higher density and less porosity in sintered parts.9) It is also the concern of this research.
On the other hand, transition-metal carbides are regarded as promising hard materials because of their high melting points, high hardness and high-temperature strength. Among the transition-metal carbides, molybdenum carbide (Mo2C) has received increased attention in recent years.10) Because of its high hardness, high melting point, and wear resistance, Mo2C has been widely applied as a reinforcement material in in steel and in metal ceramics.11,12) It is worth mentioning that Mo2C is also an effective grain growth inhibitor due to its high solubility and mobility in the metal matrix phase.13)
Our previous studies indicated that heat treatment can be used to increase the strength and hardness of as-sintered P/M alloy steel.4,6,7) However, it is still unclear whether V4 Extra steel with different amounts of Mo2C particles in the strengthening phase can obtain better mechanical properties, as well as better distribution and size of the carbides for V4 Extra composites after heat treatment. Therefore, the present research aims to explore a series of vacuum sintering processes and heat treatments for V4 Extra composites, as well as to examine the effects on the microstructure, mechanical properties and corrosion behaviors of Mo2C-strengthened V4 Extra alloy steel.
This study utilized Vanadis 4 Extra (hereinafter V4 Extra) powders as a substrate and added different ratios of Mo2C powders as a strengthening phase. In order to obtain more uniform V4 Extra powders, particle sizes smaller than 50±1 μm were sifted from the original powders. As a result, the mean particle size of the V4 Extra alloy steel powders was 30.3±0.5 μm, as shown in Fig. 1(a). The powders possess a uniform and spherical appearance; the surface morphology of Mo2C additives is shown in Fig. 1(b). The mean particle size of the Mo2C powders was about 4.7±0.5 μm; moreover, the shape of the Mo2C powders was an irregular polygon; there were no smooth or undulating surfaces. In addition, the mixed powders were milled by using the WC balls for 6 h. Figure 1(c) shows the morphology under the effect of mechanical alloying by ball milling for 6 h; the mixed alloy powders produced a significantly plastic deformation. Mo2C is clearly coated on the V4 Extra powders, as indicated by the arrows.
The SEM images of the surface morphology of (a) the sifted V4 Extra alloy steel powders, (b) the original Mo2C powders, and (c) V4 Extra- Mo2C powders after 6 h ball mixing.
In the experimental, the chemical compositions (mass%) of the V4 Extra alloy steel powders are as follows: 1.4% C, 0.4% Si, 0.4% Mn, 4.7% Cr, 3.5% Mo, 3.7% V and 85.9% Fe. Furthermore, the different amounts of Mo2C powders (1, 3 and 5 mass%) were mixed and added to V4 Extra alloy steel powders, designated as M1, M3 and M5, hereafter. After milling, the PVA (polyvinyl alcohol) as a binder was added. The green compact (40 × 6 × 6 mm3) of the powder specimen was produced under a uniaxial pressure at 300 MPa for 300 sec.
In higher carbon content steels such as cold-work tool steels, the martensite finish line temperature is below 0°C, which means that at the end of the heat treatment, a low percentage of austenite is retained at room temperature. The retained austenite as a soft phase in steels could reduce the product life in working conditions.1) Cryogenic (also known as sub-zero) treatment has been acknowledged for many decades as an effective method for increasing wear resistance in tool steels.14,15) In general, sub-zero treatment is used to transform the retained austenite into martensite. The temperature range normally is from −125 to −196°C.
To explore the effects of a series of vacuum sintering and heat treatments, the vacuum sintering temperatures were set at 1200, 1220, 1240, 1260 and 1280°C for 1 h in a 5 × 10−3 Pa, respectively. Besides, heat treatment (quenching followed by tempering, designated as HT) and sub-zero plus heat treatment steps (quenching followed by sub-zero and tempering, designated as SZ) were performed, in which the samples were heated to 1020°C and the temperature was maintained for 100 min for quenching with 0.5 MPa of N2 as the quenching media. The samples were subjected to sub-zero treatment at a temperature of −150°C for 60 min. The tempering temperature was held at 540°C for 3 h, and then cooled to room temperature, and this aging process was repeated twice.
