2016 Volume 63 Issue 7 Pages 548-554
Carbon is a key element for powder metallurgy. For example, carbon is the basic alloying element in PM sintered steel, and carbon powders are used as a carbon source for the production of hard metals. However, there are only a few studies about the mechanisms of carbon dissolution and reactivity in dependence on the carbon sources with solid metals/oxides.
This work presents the effect of the carbon source (different graphite and carbon black types) on the reactivity and efficiency of oxides reduction during the sintering of PM steels and on the synthesis of nanocrystalline WC powders.
This experimental work sets the basis for optimizing the production of PM steel parts and nano-WC powders based on raw material selection and process conditions.
Carbon is a key element for powder metallurgy. For example, carbon is the basic alloying element in PM sintered steel, and carbon powders are used as the carbon source for the production of hard metals.
There are many types of carbon allotropes with very different properties, like diamond, graphite, graphene, nanotubes, fullerenes and carbon black.
Graphite is the typical carbon source for PM steel, and it is admixed to iron or iron-based powders together with lubricant powders and possibly other additives during the preparation of the powder mix. During the sintering process, graphite is almost fully dissolved into the iron matrix. The two most commonly used grades nowadays are natural graphite and primary synthetic graphite. The main differences between these two graphite grades are purity level, crystallographic structure (crystallite size and orientation) and particle morphology. Synthetic graphite has an advantage in terms of enhanced reactivity during sintering1). Faster dissolution of synthetic graphite in Fe-C sintered steels in combination with its higher purity level is responsible for slightly higher hardness and mechanical performance. However, there are only a few studies about the mechanisms of its dissolution and reactivity in dependence on the carbon sources with solid metals/oxides2–4). In particular, there is little knowledge about the connected interactions between solid carbon, the atmosphere and solid metal/oxides.
Carbon black and graphite are the typical carbon sources for hard metals. For example, tungsten carbide powders are typically produced either by carburization of tungsten metal powder (W) or by carbothermal reduction of tungsten oxide powder (WO3), according to eq. 1 and eq. 2.
| (eq. 1) |
| (eq. 2) |
The preparation of WC powders starting from metallic W powders is the most common procedure currently used on an industrial scale. The carburization takes place through the intermediate formation of W2C and starts below 900 °C. The preparation of WC powder starting from tungsten oxide powders is in principle easier (single process for oxide reduction and carburization) but is more difficult to control on an industrial scale compared to standard carburization from metallic tungsten.
The reactions involved in reduction and carburization processes during synthesis of WC are governed by complex interactions between the material to be processed, the carbon source used, and the atmosphere. However, there are only a few studies about the influence of the carbon source on the final properties of cemented carbide parts5).
In this article, we present two case studies about the effect of the carbon source (different graphite and carbon black types) on the synthesis of nanocrystalline WC powders and on the reactivity and efficiency of oxides reduction during sintering of Cr-prealloyed PM steels.
In this first study, we show how the properties and microstructure of tungsten carbide powders (WC) can be influenced by the type of carbon source used6–9). We tested carbon blacks with different structure and surface area (BET), and synthetic graphite’s with different particle size (PSD), see Table 1. All carbon black and graphite’s used have high purity (low ash content).
| Carbon | Description | PSD (d90 in μm) | BET (m2/g) | Ash (%) |
|---|---|---|---|---|
| N991 | low BET carbon black | — | 9.3 | 0.01 |
| E150G | medium BET carbon black | — | 48.9 | 0.01 |
| E250G | high BET carbon black | — | 67.0 | 0.01 |
| KS4 | fine synthetic graphite | 3.8 | 25.9 | 0.02 |
| KS15 | medium synthetic graphite | 17.0 | 13.0 | 0.02 |
| KS44 | coarse synthetic graphite | 42.5 | 8.5 | 0.02 |
Mixes containing WO3 powders and graphite or carbon black were prepared by ball milling in a Fritsch Pulverisette planetary mill for 2 hours at 300 rpm. Carburization has been performed both in lab scale (Netzsch DIL402C dilatometer with Ar-50 %H2 atmosphere at 1100 °C) and pilot scale (industrial furnace with pure hydrogen 100 %H2 atmosphere at 1120 °C). The resulting powders have been analyzed by X-Ray Diffraction (XRD) in a Bruker D8 Advance instrument. Specific surface area (BET) has been measured with a Micromeritics ASAP 2020 physisorption analyzer.
WO3 is first reduced to metallic W, followed by the formation of W2C and finally WC.
