I. Eutectoid Steel, Hypoeutectoid Steel, and Hypereutectoid Steel
1. Eutectoid Steel
In the crystalline structure of iron, carbon dissolves to form a solid solution. The solid solution of carbon in α-iron is called ferrite, and that in γ-iron is called austenite. Both ferrite and austenite have good plasticity. When the carbon in the iron-carbon alloy cannot completely dissolve into the ferrite or austenite, the remaining carbon will combine with iron to form a compound – iron carbide (Fe3C). The crystalline structure of this compound is called cementite, which has extremely high hardness and almost zero plasticity.
From the iron-carbon equilibrium phase diagram, which reflects the relationship between the structural organization of steel and its carbon content and temperature, it can be seen that when the carbon content is exactly 0.77%, which corresponds to about 12% cementite (iron carbide) and about 88% ferrite in the alloy, the phase transformation of the alloy occurs at a constant temperature.
That is, at this specific ratio, cementite and ferrite, during a phase change, if they disappear, they disappear simultaneously (when heated); if they appear, they appear simultaneously. This behavior is similar to the phase change of pure metals.
For this reason, people consider this two-phase organization composed of specific proportions as a single structure and named it pearlite. The steel called eutectoid steel is composed of this structure. That is, the steel with a carbon content exactly equal to 0.77% is called eutectoid steel, and its structure is pearlite. (C% = 0.77%)
2. Hypoeutectoid Steel
The carbon content of commonly used structural steel is mostly less than 0.5%. As the carbon content is less than 0.77%, the amount of cementite in the structure is less than 12%. Therefore, in addition to some ferrite forming pearlite with cementite, there will be excess ferrite.
Hence, the structure of this type of steel is ferrite + pearlite. The less the carbon content, the smaller the proportion of pearlite in the steel structure, the lower the strength of the steel, but the better the plasticity. This type of steel is collectively referred to as hypoeutectoid steel. (C% < 0.77%)
3. Hypereutectoid Steel
The carbon content of tool steel often exceeds 0.77%. In this type of steel, the proportion of cementite in the structure exceeds 12%.
Therefore, in addition to forming pearlite with ferrite, there is excess cementite. Thus, the structure of this type of steel is pearlite + cementite. This type of steel is collectively referred to as hypereutectoid steel. (C% > 0.77%)
II. Terminology Related to the Mechanical Properties of Steel
1. Yield Point (σs)
The yield point is the minimum stress at which a steel material or test sample continues to exhibit significant plastic deformation during tensile stress, even when the stress no longer increases, exceeding the elastic limit.
This phenomenon is called yielding. If Ps is the external force at the yield point s, and Fo is the cross-sectional area of the test sample, then the yield point σs =Ps/Fo (MPa), where MPa is a megapascal, equivalent to N/mm2 (1MPa=106Pa, 1Pa =1N/m2).
2. Yield Strength (σ 0.2)
For some metals, the yield point is not clearly defined, which can cause measurement difficulties. To gauge the yielding characteristics of a material, the stress at which permanent residual plastic deformation equals a certain value (usually 0.2% of the original length) is defined as the conditional yield strength or simply yield strength σ0.2.
3. Tensile Strength (σb)
This is the maximum stress value that a material can reach from the start of a tensile process until fracture occurs. It signifies the steel’s ability to resist breaking. Corresponding to tensile strength are compressive strength, bending strength, and others. If Pb is the maximum tensile force before the material breaks, and Fo is the area of the sample cross-section, then the tensile strength σb= Pb/Fo (MPa).
4. Elongation Rate (δs)
After a material is stretched until it breaks, the percentage of the plastic elongation length to the original sample length is called the elongation rate or extension rate.
5. Yield Strength Ratio (σs/σb)
The ratio of the yield point (yield strength) to the tensile strength of a steel material is called the yield strength ratio. The larger the yield strength ratio, the higher the reliability of the structural components. The yield strength ratio of general carbon steel is 0.6-0.65, low-alloy structural steel is 0.65-0.75, and alloy structural steel is 0.84-0.86.
6. Hardness
Hardness indicates a material’s ability to resist the penetration of a hard object into its surface. It’s a crucial performance index for metal materials.
Generally, the higher the hardness, the better the wear resistance. Common hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness.
(1) Brinell Hardness (HB)
Brinell hardness is determined by pressing a hardened steel ball of a certain size (usually 10mm in diameter) into the surface of the material under a specific load (generally 3000kg), maintained for a period. After unloading, the ratio of the load to the indentation area is the Brinell hardness value (HB), measured in kilograms-force per square millimeter (N/mm2).
(2) Rockwell Hardness (HR)
When HB>450 or the sample size is too small, the Brinell hardness test cannot be used, and the Rockwell hardness scale is adopted instead. It measures the hardness of a material by pressing a diamond cone with a 120° apex angle or a steel ball with a diameter of 1.59 or 3.18mm into the material’s surface under a certain load, and the hardness is determined by the depth of the indentation. Depending on the hardness of the test material, it is expressed in three different scales:
HRA: Hardness derived using a 60kg load and a diamond cone indenter, used for materials of extremely high hardness (such as hard alloys).
