Temperature effect on dwell-fatigue crack propagation behavior of novel tempered martensitic ferritic steel G115

The effect of temperature and holding time on the dwell-fatigue crack propagation behavior of a novel tempered martensitic ferritic steel G115 was studied. The crack growth duration greatly increased with the increase in dwell time at all tested temperatures (625-675 degrees C). The FCG data and the da/dN for the 60-s dwell tests remained nearly constant when the temperature increased from 625 to 650 degrees C, whereas da/dN increased remarkably when the temperature increased to 675 degrees C. Moreover, da/dN increased with increasing temperature for the 600-s dwell tests. These results reflect the comprehensive effects of creep, environmental (oxidation) phenomena, and property degradation. In the (da/dt)(avg) versus (C-t)(avg) plot, the crack-propagation data under a specific dwell time fall close to each other, regardless of the temperatures. Scanning electron microscopy observation shows that the fatigue crack propagation has a transgranular cracking mode, regardless of variations in the temperature (625-675 degrees C) and load level. Meanwhile, the creep-fatigue crack growth mode is transgranular cracking at lower Delta K, whereas it gradually changes to mixed intergranular and transgranular fracture modes with increasing Delta K. The longer the holding time, the more significant the intergranular cracking. The crack growth may change to transgranular mode again when Delta K further increases. The two values of Delta K at which the crack-growth mode change all increase with decreasing dwell time and temperature.