General heat treatment methods for steel materials

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Heat treatment is a thermal processing technique in which metal materials are heated, held at a certain temperature, and then cooled while they are in the solid state, in order to alter their internal structure and thereby achieve the desired properties. For common heat treatment methods, please refer to the table below. Name: Heat Treatment Process; Purpose of Heat Treatment: 1. Tempering – Heat the steel component to a certain temperature, hold it there for a specified period of time, and then cool it slowly back to room temperature. ① Reduce the hardness of the steel and increase its plasticity, thereby facilitating machining and cold deformation processes. ② Refine the grain structure, uniformize the steel’s organization, improve its properties, and prepare it for subsequent heat treatments. ③ Eliminate internal stresses in the steel. To prevent deformation and cracking of parts after processing:
**Annealing types**
(1) **Full annealing**: The steel is heated to 30–50ºC above its critical temperature (which varies depending on the type of steel; it’s generally between 710–750ºC, while for some alloy steels it can reach 800–900ºC). It is then held at this temperature for a certain period before being cooled slowly in the furnace (or by burying it in sand) to refine the grain structure, uniformize the microstructure, reduce hardness, and completely eliminate internal stresses. Full annealing is suitable for forgings or cast steel components with a carbon content (by mass) of less than 0.8%.

(2) **Spheroidizing annealing**: The steel is heated to 20–30ºC above its critical temperature. After being held at this temperature, it is cooled slowly to below 500ºC before being taken out of the furnace and cooled in air. This process reduces the hardness of the steel, improves its machinability, and prepares it for subsequent quenching, thereby reducing deformation and cracking after quenching. Spheroidizing annealing is suitable for carbon steels and alloy tool steels with a carbon content (by mass) greater than 0.8%.

(3) **Stress-relief annealing**: The steel is heated to 500–650ºC and held at that temperature for a certain period before being cooled slowly (usually in the furnace) to eliminate internal stresses generated during welding or cold straightening, as well as those resulting from machining precision parts. This helps prevent deformation during further processing and use. Stress-relief annealing is applicable to various castings, forgings, welded parts, and cold-extracted parts.

2. **Normalizing**: The steel is heated to 40–60ºC above its critical temperature, held at that temperature for a certain period, and then cooled in air.
① It improves the microstructure and machinability of the steel.
② For parts where mechanical properties are not highly demanding, normalizing is often used as the final heat treatment method.
③ It eliminates internal stresses.

