Available online 29 October 2020
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Fabrication of chromium carbide cermets by electric resistance sintering process: Processing, microstructure and mechanical properties
M.A. Lagos
a,*, I. Agote
a, I. Leizaola
a, D. Lopez
b, J.A. Calero
baTECNALIA, Basque Research and Technology Alliance (BRTA), Mikeletegi Pasealekua 2, 20009 Donostia-San Sebasti´an, Spain
bALEACIONES DE METALES SINTERIZADOS (AMES), Ctra. Nac. 340 Km 1242, Pol. Ind. "Les Fallulles, 08620 Barcelona, Spain
A B S T R A C T
Chromium carbide-based cermets are suitable for use in abrasive and corrosive environments. This work presents the fabrication of chromium carbide-based cermets by a very fast sintering process: Electric Resistance Sintering. The thermal cycle duration was less than 1 s and without protective atmosphere.
Two different compositions were studied: Cr3C2-25NiCr (wt%) and WC-20Cr3C2-7Ni (wt%). Microstructure and crystallographic phases of the initial powders and sintered materials are presented. In addition, hardness and toughness were characterized and compared to conventional materials.
One important issue of ERS is the size and homogeneity of the pieces. This work presents the also the fabrication of a mining wear piece and some aspects about scaling up.
1. Introduction
Chromium carbide-based cermets exhibit excellent physical, anti- wear, and corrosion properties, and are suitable for use in abrasive and corrosive environments [1–3]. Compared to WC-Co, chromium carbide-based materials present superior oxidation and corrosion resis- tance and lower wear at high temperatures [1–3]. It is reported that Cr2C3 is stable up to the temperature of 950 ◦C, and therefore can protect the tool at high temperature [4]. Typically, the addition of chromium carbide is positive at temperatures higher than 600 ◦C [5].
ECAS (Electric Current Assisted Sintering), also known as FAST (Field Assisted Sintering Techniques), gathers a family of consolidation methods in which mechanical pressure is combined with electric and thermal fields to enhance particle bonding and densification. The pri- mary purpose of the imposed electric current is to provide the required amount of Joule heat. ERS (Electric Resistance Sintering) is an ultrafast ECAS technique [6] where high current density (typically >5 kA/cm2) is applied to the powder for consolidation and sintering. The processing time is typically lower than one second and no protective atmosphere is required [7]. This short duration of the process is the main difference with conventional ECAS techniques like Spark Plasma Sintering [6].
The ERS process has been used for the consolidation of different highly conductive materials like iron [8], titanium [9] and permanent magnets [10]. In addition, previous works showed the possibility of obtaining less conductive materials like WC-Co [11,12]. Materials ob- tained by ERS presented higher hardness and similar fracture toughness
than conventional materials obtained by Sinter-HIP [6,11], because of the lower grain size produced by the very short processing time.
Chromium carbide-based materials have been obtained by Spark Plasma Sintering with very high density and good mechanical properties [13,14]. However, there are not studies about the fabrication of these materials by ultrafast methods like ERS.
This work presents the fabrication of chromium carbide-based cer- mets by Electric Resistance Sintering. As the process is performed in air, no organic binders can be present in the raw material. For that reason, in this work commercial pre-sintered powders were used. These powders are typically used for thermal spray processes. Two different composi- tions were tested: Cr3C2-25NiCr (wt%) and WC-20Cr3C2-7Ni (wt%). One of them presents only chromium carbide as hard phase and the other is a combination with tungsten carbide. In both cases, the binder phase is based in nickel. The combination of chromium and tungsten carbide can produce a good compromise between mechanical properties and corrosion and oxidation resistance [15–19]. The matrix composition plays a key role in attaining the required properties of bulk cermets as well as coatings [20]. Co is the most commonly used binder used with to high wettability of WC by Co during sintering and adhesion character- istics [20]. Attempts have been made to replace Co with other matrix material such as Ni as driven by a variety of factors such as the Co is relatively scarce, it has low corrosion resistance and it is considered a critical raw material by the European Union [21]. Although an increase in oxidation and corrosion resistance was noticed in WC based cermets with Ni matrix, the mechanical properties of WC-Ni were found to be
* Corresponding author.
