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Metallographic study and wear resistance of a high-C wrought Co-based alloy Stellite 706K
Materials Science and Engineering A 407 (2005) 291–298
Metallographic study and wear resistance of a high-C wrought Co-based alloy Stellite 706K M.X. Yao a,∗ , J.B.C. Wu b , W. Xu c , R. Liu c a
b
Deloro Stellite, Inc., 471 Dundas Street East, Belleville, Ont., Canada K8N 1G2 Deloro Stellite Holdings, Inc., 555 N New Ballas, STE 305, Saint Louis, MO 63127, USA c Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, Ont., Canada K1S 5B6 Accepted 25 July 2005
Abstract The new Co Cr Mo C type wrought alloy Stellite 706K is an improved alloy over the Co Cr W C type wrought alloy Stellite 6K in wear and corrosion resistance. The microstructure of Stellite 706K is characterized by Co Cr Mo solid solution matrix with banding and occasional twinning and various carbide phases. Pin-on-disc wear resistance of Stellite 706K at room temperature is evaluated. Solution treatment at temperatures from 1235 to 1250 ◦ C on the mill annealed Stellite 706K decreases the bulk hardness and the hardness of Co Cr Mo matrix while increases the wear resistance. The excellent properties of Stellite 706K come from the combination of the high volume fraction of carbide particles and the tough Co Cr Mo matrix due to the alloying addition of Mo. © 2005 Elsevier B.V. All rights reserved. Keywords: Metallographic; Pin-on-disc; Wear resistance; Wrought; Stellite alloy
1. Introduction The conventional Co-based wrought alloys can be categorized into three types, namely, wear resistant alloys, high temperature alloys and corrosion resistant alloys. The wear resistant alloys, such as Stellite 6B and Stellite 6K, are essentially Co Cr W C quaternaries, with Cr providing strength and corrosion resistance to the Co-rich solid solution, as well as functioning as the chief carbide former during alloy solidification. W provides additional solid solution strength. The excellent wear resistance of these alloys is attributed to the high volume carbides and the unusual deformation characteristics imparted by Co-rich matrix. The relatively low stacking fault energy (SFE), which is however high enough to conserve face centered cubic (FCC) structure is thought to be important, as is the transformation to the hexagonal close packed (HCP) structure form during processing and service [1,2]. The high temperature alloys, such as alloy L-605, is considerably ductile, oxidation resistant, and microstructurally stable. Structural stability is enhanced by Ni
∗
Corresponding author. Tel.: +1 613 968 3481; fax: +1 613 966 8269. E-mail address: [email protected] (M.X. Yao).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.07.053
addition, which decreases the FCC → HCP transformation temperature [3]. The corrosion resistant alloys, such as MP35N alloy and Ultimet alloy, utilize Mo instead of W, which appear to impart a greater degree of resistance to a variety of aqueous corrosive media. C is kept low and held within the soluble range to maintain corrosion resistance. Stellite 6B has been widely used to make rock crushing rollers, cement and steel equipment, and conveyor systems, steam turbine erosion shields, half sleeves and bushings where lubrication is inaccessible or effective lubrication is impossible. Stellite 6K performs so well in a variety of cutting applications. Currently, knives made from Stellite 6K are being used to cut such materials as leather, carpets, plywood, and plastics. Stellite 6K is also used to fabricate tools for trimming earth-ware plates and scraping corn flakes off rollers. Additional information on wrought Stellite 6B and Stellite 6K can be found directly from Deloro Stellite, Inc. at Belleville, Ont., Canada or our online brochure [4]. New wrought Co Cr Mo C alloys with high-C content have been developed to combat simultaneous wear and corrosion [5]. Knives made from the new Co Cr Mo C type wrought alloy Stellite 706K are being successfully used for the cutting of viscose fibers in some corrosive environments where Stellite 6K has exhibited pitting.
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2. Experimental details
Table 1 Chemical composition of high-C Co-based wrought Stellite alloys Alloy
Co
Cr
W
Mo
C
HRC
Stellite 6B Stellite 6K Stellite 706K
Balance Balance Balance
30.0 30.0 30.0
4.0 4.0 –
– – 4.5
1.2 1.6 1.6
37 47 45
In this study, the metallographic study results on high-C wrought wear resistant alloy Stellite 706K has been reported. Solution annealing heat treatment is carried out and its effect on the hardness and wear resistance is evaluated.
2.1. Materials The nominal chemical composition and bulk hardness of wrought Stellite 706K were shown in Table 1. Table 1 also included the compositions of two comparative wrought alloys Stellite 6B and Stellite 6K. W in Stellite 6K was replaced by Mo resulting new alloy Stellite 706K. Stellite 6B had lower C content and lower bulk hardness as compared to Stellite 6K and Stellite 706K. Ingots with the composition of Stellite 706K were hot rolled into sheets of 4 mm thick. 2.2. Metallographic study Small samples were cut from hot rolled Stellite 706K sheets with a high-speed diamond saw. Samples were mounted in
Fig. 1. Optical micrographs of wrought Stellite 706K (a) planar view at 60× and (b) cross section view at 480×.
Fig. 2. EDS spectra of wrought Stellite 706K (a) Co Cr Mo matrix and (b) carbide precipitates rich in Cr and Mo.
