核电钢A508的腐蚀疲劳研究

CORROSION FATIGUE OF REACTOR PRESSURE VESSEL STEEL A508 IN DISTILLED WATER

在位移可控的悬臂梁弯曲疲劳试验机上，对核电钢Ａ５０８在２０～８０℃蒸馏水中的腐蚀疲劳特性进行了初步探讨，并结合电化学和断口分析，对其裂缝扩展机制进行了研究。结果表明，Ａ５０８钢在蒸馏水中的裂缝扩展速率比在空气中的快４～６倍。在所研究的范围内，其ｄａ／ｄＮ随温度上升而下降。这与温度升高，水中的溶解氧量降低，腐蚀电位下降，钝化膜中的Ｆｅ２Ｏ３比例减少正好相对应；也与温度升高，断口中的沿晶断裂百分比下降完全吻合。在试验范围内，Ａ５０８不会发生应力腐蚀开裂，属真腐蚀疲劳，其裂缝扩展由阳极溶解和纯疲劳断裂两个因素共同制约。最可能的模型是：在裂缝前沿，首先在某些局部微区发生侵蚀，这些微区之间便形成韧带，承受较高的有效应力强度因子幅值△Ｋｅｆｆ，从而导致机械撕裂。而这些韧带的断裂过程，控制着腐蚀疲劳的扩展。将实验结果与据此提出和韧带模型△Ｋｅｆｆ＝△Ｋａｐｐ／（１－αＣ）的估算值进行比较，两者十分吻合。

he behavior and mechanism of corrosion fatigue for A508 steel in distilled water were stydied by means of a displacement controlled cantilever bend fatigue machine as well as by fractographic and eledtrochemical techniques. The fatigue tests were performed at R=0, f=0.5Hz in ambient air distilled water at various temperatures. It was showed that the fatigue crack growth rates of A508 steel in distilled water were 4 to 6 times faster than that in ambient air, and that they decreased with increase in water temperature at 15-80℃. This was precisely related to the reduction of dissolved oxygen contents, corrosion potential and quantity of Fe2O3 in oxide film, and this agreed closely with the decrease in percentage of I.G. fracture with increasing water temperature. According to these data, it was suggested that the fatigue of A508 steel in distilled water should be considered as a true corrosion fatigue since no stress corrosion cracking occured and that the crack propagation might be controlled by anodic dissolution and simple fatigue rupture. A possible model was proposed in that local attacks occured,at the fracture front and the ligaments between these areas experienced a higher effective △K, resulting in mechanical rupture. The measured data were in reasonable agreement with the predicted curves of da/dN ～△k from this ligament model, △keff = △Kapp/(1-αC).