What is the malleability of metal materials
What is the malleability of metal materials?
The malleability of metal is the process performance to measure the difficulty of obtaining high-quality products when materials are processed under pressure. The good malleability of the metal indicates that the metal is suitable for pressure forming; The poor malleability indicates that the metal is not suitable for forming by pressure machining.
The factor that has a great influence on the malleability of metal is the shaping of metal itself. The better the plasticity, the less likely it is to crack during forging. The plasticity of metal is closely related to the structure of metal. The finer the grain and the more uniform the structure, the better the plasticity. Therefore, the malleability of metal can be improved by refining grain and homogenizing structure. Metal materials can change shape without crack during pressure machining. It includes hammer forging, rolling, stretching, extrusion and other processing in hot or cold state. The malleability is mainly related to the chemical composition of metal materials.
Malleability is usually measured by metal’s plastic and deformation resistance. The better the plasticity and the smaller the deformation resistance, the better the malleability of the metal, and vice versa. Reduction of area of metal for plasticity of metal ψ、 elongation δ And so on. Deformation resistance refers to the pressure exerted by the deformed metal on the surface of the pressing tool per unit area during pressure machining. The smaller the deformation resistance, the smaller the energy consumed in deformation.
The essence of metal
Influence of chemical composition
Metals with different chemical compositions have different malleability. In general, the malleability of pure metal is better than that of alloy; The lower the carbon content of carbon steel, the better the malleability; When steel contains more carbide forming elements (chromium, tungsten, molybdenum, vanadium, etc.), its malleability decreases significantly.
Effect of metal structure
The malleability of metals varies greatly with their microstructure. When the alloy has a single-phase solid solution structure (such as austenite), its malleability is good; When metal has metal compound structure (such as cementite), its malleability is poor. The as cast columnar structure and coarse grain are not as good as the uniform and fine structure after pressure machining.
Processing conditions
Deformation temperature
Increasing the temperature of metal deformation is an effective measure to improve the malleability of metal. In the process of metal heating, with the increase of heating temperature, the activity ability of metal atoms is enhanced, the attraction between atoms is weakened, and it is easy to slip. Therefore, the plasticity is improved, the deformation resistance is reduced, and the malleability is significantly improved. Therefore, forging is generally carried out at high temperature.
Metal heating is an important link in the whole production process. It directly affects productivity, product quality and effective utilization of metal.
The requirements for metal heating are: under the condition of uniform heat penetration of the blank, the temperature required for processing can be obtained in a short time, while maintaining the integrity of the metal and minimizing the consumption of metal and fuel. One of the important contents is to determine the forging temperature range of metal, that is, reasonable initial forging temperature and final forging temperature. The forging temperature range of carbon steel is shown in Figure 1.
Figure.1 Forging temperature range of carbon steel
The initial forging temperature is the initial forging temperature. In principle, it should be high, but there should be a limit. If it exceeds this limit, the steel will have heating defects such as oxidation, decarburization, overheating and overburning. The so-called overburning refers to that the heating temperature of the metal is too high, oxygen penetrates into the metal, oxidizes the grain boundary, forms brittle grain boundary, and is easy to break during forging, so that the forging is scrapped. The initial forging temperature of carbon steel shall be about 200 ℃ lower than that of solidus.
The final forging temperature, that is, the stop forging temperature, should be low in principle, but not too low, otherwise the metal will produce work hardening, significantly reduce its plasticity, significantly increase its strength, and it is laborious to forge, and even crack for high carbon steel and high carbon alloy tool steel.
Forging makes the temperature of metal available to be measured by instruments, and it is also commonly judged by observing the fire color. The relationship between steel temperature and fire color is shown in the table below:
|
Temperature/℃ |
1300 |
1200 |
1100 |
900 |
800 |
700 |
|
Fire color |
White |
Bright yellow |
Yellow |
Cherry red |
Crimson |
Dark red |
Deformation velocity
Deformation speed is the degree of deformation per unit time. The effect of deformation speed on metal malleability is shown in Figure 2. It can be seen from the figure that its influence on malleability is contradictory. On the one hand, with the increase of deformation speed, the recovery and recrystallization are too late to overcome the work hardening phenomenon in time, resulting in the decrease of metal plasticity, the increase of deformation resistance and the deterioration of malleability (to the left of point a in the figure). On the other hand, in the process of metal deformation, part of the energy consumed in plastic deformation is converted into heat energy, which is equivalent to heating the metal, improving the plasticity of the metal, reducing the deformation resistance and improving the malleability (to the right of point a in the figure). The higher the deformation speed, the more obvious the thermal effect.
