Properties of metallic materials
The performance characteristics of metallic materials determine the scope of the materials and the rationality of the application. The properties of metallic materials are mainly divided into four aspects, namely, mechanical properties, chemical properties, physical properties and technological properties.
1. Mechanical properties
(1) The concept of stress, the force per unit cross-sectional area inside an object is called stress. The stress caused by an external force is called the working stress, and the stress balanced inside the object in the absence of an external force is called the internal stress (for example, tissue stress, thermal stress, residual stress left after the processing process...).
(2) Mechanical properties. When metals are subjected to external forces (loads) under certain temperature conditions, the ability to resist deformation and fracture is called mechanical properties (also known as mechanical properties) of metallic materials. There are many forms of stress on metallic materials, which can be static loads or dynamic loads, including tensile stresses, compressive stresses, bending stresses, shear stresses, torsion and friction stresses, vibrations, shocks, etc., so indicators for measuring mechanical properties metal materials mainly include the following items:
This is the maximum capacity that characterizes the resistance of a material to deformation and damage under the action of an external force, and can be divided into tensile strength (σb), flexural strength (σbb), compressive strength (σbc), etc. Since the metal material has certain rules to be followed from deformation to failure by an external force, it is usually measured by a tensile test, that is, a metal material is made into a sample of a certain specification and stretched by a tensile test. machine until the destruction of the test sample, the main strength indicators:
(1) Tensile strength: the maximum stress at which a material can resist failure under an external force, usually refers to the tensile strength under tension expressed in σb, such as the tensile strength corresponding to the highest point b on the test curve in tension, the commonly used unit is megapascal (MPa), conversion ratio: 1 MPa = 1 N/m2 = (9.8) - 1 kgf/mm2 or 1 kgf/mm2 = 9.8 MPa.
(2) Yield strength: when the external force of a sample of metal material exceeds the elastic limit of the material, although the stress is no longer increased, the sample is still subject to obvious plastic deformation. This phenomenon is called yield strength, that is, the material bears When the external force reaches a certain level, its deformation is no longer proportional to the external force and causes obvious plastic deformation. The stress during yield is called the yield strength, expressed as σs, and the corresponding point S on the tensile test curve is called the yield strength. For materials with high ductility, there will be an obvious yield strength on the tensile curve, no for materials with low ductility, there is no obvious yield point, so it is difficult to find the yield point from the external force of the yield point. . Therefore, in the tensile test method, the stress at which the effective length of the sample causes a plastic deformation of 0.2% is usually specified as a conditional yield stress, expressed as σ0.2. The yield stress index can be used as a calculation basis for the requirement that parts do not cause obvious plastic deformation during operation. However, for some important parts, it is also considered that the yield strength ratio (i.e., σs/σb) should be smaller in order to improve its safety and reliability, but at this time, the material utilization ratio is also low.
(3) Elastic limit: The material will deform under the action of an external force, but the ability to return to its original shape after the removal of the external force is called elasticity. The maximum stress at which a metallic material can maintain elastic deformation is the elastic limit, which corresponds to point e on the tensile test curve, expressed in σe, and the unit is MPa: σe=Pe/Fo, where Pe is the maintenance time Elasticity Maximum external force (or maximum elastic deformation of the material load).
(4) Modulus of elasticity: it is the ratio of stress σ to strain δ (unit strain corresponding to stress) of the material within the elastic range, expressed in E, and the unit is megapascal (MPa). : E= σ/δ=tgα, where α is the angle between the o-e line on the tensile test curve and the horizontal axis o-x. Modulus of elasticity is a measure of the stiffness of metallic materials (the ability of metallic materials to resist elastic deformation under stress is called stiffness).
The maximum ability of a metallic material to permanently deform without being damaged by an external force is called ductility, usually measured by the elongation δ (%) of the calculated length of the specimen in tensile testing and the shrinkage rate of the section of the specimen ψ (%) Relative elongation δ=[(L1- L0)/L0]x100%, which is the difference between the effective length L1 and the initial effective length L0 of the sample after the sample breaks during the tensile test (growth) to the ratio L0. In actual testing, the measured elongation of specimens of the same material but with different characteristics (diameter, cross-sectional shape, e.g. square, round, rectangular and gauge length) will be different, so special notes are usually required, example For the most commonly used specimen round cross-section, the elongation measured when the initial gauge length is 5 times the sample diameter is expressed as δ5, and the elongation measured when the initial gauge length is 10 times the sample diameter. expressed as δ10. Area reduction ψ=[(F0-F1)/F0]x100%, representing theth difference between the initial cross-sectional area F0 and the minimum cross-sectional area F1 at the narrow neck of the crack (reduction in section) and F0 after the sample breaks during the tensile test Ratio. In practice, the most commonly used round specimens can usually be calculated by measuring the diameter: ψ=[1-(D1/D0)2]x100%, where: D0 is the initial diameter of the specimen; fracture of the sample after rupture The smallest diameter at the narrow neck. The larger the value of δ and ψ, the better the plasticity of the material.