E-mail address: [email protected] (M.A. Lagos).
Contents lists available at ScienceDirect
International Journal of Refractory Metals and Hard Materials
journal homepage: www.elsevier.com/locate/IJRMHM
https://doi.org/10.1016/j.ijrmhm.2020.105417
Received 28 July 2020; Received in revised form 23 October 2020; Accepted 23 October 2020
inferior to WC-Co [22]. The addition of Cr or complex binders, as well as the using of secondary carbides, improves the mechanical properties and help in the substitution of Co for some applications [23,24].
This paper presents the microstructural and mechanical character- ization of the chromium carbide based cermets obtained in a very fast sintering process. The very fast cooling process has an important effect in the microstructure and properties of the obtained materials compared to conventionally heated ones.
2. Experimental
Commercial agglomerated and sintered powders were used with two different compositions: Cr3C2-25NiCr and WC-20Cr3C2-7Ni. The chem- ical composition of the powders is presented in Table 1. In both cases, the particle size is lower than 50 μm with a d50 around 30 μm (see Table 2). This kind of powders are typically used for thermal spray processes and they do not present any kind of binder as they are pre- sintered by the supplier. In this study, the ERS process was performed in air and it is not possible to use binders (combustion gases are pro- duced during sintering).
For the ERS processing, the powder was filled in a ceramic die be- tween two copper electrodes. More details about the equipment are explained in a previous work [7]. The dimensions of the produced parts were 17 mm in diameter with a thickness of 5–8 mm. The maximum applied current density was between 4 and 6 kA/cm2 with a holding time of 500 ms. Maximum load was 200 MPa. The pressure was applied using a 15 tons electrical press (Figs. 1 and 2). In addition, rectangular samples of 20X20X10 mm were obtained in order to study the scalability of the process.
Density was measured by Archimedes method using a Mettler AE 240 weight balance and porosity was analysed according to the standard
UNE-EN ISO 4605:1978. Microstructure and semi-quantitative chemical composition were analysed by EDS in combination with SEM micro- scopy (Zeiss Nvision 40 and Zeiss EVO50). All images are presented in BES mode (backscattered electrons). Particle size of the powder measured by Laser Light Diffraction (ASTM C 1070).
Hardness Vickers (HV30) was measured according to the standard UNE-EN ISO 6507-1:2006. KIC was calculated from the length of the radial cracks originated in the corners of the Vickers indentations ac- cording to the formula proposed by Shetty et al. [25].
3. Results and discussion
The ERS samples typically consist of a dense core surrounded by a porous surface layer as it is shown in Fig. 2. It has been observed that increasing the current density, the area of the external porous layer decreases. The thickness of the outer porous layer in contact with the metallic punches (top and bottom of the sample) is thinner than that in contact with the ceramic die walls. Porosity at the core is very low (A02 according to the standard UNE-EN ISO 4605:1978). In ERS process, the electrical current passes across the material and the heating is produced by joule’s effect. Temperature in the material depends on different factors. One of the most important ones is the green density distribution Table 1
Characteristics of the raw materials (wt%).
Material Cr W Ni Fe C Particle size (microns) Supplier
Cr3C2-25NiCr Balance – 18–20 0,5 10 <53 μm AMPERIT 584.054
WC-20Cr3C2-7Ni 17–21 Balance 6,5–8 0,3 6,3-7,3 <50 μm AMPERIT 551.074
Table 2
Particle size of the powders (microns).
Particle size of the powders (microns) D10 D50 D90
Cr3C2-25NiCr 18 30 53
WC-20Cr3C2-7Ni 18 29 50
Fig. 1. ERS equipment (AMES): A) General view, B) Punches and die.
