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Overview: Metal materials refer to a general term for materials that are composed of metal elements or primarily consist of metal elements and possess metallic properties. This includes pure metals, alloys, intermetallic compounds of metallic materials, and special metallic materials, etc. (Note: Metal oxides (such as alumina) do not belong to metal materials. ”) Significance of Vol.1: The development of human civilization and social progress are closely related to metal materials. The Bronze Age and Iron Age, which followed the Stone Age, were both characterized by the use of metal materials as a prominent feature of their respective eras. In modern times, a wide variety of metal materials have become an important material foundation for the development of human society. Volume 2 Types: Metal materials are generally classified into ferrous metals, non-ferrous metals, and special metal materials. (1) Ferrous metals, also known as steel materials, include industrial pure iron with over 90% iron content, cast iron with 2%-4% carbon content, carbon steel with less than 2% carbon content, as well as various types of structural steel, stainless steel, heat-resistant steel, superalloys, precision alloys, and the like for different applications. Broadly defined, ferrous metals also include chromium, manganese, and their alloys. (2) Non-ferrous metals refer to all metals and their alloys other than iron, chromium, and manganese; they are generally classified into light metals, heavy metals, precious metals, semimetals, rare metals, and rare earth metals. Colored alloys generally have higher strength and hardness than pure metals, as well as higher electrical resistance and a lower temperature coefficient of resistance. (3) Special metal materials, including structural metal materials and functional metal materials for various applications. These include amorphous metal materials obtained through rapid solidification processes, as well as metastable crystalline, microcrystalline, and nanocrystalline metal materials ； There are also special functional alloys such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, and vibration damping, as well as metal matrix composites. The performance of Vol.3 is generally divided into two categories: process performance and service performance. The so-called process properties refer to the characteristics exhibited by metal materials under specified cold and hot processing conditions during the manufacturing of mechanical parts. The processability of metal materials determines their ability to be shaped during the manufacturing process. Due to different processing conditions, the required process properties also vary, such as castability, weldability, forgeability, heat treatment properties, and machinability. The so-called service performance refers to the properties exhibited by metal materials under operating conditions in mechanical parts, and it includes mechanical properties, physical properties, chemical properties, etc. The quality of the performance of metal materials determines their range of application and service life. In the machinery manufacturing industry, mechanical parts are generally used in normal temperature and pressure conditions as well as in highly corrosive environments, and they are subjected to various types of loads during use. The property of metal materials to resist failure under load is known as mechanical property (formerly also called mechanical strength). The mechanical properties of metal materials are the main basis for part design and material selection. Different properties of external loads (such as tension, compression, torsion, impact, cyclic loading, etc.) result in different mechanical property requirements for metal materials. Common mechanical properties include: strength, plasticity, hardness, impact toughness, resistance to multiple impacts, and fatigue limit. Properties of Metal Materials Vol.1 Fatigue Many mechanical parts and engineering components operate under alternating loads. Under alternating loads, although the stress level is below the material’s yield limit, sudden brittle fracture can still occur after prolonged cycles of stress application; this phenomenon is known as fatigue in metallic materials. The characteristics of fatigue fracture in metal materials are: (1) the load stress is alternating ； (2) The load is applied for a longer period of time ； (3) Fracture occurs instantaneously ； (4) Whether it is a plastic material or a brittle material, it is brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and dangerous form of fracture in engineering. The fatigue phenomenon of metallic materials can be classified into the following types depending on the conditions: #1 High-cycle fatigue refers to fatigue that occurs under low stress levels (where the working stress is below the material’s yield limit, or even below its elastic limit), with the number of stress cycles exceeding 100,000; it is the most common type of fatigue failure. High-cycle fatigue is generally abbreviated as fatigue. #2 Low-cycle fatigue refers to fatigue that occurs under high stress (where the operating stress is close to the material’s yield limit) or high strain conditions, with the number of stress cycles being between 10,000 and 100,000. Since alternating plastic strain plays a