Ball valves are widely used in petroleum refining, long-distance pipeline, chemical industry, water conservancy, electric power, nuclear power, and other industries. The ball is the opening and closing parts of the ball valve is the key element of the ball valve. DN80 ball manufactured with 316L is used in the nuclear power plant pipeline; the mechanical properties of the product and corrosion resistance have high requirements. At present, the main manufacturing methods for this type of ball element are the following three:
- (1) Casting: The main disadvantage of this method is that the casting is prone to internal inclusions, porosity, shrinkage cracks, and other defects, and low production efficiency and qualification rate.
- (2) Machining, by the ball diameter of the sphere using the appropriate bar specifications, and then machined. The main disadvantage of this method is that the material utilization rate is low, the production cycle is long, and the mechanical properties of the bar material are high.
- (3) For ordinary die forging, the method will be heated raw material forging into a solid sphere blank, and then through machining to complete the inner hole, the metal material utilization rate still needs to be higher, and the total production cycle is long.
Based on the advantages and disadvantages of different manufacturing methods of balls, through finite element numerical simulation, we have developed a closed extrusion process based on multi-directional die forging forming technology. In a stroke of the press, the shape of the ball and the main inner hole can be formed by heating the blank once, which not only significantly improves the utilization rate of materials, reduces the machining time, shortens the production cycle, but also improves the mechanical properties of the ball due to the deformation characteristics of closed extrusion.
1. Forging process and mold design of ball valve balls
1.1 Forging design of ball valve balls
Product specifications for DN80, material 316L, the quality of the parts 3.98kg. Figure 1 is a schematic diagram of the DN80 sphere parts, parts structure symmetry, and channel diameter Φ74mm. According to the structural form of the parts combined with the characteristics of the closed-type extrusion molding process, the design of DN80 sphere forging is shown in Figure 2. Forging parts using a horizontal die mode makes the structure conducive to billet positioning, metal flow, and convenient production operations. Forging machining allowance, fillet radius, punching even skin, etc., are designed according to the specifications. The mass of the designed forging is 5.65kg, and the material utilization rate is about 70%. The forging structure at A is designed as a compression allowance to ensure the metal fills the die chamber and reduces the convex die’s forming force.
Figure.1 Ball part drawing
Figure.2 Ball forging drawing
1.2 Mold design
According to Fig. 2, combined with the characteristics of the multi-directional die forging process of ball valve balls using SolidWorks 3D modeling software, the design of the mold structure shown in Fig. 3, which consists of the upper and lower concave die, horizontal convex die, the lower ejector rod.
Figure.3 Mold structure
2. Finite element numerical simulation
2.1 Geometric modeling
The geometric models of blank and mold shown in Fig. 3 are imported into Deform-3D pre-processing. The blank and mold models are symmetric. To reduce the amount of calculation and improve the simulation accuracy, the 1/2 model of the blank and mold is used for simulation.
2.2 Boundary condition setting
Blank material model: AISI-F316L, the material model and 316L chemical composition are almost identical. Shape: round bar material, specification: Φ85mm×125.5mm, mass: 5.65kg, the billet is defined as plastic. Mold material model: AISI-H13, concave and convex mold running speed: 25mm/s. Blank and mold heat exchange coefficient: 8kW/m2.℃, the friction between the blank and the mold selection of shear friction model, the friction factor of 0.12.
2.3 Simulation of Forming Program
In the multi-directional die forging technology, the different combinations of concave and convex mold action on the forming load forging filling have a greater impact on the forging pieces to be used in the mold action process: the upper concave mold → horizontal convex mold extrusion molding → the upper concave mold return → horizontal convex mold return. The lower ejector rod is omitted in the numerical simulation analysis to simplify the simulation process.
2.4 Analysis of simulation results
Figure 4 shows the numerical simulation of the forming process and forging forming. As can be seen from the figure, the upper concave dies downward pressure process on the blank upsetting, upsetting amount of 10mm, the end of the mold, the blank in the middle of the metal, such as free forging, upsetting the same as the formation of the drum, such as Figure 4(b). Horizontal convex mold in the opposite direction extrusion process, after contacting the blank, first upsetting the blank so that the metal toward the concave model cavity front and back direction flow, as in Figure 4(c), until the front and back direction to contact the concave model cavity, the horizontal convex mold began to counter-extrude the metal, so that the metal to the left and right ends of the cavity flow, as in Figure 4(d), until the cavity is full. According to Fig. 4(e) and Deform-3D post-processing results, the forging is filled, and no defects are generated during molding. Clamping load on the upper concave die: 11 MN, load on the horizontal convex die 5.6 MN.