Fig.2 Effect of deformation speed on plasticity and deformation resistance
Deformation mode (stress state)
The internal stress state of deformed metal is different with different deformation modes. For example, it is in three-dimensional compression state during extrusion deformation; When drawing, it is in the state of two-way compression and always tension; During upsetting, the stress state of the central part of the blank is three-dimensional compressive stress, the upper, lower and radial of the peripheral part are compressive stress, and the tangential is tensile stress, as shown in Fig. 3.
Fig.3 Stress state of several forging methods
Practice has proved that the more the number of compressive stresses in the three directions, the better the plasticity of the metal; The more the number of tensile stresses, the worse the plasticity of the metal. The deformation resistance under the same stress state is greater than that under the different stress state. Tensile stress increases the atomic spacing of metal. Especially when there are pores, microcracks and other defects in the metal, under the action of tensile stress, stress concentration is easy to occur at the defects, which makes the cracks expand and even destroy and scrap. Compressive stress reduces the atomic spacing in the metal and is not easy to expand defects, so the plasticity of the metal is improved. However, compressive stress increases the internal friction resistance and deformation resistance of metal.
Therefore, it can be concluded that the malleability of metal depends not only on the essence of metal, but also on the deformation conditions. In the process of pressure processing, we should strive to create the most favorable deformation conditions, give full play to the plasticity of the metal, reduce the deformation resistance, minimize energy consumption and fully carry out deformation, so as to achieve the best effect of processing.
Machinability of metal
In machining, judging the difficulty of material cutting and improving the machinability are of great significance to improve productivity and machining quality. This paper discusses the indexes, influencing factors and improvement methods for evaluating the machinability of metal materials.
Concept of machinability of metal materials
The machinability of metal materials usually refers to a kind of performance or quality that can be clearly defined and measured as a sign of its machinability. Generally speaking, good machinability should be: good tool durability or high cutting speed under a certain durability, low cutting force, low cutting temperature, easy to obtain good workpiece surface quality and chip shape, easy to control or easy to break chips.
The concept of machinability of materials is relative. The so-called good or bad machinability of a material is relative to another material. Generally, when discussing the machinability of steel, it is customary to take carbon structural steel 45# as the reference. For example, high strength steel is difficult to process, which is relative to 45# steel.
The cutting performance of the tool is most closely related to the machinability. The machinability of the machined material can not be discussed in isolation from the cutting performance of the tool, but should be studied together. After understanding the machinability of workpiece materials and taking effective measures, we can improve machining efficiency, ensure machining quality and reduce machining cost.
Main indexes for evaluating machinability of workpiece materials
The machinability of a material is the difficulty of guiding the machinability of a material. Its ease is generally related to the chemical composition, heat treatment state, metallographic structure, physical and mechanical properties and cutting conditions of the material. The machinability of workpiece materials is usually measured by one or more of the following indicators:
1. Measured by tool life
The allowable cutting speed for cutting a workpiece material on the premise of ensuring the same tool durability;
2. Measured by machining quality such as surface finish;
3. Measured by unit cutting force;
4. Measured by the ultimate metal removal rate;
5. Measured by chip breaking performance, including chip shape.
Factors affecting machinability of metal materials
Strength and plasticity of materials
In terms of the hardness of workpiece materials (including normal temperature hardness and high temperature hardness), generally, the machinability of similar materials with high normal temperature hardness is low. Because when the material hardness is high, the contact length between the chip and the rake face decreases, the cutting stress on the rake face increases, and the friction heat is concentrated on the smaller tool chip contact surface, which increases the cutting temperature and wear, and even causes the burning loss and edge collapse of the tool tip when the hardness is too high. Taking steel as an example, steel with moderate hardness is better processed. In addition, properly improving the hardness of the material is conducive to obtain better machined surface quality. The plasticity of a material is usually expressed in terms of elongation. Generally, the greater the plasticity of the material, the more difficult it is to process. Because of the large plastic material, the machining deformation and hardening, and the cold welding phenomenon on the tool surface are relatively serious, it is not easy to break chips, and it is not easy to obtain good machined surface quality.