M.X. Yao et al. / Materials Science and Engineering A 407 (2005) 291–298
carbon-filled phenolic with bakelite backing at 150 ◦ C under pressure of 4200 psi. The mounts were ground with SiC papers from 120, 220, 320, 400 to 600 grit size. To prevent pullout of precipitates, the mounts were further ground with SiC papers from 15, 8, 5 to 3 m size. Polishing began at 1 and 0.3 m Al2 O3 . Final polishing with 0.05 m SiO2 was performed by using vibratory polisher. The most successful etching in this metallographic study was offered by the electrolytic etching at 2 V for 20–30 s with 6 g CrO3 and 200 ml H2 O aqueous solution. Surface chemistry was studied by energy dispersive spectroscopy (EDS) X-ray microanalysis and spectral mapping of elements. The beam energy was chosen at 20 keV, approximately twice the energy required to excite L-shell electrons of heavy elements like Mo. The EDS system in use had an ultra thin window that could detect elements down to C. However, the observed C peak was associated with high uncertainty. EDS analysis was repeated five times. Due to low compositional contrast, different types of carbides were not distinguished under the magnifications in use. Higher magnification scanning electron microscope images were obtained to identify the carbides. 2.3. Solution annealing heat treatment The wrought Stellite 706K samples were solution heattreated at 1235, 1240, 1245 and 1250 ◦ C respectively, holding for 30 min, then water quenched. The mechanical behavior of Co Cr Mo matrix of alloy Stellite 706K was investigated using a load and displacement sensing indentation technique on the CSM nano-hardness machine. Values of effective Young’s modulus E* = E/(1 − ν2 ) and hardness H were calculated based on the loading/unloading curves measured with a Berkovich indenter using the Oliver–Pharr method [6], where E and ν are the Young’s modulus and Poisson ratio of the tested material, respectively. The area enclosed by the loading and unloading curves represented the energy dissipated due to plastic deformation [7]. The Vickers hardness of Stellite 706K after heat-treating was obtained under 100 g, 500 g, and 1 kg load conditions. Under each load condition, three points were selected and the average hardness was reported. 2.4. Pin-on-disc wear test The sliding wear test was conducted on a pin-on-disc tribometer on wrought Stellite 706K to study the effect of solution annealing heat treatment on wear resistance. A ball made of 94% WC and 6% Co, with the hardness of HV 1534, was wearing the specimen surface which was spinning with the rotational speed of 319 rpm for 10,000 s, under a compressive force of 10 N, resulting in a wear track or pit. The diameters of the ball and the wear track were 5 and 6 mm, respectively. The cross-section profiles of the wear track left on each sample after wear test were recorded. Based on the profiles, the wear volume loss of Stellite 706K samples was evaluated.
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3. Results 3.1. Microstructure characteristics All microstructure is revealed by electrolytic etching with CrO3 solution. Photomicrographs from optical microscope are taken and shown in Fig. 1. At low magnification (60×), Fig. 1(a) shows a planar view of banding (white) of second-phase free zone. There always are groups of parallel bands that incline or bow along the length of sample sheet. Fig. 1(b) shows a cross section view from Stellite 706K at higher magnification (480×). The matrix (M) as banding consists of equiaxed grains. Grain boundaries are revealed as the connection of small pits created by etching. Pitting in grains is observed as well. The second phase (A) appears white and is mostly situated at grain boundaries. There is also some third phase (B) existing within the matrix grains. These two precipitate phases are believed to be Cr7 C3 (A) and Cr23 C6 (B) type carbides, where Mo can replace Cr in the carbides. However, these different carbides are not readily discerned under optical microscopy at lower magnifications. It can be seen that some grains of matrix in the bands are large and some of matrix grains contain twins in alloy Stellite 706K. The preferential grain growth and carbide distribution are typical of hot rolled Co-based alloys. Fig. 2(a) is the EDS spectrum obtained from the Co Cr Mo matrix of Stellite 706K. Major peaks are Co, Cr and Mo, which show these common elements in Stellite 706K with a greater extent. Si is also detected from Co-rich matrix. Minor peaks are Mn and Ni, which could be responsible for the twin formation in Stellite 706K as seen in Fig. 1(b) despite the higher amount of Mo. Fig. 2(b) represents the EDS spectrum generated from the carbide precipitates. The precipitates are found to be Crand Mo-abundant. Since the different types of carbides are not discerned in these results, it is only a prediction that there is a higher level of Mo substitution to Cr that may result in more (Cr, Mo)7 C3 type carbide present. Although the carbide favorable to a high Mo or W contents is in the form of (Mo, W)6 C, it is less stable. Moreover, the amount of Cr is still higher than Mo. Fig. 3 shows the X-ray spectral mapping of elements Co, Cr, Mo, Mn, Ni and Si obtained from wrought Stellite 706K. It can be found that Cr and Mo signals produce a fair contrast between matrix and precipitates. Si and Mo signals detected from the same precipitate spots, indicating the existence of a Mo Si-rich phase. Fig. 4 shows the backscatter SEM micrograph at higher magnification (3100×), showing the microstructure characteristics of the dark gray Cr Mo-rich carbides and the Mo Si-rich white phase in the light gray Co Cr Mo alloy matrix. 3.2. Mechanical properties The loading and unloading curves of the nano-indentation on Co Cr Mo solid solution matrix of wrought Stellite 706K are presented in Fig. 5. The maximum load applied in each indentation is 50 mN. The ηe represents the ratio of the elastic deformation energy to the total deformation energy and ηp is the ratio of the plastic deformation energy to the total deformation energy. The mechanical properties such as Young’s modulus, hardness,
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Fig. 3. X-ray mapping of elements Co, Cr, Mo, Mn, Ni and Si in wrought Stellite 706K.
