Investigation of Causes for Thread Galling of N80 Long-round Thread Casings as Used in a Certain Oil Field

The causes for thread galling of the Φ 244.48 mm×10.03 mm N80 long-round thread casings as used in a certain oil field are investigated by means of inspecting the thread connection processing quality and material characteristics of all the trouble-ridden casings，and comparing the surface processing characteristics of different couplings by means of the make & break test.As a result，it is identified that major elements responsible for the poor anti-galling performance of the very lot of casings include rather low carbon content and hardness of the casing material，and over-thin phosphate coating of the coupling surface that leads to poor adhesion of itself.