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Powder injection molding of Stellite 6 powder: Sintering, microstructural and mechanical properties
Materials Science & Engineering A 651 (2016) 914–924
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Powder injection molding of Stellite 6 powder: Sintering, microstructural and mechanical properties H. Özkan Gülsoy a, Özgür Özgün b,n, Sezer Bilketay a a
Marmara University, Technology Faculty, Metall. and Mater. Eng., 34722 Istanbul, Turkey Bingol University, Faculty of Engineering and Architecture, Mechanical Eng. Dep., 12000 Bingol, Turkey
b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 October 2015 Received in revised form 26 October 2015 Accepted 19 November 2015
The purpose of this study was to produce Co-based Stellite 6 superalloy components by using the method of Powder Injection Molding (PIM) and to characterize the microstructural and mechanical properties of the produced components. The experimental studies were started through the formation of feedstock by mixing Stellite 6 powder with a multicomponent binder system. Prepared feedstock was formed by utilizing powder injection molding technique. Then the molded samples were subjected to the solvent and thermal debinding processes. Different sintering cycles were applied to the raw components for the purpose of determining the optimum sintering conditions. The densities of the sintered components were determined in accordance with the Archimedes' principle. The microstructural characterization was performed through scanning electron microscope (SEM) analysis, energy dispersive spectrometry (EDS) analyses, and X-ray diffraction (XRD) analysis. Hardness measurement and tensile test were conducted in order to determine the mechanical properties. The results illustrated that the injection molded Stellite 6 components were composed of fine and equiaxed grains, plenty of carbide precipitates exhibiting homogenous distribution throughout the microstructure formed at the grain boundaries and thus the mechanical properties were considerably high. & 2015 Elsevier B.V. All rights reserved.
Keywords: Powder injection molding Stellite 6 Microstructure Mechanical properties
1. Introduction The design and production of high strength materials with good wear and corrosion resistance at high temperatures are very important for the severe service conditions in the automotive, aeronautical, chemical, and petroleum industries [1]. Stellite alloys exhibite an unmatched combination of mechanical and tribological properties with superior corrosion and oxidation resistance at both room and high temperatures [2]. They are among the leading ones of Co based superalloys which are one of the three major superalloy classes [3]. While the applications of Co based superalloys are conventionally intended for nuclear industry mostly [4], the use of Stellite alloys today has spread to different industrial fields such as pulpwood, paper making, petroleum and gas processing, pharmaceutical industry, chemical processing, etc. [5]. One of the leading Stellite alloys having such widespread area of usage is undoubtedly the Stellite 6 that was the first Stellite alloy developed by Elwood Haynes during the early 1900s [6,7]. The properties of Stellite alloys are widely determined by their chemical compositions [8]. While solid solution hardeners such as n
Corresponding author. E-mail addresses: [email protected], [email protected] (Ö. Özgün).
http://dx.doi.org/10.1016/j.msea.2015.11.058 0921-5093/& 2015 Elsevier B.V. All rights reserved.
Cr, W, and Mo contribute to the strength, carbide precipitates are the main strengthening mechanism [8,9]. Type, size, and shapes of carbides affect strongly the properties of Stellite alloys. For instance, larger size carbides generally provide with the acquisition of higher strength [10]. The formation of carbides, having significant effects on the properties, is influenced by the type and amount of alloy elements (particularly carbon), the production method, sintering temperature and cooling rate and also operational parameters [2,3]. In the production of Co based alloys by using casting technique, important precautions or additional heat treatments [13] are required so as to prevent or solve problems [11,12] such as porosity, segregation, coarse grain size, eutectic carbide network between the dendrites, etc. It is difficult to form Co based alloys having high rate of carbon even at high temperatures [14]. While the industrial demands shift the usage area of Stellite 6 alloy to higher stress applications, the production processes could be adapted in such a way to provide the tribomechanical properties needed [15]. PIM is a production method which enables to produce products with high dimensional accuracy in such a way to have excellent, fine grain structure and non-anisotropic mechanical properties [16]. Despite being a powder metallurgy process, PIM enables to obtain higher rate of density than those components produced
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through conventional powder metallurgy (PM) methods and thus more superior mechanical properties. Complex shaped components that are difficult to produce by using conventional production methods such as casting, forging, machining, and die pressing can easily be transformed into products by means of PIM. The high surface quality and the fine grain structure of the components produced with high measurement precision through the PIM technique provide highly superior properties since they exhibit a homogenous distribution throughout the microstructure of the chemical compound [17]. Recently, PIM of superalloy components for various applications has attracted attention [18–21]. There is no information concerning the microstructural and mechanical properties of Stellite 6 materials produced by using PIM in the literature. For this reason, production of the component from Stellite 6 superalloy powder by using PIM technique was performed in this study. It was aimed to both determine the optimum parameters for the production of Stellite 6 alloy by using PIM and to characterize the microstructure and mechanical properties of the produced components. Hence, injection molded Stellite 6 green components were kept to different sintering cycles to characterize them. Characterization of samples was conducted by SEM, XRD, elemental analyses and mechanical tests. The experimental studies have been carried out to determine the microstructure, and mechanical properties of this alloy. Furthermore, the data obtained from the characterization were compared with properties of Stellite 6 alloy produced by using different methods.
2. Materials and method Fig. 1 illustrates the SEM image of the prealloyed Stellite 6 superalloy powder used in the experiments. It was seen in the SEM images that Stellite 6 powders had a spherical form. Fig. 2 shows the curve obtained from the particle size distribution analysis conducted by using Malvern Mastersizer instrument. While Table 1 illustrates the chemical composition of the Stellite 6 powder, Table 2 illustrates its some physical properties. The experimental studies were started by preparing the feedstock. The feedstock was obtained by mixing Stellite 6 powder through a multicomponent binder system. The mixing process was performed in a specially designed mixer under vacuum and at 170 °C for half an hour. The feedstock included 62.5 vol% Stellite 6 powder and 37.5 vol% binder. The multicomponent binder system consisted of 69 wt% paraffin wax (PW), 20 wt% polypropylene (PP), 10 wt% carnauba wax (CW), and 1 wt% stearic acid (SA). Rheological behavior of the
Fig. 1. SEM image of the Stellite 6 powder.
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Fig. 2. Particle size distribution of Stellite 6 powder.
Table 1 Chemical composition of Stellite 6 powder. Weight (%) Co
Cr
W
Ni
Mn
C
Si
Fe
Mo
P
S
58.438
30.3
4.5
2.0
1.17
1.15
1.02
0.86
0.55
0.008
0.004
Table 2 Some characteristics of Stellite 6 powder. Properties Producer Production method Particle shape Particle size (mm) D10 D50 D90
Osprey Gas atomization Spherical 5.3 13.8 27
Stellite 6 feedstocks was measured by using a rotational viscometer Physica MCR51 (Anton Paar, Austria) at shear rates from 10 to 1000 s 1 at 130–170 °C. The value of viscosity is given based on the shear stress divided by the shear rate. After the feedstock was granulated, it was molded in the form of tensile specimen at 12.5 MPa and for a holding time of 20 s. in the injection device. The temperature of the injection device's barrel and nozzle sections was adjusted to 170 °C during molding. The tensile specimens molded were subjected to a two-stage debinding process, as solvent and thermal. Being heated to 60 °C in the solvent debinding process, the samples were kept for 6 h in the heptane. The thermal debinding operation was implemented by using Al2O3 substrate in a high purity argon atmosphere based on the thermal cycle in Fig. 3. Differential scanning calorimetry (DSC) test was performed in order to determine the sintering behaviors of the debinded samples. The DSC analysis was carried out by heating the Stellite 6 powder at heating rate of 10 °C/min until 1400 °C and at 100 ml/ min flow rate under high purity Ar gas atmosphere in a Setaram Labsys Evo device. Al2O3 was used as a reference material. In the light of the data from the DSC, the samples were sintered with holding time of one hour within the temperature range of 1200– 1300 °C. The sintering processes were carried out by using Al2O3 substrate under high vacuum in Protherm tube furnace. 10 °C/min fixed heating and cooling rate was used in the sintering process. Fig. 4 illustrates the images of the samples taken after the molding, debinding, and sintering stages. The densities of the sintered samples were measured according to the Archimedes' principle by using Precisa XB 320M precision balance. The XRD analyses were carried out in the Rigaku Ultima IV X-Ray Diffractometer device by using Cu X-ray tube (λ ¼ 1,5405)
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Fig. 3. Thermal debinding cycle.
