How To Distinguish Quenching, Tempering, Normalizing, Annealing

What is tempering?

Tempering is a heat treatment process in which the quenched metal material or part is heated to a specific temperature, kept for a certain period, and then cooled in a certain way. Tempering is an operation performed immediately after quenching and is usually the last part of the heat treatment of the workpiece. The combined process of quenching and tempering is called final treatment. The primary purpose of quenching and tempering is:

1) Reduce internal stress and reduce brittleness. The quenched parts have significant stress and brittleness. They will tend to deform or even crack if not tempered in time.

2) Adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. It can be adjusted by tempering, hardness, strength, plasticity, and toughness to meet the different performance requirements of various workpieces.

3) Stabilize the size of the workpiece. The metallographic structure can be stabilized by tempering to ensure no deformation occurs during future use.

4) Improve the cutting performance of certain alloy steels.
The effect of tempering is:

① Improve the organization's stability so that the structure of the workpiece no longer changes during use so that the geometric size and performance remain stable.

② Eliminate internal stress to improve the performance of the workpiece and stabilize the geometric size of the workpiece.

③ 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 atomic activity increases. The atoms of iron, carbon, and other alloying elements in the steel can diffuse faster to realize the rearrangement and combination of particles, making it unstable. The unbalanced organization gradually transformed into a stable, balanced organization. Eliminating internal stress is also related to the decrease in metal strength when the temperature rises. When general steel is tempered, the hardness and strength decrease, and the plasticity increases. The higher the tempering temperature, the more significant the change in these mechanical properties. Some alloy steels with higher content of alloying elements will precipitate some fine particles of metal compounds when tempered in a specific temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.
Tempering requirements: Workpieces with different purposes should be tempered at various temperatures to meet the requirements for use.

① Tools, bearings, carburized and hardened parts, and surface hardened parts are usually tempered below 250°C. The hardness changes little after low-temperature tempering, the internal stress is reduced, and the toughness is slightly improved.

② The spring is tempered at a medium temperature of 350～500℃ to obtain higher elasticity and necessary toughness.

③ Parts made of medium carbon structural steel are usually tempered at high temperatures of 500～600℃ to obtain a good match of suitable strength and toughness.

 

When steel is tempered at around 300°C, it often increases its brittleness. This phenomenon is called the first type of temper brittleness. Generally, it should not be tempered in this temperature range. Certain medium-carbon alloy structural steels are also prone to become brittle if they are slowly cooled to room temperature after high-temperature tempering. This phenomenon is called the second type of temper brittleness. Adding molybdenum to steel or cooling in oil or water during tempering can prevent the second type of temper brittleness. This kind of brittleness can be eliminated by reheating the second type of tempered brittle steel to the original tempering temperature.

In production, it is often based on the workpiece's performance requirements. According to the different heating temperatures, tempering is divided into low-temperature, medium-temperature, and high-temperature. The heat treatment process that combines quenching and subsequent high-temperature tempering is called quenching and tempering, which means it has high strength and good plastic toughness.

1. Low-temperature tempering: 150-250°C, M cycles, reduces internal stress and brittleness, improves plastic toughness, and has higher hardness and wear resistance. I used to make measuring tools, cutting tools, rolling bearings, etc.

2. Intermediate temperature tempering: 350-500℃, T cycle, high elasticity, certain plasticity, and hardness. Used to make springs, forging dies, etc. CNC machining part

3. High-temperature tempering: 500-650℃, S time, with good comprehensive mechanical properties. I used to make gears, crankshafts, etc.

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 and air-cooled. The main feature is that the cooling rate is faster than annealing and lower than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly quicker cooling. Not only can satisfactory strength be obtained, but the toughness (AKV value) can also be significantly improved and reduced—the tendency of the component 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 significantly improve, and the cutting performance is also improved. aluminum part

Normalizing has the following purposes and uses:

① For hypereutectoid steels, normalizing is used to eliminate the overheated coarse-grained structure and Widmanstatten structure of cast, forging, and weldments, and the band structure in rolled materials; refine grains; and can be used as pre-heat treatment before quenching.

