Heat treatment of Heat-resistant Steel Castings

Applicable scope of heat treatment process

The heat treatment process is widely used in various metal heavy industry fields of life. For example, parts in the automotive industry, metal products in the medical industry.
Introduction of heat treatment of heat-resistant steel castings
The heat treatment of heat-resistant steel castings refers to a process that heats, heats and cools steel in a solid state to change its internal structure and obtain the required properties. The purpose of heat treatment is to significantly improve the thermal properties of steel, play the potential of steel, and improve the performance and life of the workpiece.
Method of heat treatment process

Heat-resistant Steel Castings
1. Annealing
Operation method: After heating the steel to Ac3+30~50°C or Ac1+30~50°C or below Ac1 temperature (you can consult relevant information), it is generally cooled slowly with the furnace temperature.
purpose:
1. Reduce hardness, improve plasticity, improve cutting and pressure processing performance;
2. Refine grains, improve mechanical properties, and prepare for the next step;
3. Eliminate internal stress caused by cold and hot processing.
Application points:
1. Suitable for forgings and welding parts of alloy structural steel, carbon tool steel, alloy tool steel, high-speed steel, and raw materials whose supply status is unqualified;
2. Generally, annealing is carried out in the rough state.
2. Normalizing
Operation method: heat the steel to 30~50 degrees above Ac3 or Accm, and cool it at a cooling rate slightly greater than that of annealing after heat preservation.
purpose:
1. Reduce hardness, improve plasticity, improve cutting and pressure processing performance;
2. Refine grains, improve mechanical properties, and prepare for the next step;
3. Eliminate internal stress caused by cold and hot processing.
Application points: Normalizing is usually used as a pre-heat treatment process for forgings, weldments 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 a heat treatment. For general medium and high alloy steels, air cooling can lead to complete or partial quenching, so it cannot be used as a post heat treatment process.

Heat-resistant Steel Castings
3. Quenching
Operation method: Heat the steel to the phase transition temperature Ac3 or Ac1 or above, keep it for a period of time, and then quickly cool it in water, nitrate, oil, or air.
Purpose: Quenching is generally to obtain a high-hardness martensite structure, and sometimes when quenching some high-alloy steels (such as stainless steel, wear-resistant steel), it is to obtain a single uniform austenite structure to improve wear resistance And corrosion resistance.
Application points:
1. Generally used for carbon steel and alloy steel with carbon content greater than 0.3%;
2. Quenching can give full play to the strength and wear resistance potential of steel, but at the same time it will cause great internal stress and reduce the plasticity and impact toughness of steel. Therefore, tempering is required to obtain better comprehensive mechanical properties.
4. Tempering
Operation method: reheat the quenched steel to a temperature below Ac1, and cool it in air, 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 mechanical properties required by the work;
3. Stable workpiece size.
Application points:
1. Use low temperature tempering to maintain the high hardness and wear resistance of steel after quenching; use medium temperature tempering to increase the elasticity and yield strength of steel while maintaining a certain toughness; to maintain high impact toughness and plasticity Mainly, when there is enough strength, use high temperature tempering;
2. For general steel, try to avoid tempering between 230 and 280 degrees, and stainless steel between 400 and 450 degrees, because this will produce a temper brittleness.

Heat-resistant Steel Castings
5. Tempering
Operation method: high temperature tempering after quenching is called quenching and tempering, that is, the steel parts are heated to a temperature 10-20 degrees higher than that during quenching, quenched after heat preservation, and then tempered at a temperature of 400-720 degrees.
purpose:
1. Improve the cutting performance and improve the smoothness of the processed surface;
2. Reduce deformation and cracking during quenching;
3. Obtain good comprehensive mechanical properties.
Application points:
1. Suitable for alloy structural steel, alloy tool steel and high-speed steel with high hardenability;
2. Not only can be used as a heat treatment for various important structures, but also as a pre-heat treatment for some compact parts, such as lead screws, to reduce deformation.
6. Aging
Operation method: Heat the steel to 80-200 degrees, keep it for 5-20 hours or longer, then take it out of the furnace and cool it in the air.
purpose:
1. Stabilize the structure of the steel 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.
Application points:
1. Applicable to various steel grades after quenching;
2. It is often used for compact workpieces that require no change in shape, such as compact lead screws, measuring tools, and bed cases.

Heat-resistant Steel Castings
7. Cold treatment
Operation method: Cool the quenched steel parts in a low-temperature medium (such as dry ice, liquid nitrogen) to -60 to -80 degrees or lower. After the temperature is uniform, take it out to room temperature.
purpose:
1. Convert all or most of the retained austenite in the quenched steel parts to martensite, thereby improving the hardness, strength, wear resistance and fatigue limit of the steel parts;
2. Stabilize the structure of steel to stabilize the shape and size of steel parts.
Application points:
1. Steel parts should be cold treated immediately after quenching, and then tempered at low temperature to eliminate internal stress during low temperature cooling;
2. Cold treatment is mainly suitable for compact cutting tools, measuring tools and compact parts made of alloy steel.
8. Flame heating surface hardening
Operation method: Use the flame of oxygen-acetylene mixed gas to spray on the surface of the steel, and heat it quickly. When the quenching temperature is reached, spray water to cool.
Purpose: To improve the surface hardness, wear resistance and fatigue strength of steel parts, while the core remains tough.
Application points:
1. Mostly used for medium carbon steel parts, generally the depth of hardening layer is 2~6mm;
2. It is suitable for single-piece or small batch production of large workpieces and workpieces that require partial quenching.

Heat-resistant Steel Castings
9. Induction heating surface hardening
Operation method: Put the steel part into the inductor to induce current on the surface of the steel part, heat it to the quenching temperature in a very short time, and then spray water to cool it.
Purpose: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and maintain toughness in the core.
Application points:
1. Mostly used for medium carbon steel and Zhongtang alloy structural steel parts;
2. Due to the skin effect, the high-frequency induction hardening layer is generally 1-2mm, the medium-frequency quenching is generally 3-5mm, and the high-frequency quenching is generally greater than 10mm.
10. carburization
Operation method: Put the steel part into the carburizing medium, heat it to 900-950 degrees and keep it warm, so that the steel part can obtain a certain concentration and depth of carburized layer.
Purpose: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and the core part still maintains toughness.
Application points:
1. Used for low-carbon steel and low-alloy steel parts with a carbon content of 0.15% to 0.25%, generally the depth of the carburized layer is 0.5 to 2.5 mm;
2. After carburizing, quenching must be performed to obtain martensite on the surface to achieve the purpose of carburizing.

Heat-resistant Steel Castings
11. Nitriding
Operation method: use in 5. . The active nitrogen atoms decomposed by ammonia at ~600 degrees make the surface of the steel part saturated with nitrogen and form a nitride layer.
Purpose: To improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.
Application points: It is mostly used for medium-carbon alloy structural steels containing aluminum, chromium, molybdenum and other alloying elements, as well as carbon steel and cast iron. Generally, the depth of the nitrided layer is 0.025~0.8mm.

 

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