The properties of metal materials determine the application range and rationality of materials. The performance of metal materials is mainly divided into four aspects, namely: mechanical properties, chemical properties, physical properties, process properties.
1. Mechanical properties
(1) the concept of stress, the force on the unit cross-sectional area inside the object is called stress. The stress caused by external force is called working stress, and the stress balanced inside the object under the condition of no external force is called internal stress (such as organizational stress, thermal stress, residual stress after the end of the machining process…). .
Two) mechanical properties, metal under a certain temperature conditions under the external force (load) action, resistance to deformation and fracture ability known as the mechanical properties of metal materials (also known as mechanical properties). Metal materials under load has a variety of forms, it can be static load, also can be dynamic loads, including alone or at the same time on the tensile stress and compressive stress, bending stress and shear stress, torsion stress, as well as the friction, vibration, shock, etc., so measure metal material mechanical performance indicators mainly has the following items:
It is the maximum capacity of material to resist deformation and damage under the action of external forces, which can be divided into tensile strength limit (σ B), bending strength limit (σ BB), compressive strength limit (σ BC) and so on. Due to the metal material under the action of external force from deformation to destruction has a certain rule to follow, so the tensile test is usually used for determination, that is, the metal material is made into a certain size of the sample, tensile test machine, until the sample fracture, the strength index is mainly determined:
1) Strength limit: the maximum stress that the material can resist fracture under the action of external force, generally refers to the ultimate tensile strength under the action of tensile force, represented by σb. For example, the strength limit corresponding to the highest point B in the tensile test curve is usually in the unit of MPa. The conversion relationship is as follows: 1 MPa = 1 n/m2 = KGF/was (9.8) – 1 or 1 KGF/was = 9.8 MPa.
2) yield strength limit: when the external force of the metal material sample exceeds the elastic limit of the material, although the stress is no longer increased, the sample still has obvious plastic deformation, which is called yield, that is, when the material bears the external force to a certain extent, its deformation is no longer proportional to the external force and produces obvious plastic deformation. The stress at which yield occurs is called the yield strength limit, denoted by σ S, and the point S corresponding to the tensile test curve is called the yield point. For the material with high plasticity, there will be obvious yield point in the tensile curve, but for the material with low plasticity, there is no obvious yield point, so it is difficult to calculate the yield limit according to the yield point. Therefore, in the tensile test method, the conditional yield limit is usually defined as the stress when the distance length on the specimen produces 0.2% plastic deformation, denoted by σ0.2. The yield limit index can be used as the basis for designing parts without obvious plastic deformation in operation. However, for some important parts, it is considered that the ratio of buckling strength (i.e., σ S /σ B) should be smaller to improve the safety and reliability of the parts, but the utilization of the materials is also low.
3) Elastic limit: the material will be deformed under the action of external force, but the ability to restore to its original state after removing external force is called elasticity. The maximum stress at which a metal material can retain elastic deformation is the elastic limit, which corresponds to point E in the tensile test graph, represented by σe and expressed in MPa: σe=Pe/Fo Where Pe is the maximum external force (or the load on the material at the time of the maximum elastic deformation).
4) Elastic modulus: this is the ratio of the stress σ to the strain δ (the unit deformation corresponding to the stress) within the elastic limit range of the material, denoted by E, E=σ/δ= TG in MPa: where α is the Angle between the O-E line on the tensile test curve and the horizontal axis O-x. Elastic modulus is an index reflecting the rigidity of metal materials (the ability of metal materials to resist elastic deformation under stress is called rigidity).
1.2. The plastic
The maximum capacity of metal material to produce permanent deformation without damage under the action of external force is called plasticity, and the elongation δ (%) and the reduction of sample section ψ (%) in tensile test δ=[(L1-L0)/L0]x100%, This is the ratio of the difference (increase) between the length L1 and the original length L0 of the specimen after the specimen is broken in the tensile test. In the actual test, the same material but different size (diameter, cross section shape – such as square, round, rectangular, and gauge length) of the tensile samples measured elongation will be different, so generally need special filling, such as the most commonly used sample with circular cross section, the initial gauge length of sample diameter 5 times when measured elongation is expressed as the delta 5, The elongation measured when the initial distance is 10 times the sample diameter is represented as δ10. ψ=[(f0-f1)/F0]x100%, which is the ratio of the difference between the original cross-sectional area (F0) and the minimum cross-sectional area (F1) at the thin neck of the fracture after the specimen is pulled in the tensile test. ψ=[1-(D1/D0)2]x100%, where: D0- original diameter of the sample;
D1- Minimum diameter at the thin neck of the fracture after the specimen is pulled. The larger the δ and ψ value is, the better the plasticity of the material is.
