2018 Volume 59 Issue 10 Pages 1596-1602
In this study, different amounts of vanadium carbide (VC) powders (1, 3 and 5 mass%) are mixed and added to Vanadis 4 steel powders. The composite powders are sintered at 1225, 1250, and 1275°C for 1 h, respectively. The experimental results show that the optimal sintering temperature for Vanadis 4-VC composites is 1250°C. The Vanadis 4 specimens with a 3% VC addition possess the highest transverse rupture strength (TRS) value of 1953.3 MPa, as well as the highest polarization resistance (1.84 × 104 Ω·cm2), while with a 1% VC addition obtain the highest hardness value of 82.9 HRA after sintering at 1250°C. Furthermore, a microstructural evaluation reveals that the round-shaped M23C6 carbides uniformly located on the grain boundaries are gradually increased as VC particles are added, and the fine particle-shaped (V, Fe) carbides are uniformly dispersed in the matrix. After heat treatment, the VC carbides decompose and re-precipitate refined M23C6 and (Cr, Fe) carbides around the grain boundaries and within the grains, which results in dispersion strengthening and precipitation hardening. The results clearly show that heat treatment effectively improves the microstructure and strengthens the matrix of the Vanadis 4-VC composites.
With the growing demand for metallic components in the metalworking industry, powder metallurgy (P/M) can be used to produce components with complex geometries, reduce machining costs and compete with other forming processes for large-scale production.1,2) Besides, P/M is thought to be the most common production technique for ceramic particles reinforced composites.3) One of the advantages of P/M compared to other methods is having better control on the microstructure, where better distribution of the reinforcements is possible in P/M compacts.4)
Vanadis 4 tool steel is a P/M tool steel which has high levels of vanadium, chromium, and molybdenum elements in the matrix. It has similar machining abilities and heat treatment procedures to grade AISI D2. In general, Vanadis 4 tool steels manufactured by P/M have been widely used due to their high strength, wear resistance and ductility. It is characterized by a homogeneous microstructure with uniformly a distributed carbide phases.5,6)
In addition, metal matrix composites (MMCs) are new materials with different types of ceramics as reinforcement phase dispersed in the metal matrix. Additions of carbides such as VC, NbC, or TaC etc. to the powder mixtures are often done in order to increase the wear resistance for high-speed machining operations of steel. These carbides are known to inhibit grain growth in high temperature of P/M process.7,8) Vanadium carbide (VC) have excellent hardness and high melting temperatures (2810°C), effectively inhibiting grain growth and thermal stability with Fe-based alloys; thus, they are great options for reinforcing tool steels.9) According to the above statements, this study utilizes the VC to improve the mechanical properties of the Vanadis 4 tool steels.
In higher carbon content alloyed steels, the martensite finish temperature (Mf) 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 and, in working conditions, it can be transformed into martensite. In order to resolve the problems mentioned above, the cryogenic (also known as sub-zero) treatment is used to transform the retained austenite into martensite.10) Vanadis 4 tool steels contain high amount of carbon and alloying elements (Cr, V, Mo etc.) which remarkably reduce Mf temperature of the steel. Therefore, high amount of retained austenite remains in the body of the Vanadis 4 steel after quenching. By applying sub-zero treatment after quenching to reducing the amount of the austenite is necessary.11)
Previous literature has indicated that the addition of VC not only retarded densification of the WC-ZrO2 material, but also significantly suppressed the growth of WC-grains and ZrO2-particles.12) Since minor VC is effective to increase the hardness of WC-Co hardmetals through WC grain growth inhibition.13) In recent years, Vanadis 4 tool steel manufactured by P/M, has been widely used due to its high strength and high toughness. Previous study also indicated that the hardness of as-sintered tool steels increase after heat treatment due to the secondary hardening phenomenon.14) However, the effect of VC-strengthened Vanadis 4 tool steel has not been clearly studied. The aim of this work was to explore a series of vacuum sintering by using the P/M method and heat treatments for Vanadis 4 composite materials in order to examine effects on the microstructure, hardness and transverse rupture strength, as well as corrosion resistance of VC-strengthened Vanadis 4 tool steels.
