J Mater Sci (2013) 48:140–149 DOI 10.1007/s10853-012-6805-4
Tantalum-modified Stellite 6 thick coatings: microstructure and mechanical performance A. Farnia • F. Malek Ghaini • J. C. Rao V. Ocelı´k • J. Th. M. De Hosson
•
Received: 4 June 2012 / Accepted: 8 August 2012 / Published online: 30 August 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Thick Co-based coatings with different contents of tantalum were prepared by simultaneous powder feeding laser cladding technique on 304 stainless steel substrate, with the Ta wt% being 0, 2, 7 and 12. Laser processing was carried out with a continuous 3.3 kW Yt:YAG fiber laser. Microstructural observations were executed using scanning electron microscopy, energy dispersive X-ray spectroscopy analysis, and transmission electron microscopy. Observations indicated that, with an increase in the Ta contents, the Ta-rich MC-type carbides were formed in interdendritic regions. Also, hexagonal M7C3-type carbides were formed instead of orthorhombic M7C3-type carbides. The orientation relationships between different phases and the matrix were determined by electron diffraction. Mechanical properties were determined using microhardness measurement at room temperature and wear resistance measurement at room and elevated (500 °C) temperatures. The research demonstrated that alloying any amount of tantalum, in spite of increasing the microhardness, could be detrimental for increasing the wear resistance of Stellite 6, both at room and elevated temperatures. The relationship between microstructure and mechanical properties is explained.
A. Farnia J. C. Rao V. Ocelı´k (&) J. Th. M. De Hosson Department of Applied Physics, Materials innovation institute M2i, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands e-mail:
[email protected] A. Farnia F. Malek Ghaini (&) Department of Materials Science and Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran e-mail:
[email protected]
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Introduction Laser cladding by powder injection is a process which can be used to deposit a dense, metallurgically bonded, and thick coating alloyed layer, with minimum dilution, on a comparatively soft substrate for improving surface properties. The process has advantages such as less thermal damage to the substrate and smaller distortion compared with conventional hardfacing processes [1, 2]. However, there are some disadvantages in this process. Owing to the high thermal gradients distributed over small areas and because of the mismatch in physical properties between the cladding and the substrate, severe residual stresses may cause cracking during or shortly after the process. Detailed descriptions of the laser cladding process, main characteristics of the process, and the processing conditions have been reported in the literature [3–9]. Cobalt-based alloys are widely used in wear applications [10, 11]. A group of typical cobalt hardfacing alloys are known as ‘‘Stellite.’’ These alloys are known to have high corrosion and high temperature wear resistance [12]. Stellite 6 is a hypoeutectic member of the Stellite alloy group, widely used in industry, with approximate composition of Co–28Cr–4.5W–1.1C (in wt%) [13]. The microstructure of rapidly cooled Stellite-type alloys with a carbon content lower than *2 wt% is simply characterized as hypoeutectic-containing face-centered cubic (fcc) Co-rich primary dendrites (cobalt-based solid solution) surrounded by interdendritic lamellar eutectic with W, Cr, and Co carbides. The wear resistances of this type of alloy and their laser clad coatings are attributed to the high hardness values of chromium-rich M7C3 carbides and to the formation of a protective oxidation layer at high temperatures [9, 14]. In Stellite 6 alloys, Cr provides oxidation and corrosion resistances, as well as strength by the
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formation of M7C3 and M23C6 carbides. Strengthening of Stellite alloys draws benefits from the refractory elements’ (tungsten or molybdenum) solid solution hardening and carbide precipitation [15]. In addition, alloying elements such as Ni, C, and Fe promote the stability of the fcc structure of Co-rich matrix, which is stable at high temperatures up to the melting point (1495 °C), while Cr, Mo, and W tend to stabilize the hexagonal close-packed (h.c.p) crystal structure, which is stable at temperatures below 417 °C [12]. Considerable efforts have been made to reduce wear at elevated temperatures using different approaches, such as applications of coating [16], preoxidation [17], cladding [18–20], different processing