Powder metallurgy techniques and materials

powder metallurgy

Powder metallurgy is the processing method of making metal powder and forming and sintering processes to make metal powder or a mixture of non-metallic powder into products, which can not only make special materials difficult to make with ordinary melting methods but also make a variety of precision mechanical parts, saving labor and materials. However, its mold and metal powder cost is high, and small batch or product size is too large and should not be used. Compared with traditional material processes, powder metallurgy materials and processes have the following characteristics:

1. The powder metallurgy process is carried out under the melting point of the base metal, so it can obtain a variety of metals with a wide difference in melting point and density, metals and ceramics, metals and plastics, and other heterogeneous special functional composite materials and products.

2. Improve material properties. The fine metal or alloy powder prepared by a special method has a very fast solidification rate and fine and uniform grains, which ensures the uniform organization of the material, stable performance, and good cold and hot processing properties, and the powder particles are not limited by the alloying elements and content, which can improve the content of the strengthening phase, to develop a new material system.

3. Using a variety of forming processes, the powder raw materials can be directly formed into blanks or net parts with little or no allowance, greatly reducing the processing amount. Improve material utilization and reduce costs.

There are many kinds of powder metallurgy, mainly: tungsten and other refractory metals and alloy products; Tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), and other cemented carbide with Co, Ni, etc. are used to manufacture drill bits, turning tools, milling cutters in cutting tools and wear-resistant tools, and can also manufacture molds; Porous materials such as Cu alloy, stainless steel and Ni are used in the manufacture of sintered oil-bearing, sintered metal filters and textile rings. With the development of powder metallurgy production technology, powder metallurgy and its products will be more widely used.


1 Basic knowledge of powder metallurgy

1.1. Chemical composition and properties of powder

An aggregate of discrete particles smaller than 1mm in size is usually called a powder, and its units of measurement are generally microns (μm) or nanometers (nm).

1. Chemical composition of the powder

Commonly used metal powder iron, copper, aluminum, and its alloy powder, the impurities, and gas content is required not to exceed 1% to 2%, otherwise it will affect the quality of the product.

2. Physical properties of powder

(1) Particle size and particle size distribution

The smallest entity of powder that can be separated and exist independently is a single particle. The actual powder is often aggregated particles, that is, secondary particles. Figure 7.1.1 depicts the aggregation of several primary particles into secondary particles. The percentage of different sizes in the actual powder particle body is the particle size distribution.

Particle shape

That is the appearance geometry of powder particles. The common ones are spherical, columnar, acicular, plate and flake, which can be determined by microscope observation.

Specific surface area

That is, the total surface area of the powder per unit mass can be measured by practice. The specific surface area affects the surface energy, surface adsorption and condensation of the powder.

3. Process properties of powder

The process properties of the powder include fluidity, filling properties, compressibility and formability.

Filling characteristics

Refers to the degree of tightness when the powder is freely accumulated in the absence of external conditions. Often expressed as loose or packed density. The filling properties of the powder are related to the size, shape and surface properties of the particles.

Fluidity

Refers to the flow capacity of the powder, usually 50 grams of powder from the standard funnel out of the time required to express. The fluidity is affected by particle adhesion.

(3) Compressibility

Indicates the ability of the powder to be compacted during the pressing process, expressed as the compact density achieved at a specified unit pressure, measured in a standard mold under specified lubrication conditions. The factors affecting powder compressibility are plasticity or microhardness of particles. The compressibility of plastic metal powder is better than that of hard and brittle materials. The shape and structure of the particles also affect the compressibility of the powder.

(4) Formability refers to the ability of the compact to maintain the established shape after the powder is pressed, expressed by the minimum unit pressing pressure that the powder can form, or measured by the strength of the compact. Formability is affected by particle shape and structure.

 

1.2 Mechanism of powder metallurgy

 1. Mechanism of pressing

Pressing is the process of compacting the powder in a mold or other container into a predetermined shape and size compact under the action of external forces. The cold forming process of steel die is shown in Figure 7.1.2. The powder is loaded into the negative mold and pressed through the upper and lower die punch. In the compression process, with the movement and deformation of the powder, a large void is filled, the oxide film on the surface of the particles is broken, and the contact area between the particles is increased, so that the attraction between the atoms and the mechanical wedge cooperation between the particles is enhanced, thus forming a compact with a certain density and strength.

