Valves are mainly used for the circulation and cut-off of pipeline media such as water, steam, gas and oil, flow and pressure regulation, and change of flow direction, etc. Brass valves are high-end products in valves, mostly for civilian use. The valve body is the most important and important part, and the gate valve body has the most complicated structure among many types of valves, especially the large-scale gate valve body, whose processing technology has been sand casting or die-casting blanks before machining.Both foundry and die-casting blanks have many quality problems, such as trachoma, pores, cracks, etc., and their mechanical properties are poor. The red punching process is used instead, which not only greatly improves the quality of the blank, but also improves the working conditions and saves materials and energy.
1. Process overview
Red punching is also called hot forging. In the manufacturing industry, people are more accustomed to the name “red punching”. It is a hot extrusion process that does little or no cutting of metal materials. After the metal material is heated to a certain temperature, in the cavity of the red punching die, plastic deformation occurs under the pressure of the reciprocating motion of the stamping equipment, and it is formed into a blank of the required shape and size. Under normal circumstances, it can be molded at one time, and can perform multi-directional demoulding actions at one time.
2. Material selection
Different from the method of controlling the proportion of ingredients in material smelting for foundry and casting in the past, ordinary round bars can be used directly for red punching, and special-shaped materials can also be used for blanks with large deformation for better forming effects. There are three types of brass rods: continuous casting rods, hot-rolled rods and extruded rods. Among them, extruded rods have the best performance and relatively high cost; continuous casting rods have relatively low copper density and have grain gaps, but they have an advantage in price. After continuous practice and trial, the grades of brass rods that can be used for red punching are in a wide range.
The copper content is generally required to be higher than 55%, otherwise the plasticity and forgeability of the raw material will be reduced, and the plasticity and mechanical properties of the blank after forming will drop sharply, which will easily cause cracking and fracture. Containing less than 3.5% lead in the bar is beneficial to the improvement of the cutting performance of the valve body blank in the next step of gold processing.
In lead-free or lead-less bars, silicon, bismuth and other elements will be used instead to increase cutting performance. If the lead content exceeds 3.5%, it will also cause a decrease in the plasticity and mechanical properties of the blank after molding. Generally contain a trace amount of aluminum, which will help to improve the fluidity of the material. However, if the aluminum content is higher than 0.35%, the red stamping parts of the valve body will be prone to brittleness, and even cracks will occur during valve installation and use. The most widely used grades with better performance are HPb59-1 and HPb60-2.
3. Heating and temperature control
The red punch heating method has been improved through several generations. The kerosene blowtorch at the beginning required less investment in facilities and was flexible in use, but it was difficult to measure and control the heating temperature due to uneven heating of the section materials, and the operating environment was harsh. At present, electric induction furnaces are also used for heating, but the most ideal effect is the gas automatic heating furnace, which can automatically control the temperature, heat the section material evenly, and can automatically detect and screen the qualified section material. The performance of the domestic gas automatic heating furnace is also getting better and better. good.
The purpose of section material heating is to improve the plasticity of the alloy, reduce the deformation resistance, and make the internal molecular structure of the alloy more uniform, so that the stamping parts can have good linear structure and mechanical properties. The temperature range of the red punch refers to the temperature from the beginning of the extrusion to the end of the extrusion. The higher the temperature, the better the extrusion. However, if the temperature is too high, the metal surface will be oxidized and even overburned.
Since there is a certain amount of heat loss when the section material is heated and enters the mold cavity, at the same time, a certain temperature will also be generated during the high-speed stamping process. Generally speaking, the heating temperature of the section material is generally slightly higher than the red punching temperature range. The heating temperature varies according to the characteristics of different material grades, usually 130~200°C lower than the melting point of the metal. The red flush temperature of HPb59-1 is 650~750℃, and the red flush temperature range of HPb60-2 is 670~730℃.
4. Selection of stamping equipment
The red punching of brass stamping blanks can be realized on ordinary general-purpose presses. Since the rigidity, stamping stroke, and precision of the closed press are higher than those of the open press, the stamping equipment for the gate valve body is generally a closed press, and an open press can also be used for a particularly small valve body. The selection of stamping equipment mainly lies in the reasonable selection of nominal pressure, the limitation of stamping stroke and maximum die height, and the size limitation of workbench should also be considered if necessary. The nominal pressure is the most important selection parameter, which is determined by calculating the forging pressure required for the deformation of the valve body, and the composite deformation resistance is simplified by the formula F=kσs. In the formula: F compound deformation resistance; σs the deformation resistance or yield limit of the material at the end temperature of extrusion, the end temperature of hot forging of HPb59-1 and HPb60-2 is about 650°C, and the yield limit is about 60MPa at this time; the coefficient K is actually very It is complex and involves many influencing factors, the main ones are the weight of the section material, the complexity of extrusion and the wall thickness, etc., and the gate valve body is not a simple solid stamping of upper and lower convex and concave dies, but a multiple core-pulling For extrusion, the stamping force not only provides longitudinal forging force, but also requires lateral clamping force. In addition, to carry out red punching on the crank press, it is necessary to calculate the allowable pressure at the beginning of the red punch according to the allowable compliance characteristics of the press. It is necessary to calculate the press crank angle α at the beginning of stamping, the extrusion stroke s, and the radius of the crankshaft r calculate. Cosα=[22(1-s/r)+(s/r)2]/2/(11-s/r). Of course, we have also obtained a general selection corresponding reference table from the empirical formula and practical verification.
