Complete Guide to Basic Knowledge of Metallic Materials

Overview: Metallic materials are a general term for materials composed mainly of metallic elements or having metallic properties. They include pure metals, alloys, intermetallic compounds, and special metal materials. (Note: Metal oxides (such as aluminum oxide) do not belong to metal materials.)

1. Significance
　　The development of human civilization and social progress are closely related to metallic materials. The Bronze Age and Iron Age, which appeared after the Stone Age, are marked by the application of metallic materials. In modern times, a wide variety of metallic materials have become an important material basis for the development of human society.

2. Types
　　Metallic materials are generally divided into ferrous metals, non-ferrous metals, and special metal materials.

　　(1) Ferrous metals, also known as steel materials, include industrial pure iron with more than 90% iron content, cast iron with 2%~4% carbon content, carbon steel with less than 2% carbon content, and various structural steels, stainless steels, heat-resistant steels, high-temperature alloys, stainless steels, precision alloys, etc. In a broad sense, ferrous metals also include chromium, manganese and their alloys.

　　(2) Non-ferrous metals refer to all metals and their alloys except iron, chromium, and manganese. They are usually divided into light metals, heavy metals, precious metals, semi-metals, rare metals, and rare earth metals, etc. The strength and hardness of non-ferrous alloys are generally higher than those of pure metals, and they have high resistance and low temperature coefficient of resistance.

　　(3) Special metal materials include structural metal materials and functional metal materials for different Applications. These include amorphous metallic materials obtained through rapid solidification processes, as well as quasicrystals, microcrystalline, and nanocrystalline metal materials; there are also stealth, hydrogen-resistant, superconducting, shape memory, wear-resistant, damping, and other special functional alloys, as well as metal matrix composites.

3. Properties
　　Generally divided into process properties and service properties. Process properties refer to the properties exhibited by metallic materials under specified cold and hot processing conditions during the manufacturing process of mechanical parts. The quality of the process properties of metallic materials determines its adaptability in the manufacturing process. Because of the different processing conditions, the required process properties are also different, such as castability, weldability, forgeability, heat treatment properties, machinability, etc.

　　Service properties refer to the properties exhibited by metallic materials under service conditions, including mechanical properties, physical properties, and chemical properties. The quality of the service properties of metallic materials determines its service range and service life. In the machinery manufacturing industry, general mechanical parts are used at normal temperature, normal pressure, and in very strongly corrosive media, and various mechanical parts will bear different loads during use. The ability of metallic materials to resist destruction under load is called mechanical properties (formerly also called mechanical properties). The mechanical properties of metallic materials are the main basis for part design and material selection. Different nature of external loads (such as tensile, compressive, torsional, impact, cyclic loads, etc.) will also have different requirements for the mechanical properties of metallic materials. Commonly used mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit, etc.

Metallic Material Characteristics

1. Fatigue
　　Many mechanical parts and engineering components are subjected to alternating loads. Under the action of alternating loads, even if the stress level is lower than the yield limit of the material, after a long time of stress repeated cyclic action, it will also cause sudden brittle fracture, this phenomenon is called metal material fatigue. The characteristics of fatigue fracture of metallic materials are:

　　(1) The load stress is alternating;
　　(2) The load acts for a longer time;
　　(3) The fracture occurs instantaneously;
　　(4) Whether it is a ductile material or a brittle material, it is brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and most dangerous form of fracture in engineering.

　　The fatigue phenomenon of metallic materials can be divided into the following types according to different conditions:
　　(1) High-cycle fatigue: refers to fatigue with more than 100,000 stress cycles under low stress (working stress is lower than the yield limit of the material, or even lower than the elastic limit). It is the most common type of fatigue failure. High-cycle fatigue is generally referred to as fatigue.

　　(2) Low-cycle fatigue: refers to fatigue with 10,000 to 100,000 stress cycles under high stress (working stress close to the yield limit of the material) or high strain conditions. Because alternating plastic strain plays a major role in this fatigue failure, it is also called plastic fatigue or strain fatigue.

　　(3) Thermal fatigue: refers to fatigue damage caused by the repeated action of thermal stress generated by temperature changes.

