Failure Mechanism of a Stellite Coating on Heat-Resistant Steel
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INTRODUCTION
THE operating temperature and steam pressure of boilers are being increased to improve power generation efficiency in view of the need to save energy and prevent the surge of greenhouse gas emission, which requires the materials to have large resistance to high-temperature wear and oxidation.[1,2] Heat-resistant steel is often used as the critical parts of the thermal power boiler, owing to its low thermal expansion coefficient and superior resistance to corrosion, creep deformation, and thermofatigue.[3] However, its high-temperature wear and oxidation resistance cannot meet the elevated demands of advanced boilers. There are two paths, i.e., alloying and surface coating, to improve the high-temperature resistance to
DONG WANG and HUANG WANG are with the Shanghai Key Laboratory of Materials Laser Processing and Modification and the State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. HAIXING ZHAO, YUYAN LI, and XIA LIU are with the Shanghai Electric Power Generation Equipment Co., Ltd., Shanghai Turbine Plant, Shanghai 200240, China. GUO HE is with the Shanghai Key Laboratory of Materials Laser Processing and Modification and the State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, and also with Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai 200240, China. Contact e-mail: [email protected] Manuscript submitted January 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
wear and oxidation. Alloying can be achieved by adding a small amount of transition elements, such as Ti, Cr, Nb, and W, that form dispersed intermetallic compounds or precipitation-strengthening phases in the base materials.[4–6] However, the improvement in resistance to high-temperature wear and corrosion is limited. Surface coating is an effective method to enhance the durability of the steels used in the high-temperature and high-pressure environments. The hardfacing layer formed by surface coating can prevent the base materials from wear and oxidation.[7,8] Stellite alloys, a class of cobalt-based alloys, have often been used as the coating materials due to their superior high-temperature strength and large resistance to high-temperature wear and corrosion. In such alloy systems, the carefully designed alloy components mainly contained chromium, molybdenum, and tungsten, which provided the abilities to resist high-temperature attack.[9] They could be used as the hardfacing materials for steam turbines and their components, which were made of high or low chromium alloyed base materials.[10–13] During service of the Stellite-coated steel used in steam turbines, the coatings exhibited good performance and achieved long fatigue life under 873 K (600 C); however, they severely cracked and were easily peeled off at the temperature above 873 K (600 C) for 8 years. It seems that such coating could not work above 873 K (600 C). However, the r
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