Wear, corrosion and cracking resistance of some W- or Mo-containing Stellite hardfacing alloys

Wear, corrosion and cracking resistance of some W- or Mo-containing Stellite hardfacing alloys

Materials Science and Engineering A 407 (2005) 234–244 Wear, corrosion and cracking resistance of some W- or Mo-containing Stellite hardfacing alloys...

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Materials Science and Engineering A 407 (2005) 234–244

Wear, corrosion and cracking resistance of some W- or Mo-containing Stellite hardfacing alloys M.X. Yao a,∗ , J.B.C. Wu b , Y. Xie c a 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 Solid Oxide Fuel Cell Group, NRC Institute for Fuel Cell Innovation, National Research Council Canada, 3250 East Mall, Vancouver, BC, Canada V6T 1W5 b

c

Received in revised form 30 June 2005; accepted 30 June 2005

Abstract Traditional Stellite hardfacing materials are Co–Cr–W–C type of alloys. A series of new Stellite alloys are developed based on Co–Cr–Mo–C system. Immersion–corrosion test is conducted in 10% HNO3 oxidizing acid at boiling temperature, in 10% H2 SO4 reducing acid at 66 ◦ C and in 5% HCl reducing acid at 40 ◦ C to evaluate the general corrosion resistance of the Stellite alloys. The abrasive, adhesive and erosive wear resistance is investigated on these two types of Stellite alloys. Critical plastic shear strain is obtained by conducting scratch test to evaluate the resistance to cracking. The W-containing Stellite alloys have better corrosion resistance in oxidizing acid. The Mo-containing Stellite alloys exhibit unusual combination of excellent wear resistance and corrosion resistance in reducing environments. Mo-containing Stellite alloys also have adequate cracking resistance compared with the W-containing Stellite alloys. © 2005 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Adhesive wear; Erosive wear; Corrosion; Scratch testing; Critical plastic strain; Cracking resistance; Stellite alloys

1. Introduction The commercial wear resistant Stellite alloys are derived from the Co–Cr–W–C family first investigated by Elwood Haynes in early 1900s [1]. Hardfacing alloys of the Co–Cr–W–C type exists in several modifications and it is generally held that the available range of commercial grades satisfies most of the industry requirements. However, some particular applications call for alloys with even better properties than those available. This is the case in the chemical and non-ferrous metals industries, where components of pumps, impellers, etc., must often withstand the simultaneous abrasive and corrosive action of media composed of a suspension of hard mineral particles in an aqueous solution. When highcarbon Co–Cr–W–C alloys are used in such cases, they may occasionally be inadequate, in spite of their good abrasion resistance, on account of excessive corrosion. Such failures ∗

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.2005.06.062

are encountered, for instance, in the superphosphate industry [2]. Early research [2] showed that adding Mo, singly or in combination with Ni and/or Cu, enhances greatly the corrosion and abrasion resistance of high-carbon Co–Cr–W–C cast alloys. United States Patent [3] has provided saw tip for attachment to saw blades for cutting wood. The saw tips are manufactured from Stellite alloys comprising 4–20% by weight Mo, 25–35% Cr and 0.8–3.5% C. The saw tips formed from Co–Cr–Mo–C type Stellite alloys have demonstrated better wear and corrosion resistance compared with those made from conventional Co–Cr–W–C type Stellite alloys during wood cutting services. The Co–Cr–Mo–C type Stellite 700 series alloys developed at Deloro Stellite Holdings Inc., raise the standards for wear and corrosion resistant alloys [3,4]. The product forms and typical applications of the Co–Cr–Mo–C type Stellite alloys are summarized in Table 1. These Stellite alloys have the unusual combination of excellent wear resistance and exceptional corrosion resistance in environments that are either reducing or complex. For example, a series of

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Table 1 Product forms and typical application of Co–Cr–Mo–C Stellite alloys

Table 2 Nominal compositions of Co–Cr–W–C and Co–Cr–Mo–C Stellite alloys

Product forms

Typical applications

Alloy

Type

Cr

W

Mo

C

Powders for PTA welding Powders for HVOF thermal spray Powders for spray and fuse thermal spray Coated electrodes Cast rod for TIG and oxy/acetylene welding Metal cored wires for MIG

Saw teeth Chain saw bars Rock bit journals

Stellite 6 Stellite 706 Stellite 12 Stellite 712 Stellite 1 Stellite 701 Stellite 190 Stellite 790