The ability of metallic materials to resist damage under impact load is called toughness. Impact test is usually adopted. When a metal sample of a certain size and shape is broken under impact load on a specified type of impact testing machine, the impact energy consumed on the unit cross-sectional area of the fracture is used to characterize the toughness of the material: α K =Ak/F unit J/cm2 or Kg·m/cm2, 1Kg·m/cm2=9.8J/cm2α K is known as the impact toughness of metal materials, Ak is the impact work, F is the original cross-sectional area of the fracture. 5. Fatigue strength limit of metal materials in the long-term repeated stress or under the action of alternating stress (stress is less than the yield limit strength sigma s), without significant breakage phenomenon known as the fatigue damage or deformation is fatigue fracture, this is caused by a variety of reasons make localized on the surface of the parts is greater than the sigma s even greater than that of sigma b stress (stress concentration), Plastic deformation or microcrack occurs in the part. With the increase of the number of repeated alternating stresses, the crack gradually deepens (the stress is concentrated at the crack tip) and the actual cross-sectional area of the stress at the part decreases until the local stress is greater than σb and fracture occurs. In practical application, the sample is generally under the action of repeated or alternating stress (tensile stress, compressive stress, bending or torsion stress, etc.), in the specified cycle number (generally take 106~107 times for steel, 108 times for non-ferrous metals) can withstand the maximum stress as the fatigue strength limit, expressed by σ-1, unit MPa. In addition to the above five kinds of the most commonly used mechanical performance index, especially strict with some materials, such as aerospace and nuclear industry, power plant, such as the use of metal materials, some mechanical performance index will also require the following: creep limit: under a certain temperature and constant tensile load, material slowly over time to produce plastic deformation phenomenon known as creep. Usually USES the high temperature tensile creep test, that is, under the constant temperature and constant tensile load, creep elongation of the specimen within the allotted time (total elongation or residual elongation) or a relatively constant rate in creep elongation stage, creep rate does not exceed the maximum stress when a specified value, as the creep limit, to say, unit MPa, tau in the type of test duration, t is temperature, δ is elongation, σ is stress; Or V is the creep velocity. Limit of tensile strength at high temperature: the maximum stress of the specimen without fracture at constant temperature and tensile load, expressed in unit of MPa, where τ is duration, t is temperature, σ is stress. The sensitivity coefficient of metal notch is expressed in terms of Kτ as the ratio of the stress between the notched sample and the smooth sample with the same duration (high temperature tensile endurance test) : where τ is the test duration, is the stress of the notched sample, and is the stress of the smooth sample. Or, is the ratio of the duration of notched sample to that of smooth sample under the same stress σ action. Thermal resistance: Resistance of a material to mechanical loads at high temperatures.
The properties of metals that cause chemical reactions with other substances are called the chemical properties of metals. In the practical application of the main consideration of metal corrosion resistance, oxidation resistance (also known as oxidation resistance, which is particularly refers to the metal at high temperature to oxidation resistance or stability), as well as between different metals, metal and non-metal compounds formed on mechanical properties and so on. In the chemical properties of metals, especially the corrosion resistance is of great significance to the corrosion fatigue damage of metals.
3. Physical performance
The physical properties of metals are mainly considered as follows:
1) density (specific gravity) : ρ=P/V unit gram/cubic centimeter or ton/cubic meter, where P is weight and V is volume. In practical applications, in addition to calculate the weight of the metal parts according to the density, it is important to consider the metal than strength (the ratio of sigma b strength and density rho) to help select material, and related to the nondestructive testing of acoustic detection of acoustic impedance rho (density and the product of the sound velocity C) and X-ray testing medium density of different material to ray energy absorption capacity and so on.
2) Melting point: the temperature at which metal changes from solid state to liquid state. It has a direct impact on the melting and hot processing of metal materials, and has a great relationship with the high temperature performance of materials.
3) Thermal expansibility. With the temperature change, the volume of the material also changes (expansion or contraction) phenomenon is called thermal expansion, multi-use linear expansion coefficient measurement, that is, when the temperature change 1℃, the material length of the increase and decrease of its 0℃ length ratio. Thermal expansibility is related to the specific heat of the material. In practical applications, specific capacity should also be considered (when the material is affected by temperature and other external influences, the increase or decrease of the volume of the material per unit weight, that is, the ratio of volume to mass), especially for the metal parts working in high temperature environment, or in the alternating environment of cold and heat, the impact of the expansion performance must be considered.
4) Magnetism. The property that can attract ferromagnetic objects is magnetism, which is reflected in the permeability, hysteresis loss, residual magnetic induction intensity, coercivity force and other parameters, so that metal materials can be divided into paramagnetic and inverse, soft magnetic and hard magnetic materials.
5) Electrical performance. The electrical conductivity is mainly considered, which has an effect on the electrical resistivity and eddy current loss in electromagnetic nondestructive
4. Process performance
The adaptability of metal to various processing methods is called process performance, 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: Reflecting metal material in the process of pressure to the difficulty of the molding, such as the material is heated to a certain temperature when the plasticity of the high and low (characterized by the size of the plastic deformation resistance), allows the temperature of the thermal pressure processing size, heat bilges cold shrink characteristics related to the microstructure, mechanical properties and the critical deformation limit of the thermal deformation of metal, liquidity, heat conduction performance, etc.
3) castability: reflects the difficulty of melting and casting of metal materials into castings, manifested as the fluidity, inspirability, oxidation, melting point in the melting state, the uniformity of casting microstructure, density, and cold shrinkage rate, etc.
4) weldability: Reflect the metal material in the local rapid heating, so that the combination of parts quickly melting or half melting (need to pressure), so that the combination of parts firmly together and become a whole degree of difficulty, It is manifested as melting point, inspiratory, oxidizing, thermal conductivity, thermal expansion and contraction characteristics, plasticity and the correlation with the microstructure of joints and nearby materials, and the influence on mechanical properties.
Metal materials, metal products industry development prospects
Metal products industry includes structural metal products manufacturing, metal tool manufacturing, container and metal packaging container manufacturing, container, stainless steel and similar daily metal products manufacturing, shipbuilding and offshore engineering manufacturing, etc. With the progress of society and the development of science and technology, metal products are more and more widely used in industry, agriculture and all fields of people’s life, and also create more and more value for the society.
Metal products industry in the process of development also encountered some difficulties, such as single technology, low technical level, lack of advanced equipment, talent shortage, etc., restrict the development of metal products industry. Therefore, we can improve the technical level of enterprises, introduce advanced technology and equipment, and train suitable talents to improve the development of China’s metal products industry.
The products of metal products industry will be more and more diversified, the technical level of the industry is higher and higher, the product quality will be steadily improved, and the competition and market will be further rationalized.