Mechanical properties of iron and steel
Mechanical properties of iron and steel
Round specimen:
Strength:
strength refers to the ability of a metal to resist plastic deformation and fracture under external forces.
strength refers to the ability of a metal to resist plastic deformation and fracture under external forces.
1. tensile strength (σb), unit (MPa)
When the metal specimen is stretched, the ratio between the maximum load and the original cross section area before breaking is called tensile strength.
Pb
σb＝——
Fo
Pb - maximum load before specimen breaking (N)
Fo -- the original cross section area of the sample (mm)
2. flexural strength (σbb), unit (MPa)
The maximum positive pressure of the broken section when the specimen is broken under the concentrated load located in the middle of the two supports.
Round specimen:
8PL
σbb＝——
πd3
8PL
To the rectangular specimen:
8PL
To the rectangular specimen:
8PL
σbb＝——
2bh2
P -- maximum load of specimens (N)
L -- the span between two supporting points (mm)
D - the outer diameter of the cross section of a circular specimen (mm)
B - the width of a rectangular section specimen (mm)
H - the width of a rectangular section specimen (mm)
3. compressive strength (σbc), unit (MPa)
The maximum positive pressure that a material can bear under the pressure without breaking or cracking is called compressive strength.
Pbc
σbc＝——
Fo
Medium Pbc - the maximum concentrated load of specimens (N)
Fo -- the original cross section area of the sample (mm)
4. shear strength (r、σr), unit (MPa)
The average stress of the shear section under the maximum load before shearing is:
P
Double shear:σr＝—— ；
2Fo
P
Single shear:σr＝—— ；
Fo
P - maximum shear load (N)
Fo -- the cross section area of the scissors (mm)
5. anti torsional strength (τb), unit (MPa)
The strength limit of the force is the external force.
3Mb
τb≈—— (suitable for steel)
4Wp
Mb
τb≈—— (suitable for cast iron)
Wp
Mb - torsion moment (N. mm)
Wp - the extreme section coefficient of the cross section of the specimen during torsion (mm)
6. yield point (σs), unit (MPa)
During the drawing process, the load of metal sample is no longer increased, and the phenomenon of deformation continues to be called "yielding". The stress of yielding phenomenon is called yield point or yield limit.
Ps
σs=——
Fo
Ps - yield load (N)
Fo -- the original cross section area of the sample (mm)
7. yield strength (σ0.2), unit (MPa)
For some metal materials with not obvious yield, it is difficult to determine the yield point. The stress of 0.2% permanent deformation is often defined as yield point, which is called yield strength or conditional yield limit.
P0.2
σ0.2=——
Fo
P0.2 - load of specimen with permanent deformation of 0.2% (N)
Fo -- the original cross section area of the sample (mm)
8. persistent strength (σ0.2/ time (H))
The stress of metal materials at high temperature and time of rupture is called lasting strength. Generally speaking, the rupture strength is 105h at a certain temperature.
Elasticity
Elasticity refers to a feature in which metal deforms under external force and returns to its original shape and size when the external force is removed.
1. Elastic modulus (E), unit (GPa)
In the elastic range, when the metal tensile test, the external force and the transformation ratio increase, that is, the stress and the proportional relationship should become, the proportional coefficient is called the elastic modulus, also known as the positive elastic modulus.
2. Shear modulus (G), unit (GPa)
When the metal is in the elastic range, when the torsion test is performed, the external force and the deformation are increased in proportion. That is, when the stress is proportional to the stress, the proportional coefficient is called the elastic modulus, which is also called the positive elastic modulus.
3. Elastic limit (σe), unit (MPa)
The maximum stress at which a metal can remain elastically deformed is called the elastic limit.
4. Proportional limit (σp), unit (MPa)
In the elastic deformation stage, the maximum stress that the metal material is subjected to and the strain energy is proportional to, which is called the proportional limit:
Pp
σ0.2=——
Fo
Pp in the formula - the specified proportional limit load (N)
Fo - the original cross-sectional area of the sample (mm2)
Plasticity
The so-called plasticity refers to the metal material under the action of external forces, resulting in permanent deformation without the ability to rupture.
1. Elongation (δ/%)
When the metal material is stretched, after the sample is broken, the length of the gauge length increases with the percentage of the original gauge length. Δs is the elongation at a gauge length of 5 times the diameter, and δ10 is the elongation at a gauge distance of 10 times the diameter.
2. Reduction of area (ψ/%)
The percentage of the maximum reduction of the cross-sectional area and the original cross-sectional area of the constriction after the metal specimen is broken.
