Authors: Y.K. Chung, Jong Jin Park, Cheol Hong Joo, Ik Min Park
Abstract: X20CrMoV12 1 steels were exposed at three different high temperatures and various
durations. Aging parameter, LMP s, was used to describe long-term thermal history. Creep rupture, hardness and tensile tests for aged X20CrMoV12 1 steels were carried out. Yield strength was slightly decreased and hardness was rapidly decreased afterLMP s reached 20.6. Creep strength of aged X20CrMoV12 1 steel decreased rapidly after the LMPs reached 20.6. It was shown that the creep strength decreased due to the coarsening of acicular type M23C6 at subgrain boundaries and the agglomeration of M23C6 at prior austenite grain boundaries. After LMPs reached 20.6, the assumed linear regression line of stress and Larson-Miller parameter,LMPf, also moved toward a
lower LMPf region as aging parameter increased. The dependency of creep rupture life on the microstructural degradation could be represented by CLMPs, which is LMPf at stress of 1 kg/mm2. It was found that the creep rupture life of X20CrMoV12 1 steel could be predicted by considering the material degradation during and prior to creep test.
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Authors: Grzegorz Golański
Abstract: The paper presents results of research on the influence of ageing on the microstructure and mechanical properties of T24 steel. The examined material was in the form of longitudinal samples taken from a pipe section (with the pipe size of 44.5x6.7 mm). The research is focused on degradation of the base material (T24 steel) where the steel samples, after initial heat treatment, were aged at the temperature of 580°C and time up to 6000hrs in a furnace with air atmosphere. This contribution summarizes the experimental results of mechanical and structural measurements and provides relations between them.
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Authors: Meriem Aichaoui, Ali Hadji
Abstract: A heat resistant cast steel tube from an ammonia plant made of modified HP40 steel that failed after seven years of service was investigated for damage mechanism. The assessment of material degradation was carried out using optical microscopy, scanning electron microscopy (SEM) in combination with energy dispersive spectroscopy (EDS) analysis and mechanical tests. The main cause of failure appears to be overheating and localized corrosion along the network of intergranular carbides. Failed portion of the tube showed that the interdendritic eutectic carbides had coarsened, secondary carbides were precipitate and strongly coaresned within the austenitic regions. Witch decrease the mechanical strength and ductility of the service exposed tube compared with the as cast tube. Presence of unacceptable impurities like chlorine in the internal working gas destroy the protective oxide layer leading to penetration of corroding elements and degradation of service life of the tube.
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