Dongguan Feito Industrial Technology Co., Ltd.

Dongguan Feito Industrial Technology Co., Ltd.

Difference between cold working and hot working of metal materials

Thermal processing

Thermal processing is a processing method that causes plastic deformation and recrystallization of metal materials at the same time under the condition of higher than the recrystallization temperature. Thermal processing usually includes casting, forging, welding, heat treatment and other processes. Thermal processing can change the structure of metal parts while forming or change the established state of the formed parts to improve the mechanical properties of the parts.


Smelting metal, making casting molds, pouring molten metal into casting molds, and obtaining castings with certain shapes and properties after solidification is called casting. Casting is an applied science and is widely used in the production of machine parts or blanks. Its essence It is formed by the gradual cooling and solidification of liquid metal, which has the following advantages:
(1) It can produce parts with complex shapes, especially blanks with complex inner cavities, such as various boxes, beds, racks, etc.
(2) Casting production has wide adaptability and great process flexibility. Metal materials commonly used in industry can be used for casting. The weight of castings can range from a few grams to hundreds of tons, and the wall thickness can range from 0.5 mm to 1 meter.
(3) Most of the raw materials for storage and manufacturing come from a wide range of sources, the price is low, and waste parts can be directly used, so the cost of castings is relatively low.
However, liquid forming also brings some disadvantages to the parts, such as loose casting structure, coarse grains, internal defects such as shrinkage cavity, shrinkage porosity, and pores. Therefore, the mechanical properties of castings, especially impact toughness, are lower than forgings of the same material. In addition, there are many casting processes and it is difficult to control accurately, which makes the quality of castings not stable enough, and the working conditions of casting are poor.
With the development of casting technology, in addition to machine manufacturing, various castings are also widely used in various fields of the national economy such as public facilities, daily necessities, arts and crafts, and construction. The production process of castings can be roughly divided into two categories: sand casting and special casting.
Sand casting
In sand casting, molding and core making are the most basic processes. They have a great influence on the quality, productivity and cost of castings. Molding can usually be divided into manual molding and machine molding. Manual molding is to use hand or manual tools to complete the purple sand, mold removal, and repairing processes. Manual molding is mainly suitable for single-piece, small-batch castings or large castings with complex shapes that are difficult to produce with molding machines.
With the development of modern mass production, machine molding has replaced most of the manual molding. Machine molding not only has high productivity, but also has stable quality and low labor intensity. It is the main method for mass production of castings in batches. The essence of machine molding is to use machine Complete all operations, at least complete the molding method of tight sand operation, which has high efficiency, high quality casting molds and storage parts, but the investment is relatively large. It is suitable for small and medium-sized castings produced in large quantities or in batches.
Special casting
With the development of science and technology and the improvement of production level, there are further requirements for casting quality, labor productivity, labor conditions and production cost, so the casting method has made great progress. The so-called special casting refers to other casting processes that are different from sand casting methods. There are dozens of special casting methods. Commonly used are investment casting, metal casting, centrifugal casting, pressure casting, low pressure casting, ceramic casting, solid casting, magnetic casting, graphite casting, differential pressure casting, continuous casting, squeeze casting, etc.


Welding is an important metal connection technology in modern manufacturing technology. The essence of joint forming technology is to use the method of heating and pressing simultaneously to make separated metal parts form interatomic bonds, thereby forming a new metal structure.
The essence of welding is to make two separate objects form a whole process by means of interatomic or intermolecular connection and particle diffusion under the condition of using or not filling materials by means of heating or pressure, or both. To make two separated objects form a permanent combination, first of all, the two objects must be brought close to each other to a distance of 0.3 to 0.5 nanometers, so that the force between atoms can interact, which is very easy for liquids. However, for solids, a large amount of energy is required from the outside to make the distance between the contact surfaces reach the bonding distance between atoms. However, due to the high solid hardness of the actual metal, no matter how high its surface accuracy is, it can only be partially contacted. In addition, there will be various impurities on the surface, such as oxides, grease, dust and gas molecules. These are the factors that hinder the combination of atoms of two objects. The welding technology is to use heating, pressure or both methods to overcome the factors that hinder the combination of atoms, so as to achieve the purpose of permanent semi-solid connection between the two objects. .
Advantages of welding: ① The mechanical properties and performance of the joint are good. ② Compared with riveting, the metal structure manufactured by welding process is light in weight, saves raw materials, has short manufacturing cycle and low cost.
Problems in welding: the structure and performance of welded joints will change compared with the base metal; defects such as welding cracks are prone to occur; residual stress and deformation will occur after welding. These will affect the quality of the welded structure.