Co–Cr–W–C Co–Cr–Mo–C Co–Cr–W–C Co–Cr–Mo–C Co–Cr–W–C Co–Cr–Mo–C Co–Cr–W–C Co–Cr–Mo–C

29 29 29 29 30 30 26 26

4.5 – 8 – 12 – 14 –

– 4.5 – 8 – 12 – 14

1.2 1.2 1.8 1.8 2.5 2.5 3.2 3.2

Cast components Machined components Powder metallurgy components

Sleeves and bushings Extrusion and injection screws and barrels Valve seats, slides and gates in high wear and corrosion environments Chemical, petrochemical and oil industry applications Thermowells Wear rings and impellers

electrochemical corrosion tests were performed to determine the general corrosion rate and corrosion characteristics of Stellite 712 and Stellite 12 for construction of the slurry pumps and impeller bearings [5]. Both alloys displayed passive behavior with corrosion rates <0.0254 mm/year (1 mpy) for a simulated waste solution (3 M sodium hydroxide solution and inhibited water) and should be resistant in the waste tank environment. In another report [6], Stellite 712 was again proved the preferred material for the process bearings of slurry pumps. The original coating on the shafts was removed, and the silicon carbide process bushing was replaced with a Stellite 712 press fitted bushing [6]. The good wear resistance is crucial for alloys to be used as hardfacing materials. On the other hand, the resistance to cracking is of vital importance to any hardfacing project. In order to understand the beneficial effect of Mo in Stellite hardfacing alloys, the corrosion resistance and the wear properties of some W- and Mo-containing Stellite alloys including abrasive, adhesive and erosive wear resistance have been evaluated. The resistance to sliding wear cracking has been studied by means of scratch testing. The hardness of various phase constitutions of the Stellite alloys is also studied in this report.

2. Experimental details 2.1. Materials The carbon contents of contemporary Stellite alloys vary from 0.1 to 3.2%. Co–Cr–W–C Stellite alloys essentially consist of M7 C3 and M6 C carbides in Co–Cr–W matrices. The relative amounts of M7 C3 and M6 C carbides and matrix composition depend on the Cr, W and C levels. Alloys with carbon contents less than about 2% exhibit a hypo-eutectic microstructure while alloys with a carbon content greater than about 2% possess a hyper-eutectic microstructure. Wcontaining Co–Cr–W–C type alloys Stellite 6, Stellite 12, Stellite 1 and Stellite 190, and Mo-containing Co–Cr–Mo–C

type alloys Stellite 706, Stellite 712, Stellite 701 and Stellite 790 were used in this study. These alloys and their nominal compositions were listed in Table 2. Undiluted coatings were manufactured out of these Stellite alloys by plasma transferred arc-welding method. These coatings were then used to prepare specimens for various testing conducted in this report. 2.2. General corrosion test Stellite alloys with lower carbon contents would better serve in corrosion environment. Immersion–corrosion test was conducted on Stellite 6, Stellite 706, Stellite 12 and Stellite 712 referring to ASTM G31. Strip coupons approximately 50 mm × 25 mm × 3 mm (2 in. × 1 in. × 0.125 in.) were prepared and the surfaces of each specimen were ground to 0.41 ␮m finish, cleaned in acetone and air-dried. The geometric area calculation was accurate to 1% and the mass of each specimen was measured to an accuracy of 1 mg. The test media and test temperatures were oxidizing acid 10% HNO3 at boiling temperature, reducing acids 10% H2 SO4 at 66 ◦ C and 5% HCl at 40 ◦ C. The specimens were supported in glass cradles (reference ASTM G 28-97 Fig. 2) while in the test solutions. After exposure to the test solutions for 72 h, the specimens were cleaned by brushing under warm tap water and dried with ethanol and a blow dryer. The mass of each specimen was measured, and the corrosion rate expressed in millimeters per year was obtained. 2.3. Low-stress abrasive-wear and high-stress abrasive-wear tests The low-stress abrasive-wear test was carried out on Stellite 6, Stellite 706, Stellite 12, Stellite 712, Stellite 190 and Stellite 790 according to ASTM G65 Procedure B on a Falex friction and wear test machine. The method involved feeding dry AFS 50/70 Ottawa Silica Test Sand at a controlled flow rate of 300–400 g/m, into the contact zone between a 12.7-mm wide, 229 mm diameter, rotating (200 rpm) rubber-rimmed wheel and a rectangular 76.2 mm × 25.4 mm × 12.7 mm specimen. The sample was pressed with a standard 130 N force against the wheel with a hardness of Shore Durometer A 58–62. During each test a constantly replenished layer of sand abrasive imbedded in

