Failure Analysis of T12 Boiler Re-Heater Tubes During Short-Term Service
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of T12 Boiler Re-Heater Tubes During Short-Term Service Zheng-Fei Hu • Da-Hai He • Xi-Mao Wu
Submitted: 13 January 2014 / in revised form: 8 April 2014 / Published online: 15 August 2014 ASM International 2014
Abstract The failure of T12 re-heater tubes that had been in service only for 3000 h was investigated. The wall thickness of the tubes was visibly reduced by heavy oxidation corrosion on the outer and inner walls. The original pearlite substrate completely decomposed. Uniform oxide scale appearance on the inner wall shows obvious vapor oxidation corrosion characteristics. Corrosion originated in the grain boundary, and selective oxidation occurred due to ion diffusion in the substrate. The layered oxide scale formed on the inner wall is related to the different diffusion rates for different cations. Exposure to high temperature corrosive flux accelerated the corrosion on the outer wall. The microstructure degradation and corrosion characteristics of the tubes indicate that the tubes failed primarily because of overheating, as evidenced by calculations. Keywords Steel High temperature corrosion Oxidation Microstructure
a design life of 20 years and some have been in service for much longer, tube ruptures occasionally occur in power plants; most of these accidents are tube blowouts in the tubing system. These accidents cause many forced power outages, considerably affecting the generation capacity of power plants and reducing plant productivity. The blasts are caused primarily by corrosion, stress ruptures, inadequate heat treatment, and welding defects [1–3]. Tube corrosion can be classified into two major types: waterside and fireside corrosion. Oxidative corrosion significantly reduces the thickness of tube walls, increases vapor temperature, and creates serious risks to high-temperature components in operation [4, 5]. This study investigated a boiler explosion caused by the re-heater CrMo tubes of a thermal power plant in Northeast China. The tubes failed within 3000 h of being put into service.
Examined Sample Introduction Low-alloy CrMo carbon steels, such as T12, T22, and T23, are widely used as high-temperature economiser tubes (e.g., waterwall and re-heater tubes) in thermal power plants. Although these high-temperature components have Z.-F. Hu (&) D.-H. He School of Materials Science and Engineering, Tongji University, Decai Building, 4800 Caoan Road, Room 337, Shanghai 201804, China e-mail: [email protected]; [email protected] X.-M. Wu Northeast Electric Power Research Institute Co. Ltd, Shenyang 110006, China
The examined sample is a section of a re-heater tube cut from an area of the boiler close to the blast origin. This section had been in service (nominal steam temperature = 540 C, pressure = 3.9 MPa) for only 3000 h before failure. The tube is fabricated from low-alloy steel T12 (15 CrMo) with dimensions of /38 9 4 mm2. The composition of the alloy is 0.160% C, 0.290% Si, 0.610% Mn, 0.019% S, 0.016% P, 0.950% Cr, 0.054% Ni,
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