Knowledge of steel pipe -- heat treatment process

- Mar 19, 2019-

1. annealing

      Operation method: heating steel parts to temperatures below Ac3+30~50 degrees or Ac1+30~50 degrees or less (Ac1) can be followed by slow cooling with the furnace temperature.

Objective: 1. reduce hardness, improve plasticity, improve cutting and press processing performance.

     2. refining grain and improving mechanical properties to prepare for the next step.

     3. eliminate the internal stress generated by cold and hot working.

Application points:

     1. it is suitable for alloy structural steel, carbon tool steel, alloy tool steel, forgings, welding parts of high-speed steel and raw materials with unqualified supply status.

     2. annealing is usually done in blank condition.

2. normalizing

Operation method: heat the steel parts to Ac3 or Accm above 30~50 degrees, and then cool down slightly after cooling.

Objective:

      1. reduce hardness, improve plasticity, improve cutting and press processing performance;

      2. refining grain and improving mechanical properties to prepare for the next step.

      3. eliminate the internal stress generated by cold and hot working.

      Application points: normalizing is usually used as a pre heat treatment process for forgings, weldments and carburized parts. Low carbon and medium carbon structural steels and low alloy steel parts with low performance requirements can also be used as final heat treatment. For medium and high alloy steels, air cooling can cause complete or partial quenching, so it can not be used as final heat treatment process.

 3. quenching

      Operation method: heat the steel parts to Ac3 or Ac1 above the phase change temperature for a period of time and then cool down quickly in water, nitrate, oil or air.

Purpose: quenching is generally aimed at obtaining martensite structure with high hardness, and sometimes for some high alloy steels, such as stainless steel and wear resistant steel, it is quenched to obtain a single uniform austenitic structure to improve wear resistance and corrosion resistance.

Application points:

      1. generally used for carbon and alloy steels with carbon content greater than zero point three percent;

      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 should be carried out to get better comprehensive mechanical properties. 

4. tempering

Operation method: reheat the hardened steel parts to a certain temperature below Ac1, and then cool down in air or oil, hot water and water after heat preservation.

Objective:

     1. reduce or eliminate internal stress after quenching, reduce workpiece deformation and cracking.

     2. adjusting hardness, improving plasticity and toughness, obtaining the mechanical properties required by the work.

     3. stabilize workpiece size.

Application points:

       1. keep the steel in low temperature tempering after high hardness and wear resistance after quenching. When the toughness and yield strength of steel are increased under certain toughness, tempering is tempered at medium temperature to maintain high impact toughness and plasticity, and to temper with high strength when tempered at high temperature.

       2. as far as possible, general steel should avoid tempering at 230~280 degrees and stainless steel at 400~450 degrees, because there will be a temper brittleness at this time. 

5. quenching and tempering

     Operation method: quenching and tempering is called quenching and tempering. The steel will be heated to a temperature of 10~20 degrees higher than that of quenching. After quenching, the steel will be tempered at 400~720 degree.

Objective:

    1. improve the machinability and improve the smoothness of the machined surface.

    2. reduce deformation and cracking during quenching.

    3. get good comprehensive mechanical properties.

Application points:

    1. it is suitable for alloy structural steel, alloy tool steel and high speed steel with high hardenability.

    2. not only can be used as the last heat treatment of more important structures, but also can be used as pre heat treatment of some tight parts, such as lead screw, to reduce deformation.

6. aging

     Operation method: heating steel parts to 80~200 degrees, holding for 5~20 hours or longer, then cooling with air in the air.

Objective:

     1. stabilizing steel structure after quenching, reducing deformation during storage or use.

     2. reduce the internal stress after quenching and grinding, stabilize the shape and size.

Application points:

    1. suitable for all kinds of steel after quenching.

    2., it is commonly used for tight workpieces that require no change in shape, such as tight screws, measuring tools, bed case and so on.

7. cold treatment

       Operation method: cooling the hardened steel parts in low temperature medium (such as dry ice and liquid nitrogen) to 60 to 80 degrees or lower. After uniform temperature, remove the uniform temperature to room temperature.

Objective:

      1. convert all or most of the retained austenite into martensite in quenched steel, thereby improving 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:

      After quenching, 1. steel parts should be treated immediately after cold treatment and then tempered at low temperature to eliminate internal stress at low temperature.

2. cold treatment is mainly applied to tight knives, measuring tools and tight parts made of alloy steel.

8. Flame Heating Surface Quenching

     Operation method: The flame burned by oxygen-acetylene mixture gas is sprayed onto the surface of steel parts, which is heated rapidly and cooled by water immediately after the quenching temperature is reached.

     Objective: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and to maintain the toughness of the core.