In our earlier studies, the optimal vacuum sintering for Vanadis 4 steel occurred at 1250°C for 1 h.3) The aim of this study was to continue using Vanadis 4 tool steel and adding different ratios of VC as a strengthening phase in order to explore the effects of a series of vacuum sintering and heat treatments. In this work, the morphology of the gas-atomized Vanadis 4 steel powders, used as a substrate, showed round particle characteristics. In order to obtain more uniform powders, particles smaller than 30 µm were sifted from the original powders. As a result, the mean particle size of the Vanadis 4 steel powders was 17.3 ± 1 µm. The chemical compositions (mass%) of the Vanadis 4 steel powders are as follows: 8% Cr, 4.0% V, 1.5% C, 1.5% Mo, 1.0% Si, 0.4% Mn and a balance of Fe.
Furthermore, the surface morphology of VC powders is a polygon and irregular, and there were no smooth or undulating surfaces. The mean particle size of the VC powders was about 1.7 ± 0.5 µm. The different amounts of VC powders (1, 3 and 5 mass%) were mixed and added to Vanadis 4 steel powders, designated as V1, V3 and V5, hereafter. The mixed powders were milled by using the WC balls for 6 h. The surface morphology shows the mechanical interlocking adhesion between the Vanadis 4 steel powder and VC powders by ball milling for 6 h, and the mixed alloy powders produced a significantly plastic deformation.
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 this study, the vacuum sintering was conducted at 1225, 1250 and 1275°C for 1 h in a 1.33 × 10−2 pa, respectively. In addition, a series of heat treatments (quenching followed by tempering) was performed, by which the optimal samples were heated to 1020°C and maintained at that temperature for 100 min for quenching, with 0.5 MPa of N2 for quenching media, and then subjected to sub-zero treatment (−150°C for 60 min). Sub-zero treatment is carried out in order to complete the transformation of retained austenite to martensite before tempering. Meanwhile, the tempering temperature was held at 480°C for 150 min and repeated twice.
To evaluate the sintered behavior of Vanadis 4 steels added VC powders by vacuum sintering and heat treatments, the apparent porosity, hardness, transverse rupture strength (TRS) tests, corrosion tests (Potential Stat Chi 601) and microstructure observations were performed. Microstructural observations of the specimens were performed by optical microscopy and scanning electron microscopy (Hitachi-S4700). Porosity test (apparent porosity) 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, Rbm was the transverse rupture strength, which is determined as the fracture stress in the surface zone. F was maximum fracture load, L was 30 mm, k was chamfer correction factor (normally 1.00–1.02), b and h were 5 mm in the equation Rbm = 3FLk/2bh2, respectively. The specimen dimensions of the TRS test were 5 × 5 × 40 mm3 and tests at least three pieces.
In addition, 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 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) and corrosion current (Icorr) of different sintering parameters. Finally, a comparison was conducted to determine the polarization resistance (Rp) of the various sintered and heat-treated Vanadis 4-VC composites.
Table 1 lists the volume shrinkage, apparent porosity and relative density of various VC powders added to Vanadis 4 steel as a result of different sintering temperatures. The volume shrinkage of specimens V1, V3 and V5 had a similar trend. As the sintering temperature increased from 1225°C to 1275°C, the volume shrinkage of specimens V1, V3 and V5 showed a significant increase, especially for the V5 specimen. It is reasonable to suggest that the refined VC powders (1.7 µm) were easier to rearrange and entered the interspaces of the Vanadis 4 particles (17.3 µm) during the formation process. As a result, a high density of the compact materials was acquired. As for the apparent porosity level, the porosities of specimens V1, V3 and V5 decreased as the sintering temperature increased, and the porosity levels of all specimens were less than 1% after vacuum sintering at 1250°C and 1275°C for 1 h (>98% of theoretical full density). Actually, excess VC powders can obstruct liquid phase diffusion during the sintering process. Generally speaking, when a solid skeleton develops before full densification during the sintering process, the diffusion stage of densification is often enhanced since it occurs by liquid phase diffusion. In this case, the porosity level of the V3 specimens sintered at 1275°C obviously decreased to 0.22%. It is possible to say that the V3 specimens underwent an appropriate liquid phase diffusion after being sintered at 1275°C for 1 h.
