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What is chemical heat treatment?

What is chemical heat treatment?

Chemical heat treatment is the use of chemical reactions, and sometimes physical methods to change the chemical composition and structure of the surface layer of steel parts, in order to get better technical and economic benefits than the homogeneous material metal heat treatment process. As most of the mechanical parts failure and damage are sprouted in the surface layer, especially in the parts that may cause wear, fatigue, metal corrosion, oxidation and other conditions of work, the performance of the surface layer is particularly important. After chemical heat treatment of steel parts, in essence, can be considered a special composite material. The heart is steel of original composition, and the surface layer is a material infiltrated with alloying elements. The bond between the heart and the surface layer is a tight crystalline bond, which is much stronger than the bond between the heart and the surface obtained by surface reconditioning techniques such as electroplating.

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The workpiece is placed in a medium containing active elements and heated and held so that the active atoms in the medium penetrate into the surface layer of the workpiece or form a cover layer of some compound to change the organization and chemical composition of the surface layer, thus giving the surface of the part special mechanical or physicochemical properties. Usually other suitable heat treatments are required before and after the chemical infiltration in order to maximize the potential of the infiltration layer and to achieve the best fit between the heart of the workpiece and the surface layer in terms of organization, structure and properties.

Purpose of chemical heat treatment

  • ① Improve the wear resistance of parts using steel carburizing quenching method to obtain high-carbon martensite hardening surface; alloy steel parts with nitriding method to obtain the alloy nitride dispersion hardening surface. The surface hardness of steel parts obtained by these two methods can reach HRC58-62 and HV800-1200, respectively. another way is to form wear reduction and anti-bonding film on the surface of steel parts to improve friction conditions, which can also improve wear resistance. For example, steam treatment of the surface to produce ferric tetroxide film has the effect of anti-bonding; surface sulfide to obtain ferrous sulfide film, both wear reduction and anti-bonding effect. In recent years, the development of multiple co-infiltration process, such as oxygen-nitrogen infiltration, sulfur-nitrogen co-infiltration, carbon, nitrogen, sulfur, oxygen and boron five elements co-infiltration, etc., can simultaneously form a high hardness diffusion layer and anti-adhesive or anti-wear film, effectively improve the wear resistance of the parts, especially the resistance to bonded wear.
  • ② Improve the fatigue strength of the parts carburizing, nitriding, nitrocarburizing and carbonitriding methods, can make steel parts in the surface strengthening at the same time, the formation of residual compressive stress on the surface of the parts, effectively improve the fatigue strength of the parts.
  • ③ Improve the corrosion resistance of parts and high-temperature oxidation resistance For example, nitriding can improve the parts of atmospheric corrosion resistance; steel parts after infiltration of aluminum, chromium, silicon infiltration, and oxygen or corrosive media to form a dense, stable Al2O3, Cr2O3, SiO2 protective film, improve corrosion resistance and high-temperature oxidation resistance.

Usually, the hardening of steel parts will bring embrittlement at the same time. Surface hardening method to improve the surface hardness, but still maintain the heart in a better toughness state, so it is better than the parts of the overall quenching and hardening method to resolve the contradiction between steel hardening and its toughness. Chemical heat treatment so that the chemical composition of the surface layer of the steel parts and organization change at the same time, so it is better than high, medium frequency electric induction, flame quenching and other surface quenching and hardening methods. If the infiltration element selection is appropriate, you can get a variety of performance requirements to adapt to the parts of the surface layer.

Characteristics of chemical heat treatment

Compared with surface hardening methods such as surface quenching and surface deformation strengthening, it has the following characteristics.
By infiltrating different elements, the chemical composition and organization of the workpiece surface can be effectively changed to obtain various surface properties, thus meeting the performance requirements of the workpiece under different working conditions.
Generally, the penetration depth of chemical heat treatment can be adjusted according to the technical requirements of the workpiece, and the composition, organization and performance of the penetration layer are gradually changed from the surface to the inside, and the penetration layer is metallurgically bonded with the substrate, which is firmly bonded and the surface layer is not easy to peel off.
Usually chemical heat treatment is not limited by the geometry of the workpiece, no matter how the shape of the replica can make the shell and cavity to obtain the required seepage layer or local seepage layer, unlike surface quenching, rolling, cold pressing, cold rolling and other cold hardening treatment, to be limited by the shape of the workpiece.
Most of the chemical heat treatment has the characteristics of small deformation, high precision and good dimensional stability of the workpiece. Such as nitriding, nitrocarburizing, ion nitriding and other processes, all make the workpiece maintain high precision, low surface roughness and good dimensional stability.
All chemical heat treatments can obtain the comprehensive effect of improving the surface properties of the workpiece. Most chemical heat treatments can improve the surface mechanical properties while also improving the corrosion resistance, oxidation resistance, friction reduction, anti-seize, corrosion resistance and many other properties of the surface layer of the workpiece.
General chemical heat treatment to improve the quality of mechanical products, to exploit the potential of the material, to extend the service life with more significant results, and therefore can save more expensive metal materials, reduce costs and improve economic efficiency.
Most chemical heat treatment is both a complex physicochemical process and a complex metallurgical process, which requires heating in a certain active medium and is accomplished through physicochemical reactions at the interface and metallurgical diffusion from the surface to the inside. As a result, their processes are more complex, with long treatment cycles and higher requirements for equipment.

