An oil drill pipe is an oilfield drilling tool and the main component of the oil drill string. It plays the role of connecting the drill string, conveying mud, and transmitting torque during drilling. It is subjected to complex loads such as compression, tension, torsion, and bending during work, and is subjected to strong vibration and impact. At present, oil drill pipes are made of drill pipe joints and pipe bodies welded by friction welding, and the weld zone is locally quenched and tempered after welding. Studies have found that low-stress brittle fracture in the weld zone of drill pipe is the most common phenomenon, and the weld zone becomes the weak area of the entire drill pipe. Therefore, controlling the mechanical properties of the weld zone is the key to ensuring the quality of the drill pipe, and impact toughness is the key indicator for judging the mechanical properties of the weld zone of the drill pipe.
First, the reason for low-impact toughness analysis
Except for the unqualified impact toughness, the other mechanical properties of the drill pipe weld samples meet the requirements of API SPEC 5DP:2009. The microstructure on both sides of the weld is tempered troostite + a small amount of ferrite, there is no untempered martensite and coarse overheated structure, there are many Class A non-metallic inclusions in the weld area, and the fine system reaches level 1.5. The phosphorus and sulfur content of the pipe body material is low, and the level of non-metallic inclusions is also low, but the sulfur content of the drill pipe joint material exceeds the standard, with a mass fraction of 0.016%, and the coarse system of Class A non-metallic inclusions reaches level 2.0 and the content is relatively high. Through the macroscopic and microscopic analysis of the fracture of the weld impact specimen, it can be judged that there are many mixed non-metallic inclusions of FeS and MnS at the fracture. Due to the high content of Class A non-metallic inclusions at the fracture of the impact specimen, the continuity of the organization in the weld zone is destroyed. The plasticity and elasticity of non-metallic inclusions are very different from those of steel. Non-metallic inclusions cannot be plastically deformed synchronously with steel, so more and more stress concentration is generated around the non-metallic inclusions, forming a weak bonding surface. When subjected to impact force, cracks first appear at the weak bonding surface, and eventually, the cracks extend on the welding bonding surface, causing the specimen to break. Therefore, the high content of non-metallic inclusions in the weld zone of the specimen will greatly reduce the impact toughness. The sulfur element in the drill pipe joint material is generated during the smelting process. Most of the sulfur elements gather in groups at the dendrite grain boundaries to form sulfides. The sulfides in the hot rolling process are easy to deform, often extending in the form of slender spindles and distributed in the banded organization, causing the anisotropy of the steel. During friction welding, the drill pipe joint and the pipe body are welded under high temperature and high pressure. With the plastic flow of the metal on the weld surface, the original distribution direction of the sulfide is changed from parallel to the axis of the drill pipe joint to parallel to the weld direction, that is, perpendicular to the axis direction, so the sulfide is distributed along the weld.
Due to the high content of Class A non-metallic inclusions in the weld area of the weld sample, the continuity of the organization is destroyed, resulting in the impact toughness not meeting the requirements of API SPEC 5DP:2009, but the tensile strength reaches 880MPa. Related studies show that when the sample is subjected to tensile stress, the tensile stress is perpendicular to the weld joint surface and is evenly distributed on the entire weld joint surface. Therefore, the crack propagation rate caused by non-metallic inclusions is relatively slow, but when the sample is subjected to impact force, the weld joint surface is subjected to uneven shear stress, and the crack propagation rate formed by non-metallic inclusions is relatively fast. Therefore, the weak bonding defects formed by non-metallic inclusions have little effect on the tensile strength of the weld sample, but have a great effect on the impact toughness of the weld sample.
Second, conclusions and suggestions
(1) The impact toughness of the drill pipe weld zone is low and does not meet the requirements of API SPEC5DP:2009. This is because the content of Class A non-metallic inclusions in the weld zone is high, which destroys the continuity of the structure. When subjected to impact force, cracks first appear at the weak bonding surface of the defect, and the cracks expand rapidly and eventually cause the sample to break.
(2) The high sulfur content of the drill pipe joint material and the high level and content of Class A non-metallic inclusions are the main reasons for the large number of Class A non-metallic inclusion defects in the weld zone.
(3) The weak bonding defects formed by non-metallic inclusions in the weld zone have little effect on the tensile strength but have a great impact on the impact toughness of the weld sample.
(4) It is recommended to increase the non-metallic inclusion inspection when the drill pipe joint enters the factory to ensure that the purity of the drill pipe joint material meets the requirements, thereby ensuring that the performance of the drill pipe friction welding weld zone meets the standard requirements.
Post time: Oct-15-2024