Formation of W-Cr-Phases During the Production of Cr-doped WC Powders. Z. Tükör*, W.D. Schubert*, A. Bicherl**, A. Bock**, B.

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1 Tükör, Schubert, Bicherl et al. 17th Plansee Seminar, Vol. 4 HM 44/1 Formation of W-Cr-Phases During the Production of Cr-doped WC Powders Z. Tükör*, W.D. Schubert*, A. Bicherl**, A. Bock**, B. Zeiler** *Institute of Chemical Technologies and Analytics, Vienna University of Technology, Austria **WOLFRAM Bergbau- und Hütten GmbH Nfg. KG, Austria Abstract Chromium additions to tungsten powder are known to inhibit WC particle growth during carburisation. However, the role of chromium during the processing is not clarified. To investigate the behaviour of chromium during carburization different reaction paths were investigated (W+Cr 3 C 2 +C and WC+Cr 3 C 2 +C) to synthesize the phases of the W-Cr-C system along the WC-Cr 3 C 2 -section. The results are compared to the known phase diagrams and discussed in relation to the production of chromiumdoped tungsten carbide powder. A metastable (W,Cr)C phase with higher chromium contents as hitherto reported ((W 0.85 Cr 0.15 )C; 5wt% CrC resp.) was observed. The stability of this phase was studied at different annealing temperatures. Keywords W-Cr-C-system, (W,Cr) 2 C, (W,Cr)C, Cr-pre-doping Introduction Chromium is an important additive in the hardmetal industry. It inhibits the WC particle growth during liquid phase sintering and even during the carburisation of tungsten powders, if chromium compounds are added prior to the carburisation process. Nevertheless, the mechanism of particle/grain growth inhibition is not exactly known. Two phase diagrams are presented in fig. 1 which can help to understand phase formation during production of Cr-predoped tungsten carbide powder. However, they show distinct differences in phase occurrences, in particular in the area of interest. In the work of Stecher, Benesovsky and Nowotny [1], an equilibrium between WC, (W,Cr) 2 C and C is described at 1350 C on the intersection WC-Cr 3 C 2, whereas the phase diagram by Rudy and Chang [2] shows an equilibrium between WC and (Cr,W) 3 C 2 at 1300 C. Rudy et al. [2] explained this difference in phase formation by the assumption that in the work of Stecher et al. a frozen-in equilibrium was observed rather than an equilibrium state, as the authors had tempered the specimens at first at 1600 C, and later annealed at 1350 C for too short times. This explanation would agree with thermodynamic calculations by the authors, which had shown that the equilibrium WC-C 3 C 2 is replaced by an equilibrium WC-(W,Cr) 2 C-C above approximately 1500 C.

2 HM 44/2 17th Plansee Seminar, Vol. 4 Tükör, Schubert, Bicherl et al. Based on this information the question is still open which phases form during WC powder production in the presence of chromium. These are not necessarily those formed in equilibrium. (a) T= 1350 C (b) T=1300 C Fig. 1: Phase diagrams as published by (a) Stecher et al. [1] and (b) Rudy et al. [2]. In the present work the authors tried to clarify the behaviour of chromium during the carburization of chromium doped tungsten powders. As the investigations by Stecher et al. and Rudy et al. showed the difficulty of equilibrium formation, two different reaction paths were selected to investigate the phase formation along the section WC-Cr 3 C 2 : W+Cr 3 C 2 +C and WC+Cr 3 C 2 +C. To be sure of avoiding the three phase area WC+(Cr,W) 3 C 2 +(W,Cr) 2 C described by Rudy (fig. 1) a carbon surplus was added relating to the stoichiometry. A chromium-rich (W,Cr)C phase was observed containing significantly higher chromium contents as up to now reported. It was synthesized and then annealed at various temperatures. Experimental details To investigate the behaviour of chromium during tungsten metal carburization, two reaction paths were used to synthesize the phases of the W-Cr-C system at the WC-Cr 3 C 2 -section. Both are indicated in fig. 2 in the diagrams presented above. Reaction path I: At first, (W,Cr) 2 C was prepared with different chromium contents starting from W+Cr 3 C 2 +C-mixtures at 1450 C (3h) in vacuum (first step). Then, the sub-carbides were mixed with carbon in excess and heated in vacuum at 1100 C (3h), 1300 C (3h) and 1450 C (3h) to produce the desired final products (second step). Alternatively, the mixtures were also heated in a hydrogen atmosphere at 1450 C (3-6h) and 1630 C (4-8h). In fig. 2 a reaction paths I is indicated by continuous arrows. Reaction path II: A mixtures of WC+Cr 3 C 2 +C was heated at 1450 C (6h), 1630 C (8h) and 2000 C (5h) in hydrogen atmosphere and for 300 hours at 1000 C and 1200 C in vacuum. This path is indicated in fig. 2 b by dashed arrows.

