Xu Xiaodong, Huang Zhiguo, Chengde Petroleum College, Chengde
Abstract: Through technical analysis of the service condition and failure causes of the swivel joint body, the internal factors behind the short service life of this product are examined. Without affecting the service performance of the workpiece, the structure and machining process are improved, and an important process route for increasing service life is identified.
1. Introduction
The swivel joint is an important accessory used on oilfield cementing trucks, fracturing trucks, various high-pressure manifolds, pipelines and other high-pressure equipment. It can be used to convey oil, water, fracturing fluid and mixed fluids. Because the maximum working pressure is high, the workpiece is required to have good mechanical properties. Swivel joints currently used in oilfields include imported and domestic types. Imported swivel joints have a long service life but are expensive. Most domestic swivel joints are made from forged parts, quenched and tempered, and then finish-machined. Their cost is lower, but their service life is shorter. At present, many domestic manufacturers increase wall thickness to improve service life. However, increasing wall thickness not only raises material cost but also makes handling inconvenient because of the increased weight. Therefore, to extend swivel joint service life, effective process measures must be taken to improve the comprehensive mechanical properties of the workpiece.
2. Workpiece Structure and Failure Causes
The swivel joint body is made of 35CrMo. Its structure is shown in Figure 1.
Figure 1. Workpiece structure
From the structural design of the swivel joint body, it can be seen that the body adopts an arc-shaped design to reduce the impact of high-pressure fluid. The two ends of the body are connected to two elbow subs by steel balls. The steel balls both limit axial movement between the parts and allow relative rotation. The sealing shoulder is used to place a U-shaped seal ring, and the sealing method between the body and the elbow sub is end-face sealing. During use, rotating the elbow sub can adjust the relative position between delivery pipelines to meet field requirements. The current forming process for the body is forging, normalizing, drilling, hot bending, quenching and tempering, and machining. Because the heat-treatment process is overall quenching and tempering, the comprehensive mechanical properties are good, the hardness is HRC 38-42, and machinability is acceptable. Damage to the body generally occurs in two ways. One is wear fracture at the R165 inner arc. Field technical analysis shows that when mixed fluid is conveyed during operation, particles in the mixture scour the inner wall of the workpiece under high pressure. Because the inner-wall hardness is low and wear resistance is poor, the R165 inner arc becomes the weak point. The other is that the inner surface hardness of the raceway groove is low and its accuracy cannot meet service requirements, causing serious raceway-groove wear and making the body rotation inflexible or even stuck.
3. Process Measures
In view of the above situation, improving the existing process, increasing inner-surface hardness and wear resistance, and improving machining accuracy are important routes for increasing workpiece service life.
3.1 Improvement of the Forming Process
The original process design is basically reasonable in both material selection and process route. However, it cannot meet the high wear-resistance requirement of the workpiece, which is the fundamental reason for its relatively short service life. If the heat treatment is changed to overall quenching, wear resistance will be significantly improved, but the comprehensive mechanical properties, especially impact resistance, will decrease. The failure mode of the workpiece will change, and service life still will not improve. High-frequency induction hardening is a heat-treatment method for increasing surface hardness. If the inner surface of the body is induction hardened after overall quenching and tempering, the comprehensive mechanical properties can be maintained while the hardness of the inner surface is significantly increased. Therefore, the original process is improved as follows: forging, normalizing, drilling, hot bending, overall quenching and tempering, turning, inner-surface high-frequency induction hardening, and grinding of the raceway groove.
The improved process retains the original process feature, namely the comprehensive mechanical properties of the workpiece, while adding high-frequency induction hardening to the inner surface. After induction hardening, the inner-surface hardness reaches HRC 50-55. The hardened layer is about 3 mm deep, and hardness and wear resistance are significantly improved. However, the increased hardness of the raceway groove makes turning difficult, so the raceway groove after induction hardening is finished by grinding. This not only meets the service-performance requirements of the workpiece but also reduces the surface roughness of the raceway groove from the original Ra value and significantly improves workpiece accuracy.
Because of the special structure of the workpiece, the choice of positioning datum during grinding is critical. If the axis and end face of the phi 110 outer circle are used as positioning datums, clamping during machining becomes inconvenient and cumulative error increases because the positioning datum and machining datum do not coincide. Therefore, when designing the grinding-machine fixture, and while considering clamping convenience, a process locating shoulder is designed on the end face of the body. The axis of this process locating shoulder and the axis of the raceway groove are designed as the same machining datum, as shown in Figure 2. The size of the process locating shoulder does not affect the connection between the body and the elbow sub and does not affect body strength. Using the outer end face and axis of the process locating shoulder as positioning datums can meet the grinding requirements of the raceway groove.
Figure 2. Process design diagram
3.2 Selection of Grinding Material
After high-frequency induction hardening, the hardness and wear resistance of the swivel joint body raceway groove are significantly improved. However, if the grinding-wheel material is improperly selected during grinding, grinding burn and cracking may occur, so grinding-material selection is critical.
Cubic boron nitride (CBN) is one of the hardest cutting materials. Compared with diamond, CBN has the following advantages.
First, it has excellent red hardness and can still maintain good hardness at high temperature (1200 C). Second, diamond tends to graphitize when in contact with steel, while CBN has mechanical strength twice that of diamond. More importantly, CBN crystals have exceptionally good cutting edges and generate less heat during operation, which helps reduce surface burn and cracking in the raceway groove. To give the grinding wheel a long service life, the cutting parameters should be reasonably selected. The normal cutting speed of a CBN grinding wheel is 40 m/s, the feed rate is 500-1200 mm/min, and the depth of cut can be selected according to the situation. In general, the factors determining grinding parameters are mainly heat-treatment deformation and final workpiece accuracy.
4. Summary
In summary, the main factors determining swivel joint body service life are the material of the body and the mechanical properties after heat treatment. Quenching and tempering can meet comprehensive mechanical-property requirements such as impact resistance, while high-frequency induction hardening of the inner surface can meet high-hardness and high-wear-resistance requirements. The main factor determining workpiece accuracy is the overall stiffness of the system composed of the grinding wheel, workpiece and machine-tool fixture. Meeting these requirements can significantly improve the service life of the workpiece.
