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

Heat treatment — a technological process during which metal products are subject to controlled heating, holding at a certain temperature and gradually cooling. The main goal of the process is to directly change the structure of steel or alloy, which makes it possible to improve their characteristics: strength, hardness, plasticity.

Principles of heat treatment

The process heat treatment is based on physical and chemical changes that occur within metals under the influence of temperature. As a result, the position of atoms inside crystal lattice changes, which leads to transformations in the structure and properties of the material.

To enhance the changes obtained, the following can be used:

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

This combination of methods makes it possible to create materials with unique properties that are unattainable by conventional processing methods.

The role of heat treatment in industry

In modern technology, heat treatment makes it possible to obtain materials with predetermined characteristics. It can be used as an intermediate operation to improve the machinability of metals by cutting, rolling or forging. And also be the final operation that gives products physical and mechanical propertiesnecessary for their successful operation.

Changing the structure of materials

During heating, holding and cooling, steels and alloys understand significant changes in structure. As a result of these processes, materials can be in two states:

• Equilibrium (stable) state is achieved with slow cooling, for example, together with an oven. This makes it possible to complete diffusion transformations and secondary crystallization, providing the material with uniformity and minimal residual stresses.
• Metastable state occurs when cooling rapidly, for example, in oil or water. Due to the lack of time to complete the diffusion processes, the material is fixed in an intermediate (non-equilibrium or partially non-equilibrium) state, which increases the hardness of the product, but may reduce plasticity.

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

Technical aspects and practical applications

The cooling mode 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 in conjunction with the furniture, which helps to avoid sudden temperature changes, achieve the most stable structure and reduce the risk of cracks or internal defects. This mode is used, for example, for large structures where uniform properties throughout the volume are important.
• Moderate cooling results in materials with an almost equilibrium structure. It is usually performed in air and is used for products that require a balance between strength and plasticity.
• Sharp cooling in liquids such as water or oil, it is necessary to fix non-equilibrium structures. It increases the hardness but reduces the plasticity of materials. It is widely used for tool steels and parts operating under high loads.

For spring steels, for example, 60S2A cold rolled strip according to GOST 14959-2016, or analogues of an Inconel alloy, the right choice of cooling mode ensures their reliability and durability.

The main types of heat treatment

Heat treatment processes can be divided into three main groups:

• The actual heat treatment — thermal effect to change the structure of the material.
• Thermomechanical treatment — a combination of high temperatures and plastic deformation.
• Chemical heat treatment — combines thermal effects with the saturation of the material surface with various elements (carbon, nitrogen, etc.).

The methods of actual heat treatment are the most widely used. Let us consider them in more detail below.

Annealing

It involves heating the steel or alloy to a predetermined temperature, holding it at this temperature and then slowly cooling it to avoid the formation of defects. The method is aimed at achieving optimal structural and required mechanical properties.

The main tasks of annealing are:

• Eliminating internal stresses
  • During the processing or manufacture of the product, residual stresses occur 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 sizes. It helps to increase plasticity, manufacturability and other performance characteristics.
• Achieving magnetic properties
  • For soft magnetic alloys (e.g. 49K2FA-VI, 50N, 79NM) annealing is carried out in vacuum. This makes it possible to improve the magnetic parameters required in high-precision devices.

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

Hardening

When hardening the metal is heated to a certain temperature, kept at it and then rapidly cooled in water, oil or special solutions. This makes it possible to record the structure and properties obtained during the heating stage.

The main tasks of hardening:

• Increasing hardness and strength
  • Rapid cooling fixes atoms in a given crystal lattice, creating a solid structure. This increases resistance to wear and mechanical stress.
• Improved wear resistance
  • Hardened metals are used in parts of devices and equipment that require durability and increased wear resistance.
• Structure adjustment
  • Hardening reduces the grain size, which has a positive effect on the strength of the material.

Types of hardening

Hardening with polymorphic transformation 

During this heat treatment, the crystal one changes material structure. Transformations occur when steel or alloy is heated to a temperature above a critical point, followed by rapid cooling.

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, when heated, the ferritic structure of steel is converted (volume-centered cubic lattice) into an austenitic structure (face-centered cubic lattice). When abruptly cooled, austenite turns into martensite, a structure with high hardness.

