chemical heat treatment

Chemical Heat Treatment Explained: Processes and Applications

Chemical heat treatment is an important process used to alter the physical and mechanical properties of metals and alloys. This treatment involves exposing the material to a chemical environment at high temperatures to induce changes in its microstructure. Generally, chemical environment refers to media containing active elements. The metal or alloy workpiece is heated in media, allowing active elements to penetrate the surface layer of the workpiece to alter its chemical composition, structure, and performance. Proper chemical heat treatment can enhance properties like strength, hardness, toughness, ductility, corrosion resistance, anti-fatigue performance, etc.

What is Chemical Heat Treatment?

Chemical heat treatment, also called “surface diffusion” or “thermal diffusion” treatment, is a process that involves heating metal or alloy workpieces in an active medium, resulting in one or more elements penetrating the surface layer and altering its chemical composition, structure, and performance.

Metal workpieces are placed in a solid, liquid, or gas medium containing one or more chemical elements. Then they are heated to a designated temperature in a furnace, causing the elements to enter the metal surface through decomposition, adsorption, solid solution, and combination reaction of the medium pyrolysates on the metal surface. Through thermal diffusion, the elements gradually penetrate the metal material, resulting in the formation of a penetration layer enriched with one or more alloy elements on the metal surface.

This process creates a hard outer surface and a durable internal structure, enhancing a variety of mechanical properties, such as high hardness, high strength, wear resistance, corrosion resistance, anti-seizure performance, and anti-fatigue performance. It also improves the physical and chemical properties like high-temperature oxidation resistance. Importantly, the original ductile nature of the material is retained to increase the durability of the workpiece under complex conditions. Chemical heat treatment is thus widely utilized across industries to meet specific requirements.

chemical heat treatment

The Chemical Heat Treatment Process Step-By-Step

Chemical heat treatment involves multiple steps that work in conjunction to produce the desired changes in the base metal. Here is an overview of the typical chemical heat treatment process:

Step 1 – Metal Preparation

The starting metal piece first needs to undergo several preparatory processes to ready it for chemical treatment. This includes:

  • Cleaning – Removal of dirt, grease, oxides, and other surface impurities through processes like degreasing, pickling, ultrasonic cleaning, etc. This prevents contamination of treatment baths.
  • Machining – Cutting the metal to the required dimensions and shapes if needed.
  • Stress Relieving – Heat treatment to remove internal stresses from prior cold working, welding, or machining processes. Reduces chances of warping or cracking during subsequent processing.

Step 2 – Metal Loading

The prepared metal piece is then loaded into a holding basket and immersed in a tank containing the heated chemical bath. Loading can be done manually or automatically with conveyors.

Step 3 – Treatment

With the metal immersed in the bath, the actual chemical treatment process begins. Some key aspects of this stage:

  • The bath is heated up to the process temperature, typically 400-1200°F.
  • The metal piece soaks in the hot solution for a set duration, ranging from minutes to hours based on factors like bath chemistry, metal composition, and desired results.
  • Common chemical baths used are molten salts, aqueous solutions, and other inorganic/organic liquids compatible with the base metal at high temperatures.
  • The bath chemistry induces a chemical reaction at the metal surface, modifying its composition and properties.

Step 4 – Quenching

After treatment, the hot metal piece is withdrawn from the bath and rapidly cooled or quenched, using water, oil, or air. Quenching fixes the chemical changes induced by preventing reversion.

Step 5 – Post-Treatment Processes

Additional processes may be used after quenching to finalize properties, such as:

  • Tempering – Low temperature heat treatment for reducing brittleness and improving ductility/toughness.
  • Straightening – Correction of warpage from uneven heating/cooling.
  • Quality Testing – Checks for proper treatment, dimensions, defects, etc.

Types of Chemical Heat Treatments

1. Classification Based on the Type of Infiltrated Elements

This classification has several methods, including carburizing, nitriding, boronizing, sulfurizing, aluminizing, carbonitriding, carbon-chromium composite carburizing, and so on.

