Heat Treatment 4: Quenching, Tempering, Normalizing, Annealing

- May 27, 2019-

Heat treatment 4: quenching, tempering, normalizing, annealing

 

First, quenching

1. What is quenching?

Steel quenching is one of the steel is heated to a critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) above temperature, heat preservation for a period of time, make all or part of the austenitizing, then at greater than the critical cooling rate of cold faster cold below Ms to martensite isothermal near (or Ms) or bainite transformation of heat treatment process. In general, the solid solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or heat treatment process with rapid cooling process is called quenching.

 

2. Purpose of quenching:

 

1) improve the mechanical properties of metal products or parts. For example: improve the hardness and wear resistance of tools, bearings, etc., improve the elastic limit of the spring, improve the comprehensive mechanical properties of shaft parts, etc.
2) improve the material or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel, increase the permanent magnetic steel, etc

 

Quenching cooling, in addition to the need for a reasonable choice of quenching medium, but also the correct quenching method, the common quenching methods, mainly single liquid quenching, double liquid quenching, quenching, isothermal quenching, local quenching.

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3. The steel workpiece has the following characteristics after quenching:

(1) martensite, bainite, residual austenite and other unbalanced (that is, unstable) structures were obtained.
(2) there is a large internal stress.
(3)The mechanical properties can not meet the requirements. Therefore, the steel workpiece after quenching is generally through tempering

 

Second, the tempering

1. What is tempering?

 

Tempering after quenching metal timber or parts of heating to a certain temperature, heat preservation, after a certain time in a certain way the cooling of the heat treatment process and tempering is followed by an operation, after quenching are also usually workpiece heat treatment at the end of the procedure, and thus have a combination of quenching and tempering process known as the final processing.

2. The main purposes of quenching and tempering are:

1) reduce the internal stress and reduce the brittleness, there is a great deal of stress and brittleness in the quenching parts, such as not timely tempering often produce deformation or even cracking.
2) adjust the mechanical properties of the workpiece, the workpiece after quenching, high hardness, brittleness, in order to meet the various workpiece different performance requirements, can be adjusted by tempering, hardness, strength, plasticity and toughness.
3) stabilize the workpiece size. The metallographic structure can be stabilized by tempering to ensure that deformation will not occur in the future use.
4) improve the cutting performance of some alloy steels.

 

3. The tempering effect is:

 

(1) improve the stability of the organization, so that the workpiece in the use of the process no longer occur organizational transformation, so that the workpiece geometric size and performance remain stable.
(2) eliminate the internal stress, in order to improve the workpiece performance and stability of the workpiece geometry size.
(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 activity of atoms is enhanced, and the atoms of iron, carbon and other alloying elements in steel can be diffused more quickly to achieve the rearrangement and combination of atoms, so as to gradually transform the unstable unbalanced structure into a stable equilibrium structure.The elimination of internal stress is also related to the decrease of metal strength when temperature increases. General steel temper, hardness and strength decline, plasticity improved. The higher the tempering temperature, the greater the change of these mechanical properties. Some alloy steels with higher content of alloying elements, when tempered in a certain temperature range, will precipitate some fine metal compounds, making the strength and hardness rise. This phenomenon is called secondary hardening.

 

Tempering requirements: workpieces with different purposes should be tempered at different temperatures to meet the requirements in use.

(1) cutting tools, bearing, carburizing and quenching parts, surface quenching parts often under 250 ℃ low temperature tempering. After tempering at low temperature, the hardness changes little, the internal stress decreases and the toughness increases slightly.
(2) spring in 350 ~ 500 ℃ under the tempering temperature, high elasticity and the necessary toughness can be obtained.
(3) in the carbon structural steel parts are usually made in 500 ~ 600 ℃ high temperature tempering, in order to obtain appropriate with good strength and toughness.

 

Steel at 300 tempering, often make its brittleness increases, this phenomenon is called the first kind of temper brittleness. Tempering should not normally be carried out in this temperature range. Some medium carbon alloy structural steels also tend to be brittle if they are slowly cooled to room temperature after high tempering. This phenomenon is known as the second type of temper brittleness. Adding molybdenum to steel or cooling it in oil or water during tempering can prevent the second type of temper brittleness. This brittleness can be eliminated by reheating the second type of temper brittle steel to the original tempering temperature.

 

In production, often according to the performance requirements of the workpiece. According to the different heating temperature, the tempering can be divided into low tempering, medium tempering and high tempering. The heat treatment process combining quenching and subsequent high temperature tempering is called quenching and tempering, that is, it has high strength and good ductility at the same time.

1) low temperature tempering: 150-250 ℃, M back, reduce internal stress and brittleness, improve plastic toughness, higher hardness and wear resistance. Used for making measuring tools, cutting tools and rolling bearings, etc.
Temperature tempering in 2) : 350-500 ℃, T back, has the high elasticity, has certain plasticity and hardness. Used for making springs, forging dies, etc.
3) high temperature tempering: 500-650 ℃, S back, has the good comprehensive mechanical properties. Used for making gears, crankshafts, etc.

 

 

Third, normalizing

 

1.What is normalizing?

