What is the relationship between the effect of part quenching and tempering and the hardenability of

The higher the hardenability of the material, the same structure of the surface and core of the quenched and tempered parts, the same mechanical properties, and the improved service performance.

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The hardenability of steel, what are the factors that affect the hardenability of steel.

Quenching, all want to be hardened, and the quenching of the workpiece requires that the surface and center can obtain the same high hardness martensite structure. If the surface hardness of the workpiece has reached the required level, and the hardness of the central part is low, it means that it is "not hardened". After tempering, the structure and properties of the unhardened workpiece are inconsistent, which will cause uneven stress on the surface and inside.

The mechanical properties of steel with high hardenability are evenly distributed along the cross-section, while the mechanical properties of steel with low quenchability are lower at the core, especially the impact toughness is lower.

Therefore, the magnitude of hardenability is expressed in terms of the depth of the hardened layer. The deep hardening layer indicates that the hardenability is large, and vice versa, the hardenability is small. The hardenability of steel is one of the process properties that must be considered when selecting steel.

For workpieces with large cross-sectional dimensions and high mechanical property requirements, the greater the hardenability requirements, the better.

The reason for the appearance of unhardening is mainly because the workpiece is cooled during quenching, its surface.

This is caused by the fact that the cooling rate of the surface and the center part are not the same.

The following factors also affect the hardenability:

1.The chemical composition of the steel is compared with the cylinder of 40cr steel with a diameter of 50 mm and 40 steel after quenching, and it can be seen that the depth of the hardened layer of 40cr steel is deeper than that of 40 steel, which is due to the difference in chemical composition. As the carbon content increases, the hardenability increases.

In addition, various alloying elements also have a great influence on improving hardenability, such as B (trace amount), MN, CR, SI, NI, etc. In addition to b, the more the amount is generally added, the more obvious the effect on improving the hardenability. This is due to the fact that the alloying elements reduce the speed of the critical cold cherry cracking stove of the steel.

2.Workpiece size.

The larger the size of the workpiece, the greater the internal heat capacity, and the slower the cooling rate of the ridge during quenching, so the shallower the hardening layer and the lower the surface hardness.

3.Coolant.

Under the condition that the composition and size of the steel are the same, if quenched in different coolants, the workpiece will be cooled at different rates due to their different cooling capacities, and therefore, the hardening layer depth will also be different. For example, 45 steel with a cross-sectional thickness of 4 10 mm can obtain HRC50 58 surface hardness when water quenching; The surface hardness of oil quenching is only HRC30 35. It shows that the cooling capacity of water is greater than that of oil.

The hardenability of steel is good or poor, which is often expressed by the depth of the hardened layer. The greater the depth of the hardened layer, the better the hardenability of the steel. The hardenability of steel is an intrinsic property of the steel itself, which depends only on its own internal factors and has nothing to do with external factors.

The hardenability of steel mainly depends on the chemical composition of the steel, especially the influence of alloying elements and grain size, heating temperature and holding time and other factors on the hardenability of steel. Steels with good hardenability can obtain uniform mechanical properties across the entire cross-section of the steel, and quenching agents with low quenching stress can be selected to reduce deformation and cracking.

Hardenability indicates the depth of the hardened layer (martensitic layer) that the steel achieves when it is quenched under certain conditions. It is one of the important indicators to measure the quenching ability of different steel grades.

Metals with good hardenability can meet the relevant requirements after heat treatment, so that the mechanical properties of the sample surface and core can meet the relevant requirements.

After heat treatment, the mechanical properties of the surface and core of the specimen may be different for metals with poor hardenability - the surface is qualified, and the core is unqualified.

The difference in mechanical properties is the reaction of the difference in the structure of the material, that is to say, the structure of the surface and the core of the specimen is different after heat treatment of the metal with poor hardenability.

5.Hardenability.

Hardenability refers to the ability of a material to obtain the depth of a hardened layer.

Generally, it is stipulated that the depth from the surface of the workpiece to the semi-martensitic zone (where the martensite and non-martensitic structures account for half and half of the hardness is easy to be determined) is the depth of the hardened layer. The deeper the hardened layer, the better the hardenability of the steel, and if the hardened layer reaches the core, it means that the steel is fully hardened.

The hardenability of steel has a great impact on mechanical properties, but not all mechanical parts must be fully hardened. For example, shaft parts that are subjected to bending and torsional stress, parts with surface heat treatment, etc., only need a certain depth of hardened layer to meet the use requirements.

The hardenability of steel is mainly determined by the critical cooling rate, the smaller the critical cooling rate, the better the hardenability of the steel, and vice versa, the hardenability of the steel is reduced. With the exception of CO, most alloying elements can significantly improve the hardenability of steel.

The hardenability of steel is determined by the chemical composition of the steel and the austenitization conditions

1) The chemical composition of austenite: the closer the carbon content is to the eutectic composition, the greater the hardenability, and its inversion is also true. The alloying elements dissolved in austenite, except titanium, zirconium and cobalt, can improve the hardenability of steel, and trace boron (mass fraction) can significantly improve the hardenability of steel.