According to the characteristics of the welding process, the types of welding mainly include fusion welding, pressure welding and brazing.

Manual arc welding
Manual arc welding is an arc welding method that uses manual manipulation of electrodes for welding. When the arc conducts electricity, a large amount of heat is generated and a strong arc is emitted at the same time. Manual arc welding uses the heat of the arc to melt the molten pool and electrode.

Other welding methods
Gas welding and gas cutting: Gas welding is a welding method that uses a gas flame as a heat source. An oxy-acetylene flame is commonly used as a heat source. Oxygen and acetylene are mixed in the torch and ignited to heat the wire and workpiece. Gas cutting, also known as oxygen cutting, is a widely used blanking method. The principle of gas cutting is to use a preheating flame to preheat the metal to be cut to the ignition point, and then inject an oxygen flow here. The metal, preheated to ignition point, burns in a stream of oxygen to form metal oxides. At the same time, this combustion process releases a large amount of heat. This heat melts the metal oxides into a slag. The slag is blown away by the oxygen stream, forming the incision, and then the heat of combustion and the preheating flame further heats and cuts other metals. Therefore, gas cutting is essentially the process of burning metal in oxygen. The heat released by metal combustion plays an important role in gas cutting.
Resistance Welding: In resistance welding, current passes through the welded joint to generate contact resistance heat. Resistance welding is a welding method that uses contact resistance heat to heat the joint to a plastic or molten state, and then applies pressure through electrodes to form a bond between atoms.
Brazing: The base metal does not melt during brazing. When brazing, brazing flux and solder are used, and the solder is heated to a molten state. The liquid solder wets the base metal, and fills the gap between the joints through capillary action, and then expands with the base metal, and forms a joint after cooling.


Under the action of impact force or static pressure, the processing method of making hot ingot or hot billet to produce partial or complete plastic deformation to obtain the required shape, size and performance is called forging.
Forging is generally processed by heating rolled round steel, square steel (medium and small forgings) or steel ingot (large forgings) to a high temperature state. Forging can improve the as-cast structure and casting defects (shrinkage cavity, porosity, etc.), make the segment structure compact, grain refinement, and uniform composition, thereby significantly improving the mechanical properties of the metal. Therefore, it is mainly used for important mechanical parts or blanks that are subjected to heavy loads, impact loads, and alternating loads, such as the main shaft and gears of various machine tools, crankshafts and connecting rods of automobile engines, crane hooks and various tools and molds. Wait.
Forging is divided into free forging, model making and tire mold.

Free forging
The method of using only general-purpose tools to directly deform the blank between the upper and lower whetstones of the manufacturing equipment to obtain parts is called free forging. The raw material for free forging can be rolled products (small and medium-sized forgings) or steel ingots (large pieces). The free forging process is flexible and the tools are simple. It is mainly suitable for single-piece and small-batch production of various parts, and it is also the only production method for extra-large forgings.
Membrane forging
Tire die forging is a forging method that uses movable simple dies to produce forgings on free forging equipment. Tire mold forging generally uses free blanking first, and then final forging in the tire mold. The shape and size of the forging are mainly guaranteed by the groove of the tire mold. The tire mold is not fixed on the equipment, and it is clamped by tools for forging during forging.
Model forging
Model forging, referred to as die forging for short, is a method of placing a metal billet heated to the forging temperature into a die cavity fixed on a die forging equipment, and deforming the billet under pressure to obtain a forging.