In order to evaluate the sintered behavior of V4 Extra alloy steels added Mo2C powders by vacuum sintering and heat treatments, various material characterization techniques were used, which including apparent porosity, mean grain size (FOG V1.0 software), hardness, transverse rupture strength (TRS) and corrosion tests (Potential Stat Chi 601). Microstructural observations of the specimens were performed by optical microscopy (OM) and scanning electron microscopy (SEM, Hitachi-S4700). Apparent porosity test was conducted in accordance to the ASTM C830 standard. Hardness tests were performed by Rockwell A hardness (HRA, Indentec 8150LK) with a loading of 588.4 N, which followed the ASTM B294 standard. The Hung Ta universal material test machine (HT-9501A) with a maximum load of 245 kN was used for the TRS tests (ASTMB528-05). Meanwhile, the TRS was obtained by the equation Rbm = 3FLk/2bh2, where Rbm is the TRS, which is determined as the fracture stress in the surface zone, F is maximum fracture load. Besides, L was 30 mm, k was chamfer correction factor (normally 1.00–1.02), b and h were 5 mm, respectively. The specimen dimensions of the TRS test were 5 × 5 × 40 mm3 and tests at least three pieces.
The corrosion potential analysis uses three electrodes method: the reference electrode is a saturated of silver-silver chloride electrode, auxiliary electrode is a platinum electrode, and the working electrode is connected to the test specimens (ASTM G59-97). The contact area of the specimen was 0.785 cm2. The corrosive solvent was used 3.5 mass% NaCl solution and was maintained at room temperature. A scanning speed of 0.01 v·s−1, initial potential of −2.0 V, and the final potential of 2.0 V were controlled. The polarization curve was obtained by Corr-View software to analyze and compare the corrosion potential (Ecorr), the corrosion resistance (Rcorr) and corrosion current (Icorr) of the different sintering parameters.6,7) The voltage is V (volts) vs. SCE (saturated calomel electrode). A comparison was conducted to investigate the corrosion current (Icorr) and polarization resistance (Rp) of the different sintered and heat-treated V4 Extra composites.
Figure 2 shows the apparent porosity, volume shrinkage and mean grain size at different sintering temperatures for various amounts of Mo2C added to the V4 Extra specimens. Significantly, the apparent porosity of the V4 Extra (without Mo2C added), M1, M3 and M5 specimens was higher after 1200°C sintering for 1 h, as shown in Fig. 2(a). Generally speaking, an increase in sintering temperatures can enhance the thermal energy and extend the diffusion time. When the sintering temperature increased to 1240°C, the apparent porosity of the V4 Extra, M1, M3 and M5 specimens were 4.72, 2.74, 0.18 and 0.16%, respectively. It is reasonable to suggest that all the specimens underwent sufficient diffusion time; there was no evidence of pores. Moreover, when the sintering temperature was further increased to 1260°C, the apparent porosity of the M5 specimens showed a minimum value, with porosity level value of 0.11%.
Comparison of the (a) apparent porosity and volume shrinkage, and (b) mean grain size of various mass% Mo2C added V4 Extra by the different sintering temperatures. (Online version in color.)
On the other hand, sintered V4 Extra, M1 and M3 specimens possessed the lowest apparent porosity (0.13, 0.09 and 0.15%, respectively) at 1280°C sintering for 1 h. Since the vanadium (V) content inside the V4 extra steel is higher, and the free energy of vanadium carbide (VC) is lower than the other carbides,16) Mo2C added to V4 Extra easily reacted with V in the matrix, and heat energy was released during the sintering reaction. Therefore, these composites can achieve sintering densification at a relatively lower temperature than the sintering temperature required for V4 Extra. Furthermore, by adding an increased amount of Mo2C (1 → 5 mass%), there is higher reaction energy inside the specimens; thus, densification is achieved in advance. This phenomenon is most obvious when added 5 mass% Mo2C to V4 Extra alloy steel.
