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Papers by Matthew Yao

Journal of Materials Engineering and Performance, 2021

Wear-resistant cobalt-based alloys generally contain high chromium content so that the carbides i... more Wear-resistant cobalt-based alloys generally contain high chromium content so that the carbides in these alloys are mainly Cr-rich except the alloys which contain very high tungsten content. In this research an abnormal cobalt-based with very low chromium but very high tungsten content is created, in order to avoid Cr-rich carbide precipitation. The focus of the research is to investigate the effect of chromium and tungsten contents on the formation of carbides in cobalt-based alloys. Furthermore, the influence of heat treatment on the microstructure change of the new alloy is studied by aging the alloy at a temperature of 900 °C for 24 hours or at 980 °C for 350 hours. It is found that the heat treatments can cause dissolution of W-rich carbides and promote graphite separation from the carbides in the new alloy. The dry-sliding wear tests show that the presence of graphite improves the tribological properties of the new alloy due to reduction in friction. Aging time has influence on the dissolution of the carbides in the new alloy, thus affecting the amount of graphite precipitation during the heat treatment. Stellite 80, having the same carbon content but different chromium and tungsten contents with the new alloy, is studied in parallel for comparison.

Journal of Materials Engineering and Performance, 2021

Metal-cored wire provides material selection diversity for applications where there are no commer... more Metal-cored wire provides material selection diversity for applications where there are no commercially available solid wires. This is becoming more desirable with additive manufacturing in particular. This paper investigates the application of a Stellite 6 metal-cored wire in a Wire and Arc Additive Manufacturing of thin-wall structures with a thickness of ~ 3.5 mm using the Gas Tungsten Arc Welding (GTAW) process. Preliminary experiments by continuous travel of the GTAW torch and constantly feeding the wire showed an unreliable deposition of consecutive layers. A new method was developed featuring a Coordinated Heat and Feed (CHF) printing strategy. This method provided improved control of the weld pool and minimized the nonuniform spread of the molten material. The CHF printing strategy was implemented by dividing the print path into small segments, each of which underwent three sequential steps: (1) the weld pool formation, (2) the wire insertion into the weld pool, and (3) the cooling of the molten pool. A 7-layer thin-wall structure fabricated by this CHF printing strategy was compared with a continuously printed specimen. The results showed that the CHF specimen presented a better geometrical quality and macrostructure uniformity. Both samples had similar microstructural features and properties.

Thermal Spray 2021: Proceedings from the International Thermal Spray Conference, 2021

As steam power plants continue to move towards higher operating temperatures in order to improve ... more As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have e...

Journal of Materials Science and Engineering B, 2013

Selected Stellite alloys, which are currently or potentially employed in an environment involving... more Selected Stellite alloys, which are currently or potentially employed in an environment involving erosion, are studied under solid-particle erosion test. These alloys include high-carbon Co-Cr-W system and low-carbon Co-Cr-Mo system. Two particle impact velocities (84 m/s and 98 m/s) and two impingement angles (30° and 90°) are used in the test. It is shown that Stellite alloys are more resistant to erosion at 90° impingement angle than at 30° impingement angle and the weight losses of Stellite alloys increase with the particle impact velocity. The erosion resistance of Stellite alloys is controlled mainly by their carbon content, but tungsten and molybdenum contents also play an important role in the erosion resistance of these alloys, because these elements determine the volume fractions of carbides and intermetallic compounds in the alloys. The eroded surfaces are analyzed using SEM to further understand the erosion test results.