Stamping is a processing method in which a metal or non-metal sheet is separated or deformed by a die on a punch to obtain a workpiece. Sheet metal stamping is usually carried out at room temperature, so it is also called cold stamping. The material used for stamping must have good plasticity. Commonly used metal materials such as low carbon steel, high plastic alloy steel, aluminum and aluminum alloy, copper and copper alloy And leather, plastic, bakelite and other non-metallic materials. The advantages of stamping are high productivity and low cost; the finished product has complex shape, high dimensional accuracy, good surface quality and high rigidity, high strength and light weight, and can be used without cutting. Therefore, it is widely used in the production of automobiles, tractors, motors, electrical appliances, daily necessities and national defense industries.
The basic process of stamping
The basic process of stamping mainly includes punching and blanking, bending, deep drawing and so on. The process of separating the slab along the closed contour under the action of the die edge is called punching or blanking. Bending is the process of using a bending die to bend the axis of the workpiece into a certain angle and curvature. Deep drawing is the process of using a mold to turn a flat blank into a hollow workpiece with an opening such as a cup or box.
The punching die is an essential process equipment to realize the separation or deformation of the blank.
①The main components and functions of the punching die: the working part includes punches, dies, etc., to realize the separation or deformation of the sheet metal, and complete the stamping process. The positioning part includes guide plates, positioning pins, etc., which are used to control the feeding direction and feeding distance of the blank. The unloading part includes a unloading plate, a top plate, etc., which are used to unload the slab or workpiece after stamping. The guide part includes guide pillars, guide sleeves, etc., which are used to ensure accurate clamping of the upper and lower molds. The mold body part includes upper and lower templates, mold handles, etc., which are used to connect with the punch press and transmit pressure.
② Types of punching dies: According to the nature of the process completed by the punching die, it can be divided into punching dies, blanking dies, bending dies, drawing dies, etc., and can be divided into simple dies, continuous dies and composite dies according to the degree of combination of their processes.


Cold working

In metal technology, cold working refers to the processing technology of metal at normal temperature, such as cold rolling, cutting, stamping, etc. The deformation resistance of cold working is large, and the product processing can be realized while maintaining the stable physical properties of raw materials. Usually used in the processing of precision parts.
In mechanical manufacturing technology, cold working usually refers to the cutting of metals.

Classification of cutting processing

Cutting is a processing method that uses cutting tools to cut off excess material from a workpiece. Through cutting, the workpiece is turned into a part that meets the requirements of the shape, size and surface roughness specified in the drawing.
Cutting processing is divided into two categories: mechanical processing and fitter processing.
Machining is a method of processing various workpieces by using mechanical force. It is generally processed by workers manipulating machine tool equipment, and its methods include turning, drilling, boring, milling, planing, broaching, grinding, grinding, superfinishing and polishing.
Fitter processing refers to various processing methods that generally use hand tools on the table to process workpieces. The work content of fitters generally includes scribing, sawing, filing, scraping, grinding, drilling, reaming, Reaming, tapping, threading, mechanical assembly and equipment repair, etc.

The characteristics and functions of cutting processing

①The range of cutting precision and surface roughness is wide, and high machining precision and low surface roughness can be obtained.
②The material, shape, size and weight of cutting parts have a large range. Cutting is mostly used in the processing of metal materials, and can also be used in the processing of some non-metallic materials; the shape and size of parts are generally not limited, as long as they can be clamped on the machine tool, most of them can be cut and processed. Various common shapes. Machining parts have a wide range of weights.
③ The productivity of cutting processing is high. Under normal conditions, the productivity of machining is generally higher than that of other machining methods. Only in a few special occasions, its productivity is lower than that of precision casting, precision forging and powder metallurgy.
④There is cutting force in the cutting process, both the tool and the workpiece must have a certain strength and rigidity, and the hardness of the tool material must be greater than that of the workpiece material. Therefore, the application of cutting processing in the processing of special materials such as fine structure and high hardness and high strength is limited, thus leaving room for survival and development of special processing.

