The mechanical properties of metal materials refer to the behavior of metal materials under the action of external load or the combined action of load and environmental factors (temperature, medium and loading rate).
Common mechanical properties of metals are shown in the table below:
Metal Mechanical Properties
Commonly used metal mechanical properties index
strength
Yield strength, tensile strength, breaking strength
Plasticity
Elongation, reduction of area, strain hardening index
elasticity
Elastic modulus (stiffness), elastic limit, proportional limit
hardness
Brinell hardness, Vickers hardness, Rockwell hardness
toughness
Static toughness, impact toughness, fracture toughness
fatigue
Fatigue strength, fatigue life, fatigue notch sensitivity
stress corrosion
Stress corrosion critical stress field intensity factor, stress corrosion crack growth rate
Tensile stress-strain curve of low carbon steel under uniaxial static load
picture
Mild Steel Tensile Force-Elongation Curve
1. Section oa: elastic deformation
2. Section ab: elastic deformation + plastic deformation
3. Bcd section: obvious plastic deformation, yield phenomenon, and the continuous elongation of the sample under the condition that the force remains basically unchanged
4. dB segment curve: elastic deformation + uniform plastic deformation
5. Point B: necking phenomenon occurs, the local section of the sample is obviously reduced, the bearing capacity of the sample is reduced, the tensile force reaches the maximum value, and the sample is about to break.
strength index
Strength refers to the ability of a material to resist plastic deformation and fracture.
1. Yield strength
σs = Fs/S0
Fs: the tensile force (N) that the sample bears when it yields; S0: the original cross-sectional area of the sample (mm).
2. Tensile strength
The maximum tensile stress that the sample bears before breaking reflects the maximum uniform deformation resistance of the material.
σb = Fb/S0
σb is often used as the basis for material selection and design of brittle materials.
Plastic index
Plasticity is the ability of a material to undergo plastic deformation under static load without failure.
1. Elongation after break
The percentage of the elongation of the gauge length after the sample is broken to the original gauge length.
δ=(L1-L0)/L*100%
L0: gauge length; L1: gauge length of the test piece after breaking.
2. Reduction of area
The percentage of the maximum reduction of the cross-sectional area at the retracted item of the sample to the original cross-sectional area.
Ψ=(A0-A1)/A0 *100%
A0: The original cross-sectional area of the specimen; A1: The cross-sectional area of the necking after fracture.
strength index
Strength refers to the ability of a material to resist plastic deformation and fracture.
1. Yield strength
σs = Fs/S0
Fs: the tensile force (N) that the sample bears when it yields; S0: the original cross-sectional area of the sample (mm).
2. Tensile strength
The maximum tensile stress that the sample bears before breaking reflects the maximum uniform deformation resistance of the material.
σb = Fb/S0
σb is often used as the basis for material selection and design of brittle materials.
Plastic index
Plasticity is the ability of a material to undergo plastic deformation under static load without failure.
1. Elongation after break
The percentage of the elongation of the gauge length after the sample is broken to the original gauge length.
δ=(L1-L0)/L*100%
L0: gauge length; L1: gauge length of the test piece after breaking.
picture
2. Reduction of area
The percentage of the maximum reduction of the cross-sectional area at the retracted item of the sample to the original cross-sectional area.
Ψ=(A0-A1)/A0*100%
A0: The original cross-sectional area of the specimen; A1: The cross-sectional area of the necking after fracture.
Elasticity Index
Stiffness: The ability of a material to resist elastic deformation when stressed.
E=σ/ε
σ: tensile stress; ε: tensile strain
The microstructure is not sensitive to the mechanical performance index, and alloying, heat treatment, and cold plastic deformation have little effect on it.
Important mechanical performance indicators for material selection of mechanisms and components:
►The driving beam should have sufficient rigidity, otherwise it will cause vibration due to excessive deflection when lifting heavy objects.
►Machine tool and press spindle, bed and workbench have requirements for rigidity to ensure machining accuracy.
►Main components such as internal combustion engines, centrifuges and compressors must have sufficient rigidity to prevent vibration.
hardness
The ability of the local surface of a material to resist plastic deformation and failure.
It is an index to measure the softness and hardness of the material, and its physical meaning is related to the test method.