2. Isostatic pressing

The pressure directly acts on the powder body or the elastic die sleeve, so that the powder body is evenly compressed in all directions at the same time to obtain a compact with uniform density distribution and high strength. According to its characteristics, it is divided into two categories: cold isostatic pressing and hot isostatic pressing.

(1) Cold static pressing

That is, isostatic pressing at room temperature, and liquid is the pressure transmission medium. The powder body is loaded into the elastic mold, placed in the steel body sealed container, and the liquid is pressed into the container with a high pressure pump. The powder body in the elastic mold is evenly compressed by using the characteristics of uniform liquid pressure transfer. Therefore, the cold isostatic pressing compact has high density, more uniform, better mechanical properties, large size and complex shape, and has been used in the production of bars, pipes and large-scale products.

(2) Hot isostatic pressing

The process of placing a powder compact or a powder body in a special container in a high pressure container of a hot isostatic press, applying high temperature and high pressure, so that these powder bodies are pressed and fired into dense parts or materials. The isostatic pressing at high temperature can activate the phenomenon of diffusion and creep, promote the atomic diffusion and recrystallization of the powder and plastic deformation at a very slow rate, and the gas is the pressure transfer medium. The powder body is subjected to the joint action of high temperature and high pressure in the isostatic pressure container at the same time, which strengthens the pressing and sintering process, the pressing pressure and sintering temperature of the product are lower than that of the cold isostatic pressing, the density and strength of the product are high, and the uniformity is uniform, the grain is small, the mechanical properties are high, the defects and pores between the particles inside the material are eliminated, and the shape and size are not limited. But the hot isostatic press price is high, the investment is large. Hot isostatic pressing has been used in the production of powder high speed steel, refractory metal, superalloy and cermet.

3. Powder rolling

A method of compacting powder into a continuous strip by feeding it through a funnel between a pair of rotating rollers. By feeding the metal powder into the rotating roll gap through a special funnel, the strip blank with a certain thickness, continuous length and suitable strength can be rolled out. After pre-sintering, sintering, re-rolling and heat treatment, these blanks can be made into dense powder metallurgy sheet and strip with certain porosity. The density of powder rolled products is relatively high, the length of products is not limited in principle, and the thickness and width of rolled products will be limited by the roll; The yield is as high as 80% to 90%, and the melting and casting rolling is only 60% or lower. Powder rolling is suitable for the production of porous materials, friction materials, composite materials and cemented carbide plates and strips.

4. Slurry casting

The metal powder is formed without the application of external force, the powder is mixed with water or other liquid and suspension agent to make a slurry, and then injected into the gypsum mold, and the gypsum mold is used to absorb water to dry it. The commonly used suspension agents are polyvinyl alcohol, glycerin and sodium alginate, which are used to prevent forming particles from gathering and improve wetting conditions. In order to ensure the formation of a stable colloidal suspension, the particle size is not more than 5μm ~ 10μm, and the mass content of the powder in the suspension is 40% ~ 70%. Pulp forming process refer to book 6.2.2.

5. Extrusion forming

The powder, compact or sintered body placed in the extrusion cylinder is pressed out through the specified die hole. According to different extrusion conditions, it is divided into cold extrusion and hot extrusion. Cold extrusion is the metal powder and a certain amount of organic binder mixed at a low temperature (40 ~ 200) extrusion into a blank; Powder hot extrusion refers to the metal powder compact or powder into the envelope heated to a higher temperature extrusion, hot extrusion method can produce complex shape, excellent performance of products and materials. Extrusion forming equipment is simple, high productivity, can obtain the length direction of uniform density products.

Extrusion forming can extrude the wall is very thin straight through very small micro-shaped tubes, such as the thickness of only 0.01mm, the diameter of 1mm powder metallurgy products; It can extrude dense powder materials with complex shape and excellent physical and mechanical properties, such as sintered aluminum alloy and superalloy. The transverse density of extruded products is uniform and the production continuity is high, so it is mostly used for bars, bars and spiral bars with simple cross sections (such as twist drill, etc.).

6. Loose sintering forming

The powder is sintered directly without pressing, such as the powder is loaded into the mold to vibrate, and then the mold is sintered together in the furnace for the production of porous materials; Or the powder is evenly loose on the core plate, and then the core plate is sintered together in the furnace, and then the required density is reached by repressing or rolling, which is used for the production of brake friction plates and bimetallic materials.