ηe and ηp values which are calculated from the loading and unloading curves are presented in Table 2. The nano-indentation test shows heat-treating decreases the hardness of wrought Stellite 706K. The Young’s modulus at 1235 ◦ C treated condition is with the lowest value. Heat-treating at 1235 ◦ C does not change very much in elasticity and plasticity of Co Cr Mo matrix of Stellite 706K. Increasing heat-treating temperature to 1250 ◦ C does reduce the elasticity and hardness, and increase the plasticity of the Co Cr Mo matrix of Stellite 706K. 3.3. Wear resistance evaluation
Fig. 4. High magnification (3100×) SEM micrograph showing the carbides (dark gray) and the Mo Si-rich phase (white phase) in the light gray Co Cr Mo matrix.
Images shown in Fig. 6 are the wear tracks left on the Stellite 706K samples after wear test. It is obvious that the wear track on sample treated at 1235 ◦ C (Fig. 6(b)) is smoother than those on the other samples. This implies the wear volume loss should be less for the 1235 ◦ C treated sample. The wear volume loss is calculated and averaged from three cross-sectional wear track profiles. The wear track profiles of sample treated at 1235 ◦ C are depicted in Fig. 7 as an example. The wear resistance in terms of the volume loss of Stellite 706K under different heat treatment conditions is graphed in Fig. 8. This indicates that solution annealing heat treatment has increased the wear resistance of Stellite 706K as compared to that under the mill-annealed condition. Heat treatment at 1235 ◦ C results Table 2 Nano-mechanical properties of Co Cr Mo matrix of wrought alloy Stellite 706K
Fig. 5. Loading–unloading curves of nano-indentation test from Co Cr Mo matrix of wrought Stellite 706K at different heat-treating conditions.
Heat-treating
Hardness (GPa)
E (GPa)
ηe (%)
ηp (%)
Mill annealed 1235 ◦ C 1250 ◦ C
7.6 7.1 5.7
221 205 233
20.9 19.6 13.5
79.1 80.4 86.5
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Fig. 6. Worn surfaces of wrought Stellite 706K at different heat-treating conditions.
in the highest wear resistance for the wrought alloy Stellite 706K. In order to understand why the heat treatment has enhanced the wear resistance of Stellite 706K samples, the microstructure analysis on the heat-treated 706K samples and the nontreated Stellite 706K sample is carried out using a scanning electron microscope. The images of the microstructure are shown in Fig. 9. As compared to the mill annealed microstructure (Fig. 9(a)), heat-treating at 1235 ◦ C stabilizes the rolled microstructure as evidenced by the small and large amount of carbides (Fig. 9(b)). The stabilized microstructure results in lower hardness of Co Cr Mo matrix as shown in Table 2 but significantly enhances the wear resistance of the wrought Stellite 706K alloy (Fig. 8). However, it is also noticed that the carbides grow bigger and some carbides dissolve into Co Cr Mo matrix when the treating temperatures are increased (Fig. 9(c)–(e)). The wear resistance of 706K samples heat-treated at higher temperatures is slightly decreased as compared to that treated at 1235 ◦ C due to the coarsening and dissolution of the carbides. The bulk Vickers hardness of Stellite 706K at different heattreating conditions is compared in Fig. 10. Like nano-indentation
test, Vickers hardness test also reveals that the heat-treating decreases the hardness of wrought Stellite 706K and the Vickers hardness is slightly decreased with increasing in heat-treating temperature. 4. Discussion Replacement of W in Stellite 6K by Mo in Stellite 706K results in the changes in the carbide morphology and increased volume fraction of carbides in the microstructure. Mo atoms are much lighter in weight than W atoms and with an atomic weight roughly half of that of W, there are roughly twice as many Mo atoms for a given weight percentage [8]. Mo has a great affinity for C than does W, and due to its smaller size diffuses much more quickly, thereby favoring the formation of carbides which impart abrasive and adhesive resistance. A comparison in abrasion wear resistance of wrought Stellite alloys obtained according to ASTM G65 procedure B is made in Fig. 11. Stellite 706K outperforms Stellite 6B and Stellite 6K in abrasion resistance [9]. Furthermore, Mo imparts greater corrosion resistance than does W in acidic environments [5]. Table 3 compares the corrosion
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rates of wrought alloys Stellite 706K, Stellite 6K and Stellite 6B in some acid media [9]. It shows that Stellite 706K has the corrosion resistance equivalent to Stellite 6B and has corrosion resistance much better than Stellite 6K. For the Co Cr Mo C Stellite alloys, the corrosion resistance is believed to be imparted by Cr and Mo in the Co Cr Mo solid solution matrix and the
Fig. 8. The wear resistance of wrought Stellite 706K at different heat-treating conditions.
