5-1/2FH双台肩钻杆内螺纹接头断裂原因分析

Fracture Cause Analysis on 5-1/2FH Double Shoulder Drill Pipe Box

采用宏观检验、化学成分分析、力学性能测试、金相检验和断口分析等方法对某5-1/2FH双台肩钻杆内螺纹接头的断裂原因进行了分析。结果表明:此双台肩钻杆内螺纹接头在副台肩第一完整螺纹根部发生了疲劳断裂失效。断裂具有多源特征,起源于内螺纹根部并向外壁扩展,当穿透壁厚形成通道时发生了刺漏,从而使断口表面形成了冲刷痕迹。上扣扭矩不足,同时接头承受的弯曲应力较大是造成接头疲劳断裂失效的主要原因,另外FH型螺纹固有的螺纹根部半径较小、应力集中较大,使接头失效的风险增大。

The fracture causes of a 5-1/2FH double shoulder drill pipe box were analyzed by means of maeroseopie examination, chemical eomposition analysis, mechanical property testing, metallographic examination, fracture analysis and so on. The results showed that fatigue fracture failure was happened to the first whole thread of the second shoulder on the double shoulder drill pipe box. The cracks initiated from the root of thread and developed to the outer surface to form the flowing outlets that led to the flushing of the fracture. Insufficient of make-up torque and bending stress on the box were the main reasons of the fatigue. In addition, the root radius of FH thread is small while stress eoneentration is large, which increased the risk of box fatigue.