XU Xiao-dung, HUANG Zhi-guo(Chengde Petroleum College, Chengde 067000, China

[Abstract] A very important technological way to the extension of the service Life of the products is achieved through the technical analysis of the present operation conditions and the analyis of the destructive causes of the high pressure Swivel joint.The inside.factors of the shortness of service Life of the products are presented. The structure of of workpiece and the technological processing under the maintainance of the operation functions conditions improved.

1. Introduction
High pressure Swivel joint is an important accessory oilfield cement trucks, fracturing trucks, all kinds of high pressure manifolds, pipelines and other high-voltage devices. Used to transport oil, water, fracturing fluid and mixed fluid. The maximum working pressure, thus requiring the workpiece has good mechanical properties. At present, the use of high pressure oilfield Swivel joint into two kinds of imported and domestic, imported high-pressure Swivel joint long life, but expensive. Domestic high-pressure Swivel joint after forging parts are mostly used by quenching finish forming, low cost, but life is short. Current domestic production by increasing the thickness of the Swivel joint more to improve life, but the increase in the thickness of the material not only increases costs, but also because of the increased weight of the inconvenience to the operator. Therefore, in order to prolong the life of the high-pressure Swivel joint, the process must take effective measures to improve mechanical properties of the workpiece.
2. The structure of the workpiece and the destruction reason
Swivel joint material is a high-pressure 35CrMo, the structure shown in Figure 1.

Chart 1 Workpiece structure
As can be seen from the Swivel joint structure design, the Swivel joint body uses arc-shaped design, the purpose is to alleviate the impact of high-pressure fluid. Between both ends of the two curved Swivel joint joints were connected together with a ball. Balls can limit axial movement between the two, but also allows relative rotation therebetween. Φ seal spigot for placing the U-shaped seals, body and bend the Swivel joint joint manner between the seal. When in use, can be adjusted by rotating bending joints between the relative position of pipelines in order to meet the requirements. Currently, the molding process used to bend the body: Forging - normalizing - Drilling - simmer bend - quenching - machining. Because of the heat treatment process used for the overall tone of the workpiece quality, good mechanical properties, hardness HRC38-42, good processing performance. Swivel joint body damage are generally two ways: one is to break away from the R165 wear inside the arc at the park. According to the field of technical analysis, the workpiece transport the mixture, the mixture of granular materials under high pressure on the inner wall of the erosion in the work piece, and the inner wall of the workpiece low hardness, wear resistance is poor, resulting in R165 Park arc become the weak link of the workpiece. Another is the low groove raceway surface hardness, accuracy can not meet the requirements, resulting in serious raceway groove wear Swivel joint making body rotation is not flexible or a "stuck" phenomenon.
3. Process Measures
For the above, the improvement of existing processes, improve workpiece surface hardness, wear resistance, and the workpiece machining accuracy is an important way to improve the life of the workpiece.
3.1 molding process improvement
Whether the original process design or selection process is basically reasonable, but can not meet the requirements of high wear resistance of the workpiece, which is a relatively short working life fundamental factors. Such as the heat treatment process to the overall quenching, wear will be significantly increased, but the overall mechanical properties of the workpiece, especially impact resistance will decrease, damage will change the way the work is still not improve the life of the workpiece. High-frequency quenching is a method for increasing the surface hardness of the heat treatment method, if the whole body after quenching Swivel joint induction hardening of its inner surface, can guarantee the mechanical properties of the workpiece, but also make the hardness of the inner surface of the workpiece improved significantly. Therefore, the original process to make the following improvements: Forging - normalizing - Drilling - simmer bend - overall quenching - Turning - inner surface high-frequency quenching - grinding raceway grooves.
The improved process technology, while retaining the original features, namely mechanical properties of the workpiece, the inner surface of the workpiece induction hardening. The inner surface hardness after induction hardening HRC50-55. Hardened layer depth of about 3mm, hardness and wear resistance increased significantly. But the raceway grooves hardness bring inconvenience to turning, so the raceway grooves with grinding to ensure that high-frequency quenching, so that not only meet the performance requirements of the workpiece, but also make the raceway grooves of the surface roughness Ra value is reduced to the original, so that significantly improve the accuracy of the workpiece.
Due to the particularity of the structure of the workpiece, the positioning reference selection grinding is critical. If Φ110 cylindrical axis and the end as the positioning reference, not only machining fixture will bring inconvenience, but also due to the positioning reference datum does not coincide with the process leaving the cumulative error increases. Therefore, the design jig grinder, in consideration of convenience clamping premise, a process designed to bend the body in the end face of the Mouth, the axis of the axis of the rolling groove is designed to process only the mouth of the same machining datum. As shown in Figure 2. Thus, only the mouth of the size of the process will not affect the body and bend the Swivel joint joint is connected, it will not affect the strength of the Swivel joint body. Stop by the process and the end face of the axis of the estuary as a positioning reference, you can meet the raceway groove grinding.

Figure 2 a diagram of the process design
Select 3.2 grinding materials
High pressure Swivel joint element rolling groove after induction hardening significantly improved hardness and wear resistance in the grinding process if the inappropriate choice of wheel material will be burning and cracking phenomenon so grinding grinding material selection is critical.
Cubic boron nitride (CBN) is the hardest one cutting material, as compared with diamond, CBN has the following advantages:
First, with superior red hardness. At higher temperatures (1200 ℃), to maintain good hardness; second, diamond and steel materials prone to graphitizable contact, mechanical strength is a key two times higher than diamond CBN crystal has particularly excellent cutting edge, less heat is generated during operation. Favorable raceway groove surface burns and reduce cracking phenomenon. In order to make the wheels have a higher life expectancy should be a reasonable choice of cutting parameters. CBN grinding wheels for normal cutting speed 40m / s, the feed rate of 500-1200mm / min, infeed depth can be selected at random. In general, the amount of the main determinants of the grinding is heat deformation and the final accuracy of the workpiece.
4. Summary
In summary, the decision to bend the body is a major factor in the life of the mechanical properties of the material and heat treatment after Swivel joint body. Quenching to meet the impact of the workpiece, such as mechanical properties, high frequency quenching workpiece surface of the workpiece to meet high hardness, high wear resistance. The most important factor to determine the accuracy of the workpiece is the total stiffness of the entire system of wheels, parts and machine fixture composed. Meets the above requirements can make the life of the workpiece has improved significantly.