wear resistance is imparted primarily by the formation of Cr and Mo carbides. Due to higher Mo content (in at.%) in Stellite 706K, the solid solution strengthening effect is also enhanced. Fig. 12 compares the tensile strength of the three wrought Stellite alloys. Stellite 706K has the higher yield strength and ultimate tensile strength as compared to those of Stellite 6K and Stellite 6B [9]. The hot working of Stellite 706K breaks down the eutectic carbide structure, causing the carbides to disperse. This results in a homogenous structure with uniform distribution of fine carbides in a fine-grained matrix (Figs. 1, 4 and 9). It is this large amount of carbides that allows alloy Stellite 706K to obtain a sharp edge and to hold the cutting edge better than conventional materials. It is determined that the melting range of Stellite 706K is from 1265 to 1354 ◦ C [10]. The heat treatment temperature is approaching the solidus of the alloy. Phase relationships in cast Co Cr Mo C surgical implant alloys, heat-treated at temperatures from 1180 to 1300 ◦ C, are reported [11]. Incipient melting and subsequent re-solidification of eutectic mixture accounts for observations of behavior at temperatures above 1235 ◦ C. At temperatures just below its melting point the interdendritic carbides initially break down, which subsequently dissolve into the Co Cr Mo matrix. Heat-treating at temperatures from 1235 to 1250 ◦ C of hot worked Stellite 706K also dissolves some carbide (Fig. 9) and softens the alloy (Table 2 and Fig. 10). Co imparts to its alloys an unstable FCC crystal structure with a high stacking fault energy (SFE). The instability arises from the fact that unalloyed Co exhibits a martensitic FCC → HCP phase transformation. In fact, at temperature above 417 ◦ C unalloyed Co possesses the FCC crystal structure and below this temperature the HCP crystal structure. Apart from the FCC → HCP Table 3 Immersion corrosion test conditions and general corrosion rate in mm/year
Fig. 7. Cross-sectional profiles of the wear tracks of Stellite 706K specimen tested at 1235 ◦ C.
Alloy
5% HCl at boiling
10% H2 SO4 at 65 ◦ C
10% HNO3 at boiling
Stellite 6B Stellite 6K Stellite 706K
394 488 161
0.01 47 0.04
0.02 2.9 0.09
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Fig. 9. Carbide morphology and distribution in wrought Stellite 706K after heat treatment at different temperatures.
martensitic phase transformation in Stellite 706K, twinning and slip may occur as evidenced in Fig. 1(b). The martensitic transformation and twinning processes are induced by the same transformation dislocations. As a result they create intergranular planar barriers to slip, and by additional strain hardening during sliding wear test it may provide an important contribu-
tion to the work hardening rate of the alloy. When the FCC is stabilized, hardness is decreased and deformation occurs by twinning or slip rather than by the FCC → HCP phase transformation [12]. Solution heat-treating at 1235 ◦ C has stabilized the hot rolled microstructure of wrought Stellite 706K and the excellent wear resistance is attributed to the higher work hardening of soft Co Cr Mo matrix and the large amount of carbides. Heat-treating at higher temperatures will further decrease the
Fig. 10. Vickers hardness of wrought Stellite 706K at different heat-treating conditions.
Fig. 11. Comparison in abrasive wear resistance of wrought Stellite alloys tested per ASTM G-65B.
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causes the dissolution of some carbides and coarsening of carbides yielding a lower hardness yet maintains an excellent wear resistance due to the high work hardening effect of stabilized and soft FCC Co Cr Mo matrix. Acknowledgements The authors wish to thank Mr. Kai Lorcharoensery at Lehigh University, Bethlehem, PA, USA, for conducting the microstructure observation. Ms. Heidi Lovelock at Deloro Stellite, UK is thanked for providing a high magnification SEM micrograph. Fig. 12. Comparison in ultimate tensile strength and yield strength at 0.2% offset of wrought Stellite alloys.
hardness of Stellite 706K however a high wear resistance is still maintained as compared to that obtained under mill annealed condition. 5. Conclusions The new wrought alloy Stellite 706K contains Cr and Mo as major alloying elements. It is an improved alloy over the traditional Stellite 6K in wear and corrosion resistance. It also has a higher tensile strength than that of Stellite 6K and Stellite 6B. Compared to Stellite 6B, it also has a higher hardness and equivalent corrosion resistance. The excellent properties of Stellite 706K come from the combination of the high volume fraction of carbide particles and the tough matrix due to the alloying addition of Mo. Solution treatment at 1235 ◦ C stabilizes the rolled microstructure with fine and large amount of carbides resulting in enhanced wear resistance. Heat treatment at higher temperatures on mill annealed wrought Stellite 706K
References [1] K.C. Antony, W.L. Silience, Proceedings of the Fifth International Conference on Erosion by Solid and Liquid Impact, Cambridge University Press, 1979, p. 631. [2] K.J. Bhansali, A.E. Miller, ASME (1982) 179. [3] C.T. Sims, Cobalt-Base Alloys, The Superalloys, John Wiley and Sons, 1972, p. 145. [4] Deloro Stellite Wrought Products – Stellite Alloy 6B and Stellite Alloy 6K. http://www.stellite.com/pdf/Components/wrought process.pdf. [5] J.B.C. Wu, B. McKee, I. Purvis, US Patent 6,733,603 B1 (May 11, 2004). [6] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [7] J. Musil, F. Kunc, H. Zeman, H. Polakova, Surf. Coat. Technol. 154 (2002) 304. [8] J.B.C. Wu, New alloy opportunities, Deloro Stellite Memorandum, August 13, 1992. [9] Stellite 706K – A New Wrought Stellite Alloy that Resists Wear and Corrosion, Deloro Stellite, Inc., Alloy Datasheet No. 205. [10] Wrought Alloy Stellite 706K, Stellite Alloy Database, Deloro Stellite, Inc. [11] T. Kilner, R.M. Pilliar, G.C. Weatherly, C. Allibert, J. Biomed. Mater. Res. 16 (2004) 63. [12] L. Remy, Acta Metall. 26 (1978) 443.
