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Cobalt–chromium alloys are often employed in those environments that require reliable wear and friction properties. Cold Gas Dynamic Spray offers the opportunity to obtain good quality deposits of Stellite-6, that can be successfully used in harsh environments, where good surface performance, in terms of wear resistance, is required. It is also well-known that Stellite-6 is subjected to several physical changes at the interface during dry sliding, that are often related to the loading conditions. As a consequence, wear behavior of this alloy can undergo some variations that linear models are not able to capture, since micro-structural modifications occur during operation. To better understand the wear mechanisms of cold-sprayed Stellite-6 coatings together with the occurring physical phenomena, a systematic experimental study was performed, in fact, to date, no such in-depth tribological studies have been performed. Tests were conducted under combinations of two sliding speeds (0.1 and 0.5 m/s) and four contact pressure in the range of 2-5 MPa. In low-speed tests, abrasive wear is evident, where detachment and pull-out phenomena mainly affect the worn surface of coatings. On the other hand, subsurface cracking was observed in high-speed tests, as well as evidence of plastic deformation on the wear surface. These results suggest that observed wear mechanisms are more likely a consequence of adhesive wear. Unique to this study, the cross-sectional nano-indentation tests showed how the stiffness of the coating, near to wear interface, increases significantly in the case of the lowest value of sliding speed (i.e., v = 0.1 m/s), whereas tends to decrease at high speeds, i.e., v = 0.5 m/s, as a consequence of the formation of subsurface cracks into the coating. [ABSTRACT FROM AUTHOR]

In this paper, the performance of hardsurfacing Cobalt alloy (Stellite 6) and Nickel alloy (Colmonoy 88) was tested in tribological slurry conditions by using a pot tester. Thermal spraying powders were deposited on AISI 316L substrates by using a high-velocity oxy-fuel (HVOF) technique. Wear experiments are carried out using a lab-scale pot tester at low velocities like 1.81, 2.71, 3.61, and 4.59 m/s. The high mass flux of ashes (ranging 30–60 wt%) is used to produce severe accelerated conditions. The effect of the size fraction of eroding particles was evaluated by preparing the multi-sized slurries of size fractions (ASTM: −200, −140 + 200, −100 + 140, and −60 + 100). Results show that the microhardness of AISI 316L was improved by the HVOF depositing of Cobalt alloy-6 and Nickel alloy-88. The average surface microhardness of Cobalt alloy-6 was found lower (439 ± 19 HV1000) than the Nickel alloy-88 coating (601 ± 11 HV1000). Erosion performance of coatings was found to be increased with an increase in the value of the velocity, time, mass flux, and weighted mean size of eroding particles. Results show that the Ni-alloy improved the wear resistance of AISI 316L by 2.03 ± 0.021 times in fly ash slurry conditions and 3.21 ± 0.035 times in bottom ash slurry conditions. However, the Co-alloy was beneficial in reducing the wear of AISI 316L by 1.48 ± 0.019 times in fly ash and 2.50 ± 0.032 times in bottom ash slurry conditions. Moreover, the maximum wear of AISI 316L steel was observed at an impingement angle of 30° whereas 60° and 45° for Ni-alloy and Co-alloy coatings respectively in fly ash conditions. Although, the AISI 316L and Ni-alloy coating show maximum wear at the same impingement conditions under the bottom ash slurry conditions. However, the Co-alloy coating showed maximum wear at 60° in bottom ash slurry conditions.

In this paper, the performance of hardsurfacing Cobalt alloy (Stellite 6) and Nickel alloy (Colmonoy 88) was tested in tribological slurry conditions by using a pot tester. Thermal spraying powders were deposited on AISI 316L substrates by using a high-velocity oxy-fuel (HVOF) technique. Wear experiments are carried out using a lab-scale pot tester at low velocities like 1.81, 2.71, 3.61, and 4.59 m/s. The high mass flux of ashes (ranging 30–60 wt%) is used to produce severe accelerated conditions. The effect of the size fraction of eroding particles was evaluated by preparing the multi-sized slurries of size fractions (ASTM: −200, −140 + 200, −100 + 140, and −60 + 100). Results show that the microhardness of AISI 316L was improved by the HVOF depositing of Cobalt alloy-6 and Nickel alloy-88. The average surface microhardness of Cobalt alloy-6 was found lower (439 ± 19 HV 1000) than the Nickel alloy-88 coating (601 ± 11 HV 1000). Erosion performance of coatings was found to be increased with an increase in the value of the velocity, time, mass flux, and weighted mean size of eroding particles. Results show that the Ni-alloy improved the wear resistance of AISI 316L by 2.03 ± 0.021 times in fly ash slurry conditions and 3.21 ± 0.035 times in bottom ash slurry conditions. However, the Co-alloy was beneficial in reducing the wear of AISI 316L by 1.48 ± 0.019 times in fly ash and 2.50 ± 0.032 times in bottom ash slurry conditions. Moreover, the maximum wear of AISI 316L steel was observed at an impingement angle of 30° whereas 60° and 45° for Ni-alloy and Co-alloy coatings respectively in fly ash conditions. Although, the AISI 316L and Ni-alloy coating show maximum wear at the same impingement conditions under the bottom ash slurry conditions. However, the Co-alloy coating showed maximum wear at 60° in bottom ash slurry conditions. [ABSTRACT FROM AUTHOR]

