Heat treatment summary of basic knowledge

Heat treatment refers to a kind of metal hot working process in which materials are heated, insulated and cooled in solid state to obtain the expected structure and properties.

Heat treatment

1. Normalizing: the heat treatment process of pearlite like structure is obtained by heating the steel or steel piece to a proper temperature above the critical point AC3 or ACM and keeping it in air for a certain period of time.

2. Annealing: the heat treatment process of heating hypoeutectoid steel workpiece to 20-40 degrees above AC3 and cooling slowly (or buried in sand or lime) with furnace to 500 degrees below in air after holding for a period of time.

3. Solution heat treatment: heat the alloy to a high temperature single-phase area and keep the temperature constant, so that the excess phase can be fully dissolved in the solid solution, and then rapidly cool to obtain the heat treatment process of supersaturated solid solution.

4. Aging: after solution heat treatment or cold plastic deformation, when the alloy is placed at room temperature or kept slightly higher than room temperature, its properties change with time.

5. Solution treatment: make all kinds of phases in the alloy fully dissolved, strengthen solid solution and improve toughness and corrosion resistance, eliminate stress and softening, so as to continue processing and forming.

6. Aging treatment: heating and heat preservation at the precipitation temperature of the strengthening phase to make the precipitation of the strengthening phase harden and improve the strength.

7. Quenching: a heat treatment process in which the steel is austenitized and cooled at a proper cooling rate, so that the unstable structure of the workpiece, such as martensite, changes completely or within a certain range of the cross section.

8. Tempering: a heat treatment process in which the quenched workpiece is heated to a proper temperature below the critical point AC1 for a certain period of time, and then cooled in a satisfactory way to obtain the required structure and properties.

9. Carbonitriding of steel: carbonitriding is a process of infiltration of carbon and nitrogen into the surface of steel at the same time. Traditionally, carbonitriding, also known as cyaniding, is widely used in medium temperature gas carbonitriding and low temperature gas carbonitriding. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. The main purpose of low temperature gas carbonitriding is to improve the wear resistance and bite resistance of steel.

10. Quenching and tempering: generally, the heat treatment combining quenching and high temperature tempering is called quenching and tempering. Quenching and tempering are widely used in all kinds of important structural parts, especially those connecting rods, bolts, gears and shafts working under alternating load. After quenching and tempering, tempered sorbite structure is obtained, and its mechanical properties are better than that of normalized sorbite with the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-section size of the workpiece, generally between hb200-350.

11. Brazing: a heat treatment process in which two kinds of workpieces are heated, melted and bonded together with solder.

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