Today, let's learn about the process of steel quenching and annealing

In order to make steel have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials, the heat treatment process is essential. It does not change the shape of the steel, but through heat treatment it can fully unleash the potential of the steel, give the steel the required characteristics and extend its service life. Nowadays, basically all workpieces require heat treatment. Heat treatment technology has been widely used in industry.

The heat treatment processability of steel mainly includes hardenability, hardness, temper brittleness, overheat sensitivity, temper resistance, oxidative decarburization tendency and surface state sensitivity of ultra-high strength steel. These process properties are related to the chemical properties of the material. The composition is related to the organization and is an important basis for selecting materials and formulating production processes.

1. Hardenability. The hardenability of steel refers to the ability of steel parts to obtain a hardened layer after quenching under certain conditions. The breakability of steel can generally be expressed by the quenching critical diameter, cross-section hardness distribution curve and end-quenching hardness distribution curve.

The hardenability of steel causes a size effect (also known as mass effect) in the steel. Due to the different cross-sectional sizes of the parts, the depth of the hardened layer is different, which also affects the surface hardness of the quenched parts. Therefore, the designer must pay full attention to the quenching of the material. When designing important parts with large cross-sections or complex shapes, alloy steel with good hardenability should be selected to ensure a good fit of high strength and high toughness along the entire section, while reducing heat treatment deformation and cracking. Designers must also reasonably determine the hardenability requirements based on the service conditions of the parts. For important parts (such as connecting rods, high-strength bolts, tie rods, etc.), it is required to ensure that the core obtains more than 90% martensite (volume fraction) after quenching. For parts that are generally under tension or compression in one direction, it is required that 50% martensite (volume fraction) is obtained in the center after quenching. For crankshafts with larger sizes due to stiffness considerations, they only need to be 1/4R away from the surface after quenching. Ensure to obtain more than 50 martensite (volume fraction). Spring parts generally require quenching. Rolling bearings and small bearings must be fully quenched, but large shaft spots that are subject to large impact loads should not be quenched.

The quenching critical diameter refers to the maximum diameter at which a certain amount of martensite is formed in the center of the quenched specimen, that is, the core reaches a certain critical hardness.

The hardenability of steel is an important basis for formulating heat treatment processes. When quenching steel with good hardenability, a gentler quenching medium and a slower cooling quenching process can be used to reduce the deformation and cracking tendency of parts.

2. Oxidative decarburization trend. During the heating process of steel, due to the effect of the surrounding oxidizing atmosphere, metal oxides are formed on the surface, causing the surface of the steel to lose its original luster, which is called oxidation; at the same time, all or part of the carbon on the surface of the steel is lost, which reduces the carbon content on the surface, which is called oxidation. It's called decarbonization. When heated in a reducing atmosphere, oxidation generally does not occur, but decarburization may occur if not properly controlled.

Oxidation causes the steel surface to lose its metallic luster, increase the surface roughness value, and reduce precision, which is not allowed for precision parts. At the same time, oxidation reduces the strength of steel and other mechanical properties, increasing the possibility of quenching cracking and quenching soft spots. Decarburization significantly reduces the quenching hardness, wear resistance and fatigue properties of steel, and decarburization of high-speed steel will seriously affect the thermal hardness. Among various steel types, silicon-containing steel has a greater tendency to oxidize and decarburize.

Oxidative decarburization should be avoided as much as possible in industrial production: important stress-bearing parts are not allowed to have an oxidative desulfurization layer on the final zero scale. To this end, designers must reasonably leave sufficient processing allowance based on the production process and site conditions. Technicians should appropriately arrange the processing flow. Heat treatment workers should actively adopt various heat treatment processes with less oxidative decarburization and control oxidative desulfurization. , to ensure the quality of heat treatment of parts and obtain stable and reliable performance.

3. Hardenability. Hardenability means that the steel is cooled at a rate exceeding the critical cooling rate under ideal quenching conditions, so that the formed martensite can reach the highest hardness. The hardenability of steel mainly depends on the carbon content of the steel. The higher the carbon content, the higher the hardness after quenching, and other alloying elements have less influence. When the carbon content (mass fraction) reaches 0.6%, the hardness of quenched steel is close to the maximum value. As the carbon content further increases, although the martensite hardness will increase, due to the increase in the amount of retained austenite, the hardness of carbon steel will not increase much, and the hardness of alloy steel will decrease.

4. Overheating sensitivity. When steel is heated, due to the high temperature depression, the grains will grow, causing a significant reduction in performance, which is called overheating. When the heating temperature is close to the solidus line, the grain boundaries will be oxidized and partially melted, which is called overheating. Overheated.

The important characteristic of overheating is that the grains are coarse, which will reduce the yield strength, plasticity, impact toughness and fatigue properties of the steel, and at the same time increase the brittle transition temperature of the steel; overheating will also make the quenched martensite coarse, reducing its wear resistance. Increases the tendency of quenching deformation and cracking, so the industry always refines the grains through various ways to achieve the purpose of refining the structure and improving performance. Among various steel types, manganese-containing steel is more sensitive to overheating.

For general overheated structures, they can be eliminated through multiple normalizing or annealing. For more serious overheated structures, such as stone-like fractures, they cannot be eliminated by heat treatment. They must be eliminated by a combination of high-temperature deformation and annealing. Overheated tissue cannot be saved and is an unacceptable defect.

5. Tempering brittleness. The performance of steel parts is mainly obtained through tempering, and parameters such as tempering temperature are mainly selected based on the design strength requirements. However, many steel types will experience a significant decrease in impact toughness twice as the tempering temperature increases, which is called temper brittleness.

The temper brittleness produced when steel is tempered in the temperature range of about 300°C is called the first type of temper brittleness, also known as low-temperature temper brittleness. The temper brittleness produced during tempering in the temperature range of 400 to 550°C is called the second type of temper brittleness, also known as high-temperature temper brittleness.

In design and production, try to avoid selecting strength levels that require tempering in the temper brittleness zone. Rapid cooling can eliminate the second type of temper brittleness. Select alloy steels containing molybdenum and tungsten, fine-grained steels and high-temperature steels. Pure steel reduces temper brittleness.

6. Tempering resistance. Tempering resistance refers to the ability of steel to resist softening during tempering, also known as tempering resistance, tempering resistance and tempering stability. Steel with good tempering resistance changes slowly in structure and properties during tempering and can be used after tempering at higher temperatures. The tempering resistance of alloy steel is better than that of carbon steel. Therefore, when the same tempering hardness is to be obtained for steel types with the same carbon content, the tempering temperature of alloy steel is higher than that of carbon steel, and the tempering time is longer. The stress is smaller than carbon steel, and the plasticity and toughness are also higher.

In industrial production, for parts that require complete elimination of internal stress and good combination of strength and toughness, alloy steel with good tempering resistance should be selected during design. For parts with higher operating temperatures, steel types with good tempering resistance should be selected. Generally, the maximum operating temperature is 50 degrees below the tempering temperature.

7. Surface state sensitivity of ultra-high strength steel. Ultra-high-strength steel has the characteristics of high specific strength, which can reduce the weight of parts, improve product performance, and its application scope continues to expand. However, ultra-high stiffness has greater notch sensitivity and is more sensitive to surface conditions. An incomplete surface will affect its fatigue performance. Corrosion resistance, plasticity and toughness are greatly reduced, and even cause catastrophic damage. Therefore, attention should be paid to improving notch sensitivity, maintaining surface integrity, and preventing hydrogen embrittlement and surface oxidation-decarburization.

This article introduces in detail some characteristics of the heat treatment process of steel. If you still have questions, please call our hotline immediately.