The powder, compact or sintered body placed in the extrusion cylinder is pressed out through the specified die hole. According to different extrusion conditions, it is divided into cold extrusion and hot extrusion. Cold extrusion is the metal powder and a certain amount of organic binder mixed at a low temperature (40 ~ 200) extrusion into a blank; Powder hot extrusion refers to the metal powder compact or powder into the envelope heated to a higher temperature extrusion, hot extrusion method can produce complex shape, excellent performance of products and materials. Extrusion forming equipment is simple, high productivity, can obtain the length direction of uniform density products.

7. Explosive forming

The forming method of powder consolidation by means of high energy of explosion wave. The characteristic of explosive forming is that the pressure generated during the explosion is very high, and the pressure applied to the powder body is very fast. For example, after the explosive explosion, the impact pressure produced in a few microseconds can reach 106MPa(equivalent to 107 atmospheres), which is several hundred times to several thousand times higher than the unit pressure of the powder pressed by the press. The relative density of the explosive-forming pressed compact is very high and the strength is excellent. If the electrolytic iron powder is pressed by explosive, the density of the compact is close to the theoretical density of the pure iron body.

Explosive forming can process materials that are difficult to form by ordinary pressing and sintering process, such as refractory metals, high-alloy materials, etc., and can also press large compact bodies that cannot be pressed by ordinary pressure.

In addition to the above methods, there are also new forming methods such as injection molding and hot isostatic pressing.


2 Sintering mechanism

Sintering is the heat treatment process of powder or compact at temperatures below the melting point of its main components, with the aim of increasing its strength through metallurgical bonding between particles. As the temperature increases, a series of physical and chemical changes occur in the powder or compact: evaporation or volatilization of water and organic matter, removal of adsorbed gases, stress relief, and reduction of oxides on the surface of powder particles, followed by mutual diffusion and plastic flow between atoms on the surface of the powder. With the increase of the contact surface between particles, recrystallization and grain growth will occur, and sometimes solid phase melting and recrystallization will occur. The above processes often overlap and influence each other, making the sintering process very complicated. 2 Powder metallurgy process

2.1 Powder preparation

Metal powder preparation methods are divided into two categories: mechanical method and physicochemical method. There are also newly developed mechanical alloying method, amalgam method, evaporation method, ultrasonic pulverization and other ultrafine powder manufacturing technology. The preparation method determines the particle size, shape, bulk density, chemical composition, compactability and sintering property of the powder.

2.2 Pretreatment of powder

Powder pretreatment includes powder annealing, grading, mixing, granulation, adding lubricant and so on.

Step 1: Annealing

The pre-annealing of powder can reduce the oxide, reduce the content of carbon and other impurities, and improve the purity of powder. At the same time, it can also eliminate the work hardening of the powder and stabilize the crystal structure of the powder. The annealing temperature varies according to the type of metal powder, usually 0.5 to 0.6K of the melting point of the metal. Generally, the annealing temperature of electrolytic copper powder is about 300, and that of electrolytic iron powder or electrolytic nickel powder is about 700 ° C, which cannot exceed 900 ° C. Annealing is generally done in a reducing atmosphere, and sometimes in a vacuum or inert atmosphere.

Step 2: Grading

The process of dividing powder into several stages according to particle size. Classification makes it easy to control the particle size and particle size distribution of powder during batching to meet the requirements of the forming process, and standard screen screening is commonly used for classification.

Step 3 Mix

The process of homogenizing powders of two or more different compositions. There are basically two methods of mixing: mechanical method and chemical method, and the mechanical method is widely used to mix the powder or mixture evenly without chemical reaction. Mechanical mixing can be divided into dry mixing and wet mixing, dry mixing is widely used in the production of iron based products; Wet mixing is often used to prepare cemented carbide mixtures. The liquid medium often used in wet mixing is alcohol, gasoline, acetone, water, etc. Chemical mixing is the uniform mixing of metal or compound powder and salt solution of added metal; Or all the components are mixed in a solution of some salt, and then treated by precipitation, drying and reduction to obtain a evenly distributed mixture.