Classification of chemical heat treatment

Chemical heat treatment methods are numerous, mostly to infiltrate elements or the formation of compounds to name, such as carburizing, nitriding, boron, sulfur, aluminum, chromium, silicon, carbonitriding, oxynitriding, sulfur cyanide and carbon, nitrogen, sulfur, oxygen, boron five elements, and carbon (nitride) titanium cover, etc.. The utility of various chemical heat treatment and applicable steel types are shown in the table.
Chemical heat treatment should be based on the performance requirements of the parts and the ease of process and economic indicators, the reasonable choice of process type. For example, carburizing and nitriding can improve the wear resistance of the parts; but carburizing is carried out at high temperature (900-1000 ℃), in a not too long time (6-10 hours) can obtain considerable carburizing layer, so the general requirements of the deeper hardening layer (0.9-2.5 mm) of wear-resistant parts more carburizing treatment, both to meet the performance requirements, but also more economical. When the size of the part deformation requirements are very strict, the use of low temperature (500-600 ℃) nitriding treatment, can ensure the dimensional accuracy of the parts; but the nitriding layer thickening slowly, nitriding time often takes dozens or even dozens of hours, is not an economic method.
Classification according to the type of infiltrating elements:
Can be divided into carburizing, nitriding (nitriding), boron, aluminum, sulfur, carbon and nitrogen infiltration, carbon and chromium compound infiltration, etc..
Classification according to the type and sequence of infiltrating elements:
1. Unit infiltration, infiltration of a single element
Such as carburizing (unit carburizing), boron penetration (unit boron penetration), etc.

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2. Binary co-infiltration. Simultaneous infiltration of two elements is called binary co-infiltration
Such as simultaneous infiltration of carbon, nitrogen two elements that is called carbon and nitrogen binary coextrusion (referred to as carbon and nitrogen coextrusion), simultaneous infiltration of boron, aluminum two elements that is called boron aluminum binary coextrusion (referred to as boron aluminum coextrusion), etc..

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3. Multiple coextrusion. Simultaneous infiltration of more than two elements is called multiple coextrusion
Such as the simultaneous infiltration of carbon, nitrogen, boron three elements that is called carbon, nitrogen and boron ternary coextrusion.
4. Binary compound infiltration. The infiltration of two elements is called binary compound infiltration
Such as successive infiltration of tungsten and carbon two elements that is called tungsten-carbon binary compound infiltration, etc.
5. Multi-composite infiltration. The infiltration of more than two elements is called multiple compound infiltration
Such as nitrogen, carbon and sulfur composite infiltration.
Classification according to the state of the active medium of the infiltrated elements:
1. Solid method
Including powder filling method, paste (slurry) method, electrothermal cyclone method, etc.

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2. Liquid method
Including salt bath method, electrolytic salt bath method, aqueous solution electrolysis method, etc.

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3. Gas method
Including vacuum method, solid gas method, indirect gas method, flow ion furnace method, etc.

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4. Ion bombardment method
Including ion bombardment carburization, ion bombardment nitriding, ion bombardment metal penetration, etc.

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Classified according to the characteristics of the change of the chemical composition of the surface:

  • Diffusion infiltration can be further divided into four categories.
  • Infiltration into various non-metallic elements.
  • Infiltration into various metallic elements.
  • Simultaneous infiltration of metal-nonmetal elements.
  • Diffusion elimination of impurity elements, etc.