Hardfacing is an effective method for restoring the damaged part. Herein, cobalt‐based superalloy Stellite‐6 is deposited on A36 substrate using gas metal arc welding process. The mechanical properties, microstructure, and corrosion behavior of Stellite‐6 overlays on A36 substrate are examined. Defect‐free hard overlays are obtained, and a dilution ratio of 19.9% is achieved with weaving technique. X‐ray diffraction analysis reveals the presence of M7C3, M23C6, M6C, and martensite in the Stellite‐6 hard overlays. The microstructure of the deposit contains martensite, carbides, and interdendritic precipitates while the mean hardness is 471 HV0.5. Tensile testing with specimens extracted on the longitudinal (LD) and transverse (TD) orientation of the stellite‐6 deposit shows a strength above 950 MPa along with the brittle nature of fracture. Grain growth is epitaxial and shows dendritic morphologies. The higher fraction of high‐angle grain boundaries improves the tensile strength of the deposit and interface region. The localized corrosion behavior of stellite‐6 hard overlays is found to be excellent, and the corrosion analysis exhibits a corrosion rate of 0.10 mils per year (mpy). The outcomes of the study highlight the feasibility of Stellite‐6 hard overlays for extending the service life of A36 steel in chloride environments. [ABSTRACT FROM AUTHOR]

Cladded Stellites on steel substrates are utilized vastly in harsh environments. Since the plasma nitriding process can raise the tribological behavior of the material surface. Thus, the main purpose of this paper is to investigate tribological and corrosion characteristics of nitrided Stellite 6 and 12 that were cladded through a plasma-transferred arc (PTA) method on stainless steel 410. A Pin-on-disk tribometer, Tafel polarization, and electrochemical impedance spectroscopy were utilized to study the properties of various coatings. The X-ray diffraction patterns indicated that both cladded coatings mainly included solid solution phase (γ-Co) and chromium-rich carbide phases. However, after the plasma nitriding (PN) process, nitrided phases such as CrN, CoN, and W2N were found at the surface of coatings. The PN process caused a reduction in wear rates for both Stellite coatings by about 89–98%. This reduction was due to the hardness increase and the friction coefficient decrease. However, this PN process increased the corrosion rate of Stellites in a 3.5% wt NaCl solution. Such behavior was based on forming brittle and hard nitrided phases that promote surface cracks and open the path for ion corrosion penetration.

Nowadays, the utilization of magnesium (Mg) alloys is increased due to its various beneficial characters such as better thermal conductivity, minimum density, less weight, superior strength, good dimensional, stability nature, etc. They are used in both mechanical as well as electrical industries. In the same way, it is used in electronic industries for its perfect capability of damping and electromagnetic shielding offers noise reduction. Although, they have a lot of advantages some disadvantages surround them. Mg alloy easily corroded when used at certain wet zones and the poor wear behavior may suppress the service life of Mg alloys. To overcome such issues, a surface coating approach is to be carried out on Mg alloy AZ91D using stellite powder to assist with NiCr pre-coating. The coated Mg alloys are studied for their porosity (%), corrosion rate (mm/year), and wear loss (mg). The experimental study is planned by Response Surface Methodology (RSM) using Box Behnken Design (BBD). The outcomes are validated by Hybrid Deep Belief Network (DBN) based Mayfly optimization approach in MATLAB version 2020a. The minimum porosity, corrosion rate and wear loss obtained for the coated AZ91D Mg alloy is 0.588%, 1.302 mm/year, and 0.719 mg. Besides, the hybrid DBN-based predicted outcomes are closer to the experimental outcomes.

