Common Performance Indicators Of Metal Materials
I. Overview
Commonly used properties of metal materials include: chemical properties, physical properties and mechanical properties, etc. These common properties determine the use scope and use environment of metal materials.
Engineering and technical personnel use these performance indicators as a basis for the selection and application of metal materials in the design of mechanical parts.
2. Chemical properties
The chemical properties of metal materials refer to their reactivity and stability in chemical environments. This includes the reactivity of metal materials with acids, alkalis, oxygen, water and other substances, as well as their corrosion resistance, etc. The quality of chemical properties directly affects the service life and performance stability of materials in specific environments.
Common corrosion indicators of chemical properties:
1. Uniform corrosion
Definition: The phenomenon in which chemical or electrochemical reactions occur uniformly on the entire exposed surface or a large area of a metal material, and the metal becomes macroscopically thinner, is called uniform corrosion, also called general corrosion or continuous corrosion.
Features: This kind of corrosion is evenly distributed on the entire internal and external surfaces of the metal, causing the surface to continuously reduce, and eventually destroying the stressed parts. This is the most common corrosion form of steel, which is less harmful and has little impact on the mechanical properties of the metal. .
2. Intergranular corrosion
Definition: The phenomenon of corrosion along the boundaries of metal grains is called intergranular corrosion.
Features: This kind of corrosion occurs inside the metal along the edges of the grains, and is the most dangerous type of corrosion among metal materials. After intergranular corrosion occurs, the outer dimensions of the metal are almost unchanged, and most of them can still maintain the metallic luster. However, the strength and ductility of the metal decrease, cracks appear on the surface after cold bending, and in severe cases, the metal sound is lost. When doing cross-sectional metallographic examination, it can be found that local corrosion will occur at the grain boundaries or adjacent areas, and even the grains may fall off. The corrosion spreads along the grain boundaries and is relatively uniform.
3.Selective corrosion
Definition: The phenomenon that a certain element or a certain structure in an alloy is selectively corroded during the corrosion process is called selective corrosion.
Features: Selective corrosion may occur in non-ferrous metal alloys, cast iron and stainless steel.
4. Stress corrosion cracking
Definition: The brittle cracking phenomenon that occurs in metal under the combined action of sustained tensile stress (including external load, thermal stress, cold, hot processing, residual stress after welding, etc.) and specific corrosive media is called stress corrosion cracking.
Features: When stress corrosion cracking occurs in metal, corrosion cracks or even fractures will appear. The starting point of the crack is often a small hole of pitting corrosion, or the bottom of a small corrosion pit. There are three types of crack propagation: along grain boundaries, through grains and mixed types. The main crack is usually perpendicular to the stress direction and most often has branches. The end of the crack is sharp, and the corrosion degree of the inner wall of the crack and the outer surface of the metal is usually very slight, while the expansion speed of the end of the crack is very fast, and the fracture has the characteristics of a brittle crack, which is very harmful.
5. Corrosion fatigue
Definition: The damage phenomenon caused by the combined action of corrosive media and alternating stress or pulsating stress on metal is called corrosion fatigue.
Features: Produce corrosion pits and a large number of cracks, so that the fatigue limit of the metal no longer exists. Corrosion fatigue generally has multiple crack sources. Most of the cracks penetrate the grain and are generally unbranched. The ends of the cracks are relatively pure. Most of the fracture is covered by corrosion products, and a small part shows brittle failure. The main means to eliminate this kind of corrosion is to promptly relieve stress on the metal.
6. Pitting corrosion
Definition: The corrosion phenomenon in which no corrosion occurs or the corrosion is very slight on most surfaces of metal, but small corrosion holes appear locally and develop to depth is called pitting corrosion.
Characteristics: This kind of corrosion is concentrated in a small area on the metal surface, and rapidly develops into the depth, and finally penetrates the metal. It is a more harmful corrosion damage, which often occurs in stationary media, and Usually develops in the direction of gravity.
7. Wear and corrosion
Definition: The corrosive fluid moves relative to the metal surface, especially when eddies occur and the fluid changes direction sharply. The fluid not only causes mechanical erosion and damage to the corrosion products that have been generated on the metal surface, but also chemically or electrochemically interacts with the bare metal. The reaction accelerates the corrosion of metal, which is called wear corrosion.