3 Tükör, Schubert, Bicherl et al. 17th Plansee Seminar, Vol. 4 HM 44/3 After annealing, the powders were analysed by X-ray powder diffraction. The software TOPAS (Rietveld refinements on samples containing Si as internal standard) was used to calculate the lattice parameters of some powders. (W,Cr)C: Following reaction paths I, a (W,Cr)C phase with high chromium contents [(W 0.85 Cr 0.15 )C; ~5wt% CrC )] was observed. To study the temperature stability of this phase, a larger amount of (W,Cr)C powder was synthesized and annealed. At first, tungsten, Cr 3 C 2 and carbon black were mixed and heated at 1450 C in hydrogen atmosphere to yield subcarbide (W,Cr) 2 C. This powder was then ground and mixed with carbon black. The mixture was heated at 1200 C for 6 hours under hydrogen to form (W,Cr)C, and the reaction product was analysed by X-ray diffraction. After this treatment the chromium-rich (W,Cr)C phase was heated at 1450 C (9h), 1600 C (12h), 1800 C (2h) and 2000 C (2.5h) in hydrogen atmosphere. It was also heated in vacuum for 300h at 1000 C and 1200 C. (a) T= 1350 C (b) T=1300 C Fig. 2: Phase diagrams by Stecher et al. [1] (left) and Rudy et al. [2] (right); the arrows indicate the two different reaction paths for the synthesis of the desired powder compositions: Reaction path I: continuous line (left): W+Cr 3 C 2 +C; Reaction Path II: dashed lines (right): WC+Cr 3 C 2 +C. Results Reaction path I Reaction path I consists of two individual steps: step 1 (formation of (W,Cr) 2 C) and step 2 (transformation into (W,Cr)C resp. (Cr,W) 3 C 2 ). The results of the first step are presented in table I. The first step of carburisation resulted in subcarbides (W,Cr) 2 C with significant shifts of the diffraction lines compared to the Powder Diffraction File # for pure W 2 C (fig. 3). The more chromium the powders contained in the starting powder mixture, the stronger was the shift of the lines to higher diffraction angles, which indicates the formation of continuous solid solution carbide up to (W 0.25 Cr 0.75 ) 2 C. Up to 20 mol% Cr 2 C (the hypothetical formula Cr 2 C is used to indicate the respective molar ratio) already some WC formed and also in this case the diffraction lines were shifted to higher diffraction

4 HM 44/4 17th Plansee Seminar, Vol. 4 Tükör, Schubert, Bicherl et al. angles, indicating the formation of a solid solution of Cr in the hexagonal WC: (W,Cr)C. No Cr 3 C 2 was detected in any of the annealed powders, but some (Cr,W) 7 C 3 was formed at 75 mol% Cr 2 C. The formation of a solid solution (Cr,W) 7 C 3 is indicated by a shift of the diffraction lines to lower diffraction angles compared to the Powder Diffraction File # for pure Cr 7 C 3. Table I: Results of the first step of the carburisation of W+Cr 3 C 2 +C-mixtures; reaction path I. mol proportion phases lattice parameter W 2 C sample W:Cr tempering observed a [Å] c[å] powder 1a 99: C, 3h, vacuum (W,Cr) 2 C, W, (W,Cr)C powder 1b 99: C, 6h, vacuum (W,Cr) 2 C, (W,Cr)C powder 2a 96.8: C, 3h, vacuum (W,Cr) 2 C, W, (W,Cr)C powder 2b 96.8: C, 6h, vacuum (W,Cr) 2 C, (W,Cr)C powder 3a 80: C, 3h, vacuum (W,Cr) 2 C, W powder 3b 80: C, 6h, vacuum (W,Cr) 2 C, (W,Cr)C powder 4 50: C, 3h, vacuum (W,Cr) 2 C powder 5 25: C, 3h, vacuum (W,Cr) 2 C, (Cr,W) 7 C Fig. 3: Powder diffraction patterns of synthesized (W,Cr) 2 C with 1.0mol% (0.3wt%), 3.2mol% (1.0wt%), 20.0mol% (7.1wt%), 50.0mol% (23.5wt%) and 75mol% (47.9wt%) Cr 2 C (note: Cr 2 C is a hypothetical compound which does not occur in the Cr-C system; it is used only to indicate the molar ratio). To obtain the final compositions (fig. 2a) the powders were carburised once again containing a carbon surplus (step 2). Since all carburisation conditions resulted in similar powder compositions, only