2.2 Results 2.2.1 Lab scale carburization in Ar-50 %H2On lab scale, in Ar-50 %H2, oxide reduction is complete at 900 °C for all carbon sources (see Fig. 1). XRD spectra for graphite only contains W peaks, indicating that carburization has not yet started. With carbon black, on the other hand, there are already W2C peaks, and even WC peaks for high surface area carbon black E250G (enhanced carburization). At 1100 °C transformation to WC is complete for both carbon blacks, whereas for graphite the W2C phase is still present (no big difference between fine and coarse graphite).
XRD patterns of powders obtained after lab-scale carburization of WO3 + C mixes at different temperatures in Ar-50 %H2.
On pilot scale, in 100 %H2,, the carburization process is not completed after 2 hour at 1120 °C for all carbon sources. The degree of carburization is higher for synthetic graphite KS15 and high surface area carbon black E250G compared to low surface area carbon black N991. Starting with N991, the WC phase is only 65 % and there is still > 30 % of either W2C or W. For E250G, the WC phase is already 70 % and there is < 10 % of metallic W. For KS15, the WC phase is > 80 % and there is only 5 % of metallic W (see Fig. 2). In order to check if the obtained powders have different grain size, we measured their BET specific surface area. The results indicate that high BET carbon black E250G leads to the highest BET values (lowest WC grain size), whereas low BET carbon black N991 leads to the lowest BET values (highest WC grain size). As shown in Table 2, in all cases, the grain size is < 500 nm, and in the case of E250G < 100 nm (nanocrystalline).
(left) Amount of W, W2C and WC phases estimated by XRD pattern after pilot-scale carburization of WO3 + C mixes at 1120 ℃ in H2 (right) BET of carburized WO3 with different carbon sources.
| Carbon Source | BET (m2/g) | grain size (nm) |
|---|---|---|
| N991 | 1.30 | 295 |
| ENSACO150G | 2.41 | 159 |
| ENSACO250G | 5.00 | 77 |
| KS15 | 1.75 | 219 |
These results indicate that the type of carbon powder has a strong influence on the reduction/carburization process of tungsten oxide and on the resulting WC grain size. In particular, carburization is more effective when high surface area carbon black is used compared to low surface area carbon black, and nano-sized WC powders can be obtained. This experimental work sets the basis for optimizing the production of nano-WC powders based on raw material selection and process conditions.
In this second study, we show how the sintering process rate of Cr-prealloyed PM steel can be positively affected by carbon source selection3). 3 % Chromium−0.5 % Molybdenum pre-alloyed, water-atomized powder Astaloy CrM supplied by Höganäs AB (Sweden) was used in this study. Powder mixes were made with 0.5 wt.% of carbon and 0.6 wt.% of Kenolube lubricant. Six different carbon sources were used (see Table 3): besides fine (PG10) and coarse (PG25) natural graphite, ultra-fine (KS4), fine (F10) and coarse (F25) synthetic graphite’s, we also included carbon black (E250G) in the study. Standard Charpy impact test bars (10 × 10 × 55 mm3) were uniaxially compacted at 600 MPa to a green density of ~7 g/cm3. Specimens were sintered in a 90 %Nitrogen/10 %H2 atmosphere. In order to follow the change in oxide state during the sintering process, specimens were sampled at different temperatures during heating: 900, 1120 °C as well as after sintering for 30 min at 1120 °C. Presence of oxides were evaluated on the fresh fracture surface by means of SEM (LEO Gemini 1550). All above mentioned experiments were performed at Chalmers University of Technology. The formation of inter-particles sintering necks has been discussed in the literature as a phenomenon that is possible only after reduction of the surface iron oxide layer. Hence, the efficiency of the surface iron oxide reduction can be easily followed by tracing the inter-particle necks development with increasing temperature.
| PG10 | PG25 | F10 | F25 | KS4 | E250G | |
|---|---|---|---|---|---|---|
| Ash (wt.%) | 3.5 | 3.6 | 0.6 | 0.6 | 0.07 | 0.01 |
| D10 (microns) | 2.5 | 4.0 | 2.6 | 4.2 | 1.2 | — |
| D50 (microns) | 5.5 | 10.0 | 6.3 | 11.2 | 2.4 | — |
| D90 (microns) | 11.5 | 22.0 | 12.6 | 24.6 | 4.7 | — |
Fracture surfaces clearly indicate a difference in carbon activity at 900 °C: dimple ductile fracture, associated with residual surface oxide, characteristic for material prepared with KS4, F10 and PG10, indicate good development of necks and therefore efficient reduction of the surface iron oxide during heating. In the case of F25 and especially PG25 only branched line connections between the particles were registered, indicating a much less efficient iron oxide surface reduction. The fracture surface of the compact prepared with carbon black indicated an absence of any inter-particle necks, therefore E250G is totally inert up to 900 °C.
