Meet the requirements of working conditions
1. Abrasion resistance
When the blank is plastically deformed in the mold cavity, it flows and slides along the surface of the cavity, causing violent friction between the surface of the cavity and the blank, which causes the mold to fail due to wear. Therefore, the wear resistance of the material is one of the most basic and important properties of the mold.
Hardness is the main factor affecting wear resistance. In general, the higher the hardness of the mold parts, the smaller the amount of wear and the better the wear resistance. In addition, wear resistance is also related to the type, quantity, shape, size and distribution of carbides in the material.
2. Fatigue fracture performance
In the process of mold work, under the long-term action of cyclic stress, fatigue fracture is often caused. Its forms include low-energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture, and bending fatigue fracture. The fatigue fracture performance of the mold mainly depends on its strength, toughness, hardness, and the content of inclusions in the material.
The working conditions of the molds are mostly very bad, and some of them are often subjected to large impact loads, which leads to brittle fracture. In order to prevent sudden brittle fracture of mold parts during work, the mold must have high strength and toughness. The toughness of the mold mainly depends on the carbon content, grain size and organization state of the material.
4. Corrosion resistance
When some molds such as plastic molds are in operation, due to the presence of chlorine and fluorine in the plastics, strong corrosive gases such as HCI and HF will be resolved after being heated, which will erode the surface of the mold cavity, increase its surface roughness, and aggravate wear failure.
5. Resistance to cold and heat fatigue
Some molds are in a state of repeated heating and cooling during the working process, which causes the surface of the cavity to be pulled and pressure to change the stress, causing surface cracks and peeling, increasing friction, hindering plastic deformation, and reducing dimensional accuracy. Mold failure. Hot and cold fatigue is one of the main forms of failure of hot work molds. This type of mold should have high resistance to cold and heat fatigue.
6. High temperature performance
When the working temperature of the mold is higher, the hardness and strength will decrease, leading to early wear of the mold or plastic deformation and failure. Therefore, the mold material should have high anti-tempering stability to ensure that the mold has high hardness and strength at the working temperature.
Meet process performance requirements
The manufacturing of molds generally involves several processes such as forging, cutting, and heat treatment. In order to ensure the quality of mold manufacturing and reduce production costs, the material should have good forgeability, machinability, hardenability, hardenability and grindability; it should also have small oxidation, decarburization sensitivity and quenching Deformation and cracking tendency.
Hardenability refers to the characteristics of the material characterized by the depth and hardness distribution of the sample hardenable layer under specified conditions, and it mainly depends on the critical quenching cooling rate of the material. Under specified conditions, it determines the characteristics of steel hardening depth and hardness distribution. That is, the ability of the steel to obtain the depth of the hardened layer during quenching, which indicates the ability of the steel to undergo quenching. The hardenability of steel is good and bad, and it is often expressed by the depth of the hardened layer. The greater the depth of the hardened layer, the better the hardenability of the steel. The hardenability of steel is an inherent property of the steel itself, it only depends on its own internal factors, and has nothing to do with external factors. The hardenability of steel mainly depends on its chemical composition, especially the alloying elements that increase the hardenability, grain size, heating temperature and holding time and other factors. The steel with good hardenability can make the entire section of the steel piece obtain uniform mechanical properties and the quenching agent with small quenching stress of the steel piece can be selected to reduce deformation and cracking.
Oxidation: In a narrow sense, the chemical reaction between oxygen and other material elements is called oxidation, which is also an important chemical unit process. In a broad sense, oxidation refers to the process of material losing electrons (increasing oxidation number).
Human metabolism is also like oxidation, that is, the human body rusts every day, and the rust produced is called free radicals in medicine. Free radicals are particles with unpaired electrons. Because it has a singular electron, it is very unstable and has a high degree of chemical reactivity. It is easy to react with surrounding molecules and make stable molecules become free radicals. Repeating this over and over will generate a large number of free radicals. Free radicals are very active and very restless. Just like the unwilling bachelors in our human society, if you can’t find an ideal partner, it may become a factor of social instability.
Under normal circumstances, life is inseparable from free radical activities. Our body moves from inside to outside every moment, burning energy every moment, and the porter responsible for transferring energy is free radicals. When these free radicals that help energy conversion are enclosed in cells and cannot run around, they are harmless to life. But if the activities of free radicals go out of control and exceed a certain amount, the normal order of life will be destroyed, and diseases may follow.
So free radicals are a double-edged sword. Many diseases are known to be caused by free radicals, such as rheumatoid arthritis, acute respiratory distress syndrome, AIDS, and periodontal disease. In addition to causing damage to cells, free radicals can cause a chain reaction of free radicals to further deteriorate and damage tissues in the body. This chain reaction is quite amazing: normal chemical substances are composed of atoms and molecules and need to carry two pairs of electrons to maintain the stability of the chemical state; and free radicals are molecules or atoms that contain unpaired electrons, so it must Only by snatching electrons from other molecules or atoms can it appease its wildness and maintain stability. However, if the targets of free radicals are protein, carbohydrates, carbohydrates, fats and other essential substances for the human body, these nutrients that lose electrons will not only become completely unrecognizable due to oxidation, but will further utilize their new identity of free radicals. Then grab other electrons to form a vicious circle of free radical chain reaction; therefore, the function of the human body is gradually damaged and deteriorated.