2. Classification Based on the Type and Order of Infiltrated Elements

a) Single Element Infiltration

Examples of single element infiltration include carburizing (also known as unit carburizing) and boronizing (also known as unit boronizing).

b) Dual Element Infiltration

The simultaneous infiltration of two elements is known as dual Element infiltration. When carbon and nitrogen are infiltrated together, it is called carbonitriding. Similarly, the simultaneous infiltration of boron and aluminum is referred to as boron aluminizing.

c) Multi Element Infiltration

Multi-element infiltration involves the simultaneous infiltration of more than two elements. 

d) Dual Element Composite Infiltration

Infiltrating a material with two elements is known as dual element composite infiltration. For example, if tungsten and carbon are infiltrated sequentially, the process is called tungsten-carbon dual composite infiltration.

e) Multi Element Composite Infiltration

Multi-element composite infiltration refers to the sequential infiltration of more than two elements. For instance, the ternary composite infiltration of nitrogen, carbon, and sulfur is a type of multi-element composite infiltration.

3. Classification Based on the Active Medium State of the Infiltrated Elements

a) Liquid Method

Liquid methods include the salt bath method, electrolytic salt bath method, aqueous solution electrolysis method, etc.

b) Gas Method

Gas methods include the vacuum method, solid gas method, indirect gas method, mobile ion furnace method, etc.

c) Solid Method

Solid methods include powder filling method, paste (slurry) method, electric cyclone method, etc.

d) Ion Bombardment Method

The ion bombardment method includes ion bombardment carburizing, ion bombardment nitriding, ion bombardment metalizing, etc.

4. Classification Based on the Change Characteristics of Surface Chemical Composition

Diffusion infiltration can be further divided into four categories:

  • Infiltration of various metal elements.
  • Infiltration of various non-metallic elements.
  • Simultaneous infiltration of both metal and non-metal elements.
  • Diffusion elimination of impurity elements, etc.

5. Classification Based on the Phase Structure Formed by the Infiltrated Elements and the Elements in the Steel

a) The first type is that the infiltrating element is dissolved in the lattice of the solvent element to form a solid solution. This type includes carburizing, carbonitriding, etc.

b) The second type is reaction diffusion. This type can be divided into two subtypes: the first is that the infiltrated element reacts with the steel element to form an ordered phase (metal compound). This subtype includes nitriding. The second is that the solubility of the infiltrated element in the solvent element lattice is very low. Hence, the infiltrated element reacts with the steel element to form a compound phase, such as boronizing.

6. Classification Based on the Role/Purpose of Infiltrated Elements on the Surface Properties of Steel Parts

a) For increasing the hardness, strength, fatigue strength, and wear resistance of the workpiece surface, consider carburizing, nitriding, carbonitriding, and other methods.

b) To improve the hardness and wear resistance of the workpiece surface, consider techniques like boronizing, vanadizing, niobium, etc.

c) Lower the friction coefficient and improve the anti-seizure and scratch resistance properties of the workpiece surface through methods such as sulfurizing, oxynitriding, and sulfur-nitrogen penetration treatment.

d) The corrosion resistance of the workpiece surface is enhanced through techniques like siliconizing, chromizing, nitriding, and others.

e) Improving high-temperature oxidation resistance of the workpiece surface can be achieved by techniques like aluminizing, chromizing, siliconizing, etc.

