In oil and gas wells, the thread joint of tubing and tubing bear the test of complex stress caused by stretching, compression, bending, internal pressure, external pressure and temperature for a long time. The quality of thread joint in the weakest part of tubing directly affects the service life of oil and gas wells. Some threaded steel pipes have the following problems in use:
1. Due to improper selection of thread interference, the torque shoulder of the joint does not contact, and the contact pressure of the main sealing surface is too small, resulting in insufficient air tightness.
2. It is easy to be glued when making up, and the connection strength and bearing capacity of the tubing joint are greatly reduced after serious gluing and staggering, resulting in tripping. 64% of the failure accidents of tubings in foreign countries occur at screw joints, while 86% in China.
J55 commonly used for tubing is high-quality carbon structural steel with high strength and wear resistance, in which C content is 0.1552% ~ 0.16%, Si content is 0.117% ~ 0.137%, Mn content is 0.15% ~ 0.18%. The surface is pretreated to form a hard wear-resistant layer, and then coated with a molybdenum disulfide bonded solid lubricating film, which can achieve both sealing and anti viscosity. Surface pretreatment greatly affects the bonding performance and service life of the dry film and the substrate. First, degrease and remove rust, and then use chemical or physical treatment, such as sandblasting, phosphating, nitriding, anodizing, electroplating, chemical oxidation and sulfuration. During sandblasting, fine sand with a particle size of 75 μm is used for hard metal, coarse sand with a particle size of 150 μm is used for soft metal, and phosphating is generally not suitable for elastic deformation parts. Sand blasting and phosphating combined with solid lubrication and wear-resistant dry film can significantly improve the life of dry film.
Some form a 5-30 μm manganese phosphating film on the surface of threaded joints or metal sealing parts, or a 1-20 μm nitriding layer is added with a phosphating film, and then coated with molybdenum disulfide or tungsten disulfide powder, and a solid lubricant composed of a resin is heated and solidified to form a 10-45 μ m solid lubricating dry film. The composite film can increase the use times of oil pipe and prevent the joint from burn and wear.