In addition, OM images of the Vanadis 4-VC samples showed the similar evolution of microstructure under the different sintering temperatures. Taking specimens sintered at 1250°C for 1 h as an example, the OM images of specimens V1, V3 and V5 are shown in Fig. 1. Significantly, there were some small particles produced in the grain boundaries, as indicated by the arrow in Figs. 1(a)–1(c). Our previous study indicated that the refined carbides in the grain boundaries possibly were Fe-rich, Cr-rich and a small amount of Mo-rich and V-rich carbides (M23C6 types).14) The result will be further examined in subsequent discussions. A comparison of Figs. 1(a), 1(b) and 1(c) shows that the amount and size of the residual porosity were significantly increased as the amount of VC added powders increased. V5 specimens had more continuous pores in the grain boundaries resulting from the restricted diffusion of the VC particles under the 1250°C sintering temperatures. Besides, the residual porosity of V5 specimens displayed an obvious decrease as the sintered temperature increased to 1275°C (Fig. 1(d)). Figure 1 also reveals that the specimens with more VC powders added required a higher temperature to provide sufficient energy for a full density of the Vanadis 4-VC materials. Thus, the relative density of V5 specimens was lower than that of specimens V1 and V3 after sintering at 1250°C.
The OM images of V1, V3 and V5 specimens by sintering at 1250°C and 1275°C (a) V1 sintered at 1250°C, (b) V3 sintered at 1250°C, (c) V5 sintered at 1250°C, and (d) V5 sintered at 1275°C.
In the present research, the relative density of specimens V1 and V3 rapidly increased to over 7.2 g·cm−3. However, the relative density of V5 specimens showed a slight increase (6.0 → 6.4 g·cm−3) as the sintering temperature increased to 1250°C. After further increasing the sintering temperatures to 1275°C, the relative density of V5 specimens (Table 1, Fig. 1(d)) displayed a significant improvement (from 6.4 to 7.5 g·cm−3). The results suggest that the sintering temperature of 1250°C corresponded to the ideal sintering temperature for V1 and V3 sintered specimens. As for the higher amount of VC powders added, the relative density of the V5 specimen was relatively low after sintering at 1250°C, but it increased to 7.5 g·cm−3 as the sintering temperature increased to 1275°C. This can be ascribed to draining off the pores from the high-energy grain boundaries and increasing the relative density. Consequently, the near-full density of Vanadis 4-VC composite materials was acquired.
Figure 2 shows the SEM images and EDS analysis of the Vanadis 4-VC specimens after sintering at different temperatures. The EDS results are listed in Table 2. Compared with the specimens with different amounts of VC added, the distribution of the carbides was similar, as seen by taking a sample from each under different sintering temperatures. As Fig. 2(a) shows, the carbides (gray color) were uniformly distributed around the grain boundaries and within the grains. The EDS analysis results revealed that the round-like carbides (Location 1) in the grain boundaries and the particle-shaped carbides (Location 2) within the grains were V-rich and Cr-rich M23C6 carbides and (V, Fe) carbides, respectively. After increasing the sintering temperatures to 1250°C, particle-shaped carbides (Location 2) appeared within the grains, which became fewer and finer. Significantly, the round-like carbides (Location 1) in the grain boundaries gradually grew larger, as shown in Fig. 2(b). The EDS analysis results reveal that the round-like carbides in the grain boundaries were V-rich and Cr-rich M23C6 carbides, and the particle-shaped carbides located within the grains were V-rich and Fe-rich (V, Fe) carbides. It is reasonable to infer that more uniformly dispersed carbides within the matrix are advantageous to dispersion hardening which affects the mechanical properties.
