Heat Treatment of Stainless Steel

Heat Treatment Explained

Heat treatment is a collection of industrial and metalworking processes used to alter the physical and sometimes chemical properties of a material. By carefully applying controlled heating and cooling, these processes aim to achieve desired characteristics in the treated material. Heat treatment is essential in various industries to enhance the performance and durability of materials.

Several key heat treatment processes include:

• Annealing: Heating the material to a specific temperature, holding it, and then cooling it slowly. This process reduces hardness, increases ductility, and helps eliminate internal stresses.
• Quenching: Heating the workpiece to a high temperature and then cooling it rapidly, usually in water, oil, or air. This rapid cooling sets the material’s structure, increasing its hardness.
• Tempering: Performed after quenching, tempering involves reheating the quenched workpiece to a lower temperature and then cooling it again. This process reduces brittleness while maintaining hardness.

Types of Heat Treatments

Stainless steel heat treatments come in various forms, each designed to achieve specific properties in stainless steel. Understanding these different processes is crucial for selecting the appropriate treatment to meet particular performance requirements. Here, we explore some of the most common heat treatment methods and their applications in more detail.

Annealing

Definition: Annealing is a heat treatment process that reduces hardness, increases ductility, and helps eliminate internal stresses.

Process: The metal is heated to a certain temperature, held at this temperature for a set time, and then slowly cooled. This gradual cooling allows the steel to rearrange its microstructure, relieve internal stresses and increase its ductility.

Applications: Annealing is used to improve machinability and prepare the steel for subsequent processing operations.

Quenching

Definition: Quenching involves the rapid cooling of a material to change its lattice structure and thereby increase its hardness.

Process: The metal is heated to a high temperature, usually to austenitisation temperature, and then rapidly cooled in a medium such as water, oil or air. This sudden cooling ensures that the carbon is forcibly dissolved and transforms from austenite to martensite. This makes the material very hard but brittle.

Applications: Quenching is typically used to increase the hardness and strength of metal components and make them more resistant to wear. However, it also increases brittleness, which is why further heat treatment such as tempering is often required.

Tempering

Definition: Tempering is a heat treatment process aimed at achieving greater toughness by decreasing the hardness of quenched steel.

Process: Quenched steel is reheated to a lower temperature and held there for a specified time before being cooled again (air). This reheating allows some of the internal stresses to be relieved and reduces brittleness while maintaining a degree of hardness.

Applications: Tempering is used to reduce the brittleness of quenched steel, providing a balance between hardness and toughness. This makes the material more durable and less prone to breaking under stress.

Normalizing

Definition:  During normalizing, the steel is heated to a temperature above the GSK line and then slowly cooled in the air. This process refines and homogenizes the structure.

Process: The steel is heated to a temperature of approximately between 800 and 950°C, held at this temperature to allow the transformation to austenite, and then cooled in air. The slower cooling rate compared to quenching results in a homogeneous and refined microstructure that is “normalized”.

Applications: Normalizing is used to improve the machinability of steel, increase mechanical properties such as toughness and strength, reduce internal stresses and prepare the steel for subsequent heat treatments.

Nitriding and Nitrocarburizing

Definition: Nitriding and nitrocarburizing are surface hardening techniques that involve diffusing nitrogen and carbon into the surface of the metal.

Process: These treatments are conducted at relatively low temperatures compared to other heat treatments. Nitriding introduces nitrogen into the metal, forming a hard, wear-resistant surface layer. Nitrocarburizing introduces both nitrogen and carbon, providing similar benefits with added resistance to corrosion and fatigue.

Applications: These methods enhance wear resistance, fatigue strength, and corrosion resistance, making them ideal for components exposed to high stress and harsh environments. Due to the low process temperatures, the process is virtually distortion-free.

BorTec’s expertise in nitriding and nitrocarburizing ensures optimal results, providing superior wear resistance and extended lifespan for stainless steel components.