Toughness of materials
Toughness is represented by impact value. The higher the toughness of the material, the more energy is consumed during cutting, the higher the cutting force and cutting temperature, and it is not easy to break chips, so the machinability is poor. Some alloy structural steels not only have higher strength than carbon structural steels, but also have higher impact value, so they are difficult to process.
Other physical and mechanical properties also have a certain impact on machinability. For example, for materials with large linear expansion coefficient, the workpiece size changes greatly due to thermal expansion and cold contraction during processing, so it is not easy to control the accuracy. Materials with small elastic modulus have large elastic recovery during the formation of machined surface, and are easy to have strong friction with the flank.
The chemical properties of some materials also affect the machinability to a certain extent. For example, when cutting magnesium alloy, powdered debris is easy to combine with oxidation and burn. When cutting titanium alloy, it is easy to absorb oxygen and nitrogen from the atmosphere at high temperature to form hard and brittle compounds, which makes the chips become short fragments. The cutting force and cutting heat are concentrated near the cutting edge, thus accelerating the wear of the tool.
Metallographic structure and heat treatment method of materials
Metal heat treatment process can be divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different structures can be obtained by different heat treatment processes for the same metal, so it has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of steel heat treatment processes.
Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to change its overall mechanical properties. There are four basic processes for overall heat treatment of iron and steel: annealing, normalizing, quenching and tempering.
Annealing is to heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and service performance, or prepare the structure for further quenching. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as the final heat treatment.
Quenching is the rapid cooling of the workpiece in water, oil or other inorganic salts, organic aqueous solutions and other quenching media after heating and insulation. After quenching, the steel becomes hard but brittle at the same time. In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature higher than room temperature and lower than 650 ℃, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the “four fires” in the overall heat treatment. Among them, quenching and tempering are closely related and often used together.
“Four fires” evolve different heat treatment processes with different heating temperatures and cooling methods. In order to obtain certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. After some alloys are quenched to form supersaturated solid solutions, they are kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloys. Such heat treatment process is called aging treatment.
The method of effectively and closely combining pressure machining deformation and heat treatment to make the workpiece obtain a good combination of strength and toughness is called thermomechanical treatment; The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only prevent the workpiece from oxidation and decarburization, maintain the surface of the workpiece after treatment and improve the performance of the workpiece, but also carry out chemical heat treatment with infiltrating agent.
Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece and not transfer too much heat into the interior of the workpiece, the heat source used must have high energy density, that is, large heat energy is given to the workpiece per unit area, so that the surface layer or part of the workpiece can reach high temperature in a short time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. The common heat sources include oxyacetylene or oxypropane flame, induced current, laser and electron beam.
Mechanical characteristics
Different machine tools and different parameters of machine tools will also have different degrees of influence on the cutting performance of metal materials.
Ways to improve the machinability of materials
Improve the chemical composition of materials
Taking common metals as an example, 1% ~ 3% lead is added to brass and 0.1% ~ 0.25% lead is added to steel. Lead spherical particles can exist in the metallographic structure of the material, which can play a good role in lubrication during cutting, reduce friction, and improve the tool durability and surface quality. MNS is added to carbon steel, which is distributed in pearlite and plays a lubricating role, so as to improve tool durability and surface quality after cutting, increase brittleness, and chips are easy to break.
Appropriate heat treatment shall be carried out before material processing
- After normalizing, the grain of low carbon steel is refined, the hardness is improved and the plasticity is reduced, which is conducive to reducing the bonding wear of tools, reducing chip deposition and improving the surface roughness of workpieces; After spheroidizing annealing of high carbon steel, the hardness decreases and the tool wear can be reduced; Stainless steel should be quenched and tempered to HRC28. If the hardness is too low, the plasticity is large, the surface roughness of the workpiece is poor, and if the hardness is high, the tool is easy to wear;
- White cast iron can be annealed in the range of 950 ~ 1000 ℃ for a long time to form malleable cast iron, which is easier to cut.
Select the material with good processability
- After cold drawing, the plasticity of low carbon steel decreases and the machinability is good;
- The forging blank has uneven allowance, hard skin and poor machinability. After hot rolling, the machinability can be improved.
Others
Using appropriate tool materials, selecting reasonable tool geometric parameters, reasonably formulating cutting parameters and selecting cutting fluid can also affect the cutting performance of materials.
Source: China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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