The SEM images and EDS analysis of V1, V3 and V5 specimens by various sintering temperatures (a) V1 sintered at 1225°C, (b) V3 sintered at 1250°C, (c) V5 sintered at 1275°C, and (d) Vanadis 4 sintered at 1250°C.
When the sintering temperature was raised to 1275°C, the particle-shaped carbides within the grains obviously decreased or disappeared, as shown in Fig. 2(c). Meanwhile, the M23C6 carbides (Location 2) not only appeared in the grain boundaries but also a few precipitated into the grains. Besides, threadlike carbides (Location 1) in the grain boundaries can be observed. The EDS analysis results reveal that the round-like carbides and threadlike carbides in the grain boundaries were V-rich and Cr-rich M23C6 carbides and V-rich M7C3 carbides, respectively. As a result we found that more VC powders added to Vanadis 4 resulted in two kinds of strengthening carbide precipitation in the grain boundaries. The results, further compared with Vanadis 4 specimens, are shown in Fig. 2(d). The carbide distribution of Vanadis 4 revealed that the threadlike carbides in the grain boundaries were V-rich M7C3 carbides (Location 2), and the particle-shaped carbides within the grain were V-rich VC carbides (Location 1). Thus, the strengthening effect of the Vanadis 4 specimens resulted from two kinds of carbides (M7C3 carbides and VC carbides). However, the carbides (M7C3) in the grain boundaries tended to disappear, and the carbides (M23C6) in the grain boundaries re-precipitated as VC powders (1% and 3%) were added to the Vanadis 4 steel.
Besides, Table 3 displays that the average grain size obviously increased as the sintering temperature increased. The higher sintering temperature easily caused a grain-coarsening phenomenon. According to a previous study the grain coarsening was disadvantageous to the TRS.15) The results, when further compared with those in Fig. 1, showed that the average grain size decreased as the amount of added VC powders increased. The average grain size showed a significant decrease (24.8 → 13.0 → 8.7 µm), as shown in Figs. 1(a), 1(b) and 1(c). Apparently, the VC played an important role in inhibiting grain growth during the vacuum sintering process. Our previous study indicated that the average grain sizes for different NbC added to the Vanadis 4 specimens were from 16.96 to 13.49 µm after the optimal sintering temperature.16) As a result, the VC added to the Vanadis 4 steel possessed a better effect on inhibiting grain growth.
Figure 3 shows the hardness and TRS test results of specimens V1, V3 and V5 after using different sintering temperatures. As Fig. 3(a) shows, the hardness of specimens V1, V3 and V5 was in reverse proportion to the porosity level (Table 1) and was lower than that of the Vanadis 4 steel after sintering at 1225°C for 1 h. The result showed that a sintering temperature of 1225°C for Vanadis 4-VC composite materials was not high enough. Moreover, the hardness values of specimens V1 and V3 increased to higher values (82.9 and 82.1 HRA, respectively) than that of Vanadis 4 specimens after the sintering temperature reached 1250°C. However, the hardness of V5 specimens obviously improved when the sintering temperature was raised to 1275°C. The results suggest that specimens V1 and V3 reached an optimal sintering result after being sintered at 1250°C, and V5 specimens possessed a better hardness at 1275°C. Meanwhile, the hardness values of specimens V1 and V3 tended to decrease slightly resulting from the grain coarsening after sintering at 1275°C. In addition, the (V, Fe) carbides within the grains tended to decrease, and re-precipitated carbides (M23C6) in the grain boundaries displayed an increase in the volume fraction of the carbides as the amount of added VC powders increased. Our previous study indicated that the carbide distribution, size and type will directly affect the hardness and TRS values.17) Therefore, it is reasonable to suggest that the porosity and carbides are main factors affecting the hardness.
Comparison of the hardness and TRS of various mass% VC added Vanadis 4 by the different sintering temperatures.
