Toshio Ohba

Life prediction by the iso-stress method of boiler tubes after prolonged service

Abstract Internal pressure creep rupture strength was investigated on 1·25Cr-0·5Mo, 2·25Cr-1Mo and 9Cr-1Mo steel tubes which had been used for 153 500 h as the high temperature reheater tubes in a power station boiler. The applicability of an Iso-stress method to predict the remaining life was discussed. A linear relationship between the testing temperature and the logarithm of the time to rupture for 1·25Cr-0·5Mo and 2.25Cr-1Mo steels after prolonged service was found. The remaining life for these two steel tubes can be predicted using the Iso-stress method. However, 9Cr-1Mo steel after prolonged service was strengthened at higher temperatures in Iso-stress tests, and the relationship between testing temperature and logarithm of time to rupture was not linear. The strengthening during the Iso-stress tests at higher temperatures was caused by microstructural change of (Cr, Fe)2N nitrides precipitated during prolonged service.

Degradation in tensile and creep properties of 2.25Cr-1Mo steel by long term service in plant

Abstract Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 105 h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.

Elastic properties of heat resistant steels after long-term creep exposure

Abstract In order to clarify the effect of microstructural change during ageing and creep on elastic modulus, the elastic moduli of aged, crept and serviced (service-exposed) materials were investigated. In ferritic steels, the elastic modulus obtained by ultrasonic pulse does not change after long-term ageing and creep although the formation of creep voids slightly decreases the elastic modulus. In SUS304 and SUS316, the elastic modulus evaluated by ultrasonic pulse increases during ageing due to precipitation of M23C6 carbides, Laves phase and σ phase though the elastic modulus decreases during creep because of the formation of creep voids on grain boundaries. The increased elastic modulus of aged material with a large amount of σ phase was regarded as that of composite consists of matrix and σ phase. For STBA24, the elastic modulus obtained by a high temperature tensile test decreases after long-term service. The tensile test at a higher strain rate was proposed to avoid underestimation of elastic modulus of serviced materials at high temperatures.

Life Prediction Under Creep-Fatigue Condition for 1Cr-Mo-V Steel

Damage evaluation and life prediction for creep-fatigue loading conditions should be related to creep damage mechanism. In order to examine the connection between creep damage mode and creep-fatigue interaction, a“combined creep-fatigue loading test” was carried out on lCr-Mo-V steel. The fracture criterion under combined loading showed the similar trend as that for stainless steels, but the amount of creep damage permitted to rupture for lCr-Mo-V steel was less. The fracture criterion obtained from fatigue tests with tensile strain-hold wave form was different from that obtained from combined creep-fatigue loading tests.