In addition, the previous literature indicated that the green density increases as the compacting pressure was increased and levels out at higher compacting pressures. Powder particles work harden as the result of plastic deformation and it requires higher pressures to cause further plastic flow.17) Moreover, a higher green density showed a more homogeneous microstructure and resulted in a higher sintered density.18) In this work, the green compact of the powder specimen was produced under a higher pressure at 300 MPa and continuously maintained for 300 sec. It is reasonable to suggest that the higher green density (the green density of V4 Extra, M1, M3 and M5 specimens is 5.52, 4.85, 4.95 and 5.04 g·cm−3, respectively) is effective in improving the apparent porosity (Fig. 2(a)) and sintering densification of the sintered Vanadis 4 extra composites.
In the present research, the volume shrinkage of the V4 Extra, M1, M3 and M5 specimens showed a similar trend in regard to apparent porosity, as shown in Fig. 2(a). When the sintering temperature was 1200°C, all the specimens had relatively low volume shrinkage resulting from the restricted diffusion of the Mo2C particles. As the sintering temperature increased to 1220°C, the volume shrinkage of all the specimens showed a significant increase. The volume shrinkage of the V4 Extra, M1, M3 and M5 specimens increased rapidly to 23.37, 32.16, 32.49, and 35.54%, respectively. When the sintering temperature was further increased to 1260°C, the volume shrinkage of the M5 specimens presented a slowly increasing trend (37.99%). Meanwhile, the M1 and M3 specimens showed a similarly slowly increasing trend after 1280°C sintering for 1 h. According to the above test results, it was possibly easier for a high amount (5 mass%) of added Mo2C powders to be rearranged and enter the interspaces of the V4 Extra particles during the sintering process; thus, a high density of the sintering composites was obtained.
Figure 2(b) shows the mean grain size at different sintering temperatures for various amounts of Mo2C added to the V4 Extra specimens. The mean grain sizes of the V4 Extra, M1, M3 and M5 specimens were 6.1, 5.8, 6.6 and 6.9 μm, respectively, after sintering at 1200°C for 1 h. As the sintering temperature increased, the mean grain size increased rapidly. Aa a result, the mean grain sizes for the V4 Extra, M1 and M3 specimens showed a significant increase to 77.3, 68.2 and 61.2 μm, respectively, after sintering at 1280°C for 1 h, whereas the M5 specimens (57.9 μm) appeared at 1260°C. In addition, the values for the V4 Extra, M1, M3 and M5 specimens after sintering at 1220°C for 1 h were 8.4, 7.9, 10.2 and 15.1 μm, respectively. Obviously, the mean grain size is independent of the amount of Mo2C added, but is related to the degree of sintering temperature. In other words, high-temperature sintering causes coarsening to the grain sized specimens. Consequently, the Mo2C additives seem to be insignificant in inhibiting grain growth for the V4 Extra alloy steels.
Figure 3 reveals the hardness and TRS test results of the V4 Extra, M1, M3 and M5 specimens after the different sintering temperatures. All of the experimental values for hardness and TRS are listed in Tables 1 and 2. The hardness of the V4 Extra, M1, M3 and M5 specimens was relatively low when the sintering temperature was at 1200°C for 1 h (as seen in Fig. 3(a)), which indicates that the specimens did not reach complete liquid phase sintering (LPS). As the sintering temperature increased to 1220°C, the apparent porosity decreased rapidly, as shown in Fig. 2(a). The hardness of the V4 Extra, M1, M3 and M5 specimens were significantly enhanced (hardness values were 65.8, 66.2, 70.2 and 79.2 HRA, respectively). Besides, the hardness of M3 specimens can be further increased when the sintering temperature is raised to 1240°C; the hardness of the V4 Extra and M1 specimens can also be enhanced when the sintering temperature is raised to 1260°C. In the present research, the M5 specimens possessed the highest hardness after 1220°C sintering for 1 h. It is reasonable to suggest that the 1220°C-sintered M5, 1240°C-sintered M3, 1260°C-sintered V4 Extra and M1 specimens seemed to have already reached a complete LPS, possessing an optimal hardness value. Moreover, the hardness values of the specimens improved slightly when the added amount of Mo2C powder was increased. As a result, suitably reinforced Mo2C powders added to the V4 Extra alloy steel matrix effectively enhanced the hardness.