Key points of application:

     1. Most of them are used for medium carbon steel parts, and the depth of hardened layer is 2-6 mm.

     2. Suitable for single or small batch production of large workpieces and parts requiring local quenching.

9. induction heating surface quenching

      Operation method: put steel parts into inductor, make the surface of steel surface produce induced current, heat it to quenching temperature in a very short time, then spray water to cool down.

Objective: to improve the surface hardness, wear resistance and fatigue strength of steel parts, and maintain the toughness of the heart.

Application points:

     1. more for medium carbon steel and central hall alloy structural steel parts;

     2. as a result of skin effect, high frequency induction quenching quench layer is generally 1 to 2mm, medium frequency quenching is generally 3 to 5mm, high frequency quenching is generally greater than 10mm.

10. carburizing

      Operation method: put steel parts into carburizing medium, heat up to 900~950 degrees and heat preservation, so that the steel surface can obtain a certain concentration and depth of carburized layer.

Objective: to improve the surface hardness, wear resistance and fatigue strength of steel parts, and the heart part still maintains toughness.

Application points:

     1. for low carbon steel and low alloy steel with carbon content ranging from 0.15% to 0.25%, the carburizing depth is generally 0.5 to 2.5mm.

     2. carburizing must be quenched so that martensite can be obtained on the surface so as to achieve carburizing purposes.

11. nitriding

       Operating method: The active nitrogen atom decomposed from ammonia at 5. ~600 degrees can saturate the steel surface with nitrogen and form a nitriding layer.

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

        Application Points: Most of them are used in medium carbon alloy structural steels containing Al, Cr, Mo and other alloying elements, as well as carbon steels and cast iron. Generally, the depth of nitriding layer is 0.025-0.8mm.

12. Nitrocarburizing

      Operation method: carburizing and nitriding on the surface of steel parts.

Objective: to improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel surface.

Application points:

      1. more for low carbon steel, low alloy structural steel and tool steel parts. The general nitrided layer is 0.02 ~ 3mm deep.

     2. after nitriding, quenching and low temperature tempering are needed. 

Example 1: Several heat treatment schemes for commonly used steels

     Q345: This steel is used for building, bridge and so on. Its yield strength is about 345 MPa. Normalizing treatment is usually adopted, and its carbon content is about 0.12-0.2%.

     20Cr: Carbon content of about 0.2%, can be used as building steel, normalizing can be used; can also be used as carburizing steel, carburizing and quenching.

      GCr15: Bearing steel, carbon content about 1%, quenching temperature is generally 860C, 180C low temperature tempering.

       60Si2Mn: spring steel, about 0.6% carbon content, quenching temperature 860C, tempering at medium temperature (about 450C).

       0Cr18Ni9Ti: Austenitic stainless steel, carbon content is less than 0.06%, generally using about 1050C solution treatment (that is, heating water cooling) to improve corrosion resistance.

Example 2: How does 45#, 42CrMo and 20CrMo meet the hardness requirement of 45HRC

Both 45 # steel and 42CrMo can be quenched and tempered directly to meet the hardness requirement of 45HRC.

       20CrMo can also meet the hardness requirement of 45HRC after carburizing, quenching and tempering treatment.  

40CrMo.45

Example 3: Heat treatment process parameters of GCr15 steel

        GCr15 steel is a kind of high carbon chromium bearing steel with less alloy content, good properties and the most widely used. After quenching and tempering, it has high and uniform hardness, good wear resistance and high contact fatigue performance. The steel has medium plasticity in cold working, general cutting performance, poor welding performance, high sensitivity to white spot formation and temper brittleness.

Chemical Composition/Element Content (%)

C:0.95-1.05 Mn:0.20-0.40 Si:0.15-0.35 S:<=0.020 P:<=0.027 Cr:1.30-1.65

     Its heat treatment system is: steel bar annealing, steel wire annealing or 830-840 degree oil quenching.

Heat treatment process parameters:

1. General annealing: 790-810 degree heating, furnace cooling to 650 degree, air cooling - HB170-207

2. Isothermal annealing: 790-810 degree heating, 710-720 degree isothermal, air cooling - HB207-229

3. Normalizing: 900-920 degree heating, air cooling - HB270-390

4. Tempering at high temperature: 650-700 degrees heating, air cooling - HB229-285

5. Quenching: 860 degree heating, oil quenching - HRC62-66

6. Low Tempering: 150-170 degrees tempering, air cooling - HRC61-66

7. Carbonitriding: 820-830 degree co-permeation for 1.5-3 hours, oil quenching, - 60 degree to - 70 degree cryogenic treatment + 150 degree to + 160 tempering, air cooling - HRC_67

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