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Common heat treatment processes for metal material


Heat treatment is a very important step in the processing of metal materials. Heat treatment can change the physical and mechanical properties of metal materials, improve their hardness, strength, toughness, and other properties.

In order to ensure that the structure of product design is safe, reliable, economical, and efficient, structural engineers generally need to understand the mechanical properties of materials, select appropriate heat treatment processes based on design requirements and material characteristics, and improve their performance and lifespan. The following are 13 heat treatment processes related to metal materials, hoping to be helpful to everyone.

1. Annealing

A heat treatment process in which metal materials are heated to an appropriate temperature, maintained for a certain period of time, and then slowly cooled. The purpose of annealing is mainly to reduce the hardness of metal materials, improve plasticity, facilitate cutting or pressure processing, reduce residual stress, improve the uniformity of microstructure and composition, or prepare microstructure for subsequent heat treatment. Common annealing processes include recrystallization annealing, complete annealing, spheroidization annealing, and stress relieving annealing.

Complete annealing: Refine grain size, uniform structure, reduce hardness, fully eliminate internal stress. Complete annealing is suitable for forgings or steel castings with carbon content (mass fraction) below 0.8%.

Spheroidizing annealing: reduces the hardness of steel, improves cutting performance, and prepares for future quenching to reduce deformation and cracking after quenching. Spheroidizing annealing is suitable for carbon steel and alloy tool steel with a carbon content (mass fraction) greater than 0.8%.

Stress relieving annealing: It eliminates the internal stress generated during welding and cold straightening of steel parts, eliminates the internal stress generated during precision machining of parts, and prevents deformation during subsequent processing and use. Stress relieving annealing is suitable for various castings, forgings, welded parts, and cold extruded parts.

2. Normalization

It refers to the heat treatment process of heating steel or steel components to a temperature of 30-50 ℃ above Ac3 or Acm (the upper critical point temperature of steel), holding them for an appropriate time, and cooling them in still air. The purpose of normalizing is mainly to improve the mechanical properties of low-carbon steel, improve machinability, refine grain size, eliminate structural defects, and prepare the structure for subsequent heat treatment.

3. Quenching

It refers to the heat treatment process of heating a steel component to a temperature above Ac3 or Ac1 (the lower critical point temperature of the steel), holding it for a certain period of time, and then obtaining martensite (or bainite) structure at an appropriate cooling rate. The purpose of quenching is to obtain the required martensitic structure for steel parts, improve the hardness, strength, and wear resistance of the workpiece, and prepare the structure for subsequent heat treatment.

Common quenching processes include salt bath quenching, martensitic graded quenching, bainite isothermal quenching, surface quenching, and local quenching.

Single liquid quenching: Single liquid quenching is only applicable to carbon steel and alloy steel parts with relatively simple shapes and low technical requirements. During quenching, for carbon steel parts with a diameter or thickness greater than 5-8mm, salt water or water cooling should be used; Alloy steel parts are cooled with oil.

Double liquid quenching: Heat the steel parts to the quenching temperature, after insulation, quickly cool them in water to 300-400 º C, and then transfer them to oil for cooling.

Flame surface quenching: Flame surface quenching is suitable for large medium carbon steel and medium carbon alloy steel parts, such as crankshafts, gears, and guide rails, that require hard and wear-resistant surfaces and can withstand impact loads in single or small batch production.

Surface induction hardening: Parts that have undergone surface induction hardening have a hard and wear-resistant surface, while maintaining good strength and toughness at the core. Surface induction hardening is suitable for medium carbon steel and alloy steel parts with moderate carbon content.

4. Tempering

It refers to the heat treatment process where steel parts are quenched and then heated to a temperature below Ac1, held for a certain period of time, and then cooled to room temperature. The purpose of tempering is mainly to eliminate the stress generated by steel parts during quenching, so that the steel parts have high hardness and wear resistance, as well as the required plasticity and toughness. Common tempering processes include low temperature tempering, medium temperature tempering, high temperature tempering, etc.

Low temperature tempering: Low temperature tempering eliminates internal stress caused by quenching in steel parts, and is commonly used for cutting tools, measuring tools, molds, rolling bearings, and carburized parts.

Medium temperature tempering: Medium temperature tempering enables steel parts to achieve high elasticity, certain toughness, and hardness, and is generally used for various types of springs, hot stamping dies, and other parts.

High temperature tempering: High temperature tempering enables steel parts to achieve good comprehensive mechanical properties, namely high strength, toughness, and sufficient hardness, eliminating internal stress caused by quenching. It is mainly used for important structural parts that require high strength and toughness, such as spindles, crankshafts, cams, gears, and connecting rods.