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the conforming rubber wheel surface and was transported over the test coupon, continuously removing material from the contact surface. The high-stress abrasive-wear test on Stellite 12 and Stellite 712 was also carried out on the Falex friction and wear test machine modified to generate high-stress conditions. A smaller diameter, soft-steel wheel (150 HB) replaced the rubber-rimmed steel wheel used in the G65 method. The test duration was 750 revolutions during which the rotating wheel crushed the AFS 50–70 silica sand between itself and the coupon. The volume loss was obtained to represent the abrasive-wear resistance. 2.4. Adhesive-wear test Adhesive-wear test was conducted in accordance with ASTM G77 on a Falex block on ring tester at room temperature under 68.2 kg (150 lbs) load. The S10 type ring specimens with hardness of HRC 58–63 and surface finish of 6–12 rms were used. The block specimens were made from Stellite 12, Stellite 712, Stellite 1, Stellite 701, Stellite 190 and Stellite 790. Two tests were run for each alloy and the test duration was 2003 cycles. The volume loss of block specimens was measured as the adhesive-wear resistance. 2.5. Erosive-wear test Erosive-wear test was conducted according ASTM G76 on Stellite 12 and Stellite 712 at 700 ◦ C per sample for 5 min. The test samples had the dimensions of 60 mm × 18 mm × 6 mm. A real industry sand mixture was used as abrasive medium. Sand flux was about 300 g/min. Jet velocity was about 100 m/sec and the impingement angle was 60◦ . Erosive-wear test was also conducted on Stellite 12 and Stellite 712 at room temperature. The test was known as slurry jet erosion and the test conditions were as follows. A 1:10 (by weight) AFS 50/70 silica sand and water slurry was recirculated through the piping loop via a mixing tank/reservoir, and emitted in a high-speed jet at a velocity of 16 m/s from a tungsten carbide nozzle with a 5 mm diameter orifice located above the reservoir. The nozzle to specimen standoff distance was 10 cm. Test duration was 120 min and fresh abrasive was used each time. The slurry jet stroked the flat test specimen at impingement angles of 20◦ , 45◦ and 90◦ .

load. At relatively small load, cracking was hard to induce. When the load exceeded a critical value, significant microcracking/fracture occurred at the sides of the scratch. Because the cracking/fracture released elastic energy which could be recorded by an acoustic emission (AE) detector mounted on the indenter holder, the initiation of the cracking/fracture was accompanied by a sudden increase in the recorded AE signal. From the indenter radius and the width of the scratch where the cracking/fracture initiated, the critical plastic shear strain to initiate the micro-cracking/fracture could be calculated. Therefore, the scratch test could be used to evaluate the cracking-resistance of a material in sliding contact. Details of this method had been described in a study conducted by Xie and Hawthorne [7]. Stellite 190, Stellite 790, Stellite 1 and Stellite 701 four alloys were tested. There were two specimens for each alloy. The scratch and indentation tests were performed on polished and etched top surfaces. The specimens were first ground by 240 grit silicon carbide abrasive paper, then lapped by 6, 3 and 1 ␮m diamond particle slurry and finally, polished with 0.06 ␮m silica particle slurry. The roughness, Ra , of the polished surface was about 0.03 ␮m. The polished surfaces were then etched by HCl:H2 O2 :H2 O (20:1:20) for a few seconds to reveal the microstructure. The specimen test surface and the indenter tip were wiped with ethanol prior to each test, which was carried out in an unlubricated condition. Since the cracking resistance of Stellite 790 and Stellite 1 could not be discriminated by the single scratch test, a parallel scratch test was performed on these two alloys. Nano-indentation test was carried out on Fisherscope H 100B, Helmut Fischer, Germany. The test was run under 7 mN load with a three-sided pyramid diamond Berkovich indenter. The hardness and elastic modulus of different phases for the four alloys were measured by the depth sensing nano-indentation method. The measured hardness value was given by the ratio of indentation load to the projected area of the resulting permanent impression [8]. The measured elastic modulus was in the form of E/(1 − ν2 ), where E is Young’s modulus and ν is Poisson’s ratio. The Young’s modulus of the material could be estimated by assuming ν = 0.3 since most metallic materials had Poisson’s ratio around 0.3.