Comparison of the hardness and TRS of various mass% Mo2C added V4 Extra by the different sintering temperatures (a) hardness, and (b) TRS. (Online version in color.)
| Temperature (°C) | V4 Extra | M1 | M3 | M5 | M5+HT | M5+SZ |
|---|---|---|---|---|---|---|
| 1200 | 59.7±0.5 | 61.4±0.4 | 62.5±0.5 | 63.9±0.3 | – | – |
| 1220 | 65.8±1.2 | 66.2±0.9 | 70.2±1.4 | 79.2±1.1 | 80.6±0.5 | 81.1±0.3 |
| 1240 | 73.7±1.1 | 74.4±1.2 | 78.1±0.4 | 77.6±0.4 | – | – |
| 1260 | 78.3±0.7 | 78.6±0.6 | 76.9±0.6 | 76.3±0.7 | – | – |
| 1280 | 77.5±0.9 | 76.6±0.5 | 75.9±0.7 | – | – | – |
| Temperature (°C) | V4 Extra | M1 | M3 | M5 | M5+HT | M5+SZ |
|---|---|---|---|---|---|---|
| 1200 | 491.5±11.6 | 975.6±58.2 | 1217.6±20.1 | 1313.2±17.4 | – | – |
| 1220 | 892.1±14.3 | 1167.7±45.1 | 1649.4±48.3 | 2281.3±28.5 | 2437.6±23.1 | 2491.4±84.2 |
| 1240 | 1202.2±49.8 | 1765.7±97.7 | 1973.7±38.6 | 1689.4±43.9 | – | – |
| 1260 | 1521.8±93.4 | 1874.5±28.1 | 1340.4±73.9 | 1333.7±81.5 | – | – |
| 1280 | 1235.1±28.9 | 1516.8±26.9 | 1241.7±23.1 | – | – | – |
Figure 3(b) shows the transverse rupture strength (TRS) test results for the V4 Extra, M1, M3 and M5 specimens at different sintering temperatures. As the sintering temperature increased, most of the TRS value obviously increased, and then declined. Among these values, the highest TRS value of V4 Extra and M1 specimens appeared at 1260°C sintering for 1 h; they were 1521.8 and 1874.5 MPa, respectively. However, the highest TRS value of M3 and M5 specimens appeared at 1240 and 1220°C sintering for 1 h: they were 1973.7 and 2281.3 MPa, respectively. This result is consistent with the trend of hardness tests. The highest hardness and TRS occur at the optimum sintering temperature of each test specimen. Our previous studies showed precipitated carbides and porosity levels of Vanadis 4 composite materials to be important factors directly affecting the TRS.4,6) As previously stated, Mo2C additives are not significant in inhibiting grain growth for V4 Extra alloy steels. Apparently, while the effect of grain size is not obvious, the carbide precipitates play an important role in TRS tests. As shown in Fig. 3(b), the TRS value of the M5 specimen dramatically improved after sintering at 1220°C for 1 h. However, the high temperature sintering (1280°C) caused a significant grain-coarsening phenomenon, which was disadvantageous to the TRS values of the V4 Extra, M1 and M3 specimens. In this study, the highest hardness (79.2 HRA) and TRS (2281.3 MPa) values appeared at 1220°C-sintered M5 specimens.
