In order for metal workpiece to have required working properties, a heat treatment process is often necessary. Heat treatment process generally includes three processes of heating, heat preservation and cooling. It is divided into quenching, tempering, normalizing, annealing, etc. depending on process. Can you distinguish it?

Quenching of steel is to heat steel to a temperature above critical temperature Ac3 (hypegmatized steel) or Ac1 (hyper-eutectoid steel), hold it for a period of time, make it all or part austenitized, and then rapidly cooling to a temperature below Ms (or isothermal near Ms) at a cooling rate greater than critical cooling rate. Solution treatment of materials such as aluminum alloys, copper alloys, titanium alloys, tempered glass, or heat treatment process with a rapid cooling process is also referred to as quenching.

1) Improve mechanical properties of metal materials or parts. For example, improve hardness and wear resistance of tools, bearings, etc., improve elastic limit of spring, and improve comprehensive mechanical properties of shaft parts.
2) Improve material properties or chemical properties of certain special steels. Such as improving corrosion resistance of stainless steel, increasing permanent magnetity of magnetic steel.
When quenching and cooling, in addition to reasonable selection of quenching medium, there must be a correct quenching method. Commonly used quenching methods mainly include single-liquid quenching, two-liquid quenching, fractional quenching, austempering, and local quenching.

 

1 Unbalanced(unstable) structures such as martensite, bainite, and retained austenite are obtained.
2 There is a large internal stress.
3 Mechanical properties can not meet requirements. Therefore, steel workpieces are generally tempered after quenching

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

1) Reduce internal stress and reduce brittleness. Quenching parts have great stress and brittleness. If they are not tempered in time, they will often deform or even crack.
2) Adjust mechanical properties of workpiece. After workpiece is quenched, hardness is high and brittleness is large. In order to meet different performance requirements of various workpieces, hardness, strength, ductility and toughness can be adjusted by tempering.
3) Stabilize workpiece size. By tempering, metallographic structure can be stabilized to ensure that deformation does not occur during subsequent use.
4) Improve cutting performance of certain alloy steels.

 

1 Improve stability of structure, so that workpiece no longer undergoes tissue transformation during use, workpiece geometry and performance are stable.
2 Eliminate internal stress to improve performance of workpiece and stabilize geometry of workpiece.
3 Adjust mechanical properties of steel to meet requirements of use.
Reason why tempering has these effects is because atomic activity is enhanced when temperature is raised, atoms of iron, carbon and other alloying elements in steel can be diffused faster, realizing rearrangement and combination of atoms, thereby unstable unbalanced tissue is gradually transformed into a stable equilibrium organization. Elimination of internal stress is also related to decrease in metal strength at elevated temperatures. In general, when steel is tempered, hardness and strength are lowered, and plasticity is improved. The higher tempering temperature, the greater change in these mechanical properties. Some alloy steels with a high content of alloying elements will precipitate some fine metal compounds when tempered in a certain temperature range, which will increase strength and hardness. This phenomenon is called secondary hardening.

Workpieces with different uses should be tempered at different temperatures to meet requirements of use.
1 Tools, bearings, carburized and quenched parts, and surface hardened parts are usually tempered at a low temperature below 250℃. After low temperature tempering, hardness does not change much, internal stress decreases, and toughness is slightly improved.
2 Spring is tempered at 350-500℃ at medium temperature to obtain high elasticity and necessary toughness.
3 Parts made of medium carbon structural steel are usually tempered at a high temperature of 500 to 600℃ to obtain a good fit of strength and toughness.
When steel is tempered at around 300℃, its brittleness is often increased. This phenomenon is called the first type of temper brittleness. Generally should not temper in this temperature range. Some medium carbon alloy structural steels tend to become brittle if they are slowly cooled to room temperature after tempering at high temperatures. This phenomenon is called second type of temper brittleness. Addition of molybdenum to steel, cooling in oil or water during tempering prevents second type of temper brittleness. This brittleness can be eliminated by reheating second type of tempered brittle steel to original tempering temperature.
In production, tempering is often divided into low temperature tempering, medium temperature tempering, and high temperature tempering according to requirements of performance of workpiece and heating temperature. Heat treatment process combined with quenching and subsequent high temperature tempering is called quenching and tempering, that is, it has high strength and good plastic toughness.
1) Low temperature tempering: 150-250℃, M tempering, reduce internal stress and brittleness, improve plastic toughness, have high hardness and wear resistance. Used in production of measuring tools, tools and rolling bearings.
2) Medium temperature tempering: 350-500℃, T tempering, with high elasticity, a certain plasticity and hardness. Used to make springs, forging dies, etc.
3) High temperature tempering: 500-650℃, S tempering, with good comprehensive mechanical properties. Used to make gears, crankshafts, etc.