3. Results and discussion 2.6. Scratch test and nano-indentation test 3.1. Abrasive-wear resistance Stellite alloys with higher carbon contents are prone to crack. The scratch test was performed on an automated scratch tester with a diamond Rockwell C type indenter, REVETEST, CSEM, Switzerland. The conical indenter was with an included angle of 120◦ . The actual tip radius was measured to be 230 ␮m by a 3-D surface imager. In a scratch test, a plastic strain was induced by the indenter. The extent of the plastic strain increased with an increase in the applied normal

Abrasive wear is encountered when hard particles, or hard projections on a counter-face are forced against, and moved relatively to a surface. The terms high-stress abrasion and low-stress abrasion relate to the condition of the abrasive medium, be it hard particles or projections, after interaction with the surface. If the abrasive medium is crushed, then the high-stress condition is said to prevail. If the abrasive medium

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obtained. It is found from Table 3 that the average COF varies from 0.40 to 0.48. It seems that the COF of Mo-containing Stellite alloys is a little bit higher than that of W-containing alloys and yet the adhesive-wear resistance of Co–Cr–Mo–C alloys is still better. 3.3. Erosive wear resistance

Fig. 1. Low-stress abrasive and high-stress abrasive-wear resistance.

remains intact, the process is described as low-stress abrasion. Typically, high-stress abrasion results from the entrapment of hard particles between metallic surfaces in relative motion, while low-stress abrasion is encountered when moving surfaces come into contact with packed abrasives, such as soil and sand. As examples, the low- and high-stress abrasive resistance of Stellite alloys is shown in Fig. 1. For cobaltbased Stellite alloys, which contain a hard carbide phase, the abrasion resistance generally increases as the volume fraction of the hard phase increases. Stellite 190 and Stellite 790 contain larger amount of carbides than do Stellite 6, Stellite 706, Stellite 12 and Stellite 712, hence with higher abrasive-wear resistance. It is found in Fig. 1 that the Mo-containing Stellite alloys have higher abrasive resistance than W-containing Stellite alloys under both low and high-stress abrasive-wear conditions. 3.2. Adhesive-wear resistance Conditions that increase adhesive wear are high friction and heavy loading. The origin of adhesive wear is the formation of microwelds between localized contact points on the moving contact surfaces. The relative movement of the contact surfaces causes these microwelds broken up. If breakage takes place, small fragments will be torn out, which leads to a gradual loss of material. Even if the microwelds do not break, repeated tearing will lead to the formation of microcracks. Over time, these cracks will grow together, and when they reach a critical size, larger pieces of the edge will start to break out. Materials properties such as low coefficient of friction (COF), high yield strength, high ductility and hard carbides in the matrix will reduce adhesive wear. W and Mo together with Cr and C play an important role in these properties. The coefficients of friction obtained from the typical block-on-ring wear test for Stellite 190 and Stellite 790 blocks against hardened steel rings are demonstrated in Fig. 2. The adhesive-wear resistance of six Stellite alloys studied is graphically described in Fig. 3. The COF was also recorded and shown in Table 3. The value of the coefficient of friction during adhesive-wear test is obtained by an average at three load levels. The average of the COF of the two tests is then