According to the phase structure formed by the infiltrated elements and the elements in the steel:
1. The first category is the infiltrating element dissolved in the lattice of the solvent element to form a solid solution.
Such as carburizing, carbonitriding, etc.
2. The second category is reaction diffusion
This category can be divided into two types: the first is the infiltrating element and the steel element reaction to form an ordered phase (metal compounds), such as nitriding (commonly known as nitriding); the second is the infiltrating element in the solvent element lattice solubility is very small, the infiltrating element and the steel element reaction to form a compound phase, such as boron infiltration.
Classification according to the role/purpose of infiltrating elements on the surface properties of steel parts:

  • 1. To improve the hardness, strength, fatigue strength and wear resistance of the surface of the workpiece. Such as carburizing, nitriding, carbonitriding, etc.
  • 2. To improve the hardness and wear resistance of the surface of the workpiece. Such as boron infiltration, vanadium infiltration, niobium infiltration, etc.
  • 3. Reduce the coefficient of friction, improve the resistance to bite, anti-scuffing. Such as sulfurization, oxynitriding, sulfur-nitriding treatment, etc.
  • 4. Improve corrosion resistance. Such as silicon penetration, chromium penetration, nitriding, etc.
  • 5. Improve the resistance to high temperature oxidation. Such as aluminizing, chromium, silicon, etc.

Classification of steel according to the state of the organization when chemical heat treatment:
Table.1 Classification table formed according to the state of steel organization

Austenitic state chemical heat treatment Ferritic state chemical heat treatment
Carburizing Nitriding
Carbonitriding Nitrogen and carbon altogether permeability
Boronizing, boroaluminizing, borosilicate, borozirconium, boron-carbon composite, boron-carbon-nitrogen composite, etc Oxygen-nitriding, oxygen-nitrocarburizing
Chromizing, chromium-aluminum, chromium-silicon, chromium-nitrogen, chromium-titanium Sulfurizing
Aluminizing, aluminizing nickel, aluminizing rare earth, etc Sulfur-nitriding, sulfur-nitrocarburizing
Siliconizing Zinc impregnation
Vanadium infiltration, infiltration niobium, infiltration titanium and so on

As shown in Table 1, because the steel in the ferrite state, chemical heat treatment temperature is generally lower than 600 ℃, so the chemical heat treatment in the ferrite state, also known as low-temperature chemical heat treatment; while the steel in the austenite state, chemical heat treatment temperature is generally higher than 600 ℃, is called high-temperature chemical heat treatment.
Low-temperature chemical heat treatment process has the advantages of low processing temperature, energy saving, small distortion of the workpiece, corrosion resistance and good anti-seize, high hardness, wear resistance, good friction reduction performance.
At the same time, from Table 1 can also see that the chemical heat treatment of steel is usually named after the infiltration of different elements, such as carburizing, nitriding, carbonitriding, etc.

Table.2 main application characteristics of common chemical surface heat treatment methods

Process method
Thermal effect on matrix
Microstructure and thickness of strengthening layer
Other features
Performance and Application
Carburizing (solid, gas, fluidized bed, vacuum, ion carburizing, etc.) and carbonitriding
The heating temperature is usually 880 – 1050 degrees, and the nitriding temperature is low
Martensite, carburized layer thickness is 1 – 2 mm, carburizing to form a thin layer of carbon nitrogen conjugate, the layer thickness is less than 0.8 mm
The plasma carburizing speed is fast and the surface structure is excellent
The hardness, wear resistance and fatigue strength are improved by increasing the carbon content on the surface. Medium carbon steel with high carbon content can be used for carbonitriding
Nitriding, gas nitriding and gas nitriding
Gas nitriding temperature is generally 500 – 580 degrees, carbonitriding temperature is usually 530 – 570 degrees
The depth of nitriding layer is equal to 0.6 mm, and the depth of nitriding layer is 0.01 – 0.06 mm
The range of plasma carburizing is wide and can be carried out at 400 ℃. The deformation of workpiece is small, but the nitriding speed is low
High hardness, high wear resistance and high fatigue strength. For carbon steel and alloy steel containing Cr, Mo, Al, W, V, Ni, Ti and other elements. The toughness and fatigue strength of the layer increased
Aluminizing and compound permeating
The temperature of Al Si, Al Cr, Al Ti powder infiltration and composite infiltration is usually 1000 ℃
Al Si powder method was used for 8h, the carburizing layer of 20# steel was 0.23mm, and that of 45# steel was 0.18mm; After 10h of Al Cr powder method, 1Cr18Ni9Ti layer was 0.22mm
Compared with aluminizing alone, aluminizing and compound aluminizing can obtain higher thermal stability and corrosion resistance in some corrosive media
Improve thermal stability and corrosion resistance in some corrosive media. For example, carbon steel, low alloy steel, Al Si composite infiltration are used to replace high alloy heat-resistant steel, and low-cost steel is used to replace high-alloy steel by Al Cr infiltration
Chromizing and compound permeating
The temperature of Cr Si is 1000 ℃, that of Cr ti is 1100 ℃, and that of Cr re is 950 ℃
Chromium and other compounds were obtained, and Cr Si was infiltrated for 10 h; The thickness of Cr ti is 0.03 – 0.06mm for 4H; The thickness of Cr re is 0.01-0.015 mm after 4-8h
In the compound of the permeating layer
,Cr7C3 Hardness: 2300hv – 1800; VC hardness 3000 – 3300hv
Improve corrosion resistance (gas corrosion, electrochemical corrosion), wear resistance, oxidation resistance. Adding proper amount of rare earth can increase the chromizing speed and improve the quality of Chromizing Layer
Including scanning and composite permeating
B-Al powder, B-Si powder method 1050 degrees, C-N-B salt bath method 730 degrees
The results show that the coating thickness of 45# steel is 0.36mm after aluminizing for 6h; The thickness of siliconizing layer is 0.24mm
The hardness of 20# carbon steel is 1800 – 2500hv
Improve wear resistance. The oxidation resistance can also be improved by scanning Al, B-Si and composite infiltration. Five element infiltration is mainly used for high speed steel cutting tools, and the service life is increased by 1 – 2 times

