Effect of Ageing on Microstructure, Mechanical Properties and Creep Behavior of Alloy 740H
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THE development of advanced ultra supercritical (AUSC) power plants, which are more efficient and greener as compared to the sub-critical and supercritical power plants, mandates the identification of new materials that can retain their structural integrity at temperatures and pressures in excess of 700 C and 31 MPa, respectively.[1–4] The usage of Ni-based superalloys for the fabrication of high temperature AUSC components is inevitable as steels cannot sustain prolonged service temperatures beyond 700 C. Various Ni-based superalloys such as Alloy 230, HR6W, Alloy 617, Alloy 263,
A.H.V. PAVAN and KULVIR SINGH are with Metallurgy Department, Corporate R&D Division, Bharat Heavy Electricals Limited, Vikasnagar, Hyderabad 500 093, India. Contact e-mail: [email protected] R.L. NARAYAN is with the Department of Materials Science and Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India. Contact e-mail: [email protected] U. RAMAMURTY is with the School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798, Singapore. Manuscript submitted May 8, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Alloy 740/740H, Alloy 282, etc. have been developed for meeting AUSC power plants operation criteria.[5–10] The Ni based superalloy, Alloy 740, which exhibits high strength and toughness even at a temperature of 760 C, appears to be an ideal candidate material for making high temperature AUSC boiler components.[5,6] Detailed studies on this alloy[11–14] confirmed its superior creep strength and also provided insight into microstructural changes occurring and precipitation of g, G phases on creep deformation. Elevated temperature ageing studies in the range of 750 C to 810 C for durations of 100 to 5000 hours reported by References 15–18 indicate that a stable microstructure evolves with an increase in the size of the principal strengthening phase, c¢, leading to a marginal lowering of the strength and precipitation of various carbides along with g and G phases. Additionally, Alloy 740 also possesses excellent fireside corrosion resistance as reported by References 5, 19, 20. As a result, it meets the prescribed standards for AUSC applications. Despite the above mentioned advantages, making boiler components using Alloy 740 is a challenge as it has weldability issues due to the formation of cracks and fissures in the heat affected zones and degradation of mechanical properties, which is attributed to the
precipitation of brittle g and G phases.[5,11,12,15,20,21] Such brittle phases, primarily composed of Ti and Si, may also form when a component is subjected to elevated service temperatures in AUSC boilers. To circumvent this problem, both the Ti and Si contents were reduced in Alloy 740 and a new alloy, commercially known as Alloy 740H, was developed. Studies have shown that it possesses all the good attributes of Alloy 740 in addition to being weldable.[5] Data reported on the high temperature mechanical behavior of tubes, pipes and h
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