Erosion is a form of wear caused by sharp hard particles entrained in a fluid stream impinging on the surface of a material. Various methods of measuring particle impingement erosion have been proposed. The slurry jet erosion wear resistance at room temperature and particle erosion wear resistance at 700 ◦ C for Stellite 12 and Stellite 712 are summarized in Fig. 4. The data suggest that under low angle impingement conditions, the erosion resistance of Wcontaining alloy Stellite 12 is enhanced compared with the Mo-containing Stellite 712. On the other hand, the erosion resistance of Mo-containing Stellite 712 is better than that of W-containing Stellite 12 under high-angle impingement conditions. 3.4. Resistance to cracking In the single scratch test, the normal load applied on the indenter is linearly increased from 20 N to a value beyond the critical load at which a sudden increase in AE signal is detected. The loading rate is 32 N/min and scratch speed is 3.2 mm/min. Analysis of the recorded AE signals and microscopic examination of the scratches has revealed that, at relatively light normal load in Fig. 5, the AE signal is very low, and few cracks are observed in Fig. 6. When the load exceeds a critical value, a sudden increase in AE signal is detected (Fig. 5). After the normal load is increased beyond the critical value, obvious micro-cracking/fracture begins to appear on the scratch sides. With further increase in the normal load, progressively more micro-cracking/fracture appears, which is shown in Fig. 7. The calculated critical plastic shear strain from each test is given in Fig. 8. The critical plastic strain for Stellite 190 is much higher than those of the other alloys. This indicates that Stellite 190 has the best cracking resistance among the four alloys. Careful examination of the scratches and recorded AE signal reveals that, although the critical plastic strain of Stellite 701 is slightly higher than those of Stellite 790 and Stellite 1, the AE signal of Stellite 701 is always higher than those of other alloys (see Fig. 5—the AE signal of Stellite 701 is beyond the plotting range when normal load is over about 66 N), and there are more cracks on the scratch sides of Stellite 701 (see Fig. 7). Even when the normal load is below the critical value, there are some cracks on the scratch sides of Stellite 701 while there is no observable cracking on the other alloys (see Fig. 6). The examination has indicted that the cracking resistance of Stellite 701 is inferior to other alloys. The cracking resistance of Stellite 790 and Stellite 1 cannot be discriminated by the single scratch test since they have

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Fig. 2. Coefficient of friction of (a) Stellite 190 vs. steel and (b) Stellite 790 vs steel.

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Table 3 Coefficient of friction obtained during adhesive test Alloy

Stellite 12

Stellite 712

Stellite 1

Stellite 701

Stellite 190

Stellite 790

Run 1 Run 2 Average

0.363 0.445 0.404

0.402 0.465 0.434

0.432 0.463 0.448

0.465 0.462 0.464

0.421 0.448 0.435

0.483 0.472 0.478

Fig. 3. Adhesive-wear resistance of W-containing Stellite 12, Stellite 1 and Stellite 190, and Mo-containing Stellite 712, Stellite 701 and Stellite 790.

the same critical plastic strain value, and similar AE signal and scratch appearance. As a method to further investigate micro-cracking/fracture in sliding contact, parallel scratch test is performed on these two coatings. In the parallel scratch test, an initial scratch is made on the specimen test surface at a constant load, which is below the critical normal load mea-

Fig. 4. Open points represent slurry jet erosion wear resistance at room temperature. The solid point data represent erosion resistance at 700 ◦ C.

Fig. 5. AE signal as a function of normal applied load.

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Fig. 6. SEM images of the scratches made below the critical load. Scratch direction was from left to right. (a) Stellite 790 at normal load of 32 N with plastic strain of 0.20; (b) Stellite 190 at normal load of 29 N with plastic strain of 0.21; (c) Stellite 1 at normal load of 33 N with plastic strain of 0.21, and (d) Stellite 701 at normal load of 32 N with plastic strain of 0.22.

sured in the single scratch test. Then, a subsequent scratch is made parallel to the previous one but displaced from it by a fixed separating distance at increasing load starting from a small value until a critical normal load is reached at which AE signal jump has been detected. Fig. 9 is the scanning electron microscope (SEM) image of the parallel scratches made on Stellite 790. Since the second scratch is made on prestrained surface, micro-cracking/fracture is easier to induce and thus, the cracking resistance of different materials can be discriminated. Since the two alloys have different hardness, the constant normal load used in the first scratch is 50 N on Stellite 790 and 42 N on Stellite 1, respectively, to cause the same plastic strain about 0.26 on both specimen surfaces. The separating distance between the parallel scratches is 50 ␮m. The normal load in the second scratch is linearly increased from 20 N, the loading rate is 32 N/min and scratch speed is 3.2 mm/min, the same as those in the single scratch test. The change of AE signal and observed cracking pattern in

the second scratch are similar to those in the single scratch test but the AE signal jump and micro-cracking/fracture have occurred at lower normal load. The measured critical normal load and the calculated critical plastic shear strain from each test are given in Table 4. The results indicate that Stellite 790 has better cracking resistance than Stellite 1. According to the results from the single and parallel scratch tests, it is concluded that the order of the crack resistance of the four Stellite alloys, from the best to the worst is: Stellite 190, Stellite 790, Stellite 1, and Stellite 701. It has been reported that the critical plastic shear strains of laser clad coating 50Cr–39Fe–8B–3Si and HVOF sprayed coating Co–26Cr–9Ni–5Mo–0.06C are 0.29 and 0.33, respectively [7]. PVD CrN/Cr coating and unbalanced magnetron sputtering TiN coating have a critical plastic shear strain of 0.23–0.29 and 0.22–0.31, respectively [9]. It can be concluded that both W- and Mo-containing Stellite alloys have adequate cracking resistance for hardfacing applications.

