| 25Cr35Ni heat-resistant steel was widely used in high temperature equipment and components, such as cracking tube, reformer tube, radiant tube and so on. Materials were exposed to high temperature, pressure and oxidation environment, and the operating parameters still fluctuate. Even though most of reformer tube and radiant tube had a specification design life of 10 to 20 years, the state analysis and life prediction were extraordinary necessary to eliminate security vulnerabilities and explore the using potential of materials. This work mainly focused on two kinds of components, on the one hand, the paper researched on creep rupture properties of the radiant tube of continuous annealing furnace obtained from a subsidiary of Baosteel Group Corporation. On the other hand, this paper aimed at the state analysis and remnant life prediction of primary reformer tubes provided by a subsidiary of China National Offshore Oil Corporation. In engineering, the TTP parameter method was usually used for predicting remnant life, which synthesized rupture time and temperature as a complementary parameter, namely, P(σ)= (tr,T). The parameter was utilized to normalize the creep rupture data and assess stress rupture property. However, creep rupture test took more money and time, which restricted quick life prediction. Thus it was important to study the accurate and quick method. Stress relaxation test could be used to evaluate rupture life and endurance strength. The creep rate of different temperature and stress could be expressed by the relaxation creep curve normalized by the parameter. Combined with the relaxation creep rate and fracture time relationship, the life prediction could be realized through relaxation test and creep test.The main conclusions were drawn as follows:(1) For reformer tube, inter-dendritic eutectic carbides keep the skeleton-like shape or network chain, and secondary carbides in austenitic matrix are small and dispersed. Besides that, the strength index is higher, and plasticity index is lower. The material of reformer tube in general is in good state. For radiant tube, primary carbides in the grain boundaries appear as the trend of network, and the precipitation of carbides in the austenitic matrix tend to get coarsened to a blocky shape. If there is obvious difference on microstructure among materials, rupture strength can be clearly identified by microstructure, otherwise will not be able to distinguish.(2) Combined with the Larson-Miller parameter and Z parameter, the remnant life of reformer tube is predicted. The average rupture time is 109 thousand hours in the design environment. By contrast with this, the remnant life is about 64 thousand hours at the 95% confidence level. The high temperature performance of radiant tube is evaluated by Manson-Haferd and Z parameter, which consist well with the experimental results.(3) The different of creep rupture property among materials can be characterized by stress relaxation test. The endurance strength increase with the increase of stress relaxation limit and the decrease of relaxation creep rate. The relaxation behavior of the material is well described by constitutive relation, namely, εp=Aexp(-Q/RT)(sinh Bσ)n. Combined with ABAQUS software and constitutive equation can predict and simulate stress relaxation behavior. In the paper, we research on high temperature performance of radiant tube based on RTP parameter and the relationship between relaxation stress and rupture time. The result are in good accord with the experimental data, and stress relaxation test can, so stress relaxation test can be used as a reference method to evaluate creep rupture property. |
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