Why does steel have to be tempered after quenching?

Tempering. It is generally followed by quenching.

carried out, the purpose of which is to:

1. Eliminate the residual stress generated during the quenching of the workpiece to prevent deformation and cracking;

2. Adjust the hardness, strength, plasticity and toughness of the workpiece to meet the performance requirements;

3. Stabilize the structure and size to ensure accuracy;

4. Improve and enhance processing performance. Therefore, tempering is the last important step in the process of obtaining the required performance of the workpiece. Through the combination of quenching and tempering, the required mechanical properties can be obtained.

According to the tempering temperature range, tempering can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.

Tempering is an indispensable follow-up process after quenching parts, it is a heat treatment process method that heats the quenched steel to a certain temperature below A1 and cools it to room temperature after heat preservation. Quenched steel generally cannot be used directly, this is due to: The parts are in a state of high stress, which is easy to cause deformation and cracking when placed or used at room temperature The quenched state (m+a) is a metastable state, and changes in structure, properties and dimensions will occur during use The flaky martensite in the quenched structure is hard and brittle, which cannot meet the requirements of the use of parts.

Ignition can improve these conditions and obtain the required mechanical properties. Due to the gradual changes in various microstructures with the increase of temperature during the tempering process, the properties of the steel will also gradually change.

Tempering. tempering；temper

It is also known as matching fire. A type of metal heat treatment process. Metal heat treatment in which the quenched workpiece is reheated to an appropriate temperature below the lower critical temperature, and then cooled in air or water or oil after holding it for a period of time.

Or the quenched alloy workpiece is heated to the appropriate temperature, kept warm for a certain period of time, and then cooled slowly or quickly. It is generally used to reduce or eliminate the internal stress in the hardened steel, or reduce its hardness and strength to improve its ductility or toughness. According to different requirements, low temperature tempering, medium temperature tempering or high temperature tempering can be used.

Generally, as the tempering temperature increases, the hardness and strength decrease, and the ductility or toughness gradually increases.

After quenching, the steel workpiece has the following characteristics: Unbalanced (i.e., unstable) structures such as martensite, bainite, and residual austenite are obtained. There are large internal stresses. The mechanical properties can not meet the requirements. Therefore, steel workpieces are generally tempered after quenching.

The function of tempering is to improve the stability of the tissue, so that the workpiece will no longer undergo tissue transformation during use, so that the geometric size and performance of the workpiece remain stable. Internal stresses are relieved in order to improve the performance of the workpiece and stabilize the workpiece geometry.

Adjust the mechanical properties of steel to meet the requirements of use.

The reason why tempering has these effects is that when the temperature rises, the activity ability of the atoms increases, and the atoms of iron, carbon and other alloying elements in the steel can diffuse relatively quickly, realizing the rearrangement and combination of atoms, so that the unstable unbalanced organization is gradually transformed into a stable equilibrium organization. The elimination of internal stresses is also associated with a decrease in the strength of the metal as the temperature increases. Generally, when steel is tempered, the hardness and strength decrease, and the plasticity increases.

The higher the tempering temperature, the greater the variation in these mechanical properties. Some alloy steels with high alloying element content will precipitate some fine-grained metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.

Requirements Workpieces with different uses should be tempered at different temperatures to meet the requirements in use. Tools, bearings, carburizing and quenching parts, and case hardened parts are usually tempered at low temperatures below 250. After low temperature tempering, the hardness changes little, the internal stress decreases, and the toughness is slightly improved.

The springs are tempered at medium temperature at 350 500 degrees Celsius to obtain high elasticity and the necessary toughness. Parts made of medium carbon structural steel are usually tempered at high temperatures at 500 600 degrees Celsius to obtain a good combination of appropriate strength and toughness. The heat treatment process of quenching and high temperature tempering is generally called quenching and tempering.

When steel is tempered at about 300, it often increases its brittleness, and this phenomenon is called the first type of tempering brittleness. In general, tempering should not be carried out in this temperature range. Some medium carbon alloy structural steels are also prone to brittleness if they are slowly cooled to room temperature after tempering at high temperatures.

This phenomenon is called type II tempering brittleness. The addition of molybdenum to steel, or cooling in oil or water when tempered, can prevent Type II tempering brittleness. This brittleness can be eliminated by reheating the steel of the second type of tempered brittleness to the original tempering temperature.

Because after quenching, although the hardness of the steel is greatly improved, it lacks flexibility, so it is easy to crack and cause a gap when used, so the hardness can be slightly reduced by backfire, so that it is rigid and soft, and it is not easy to be damaged.

Here's why:

First, the quenched steel is in a state of high stress, which is easy to cause deformation and cracking; Second, the quenching structure is in a metastable state, which is prone to microstructure and size changes. Thirdly, the flaky martensite in the quenched structure is hard and brittle, which cannot meet the requirements of the workpiece, and tempering can improve these conditions.

Tempering is generally required after quenching. This is because the internal stress of the steel caused by quenching, and it is the structural distortion and internal stress that can improve the hardness of the steel through quenching, and the proper tempering is to reduce the internal stress and prevent the deformation and even cracking of the material due to the continuous high stress after the steel is quenched.

Quenching is to increase the hardness and wear resistance of steel, but steel that is too hard is brittle and prone to breakage, so it is necessary to reduce the hardness of steel by tempering and increase its toughness and ductility. Tempering also reduces residual stresses and deformations after quenching. Tempering is to heat the quenched steel parts again to a proper temperature and return to a moderate degree, and keep the socks warm for a period of time, and then cool to room temperature.

During tempering, the hardness of the steel gradually decreases, while the toughness and ductility increase. In this way, it can improve the toughness and ductility of the steel while maintaining a certain hardness, so as to make it more valuable.

Quenching: Heat the steel to 30-50 above the AC3 or AC1 line, keep it warm for a certain time, and then cool it quickly in water or oil to obtain a martensitic structure.

Purpose: Mainly to obtain martensite and improve the hardness and wear resistance of steel.

Two concepts: hardenability, hardenability.

After quenching, the strength and hardness have been greatly improved, but the plasticity and toughness have been significantly reduced, in addition, there is a large internal stress inside the quenched workpiece, if not treated in time, it will be further deformed to cracking, for this reason, it is necessary to temper in time after quenching.

Tempering: The quenched steel is heated to a certain temperature below the AC1 line, kept warm at this temperature for a certain period of time (2-4 hours), and then taken out to cool in air or oil.

Tempering is usually the last step in the heat treatment of steel parts, so the combined process of quenching and tempering is called the final heat treatment.

Objectives: 1) Reduce brittleness, reduce internal stress, and prevent deformation and cracking.

2) Adjust the mechanical properties of the steel.

3) Stabilize the organization to ensure the stability of the size and shape of the workpiece.

Low temperature tempering: heat to 150-250, keep warm for 1-3 hours and then air cool, to obtain tempered martensite. (High hardness is guaranteed, such as cutting tools, measuring tools).

Medium temperature tempering: heating to 350-450, air cooling after heat preservation, to obtain tempered doxylite. (High elastic limit, with a certain toughness and hardness, such as springs).

High temperature tempering: heating to 500-650, air cooling after heat preservation, to obtain tempered sostenite. (There is a certain strength and hardness, and good plasticity and toughness, such as crankshafts, gears).

Quenching High temperature tempering = quenching and tempering treatment.