The heat treatment of steel includes normalizing, annealing, tempering and quenching. Among them, annealing and normalizing are mainly used for preliminary heat treatment, and are only used as final heat treatment when the performance requirements of the workpiece are not high.
The purpose of quenching is to obtain martensite structure and improve the properties of steel. Tempering is mainly to reduce or eliminate quenching internal stress, prevent deformation or cracking, and obtain the required mechanical properties at the same time.
Let’s learn it through this article!
1.What is quenching?
The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), keep it warm for a period of time to make it fully or partially austenitized, and then cool it at a temperature greater than the critical cooling rate. A heat treatment process that rapidly cools to below Ms (or isothermally near Ms) to transform martensite (or bainite). The solid solution treatment or heat treatment process with rapid cooling process of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials is usually called quenching.
Purpose of quenching:
1) Improve the mechanical properties of metal products or parts. For example: improving the hardness and wear resistance of tools, bearings, etc., increasing the elastic limit of springs, improving the comprehensive mechanical properties of shaft parts, etc. 2) Improve the material properties or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel and increasing the permanent magnetism of magnetic steel. When quenching and cooling, in addition to the reasonable selection of quenching media, the correct quenching method is also required. Commonly used quenching methods mainly include single-liquid quenching, double-liquid quenching, graded quenching, isothermal quenching, partial quenching, etc.
2.What is tempering?
Tempering is a heat treatment process that heats quenched metal products or parts to a certain temperature, holds them for a certain period of time, and then cools them in a certain way. Tempering is an operation performed immediately after quenching, and is usually the last step in the heat treatment of the workpiece. A process, so the combined process of quenching and tempering is called the final treatment.
The main purposes of quenching and tempering are:
1) Reduce internal stress and brittleness. Quenched parts have great stress and brittleness. If they are not tempered in time, they will often deform or even crack.
2) Adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and brittleness. In order to meet the different performance requirements of various workpieces, the hardness, strength, plasticity and toughness can be adjusted through tempering.
3) Stable workpiece size. Tempering can stabilize the metallographic structure to ensure that it will no longer deform during subsequent use.
4) Improve the cutting performance of certain alloy steels.
The function of tempering is:
① Improve the structural stability so that the workpiece will no longer undergo structural transformation during use, thereby maintaining stable geometric dimensions and performance of the workpiece.
② Eliminate internal stress to improve the performance of the workpiece and stabilize the geometric dimensions of the workpiece.
③ Adjust the mechanical properties of steel to meet usage requirements.
The reason why tempering has these effects is that when the temperature rises, the atomic mobility increases, and the atoms of iron, carbon and other alloying elements in the steel can diffuse faster, realizing the rearrangement and combination of atoms, thereby making the instability The unbalanced organization gradually transforms into a stable balanced organization. The elimination of internal stress is also related to the decrease in metal strength when 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 change in these mechanical properties. Some alloy steels with higher alloy element content will precipitate some fine metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.
3.What is annealing?
Annealing is a metal heat treatment process that slowly heats metal to a certain temperature, maintains it for a sufficient time, and then cools it at an appropriate speed. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile testing or hardness testing. Many steels are supplied in an annealed and heat-treated state. The hardness of steel can be tested using a Rockwell hardness tester to test the HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, a surface Rockwell hardness tester can be used to test the HRT hardness. .
The purpose of annealing is to:
① Improve or eliminate various structural defects and residual stress caused by steel casting, forging, rolling and welding processes to prevent workpiece deformation and cracking.
② Soften the workpiece for cutting.
③ Refine the grains and improve the structure to improve the mechanical properties of the workpiece.
④ Prepare the structure for final heat treatment (quenching and tempering).
4.What is normalizing?
Normalizing is a heat treatment that improves the toughness of steel. After the steel component is heated to 30~50°C above the Ac3 temperature, it is kept warm for a period of time and then air-cooled. The main feature is that the cooling speed is faster than annealing and lower than quenching. During normalizing, the crystal grains of the steel can be refined in slightly faster cooling. Not only can satisfactory strength be obtained, but also the toughness (AKV value) can be significantly improved and the steel can be reduced. Tendency of components to crack. After normalizing treatment of some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting performance is also improved.
The purpose of normalizing
(1) Remove the internal stress of the material
(2) Increase the hardness of the material
This is to prepare for subsequent processing. It has the same effect as annealing, but is just to improve efficiency and reduce costs.
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