The forgeability of metals is a measure of the technological properties of materials to obtain high quality products when they are processed under pressure. The forgeability of the metal is good, which indicates that the metal is suitable for pressure forming, and the poor forgeability indicates that the metal is not suitable for pressure forming. The plastic shape of the metal itself is the most important factor to the metal forgeability. The better the plasticity, the less easy to crack when forging. The plasticity of metal is closely related to the structure of metal, the smaller the grain size and the more uniform the structure, the better the plasticity. Therefore, the forgeability of metal can be improved by refining grain and homogenizing structure. The ability of a metal material to change shape without cracking during pressure processing. It includes in hot or cold state can carry out hammer forging, rolling, drawing, extrusion and other processing. The forgeability is mainly related to the chemical composition of metal materials. Malleability is often measured in terms of both the plasticity and the deformation resistance of the metal. The better the plasticity, the less the resistance to deformation, the better the malleability of the metal, and vice versa. The plasticity of a metal is expressed in terms of its sectional shrinkage ψ, Elongation δ, etc. . Deformation resistance refers to the pressure exerted by a deformed metal on the surface of a press tool during pressure processing. The smaller the deformation resistance, the smaller the energy consumed in deformation. The nature of metals 1.1 the effect of chemical composition the forgeability of metals with different chemical composition is different. In general, the forgeability of pure metal is better than that of alloy, the lower the carbon content of carbon steel, the better the forgeability, and the forgeability of steel containing more carbide forming elements (CR, W, Mo, V, etc. . 1.2 the effect of metal structure on the malleability of metal is very different due to the different structure of metal. Alloys with single-phase solid solution structure (such as Austenite) have good malleability, while metals with metal compound structure (such as cementite) have poor malleability. The as-cast Columnar structure and coarse grain are not as malleable as the homogeneous and fine structure after pressure processing. 2. Processing condition 2.1. Raising the temperature of metal deformation is an effective measure to improve the metal forgeability. During the heating process, with the increase of the heating temperature, the mobility of the metal atoms is enhanced, the attraction between the atoms is weakened, and it is easy to slip, so the plasticity is increased, the deformation resistance is reduced, and the forgeability is improved obviously, therefore, forging is generally carried out at high temperatures. The heating of metal is an important link in the whole production process, which directly affects the productivity, product quality and the effective utilization of metal and so on. The requirement of metal heating is that under the condition of uniform heat penetration of the billet, the required temperature can be obtained in a short time, the metal integrity can be kept and the consumption of metal and fuel can be minimized. One of the important contents is to determine the forging temperature range of the metal, that is, a reasonable starting and finishing forging temperature. The forging temperature range of carbon steel is shown in Fig. 1.

The starting forging temperature, that is, the starting forging temperature, is in principle high, but there is a limit beyond which oxidation, decarburization, overheating and overheating of the steel will occur. The so-called overheating refers to the metal heating temperature is too high, oxygen infiltration of the metal, so that the grain boundary oxidation, forming brittle grain boundary, easy to break when forging, so that the forging scrap. The starting forging temperature of carbon steel should be about 200 °c lower than the solid phase line. The final forging temperature is the stop forging temperature. In principle, the forging temperature should be low, but not too low. Otherwise, the metal will produce work hardening and its plasticity will be significantly reduced, even cracked for high carbon steel and high carbon alloy tool steels. The forging makes the temperature of the metal measurable by means of an instrument and judged by means of the observation of the flame. The relationship between the temperature and the fire colour of steel is shown in the following table:

22.2 rate of deformation rate of deformation is the degree of deformation per unit time. The effect of deformation velocity on metal forgeability is shown in Fig. 2. As can be seen in FIG. , Its effect on forgeability is contradictory. On the one hand, with the increase of deformation speed, recovery and recrystallization can not be carried out in time, can not overcome the work hardening phenomenon, so that the metal plastic decline, deformation resistance increase, forgeability deterioration (a point left in the figure) . On the other hand, in the process of metal deformation, some of the energy consumed in plastic deformation is converted into heat energy, which is equivalent to heating the metal. The higher the deformation velocity is, the more obvious the thermal effect is.

2.3 The internal stress state of the deformed metal is different according to the deformation mode (stress state) . For example, the state of extrusion deformation is in three-dimensional compression, while that of drawing is in two-dimensional compression and one-dimensional tension; the state of stress in the center of the billet is in three-dimensional compression, while the upper, lower and radial stress of the peripheral parts are in three-dimensional compression, tangential is the tensile stress, as shown in figure 3.