Metallographic study and wear resistance of a high-C wrought Co-based alloy Stellite 706K M.X. Yao a,∗ , J.B.C. Wu b , W. Xu c , R. Liu c a
b
Deloro Stellite, Inc., 471 Dundas Street East, Belleville, Ont., Canada K8N 1G2 Deloro Stellite Holdings, Inc., 555 N New Ballas, STE 305, Saint Louis, MO 63127, USA c Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, Ont., Canada K1S 5B6 Accepted 25 July 2005
Abstract The new Co Cr Mo C type wrought alloy Stellite 706K is an improved alloy over the Co Cr W C type wrought alloy Stellite 6K in wear and corrosion resistance. The microstructure of Stellite 706K is characterized by Co Cr Mo solid solution matrix with banding and occasional twinning and various carbide phases. Pin-on-disc wear resistance of Stellite 706K at room temperature is evaluated. Solution treatment at temperatures from 1235 to 1250 ◦ C on the mill annealed Stellite 706K decreases the bulk hardness and the hardness of Co Cr Mo matrix while increases the wear resistance. The excellent properties of Stellite 706K come from the combination of the high volume fraction of carbide particles and the tough Co Cr Mo matrix due to the alloying addition of Mo. © 2005 Elsevier B.V. All rights reserved. Keywords: Metallographic; Pin-on-disc; Wear resistance; Wrought; Stellite alloy
1. Introduction The conventional Co-based wrought alloys can be categorized into three types, namely, wear resistant alloys, high temperature alloys and corrosion resistant alloys. The wear resistant alloys, such as Stellite 6B and Stellite 6K, are essentially Co Cr W C quaternaries, with Cr providing strength and corrosion resistance to the Co-rich solid solution, as well as functioning as the chief carbide former during alloy solidification. W provides additional solid solution strength. The excellent wear resistance of these alloys is attributed to the high volume carbides and the unusual deformation characteristics imparted by Co-rich matrix. The relatively low stacking fault energy (SFE), which is however high enough to conserve face centered cubic (FCC) structure is thought to be important, as is the transformation to the hexagonal close packed (HCP) structure form during processing and service [1,2]. The high temperature alloys, such as alloy L-605, is considerably ductile, oxidation resistant, and microstructurally stable. Structural stability is enhanced by Ni
∗
Corresponding author. Tel.: +1 613 968 3481; fax: +1 613 966 8269. E-mail address: [email protected] (M.X. Yao).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.07.053
addition, which decreases the FCC → HCP transformation temperature [3]. The corrosion resistant alloys, such as MP35N alloy and Ultimet alloy, utilize Mo instead of W, which appear to impart a greater degree of resistance to a variety of aqueous corrosive media. C is kept low and held within the soluble range to maintain corrosion resistance. Stellite 6B has been widely used to make rock crushing rollers, cement and steel equipment, and conveyor systems, steam turbine erosion shields, half sleeves and bushings where lubrication is inaccessible or effective lubrication is impossible. Stellite 6K performs so well in a variety of cutting applications. Currently, knives made from Stellite 6K are being used to cut such materials as leather, carpets, plywood, and plastics. Stellite 6K is also used to fabricate tools for trimming earth-ware plates and scraping corn flakes off rollers. Additional information on wrought Stellite 6B and Stellite 6K can be found directly from Deloro Stellite, Inc. at Belleville, Ont., Canada or our online brochure [4]. New wrought Co Cr Mo C alloys with high-C content have been developed to combat simultaneous wear and corrosion [5]. Knives made from the new Co Cr Mo C type wrought alloy Stellite 706K are being successfully used for the cutting of viscose fibers in some corrosive environments where Stellite 6K has exhibited pitting.
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2. Experimental details
Table 1 Chemical composition of high-C Co-based wrought Stellite alloys Alloy
Co
Cr
W
Mo
C
HRC
Stellite 6B Stellite 6K Stellite 706K
Balance Balance Balance
30.0 30.0 30.0
4.0 4.0 –
– – 4.5
1.2 1.6 1.6
37 47 45
In this study, the metallographic study results on high-C wrought wear resistant alloy Stellite 706K has been reported. Solution annealing heat treatment is carried out and its effect on the hardness and wear resistance is evaluated.
2.1. Materials The nominal chemical composition and bulk hardness of wrought Stellite 706K were shown in Table 1. Table 1 also included the compositions of two comparative wrought alloys Stellite 6B and Stellite 6K. W in Stellite 6K was replaced by Mo resulting new alloy Stellite 706K. Stellite 6B had lower C content and lower bulk hardness as compared to Stellite 6K and Stellite 706K. Ingots with the composition of Stellite 706K were hot rolled into sheets of 4 mm thick. 2.2. Metallographic study Small samples were cut from hot rolled Stellite 706K sheets with a high-speed diamond saw. Samples were mounted in
Fig. 1. Optical micrographs of wrought Stellite 706K (a) planar view at 60× and (b) cross section view at 480×.
Fig. 2. EDS spectra of wrought Stellite 706K (a) Co Cr Mo matrix and (b) carbide precipitates rich in Cr and Mo.