According to the above discussion and test results, the optimal sintering temperature of V4 Extra and M1 specimens was 1260°C, while the M3 and M5 samples was 1240°C and 1220°C, respectively. Hence, the subsequent research used the optimal sintered specimens for further analysis and observation. Figure 4 shows the OM images of 1260°C-sintered V4 Extra and M1, 1240°C-sintered M3, and 1220°C-sintered M5 specimens, respectively. As seen in Fig. 4(a), there is obvious carbide precipitation in the microstructure; at the same time, the round carbides are distributed in the grain and grain boundaries (as indicated by the arrows). Since the V4 Extra alloy composition contains high vanadium content, it is reasonable to speculate that the precipitate is dominated by VC-based MC type carbide. Subsequent EDS analysis will provide further verification. Figure 4(b) represents the OM image of M1 specimens after 1260°C sintering for 1 h. It is found that there are more fine carbides distributed in the grains. Meanwhile, there are also fine round carbides precipitated in the grain boundaries. It is worth noting that the V4 Extra and M1 specimens were sintered at the same temperature (1260°C); they almost possess the same grain size (42.9 and 38.7 μm, respectively). The strengthening mechanism of adding Mo2C is related to the amount and distribution of carbides. Furthermore, the 1240°C-sintered M3 specimens obviously possess two different carbide precipitations, as shown in Fig. 4(c). The internal precipitation of round carbides and the bigger carbides (MC and M6C, as indicated by the arrows) are simultaneously precipitated onto the grain boundaries. Besides, Fig. 4(d) also shows that the round carbides (MC) are distributed in the grain and plate-shaped carbides (M6C) in the grain boundaries obviously appear (as indicated by the arrows). As a result, it is reasonable to surmise that the different kinds of carbides are also an important factor in affecting the strengthening mechanism of V4 Extra composite materials.
The OM images of (a) 1260°C-sintered V4 Extra, (b) 1260°C-sintered M1, (c) 1240°C-sintered M3, and (d) 1220°C-sintered M5, respectively.
As previously discussed, the primary strengthening mechanisms of V4 Extra composites include solid-solution strengthening and precipitation hardening by vanadium, molybdenum and chromium carbides, respectively. In addition, no obvious internal porosities remained; it shows that the specimen has reached complete sintering densification. Furthermore, the precipitated carbides of the 1220°C-sintered M5 specimen on the grain boundaries are very uniform and widespread, and the size of the carbide is also relatively small. Hence, the optimal mechanical properties will be obtained after suitable sintering temperature (1220°C) and adding amount (5 mass% Mo2C).
Figure 5(a) shows the SEM images of the M5 specimen sintered at 1220°C. In this study, the EDS analysis showed that a few refined and spherical-shaped carbides (Location 1) in the grain and grain boundaries had a high content of V-rich and Mo-rich (21.2 at% V and 11.31 at% Mo, respectively), as listed in Table 3 ((a)-1). It is reasonable to surmise that the refined and spherical-shaped carbides mainly belong to V-rich MC carbides. On the other hand, the plate-shaped carbides (Location 2) only distributed in the grain boundaries had a high content of Mo-rich and V-rich (24.56 at% Mo and 10.55 at% V, respectively), as listed in Table 3 ((a)-2). Therefore, it is possible that the carbides mainly belong to Mo-rich M6C carbides. Generally speaking, the morphology and sizes of carbides are used to roughly judge the type of carbides. Moreover, the previous literature indicated that the identification of these MC and M6C carbides was carried out by a combination of EDS and XRD measurements.19) In the research, EDS analysis results (as shown in Table 3) revealed that the spherical-shaped carbides mainly belong to V-rich MC carbides and distributed in the grain boundaries was Mo-rich M6C carbides, respectively. The EDS analysis results is consistent with previous study results, also agreed with our XRD findings.6,7,20)
The SEM images of the optimal sintered M5, M5+HT and M5+SZ specimens (a) optimal sintered M5, (b) M5+HT, and (c) M5+SZ.