As Fig. 3(b) shows, the TRS values of specimens V1 and V3 first increased and then decreased. Besides, V5 specimens tended to increase as the sintering temperature increased from 1225 to 1275°C. Specimens V1 and V3 possessed better TRS after sintering at 1250°C, where V3 specimens have the highest TRS (1953.3 MPa). Our previous study indicated that the precipitated refined carbides and porosity levels of Vanadis 4 composite materials are important factors that directly affect the TRS.6,16) If the sintered specimen was not fully densified, the materials possessed numerous small pores. When the specimen was subjected to extra stress during the TRS tests, cracks grew along the internal pores, resulting in rapid fracturing.
In addition, Fig. 2(b) shows that the round-like carbides in the grain boundaries and the small particle-shaped carbides within the grains show uniform distribution. The carbides are able to resist the movement of dislocation, which is advantageous to TRS. From the above-mentioned discussion, we can see that the higher content of V5 specimens must reach 1275°C to gain a fully densified microstructure (Fig. 2(d)) resulting in a relatively high TRS value. Moreover, the TRS values of specimens V1 and V3 obviously decreased as the sintering temperature increased to 1275°C. Although the porosity of the V3 specimens decreased less than 0.3%, the higher sintering temperature (1275°C) caused a grain-coarsening phenomenon which seemed to significantly affect the TRS values. However, the grain coarsening phenomenon is not the only factor that affects the TRS value. In this study, VC dissolving into the matrix and M23C6 carbides precipitating in the grain boundaries also affected the TRS values. When the sintering temperature was raised to 1275°C, the carbides (M23C6) precipitation was too large to contribute to the TRS properties that were disadvantageous to the TRS. It is reasonable to infer that a suitable amount of carbides uniformly distributed in the matrix and a lower porosity (0.27%) are effective in improving the TRS. Consequently, the V3 specimens sintered at 1250°C for 1 h had the optimal TRS value and a suitable hardness.
Figure 4 shows the fracture feature of specimens V3 and V5 after sintering at different temperatures for 1 h. The fracture feature of specimens with the same amount of VC added was similar after sintering at 1225, 1250 and 1275°C, respectively. Taking the V3 specimen as an example, many pores existed in the fracture surface. It was confirmed that a sintering temperature of 1225°C is not enough for it to become fully densified, as shown in Fig. 4(a). When the sintering temperature is raised to 1250°C, many dimples and some small cleavages could be observed, as shown in Fig. 4(b). The presence of a greater number of dislocations was generated by the plastic extension when the fracture was under continually increasing loads. The carbides in the matrix resisted the movement of dislocation, and then, the strength improved.18) As the load increased, the general aspect of the fracture showed dimpled ruptures. Meanwhile, the carbides broke up into smaller particles during deformation resulting from the ductile behavior.
The fracture surfaces of V3 and V5 specimens after sintering at different temperatures (a) V3 sintered at 1225°C, (b) V3 sintered at 1250°C, (c) V3 sintered at 1275°C, and (d) V5 sintered at 1275°C.
When the sintering temperature increased to 1275°C, the V3 specimens caused a grain-coarsening phenomenon, and the excess round-like M23C6 carbides re-precipitating in the grain boundaries contributed to the brittleness. Although the fracture surfaces is a little difficult to distinguish. However, the fracture surface observation was in agreement with the findings that brittle cleavage was easily generated on the grain boundaries, as shown in Fig. 4(c). In addition, many pores and cleavage features appeared on the fracture surface of the V5 specimen resulting from higher amounts of VC being added after sintering at 1250°C. However, Fig. 4(d) shows many dimple fracture features in the V5 specimens after sintering at 1275°C. The ductile behavior is the main fracture characteristic, which is ascribed to a full density of V5 specimens. Although the V3 sintered at 1250°C and V5 sintered at 1275°C have the similar fracture features, the average grain size for V3 specimens (13.01 µm) sintered at 1250°C is smaller than that of V5 specimens (29.68 µm) sintered at 1275°C. Thus, the V3 specimens possess the highest TRS value.
