Date:2019-06-17 View:4584 Tags:Steel pipe knowledge---heat treatment process
Heat treatment process
Annealing
Method of operation: After heating the steel to Ac3+30~50 degrees or Ac1+30~50 degrees or below Ac1 (you can refer to the relevant information), it will generally cool slowly with the furnace temperature.
Objectives: 1. Reduce hardness, improve plasticity, improve cutting and pressure processing performance; 2. Refine grain, improve mechanical properties, prepare for the next step; 3. Eliminate internal stress generated by cold and hot processing.
Application points: 1. Applicable to alloy structural steel, carbon tool steel, alloy tool steel, high-speed steel forgings, welded parts and raw materials with unsatisfactory supply conditions; 2. Generally annealing in the blank state.
2. Normalizing
Method of operation: The steel is heated to 30 to 50 degrees above Ac3 or Accm, and then cooled to a temperature slightly larger than the annealing rate.
Objectives: 1. Reduce hardness, improve plasticity, improve cutting and pressure processing performance; 2. Refine grain, improve mechanical properties, prepare for the next step; 3. Eliminate internal stress generated by cold and hot processing.
Application points: Normalizing is usually used as a pre-heat treatment process for forgings, welded parts and carburized parts. For low-carbon and medium-carbon carbon structural steels and low-alloy steels with low performance requirements, they can also be used as final heat treatment. For general medium and high alloy steels, air cooling can lead to complete or partial quenching and therefore 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, held for a period of time, and then rapidly cooled in water, nitrate, oil, or air.
Purpose: Quenching is generally used to obtain high-hardness martensite structure. Sometimes, when quenching some 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 points: 1. Generally 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 at the same time it will cause great internal stress. To reduce the plasticity and impact toughness of steel, it is necessary to temper to obtain better comprehensive mechanical properties.
4. tempering
Operation method: reheat the quenched steel to a temperature below Ac1, and after cooling, cool in air or oil, hot water and water.
Objective: 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 for the work; 3. Stabilize the workpiece size.
Application points: 1. Keep the steel high temperature and wear resistance after quenching with low temperature tempering; use medium temperature tempering to improve the elasticity and yield strength of steel under the condition of maintaining certain toughness; to maintain high impact toughness Degree and plasticity, and high temperature tempering when there is sufficient strength; 2. General steel should avoid tempering between 230~280 degrees and stainless steel between 400~450 degrees, because there will be a temper brittleness.
5. Tempering
Operation method: high-temperature tempering after quenching is called quenching and tempering, that is, the steel piece is heated to a temperature 10 to 20 degrees higher than that during quenching, quenched after heat preservation, and then tempered at a temperature of 400 to 720 degrees.
Objective: 1. Improve the cutting performance and improve the smoothness of the machined surface; 2. Reduce the 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 the final heat treatment of various important structures, but also as some compact parts, such as lead screw Pre-heat treatment, etc., to reduce deformation.
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6. Timeliness
Method of operation: Heat the steel to 80~200 degrees, keep it for 5~20 hours or longer, and then take it out in the air and cool it.
Purpose: 1. Stabilize the microstructure of the steel after quenching, 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. Suitable for quenched steel grades; 2. Commonly used for tight workpieces that require no change in shape, such as tight screws, measuring tools, bed chassis, etc.
7. Cold treatment
Operation method: The quenched steel piece is cooled to -60 to -80 degrees 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.
purpose:
1. Reducing all or most of the retained austenite in the hardened steel to martensite, thereby increasing 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.
Application points:
1. After quenching, the steel parts shall be cold-treated immediately and then tempered at low temperature to eliminate internal stress during low-temperature cooling;
2. Cold treatment is mainly applied to compact tools, gauges and compact parts made of alloy steel.
8. Flame heating surface quenching
Method of operation: a flame that is burned with an oxygen-acetylene mixed gas is sprayed onto the surface of the steel member, heated rapidly, and sprayed to cool immediately after reaching the quenching temperature.
Objective: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and the core still maintains a tough state.
Application points:
1. Mostly used for medium carbon steel parts, the depth of the hardened layer is generally 2 ~ 6mm;
2. Suitable for large workpieces in single or small batch production and workpieces requiring partial quenching.
9. Induction heating surface quenching
Operation method: The steel piece is placed in the inductor, so that the surface of the steel piece generates an induced current, which is heated to the quenching temperature in a very short time, and then sprayed and cooled.
Objective: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and maintain the toughness of the heart.
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 quenching and hardening layer is generally 1 to 2 mm, the intermediate frequency quenching is generally 3 to 5 mm, and the high frequency quenching is generally greater than 10 mm.
10. carburization
Operation method: the steel piece is placed in a carburizing medium, heated to 900-950 degrees and insulated, so that the steel part can obtain a carburizing layer of a certain concentration and depth.
Objective: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and the core still maintains a tough state.
Application points:
1. For low carbon steel and low alloy steel parts with a carbon content of 0.15% to 0.25%, the depth of the carburized layer is generally 0.5 to 2.5 mm;
2. After carburizing, quenching must be carried out to obtain martensite on the surface to achieve the purpose of carburizing.
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11. Nitriding
Method of operation: use at 5. . The active nitrogen atom decomposed by ammonia gas at ~600 degrees saturates the surface of the steel to form a nitride layer.
Purpose: To improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.
Application points: mostly used for medium carbon alloy structural steel containing aluminum, chromium, molybdenum and other alloying elements, as well as carbon steel and cast iron, the general nitride layer depth is 0.025 ~ 0.8mm.
12. Nitrocarburizing
Method of operation: simultaneous carburizing and nitriding to the surface of the steel.
Purpose: To improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.
Application points:
1. Mostly used for low carbon steel, low alloy structural steel and tool steel parts, generally nitriding layer depth 0.02 ~ 3mm;
2. Quenching and low temperature tempering after nitriding.
Example 1: Heat treatment scheme for several common steels
Q345: The steel is steel for construction, bridges, etc., and the yield strength is about 345 MPa. It is generally normalized and the carbon content is about 0.12-0.2%.
20Cr: The carbon content is about 0.2%, which can be used as construction steel, which can be used for normalizing. It can also be used as carburized steel, carburizing and quenching.
GCr15: Bearing steel, carbon content of about 1%, quenching temperature is generally 860C, low temperature tempering around 180C.
60Si2Mn: spring steel, carbon content of about 0.6%, quenching temperature of 860C, medium temperature tempering (about 450C).
0Cr18Ni9Ti: austenitic stainless steel, carbon content is less than 0.06%, generally used around 1050C solution treatment (ie heating water cooling), improve corrosion resistance.
Example 2: 45#, 42CrMo, 20CrMo how to achieve 45HRC hardness requirements
45# steel and 42CrMo can be directly quenched + tempered to meet the 45HRC hardness requirements;
20CrMo can also meet the 45HRC hardness requirements after carburizing and quenching + tempering.
Gears made with 40CrMo.45# are normally normalized [or tempered] with surface hardening. 20CrMo is carburized and quenched. It not only meets the hardness requirements. This type of parts requires a certain contact fatigue strength.
Example 3: Heat treatment process parameters of GCr15 steel
GCr15 steel is a high carbon chromium bearing steel with less alloy content, good performance and wide application. After quenching and tempering, it has high and uniform hardness, good wear resistance and high contact fatigue performance. The cold working of the steel is medium in plasticity, the cutting performance is general, the welding performance is poor, the sensitivity to white spots is large, and the temper brittleness is obtained.
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