Journal of Materials Engineering and Performance, 2020

Elements tungsten and molybdenum are the key constituents of Stellite alloys as solutes to streng... more Elements tungsten and molybdenum are the key constituents of Stellite alloys as solutes to strengthen the solid solution matrix. Meanwhile, they also promote formation of W-rich and Mo-rich carbides when present in large quantities in high-carbon Stellite alloys. Extensive studies on tungsten Stellite alloys, in particular, Stellite ® 6, have been reported in the literature; however, molybdenum Stellite alloys, especially, the influence of Mo content on the performance of the alloys, have been rarely investigated. In this research, the tribological performance of three molybdenum Stellite alloys with varying C and Mo contents is studied using pin-on-disk dry-sliding wear test at ambient temperature, focusing on the effect of test duration and interrupted test on the wear behavior of the alloys. The worn surfaces are examined using SEM/EDS, and the analyses of the surface morphologies help explore the wear mechanisms of these alloys in dry-sliding condition. A wear model is developed for molybdenum Stellite alloys, based on the Archard equation in check with the wear test data, using hardness as a controlling factor.

Materials at High Temperatures, 2019

ABSTRACT The performance of S16, a CoCrWC alloy, was evaluated in superheated steam (SHS) at 625°... more ABSTRACT The performance of S16, a CoCrWC alloy, was evaluated in superheated steam (SHS) at 625°C and 800°C (at 0.1 MPa), and in supercritical water (SCW) at 625°C and 26 MPa for 500 hours. S16 experienced weight loss under the SHS conditions (625°C and 800°C) and weight gain under the SCW condition. Weight loss was associated with scale spallation and pitting while oxide growth on the surface contributed to weight gain. Formation of CoCr2O4 and Cr2O3 was observed in the oxide layer of samples tested under the three conditions, however, the Cr-rich oxide on SCW tested samples was found to be continuous. A thin, continuous Co3W subscale was believed to have provided an oxidation barrier under the SCW test condition. Based on the limited weight change and surface oxide formation, S16 has the potential to be used as a coating material for ultra-supercritical power generation systems at temperatures up to 625°C.

Surface and Coatings Technology, 2019

Abstract In this study, the oxidation behavior of alloy CoCrMoSi (T14) and alloy CoCrNiMoSi (T19)... more Abstract In this study, the oxidation behavior of alloy CoCrMoSi (T14) and alloy CoCrNiMoSi (T19) was evaluated in superheated steam (SHS, 0.1 MPa) and supercritical water (SCW, 26 MPa), at temperature of 625 °C for 500 h. After exposure to both conditions, the as-exposed surfaces and cross sections were evaluated using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). The identification of phases was carried out based on combined EDS and X-Ray Diffraction (XRD) analyses. The results showed that both T14 and T19 formed very complex oxides on the surface after exposure. And they experienced weight gain under SHS condition, and weight loss under SCW condition. However, T19 had much smaller weight changes after both tests. Weight loss was due to the oxide scale spallation and possibly volatilization of MoO3 and Cr2O3, in addition to preferential attack and subsequent degradation of Laves phase in both alloys. Due to Ni addition acting as FCC stabilizer, the possible SiO2 formation as a subscale, and smaller Laves phase, alloy T19 was found to be superior to T14 as potential coating material for steam power generation system at temperatures up to 625 °C, under SHS and SCW conditions.

Metallography, Microstructure, and Analysis, 2015

ABSTRACT

Wear, 2015

A high-tungsten Stellite alloy is developed for wear and erosion resistance application in this r... more A high-tungsten Stellite alloy is developed for wear and erosion resistance application in this research, taking the advantage of tungsten in Stellite alloys. The microstructure of this alloy is analyzed using SEM/EDX and XRD. The sliding wear resistance of the alloy is evaluated on a pin-on-disc tribometer and the solid-particle erosion behavior is investigated using a micro-blasting jet machine at two particle impact velocities (84 and 98 m⋅s-1) and two impingement angles (30° and 90°). The experimental results of the alloy are compared with those of well-known wear-resistant Stellite 3 and Stellite 6. The worn and eroded surfaces of the specimens are studied using SEM/EDX to explore the wear and erosion mechanisms of this new alloy. The experimental results show that this novel high-tungsten Stellite alloy has superior sliding wear and solid-particle erosion resistance to Stellite 3 and Stellite 6, owing to the formation of large amounts of large-size W-rich carbides in the alloy.