The basic process of chemical heat treatment

Chemical heat treatment consists of three basic processes, namely:

  • ① The decomposition process in which the chemical penetrant is decomposed into active atoms or ions
  • ② The absorption process in which the active atoms or ions are absorbed and solidified on the surface of the steel part.
  • ③ Diffusion process in which the atoms of the infiltrated elements continuously diffuse to the inside.

Decomposition process

The chemical infiltrant is a substance containing the infiltrated element. It must be decomposed into active atoms or ions before it can be absorbed and solidified on the surface of the steel. 

2NH3→3H2+2 [N]

According to the thermodynamics of chemical reactions, the free energy of decomposition reaction products must be lower than the free energy of reactants for decomposition reactions to occur. However, it is not enough to meet the thermodynamic conditions alone, and the kinetic conditions, i.e. reaction speed, must be considered for practical application in production; increasing the concentration of reactants and reaction temperature, although both can accelerate the decomposition of percolating agents, are limited by factors such as materials or processes. In actual production, the use of catalysts to reduce the activation energy of the reaction process can make a single reaction process with high activation energy into an intermediate transitional reaction process consisting of several low activation energies, thus accelerating the decomposition reaction. Iron, nickel, cobalt, platinum and other metals are effective catalysts for the decomposition of ammonia or organic hydrocarbons, so the surface of the steel itself is a very good catalyst, and the decomposition rate of the permeate on the surface of the steel can be increased several times than its decomposition rate when it is present alone.

Absorption process

The surface of the workpiece has the ability to adsorb the surrounding gas molecules, ions or active atoms, and this physical or chemical effect of the surface is called solid adsorption effect (see crystal surface. Gas molecules are either adsorbed on the steel surface and accelerated to active atoms due to the catalytic effect of iron, or they are first decomposed to active atoms or ions and then adsorbed on the steel surface. Which of the above two cases is dominant depends on the process. The adsorbed active atoms or ions are dissolved on the surface of the steel part into the crystal puncta of iron to form a solid solution; if the concentration of the infiltrated element exceeds the solid solution of the element in iron, the corresponding intermetallic compound is formed (see alloy phase), and these processes are called absorption processes.

Diffusion processes

The active atoms or ions of the infiltrated element are absorbed and dissolved on the surface of the steel part, which inevitably increases the concentration of the infiltrated element on the surface, forming a concentration gradient between the heart and the surface. Driven by the concentration gradient between the heart and the surface, the infiltrated atoms will diffuse from the surface to the heart. The rate of diffusion of atoms in solid crystals is much lower than the rate of decomposition and absorption of the penetrant, so the diffusion process is often the main controlling factor in chemical heat treatment. This means that the enhanced diffusion process is the main direction of the enhanced chemical heat treatment production process. From the diffusion equation (see diffusion in metals), it is clear that increasing the temperature, increasing the diffusion constant of the infiltrating element in the metal, and reducing its diffusion activation energy factors can accelerate the diffusion process. Since the three processes of chemical heat treatment are interconnected, the decomposition and absorption processes may also become the main controlling factors under certain specific conditions.

