1. Annealing
Operation method:
The steel is heated to a temperature of Ac3 + 30 – 50 degrees, Ac1 + 30 – 50 degrees, or below Ac1 (as recommended by the relevant materials) and then is slowly cooled inside the furnace.
Goals.
- To reduce hardness and improve plasticity, cutting and pressure processing capabilities.
- To refine grain structure and improve mechanical properties, as well as prepare for subsequent steps.
- To relieve internal stresses that occur during cold and hot processing.
Main applications:
(1) This method is suitable for treating structured alloy steel, east-west carbon steel, east-west alloy steel, high-speed steel forgings, welding components and raw materials with suboptimal supply conditions.
(2) Typically, this process is used under raw conditions and is called “annealing”.
2. Normalizing
Operation method:
To perform normalization, heat the steel to a temperature of 30 to 50 degrees above Ac3 or Accm and, after soaking, cool it at a rate slightly faster than during annealing.
Goals.
The purpose of normalization is to reduce hardness, improve plasticity, and enhance cutting and pressure processing capabilities. It also helps refine grain structure, improve mechanical properties and prepare the material for further processing. Normalization also helps eliminate internal stresses that may have been caused by cold or hot working.
Main applications:
Normalizing is commonly used as a pretreatment process for forged, welded, and carburized parts. For low to medium carbon steels and low alloy steel components with low functional requirements, normalization can be carried out as a final heat treatment process. However, for common medium and high alloy steels, air cooling may result in full or partial hardening, so it cannot be used as a final heat treatment process.
3. Tempera
Operation method:
The steel part is heated to a temperature above the Ac3 or Ac1 phase transition temperature, held for a specified period, and then rapidly cooled in water, nitrate, oil, or air.
Goals.
Quenching is normally carried out to obtain a martensitic structure with high hardness.
In some cases, quenching of high-alloy steel (such as stainless steel or wear-resistant steel) is done to obtain a unique and uniform austenitic structure to increase wear and corrosion resistance.
Main applications:
(1) Generally applied to carbon steel and alloy steel with carbon content greater than 0.3%.
(2) Quenching maximizes the strength and abrasion resistance of the steel, but also results in high internal stress that reduces the plasticity and impact resistance of the steel.
Therefore, tempering is necessary to obtain better mechanical properties.
4. Temperament
Operation method:
Hardened steel parts are heated to a temperature below Ac1, held for a period of time, and then cooled in hot air, oil, or water.
Goals.
To reduce or eliminate internal stress after quenching, minimize workpiece deformation and cracking.
To adjust hardness, improve plasticity and toughness, and achieve the mechanical properties required for the application.
To stabilize the size of the workpiece.
Main applications:
(1) Low temperature tempering is used when high hardness and wear resistance are desired in quenched steel.
(2) Medium temperature tempering is used to improve the elasticity and yield strength of steel while maintaining a certain degree of toughness.
(3) High-temperature tempering is used to prioritize high-impact strength and plasticity, and is used when there is sufficient strength.
It is generally advisable to avoid tempering between 230-280 degrees for steel and 400-450 degrees for stainless steel, as this range can cause temper brittleness.
5. Quenching and tempering
Operation procedure:
The process of heating steel to a temperature 10-20 degrees higher than during quenching, after which quenching is carried out, is called quenching and tempering.
After being held at high temperature, the steel is quenched and then tempered at a temperature range of 400-720 degrees.
Goals.
- To increase cutting capacity and improve the finish of the processed item.
- To minimize deformations and cracks during the tempering process.
- To achieve exceptional mechanical properties through induction.
Main applications:
This process is suitable for high hardenability alloys such as tool steel alloys, high-speed alloy steels and structural steel alloys.
It can serve as final heat treatment for critical components and also as pre-heat treatment for tight parts such as screws to reduce deformation during processing.
6. Aging
Operation procedure:
The steel is heated to a temperature range of 80 to 200 degrees and held at that temperature for 5 to 20 hours or more. After that, it is removed from the oven and cooled in air.