Figure 5 shows the Tafel slope results for sintered specimens V1, V3 and V5 at 1250°C as well as 1250°C-sintered Vanadis 4 after the 3.5 mass% NaCl corrosion test. Where, the voltage of vertical axis is shown E/Volts, and the voltage is V (volts) vs. SCE (saturated calomel electrode). All the specimens possessed a significant passivation phenomenon, especially for V3 specimens. Since the passivation surface possessed a protective effect, this showed that the sintered V3 specimens had the best corrosion resistance. Additionally, all the experimental values for the corrosion tests are shown in Table 4. Actually, in an electrochemical reaction, higher polarization resistance usually means better corrosion resistance. The apparent porosity level should be the main factor affecting corrosion resistance. In addition, the higher the amount of VC added to Vanadis 4, the more carbides that precipitated in the grain boundaries. In poly-crystalline materials, the grain boundaries also act as a site where not only orientation but also chemistry undergoes an abrupt transition. Also, segregation of a phase active in certain atmospheres also raises the energy at the grain boundaries. Excess carbides precipitated in the grain boundaries easily caused electrochemical non-uniformity resulting in intergranular corrosion.19) This is why V5 specimens possess the lowest corrosion resistance.
Tafel results of the V1, V3, V5 and Vanadis 4 specimens after the optimal sintering (sintered at 1250°C for 1 h) after 3.5 mass% NaCl corrosion tests.
On the other hand, the polarization current must be considered. The current value represents the diversification of the equilibrium constants in the oxidation reaction. If the current value is higher, it leads to an increase in the equilibrium constant and a fast oxidation. In this work, adding VC to Vanadis 4 led to a significant decline in the corrosion current. The polarization current of specimen V3 possessed the lowest current value. Clearly, specimen V3 possessed the minimum corrosion current (Icorr = 1.8 × 10−4 A·cm−2) and the highest polarization resistance (Rp = 18.42 × 103 Ω·cm2). Compared to the original Vanadis 4 specimens (Icorr = 61.2 × 10−4 A·cm−2 and Rp = 2.19 × 103 Ω·cm2), the addition of an appropriate amount of VC is helpful in decreasing the oxidation rate. According to the above discussion and results, it is suitable to say that specimen V3 had the optimal microstructure, corrosion resistance and mechanical properties after sintering at 1250°C for 1 h.
3.3 Effect of the sub-zero heat treatment on the optimal Vanadis 4-VC composite materialsFigure 6 shows the SEM images, fracture surface and EDS analysis of the optimal V3 specimens after heat treatment. As compared with Fig. 2(b), the SZ-V3 specimens (sub-zero heat treatment and sintered at 1250°C for 1 h) clearly showed that the microstructure is more densified (the porosity is a slight decrease). The round-like black carbides (Location 1) and the grey carbides (Location 2) around the grain boundaries and within the grains have a more uniform distribution, as shown in Fig. 6(a). A reasonable explanation for this effect is that a large number of VC carbides dissolved into the matrix, while the round-like black carbides and refined carbides (grey) around the grain boundaries and within the grains uniformly re-precipitated. The EDS analysis results reveal that the round-like black carbides (Location 1) were V-rich M23C6 carbides, and refined grey carbides (Location 2) were Cr and Fe rich (Cr, Fe) carbides. It is well known that sub-zero treatment increases hardness by decreasing the retained austenite.
The SEM images, fracture surface and EDS analysis of SZ-V3 specimens (a) SEM image, (b) fracture surface and (c) EDS analysis of the Fig. 6(a).
In our present study, it was found that the hardness of the V3 sample was about 82.1 HRA whereas the hardness of the SZ-V3 sample was about 82.8 HRA. It is possible to say that the high hardness of the V3 sample is due to martensite morphology and dissolved alloying elements in martensite and austenite. Moreover, sub-zero heat treatment could have encouraged carbide formation during tempering, leading to increased hardness.10) Otherwise, a previous study also indicated that in the sub-zero and heat treatment, the variation in hardness was dominated by the tempering process, which released the internal stress from the tempering treatment. As a result, a slight increase in hardness could be ascribed to the more dense microstructures and re-precipitated carbides uniform distribution in the matrix.