3. Poisson's ratio (μ)
For isotropic materials, Poisson's ratio means the ratio of the relative amount of lateral shrinkage to the axial relative elongation when the sample is stretched in a single phase:
E
μ=— - 1
2G
E is the modulus of elasticity (GPa)
G——shear modulus (GPa)
Toughness
The term "toughness" refers to the ability of a metal material to be destroyed by the impact force (dynamic load).
1. Impact toughness (αKU), unit (J/cm2)
Impact toughness is a mechanical property index for assessing the impact resistance of metallic materials under dynamic loads. It is usually based on a single energy impact value (αKU or αKV) of a large energy. It is a standard sample of a certain size and shape. The test was carried out on a pendulum type impact tester. The test results are measured by the ratio of the work (AKU or AKV) consumed on the test piece to the cross-sectional area (F) at the cross section.
2. Impact absorption work (AKU), unit (J)
Since the value of αK depends not only on the material itself, but also on the size and shape of the sample and the temperature of the test, the αK value is only a relative index. At present, many countries in the world directly use the impact absorption work AK as an indicator of impact toughness.
AKU
αKU = ——；
F
AKU
αKV= ——；
F
αKU - Charpy U-notch specimen impact value (J/cm²)
αKV - Charpy V-notch impact value (J/cm²)
AKU - the work consumed by the Charpy U-notch specimen when it is broken (J)
AKV - Work performed when Charpy V-notch specimens are punched (J)
F - cross-sectional area at the sample notch (cm²)
Fatigue
The phenomenon that the metal material is below the ultimate strength and is subjected to an alternating load (that is, a load whose shape and direction are repeatedly changed) for a long period of time, and suddenly breaks without significant plastic deformation, is called fatigue.
1. Fatigue limit (σ-1), unit (MPa)
The maximum stress that a metal material can withstand after repeated (N) stress cycles without repeated fractures is called the fatigue limit.
2. Fatigue strength (σN), unit (MPa)
The maximum stress that a metal material can withstand after repeated (N) fractures under repeated or alternating stress is called fatigue strength. At this time, N is called the fatigue life of the material. Some metal materials have no obvious fatigue limit under repeated or alternating stress, and are usually expressed by fatigue strength.
Hardness
Hardness refers to the ability of a metal to resist pushing a harder object onto its surface. Hardness is not a simple physical quantity, but a comprehensive performance index reflecting elasticity, strength, and plasticity.
1. Brinell hardness (HBS)
Using a ball of a certain diameter (steel ball or cemented carbide ball pressed into the surface of the sample with the corresponding test force, after the specified holding time, remove the test force and measure the hardness value calculated from the surface indentation diameter. Hardness less than or equal to 450HBS; Use hard alloy ball to measure hardness greater than 450HBW
2. Rockwell hardness (HRA, HRB, HRC, HRD, HRE, HRF, HRG, HRH, HRK)
The diamond cone or the steel ball indenter is pressed into the surface of the sample under the initial test force and the total test force. After the specified holding time, the main test force is removed, and the hardness value calculated by the depth increment of the residual indentation is measured.
The Rockwell hardness test is divided into A, B, C, D, E, F, G, H, K scales.
3. Vickers hardness (HV)
The surface of the sample was pressed with a diamond positive quadrangular indenter at a test force of 49.03-980.7 N. After a predetermined holding time, the test force was removed and the calculated hardness value of the diagonal length of the indentation was measured.
4. Shore hardness (HSC, HSD)
With a diamond or steel ball to a certain height fell to the surface of the sample, measuring punch rebound height calculation of the hardness value. The hardness code of the visual hardness tester is HSC, and the hardness code of the indicator type hardness tester is HSD.
Friction reduction, wear resistance
1. Friction factor (μ)
Objects that come into contact with each other cause friction when they move relative to each other, and the resistance that causes friction is called friction. According to the law of friction, the ratio of the frictional force (F) to the vertical load (N) applied to the frictional area is usually referred to as the frictional factor.
F
μ=—
N
F - friction (N)
N—vertical load (N) applied to the friction member
2. Abrasion (W/g, V/cm 3)
After the sample has been rubbed for a certain period of time or a certain distance under the specified test conditions, the amount (g) or volume (cm 3) of the sample to be ground is called the wear amount (or wear amount) to remove the volume. The indicator is called volume wear V.
3. Relative wear coefficient (ε)
On the simulated wear tester, 65Mn (52-53HRC) is used as the standard sample. Under the same conditions, the ratio of the wear of the standard sample to the absolute wear of the material to be measured is called the relative wear coefficient of the tested material. .