M.X. Yao et al. / Materials Science and Engineering A 407 (2005) 291–298
carbon-filled phenolic with bakelite backing at 150 ◦ C under pressure of 4200 psi. The mounts were ground with SiC papers from 120, 220, 320, 400 to 600 grit size. To prevent pullout of precipitates, the mounts were further ground with SiC papers from 15, 8, 5 to 3 m size. Polishing began at 1 and 0.3 m Al2 O3 . Final polishing with 0.05 m SiO2 was performed by using vibratory polisher. The most successful etching in this metallographic study was offered by the electrolytic etching at 2 V for 20–30 s with 6 g CrO3 and 200 ml H2 O aqueous solution. Surface chemistry was studied by energy dispersive spectroscopy (EDS) X-ray microanalysis and spectral mapping of elements. The beam energy was chosen at 20 keV, approximately twice the energy required to excite L-shell electrons of heavy elements like Mo. The EDS system in use had an ultra thin window that could detect elements down to C. However, the observed C peak was associated with high uncertainty. EDS analysis was repeated five times. Due to low compositional contrast, different types of carbides were not distinguished under the magnifications in use. Higher magnification scanning electron microscope images were obtained to identify the carbides. 2.3. Solution annealing heat treatment The wrought Stellite 706K samples were solution heattreated at 1235, 1240, 1245 and 1250 ◦ C respectively, holding for 30 min, then water quenched. The mechanical behavior of Co Cr Mo matrix of alloy Stellite 706K was investigated using a load and displacement sensing indentation technique on the CSM nano-hardness machine. Values of effective Young’s modulus E* = E/(1 − ν2 ) and hardness H were calculated based on the loading/unloading curves measured with a Berkovich indenter using the Oliver–Pharr method [6], where E and ν are the Young’s modulus and Poisson ratio of the tested material, respectively. The area enclosed by the loading and unloading curves represented the energy dissipated due to plastic deformation [7]. The Vickers hardness of Stellite 706K after heat-treating was obtained under 100 g, 500 g, and 1 kg load conditions. Under each load condition, three points were selected and the average hardness was reported. 2.4. Pin-on-disc wear test The sliding wear test was conducted on a pin-on-disc tribometer on wrought Stellite 706K to study the effect of solution annealing heat treatment on wear resistance. A ball made of 94% WC and 6% Co, with the hardness of HV 1534, was wearing the specimen surface which was spinning with the rotational speed of 319 rpm for 10,000 s, under a compressive force of 10 N, resulting in a wear track or pit. The diameters of the ball and the wear track were 5 and 6 mm, respectively. The cross-section profiles of the wear track left on each sample after wear test were recorded. Based on the profiles, the wear volume loss of Stellite 706K samples was evaluated.
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3. Results 3.1. Microstructure characteristics All microstructure is revealed by electrolytic etching with CrO3 solution. Photomicrographs from optical microscope are taken and shown in Fig. 1. At low magnification (60×), Fig. 1(a) shows a planar view of banding (white) of second-phase free zone. There always are groups of parallel bands that incline or bow along the length of sample sheet. Fig. 1(b) shows a cross section view from Stellite 706K at higher magnification (480×). The matrix (M) as banding consists of equiaxed grains. Grain boundaries are revealed as the connection of small pits created by etching. Pitting in grains is observed as well. The second phase (A) appears white and is mostly situated at grain boundaries. There is also some third phase (B) existing within the matrix grains. These two precipitate phases are believed to be Cr7 C3 (A) and Cr23 C6 (B) type carbides, where Mo can replace Cr in the carbides. However, these different carbides are not readily discerned under optical microscopy at lower magnifications. It can be seen that some grains of matrix in the bands are large and some of matrix grains contain twins in alloy Stellite 706K. The preferential grain growth and carbide distribution are typical of hot rolled Co-based alloys. Fig. 2(a) is the EDS spectrum obtained from the Co Cr Mo matrix of Stellite 706K. Major peaks are Co, Cr and Mo, which show these common elements in Stellite 706K with a greater extent. Si is also detected from Co-rich matrix. Minor peaks are Mn and Ni, which could be responsible for the twin formation in Stellite 706K as seen in Fig. 1(b) despite the higher amount of Mo. Fig. 2(b) represents the EDS spectrum generated from the carbide precipitates. The precipitates are found to be Crand Mo-abundant. Since the different types of carbides are not discerned in these results, it is only a prediction that there is a higher level of Mo substitution to Cr that may result in more (Cr, Mo)7 C3 type carbide present. Although the carbide favorable to a high Mo or W contents is in the form of (Mo, W)6 C, it is less stable. Moreover, the amount of Cr is still higher than Mo. Fig. 3 shows the X-ray spectral mapping of elements Co, Cr, Mo, Mn, Ni and Si obtained from wrought Stellite 706K. It can be found that Cr and Mo signals produce a fair contrast between matrix and precipitates. Si and Mo signals detected from the same precipitate spots, indicating the existence of a Mo Si-rich phase. Fig. 4 shows the backscatter SEM micrograph at higher magnification (3100×), showing the microstructure characteristics of the dark gray Cr Mo-rich carbides and the Mo Si-rich white phase in the light gray Co Cr Mo alloy matrix. 3.2. Mechanical properties The loading and unloading curves of the nano-indentation on Co Cr Mo solid solution matrix of wrought Stellite 706K are presented in Fig. 5. The maximum load applied in each indentation is 50 mN. The ηe represents the ratio of the elastic deformation energy to the total deformation energy and ηp is the ratio of the plastic deformation energy to the total deformation energy. The mechanical properties such as Young’s modulus, hardness,
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Fig. 3. X-ray mapping of elements Co, Cr, Mo, Mn, Ni and Si in wrought Stellite 706K.
