Introduction to Heat Treatment Processes

Procedure: Heat the steel part to a temperature 30–50 degrees above Ac3, or 30–50 degrees above Ac1, or below Ac1 (refer to relevant materials for specific temperatures). Then, typically allow it to cool slowly along with the furnace temperature.

1 Annealing

　　Operating procedure: Heat the steel part to... Ac3+30~50 degree or Ac1+30~50 degree or Ac1 After reaching the following temperatures (which can be found in the relevant materials), the material is typically allowed to cool slowly along with the furnace temperature.

　　Objective: 1. Reduce hardness, enhance ductility, and improve machinability and formability; 2. Refine the grain size and improve mechanical properties to prepare for the next processing step. 3. Eliminate internal stresses caused by cold and hot working.

　　Key application: 1. Suitable for forgings, welded components, and raw materials in non-conforming supply conditions of alloy structural steels, carbon tool steels, alloy tool steels, and high-speed steels; 2. Annealing is typically performed in the as-cast condition. 。

2 Normalizing

　　Operating procedure: Heat the steel part to... Ac3 or Accm The above 30~50 After annealing, cool at a rate slightly faster than the cooling rate used for annealing.

　　Objective: 1. Reduce hardness, enhance ductility, and improve machinability and formability; 2. Refine the grain size and improve mechanical properties to prepare for the next processing step. 3. Eliminate internal stresses caused by cold and hot working.

　　Key application: Normalizing is typically used as a preliminary heat treatment process for forgings, welded components, and carburized parts. For low-carbon and medium-carbon carbon structural steels and low-alloy steel parts with modest functional requirements, normalizing can also serve as the final heat treatment. Heat treatment For commonly used medium- and high-alloy steels, air cooling can lead to either complete or partial quenching; therefore, it cannot be used as the final heat treatment process.

3 Quenching

　　Operating procedure: Heat the steel part to the phase transformation temperature. Ac3 or Ac1 Above, allow to sit for a period of time to retain heat, then rapidly cool in water, nitrate salt, oil, or air.

　　Objective: Quenching is typically performed to obtain a martensitic structure with high hardness. However, for certain high-alloy steels (such as stainless steel and wear-resistant steel), quenching is sometimes carried out to produce a single, uniform austenitic structure, thereby enhancing wear resistance and corrosion resistance.

　　Key application: 1. Typically used for carbon steels and alloy steels with a carbon content greater than 0.3 percent; 2. Quenching can fully exploit the strength and wear resistance potential of steel; however, it also introduces significant internal stresses, reducing the steel's ductility and impact toughness. Therefore, tempering is necessary to achieve better overall mechanical properties.

4 Tempering

　　Operating procedure: Heat the quenched steel part from the beginning to... Ac1 At a certain temperature below, after being held at that temperature, cool it in air or in oil, hot water, or water.

　　Objective: 1. Reduce or eliminate internal stresses after quenching, thereby minimizing workpiece deformation and cracking. 2. Adjust the hardness, improve ductility and toughness, and achieve the mechanical properties required for the job; 3. Stable workpiece dimensions.

　　Key application: 1. For steels that require high hardness and wear resistance after quenching, use low-temperature tempering. When aiming to enhance the elasticity and yield strength of steel while maintaining a certain level of toughness, use medium-temperature tempering. If the primary goal is to achieve high impact toughness and ductility along with sufficient strength, use high-temperature tempering. 2. Steel is usually kept away as much as possible from 230~280 Degree, stainless steel in 400~450 Temper between these ranges, because temper embrittlement occurs at this point.

5 Tempering

　　Method of operation: After quenching, high-temperature tempering is referred to as "tempering." This involves heating the steel part to a temperature higher than that used during quenching. 10~20 at the specified temperature, followed by quenching after insulation, and then... 400~720 Temper at the specified temperature.

　　Objective: 1. Improve cutting performance and enhance the surface finish of machined parts. 2. Reduce deformation and cracking during quenching; 3. Achieve outstanding comprehensive mechanical performance.

　　Key application: 1. Suitable for alloy structural steels with high hardenability, alloy tool steels, and high-speed steels; 2. It can not only serve as the final heat treatment for various critical components, but also function as a preliminary heat treatment for certain precision parts, such as lead screws, to minimize deformation.