techniques [21], and alloying treatment [22–24]. It is known that the mechanical properties of Co-base Stellite hardfacing alloys depend on chemical composition, microstructures, and their manufacturing processes. Kuzucu et al. [25] reported that the Cr-rich carbide phase changes from M7C3 to M23C6 and its morphology from lamellar to granular shape in interdendritic regions when 6 wt% Mo was added to the Stellite 6 alloy. Shin et al. [12] showed the detailed relationship between the microstructure and mechanical properties in the Mo-modified Stellite alloy using plasma-transferred arc (PTA) process. Radu et al. [15, 26] investigated the effect of addition of yttrium to Stellite 712 alloys. They showed that alloying small amounts of yttrium (less than 1 %) enhanced the mechanical properties of the oxide scale that can improve the high-temperature wear properties. Tantalum is a strong carbide former element, which can easily react with C to form tantalum carbide (TaC). TaC belongs to the cubic crystal system, with the high melting point of 3880 °C and the density of 13.9 g/cm3 [27]. TaC has some excellent properties such as high hardness, high chemical stability, good resistance to chemical attack and thermal shock, and good corrosion resistance [28]. TaC can be not only an effective inhibitor to control grain growth (for example in WC–Co alloys [29]) but also it can increase hardness, thermal shock resistance and oxidation resistance (e.g., in carbon/carbon composites with C–TaC–C multiinterlayer [30]). Chao et al. [27] synthesized TaC particles in a Ni-based composite coating by reacting Ta2O5 with graphite by in situ laser cladding. In another study Yu et al. [31], synthesized fine dispersed TaC particles to increase the hardness of the Ni-based (NiCrBSi) laser clad composite coating through in situ reaction between Ta and C. However, to our knowledge, there is no report in literature concerning in situ synthesis of TaC in Stellite 6 Co-based alloy. The effect of Ta addition on microstructural features of Stellite 6 alloy and the relationship between microstructure and mechanical properties such as hardness and wear resistance in the Ta-modified Stellite 6 alloy have not been established until now. In this study,
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Stellite 6 alloy is deposited on stainless steel with various Ta additions, using simultaneous powder feeding technique and a side laser cladding process. The prime objective of the investigation is to establish the structure–property relationship of the effect of Ta addition on the hardness at room temperature and the dry sliding wear properties of the laser clads at the room and high temperatures.
Experimental Stellite 6 powder with tantalum additions were produced by simultaneous powder feeding system and deposited on a stainless steel substrate using laser cladding technique. The powders were commercial Stellite 6 with the mean size of 140 lm and pure (99.95 %) tantalum powder with the mean size of 50 lm. The substrate was 40-mm diameter, 304 stainless steel bar. The bar surface was sand blasted before cladding process. Chemical compositions of the materials used in this study are summarized in Table 1. The 3.3 kW Yt:YAG fiber laser, with the wavelength of 1.07 lm, was set at 550 W and was focused at 24 mm above the substrate surface to obtain a circular beam spot size of 3 mm in diameter with a Gaussian distribution of energy density. A CNC (4 axes) table was used to rotate and horizontally move the substrate bar under the laser head. The rotational speed of the bar was set so as to obtain a travel speed of 2 mm/s. At the same time, the substrate bar was slowly but continuously moved transversely in the direction of the bar axis to obtain about 30 % overlapping between subsequent tracks. Figure 1 illustrates the schematic of the process configuration. Stellite 6 claddings alloyed with four different levels of tantalum additions were deposited in three layers resulting in the average coating thickness of about 3 mm. Tantalum contents were targeted as 0 wt% (no addition of Ta powder), 2, 7, and 12 wt%, respectively. The chemical compositions of different coatings were measured after cladding using energy dispersive X-ray spectroscopy (EDS) technique. The powder feeding system consisted of a Metco Twin 10C powder feeder with two separate containers that contained Stellite 6 and Ta powder, respectively. The feeding Table 1 Chemical composition of used materials in wt% Material
C
Cr
Si
Mn
Ni
W
Co
Fe
Ta
Base powder Stellite 6
1.1
28.3
1.2
–
–
4.5
Bal.
–
–
Substrate AISI 304
0.08
18–20
1
2
8–10.5
–
–
Bal.