 

Normalizing is a heat treatment that improves the toughness of steel. Heat the steel members to Ac3 after more than 30 ~ 50 temperature, heat preservation for a period of time from air cooling. The main characteristic is that the cooling speed is faster than that of annealing but lower than that of quenching. When normalizing, the crystallized grains of steel can be refined in slightly faster cooling, which not only can obtain satisfactory strength, but also can significantly improve the toughness (AKV value) and reduce the cracking tendency of components. After normalizing some low-alloy hot rolled steel plates, forging and casting parts, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting properties can also be improved.

 

 

2. Normalizing has the following purposes and USES:

 

(1) for hypoeutectoid steel, normalizing to eliminate the casting, forging, welding parts of the superheated coarse crystal structure and wechsler structure, rolled in the ribbon structure; Refining grain; And can be used as a pre-heat treatment before quenching.
(2) for hypereutectoid steel, normalizing can eliminate the network secondary cementite, and make the pearlite refining, not only improve the mechanical properties, but also conducive to the future spheroidization annealing.
(3) for low carbon deep drawing thin steel plate, normalizing can eliminate the grain boundary free cementite, in order to improve its deep drawing performance.

(4) for low carbon steel and low carbon low alloy steel, using normalizing, can get more fine flake pearlite structure, make the hardness increased to hb140-190, avoid the "stick knife" phenomenon when cutting, improve cutting processing. For medium carbon steel, normalizing is more economical and convenient when both normalizing and annealing are available.
(5) for ordinary medium carbon structure steel, in the mechanical properties of the occasion is not high, can be used instead of quenching and high temperature tempering, not only easy to operate, and make the steel structure and size stability.
(6) high temperature normalizing (150 ~ 200 ℃ above Ac3) due to the high temperature diffusion velocity is higher, can reduce the composition segregation of castings and forgings. The coarse grains after high temperature normalizing can be refined by the second subsequent normalizing at lower temperature.

(7)All landowners to some used for steam turbine and boiler of low and medium carbon alloy steel, normalizing is usually utilized to obtain bainite structure, then through high temperature tempering, when used in 400 ~ 550 ℃ has the good ability to resist creep.
(8)In addition to steel and steel, normalizing is also widely used in the heat treatment of nodular cast iron, so that it has pearlite matrix, improve the strength of nodular cast iron.

 

Because normalizing is characterized by air cooling, ambient temperature, stacking mode, airflow and workpiece size have an impact on the structure and performance after normalizing. Normalizing microstructure can also be used as a classification method for alloy steels. Usually according to the sample is heated to 900 for 25 millimeters in diameter, air cooling of organization, alloy steel can be divided into pearlite, bainite steel, steel martensitic steel and austenitic steel.

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Four, annealing

1. What is annealing?

Annealing is a heat treatment process in which metal is slowly heated to a certain temperature, kept for a sufficient time, and then cooled at an appropriate rate. 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 test or hardness test. Many steels are supplied in the state of annealing heat treatment. Rockwell hardness tester can be used to test HRB hardness for steel hardness testing. For thinner steel plates, steel belts and thin-walled steel tubes, surface rockwell hardness tester can be used to test HRT hardness.

2. The purpose of annealing is:

 

(1) improve or eliminate steel in casting, forging, rolling and welding process caused by a variety of tissue defects and residual stress, to prevent the workpiece deformation, cracking.
(2) to soften the workpiece for cutting.
(3) refine the grain, improve the organization to improve the mechanical properties of the workpiece.
(4) for the final heat treatment (quenching, tempering) to prepare for the organization.

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3. Common annealing processes include:

(1) Complete annealing. Used to fine medium and low carbon steel casting, forging and welding after the mechanical properties of the poor coarse overheating structure. All into the workpiece heated to ferrite austenite temperature more than 30 ~ 50 , the heat preservation for a period of time, and then with the slow cooling furnace, austenite during cooling to change again, can make the organization of the steel.

(2) Spheroidizing annealing. To reduce the high hardness of tool steel and bearing steel after forging. The workpiece heated to begin to form austenitic steel temperature above 20 ~ 40 , slow cooling after heat preservation, in the process of cooling of pearlite lamellar cementite into a ball, reducing the hardness.

(3) Isothermal annealing. To reduce the high hardness of some alloy structural steels with high content of nickel and chromium for cutting. Generally, the austenite is cooled to the most unstable temperature of the austenite at a relatively fast speed first, and the hardness can be reduced when the austenite is transformed into tortuoite or sorbite after proper heat preservation.

(4) Recrystallization annealing. To eliminate the metal wire, thin plate in the process of cold drawing, cold rolling hardening phenomenon (hardness increase, plastic decline). Heating temperature is commonly steel began to form the austenitic temperature below 50 ~ 150 , only in this way can eliminate the strain hardening effect to soften a metal.

(5) Graphitization annealing. It is used to transform cast iron containing a large amount of cementite into malleable iron with good plasticity. Casting process is be heated to 950 or so, appropriate cooling after heat preservation time, make cementite decomposition form flocculent mass graphite.

(6) Diffusion annealing. It is used to homogenize the chemical composition of alloy casting and improve its service performance. The method is to heat the casting to the highest temperature possible without melting and keep it warm for a long time, and then cool it slowly after all elements in the alloy are diffused and evenly distributed.

(7) Stress relief annealing. To eliminate the internal stress of steel castings and welding parts. For began after the formation of austenitic steel heating temperature under 100 ~ 200 , holding in the air after cooling, can eliminate internal stress.

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