Summary of basic knowledge of heat treatment

Heat treatment refers to a metal thermal processing technology in which the material is in a solid state, through heating, heat preservation and cooling, to obtain the expected structure and performance.

1. Heat treatment

1. Normalizing: Heat the steel or steel parts to an appropriate temperature above the critical point AC3 or ACM for a certain period of time and then cool in the air to obtain a heat treatment process for pearlite-like structure.

2. Annealing: The hypoeutectoid steel workpiece is heated to 20-40 degrees above AC3, and after holding for a period of time, it is slowly cooled with the furnace (or buried in sand or lime) to below 500 degrees and cooled in the air. .

3. Solution heat treatment: heat the alloy to a high-temperature single-phase zone and keep it at a constant temperature, so that the excess phase is fully dissolved into the solid solution, and then rapidly cooled to obtain a supersaturated solid solution heat treatment process.

4. Aging: The performance of the alloy changes with time when it is placed at room temperature or kept slightly higher than room temperature after solution heat treatment or cold plastic deformation.

5. Solid solution treatment: Fully dissolve various phases in the alloy, strengthen solid solution and improve toughness and corrosion resistance, eliminate stress and soften, so as to continue processing and forming.

6. Aging treatment: heat and keep warm at the temperature at which the strengthening phase precipitates, so that the strengthening phase will precipitate out, harden and increase the strength.

7. Quenching: A heat treatment process in which the steel is austenitized and cooled at an appropriate cooling rate to cause the workpiece to undergo martensite and other unstable structural transformations in the cross section or within a certain range.

8. Tempering: Heat the quenched workpiece to an appropriate temperature below the critical point AC1 for a certain period of time, and then cool it with a method that meets the requirements to obtain the required structure and performance of the heat treatment process.

9. Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface of steel. Conventionally, carbonitriding is also called cyanidation, and it is widely used for medium temperature gas carbonitriding and low temperature gas carbonitriding (ie, gas nitrocarburizing). The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low temperature gas carbonitriding is mainly based on nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel.

10. Quenching and tempering: It is generally customary to combine the heat treatment of quenching and high temperature tempering as quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized sorbite structure of the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the size of the workpiece section, generally between HB200-350.

11. Brazing: a heat treatment process in which two workpieces are heated and melted and bonded together with brazing filler metal.

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