Fig. 4. Image of the tensile specimens after molding, debinding and sintering stages.
at the scanning rate of 0.02/0.4°/s. After the metallographic preparations such as grinding and polishing, the microstructures of the samples etched through 50 ml HCl (32%) þ10 ml HNO3 (65%) þ 10 g FeCl3 þ 100 ml H2O solution were examined through SEM and EDS analyses. While SEM analysis was performed with JEOL JSM 6510 scanning electron microscope, SEM/EDS analysis were carried out with IXRF 550 brand EDS system connected to this device. The hardness measurements were carried out by using Vickers scale through the application of 500 g load for 10 s in the EmcoTest brand (M1C 010) device. The tensile tests were performed in Shimadzu AG-X 50 kN tensile device at the constant rate of 1 mm/ min. At least three samples were tested under the same conditions in terms of the reliability of the results.
3. Results and discussion The evaluation of the feedstock rheological properties is based on viscosity and its shear sensitivity and temperature sensitivity. Fig. 5 illustrates the shear stress and viscosities of the feedstock at different shear rates and different temperatures of 130–180 °C. The viscosity data indicate the flowability of the feedstock. As the value of the viscosity decreases, it is easier for a feedstock to flow. Fig. 5 shows that the viscosity decreases at all shear rates. With the increase of temperature, viscosity of feedstock decreases. PIM
Fig. 5. Temperature-dependent viscosity versus shear rate of Stellite 6 feedstock.
feedstock is carried out under pressure and temperature. It is desirable that the viscosity of the feedstock should decrease quickly with increasing shear rate during molding. This high shear sensitivity is especially important in producing complex parts. The results indicate that the feedstocks have pseudo-plastic rheological behavior and increasing temperature leads to a decrease in the viscosity. Fig. 6 shows the curve obtained from the DSC analysis applied to Stellite 6 powder. According to this curve, it was determined that the solidus temperature of Stellite 6 alloy was 1235 °C, and its liquidus temperature was 1326 °C. In the literature, the solidification range of the Stellite 6 alloy is reported to be 1260–1357 °C [22]. The solidus and the liquidus temperatures in this study were lower than the data in the literature. This situation was thought to be associated with the fact that the material was in the powder form and accordingly had a high surface energy as well as possible chemical composition. Moreover, in the DSC curve, endothermic peaks were observed at 855 and 1076 °C. It was considered that these peaks were related to formation and transformation of carbides. It is reported that in Co based superalloys, the M23C6 type carbides formed secondary precipitates in the form of fine grains
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Fig. 6. The curve of the DSC analysis of the Stellite 6 powder.
throughout the alloy matrix within the temperature range of 704– 871 °C [23]. It can be asserted that the peak occurring at 855 °C referred to the dissolving of these fine grains. It is known that M23C6 carbides form as primary precipitates during solidification in the Co based alloys produced in the casting. It was reported that the heat treatment made within the range of 1150–1204 °C caused the dissolving of the coarse carbides at the grain boundaries [23]. Taking into account the fact that the DSC analysis was applied to the Stellite 6 powder consisting of small sized particles with high surface energy (D50 ¼13.8 mm) and with reference to the aforementioned descriptions, it was that the endothermic peak at 1076 °C referred to the fact that the carbide precipitates were dissolved. Fig. 7 illustrates the relative density values obtained in Stellite 6 components depending on the sintering temperature. The relative density values increased with the increase in the sintering temperature. While the highest relative density value was obtained with the sintering process made at 1275 °C, this value was found as 98.27%. In fact, by taking the DSC analysis into account, the samples formed with PIM were sintered at 1300 °C, as well. However, it was observed that the samples were deformed depending on the formation of excessive liquid phase at this temperature. Therefore, the information concerning the sample sintered at 1300 °C was not present in the graphic and it was accepted that the highest density value was obtained from the sintering process made at 1275 °C. It is indicated in the literature that the Co-based alloys produced by using PM technique can condense between 97% and 100% [24]. On the other hand, in numerous studies conducted regarding Stellite alloys, hot isostatic pressing (HIP) method was used in order to reach full density [1,5,15,25,26]. In some studies, the materials produced by using HIP were subjected to a secondary HIP process [15]. Even in some studies, relative density of 100% was not reached although the HIP was applied [1].
Fig. 7. Change in the relative density values depending on different sintering temperatures for Stellite 6 samples.
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Fig. 8 illustrates the SEM images of the samples taken after the polishing process for the purpose of getting information about the pore structure and amount of the sintered samples. When the images were examined, it was understood that as sintering temperature increased, the amount of interconnected pores decreased. This was accompanied by the shrinkage in the size of the pores. In accordance with the results obtained from the density measurements, the least porosity was seen in the image of the material sintered at 1275 °C. In addition, it can be seen that the pores in the sample were not interconnected and the pore morphology was spherical. The fact that the pore geometry in the parts produced by using PM techniques had such spherical pores instead of shapeless pores including sharp edges is a desired condition in terms of strength [27]. The fact that the pores remained in the form of noninterconnected spherical pores in this way is one of the advantages of PIM method [28]. From the DSC analysis, the solidus temperature of the material was determined as 1235 °C. This situation showed that the sintering process at 1275 °C was performed by using supersolidus liquid phase sintering (SLPS) mechanism. It can be seen from the SEM images in Fig. 8 that the microstructure included light colored and dark colored phases although no etching process was performed. In the previous studies conducted concerning Stellite alloys, it is reported that the Co matrix in the microstructure was light gray and the carbides rich in Cr were dark gray [3,5,15,26,29]. It is indicated that W element also forms carbides in Co based superalloys and the carbides formed by W were of a brilliant white color in the microstructure [26]. Based on these explanations, it can be asserted that the dark precipitates homogenously distributed in the microstructure in the SEM images of the samples taken without etching them in this study were the carbides formed by Cr. As the sintering temperature increased, even slightly coarse was observed in sizes of the dark carbides. In a previous study using the HIP method, it was reported that the increase in the sintering temperature caused coarsening in the size of carbides similar to the present study [29]. This situation was associated with the fact that the atom movements occurring easier high temperatures facilitate the precipitation and growth of the carbide [29]. Fig. 9 illustrates the SEM images of the sintered samples taken after the etching process in order to understand their grain structures, the grain boundary – pore relationship, and the morphology and distribution of the phases contained in them. It can be seen from the images that the pores were located in intergranular regions. In order to obtain the appropriate properties in superalloys, it is preferred that the material contains randomly oriented equiaxed grains [13]. The SEM images show that the samples produced consisted of equiaxed grains. The PIM method generally enables the materials produced without requiring any precaution or additional process to consist of equiaxed grains [28]. Numerous previous studies have reported that the materials produced by using PIM consist of equiaxed grains as in this study [18– 21]. The controlling of the grain size is very significant to develop and sustain the physical and mechanical properties among the materials. Finer grain size generally improves the properties of tensile, fatigue, and creep strength for service conditions with low and middle temperatures [13]. It is known that the grain size in the material production by sintering increases with the increasing sintering temperature and duration [27,30]. The SEM images show that a slight coarsening occurred in the grain size due to the increasing sintering temperature. While the sample sintered at 1200 °C had an average grain size of 3–4 mm, the sample sintered at 1275 °C, where the highest density was obtained, had an average grain size of 5–6 μm (ASTM grain size number is 12). Actually, despite an increase of 75 °C in temperatures in the sintering processes, the increase in the grain size is at a hard-to-distinguish level. This situation makes us think that the carbides precipitating
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Fig. 8. SEM images of the sintered samples at different temperatures before the etching process; (a) 1200 °C, (b) 1225 °C, (c) 1250 °C and (d) 1275 °C.
