碳钢氢腐蚀裂纹愈合的TEM观察

TEM STUDY ON THE HEALING OF CRACK INDUCED BY HYDROGEN IN CARBON STEEL

对含氢腐蚀裂纹的碳钢进行 10 0 0℃、10h的再热循环处理后 ,SEM和TEM观察表明 ,氢蚀裂纹发生完全愈合 .Fe和C在γ -Fe中的快速扩散是碳钢中氢蚀裂纹愈合的必要条件 .氢蚀裂纹愈合的动力是氢蚀气泡长大导致的塑性变形能Es,在Fe、C和氢原子扩散都足够快的情况下 ,氢蚀裂纹的愈合条件是Es 2γ/r(γ为界面表面张力 ,r为裂纹半长 ) .TEM观察表明 ,氢蚀裂纹形成使晶界发生严重的塑性变形 ,位错密度高且相互缠绕 ,氢蚀裂纹的愈合过程伴随着亚晶长大、晶界滑动的高温回复过程甚至再结晶 ,位错回复低密度、整齐排列的低能态 。

The microstructures of the crack induced by hydrogen and crack healing have been studied by using the transmission electron microscopy(TEM).These specimens of carbon steel with the hydrogen induced cracks were cyclically annealed at 1000℃ for 10 h.The SEM and TEM observations on the specimens before and after the heat treatment showed that the cracks could be healed completely by heat treatment.The healing of hydrogen induced cracks was closely related to heat diffusion of Fe and C atoms.The driving force of crack healing resulted from the plastic deformation energy E s created by the growth of hydrogen bubbles or cracks.The critical condition of healing of bubbles or cracks is E s≥2 γ/r (where γ is the surface tension, r is the half length of crack).During healing of the hydrogen induced cracks,the dislocation density around grain boundaries decreased and the dislocation type transformed from tangle dislocation with high energy to vertical and regular dislocation with low energy.The formation of pearlite in distinct layers during healing process indicated the structure of carbon steel recovered too.