 

Normalizing is a heat treatment that improves toughness of steel. After heating steel member to 30~50℃ above Ac3 temperature, steel is cooled for a while. Main feature is that cooling rate is faster than annealing and less than quenching. During normalizing, crystal grains of steel can be refined in a slightly faster cooling, which not only can obtain satisfactory strength, but also can significantly improve toughness (AKV value) and reduce tendency of member to crack. After normalized treatment of some low-alloy hot-rolled steel sheets, low-alloy steel forgings and castings, comprehensive mechanical properties of material can be greatly improved, and cutting performance is also improved.

1 For sub-eutectic steel, normalizing is used to eliminate superheated coarse-grained structure and Wei's structure of cast, forged and welded parts, banded structure in rolled material, and grain refinement, and it can be used as a pre-heat treatment before quenching.
2 For hypereutectoid steel, normalizing can eliminate network secondary cementite and refine pearlite, which not only improves mechanical properties, but also facilitates subsequent spheroidizing annealing.
3 For low-carbon deep-drawn thin steel sheets, normalizing can eliminate free cementite at grain boundary to improve its deep-drawing performance.
4 For low-carbon steel and low-carbon low-alloy steel, normalizing can obtain more fine-grained pearlite structure, so that hardness is increased to HB140-190, avoiding "sticking knife" phenomenon during cutting and improving machinability. For medium carbon steel, it is more economical and convenient to use normalizing in the case where both normalizing and annealing can be used.
5 For ordinary medium carbon structural steel, in the case where mechanical properties are not high, normalizing can be used instead of quenching and high temperature tempering, which is not only easy to operate, but also makes steel structure and size stable.
6 High-temperature normalizing (Ac-3 or more 150 to 200℃). Due to high diffusion rate at high temperatures, composition segregation of castings and forgings can be reduced. Coarse grains after high temperature normalizing can be refined by a second lower temperature normalizing.
7 For some low and medium carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure, then tempered at high temperature for good creep resistance at 400-550℃.
8 In addition to steel, normalizing is also widely used in heat treatment of ductile iron to obtain a pearlite matrix and improve strength of ductile iron.
Since normalizing is characterized by air cooling, ambient temperature, stacking mode, airflow, and workpiece size all have an impact on organization and performance after normalizing. Normalizing tissue can also be used as a classification method for alloy steel. Generally, according to structure obtained by air cooling after heating a sample with 25 mm diameter to 900℃,   alloy steel is divided into pearlitic steel, bainitic steel, martensitic steel and austenitic steel.

 

Annealing is a metal heat treatment process in which metal is slowly heated to a certain temperature for a sufficient period of time and then cooled at a suitable rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. Mechanical properties of annealed material can be tested by tensile testing or by hardness testing. Many steels are supplied in an annealed heat treatment state. Hardness test of steel can be tested by Rockwell hardness tester. For thinner steel plates, steel strips and thin-walled steel pipes, a surface Rockwell hardness tester can be used to detect HRT hardness. .

1 Improve or eliminate various structural defects and residual stresses caused by steel during casting, forging, rolling and welding, prevent deformation and cracking of workpieces.
2 Soften workpiece for cutting.
3 Refine grains and improve structure to improve mechanical properties of workpiece.
4 Prepare organization for final heat treatment (quenching, tempering).

 

1 Fully annealing. It is used to refine coarse superheated structure of medium and low carbon steel which has poor mechanical properties after casting, forging and welding. Workpiece is heated above a temperature of 30 to 50℃ at which all of ferrite is transformed into austenite, kept for a period of time, slowly cooled with furnace, and austenite is again transformed during cooling process, so that microstructure of steel is thinned. .
2 Spheroidizing annealing. It is used to reduce high hardness of tool steel and bearing steel after forging. Workpiece is heated above a temperature of 20 to 40℃ at which steel begins to form austenite, and is slowly cooled after heat preservation. During cooling process, lamellar cementite in pearlite becomes spherical, thereby lowering hardness.
3 Isothermal annealing. It is used to reduce high hardness of certain alloy structural steels with high nickel and chromium content for cutting. Generally, it is cooled to the most unstable temperature of austenite at a relatively rapid rate, austenite is transformed into torsite or sorbite at a suitable temperature for a suitable period of time, and hardness can be lowered.
4 Recrystallization annealing. It is used to eliminate hardening phenomenon (hardness increase, plasticity drop) of metal wire and sheet during cold drawing and cold rolling. Heating temperature is generally 50 to 150℃ below temperature at which steel begins to form austenite. Only in this way can work hardening effect be eliminated and metal softened.

 

5 Graphitization annealing. It is used to make cast iron containing a large amount of cementite into a malleable cast iron with good plasticity. Process operation is to heat casting to about 950℃, and after proper cooling for a certain period of time, cementite is decomposed to form a group of flocculent graphite.
6 Diffusion annealing. It is used to homogenize chemical composition of alloy castings and improve their performance. Method is to heat casting to the highest possible temperature without melting, keep for a long time, various elements in alloy tend to be uniformly distributed and then slowly cooled.
7 Stress relief annealing. Used to eliminate internal stress of steel castings and welded parts. After steel product is heated, austenite is formed at a temperature of 100 to 200℃ below, and after cooling, it is cooled in the air to eliminate internal stress.