In this study, Stellite 6 and Stellite 12 claddings were deposited on 56NiCrMoV7 hot-worked die steel using the Tungsten Inert Gas (TIG) welding process. Initially, single tracks were deposited using a full factorial design varying two process parameters - current (65, 75, and 85 A) and scan speed (400, 600, and 800 mm/min). The single tracks were then optimized based on dilution rate and microhardness. The optimized parameters were used to develop Stellite 6 and 12 multi-pass clad layers on 56NiCrMoV7. The evaluation of the microstructure of the claddings was done using an optical microscope, XRD, FESEM, EDX, and EBSD. The microstructure of the claddings consists of α-Co matric with M 7 C 3 , M 23 C6, and W 2 C carbides precipitated along the grain boundaries. Further microhardness and tribological performance were analyzed using Vicker's hardness tester and ball-on-disc tribometer, respectively. Stellite 12 claddings exhibited superior wear resistance and indicated reduced wear volume by 51.3 % at room temperature, 69.2 % at 450 °C, and 32 % at 550 °C compared to the base material and 53.2 % at room temperature, 37.3 % at 450 °C, and 24.1 % at 550 °C compared to Stellite 6 claddings. The KAM mapping showed higher dislocation density for Stellite 12 cladding. The combination of finer grains, higher hardness, and higher dislocation density resulted in higher wear resistance. • Stellite 6 and Stellite 12 claddings were fabricated with optimized process parameters on 56NiCrMoV7 die-steel substrate using a TIG cladding. • Variation in hardness occurred in multi-track clads compared to single tracks. • The friction coefficients of the cladded layers were reduced with an increase in temperatures. • Stellite 12 claddings exhibited reduced wear volume by 51.3 % RT, 69.2 % at 450 °C, and 32 % at 550 °C compared to the base material. • The FESEM images of worn-out surfaces revealed that adhesive and abrasive wear takes place and is further aggregated by oxidation at higher temperatures. [ABSTRACT FROM AUTHOR]

The accuracy of a mathematical model describing the influence of the cutting conditions on the surface roughness of stellite in turning is investigated. The literature on the roughness of stellite is analyzed. Experimental data are selected to verify the accuracy of the proposed model. Criteria for assessing the accuracy are determined. The influence of the cutter's vertex angle on the surface roughness of the machined alloy is compensated. On the basis of the results, the deficiencies of the model are identified and directions for improvement are noted. [ABSTRACT FROM AUTHOR]

Stellite-21/WC nanopowders were deposited on Inconel through vibration-assisted laser cladding with different laser parameters. Optical and scanning electron microscopy, hardness measurements, and wear characterizations were employed to understand the microstructural and mechanical behaviors of the nanocomposites. Results showed that varying the cooling rate exerted remarkable effects on the microstructure of the as-cladded composites. Moreover, increasing the laser power from 150 W to 250 W increased the heat input and the dilutions. Dilution was affected by the scanning rate and powder feeding rate at a high laser power of 250 W. When WC nanoparticles were added as reinforcement, the dilution magnitude intensified while the hardness value increased from HV 350 to HV 700. The wear characterizations indicated that the composites containing 3wt% WC nanoparticles possessed the highest wear resistance. [ABSTRACT FROM AUTHOR]

The paper presents the results of structural investigations of a laser-welded layer in the Inconel 625 and Stellite 6 alloy matrix with the addition of WC. The tests involved the use of on plates made of low-alloy steel S355J2C+N. The filler materials used in the tests were nickelbased (Inconel 625) or co-balt-based powders (Stellite 6) having various contents (0-25%) of spherical tung-sten carbide (WC) used as the reinforcing phase. The granularity of the powders was re-stricted within the range of 45 µm to 90 µm. The cladding tests involved making multi-layer overlay welds. Next, non-destructive (VT and PT) and destructive (macro, hardness) tests were carried out. The macroscopic metallographic tests revealed that the three-layer Inconel 625-based overlay welds (with a WC content of up to 15%) had the correct shape and were characterized by full fusion into the base material, ensuring the continuity of the overlay weld. In the case of the Stellite 6-based overlay welds containing 10% of WC, it was possible to observe a throughcrack of the overlay welds. The crack was formed in the first layer (as a result of stresses during the solidification of the liquid metal) and developed during the deposition of subsequent layers. The macrostructural tests were supplemented with microstructural observations and microanalysis of the chemical composition (EDS). The above-named tests revealed the good stirring of alloying elements in the overlay weld, indicating the high homogeneity of the overlay weld. The hardness obtained on the surface of the padding weld was 600 HV1. [ABSTRACT FROM AUTHOR]