–
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Fig. 1 Detail of laser cladding set-up
rates from each container were controlled independently. Powders were mixed upon being fed into a cyclone as shown schematically in Fig. 2a and delivered simultaneously by a carrier gas (argon) from the side opposite to the laser track using the ALOtec Dresden GmbH Cu-based side cladding nozzle, with the nozzle opening of 2 mm, mounted at an angle of 38° normal to the surface. Figure 2b illustrates the configuration of laser cladding devices. Constant feeding rate of 85 mg/s was employed for Stellite 6 powder in all experiments. In order to obtain different compositions with Ta contents of 12, 7, and 2 wt%, Ta powder was fed with different feeding rates of 15.7, 7.7, and 1.7 mg/s, respectively. The microstructural features of coatings were studied using scanning electron microscopy (SEM) (Philips XL30 FEG with EDS) and transmission electron microscopy (TEM). Several thin foils for TEM observations were prepared from the middle of the coating by conventional mechanical grinding and polishing with SiC abrasive papers and finalized by ion milling at 4 kV in PIPS 691 system (Gatan Inc., USA). A JEM 2010F transmission electron microscope, operating at 200 kV, equipped with an EDS system (127 eV resolution, Bruker Co., USA), was used for selected area electron diffraction (SAED) analysis, morphology as well as highresolution TEM (HRTEM) observations. Micro Vickers indentation hardness measurement and pin-on-disk dry sliding test were performed using CSM Revetest machine and CSM HT Tribometer, respectively. The load of 4 N with 15 s holding time was used for microhardness indentations. The pin-on-disk dry sliding tests were performed at room (25 °C) and at elevated (500 °C) temperatures. The disk was a tool steel with the hardness of 700 HV. The pins were made from the coatings under study. The pins were machined from the clad
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Fig. 2 Laser cladding devices: a top view of the cyclone used as powders mixer, b overview of the experimental set-up
samples, and their coated sides were rounded to a ball shape with the ball diameter of 6 mm [9]. During the test, the pin was fixed in a pin holder, and the pin axis made an angle of 45° to the normal of the disk surface. Surface of the disks were polished using 1 lm diamond polish suspension and then thoroughly cleaned with alcohol and acetone before testing. All the tests were performed in air at the sliding speed of 10 cm/s along a circular path under the load of 10 N. The number of rotations was fixed to obtain the sliding distance of 500 m.
Results Microstructural studies In order to determine the chemical compositions of different coatings, they were analyzed employing EDS analysis. EDS measurements were performed on transversal
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Fig. 3 Microstructural changes in Stellite 6 alloy coating with Ta addition as detected by SEM with BSE detector: a Ta-free Stellite 6, b 2 wt% Ta-containing Stellite 6, c 7 wt% Ta-containing Stellite 6, d 12 wt% Ta-containing Stellite 6
cuts from larger areas (500 9 500 lm) to characterize average amount of Ta in clad layer. The calculated amounts of Ta powder fed together with Stellite 6 do not correspond to the final compositions of the clad layer measured by EDS because of different powder efficiencies for these two powders. From comparison of expected and measured compositions of the clad layer, it is clear that a higher fraction of Ta powder is lost during the side powder feeding than it is for Stellite 6 powder particles. Owing to the differences between Stellite 6 and tantalum powders in densities (8.5 and 16.7 g/cm3, respectively) and mean particle sizes (140 and 50 lm, respectively), these two powder particles behave differently after leaving the side nozzle opening, which results in the powder segregation effect before particles enter the laser beam and join the melt-pool. Details are beyond the scope of this article and will be the subject of another article submitted elsewhere. Backscattered electron images of the microstructures are shown in Fig. 3. As expected the microstructure of the straight Stellite 6 cladding consisted of Co-rich primary dendrites (cobalt-based solid solution) surrounded by interdendritic lamellar eutectic with W, Cr, and Co
carbides, see Fig. 3a. In the alloy containing 2 wt% Ta microstructure, a new phase appeared in eutectic area (bright regions), Fig 3b. By increasing the Ta amount to 7 wt%, Fig. 3c, a separate eutectic phase was identifiable alongside the Cr-rich eutectic. Table 2 shows the result of analysis by SEM–EDS of the three different regions marked in the Fig. 3c, confirming that this new phase is rich in Ta. At 12 wt% Ta, Fig. 3d, the volume fraction of Ta-rich eutectic has increased further and formed a network, while the primary dendrites and the eutectics become much finer. It is not easy to identify the phases from the simple study of XRD results as the wide solubility limits of most carbides exist. Precise value of lattice parameter could vary Table 2 Amount of main alloying elements (in wt%) in the regions marked in Fig. 3c as detected by SEM–EDS Cr