7. Classification Based on the Structure State of Steel During Chemical Heat Treatment

a) Austenitic Chemical Heat Treatment:

  • Carburizing
  • Carbonitriding
  • Boronizing, boroaluminizing, borosiliconizing, borozirconizing, boron carbon composite carburizing, boron carbon ammonia composite carburizing, etc.
  • Chromizing, chromizing aluminizing, chromizing siliconizing, chromizing nitriding, chromizing titanizing.
  • Aluminizing, aluminum nickel penetration, aluminum rare earth penetration, etc.
  • Siliconizing
  • Vanadium, niobium, titanium, etc.

b) Ferrite Chemical Heat Treatment: 

  • Nitriding
  • Nitrocarburizing
  • Oxynitriding, Oxynitrocarburizing
  • Sulphurization
  • Oxynitriding, Oxynitrocarburizing
  • Zincizing

Since steel is in the ferrite state, the chemical heat treatment temperature is generally lower than 600°C. Therefore, chemical heat treatment in the ferrite state is also called low-temperature chemical heat treatment. However, for steel in the austenitic state, the chemical heat treatment temperature is generally higher than 600°C, which is called high-temperature chemical heat treatment.

Generally speaking, the low-temperature chemical heat treatment process has the advantages of low processing temperature, energy saving, small deformation of the workpiece, good corrosion resistance and anti-seize, high hardness, wear resistance, and good friction reduction performance.

chemical heat treatment part

Advantages and Characteristics of Chemical Heat Treatments

In comparison to surface hardening techniques like surface quenching and surface deformation strengthening, it demonstrates the following benefits and characteristics.

Not Limited by Geometry

Generally, chemical heat treatment is not limited by the geometry of the workpiece. Regardless of the shape, both the shell and inner cavity can achieve the required penetration layer or local penetration layer. Unlike surface quenching, rolling, cold pressing, and cold rolling, they are constrained by the shape of the workpiece.

Achieve Diverse Surface Properties

Diverse surface properties can be achieved by effectively changing the chemical composition and structure of the workpiece surface through the introduction of various elements. This allows the workpiece to meet the performance requirements under different operating conditions.

Can Adjust Depth of the Carburizing Layer 

In typical chemical heat treatment, the depth of the carburizing layer can be adjusted according to the technical requirements of the workpiece. The composition, structure, and performance of the carburizing layer exhibit gradual variations from the surface to the interior. The carburizing layer forms a metallurgical bond with the matrix, ensuring a firm bond and preventing any peeling of the surface layer.

Enhance the Surface Properties 

Chemical heat treatment can significantly enhance the surface properties of the workpiece. The majority of chemical heat treatments can effectively improve the surface layer of the workpiece, enhancing properties such as corrosion resistance, oxidation resistance, friction reduction, anti-seizure capability, and overall surface mechanical properties.

High Precision and Excellent Dimensional Stability

Chemical heat treatments, for the most part, exhibit minimal workpiece deformation and offer several advantages such as high precision and excellent dimensional stability. Techniques like nitriding, soft nitriding, ion nitriding, and others enable the workpiece to retain its high precision, low surface roughness, and good dimensional stability.

Complex Process

The majority of chemical heat treatment involves a complex combination of physical, chemical, and metallurgical processes. It requires heating in a certain active medium and is completed through physical and chemical reactions at the interface and metallurgical diffusion from the exterior to the interior of the workpiece. Consequently, the process is intricate, the treatment cycle is lengthy, and there are stringent equipment requirements.

Enhance Economic Benefits

Chemical heat treatment in general has a notable impact on improving the quality of mechanical products, unleashing the potential of materials, and prolonging their service life. As a result, it contributes to the conservation of precious metal materials, reduces expenses, and enhances economic benefits.

Typical Properties Obtained from Chemical Heat Treatments

Chemical heat treatments can yield a wide range of properties depending on the specific treatment employed. Here are some typical properties obtained from chemical heat treatments:

Increased Hardness: Chemical heat treatments can significantly increase the hardness of materials, making them more resistant to wear, indentation, and deformation. This is particularly important for applications where materials need to withstand heavy loads or abrasive conditions.

Improved Strength: Chemical heat treatments can enhance the strength of materials, making them capable of withstanding higher stresses and loads. This is crucial for applications where structural integrity and load-bearing capacity are essential, such as in the automotive and aerospace industries.

