Improvement of Forging Process for Valve Body
Analysis and discussion are conducted on the quality problems existing after forging the valve body, such as cracks, end face shrinkage, folding, and uneven defects. It is believed that during the forging process, due to the relatively low-end face temperature of the valve body forging, significant cross-sectional changes, and limited material distribution, the main reason for the quality problems is the difficulty in metal flow. By improving its process by adopting a “two in one” forging process, the middle round bar is pulled out and then split in half by gas cutting. After the process improvement, the use of materials has been reduced, and the existing quality problems of cracks, shrinkage, and uneven joints have been thoroughly solved. The experiment has proven that this plan is a practical and effective method.
Table of Contents
- Valve body quality problem analysis
- Valve body quality problems of process improvement
The valve body is a pipeline connection used in large quantities in the chemical industry. Its working environment is very poor; it must withstand high pressure and temperature, resist corrosion and impact, and have high mechanical strength and good welding performance. After forging and normalizing treatment, the valve body is usually selected from 20# steel. This paper studies the shape of the valve body is larger, requiring more than 30kN free forging hammer production, belonging to the typical branch forgings; the valve body of the two connected interfaces often cracks, end shrinkage, folding, and uneven and other quality problems caused by product scrap.
Valve body quality problem analysis
The valve body weight is 420kg, and the material selected is 20# steel, Figure 1 for its forging diagram is shown. The forging process for a fire one, time 30min, equipment for 50kN free steam hammer. Valve body common quality problems are: the two-even interface often appears cracks, end shrinkage, folding, and unevenness.
Temperature unevenness caused by the impact
Deformation temperature significantly impacts the plasticity of the metal; production due to improper control of deformation temperature and cracking of the workpiece is not uncommon. Plasticity increases as the temperature rises for most metals, but this increase is not simply a linear rise. In the heating process of certain temperature intervals, often due to changes in the phase or grain boundary state and the emergence of brittle areas, the plasticity of the metal is reduced. For example, when the metal temperature rises to about 800-950℃, the opposite situation of slightly decreasing plasticity appears again (see area III in Fig. 2). During this period, the pearlite transformation into austenite, the formation of ferrite and austenite two-phase coexistence, but also may also appear with the grain boundary FeS-FeO low melting eutectic (melting point of 910 ℃), which is a common hot brittle area, forging in this, there may be cracks. In plastic processing, we should avoid the various brittle areas mentioned above.
Figure.1 Valve body forging diagram
Figure.2 Plasticity of carbon steel with the change of temperature curve
The effect of strain rate
As shown in Figure 3, thermal deformation, with the increase in strain rate, the beginning of plasticity usually has a more significant reduction; after that, due to the enhancement of the temperature effect, plasticity has rebounded, but if the temperature effect is too large, so that the actual deformation temperature from the plastic zone into the high-temperature brittle zone, the plasticity of the metal and a sharp decline (such as the dashed line in Figure 3 DE). Regarding material properties, the more complex the chemical composition, the higher the content of alloying elements, the lower the recrystallization rate, and the slower the softening process. Therefore, the relationship between strain rate and plasticity is more sensitive when increasing the strain rate will cause a significant decrease in plasticity.
From the point of view of process performance, increasing the strain rate will play a beneficial role in the following two aspects: first, reduce the friction coefficient, thus reducing the flow resistance of the metal, improving the filling of the metal and the unevenness of the deformation; second, reduce the heat loss during hot forming, thus reducing the drop of the skin temperature and the unevenness of the temperature distribution, which is conducive to the control of forging temperature.
Figure.3 Schematic curve of the effect of strain rate on plasticity
An appropriate increase in strain rate will increase the temperature effect and the metal temperature. In contrast, the increase in temperature can promote the readjustment of dislocations during the deformation process, which is conducive to the merging of dislocations and the reduction of dislocation density. Especially for thermal deformation, the increase in temperature can promote recovery and recrystallization, thus promoting the repair of microcracks.
Influence of the stress state at the joint
The stress state at the forging of the valve body with the interface, usually using a flat anvil, the stress state of the three-way stress state, compressive stress, and two-way tensile stress, such a stress state is not conducive to product forming, easy to cause cracks in the core of the blank, as shown in Figure 4.
Figure.4 The stress situation of the round section of the blank and the longitudinal cracks produced when the anvil is drawn long
Influence of inconsistent internal and external deformation at the connecting interface
As can be seen from Figure 1, the Φ200mm×160mm long cylindrical material in the figure is only 46mm thick when folded into a 330mm×330mm square. In the forging, the material is first forged into a rectangle, pressed into a step, and then dropped into a round bar part. Because the material’s surface is easier to flow, the degree of deformation is large, while the core of the material deformation is small; even if part of the material is increased, it is difficult to flow outward together. This is the main reason for the shrinkage of the end face.
Valve body quality problems of process improvement
Valve body interface at one end of the “two a company” forging process
Branch valve body forgings usually use a single forging, time-consuming and labor-intensive; forging quality could be better; forgings with the interface are very easy to produce cracks, end cratering, and folding defects. Therefore, the key to process improvement is improving the quality of the interface.
Figure 5 is the valve body forging billet diagram.
Figure.5 Valve body forging billet diagram
As shown in Figure 5, the use of an improved “two in a row” forging process, that is, in the forging, the first billet forging into 330mm × 330mm, the total length of 760mm rectangular body. In the figure, the median length of the rectangular body is 100mm, which is the volume of material needed for two Φ200mm×160mm long cylinders, and then forge this part into Φ200mm×320mm long cylinders, as shown in Figure 6. Then the middle of the blank is cut off by gas cutting, and two valve body forgings are obtained. This is the first fire.
As the interface of one end of the valve body adopts the forging process of “two one even,” the excess material of the blank is concentrated in the interface of the other end of the valve body blank. The second fire is to press the other end of the interface step, chamfering, and finally use the V-shaped wrestling stem to achieve continuous forging.
Figure.6 Valve body “two in a row” blank diagram
The other end of the valve body at the mouth using a V-shaped wrestling rod part
In plastic processing, the plasticity of the metal can be improved by changing the stress state. After changing to a V-shaped anvil, the tensile stress in the heart of the blank is reduced due to the action of the lateral pressure of the tool, thus avoiding the generation of cracks (Figure 7).
Figure.7 V-shaped anvil drawing long round section blank
Prevention of temperature reduction
Using a continuous forging process, the first fire first forges the billet into forgings, as shown in Figure 6, followed by gas cutting; the second fire directly drops the cylindrical part, and the forging temperature reaches more than 1000℃ after forging, which effectively prevents the drop of forging temperature and controls the generation of cracks.
The original forging material is 420kg, changed to “two in a row” forging; the weight of the material is 830kg so that each valve body forgings save about 5kg of material.
The product quality of one end of the forging is improved, and the quality problem of the gas-cutting end is greatly improved. The quality of the other end is also greatly improved due to the guarantee of temperature, the collection of excess material, and the application of V-wrestling; only a few forgings still have shrinkage in the core, which is inevitable. However, the machining of the middle part has increased.
Through the improvement of the free forging process and trial production of the valve body, it can be concluded that the uneven forging temperature and the deformation method change greatly impact the forging quality. For long rod forgings, fork forgings, and some branch forgings in forging, you can use continuous forging and then the auxiliary gas cutting method, which can reduce the quality problems caused by cracks, end shrinkage, folding, and uneven material distribution.
Author: Li Zengguo