Ta
W
Co 34.8
Region A
33.0
1.9
6.8
Region B
15.9
35.5
5.6
30.4
Region C
26.0
3.8
4.0
55.5
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Fig. 4 TEM bright field image of the eutectic microstructure of the Stellite 6 alloy and related electron diffraction patterns
Table 3 Lattice parameters of phases in the Stellite 6 alloy Phases
Lattice parameters Theoretical values (nm)
Orthorhombic Cr7C3
fcc Co solid solution
Measured values (nm)
a = 0.4528
a = 0.4701
b = 0.7010
b = 0.7090
c = 1.2142 aCo = 0.3545
c = 1.2200 a = 0.3630
with compositional changes and processing. Besides, the internal stresses always presenting in laser clad coatings can affect the location of the XRD peaks. Therefore, in order to identify the phases, TEM was used, instead. Figure 4 shows the TEM bright field micrograph of the Cr-rich eutectic in straight Stellite 6 coating and related EDPs (electron diffraction patterns). The typical lamellar structure of the eutectics is shown. The EDPs from different lamellae revealed the orthorhombic M7C3 chromium carbide lamellae and fcc cobalt solid solution. The measured lattice parameter values of both phases are larger than the theoretical ones as summarized in Table 3. TEM–EDS analysis indicated that the difference between the measured lattice parameters and the theoretical values could be due to dissolution of some elements with larger atomic radius, e.g., tungsten, in these two phases. The presence of stacking faults in Cr7C3 and their effects on electron diffraction pattern have been reported before [32]. The stacking fault can cause a string shape in electron pattern, as can be seen in Fig. 4. Figure 5 shows clearly these
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stacking faults inside the Cr7C3 carbide. Figure 5a shows the TEM bright field image of the carbide, and Fig. 5b shows HRTEM image from the interior of the carbide. The atomic rows and repeated planes of stacking faults are obvious. It is known that cobalt-based solid solutions have a low stacking fault energy (SFE), and several slips systems can be activated. When high thermal stresses are generated by the fast cooling of the laser cladding process, stacking faults and dislocations are easily formed in the a-Co phase. This effect is pronounced in regions with high amount of hard carbides [14, 33], see Fig. 6. HRTEM is a strong method to study the interface and the orientation relationship (OR) between phases. Figure 7 reveals the OR between Co solid solution and orthorhombic Cr7C3 in laser cladded Stellite 6 using HRTEM technique. It is shown that (200) planes of Co are approximately parallel to (21–2) planes of orthorhombic Cr7C3 carbide. The typical moire´ fringes can be seen at the interface of two lattices, as well. Figure 8 indicates the bright field TEM image of microstructure of 7 wt% Ta-containing Stellite 6 coating. EDPs from the dark lamellae reveal the formation of TaC in this microstructure. As seen in EDPs in Fig. 8, the crystallographic orientations of all TaC lamellas are almost the same. So it can be concluded that these lamellae form a eutectic structure with cobalt solid solution. The measurements were done on EDPs, and the results showed that TaC is fcc crystal carbide with the lattice parameter of 0.4434 nm which is smaller than the theoretical value (i.e., 0.4455 nm). TEM–EDS analysis revealed that dissolution of some elements with smaller atomic radius than tantalum
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Fig. 5 TEM images of a Cr7C3 carbide in the Stellite 6 alloy microstructure: a stacking faults inside the Cr7C3, b high-resolution TEM image of the Cr7C3 showing crystallographic planes and the repeated stacking faults on atomic scale
Fig. 6 TEM bright field view of microstructure of eutectic in Stellite 6 showing stacking faults and traces of dislocations in the cobalt solid solution phase located between hard chromium-rich carbides
in TaC carbide, e.g., W and Cr, is the reason for the difference between measured and theoretical values for TaC lattice parameter. Figure 9 indicates the OR between TaC and the Co solid solution phases. Figure 9a is the combined electron diffraction pattern from their interface. It reveals the OR of {111}k{111} between these two phases. Figure 9b is HRTEM image showing the interface of these two phases on atomic scale. With a closer look, it can be seen that there are some dislocations at the interface which are repeated periodically after each four atomic planes in TaC side. Therefore, it can be concluded that the mismatch
Fig. 7 Orientation relationship between Co solid solution and orthorhombic Cr7C3 in Stellite 6, with typical moire´ fringe interference
factor is 0.25, and this interface can be categorized as a semi-coherent interface. Figure 10a indicates the TEM bright field image of the morphology of Cr7C3–Co solid solution eutectic in the 7 wt% tantalum-containing Stellite 6 alloy. High density of stacking faults is clearly visible as well as dislocations. Electron diffraction pattern, Fig. 10b, revealed that this eutectic consists of hexagonal Cr7C3 and fcc Co solid solution lamellae, while the orthorhombic Cr7C3 was detected in Ta-free Stellite 6 alloy microstructure.
