Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape of the workpiece and the overall chemical[TJC STEEL] composition, but by changing the internal microstructure of the workpiece, or changing the chemical composition of the workpiece surface, giving or improving the performance of the workpiece. It is characterized by improving the intrinsic quality of the workpiece, which is generally not visible to the naked eye. As some people say, machining is surgery, heat treatment is internal medicine, representing the core competitiveness of a country's manufacturing industry.

 

Technological Process

Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterruptible.

When the metal is heated, the workpiece is exposed to the air, and often occurs oxidation and decarbonization (that is, the carbon content on the surface of the steel parts is reduced), which has a very adverse effect on the surface performance of the parts after heat treatment. Therefore, the metal should usually be heated in a controlled or protective atmosphere, molten salt and vacuum, and can also be used for protective heating [TJC STEEL]by coating or packaging methods.

The heating temperature is one of the important process parameters of heat treatment technology. The selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain high temperature microstructure. In addition, the transformation needs a certain amount of time, so when the surface of the metal workpiece to meet the requirements of the heating temperature, also must be maintained at this temperature for a certain time, so that the internal and external temperature is consistent, so that the microstructure transformation is complete, this period of time is called insulation time.

When high energy density heating and surface heat treatment are used, the heating speed is very fast, and generally there is no holding time, while the[TJC STEEL] holding time of chemical heat treatment is often longer.

 

Process Classification

Metal heat treatment process can be divided into integral heat treatment, surface heat treatment and chemical heat treatment. According to the heating medium, heating temperature and cooling methods, each category can be divided into several different heat treatment processes. The same metal with different heat treatment process, can obtain different structure, thus has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many[TJC STEEL] kinds of steel heat treatment processes.

Integral heat treatment is a metal heat treatment process in which the workpiece is heated as a whole and then cooled at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel has annealing, normalizing, quenching and tempering four[TJC STEEL] basic processes, that is, the "four fire" of heat treatment.

 

Quenching Process

The quenching of steel is to heat the steel to the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) above the temperature, heat preservation for a period of time, so that all or part of the austenitizing, and then with more than the critical cooling speed to Ms below fast cold (or Ms near isothermal) martensitic (or bainite) transformation of heat treatment process.

Process: heating, insulation, cooling.

The essence of quenching: the transformation of supercooled austenite to martensite or bainite to obtain martensite or bainite structure.

The purpose of quenching: (1) greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of steel, so as to meet the different requirements of various mechanical parts and tools; (2) Meet the ferromagnetic, corrosion resistance and other special physical and chemical properties of some special steels by[TJC STEEL] quenching.

Application: Quenching process is the most widely used, such as tools, measuring tools, molds, bearings, springs and automobiles, tractors, diesel engines, cutting machine tools, pneumatic tools, drilling machinery, agricultural machinery, petroleum machinery, chemical machinery, textile machinery, aircraft and other parts are using quenching process.

 

Quenching Medium

The medium used for workpiece quenching cooling is called quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress.

The commonly used quenching media are water, water solution, mineral oil, molten salt, molten alkali, etc.

 

● Water

Water is a quenching medium with strong cooling capacity.

Advantages: wide source, low price, stable composition is not easy to deteriorate.

Disadvantages: unstable cooling ability, easy to make the workpiece deformation or cracking. In the "nose" area of C curve (about 500 ~ 600℃), the water is in the steam film stage, cooling is not fast enough, will form a "soft spot"; And in the martensitic transition temperature zone (300 ~ 100℃), water is in the boiling stage, cooling too fast, easy to make the martensitic transition speed is too fast and produce great internal stress, resulting in workpiece deformation and even cracking. When the water temperature rises, [TJC STEEL]the water contains more gas or the water is mixed with insoluble impurities (such as oil, soap, mud, etc.), will significantly reduce its cooling capacity.

Application: It is suitable for quenching and cooling of carbon steel workpiece with small section size and simple shape.