at the grain boundaries played an inhibitive role in the grain coarsening. This thought becomes more convincing by considering the fact that in Ni based superalloys, the δ phase [31] preferring to form at the grain boundaries prevents the grain coarsening during the hot forging operations [32]. In this study, the superior properties such as homogenous microstructure, fine and equiaxed grains easily obtained by using PIM technique without needing any precaution were rather difficult to be obtained by using casting method. Obtaining the aforementioned superior properties by using the casting method requires additional precautions such as the coating of the mold surface with nucleants for the purpose of accelerating the grain nucleation and solidification, proper selection of mold and metal temperature and the selection of some other parameters properly [13]. A coarsening similar to the ambiguous coarsening occurring in the grain size of the material with the increase in sintering temperature was observed in the carbides rich in Cr at the grain boundaries exhibiting homogenous distribution throughout the microstructure. The carbide sizes were observed to range between 1 and 3 μm in the SEM image of the sample sintered at 1275 °C. It was reported that the carbide sizes ranged between 1 and 3 μm also in the Stellite 6 alloys produced by using HIP [5,15]. Like their type, carbide sizes and shapes strongly affected the properties of Stellite alloys. There are studies reporting that larger size carbides generally contribute to higher strength and wear resistance [10]. The microstructure in the Stellite alloys produced by using casting method consisted of the dendrites formed by the Co solid solution and the eutectic carbide phases between these dendrites. The carbides were in the form of continuous films surrounding the grains and had a large size [5,15,25,26,33]. In the alloys produced by casting, this shape, size, and distribution style of the carbides decreased ductility and fatigue strength of the material [34]. In the
present study, it was observed in the material produced by using PIM that the non-interconnected carbides with the block morphology had a homogenous distribution on the grain boundaries throughout the microstructure instead of large size eutectic carbides surrounding the dendrites. Various studies have reported that the carbides exhibited such a distribution in the Stellite alloys produced by using HIP and they had a spherical-like form [5,15,25]. The morphology of the carbide precipitates in superalloys has significant effects on the properties of alloys. The fact that carbide precipitates at the grain boundaries in the form of a continuous film sets a ground for the formation of cracks and decreases significantly the impact and rupture properties of the alloy. Since the formation of precipitates that are large and independent from each other (discrete) at the grain boundaries, instead of a continuous film, prevents dramatically the grain boundary sliding, it is useful [23,35]. Additional heat treatments are needed in Co alloys produced by casting in order to obtain a more homogenous structure by dissolving the large carbide network and thus to improve the mechanical properties [34]. Taking into account these explanations and in contrast to the Stellite alloys produced by casting, it is also expected for the PIM method, which provides the acquiring of materials containing carbides exhibiting finer and more homogenous distribution without needing additional heat treatments, to provide more superior mechanical properties. Fig. 10 illustrates the images of SEM elemental mapping analysis taken from the sample sintered at 1275 °C in order to determine the distributions of the elements inside the material. It is seen in the SEM image that abundantly discrete precipitates with block morphology formed at the grain boundaries. A significant decrease was observed in the amount of Co element forming the matrix in regions where these precipitates were located. Co based
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Fig. 9. The SEM microstructure images of the samples sintered at (a) 1200 °C, (b) 1225 °C, (c) 1250 °C and (d) 1275 °C.
superalloys generally gain their strength from the solid solution hardening and the carbide precipitates [14]. Considering that the main strengthening phase is the carbides, it can be asserted that Cr is the most significant alloying element for Co based superalloys because Cr is not only a predominant carbide former but also contributes to the solid solution hardening [23,36]. In addition, another significant role of Cr is to increase the corrosion and oxidation resistance of the alloy [23]. When the SEM elemental mapping analysis was examined, the Cr element exhibited a serious clustering in the precipitates seen in the microstructure. The fact that C element also had high amounts in these regions where Cr exhibits clustering signified that these precipitates were the carbides formed by Cr. The carbides formed by Cr in Co based superalloys are the carbides of M3C2, M7C3, and M23C6. It is indicated that M3C2 among them is a type of carbide seen in previous superalloys with a low rate of Cr. The M7C3 carbide also consists of low Cr–C alloy rates. This carbide forms more in intragranular regions and sometimes intergranular regions. Being a metastable carbide M7C3 transforms into the carbide of M23C6 with heat treatment or under high temperature service conditions [3,23]. It is indicated that the M23C6 type carbides can precipitate as both the primary and the secondary carbides in Co based alloys containing a high level of Cr [23]. It is known that this carbide generally forms at the grain boundaries of the multi-crystalline materials and provides the grain boundary strength and the fracture resistance required for long service conditions when found as discontinuous precipitates [13]. The Cr ratio of the Stellite 6 powder used in the present study is 30.3 wt%, which is considerably high. The SEM images of Fig. 10 shows that all of the carbides formed at the grain boundaries. In addition, it was thought that since all the precipitates in the SEM images had approximately the same size and morphology, they were the same phase. All these
explanations indicated that these precipitates seen in the microstructure were M23C6 type carbides formed by Cr. In the literature, it is reported that Co, W or Mo can substitute for a little amount of Cr in the M23C6 type carbides [23]. In a study conducted by Rosalbino and Scavino [5], it was found that the M23C6 carbide contained 69.6% Cr, 16% Co, 6.3% W, 2.3% Si, 1.8% Fe, and 1.3% Ni [5]. In the images of the elemental mapping analysis, Mn, Si, and O elements, along with Cr and C, were higher in amount in the regions having carbide precipitates when compared to the matrix. Additionally, it was observed that even though presence of Co, W, Ni, and Fe elements exhibited a decrease in these regions, these elements had low amounts inside the carbides. W and Mo elements in the Co based superalloys were the elements that provided the hardening of the matrix with solid solution by means of its large atom sizes. When amounts of these two elements were high in the alloy, this provided the formation of carbides or intermetallic compounds [36]. The carbides formed by W and Mo were M6C type carbides [5,23,36]. When the images of W and Mo were examined in the mapping analysis, it was seen that these two elements did not cluster in any region in a manner that will form a precipitate. For this reason, it became definite that there was no carbide or intermetallic precipitate formed by W and Mo in the microstructure. While Mo had a homogenous distribution in all the regions in the microstructure including the carbides formed by Cr, the amount of W decreased in the regions with carbides as mentioned above. From this point of view, it can be asserted in this study that W and Mo would only contribute to the material strength by means of solid solution hardening in the components produced by using PIM. The elemental mapping analysis showed that all the precipitates forming in the microstructure contained C. This situation signified that topologically close packed (TCP) phases, having negative effects on the material
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Fig. 10. Images of SEM/EDS elemental mapping analysis taken from the sample sintered at 1275 °C.
strength and especially low temperature ductility did not form. In Co based superalloys, Ni element is added in order to stabilize the fcc crystalline structure from room temperature to the melting temperature. It is also known that Fe also stabilises the fcc structure like Ni [24]. According to images of the elemental mapping analysis of Ni and Fe, they exhibited a homogenous distribution within the matrix. In the elemental mapping analysis,
the amount of oxygen element increased significantly in the precipitates stated as M23C6 type carbides. It was reported that M7C3 and MC type carbides preferentially oxidized when exposed to high temperature [37]. Similarly, it was understood in the present study that M23C6 type carbides also oxidized during the sintering process. Fig. 11 illustrates the images of SEM/EDS point analysis taken
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Fig. 11. SEM/EDS analysis taken from the sample sintered at 1275 °C.
from the sample sintered at 1275 °C. The amount of Cr element was considerably high in point 1 on a precipitate located at the grain boundary. The fact that Cr was accompanied by C element showed that this precipitate was Cr carbide. Together with Cr, Co element also existed within the carbide precipitate. Moreover, W, Mo, Fe, and Ni elements were also present in low amounts in the carbide precipitate. The said carbide precipitate contained no Si
element. Element rates close to the chemical composition of the initial powder in point 2 received from Co matrix were obtained. Only the Cr element exhibited a lower rate than its rate in the initial powder. This situation was caused by the fact that Cr formed a significant amount of carbide as is understood from the SEM examinations up to now. It was observed that the amount of Cr in point 3 at the grain boundary was higher compared to the matrix.
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Fig. 12. XRD patterns taken from the initial powder and the samples sintered at 1200 °C and 1275 °C.