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Fig. 8 TEM bright field image of microstructure of TaC–Co eutectic and related electron diffraction patterns from TaC branches (sample contains 7 wt% Ta)
Fig. 9 Orientation relationship between Co solid solution and fcc TaC: a electron diffraction pattern from interface, b high-resolution TEM of interface indicating a semi-coherent interface (white arrows indicate the interface dislocations in TaC side, sample contains 7 wt% Ta)
Hardness and wear studies Figure 11 shows the results of hardness profile measurements performed at the room temperature. As expected, the additions of tantalum to Stellite 6 have resulted in increasing the hardness. The average hardness measured for clad layers for the straight Stellite (no Ta addition) was 500 HV which was increased to 650 HV as Ta increased to
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12 wt%. One may also conclude that the hardness is not homogenous throughout the whole coating thickness and that the coatings with higher content of Ta clearly revealed a drop of hardness at the individual layers interfaces. Dry sliding wear tests were performed on a pin-on-disk tribometer at both room and high temperatures (500 °C). The wear rates were determined by measuring the contact area on the pin using an optical confocal microscope. SEM
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Fig. 10 TEM bright field image of the eutectic microstructure of the 7 wt% Ta-containing Stellite 6 alloy showing the traces of dislocations and stacking faults in the Co solid solution phase and the electron diffraction pattern related to chromium carbide phase in Co solid solution
was used to observe morphologies of worn surfaces, and EDS was employed to determine the local composition of the worn surfaces. On each coating, two wear tests were performed, the results of which are shown in Fig. 12. It is seen that alloying of Stellite 6 with tantalum, in addition to increasing the hardness, has increased the wear rates both at room and elevated temperatures. The loss in wear resistance is dramatically increased at elevated temperature.