Hardening process:

1. Heating
   • The material is heated to a temperature 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 conversion temperature is generally 730—910°C.
2. Excerpt
   • At this stage, the initial structure is uniformly transformed into the austenitic phase.
3. Fast cooling
   • It is 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.

The result of hardening

In steels and alloys, it increases:

• Hardness: due to the formation of martensite.
• Durability: resistance to static and dynamic loads increases.
• Wear resistance: After processing, the material can operate under friction conditions for a long time.

Application

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

• 60S2A, 65G, 70S2HA — for springs and springs.
• U8A, U10A — for tools such as knives and drills.

Hardening without polymorphic transformation

As a result of heat treatment, the material does not experience phase changes in the crystal structure. In contrast to polymorphic hardening, this method is aimed at fixing 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. Excerpt
   • At this stage, alloying elements are evenly distributed in the crystal structure.
3. Cooling
   • Rapid cooling captures the achieved phase, preventing the formation of other structural states.

The result of hardening

This method slightly improves toughness material due to the dissolution of alloying elements in solid solution.

Application

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

• Austenitic corrosion-resistant steels:
  • 12X18N9, 12X18H10T — for the chemical and food industries.
• Non-ferrous metals:
  • For example, aluminum and magnesium alloys, where hardening improves plasticity and stability of mechanical properties.

Comparison of two hardening methods

Polymorphic hardening is used to improve hardness and wear resistance, which makes it indispensable for tools and machine parts. Hardening without polymorphic transformation, in turn, plays an important role in increasing the corrosion resistance and stability of non-ferrous metals and stainless steels. The choice of method depends on the properties of the material and the requirements for the final product.

Vacation

It is carried out after hardening to eliminate its negative effects, such as excessive brittleness and internal stress. It includes heating the hardened metal to a temperature below a critical point, holding and slowly cooling.

Depending on the temperature, they are released three types of vacation:

• Low-temperature tempering (up to 250°C)
  • Eliminates residual stresses and maintains high hardness. It is used for springs, tools and parts that require increased hardness and strength.
• Medium temperature vacation (250—450°C)
  • Improves impact strength and plasticity. It is used for parts operating under dynamic loads.
• High temperature release (450°C and higher)
  • It reduces brittleness, making the material more ductile. It is used for shafts, gears and other loaded parts.

The main objectives of the vacation are:

• Reducing internal stresses
  • This step prevents deformations, increasing the stability of the product.
• Increased plasticity
  • The metal becomes less brittle, which makes it possible to use it under dynamic loads.
• Increased impact strength
  • The parts become resistant to sudden mechanical effects.

Aging

A heat treatment method in which the product is kept at normal or elevated temperatures in order to change its properties. It allows you to stabilize the structure and properties of the material.

The main types of aging are:

• Natural aging
  • It is carried out at room temperature for a long time. It is usually used for aluminum alloys.
• Artificial aging
  • It is carried out at elevated temperatures, which speeds up the process and improves the performance characteristics of metals. It is used for nickel and iron alloys, such as Inconel 718.

Heat-resistant materials, such as KhN78t, analogues of foreign alloys Inconel 625, Inconel 718, Inconel S-276, are aged to improve resistance to high temperatures. During aging, special carbides are released that provide the required properties of finished products.

Benefits of heat treatment

Heat treatment makes it possible to achieve:

• optimal mechanical strength and wear resistance;
• increasing plasticity and impact strength for difficult operating conditions.

Each of the considered types of heat treatment has its own characteristics and goals, which makes them irreplaceable in industry. Annealing eliminates defects and stresses, hardening improves hardness and strength, tempering optimizes properties after quenching, and aging makes it possible to further stabilize the structure of metals. The choice of a particular method depends on the properties of the material and the tasks faced by the manufacturer.

Why do they choose PZPS

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

The advantages of working with PZPS:

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

Here you can buy cold-rolled strip made of 49K2FA-VI alloys, 27KH, manufactured in accordance with GOST 10160-75, alloy tape X20N80 according to GOST 12766.1-90, cold-rolled low-carbon steel strip in accordance with GOST 503-81, as well as analogues of Inconel alloys and other foreign materials. To order, please contact specified phones or leave application on the site. Our experts will contact you as soon as possible.

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