The development direction of cutting processing

With the rapid development of science and technology and modern industry, cutting processing is also developing in the direction of high precision, high efficiency, automation, flexibility and intelligence, which is mainly reflected in the following three aspects: processing equipment is moving towards digital, precision and super Precision and high-speed and ultra-high-speed development.
At present, the accuracy of ordinary machining, precision machining and high-precision machining has reached 1 micron, 0.01 micron and 0.001 micron (nanometer, that is, nano), and is approaching atomic-level machining; tool materials are developing towards superhard tool materials; production The scale is developing from the current small batch and large batch of single variety to the direction of multi-variety variable batch, and the production method is developing from the current manual operation, mechanization, stand-alone automation, and rigid assembly line automation to flexible automation and intelligent automation.
① Turning: Turning is the most commonly used processing method in mechanical parts processing. It uses the turning tool to complete the processing on the lathe. During processing, the workpiece rotates, and the turning tool moves in a straight line or a curve in the plane.
②Milling: Milling is the cutting process that uses a rotating milling cutter as a tool. Milling is generally performed on horizontal milling machines (referred to as horizontal milling), vertical milling machines (referred to as vertical milling), gantry milling machines, tool milling machines and various special milling machines or on a boring machine.
③ Grinding: Using high-speed rotating grinding wheels and other abrasive tools to process the surface of the workpiece is called grinding, and grinding is generally carried out on a grinding machine.
④ Drilling: The method of machining holes on workpieces with drills, reamers, and countersinks is collectively referred to as drilling. It is mainly used for drilling, reaming, reaming, countersinking, center hole drilling, and tapping.
⑤ Boring: Boring is a process completed on a boring machine using a boring tool. During boring, the bed spindle drives the boring tool to rotate, and the workpiece or the boring tool is fed to complete the cutting process. It is commonly used in hole processing. one of the methods.
⑥Broaching: The cutting method of using a broach as a tool to process through holes, planes and formed surfaces of workpieces is called broaching. Broaching can obtain high dimensional accuracy and small surface roughness, and has high productivity. It is suitable for Mass production in batches. In most broaching processes, the broaching machine only has the main motion of the broach for linear broaching, but no feed motion.
⑦ Planing: The cutting process in which the planer is used to reciprocate horizontally and relatively linearly on the workpiece is called planing. Planing is one of the surface processing methods, which can be carried out on planers and planers. The former is suitable for processing small and medium-sized workpieces, while the latter is suitable for processing large-scale workpieces or processing multiple medium-sized workpieces at the same time.

Advantages of cold working

1. The desired shape can be obtained while strengthening the metal;
2. Good dimensional tolerance and surface roughness can be obtained;
3. cheap;
4. Some metals can only be cold worked to a limited extent because they are brittle at room temperature;
5. Cold working reduces ductility, electrical conductivity and corrosion resistance. However, the degree of electrical conductivity reduction due to cold working is less than that of other strengthening processes, so cold working is also used to strengthen conductive materials, such as copper wire;
6. Anisotropic properties and residual stresses can also bring benefits if they are properly controlled. If not properly controlled, it will greatly weaken the material performance;
7. Since the effect of cold working will decrease or even disappear at high temperature, cold working strengthening is not suitable for those parts working in high temperature environment;

Cold Working Limitations

Cold working can cause some unwanted effects. Such as the reduction of ductility and the increase of residual stress. Since the mechanism of cold working or work hardening is to increase the dislocation density, any treatment that can rearrange or eliminate dislocations will negate the effect of cold working.
Work annealing is a heat treatment method used to remove some or all of the effects of cold working.
Processing annealing is divided into three stages: Recovery, Recrystallization and Grain Growth. Recovery occurs at lower temperatures. It can eliminate residual stress without changing the dislocation density. Recrystallization occurs at moderate temperatures. After heating above a certain temperature, new grains with very low dislocation density will be formed on the grain boundaries. Recrystallized metals are therefore less strong and more ductile due to the greatly reduced number of dislocations. The heating temperature continues to rise, and the grain growth stage is reached. During this stage, all effects of cold working are eliminated. So this stage is not expected to be reached.


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