Investigations on servicing damage mechanisms of Cr35Ni45Nb alloy under complex conditions
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The damage induced by oxidation and stress during the service process of Cr35Ni45Nb alloy and their influences on the microstructure evolution and creep rupture life of this alloy were studied. Continuity, compactness, and high temperature stability of the oxide layers are conductive to the performance of the alloy. A slight but complete layer of oxidation on surface improves the creep rupture life of the specimens. However, being exposed to long-term oxidation is observed to be counter-productive. Temperature stress generated during the air-cooling process creates large carbides near the surface of the specimen’s cracks. Moreover, tiny carbides are precipitated from c matrix and interfacial defects due to creep stress, but strip carbides are dissolved due to creep stress and the Gibbs–Thomson effect.
I. INTRODUCTION
Ethylene cracking furnace tube is a core component in petrochemical plants. Cracking furnace tubes are operated under stress for a long term in the complex condition like high temperature, oxidation, and carburization. As austenitic heat resistant steel is capable of withstanding high temperature and has good properties of antioxidation and carburization, it becomes the first choice.1,2 Cr35Ni45Nb alloy is widely used for manufacturing ethylene cracking furnace tubes due to its excellent mechanical and anticorrosion performances. The cracking furnace tubes are easy to be damaged while being exposed in an environment of oxidation and carburizing for a long time.3 Besides, the presence of thermal stress is one of the main reasons for creep failure of the tubes during long-term services. Industrial observation has shown that surface oxides of cracking tubes are widespread in the process of servicing.4,5 Under most industrial conditions, protection against carburization and catalytic coke formation is provided by means of an oxide layer and several studies have focused on the formation of such protective oxide layers during in situ or preoxidation.6–8 These layers are strong barriers against diffusion of carbon.5 However, the fundamental problem is that those protective oxide layers show little thermal stability.9 Furthermore, with the formation of the oxide layer, diffusion of elements like chromium onto the surface for forming chromium oxides leads to the depletion of carbide within subsurface regions of the alloy.10
J. Mater. Res., Vol. 31, No. 14, Jul 28, 2016
http://journals.cambridge.org
II. EXPERIMENTAL PROCEDURES
A Cr35Ni45Nb tube, serviced for 2.5 years, was divided into three parts for different further treatments. The specimens of 3 mm in diameter of the alloy for high temperature creep rupture tests are cut from the heat treated tubes. The chemical components of the alloy are listed in Table I.
Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.175 2156
The loss of carbides on the grain boundary would weaken the mechanical properties of the alloy such as strength, plasticity, a
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