Features: When wear and corrosion occurs, the metal detaches from the metal surface in the form of corrosion products instead of falling off in the form of solid metal powder like pure mechanical wear. Directional grooves, channels, ripples, etc. often appear on the metal surface. Corrosion shapes such as round holes.
8. Crevice corrosion
Definition: In the electrolyte, a narrow gap is formed between metal and metal or metal and non-metal surfaces. The movement of relevant substances in the gap is blocked, forming a concentration cell, resulting in local corrosion. This corrosion is called Crevice corrosion.
Features: This kind of corrosion often occurs at the joints of gaskets, rivets and screw connections, overlapping welded joints, valve seats, accumulated metal sheets, etc. Crevice corrosion occurs in almost all metals and alloys, as well as in connections between metals and non-metals. Almost all corrosive media (including fresh water) can induce crevice corrosion, and solutions containing chloride ions are most likely to cause crevice corrosion. The higher the temperature, the greater the risk of crevice corrosion.
9. Hydrogen embrittlement
Definition: A brittle failure caused by a reduction in the strength of the metal material due to the interaction between hydrogen and metal produced during corrosion, which is called hydrogen embrittlement.
Features: It is the result of the combined action of hydrogen and stress. The hydrogen produced by corrosion usually exists in an atomic state and is concentrated along the grain boundaries in the metal toward the largest two-dimensional stress concentration area. Once given the opportunity, it may form molecules and generate huge internal stress in the metal, leading to brittle failure of the material. Hydrogen embrittlement fracture may be intergranular or transgranular. The bifurcation phenomenon of hydrogen embrittlement cracks is much smaller than that of stress corrosion, and the cracks are accompanied by decarburization.
3. Physical properties
The physical properties of metallic materials refer to their characteristics when interacting with the physical environment. This includes electrical conductivity, thermal conductivity, magnetism, density, thermal expansion coefficient and other indicators. The quality of physical properties will affect the application of metal materials in electronics, heat conduction, magnetism, etc.
Common indicators of physical properties:
1. Elastic modulus E (MPa)
Definition: Within the elastic deformation range of a material, the ratio of stress to strain is called the elastic modulus, which characterizes the material’s ability to resist elastic deformation.
Note: The value reflects the ease of elastic deformation of the material, which is equivalent to the stress required to produce unit elastic deformation of the material. For parts requiring smaller elastic deformation in engineering applications, materials with high elastic modulus must be selected. The elastic modulus can be determined by tensile testing.
2. Shear modulus G (MPa)
Definition: The ratio of shear stress to shear strain within the elastic deformation range of a material is called shear modulus.
Description: It is a material constant that characterizes the material’s ability to resist shear strain. It is sometimes also called shear modulus or rigid modulus. In isotropic materials, it has the following relationship with the elastic modulus E and Poisson’s ratio: G=E/[2(1+y)]. The laboratory often uses torsion tests to measure the shear modulus of materials.
3. Poisson’s ratio v
Definition: Under the action of uniformly distributed axial stress, within the proportional limit of elastic deformation of a material, the absolute value of the ratio of transverse strain to longitudinal strain is called Poisson’s ratio, also known as transverse deformation coefficient.
Note: For isotropic materials, within the proportional limit of elastic deformation, this value is a constant. Beyond this range, this value changes with the average stress and the used stress range, and is no longer called Poisson’s ratio. For anisotropic materials, there are multiple Poisson’s ratios. The Poisson’s ratio of commonly used carbon steel materials ranges from 0.24 to 0.28. There is the following relationship between Poisson’s ratio, elastic modulus E and shear modulus G u=E/2G-1.
4. Density p(t/m³)
Definition: Represents the mass of metal per unit volume.
Note: The density of different metal materials is different. The density value of the material is directly related to the weight and compactness of the parts made of it.
5. Melting point T melting (°C)
Definition: The temperature at which the crystalline and liquid states of a substance coexist in equilibrium is called the melting point.
Explanation: The melting point of a crystal is related to the pressure it is subjected to. Under a certain pressure, the melting point of a crystal is the same as its freezing point. Melting point is one of the important basis for formulating material thermal processing technology specifications. For amorphous materials such as glass, there is no melting point, only a softening temperature range.
6.Specific heat capacity c [J/(kg·K)]
Definition: The heat absorbed by an object per unit mass for every 1°C increase or the heat released for every 1°C decrease becomes the specific heat capacity of the substance.