It is proved that the more the compressive stress is, the better the plasticity of the metal is, and the more the tensile stress is, the worse the plasticity of the metal is. The deformation resistance under the same stress state is greater than that under the different stress state. The tensile stress makes the atomic spacing of metal increase, especially when there are some defects such as air hole and micro-crack in the metal, under the tensile stress, the stress concentration is easy to occur at the defect, which makes the crack expand, even to the extent of failure. The compressive stress reduces the interatomic distance of the metal, and the defects are not easy to spread, so the plasticity of the metal is improved. But the compressive stress makes the internal friction resistance of the metal increase, and the deformation resistance also increases. Therefore, it can be concluded that the malleability of a metal depends on both the nature of the metal and the deformation conditions. In the process of pressure processing, it is necessary to create the most favorable deformation conditions, give full play to the metal plasticity, reduce the deformation resistance, make the least energy consumption, fully carry out the deformation, to achieve the best effect of processing. 3. The machinability of metal in the process of cutting, it is important to judge the degree of difficulty of material cutting and improve the machinability to improve productivity and processing quality. In this paper, the index, influence factors and improving methods of evaluating machinability of metal materials are discussed. The concept of machinability of metallic materials machinability of metallic materials generally refers to the properties or qualities of metallic materials that can be clearly defined and measured as a sign of their machinability. In general, good machinability should be: tool durability is better or under certain durability cutting speed is higher, cutting force is lower, cutting temperature is lower, easy to get better surface quality and chip shape is easy to control or easy to break chip. The concept of machinability of materials is relative. The so-called good or bad machinability of one material is relative to another material. When discussing the machinability of steel, it is customary to refer to carbon structural steel 45. For example, high strength steel is more difficult to work with, as opposed to 45 steel. The cutting performance of the cutting tool is closely related to the cutting machinability of the cutting tool. The cutting performance of the cutting tool can not be separated from the cutting performance of the cutting tool to discuss the cutting machinability of the processed material in isolation. After knowing the machinability of the workpiece material and taking effective measures, we can improve the processing efficiency, guarantee the processing quality and reduce the processing cost. The main index for evaluating the machinability of workpiece materials is the degree of difficulty, the degree of difficulty, the general degree and the chemical composition of the material, the mechanical properties and cutting conditions of heat-treated metallographic structure are related. The machinability of the workpiece material is usually measured by one or more of the following indexes: 1. The cutting speed of the workpiece material is measured by the tool life under the premise of the same tool durability; 2. Measure by machining quality such as Surface finish, measured by unit cutting force, measured by ultimate metal removal rate, measured by chip breaking performance, including chip shape

Factors affecting the machinability of Metal Materials 1. The strength and plasticity of the material in terms of the hardness of the workpiece material (including room temperature hardness and high temperature hardness) , in general, similar materials in the processing of high hardness at room temperature low. Because the contact length between the chip and the tool face decreases when the material hardness is high, the cutting stress on the tool face increases, and the friction heat concentrates on the smaller tool-chip contact surface, which leads to the increase of cutting temperature and the aggravation of wear, when the hardness is too high, it may even cause the burning of the tip and the edge collapse. Take steel as an example, steel with moderate hardness is better processed. In addition, properly increasing the hardness of the material is beneficial to obtain better machined surface quality. The plasticity of a material is usually expressed in terms of its elongation. In general, the greater the plasticity of the material, the more difficult it is to process. Because the plastic material, processing deformation and hardening, tool surface cold welding phenomenon is more serious, not easy to break chip, not easy to obtain a good surface quality has been processed. 2. The toughness of a material is expressed as an impact value. The higher the toughness of the material, the more energy is consumed in cutting, the higher the cutting force and cutting temperature are, and the chip is not easy to break, so the machinability is poor. Some alloy structural steels are not only stronger than carbon structural steels, but also have higher impact values, so they are more difficult to process. Other physical and mechanical properties also have an effect on the machinability. Such as linear expansion coefficient of large materials, processing thermal expansion cold contraction, workpiece size changes greatly, it is not easy to control accuracy. The material with small elastic modulus has large elastic recovery during the formation of machined surface, and is easy to have strong friction with the flank. The chemical property of some materials also affects the machinability to a certain extent. Such as cutting magnesium alloy, powder debris and oxidation and combustion. When cutting titanium alloy, it is easy to absorb oxygen and nitrogen from atmosphere at high temperature, forming hard and brittle compound, which makes chip become short chip, cutting force and cutting heat are concentrated near cutting edge, thus accelerating tool wear.