Figure.4 Simulation process
3. Test verification
The closed extrusion test of the sphere forging is carried out on our 40MN multi-directional die forging production line and the supporting equipment; the process flow is as follows: unloading → billet heating → die preparation → removal of oxidized skin → upsetting and extruding → non-destructive testing → forging heat treatment → forging shot blasting.
3.1 Billet and mold preparation
A band discharges the billet saw; the specification of discharging is Φ85mm×125.5mm, and the length deviation is ±0.5mm. A box-type resistance furnace heats the billet, the empty furnace is warmed up to 1180℃, the billet is put into the furnace, and the heat preservation is 1.5h. The temperature of the beginning forging is 1180℃, and the preheating temperature of the mold is 200-300℃; to ensure that the mold is warmed up, the preheating time is no less than 5h. lubrication is made by Atchison F568, which is made by proportion 1 to 1, and Atchison F568 makes the lubrication. F568, according to the ratio of 1:3 mixed with water.
3.2 Process program
To obtain the optimal forming results, the extrusion process should ensure the synchronization of the operation of the horizontal convex dies; the specific horizontal convex die control mode takes the displacement control, the upper concave dies takes the pressure control, and the pressure of 40MPa. The specific forming process of the forging is as follows: the upper concave die mold (displacement 250mm) → horizontal convex die opposite piercing to the end position (displacement 150mm) → horizontal convex die unloading → the upper concave die return trip → horizontal Convex die return → lower ejector upward ejection of forgings.
3.3 Forging quality
According to the above technology program, in the 40MN multi-directional die forging hydraulic press production line for the trial test, Figure 5 (a) for the trial forgings, Figure 5 (b) for the processed parts. As can be seen in Figure 5, the metal has flowed into the forging at the press residue, the forging is filled to the brim, there are no cracks, folding and other defects on the surface, there are no burrs or wrong molds at the parting surface, and the forging bore is smooth. Ultrasonic testing results show no folding, cracks, or other defects inside the forging.
Figure.5 Forging molding quality
3.4 Mechanical properties of forging
The forging body is sampled after solid solution plus quenching heat treatment. Figure 6 is the forging tensile specimen sampling location map. Mechanical properties of the forging and the standard requirements of the comparison of the value are shown in Table 1; Table 1 shows that the closed extrusion process of ball forgings formed by the mechanical properties are better than the standard requirements of the value of the yield strength increased by 37.1%, the tensile strength increased by 8.1%, the elongation increased by 36.7%.
3.5 Forming load
The hydraulic cylinder of the press used in the test has a built-in pressure sensor, which can monitor the pressure of the hydraulic cylinder in real-time. Since the upper concave die adopts pressure control and the pressure is a set value, this paper only compares the measured forming load and numerical simulation forming load of the horizontal convex die, see Table 2. The simulated load is 16.7% larger than the measured load. This parameter can correct the forming force of the horizontal convex die of the forging pieces of the same type and material in the future.
Fig.6 Sampling diagram of tensile specimen
Table.1 Comparison table of mechanical properties
|Project||Yield strength (MPa)||Tensile strength (MPa)||Elongation rate (%)|
|Standard requirement value||≥ 175||≥ 490||≥ 45|
Table.2 Comparison table of forming loads.
|Simulated load||Measured load||Difference|
4. Analysis of Results
Based on the multidirectional die forging forming technology, the DN80 ball valve ball bore. The upper and lower end surfaces of a fire forging forming, compared with the use of Φ135mm bar machining of the part, the material utilization rate can be doubled, significantly shorten the machining time, greatly reduce the production cost, and improve production efficiency. The use of multi-directional die forging technology manufactured forgings dense organization, excellent quality, in three-way hydrostatic pressure, after intense deformation, forgings grain refinement, performance significantly improved.
The following conclusions were obtained through the DN80 ball valve body closed extrusion molding process of finite element numerical simulation and experimental verification.
- (1) DN80 ball forgings obtained through the closed extrusion molding process, the forging surface quality is good, with no folding, cracks, and other defects, and the mechanical properties have been greatly improved compared with the standard value.
(2) The simulated load of the horizontal convex die is 0.8MN (16.7%) larger than the measured value, which provides a reliable basis for calculating the forming force of forging parts of the same type and material.
(3) The finite element numerical simulation of the metal flow law, the filling of the forging, coincides with the test, and the finite element numerical simulation can be used as an analytical tool for the multidirectional die forging and forming process and provide technical support for the subsequent production.
Author: Xu Wencui, Wu Yanli, Zhao Wencheng