12 Heat Treatment Processes: Annealing, Normalizing, Quenching, Tempering …

Annealing, normalizing, quenching, tempering, quenching and tempering … totally 12 heat treatment processes. This article will help you sort out.

Heat Treatment Processes

01 Annealing

Operation method:

After heating the steel to a temperature of Ac3 + 30 -50 degrees or Ac1 + 30 – 50 degrees or below Ac1 (you can consult the relevant materials), it is usually slowly cooled with the furnace temperature.

Intentions:

  • To reduce hardness, improve plasticity, cutting and pressure processing functions.
  • To refine grains, improve mechanical functions, and prepare for next steps.
  • To eliminate internal stresses during cold and hot processing.

Application key:

(1) Suitable for alloy layout steel, carbon east-west steel, alloy east-west steel, high-speed steel forgings, welding parts and raw materials with unsatisfactory supply conditions.

(2) Usually anneals in the rough condition.

02 Normalizing

Operation method:

Heat the steel to 30 – 50 degrees above Ac3 or Accm, after soaking, cool it at a cooling rate slightly larger than that of annealing.

Intentions:

  • To reduce hardness, improve plasticity, cutting and pressure processing functions.
  • To refinesgrains, improve mechanical functions, and prepare for next steps.
  • To eliminate internal stresses during cold and hot processing.

Application key:

Normalizing is usually used as a pre-treatment process for forgings, weldments and carburized parts. For low- and medium-carbon carbon layout steels and low-alloy steel parts with low functional requirements can be performed with the final heat treatment. For ordinary medium and high alloy steels, air cooling can cause complete or partial hardening, so it cannot be used as the final heat treatment process.

03 Quenching

Operation method:

The steel piece is heated to a temperature above the phase transition temperature Ac3 or Ac1, hold for a certain period of time, and then rapidly cooled in water, nitrate, oil, or air.

Intention:

Quenching is usually done to obtain a martensitic arrangement with high hardness. Sometimes when high-alloy steel (such as stainless steel, wear-resistant steel) is quenched, it is to obtain a single uniform austenite arrangement to improve wear resistance and corrosion resistance.

Application key:

(1) Usually used for carbon steel and alloy steel with carbon content greater than 0.3%;

(2) Quenching can give full play to the strength and abrasion resistance potential of steel, but together they will constitute a large internal stress and reduce the plasticity and impact toughness of the steel. Therefore, tempering is required to obtain better induction mechanical functions.

04 Tempering

Operation method:

The quenched steel parts are heated from the beginning to a temperature below Ac1, and after heat preservation, cool them in air, oil or hot water.

Intentions:

  • To reduce or eliminate the internal stress after quenching, reduce the deformation and cracking of the workpiece.
  • To adjust the hardness, improve the plasticity and resistance, and obtain the mechanical functions required by the operation.
  • To stabilize the workpiece size.

Application key:

(1)Tempering with low temperature when insisting on high hardness and wear resistance of steel after quenching.

(2)Under the condition of insisting on a certain toughness, tempering at medium temperature is used to improve the elasticity and yield strength of steel.

(3)Mainly insist on high impact toughness and plasticity, and use high temperature tempering when there is sufficient strength.

Generally, steel should be prevented from tempering between 230 – 280 degrees and stainless steel between 400 – 450 degrees, because a tempering brittleness occurs at this time.

05 Quenching and tempering

Operation method:

After quenching, high temperature tempering is called quenching and tempering. Steel is heated to a temperature that is 10-20 degrees higher than that during quenching. After heat preservation, quenching is performed, and then tempered at a temperature of 400-720 degrees.

Intentions:

  • To improve the cutting function and improve the appearance of processing;
  • To reduce deformation and cracking during quenching;
  • To get outstanding inductive mechanics.

Application key:

  • Suitable for alloy layout steel, alloy east-west steel and high-speed steel with high hardenability.
  • Not only can it be used as the final heat treatment of various more important layouts, but also it can be used as a pre-heat treatment of certain tight parts, such as screws, to reduce deformation.

06 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.

Intentions:

  • To arrange the steel parts after quenching to reduce the deformation during storage or use time.
  • To reduce internal stress after quenching and grinding, and stabilize shape and size.

Application key:

(1) Suitable for all steel types after quenching;

(2) It is often used for tight workpieces whose shapes are no longer changed, such as tight screws, measuring things, bed chassis, etc.

07 Cold treatment

Operation method:

The quenched steel parts are cooled in a low-temperature medium (such as dry ice, liquid nitrogen) to -60 to -80 degrees or lower, and the temperature is uniformly taken out and then allowed to reach room temperature.

Intentions:

(1)To make all or most of the remaining austenite in the quenched steel part into martensite, and then improve the hardness, strength, wear resistance and fatigue limit of the steel part;

(2) To ensure the steel arrangement to stabilize the shape and size of steel pieces.

Application key:

(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 applicable to tight tools, measuring tools and tight parts made of alloy steel.

08 Flame-heated surface quenching

Operation method:

The flame incinerated with oxygen-acetylene mixed gas is sprayed onto the surface of the steel part, and the steel is heated rapidly. When it reaches the quenching temperature, to  spray with water to cool the steel immediately.

Intention:

The hardness, wear resistance and fatigue strength of steel parts are improved, and the heart still adheres to the resistance status.

Application key:

(1) Mostly used for medium carbon steel parts, usually the depth of hardened layer is 2-6mm;

(2) Suitable for single-piece or small-batch production of large workpieces and workpieces requiring partial hardening.

09 Induction heating surface quenching

Operation method:

Put the steel parts into the inductor, make the surface of the steel parts induce current, heat to the quenching temperature in a very short time, and then spray water to cool.

Intention:

To improve the appearance hardness, wear resistance and fatigue strength of steel parts, and adhere to the endurance of the heart.

Application key:

(1) Mostly used for medium carbon steel and middle hall alloy layout steel parts;

(2) Because of the skin effect, the high-frequency induction hardened hardened layer is usually 1 to 2 mm, the intermediate frequency hardened is usually 3 to 5 mm, and the high frequency hardened is usually greater than 10 mm.

10 Carburizing

Operation method:

Put the steel parts in the carburizing medium, heat it to 900-950 degrees and keep it warm, so that the surface of the steel parts can obtain a carburizing layer with a certain concentration and depth.

Intention:

To improve the external hardness, wear resistance and fatigue strength of steel parts, and the heart still adheres to the resistance status.

Application key:

(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 usually 0.5 to 2.5mm;

(2) After carburizing, it is necessary to perform quenching to obtain martensite on the surface before carburizing intention is completed.

11 Nitriding

Operation method:

By using active nitrogen atoms that are separated out by ammonia gas at 500-600 degrees, the appearance of the steel is saturated with nitrogen to form a nitrided layer.

Intention:

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

Application key:

It is mostly used for medium-carbon alloy layout steels that are rich in alloy elements such as aluminum, chromium, molybdenum, carbon steel and cast iron. The depth of the nitrided layer is usually 0.025 to 0.8 mm.

12 Nitrocarburizing

Operation method:

Carburizing and nitriding the steel surface together.

Intention:

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

Application key:

(1) Mostly used for low-carbon steel, low-alloy layout steel, and east-west steel parts, usually with a nitriding layer depth of 0.02 – 3mm;

(2) Quenching and low temperature tempering are required after nitriding.

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