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Metal materials: types, properties, characteristics

April 12, 2024

Types of metal materials
Metal materials are usually divided into ferrous metals, non-ferrous metals, and special metal materials.

(1) Black metal, also known as steel materials, includes 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 structural steel, stainless steel, heat-resistant steel, high-temperature alloys, stainless steel, precision alloys, etc. for various purposes. Broadly speaking, black metals also include chromium, manganese, and their alloys.

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

(3) Special metal materials, including structural and functional metal materials for different purposes. Among them are amorphous metal materials obtained through rapid condensation process, as well as quasicrystalline, microcrystalline, nanocrystalline metal materials, etc; There are also special functional alloys such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, vibration reduction and damping, as well as metal matrix composite materials.


Metal material properties
It is generally divided into two categories: process performance and usage performance. The so-called process performance refers to the performance of metal materials under specified cold and hot working conditions during the machining and manufacturing process of mechanical parts. The quality of the technological performance of metal materials determines their adaptability to processing and forming during the manufacturing process. Due to different processing conditions, the required process performance also varies, such as casting performance, weldability, malleability, heat treatment performance, cutting performance, etc.

The so-called performance refers to the performance of metal materials exhibited by mechanical parts under usage conditions, including mechanical properties, physical properties, chemical properties, etc. The performance of metal materials determines their range of use and service life. In the mechanical manufacturing industry, mechanical parts are generally used in normal temperature, normal pressure, and highly corrosive media, and each mechanical part will bear different loads during use. The resistance of metal materials to failure under load is called mechanical properties (formerly also known as mechanical properties). The mechanical properties of metal materials are the main basis for the design and material selection of parts. The mechanical properties required for metal materials will also vary depending on the nature of the applied loads (such as tension, compression, torsion, impact, cyclic loading, etc.). Common mechanical properties include strength, plasticity, hardness, impact toughness, multiple impact resistance, and fatigue limit.


Characteristics of Metal Materials
1. Fatigue
Many mechanical parts and engineering components work under alternating loads. Under the action of alternating loads, although the stress level is lower than the yield limit of the material, after a long period of repeated stress cycles, sudden brittle fracture can also occur, which is called fatigue of metal materials. The characteristics of fatigue fracture of metal materials are:
(1) The load stress is alternating.
(2) The action time of the load is relatively long.
(3) Fracture occurs instantaneously.
(4) Both plastic and brittle materials are brittle in the fatigue fracture zone. So, fatigue fracture is the most common and dangerous form of fracture in engineering.

The fatigue phenomenon of metal materials can be divided into the following types according to different conditions:

#1. High cycle fatigue
It refers to fatigue failure with stress cycles of over 100000 under low stress conditions (working stress 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 stress cycles below 10000 to 100000 under high stress (working stress close to the yield limit of the material) or high strain conditions. Due to the significant role of alternating plastic strain in this type of fatigue failure, it is also known as plastic fatigue or strain fatigue.

#3. Thermal fatigue
The fatigue damage caused by the repeated action of thermal stress caused by temperature changes.

#4 Corrosion fatigue
The fatigue failure of machine components under the combined action of alternating loads and corrosive media (such as acid, alkali, seawater, active gases, etc.).

#5 Contact fatigue
This refers to the occurrence of pitting peeling or surface crushing peeling on the contact surface of machine parts under the repeated action of contact stress, resulting in the failure and damage of the parts.


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

The higher the elongation and cross-sectional shrinkage of a metal material, the better its plasticity, meaning that the material can withstand significant plastic deformation without damage. Metal materials with an elongation greater than 5% are generally referred to as plastic materials (such as low-carbon steel), while metal materials with an elongation less than 5% are referred to as brittle materials (such as gray cast iron). A material with good plasticity can generate plastic deformation within a large macroscopic range, and at the same time, it strengthens the metal material due to plastic deformation, thereby improving the strength of the material and ensuring the safe use of the parts. In addition, materials with good plasticity can smoothly undergo certain forming processes, such as stamping, cold bending, cold drawing, straightening, etc. Therefore, when selecting metal materials as mechanical parts, certain plasticity indicators must be met.