Advanced Engineering Materials, Jan 29, 2007

Journal of Materials Engineering and Performance, 2021

Wear-resistant cobalt-based alloys generally contain high chromium content so that the carbides i... more Wear-resistant cobalt-based alloys generally contain high chromium content so that the carbides in these alloys are mainly Cr-rich except the alloys which contain very high tungsten content. In this research an abnormal cobalt-based with very low chromium but very high tungsten content is created, in order to avoid Cr-rich carbide precipitation. The focus of the research is to investigate the effect of chromium and tungsten contents on the formation of carbides in cobalt-based alloys. Furthermore, the influence of heat treatment on the microstructure change of the new alloy is studied by aging the alloy at a temperature of 900 °C for 24 hours or at 980 °C for 350 hours. It is found that the heat treatments can cause dissolution of W-rich carbides and promote graphite separation from the carbides in the new alloy. The dry-sliding wear tests show that the presence of graphite improves the tribological properties of the new alloy due to reduction in friction. Aging time has influence on the dissolution of the carbides in the new alloy, thus affecting the amount of graphite precipitation during the heat treatment. Stellite 80, having the same carbon content but different chromium and tungsten contents with the new alloy, is studied in parallel for comparison.

Journal of Materials Engineering and Performance, 2021

Metal-cored wire provides material selection diversity for applications where there are no commer... more Metal-cored wire provides material selection diversity for applications where there are no commercially available solid wires. This is becoming more desirable with additive manufacturing in particular. This paper investigates the application of a Stellite 6 metal-cored wire in a Wire and Arc Additive Manufacturing of thin-wall structures with a thickness of ~ 3.5 mm using the Gas Tungsten Arc Welding (GTAW) process. Preliminary experiments by continuous travel of the GTAW torch and constantly feeding the wire showed an unreliable deposition of consecutive layers. A new method was developed featuring a Coordinated Heat and Feed (CHF) printing strategy. This method provided improved control of the weld pool and minimized the nonuniform spread of the molten material. The CHF printing strategy was implemented by dividing the print path into small segments, each of which underwent three sequential steps: (1) the weld pool formation, (2) the wire insertion into the weld pool, and (3) the cooling of the molten pool. A 7-layer thin-wall structure fabricated by this CHF printing strategy was compared with a continuously printed specimen. The results showed that the CHF specimen presented a better geometrical quality and macrostructure uniformity. Both samples had similar microstructural features and properties.

Thermal Spray 2021: Proceedings from the International Thermal Spray Conference, 2021

As steam power plants continue to move towards higher operating temperatures in order to improve ... more As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have e...

Journal of Materials Science and Engineering B, 2013

Selected Stellite alloys, which are currently or potentially employed in an environment involving... more Selected Stellite alloys, which are currently or potentially employed in an environment involving erosion, are studied under solid-particle erosion test. These alloys include high-carbon Co-Cr-W system and low-carbon Co-Cr-Mo system. Two particle impact velocities (84 m/s and 98 m/s) and two impingement angles (30° and 90°) are used in the test. It is shown that Stellite alloys are more resistant to erosion at 90° impingement angle than at 30° impingement angle and the weight losses of Stellite alloys increase with the particle impact velocity. The erosion resistance of Stellite alloys is controlled mainly by their carbon content, but tungsten and molybdenum contents also play an important role in the erosion resistance of these alloys, because these elements determine the volume fractions of carbides and intermetallic compounds in the alloys. The eroded surfaces are analyzed using SEM to further understand the erosion test results.

