Mechanical properties of metal materials
1. Strength
Strength refers to the ability of a material to resist plastic deformation or fracture under load. The higher the strength of the material, the greater the load it can bear. According to the different modes of loading, the strength can be divided into tensile strength, compressive strength, bending strength and shear strength. In engineering, yield strength and tensile strength at room temperature are often used as strength indicators.
①Yield point σs The minimum stress when the material yields, in MPa.
The yield point indicates the ability of a material to resist plastic deformation. However, most mechanical parts and engineering components are not allowed to produce obvious plastic deformation during work. Therefore, it is the main basis for mechanical design and one of the important indicators for evaluating the quality of materials. For metal materials with no obvious yield phenomenon (such as cast iron, high carbon steel, etc.), it is difficult to measure, and it is usually specified that the stress at 0.2% plastic deformation is used as the conditional yield point, which is expressed by σ0.2.
②Tensile strength σb The maximum stress that a material can withstand before breaking, in MPa.
Tensile strength represents the maximum ability of a material to resist uniform plastic deformation. It indicates that the material can withstand the maximum load per unit cross-sectional area under tensile conditions. It is the main basis for designing mechanical parts and selecting materials.
For brittle materials, σ0.2 is usually difficult to measure. Therefore, when brittle materials are used to make parts, σb is generally used as the basis for material selection and design. When parts work at higher temperatures (such as hot work molds), the plastic deformation resistance index of the part material is the high temperature yield point or high temperature tensile strength, and the impact of fracture toughness should also be considered.
There are many factors that affect the intensity. Steel composition, grain size, metallographic structure, carbide type, shape, size and distribution, amount of retained austenite, internal stress state, etc., all have a significant impact on strength.
2. Plasticity
The ability of a material to produce plastic deformation without breaking under a load is called plasticity, and the plasticity index is also measured by a tensile test. Commonly used plasticity indicators are elongation after fracture and reduction of area.
①The elongation after breaking δ The relative elongation value of the gauge length after the tensile test specimen is broken, namely
δ=(L1-L0)/L0×100%
Where L0---the original gauge length of the sample, mm;
L1---The length of the gauge length when the sample is broken, mm.
②The reduction of area ψ is the percentage of the maximum reduction of the cross-sectional area at the neck of the specimen after the sample is broken to the original cross-sectional area, namely
ψ=(A0-A1)/A0X100%
Where A0---the original cross-sectional area of the sample, mm2;
A1---The minimum cross-sectional area of the neck after the sample is broken, mm2.
It is generally considered that the elongation after fracture δ ≥ 5%, or the reduction of area ψ ≥ 10% is a plastic material; on the contrary, it is called a brittle material.
3. Hardness
Hardness is an index to measure the hardness of a material, and its physical meaning is related to the test method. The hardness value is actually an engineering quantity or a technical quantity rather than a physical quantity. It is generally believed that hardness refers to the ability of a material's local surface to resist plastic deformation and destruction.
The static load indentation hardness test is widely used in engineering, that is, the indenter is pressed into the surface of the material under a specified static test force, and the hardness is evaluated by the indentation depth or the indentation surface area. Commonly used are Brinell hardness, Rockwell hardness, Vickers hardness and so on.
(1) Brinell hardness
Brinell hardness is to use a quenched steel ball or cemented carbide ball of a certain diameter, press it into the surface of the sample with the corresponding test force, and keep it for a specified time, and then remove the test force; use a reading microscope to measure the indentation on the surface of the sample Diameter, the test force divided by the indentation spherical surface area is the Brinell hardness.
When the indenter is a hardened steel ball, the Brinell hardness is indicated by the symbol HBS, which is suitable for materials with a Brinell hardness value below 450; when the indenter is a cemented carbide ball, it is indicated by HBW, and it is suitable for the Brinell hardness value. Materials below 650. It is usually represented by the hardness value + symbol (HBS or HBW). For example, 217HBS indicates that the Brinell hardness value measured by quenched steel balls is 217.
(2) Rockwell hardness
Use a diamond cone with an apex angle of 120° or a molten steel ball with a diameter of 1.588m as the indenter, press it into the surface of the sample with a prescribed test force, and determine the hardness of the metal to be tested according to the depth of the indentation.
According to the applied load and indenter, the Rockwell hardness value has three scales: HRA, HRB, and HRC. HRA is usually used to measure cemented carbide, surface hardened steel, carburized steel, etc.; HRB is usually used to measure non-ferrous metals, annealed steel, normalized steel, etc.; HRC is usually used to measure quenched and tempered steel, quenched steel, etc.