Treatment of workpiece distortion

The chemical composition and organization of the surface and heart of the workpiece after chemical heat treatment are different, so they have different specific volume and different austenite isothermal transformation curves, and their heat treatment distortion characteristics and laws are different from the general workpiece. Chemical heat treatment workpiece distortion correction work is more difficult to carry out. Chemical heat treatment can be divided into two categories: a class of carburizing in the high temperature austenite state, the heat treatment process has a phase change, the workpiece distortion is larger; the other class of nitriding in the low temperature ferrite state, the heat treatment process in addition to the infiltration of elements into the formation of new phase of the percolation layer, no phase change, the workpiece distortion is small.
Carburized workpiece distortion: Carburized workpiece is usually made of low carbon steel and low carbon alloy steel, its original organization is ferrite and a small amount of pearlite, according to the service requirements of the workpiece, the workpiece needs to be quenched directly after carburizing, slow cooling reheating quenching or secondary quenching. The distortion of the carburized workpiece occurs in the process of slow cooling and carburizing quenching after carburizing due to tissue stress and thermal stress, and the size of the distortion and the law of distortion depend on the chemical composition of the carburized steel, the depth of the carburizing layer, the geometry and size of the workpiece, and the parameters of the carburizing and post-carburizing heat treatment process. The workpiece can be divided into slender, flat and cubic parts according to the relative dimensions of its length, width and height (thickness). The length of slender parts is much larger than their cross-sectional size, the length and width of flat parts are much larger than their height (thickness), and the size of cubic parts in three directions is not much different. The maximum heat treatment internal stress is generally always generated in the direction of the largest dimension. If the direction is called the dominant stress direction, the workpiece made of low-carbon steel and low-carbon alloy steel, after carburizing and slow cooling or air-cooling the core to form ferrite and pearlite, generally along the dominant stress direction to show shrinkage deformation. When the alloying element content of the steel increases and the cross-sectional size of the workpiece decreases, the distortion rate decreases, and even distension distortion occurs.
Asymmetric shape of slender rod with large difference in cross-sectional thickness, bending distortion is easily produced after carburizing and air cooling. The direction of bending distortion depends on the material. Low-carbon steel carburizing workpiece cooling fast side of the thin section is mostly concave, while 12CrN3A steel, 18CrMnTi steel and other alloy elements of high low-carbon alloy steel carburizing workpiece, cooling fast side of the thin section is often convex. After the workpiece made of low-carbon steel and low-carbon alloy steel is carburized at 920-940 degrees, the mass fraction of carbon in the carburized layer increases to 0.6%-1.0%.
The high-carbon austenite of the carburizing layer in the air cooling or slow cooling to subcooling to the following (600 degrees or so) before the transformation to pearlite, while the heart of the low-carbon austenite in about 900 degrees that began to precipitate ferrite, the remaining austenite subcooling to the Arl temperature below also occurred in the eutectic decomposition, transformation to pearlite. From the carburizing temperature subcooling to Arl below, the carburizing layer of the eutectic composition has not undergone phase transformation, high-carbon austenite only with the reduction of temperature and thermal contraction.
At the same time, the heart of the low-carbon austenite is due to the precipitation of ferrite than the volume of the increase in expansion, the result of the heart by the compressive stress, the carburized layer is subject to tensile stress. When the γ to α transition occurs in the heart, the effect of phase change stress reduces its yield strength, resulting in compressive distortion of the heart. The strength of low-carbon alloy steel is higher, and the amount of compressive plastic distortion in the core is smaller under the same conditions. Asymmetric shape of the carburized workpiece air-cooling, cooling faster than the side of the austenite line length shrinkage than the side of the slow cooling, thus generating bending stress. When the bending stress is greater than the yield strength of the slow-cooling side, the workpiece bends to the fast-cooling side. For low-carbon alloy steel with high alloy element content, after carburizing, the surface layer has the composition of high-carbon alloy steel, and phase transformation occurs on the fast cooling side during air cooling, forming a new phase with higher hardness and larger volume than the organization, while the other side is cooled more slowly to form a new phase with lower hardness, so the opposite bending distortion occurs. The quenching distortion law of the carburized workpiece can be divided and folded in the same way. The quenching temperature of the carburized parts is usually 800-820 degrees, quenching the carburized layer of high carbon austenite from the carburizing temperature cooling to Ms point temperature interval will occur obvious thermal contraction; at the same time the heart of low carbon austenite transformation to ferrite, pearlite, low carbon bainite or low carbon martensite. Regardless of the transformation into what kind of organization, the core is due to the increase in tissue volume than the volume expansion, the result in the carburized layer and the core to produce a large internal stress. Generally speaking, in the case of unhardened, because the core of the phase transformation products for the lower yield strength of ferrite and pearlite, so the core in the carburizing layer of thermal contraction compressive stress, along the direction of the dominant stress shrinkage deformation; when the core of the phase transformation products for the higher strength of low-carbon bainite and low-carbon martensite, the surface layer of high-carbon austenite in the heart of the plastic deformation under the action of expansion stress.

Source: China Forgings Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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