　　(4) Corrosion fatigue: refers to fatigue damage produced under the combined action of alternating loads and corrosive media (such as acids, alkalis, seawater, active gases, etc.).

　　(5) Contact fatigue: This refers to the repeated action of contact stress on the contact surfaces of machine parts, resulting in pitting or surface crushing and spalling, resulting in component failure.

2. Plasticity
　　Plasticity refers to the ability of metallic materials to produce permanent deformation (plastic deformation) without being destroyed under the action of external loads. When a metallic material is subjected to tension, both the length and cross-sectional area change. Therefore, the plasticity of a metal can be measured by two indicators: elongation and reduction of area.

　　The larger the elongation and reduction of area of metallic materials, the better the plasticity of the material, that is, the material can withstand larger plastic deformation without being destroyed. Generally, metallic materials with elongation greater than 5% are called ductile materials (such as low-carbon steel), while metallic materials with elongation less than 5% are called brittle materials (such as gray cast iron). Ductile materials can produce plastic deformation in a relatively large macroscopic range, and at the same time, the metallic materials are strengthened by plastic deformation, thereby improving the strength of the materials and ensuring the safe use of parts. In addition, ductile materials can be smoothly processed by certain forming processes, such as stamping, cold bending, cold drawing, and straightening. Therefore, when selecting metallic materials for mechanical parts, certain plasticity indicators must be met.

3. Durability
　　Main forms of metal corrosion in buildings:
　　(1) Uniform corrosion. Corrosion of the metal surface causes uniform thinning of the cross-section. Therefore, the average annual thickness loss is often used as an indicator of corrosion performance (corrosion rate). Steel in the atmosphere generally exhibits uniform corrosion.

　　(2) Pitting corrosion. Metal corrosion is point-like and forms deep pits. The occurrence of pitting corrosion is related to the nature of the metal and the medium in which it is located. Pitting corrosion is prone to occur in media containing chloride salts. The maximum pit depth is often used as an evaluation indicator. Pipeline corrosion often considers pitting corrosion problems.

　　(3) Galvanic corrosion. Corrosion produced at the contact point of different metals due to their different potentials.

　　(4) Crevice corrosion. Local corrosion caused by differences in the composition and concentration of the medium between different parts often occurs in crevices or other concealed areas on the metal surface.

　　(5) Stress corrosion. Under the combined action of a corrosive medium and high tensile stress, corrosion occurs on the metal surface and extends inward to form microcracks, often leading to sudden fracture. High-strength reinforcement (steel wire) in concrete may experience this type of damage.

4. Hardness
　　Hardness represents the ability of a material to resist the indentation of a hard object into its surface. It is one of the important performance indicators of metallic materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness.

　　Brinell Hardness (HB): A hardened steel ball of a certain size (generally 10 mm in diameter) is pressed into the surface of the material under a certain load (generally 3000 kg) and held for a period of time. After unloading, the ratio of the load to the indentation area is the Brinell hardness value (HB), with the unit being kgf/mm2 (N/mm2).

　　Rockwell Hardness (HR): When HB>450 or the sample is too small, the Brinell hardness test cannot be used, and the Rockwell hardness meter is used instead. It uses a diamond cone with a 120° apex angle or a steel ball with a diameter of 1.59 or 3.18 mm to press into the surface of the material to be tested under a certain load, and the hardness of the material is determined from the depth of the indentation. According to the different hardness of the test material, different indenters and total test pressures can be used to form several different Rockwell hardness scales. Each scale is indicated by a letter after 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: Hardness obtained using a 60 kg load diamond cone indenter, used for materials with extremely high hardness (such as cemented carbide).

　　HRB: Hardness obtained using a 100 kg load and a 1.58 mm diameter hardened steel ball, used for materials with lower hardness (such as annealed steel and cast iron).

　　HRC: Hardness obtained using a 150 kg load and a diamond cone indenter, used for materials with very high hardness (such as quenched steel).

　　Vickers Hardness (HV): A diamond square pyramid indenter with an apex angle of 136° is pressed into the material surface with a load of 120 kg or less. The Vickers hardness value (HV) is obtained by dividing the load by the surface area of the indentation. Hardness testing is the simplest and most convenient test method in mechanical property testing. In order to replace some mechanical property tests with hardness tests, a relatively accurate conversion relationship between hardness and strength is needed in production. Practice has proved that there is an approximate corresponding relationship between various hardness values of metallic materials and between hardness values and strength values. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value.