5.  Quenching&Tempering

Refers to the composite heat treatment process of quenching and tempering steel or steel components. The steel used for quenching and tempering treatment is called quenched and tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.

6. Chemical heat treatment

A heat treatment process in which a metal or alloy workpiece is placed in an active medium at a certain temperature for insulation, allowing one or more elements to penetrate its surface to change its chemical composition, structure, and performance. The purpose of chemical heat treatment is mainly to improve the surface hardness, wear resistance, corrosion resistance, fatigue strength, and oxidation resistance of steel parts. Common chemical heat treatment processes include carburization, nitriding, carbonitriding, etc.

Carburization: To achieve high hardness (HRC60-65) and wear resistance on the surface, while maintaining high toughness at the center. It is commonly used for wear-resistant and impact resistant parts such as wheels, gears, shafts, piston pins, etc.

Nitriding: Improving the hardness, wear resistance, and corrosion resistance of the surface layer of steel parts, commonly used in important parts such as bolts, nuts, and pins.

Carbonitriding: improves the hardness and wear resistance of the surface layer of steel parts, suitable for low carbon steel, medium carbon steel, or alloy steel parts, and can also be used for high-speed steel cutting tools.

7. Solid solution treatment

It refers to the heat treatment process of heating an alloy to a high-temperature single-phase zone and maintaining a constant temperature, allowing the excess phase to fully dissolve in the solid solution and then rapidly cool to obtain a supersaturated solid solution. The purpose of solution treatment is mainly to improve the plasticity and toughness of steel and alloys, and to prepare for precipitation hardening treatment.

8. Precipitation hardening (precipitation strengthening)

A heat treatment process in which a metal undergoes hardening due to the segregation of solute atoms in a supersaturated solid solution and/or the dispersion of dissolved particles in the matrix. If austenitic precipitation stainless steel is subjected to precipitation hardening treatment at 400-500 ℃ or 700-800 ℃ after solid solution treatment or cold working, it can achieve high strength.

9. Timeliness treatment

It refers to the heat treatment process in which alloy workpieces undergo solid solution treatment, cold plastic deformation or casting, and are then forged, placed at a higher temperature or maintained at room temperature, and their properties, shape, and size change over time.

If the aging treatment process of heating the workpiece to a higher temperature and conducting aging treatment for a longer time is adopted, it is called artificial aging treatment; The aging phenomenon that occurs when the workpiece is stored at room temperature or natural conditions for a long time is called natural aging treatment. The purpose of aging treatment is to eliminate internal stress in the workpiece, stabilize the structure and size, and improve mechanical properties.

10. Hardenability

Refers to the characteristics that determine the quenching depth and hardness distribution of steel under specified conditions. The good or poor hardenability of steel is often represented by the depth of the hardened layer. The greater the depth of the hardening layer, the better the hardenability of the steel. The hardenability of steel mainly depends on its chemical composition, especially the alloy elements and grain size that increase the hardenability, heating temperature, and holding time. Steel with good hardenability can achieve uniform and consistent mechanical properties throughout the entire section of the steel, and quenching agents with low quenching stress can be selected to reduce deformation and cracking.

11. Critical diameter (critical quenching diameter)

The critical diameter refers to the maximum diameter of a steel when all martensite or 50% martensite structure is obtained at the center after quenching in a certain medium. The critical diameter of some steels can generally be obtained through hardenability tests in oil or water.

12. Secondary hardening

Some iron-carbon alloys (such as high-speed steel) require multiple tempering cycles to further increase their hardness. This hardening phenomenon, known as secondary hardening, is caused by the precipitation of special carbides and/or the transformation of austenite into martensite or bainite.

13. Tempering brittleness

Refers to the embrittlement phenomenon of quenched steel tempered in certain temperature ranges or slowly cooled from the tempering temperature through this temperature range. Temper brittleness can be divided into the first type of temper brittleness and the second type of temper brittleness.

The first type of temper brittleness, also known as irreversible temper brittleness, mainly occurs at a tempering temperature of 250-400 ℃. After the brittleness disappears after reheating, the brittleness is repeated in this range and no longer occurs;

The second type of temper brittleness, also known as reversible temper brittleness, occurs at temperatures ranging from 400 to 650 ℃. When the brittleness disappears after reheating, it should be quickly cooled and should not stay for a long time or slow cooled in the range of 400 to 650 ℃, otherwise catalytic phenomena will occur again.

The occurrence of temper brittleness is related to the alloy elements contained in steel, such as manganese, chromium, silicon, and nickel, which tend to develop temper brittleness, while molybdenum and tungsten have a tendency to weaken temper brittleness.

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Post time: Nov-23-2023