S135钻杆腐蚀孔洞形成原因分析

Cause Analysis of Corrosion Holes on S135 Drill Pipe

目的对某S135钢级钻杆的腐蚀孔洞形成原因进行分析。方法通过化学成分分析、金相组织分析、钻杆宏观外貌分析、力学性能测试及扫描电镜与能谱分析等一系列分析试验,对该S135钻杆材料的力学性能、化学成分、金相组织、非金属夹杂物、晶粒度、钻杆的外圆面宏观形貌、钻杆的腐蚀孔洞特征和腐蚀产物进行分析。结果该S135钢级钻杆的化学成分合格,符合产品技术条件要求;钻杆的强度、塑性、冲击韧性等力学性能指标均合格,均符合产品技术条件要求,其中冲击韧性值达到97 J,超出技术条件要求值一倍以上;钻杆的金相组织为回火索氏体,为正常的调质热处理组织;钻杆的晶粒度级别为7级,符合产品技术条件要求;钻杆的各类非金属夹杂物级别均未超过1.0级,符合产品技术条件要求,钻杆外圆面存在大量的麻点、麻坑,腐蚀孔洞壁存在含氧元素的腐蚀产物,即存在氧腐蚀。结论该S135钢级钻杆本体孔洞为应力腐蚀疲劳所致的孔洞,其产生因素有:第一,钻杆本体外圆面上存在麻点、麻坑;第二,钻杆承受交变弯曲应力;第三,钻杆外圆面局部氧腐蚀作用。

Objective To analyze the cause for corrosion holes on S135 drill pipe. Methods In this paper,the chemical composition analysis,metallographic test,macro morphological analysis,mechanical performance testing,scanning electron microscopy（ SEM） and energy spectrum analysis were used to analyze the mechanical property,chemical composition,metallurgical structure,non-metallic inclusion,grain size,and drill pipe＇s top circle surface,and corrosion holes＇ s characteristics and corrosion products of S135 drill pipe material. Results The drill pipe＇ s chemical composition was qualified,meeting the technical requirements of the product. The mechanical performance indexes of the drill pipe such has strength,plasticity and impact toughness were all qualified,meeting the technical requirements of the product,among which,the impact toughness reached 97 J,which exceeded the technical requirement value by more than 1 fold. The metallographic structure of the drill pipe was tempering sorbite,which was a normal quenched heat treatment structure. The grain size of the drill pipe was Grade 7,meeting the technical requirements of the product. The grade of all kinds of non-metallic inclusion did not exceed 1. 0,which met the technical requirements of the product.There were plenty of pits and pockmarks on the top circle surface of the drill pipe,and there were corrosion products containing oxygen element on the wall of the corrosion holes,i. e.,there was oxygen corrosion. Conclusion The analysis results showed that the hole on the S135 drill pipe was caused by the stress corrosion fatigue. There were three factors leading to its generation： first,the pits and pockmarks existed on the top circle surface of the drill pipe; second,the alternating bending stress borne by drill pipe;third,the local oxygen corrosion on the top circle surface of the drill pipe.