Additives often needed, plasticizers (gasoline, rubber solution, paraffin, etc.) to improve the strength of the compact or to prevent segregation of the powder components, lubricants (zinc carbide, molybdenum disulfide, etc.) to reduce friction between particles and between the compact and the mold wall.

4. Granulation

The process of making small particles of powder into large particles or aggregates is often used to improve the fluidity of the powder. Commonly used granulation equipment are vibrating screen, drum granulation machine, disc granulation machine and so on.

2.3 Forming

Forming is the process of transforming a powder into a condensate with the desired shape. The commonly used forming methods are die pressing, rolling, extrusion, isostatic pressing, loose sintering forming, slurry casting and explosion forming.

(1) Molding

That is, the powder is pressed in the die. Room temperature pressing generally requires more than 1 ton/cm 2 pressure, pressing pressure is too large, affect the pressure tool; And sometimes the billet has layered cracks, scars and defects. The maximum pressing pressure is 12-15 t/cm 2. When the ultimate strength is exceeded, the powder particles are crushed.

two-way pressing

The method for pressing powder in the negative mold between die strokes moving opposite each other is shown in Figure 7.2.1(b). Bidirectional pressing is more suitable for products with larger height or thickness. The density of billet pressing is more uniform than that of unidirectional pressing, but the density of the middle part of billet thickness is lower when the billet is pressed simultaneously.

Floating pressing

The powder in the floating die is pressed between a moving die and a fixed die, as shown in Figure 7.2.1(c). The negative mold is supported by a spring and is in a floating state. When the powder is pressurized, the friction between the powder and the negative mold wall is less than the spring supporting force, and only the upper mold rushes down. With the increase of pressure, when the friction force of the two is greater than the spring supporting force, the female die and the upper die punch go down together, and the relative movement between the lower die punch occurs, so that the one-way pressing is changed into the bidirectional pressure of the compact, and the compact is not bidirectional at the same time, so that the density of the compact is more uniform.

2.4 Sintering

1. Sintering method

Different products, and different properties of sintering methods are not the same.

(1) Classification according to the composition of raw materials. Sintering can be divided into unit system sintering, multicomponent system solid phase sintering and multicomponent liquid phase sintering. Unit sintering is solid phase sintering at temperatures below the melting point of pure metals (such as refractory metals and pure iron soft magnetic materials) or compounds (Al2O3, B4C, BeO, MoSi2, etc.). Multicomponent solid-phase sintering is a sintering system consisting of two or more components, in which the low-melting component of the solid phase sintering below the melting point temperature. Powder sintered alloys mostly belong to this category. Such as Cu-Ni, Fe-Ni, Cu-Au, W-Mo, Ag-Au, Fe-Cu, W-Ni, Fe-C, Cu-C, Cu-W, Ag-W, etc. Multicomponent liquid phase sintering sintering at a temperature that exceeds the melting point of the low melting components in the system. Such as W-Cu-Ni, W-Cu, WC-Co, TiC-Ni, Fe-Cu(Cu>10%, Fe-Ni-Al, Cu-Pb, Cu-Sn, Fe-Cu(Cu<10%) and so on

(2) Classification according to different feeding methods. It can be divided into continuous sintering and intermittent sintering.

        Continuous sintering

Sintering furnace has dewaxing, pre-burning, sintering, cooling each function section, sintering materials continuously or smoothly, stage to complete the sintering. Continuous sintering has high production efficiency and is suitable for mass production. The commonly used feeding methods are push-rod type, roller table type and mesh belt conveyor type.

Batch sintering

The parts are placed stationary in the furnace. Through the temperature control equipment, the sintering furnace is preheated, heated and cooled to complete the sintering process of the sintered material. Intermittent sintering can determine the appropriate sintering system according to the performance of the sintering material in the furnace, but the production efficiency is low, suitable for single piece, small batch production, commonly used sintering furnaces are bell furnace, box furnace and so on.

In addition to the above classification methods. According to whether there is liquid phase at the sintering temperature, it can be divided into solid phase sintering and liquid phase sintering. According to the sintering temperature is divided into medium temperature sintering and high temperature sintering (1100 ~ 1700), according to the different sintering atmosphere is divided into air sintering, hydrogen protection sintering (such as molybdenum wire furnace, stainless steel tube and hydrogen furnace, etc.) and vacuum sintering. In addition, there are new sintering technologies such as ultra-high pressure sintering and activated hot press sintering.

