What Is Chemical Heat Treatment? (Definition, Classification and Characteristics)

Definition of chemical heat treatment

Chemical heat treatment is a heat treatment process that places metal or alloy workpieces in an appropriate active medium for heating and insulation, so that one or several elements can penetrate into its surface layer to change its chemical composition, structure and performance.

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Chemical heat treatment is also called “surface diffusion” or “thermal diffusion” treatment.

In fact, chemical heat treatment is to place metal materials or parts in a solid, liquid or gas medium containing one or more chemical elements, heat them to a certain temperature in the furnace, and make these elements enter the metal surface through the decomposition, adsorption, solid solution and combination reaction of medium pyrolysates on the metal surface, and gradually penetrate into the metal material through thermal diffusion, forming a penetration layer rich in one or more alloy elements on the metal surface.

Chemical heat treatment technology accounts for a high proportion in modern industry.

The reason why it is widely used is that it can greatly improve the performance requirements of “hard surface and tough internal surface” of workpieces, such as requiring high strength, high hardness, high wear resistance and other mechanical properties, anti seizure performance, anti fatigue performance, special corrosion resistance, high temperature oxidation resistance and other physical and chemical properties of the workpiece surface.

At the same time, the workpiece itself should also maintain its original good plastic and ductile properties, so as to improve the durability of machine parts under various complex working conditions.

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The main characteristics of chemical heat treatment are:

Solid diffusion infiltration not only changes the chemical composition of the surface layer of the workpiece, but also changes its structure.

There is a diffusion layer between the infiltration layer and the matrix, which can obtain properties that are difficult to obtain with a single material or further improve the working performance of the workpiece.

The driving force of chemical heat treatment is concentration gradient.

The structure of the infiltration layer formed by chemical heat treatment conforms to the phase diagram.

Its structure is continuous and belongs to metallurgical combination.

Classification of chemical heat treatment

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1. Classification according to the type of infiltrated elements

It can be divided into carburizing, nitriding (nitriding), boronizing, aluminizing, sulfurizing, carbonitriding, carbon chromium composite carburizing, etc.

2. Classification according to the type and order of the elements infiltrated

1. Single element infiltration

Such as carburizing (unit carburizing), boronizing (unit boronizing), etc.

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2. Binary penetration.

Two element penetration is called dual penetration.

The simultaneous infiltration of carbon and nitrogen is called carbonitriding (carbonitriding for short), and the simultaneous infiltration of boron and aluminum is called boron aluminizing (boron aluminizing for short).

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3. Multi element penetration.

The simultaneous infiltration of more than two elements is called multi-element penetration.

In the same time, the infiltration of carbon, nitrogen, and boron is called carbonitriding.

4. Binary composite infiltration.

Two element composite infiltration.

For example, tungsten and carbon are infiltrated successively, which is called tungsten carbon binary composite infiltration.

5. Multi element composite infiltration.

Multi element composite infiltration refers to the infiltration of more than two elements successively.

Such as nitrogen, carbon and sulfur ternary composite infiltration.

3. Classification according to the state of active medium penetrating elements

1. Solid method

Including powder filling method, paste (slurry) method, electric 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, mobile ion furnace method, etc.

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4. Ion bombardment method

It includes ion bombardment carburizing, ion bombardment nitriding, ion bombardment metallizing, etc.

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4. Classification according to the change characteristics of surface chemical composition

Diffusion infiltration can be divided into four categories:

  • Infiltration of various non-metallic elements;
  • Infiltration of various metal elements;
  • Simultaneous infiltration of metal nonmetal elements;
  • Diffusion to eliminate impurity elements, etc.

5. Classification according to the phase structure formed by the infiltrating elements and the elements in the steel

The first is that the infiltrating elements dissolve in the lattice of solvent elements to form solid solutions. 

Such as carburizing, carbonitriding, etc.

The second is reaction diffusion.

There are two types:

The first is that the infiltrating elements react with the elements in the steel to form ordered phases (metal compounds), such as nitriding (commonly known as nitriding);

The second is that the solubility of the infiltrating elements in the lattice of solvent elements is very small, and the infiltrating elements react with the elements in steel to form compound phases, such as boronizing.

6. Classify according to the effect/purpose of infiltration elements on surface properties of steel parts

1.Improve the hardness, strength, fatigue strength and wear resistance of the workpiece surface.

