Abstract 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 °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 °C, whereas da/dN increased remarkably when the temperature increased to 675 °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 (Ct)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 °C) and load level. Meanwhile, the creep-fatigue crack growth mode is transgranular cracking at lower Δ K , whereas it gradually changes to mixed intergranular and transgranular fracture modes with increasing Δ K . The longer the holding time, the more significant the intergranular cracking. The crack growth may change to transgranular mode again when Δ K further increases. The two values of Δ K at which the crack-growth mode change all increase with decreasing dwell time and temperature.

中文翻译：

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温度对新型回火马氏体铁素体钢 G115 驻留疲劳裂纹扩展行为的影响

摘要 研究了温度和保温时间对新型回火马氏体铁素体钢 G115 的驻留疲劳裂纹扩展行为的影响。在所有测试温度（625-675°C）下，随着停留时间的增加，裂纹扩展持续时间大大增加。当温度从 625°C 增加到 650°C 时，60 秒停留测试的 FCG 数据和 da/dN 几乎保持不变，而当温度增加到 675°C 时，da/dN 显着增加。此外，对于 600 秒的驻留测试，da/dN 随着温度的升高而增加。这些结果反映了蠕变、环境（氧化）现象和性能退化的综合影响。在 (da/dt)avg 与 (Ct)avg 图中，特定停留时间下的裂纹扩展数据彼此接近，而与温度无关。扫描电子显微镜观察表明，无论温度（625-675°C）和载荷水平如何变化，疲劳裂纹扩展都具有穿晶裂纹模式。同时，蠕变疲劳裂纹扩展模式在 Δ K 较低时为穿晶开裂，而随着 Δ K 的增加逐渐转变为沿晶和穿晶混合断裂模式。保温时间越长，晶间开裂越显着。当 Δ K 进一步增加时，裂纹扩展可能再次转变为穿晶模式。裂纹生长模式改变的两个Δ K 值都随着停留时间和温度的降低而增加。蠕变-疲劳裂纹扩展模式在 Δ K 较低时为穿晶开裂，而随着 Δ K 的增加逐渐转变为沿晶和穿晶混合断裂模式。保温时间越长，晶间开裂越显着。当 Δ K 进一步增加时，裂纹扩展可能再次转变为穿晶模式。裂纹生长模式改变的两个Δ K 值都随着停留时间和温度的降低而增加。蠕变-疲劳裂纹扩展模式在 Δ K 较低时为穿晶开裂，而随着 Δ K 的增加逐渐转变为沿晶和穿晶混合断裂模式。保温时间越长，晶间开裂越显着。当 Δ K 进一步增加时，裂纹扩展可能再次转变为穿晶模式。裂纹生长模式改变的两个Δ K 值都随着停留时间和温度的降低而增加。