2. Factors affecting the sintering quality of powder products

There are many factors affecting the properties of sintered body, mainly the properties of powder body, forming conditions and sintering conditions. The factors of sintering conditions include heating rate, sintering temperature and time, cooling rate, sintering atmosphere and sintering pressure.

Sintering temperature and time

The temperature and time of sintering affect the porosity, density, strength and hardness of the sintered body. If the sintering temperature is too high and the sintering time is too long, the product performance will be reduced, and even the product overburning defect will appear. If the sintering temperature is too low or the time is too short, the performance of the product will be decreased due to underfiring.

Sintering atmosphere

The sintering atmosphere commonly used in powder metallurgy is reduction atmosphere, vacuum, hydrogen atmosphere and so on. The sintering atmosphere also directly affects the properties of the sintered body. Sintering in reducing atmosphere can prevent the burning of compact and reduce the surface oxide. Such as iron, copper based products often use producer gas or decomposed ammonia, tungsten carbide, stainless steel often use pure hydrogen. Active or refractory metals (such as beryllium, titanium, zirconium, tantalum), TiC containing cemented carbide and stainless steel can be sintered by vacuum. Vacuum sintering can avoid the adverse effects of harmful components in the atmosphere (H2O, O2, H2), etc., and can also reduce the sintering temperature (generally reduced by 100 ~ 150).

2.5 Post-Processing

The further treatment of the compact after sintering, according to the specific requirements of the product to determine whether post-treatment is required. Common post-treatment methods include repressing, dipping, heat treatment, surface treatment and cutting.

1. Repress

Pressure treatment to improve the physical and mechanical properties of sintered bodies, including finishing and shaping. Finishing is the re-pressing to achieve the desired size by applying pressure to the sintered body through a finishing die to improve accuracy. Shaping is repressing to achieve a specific surface shape by applying pressure to the product through a shaping die to correct the deformation and reduce the surface roughness value. Repressing is suitable for products with high requirements and good plasticity, such as iron base and copper base products.

Step 2: Dip

Method of filling the pores of the sintered body with non-metallic substances such as oil, paraffin, resin, etc. The commonly used impregnation methods are oil, plastic, molten metal, etc. Oil immersion is to immerse lubricating oil in the sintering body to improve its self-lubricating performance and rust prevention, which is often used in iron and copper based oil bearing. Impregnated plastic is the use of polytetrafluoroethylene dispersion, after curing, to achieve oil-free lubrication, often used in metal plastic anti-friction parts. Impregnation of molten metal can improve strength and wear resistance, and ferrous materials are often impregnated with copper or lead.

3. Heat treatment

The sintered body is heated to a certain temperature, and then treated by controlled cooling methods to improve the performance of the product. Commonly used heat treatment methods are quenching, chemical heat treatment, thermal mechanical treatment, etc., the process method is generally similar to dense materials. For iron-based parts that are not subject to impact and require wear resistance, integral quenching can be used, and the internal stress can be reduced due to the existence of pores, which can generally not be tempered. The iron based parts that require external hardness and internal toughness can be quenched or carburized. Hot forging is a common method to obtain dense parts. The products produced by hot forging have small grain size and high strength and toughness. 

4. Surface treatment

Common surface treatment methods include steam treatment, electroplating, zinc dipping and so on. Steam treatment is a surface process in which the workpiece is heated in hot steam at 500 ~ 560 ° C and maintained for a certain time, so that its surface and pores form a dense oxide film, which is used for iron-based parts requiring rust prevention, wear resistance or high pressure penetration. Electroplating applies the principle of electrochemistry to deposit a solid coating on the surface of the product, and its process is the same as that of dense materials. Electroplating is used for parts requiring rust prevention, wear resistance and decoration.

In addition, the shape of the sintered body can be further changed or the accuracy can be improved by forging, welding, cutting, special processing and other methods to meet the final requirements of the parts. Special processing methods such as EDM, electron beam processing, laser processing, as well as surface engineering technologies such as ion nitriding, ion implantation, vapor deposition, and thermal spraying have been used for post-treatment of powder metallurgy products, further improving production efficiency and product quality.


