Several common heat treatment concepts

1. Normalizing: The heat treatment process of heating steel or steel parts to an appropriate temperature above the critical point Ac3 or Acm for a certain period of time and then cooling in the air to obtain a pearlite structure.

2. Annealing: heat the hypoeutectoid steel workpiece to 30-50 degrees above Ac3, after holding for a period of time, slowly cool with the furnace (or be buried in sand or lime) to below 500 degrees and cool in the air. Craft

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

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

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 precipitates out, hardened and improved strength

7. Quenching: After austenitizing the steel, it is cooled at an appropriate cooling rate to make the workpiece dissolve into the solid solution in all or a certain range in the cross section, and then heat treatment for the transformation of unstable structure such as martensite. Craft

8. Tempering: heat the quenched workpiece to an appropriate temperature below the critical point Ac1 for a certain period of time, and then use a method that meets the requirements to cool it to obtain the required structure and performance of the heat treatment process

9. Nitriding and carbonitriding of steel

(1). Nitriding of steel (gas nitriding)

Concept: Nitriding is a process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve surface hardness and wear resistance, as well as fatigue strength and corrosion resistance.

It uses ammonia gas to decompose active nitrogen atoms when heated, and forms a nitride layer on the surface after being absorbed by the steel, while diffusing to the center.

Nitriding is usually carried out using specialized equipment or pit carburizing furnace. Suitable for various high-speed transmission precision gears, machine tool spindles (such as boring bars, grinder spindles), high-speed diesel engine crankshafts, valves, etc.

Process route of nitriding workpiece: forging-annealing-rough machining-quenching and tempering-finishing-stress removal-rough grinding-nitriding-fine grinding or grinding.

Because the nitrided layer is thin and brittle, it requires a high-strength core structure. Therefore, it is necessary to perform quenching and tempering heat treatment first to obtain tempered sorbite and improve the mechanical properties of the core and the quality of the nitrided layer.

After nitriding, the steel no longer needs to be quenched and has high surface hardness and wear resistance.

The nitriding treatment temperature is low and the deformation is small. Compared with carburizing and induction surface quenching, the deformation is much smaller

(2) Carbonitriding of steel: Carbonitriding is a process of simultaneously infiltrating carbon and nitrogen into the surface of the steel. Carbonitriding is customarily called cyanidation. At present, medium temperature gas carbonitriding and low temperature gas nitrocarburizing (ie, gas nitrocarburizing) are widely used. 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 nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel.

10. Quenching and tempering: It is generally used 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 for bonding two workpieces together with brazing filler metal