| Elements (at %) | (a)-1 | (a)-2 | (c)-1 | (c)-2 |
|---|---|---|---|---|
| C | 63.63 | 53.75 | 62.02 | 62.53 |
| V | 21.20 | 10.55 | 9.74 | 22.28 |
| Cr | 2.56 | 4.78 | 4.42 | 2.46 |
| Mo | 11.31 | 24.56 | 21.32 | 11.53 |
| Fe | 1.30 | 6.36 | 2.50 | 1.21 |
Figure 5(b) represents the SEM images of the 1220°C-sintered M5 specimens after heat treatment (M5+HT). As mentioned previously, the spherical-shaped carbides should be the V-rich MC carbides. Although the MC carbides produced on grain boundaries are dramatically larger than after sintering treatment, there are more evenly distributed fine carbides on the grains. It is speculated that after the quenching and tempering processes, a good dispersion strengthening effect results; thus, it has a positive influence on the subsequent mechanical properties. Further observation of the M5+SZ (quenching, sub-zero and tempering) specimen is shown in Fig. 5(c). The plate-shaped carbides (Location 1) distributed in the grain boundaries had a high content of Mo-rich and V-rich (21.32 at% Mo and 9.74 at% V, respectively) carbides, as listed in Table 3 ((c)-1). It is reasonable to suggest that the carbides mainly belong to Mo-rich M6C carbides. Additionally, the spherical-shaped carbides (Location 2) in the grain and grain boundaries had a high content of V-rich and Mo-rich (22.28 at% V and 11.53 at% Mo, respectively) carbides, as listed in Table 3 ((c)-2). Similarly, the carbides mainly belong to V-rich MC carbides. Meanwhile, there are many refined carbides uniformly distributed on the grains; these are mainly VC and Fe3C carbides.6) It is worth noting that Fig. 5(c) displays more refined carbides in the grains as compared with Fig. 5(b). This should be a great help to the improvement of its mechanical properties; this will be verified in subsequent tests.
A further comparison of the mechanical properties for the different treatments of V4 Extra alloy steel is shown in Fig. 6(a). The complete data are also shown in Table 1. As seen in Fig. 6(a), the hardness has a slight increasing trend after HT and SZ treatments. Among them, the hardness of 1260°C-sintered V4 Extra was about 78.3 HRA, whereas the hardness of M5 (sintered), M5+HT and M5+SZ was about 79.2, 80.6 and 81.1 HRA, respectively. It is possible that the high hardness of the sintered M5+HT specimen was owing to the martensite morphology and the alloying elements solid solutions in the martensite and austenite. Generally speaking, the sintered M5 specimen was not a fully tempered martensite microstructure. It is necessary to undergo sub-zero and heat treatments; the completely tempered martensite structure can then be obtained. Previous studies indicated that sub-zero treatment can encourage carbide formation during tempering, leading to increased hardness.6) As a result, the hardness values of M5+SZ specimens were slightly higher than that of M5 and M5+HT.
Comparison of the (a) hardness and TRS, (b) Tafel results of 1260°C-sintered V4 Extra, and optimal sintered M5, M5+HT and M5+SZ specimens after 3.5 mass% NaCl corrosion tests, respectively. (Online version in color.)
In addition, a comparison of the TRS of the different treatments of V4 Extra specimens is shown in Fig. 6(a) and Table 2. It is worth mentioning that the TRS has a dramatically increasing trend after sintering, sub-zero and heat treatments. Our previous study showed that heat treatment effectively improved the size of the carbides of the alloy steel composite.4,6,20) In the present research, the TRS value of the M5+HT and M5+SZ showed a significant increase, and reached 2437.6 and 2491.4 MPa, respectively. As mentioned previously, the M5+SZ possessed more refined carbides uniformly distributed in the grains (as seen in Fig. 5(c)). It is reasonable to conclude that the plate-shaped carbides in the grain-boundaries and the refined VC and Fe3C carbides reprecipitating within the grains resulted in the increase in TRS. According to the above discussion and test results, the M5 specimen sintered at 1220°C for 1 h, followed by a quenching, sub-zero and tempering treatment, possessed optimal microstructures and mechanical properties.
