The oil drill pipe is a kind of 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, bending, etc. during operation, and is subjected to strong vibration and impact. At present, oil drill pipes are all made of drill pipe joints and pipe bodies by friction welding. After welding, the weld area is locally quenched and tempered. Studies have found that the low-stress brittle fracture phenomenon in the weld area of ​​the drill pipe is the most common, and the weld area becomes the weak area of ​​the entire drill pipe. Therefore, controlling the mechanical properties of the weld area is the key to ensuring the quality of the drill pipe, and the impact toughness is the key indicator for judging the mechanical properties of the weld area of ​​the drill pipe.

First, impact test and results
According to the requirements of API SPEC 5DP:2009, a drill pipe was randomly selected to inspect the impact toughness of the weld area of ​​the batch of drill pipes. Three impact specimens with a size of 10mm×10mm×55mm were taken along the longitudinal direction of the weld area. The number of this group of specimens was A1, and the test temperature was -20℃. According to the technical requirements of ASTM E23:2018 S, a pendulum impact tester was used to perform the Charpy V-notch impact test. From the test process, the test method and test equipment are the same, the test temperature is -20℃, and the impact energy of the A1 group of specimens is basically the same as that of the A2 and A3 groups of specimens. Excluding the factors that may be confused with other specimens, the inspection of the specimen processing size and V-notch also meets the requirements of ASTM E23:2018, so there is no problem with the impact test itself.

Second, physical and chemical tests and results
2.1 Tensile test: According to the technical requirements of API SPEC 5DP:2009, a round bar tensile specimen with a diameter of 12.5mm was taken from the same drill pipe weld. According to the technical requirements of ASTM E8/E8M:2016, an electro-hydraulic servo universal testing machine was used to perform a tensile test. It can be seen that the tensile performance of the weld area meets the requirements of API SPEC 5DP:2009 and is within the normal control range without abnormalities.
2.2 Hardness test: According to the technical requirements of API SPEC 5DP:2009, a longitudinal full-section hardness specimen was taken along the drill pipe weld area, and a Rockwell hardness test was performed on the heat-affected zones on both sides according to the requirements of ASTM E18:2019. It can be seen that the hardness of the drill pipe weld area meets the requirements of API SPEC 5DP:2009.
2.3 Transverse bending test: According to the technical requirements of API SPEC 5DP:2009, the weld zone was subjected to a transverse bending test. Two full-wall thickness specimens with a length of 200 mm and a width of 9.5 mm were taken, and a 38.1 mm bending core was used for the transverse bending test. When the angle formed by the two wings of the specimens in the clockwise and counterclockwise directions was not greater than 40°, there were no cracks, which met the requirements of the standard.
2.4 Metallographic inspection: According to the technical requirements of GB/T 13298-2015 “Methods for the inspection of metal microstructures” and GB/T 6394-2017 “Methods for the determination of average grain size of metals”, two longitudinal section metallographic specimens were taken perpendicular to the weld, and the specimens were observed using a metallographic microscope. The results are shown in Figure 2. It can be seen that the microstructures on the drill pipe joint side and the pipe body side are mainly tempered troostite + a small amount of ferrite, and there is no untempered martensite and coarse overheated structure. The grain size grade of the drill pipe joint side and the pipe body side is 9.0, which is higher than the internal control standard of 7.0.
2.5 Non-metallic inclusion inspection: According to the technical requirements of ASTM E45:2018, a longitudinal section sample is taken perpendicular to the weld, and a metallographic microscope is used to inspect non-metallic inclusions. It can be seen that the drill pipe joint side and the weld area have a large content of Class A non-metallic inclusions, the weld area is rated as 1.5, and the joint side is rated as 2.0.
2.6 Chemical composition analysis: According to the requirements of ASTM A751:2014, the chemical composition analysis of the drill pipe joint and the pipe body is carried out, and it is known that the sulfur content of the drill pipe joint exceeds the required value of API SPEC 5DP:2009.
2.7 Scanning electron microscope and energy spectrum analysis: Scanning electron microscope (SEM) analysis was performed on the fracture of one of the impact specimens in group A1. The fracture was densely grooved; there were small black block substances in some grooves and small dimples, and there were a large number of black block substances on the local plane. Some long black block substances were up to 100μm. Energy spectrum (EDS) analysis was performed on these black block substances. It can be seen that the main components of the black block substances are sulfur, manganese, and iron, with mass fractions of 35.43%, 36.88%, and 17.82% respectively, and the remaining elements are carbon, calcium, and chromium. It can be inferred that the black block substances are mixed inclusions of FeS and MnS.

Third, an analysis of the reasons for low-impact toughness
Except for the unqualified impact toughness, the other mechanical properties of this batch of drill pipe weld specimens 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 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 welding 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, 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 evenly distributed on the entire weld joint surface. Therefore, the crack propagation speed 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 speed 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.

Fourth, the conclusion 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 organization. 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 specimen 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 specimen.
(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.