About heat treatment

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. In the process from the stone age to the bronze age and iron age, the role of heat treatment is gradually recognized.

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

3. Solution heat treatment: heat the alloy to the 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 or slightly higher than room temperature, its properties change with time.

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

6. Aging treatment: heat and keep warm at the precipitation temperature of the strengthened phase to make the precipitation of the strengthened 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 workpiece can undergo the transformation of unstable structure such as martensite in the whole or a certain range of cross section.

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

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. Tempering treatment: generally, the heat treatment combining quenching and high temperature tempering is called tempering treatment. 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.

Characteristics of heat treatment process

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the performance of the workpiece by changing the microstructure inside the workpiece or the chemical composition on the surface of the workpiece. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye. In order to make the metal parts have the required mechanical, physical and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in mechanical industry. The microstructure of steel is complex and can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different service properties.