Hardness testing methods: Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, Leeb hardness, Mohs hardness
(1) Brinell hardness
The average stress per unit area, that is, the quotient of the test force p and the spherical surface area of the indentation.
picture
< 450HB: The test indenter is a quenched steel ball, the hardness symbol is HBS;
<650HB: The test indenter is cemented carbide, and the hardness symbol is HBW.
Empirical formula:
Low carbon steel: σb≈3.6HBS;
High carbon steel: σb≈3.4HBS.
Scope of application: used to measure gray cast iron, structural steel, non-ferrous metals and non-metallic materials, etc.
Advantages and disadvantages:
The measured value is more accurate and repeatable;
Measurable tissue inhomogeneous materials;
Not suitable for testing finished products and thin parts;
Measurement is time-consuming and inefficient.
(2) Rockwell hardness
The hardness value of the material is expressed by measuring the indentation depth, and every 0.002mm is equivalent to 1 Rockwell hardness unit.
There are two types of indenters:
1. Diamond cone with cone angle α=120°,
2. A small quenched steel ball with a diameter of Φ1.588mm.
Rockwell hardness calculation formula:
HR=(k-h)/0.002
Indenter 1: k=0.2mm; Indenter 2: k=0.26mm.
ruler
hardness symbol
Head type
Total test force F/N
Measuring hardness range
Application examples
C
HRC
Diamond cone
1471
20-70
Hardened steel, high hardness cast iron, pearlitic malleable cast iron
B
HRB
Φ1.588mm steel ball
980.7
20-100
Mild steel, copper alloy, ferritic malleable iron
A
HRA
Diamond cone
588.4
20-88
Carbide, Hardened Sheet Steel, Case Hardened Steel
Advantages and disadvantages:
The test is simple, convenient and fast;
The indentation is small, and the finished product and thin parts can be measured;
The data is not accurate enough, three points should be measured to take the average value;
Inhomogeneous materials such as cast iron should not be tested.
(3) Vickers hardness
The hardness value is calculated according to the test force per unit area of the indentation.
The indenter is a diamond quadrangular pyramid with an included angle of 136° between two opposite surfaces.
Measuring range :
It is often used to measure thin parts, coatings, surface layers after chemical heat treatment, etc.
Advantages and disadvantages:
Accurate measurement and wide range of applications (hardness from extremely soft to extremely hard);
Measurable finished products and thin parts;
The surface requirements of the sample are high and labor-intensive.
Impact toughness
The ability of a material to resist damage under impact loads.
The impact energy Ak consumed when the sample breaks is:
Ak = mgH – mgh (J)
The impact toughness value ak is the impact energy consumed per unit cross-sectional area at the notch of the sample.
ak = Ak / S0 (J/cm²)
Low ak value - brittle material:
No obvious deformation when broken, metallic luster, crystalline.
High ak value - tough material:
Obvious plastic change, the fracture is gray and fibrous, dull.
picture
Fracture toughness
Fracture Mechanics: On the premise of acknowledging the existence of macroscopic cracks in machine parts, various new mechanical parameters of crack propagation are established, and the fracture criterion and material fracture toughness of cracked bodies are proposed.
picture
fatigue
Fatigue phenomenon:
The fracture phenomenon caused by cumulative damage of metal parts or components under the long-term action of fluctuating stress and strain.
Fatigue Features:
(1) Fatigue is a low-stress cycle time-delayed fracture, and the fracture stress is often lower than the tensile strength of the material, or even the yield strength;
(2) Fatigue is a brittle and sudden fracture, and there will be no obvious signs of deformation before the fracture, which is very dangerous;
(3) Fatigue is very sensitive to notches, cracks and structural defects, and is highly selective.
Fatigue limit σ-1:
The highest stress value at which a material undergoes numerous stress cycles without fatigue fracture.
Condition fatigue limit:
The maximum stress value that can withstand 107 stress cycles without breaking.
Empirical formula of steel fatigue strength:
σ-1= (0.45～0.55)σb
or σ-1= 0.27(σs+σb)
σ-1p= 0.23(σs+σb）
02
heat treatment process
Definition: The process of changing the internal structure of solid metal or alloy through heating, heat preservation and cooling to obtain the required properties.
picture
Purpose: One is to improve the process performance of materials and ensure the smooth progress of subsequent processing. This heat treatment is called pre-heat treatment; the other is to improve the performance of materials and prolong the service life of parts. This heat treatment is called final heat treatment.