6 , timeliness

　　Operating procedure: Heat the steel part to... 80~200 Degree, heat preservation 5~20 For hours or longer, then remove from the furnace and allow to cool in air.

　　Objective: 1. The microstructure of stabilized steel parts after quenching, which reduces deformation during storage or use; 2. Relieve internal stresses after quenching and grinding, and stabilize shape and dimensions.

　　Key application: 1. Suitable for all types of steels after quenching; 2. Commonly used for precision parts whose shapes no longer change, such as precision lead screws, measuring instruments, machine bed housings, and the like.

7 Cold treatment

　　Operating procedure: After quenching, cool the steel part in a low-temperature medium (such as dry ice or liquid nitrogen) down to – 60 ~－ 80 At or below the specified temperature, remove the item once the temperature is uniformly stabilized and allow it to cool to room temperature.

　　Objective: 1 To completely or largely transform the residual austenite within quenched steel parts into martensite, thereby enhancing the steel parts' hardness, strength, wear resistance, and fatigue limit. 2 . Microstructure of stable steel To maintain the shape and dimensions of the steel parts.

　　Key application: 1 After quenching, steel parts should immediately undergo cryogenic treatment and then be subjected to low-temperature tempering to relieve internal stresses induced during the low-temperature cooling process. 2 Cold treatment is primarily suitable for precision cutting tools, measuring instruments, and precision parts made of alloy steel.

8 Flame Heating Surface Hardening

　　Operating procedure: A flame produced by burning an oxy-acetylene mixed gas is sprayed onto the surface of the steel part, rapidly heating it. Once the quenching temperature is reached, water is immediately sprayed for cooling.

　　Objective: To improve the surface hardness, wear resistance, and fatigue strength of steel parts while maintaining toughness in the core.

　　Key application: 1 ．Primarily used for parts made of medium-carbon steel, the typical hardening depth is: 2 ~ 6mm；2 Suitable for large workpieces produced in single pieces or small batches, as well as for workpieces requiring partial quenching.

9 Induction heating surface quenching

　　Operating procedure: Place the steel part into the induction coil, inducing eddy currents on the surface of the steel part. Heat it to the quenching temperature within an extremely short period of time, then cool it by spraying water.

　　Objective: To improve the surface hardness, wear resistance, and fatigue strength of steel parts while maintaining toughness in the core.

　　Key application: 1 ．Primarily used for parts made of medium-carbon steel and medium-alloy structural steel; 2 . Due to the skin effect, the hardening layer in high-frequency induction quenching is typically: 1 ~ 2mm , medium-frequency quenching is typically... 3 ~ 5mm , high-frequency quenching is usually greater than 10mm。

10 Carburizing

　　Operating procedure: Place the steel part into the carburizing medium and heat it to... 900 ~ 950 Carburize and hold at temperature to ensure that the steel part’s surface achieves a carburized layer of a certain concentration and depth.

　　Objective: To improve the surface hardness, wear resistance, and fatigue strength of steel parts while maintaining toughness in the core.

　　Key application: 1 . Used for carbon content of 0.15 %~ 0.25 For parts made of low-carbon steel and low-alloy steel, the typical case-hardened layer depth is: 0.5 ~ 2.5mm；2 After carburizing, it is necessary to perform quenching to obtain martensite on the surface, thereby achieving the purpose of carburizing.

11 Nitriding

　　Operating method: Utilize in 500 ~ 600 The active nitrogen atoms released during the ammonia decomposition process saturate the steel surface, forming a nitrided layer.

　　Objective: To improve the hardness, wear resistance, fatigue strength, and corrosion resistance of steel surfaces.

　　Key application: Primarily used for medium-carbon alloy structural steels rich in alloying elements such as aluminum, chromium, and molybdenum, as well as carbon steels and cast irons; typically, the nitrided layer depth is... 0.025 ~ 0.8mm .

12 Nitrocarburizing

　　Method of operation: Simultaneously diffuse carbon and nitrogen atoms into the surface of the steel component.

　　Objective: To improve the hardness, wear resistance, fatigue strength, and corrosion resistance of steel surfaces.

　　Key application: 1 It is commonly used for parts made of low-carbon steel, low-alloy structural steel, and tool steel, typically resulting in a deep nitrided layer. 0.02 ~ 3mm；2. After nitriding, it also requires quenching and low-temperature tempering.