Goals.
- To relieve internal stress after quenching and grinding and stabilize the shape and size of steel parts.
- To minimize deformation during storage or use.
Main applications:
This process is suitable for all types of steel after quenching.
It is commonly used for tight components whose shape does not change, such as tight screws, measuring instruments, bed frames, etc.
7. Cold treatment
Operation procedure:
Hardened steel components are cooled in a low-temperature medium, such as dry ice or liquid nitrogen, to a temperature of -60 to -80 degrees or lower. The temperature is then removed evenly and the parts are allowed to reach room temperature.
Goals.
- Converting most or all of the remaining austenite in the hardened steel component to martensite, thereby improving the hardness, strength, wear resistance and fatigue limit of the component.
- Stabilize the shape and size of steel components by arranging the steel structure.
Main applications:
Steel components must undergo cold treatment immediately after quenching and then be tempered at low temperature to eliminate internal stresses during low-temperature cooling.
Cold treatment is mainly suitable for watertight tools, measuring tools and watertight components made of alloy steel.
8. Flame-heated surface quenching
Operation procedure:
A flame produced by a mixture of oxygen and acetylene gas is directed to the surface of the steel component, quickly heating it. When the desired quenching temperature is reached, the steel is immediately cooled by spraying with water.
Goals.
To improve the hardness, wear resistance and fatigue resistance of the steel component while maintaining its toughness.
Main applications:
- This process is mainly used for medium carbon steel components, and the depth of the hardened layer typically ranges from 2 to 6 mm.
- It is suitable for single-piece or small-batch production of large components and components that require partial hardening.
9. Induction heating surface quenching
Operation procedure:
The steel components are placed in an inductor, where the surface of the components is subjected to an electrical current. The steel is heated to the desired quenching temperature in a very short period of time and then cooled by spraying with water.
Goals.
To increase the hardness, wear resistance and fatigue resistance of steel components while maintaining their toughness.
Main applications:
This process is mainly used for medium carbon steel and medium alloy steel components.
The depth of the induction hardened layer depends on the frequency of the electrical current used: high frequency induction hardening typically results in a layer 1 to 2 mm deep, intermediate frequency hardening typically results in a layer 3 to 5 mm deep, and high-frequency hardening typically results in a layer greater than 10 mm deep. This is due to the “skin effect”, where the electrical current is concentrated in the outermost layer of the component.
10. Carburization
Operation method:
Place the steel parts in a carburizing medium, heat it to a temperature between 900-950 degrees and keep it there. This allows the surface of the steel parts to form a carburizing layer of specific concentration and depth.
Goals.
To improve the external hardness, wear resistance and fatigue resistance of steel parts while maintaining their strength.
Main applications:
(1) This method is mainly used for low-carbon steel and low-alloy steel parts with carbon content ranging from 0.15% to 0.25%. The depth of the carburized layer is normally between 0.5 mm and 2.5 mm.
(2) After carburizing, it is necessary to undergo quenching to reach the martensite on the surface and complete the carburizing process.
11. Nitriding
Operation method:
The surface of the steel is saturated with nitrogen through the use of active nitrogen atoms separated by ammonia gas at temperatures between 500-600 degrees.
Goals.
The hardness, wear resistance, fatigue resistance and corrosion resistance of steel parts are improved.
Main applications:
This method is mainly used for medium-carbon alloy steels rich in alloying elements such as aluminum, chromium, molybdenum, carbon steel and cast iron. The depth of the nitrided layer is normally between 0.025 and 0.8 mm.
12. Nitrocarburizing
Operation method:
The steel surface is treated through a combination of carburizing and nitriding.
Goals.
To increase the hardness, wear resistance, fatigue resistance and corrosion resistance of steel parts.
Main applications:
(1) Mainly used for low-carbon steel, low-alloy structured steel and cast steel parts, with a typical nitriding layer depth of 0.02 to 3 mm;
(2) After nitriding, low temperature quenching and tempering are required.