Fig. 12 shows XRD patterns of the initial powder and samples sintered at different temperatures. Only peaks of fcc-Co matrix were seen in the XRD pattern of the initial powder. However, the initial powder was a prealloyed powder produced by gas atomization. For this reason, it was considered that it had to contain carbides. The elements having property of forming the carbide in this powder were Mo, W, and Cr [3,23]. However, it is not possible for Mo and W having large atomic radii and therefore having considerably high diffusion coefficients to find sufficient time to form carbide during the gas atomization. The carbides that Cr can form in Co based superalloys are M3C2, M7C3 and M23C6 carbides [3,23]. From the XRD cards of these carbides, it was observed that the peaks of the M23C6 carbide showed coincidence with the peaks of Co matrix. The peaks of M23C6 carbides in the sintered material became more apparent. Although the sintering process was carried out in high vacuum values, it was observed in the XRD pattern of the sintered materials that CrO phase formed. The increasing sintering temperature led a slight increase in the severity of the peak of CrO phase. In a study conducted by Rosalbino and Scavino [5], it was determined that M23C6 type carbides formed in the XRD analysis of the materials produced by using HIP method from Stellite 6 powder as well. The amount of Cr in the Stellite 6 powder used in this study was higher than Stellite 6 powder used in Rosalbino and Scavino's study [5]. It was stated that M23C6 type carbides can precipitate as both the primary and secondary carbides in high Cr content [23]. The values obtained as a result of the hardness measurements are shown in the graphic in Fig. 13. It is seen from the graphic that the hardness also increased with the increase in sintering temperature. This situation was associated with the fact that the increasing sintering temperature caused the density of the material to increase. The average hardness value measured in the sample sintered at 1275 °C, where the highest density was obtained, was 458.25 HV0.5. In the literature, it is reported that the hardness of
Fig. 13. Hardness values obtained in the samples depending on the sintering temperature.
produced Stellite 6 alloys produced by casting and conventionally PM is 40 HRC [24,38]. This value approximately corresponds to 400 HV. In a study conducted by Yu et al., [26], it was reported that the hardness of the Stellite 6 materials produced through sand casting was 402.6 720.9 HV(2.94 N). Also, in the same study, the hardness value obtained in the materials produced by using HIP process for 4 h under 100 MPa pressure at 1200 °C from the Stellite 6 powder, whose grain size ranged predominantly between 45 and 180 mm and which was produced by gas atomization was 419.1 79.90 HV(2.94 N). Since the same material was subject to a secondary HIP treatment with the same parameters, this hardness value reached 459.3 710.4 HV(2.94 N) [15]. In the present study, the hardness of the materials produced by using PIM from the small sized prealloyed Stellite 6 powders is higher than the Stellite 6 materials produced by using casting and HIP methods. This situation was caused by the grain structure, grain size of the materials produced by using PIM as well as morphology and distributions of carbides. Ahmed et al., [15] reached the hardness value reached in the Stellite 6 samples produced by using PIM but by the re-HIPing process. Fig. 14 illustrates average tensile curves obtained from the Stellite 6 samples depending on the sintering temperature. The increase occurring in the density due to the increase in the sintering temperature affected both the tensile strength and hardness. The highest tensile strength was 1230 724 MPa and obtained in the sample sintered at 1275 °C. Table 3 compares the tensile strength obtained from the sintered tensile samples and % elongation values with casting and forging Stellite 6 alloys. In the Co based alloys produced by casting, the large sized carbides were enabled to be dissolved by performing a heat treatment within the temperature range of 1149–1204 °C. The solution process was followed by the aging treatment made within the temperature range of 760 and 982 °C. Therefore, the fine M23C6 precipitates exhibiting a more homogenous distribution throughout the structure precipitated again and thus the tensile strength significantly increased [23]. In the present study, the components produced by using PIM had a higher tensile strength than the materials produced by casting and forging without making any additional heat treatment similar to above-mentioned one. The reason for this was associated with the fact that the PIM method provides to obtain components consisting of directly fine and equiaxed grains, having small-sized carbides and exhibiting a homogenous distribution throughout the structure. As in the Nickel based superalloys, the fact that carbides precipitated at the grain boundaries in Co based superalloys prevented the grain boundary sliding or replacement. Optimum mechanical properties could be obtained by a careful balancing of the carbides at the grain boundaries and matrix. When the carbides were in sufficient
Fig. 14. Average tensile curves depending on the sintering temperatures of the Stellite 6 samples.
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Table 3 Comparison of average mechanical properties of Stellite 6 components produced by using PIM, casting, forging, and HIP. Production method
PIM (sintered at PIM (sintered at PIM (sintered at PIM (sintered at Casting Forging (sheet) HIP'ing
1200 °C) 1225 °C) 1250 °C) 1275 °C)
Tensile strength (MPa)
Elongation (%)
Hardness (HV)
688 792 945 1230 896 [38] 1010 [24] 41200 [39]
8.84 9.37 10.31 20.22 o 1 [38] 11 [24] 10 [39]
256.75 296.25 436.75 458.25 402.6 [26] – 419.1 [26]
amount, the skeletal carbide network contributed to the strength of the material as in the composites [13]. The microstructure obtained in the present study was in accordance with the abovementioned properties. The fact that no TCP phase formed in the microstructure as stated in SEM examinations also contributed to the produced components' exhibiting a highly ductile fracture. Fig. 15 illustrates the SEM images taken from the fracture surfaces after the tensile test. It is understood from the images that the fracture surfaces of the materials had a ductile character. With the increase in the sintering temperature, this character became more apparent. Pores and partly non-sintered regions were present on the fracture surfaces of the samples sintered at 1200 and 1225 °C. Formations of dimple were observed in the components that were fully sintered on these samples. As the sintering temperature increased, the formation of dimple in subject increased. Almost no pore was observed on the fracture surface of the sample sintered at 1275 °C in accordance with the SEM images and there was a considerably ductile fracture surface. In this image, it was observed that carbide precipitates and partially formations of
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cracks occurred in the dimples. In addition, it can be asserted that the fracture mostly occurred from the interfaces of the carbides with matrix. In a study conducted by Silvia et al., [40], it was reported that the fracture occurred in the interface of carbides formed by Cr with the matrix, as similar with this study.
4. Conclusions In this study, superalloy components were produced from Stellite 6 powder by using PIM technique. The microstructure and mechanical properties of Stellite 6 superalloy components produced by using PIM at different sintering temperatures were characterized and the obtained results were compared with the properties of the components produced by using casting, forging and HIP techniques. The obtained results are summarized as below. 1. A feedstock with 62.5 vol% powder loading will have good rheological properties for molding, small distortion, and good mechanical properties after debinding and sintering. 2. The highest densification in the raw parts formed through PIM was obtained with the sintering process performed at 1275 °C. The average relative density value of the sample sintered at 1275 °C was measured as 98.27%. 3. The components produced by using PIM consisted of fine and equiaxed grains. The pores of low amount left in the sample produced at 1275 °C were discrete spherical pores. The grain structure and grain size obtained in this study were superior according to the casting and forging methods and displayed similarity with the materials produced by using HIP. 4. M23C6 type discrete carbide precipitates with block morphology
Fig. 15. Fractured surface morphologies of the Stellite 6 samples sintered at different temperatures; sintered at (a) 1200 °C, (b) 1225 °C, (c) 1250 °C and (d) 1275 °C.
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abundantly formed at the grain boundaries of the components produced without needing any precaution, additional heat treatment or a thermomechanical treatment. These carbides exhibited a homogenous distribution throughout the microstructure. Such microstructure is difficult to be obtained by using especially casting method in particular, and requires special precautions or additional treatments. Other than M23C6 type carbide, no other carbide or TCP phase formation was found in the microstructures of the components produced by using PIM. 5. The increase in density caused by the increase in the sintering temperature led to the increase of hardness, as well. The hardness value obtained in the sample sintered at 1275 °C was 458.25 HV0.5. This value was considerably higher than the Stellite 6 alloy produced by casting and corresponded to the hardness of the components produced by using HIP. 6. Higher tensile strength values were obtained in the components produced by using PIM than those materials produced through casting or forging methods. The tensile strength of the sample sintered at 1275 °C was 1230 722 MPa. This superiority was associated the fact that the microstructure contained fine and equiaxed grains and the element and precipitate phases had the homogenous distribution. The tensile strength obtained in the sample sintered at 1275 °C was close to that of the materials produced by using HIP. 7. The components produced by using PIM in the tensile test exhibited considerably higher elongation values compared to the components of casting and forging. The microstructure examinations showed that no TCP phase formed in the alloy. The small-sized carbide precipitates provided an increase in strength, but did not decrease the ductility.