3. The manufacturability of powder metallurgy parts structure

The common forming method of powder metallurgy materials is to compress the metal powder into a rigid closed mold, and the mold cost is high. Due to the poor fluidity of the powder and the influence of friction, the compact density is generally low and the distribution is not uniform, the strength is not high, and the products with thin walls, slender shapes and variable sections along the pressing direction are difficult to form. Therefore, the following problems should be paid attention to in the design of the parts structure using press forming.

(1) Try to use a simple, symmetrical shape to avoid excessive changes in section and narrow slots, spheres, etc., in order to facilitate mold making and compaction.

(2) Avoid local thin walls to compact powder and prevent cracks.

(3) Avoid grooves and holes on the side walls to facilitate compaction or reduce residual blocks.

(4) Avoid increasing cross-sectional area along the pressing direction to facilitate compaction. The intersection of the walls should be rounded or chamfered to avoid sharp corners to facilitate compaction and prevent stress concentration in the mold or compact.


4. Powder metallurgy materials

Powder metallurgy is a very developed new technology, new process, has been widely used in agricultural machinery, automobiles, machine tools, metallurgy, chemical industry, light industry, geological exploration, transportation and other aspects. Powder metallurgy materials are tool materials and mechanical parts and structural materials. Tool materials are roughly powder high-speed steel, cemented carbide, superhard materials, ceramic tool materials and composite materials. Mechanical parts and structural materials are powder anti-friction materials, including porous anti-friction materials and dense anti-friction materials; Powder metallurgy iron based parts and powder metallurgy non-ferrous metal parts.

  1. Cemented carbide

Cemented carbide is composed of a hard matrix (mass fraction of 70% ~ 97%) and bonded metal. The hard matrix is the carbide of refractory metal, such as tungsten carbide and titanium carbide; The bonding metals are ferrous group metals and alloys, mainly cobalt.

(1) Types and grades of cemented carbide

Carbide is an excellent tool material, mainly used as cutting tools, metal forming tools, mining tools, surface wear-resistant materials and high-rigidity structural components. Types include tungsten carbide, steel bonded carbide, coated carbide, fine grain carbide and so on. Steel-bonded cemented carbide is a new type of tool material, performance between high-speed tool steel and cemented carbide, is one or several carbides (such as WC, TiC) as hardening phase, carbon steel or alloy steel (such as high-speed tool steel, chromium molybdenum steel, etc.) powder binder, by batching, pressing, sintering and made of powder metallurgy materials. After annealing treatment, it can be cut; After quenching and tempering treatment, it has high hardness and wear resistance equivalent to cemented carbide, and certain heat resistance, corrosion resistance and oxidation resistance. Suitable for manufacturing twist drill, milling cutter and other shapes of complex tools, molds and wear-resistant parts.

According to its composition and performance characteristics, tungsten carbide is divided into tungsten cobalt (WC-Co series), tungsten titanium cobalt (WC-TiC-Co series), tungsten titanium tantalum (niobium) [WC-TiC-TaC(NbC)-Co series, WC-TAC (NbC)-Co series]. The main chemical components of tungsten-cobalt cemented carbide are tungsten carbide (WC) and cobalt. The brand is “YG+ number” (YG is the first word of “hard cobalt” Chinese pinyin), and the number indicates the average quality fraction of cobalt. For example, YG6 means that the average mass fraction of cobalt is 6%, and the margin is tungsten carbide tungsten cobalt carbide. This kind of alloy has high bending strength, can withstand large impact, good grinding workability, but low thermal hardness (800 ~ 900℃), poor wear resistance, mainly used for processing cast iron and non-ferrous metal cutting tools.

The main chemical composition of tungsten titanium cobalt carbide is tungsten carbide, titanium carbide (TiC) and cobalt. The grade is “YT+ number” (YT is the Chinese pinyin prefix of “hard titanium”), and the number indicates the average quality fraction of titanium carbide. For example, YT15 means that TiC is 15%, and the rest is tungsten carbide of WC and Co. This kind of cemented carbide has high thermal hardness (900 ~ 1100℃), good wear resistance, but low bending strength, can not withstand large impact, poor grinding workability, mainly used for processing steel.