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Fig. 7. SEM images of the scratches made above the critical load. Scratch direction was from left to right. (a) Stellite 790 at normal load of 95 N with plastic strain of 0.32; (b) Stellite 190 at normal load of 110 N with plastic strain of 0.40; (c) Stellite 1 at normal load of 88 N with plastic strain of 0.34, and (d) Stellite 701 at normal load of 62 N with plastic strain of 0.31.

3.5. Corrosion resistance Both W and Mo enhance corrosion resistance, especially in reducing or non-oxidizing conditions. Mo has a strong effect to resist corrosion in reducing acids and complex or pitting environments. However, it weakens the corrosion resistance in purely oxidizing acids. In a complex pitting environment, Mo has a strong beneficial effect as evidenced in the commonly adopted pitting resistance equivalent number (PREN) formula for iron and nickel alloys. The corrosion rate of Stellite 6, Stellite 706, Stellite 12 and Stellite 712 is summarized in Fig. 10. It can be clearly seen that the Wcontaining Stellite alloys have better corrosion resistance in oxidizing 10% HNO3 acid at boiling temperature and the Mo-containing Stellite alloys exhibit exceptional corrosion resistance in reducing 5% HCl acid at 40 ◦ C and 10% H2 SO4 at 66 ◦ C. The data suggest that Mo tends to have a stronger effect on corrosion resistance than W at the same weight percentage in the alloy.

Fig. 8. Cracking resistance in terms of critical plastic shear strain.

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Fig. 9. SEM image of parallel scratches made on Stellite 790. The second scratch ended at 80 N at which the plastic strain induced by the scratch was about 0.30.

3.6. W and Mo alloying effects in Stellite alloys Cr and C are inevitably present in Stellite hardfacing alloys and they form the needed “Cr carbides” for wear resistance. They are typically in the form of M7 C3 and M23 C6 carbides [10,11]. The primary M7 C3 carbide, found mostly in hypereutectic alloys, has a high melting point among the carbides and therefore, precipitates first during solidification if the carbon content is high enough to favor its formation. Other elements, such as, W and Mo, are added to these alloys for improving other properties. W is known to partition in the carbides but not significantly due to the slow kinetics of diffusion. Most of the W remains in solid solution. Replacement of W by Mo results in the changes in the carbide morphology and increased volume fraction of carbides in the microstructure, compared with the W-containing counterparts, since Mo atoms are much lighter in weight than W atoms [12]. The carbide formed by W and Mo tends to be the M6 C carbide whereas Cr tends to form M7 C3 carbide [10,11]. Under scanning electron microscope of alloy Stellite 190, it can be clearly seen that the Cr-rich carbide and W-rich carbide are separate in significant amounts as illustrated in Fig. 11.

The separation of the Cr-rich carbide and Mo-rich carbide in Stellite 790 can also be seen in Fig. 12. Mo atoms are much smaller 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. 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. Furthermore, Mo imparts greater corrosion resistance than does W in acidic environments of a reducing nature, which are often encountered in wood cutting applications [3]. In Co–Cr–Mo–C Stellite alloys, the corrosion resistance is believed to be imparted by Mo in solid solution and the wear resistance is imparted primarily by the formation of Mo carbides. In an indentation test, the plastic deformation zone induced by the indenter is much smaller than the dimension of the measured phase, so the hardness differences between the measured phase and the surrounding phases have only a small effect on the plastic deformation. Thus, the measured hardness value can reflect the real hardness of the phase. However, the elastic deformation zone induced by the indenter is always beyond the measured phase. As a result, the

Table 4 Critical normal load and plastic shear strain in parallel scratch test Coating

Test

Critical load (N)

Critical plastic shear strain

Stellite 790

Run 1

65 71 79 77

0.27 0.28 0.30 0.29 0.28

56 57 49 47

0.27 0.27 0.25 0.25 0.26

Run 2 Average Stellite 1

Run 1 Run 2

Average

Fig. 10. Corrosion resistance of W-containing Stellite 6 and Stellite 12 and Mo-containing Stellite 706 and Stellite 712.