Fracture surface of AstCrM + 0.5 %C compacts utilizing different carbon sources, heated in N2/10 %H2 atmosphere to 900 °C, showing development of inter-particle necks.
As expected, a considerable improvement in the amount and strength of the inter-particle necks was registered after further heating to 1120 °C in the case of all carbon sources. Inter-particle dimple ductile fracture is the main failure mechanism for all of the compacts with admixed graphite. Deeper dimples of inter-particle failure were registered in the case of the compacts prepared with finer graphite grades, especially with KS4. For the compacts prepared with coarse graphite grades PG25 and F25 branched inter-particle connections only start to form a network. In the case of all compacts, admixed with graphite, its particles were registered on the fracture surface, with larger amount and size in the case of PG25 and F25. In the case of KS4, the observed amount of undissolved graphite particles was much lower. The largest progress in oxide reduction, inter-particle neck development and carbon dissolution was registered for compacts prepared with carbon black E250G. There are no residues of the carbon black observed on the fracture surface, indicating its full dissolution during heating between 900 and 1120 °C. Additionally, pore surface inside the compact was totally oxide-free, meaning the full reduction of even thermodynamically stable surface oxides. A Large fraction of cleavage facets on the fracture surface also indicates much stronger developed inter-particle necks. All of these facts indicates the highest temperature enforcement of the carbon activity in the case of carbon black, which turns from the totally inert at 900 °C to extremely active at above 1100 °C. This is connected to the amorphous structure of E250G that results in its inactivity at low temperature. However, when thermal activation starts, its activity is boosted due to its nanometric size, resulting in the observed rapid dissolution and reduction of the surface oxides.
Fracture surface of AstCrM + 0.5 %C compacts utilizing different carbon sources, heated in N2/10 %H2 atmosphere to 1120 °C, showing development of inter-particle necks.
Hence, such enforced Carbon activity close to the sintering temperature is very positive due to the enhanced reduction of the surface oxides residues at above 900 °C, when the formation of inter-particle necks starts and there is a great risk of massive oxide enclosure. On the other hand, fine graphite grades are more active at lower temperatures than carbon black, but their activity is not as boosted as in the case of carbon black with the temperature increase.
3.2.3 After Sintering at 1120 °C, 30 min (Fig. 5)In the case of carbon black and fine graphite grades there is a much lower amount of oxide inclusions inside the inter-particle necks. In the case of compacts with coarser graphite, larger total amounts of oxide and coarser oxide inclusions were registered. No residues of un-dissolved carbon were observed after sintering except for the compact admixed with coarse natural graphite PG25, where graphite flakes were observed in a random pattern in the center of the compact.
Fracture surface of AstCrM + 0.5 %C compacts utilizing different carbon sources, sintered in N2/10 %H2 atmosphere to 1120 °C, 30 min.
These results indicate that the type of carbon powder can influence the oxide reduction, inter-particle neck development and carbon dissolution of Cr-prealloyed PM steel. Particle size distribution is a highly significant property of the graphite sources: the finer the particle size the higher the carbon reactivity. Carbon black shows fully inert behavior until ~900 °C, after which its activity is “boosted” exhibiting highest reactivity between 900 and 1120 °C.
The effect of the carbon source (different graphite and carbon black types) on the synthesis of nanocrystalline WC powders and on the reactivity and efficiency of oxides reduction during sintering of PM steels has been discussed based on two case studies. In both cases, carbon black and graphite show different reactivity. The surface area of carbon black and particle size of graphite are two important properties that strongly influence the reactivity of carbon.
Our experimental tests indicate that both high purity synthetic graphite and high surface area carbon black can be used as a carbon source for the production of tungsten carbide. In particular, high surface area carbon black can be used to produce nano-sized WC powders. The sintering process rate of Cr-prealloyed PM steel can be influenced by the carbon source: fine synthetic graphite is more active at low temperatures, whereas high surface area carbon black is activated only at high temperatures. These results indicate that an efficient reduction of oxides and effective carbon dissolution could be best achieved by a combination of graphite and carbon black.
This work has been partially funded by Eureka Project 9156. The authors would like to acknowledge Mrs. Gabriele Kremser from Wolfram Bergbau und Hütten (Austria) for providing the tungsten powders.