ηe and ηp values which are calculated from the loading and unloading curves are presented in Table 2. The nano-indentation test shows heat-treating decreases the hardness of wrought Stellite 706K. The Young’s modulus at 1235 ◦ C treated condition is with the lowest value. Heat-treating at 1235 ◦ C does not change very much in elasticity and plasticity of Co Cr Mo matrix of Stellite 706K. Increasing heat-treating temperature to 1250 ◦ C does reduce the elasticity and hardness, and increase the plasticity of the Co Cr Mo matrix of Stellite 706K. 3.3. Wear resistance evaluation
Fig. 4. High magnification (3100×) SEM micrograph showing the carbides (dark gray) and the Mo Si-rich phase (white phase) in the light gray Co Cr Mo matrix.
Images shown in Fig. 6 are the wear tracks left on the Stellite 706K samples after wear test. It is obvious that the wear track on sample treated at 1235 ◦ C (Fig. 6(b)) is smoother than those on the other samples. This implies the wear volume loss should be less for the 1235 ◦ C treated sample. The wear volume loss is calculated and averaged from three cross-sectional wear track profiles. The wear track profiles of sample treated at 1235 ◦ C are depicted in Fig. 7 as an example. The wear resistance in terms of the volume loss of Stellite 706K under different heat treatment conditions is graphed in Fig. 8. This indicates that solution annealing heat treatment has increased the wear resistance of Stellite 706K as compared to that under the mill-annealed condition. Heat treatment at 1235 ◦ C results Table 2 Nano-mechanical properties of Co Cr Mo matrix of wrought alloy Stellite 706K
Fig. 5. Loading–unloading curves of nano-indentation test from Co Cr Mo matrix of wrought Stellite 706K at different heat-treating conditions.
Heat-treating
Hardness (GPa)
E (GPa)
ηe (%)
ηp (%)
Mill annealed 1235 ◦ C 1250 ◦ C
7.6 7.1 5.7
221 205 233
20.9 19.6 13.5
79.1 80.4 86.5
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Fig. 6. Worn surfaces of wrought Stellite 706K at different heat-treating conditions.
in the highest wear resistance for the wrought alloy Stellite 706K. In order to understand why the heat treatment has enhanced the wear resistance of Stellite 706K samples, the microstructure analysis on the heat-treated 706K samples and the nontreated Stellite 706K sample is carried out using a scanning electron microscope. The images of the microstructure are shown in Fig. 9. As compared to the mill annealed microstructure (Fig. 9(a)), heat-treating at 1235 ◦ C stabilizes the rolled microstructure as evidenced by the small and large amount of carbides (Fig. 9(b)). The stabilized microstructure results in lower hardness of Co Cr Mo matrix as shown in Table 2 but significantly enhances the wear resistance of the wrought Stellite 706K alloy (Fig. 8). However, it is also noticed that the carbides grow bigger and some carbides dissolve into Co Cr Mo matrix when the treating temperatures are increased (Fig. 9(c)–(e)). The wear resistance of 706K samples heat-treated at higher temperatures is slightly decreased as compared to that treated at 1235 ◦ C due to the coarsening and dissolution of the carbides. The bulk Vickers hardness of Stellite 706K at different heattreating conditions is compared in Fig. 10. Like nano-indentation
test, Vickers hardness test also reveals that the heat-treating decreases the hardness of wrought Stellite 706K and the Vickers hardness is slightly decreased with increasing in heat-treating temperature. 4. Discussion Replacement of W in Stellite 6K by Mo in Stellite 706K results in the changes in the carbide morphology and increased volume fraction of carbides in the microstructure. Mo atoms are much lighter in weight than W atoms and with an atomic weight roughly half of that of W, there are roughly twice as many Mo atoms for a given weight percentage [8]. Mo has a great affinity for C than does W, and due to its smaller size diffuses much more quickly, thereby favoring the formation of carbides which impart abrasive and adhesive resistance. A comparison in abrasion wear resistance of wrought Stellite alloys obtained according to ASTM G65 procedure B is made in Fig. 11. Stellite 706K outperforms Stellite 6B and Stellite 6K in abrasion resistance [9]. Furthermore, Mo imparts greater corrosion resistance than does W in acidic environments [5]. Table 3 compares the corrosion
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rates of wrought alloys Stellite 706K, Stellite 6K and Stellite 6B in some acid media [9]. It shows that Stellite 706K has the corrosion resistance equivalent to Stellite 6B and has corrosion resistance much better than Stellite 6K. For the Co Cr Mo C Stellite alloys, the corrosion resistance is believed to be imparted by Cr and Mo in the Co Cr Mo solid solution matrix and the
Fig. 8. The wear resistance of wrought Stellite 706K at different heat-treating conditions.