② For hypereutectoid steels, normalizing can eliminate the reticulated secondary cementite and refine the pearlite, improving the mechanical properties and facilitating the subsequent spheroidizing annealing.

③ For low-carbon deep-drawing thin steel sheets, normalizing can eliminate the free cementite in the grain boundary to improve its deep-drawing performance.

④ For low-carbon steel and low-carbon low-alloy steel, normalizing can obtain more flake pearlite structure, increase the hardness to HB140-190, avoid the phenomenon of "sticking knife" during cutting, and improve the machinability. Normalizing is more economical and convenient for medium carbon steel when normalizing and annealing are available. Five axes machined part

⑤ For ordinary medium carbon structural steels, where the mechanical properties are not high, normalizing can be used instead of quenching and high-temperature tempering, which is easy to operate and stable in the structure and size of the steel.

⑥ High temperature normalizing (150～200℃ above Ac3) can reduce the composition segregation of castings and forgings due to the high diffusion rate at high temperatures. After high-temperature normalization, a second lower-temperature normalization can refine the coarse grains. 

⑦ For some low- and medium-carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure. Then, after high-temperature tempering, it has good creep resistance when used at 400-550℃.

⑧ In addition to steel parts and steel, normalizing is also widely used in the heat treatment of ductile iron to obtain a pearlite matrix and improve the strength of ductile iron.

Since the characteristic of normalizing is air cooling, the ambient temperature, stacking method, airflow, and workpiece size all affect the organization and performance after normalizing. The normalizing structure can also be used as a classification method for alloy steel. Generally, alloy steels are divided into pearlite, bainite, martensitic, and austenitic steel based on the structure obtained by air cooling after a sample with a diameter of 25 mm is heated to 900°C.

What is annealing?

Annealing is a metal heat treatment process that slowly heats the metal to a specific temperature, keeps it for a sufficient time, and then cools it at an appropriate speed. Annealing heat treatment is divided into incomplete,g, and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile or hardness tests. Many steels are supplied in an annealed heat treatment state. A Rockwell hardness tester can test the hardness of steel to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, the surface Rockwell hardness tester can be used to test HRT hardness. .

The purpose of annealing is to:

① Improve or eliminate structural defects and residual stresses caused by steel casting, forging, rolling, and welding, and prevent deformation and cracking of the workpiece.

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 organization for the final heat treatment (quenching, tempering).

Commonly used annealing processes are:

① Completely annealed. It is used to refine the coarse superheated structure with poor mechanical properties after casting, forging,g, and welding medium and low carbon steel. Heat the workpiece to 30-50℃ above the temperature at which all ferrite is transformed into austenite, keep it for some time, then slowly cool down with the furnace. During the cooling process, the austenite transforms again to make the steel structure finer.

② Spheroidizing annealing. They are used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20-40°C above the temperature at which the steel forms austenite and then slowly cools after holding the temperature. During the cooling process, the lamellar cementite in the pearlite becomes spherical, reducing the hardness.

③ Isothermal annealing. It reduces the hardness of some alloy structural steels with higher nickel and chromium content for cutting. Generally, it is cooled to the most unstable temperature of austenite at a relatively rapid rate. After holding for a proper time, the austenite is transformed into troostite or sorbite, and the hardness can be reduced.

④ Recrystallization annealing. It eliminates the hardening phenomenon (increase in hardness and decrease in plasticity) of metal wire and sheet during cold drawing and rolling. The heating temperature is generally 50 to 150°C below the temperature at which the steel begins to form austenite. Only in this way can the work-hardening effect be eliminated, and the metal can be softened.

⑤ Graphitization annealing. It is used to make cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950°C, keep it warm for a certain period, and then cool it appropriately to decompose the cementite to form flocculent graphite.

⑥ Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve its performance. The method is to heat the casting to the highest possible temperature without melting it for a long time and slowly cooling down after the diffusion of various elements in the alloy, which tends to be evenly distributed.

⑦ Stress relief annealing. It eliminates the internal stress of steel castings and welding parts. For steel products, the temperature at which austenite begins to form after heating is 100-200℃, and the internal stress can be eliminated by cooling in the air after holding the temperature.