Acknowledgments The authors are grateful to Bingol University Central Laboratory due to provisional experimental facilities, Bingol University (Project no: BAP-376-205-2014), Marmara University (Project no: FEN-C-YLP-080415-0122), and Sandvik Osprey Ltd. for supplying the powders.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Powder injection molding of Stellite 6 powder: Sintering, microstructural and mechanical properties H. Özkan Gülsoy a, Özgür Özgün b,n, Sezer Bilketay a a
Marmara University, Technology Faculty, Metall. and Mater. Eng., 34722 Istanbul, Turkey Bingol University, Faculty of Engineering and Architecture, Mechanical Eng. Dep., 12000 Bingol, Turkey
b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 October 2015 Received in revised form 26 October 2015 Accepted 19 November 2015
The purpose of this study was to produce Co-based Stellite 6 superalloy components by using the method of Powder Injection Molding (PIM) and to characterize the microstructural and mechanical properties of the produced components. The experimental studies were started through the formation of feedstock by mixing Stellite 6 powder with a multicomponent binder system. Prepared feedstock was formed by utilizing powder injection molding technique. Then the molded samples were subjected to the solvent and thermal debinding processes. Different sintering cycles were applied to the raw components for the purpose of determining the optimum sintering conditions. The densities of the sintered components were determined in accordance with the Archimedes' principle. The microstructural characterization was performed through scanning electron microscope (SEM) analysis, energy dispersive spectrometry (EDS) analyses, and X-ray diffraction (XRD) analysis. Hardness measurement and tensile test were conducted in order to determine the mechanical properties. The results illustrated that the injection molded Stellite 6 components were composed of fine and equiaxed grains, plenty of carbide precipitates exhibiting homogenous distribution throughout the microstructure formed at the grain boundaries and thus the mechanical properties were considerably high. & 2015 Elsevier B.V. All rights reserved.
Keywords: Powder injection molding Stellite 6 Microstructure Mechanical properties
1. Introduction The design and production of high strength materials with good wear and corrosion resistance at high temperatures are very important for the severe service conditions in the automotive, aeronautical, chemical, and petroleum industries [1]. Stellite alloys exhibite an unmatched combination of mechanical and tribological properties with superior corrosion and oxidation resistance at both room and high temperatures [2]. They are among the leading ones of Co based superalloys which are one of the three major superalloy classes [3]. While the applications of Co based superalloys are conventionally intended for nuclear industry mostly [4], the use of Stellite alloys today has spread to different industrial fields such as pulpwood, paper making, petroleum and gas processing, pharmaceutical industry, chemical processing, etc. [5]. One of the leading Stellite alloys having such widespread area of usage is undoubtedly the Stellite 6 that was the first Stellite alloy developed by Elwood Haynes during the early 1900s [6,7]. The properties of Stellite alloys are widely determined by their chemical compositions [8]. While solid solution hardeners such as n
Corresponding author. E-mail addresses: [email protected], [email protected] (Ö. Özgün).
http://dx.doi.org/10.1016/j.msea.2015.11.058 0921-5093/& 2015 Elsevier B.V. All rights reserved.
Cr, W, and Mo contribute to the strength, carbide precipitates are the main strengthening mechanism [8,9]. Type, size, and shapes of carbides affect strongly the properties of Stellite alloys. For instance, larger size carbides generally provide with the acquisition of higher strength [10]. The formation of carbides, having significant effects on the properties, is influenced by the type and amount of alloy elements (particularly carbon), the production method, sintering temperature and cooling rate and also operational parameters [2,3]. In the production of Co based alloys by using casting technique, important precautions or additional heat treatments [13] are required so as to prevent or solve problems [11,12] such as porosity, segregation, coarse grain size, eutectic carbide network between the dendrites, etc. It is difficult to form Co based alloys having high rate of carbon even at high temperatures [14]. While the industrial demands shift the usage area of Stellite 6 alloy to higher stress applications, the production processes could be adapted in such a way to provide the tribomechanical properties needed [15]. PIM is a production method which enables to produce products with high dimensional accuracy in such a way to have excellent, fine grain structure and non-anisotropic mechanical properties [16]. Despite being a powder metallurgy process, PIM enables to obtain higher rate of density than those components produced
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through conventional powder metallurgy (PM) methods and thus more superior mechanical properties. Complex shaped components that are difficult to produce by using conventional production methods such as casting, forging, machining, and die pressing can easily be transformed into products by means of PIM. The high surface quality and the fine grain structure of the components produced with high measurement precision through the PIM technique provide highly superior properties since they exhibit a homogenous distribution throughout the microstructure of the chemical compound [17]. Recently, PIM of superalloy components for various applications has attracted attention [18–21]. There is no information concerning the microstructural and mechanical properties of Stellite 6 materials produced by using PIM in the literature. For this reason, production of the component from Stellite 6 superalloy powder by using PIM technique was performed in this study. It was aimed to both determine the optimum parameters for the production of Stellite 6 alloy by using PIM and to characterize the microstructure and mechanical properties of the produced components. Hence, injection molded Stellite 6 green components were kept to different sintering cycles to characterize them. Characterization of samples was conducted by SEM, XRD, elemental analyses and mechanical tests. The experimental studies have been carried out to determine the microstructure, and mechanical properties of this alloy. Furthermore, the data obtained from the characterization were compared with properties of Stellite 6 alloy produced by using different methods.
2. Materials and method Fig. 1 illustrates the SEM image of the prealloyed Stellite 6 superalloy powder used in the experiments. It was seen in the SEM images that Stellite 6 powders had a spherical form. Fig. 2 shows the curve obtained from the particle size distribution analysis conducted by using Malvern Mastersizer instrument. While Table 1 illustrates the chemical composition of the Stellite 6 powder, Table 2 illustrates its some physical properties. The experimental studies were started by preparing the feedstock. The feedstock was obtained by mixing Stellite 6 powder through a multicomponent binder system. The mixing process was performed in a specially designed mixer under vacuum and at 170 °C for half an hour. The feedstock included 62.5 vol% Stellite 6 powder and 37.5 vol% binder. The multicomponent binder system consisted of 69 wt% paraffin wax (PW), 20 wt% polypropylene (PP), 10 wt% carnauba wax (CW), and 1 wt% stearic acid (SA). Rheological behavior of the
Fig. 1. SEM image of the Stellite 6 powder.
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Fig. 2. Particle size distribution of Stellite 6 powder.
Table 1 Chemical composition of Stellite 6 powder. Weight (%) Co
Cr
W
Ni
Mn
C
Si
Fe
Mo
P
S
58.438
30.3
4.5
2.0
1.17
1.15
1.02
0.86
0.55
0.008
0.004
Table 2 Some characteristics of Stellite 6 powder. Properties Producer Production method Particle shape Particle size (mm) D10 D50 D90
Osprey Gas atomization Spherical 5.3 13.8 27
Stellite 6 feedstocks was measured by using a rotational viscometer Physica MCR51 (Anton Paar, Austria) at shear rates from 10 to 1000 s 1 at 130–170 °C. The value of viscosity is given based on the shear stress divided by the shear rate. After the feedstock was granulated, it was molded in the form of tensile specimen at 12.5 MPa and for a holding time of 20 s. in the injection device. The temperature of the injection device's barrel and nozzle sections was adjusted to 170 °C during molding. The tensile specimens molded were subjected to a two-stage debinding process, as solvent and thermal. Being heated to 60 °C in the solvent debinding process, the samples were kept for 6 h in the heptane. The thermal debinding operation was implemented by using Al2O3 substrate in a high purity argon atmosphere based on the thermal cycle in Fig. 3. Differential scanning calorimetry (DSC) test was performed in order to determine the sintering behaviors of the debinded samples. The DSC analysis was carried out by heating the Stellite 6 powder at heating rate of 10 °C/min until 1400 °C and at 100 ml/ min flow rate under high purity Ar gas atmosphere in a Setaram Labsys Evo device. Al2O3 was used as a reference material. In the light of the data from the DSC, the samples were sintered with holding time of one hour within the temperature range of 1200– 1300 °C. The sintering processes were carried out by using Al2O3 substrate under high vacuum in Protherm tube furnace. 10 °C/min fixed heating and cooling rate was used in the sintering process. Fig. 4 illustrates the images of the samples taken after the molding, debinding, and sintering stages. The densities of the sintered samples were measured according to the Archimedes' principle by using Precisa XB 320M precision balance. The XRD analyses were carried out in the Rigaku Ultima IV X-Ray Diffractometer device by using Cu X-ray tube (λ ¼ 1,5405)
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Fig. 3. Thermal debinding cycle.