Rockwell hardness is carried out on the Rockwell hardness testing machine, and its hardness value can be read directly from the dial. The number in front of the Rockwell hardness symbol HR is the hardness value, and the letter at the back indicates the grade. For example, 60HRC means the Rockwell hardness value measured by the C scale is 60.
(3) Vickers hardness
The principle of Vickers hardness test is the same as that of Brinell hardness, and the hardness value is also measured according to the average load on the unit area of the indentation. The difference is that the Vickers hardness indenter uses a cone made of diamond with an angle of 136° Regular quadrangular pyramid.
The Vickers hardness test is carried out on the Vickers hardness testing machine, represented by the symbol HV, and the marking method is the same as the Brinell hardness. Vickers hardness has a wide range of applications, from very soft materials to very hard materials can be measured, can better measure the hardness of very thin specimens, especially the hardness of the infiltration layer of chemical heat treatment.
The hardness of the material is mainly determined by the chemical composition and organization of the material. For example, the hardness of steel when it is completely quenched into martensite depends on the carbon content in martensite, while the content of alloying elements has little effect; when the amount of retained austenite (volume fraction) in the quenched structure is> 10%, quenching The hardness is significantly reduced; when the hardenability of the steel is insufficient, and the incomplete fire is caused, pearlite or lower bainite will appear in the structure, which will cause the quenching hardness to decrease; if the cementite after spheroidizing annealing is coarse, it will be heated during quenching. , It is difficult to dissolve into austenite, and it also reduces the quenching hardness. In fact, the hardness is a comprehensive performance index of a series of different physical quantities such as elasticity, plasticity, deformation strengthening rate, strength and toughness of the material, hardness and other mechanical properties. There is a certain relationship between the indicators. Because it is difficult to determine the main mechanical properties, the hardness is often used to indirectly reflect the strength, plasticity, toughness, fatigue resistance and wear resistance of the parts.
4. Impact toughness
Many mechanical parts work under superb load, for example, hammer rods of forging hammers, punches of punching machines, etc. The impact load is greater than the static load's destructive capacity. For the material to withstand the impact load, not only high strength and a certain degree of plasticity are required, but also sufficient impact toughness is required. The ability of metal materials to resist impact load without breaking is called impact toughness. Impact toughness is usually measured by a pendulum impact test. αk is used to represent impact toughness, and the unit is J/cm2. The larger the value of impact toughness αk, the better the toughness of the material, and it is not easy to break when subjected to impact. Therefore, the impact toughness value is generally only used as a reference when selecting materials and not as a basis for calculation. Impact toughness is the comprehensive performance of the strength and plasticity of the material, and its influencing factors are the chemical composition, organization and metallurgical quality of the material. The lower the carbon content, the fewer impurities, and the finer the grains, the higher the toughness of the material.
In engineering practice, mechanical parts working under impact load are rarely damaged by a single impact with high energy, and most of them are broken by repeated impacts of millions of times with small energy. For example, the punch of the die, the piston on the rock drill, etc., so the αk value is used to measure the impact resistance of the material, which does not conform to the actual situation, but should be measured by repeated impact tests with small energy. The test proves that the failure process of the material under multiple impacts is the process of crack generation and propagation, which is the result of the accumulation and development of multiple impact damage. Therefore, the multiple impact resistance of a material is a comprehensive index that depends on the strength and plasticity of the material. When the impact energy is high, the multiple impact resistance of the material mainly depends on the plasticity; when the magical energy is low, it mainly depends on the strength. In addition to using αk to express the toughness of the shock, static bending deflection f and fracture toughness Kic are used to express the ability of the material to resist impact.
5. Abrasion resistance
Abrasion resistance refers to the ability of a material to resist damage, which is usually expressed by the amount of wear or relative wear resistance at room temperature.
The main influencing factors of wear resistance are hardness, organization and metallurgical quality. When the impact load is small, the wear resistance is proportional to the hardness, and the hardness can be used to judge the wear resistance of the material; when the impact load is large, the wear resistance is affected by the strength and toughness, and the surface hardness is not the higher at this time. The higher the better, but there is a suitable range, and the wear resistance decreases after the hardness exceeds a certain value.
The nature, quantity and distribution of carbides in the material structure also have a great influence on the wear resistance. In the matrix structure of steel, ferrite has the worst wear resistance, martensite has better wear resistance, and lower bainite has the best wear resistance; for quenched and tempered steel, it is generally considered to contain a small amount of residual The austenite tempered martensite has a structure of fine carbides uniformly distributed on the matrix, and its wear resistance is the best; in the case of high magical force, the fine-grained martensite has high strength and toughness, so Good abrasion resistance.