major role in this type of fatigue failure, it is also known as plastic fatigue or strain fatigue. #3 Thermal fatigue refers to the fatigue failure caused by the repeated action of thermal stresses resulting from temperature changes. #4 Corrosion fatigue refers to the fatigue failure of machine components resulting from the combined action of alternating loads and corrosive agents such as acids, alkalis, seawater, and reactive gases. #5 Contact fatigue refers to the condition in which, under the repeated action of contact stress on the contact surfaces of machine parts, pitting, spalling, or surface crushing and spalling occurs, leading to the failure and damage of those parts. Vol.2 Plasticity Plasticity refers to the ability of metal materials to undergo permanent deformation (plastic deformation) under the action of external loads without being damaged. When metal materials are subjected to tension, both their length and cross-sectional area change. Therefore, the plasticity of metals can be measured using two indicators: the elongation of the length (elongation percentage) and the reduction in the cross-sectional area (reduction in area percentage). The greater the elongation and reduction of area of a metal material, the better its plasticity, meaning that the material can undergo significant plastic deformation without breaking. Metal materials with an elongation of more than 5% are generally referred to as ductile materials (such as low-carbon steel), whereas those with an elongation of less than 5% are called brittle materials (such as gray cast iron). Materials with good plasticity can undergo plastic deformation over a large macroscopic scale; during this plastic deformation, the metal material becomes stronger as a result, thereby increasing its strength and ensuring the safe use of the components. Furthermore, materials with good plasticity can be easily processed using various forming techniques such as stamping, cold bending, cold drawing, and straightening. Therefore, when selecting metal materials for mechanical parts, certain plasticity criteria must be met. Vol.3 Durability: Main forms of corrosion in building metals: (1) Uniform corrosion. Corrosion on the metal surface causes the cross-section to thin uniformly. Therefore, the average annual thickness loss is commonly used as an indicator of corrosion performance (corrosion rate). Steel generally undergoes uniform corrosion in the atmosphere. (2) Pitting corrosion. Metal corrosion appears as spots and forms deep pits. The formation of pitting is related to the nature of the metal and the medium in which it is found. Pitting corrosion occurs easily in media containing chlorides. For pitting, the maximum pit depth is commonly used as an evaluation criterion. Corrosion in pipelines is mostly considered in terms of pitting corrosion. (3) Galvanic corrosion. Corrosion that occurs at the junctions of different metals due to their differing potentials. (4) Crevice corrosion. Local corrosion often occurs on metal surfaces in gaps or other concealed areas due to differences in the composition and concentration of the media in different areas. (5) Stress corrosion. Under the combined action of corrosive media and high tensile stress, corrosion occurs on the metal surface and spreads inward to form microcracks, often leading to sudden fracture. High-strength steel bars (wires) in concrete can suffer from this type of failure. Vol.4 Hardness Hardness refers to a material’s ability to resist the indentation of a hard object on its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Common hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness. Brinell hardness (HB): With a certain load (usually 3000 kg), a hardened steel ball of a specific size (with a diameter of generally 10 mm) is pressed into the surface of the material. After holding it there for a certain period of time and removing the load, the ratio of the load to the area of the indentation gives the Brinell hardness value (HB), which is expressed in kilogram-force per mm2 (N/mm2). Rockwell hardness (HR): When HB>450 or the specimen is too small, the Brinell hardness test cannot be used; instead, Rockwell hardness should be employed. It involves using a diamond cone with an apex angle of 120° or steel balls with diameters of 1.59 and 3.18 mm to press against the surface of the material under test; the hardness of the material is determined from the depth of the indentation. Depending on the hardness of the test material, different indents and total test pressures can be used to create several different Rockwell hardness scales, each of which is indicated by a letter following the Rockwell hardness symbol HR. The commonly used Rockwell hardness scales are A, B, and C (HRA, HRB, HRC). Among them, the C scale is the most widely used. HRA: It is the hardness determined using a diamond cone indenter with a load of 60 kg, and it is used for materials with extremely high hardness (such as cemented carbides). HRB: It is the hardness obtained using a load of 100 kg and hardened steel balls with a diameter of 1.58 mm; it is used for materials with lower hardness, such as annealed steel and cast iron. HRC: It is the hardness determined using a 150 kg load and a diamond cone indenter, and it is used for materials with very high hardness (such as quenched steel). Vickers hardness (HV): A diamond