Case Hardening

Definition: Case hardening is a heat treatment process in which the edge zone of the steel is enriched with carbon and then hardened. This produces a hard edge zone and a soft, ductile core. Case-hardening is further subdivided into three processes: Single hardening; direct hardening and double hardening

Process: The steel is exposed to a carbon-rich environment (gaseous, liquid or solid) at high temperatures, allowing the carbon to diffuse into the surface layer. Direct hardening is followed by quenching, which hardens the outer layer while the core remains tough and ductile. The reason for this is the difference in carbon content in the surface and core.

Applications: Case hardening is used for components that require a wear-resistant surface and a tough core, such as gears, camshafts and other highly stressed parts.

Recrystallization Annealing

Definition: Recrystallization annealing is a heat treatment method that eliminates the effects of cold forming, restoring the ductility of the material.

Process: The steel is heated to a temperature in the recrystallization range without causing structural changes. This process is often carried out after cold working to prepare the steel for further forming processes.

Applications: This method is used to restore ductility and reduce hardness, making it easier to perform additional forming operations without cracking. It is particularly beneficial for steels with an austenitic structure, as it helps maintain their mechanical properties and formability.

Additional Heat Treatments

In addition to the common heat treatments, there are several other specialized techniques that are crucial for specific applications and material properties. These include flame hardening, induction hardening, and carburizing.

Flame Hardening

Definition: Flame hardening is a surface hardening technique that uses a high-temperature flame to heat the steel’s surface, followed by rapid cooling (quenching).

Process: The steel’s surface is heated with an oxy-gas flame until it reaches the required temperature. The heated area is then quenched rapidly, usually with water, to harden the surface layer while the core remains softer and ductile.

Applications: Flame hardening is commonly used for large parts such as machine tools, crankshafts, and railway wheels, where only the surface needs to be hardened for wear resistance.

Induction Hardening

Definition: Induction hardening is a heat treatment process that uses electromagnetic induction to heat the steel, followed by quenching.

Process: An alternating magnetic field induces an electric current (eddy current) in the steel, rapidly heating it to the desired temperature. The steel is then immediately quenched to harden the surface while maintaining a tough core.

Applications: Induction hardening is ideal for components that require a hard surface and a tough core, such as gears, shafts, and bearings.

Carburizing

Definition: Carburizing is a heat treatment process in which carbon is introduced into the surface layer of low-carbon steel to make it hardenable (C content > 0.2%).

Process: The steel is exposed to a high-carbon environment at high temperatures, causing carbon atoms to diffuse into the surface.

Applications: Carburizing is used for parts that require a wear-resistant surface and a tough core, such as gears, camshafts and fasteners, and have a carbon content of less than 0.2%

Specialized Processes at BorTec

BorTec employs advanced stainless steel heat treatment processes to enhance the properties of stainless steel, ensuring superior performance and durability. These specialized techniques — BORINOX®, BOROCOAT®, and NICKELCOAT® — are designed to meet the highest standards of hardness, wear resistance, and corrosion protection, catering to a wide range of industrial applications.

BORINOX® Process

The BORINOX® process is a low-temperature diffusion method specifically developed for hardening stainless steel without compromising its corrosion resistance. This process involves the diffusion of carbon and nitrogen into the surface of the steel at low temperatures, which prevents the formation of chromium carbides and nitrides that could reduce corrosion resistance.

Suitability for Complex Geometries: The BORINOX® process is particularly suitable for components with complex geometries, ensuring uniform hardening without altering the material’s dimensions. This makes it an ideal choice for precision parts that require high surface hardness and excellent corrosion resistance.

BOROCOAT® Process

The BOROCOAT® process is a boronizing technique designed to significantly increase the wear resistance of steel. During this process, boron atoms diffuse into the steel surface, forming a hard, wear-resistant layer that is strongly bonded to the substrate.

High Thermal Stability and Chemical Resistance: BOROCOAT® provides exceptional thermal stability and resistance to various chemicals, including strong acids. This makes it suitable for applications in environments where high temperatures and corrosive substances are present, such as in the chemical and petrochemical industries.