Fig. 4. Image of the tensile specimens after molding, debinding and sintering stages.
at the scanning rate of 0.02/0.4°/s. After the metallographic preparations such as grinding and polishing, the microstructures of the samples etched through 50 ml HCl (32%) þ10 ml HNO3 (65%) þ 10 g FeCl3 þ 100 ml H2O solution were examined through SEM and EDS analyses. While SEM analysis was performed with JEOL JSM 6510 scanning electron microscope, SEM/EDS analysis were carried out with IXRF 550 brand EDS system connected to this device. The hardness measurements were carried out by using Vickers scale through the application of 500 g load for 10 s in the EmcoTest brand (M1C 010) device. The tensile tests were performed in Shimadzu AG-X 50 kN tensile device at the constant rate of 1 mm/ min. At least three samples were tested under the same conditions in terms of the reliability of the results.
3. Results and discussion The evaluation of the feedstock rheological properties is based on viscosity and its shear sensitivity and temperature sensitivity. Fig. 5 illustrates the shear stress and viscosities of the feedstock at different shear rates and different temperatures of 130–180 °C. The viscosity data indicate the flowability of the feedstock. As the value of the viscosity decreases, it is easier for a feedstock to flow. Fig. 5 shows that the viscosity decreases at all shear rates. With the increase of temperature, viscosity of feedstock decreases. PIM
Fig. 5. Temperature-dependent viscosity versus shear rate of Stellite 6 feedstock.
feedstock is carried out under pressure and temperature. It is desirable that the viscosity of the feedstock should decrease quickly with increasing shear rate during molding. This high shear sensitivity is especially important in producing complex parts. The results indicate that the feedstocks have pseudo-plastic rheological behavior and increasing temperature leads to a decrease in the viscosity. Fig. 6 shows the curve obtained from the DSC analysis applied to Stellite 6 powder. According to this curve, it was determined that the solidus temperature of Stellite 6 alloy was 1235 °C, and its liquidus temperature was 1326 °C. In the literature, the solidification range of the Stellite 6 alloy is reported to be 1260–1357 °C [22]. The solidus and the liquidus temperatures in this study were lower than the data in the literature. This situation was thought to be associated with the fact that the material was in the powder form and accordingly had a high surface energy as well as possible chemical composition. Moreover, in the DSC curve, endothermic peaks were observed at 855 and 1076 °C. It was considered that these peaks were related to formation and transformation of carbides. It is reported that in Co based superalloys, the M23C6 type carbides formed secondary precipitates in the form of fine grains
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Fig. 6. The curve of the DSC analysis of the Stellite 6 powder.
throughout the alloy matrix within the temperature range of 704– 871 °C [23]. It can be asserted that the peak occurring at 855 °C referred to the dissolving of these fine grains. It is known that M23C6 carbides form as primary precipitates during solidification in the Co based alloys produced in the casting. It was reported that the heat treatment made within the range of 1150–1204 °C caused the dissolving of the coarse carbides at the grain boundaries [23]. Taking into account the fact that the DSC analysis was applied to the Stellite 6 powder consisting of small sized particles with high surface energy (D50 ¼13.8 mm) and with reference to the aforementioned descriptions, it was that the endothermic peak at 1076 °C referred to the fact that the carbide precipitates were dissolved. Fig. 7 illustrates the relative density values obtained in Stellite 6 components depending on the sintering temperature. The relative density values increased with the increase in the sintering temperature. While the highest relative density value was obtained with the sintering process made at 1275 °C, this value was found as 98.27%. In fact, by taking the DSC analysis into account, the samples formed with PIM were sintered at 1300 °C, as well. However, it was observed that the samples were deformed depending on the formation of excessive liquid phase at this temperature. Therefore, the information concerning the sample sintered at 1300 °C was not present in the graphic and it was accepted that the highest density value was obtained from the sintering process made at 1275 °C. It is indicated in the literature that the Co-based alloys produced by using PM technique can condense between 97% and 100% [24]. On the other hand, in numerous studies conducted regarding Stellite alloys, hot isostatic pressing (HIP) method was used in order to reach full density [1,5,15,25,26]. In some studies, the materials produced by using HIP were subjected to a secondary HIP process [15]. Even in some studies, relative density of 100% was not reached although the HIP was applied [1].
Fig. 7. Change in the relative density values depending on different sintering temperatures for Stellite 6 samples.
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Fig. 8 illustrates the SEM images of the samples taken after the polishing process for the purpose of getting information about the pore structure and amount of the sintered samples. When the images were examined, it was understood that as sintering temperature increased, the amount of interconnected pores decreased. This was accompanied by the shrinkage in the size of the pores. In accordance with the results obtained from the density measurements, the least porosity was seen in the image of the material sintered at 1275 °C. In addition, it can be seen that the pores in the sample were not interconnected and the pore morphology was spherical. The fact that the pore geometry in the parts produced by using PM techniques had such spherical pores instead of shapeless pores including sharp edges is a desired condition in terms of strength [27]. The fact that the pores remained in the form of noninterconnected spherical pores in this way is one of the advantages of PIM method [28]. From the DSC analysis, the solidus temperature of the material was determined as 1235 °C. This situation showed that the sintering process at 1275 °C was performed by using supersolidus liquid phase sintering (SLPS) mechanism. It can be seen from the SEM images in Fig. 8 that the microstructure included light colored and dark colored phases although no etching process was performed. In the previous studies conducted concerning Stellite alloys, it is reported that the Co matrix in the microstructure was light gray and the carbides rich in Cr were dark gray [3,5,15,26,29]. It is indicated that W element also forms carbides in Co based superalloys and the carbides formed by W were of a brilliant white color in the microstructure [26]. Based on these explanations, it can be asserted that the dark precipitates homogenously distributed in the microstructure in the SEM images of the samples taken without etching them in this study were the carbides formed by Cr. As the sintering temperature increased, even slightly coarse was observed in sizes of the dark carbides. In a previous study using the HIP method, it was reported that the increase in the sintering temperature caused coarsening in the size of carbides similar to the present study [29]. This situation was associated with the fact that the atom movements occurring easier high temperatures facilitate the precipitation and growth of the carbide [29]. Fig. 9 illustrates the SEM images of the sintered samples taken after the etching process in order to understand their grain structures, the grain boundary – pore relationship, and the morphology and distribution of the phases contained in them. It can be seen from the images that the pores were located in intergranular regions. In order to obtain the appropriate properties in superalloys, it is preferred that the material contains randomly oriented equiaxed grains [13]. The SEM images show that the samples produced consisted of equiaxed grains. The PIM method generally enables the materials produced without requiring any precaution or additional process to consist of equiaxed grains [28]. Numerous previous studies have reported that the materials produced by using PIM consist of equiaxed grains as in this study [18– 21]. The controlling of the grain size is very significant to develop and sustain the physical and mechanical properties among the materials. Finer grain size generally improves the properties of tensile, fatigue, and creep strength for service conditions with low and middle temperatures [13]. It is known that the grain size in the material production by sintering increases with the increasing sintering temperature and duration [27,30]. The SEM images show that a slight coarsening occurred in the grain size due to the increasing sintering temperature. While the sample sintered at 1200 °C had an average grain size of 3–4 mm, the sample sintered at 1275 °C, where the highest density was obtained, had an average grain size of 5–6 μm (ASTM grain size number is 12). Actually, despite an increase of 75 °C in temperatures in the sintering processes, the increase in the grain size is at a hard-to-distinguish level. This situation makes us think that the carbides precipitating
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Fig. 8. SEM images of the sintered samples at different temperatures before the etching process; (a) 1200 °C, (b) 1225 °C, (c) 1250 °C and (d) 1275 °C.