Heat treatment classification:
Ordinary heat treatment (four fires: annealing, normalizing, quenching, tempering)
Surface heat treatment (surface quenching, chemical heat treatment)
Other heat treatment (vacuum heat treatment, deformation heat treatment, etc.)
Microstructural transformation of eutectoid steel during heating
Four steps in the transformation process of pearlite to austenite:
(1) Austenite nucleation;
(2) Austenite growth;
(3) The remaining Fe3C dissolves;
(4) Homogenization of austenite.
picture
picture
Structural transformation of steel during cooling
Cooling transformation of austenite: Austenite is a stable phase above the critical point A1, and it becomes an unstable phase when it is cooled below A1, and the structure transformation will occur.
Importance: Determines the structure and properties of steel after heat treatment. For the same steel, the heating temperature and holding time are the same, but the cooling method is different, and the properties after heat treatment are completely different.
picture
Mechanical properties of 45 steel heated to 840°C and cooled under different cooling conditions
cooling method
σb/Mpa
σs/Mpa
δ/%
ψ/%
HRC
Cooling with the furnace
519
272
32.5
49
15~18
air cooling
657~706
333
15~18
45~50
18~24
cooling in oil
882
608
18~20
48
40~50
water cooling
1078
706
7~8
12~14
52~60
Establishment of isothermal transformation curve of supercooled austenite in eutectoid steel (metallographic hardness method)
Also known as "TTT curve" (Time-Temperature-Transformation Curve), because the shape is similar to "C", it is often called "C curve".
picture
With the help of the "C curve", it is possible to understand what kind of structure austenite transforms into under different cooling conditions and the properties of the transformed products, providing a theoretical basis for the correct formulation and selection of heat treatment processes.
Eutectoid steel C curve and transformation products
picture
1) Pearlite type transformation (also known as high temperature transformation)
Transformation temperature: A1~550℃; transformation product: pearlite
A1~6500℃: the pearlite sheet is thicker, P (pearlite-pearlite)
6500℃~6000℃: Pearlite layer is thinner, S (Sorbite-sorbite)
6000℃~5500℃: the pearlite layer is very fine, T (troolstite)
picture
The thickness of the ferrite and cementite lamellar layers of pearlite is related to the transformation temperature. The lower the temperature, the finer the pearlite lamellae. The layers become thinner, the strength and hardness increase, and the plastic toughness increases.
2) Bainitic transformation (also known as medium temperature transformation)
Transition temperature: 550-Ms (230°C)
Transformation product: Bainite B (bainite) - a mixture of supersaturated F and cementite.
picture
550~350℃: upper bainite (upper B) feathery structure, low strength and plasticity, high brittleness.
350℃~ Ms: lower bainite (lower B) needle-like structure, good comprehensive performance.
picture
3) Martensitic transformation (also known as low temperature transformation)
Transition temperature: Ms (230°C) ~ Mf
Transformation product: martensite (martensite) + A'(residual austenite)
Martensite: A supersaturated solid solution of carbon formed in α-Fe, represented by M.
Classification:
Low carbon martensite (low carbon martensite): Lath-like, with high strength and ductility. Also known as lath M (lath martensite).
High carbon martensite (high carbon martensite): lenticular, sheet-like, with ridges in the middle. It has high strength, but poor ductility and high brittleness.
Image] [image
C curve of hypoeutectoid steel
picture
C curve of hypereutectoid steel
picture
Supercooled austenite continuous transformation cooling curve (CCT curve) (Continuous Cooling Transformation)
picture
annealing
Definition: Heating metal to a certain temperature, maintaining it for a sufficient time, and then cooling it at an appropriate rate
Purpose:
refine grains;
Reduce the hardness and improve the forming and cutting performance of steel;
Eliminate internal stress.
Classification: According to the purpose and process characteristics of annealing, it can be divided into complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing, stress relief annealing, etc.
full annealing
l Scope of application: hypoeutectoid steel
lHeating temperature: Ac3+30～50℃
l Purpose: to refine the structure, reduce the hardness, improve machinability,
Eliminate internal stress
l Room temperature tissue: F+P
picture
Spheroidizing annealing
Scope of application: eutectoid steel and hypereutectoid steel
Heating temperature: Ac1+20~30℃
Purpose: to spheroidize reticular or flake Fe3CⅡ
Organization: spherical pearlite
picture
isothermal annealing
Process: Heating to Ac1+30~50°C or Ac3+30~50°C, after keeping warm, quickly cooling to a temperature below Ar1, when A has turned into P-type tissue, take it out of the furnace and air-cool.