Tungsten titanium tantalum (niobium) tungsten carbide is also known as universal carbide or universal carbide. It is composed of tungsten carbide, titanium carbide, tantalum carbide (TaC) or niobium carbide (NbC) and cobalt. The brand is “YW+ serial number” (YW means “hard million” Chinese pinyin prefix), such as YW1 means universal carbide. This kind of cemented carbide is to add TaC or NbC in the above cemented carbide, its high thermal hardness (>1000 ° C), other properties between tungsten cobalt and tungsten titanium cobalt, it can process steel, but also can process non-ferrous metals.

⑵ Performance and application of cemented carbide

1) Performance

The hardness of cemented carbide is high, reaching 86 ~ 93HRA at room temperature, good wear resistance, cutting speed is 4 ~ 7 times higher than high-speed tool steel, tool life is 5 ~ 80 times higher, and hard materials of about 50HRC can be cut; The bending strength is high, up to 6000MPa, but the bending strength is low, about 1/3 to 1/2 of the high-speed tool steel, and the toughness is poor, about 30% to 50% of the hardened steel; Good corrosion resistance and oxidation resistance; The linear expansion coefficient is small, but the thermal conductivity is poor.

2) Application

Carbide is mainly used to manufacture cutting tools for high-speed cutting or processing high-hardness materials, such as turning tools, milling cutters, etc. Also used as mold materials (such as cold drawing die, cold punching die, cold extrusion die, etc.) and measuring tools and wear-resistant materials. According to the provisions of GB2075-87, cutting cemented carbide according to the cutting discharge form and processing object range is different, divided into P, M, K three categories, and at the same time according to the processing material and processing conditions are different, according to the use of the group, add a group of numbers behind the category to form a code. For example, P01, P10, P20…… In each category, the larger the number, the better the toughness and the lower the wear resistance.

  1. Powder high speed steel

The high content of alloying elements in high speed steel can cause serious segregation and decrease the mechanical properties when adopting the casting process. The loss of metal is also large, up to 30% to 50% of the ingot weight. Powder high speed steel can reduce or eliminate segregation, obtain uniform distribution of fine carbide, with greater bending strength and impact strength; Toughness increased by 50%, grinding is also greatly improved; The amount of distortion during heat treatment is about one-tenth of that of melted high-speed steel, and the tool life is increased by 1 to 2 times.

Powder metallurgy method can also be used to further increase the alloying element content to produce some special components of steel. Such as the composition of 9W-6Mo-7Cr-8V-8Co-2.6C of A32 high-speed steel, the cutting performance is 1 to 4 times that of melting high-speed steel.

Commonly used high-speed steel grades are W18Cr4V and W6Mo5Cr4V2, containing 0.7% ~ 0.9%C, and >10% tungsten, chromium, molybdenum, vanadium and other alloying elements. Among them, carbon ensures that high-speed steel has high hardness and high wear resistance, tungsten and molybdenum improve the thermal hardness of steel, chromium improves the hardenability of steel, and vanadium improves the wear resistance of steel.

  1. Powder metallurgy of iron and ferroalloys

In powder metallurgy production, the amount of iron powder is much larger than its metal powder. 60% to 70% of iron powder is used to make powder metallurgy parts. The main types are iron-based materials, iron-nickel alloys, iron-copper alloys ferroalloys, and steel. Powder metallurgy iron-based structural parts have the characteristics of high precision, small surface roughness value, no need or only a small amount of cutting, saving materials, high productivity, porous products, impregnable lubricating oil, friction reduction, vibration reduction, noise reduction and so on. Widely used in the manufacture of mechanical parts, such as adjusting gaskets on machine tools, adjusting rings, end caps, sliders, bases, eccentric wheels, oil pump gears, piston rings in cars, transmission gears, piston rings on tractors, as well as joints, spacers, oil pump rotors, guard sleeves, rollers, etc.

The grade of powder metallurgy iron-based structural materials is composed of FTG, which is the Chinese phonetic prefix of the words “powder”, “iron” and “structure”, and the fraction of the combined carbon content, the symbol of the main alloying element and its percentage content, the symbol of the auxiliary alloying element and its percentage content and the tensile strength. As FTG60-20, it means powder metallurgy iron-based structural materials with a combined carbon content of 0.4% to 0.7% and tensile strength of 200MPa; FTG60Cu3Mo-40 is powder metallurgy iron-based structural material with a combined carbon content of 0.4% ~ 0.7%, alloying element content of Cu2% ~ 4%, Mo0.5% ~ 1.0%, and tensile strength of 400MPa.

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