Creep-fatigue interaction of several ferritic steels such as Mod. 9Cr-1Mo, 9∼12Cr-2W, newly produced Pd-added 9Cr steels, extremely high strength oxide dispersion strengthened (ODS) ferritic steels and a precipitation hardened carbon-free martensitic steel, were investigated. From these results it was clarified that the points to improve the creep-fatigue life are to keep the high creep rupture ductility or to decrease the plastic strain of cyclic deformation by making steel creep rupture strength extremely high. From the viewpoint of an evaluation of the creep-fatigue life, the analysis by ductility normalized-strainrange partitioning method explains well the effect of creep ductility and strength of the materials on the creep-fatigue life.

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Effects of preloading on fracture toughness are examined on the basis of a simple model which assumes the Beremin model for a fracture criterion and the Dugdale model for an estimate of stress distribution near a crack tip on the small scale yielding condition. The present model is shown to predict overall behaviour of the effect of preloading fairly well. Prediction on the modified Beremin model is also given.

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Strain-controlled, and load-controlled fatigue tests for a 12Cr-2W ferritic heat-resistant steel were carried out at high temperatures in order to obtain the fatigue life in a wide range from 10² to 10⁹ with loading alignment-adjusted testing machine. There was scarcely the scatter in a wide range from 10² to 10⁷ cycles by strain-controlled tests and from 10³ to 10⁹ cycles by load-controlled tests. As for strain-controlled data, the Manson-Coffin relationship (Δε_p vs. N_f, where Δε_p is plastic strain range and is number of cycles to failure) was not linear but curved near 10⁴ or 10&⁵ cycles at 600, 650 and 700°C. Fish-eye fracture was observed at 400°C over 10⁴ cycles, and at 650°C over 10⁶ cycles. The fish-eye fracture seemed to reduce the fatigue strength in high-cycle region.

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