Heat treatment of steels and alloys: types, features, and applications

Heat treatment is a technological process during which metal products undergo controlled heating, holding at a given temperature, and gradual cooling. The main goal of the process is a directed change in the structure of steel or alloy, which improves their characteristics: strength, hardness, and ductility.

Principles of heat treatment

The process of heat treatment is based on physical and chemical changes that occur inside metals under the influence of temperature. As a result, the positions of atoms within the crystal lattice change, leading to transformations of the material’s structure and properties.

To intensify the resulting changes, the following may be used:

• chemical saturation of the surface;
• plastic deformation;
• exposure to a magnetic field. 

Such a combination of methods makes it possible to create materials with unique properties unattainable by ordinary processing methods.

The role of heat treatment in industry

In modern engineering, heat treatment makes it possible to obtain materials with predetermined characteristics. It may be used as an intermediate operation to improve the machinability of metals by cutting, rolling, or forging. It may also be a finishing operation that imparts to products the physical and mechanical properties required for successful service.

Changes in material structure 

During heating, holding, and cooling, steels and alloys undergo substantial structural changes. As a result of these processes, materials may be in two states:

• Equilibrium (stable) state is achieved with slow cooling, for example together with the furnace. This allows diffusion transformations and secondary crystallization to complete, giving the material uniformity and minimal residual stresses.
• Metastable state arises with rapid cooling, for example in oil or water. Because there is insufficient time to complete diffusion processes, the material is fixed in an intermediate (nonequilibrium or partially nonequilibrium) state, which increases product hardness but may reduce ductility.

These states affect the service properties of the product, determining its suitability for various operating conditions.

Technical aspects and practical application

The cooling regime is a key factor in heat treatment on which the final state of the material depends.

• Slow cooling provides a state close to equilibrium. As a rule it is carried out together with the furnace, which avoids sharp temperature differentials, achieves the most stable structure, and reduces the risk of cracks or internal defects. This regime is used, for example, for large structures where uniformity of properties throughout the volume is important.
• Moderate cooling yields materials with a nearly equilibrium structure. It is usually performed in air and is used for products that require a balance between strength and ductility.
• Rapid cooling in liquids such as water or oil is necessary to fix nonequilibrium structures. It increases hardness but reduces material ductility. It is widely used for tool steels and parts operating under high loads.

For spring steels, for example cold-rolled strip 60S2A per GOST 14959-2016, or analogues of alloys of the Inconel type, the correct choice of cooling regime ensures their reliability and durability.

Main types of heat treatment

Heat-treatment processes can be divided into three main groups:

• Heat treatment proper — temperature exposure to change the material structure.
• Thermomechanical treatment — a combination of high temperatures and plastic deformation.
• Chemico-thermal treatment — combines thermal exposure with saturation of the material surface by various elements (carbon, nitrogen, etc.).

Methods of heat treatment proper have become the most widespread. Below we examine them in more detail.

Annealing

It includes heating steel or alloy to a set temperature, holding at that temperature, and subsequent slow cooling, which avoids defect formation. The method aims at achieving optimal structural and required mechanical properties.

Main tasks of annealing:

• Relief of internal stresses
  • During processing or manufacture, residual stresses arise inside the material. Annealing relieves these stresses, reducing the risk of cracking.
• Structure modification
  • Annealing makes it possible to transform the structure of steel or alloy, improving uniformity and changing grain size. It promotes increased ductility, processability, and improvement of other service characteristics.
• Achievement of magnetic properties
  • For soft magnetic alloys (for example 49K2FA-VI, 50N, 79NM) annealing is carried out in vacuum. This improves the magnetic parameters needed in high-precision devices.

Annealing is usually carried out at temperatures close to the critical ones, followed by slow cooling, which excludes sharp phase transitions.

Quenching

In quenching, the metal is heated to a certain temperature, held there, and then rapidly cooled in water, oil, or special solutions. This fixes the structure and properties obtained at the heating stage.

Main tasks of quenching:

• Increase of hardness and strength
  • Rapid cooling fixes atoms in a given crystal lattice, creating a strong structure. This increases resistance to wear and mechanical loads.
• Improvement of wear resistance
  • Quenched metals are used in parts of devices and equipment where durability and elevated wear resistance are required.
• Structure control
  • Quenching makes it possible to reduce grain size, which positively affects material strength.

Types of quenching

Quenching with polymorphic transformation 

In such heat treatment the crystalline structure of the material changes. Transformations occur when steel or alloy is heated above the critical point and then rapidly cooled.

Polymorphic transformation: what is it?

Polymorphism in metals is the ability of a material to change its crystal lattice under certain temperature conditions. For example, on heating the ferritic structure of steel (body-centered cubic lattice) transforms into austenite (face-centered cubic lattice). On rapid cooling, austenite transforms into martensite — a structure of high hardness.