3. Durability
The main forms of corrosion of building metals:
(1) Uniform corrosion. The corrosion on the metal surface causes a uniform thinning of the cross-section. Therefore, the annual average thickness loss value is commonly used as an indicator of corrosion performance (corrosion rate). Steel generally exhibits uniform corrosion in the atmosphere.
(2) Pore erosion. The metal corrodes in a dotted pattern and forms deep pits. The occurrence of pitting corrosion is related to the nature of the metal and its medium. Pore corrosion is prone to occur in media containing chloride salts. The maximum hole depth is commonly used as an evaluation index for pitting corrosion. The corrosion of pipelines often considers the issue of pitting corrosion.
(3) Galvanic corrosion. Corrosion caused by different potentials at the contact points of different metals.
(4) Gap corrosion. Local corrosion on metal surfaces often occurs in crevices or other concealed areas due to differences in the composition and concentration of media between different parts.
(5) Stress corrosion. Under the combined action of corrosive media and high tensile stress, the metal surface undergoes corrosion and expands inward into microcracks, often leading to sudden fracture. High strength steel bars (steel wires) in concrete may experience such damage.


4. Hardness
Hardness represents the ability of a material to resist hard objects pressing into its surface. It is one of the important performance indicators of metal materials. The higher the hardness, the better the wear resistance. The commonly used hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness.

Brinell hardness (HB): a hardened steel ball of a certain size (usually 10mm in diameter) is pressed into the surface of the material under a certain load (usually 3000kg), maintained for a period of time, and after unloading, the ratio of the load to the indentation area is the Brinell hardness value (HB), measured in kilograms per square meter (N/mm2).

Rockwell hardness (HR): when HB>450 or the sample is too small, the Brinell hardness test cannot be used and Rockwell hardness measurement should be used instead. It is a diamond cone with a top angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18mm, pressed into the surface of the tested material under a certain load, and the hardness of the material is calculated from the depth of the indentation. According to the different hardness of the test materials, different indenters and total test pressure can be used to form several different Rockwell hardness scales. Each scale is indicated with a letter after the Rockwell hardness symbol HR. The commonly used Rockwell hardness scales are A, B, and C (HRA, HRB, HRC). The C scale is the most widely used among them.

HRA: it's the hardness obtained using a 60kg load diamond cone indenter, used for materials with extremely high hardness (such as hard alloys).
HRB: it's the hardness obtained using a 100kg load and a 1.58mm diameter quenched steel ball, used for materials with lower hardness (such as annealed steel, cast iron, etc.).
HRC: it's a hardness obtained using a 150kg load and a diamond cone indenter, used for materials with high hardness (such as quenched steel).

Vickers hardness (HV): a diamond square cone indenter with a top angle of 136 ° and a load of up to 120kg is used to press into the surface of the material. The Vickers hardness value (HV) is obtained by dividing the surface product of the material's indentation pits by the load value. Hardness test is the simplest and most feasible testing method in mechanical performance testing. In order to replace certain mechanical performance tests with hardness tests, a more accurate conversion relationship between hardness and strength is needed in production. Practice has proven that there is an approximate corresponding relationship between various hardness values of metal materials, as well as 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.


The properties of metal materials
The performance of metal materials determines their applicability and rationality of application. The performance of metal materials is mainly divided into four aspects, namely: mechanical performance, chemical performance, physical performance, and process performance.

1. Mechanical property
Stress: The force borne per unit cross-sectional area inside an object is called stress. The stress caused by external forces is called working stress, and the stress that is balanced inside the object under the condition of no external force is called internal stress (such as tissue stress, thermal stress, residual stress remaining after the processing is completed).

Mechanical properties: The ability of a metal to resist deformation and fracture under external forces (loads) under certain temperature conditions is called the mechanical properties of the metal material (also known as mechanical properties). There are various forms of loads that metal materials can bear, which can be static loads or dynamic loads, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, as well as friction, vibration, impact, and so on, which can be borne alone or simultaneously. Therefore, the main indicators for measuring the mechanical properties of metal materials are as follows.


1. Strength
This is the maximum ability of a material to resist deformation and failure under external forces, which can be divided into tensile strength limits( σ b) Bending strength limit( σ Bb) Ultimate compressive strength( σ BC), etc. Due to the certain regularity of deformation to failure of metal materials under external forces, tensile tests are usually used for measurement. That is, metal materials are made into specimens of certain specifications and stretched on a tensile testing machine until the specimen fractures. The strength indicators measured mainly include:

(1) Strength limit: the maximum stress that a material can resist fracture under external forces, generally referring to the ultimate tensile strength under tensile force σ B represents the strength limit corresponding to the highest point b in the tensile test curve, commonly measured in megapascals (MPa). The conversion relationship is: 1MPa=1N/m2=(9.8) -1kgf/mm2 or 1kgf/mm2=9.8MPa.