Figure 6(b) represents the Tafel slope results of 1220°C-sintered V4 Extra, M5, M5+HT and M5+SZ after the 3.5 mass% NaCl corrosion test. All of the specimens exhibited a significant passivation phenomenon, signifying that the passivation surface possessed a protective effect. All the experimental data for the corrosion tests are shown in Table 4. In this work, adding 5 mass% Mo2C powders to V4 Extra did not significantly improve the corrosion resistance, which can be ascribed to the occurrence of intergranular corrosion. The corrosion current slightly decreased (3.95 → 3.12 × 10−4 A·cm−2), and polarization resistance also shows a declining trend (3.04 → 3.01 × 102 Ω·cm2). It is possible that with the increased carbide content, the activity of grain boundaries will be enhanced; as a result, the molybdenum carbide precipitates at the grain boundaries, which easily results in intergranular corrosion.
| V4 Extra | M5 | M5+HT | M5+SZ | |
|---|---|---|---|---|
| Icorr (×10−5 A·cm−2) | 3.95 | 3.12 | 1.21 | 6.07 |
| Ecorr (Volts) | −0.80 | −0.82 | −0.79 | −0.87 |
| Rp (×102 Ω·cm2) | 3.04 | 3.01 | 5.87 | 9.06 |
While the corrosion resistance is not significantly improved after vacuum sintering process, the corrosion resistance is effectively improved after heat treatment and sub-zero plus heat treatments. Generally speaking, higher polarization resistance usually means better corrosion resistance.6,21) As seen in Table 4, the difference in corrosion current is not obvious, but the polarization resistance shows an obviously rise trend (3.04 → 5.87 → 9.06 × 102 Ω·cm2) after the HT and SZ treatments. In this study, the microstructure is more densified after HT and SZ treatments. Meanwhile, all the carbides around the grain boundaries and within the grains have a more uniform distribution, as shown Fig. 4. Significantly, a large number of carbides dissolved into the matrix, while the refined carbides around the grain boundaries and within the grains uniformly re-precipitated. It is possible to say that the HT and SZ treatments effectively improved the size of the carbides and mechanical properties of the Vanadis 4 extra composite materials, which resulted in a significant increase in the polarization resistance. Therefore, a reasonable explanation is that the uniform carbide precipitates of HT and SZ treatments are helpful in decreasing the oxidation rate. In other words, it showed that the HT and SZ treatments can effectively improve the corrosion resistance of sintered V4 Extra composites. Consequently, according to the above discussion and experimental results, it is reasonable to say that the M5+SZ specimen had good corrosion resistance, optimal microstructure and mechanical properties after 1220°C sintering, quenching, sub-zero and tempering treatments.
In this study, adding the appropriate amount of molybdenum carbide proved effective in enhancing the mechanical properties of V4 Extra alloy steel. The M5 specimen (adding 5% mass Mo2C powders) possessed the greatest hardness (79.2 HRA) and TRS value (2281.3 MPa) after sintering at 1220°C for 1 h. The increase in Mo2C content did not significantly improve the corrosion resistance of V4 Extra alloy steel; however, the M5 specimen clearly exhibited a significant passivation phenomenon, indicating that it still had good corrosion resistance.
In addition, the 1220°C-sintered M5 specimens underwent a series of quenching, sub-zero and tempering treatments; this represented that the plate-shaped carbides were distributed in the grain boundaries, which indicated that the carbides mainly belong to Mo-rich M6C carbides. Furthermore, the round-shaped carbides in the grain and grain boundaries showed that the carbides mainly belong to V-rich MC carbides. Meanwhile, there are many refined carbides uniformly distributed on the grains; these are mainly VC and Fe3C carbides. As a result, the highest TRS (2491.4 MPa) and hardness (81.1 HRA), and optimal corrosion resistance (Rp = 9.06 × 102 Ω·cm2) were acquired by adding 5 mass% Mo2C to V4 Extra alloy steel after sintering at 1220°C for 1 h, followed by the quenching, sub-zero and tempering treatments.
This research is supported by the VOESTALPINE HIGH PERFORMANCE METALS PACIFIC PTE. LTD. and ASSAB STEELS TAIWAN CO., LTD. The authors would like to express their appreciation for Dr. Harvard Chen, Michael Liao and Mr. Meng-Yu Liu.