Fig. 2. Cross section of a part of Cr3C2-25NiCr produced by ERS, distribution of the porosity (scale bar 1 mm).
of the parts. It is known that in a double sided pressing, green density of the parts is lower at the core. Where porosity is higher, electrical resistance is also higher. This fact produces higher temperature at the core of the material during resistance heating. Another important factor is the heat losses produced by conduction from the material to the dies and electrodes; this also increases the temperature gradient from the core of the parts to the borders. For that reason, external porosity is produced by the fast cooling of the material in contact to the copper punches and ceramic die.
Densities of pieces obtained at different values of current density are presented in Table 3. Both materials present similar values of densifi- cation. In ERS, the densification is affected by thermo-mechanical and electrical aspects. In general, the more electrically and thermally conductive the material is, the more homogenous the heating is. In this case, both materials are different in terms of the amount of metallic binder. Cr3C2-25NiCr presents around 9 vol% of metallic binder phase
and WC-20Cr3C2-7Ni around the 20 vol%. However, there are other aspects that can affect the densification like the thermal conductivity of the ceramic phase, the size and shape of the ceramic particles and the homogeneity in the distribution of the reinforcement/binder. The dif- ferences between the powders will be described in this paragraph but not substantial effects were found with regards to the sintering parameters.
The density values presented in Table 3 show values between 90 and 96% of the theoretical density since the outer porous layer is included.
As indicated in Fig. 2, there is a porous layer surrounding the sample.
Fig. 3 shows the microstructure at the core of the ERS sample for Cr3C2-25NiCr. In addition, Fig. 4 shows the microstructure of the initial powder before sintering at two different magnifications (images of the cross section of the powder). Each grain of powder is porous, and it is composed of carbides and binder. These commercial powders are ob- tained by agglomeration of a suspension and a sintering process.
In the powder and ERS sample, three different regions can be iden- tified (labelled as 1, 2 and 3 at the images). The EDX analysis of the different regions is also presented in Table 4. Region 1 (dark grey) corresponds to the chromium rich phase and the region 3 (light grey) corresponds to the nickel‑chromium area. Region 2 is an intermediate phase rich in chromium with a residual amount of nickel.
It is interesting to observe that grain growth was not very significant during the ERS process. Carbide grains are in the range of 5 μm for the initial powder and the sintered sample.
Chromium carbides exhibit three different crystallographic struc- tures: cubic Cr23C6, orthorhombic Cr3C2, and hexagonal Cr7C3. Among these, Cr3C2 is the most stable structure [26]. The XRD patterns of the initial powder and ERS sample are compared in Fig. 5. In both cases, Cr3C2 was identified as the main phase with Cr7C3 as a secondary phase.
NiCr binder can be observed also in both cases. Chromium carbides could correspond to the grey areas observed in the SEM images (see Fig. 3) and the light areas to the NiCr binder. Cr3C2 is not stable against pure nickel, the reaction forms Cr7C3, a NiCr metallic binder and some free carbon [27].
Regarding the microstructure of WC-20Cr3C2-7Ni, Fig. 6 shows the microstructure of the core of an ERS sintered sample. For comparison, Fig. 7 shows the microstructure of the initial powder before sintering at two different magnifications (images of the cross section of the powder).
The EDX analysis of the different regions is also presented in Table 5.
In the sintered sample (Fig. 6), small white grains (region 3) are regions rich in tungsten (surely tungsten carbide grains). Grey areas (1, 2 and 4) are areas with different amounts of chromium, tungsten and nickel (the lighter the area, the higher the amount of tungsten). Spe- cifically, region 4 is a region with an important amount of nickel binder.
The microstructure of the powder (Fig. 7) is similar but it is more difficult to see differences within the grey areas (regions 1, 2 and 4).
Grain growth was not very significant during the ERS process.
Table 3
ERS tests: current vs density.
Ref Composition Current (KA/cm2) % of theoretical densitya
A Cr3C2-25NiCr 4 93.2 ± 0,5
B Cr3C2-25NiCr 5 96.1 ± 0,4
C Cr3C2-25NiCrb 6 95.6 ± 0,4
D WC-20Cr3C2-7Ni 4 90.0 ± 0,7
E WC-20Cr3C2-7Ni 5 95.3 ± 0,4
F WC-20Cr3C2-7Ni 6 96.0 ± 0,5
aFive identical samples were obtained for each condition and the average value and standard deviation are presented.
b Sample welded to the electrodes.