3. Sensitivity to decarburization
Decarbonization is a process of purifying gas, which refers to the removal of carbon dioxide from mixed gas, which is mainly seen in the treatment of raw gas or coal gas for the production of synthetic ammonia. The methods for removing carbon dioxide in the feed gas are divided into three categories.
(1) The first physical absorption method used pressurized water to remove carbon dioxide and regenerate the water after decompression.
The equipment of this method is simple, but the degree of carbon dioxide removal is poor, the outlet carbon dioxide is generally below 2% (volume), and the power consumption is also high. In the past 20 years, methanol washing method, propylene carbonate method, polyethylene glycol dimethyl ether method, etc. have been developed. Compared with pressurized water decarbonization method, they have the advantages of high purification, low energy consumption, and high purity of recovered carbon dioxide. It can also selectively remove hydrogen sulfide, which is a widely used decarbonization method in industry.
(2) The chemical absorption method has the advantages of good absorption effect, easy regeneration, and hydrogen sulfide removal.
The main methods are ethanolamine method and catalytic hot potash method. The latter decarburization reaction formula is:
In order to increase the rate of carbon dioxide absorption and regeneration, some inorganic or organic substances can be added to the potassium carbonate solution as activators, and corrosion inhibitors can be added to reduce the corrosion of the equipment to the solution. Among them, there are many methods widely used in industry (Table 5-9).
Machinability (machinability, machinability): refers to the degree of difficulty for a metal material to become a qualified workpiece after being cut by a tool. The machinability is often measured by the surface roughness of the workpiece after processing, the allowable cutting speed and the degree of tool wear. It is related to many factors such as the chemical composition, mechanical properties, thermal conductivity and work hardening degree of metal materials. Hardness and toughness are usually used as a rough judgment of the machinability. Generally speaking, the higher the hardness of metal materials, the more difficult it is to cut. Although the hardness is not high, the toughness is greater and the cutting is more difficult. Generally, non-ferrous metals (non-ferrous metals) have better machinability than ferrous metals, and cast iron is better than steel.
Annealing is a metal heat treatment process, which refers to slowly heating the metal to a certain temperature, keeping it for a sufficient time, and then cooling it at an appropriate speed. The purpose is to reduce the hardness, improve the machinability; eliminate residual stress, stabilize the size, reduce deformation and cracking tendency; refine the grain, adjust the structure, and eliminate the structure defect. To be precise, annealing is a heat treatment process for materials, including metallic materials and non-metallic materials. Moreover, the annealing purpose of the new material is similar to that of the traditional metal annealing.
The ability of steel to obtain hardness after quenching depends on the WC% in M. The hardenability and hardenability of steel are two completely different concepts. The hardenability of steel refers to the ability of steel to reach the highest hardness by quenching under ideal conditions, and it mainly depends on the carbon content of martensite. The hardenability of steel with good hardenability is not necessarily high. For example, the hardenability of low-carbon alloy steel is quite good, but its hardenability is not high, and the hardenability of high-carbon tool steel is poor, but its hardenability is high.
Metal has thermoplastic properties and can be press processed in a heated state (various metals require different temperatures), which is called malleability. Refers to the performance of metal materials that can change shape without cracking during press processing. It includes the ability to perform hammer forging, rolling, stretching, extrusion and other processing in hot or cold state. The quality of forgeability is mainly related to the chemical composition of metal materials.
The ability of metal materials to withstand plastic deformation without breaking during forging and pressing is also called process plasticity. The malleability index is usually expressed by the amount of deformation of the metal material when cracks begin to appear on the surface under a certain plastic deformation method. This amount of deformation is called the critical amount of deformation. The deformation methods of various forging processes are different, and the indicators of forgeability are also different. Upsetting is expressed by compression rate, elongation is expressed by elongation or cross-sectional reduction rate, and torsion is expressed by torsion angle.
(7) Tendency of quenching deformation and cracking
Conventional quenching has small volume change, shape warping, slight distortion, and low tendency of abnormal deformation. Conventional quenching has low sensitivity to cracking, and is insensitive to quenching temperature and workpiece shape.
The relative wear of the grinding wheel is small, and the limit of grinding without burn is large. It is not sensitive to the quality of the grinding wheel and cooling conditions, and it is not easy to cause abrasion and grinding cracks.
Meeting economic requirements
When selecting materials for molds, the principle of economy must be considered to reduce manufacturing costs as much as possible. Therefore, under the premise of satisfying the performance, the first choice is the lower price, carbon steel can be used without alloy steel, and domestic materials can be used without imported materials.
I will share with you how to choose mold materials. Finally, I remind you that when selecting materials, you should also consider the production and supply of the market. The selected steel grades should be as few and concentrated as possible for easy purchase.
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