Enhanced Toughness: Chemical heat treatments can improve the toughness of materials, making them more resistant to fracture and impact. Toughness is particularly important in applications where materials are subjected to sudden or repeated loads, such as in tooling and machinery components.

Increased Wear Resistance: Chemical heat treatments can significantly enhance the wear resistance of materials, reducing the rate of wear and extending their lifespan. This is beneficial in applications where materials come into contact with abrasive surfaces or undergo sliding, rolling, or impact wear, such as in gears, bearings, and cutting tools.

Enhanced Corrosion Resistance: Chemical heat treatments can improve the corrosion resistance of materials, making them more resistant to degradation caused by chemical reactions to their environment. This is particularly important in industries such as oil and gas, where materials are exposed to corrosive substances or harsh environments.

Greater Dimensional Stability: Chemical heat treatments can increase the dimensional stability of materials, reducing the risk of dimensional changes due to temperature variations or stress. This is crucial in applications where tight tolerances and precise dimensions are required, such as in precision engineering and tooling.

Reduced Internal Stresses: Chemical heat treatments can relieve internal stresses within materials, improving their stability and reducing the risk of distortion or cracking. This is beneficial in applications where materials undergo significant thermal or mechanical loading, such as in welded structures or heat-treated components.

Enhanced Fatigue Resistance: Chemical heat treatments can improve the fatigue resistance of materials, enabling them to withstand cyclic loading without failure. This is important in applications where materials are subjected to repeated stress cycles, such as in automotive and aerospace components.

Controlled Electrical and Magnetic Properties: Chemical heat treatments can be used to modify the electrical and magnetic properties of certain materials, making them suitable for specific applications in electronics, electrical engineering, and magnetic devices.

Tailored Microstructure: Chemical heat treatments can be employed to achieve specific microstructures within materials, such as fine-grained structures, precipitate formations, or phase transformations. These microstructural changes can impart unique properties to materials, enabling them to meet specific performance requirements.

chemical heat treatment equipment

Applications of Chemical Heat Treatment Across Industries

Owing to the broad range of achievable properties, chemical heat treatments are widely used across practically every industry that employs metal components. Some examples include:

Automotive – Gears, drive shafts, valves, tappets, pistons, etc.

Aerospace – Turbine blades, landing gear parts, engine components, airframes, etc.

Oil & Gas – Drill bits, casings, offshore platform equipment, etc.

Construction & Mining – Excavator teeth, bulldozer blades, crushers, conveyors, etc.

Power Generation – Turbine components, fasteners, distribution hardware, etc.

Railways – Wheels, axles, couplings, rails, locomotive parts, etc.

Electronics – Semiconductor wafer hardware, vacuum/cryogenic equipment parts, etc.

Food Processing – Conveyor components, bottling machinery parts, valves, presses, etc.

Medical – Surgical instruments, dental tools, medical equipment parts, etc.

Defense – Firearm components, armor plates, aerospace & marine hardware, etc.

Summary

Chemical heat treatment is a versatile process that brings about significant improvements in the properties of materials, making them suitable for diverse applications. This process can enhance the strength, hardness, toughness, and corrosion resistance of metals and alloys. The wide-ranging applications of chemical heat treatment across industries highlight its importance in modern manufacturing processes. With its ability to tailor material properties and improve performance, chemical heat treatment continues to be a vital technique in the pursuit of advanced materials and enhanced product quality.

 

FAQs

Conventional heat treatment relies solely on heating/cooling to alter metal properties. Chemical heat treatment utilizes thermochemical reactions between the heated metal and processing bath to induce changes. This enables modifying surface properties independently of the core.

Yes, chemical heat treatment can be applied to certain non-metallic materials such as ceramics and polymers to modify their properties.

Chemical heat treatment sees widespread use in automotive, aerospace, oil and gas, construction, power generation, railways, electronics, and medical device manufacturing. Almost any industry dealing with metal components can benefit from chemical treatment.

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