 

 

● Salt Water and Lye Water

Add the appropriate amount of salt and alkali in the water, so that high temperature workpiece immersed in the cooling medium, in the steam film stage precipitation of salt and alkali crystals and immediately burst, the steam film is[TJC STEEL] destroyed, the workpiece surface of the oxide skin is also broken, so as to improve the cooling capacity of the medium in the high temperature zone, its disadvantage is the corrosion of the medium.

Application: Under normal circumstances, the concentration of salt water is 10%, caustic soda solution concentration is 10% ~ 15%. Can be used as the quenching medium of carbon steel and low alloy structural steel workpiece, the use temperature should not exceed 60℃, after quenching should be cleaned in time and rust prevention treatment.

 

● Oil

The cooling medium is usually mineral oil (mineral oil). Such as oil, transformer oil and diesel oil. Oil generally uses No. 10, No. 20, No. 30 oil, the larger the oil, the greater the viscosity, the higher the flash point, the lower the cooling capacity, [TJC STEEL]the use of temperature increases accordingly.

 

Quenching Mode

● Single liquid quenching

It is a quenching operation method in which austenitic chemical components are immersed in a quenching medium and cooled to room temperature. The single liquid quenching medium includes water, salt water, alkali water, oil and specially prepared quenching agent.

Advantages: Simple operation, conducive to mechanization and automation.

Disadvantages: The cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality.

Application: Single liquid quenching for carbon steel is only suitable for simple shape of the workpiece.

 

● Double Liquid Quenching

Is the austenitic chemical parts first immersed in a kind of cooling ability of the medium, in the steel parts have not reached the quenching [TJC STEEL]medium temperature is removed, immediately immersed in another kind of cooling ability of the medium, such as first water after oil, first water after air, etc. Double liquid quenching reduces deformation and cracking tendency, it is difficult to master the operation, and has certain limitations in application.

 

● Martensite grading quenching

Is the austenitic chemical parts first immersed in the temperature slightly higher or slightly lower than the steel martensitic point of the liquid medium (salt bath or alkali bath), maintain the appropriate time, until the steel parts inside and outside reach the medium temperature after the air cooling, in order to obtain the quenching [TJC STEEL]process of martensitic structure, also known as fractional quenching.

Advantages: Fractional quenching can effectively reduce the phase change stress and thermal stress, and reduce the tendency of quenching deformation and cracking because of the air cooling after the fractional temperature stays to the same temperature inside and outside the workpiece.

Application: It is suitable for alloy steel and high [TJC STEEL]alloy steel workpiece with high deformation requirement. It can also be used for carbon steel workpiece with small section size and complex shape.

 

● Bainite Isothermal Quenching

It is the quenching process of austenitizing steel parts, making it fast cold to the bainite transition temperature range (260 ~ 400℃) and isothermal maintenance, so that austenite into bainite, sometimes also called isothermal quenching, the general holding time is 30 ~ 60min.

 

● Compound hardening

The workpiece was quenched below Ms to obtain 10%-20% martensite, and then isothermal in the lower bainite temperature zone. With this cooling method, M+B structure can be obtained in workpiece with large cross-section. The martensite formed during pre-quenching can promote bainite transformation, and temper martensite[TJC STEEL] at isotherm. Composite quenching for alloy tool steel workpiece can avoid the first type of tempering brittleness and reduce the residual Austenitic volume, namely the deformation and cracking tendency.

 

Tempering

Tempering Process

Tempering is a heat treatment process in which the quenched workpiece is reheated to an appropriate temperature below the lower critical temperature and then cooled to room temperature in air, water, oil and other media after holding it for a period of time.

The purpose of tempering is: (1) to eliminate the residual stress generated during quenching and prevent deformation and cracking; (2) Adjust the hardness, strength, plasticity and toughness of the workpiece to meet the performance requirements; (3) Stable structure and size to ensure accuracy; (4) Improve and enhance the processing performance.

 

Tempering Classification

● Low Temperature Tempering

Refers to the workpiece at 150~250℃ tempering.

Objective: To maintain high hardness and wear [TJC STEEL]resistance of quenched workpiece and reduce quenching residual stress and brittleness.

Tempered martensite is obtained after tempering, which refers to the microstructure obtained by tempering quenched martensite at low temperature.

