疲劳载荷作用下X80管线钢夹杂物的微观行为

Micro-behaviors of inclusions in the X80 pipeline steel under fatigue loading

X80管线钢是我国长输管线的主要用钢,其安全性评价直接关系到管线的经济性和安全性。采用扫描电镜(SEM)原位观测的方法,动态跟踪观察了拉-拉疲劳载荷作用下国产X80管线钢中不同形状和尺寸的夹杂物导致裂纹萌生、扩展乃至试样断裂的全过程,并计算了影响X80管线钢疲劳性能的临界夹杂尺寸。结果表明,非金属夹杂物能够导致X80管线钢疲劳裂纹的萌生与扩展,疲劳裂纹首先在夹杂/基体界面的基体一侧萌生,然后向远离夹杂的基体中扩展;在拉-拉疲劳载荷作用下,X80管线钢薄板疲劳裂纹的稳态扩展速率为10-10 m/cycle量级;夹杂物尺寸越大,疲劳裂纹萌生越早,试样寿命越短。为了避免夹杂物对管线钢疲劳性能的危害,管线钢中夹杂物的尺寸应尽量控制在临界尺寸之内,尤其应避免出现带状夹杂物。

Specially designed SEM in-situ fatigue tests were conducted to trace an entire process of crack initiation and propagation induced by inclusions in the X80 pipeline steel.Influences of inclusions with different shape and size on the crack initiation were studied,the critical size of inclusions which does not reduce the fatigue strength of the X80 pipeline steel was then calculated.Experimental results show that the non-metallic inclusion can lead to fatigue crack initiation and propagation in the X80 pipeline steel.Fatigue cracks initiate firstly in the matrix adjacent to an inclusion and then propagate into the matrix far away from the inclusion.The steady growth rate of fatigue crack in the X80 pipe steel sheet is at 10-10m/cycle under the stretch-stretch fatigue loading.The larger the inclusion size in the X80 pipeline steel,the earlier the initiation of fatigue crack.In order to avoid the harm of inclusions to fatigue properties,the size of inclusions in the X80 pipeline steel should be smaller at least than the critical size,and especially a string of inclusions should be eliminated.