3. **Quenching**: The steel is heated to the quenching temperature, held there for a while, and then rapidly cooled in water, saltwater, or oil (for some materials, in air).
① This gives the steel higher hardness and wear resistance.
② It enables the steel to acquire certain special properties after tempering, such as higher strength, elasticity, and toughness.
**Types of quenching**
(1) **Single-fluid quenching**: The steel is heated to the quenching temperature, held there for a while, and then cooled in a single quenching medium. Single-fluid quenching is only suitable for carbon steels and alloy steels with relatively simple shapes and lower technical requirements. During quenching, for carbon steel parts with a diameter or thickness greater than 5–8 mm, salt water or water is used for cooling ； For alloy steel parts, oil cooling is used. (2) Two-fluid quenching involves heating the steel part to the quenching temperature; after holding it at that temperature, it is first cooled rapidly in water to 300–400ºC, and then transferred to oil for further cooling. (3) Flame surface quenching uses a flame generated by the combustion of acetylene and oxygen to heat the surface of the part quickly to the quenching temperature, after which water is sprayed onto the surface immediately. This method is suitable for single-piece or small-batch production of large medium-carbon steel and medium-carbon alloy steel parts that require a hard, wear-resistant surface as well as the ability to withstand impact loads, such as crankshafts, gears, and guide rails. (4) Surface induction quenching involves placing the steel part inside an inductor; the inductor generates a magnetic field under the influence of alternating current of a certain frequency, and the steel part develops induced currents as a result, allowing its surface to be heated rapidly (within 2–10 minutes) to the quenching temperature. Water is then sprayed onto the surface of the part immediately. Parts subjected to surface induction hardening have a hard and wear-resistant surface, while the core retains good strength and toughness. Surface induction hardening is suitable for medium-carbon steels and alloy steels with moderate carbon content. 4. Tempering involves heating the steel parts that have been quenched below their critical temperature, holding them at that temperature for a certain period of time, and then cooling them in air or oil. Tempering is carried out immediately after quenching and represents the final step in the heat treatment process; it is done to achieve the desired mechanical properties. Under normal circumstances, the strength and hardness of parts increase significantly after quenching, but their plasticity and toughness decrease markedly; yet the actual operating conditions of these parts require good strength and toughness. After selecting an appropriate tempering temperature for tempering, the desired mechanical properties can be achieved: ② a stable microstructure and consistent dimensions; ③ elimination of internal stresses. (1) Low-temperature tempering involves heating the quenched steel parts to 150–50ºC, holding them at this temperature for a certain period of time, and then cooling them in air. This method is often used for cutting tools, measuring instruments, molds, rolling bearings, and carburized parts, in order to eliminate the internal stresses resulting from quenching. (2) Medium-temperature tempering involves heating the quenched steel parts to 350–450ºC, holding them at that temperature for a while before cooling them. It is generally used for various springs and hot stamping dies, etc., to give the steel parts higher elasticity, as well as certain levels of toughness and hardness. (3) High-temperature tempering involves heating the quenched steel parts to 500–650ºC, holding them at that temperature before cooling them. This method is mainly used for important structural parts that require high strength and toughness, such as spindles, crankshafts, cams, gears, and connecting rods. It helps to achieve good overall mechanical properties, namely high strength and toughness, along with sufficient hardness, while also eliminating the internal stresses resulting from quenching. 5. Quenching and tempering involves subjecting the quenched steel parts to high-temperature tempering at 500–600ºC. This method is commonly used for important structural parts such as shafts, gears, and connecting rods. Quenching and tempering is usually carried out after rough machining, in order to refine the grain structure of the steel, thereby giving it higher toughness and sufficient strength, and thus good overall mechanical properties. 6. Aging treatment: (1) Artificial aging involves heating the quenched steel parts to 100–160ºC, holding them at that temperature for a long time, and then cooling them to eliminate internal stresses, reduce deformation of the parts, and stabilize their dimensions. This is particularly important for parts that require high precision. (2) Natural aging involves leaving the castings outdoors ； Steel parts such as long shafts and screws that are to be placed in seawater, left hanging for extended periods, or subjected to light impacts and require natural aging should first undergo rough machining. 7. Chemical heat treatment involves placing steel parts in a chemical medium containing certain active atoms such as carbon, nitrogen, and chromium. Through processes like heating, holding at a certain temperature, and cooling, these atoms are induced to penetrate into the surface layer of the steel, thereby changing the chemical composition of that surface layer and endowing it with specific properties. Chemical heat treatment: (1) Carbon infiltration – Carbon atoms are introduced into the surface layer of the steel; this method is used for parts that are subject to wear and impact, such as wheels, gears, shafts, and piston pins. It results in a high hardness level on the surface (HRC 60–65) along with good wear resistance, while the core of the part retains high toughness. (2) Nitrogen infiltration – Nitrogen atoms are introduced into the surface layer of the steel; this is used for important components such as bolts, nuts, and pins, to improve the hardness, wear resistance, and corrosion resistance of their surfaces. (3) Cyaniding – Both carbon and nitrogen atoms are introduced into the surface layer of the steel; this method is suitable for low-carbon steel, medium-carbon steel, or alloy steel parts, as well as for high-speed steel tools, to enhance the hardness and wear resistance of their surfaces. 8. Blackening involves heating metal parts in a solution containing strong alkalis and oxidizing agents, which causes an oxidation reaction that results in the formation of a magnetic layer of magnetite on the surface of the metal parts. This process is commonly used for low-carbon steel and low-carbon alloy tool steels. Due to factors related to the material and other conditions, the color of the blackening layer can range from blue-black to black, red-brown, or brownish-gray. Its thickness is typically between 0.6 and 0.8 µm. Blackening helps to prevent rusting, improves the appearance and luster of the metal surface, and reduces stresses resulting from the quenching process