Properties of metallic materials: The properties of metallic materials determine the applicable range and rationality of the application of the materials. The properties of metallic materials are mainly divided into four aspects: mechanical properties, chemical properties, physical properties, and process properties.

1. Mechanical Properties
　　(A) Concept of stress: The force per unit area on a cross-section inside an object is called stress. Stress caused by external forces is called working stress, and stress balanced inside an object without external forces is called internal stress (such as organizational stress, thermal stress, residual stress remaining after the processing process...).

　　(B) Mechanical properties: Under certain temperature conditions, the ability of a metal to resist deformation and fracture when subjected to external forces (loads) is called the mechanical properties (also called mechanical properties) of the metallic material. The loads borne by metallic materials have various forms. They can be static loads or dynamic loads, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, and friction, vibration, impact, etc., which are borne alone or simultaneously. Therefore, the main indicators for measuring the mechanical properties of metallic materials are as follows:

1.1. Strength
　　This represents the maximum ability of a material to resist deformation and failure under external force. It can be divided into ultimate tensile strength (σb), ultimate bending strength (σbb), ultimate compressive strength (σbc), etc. Since metallic materials have certain laws from deformation to failure under external force, tensile tests are usually used for determination. That is, metallic materials are made into test samples of certain specifications and stretched on a tensile testing machine until the samples break. The main strength indicators measured are:

　　(1) Strength limit: The maximum stress that a material can withstand before fracture under external force, generally referring to the ultimate tensile strength under tensile force, represented by σb, such as the strength limit corresponding to the highest point b in the tensile test curve. The commonly used unit is megapascals (MPa), and the conversion relationship is: 1 MPa = 1 N/m2 = (9.8)-1 kgf/mm2 or 1 kgf/mm2 = 9.8 MPa.

　　(2) Yield Strength Limit: When the external force applied to a metal material specimen exceeds the elastic limit of the material, although the stress no longer increases, the specimen still undergoes significant plastic deformation. This phenomenon is called yielding, that is, when the material withstands external force to a certain extent, its deformation is no longer proportional to the external force, and significant plastic deformation occurs. The stress at which yielding occurs is called the yield strength limit, denoted by σs, and the corresponding point S in the tensile test curve is called the yield point. For materials with high plasticity, a clear yield point will appear on the tensile curve, while for materials with low plasticity, there is no clear yield point, making it difficult to determine the yield limit based on the external force at the yield point. Therefore, in the tensile test method, the stress at which the gauge length of the specimen undergoes a 0.2% plastic deformation is usually specified as the conditional yield limit, denoted by σ0.2. The yield limit index can be used as the basis for the design that requires parts not to produce significant plastic deformation during operation. However, for some important parts, the yield strength ratio (i.e., σs/σb) is also required to be small to improve their safety and reliability; however, the utilization rate of the material is also lower at this time.

　　(3) Elastic Limit: When a material is subjected to an external force, it will produce deformation, but the ability to recover its original state after the removal of the external force is called elasticity. The maximum stress at which a metal material can maintain elastic deformation is the elastic limit, corresponding to point e in the tensile test curve, denoted by σe, and the unit is megapascals (MPa): σe=Pe/Fo, where Pe is the maximum external force maintained during elasticity (or the load when the material has maximum elastic deformation).

　　(4) Elastic Modulus: This is the ratio of stress σ to strain δ (unit deformation corresponding to stress) within the elastic limit range of the material, denoted by E, with the unit being megapascals (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. The elastic modulus is an indicator reflecting the rigidity of metal materials (the ability of a metal material to resist elastic deformation when subjected to force is called rigidity).