Quenching process:

1. Heating
   • The material is heated above the critical point of the phase transition. For carbon steels this value is determined by their chemical composition and carbon content.
   • For example, for carbon steel the transformation temperature is usually 730–910°C.
2. Holding
   • At this stage uniform conversion of the initial structure into the austenitic phase is achieved.
3. Rapid cooling
   • Carried out in water, oil, or a special solution. The cooling rate must be high enough to prevent reverse diffusion processes and to fix martensite.

Result of quenching

In steels and alloys it increases:

• Hardness: thanks to formation of martensite.
• Strength: resistance to static and dynamic loads increases.
• Wear resistance: after treatment the material can operate for a long time under friction.

Application

This type of quenching is used for carbon and low-alloy steels, for example:

• 60S2A, 65G, 70S2KhA — for springs and leaf springs.
• U8A, U10A — for tools such as knives and drills.

Quenching without polymorphic transformation

As a result of heat treatment, no phase changes of the crystal structure occur in the material. Unlike polymorphic quenching, this method aims to fix the existing state of the material after heating.

Process features:

1. Heating
   • The heating temperature must be below the point at which phase changes occur. 
2. Holding
   • At this stage alloying elements are uniformly distributed in the crystal structure.
3. Cooling
   • Rapid cooling fixes the attained phase, preventing formation of other structural states.

Result of quenching

This method slightly increases the strength of the material thanks to dissolution of alloying elements in the solid solution.

Application

This method is widely used for materials not prone to polymorphic transformations, such as:

• Austenitic corrosion-resistant steels:
  • 12Kh18N9, 12Kh18N10T — for the chemical and food industries.
• Nonferrous metals:
  • For example, aluminum and magnesium alloys, where quenching improves ductility and stability of mechanical properties.

Comparison of the two quenching methods

Quenching with polymorphic transformation is used to improve hardness and wear resistance, making it indispensable for tools and machine parts. Quenching without polymorphic transformation, in turn, plays an important role in increasing corrosion resistance and stability of nonferrous metals and stainless steels. The choice of method depends on the material properties and requirements for the final product.

Tempering

It is carried out after quenching to eliminate its adverse effects, such as excessive brittleness and internal stresses. It includes heating the quenched metal to a temperature below the critical point, holding, and slow cooling.

Depending on temperature, three types of tempering are distinguished:

• Low-temperature tempering (up to 250°C)
  • Relieves residual stresses and preserves high hardness. Used for springs, tools, and parts that require elevated hardness and strength. 
• Medium-temperature tempering (250–450°C)
  • Improves impact toughness and ductility. Used for parts operating under dynamic loads.
• High-temperature tempering (450°C and above)
  • Reduces brittleness, making the material more ductile. Used for shafts, gear transmissions, and other loaded parts.

Main tasks of tempering:

• Reduction of internal stresses
  • This stage prevents deformation, increasing product stability.
• Increase of ductility
  • The metal becomes less brittle, allowing use under dynamic loads.
• Increase of impact toughness
  • Parts become resistant to sudden mechanical impacts.

Aging

A heat-treatment method in which the product is held at normal or elevated temperature to change its properties. It stabilizes the structure and properties of the material. 

Main types of aging:

• Natural aging
  • Carried out at room temperature over a long time. Usually used for aluminum alloys.
• Artificial aging
  • Carried out at elevated temperatures, which accelerates the process and improves service characteristics of metals. Used for nickel- and iron-based alloys such as Inconel 718.

Heat-resistant materials such as KhN78T, analogues of the foreign alloys Inconel 625, Inconel 718, and Inconel C-276, undergo aging to improve resistance to high temperatures. During aging, special carbides precipitate that provide the required properties of finished products.

Advantages of heat treatment

Heat treatment makes it possible to achieve:

• optimal mechanical strength and wear resistance;
• increased ductility and impact toughness for demanding service conditions.

Each of the heat-treatment types considered has its own features and goals, making them indispensable in industry. Annealing removes defects and stresses, quenching improves hardness and strength, tempering optimizes properties after quenching, and aging additionally stabilizes the metal structure. The choice of a specific method depends on the material properties and the tasks facing the manufacturer.

Why choose PZPS

The Saint Petersburg Precision Alloys Plant offers a wide range of high-quality products manufactured using modern heat-treatment technologies. 

Advantages of working with PZPS:

• Modern equipment that guarantees processing accuracy.
• Quality control at every production stage.
• Individual approach to orders.

Here you can buy cold-rolled strip of alloy grades 49K2FA-VI, 27KKh, manufactured per GOST 10160-75, strip of alloy Kh20N80 per GOST 12766.1-90, cold-rolled strip of low-carbon steel per GOST 503-81, as well as analogues of Inconel-type alloys and other foreign materials. To place an order, contact us at the numbers listed or submit a request on the website. Our specialists will contact you as soon as possible.

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