The article presents the current state of the use of cobalt in various industries and its possible elimination in the field of welding. In this area, it is used in the form of cobalt alloys which are affected by various mechanical and chemical influences. These alloys are produced by the secondary metallurgical area. The primary area is the chemical industry which reserves all the use of cobalt for the production of Li-ion batteries for electric vehicles at an increasing price. The primary area is the chemical industry which reserves all the use of cobalt for the production of Li-ion batteries for electric vehicles at an increasing price. As it becomes unprofitable for other areas, its savings must be sought. From the point of view of welding technology, the solution could be the use of unconventional and hybrid methods that do not require filler material for the connection of elements. Based on these aspects, the methods of a laser beam, electron beam, and combination of a laser beam with TIG method were selected. Verification of the suitability of the use of the mentioned methods was performed on the abrasion-resistant alloy Stellite 6B. The quality of the joints was evaluated by destructive methods such as microstructure, macrostructure, microhardness, and EDS. After evaluating the results of all samples, the hybrid method in the combination of a laser beam and TIG method seems to be the best. [ABSTRACT FROM AUTHOR]

Plasma transferred arc welding (PTAW) is one of the outstanding overlay coating technique used in numerous industries including pressure vessel, automotive, chemical, nuclear, etc. owing to its superior characteristics, low dilution and high efficiency in the coating. In the present investigations, an effort is made to investigate the multitrack overlay deposition by PTAW on 16 mm thick 316 L plate by CO-Cr alloy. Samples are produced under different processing conditions, as per full factorial central composite design of experiment with preheating of 200°C by maintaining the interpass temperature 250 °C. Effects of transferred arc current, welding travel speed, powder feed rate, welding oscillation speed and stand-off distance on weld bead shape parameters comprising of width of deposition and reinforcement are presented and discussed based on experimental observations and fitted model. Relationship between the input process parameters with deposition characteristics is also presented in the form of regression equation. Lower current (100–120 A), intermediate travel speed (120–140 mm/min), intermediate powder feed rate (12–14 gms/min), lower oscillating speed (450–550 mm/min) and lower stand-off distance (6–8 mm) would give better deposition characteristics with minimum distortion, less residual stresses, no surface defects with strong metallurgical bond.

AM (Additive Manufacturing) is an increasingly widespread technology in the world. This technique was used in the present work to deposit material on the baseplate in order to find out its ability to repair damaged parts (baseplate material). The material of the baseplate was AKT1TD martensitic steel used in the aviation and energy industry. The steel could suffer from corrosion and erosion, and AM could be the potential method for damaged parts repairing. As a first step, a material similar to the baseplate was deposited. After that, a deposition of a stellite protective layer took place to increase the corrosionerosion resistance. The whole build was observed by OM (optical microscopy) and SEM (scanning electron microscopy) to evaluate the quality of deposition. [ABSTRACT FROM AUTHOR]

To ensure the required surface roughness when machining the Stellite facing of a housing, research attention focuses on the roughing and finishing of Stellite samples by means of a state-of-the-art tool. Practical recommendations are made regarding the manufacture of housings. Deform-3D software is used for experimental determination of the relevant formulas: cutting is simulated by means of the built-in Machining-3D application. [ABSTRACT FROM AUTHOR]

This paper studies establishing an effective deposition technique of cobalt-based Stellite1 on austenitic stainless steel (SS 304) orifice plate component, used in offshore application. Stellite1 hardfacing was done using gas tungsten arc welding (GTAW) and flux-cored arc welding (FCAW) techniques, which resulted in intergranular and longitudinal cracks. In finite element analysis (FEA), a three-dimensional solid model was generated and simulated for spot heat welding using ANSYS software. After simulation for heat distribution with and without weld bordering, crack-free hardfacing of Stellite1 was carried out and validated experimentally using FCAW technique. A control on substrate preheating, thermal insulation, job clamping and cooling time reduced the intersecting cracks. Metallurgical analysis using Optical microscopy, SEM, EDS and XRD along with hardness measurements was done to investigate the mechanisms causing failure. Crack area measurements were done in gradient edge detection technique using MATLAB software and quantified. [ABSTRACT FROM AUTHOR]

The presented results significantly expand the information on the modification of sputtered materials (PG-10K-01 + CrB2 or TiB2) in a plasma stream, which makes it possible to vary the operational parameters of the process for shaping the necessary structure of the protective layer with enhanced performance properties. Thus, if the parts are subject to impact loads, the preferred option is fused coatings in the form of gradient-layer composites. If the parts operate under ordinary abrasive friction, the fusion operation can be excluded from the production process provided that particles are ground during sputtering; in addition, an evenly strengthened coating structure is formed. New compositions of microcomposites for plasma sputtering of coatings are developed for being crushed in the plasma stream, which is followed by the spheroidization of dispersed components; this allows excluding the use of submicron powders during plasma sputtering for they are somewhat difficult to supply along powder-feeding lines and improving the wear resistance of the composites by 7.35–27.33%. [ABSTRACT FROM AUTHOR]