Application: cutting tools, measuring tools, die, rolling bearing, carburizing and surface hardening parts, etc.

 

 

● Moderate heat

Refers to the workpiece between 350 ~ 500℃ tempering.

Objective: To obtain high elasticity and yield point, appropriate toughness. Tempered tretinite refers to a complex structure of extremely[TJC STEEL] fine spherical carbide (or cementite) distributed in the ferrite matrix formed during the tempering of martensite.

Application: spring, forging die, impact tool, etc.

 

● Tempering at high temperature

It refers to the tempering of the workpiece at more than 500℃.

Objective: To obtain comprehensive mechanical properties with good strength, plasticity and toughness.

Tempered soetensite refers to the complex phase structure of ferrite matrix formed during martensite tempering with fine spherical carbides (including cementite) distributed in the matrix.

Application: Widely used in a variety of[TJC STEEL] important mechanical structural parts, such as connecting rod, bolt, gear and shaft parts.

 

 

Normalizing

Normalizing Process

Normalizing is a metal heat treatment process in which steel parts are heated to 30-50℃ above the critical temperature (the temperature of complete austenitization) and taken out of the furnace in the air or cooled by water spray, spray or blow after holding for an appropriate time.

Objective: (1) To refine grain size and homogenize [TJC STEEL]carbide distribution; (2) Remove the internal stress of the material; (3) Increase the hardness of the material.

 

Material: Subeutectic Tinplate

Status: Normal

Structure: Graphite Brown, Martensite Light Yellow, Pearlite Green and Dark Yellow, Cementite Brown

 

Material: A-299

Status: Normal

Tissue: Brown ferrite yellow, blue, white pearlite brown

 

Material: 13MnNiMoNb

Status: Normal

Tissue: light brown pearlite grayish brown, ferrite red, yellow, blue

 

Material: 45# Steel

Status: normal

Structure: light blue ferrite, pearlite variety of colors

 

 

Advantages: (1) normalizing cooling rate is slightly faster than annealing cooling rate, so the obtained pearlite laminate spacing is smaller, normalizing structure is finer than annealing structure, so its hardness and strength is higher; (2)[TJC STEEL] The cooling outside the normal furnace does not occupy the equipment, and the productivity is higher.

Application: Only applicable to carbon steel and low and medium alloy steel, but not applicable to high alloy steel. Because the austenite of high alloy steel is very stable, cooling in air will also result in martensitic structure.

 

 

Specific Use

(1) For low carbon steel and low alloy steel, normalizing can increase its hardness to improve the machinability;

(2) For medium carbon steel, normalizing can replace tempering treatment, prepare the microstructure for high-frequency quenching, and reduce the deformation of steel parts and reduce the processing cost;

(3) For high carbon steel, normalizing can eliminate the mesh cementite structure and facilitate spheroidizing annealing;

(4) For large steel forgings or steel castings with sharp changes in section, normalizing can be used instead of quenching, in order to reduce the deformation and cracking tendency, or prepare the organization for quenching;

(5) For the quenching repair parts of steel, the [TJC STEEL]influence of overheating can be eliminated by normalizing, so as to be re-quenched;

(6) Used for cast iron to increase the pearlescent volume of matrix and improve the strength and wear resistance of castings.

 

Annealing

Annealing Process

Annealing is the process by which a metal or alloy is heated to a suitable temperature, held for a certain period of time, and then cooled slowly (usually with the furnace).

 

Purpose of Annealing:

(1) reduce the hardness of steel, improve plasticity, easy machining and cold deformation processing;

(2) To uniform the chemical composition and microstructure of steel, refine the grain, improve the performance of steel or prepare the microstructure for quenching;

(3) Eliminate internal stress and work hardening to [TJC STEEL]prevent deformation and cracking.

 

 

Annealing Method

1. Complete Annealing

Objective: To refine grain, homogenize microstructure, eliminate internal stress, reduce hardness and improve machinability of steel. The microstructure of fully annealed hypoeutectoid steel is F+P.