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Fig. 11. Positions of measuring hardness and elastic modulus for Stellite 190.

Table 5 Hardness measured from various phases and the Young’s modulus of Stellite alloys Alloy

Co matrix hardness (kg/mm2 )

Eutectic hardness (kg/mm2 )

Carbide hardness (kg/mm2 )

Young’s modulus (GPa)

Stellite 1 Stellite 701 Stellite 190 Stellite 790

645 550 490 600

970 915 1030 910

1485 1555 1760 1535

250 235 240 235

Fig. 12. Positions of measuring hardness and elastic modulus for Stellite 790.

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differences in elastic moduli between the measured phase and the surrounding phases have a significant effect on the elastic deformation, thus, the measured elastic modulus value reflects not only the elastic modulus of the measured phase but also those of the surrounding phases [8]. In our study, the locations of each indentation measurement are indicated by the tip of the arrow with corresponding number in the SEM image (Figs. 11 and 12). The measured results are given in Table 5. The W- or Mo-rich carbides have hardness from 1450 to 1750 kg/mm2 . The hardness of cobalt solid solution ranges from 500 to 650 kg/mm2 . The eutectic phase consisted Cr-rich carbide and cobalt solid solution have hardness about 900–1050 kg/mm2 . The high hardness of the various phase constitutions in the Stellite alloys is important in wear situations. The Young’s modulus of the four alloys is calculated to be about 235–250 GPa indicating the high strength of the Stellite alloys, which is also beneficial for the high wear resistance.

4. Conclusions Diffusion of Mo in cobalt alloys is less sluggish than W since Mo atoms are relatively smaller and much lighter than W atoms. Mo in replace of W in Stellite alloys results in changes in the carbide morphology and increased volume fraction of carbides in the microstructure, compared with the W-containing alloys. Large amount of carbides, high hardness and high Young’s modulus are important to the wear resistance of Stellite alloys. The Mo-containing Stellite alloys exhibit excellent abrasive, and adhesive-wear resistance. Under low-angle impingement conditions, the erosion resistance of W-containing Stellite alloy is enhanced com-

pared with the Mo-containing alloy. On the other hand, the erosion resistance of Mo-containing Stellite alloy is better than that of W-containing alloy under high angle impingement conditions. The W-containing Stellite alloys have better corrosion resistance in oxidizing 10% HNO3 acid at boiling temperature and the Mo-containing Stellite alloys exhibit exceptional corrosion resistance in reducing 5% HCl acid at 40 ◦ C and 10% H2 SO4 at 66 ◦ C. The critical plastic shear strain obtained by scratch test shows that the Mo-containing Stellite alloys have adequate cracking resistance compared with their counterpart W-containing Stellite alloys. References [1] Elwood Haynes, Metal Alloy, US Patent no. 873,745, December 17, 1907. [2] J.M. Drapier, A. Davin, A. Magnee, D. Coutsouradis, L. Habraken, Wear 33 (1975) 271–282. [3] J.B.C. Wu, D. Raghu, B. McKee, US Patent no. 6,479,014, November 12, 2002. [4] Stellite 700 Alloys—A new Stellite alloy series that resists wear and corrosion, Deloro Stellite Alloy Datasheet No. 2050. [5] J.I. Mickalonis, A.W. Bowser, Report WSRC-TR-2000-00289, Westinghouse Savannah River Company, Aiken, SC 29808, October 30, 2000. [6] R.A. Leishear, D.B. Stefanko, Report WSRC-RP-2001-00605, Westinghouse Savannah River Company, Aiken, SC 29808, May 10, 2002. [7] Y. Xie, H.M. Hawthorne, Wear 240 (2000) 65–71. [8] Y. Xie, H.M. Hawthorne, Surf. Coat. Tech. 141 (2001) 15–25. [9] Y. Xie, H.M. Hawthorne, Surf. Coat. Tech. 155 (2002) 121–129. [10] D. Klarstrom, P. Crook, J. Wu (Eds.), Metallography and Microstructures of Cobalt and Cobalt Alloys, ASM Metals Handbook, vol. 9, ASM International, Metals Park, OH, USA, 2004. [11] K.C. Antony, J. Metals (February) (1983) 52–60. [12] J.B.C. Wu, New alloy opportunities, Deloro Stellite Memorandum, August 13, 1992.