at the grain boundaries played an inhibitive role in the grain coarsening. This thought becomes more convincing by considering the fact that in Ni based superalloys, the δ phase [31] preferring to form at the grain boundaries prevents the grain coarsening during the hot forging operations [32]. In this study, the superior properties such as homogenous microstructure, fine and equiaxed grains easily obtained by using PIM technique without needing any precaution were rather difficult to be obtained by using casting method. Obtaining the aforementioned superior properties by using the casting method requires additional precautions such as the coating of the mold surface with nucleants for the purpose of accelerating the grain nucleation and solidification, proper selection of mold and metal temperature and the selection of some other parameters properly [13]. A coarsening similar to the ambiguous coarsening occurring in the grain size of the material with the increase in sintering temperature was observed in the carbides rich in Cr at the grain boundaries exhibiting homogenous distribution throughout the microstructure. The carbide sizes were observed to range between 1 and 3 μm in the SEM image of the sample sintered at 1275 °C. It was reported that the carbide sizes ranged between 1 and 3 μm also in the Stellite 6 alloys produced by using HIP [5,15]. Like their type, carbide sizes and shapes strongly affected the properties of Stellite alloys. There are studies reporting that larger size carbides generally contribute to higher strength and wear resistance [10]. The microstructure in the Stellite alloys produced by using casting method consisted of the dendrites formed by the Co solid solution and the eutectic carbide phases between these dendrites. The carbides were in the form of continuous films surrounding the grains and had a large size [5,15,25,26,33]. In the alloys produced by casting, this shape, size, and distribution style of the carbides decreased ductility and fatigue strength of the material [34]. In the
present study, it was observed in the material produced by using PIM that the non-interconnected carbides with the block morphology had a homogenous distribution on the grain boundaries throughout the microstructure instead of large size eutectic carbides surrounding the dendrites. Various studies have reported that the carbides exhibited such a distribution in the Stellite alloys produced by using HIP and they had a spherical-like form [5,15,25]. The morphology of the carbide precipitates in superalloys has significant effects on the properties of alloys. The fact that carbide precipitates at the grain boundaries in the form of a continuous film sets a ground for the formation of cracks and decreases significantly the impact and rupture properties of the alloy. Since the formation of precipitates that are large and independent from each other (discrete) at the grain boundaries, instead of a continuous film, prevents dramatically the grain boundary sliding, it is useful [23,35]. Additional heat treatments are needed in Co alloys produced by casting in order to obtain a more homogenous structure by dissolving the large carbide network and thus to improve the mechanical properties [34]. Taking into account these explanations and in contrast to the Stellite alloys produced by casting, it is also expected for the PIM method, which provides the acquiring of materials containing carbides exhibiting finer and more homogenous distribution without needing additional heat treatments, to provide more superior mechanical properties. Fig. 10 illustrates the images of SEM elemental mapping analysis taken from the sample sintered at 1275 °C in order to determine the distributions of the elements inside the material. It is seen in the SEM image that abundantly discrete precipitates with block morphology formed at the grain boundaries. A significant decrease was observed in the amount of Co element forming the matrix in regions where these precipitates were located. Co based
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Fig. 9. The SEM microstructure images of the samples sintered at (a) 1200 °C, (b) 1225 °C, (c) 1250 °C and (d) 1275 °C.
superalloys generally gain their strength from the solid solution hardening and the carbide precipitates [14]. Considering that the main strengthening phase is the carbides, it can be asserted that Cr is the most significant alloying element for Co based superalloys because Cr is not only a predominant carbide former but also contributes to the solid solution hardening [23,36]. In addition, another significant role of Cr is to increase the corrosion and oxidation resistance of the alloy [23]. When the SEM elemental mapping analysis was examined, the Cr element exhibited a serious clustering in the precipitates seen in the microstructure. The fact that C element also had high amounts in these regions where Cr exhibits clustering signified that these precipitates were the carbides formed by Cr. The carbides formed by Cr in Co based superalloys are the carbides of M3C2, M7C3, and M23C6. It is indicated that M3C2 among them is a type of carbide seen in previous superalloys with a low rate of Cr. The M7C3 carbide also consists of low Cr–C alloy rates. This carbide forms more in intragranular regions and sometimes intergranular regions. Being a metastable carbide M7C3 transforms into the carbide of M23C6 with heat treatment or under high temperature service conditions [3,23]. It is indicated that the M23C6 type carbides can precipitate as both the primary and the secondary carbides in Co based alloys containing a high level of Cr [23]. It is known that this carbide generally forms at the grain boundaries of the multi-crystalline materials and provides the grain boundary strength and the fracture resistance required for long service conditions when found as discontinuous precipitates [13]. The Cr ratio of the Stellite 6 powder used in the present study is 30.3 wt%, which is considerably high. The SEM images of Fig. 10 shows that all of the carbides formed at the grain boundaries. In addition, it was thought that since all the precipitates in the SEM images had approximately the same size and morphology, they were the same phase. All these
explanations indicated that these precipitates seen in the microstructure were M23C6 type carbides formed by Cr. In the literature, it is reported that Co, W or Mo can substitute for a little amount of Cr in the M23C6 type carbides [23]. In a study conducted by Rosalbino and Scavino [5], it was found that the M23C6 carbide contained 69.6% Cr, 16% Co, 6.3% W, 2.3% Si, 1.8% Fe, and 1.3% Ni [5]. In the images of the elemental mapping analysis, Mn, Si, and O elements, along with Cr and C, were higher in amount in the regions having carbide precipitates when compared to the matrix. Additionally, it was observed that even though presence of Co, W, Ni, and Fe elements exhibited a decrease in these regions, these elements had low amounts inside the carbides. W and Mo elements in the Co based superalloys were the elements that provided the hardening of the matrix with solid solution by means of its large atom sizes. When amounts of these two elements were high in the alloy, this provided the formation of carbides or intermetallic compounds [36]. The carbides formed by W and Mo were M6C type carbides [5,23,36]. When the images of W and Mo were examined in the mapping analysis, it was seen that these two elements did not cluster in any region in a manner that will form a precipitate. For this reason, it became definite that there was no carbide or intermetallic precipitate formed by W and Mo in the microstructure. While Mo had a homogenous distribution in all the regions in the microstructure including the carbides formed by Cr, the amount of W decreased in the regions with carbides as mentioned above. From this point of view, it can be asserted in this study that W and Mo would only contribute to the material strength by means of solid solution hardening in the components produced by using PIM. The elemental mapping analysis showed that all the precipitates forming in the microstructure contained C. This situation signified that topologically close packed (TCP) phases, having negative effects on the material
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Fig. 10. Images of SEM/EDS elemental mapping analysis taken from the sample sintered at 1275 °C.
strength and especially low temperature ductility did not form. In Co based superalloys, Ni element is added in order to stabilize the fcc crystalline structure from room temperature to the melting temperature. It is also known that Fe also stabilises the fcc structure like Ni [24]. According to images of the elemental mapping analysis of Ni and Fe, they exhibited a homogenous distribution within the matrix. In the elemental mapping analysis,
the amount of oxygen element increased significantly in the precipitates stated as M23C6 type carbides. It was reported that M7C3 and MC type carbides preferentially oxidized when exposed to high temperature [37]. Similarly, it was understood in the present study that M23C6 type carbides also oxidized during the sintering process. Fig. 11 illustrates the images of SEM/EDS point analysis taken
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Fig. 11. SEM/EDS analysis taken from the sample sintered at 1275 °C.
from the sample sintered at 1275 °C. The amount of Cr element was considerably high in point 1 on a precipitate located at the grain boundary. The fact that Cr was accompanied by C element showed that this precipitate was Cr carbide. Together with Cr, Co element also existed within the carbide precipitate. Moreover, W, Mo, Fe, and Ni elements were also present in low amounts in the carbide precipitate. The said carbide precipitate contained no Si
element. Element rates close to the chemical composition of the initial powder in point 2 received from Co matrix were obtained. Only the Cr element exhibited a lower rate than its rate in the initial powder. This situation was caused by the fact that Cr formed a significant amount of carbide as is understood from the SEM examinations up to now. It was observed that the amount of Cr in point 3 at the grain boundary was higher compared to the matrix.
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Fig. 12. XRD patterns taken from the initial powder and the samples sintered at 1200 °C and 1275 °C.