Such as carburizing, nitriding, carbonitriding, etc.

2. Improve the hardness and wear resistance of the workpiece surface. Such as boronizing, vanadizing, niobium, etc.

3. Reduce the friction coefficient and improve the anti seizure and scratch resistance. Such as sulfurizing, oxynitriding, and sulfur nitrogen penetration treatment.

4. Improve corrosion resistance. Such as siliconizing, chromizing, nitriding, etc.

5. Improve high-temperature oxidation resistance. Such as aluminizing, chromizing, siliconizing, etc.

7. Classification according to the structure state of steel during chemical heat treatment

Table 1 Classification table formed according to the structure state of steel

Austenitic state chemical heat treatmentChemical heat treatment in ferrite state
Carburization  Nitriding
CarbonitridingNitrocarbonization
Boronizing, boroaluminizing, borosiliconizing, borozirconizing, boron carbon composite carburizing, boron carbon ammonia composite carburizing, etcOxynitriding, Oxynitrocarburizing
Chromizing, chromizing aluminizing, chromizing siliconizing, chromizing nitriding, chromizing titanizingSulphurization
Aluminizing, aluminum nickel penetration, aluminum rare earth penetration, etcOxynitriding, Oxynitrocarburizing
SiliconizingZincizing
Vanadium, niobium, titanium, etc 

As shown in Table 1, since the chemical heat treatment temperature of steel in ferrite state is generally lower than 600 ℃, the chemical heat treatment in ferrite state is also called low-temperature chemical heat treatment;

The chemical heat treatment temperature of steel in the austenitic state is generally higher than 600 ℃, which is called high temperature chemical heat treatment.

Low temperature chemical heat treatment process has the advantages of low treatment temperature, energy saving, small distortion of workpiece, good corrosion resistance and anti seizure, high hardness, good wear resistance and antifriction performance.

At the same time, it can be seen from Table 1 that the chemical heat treatment of steel is usually named after the infiltration of different elements, such as carburizing, nitriding, carbonitriding, etc.

Characteristics of chemical heat treatment

Compared with surface hardening methods such as surface quenching and surface deformation strengthening, it has the following characteristics.

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1. By infiltrating different elements, the chemical composition and structure of the workpiece surface can be effectively changed to obtain various surface properties, so as to meet the performance requirements of the workpiece under different working conditions.

2. The depth of the carburizing layer of general chemical heat treatment can be adjusted according to the technical requirements of the workpiece, and the composition, structure and performance of the carburizing layer change gradually from the surface to the inside.

The carburizing layer and the matrix belong to metallurgical bonding, and the bonding is firm, and the surface layer is not easy to peel off.

3. Generally, chemical heat treatment is not limited by the geometric shape of the workpiece.

No matter how the shape is copied, the shell and inner cavity can obtain the required penetration layer or local penetration layer.

Unlike surface hardening, rolling, cold pressing, cold rolling and other cold work hardening treatments, it is limited by the shape of the workpiece.

4. The vast majority of chemical heat treatment has the characteristics of small deformation of workpiece, high precision, good dimensional stability, etc.

Such as nitriding, soft nitriding, ion nitriding and other processes, all enable the workpiece to maintain high accuracy, low surface roughness and good dimensional stability.

5. All chemical heat treatments can achieve the comprehensive effect of improving the surface properties of the workpiece.

Most chemical heat treatments can improve the corrosion resistance, oxidation resistance, friction reduction, anti seizure, corrosion resistance and other properties of the workpiece surface layer while improving the surface mechanical properties.

6. General chemical heat treatment has a more significant effect on improving the quality of mechanical products, tapping the potential of materials, and extending the service life.

Therefore, it can save more valuable metal materials, reduce costs, and improve economic benefits.

7. Most chemical heat treatment is not only a complex physical and chemical process, but also a complex metallurgical process.

It needs to be heated in a certain active medium through physical and chemical reactions on the interface and metallurgical diffusion from the outside to the inside.

Therefore, the process is complex, the treatment cycle is long, and the requirements for equipment are high.

Conclusion

This article mainly shares the definition, classification and characteristics of chemical heat treatment.

I believe that through this basic information introduction, we have a further understanding of chemical heat treatment.

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