1.Mold frame design
The brass gate valve body requires core pulling in three directions. If a flat-opening mold is used, the requirements for the mold will be higher, and the tonnage requirements of the press will also increase a lot. The vertical half open mold is only suitable for small-sized valve bodies, and because the half mold and the horizontal cores on both sides are first clamped and fixed in the stamping process, and then put into the segment material for stamping, so the size of the horizontal core entering the cavity It can only be designed very short and must not exceed the outer dimension of the valve body neck in the cavity. As shown in Figure 1, the head of the transverse core cannot exceed the position of the straight line aa. Otherwise, the segment material will be blocked by the horizontal core and cannot be completely put into the cavity, and the deformation resistance on both sides of the water hole of the valve body is too large, which will easily cause a large number of cracks. The bell mold design shown in Figure 2 is the most suitable for the large-scale gate valve body, which can be installed on a general closed single-point press.
Utilize the tapered clamping ring 4 of the upper mold, press the 4 oblique sliders 14, 15, 2 (2 pieces) of the lower mold to drive the respective four modules to lock each other, and the front and rear 2 cavity modules 20 are fixed on the front and rear On the inclined sliders 14 and 15, two horizontal cores 3 are fixed on the left and right inclined sliders 2, the upper core is fixed on the upper template 1 by the upper core fixing ring 12, and the lower core 10 is fixed on the lower template 9. When working, the tapered clamping ring 4 covers the 4 slanted sliders of the lower mold, and with the interaction of the slanted surfaces of the two and the downward movement of the press slider, the 4 slanted sliders and the upper and lower cores are driven to simultaneously extrude , when the press slider moves to the bottom dead center, the billet is forged and formed. The press slider returns, and the conical clamping ring 4 goes up. At this time, the four demoulding ramps 5 are opened again, and the front and rear cavity modules 20 and the left and right horizontal cores 3 perform demoulding and core-pulling movements at the same time.
The moving distance is limited by the guide post 11. During the demoulding process, the front slide block is further pulled outwards, and under the action of the beating rod 16, the blank will be left on the rear insert, which is also convenient for the taking and unloading of the blank. The bell jar mold frame has 4 inserts, wherein the left and right inserts are used to install the left and right horizontal cores, and the upper and lower templates are used to install the upper and lower cores, so there are four directions of core pulling: up, down, left, and right. And the gate valve body is a three-way form, only need 3 direction core-pulling, only need to utilize wherein in upper and lower mold core just, collectively referred to as middle mold core, has adopted the mode of lower mold in Fig. 2. One on each mold slide plate) 6 slide plates (4 pieces) 7 slide plate pads 8 base plates 9 lower templates 10 lower cores 11 guide pillars (4 pieces) 12 upper core fixing rings 13 upper core pads 14 inserts for fixing Slider (without driving rod) 15 Rear slider for insert fixing (with driving rod) 16 Driving rod 17 Driving rod fixing assembly 18 Lower core fixing ring 19 Lower core spacer 20 Cavity insert (2 pieces)
2. After the design of the mold frame is determined
The outer dimensions of the inserts are basically fixed, and the design of the inserts is mainly the design of the mold cavity and core. If the cavity and core are designed according to the blank drawing, only the shrinkage rate of the metal needs to be considered. According to the design of the valve body parts, it is also necessary to consider the machining allowance for design, so as to minimize the machining surface and processing difficulty.
As shown in Figure 1, the shape of the valve body is basically not machined, and only about 1mm of machining allowance is left on the upper, left, and right end faces. Of course, there will be adjustments according to the specifications, and a certain demoulding slope, generally about 15o. The transition position adopts rounded corners, the larger the rounded corners, the easier it is to form, but the principle is not to damage the overall shape of the valve body. Inner cavity core pulling:
- a.Leave a processing allowance of about 1mm in the inner diameter of the three ends according to the small diameter of the valve body thread;
- b.Other parts of the medium core are designed for inorganic brass machining according to the shape of the valve body and to ensure the minimum wall thickness of the valve body;
- c.Except for point a, the horizontal core needs to be left at the end position of extrusion and the sealing surface of the medium core in order to reduce deformation resistance There is a certain margin between the positions, the head adopts large rounded corners, and the multi-section form is used when the core pulls for a long time.
The draft angle is generally about 3o, and the transition position should be rounded.
3.Selection of segment material size
Calculate the weight of the segment material according to the blank or cavity, plus the flash margin. If the length direction of the segment material is placed in the same direction as the height of the valve body (the bell mold should adopt this placement method), the diameter of the segment material is generally 1mm smaller than the minimum radial dimension (commonly known as the neck) in the height direction of the valve body. Length must not exceed insert height. If the length direction of the section material is vertical, the diameter of the section material is generally 1 mm smaller than the minimum radial dimension of the valve body in the direction of the water hole, which is generally the middle position of the lower part of the valve body (commonly known as the belly) as shown in Figure 1. The length of the section material must not exceed Insert length.
Six Points For Attention In The Process
- Before the red punching starts, the mold cavity should be preheated, and the temperature should be controlled at about 250~300°C to reduce the scrapping of cold molds.
- The joint surface between the various parts of the mold should be within the design tolerance range to ensure that it is in a good sliding state during movement.
- The grade of the mold material must be reasonable, the module blank needs to be repeatedly upset and drawn, and the heat treatment before the mold is used is also very important.
- Inserts and cores should be repaired regularly. The cavity of the inserts is designed according to the tolerance of the valve body shape. During the mold repair process, the size of the cavity will be increased, and the flash between the inserts should be considered.
- Guarantee the rigidity of the core, especially the medium core, once deformed, the machining allowance needs to be increased if it is light, and the blank will be scrapped if it is heavy.
- After a certain modulus is produced, it is necessary to spray the demoulding machine on the cavity part in time to ensure the smooth demoulding of the blank.