HRB: Hardness derived using a 100kg load and a 1.58mm diameter hardened steel ball, used for materials of lower hardness (such as annealed steel, cast iron, etc.).
HRC: Hardness derived using a 150kg load and a diamond cone indenter, used for materials of very high hardness (such as quenched steel).
(3) Vickers Hardness (HV)
Vickers hardness is determined by pressing a square-based diamond pyramid indenter with a 136° apex angle into the material’s surface under a load of 120kg or less. The Vickers hardness value (HV) is the quotient of the surface area of the material indentation pit to the load value.
III. Terminology Related to the Heat Treatment of Steel
1. Annealing of Steel
Annealing refers to the process of heating steel to a specific temperature, maintaining it for a period, and then allowing it to cool slowly. In this heat treatment method, the steel is heated until phase transformations, or partial transformations, occur. After maintaining this temperature, it is cooled slowly.
The purpose of annealing is to eliminate structural defects, improve the uniformity of the structure, refine grains, enhance the mechanical properties of the steel, and reduce residual stress. Simultaneously, it also reduces hardness, increases ductility and toughness, and improves machinability.
Therefore, annealing not only eliminates and improves the structural defects and internal stress left by the previous processes, but also prepares for subsequent processes. Hence, annealing is considered a semi-finished heat treatment, also known as pre-heat treatment.
2. Normalizing of Steel
Normalizing is a heat treatment method where steel is heated above the critical temperature, transforming it entirely into uniform austenite, and then naturally cooled in the air. It eliminates the network of cementite in hypereutectoid steels.
For hypo-eutectoid steels, normalizing can refine the lattice, improving overall mechanical properties. For parts with lower requirements, using normalizing instead of annealing is a more economical choice.
3. Quenching of Steel
Quenching is a heat treatment method where steel is heated above the critical temperature, held for a period, and then quickly immersed in a quenching medium, causing a sudden drop in temperature. This rapid cooling at speeds greater than the critical cooling rate results in a predominantly martensitic, unbalanced structure.
Quenching increases the strength and hardness of steel but reduces its plasticity. Common quenching mediums include water, oil, alkali water, and salt solutions.
4. Tempering of Steel
Tempering is the process of reheating quenched steel to a certain temperature, then cooling it using a particular method. The purpose of this process is to eliminate the internal stresses caused by quenching, reduce hardness and brittleness, and achieve the desired mechanical properties.
Tempering can be classified into high-temperature tempering, medium-temperature tempering, and low-temperature tempering, often used in conjunction with quenching and normalizing.
1. Quench and Temper Treatment:
This heat treatment technique involves high-temperature tempering after quenching, referred to as quench and temper treatment. High-temperature tempering is performed between 500-650℃. This process significantly modifies the properties and material of the steel, resulting in good strength, plasticity and toughness, and overall excellent mechanical performance.
2. Aging Treatment:
To prevent changes in size and shape of precision tools, molds, and parts during long-term use, parts are often re-heated to 100-150℃ before precision machining, following low-temperature tempering (at a temperature of 150-250℃), and held for 5-20 hours.
This treatment, known as aging, is used to stabilize the quality of precision parts. Aging treatment of steel components under low temperature or dynamic load conditions to eliminate residual stress and stabilize the steel structure and dimensions is especially important.
5. Surface Heat Treatment of Steel
This involves rapidly heating the surface of a steel part to above the critical temperature and then quickly cooling it before the heat can penetrate to the core. This process hardens the surface layer into a martensitic structure, without causing phase changes in the core, achieving surface hardening while the core remains unchanged. This process is suitable for medium carbon steel.
2. Chemical Heat Treatment:
This refers to a heat treatment process that uses the diffusivity of atoms at high temperatures to infiltrate chemical elements into the surface layer of the part, thereby changing its chemical composition and structure. This achieves the required organization and performance of the steel’s surface layer. Depending on the type of infiltrating element, chemical heat treatments can be divided into carburizing, nitriding, cyaniding, and metalizing.
(1) Carburizing:
Carburizing is the process of infiltrating carbon atoms into the steel surface layer. This gives low carbon steel parts a high carbon steel surface layer. After quenching and low-temperature tempering, the surface layer of the part has high hardness and wear resistance, while the core of the part maintains the toughness and plasticity of low carbon steel.
(2) Nitriding:
Also known as nitridation, this process involves infiltrating nitrogen atoms into the steel surface layer. The purpose is to enhance the hardness, wear resistance of the surface layer, as well as improve fatigue strength and corrosion resistance. Gas nitriding is commonly used in production.
(3) Cyaniding:
Also known as carbonitriding, this process involves infiltrating both carbon and nitrogen atoms into the steel. It gives the steel surface the characteristics of both carburizing and nitriding.
(4) Metalizing:
This process involves infiltrating metal atoms into the steel surface layer. It alloys the surface layer of the steel, giving the part’s surface the properties of certain alloy steels and special steels, such as heat resistance, wear resistance, anti-oxidation, and corrosion resistance. Commonly used methods include aluminizing, chromizing, borizing, and siliconizing.