1.2. Plasticity
　　The maximum ability of a metal material to produce permanent deformation without being destroyed under external force is called plasticity, usually expressed by the elongation δ (%) and the reduction of area ψ (%) of the specimen gauge length during the tensile test. Elongation δ = [(L1-L0)/L0] x 100%, which is the ratio of the difference (increase) between the gauge length L1 after the specimen is broken in the tensile test and the original gauge length L0 of the specimen to L0. In actual tests, the elongation measured for tensile specimens of the same material but different specifications (diameter, cross-sectional shape—such as square, round, rectangular, and gauge length) will be different. Therefore, it is generally necessary to add special notes. For example, for the most commonly used circular cross-sectional specimens, the elongation measured when the initial gauge length is 5 times the specimen diameter is represented as δ5, while the elongation measured when the initial gauge length is 10 times the specimen diameter is represented as δ10. Reduction of area ψ = [(F0-F1)/F0] x 100%, which is the ratio of the difference (reduction of area) between the original cross-sectional area F0 and the minimum cross-sectional area F1 at the neck of the fracture after the specimen is broken in the tensile test to F0. In practice, for the most commonly used circular cross-sectional specimens, it can usually be calculated by diameter measurement: ψ = [1-(D1/D0)2] x 100%, where: D0—original diameter of the specimen; D1—minimum diameter at the neck of the fracture after the specimen is broken. The larger the values of δ and ψ, the better the plasticity of the material.

1.3. Toughness
　　The ability of a metal material to resist fracture under impact load is called toughness. Usually, impact tests are used, that is, when a metal specimen of a certain size and shape is subjected to impact load on a specified type of impact testing machine and broken, the impact work consumed per unit cross-sectional area on the fracture represents the toughness of the material: αk=Ak/F unit J/cm2 or Kg·m/cm2, 1Kg·m/cm2=9.8J/cm2 αk is called the impact toughness of the metal material, Ak is the impact work, and F is the original cross-sectional area of the fracture. 5. Fatigue Strength Limit The phenomenon that a metal material fractures without significant deformation under long-term repeated stress or alternating stress (stress is generally less than the yield strength σs) is called fatigue failure or fatigue fracture. This is due to various reasons that cause local areas on the surface of the part to have stress greater than σs or even greater than σb (stress concentration), causing plastic deformation or microcracks in that area. As the number of repeated alternating stress cycles increases, the cracks gradually expand and deepen (stress concentration at the crack tip), leading to a reduction in the actual cross-sectional area of that area that bears the stress until the local stress is greater than σb, resulting in fracture. In practical applications, the maximum stress that a specimen can withstand without fracture within a specified number of cycles (generally 106～107 cycles for steel and 108 cycles for non-ferrous metals) under repeated or alternating stress (tensile stress, compressive stress, bending or torsional stress, etc.) is generally used as the fatigue strength limit, denoted by σ-1, unit MPa. In addition to the above five commonly used mechanical properties indicators, some mechanical properties indicators are also required for materials with particularly strict requirements, such as aerospace, nuclear industry, and power plants: Creep Limit: Under a certain temperature and constant tensile load, the phenomenon that the material slowly produces plastic deformation over time is called creep. Usually, high-temperature tensile creep tests are used, that is, under constant temperature and constant tensile load, the creep elongation rate (total elongation or residual elongation) of the specimen within a specified time or the maximum stress when the creep elongation rate does not exceed a specified value in the stage where the creep elongation rate is relatively constant, is used as the creep limit, expressed as MPa, where τ is the test duration, t is the temperature, δ is the elongation, and σ is the stress; or expressed as, V is the creep rate. High-Temperature Tensile Endurance Strength Limit: The maximum stress that a specimen can withstand without fracture under constant temperature and constant tensile load for a specified duration, expressed as MPa, where τ is the duration, t is the temperature, and σ is the stress. Metal Notch Sensitivity Coefficient: Denoted by Kτ, it is the ratio of stress of notched specimens to stress of smooth specimens without notches at the same duration (high-temperature tensile endurance test): Where τ is the test duration, is the stress of the notched specimen, and is the stress of the smooth specimen. Or expressed as: that is, under the same stress σ, the ratio of the duration of the notched specimen to the duration of the smooth specimen. Heat Resistance: The resistance of a material to mechanical loads at high temperatures.

2. Chemical Properties

　　The characteristics of a metal that cause chemical reactions with other substances are called the chemical properties of the metal. In practical applications, the main considerations include the corrosion resistance, oxidation resistance (also known as oxidation resistance, which specifically refers to the resistance or stability of a metal to oxidation at high temperatures), and the influence of compounds formed between different metals, metals, and non-metals on mechanical properties. In the chemical properties of metals, corrosion resistance, in particular, has a significant impact on corrosion fatigue damage of metals.