Fig. 3. SEM image of a Cr3C2-25NiCr ERS sintered sample (ref B), scale bar 5 μm (cross section).
Fig. 4. SEM images of the cross sections of the Cr3C2-25NiCr powders at two different magnifications: A) Scale bar 50 μm, B) Scale bar 5 μm.
Tungsten carbide grains are below 2 μm and chromium carbide regions in the range of 5 μm for the initial powder and the sintered sample.
The XRD patterns of the initial powder and ERS sample are compared in Fig. 8. In both cases, main phase is the hexagonal WC. This phase could correspond to the white grains observed in the microstructural analysis. In addition, the nickel/chromium binder and the Cr3C2 were also identified. There are other small picks that could correspond to complex carbides: (W,Cr)2C. The formation of complex carbides during the thermal deposition of WC-20Cr3C2-7Ni powder was also observed by Bolelli et al. [27]. The different carbides mixed with the binder corre- spond to the grey areas observed in the SEM analysis (see Fig. 6). Nickel
presents an eutectic temperature of 1255 ◦C in the Cr3C2–Ni system [28].
The solubility of WC and Cr3C2 in nickel at 1250 ◦C is high and corre- sponds to 22% and 12%, respectively. This leads to the increased overall volume fraction of matrix phase.
Microstructures observed after ERS are similar to the obtained ones by thermal spray processes [28], typically used to obtain coatings. ERS is a sintering process to obtain bulk materials, but processing time is very short and cooling rates very fast. For that reason, grain growth is nor- mally very limited and non-equilibrium structures are possible [10].
Hardness and toughness of the ERS samples is presented in Table 6.
Both materials present a hardness around to 1100 HV30. Hardness of Table 4
EDX patterns for Cr3C2-25NiCr: powder and ERS sample.
Spot Powder ERS densified sample
1
2
3
this kind of materials is normally limited by the grain size of the chro- mium carbides [29].
Cr3C2-25NiCr presented higher toughness (13,5 vs 10). It is impor- tant to consider that the amount of metallic phase is very different in both cases, the amount in the case of Cr3C2-25NiCr is almost double and
this significantly affects the fracture behavior of the material.
Table 7 presents hardness and toughness of materials with similar compositions obtained by other processing routes. Cr3C2-NiCr obtained by ERS shows similar values of hardness and toughness to the materials obtained by conventional PM route [30]. In the case of WC-Cr3C2-Ni, the comparison is more difficult as this material is typically used in coatings.
Almost fully dense coatings present a similar value of harness but lower toughness than the material obtained by ERS.
The scalability is an important issue in the ERS technology. Typi- cally, samples of very few millimetres are obtained [7,8,10]. This nor- mally limits the practical application of the technology.
Main limitation is the homogeneity in large pieces. Increasing the size of the pieces, the gradient of density within the samples is also increased. This different in density produces a large gradient in elec- trical resistance and this means a difference in temperature during processing. The homogeneity of the final pieces depends on the electric and thermal conductivity of the powder, but also on the electrical con- ductivity of punches and dies. The electrical capacity of the existing equipment also limits the possibility of increasing the size. In this case, a pilot plant equipment with 500 KVA was used to try to increase the size of the pieces. Rectangular samples of 20X10X10 mm were obtained to see the final homogeneity of the samples.
Fig. 9 shows the cross section of a rectangular piece obtained by ERS.
It is possible to observe the external porosity, but the core of the piece present high density.
One possible application for this kind of pieces are wear parts for stone crushers. These parts are typically rectangular and manufactured with hardmetal (WC-Co) and cermets. In this case, a demonstrator was obtained gluing five pieces to a steel plate. The external porous layer was removed to ensure a good hardness at the surface. This is a proof of concept of possible applications for the ERS technology (Fig. 10).