Explanation: It is an important process parameter for formulating material thermal processing process specifications.
7. Thermal conductivity λ [W/(m ·K)]
Definition: In unit time, when the temperature difference along the unit length along the heat flow direction is 1°C, the heat allowed to be conducted per unit area is called the thermal conductivity of the material.
Description: A physical quantity that characterizes the thermal conduction speed of metal materials. A material with a large thermal conductivity value has good thermal conductivity; otherwise, it has poor thermal conductivity. It is an important performance indicator to measure the thermal conductivity of materials.
4. Mechanical properties
The mechanical properties of metallic materials refer to their behavior and performance under the action of external forces. This includes strength, toughness, hardness, ductility, tensile strength and other indicators. The quality of mechanical properties determines the load-bearing capacity, deformation resistance and damage resistance of metal materials.
Common indicators of mechanical properties:
1. Tensile strength Rm (MPa)
Definition: The maximum stress that characterizes a metal material’s resistance to tensile fracture is called tensile strength, also known as strength limit.
Note: It can be measured by tensile test. For plastic materials, it represents the maximum uniform deformation resistance of the material, but does not represent the material’s true fracture resistance; for brittle materials with no or only small plastic deformation, it reflects the material’s true fracture resistance.
2. Compressive strength Rmc (MPa)
Definition: The maximum stress that characterizes a metal material’s resistance to compressive load without failure is called compressive strength, also known as compressive strength.
Note: It can be measured by compression test. For brittle or low plasticity materials, rupture occurs under pressure, and the compressive strength has a clear value at this time; while for plastic materials, brittle fracture will not occur during compression, and the compressive strength at this time can be used to produce a certain compression deformation. Compressive stress is defined.
3.Bending strength σbb(MPa)
Definition: The ability of a metal material to resist bending moments without failure is called bending strength, also known as bending strength.
Note: It can be measured by bending test. For brittle materials, the flexural strength can be measured when fracture occurs during bending; for plastic materials, the sample does not fracture during bending, so the bending test is only used to compare the plastic deformation capabilities of various materials under certain bending conditions or for Identify the surface quality of parts.
4. Torsional strength τb (MPa)
Definition: The ability of a metal material to resist torque without failure is called torsional strength, also known as torsional strength.
Note: It can be measured by torsion test.
5. Shear strength τ(MPa)
Definition: The ability of a metal material to resist shear load without failure is called shear strength.
Note: For brittle materials, the shear test can be used to directly measure. For plastic materials, since large plastic deformation occurs during shearing, the torsion test is used for measurement.
6. Yield point / conditional yield strength Rp0.2 (MPa)
Definition: Characterizes the ability of a metal material to resist plastic deformation. When a metal material is subjected to a tensile load, the phenomenon that the load no longer increases but the deformation continues to increase is called yielding. The stress when yielding occurs is called the yield point.
Note: The maximum stress before the yield stress of the material decreases for the first time is the upper yield point; when the initial instantaneous effect is not considered, the minimum stress in the yield stage is the lower yield point. For materials with an obvious yield point, the yield strength is equal to the stress corresponding to the yield point; for materials without an obvious yield point, the stress when the plastic deformation is 0.2% is specified as the conditional yield strength.
7. Fatigue strength/conditional fatigue strength σN (MPa)
Definition: Fatigue life N is the number of times a material fails due to stress cycles under a specific environment. Fatigue strength S is the stress level S value when the specimen fails under the specified fatigue life. The conditional fatigue strength σw is the stress amplitude of the sample with N cycles under the specified stress ratio, and σN is the fatigue strength in N cycles.
Note: Stress level S is the stress intensity under test control conditions, such as stress amplitude, maximum stress and stress range. The conditional fatigue strength σN is the stress amplitude of a specific stress ratio. In this case, the specimen has a life of N cycles. The stress ratio is the algebraic ratio of the minimum stress to the maximum stress.
8.Fatigue limit σD(MPa)
Definition: The fatigue limit σD is a stress amplitude value at which the specimen is expected to undergo an infinite number of stress cycles at a given probability.
Note: National standards point out that some materials have no fatigue limit; other materials will show fatigue strength under certain environments.
9. Torsional fatigue limit τD (MPa)
Definition: The torsional fatigue limit is the median torsional fatigue strength at a specified base number of cycles.
Note: The cycle base is generally 10⁷ or higher.