wear resistance is imparted primarily by the formation of Cr and Mo carbides. Due to higher Mo content (in at.%) in Stellite 706K, the solid solution strengthening effect is also enhanced. Fig. 12 compares the tensile strength of the three wrought Stellite alloys. Stellite 706K has the higher yield strength and ultimate tensile strength as compared to those of Stellite 6K and Stellite 6B [9]. The hot working of Stellite 706K breaks down the eutectic carbide structure, causing the carbides to disperse. This results in a homogenous structure with uniform distribution of fine carbides in a fine-grained matrix (Figs. 1, 4 and 9). It is this large amount of carbides that allows alloy Stellite 706K to obtain a sharp edge and to hold the cutting edge better than conventional materials. It is determined that the melting range of Stellite 706K is from 1265 to 1354 ◦ C [10]. The heat treatment temperature is approaching the solidus of the alloy. Phase relationships in cast Co Cr Mo C surgical implant alloys, heat-treated at temperatures from 1180 to 1300 ◦ C, are reported [11]. Incipient melting and subsequent re-solidification of eutectic mixture accounts for observations of behavior at temperatures above 1235 ◦ C. At temperatures just below its melting point the interdendritic carbides initially break down, which subsequently dissolve into the Co Cr Mo matrix. Heat-treating at temperatures from 1235 to 1250 ◦ C of hot worked Stellite 706K also dissolves some carbide (Fig. 9) and softens the alloy (Table 2 and Fig. 10). Co imparts to its alloys an unstable FCC crystal structure with a high stacking fault energy (SFE). The instability arises from the fact that unalloyed Co exhibits a martensitic FCC → HCP phase transformation. In fact, at temperature above 417 ◦ C unalloyed Co possesses the FCC crystal structure and below this temperature the HCP crystal structure. Apart from the FCC → HCP Table 3 Immersion corrosion test conditions and general corrosion rate in mm/year
Fig. 7. Cross-sectional profiles of the wear tracks of Stellite 706K specimen tested at 1235 ◦ C.
Alloy
5% HCl at boiling
10% H2 SO4 at 65 ◦ C
10% HNO3 at boiling
Stellite 6B Stellite 6K Stellite 706K
394 488 161
0.01 47 0.04
0.02 2.9 0.09
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Fig. 9. Carbide morphology and distribution in wrought Stellite 706K after heat treatment at different temperatures.
martensitic phase transformation in Stellite 706K, twinning and slip may occur as evidenced in Fig. 1(b). The martensitic transformation and twinning processes are induced by the same transformation dislocations. As a result they create intergranular planar barriers to slip, and by additional strain hardening during sliding wear test it may provide an important contribu-
tion to the work hardening rate of the alloy. When the FCC is stabilized, hardness is decreased and deformation occurs by twinning or slip rather than by the FCC → HCP phase transformation [12]. Solution heat-treating at 1235 ◦ C has stabilized the hot rolled microstructure of wrought Stellite 706K and the excellent wear resistance is attributed to the higher work hardening of soft Co Cr Mo matrix and the large amount of carbides. Heat-treating at higher temperatures will further decrease the
Fig. 10. Vickers hardness of wrought Stellite 706K at different heat-treating conditions.
Fig. 11. Comparison in abrasive wear resistance of wrought Stellite alloys tested per ASTM G-65B.
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causes the dissolution of some carbides and coarsening of carbides yielding a lower hardness yet maintains an excellent wear resistance due to the high work hardening effect of stabilized and soft FCC Co Cr Mo matrix. Acknowledgements The authors wish to thank Mr. Kai Lorcharoensery at Lehigh University, Bethlehem, PA, USA, for conducting the microstructure observation. Ms. Heidi Lovelock at Deloro Stellite, UK is thanked for providing a high magnification SEM micrograph. Fig. 12. Comparison in ultimate tensile strength and yield strength at 0.2% offset of wrought Stellite alloys.
hardness of Stellite 706K however a high wear resistance is still maintained as compared to that obtained under mill annealed condition. 5. Conclusions The new wrought alloy Stellite 706K contains Cr and Mo as major alloying elements. It is an improved alloy over the traditional Stellite 6K in wear and corrosion resistance. It also has a higher tensile strength than that of Stellite 6K and Stellite 6B. Compared to Stellite 6B, it also has a higher hardness and equivalent corrosion resistance. The excellent properties of Stellite 706K come from the combination of the high volume fraction of carbide particles and the tough matrix due to the alloying addition of Mo. Solution treatment at 1235 ◦ C stabilizes the rolled microstructure with fine and large amount of carbides resulting in enhanced wear resistance. Heat treatment at higher temperatures on mill annealed wrought Stellite 706K
References [1] K.C. Antony, W.L. Silience, Proceedings of the Fifth International Conference on Erosion by Solid and Liquid Impact, Cambridge University Press, 1979, p. 631. [2] K.J. Bhansali, A.E. Miller, ASME (1982) 179. [3] C.T. Sims, Cobalt-Base Alloys, The Superalloys, John Wiley and Sons, 1972, p. 145. [4] Deloro Stellite Wrought Products – Stellite Alloy 6B and Stellite Alloy 6K. http://www.stellite.com/pdf/Components/wrought process.pdf. [5] J.B.C. Wu, B. McKee, I. Purvis, US Patent 6,733,603 B1 (May 11, 2004). [6] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [7] J. Musil, F. Kunc, H. Zeman, H. Polakova, Surf. Coat. Technol. 154 (2002) 304. [8] J.B.C. Wu, New alloy opportunities, Deloro Stellite Memorandum, August 13, 1992. [9] Stellite 706K – A New Wrought Stellite Alloy that Resists Wear and Corrosion, Deloro Stellite, Inc., Alloy Datasheet No. 205. [10] Wrought Alloy Stellite 706K, Stellite Alloy Database, Deloro Stellite, Inc. [11] T. Kilner, R.M. Pilliar, G.C. Weatherly, C. Allibert, J. Biomed. Mater. Res. 16 (2004) 63. [12] L. Remy, Acta Metall. 26 (1978) 443.