4. Conclusions
This work presented the fabrication of chromium carbide-based cermets by Electric Resistance Sintering. Two different compositions were tested and characterized: Cr3C2-25NiCr (wt%) and WC-20Cr3C2- 7Ni (wt%). The matrix composition was based in nickel. The addition of Cr or complex binders, as well as the using of secondary carbides, im- proves the mechanical properties of chromium carbide cermets and this can help in the substitution of WC-Co for some applications [23,24].
The combination of chromium and tungsten carbide can produce a good compromise between mechanical properties and corrosion and oxidation resistance.
The use of commercial agglomerated and pre-sintered powders is Fig. 5. XRD pattern of Cr3C2-25NiCr: Powder (up), ERS sample (down).
Fig. 6. SEM image of a WC-20Cr3C2-7Ni ERS sintered sample (ref B), scale bar 5 μm (cross section).
Fig. 7. SEM images of the cross sections of the WC-20Cr3C2-7Ni powders at two different magnifications: A) Scale bar 50 μm, B) Scale bar 5 μm.
Table 5
EDX patterns for WC-20Cr3C2-7Ni: powder and ERS sample.
Spot Powder ERS densified sample
1
2
3
4
(continued on next page)
interesting for ERS, as they present good flowability and no organic binder. In all cases, materials present a dense core surrounded by a porous area at the surface. Microstructures of the initial powders and densified samples show that grain growth during sintering was limited, due to the fast processing time.
Hardness and toughness obtained mainly depend on the amount of
metallic binder and the size of the carbides. Results were compared with conventional materials obtained by other methods. Values are similar to conventional processing, but hardness is not very high (around 1000–1100 HV30). One possible reason is the size of chromium carbide.
Reducing the size of chromium carbide, properties could be improved.
In this work, commercial plasma spray powders were used with a chromium carbide size not very fine. Optimizing the initial powder, it is believed that mechanical properties will improve.
In addition, this work considered only the ERS densification of the materials, but not possible post processing technologies to improve the properties of chromium carbide materials. A posterior thermal treat- ment could improve homogeneity and hardness [1].
One of the main limitations for the industrial appication of the ERS technology is the limited size of the pieces. The development of more powerful machines, like the used in this work, can help in the scaling up of the techlonogy. The sizes obtained in this work are still small, but they can fit with some wear tools. One possible example was presented.
Table 5 (continued)
Spot Powder ERS densified sample
Fig. 8. XRD pattern of WC-20Cr3C2-7Ni: Powder (up), ERS sample (down).
Table 6
Hardness and toughness of the ERS materials.
Composition (wt
%) Process Hardness
HV30 Fracture toughness KIC
(MPa√m)
vol% of metallic phase Cr3C2-25NiCr
(ref C) ERS 1060 ± 42 13,5 ± 0.5 20
WC-20Cr3C2-7Ni
(ref F) ERS 1110 ± 30 10.0 ± 0.6 9
Table 7
Hardness and toughness in bibliography.
Composition (wt%) Process Hardness Vickers Fracture toughness KIC (MPa√m) vol% of metallic phase Ref
Cr3C2-30NiCr Conventional PM 920 (HV10) 18 [31]
Cr3C2-30NiCr Reactive sintering 990 (HV10) 18,5 [31]
WC-20Cr3C2-7Ni Thermal spray 1151a (microhardness) 6 9 [32]
aCoating.
Fig. 9. Cross section of a part of Cr3C2-25NiCr produced by ERS, distribution of the porosity (scale bar 5 mm).
Chromium carbide cermets are widely used as a coating but they are also interesting for bulk applications e.g. turbomachinery at high tem- perature and/or in corrosive environments [33].
Declaration of Competing Interest None.
Acknowledgements
This work is financially supported by the European Institute of Innovation and Technology (EIT Raw Materials), a body of European Union (Horizon 2020 Framework Programme) under the project FASTRAM.
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Fig. 10. Stone crusher wear part: 5 pieces of Cr3C2-25NiCr produced by ERS joined to steel.