10. Creep limit σv (MPa)
Definition: The ability of a metallic material to resist creep deformation. It is divided into physical creep limit and conditional creep limit. The physical creep limit refers to the ability of a metal material not to creep at a certain temperature. The conditional creep limit is the stress that causes a metal material to produce a specified creep rate at a given temperature or a specified total plastic deformation within a specified time.
Note: The physical creep limit depends on the ability of the deformation testing equipment to detect the smallest deformation. Commonly used in engineering is the conditional creep limit.
11. Elongation after break A (%)
Definition: The percentage of the actual elongation of the gauge length part of the specimen after breaking to the original gauge length is called the post-break elongation, represented by A.
Note: The index characterizing the plastic deformation ability of metal materials can be measured through tensile testing. For circular specimens whose gauge length is 10 times the diameter and rectangular cross-section specimens with L=11.3√S (S is the cross-sectional area of the specimen), the post-fracture elongation is recorded as A11.3; for L=5do The post-fracture elongation of the cylindrical sample and the rectangular cross-section sample with L=5.65 √S is recorded as A. The higher the A value, the better the plasticity of the material.
12.Reduction of area Z (%)
Definition: After the specimen is broken, the percentage of the maximum reduction in the cross-sectional area at the necking area to the original cross-sectional area is called the area shrinkage, represented by Z.
Note: The index characterizing the plastic deformation ability of metal materials can be measured through tensile testing. The higher the Z value, the more plastic the material is.
13. Durable plasticity δ(%)
Definition: Characterized by the elongation A and area shrinkage Z of the specimen after creep rupture.
Explanation: It reflects the plastic properties of the material under the long-term action of temperature stress and is an important indicator of the creep brittleness of the material.
14. Resilience
Definition: Characterizes the ability of metal materials to absorb energy during plastic deformation and crack expansion before fracture. It is a comprehensive performance index of the strength and plasticity of metal materials.
Explanation: The main parameters characterizing the toughness of materials include impact absorbed energy, impact toughness, brittle transition temperature, non-plastic transition temperature, and fracture toughness.
15. Impact absorption energy KV, KU(J)
Definition: Using a V-shaped or U-shaped notch specimen of specified shape and size, under the action of impact test force, the energy required to produce two new free surfaces and a part of the volume plastic deformation during one break is the impact absorption energy. .
Note: The higher the value, the better the toughness of the material and the stronger its ability to resist impact damage.
16.Hardness
Definition: A mechanical property index that characterizes the relative softness and hardness of metal materials.
Note: There are three commonly used measurement methods: indentation method, dynamic method and scratch method. Indentation hardness represents the ability of a metal material to resist plastic deformation; dynamic hardness represents the deformation work of the material; scratch hardness represents the material’s ability to resist grinding. Generally, the higher the hardness of metal materials, the higher the strength, the higher the wear resistance, but the worse the plasticity and toughness.
17. Brinell hardness HB (HBS/HBW)
Definition: The Brinell hardness is measured by the pressing method, which uses a hardened steel ball or carbide ball to press into the metal surface. The quotient obtained by dividing the indentation area by the load applied to the steel ball is the Brinell hardness value HB of the metal.
Description: First proposed by the Swede J.A. Brinell. When the indenter is a steel ball (applicable to HB<450), Brinell hardness is expressed as HBS; when the indenter is a cemented carbide ball (applicable to HB<650), it is expressed as HBW.
18. Rockwell hardness HR (HRA/HRB/HRC)
Definition: Determination of Rockwell hardness by pressing in. Using a diamond cone with a cone angle of 120° or a steel ball with a diameter of 1.588mm as the indenter, first press into the surface of the specimen with the initial load Fo, then apply the main load F₁, hold for a certain period of time and then remove the main load. Measure the remaining indentation depth and calculate the hardness value based on the depth of the indentation.
Description: Proposed by American S.P. Rockwell. Rockwell hardness can be obtained in a variety of hardness scales according to the combination of different types of indenter and load, among which the commonly used ones are HRA, HRB and HRC.
19.Vickers hardness HV
Definition: Determination of Vickers hardness by pressing in. A diamond square pyramid with a relative angle of 136° is used as the indenter, which is pressed into the surface of the specimen under the action of load F, and then the indentation surface area is calculated based on the average diagonal length of the indentation. The quotient of the indentation area divided by the load is the Vickers hardness value.
Description: Proposed by British Vickers Company.