Application: Full annealing is mainly used for hypoeutectoid steel (wc=0.3~0.6%), generally medium carbon steel and low and medium carbon alloy steel castings, forgings and hot rolled profiles, and sometimes for their welding parts.

 

2. Incomplete Annealing

Process: The steel is heated to Ac1~Ac3(hypoeutectoid steel) or Ac1~Accm(hypoeutectoid steel) after insulation slowly cooling to obtain the heat [TJC STEEL]treatment process close to the equilibrium structure.

Application: Mainly used in eutectoid steel to obtain spherical pearlite structure, in order to eliminate internal stress, reduce hardness, improve machinability.

 

3, isothermal Annealing

Process: Heat treatment process in which steel is heated to a temperature higher than Ac3(or Ac1), and cooled to a certain temperature in the pearlite region quickly after holding for an appropriate time, and isothermal maintenance, so that austenite is transformed into pearlite, and then air cooled to room temperature.

Objective: Same as full annealing, the transition is easier to control.

Application: It is suitable for stable steel: high carbon steel (wc> 0.6%), alloy tool steel, high alloy steel (total alloying element > 10%). Isothermal annealing is also beneficial to obtain uniform microstructure and properties. But it is not suitable for large section steel and large quantities of charge, because it is not easy to make the workpiece internal or batch workpiece isothermal temperature.

 

4. Spheroidizing Annealing

Process: A heat treatment process to spheroidize carbide in steel and obtain granular pearlite. Heating to Ac1 above 20~30℃ temperature, holding[TJC STEEL] time should not be too long, generally 2~4h is appropriate, cooling method is usually used furnace cooling, or Ar1 below 20℃ isothermal for a long time.

Objective: To reduce hardness, uniform microstructure and improve machinability to prepare microstructure for quenching.

Application: Mainly used in eutectoid steel and eutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. Spheroidization annealing to obtain spherical pearlite, in the spherical pearlite, cementite is spherical fine particles, dispersed on the ferritic matrix. Spherical pearlite compared with sheet pearlite, not only low hardness, easy to cut, and in quenching heating, austenite grain is not easy to coarse, cooling deformation and cracking tendency is small.

 

5. Diffusion Annealing (homogenizing annealing)

Process: Heat treatment process in which steel ingot, castings or forging billets are heated to a temperature slightly below the solid-phase line to hold them for a long time and then cooled slowly to eliminate chemical composition inhomogeneity.

Objective: To eliminate dendrite segregation and regional segregation during solidification of ingot and homogenize composition and structure.

Application: Used in some high quality alloy steel and alloy steel castings and ingot with severe segregation. The heating temperature of diffusion [TJC STEEL]annealing is very high, usually 100~200℃ above Ac3 or Accm. The specific temperature depends on the degree of segregation and steel type, and the holding time is generally 10~15 hours. After diffusion annealing, complete annealing and normalizing treatment are needed to refine the structure.

 

6, Stress Removal Annealing

Process: the steel parts are heated to a temperature lower than Ac1 (generally 500~650℃), insulation, and then cooled with the furnace.

The stress removal annealing temperature is lower than A1, so no microstructure change is caused by stress removal annealing.

Objective: To eliminate residual internal stress.

Application: Mainly used to eliminate the residual stress of castings, forgings, welding parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, they will cause deformation or cracks in the steel parts after a certain time, or in the subsequent cutting process.

 

7. Recrystallization Annealing

Recrystallization annealing, also known as intermediate annealing, is a heat treatment process to heat the metal after cold deformation to the recrystallization temperature above the appropriate time, so that the deformed grains are transformed into uniform equiaxed grains and eliminate work hardening and residual stress.

The recrystallization phenomenon must first have a certain amount of cold plastic deformation, and then must be heated to a certain [TJC STEEL]temperature. The lowest temperature at which recrystallization occurs is called the lowest recrystallization temperature. The minimum recrystallization temperature of general metal materials is: T =0.4T.

The heating temperature of recrystallization annealing should be 100~200℃ higher than the minimum recrystallization temperature (the minimum recrystallization temperature of steel is about 450℃), and the temperature should be cooled slowly after appropriate insulation.