Metal Heat treatment technology can be divided into three categories: integral 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 a number of different heat treatment process. The same metal with different heat treatment process, can obtain different structure, thus have different performance. Steel is the most widely used metal in industry, and its microstructure is the most complex, so there are many kinds of heat treatment processes. Integral heat treatment is a metal heat treatment process in which the workpiece is heated as a whole and then cooled at an appropriate rate to change its overall mechanical properties. There are four basic processes for the integral heat treatment of steel: Annealing, normalizing, quenching and tempering. The purpose of annealing is to heat the workpiece to a suitable temperature, to apply different holding time according to the material and the size of the workpiece, and then to cool it slowly so that the internal structure of the metal reaches or approaches the equilibrium state, to obtain good processing and service properties, or to prepare the structure for further quenching. Normalizing is heating the workpiece to a suitable temperature and then cooling it in the air. The effect of normalizing is similar to that of annealing, except that the obtained microstructure is finer and is often used to improve the cutting performance of materials, it is also sometimes used as final heat treatment for less demanding parts. Quenching is the work-piece heating insulation, in water, oil or other inorganic salts, organic water solution, such as rapid cooling medium. The hardened steel becomes brittle at the same time. In order to reduce the Brittleness of the steel parts, the steel parts after quenching are kept warm for a long time at an appropriate temperature higher than room temperature but lower than 650 °c, and then cooled. Annealing, normalizing, quenching, tempering is the overall heat treatment of the “Four”, in which quenching and tempering is closely related, often used together, one can not. “Four torches”with different heating temperature and cooling methods, and evolved into different heat treatment process. In order to obtain a certain strength and toughness, the quenching and high temperature tempering combined process, known as quenching and tempering. Some alloys are quenched to form supersaturated solid solutions, which are kept at room temperature or slightly higher temperature for a long time to improve the hardness, strength or electromagnetic properties of the alloys. Such heat treatment process is called aging treatment. The effective and close combination of pressure processing deformation and heat treatment to obtain a good combination of strength and toughness of the workpiece is called deformation heat treatment, and the heat treatment under negative pressure or vacuum is called vacuum heat treatment, it can not only make the workpiece non-oxidation, non-decarbonization, keep the surface of the workpiece after treatment, improve the performance of the workpiece, but also through the infiltration agent for chemical heat treatment. Surface Heat treatment is a metal heat treatment process which only heats the surface layer of the workpiece to change its mechanical properties. In order to heat only the surface layer of the work piece without excessive heat transfer to the inside of the work piece, the heat source used must have a high energy density, that is to say, a large amount of heat is given to the work piece per unit area, 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 are flame quenching and induction heating, common heat sources are oxyacetylene or oxypropane flame, induction current, laser and electron beam, etc. .

4. Machine tools with different mechanical properties and different parameters of machine tools also have different effects on the machinability of metal materials. Ways to improve the machinability of materials 1. To improve the chemical composition of the materials, for example, adding 1% ~ 3% lead to brass and 0.1% ~ 0.25% lead to steel. Lead can exist in spherical particles in the microstructure of the material, which can play a good role in lubrication, reduce friction, tool durability and surface quality can be improved. MNS is added to Carbon Steel, which distributes in pearlite and acts as lubrication, which improves tool durability and surface quality after cutting, increases Brittleness and chips are easy to break. 2. After normalizing, the grain of low carbon steel is refined, the hardness is increased and the plasticity is decreased, which is beneficial to reduce the tool’s adhesive wear, reduce the accumulated debris and improve the workpiece Surface roughness, stainless steel should be tempered to HRC28, the hardness is too low, the plasticity is too big, the work piece is Surface roughness, the hardness is high, then the tool is easy to wear; white cast iron can be annealed in the range of 950 ~ 1000 °C for a long time to become malleable cast iron, it’s easier to cut. 3. After cold drawing, the ductility of low carbon steel with good workability is reduced and the workability is improved. The forging billet has uneven allowance, hard skin and poor workability. 4. The cutting performance of other materials can also be affected by using proper cutting tool materials, choosing reasonable tool geometry parameters, making reasonable cutting parameters and selecting cutting fluid.

 Source: Caitong