Journal of Materials Engineering and Performance, 2020

Elements tungsten and molybdenum are the key constituents of Stellite alloys as solutes to streng... more Elements tungsten and molybdenum are the key constituents of Stellite alloys as solutes to strengthen the solid solution matrix. Meanwhile, they also promote formation of W-rich and Mo-rich carbides when present in large quantities in high-carbon Stellite alloys. Extensive studies on tungsten Stellite alloys, in particular, Stellite ® 6, have been reported in the literature; however, molybdenum Stellite alloys, especially, the influence of Mo content on the performance of the alloys, have been rarely investigated. In this research, the tribological performance of three molybdenum Stellite alloys with varying C and Mo contents is studied using pin-on-disk dry-sliding wear test at ambient temperature, focusing on the effect of test duration and interrupted test on the wear behavior of the alloys. The worn surfaces are examined using SEM/EDS, and the analyses of the surface morphologies help explore the wear mechanisms of these alloys in dry-sliding condition. A wear model is developed for molybdenum Stellite alloys, based on the Archard equation in check with the wear test data, using hardness as a controlling factor.

Materials at High Temperatures, 2019

ABSTRACT The performance of S16, a CoCrWC alloy, was evaluated in superheated steam (SHS) at 625°... more ABSTRACT The performance of S16, a CoCrWC alloy, was evaluated in superheated steam (SHS) at 625°C and 800°C (at 0.1 MPa), and in supercritical water (SCW) at 625°C and 26 MPa for 500 hours. S16 experienced weight loss under the SHS conditions (625°C and 800°C) and weight gain under the SCW condition. Weight loss was associated with scale spallation and pitting while oxide growth on the surface contributed to weight gain. Formation of CoCr2O4 and Cr2O3 was observed in the oxide layer of samples tested under the three conditions, however, the Cr-rich oxide on SCW tested samples was found to be continuous. A thin, continuous Co3W subscale was believed to have provided an oxidation barrier under the SCW test condition. Based on the limited weight change and surface oxide formation, S16 has the potential to be used as a coating material for ultra-supercritical power generation systems at temperatures up to 625°C.

Surface and Coatings Technology, 2019

Abstract In this study, the oxidation behavior of alloy CoCrMoSi (T14) and alloy CoCrNiMoSi (T19)... more Abstract In this study, the oxidation behavior of alloy CoCrMoSi (T14) and alloy CoCrNiMoSi (T19) was evaluated in superheated steam (SHS, 0.1 MPa) and supercritical water (SCW, 26 MPa), at temperature of 625 °C for 500 h. After exposure to both conditions, the as-exposed surfaces and cross sections were evaluated using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). The identification of phases was carried out based on combined EDS and X-Ray Diffraction (XRD) analyses. The results showed that both T14 and T19 formed very complex oxides on the surface after exposure. And they experienced weight gain under SHS condition, and weight loss under SCW condition. However, T19 had much smaller weight changes after both tests. Weight loss was due to the oxide scale spallation and possibly volatilization of MoO3 and Cr2O3, in addition to preferential attack and subsequent degradation of Laves phase in both alloys. Due to Ni addition acting as FCC stabilizer, the possible SiO2 formation as a subscale, and smaller Laves phase, alloy T19 was found to be superior to T14 as potential coating material for steam power generation system at temperatures up to 625 °C, under SHS and SCW conditions.

Metallography, Microstructure, and Analysis, 2015

ABSTRACT

Wear, 2015

A high-tungsten Stellite alloy is developed for wear and erosion resistance application in this r... more A high-tungsten Stellite alloy is developed for wear and erosion resistance application in this research, taking the advantage of tungsten in Stellite alloys. The microstructure of this alloy is analyzed using SEM/EDX and XRD. The sliding wear resistance of the alloy is evaluated on a pin-on-disc tribometer and the solid-particle erosion behavior is investigated using a micro-blasting jet machine at two particle impact velocities (84 and 98 m⋅s-1) and two impingement angles (30° and 90°). The experimental results of the alloy are compared with those of well-known wear-resistant Stellite 3 and Stellite 6. The worn and eroded surfaces of the specimens are studied using SEM/EDX to explore the wear and erosion mechanisms of this new alloy. The experimental results show that this novel high-tungsten Stellite alloy has superior sliding wear and solid-particle erosion resistance to Stellite 3 and Stellite 6, owing to the formation of large amounts of large-size W-rich carbides in the alloy.

Advanced Engineering Materials, Jan 29, 2007