Heat treatment of metallic materials
Heat treatment is the process of heating the metal to a certain temperature, staying at this temperature for a period of time, and then cooling to a certain temperature. The process of heat treatment generally goes through three stages: heating, insulation and cooling.
Although heat treatment can not change the shape and size of metal parts, but can change the internal organization of the metal, so as to improve the performance of the metal, improve product quality, extend the service life. The commonly used types of heat treatment are shown in Table 1-31.
Table 1-31 Common types of heat treatment
| Type | operation Process | Purpose and application description |
| anneal | The steel parts are heated to a temperature below Ac3+(30~50)℃ or 4c+(30~50)℃ or Ac1, and are generally cooled slowly with the furnace after thorough burning and heat preservation | Purpose: ① Reduce hardness, improve plasticity, improve cutting and pressure processing performance ② Fine grain, improve mechanical properties, prepare for the next step ③ Eliminate the internal stress caused by hot and cold processing Applications: ① Live for alloy structural steel, carbon tool steel, alloy tool steel, high-speed steel and other silver parts, welding parts and supply of unqualified raw materials ② Annealing is generally carried out in the wool return state |
| normalizing | The steel parts are heated to 30~50℃ above Ac3 or ACcm, and the cooling rate is slightly greater than that of annealing after insulation | Purpose: The purpose of normalizing is similar to annealingApplications: Normalizing is usually used as a pre-heat treatment process for forgings, welds and carburized parts. For low carbon and medium carbon carbon structural steel and low alloy steel parts with low performance requirements, it can also be used as the final heat treatment. For general medium and high alloy steel, air cooling can lead to complete or local quenching, so it cannot be used as the final heat treatment process |
| quench | The steel is heated to a phase change temperature of Ac3 or Ac1, held for a certain time, and then quickly cooled in water, nitrate, oil or air | Purpose: Quenching is generally to obtain a high hardness of martensitic structure, and sometimes for some high alloy steels (such as stainless steel, wear-resistant steel) quenching, it is to obtain a single uniform austenitic structure to improve its corrosion resistance and wear resistance Applications: ① It is generally used for carbon steel and alloy steel with carbon content greater than 0.30% ②Quenching can give full play to the strength and corrosion resistance potential of steel, but at the same time it will cause a lot of internal stress, reduce the plasticity and impact toughness of steel, so it is necessary to temper in order to obtain better comprehensive mechanical properties |
| temper | The quenched steel is reheated to a temperature below AC1, and then cooled in air or oil, hot water or water after heat preservation | Purpose: ① Reduce or eliminate the internal stress after quenching, reduce the deformation and cracking of the workpiece ② Adjust the hardness, improve the plasticity and toughness, and obtain the required mechanical properties ③ Stabilize the workpiece size Applications: ① Low temperature tempering is used to maintain the high hardness and wear resistance of the steel after quenching; Moderate temperature tempering is used to improve the elasticity and yield strength under the condition of maintaining a certain toughness; To maintain high impact toughness and plasticity, but also have enough strength with high temperature tempering ② General steel try to avoid tempering between 230~280 ° C, stainless steel between 400~450 ° C, because this will produce a tempering brittleness |
| Conditioning and tempering | High temperature tempering after quenching is called tempering, that is, the steel is heated to a temperature of 10 to 20 ° C higher than the quenching, the quenching after holding the heat, and then the tempering at a temperature of 400 to 720 ° C | Purpose: ① Improve the cutting performance and improve the smoothness of the machined surface ② Reduce the deformation and cracking during quenching ③ Good comprehensive mechanical properties are obtainedApplications: ① Suitable for alloy structural steel with high hardenability, alloy tool steel and high-speed steel ② It can not only be used as the final heat treatment of various more important structural parts, but also as the pre-heat treatment of some precision parts, such as lead screws, to reduce deformation |
| ageing | The steel parts are heated to 80~200℃, held for 5~20h or longer, and then taken out to cool in the air | Purpose: ① Stabilize the microstructure of steel parts after quenching and reduce the deformation during storage or use ② Reduce the internal stress after quenching and grinding, and stabilize the shape and size Applications: ① Suitable for all kinds of steel after quenching ② It is often used for precision workpieces requiring no deformation of shape, such as precision screw, measuring tools, bed, box, etc |
| cold treatment | Cool the quenched steel parts to 60~- 80 ℃ or lower in low temperature medium (such as dry ice and liquid nitrogen), and take them out to room temperature after the temperature is uniform | Purpose: ① All or most of the residual austenite in the hardened steel is transformed into martensite, thereby improving the hardness, strength, wear resistance and fatigue limit of the steel ② Stabilize the structure of the steel to stabilize the shape and size of the steel parts Applications: ① Steel parts should be cold treated immediately after quenching, and then tempered at low temperature to eliminate the internal stress during low temperature cooling ② Cold treatment is mainly suitable for precision cutting tools, measuring tools and precision parts made of alloy steel |
| Flame heating surface quenching | The flame of oxygen-acetylene gas mixture combustion is sprayed onto the surface of the steel, rapidly heated, and immediately sprayed with water to cool when the quenching temperature is reached | Purpose: Improve the surface hardness, wear resistance and fatigue strength of steel parts, and the heart remains ductile Applications: ① It is mostly used in the production of medium carbon steel, and the depth of the quenched layer is generally 2~6mm It is suitable for large workpieces produced in a single piece or small batch and workpieces requiring local quenching |
| Induction heating surface hardening | The steel is placed in the sensor, so that the surface of the steel generates an induced current, which is heated to the quenching temperature in a very short time, and then immediately sprayed with water to cool | Objective: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and to maintain the toughness of the heart. ① Mostly used in medium carbon steel and medium carbon alloy structural steel parts ② Due to the skin effect, the high-frequency induction heating quenching layer is generally 1~2mm, the mid-frequency induction heating quenching is generally 3~5mm, and the low-frequency induction heating quenching is generally greater than 10mm |
| carburizing | The steel parts are put into the carburizing medium, heated to 900~950℃ and heat preservation, so that the surface of the steel parts to obtain a certain concentration and depth of carburizing layer | Purpose: Improve the surface hardness, wear resistance and fatigue strength of steel parts, and the heart remains ductile Applications: ① It is mostly used for low carbon steel and low alloy steel with carbon content of 0.15% to 0.25%. General carburizing layer depth 0.5~2.5mm ②After carburizing, the surface must be quenched to obtain martensite in order to achieve the purpose of carburizing |
| ammoniating | The active nitrogen atoms decomposed by ammonia at 500~600℃ are used to saturate the surface of the steel with nitrogen and form a nitriding layer | Purpose: Improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel surface Applications: It is mostly used in medium carbon alloy structural steel containing aluminum, chromium, molybdenum and other alloying elements, as well as carbon steel and cast iron. Generally, the depth of the nitriding layer is 0.025~0.8mm |
| nitrocarburizing | Carburizing and nitriding the steel surface simultaneously | Purpose: Improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel surface Applications: ① Mostly used in low carbon steel, low alloy structural steel and tool steel parts. Generally, the depth of nitriding layer is 0.02~3mm ② After nitriding, quenching and low temperature tempering are also required |
Note: Ac1, Ac3 and ACcm in the table refer to the heating critical point of the steel.