6. Fatigue strength
Many mechanical parts work under the action of alternating stress, such as shafts, springs, gears, rolling bearings and so on. Although the alternating stress value of the part is less than the yield strength of the material, it will break after long-term operation. This phenomenon is called fatigue fracture. Fatigue fracture occurs suddenly, whether it is brittle or ductile materials, without obvious plastic deformation beforehand, which is very dangerous and often causes serious accidents. According to statistics, 80% of mechanical parts fractures are caused by fatigue. Therefore, studying the fatigue phenomenon is of great significance for the correct use of materials and rational design of parts.
According to engineering regulations, the maximum stress at which a material does not break after countless repeated alternating loads is called the fatigue strength. When the stress is lower than a certain value, the sample will not be destroyed after infinite cycles. This stress value is called the fatigue strength of the material, which is usually expressed by σr-, such as symmetrical cycle r=-1, which is used for fatigue strength. σ-1 means. There is a certain empirical relationship between the fatigue limit of a material and its tensile strength, such as carbon steel σ-1≈(0.4~0.5)σb, gray cast iron o1≈0.4σb, and non-ferrous metals σ-1≈(0.3~0.4) σb, so under the same other conditions, the fatigue strength of the material increases with the increase of its tensile strength.
Studies have shown that the cause of fatigue fracture is the occurrence of micro-cracks in the stress concentration part, surface scratches, residual internal stress or a weak part (such as inclusions, pores, looseness, etc.) , The cracks continue to expand in depth, so that the effective load-bearing section of the part is continuously reduced, and finally when it is reduced to the extent that it cannot withstand the applied load, a sudden fracture occurs.
7. Process performance
The process performance of a material is a combination of physical, chemical and mechanical properties, which refers to the ability of the material to adapt to various processing techniques. It includes casting performance, forging performance, welding performance, cutting performance and heat treatment performance. The quality of the process performance directly affects the processing quality and production cost of the parts, so the material selection and part processing technology must be considered in the design. Different materials correspond to different processing techniques, and the quality of the material's process performance plays a decisive role in the difficulty of parts processing, production efficiency, and production costs. Therefore, process performance is another important factor that must be considered at the same time when selecting materials. The process performance of the material mainly includes the following aspects.
① Casting performance refers to the ability of metals to obtain qualified castings by casting methods. Generally, a comprehensive assessment is based on fluidity, shrinkage and segregation tendency. Different materials have different casting properties. Among several commonly used casting alloys, the casting performance of cast aluminum alloy and cast copper alloy is better than that of cast iron, and the casting performance of cast iron is better than that of cast steel. Among the cast irons, gray cast iron (wc=2.7%~3.6%) has the best casting performance.
②Welding performance refers to the difficulty of obtaining high-quality welded joints under certain welding conditions. Generally speaking, the tendency of cracks, brittleness, porosity or other defects to be used to measure welding performance. The material with excellent welding performance is not easy to produce various defects during welding, its welding process is simple, and the weld has sufficient strength and toughness. Generally, low-carbon steel and low-alloy steel (equivalent carbon≤0.4%) have good welding performance; high-carbon steel, high-alloy steel, copper alloy and aluminum alloy have poor welding performance; cast iron basically cannot be welded.
③Pressure processing performance includes forging performance, cold stamping performance, etc. The material has high plasticity and good formability, the surface quality after pressure processing is good, and it is not easy to produce cracks; the deformation resistance is low, the deformation is easier, the metal is easy to flow in the solid state, it is easy to fill the cavity, and it is not easy to produce defects. Generally, low-carbon steel has better pressure processing performance than high-carbon steel (the lower the wc, the better the pressure processing performance), and the pressure processing performance of non-alloy steel is better than that of alloy steel (the lower the alloy content, the better the pressure processing performance).
④Machining performance refers to the difficulty of the material to be cut and become a qualified work piece. Generally use the size of the cutting resistance, the surface roughness value of the part, the difficulty of chip removal during processing, and the size of the tool wear to measure its performance. In general, carbon steel with wc≈0.4% has the best cutting performance, and the cutting performance is better when the hardness of the material is between 170~230HBS.
⑤ Heat treatment process performance mainly includes hardenability, hardenability, deformation cracking tendency, temper brittleness, tempering stability, tendency to oxidative decarburization, etc. The heat treatment operation of the material is easy (that is, the quenching temperature range is wide), the hardenability and hardenability are good, the quenching deformation is small, and the processing cost is low, that is, the heat treatment process performance of the material is good.