Fig. 12 shows XRD patterns of the initial powder and samples sintered at different temperatures. Only peaks of fcc-Co matrix were seen in the XRD pattern of the initial powder. However, the initial powder was a prealloyed powder produced by gas atomization. For this reason, it was considered that it had to contain carbides. The elements having property of forming the carbide in this powder were Mo, W, and Cr [3,23]. However, it is not possible for Mo and W having large atomic radii and therefore having considerably high diffusion coefficients to find sufficient time to form carbide during the gas atomization. The carbides that Cr can form in Co based superalloys are M3C2, M7C3 and M23C6 carbides [3,23]. From the XRD cards of these carbides, it was observed that the peaks of the M23C6 carbide showed coincidence with the peaks of Co matrix. The peaks of M23C6 carbides in the sintered material became more apparent. Although the sintering process was carried out in high vacuum values, it was observed in the XRD pattern of the sintered materials that CrO phase formed. The increasing sintering temperature led a slight increase in the severity of the peak of CrO phase. In a study conducted by Rosalbino and Scavino [5], it was determined that M23C6 type carbides formed in the XRD analysis of the materials produced by using HIP method from Stellite 6 powder as well. The amount of Cr in the Stellite 6 powder used in this study was higher than Stellite 6 powder used in Rosalbino and Scavino's study [5]. It was stated that M23C6 type carbides can precipitate as both the primary and secondary carbides in high Cr content [23]. The values obtained as a result of the hardness measurements are shown in the graphic in Fig. 13. It is seen from the graphic that the hardness also increased with the increase in sintering temperature. This situation was associated with the fact that the increasing sintering temperature caused the density of the material to increase. The average hardness value measured in the sample sintered at 1275 °C, where the highest density was obtained, was 458.25 HV0.5. In the literature, it is reported that the hardness of
Fig. 13. Hardness values obtained in the samples depending on the sintering temperature.
produced Stellite 6 alloys produced by casting and conventionally PM is 40 HRC [24,38]. This value approximately corresponds to 400 HV. In a study conducted by Yu et al., [26], it was reported that the hardness of the Stellite 6 materials produced through sand casting was 402.6 720.9 HV(2.94 N). Also, in the same study, the hardness value obtained in the materials produced by using HIP process for 4 h under 100 MPa pressure at 1200 °C from the Stellite 6 powder, whose grain size ranged predominantly between 45 and 180 mm and which was produced by gas atomization was 419.1 79.90 HV(2.94 N). Since the same material was subject to a secondary HIP treatment with the same parameters, this hardness value reached 459.3 710.4 HV(2.94 N) [15]. In the present study, the hardness of the materials produced by using PIM from the small sized prealloyed Stellite 6 powders is higher than the Stellite 6 materials produced by using casting and HIP methods. This situation was caused by the grain structure, grain size of the materials produced by using PIM as well as morphology and distributions of carbides. Ahmed et al., [15] reached the hardness value reached in the Stellite 6 samples produced by using PIM but by the re-HIPing process. Fig. 14 illustrates average tensile curves obtained from the Stellite 6 samples depending on the sintering temperature. The increase occurring in the density due to the increase in the sintering temperature affected both the tensile strength and hardness. The highest tensile strength was 1230 724 MPa and obtained in the sample sintered at 1275 °C. Table 3 compares the tensile strength obtained from the sintered tensile samples and % elongation values with casting and forging Stellite 6 alloys. In the Co based alloys produced by casting, the large sized carbides were enabled to be dissolved by performing a heat treatment within the temperature range of 1149–1204 °C. The solution process was followed by the aging treatment made within the temperature range of 760 and 982 °C. Therefore, the fine M23C6 precipitates exhibiting a more homogenous distribution throughout the structure precipitated again and thus the tensile strength significantly increased [23]. In the present study, the components produced by using PIM had a higher tensile strength than the materials produced by casting and forging without making any additional heat treatment similar to above-mentioned one. The reason for this was associated with the fact that the PIM method provides to obtain components consisting of directly fine and equiaxed grains, having small-sized carbides and exhibiting a homogenous distribution throughout the structure. As in the Nickel based superalloys, the fact that carbides precipitated at the grain boundaries in Co based superalloys prevented the grain boundary sliding or replacement. Optimum mechanical properties could be obtained by a careful balancing of the carbides at the grain boundaries and matrix. When the carbides were in sufficient
Fig. 14. Average tensile curves depending on the sintering temperatures of the Stellite 6 samples.
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Table 3 Comparison of average mechanical properties of Stellite 6 components produced by using PIM, casting, forging, and HIP. Production method
PIM (sintered at PIM (sintered at PIM (sintered at PIM (sintered at Casting Forging (sheet) HIP'ing
1200 °C) 1225 °C) 1250 °C) 1275 °C)
Tensile strength (MPa)
Elongation (%)
Hardness (HV)
688 792 945 1230 896 [38] 1010 [24] 41200 [39]
8.84 9.37 10.31 20.22 o 1 [38] 11 [24] 10 [39]
256.75 296.25 436.75 458.25 402.6 [26] – 419.1 [26]
amount, the skeletal carbide network contributed to the strength of the material as in the composites [13]. The microstructure obtained in the present study was in accordance with the abovementioned properties. The fact that no TCP phase formed in the microstructure as stated in SEM examinations also contributed to the produced components' exhibiting a highly ductile fracture. Fig. 15 illustrates the SEM images taken from the fracture surfaces after the tensile test. It is understood from the images that the fracture surfaces of the materials had a ductile character. With the increase in the sintering temperature, this character became more apparent. Pores and partly non-sintered regions were present on the fracture surfaces of the samples sintered at 1200 and 1225 °C. Formations of dimple were observed in the components that were fully sintered on these samples. As the sintering temperature increased, the formation of dimple in subject increased. Almost no pore was observed on the fracture surface of the sample sintered at 1275 °C in accordance with the SEM images and there was a considerably ductile fracture surface. In this image, it was observed that carbide precipitates and partially formations of
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cracks occurred in the dimples. In addition, it can be asserted that the fracture mostly occurred from the interfaces of the carbides with matrix. In a study conducted by Silvia et al., [40], it was reported that the fracture occurred in the interface of carbides formed by Cr with the matrix, as similar with this study.
4. Conclusions In this study, superalloy components were produced from Stellite 6 powder by using PIM technique. The microstructure and mechanical properties of Stellite 6 superalloy components produced by using PIM at different sintering temperatures were characterized and the obtained results were compared with the properties of the components produced by using casting, forging and HIP techniques. The obtained results are summarized as below. 1. A feedstock with 62.5 vol% powder loading will have good rheological properties for molding, small distortion, and good mechanical properties after debinding and sintering. 2. The highest densification in the raw parts formed through PIM was obtained with the sintering process performed at 1275 °C. The average relative density value of the sample sintered at 1275 °C was measured as 98.27%. 3. The components produced by using PIM consisted of fine and equiaxed grains. The pores of low amount left in the sample produced at 1275 °C were discrete spherical pores. The grain structure and grain size obtained in this study were superior according to the casting and forging methods and displayed similarity with the materials produced by using HIP. 4. M23C6 type discrete carbide precipitates with block morphology
Fig. 15. Fractured surface morphologies of the Stellite 6 samples sintered at different temperatures; sintered at (a) 1200 °C, (b) 1225 °C, (c) 1250 °C and (d) 1275 °C.
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abundantly formed at the grain boundaries of the components produced without needing any precaution, additional heat treatment or a thermomechanical treatment. These carbides exhibited a homogenous distribution throughout the microstructure. Such microstructure is difficult to be obtained by using especially casting method in particular, and requires special precautions or additional treatments. Other than M23C6 type carbide, no other carbide or TCP phase formation was found in the microstructures of the components produced by using PIM. 5. The increase in density caused by the increase in the sintering temperature led to the increase of hardness, as well. The hardness value obtained in the sample sintered at 1275 °C was 458.25 HV0.5. This value was considerably higher than the Stellite 6 alloy produced by casting and corresponded to the hardness of the components produced by using HIP. 6. Higher tensile strength values were obtained in the components produced by using PIM than those materials produced through casting or forging methods. The tensile strength of the sample sintered at 1275 °C was 1230 722 MPa. This superiority was associated the fact that the microstructure contained fine and equiaxed grains and the element and precipitate phases had the homogenous distribution. The tensile strength obtained in the sample sintered at 1275 °C was close to that of the materials produced by using HIP. 7. The components produced by using PIM in the tensile test exhibited considerably higher elongation values compared to the components of casting and forging. The microstructure examinations showed that no TCP phase formed in the alloy. The small-sized carbide precipitates provided an increase in strength, but did not decrease the ductility.
Acknowledgments The authors are grateful to Bingol University Central Laboratory due to provisional experimental facilities, Bingol University (Project no: BAP-376-205-2014), Marmara University (Project no: FEN-C-YLP-080415-0122), and Sandvik Osprey Ltd. for supplying the powders.
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