液氨储罐氨应力腐蚀开裂分析

ANALYSIS OF AMMONIA STRESS CORROSION CRACKING IN LIQUID AMMONIA STORAGE TANK

某制冰企业1台液氨储罐投用3年后,首次定期检验发现其气相空间封头直边段有1处开裂,导致氨液泄漏。通过相控阵检测,发现其两侧封头直边段存在多处横向裂纹缺陷,缺陷均起源于焊缝热影响区,且与母材表面基本垂直。利用金相检验发现,裂纹有沿晶和穿晶两种微观形貌;通过硬度测定发现,越靠近热影响区的硬度值越高;此外,测厚未发现严重减薄。综合检测结果和液氨储罐的使用工况,判断该台液氨储罐封头直边段的多处缺陷为内壁在液氨腐蚀环境和残余应力共同作用下发生了氨应力腐蚀开裂并扩展至外壁所致。针对氨应力腐蚀开裂的成因,提出了一些防范措施,并对此类设备的定期检验提出了一些建议。

Three years after the commissioning of a liquid ammonia storage tank in an ice-making enterprise,one crack was found in the straight edge section of the head in the gas-phase space during the first periodic inspection,resulting in ammonia leakage.Through phased array inspection,it was found that there were many transverse cracks in the straight edge of the head on both sides of the head.The defects originated from the heat affected zone of the weld and were basically perpendicular to the surface of the base metal.Through metallographic examination,it was found that there were two kinds of microscopic morphology coveringintergranular and transgranularin the cracks.Through hardness measurement,it was found that the hardness value was higher near the heat affected zone;in addition,no serious thinning was found during thickness measurement.Based on the comprehensive inspection results and the working conditions of the liquid ammonia storage tank,it was determined that the multiple defects of the straight side section of the head of the liquid ammonia storage tank were caused by the ammonia stress corrosion cracking of the inner wall under the combined action of the liquid ammonia corrosion environment and residual stress and the expansion to the outer wall.Aiming at the causes of ammonia stress corrosion cracking,some preventive measures were put forward and some suggestions were put forward for the regular inspection of such equipment.