We report on the fabrication of rotationally symmetrical geometries with ultrashort laser pulses impinging the constant rotating workpiece tangentially. This particular micro machining of geometry is referred to as laser turning, upon which the laser beam is, in general, oriented perpendicularly to the rotational axis and tangentially to the work piece. The target geometry is realized by moving the constantly rotating specimen according to the specified geometry along the focused laser spot. In contrast to mechanical turning, laser turning induces a minimized amount of lateral forces through friction that might deteriorate the geometry. In this study, laser turning is examined for stellite as a function of pulse energy and pulse length in the range between 240 fs and 10 ps. Shorter pulse lengths and higher pulse energies increase the achievable ablation rate, while in the pulse length regime of below 1 ps lower roughness is achieved. With a maximum ablation rate of 1.27 mm?/min and a minimum roughness of 0.17 µm, laser turning combines both, fast processing and high surface quality. [ABSTRACT FROM AUTHOR]

The article considers the results of metallographic studies and endurance tests with regards to droplet impact erosion of nitrided samples of 12Kh13, 20Kh13, 15Kh11MF, and EI961 blade steels, as well as stellite. Data on the effect of ion-plasma nitriding of the surface on the erosion resistance of the tested blade steels are presented. The authors consider the prospects of using the applied hardening method as alternative passive protection of the rotor blades of wet-steam turbine stages or a method of a joint application of nitriding with widely used stellite overlays. [ABSTRACT FROM AUTHOR]

The protection system against material degradation at elevated temperatures is gaining technological importance. Three cobalt-based coatings were developed on SAE213-T91 boiler steel using the detonation gun method. The oxidation performance of all the specimens was examined for 50 cycles of 1 h heating at 900 °C in a thermal cyclic furnace and 20 min cooling at 25 °C outside the furnace. All the oxidised samples were investigated through SEM/X-ray mapping and an X-ray diffraction analyser. All the specimens obeyed the parabolic rate law of oxidation approximately. The oxidation resistance of Stellite 6 coating was found to be superior among all coated and uncoated specimens due to the growth of protective oxide Cr 2 O 3 and CoCr 2 O 4 spinel in it. The development of Ni, Cr and Co oxides in Stellite 21 coating resulted into its slow oxidation rate, whereas the growth of the non-protective WO 3 and CoWO 4 phases in WC-Co coating caused its rapid oxidation. • Cyclic oxidation behaviors of three Co-based coatings have been studied using a thermal cyclic furnace at 900 °C. • Co with WC (WC-Co); Co with Cr, W, Si, C (Stellite 6); Co with Cr, Mo, Ni, Fe, C, Si (Stellite 21) have been studied. • Stellite 6 lowers the oxidation rate due to the growth of protective Cr 2 O 3 and CoCr 2 O 4 phases. • The growth of Ni, Cr and Co oxides in stellite 21 coating enabled the slow rate of oxidation of this coating. • The oxidation of the WC-Co coating was rapid due to the formation of the non-protective CoWO 4 phase. [ABSTRACT FROM AUTHOR]

This work is based on the cyclic oxidation of the composites deposited on a TA10 titanium alloy by means of the laser alloying of the Stellite SF 12-SiC-TiC-Y2O3 mixed powders. It was noted that an improvement of oxidation resistance was obtained for such laser alloyed composites that compared with the TA10 substrate. Scanning electron microscope (SEM) images indicated that lots of the amorphous/nanocrystalline phases were produced, improving the oxidation resistance of such laser alloyed composites, which owned the better oxidation resistance than that of the laser alloyed composites free of Y2O3 due to the formation of the denser and finer-grain scale. The effect of Y2O3 on the microstructures and oxidation behavior of the laser alloyed composites on the TA10 titanium alloy is discussed in detail. [ABSTRACT FROM AUTHOR]

Valve seats of internal combustion engines are used in cylinder heads, due to increase the strength and abrasion resistance of the intake and exhaust valve seats. Valve seats are made from alloy cast iron, high-alloy steels, wear-resistant alloys and seats made using powder metallurgy. The valve seats are pressed into the head. Some materials must be deep-frozen before pressing to reduce the pressing overlap. The overlap is large, in the order of tenths of a millimetre, due to the guaranteed seating of the seat in the cylinder head. The amount of overlap depends also on the material of the cylinder head. Liquid nitrogen (about -196 °C) is used for freezing. In our case, these are monometallic seats, made by technology of precision casting from cobalt alloys. A particular type of cobalt alloy uses the trade name STELLITE. [ABSTRACT FROM AUTHOR]