Discussion
Fig. 11 Hardness profile of coatings with different wt% of Ta. Vertical dash line indicates the position of interface between coatings and substrate
Fig. 12 Wear rates of different coatings with different wt% of Ta at room and high temperatures
With addition of Ta to the Stellite 6 alloy, one of the most significant changes in microstructural features is the change of the chromium carbides crystallography, i.e., from orthorhombic structure in Ta-free Stellite 6 to hexagonal structure in 7 wt% Ta-containing Stellite 6 alloy. According to the previous results, five polymorphs of Cr7C3 exist. Two of them have hexagonal structures represented by rhombohedral cell (space group P31c), the other two structures designated as orthorhombic crystals (space group Pnma and Pmcn), and the last one has the hexagonal cell with space group of P63mc [34]. In the current study, as mentioned above, EDPs showed that the chromium carbide formed in Ta-free Stellite 6 is orthorhombic, see Fig. 4, and the chromium carbide formed in Ta-containing Stellite 6 is hexagonal with the space group of P31c, see Fig. 10b. It is reported that the orthorhombic structure is a hightemperature structure of Cr7C3 referred to as the stable phase. The hexagonal structure is reported to be a poorly crystallized structure, called the low temperature form, observed in carbides formed at temperatures lower than 1200 °C [32]. In the case of the current investigation,
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Fig. 13 Decomposition of chromium carbides in presence of tantalum after wear test as detected by SEM with BSE detector: worn surface of the: a Ta-free Stellite 6 alloy at room temperature,
b 7 wt% Ta-containing Stellite 6 alloy at room temperature, c Ta-free Stellite 6 alloy at high temperature, d 7 wt% Ta-containing Stellite 6 alloy at high temperature
because of the high affinity of carbon for reaction with Ta in comparison with chromium [35], the chromium carbide becomes less stable. Figure 13 reveals the microstructure of the worn surfaces of Ta-free and 7 wt% Ta-containing Stellite 6 coating both at room and high temperatures. It can be seen that the chromium carbides are decomposed after the test both in room and high temperatures. Therefore, it can be concluded that the stability of chromium carbides play the main role in high wear performance of the Stellite 6 alloy, not the hardness. High wear resistance of Stellite 6 is thought to be because of the fcc structure of the cobalt transforming to the low temperature h.c.p structure on the application of high shear stresses during sliding [36]. A thin, easily sheared layer can form at the sliding surface because of the shear-induced alignment of the h.c.p basal plane parallel to the direction of sliding. This alignment significantly reduces friction and improves galling resistance, with shear and adhesive transfers restricted to this layer. This layer easily reforms when removed [10]. Study by Koster [37] showed that tantalum, niobium, and titanium raise the SFE of
cobalt and stabilize the ductile fcc structure. Therefore, during wear test, the formation of transforming layer is suppressed. Change of fracture toughness (K1C) is another feature that occurs when tantalum is added to Stellite 6 alloy. It has been shown that while the hardness of the Stellites increases, the wear resistance of Stellites strongly depends on fracture toughness (K1C) of the material [38]. In this case, addition of Ta increases the eutectic phase in the structure, as shown in Fig. 3, which can decrease the toughness of material, and, therefore, the wear resistance decreases. The wear surface examinations and observations suggested that the dominant wear mechanism was oxidation wear. The type and physical characteristics of the oxide layers produced on the surfaces contributed to the wear resistance. EDS analysis revealed the presence of oxygen on the surface which indicates an oxidation process due to the heat generated during wear. It is known that the good wear resistance of Stellites at high temperatures is mainly due to formation of a protective oxide layer [10, 13]. The
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oxides generated from Stellite 6 during sliding are very good ‘glaze’ formers. ‘‘Glazes’’ are formed via wear debris particles which are oxidized, fractured into small particles (some down to 5 nm), and consolidated together onto the contacting surfaces. ‘‘Glazes’’ are very wear resistant and provide low friction surfaces [10, 39]. On the other hand, TaC can increase oxidation resistance [30]. From the SEM–EDS observations of wear surfaces, it was concluded that the addition of Ta to Stellite 6 suppresses the formation of oxidation layer. This behavior could be a reason for worse wear resistance of Ta-containing coatings. However, a detailed explanation of the sliding behavior and wear mechanism of these coatings at room and elevated temperatures are out of the scope of this study and will be discussed elsewhere.
Conclusions The alloying effect of Ta in the Stellite 6 hardfacing alloy deposited on 304 stainless steel using laser cladding process on the microstructure and wear resistance was investigated. It was shown that with increasing Ta content, Ta-rich MC-type carbides were formed in the interdenritic regions with a semi-coherent interface with Co solid solution. Besides, the orthorhombic chromium-rich M7 C3-type carbides change to hexagonal chromium-rich M7C3-type carbides which are less stable. On the other hand, the SFE of the alloy is increased in presence of tantalum in the composition. Therefore, alloying of Stellite 6 with tantalum, in spite of increasing the hardness, has increased the wear rates both at room and at elevated temperatures. The loss in wear resistance is dramatically increased at elevated temperature because oxide layers are not formed. It was shown that the wear performance is not only dependent on the hardness, but also strongly depends on different structural features. Acknowledgements This research was a collaborative effort of the Tarbiat Modares University in Tehran and the Materials innovation institute M2i (www.m2i.nl) where it was carried out under project number MC7.06259.
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