3. Physical Properties
　　The physical properties of metals mainly consider:
　　(1) Density (Specific Gravity): ρ=P/V Unit g/cm³ or t/m³, 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 important to consider the specific strength of the metal (the ratio of strength σb to density ρ) to assist in material selection, as well as the acoustic impedance (the product of density ρ and sound velocity C) in acoustic testing related to non-destructive testing, and the different absorption capacities of different density substances for ray energy in ray detection, etc.

　　(2) Melting Point: The temperature at which a metal changes from a solid to a liquid state, which directly affects the smelting and hot processing of metal materials and is closely related to the high-temperature performance of the material.

　　(3) Thermal Expansion. The phenomenon that the volume of a material changes (expands or contracts) with temperature changes is called thermal expansion, which is often measured by the linear expansion coefficient, that is, the ratio of the increase or decrease in the length of the material to its length at 0℃ when the temperature changes by 1℃. Thermal expansion is related to the specific heat of the material. In practical applications, it is also necessary to consider the specific volume (the increase or decrease in the volume of a unit weight of material when affected by external factors such as temperature, that is, the ratio of volume to mass), especially for metal parts working in high-temperature environments or in cold and hot alternating environments, the influence of their expansion performance must be considered.

　　(4) Magnetism. The property of attracting ferromagnetic substances is magnetism, which is reflected in parameters such as magnetic permeability, hysteresis loss, residual magnetic induction intensity, and coercive force, so that metal materials can be divided into paramagnetic and diamagnetic, soft magnetic and hard magnetic materials.

　　(5) Electrical Properties. Mainly consider its conductivity, which affects its resistivity and eddy current loss in electromagnetic non-destructive testing.

4. Processability
　　The adaptability of metals to various processing methods is called processability, which mainly includes the following four aspects:
　　(1) Machinability: Reflects the ease with which metal materials can be machined using cutting tools (such as turning, milling, planing, grinding, etc.).

　　(2) Forgeability: Reflects the ease with which metal materials can be formed during pressure processing, such as the high or low plasticity of the material when heated to a certain temperature (expressed by the magnitude of the plastic deformation resistance), the size of the temperature range allowed for hot pressure processing, the characteristics of thermal expansion and contraction, and the critical deformation limits related to microstructure and mechanical properties, the fluidity of the metal during hot deformation, thermal conductivity, etc.

　　(3) Castability: Reflects the ease with which metal materials can be melted and cast into castings, manifested in fluidity, gas absorption, oxidation, melting point in the molten state, uniformity and density of the microstructure of the castings, and shrinkage rate, etc.

　　(4) Weldability: Reflects the ease with which metal materials can be locally and rapidly heated to make the bonding parts quickly melt or semi-melt (pressure is required), so that the bonding parts are firmly bonded together to form a whole, manifested in the melting point, gas absorption and oxidation during melting, thermal conductivity, thermal expansion and contraction characteristics, plasticity, and the correlation with the microstructure of the materials in and near the joint, and the influence on mechanical properties, etc.

Development Prospects of Metal Materials and Metal Products Industry
　　The metal products industry includes structural metal product manufacturing, metal tool manufacturing, container and metal packaging container manufacturing, containers, stainless steel and similar daily metal product manufacturing, shipbuilding and marine engineering manufacturing, etc. With the progress of society and the development of science and technology, metal products are increasingly widely used in various fields of industry, agriculture, and people's lives, creating greater and greater value for society.

　　The metal products industry has also encountered some difficulties in its development, such as single technology, low technical level, lack of advanced equipment, and shortage of talents, which have restricted the development of the metal products industry. To this end, measures can be taken to improve the technical level of enterprises, introduce advanced technical equipment, and cultivate applicable talents to improve the development of China's metal products industry.

　　In 2009, the products of the metal products industry will become increasingly diversified, the technical level of the industry will become increasingly high, product quality will steadily improve, and competition and the market will be further rationalized. Coupled with the further standardization of the industry by the state and the implementation of relevant industry preferential policies, from 2009 to 2012, the metal products industry will have huge development space.