NICKELCOAT® Process

NICKELCOAT® is an electroless nickel plating process that offers excellent corrosion resistance and a uniform coating, even on complex parts. This process involves an auto-catalytic reaction that deposits a layer of nickel-phosphorus alloy onto the steel surface without the need for an electric current.

Applications in Highly Corrosive Environments: The uniform and low-porosity coating provided by NICKELCOAT® makes it ideal for use in highly corrosive environments, such as the oil and gas industry. Components treated with NICKELCOAT® exhibit excellent resistance to corrosive gases, CO2 injection wells, and other harsh conditions, ensuring long-term durability and reliability.

Suitable Heat Treatments for Different Stainless Steel Types

Different types of metal require specific heat treatments to optimize their properties for various applications. Understanding which heat treatment is best suited for each type of steel is crucial for achieving desired performance characteristics. BorTec’s advanced heat treatment processes, including BORINOX®, BOROCOAT®, and NICKELCOAT®, are designed to provide superior results for a wide range of steel types.

Austenitic Stainless Steel

Austenitic stainless steels are known for their excellent corrosion resistance and good mechanical properties. Common grades include 304 and 316.

Suitable Heat Treatments:

• Solution Annealing: Heating to around 950 – 1200°C followed by rapid cooling. This treatment dissolves carbides and restores corrosion resistance and ductility.
• Stress Relieving: Heating to 550 – 650°C to relieve residual stresses without significantly affecting corrosion resistance.

The BORINOX® process is ideal for hardening austenitic stainless steels while maintaining their superior corrosion resistance, making it suitable for applications requiring high surface hardness and durability.

Martensitic Stainless Steel

Martensitic steels are characterized by their high hardness and strength. Common grades include 410 and 420.

Suitable Heat Treatments:

• Hardening (Quenching): Heating to 950-1050°C followed by rapid cooling in oil or air to achieve high hardness.
• Tempering: Reheating quenched steel to adjust the hardness and improve toughness.

The BORINOX® process provides enhanced wear resistance without compromising the steel’s mechanical properties, making it an excellent choice for components requiring high strength and hardness.

Ferritic Stainless Steel

Ferritic stainless steels are magnetic and have moderate corrosion resistance. Common grades include 430 and 446. These steels typically have a lower carbon content compared to other types of stainless steel, which influences their heat treatment requirements.

Suitable Heat Treatments:

• Annealing: Heating to 700-800°C followed by slow cooling to improve ductility and reduce brittleness.
• Stress Relieving: Heating to 200-300°C to relieve residual stresses.

The BOROCOAT® process enhances the wear resistance of ferritic stainless steels, providing a hard, wear-resistant layer that withstands high temperatures and corrosive environments.

Duplex Stainless Steel

Duplex stainless steels have a mixed microstructure of austenite and ferrite, offering high strength and excellent corrosion resistance. Common grades include 2205 and 2507.

Suitable Heat Treatments:

• Solution Annealing: Heating to a temperature range between 1020 and 1100°C followed by rapid cooling to prevent the formation of detrimental phases and restore balance between austenite and ferrite.

The BORINOX® process is particularly effective for duplex stainless steels, ensuring uniform hardening and maintaining corrosion resistance, which is critical for applications in aggressive environments.

Precipitation-Hardening Stainless Steel

Precipitation-hardening stainless steels can achieve high strength through heat treatment. Common grades include 17-4 PH and 15-5 PH.

Suitable Heat Treatments:

• Solution Treating: Heating to 1040-1065°C followed by rapid cooling.
• Aging: Heating to 480-620°C to precipitate secondary phases that increase strength.

The BORINOX® process provides excellent wear resistance and hardness, making it ideal for precipitation-hardening stainless steels used in demanding environments such as aerospace and heavy machinery. Additionally, BorTec’s processes are highly effective for tool steel, offering enhanced wear resistance and durability.

Benefits and Applications of Heat Treatment in Stainless Steel