2) Austenitic conditions: the higher the austenite temperature, the longer the holding time, due to the large austenite grains, uniform composition, and the complete dissolution of various carbides, the supercooled austenite is stable, and the quenching critical speed is small, so the hardenability of the steel increases. It is important to note that grain size is not suitable because it will lead to a decrease in toughness and plasticity, and a greater tendency to crack.

The addition of alloying elements such as MO, CR, MN, and Ni can significantly reduce the critical cooling rate of steel, so the addition of these elements can make the transformation of martensite more complete during quenching (less untransformed austenite residue), so that alloying elements such as MO, CR, MN, and Ni can improve the hardenability of steel.

Quenching + tempering = quenching and tempering... Heating the steel to a critical temperature and cooling it quickly is called quenching.

Quenching and tempering = quenching + high temperature tempering.

Quenching = rapid cooling + tempering after heating to the austenitization temperature (there are also non-tempering ones, depending on the material and requirements).

Generally, medium carbon steel and high carbon steel materials need to be quenched, and low carbon steel materials need to be quenched.

Quenching is a heat treatment process in which the steel is heated to a temperature above the critical temperature AC3 (sub-eutectic steel) or AC1 (super-eutectic steel), kept warm for a period of time to make it fully or partially austenitized, and then cooled to below ms (or isothermal near ms) at a cooling rate greater than the critical cooling rate for martensite (or bainite) transformation.

Quenching and tempering treatment: The heat treatment method of high-temperature tempering after quenching is called quenching and tempering treatment. High temperature tempering refers to tempering between 500-650.

Quenching and tempering can make the performance and material of steel be adjusted to a great extent, and its strength, plasticity and toughness are good, and it has good comprehensive mechanical properties.

Quenching + high temperature tempering = quenching and tempering, quenching and tempering is the dual heat treatment of quenching and high temperature tempering, the purpose of which is to make the workpiece have good comprehensive mechanical properties.

Quenching: The heat treatment process in which steel parts are heated to an austenitizing temperature and held for a certain period of time, and then cooled at a cooling rate greater than the critical cooling rate to obtain non-diffusion transformation structures, such as martensitic, bainite and austenite.

The heat treatment process of steel includes methods such as annealing, normalizing, quenching, tempering, and surface heat treatment. Among them, tempering includes high-temperature tempering, medium-temperature tempering and low-temperature tempering. Reheating the quenched steel to a certain temperature and then cooling it in a certain way is called tempering.

Its purpose is to eliminate the internal stresses caused by quenching and reduce hardness and brittleness in order to achieve the desired mechanical properties.

Quenching and tempering usually refers to the heat treatment process of quenching + high temperature tempering to obtain tempered sostene. The method is to quench first, and the quenching temperature: AC3+30 50 for sub-eutectic steel; Ac1+30 50 for eutectic steel; Alloy steel can be slightly higher than carbon steel.

After quenching, tempering at 500 650 can be done.

Quenching is the first step of the process, the heating temperature depends on the composition of the steel, and the quenching medium is selected according to the hardenability of the steel and the size of the steel. After quenching, the internal stress of steel is large and brittle, and it must be tempered in order to eliminate stress, increase toughness, and adjust strength. Tempering is the most important process to finalize the mechanical properties of quenched and tempered steel.

The curve of the mechanical properties of various steels with the tempering temperature, also known as the tempering curve of steel, can be used as the basis for selecting the tempering temperature. For the high-temperature tempering of some alloy quenched and tempered steels, it is necessary to pay attention to prevent the occurrence of the second type of tempering brittleness to ensure the performance of the steel.

Hardenability refers to the ability to obtain the depth of a hardened layer, and the hardenability of steel depends on the chemical composition in the steel and how it is quenched and cooled. Hardenability, on the other hand, refers to the highest hardness that can be achieved by quenching, which mainly depends on the carbon content in the steel.

The depth of the hardening layer is related to the hardenability of the steel, the shape and size of the workpiece, and the cooling capacity of the quenching medium.

Steel refers to an iron-carbon alloy with a carbon content of less than 2%. According to the different compositions, it can be divided into carbon steel and alloy steel. According to different properties and uses, it can be divided into structural steel, tool steel and special performance steel.

Hardenability: The hardenability of steel refers to the ability of austenitized steel to obtain martensitic when quenched, which reflects the stability of supercooled austenite and is related to the critical cooling rate of steel. Its size is expressed by the depth of the hardened layer and the hardness distribution obtained by quenching the steel under certain conditions.

Hardenability: The hardenability of steel refers to the ability of austenitized steel to harden when quenched, mainly depending on the carbon content in the martensitic and expressed in terms of the highest hardness that can be achieved by quenched martensite. Hardened Layer Depth:

The depth of the hardened layer refers to the depth from the semi-martensitic zone to the surface of the workpiece, as determined when the steel is quenched under specific conditions. It is related to the hardenability of the steel, the shape and size of the workpiece, and the cooling capacity of the quenching medium.