Stellite 3 is an excellent wear-resistant material owing to its high carbon content, but has the serious problem of cracking during laser processing due to its extreme brittleness. To solve this problem a novel composite Stellite alloy which has comparable hardness and sliding wear resistance to Stellite 3 was prepared on 316 stainless steel by laser cladding using a mixed powder of major Stellite 3 and minor Stellite 21. The microstructure and phase of the new alloy was analysed by using a scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The hardness, wear resistance and single-tip scratching behavior of the specimens were investigated experimentally. The results showed that no pores and cracks were formed in the new Stellite alloy laser clad layer. The main reinforcement phases in the microstructure consist of (Co,W)3C, Cr23C6, Cr7C3, Co3Mo. The hardness and wear resistance were improved not only by the carbides, but also by the Co3Mo in this new Stellite alloy. The butterfly valve switch test demonstrated that the sealing properties of the new alloy were significantly better than the existing Stellite 6 alloy. [ABSTRACT FROM AUTHOR]

Abstract Cold spraying of Stellite 21 powder on low carbon steel is performed to investigate the effect of traverse speed on the deposition efficiency (DE) of high-temperature alloys. Based on the simulation of particles' impact temperature and velocity, the initial experiments are performed at different gas pressures (32 and 40 bar), temperatures (800 and 730 °C), and stand-off distances (10, 25, and 40 mm) at a constant traverse speed of 20 mm·s−1. The experiments showed that high pressure, temperature, and short stand-off distance are preferred. The wipe test results indicated a potential of high DE at lower deposition flux over the substrate surface area at the first layer deposition. Thus, new tests are carried out at different traverse speeds ranging from 20 to 400 mm·s−1 to adjust the deposition flux over the surface area of the substrate. The effect of traverse speed on DE and coating characteristics such as porosity and microhardness is studied as well. The results showed that the lower DE at lower traverse speed is related to the erosion of bonded particles due to the subsequent particles' impact. By increasing the traverse speed from 20 to 300 mm·s−1, the DE increased more than twice. Induction time extension at higher traverse speed led to lower DE at 400 mm·s−1. Graphical abstract Unlabelled Image Highlights • Stellite 21 coating was successfully cold sprayed onto the steel substrate. • The high temperature of the gas was more favorable for Stellite 21 deposition. • Varying the traverse speed influences the DE, microhardness, porosity, and roughness. • Lower DE at low traverse speed was related to the erosion of bonded particle. [ABSTRACT FROM AUTHOR]

Cobalt-based alloys such as Stellite 3 and Stellite 6 are widely used to protect stainless steel surfaces in primary circuit nuclear reactor applications where high resistance to wear and corrosion are required. In this study, self-mated sliding wear of Stellite 3 and Stellite 6 consolidated by hot isostatic pressing were compared. Tests were performed with a pin-on-disc apparatus enclosed in a water-submerged autoclave environment and wear was measured from room temperature up to 250 °C (a representative pressurized water reactor environment). Both alloys exhibit a microstructure of micron-sized carbides embedded in a cobalt-rich matrix. Stellite 3 (higher tungsten and carbon content) contains M 7 C 3 and an eta (η) -carbide whereas Stellite 6 contains only M 7 C 3. Furthermore, the former has a significantly higher carbide volume fraction and hardness than the latter. Both alloys show a significant increase in the wear rate as the temperature is increased but Stellite 3 has a higher wear resistance over the entire range; at 250 °C the wear rate of Stellite 6 is more than five times that of Stellite 3. There is only a minimal formation of a transfer layer on the sliding surfaces but electron backscatter diffraction on cross-sections through the wear scar revealed that wear causes partial transformation of the cobalt matrix from fcc to hcp in both alloys over the entire temperature range. It is proposed that the acceleration of wear with increasing temperature in the range studied is associated with a tribocorrosion mechanism and that the higher carbide fraction in Stellite 3 resulted in its reduced wear rate compared to Stellite 6. • Sliding wear tests were conducted at 20–250 °C in lithiated deionised water. • Stellite 3 exhibited much higher wear resistance in comparison to Stellite 6. • Wear resistance was increased through higher carbide content and hardness. • Both alloys underwent fcc-hcp transformation in the cobalt matrix at 20–250 °C. • Tribocorrosion is proposed to cause steep increase of wear at above 150–200 °C. [ABSTRACT FROM AUTHOR]