(2) Yield strength limit: when the external force borne by 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 yield, which means that when the material bears an 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 yield occurs is called the yield strength limit, which is determined by σ S represents the yield point corresponding to the S point in the tensile test curve. For materials with high plasticity, there is a clear yield point 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 testing method, the stress at which the gauge length on the specimen produces 0.2% plastic deformation is usually specified as the conditional yield limit, using σ 0.2 represents. The yield limit index can be used as a design basis for requiring parts to not undergo significant plastic deformation during operation. However, for some important parts, it is also considered to require a flexural strength ratio (i.e σ S/ σ b) It should be small to improve its safety and reliability, but at this time, the utilization rate of materials is also low.

(3) Elastic limit: the ability of a material to deform under external forces, but still recover to its original state after removing the external force, is called elasticity. The maximum stress at which metal materials can maintain elastic deformation is the elastic limit, corresponding to point e in the tensile test curve σ E represents, in megapascals (MPa): σ In the equation e=Pe/Fo, Pe represents the maximum external force while maintaining elasticity (or the load at the maximum elastic deformation of the material).

(4) Elastic modulus: this is the stress of the material within the elastic limit range σ And strain δ The ratio of unit deformation corresponding to stress, expressed in E, in megapascals (MPa): E= σ/δ= TG α。 In the formula α The angle between the o-e line on the tensile test curve and the horizontal axis o-x. The elastic modulus is an indicator that reflects the rigidity of metal materials (the ability of metal materials to resist elastic deformation when subjected to force is called rigidity).


2. Plasticity
The maximum ability of metal materials to undergo permanent deformation without damage under external forces is called plasticity, usually measured by the elongation of the gauge length of the specimen during tensile testing δ (%) and sample area reduction rate ψ Elongation rate (%) δ= [(L1-L0)/L0] x100%, which is the ratio of the difference (increase) between the gauge length L1 and the original gauge length L0 of the specimen after the fracture surface of the specimen is aligned during the tensile test. In actual testing, the elongation measured by tensile specimens of the same material but with different specifications (diameter, cross-sectional shape - such as square, circular, rectangular, and gauge length) may vary, so special notes are generally needed. For example, the elongation measured when the initial gauge length of the most commonly used circular cross-section specimen is 5 times the diameter of the specimen is expressed as: δ 5, and the elongation measured when the initial gauge length is 10 times the diameter of the specimen is expressed as δ 10. Reduction of area ψ= [(F0-F1)/F0] x100%, which is the ratio of the difference (reduction in cross-section) between the original cross-sectional area F0 of the specimen after fracture and the minimum cross-sectional area F1 at the fracture neck during tensile testing to F0. In practice, the most commonly used circular cross-section specimens can usually be calculated through diameter measurement: ψ= [1- (D1/D0) 2] x 100%, where: D0- original diameter of the sample; D1- The minimum diameter at the neck of the fracture after the specimen is pulled apart. δ Related to ψ The larger the value, the better the plasticity of the material.


3. Resilience
The ability of metal materials to resist damage under impact loads is called toughness. Usually, impact testing is used, which characterizes the toughness of a material by the impact energy consumed per unit cross-sectional area on the fracture surface when a metal specimen of a certain size and shape is subjected to an impact load and fractured on a specified type of impact testing machine α K=Ak/F. Unit J/cm2 or Kg · m/cm2, 1Kg · m/cm2=9.8J/cm2. α K is called the impact toughness of metal materials, Ak is the impact energy, and F is the original cross-sectional area of the fracture.


4. Fatigue performance
The ultimate fatigue strength of metal materials is generally lower than the yield ultimate strength under long-term repeated stress or alternating stress σ s) The phenomenon of fracture occurring without significant deformation is called fatigue failure or fatigue fracture, which is caused by various reasons that cause localized damage to the surface of the part σ S is even greater than σ The stress of b (stress concentration) causes plastic deformation or microcracks to occur in the local area. As the number of repeated alternating stresses increases, the cracks gradually expand and deepen (stress concentration at the crack tip), resulting in a decrease in the actual cross-sectional area of the stress bearing area in the local area until the local stress is greater than σ B causes fracture. In practical applications, the maximum stress that a specimen can withstand without fracture within a specified number of cycles (usually 106-107 times for steel and 108 times for non-ferrous metals) under repeated or alternating stresses (such as tensile stress, compressive stress, bending or torsional stress, etc.) is generally taken as the fatigue strength limit σ- 1 represents in MPa.