5 Tükör, Schubert, Bicherl et al. 17th Plansee Seminar, Vol. 4 HM 44/5 the carburisation at 1450 C in a hydrogen atmosphere are presented in this work (table II). In this step (W,Cr)C phase was formed up to 20 mol% CrC with a strong shift of the diffraction lines towards higher angles compared to the Powder Diffraction File # for pure WC (fig. 4). At 50 and 75 mol% CrC, (W,Cr) 2 C was observed with strong shifts of the diffraction lines to higher angles as compared to the PDF. In case of 50 mol% a small amount of (W,Cr)C was formed as well, whereas at 75 mol% CrC a (Cr,W) 3 C 2 was detected besides the subcarbide. Table II: Results of the second step of the carburisation of W+Cr 3 C 2 +C-mixtures (W 2 C -> WC); Reaction path I; * no lattice parameter calculation was performed due to the small amount of (W,Cr)C. mol proportion tempering observed lattice parameter WC sample W:Cr atmosphere T [ C] t [h] phases a [Å] c [Å] powder 1c 99:1 H (W,Cr)C powder 2c 96.8:3.2 H (W,Cr)C, (W,Cr) 2 C powder 3c 80:20 H (W,Cr)C, (W,Cr) 2 C powder 4c 50:50 H (W,Cr) 2 C, (W,Cr)C * * powder 5c 25:75 H (W,Cr) 2 C, (Cr,W) 3 C 2 Fig. 4: Powder diffraction patterns of carburised (W,Cr) 2 C (second step, 1450 C, hydrogen atmpsphere) with 1.0mol% (0.3wt%), 3.2mol% (1.0wt%), 20.0mol% (7.1wt%), 50.0mol% (23.5wt%) and 75mol% (47.9wt%) CrC. Especially striking is the strong shift of the diffraction lines of the tungsten monocarbide phase towards higher diffraction angles in the sample containing 20 mol% CrC (~ 7 wt%). It indicates a much higher amount of chromium in solid solution as one would expect from the phase diagrams presented above. In addition, no chromium carbide ((Cr,W) 3 C 2 ) was detected in this sample.

6 HM 44/6 17th Plansee Seminar, Vol. 4 Tükör, Schubert, Bicherl et al. Reaction path II Reaction path II consists of a heat treatment of a WC, Cr 3 C 2 and carbon black powder mixture within the temperature range of 1000 to 2000 C. Following the phase diagrams [1,2] one would expect the formation of a (W,Cr)C mixed crystal carbide besides either (Cr,W) 3 C 2 - at temperatures <1500 C -, or (W,Cr) 2 C at >1500 C. Table III shows the lattice parameters of the pure WC as compared to those of the annealed WC-Cr 3 C 2 -C mixtures. Fig. 5 demonstrates the respective diffraction patterns. No change in lattice parameters of the hexagonal WC was observed up to 1630 C, which indicates that the chromium did not form a solid solution. In addition, no (W,Cr) 2 C was formed at 1630 C as one would expect from the phase diagrams. Only at 2000 C, a solid solution (W,Cr) 2 C was detected (fig. 5) and the diffraction lines of the (Cr,W) 3 C 2 phase disappeared. In this case also a small amount of chromium dissolved into the WC lattice. Table III: Lattice parameters of PDF standard and annealed WC-Cr 3 C 2 -mixtures (powder 6b-e); reaction path II. annealing lattice parameters sample atmosphere T [ C] t [h] a [Å] c [Å] WC PDF # (pure tungsten carbide) powder 6b vacuum powder 6c vacuum powder 6d H powder 6e H Fig. 5: Powder pattern of annealed WC+Cr 3 C 2 +C-mixtures; the annealing conditions are listed in Table 3.

7 Tükör, Schubert, Bicherl et al. 17th Plansee Seminar, Vol. 4 HM 44/7 Chromium-rich (W,Cr)C At first a chromium-rich (W 0.85 Cr 0.15 )C (~5 wt% CrC ) was synthesized by reaction path I, which was heat treated between 1200 C and 2000 C to investigate the thermal stability of the solid solution. The results are demonstrated in table IV. Table 4: Lattice parameters of as-synthesized and annealed (W,Cr)C. tempering lattice parameter sample T [ C] t [h] a [Å] c [Å] WC PDF # (pure tungsten carbide) (W,Cr)C as-synthesized (W,Cr)C/ (W,Cr)C/ (W,Cr)C/ (W,Cr)C/ ,8185 (W,Cr)C/ (W,Cr)C/ After synthesis of the solid solution at 1200 C the powder diffraction pattern showed a WC with a strong shift of the diffraction lines towards higher diffraction angles (compared to the PDF). Apart from this only a small amount of (W,Cr) 2 C was detected. Fig. 6: Powder patterns of as-synthesised and annealed (W,Cr)C containing 15mol% CrC.