Abstract Thermally sprayed Stellite 6 coatings offer high wear and corrosion-erosion resistance at high temperature but the creep behaviour of such coatings is not well understood. In this paper, the microstructure and creep behaviour of a HVOF Stellite 6 coating was investigated. The coating was tested in the as-sprayed condition and after an isothermal heat-treatment of 1 h at 1050 °C. The as-sprayed coating comprised FCC Co-Cr-W solid solution with 2–3 wt% HCP phase, whereas the heat-treated coating comprised FCC Co-Cr-W solid solution, 11 wt% M 23 C 6 carbides and 7 wt% M 7 C 3 carbides. Small punch creep tests were conducted at 700 °C on the as-sprayed and heat-treated coatings. The heat-treated coating exhibited minimum steady-state strain rates approximately 2 orders of magnitude lower than the as-sprayed coating when tested at similar loads, leading to times to failure approximately two orders of magnitude longer for the heat-treated coating within the selected load range. Fracture analysis showed cracking along powder particle boundaries was the main mode of cracking in the as-sprayed coating whereas for the heat-treated coating, fracture along the carbide / matrix interface was the main fracture mechanism. [ABSTRACT FROM AUTHOR]

Stellite 6 alloy coatings were produced by the plasma arc cladding process, and the as-cladded samples were treated with isothermal aging at 700 °C for up to 1000 hours to evaluate the effect of long-term isothermal aging on microstructural evolution. The results show that the microstructure of the cladding consists of γ-Co solid solution and carbide-based eutectics. During aging, γ-Co gradually transformed to ε-Co, and the volume fraction of ε-Co reached to about 75 pct as the aging time was 1000 hours. On other hand, the M23C6 fine particles were found to gradually precipitate in the solid solution during aging. Approximately pairwise proportional relationships were found among the fraction of precipitated M23C6 carbides, the volume fraction of martensite transformation, and the stacking fault energy, and they were verified by the statistical results of phase fraction and change of SF possibility calculated through X-ray diffraction patterns. A kinetic model for martensite transformation was also established to explain the change of martensite volume fraction during the aging period and to predict the transformation degree during high-temperature aging. [ABSTRACT FROM AUTHOR]

Abstract Cobalt-based Stellite alloys are widely used in the primary circuit of pressurised water reactors (PWR) to protect valve surfaces against wear and galling in a corrosive environment. In this study, self-mated sliding wear of HIP-consolidated (Hot Isostatically Pressed) Stellite 6 (Co - 27.1 Cr - 1.5 Si - 5.0 W - 0.96 C, in wt%) was investigated. A pin-on-disc apparatus was enclosed in an autoclave for wear testing, which was conducted in lithiated water from room temperature up to 250 °C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction and scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD). The bulk HIPed alloy is predominantly two phase and comprises a cobalt-rich fcc matrix and an M 7 C 3 carbide phase. However, surface grinding prior to wear testing causes a surface layer of the matrix to partially transform to a hcp cobalt-rich phase. The wear (mass loss) is very low below 150 °C but increases by approximately an order of magnitude when the temperature is increased from 150° to 250°C. SEM/EBSD reveals sub-surface damage and partial fcc to hcp transformation of the Co-rich matrix phase to a depth of ~ 15 µm in the disc. However, there is little change in transformation behaviour and depth with temperature and this is not regarded as a significant cause of the increased wear. The order of magnitude increase in wear is instead ascribed to a tribocorrosion mechanism associated with significantly higher corrosion rates at 250 °C than at 150 °C. As the material removal and factors affecting it are found to be significantly dependent on temperature, this work demonstrates the necessity of conducting assessments of materials for use in PWR environments under representative conditions. Highlights • Wear rate increased substantially at above 150 °C in PWR simulating conditions. • Martensitic phase transformation due to wear was found to occur at 20–250 °C. • Wear rate is proposed to increase due to tribocorrosion, i.e., oxide film removal. [ABSTRACT FROM AUTHOR]

Abstract In order to reveal the mechanism of microstructure evolution and crack propagation in laser-deposited Stellite 6 alloys, a quenching thermal fatigue test was conducted. Various detection methods were applied to observe differences between the coatings as deposited and after thermal fatigue. The results showed that the γ → ε martensitic transformation occurred in the as-deposited γ -Co matrix during the thermal fatigue process, driven by a fast cooling and thermal stress. The generated ε -Co phase presented variant selection, obeying Schmidt's law. In the ε -Co phase, the slip activity derived from different 1 1 1 γ 1 1 2 ¯ γ slipping systems that produced stacking faults and planar defects during the phase transformation. In addition, the stacking faults on {1 1 1} γ planes promoted the precipitation of directional M 7 C 3 fine particle carbides. The net-like eutectic structures and γ / ε interfaces acted as paths for thermal crack propagation. Highlights • The mechanism of γ → ε martensitic transformation in Co matrix of Stellite 6 coating during thermal fatigue was revealed. • The crack propagation features of Stellite 6 coating during thermal fatigue were characterized. • The type and formation mechanism of carbides precipitated in Co matrix during thermal fatigue was revealed. [ABSTRACT FROM AUTHOR]