square pyramid indenter with a load of 120 kg or less and an apex angle of 136° is pressed into the surface of the material; the Vickers hardness value (HV) is obtained by dividing the surface area of the indentation formed in the material by the load value. The hardness test is the simplest and most straightforward method among mechanical property tests. In order to use hardness tests as a substitute for certain mechanical property tests, it is necessary in production to have a relatively accurate conversion relationship between hardness and strength. Practice has shown that there is an approximate corresponding relationship among various hardness values of metal materials, as well as between hardness values and strength values. Since the hardness value is determined by the resistance to initial plastic deformation and the resistance to further plastic deformation, the higher the strength of a material, the greater its resistance to plastic deformation, and thus the higher its hardness value. Properties of metal materials: The properties of metal materials determine the range of applications for these materials and the suitability of their use. The properties of metal materials are mainly divided into four aspects, namely: mechanical properties, chemical properties, physical properties, and process properties. Vol.1 Mechanical Properties Stress: The force exerted per unit cross-sectional area within a material is called stress. Stress caused by external forces is called working stress, while the stress that exists within an object in the absence of external forces is known as internal stress (such as structural stress, thermal stress, and residual stress remaining after a processing step is completed). Mechanical properties: The ability of a metal to resist deformation and fracture when subjected to external forces (loads) at certain temperatures is known as the mechanical properties of that metal material (also referred to as mechanical properties). Metal materials are subjected to various types of loads; these can be static or dynamic loads, including tensile stress, compressive stress, bending stress, shear stress, and torsional stress, either alone or simultaneously, as well as factors such as friction, vibration, and impact. Therefore, the key indicators used to assess the mechanical properties of metal materials are as follows. 1.1 Strength: This refers to the maximum capacity of a material to resist deformation and failure under external forces, and it can be divided into ultimate tensile strength (σb), ultimate bending strength (σbb), ultimate compressive strength (σbc), etc. Since there is a certain pattern to the transformation of metal materials from deformation to failure under external forces, tensile testing is commonly used for measurement. In this process, the metal material is turned into specimens of specific dimensions, which are then stretched on a tensile testing machine until they break. The main strength indicators obtained through this testing include: (1) Ultimate strength: the maximum stress that a material can withstand before breaking under external forces; this generally refers to the tensile strength limit under tension, denoted as σb – it corresponds to the highest point b on the tensile test curve. The common unit for expressing this value is megapascal (MPa). The conversion relationship is as follows: 1 MPa = 1 N/m2 = 9.8 kgf/mm2, or 1 kgf/mm2 = 9.8 MPa. (2) Yield strength limit: When the external force applied to a metal material specimen exceeds its elastic limit, although the stress no longer increases, the specimen still undergoes significant plastic deformation. This phenomenon is known as yield; that is, when a material is subjected to a certain level of external force, its deformation ceases to be proportional to that force and significant plastic deformation occurs. The stress at which yield occurs is called the yield strength limit, denoted by σs; the point corresponding to this on the tensile test curve is known as the yield point. For materials with high plasticity, a distinct yield point appears on the tensile curve, whereas materials with low plasticity do not have a distinct yield point, making it difficult to determine the yield strength based on the external force at the yield point. Therefore, in the tensile testing method, the stress at which a 0.2% plastic deformation occurs in the gauge length of the specimen is typically specified as the conditional yield limit, denoted as σ0.2. The yield limit criterion can be used as a design basis to ensure that parts do not undergo significant plastic deformation during operation. However, for some critical components, it is also considered necessary to have a low yield strength ratio (i.e., σs/σb) in order to improve their safety and reliability; yet this results in a lower material utilization rate. (3) Elastic limit: The ability of a material to deform under external forces but to return to its original state once the forces are removed is known as elasticity. The maximum stress at which a metal material can maintain elastic deformation is known as the elastic limit; it corresponds to point e on the tensile test curve and is denoted by σe, with units of megapascals (MPa): σe = Pe/Fo, where Pe is the maximum external force required to maintain elasticity (or in other words, the load at which the material undergoes its maximum elastic deformation). (4) Elastic modulus: This is the ratio of stress σ to strain δ (the unit amount of deformation corresponding to the stress) within the elastic limit range of a material, denoted by E, with the unit being megapascals (MPa): E=σ/δ=tgα. In the formula, α is the angle between the o-e line on the tensile test curve and the horizontal axis o-x. The elastic modulus is an indicator of the stiffness of metal materials (the ability of a metal material to resist elastic deformation under stress is referred to as stiffness). 