Organization: Class P
Advantages: short annealing time, uniform structure
picture
Relief annealing
Purpose: to remove residual stress
heating
Temperature: T heating < AC1 (500 ~ 600 ℃)
Application: Eliminate residual internal stress of castings, forgings, weldments, etc.
picture
Homogenization annealing (diffusion annealing)
Purpose: Eliminate segregation; uniform composition, organization
Heating temperature: AC3＋150～250℃
Organization: hypoeutectoid steel is P+F.
Application: Mainly used for alloy steel ingots, castings and forgings with high quality requirements.
Recrystallization annealing
Process: Heating to 50-150°C below Ac1, or T+30-50°C, keeping warm and cooling slowly.
Purpose: Eliminate work hardening and restore the plasticity and toughness of steel.
Application: Eliminate work hardening of workpieces after cold working. Such as the annealing in the middle of the steel wire drawing process.
Normalizing
Definition: A heat treatment process in which the workpiece is heated to 30-50°C above Ac3 or Accm, taken out of the furnace after heat preservation, and cooled in air.
Purpose:
Low carbon steel: increase hardness and facilitate cutting.
Hypereutectoid steel: Eliminate reticular secondary cementite, which is beneficial to P spheroidization.
Medium-carbon steel and medium-carbon low-alloy steel: the stress is not large, and the performance requirements are not high, which can be used as the final heat treatment.
picture
Quenching
picture
Purpose: To obtain the structure under M or B, and improve the hardness and wear resistance of steel.
Selection of quenching temperature
Hypoeutectoid steel: AC3+30～50 ℃;
Eutectoid steel and hypereutectoid steel: AC1+30～50℃.
picture
Quenching cooling is the key to determining the quality of quenching, and the ideal cooling rate should be as shown in the figure.
Above 650℃, slow, reduce thermal stress
650-400 ℃, fast, avoid C curve
Below 400 ℃, slow, reduce phase transition stress
picture
Commonly used quenching medium
At present, the commonly used cooling media in production are oil, water, and brine, and their cooling capacity increases sequentially.
Water: strong quenching ability, but there are soft spots on the surface of the workpiece, which are easy to deform and crack.
Salt water: the quenching ability is stronger, the surface of the workpiece is smooth and clean, without soft spots, but it is easier to deform and crack;
Oil: The quenching ability is weak, but the workpiece is not easy to deform and crack
Common quenching cooling method (quench cooling method)
picture
Temper
Definition: picture
The main purpose of tempering
Eliminate internal stress and reduce brittleness
Stable tissue and workpiece dimensions
Reduce hardness, improve plasticity
Changes in structure and properties of tempering
The structural transformation of quenched steel during tempering mainly occurs in the heating stage. As the heating temperature increases, the structure of quenched steel undergoes four stages of change.
1. Decomposition of martensite
Tempering stage: When tempering at <100°C, the structure does not change; when heating at 100~200°C, martensite will decompose.
Obtained organization: tempered martensite M times (supersaturated α solid solution).
Performance changes: the internal stress gradually decreases, and the performance basically remains the same.
2. Decomposition of retained austenite
Tempering stage: 200-300°C. A' decomposes and transforms into B.
Obtained organization: M (Tempered Martensite) indicates
Performance changes: The stress is further reduced, and the strength and hardness are slightly reduced.
3. The decomposition of martensite is completed and the formation of cementite
Tempering stage: 300-400°C. ε carbides transform into stable cementite.
Obtained organization: Tempered Troostite, represented by T (Tempered Troostite).
Performance changes: the internal stress is basically eliminated, the hardness decreases, and the plastic toughness increases.
4. Fe3C aggregate growth and recovery and recrystallization of α solid solution
Tempering stage: above 400°C. The α phase begins to recover, and recrystallization occurs above 500°C;
Obtained organization: Tempered Sorbite, represented by S (Tempered Sorbite).
Performance changes: good overall performance is obtained.