The ability of a metallic material to resist fracture under impact loading is called toughness. The impact method is usually used, i.e., when a metal sample of a certain size and shape is destroyed under the action of an impact load on an impact machine of a given type, the impact energy expended per unit area of the cross-section on the fracture surface characterizes the impact strength of the material: αk = Ak / Unit F equal to J/cm2 or kg m/cm2, 1 kg m/cm2=9.8 J/cm2αk is called the impact strength of metallic materials, Ak is the impact energy, and F is the initial fracture cross-sectional area. 5. Fatigue limit The phenomenon in which metallic materials fail without significant deformation under long-term repetitive stress or alternating stress (stress is usually less than the yield strength σs) is called fatigue failure or fatigue failure, due to a variety of reasons causing in the local area surface of the part, the occurrence of a stress (stress concentration) greater than σs or even greater than σb, causing plastic deformation or the appearance of microcracks in a local area. As the number of repeated alternating stresses increases, the crack gradually expands and deepens (stress concentration at the crack tip in this place) leads to decrease in the actual cross-sectional area bearing the stress at that location until the local stress exceeds σb and failure occurs. In practical applications, the specimen is usually subjected to repeated or alternating loads (tensile, compression, bending or torsion, etc.) for a certain number of cycles (typically 106 to 107 times for steel and 108 times). the maximum stress that can withstand without destruction, expressed in terms of σ-1, is taken as the fatigue strength limit, and MPa is taken as a unit. In addition to the above five most commonly used mechanical properties, some materials with particularly stringent requirements, such as metallic materials used in aerospace, nuclear industry, power plants, etc., also require the following mechanical properties: Creep strength: at a certain creep is a slow plastic deformation of a material over time under the influence of temperature and a constant tensile load. Commonly used high temperature testTensile creep resistance, i.e. at constant temperature and constant tensile load, the creep elongation (total elongation or permanent elongation) of the specimen for a given time or the rate of creep elongation is relatively constant. The maximum stress is taken as the creep limit when the creep rate does not exceed a certain value, expressed in MPa, where τ is the test duration, t is the temperature, δ is the relative elongation, σ is the stress; V is the creep rate. High Temperature Tensile Strength: The maximum stress on a specimen that reaches a given duration without failure under constant temperature and constant tensile load, expressed in MPa, where τ is duration, t is temperature, σ is for stress. Metal notch sensitivity ratio: expressed in Kτ at the same duration (tensile endurance test at high temperature), the ratio of the stress of a notched specimen to a smooth unnotched specimen: where τ is the duration of the test and a is the notched test. sample stress is the stress of a smooth sample. Or use: to express, i.e., at the same stress σ, the ratio of the duration of a notched specimen to that of a smooth specimen. Heat resistance: The resistance of a material to mechanical stress at elevated temperatures.
2. Chemical Properties
The properties of metals that cause chemical reactions with other substances are called the chemical properties of metals. In practical applications, corrosion resistance and oxidation resistance of metals (also known as oxidation resistance, which especially refers to the resistance or stability of metals to oxidation at high temperatures), as well as the relationship between various metals, metals and the effect of the formation of compounds between non-metals on mechanical properties etc. Among the chemical properties of metals, especially corrosion resistance, corrosion-fatigue damage to metals is of great importance.
3. Physical Properties
The physical properties of metals are mainly considered:
(1) Density (specific gravity): ρ=P/V in grams per cubic centimeter or tons per cubic meter, where P is weight and V is volume. In practical applications, in addition to calculating the weight of metal parts based on density, it is very important to consider the specific strength of the metal (ratio of strength σb to density ρ) to help in material selection, as well as acoustic impedance under acoustic conditions. tests related to non-destructive testing (the product of density ρ and the speed of sound C), and materials with different densities when detecting beams have different beam energy absorption abilities, and so on.
(2) Melting point: The temperature at which the metal changes from solid to liquid, which directly affects the melting and heat treatment of metal materials and is closely related to the high temperature performance of materials.
(3) Thermal expansion. When the temperature changes, the volume of the material (expansion or contraction) also changes (expansion or contraction) is called thermal expansion, which is usually measured by the coefficient of linear expansion, that is, when the temperature changes by 1 ° C, the ratio of increase or decrease in the length of the material to its length at 0 ° C. Thermal expansion is related to the specific heat capacity of the material. In practical applications, specific volume (when the material is subjected to external influences such as temperature, increase or decrease in material volume per unit weight, i.e. volume to mass ratio) should also be considered, especially for high temperature or cold or hot conditions. .For metal parts operating in variable environments, the influence of their expansion properties must be taken into account.
(4) Magnetic. The property that can attract ferromagnetic objects is magnetism, which is reflected in parameters such as magnetic permeability, hysteresis loss, remanent magnetic induction, coercive force, etc., so that metal materials can be divided into paramagnetic and diamagnetic, soft magnetic and hard magnetic . materials .
(5) Electrical properties. The main consideration is its electrical conductivity, which affects its resistivity and eddy current losses in electromagnetic non-destructive testing.
4. Process Performance
The adaptability of metals to different processing methods is called process productivity, which mainly includes the following four aspects:
(1) Cutting performance: reflects the difficulty of cutting metal materials with cutting tools (such as turning, milling, planing, grinding, etc.).
(2) Malleability: Indicates the difficulty of forming metal materials in the forming process, such as the level of ductility (expressed by the amount of resistance to plastic deformation) when the material is heated to a certain temperature, allowing hot forming. Temperature range value, thermal expansion and contraction characteristics, limit critical deformation associated with the microstructure and mechanical properties, fluidity and thermal conductivity of metals during thermal deformation, etc.
(3) Castability: Reflects the difficulty of melting and casting metal materials into castings, which manifests itself in fluidity, gas absorption, oxidation, melting point, uniformity and compactness of the casting microstructure in the molten state and cold. shrinkage, etc.
(4) Weldability: reflects the difficulty of locally heating the metal material to quickly melt or semi-melt the connecting part (pressure is needed) so that the connected part can be firmly connected and become a whole. These are the melting temperature, gas absorption during melting, oxidation, thermal conductivity, thermal expansion and contraction characteristics, plasticity, relationship with the microstructure of compounds and nearby materials, and the effect on mechanical properties.