Abstract A design strategy is presented for the development of a laser-deposited Stellite 6 coating with high ductility and low thermal fatigue crack propagation rate. The strategy involved adding titanium to modify carbide morphology, accompanied by an increase in the nickel content to stabilize the γ-Co phase in the matrix. The molar ratios of the titanium to the carbon in coatings were 0.5, 1.0, 1.5, and 2.0. The amount of nickel added to the titanium-modified coatings was 10 and 20 wt%. The net-like M 23 C 6 eutectic carbides in the Stellite 6 coating transformed to isolated TiC particles with increasing molar ratio of titanium to carbon. Adding nickel to the coatings resulted in an increase in the stacking fault energy and the stabilization of the γ-Co phase. Accordingly, the ductility increased with increasing nickel concentration. The modified coating, named 1.0Ti20Ni, in which the molar ratio of titanium to carbon was 1.0 and the nickel concentration was 20 wt%, exhibited the highest elongation to fracture (~12%), along with a high strength (1230 MPa) in all coatings, Besides, the 1.0Ti20Ni coating revealed the lowest thermal fatigue crack propagation rate. Graphical abstract Unlabelled Image Highlights • A strategy is given to improve the ductility and thermal fatigue cracking feature of Stellite 6 coating by adding Ti and Ni. • Ti addition made the M 23 C 6 eutectic carbide change to the TiC carbide; Ni addition stabilized the γ-Co in matrix. • 1.0Ti20Ni coating exhibited the highest elongation to fracture and the lowest thermal fatigue crack propagation rate. [ABSTRACT FROM AUTHOR]

Abstract The effects of different cladding materials, processing parameters and heating regimes on the underlying microstructural features and mechanical properties of laser-cladded premium rails were investigated by using a hypereutectoid rail grade as a substrate, which is extensively used in heavy-haul rail systems. Cladding materials of 410L, 420SS, Stellite 6 and Stellite 21 with single and double depositions were considered for the comparative study of different cladding materials and processing parameters. To ensure the constant thickness of the claddings for comparison purposes, transverse speed and powder feed rate were modified concurrently in the ranges of 1000–1200 mm/min and 3–4 RPM, respectively. Two heating conditions, i.e. preheating only (HTA) and a combination (HTB) of preheating and post weld heat treatment (PWHT) were applied after the preferable parameters for each cladding material were obtained. The most suitable cladding material for rail-wheel contact was established by assessing all crucial aspects, i.e. surface defects, hardness, microstructural and mechanical properties. Process parameters for each considered cladding material were determined to achieve no surface defects. For cladding layers, application of HTA was not able to significantly modify the microstructures of the deposits, whereas HTB was observed to cause severe cracks in Co-base alloys, i.e. Stellite 6 and Stellite 21. In the heat affected zones (HAZs), irrespective of the cladding materials, the formation of untempered martensite was not avoided by the application of preheating at 350 °C. Consequentially, cracking in the HAZ was observed. An uncracked and desirable microstructure in the HAZs was established using HTB, regardless of the depositing materials. The addition of a second layer did not change the thickness of the HAZs but refined the HAZ’s microstructures. Shear punch testing (SPT) and Vickers hardness testing were utilized to characterize mechanical properties for the considered cladding materials and good correlations with the obtained microstructural morphologies were shown. [ABSTRACT FROM AUTHOR]

This study aims to understand the oxide scale formation on the surfaces of commercial and Mo (10 wt.%)-modified Stellite 12 hardfacings deposited on 4140 steel. For this purpose, samples of the hardfacings were isothermally oxidized in air at various temperatures in between 500 and 900 °C for 30 h. As a result of changes in the microstructure upon Mo addition, about 50% higher mass gain at all oxidation temperatures was observed. The characteristics of the developed oxide scales were examined by utilizing Raman spectroscopy, scanning electron microscopy and x-ray diffraction analysis. The oxide scale formed over Stellite 12 consisted of CoO-, Co3O4- and Cr2O3-type oxides, while additionally CoMoO4-type complex oxide was detected on Mo-modified hardfacing, more likely at the alloy-scale interface. On the basis of findings of isothermal oxidation tests, the formation mechanisms of this complex oxide imposing better wear resistance as reported in our previous studies were explained during high-temperature wear testing, i.e., dynamic oxidation conditions. [ABSTRACT FROM AUTHOR]