1.2 Plasticity: The maximum capacity of a metal material to undergo permanent deformation under external forces without breaking is known as plasticity. It is usually expressed as the elongation δ (% ) of the gauge length of the specimen during tensile testing, and the reduction in cross-sectional area ψ (%). Elongation δ = x100%, where it represents the ratio of the difference (the increase) between the gauge length L1 after the specimen breaks and is recombined, to the original gauge length L0 of the specimen. In actual tests, tensile specimens made of the same material but with different specifications (diameter, cross-sectional shape – such as square, circular, rectangular – and gauge length) will exhibit varying elongation values. Therefore, it is necessary to specify these details clearly. For example, in the case of the most commonly used circular cross-section specimens, the elongation measured when the initial gauge length is 5 times the specimen’s diameter is denoted as δ5, while the elongation measured when the initial gauge length is 10 times the specimen’s diameter is denoted as δ10. The reduction of area ψ = x100% is, in a tensile test, the ratio of the difference between the original cross-sectional area F0 of the specimen and the minimum cross-sectional area F1 at the narrowest part of the fracture (the amount of area reduction) to F0. In practice, for the most commonly used circular cross-section specimens, calculations can usually be carried out by measuring the diameter: ψ=x100%, where D0 is the original diameter of the specimen ； D1 – Minimum diameter at the neck of the fracture surface after the D1 sample is pulled apart. The larger the values of δ and ψ, the better the plasticity of the material. 1.3 Toughness: The ability of metal materials to resist failure under impact loads is known as toughness. Impact testing is commonly used; in this method, a metal specimen of certain size and shape is subjected to an impact load on a specified type of impact testing machine, and the toughness of the material is characterized by the amount of impact energy absorbed per unit cross-sectional area at the fracture surface: αk = Ak/F. The unit is J/cm2 or Kg·m/cm2, with 1 Kg·m/cm2 = 9.8 J/cm2. αk is referred to as the impact toughness of the metal material, Ak is the impact energy, and F is the original cross-sectional area of the fracture surface. 1.4 Fatigue Performance: The fatigue strength limit refers to the phenomenon in which a metal material fractures without significant deformation under long-term exposure to repeated stresses or alternating stresses (where the stresses are generally below the yield strength σs). This is known as fatigue failure or fatigue fracture. It occurs due to various factors that cause local areas on the surface of the part to experience stresses greater than σs, or even greater than σb (stress concentration), leading to plastic deformation or the formation of microcracks in those areas. As the number of cycles of alternating stress increases, these cracks gradually expand and deepen (due to stress concentration at the crack tips), resulting in a reduction in the actual cross-sectional area subjected to stress in that area, until the local stress exceeds σb and fracture occurs. In practical applications, the fatigue strength limit is generally defined as the maximum stress that a specimen can withstand under repeated or alternating stresses (such as tensile stress, compressive stress, bending or torsional stress, etc.) without breaking, over a specified number of cycles (typically 106–107 cycles for steel, and 108 cycles for non-ferrous metals), and it is denoted by σ-1, with the unit being MPa. In addition to the aforementioned most commonly used mechanical property indicators, for materials with particularly stringent requirements, such as metal materials used in the aerospace, nuclear industry, power plants, etc., some additional mechanical property indicators are also required. Creep limit: The phenomenon in which a material undergoes slow plastic deformation over time under a certain temperature and constant tensile load is known as creep. The high-temperature tensile creep test is commonly used; under constant temperature and constant tensile load, the creep elongation rate of the specimen over a specified time period (total elongation or residual elongation), or the maximum stress when the creep elongation rate remains relatively constant and does not exceed a certain specified value, is taken as the creep limit, expressed in MPa. Here, τ represents the duration of the test, t represents the temperature, δ represents the elongation rate, and σ represents the stress ； Or it can be expressed as V being the creep velocity. High-temperature tensile endurance strength limit: The maximum stress that the