As for the TRS, the SZ-V3 specimen revealed a significant increase, as shown in Fig. 7. The TRS value increased from 1953.3 to 2066.3 MPa. Generally speaking, the addition of an excess amount of carbide made the carbide particles pile up and cluster in the grain boundaries, which was detrimental to the TRS. Moreover, a suitable amount of carbides uniformly re-precipitated within the grains resulted in the increased TRS. In this study, it is reasonable to infer that a large number of VC carbides dissolving and refined M23C6 and (Fe, Cr) carbides re-precipitating around the grain boundaries and within the grains resulted in the increase in the TRS. Thus, the TRS of the SZ-V3 specimens revealed a significant increase resulting from the dispersion strengthening and precipitation hardening effects. In the TRS test, the uniformly refined carbides resisted the movement of dislocation, and then, the strength improved as the load increased. The fracture surface of such a ductile fracture appears as spider web-like dimpled ruptures when observed on SEM images, as shown in Fig. 6(b). From the above results and discussion, it is possible that the V3 specimens sintered at 1250°C for 1 h, followed by heat treatment, possessed optimal mechanical properties.
Comparison of the hardness and TRS of Vanadis 4-VC specimen after the optimally sintering temperature (1250°C) and heat treatment.
Table 4 also lists the Tafel slope results for SZ-V3 specimens after 3.5 mass% NaCl corrosion tests. The corrosion resistance of the V3 specimens had a significant variation after sub-zero heat treatment. The polarization current of the SZ-V3 specimens is the highest current value and close to that of Vanadis 4 specimens. Moreover, the polarization resistance of the SZ-V3 specimens was the lowest resistance value. It is possible to say that VC carbides dissolving and too many refined M23C6 and (Cr, Fe) carbides re-precipitating in the grain boundaries after the sub-zero heat treatment resulted in a significant decrease in corrosion resistance. This could be ascribed to more refined carbides re-precipitating in the grain boundaries, which resulted in intergranular corrosion. Although heat treatment did not improve the corrosion resistance of the V3 composite materials, it is possible to suggest that the V3 specimens sintered at 1250°C for 1 h, followed by an optimal heat treatment, which possessed optimal mechanical properties and suitable anti-corrosion ability in the 3.5 mass% NaCl solution.
In this study, the V3 specimen possessed the highest TRS value (1953.3 MPa), and the hardness reached 82.1 HRA after sintering at 1250°C for 1 h. The corrosion resistance of the V3 specimen was significantly improved compared with that of Vanadis 4 tool steels. The polarization resistance (Rp) of the V3 specimens increased from 2.19 × 103 to 1.84 × 104 Ω·cm2. The hardness, TRS and corrosion resistance of the composite materials significantly increased after adding 3% VC powder to Vanadis 4 tool steels.
For the V3 specimens, round-like M23C6 carbides were uniformly dispersed in the grain boundaries, and refined particle-shaped (V, Fe) carbides uniformly appeared within the grains after optimal sintering temperatures. When the optimally sintered specimens underwent a series of sub-zero heat treatments, the VC carbides decomposed and re-precipitated as refined M23C6 and (Cr, Fe) carbides around the grain boundaries and within the grains, which resulted in dispersion strengthening and precipitation hardening, thereby increasing the hardness and TRS of the Vanadis 4 composites.
Sub-zero heat treatment effectively improved the size of the carbides and mechanical properties of the Vanadis 4 composite materials while the corrosive resistance displayed a significant decrease. The optimal TRS (2066.3 MPa) and hardness (82.8 HRA) were obtained for Vanadis 4 tool steel by adding 3% VC after sintering at 1250°C for 1 h, as well as undergoing a series of heat treatments.
This research is supported by the Ministry of Science and Technology of the Republic of China under Grant # MOST 106-2221-E-027-035-. The authors would like to express their appreciations for VOESTALPINE HIGH PERFORMANCE METALS PACIFIC PTE. LTD. and ASSAB STEELS TAIWAN CO., LTD.
