The following is a brief summary of the 12 processes for the heat treatment process:
1. Annealing
Method of operation: After heating the steel to Ac3+30~50°C or Ac1+30~50°C or below Ac1 (refer to the relevant materials), it is usually cooled slowly with the furnace temperature.
Intention: 1. Reduce hardness, improve plasticity, improve cutting performance; 2. Refine grain, improve mechanical properties, prepare for the next step; 3. Eliminate internal stress generated by cold and hot processing.
Application key: 1. Applicable to alloy stainless steel, carbon structural steel, alloy structural steel, high speed steel forgings, welded parts and raw materials with unqualified supply conditions; 2. Usually annealing in the condition of the blank.
2, normalizing
Method of operation: Heat the steel to 30~50 °C above Ac3 or Accm, and then cool it at a cooling rate slightly larger than the annealing temperature.
Intention: 1. Reduce hardness, improve plasticity, improve cutting performance; 2. Refine grain, improve mechanical properties, prepare for the next step; 3. Eliminate internal stress generated by cold and hot processing.
Application key: Normalizing is usually used as a pre-heat treatment process for forgings, welded parts and carburized parts. For low-carbon and medium-carbon carbon stainless steels and low-alloy steels that are not in demand, they can also be used as final heat treatment. With regard to the usual medium and high alloy steels, air cooling can cause complete or partial quenching and thus cannot be used as the final heat treatment process.
3, quenching
Method of operation: The steel is heated to a phase transition temperature of Ac3 or Ac1, kept for a while, and then rapidly cooled in water, nitrate, oil, or air.
Intent: Quenching is usually used to obtain high-hardness martensite structure. Sometimes, when quenching certain high-alloy steels (such as stainless steel and wear-resistant steel), it is to obtain a single uniform austenite structure to improve wear resistance. And corrosion resistance.
Application key: 1. Commonly used for carbon steel and alloy steel with carbon content greater than 0.3%; 2. Quenching can fully exert the strength and wear resistance potential of steel, but it will produce great internal stress and reduce Steel plasticity and impact toughness, so tempering is required to obtain better mechanical properties.
4, tempering
Operation method: The quenched steel piece is heated from the head to a temperature below Ac1, and after being kept warm, it is cooled in air or oil, hot water and water.
Intentions: 1. Reduce or eliminate the internal stress after quenching to prevent deformation and cracking of the workpiece; 2. Adjust the hardness, improve the plasticity and durability, and obtain the mechanical properties required by the workpiece; 3. Stabilize the workpiece scale.
Key to use: 1. Use low temperature tempering to maintain high hardness and wear resistance of steel after quenching; use medium temperature tempering to improve steel elasticity and yield strength while maintaining certain toughness; to maintain high impact toughness Degree and plasticity are dominant, and high temperature tempering is used when there is satisfactory strength; 2. Normal steel should avoid tempering between 230~280°C and stainless steel between 400~450°C, because a temper brittleness will occur at this time.
5, tempering
Operation method: After quenching, high temperature tempering is called quenching and tempering, and the steel piece is heated to a temperature 10 to 20 degrees higher than quenching, quenched after heat preservation, and then tempered at a temperature of 400 to 720 °C.
Intention: 1. Improve the cutting performance, improve the smoothness of the processing appearance; 2. Reduce the deformation and cracking during quenching; 3. Obtain outstanding structural mechanical properties.
Application key: 1. Suitable for alloy stainless steel, alloy structural steel and high speed steel with high hardenability; 2. Not only can be used as the final heat treatment of various important processes, but also can be used as some precision parts, such as screw Pre-heat treatment to reduce deformation.
6, timeliness
Method of operation: Heat the steel to 80~200 °C, keep it for 5~20 hours or longer, then take it out in the air and cool it.
Intention: 1. Stabilize the microstructure of the steel after quenching, reduce the aging or long-term deformation; 2. Reduce the internal stress after quenching and grinding, and stabilize the shape and scale.
Application key: 1. Applicable to various steel grades after quenching; 2. Commonly used for precision workpieces where the shape of the demand does not change, such as precision lead screws, bed chassis, etc.
7, cold treatment
Method of operation: The quenched steel is cooled to -60 to -80 ° C or lower in a low temperature medium (such as dry ice, liquid nitrogen), and the temperature is uniform, and then the temperature is taken to room temperature.
Intention: 1. Converting the remaining austenite in the hardened steel to all or part of martensite, and then increasing the hardness, strength, wear resistance and fatigue limit of the steel; Stabilize the structure of the steel to maintain the shape and dimensions of the steel.
The key to use: 1. After quenching, the steel parts shall be cold treated and then tempered at low temperature to eliminate the internal stress during low temperature cooling; Cold treatment is primarily suitable for precision tools, gauges and precision parts made of alloy steel.
8, flame heating appearance quenching
Operation method: The flame incinerated with the oxygen-acetylene mixed gas is sprayed onto the outer surface of the steel piece, and is rapidly heated. When the quenching temperature is reached, the water is sprayed and cooled.
Intention: To improve the hardness, wear resistance and fatigue strength of the steel parts, the core still maintains a certain degree of toughness.
The key to use: 1. Mostly used for medium carbon steel parts, usually the depth of the hardened layer is 2 ~ 6mm; Suitable for large workpieces in single or small batch production and parts requiring partial quenching.
9, induction heating surface quenching
Operation method: The steel piece is placed in the inductor, the induced current is generated on the surface of the steel piece, heated to the quenching temperature in a very short time, and then sprayed and cooled.
Intention: Improve the hardness, wear resistance and fatigue strength of steel parts, and maintain a certain degree of toughness in the heart.
The key to use: 1. Mostly used for medium carbon steel and medium carbon alloy stainless steel parts; 2. Because of the stimuli effect, the high frequency induction hardening layer is usually 1 to 2 mm, the medium frequency quenching is usually 3 to 5 mm, and the high frequency quenching is usually greater than 10 mm.
10, carburizing
Operation method: The steel piece is placed in a carburizing medium, heated to 900-950 ° C and kept warm, so that the steel part can obtain a carburized layer of a certain concentration and depth.
Intention: Improve the hardness, wear resistance and fatigue strength of steel parts, and maintain a certain degree of toughness in the heart.
The key to use: 1. For low carbon steel and low alloy steel parts with carbon content of 0.15% to 0.25%, usually the carburized layer depth is 0.5 ~ 2.5mm; After carburizing, it is necessary to quench the surface to obtain the desired martensite to achieve the purpose of carburizing.
11, nitriding
Method of operation: The active nitrogen atom differentiated by ammonia gas at 500-600 ° C is used to make the steel piece rich in nitrogen atoms to form a nitride layer.
Intention: Improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.
Key to application: It is mainly used for medium carbon alloy stainless steel rich in alloying elements such as aluminum, chromium and molybdenum, as well as carbon steel and cast iron. Generally, the depth of the nitride layer is 0.025-0.8mm.
12. Nitrocarburon
Method of operation: carburizing and nitriding together to the exterior of the steel.
Intention: Improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.
Application key: (1) It is mostly used in low carbon steel, low alloy stainless steel and structural steel parts, usually nitriding layer depth 0.02 ~ 3mm; (2) quenching and low temperature tempering after nitriding.