8 HM 44/8 17th Plansee Seminar, Vol. 4 Tükör, Schubert, Bicherl et al. Annealing the (W,Cr)C powder up to 1450 C hardly changed the lattice parameters of the monocarbide (fig. 6). At 1450 C, even the small amounts of (W,Cr) 2 C disappeared. Significant diffusion of chromium out of the WC lattice (larger lattice parameters) started at 1600 C and at the same time a chromium-rich (W,Cr) 2 C was observed. The shift of the diffraction lines of (W,Cr) 2 C to higher angles is even stronger in this powder than in the starting (W,Cr)C powder. It clearly indicates the partial decomposition of the solid solution into a chromium-poorer (W,Cr)C and a chromium-rich (W,Cr) 2 C. Rapid decomposition occurred at 2000 C. Discussion The importance of the reaction paths on phase formation The present investigation on phase formation in the W-Cr-C system confirmed the difficulty in obtaining equilibrium conditions, in particular when using carbide powder mixtures. Even at 2000 C and annealing times of 2 hours the results of the heat treatments on samples of the same nominal composition but different reaction paths were not consistent. This peculiarity of the system makes a final statement about equilibrium phases and their compositions, especially along the WC-Cr 3 C 2 intersection, impossible. Within the temperature range studied (1200 C-2000 C) phase formation was determined strongly by the starting raw material and the choice of the reaction path. If the reaction proceeded via the subcarbide phase (W 2 C), this phase acted as a collector for the chromium addition, and a solid solution was formed up to 75 mol % Cr 2 C : (W 0.25 Cr 0.75 ) 2 C. During further carburisation at C a chromium-rich (W,Cr)C mixed-crystal carbide occurred with chromium fractions up to 20 mol% CrC [(W 0.80 Cr 0.20 ); 7 wt% CrC ). This amount is much higher than hitherto reported in literature (~1.5 wt%; [2,3]). Above this percentage, the chromium was present as both (W,Cr)C and (W,Cr) 2 C, but only at 75mol% CrC also (Cr,W) 3 C 2 was detected as one would expect from the phase diagram established by Rudy [2]. On the other hand, no (W,Cr) 2 C formed at 1630 C on annealing WC-Cr 3 C 2 -C powder mixtures for 8 hours as predicted by Rudy through thermodynamic calculations [2]. Only at 2000 C, the (Cr,W) 3 C 2 - phase disappeared and a chromium-rich (W,Cr) 2 C phase was detected. These results indicate that phase formation is obviously determined by the reaction kinetics rather than merely by thermodynamic driving forces (differences in Gibbs free energy). Thermal stability of the (W,Cr)C Phase Annealing of a (W 0.85 Cr 0.15 )C phase at 1200 C to 1450 C did not change the lattice parameters of the hexagonal lattice. Above, diffusion of chromium out of the lattice occurred, and a chromium-rich (W,Cr) 2 C was formed. However, even at 2000 C a detectable amount of chromium still remained in the monocarbide phase. Two explanations exist for the observed behaviour: The chromium-rich (W,Cr)C is thermodynamically stable at temperatures 1450 C, but decomposes above 1600 C into a chromium-poorer (W,Cr)C and a chromium-rich (W,Cr) 2 C. In this case, the solubility of chromium in the hexagonal WC would be much higher than previously reported (5-7 wt% 1.5 wt%).