specimen can withstand under constant temperature and constant tensile load without fracturing over a specified duration of time. Metal notch sensitivity coefficient: Denoted by Kτ, it represents the ratio of stress in a notched specimen to that in a smooth specimen under the same duration of testing (high-temperature tensile endurance test). Heat resistance: The material’s ability to withstand mechanical loads at high temperatures. Vol.2 Chemical Properties: The property of metals to undergo chemical reactions with other substances is known as the chemical properties of metals. In practical applications, the main considerations are the corrosion resistance of metals, their oxidation resistance (also known as resistance to oxidation, which refers specifically to a metal’s ability to resist oxidation or its stability at high temperatures), as well as the impact of compounds formed between different metals or between metals and non-metals on their mechanical properties. Among the chemical properties of metals, corrosion resistance in particular plays a significant role in preventing corrosion fatigue damage to metals. Volume 3: Physical Properties. The physical properties of metals mainly include: (1) Density (specific gravity): ρ=P/V, with units of grams per cubic centimeter or tons per cubic meter, where P is the weight and V is the volume. In practical applications, in addition to calculating the weight of metal parts based on their density, it is important to consider the specific strength of the metal (the ratio of strength σb to density ρ) to assist in material selection. Additionally, factors such as acoustic impedance in acoustic testing related to non-destructive testing (the product of density ρ and sound speed C), and the different absorption capacities of various materials for radiation energy in radiographic testing, are also important considerations. (2) Melting point: The temperature at which a metal changes from a solid state to a liquid state. It has a direct impact on the melting and heat treatment of metallic materials, and is closely related to the material’s properties at high temperatures. (3) Thermal expansion: The phenomenon in which a material’s volume changes (expands or contracts) as the temperature changes is known as thermal expansion. It is usually measured by the coefficient of linear expansion, which is the ratio of the change in the material’s length when the temperature changes by 1°C to its length at 0°C. Thermal expansion is related to the specific heat of the material. In practical applications, specific volume must also be taken into account (the change in the volume per unit weight of a material when affected by external factors such as temperature; in other words, the ratio of volume to mass). This is especially important for metal parts that operate in high-temperature environments or in environments with alternating hot and cold conditions, as the effects of their expansion properties must be considered. (4) Magnetism: The property of attracting ferromagnetic objects is known as magnetism. It is reflected in parameters such as magnetic permeability, hysteresis loss, residual magnetic induction, and coercive force, which allows metal materials to be classified into paramagnetic and diamagnetic materials, as well as soft magnetic and hard magnetic materials. (5) Electrical properties: Its electrical conductivity is the main factor to consider, as it affects parameters such as resistivity and eddy current loss in electromagnetic non-destructive testing. Vol.4 Processability The adaptability of metals to various processing methods is referred to as processability, which mainly includes the following four aspects: (1) Machinability: It indicates the ease with which metal materials can be machined using cutting tools such as turning, milling, planing, and grinding. (2) Ductility: It reflects the ease with which a metal material can be shaped during pressure processing. This includes the level of plasticity of the material when heated to a certain temperature (as indicated by the resistance to plastic deformation), the temperature range within which heat-based pressure processing is possible, the properties related to thermal expansion and contraction, as well as the limits of critical deformation associated with the microstructure and mechanical properties. It also encompasses the fluidity of the metal during thermal deformation and its thermal conductivity. (3) Castability: It reflects the ease with which a metal material can be melted and poured into a casting. It is characterized by the fluidity, gas absorption tendency, oxidization tendency, and melting point of the material in its molten state, as well as the uniformity and density of the microstructure of the casting, as well as the rate of cold shrinkage. (4) Weldability: It reflects the ease with which a metal material can be joined together by rapid local heating, causing the joint area to melt or semi-melt (under pressure), thereby forming a solid bond. This is determined by factors such as the melting point, the tendency to absorb gases during melting, oxidation properties, thermal conductivity, thermal expansion and contraction characteristics, plasticity, as well as the relationship between the microstructure of the joint area and that of the surrounding materials, and the impact on mechanical properties.