In addition to the most commonly used mechanical performance indicators mentioned above, for some materials with particularly strict requirements, such as metal materials used in aerospace, nuclear industry, power plants, etc., the following mechanical performance indicators will also be required.


Creep limit: the phenomenon in which a material slowly undergoes plastic deformation over time at a certain temperature and constant tensile load is called creep. High temperature tensile creep test is usually used, which refers to the maximum stress at which the creep elongation (total elongation or residual elongation) of the specimen within a specified time under constant temperature and constant tensile load, or at a stage where the creep elongation rate is relatively constant and does not exceed a certain specified value, as the creep limit, expressed in MPa, where τ Is the duration of the experiment, t is the temperature, δ For elongation, σ For stress; Alternatively, V represents the creep rate.
High temperature tensile endurance strength limit: The maximum stress at which a specimen reaches a specified duration without fracture under constant temperature and constant tensile load.

Metal notch sensitivity coefficient: in K τ The ratio of stress between a notched specimen and a smooth specimen without notches for the same duration (high-temperature tensile endurance test).

Heat resistance: the resistance of a material to mechanical loads at high temperatures.


2. Chemical properties
The characteristic of a metal causing chemical reactions with other substances is called its chemical properties. In practical applications, the main considerations are the corrosion resistance and oxidation resistance of metals (also known as oxidation resistance, which specifically refers to the resistance or stability of metals to oxidation at high temperatures), as well as the influence of compounds formed between different metals and between metals and non-metals on mechanical properties. The chemical properties of metals, especially their corrosion resistance, have significant implications for the corrosion fatigue damage of metals.


3. Physical property
The physical properties of metals mainly consider:
(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, besides calculating the weight of metal parts based on density, it is important to consider the specific strength of the metal (strength σ B and density ρ To assist in material selection and acoustic impedance (density) in non-destructive testing related acoustic testing ρ The product of the speed of sound C and the fact that substances with different densities in radiographic testing have different absorption abilities for radiation energy, etc.

(2) Melting point: the temperature at which a metal transforms from solid to liquid, which has a direct impact on the melting and hot working of metal materials, and is closely related to the high-temperature performance of the material.

(3) Thermal expansion: the phenomenon where the volume of a material also changes (expands or shrinks) with temperature changes is called thermal expansion, which is often measured by the coefficient of linear expansion, that is, the ratio of the increase or decrease in the length of the material when the temperature changes by 1 ℃ to its length at 0 ℃. The thermal expansion is related to the specific heat of the material. In practical applications, specific volume (the increase or decrease in volume per unit weight of a material due to external influences such as temperature, i.e. the ratio of volume to mass) must also be considered, especially for metal parts working in high-temperature environments or alternating cold and hot environments, the impact of their expansion performance must be taken into account.

(4) Magnetism: the property that can attract ferromagnetic objects is called magnetism, which is reflected in parameters such as permeability, hysteresis loss, residual magnetic induction strength, coercive force, etc. Therefore, metal materials can be divided into paramagnetic and demagnetic, soft magnetic and hard magnetic materials.

(5) Electrical performance: mainly considering its conductivity, which has an impact on its resistivity and eddy current loss in electromagnetic non-destructive testing.


4. Process performance
The adaptability of metals to various processing methods is called process performance, which mainly includes the following four aspects:
(1) Cutting performance: reflects the difficulty of using cutting tools (such as turning, milling, planing, grinding, etc.) to cut metal materials.

(2) Forgeability: reflects the difficulty of forming metal materials during pressure processing, such as the level of plasticity of the material when heated to a certain temperature (manifested as the resistance to plastic deformation), the temperature range allowed for hot pressure processing, the characteristics of thermal expansion and contraction, and the boundaries of critical deformation related to microstructure and mechanical properties, as well as the fluidity and thermal conductivity of the metal during hot deformation.

(3) Castability: reflects the difficulty of melting and casting metal materials into castings, manifested in fluidity, gas absorption, oxidation, melting point in the molten state, uniformity and density of casting microstructure, as well as cold shrinkage rate.

(4) Weldability: reflects the difficulty of metal materials being rapidly heated locally, causing rapid melting or semi melting of the bonding area (requiring pressure), thereby firmly bonding the bonding area together and forming a whole. It is manifested in melting point, gas absorption during melting, oxidation, thermal conductivity, thermal expansion and contraction characteristics, plasticity, correlation with the microstructure of the joint and nearby materials, and its impact on mechanical properties.