9 Tükör, Schubert, Bicherl et al. 17th Plansee Seminar, Vol. 4 HM 44/9 The chromium-rich (W,Cr)C is a metastable phase, which can form only out of the structurally related (W,Cr) 2 C, and the transformation into a chromium-poorer (W,Cr)C and chromium-rich (W,Cr) 2 C is inhibited. As the annealing of WC+Cr 3 C 2 +C powder mixes did not result in a significant change of lattice parameters up to 1630 C, the latter explanation seems more likely. The behaviour of chromium additions during WC powder manufacture By the analysis of Cr pre-doped WC powders, where chromium compounds are added prior to carburization, it was demonstrated that a certain amount of chromium is present in the hexagonal WC lattice [4]. This amount increased with increasing chromium additions and, as a result of this solid solution, both the a and c parameter of the unit cell changed (fig. 7). In industrial practice, up to 1 wt% of chromium is added to the W+C+Cr batch, either as Cr 3 C 2, or, as Cr 2 O 3. Depending on the carburisation process (in particular the carburisation temperature and the chromium compound used), more or less chromium will dissolve in the (W,Cr) 2 C phase formed intermediately during the reaction process. During final carburisation, chromium will remain in solid solution as (W,Cr)C, at least up to 1 wt%. This amount is in the range of the equilibrium value of chromium dissolved in the WC as stated by Rudy at 1350 C [2]; i.e. ~ 1.5 wt% CrC. If chromium is inhomogeneously distributed, and higher chromium values are locally present in the carburisation batch, a chromium richer (W,Cr) 2 C will form on carburisation. Depending on the exact composition this chromium-rich (W,Cr) 2 C might not transform into the monocarbide. In this case (>7 wt% Cr) the chromium contained in the sub-carbide will remain within the (W,Cr) 2 C, as it is not transformed into WC and Cr 3 C 2, as demonstrated in the present investigation (table III). Such irregularities have to be avoided by a proper mixing process prior to carburisation; otherwise a local WC growth might occur on subsequent sintering [5]. 2,846 2,844 2,842 undoped 0.5% Cr-predoped 1% Cr-predoped tempered WC-Cr3C2-C ( C) tempered WC-Cr3C2-C (2000 C) tempered (W,Cr)C (2000 C) c [Å] 2,84 2, C Reaction path II 2,836 2,834 2, C Reaction path I 2000 C Reaction path II 2,83 2,901 2,902 2,903 2,904 2,905 2,906 2,907 2,908 2,909 a [Å] Fig. 7: Lattice parameters of annealed W-Cr-C-mixtures compared to industrial pre-doped and undoped WC powders.

10 HM 44/10 17th Plansee Seminar, Vol. 4 Tükör, Schubert, Bicherl et al. On plotting the lattice parameter values of the (W,Cr)C phase as obtained by the two different reaction paths at 2000 C in fig. 7 it becomes obvious that even at 2000 C a final equilibrium value of chromium in WC cannot be concluded. Much more chromium remained in the WC lattice in case of reaction path I compared to reaction path II. Up to 1630 C, only a slight change in lattice parameters occurred in case of route II, indicating that the diffusion of chromium into the WC lattice is heavily inhibited. In addition, no (W,Cr) 2 C formed in this case as one would expect from the equilibrium diagrams. Conclusions Phase formation within the W-Cr-C system at temperatures between 1200 C and 2000 C is crucially determined by reaction kinetics of phase transformations. Strong differences exist in phase occurrences depending on whether or not (W,Cr) 2 C is formed intermediately. This phase acts as a collector for chromium within the tungsten matrix and a most likely metastable (W,Cr)C phase is formed, with much higher chromium contents as hitherto reported. It shows a high thermal stability up to 1450 C. At higher chromium contents (W,Cr) 2 C is formed which does not further transform into (W,Cr)C and (Cr,W) 3 C 2 at 1450 C as expected from equilibrium diagrams. In contrast, no (W,Cr) 2 C forms on annealing of WC+Cr 3 C 2 +C powder mixes up to 1630 C, and the amount of chromium diffusing into the WC is strongly inhibited. Only at 2000 C, a certain amount of chromium is incorporated into the WC lattice. During industrial production of Cr-predoped WC powders carburisation occurs via intermediately formed (W,Cr) 2 C and the main part of chromium is built into the hexagonal WC lattice. The residual part of chromium is present either as (W,Cr) 2 C or (Cr,W) 3 C 2, depending on the processing conditions. References 1. P. Stecher, F. Benesovsky and H. Nowotny, Planseeberichte für Pulvermetallurgie 12 [2], pp , (1964). 2. E. Rudy and Y. A. Chang, 5 th Plansee Seminar, F. Benesovsky, Reute/Tirol, pp , (1964). 3. V.N. Eremenko, T.Ya: Velikanova and A.A. Bondar, Fiziko-Matematichni ta Tekhnichni Nauk 11, 74-78, (1986). 4. Z. Tükör, E. Halwax, W.D. Schubert, A. Bicherl, A. Bock and B. Zeiler, 17 th Plansee Seminar, (2009). 5. A. Adorjan, W.D. Schubert, A. Schön, A. Bock, B. Zeiler, 16 th International Plansee Seminar 2005, in: Powder Metallurgical High Performance Materials, RWF, Wattens, Vol.2, pp (2005).