Microstructure and mechanical properties of tempered steel
craft
tempering temperature
(℃)
Tissue after tempering
Hardness after tempering (HRC)
Features
use
low temperature tempering
150～250
M back
58～64
High hardness, high wear resistance; brittleness, reduced internal stress
tool steel,
Rolling bearings, carburized parts, etc.
Medium temperature tempering
250～500
T back
35～50
Higher elastic limit and yield limit, with certain plasticity and toughness
spring steel,
Hot work mold
high temperature tempering
500～600
S back
25～35
good overall performance
important structural parts
The general trend of mechanical properties changes during tempering: With the increase of tempering temperature, the strength and hardness of steel decrease, and the plasticity and toughness increase.
Surface Heat Treatment (Surface Heat Treatment)
Surface heat treatment: a heat treatment process that only heats the surface of the workpiece to change its structure and properties.
Classification: surface quenching and chemical heat treatment.
In production, there are many parts that require the surface and the core to have different properties. Generally, the surface has high hardness, high wear resistance and fatigue strength; while the core requires better plasticity and toughness.
In this case, starting from material selection alone or using ordinary heat treatment methods cannot meet its requirements. The way to solve this problem is surface heat treatment.
surface quenching
Definition: A heat treatment process that only quenches (+ tempers) the surface of the workpiece
Purpose: To make the surface of the workpiece hard and tough.
Steel for surface hardening: medium carbon structural steel (0.4%-0.5% carbon content)
Methods: surface hardening by induction heating and surface hardening by flame heating.
Induction surface quenching
Basic principle: The induction coil is fed with alternating current → forms an eddy current (skin effect) → obtains A on the surface → obtains M by water cooling.
Classification:
High frequency induction heating:
200~300kHz, 0.5~2.5mm;
Medium frequency induction heating:
0.5~10kHz, 2~10mm;
Power frequency induction heating:
50Hz, 10-20mm.
Rule: The greater the current frequency, the shallower the depth of the hardened layer.
flame heating surface quenching
Definition: Flame heating surface quenching is the application of oxy-acetylene (or other combustible gas) flames to heat the surface of parts and then quench them rapidly. The depth of the hardened layer is generally 2 to 6mm.
Application: suitable for single piece and small batch production.
Chemical heat treatment of steel
Definition: A heat treatment process in which a steel part is kept in an active medium at a certain temperature to allow one or several elements to penetrate into its surface to change its chemical composition, structure and performance.
Classification: According to different infiltrated elements, chemical heat treatment can be divided into carburizing, nitriding, carbonitriding, boronizing, aluminizing, etc.
Basic process:
① Decomposition: Make the chemical medium decompose the active atoms that penetrate into the elements during the heating and heat preservation process;
② Absorption: Active atoms are adsorbed by the surface of the workpiece to form solid solutions or special compounds;
③ Diffusion: The infiltrated atoms diffuse inwardly from the surface of the workpiece to form a diffusion layer with a certain depth, that is, the infiltrated layer
Carburizing of steel (Carburize of steel)
picture
Purpose: To improve the hardness and wear resistance of the workpiece surface
Steel for carburizing: low carbon steel or low carbon alloy steel
Medium: most commonly used gases (kerosene, benzene, etc.), with activated carbon atoms.
Temperature: in the austenite zone, 900-950°C
Time: Depending on the depth of the seepage layer, about 10 hours.
Other chemical heat treatment methods
Nitriding: A heat treatment process that infiltrates active nitrogen atoms into the surface of a workpiece at a certain temperature. Improve the surface hardness, wear resistance, fatigue strength, thermal hardness and corrosion resistance of parts.
Carbonitriding (carbonitriding): Carbon and nitrogen penetrate into the surface of the workpiece at the same time. Improve surface hardness, fatigue resistance and wear resistance, and combine the advantages of carburizing and nitriding.
Chromizing: It has good corrosion resistance and excellent oxidation resistance, hardness and wear resistance, and can replace stainless steel and heat-resistant steel for tool manufacturing.
Boronizing: very excellent wear resistance, corrosion resistance and mud wear resistance, wear resistance is obviously better than nitriding, carbon and carbonitriding layers, but not resistant to atmospheric and water corrosion. Mainly used for mud pump parts, hot work dies and workpiece fixtures.
