1、 The first fire of metal heat treatment annealing:

1. Annealing is to heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly, so as to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and use performance, or make organization preparation for further quenching.

2. Purpose of annealing:

① Improve or eliminate all kinds of structural defects and residual stress caused by steel in the process of casting, forging, rolling and welding, and prevent deformation and cracking of workpiece.

② Soften the workpiece for machining.

③ Refine the grain and improve the structure to improve the mechanical properties of the workpiece.

④ Make organization preparation for final heat treatment (quenching and tempering).

2、 The second fire of metal heat treatment - normalizing:

1. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, except that the microstructure obtained is finer, which is often used to improve the cutting performance of materials, and sometimes used for one-to-one

Some parts with low requirements shall be treated as final heat treatment.

2. Purpose of normalization:

① It can eliminate the overheated coarse grain structure and widmanstatten structure of castings, forgings and weldments, strip structure of rolled products, refine grain, and be used as pre heat treatment before quenching.

② The net secondary cementite can be eliminated and the pearlite can be refined, which not only improves the mechanical properties, but also benefits the spheroidizing annealing in the future.

③ The free cementite at grain boundary can be eliminated to improve its deep drawing property.

3、 The third fire of metal heat treatment - quenching:

1. Quenching is the rapid cooling of workpiece in water, oil or other inorganic salt, organic water solution and other quenching medium after heating and heat preservation. After quenching, the steel becomes hard but brittle at the same time.

2. Purpose of quenching:

① Improve the mechanical properties of metal products or parts. For example: improve the hardness and wear resistance of tools and bearings, increase the elastic limit of springs, and improve the comprehensive mechanical properties of shaft parts

② Improve the material or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel and so on.

4、 The fourth fire of metal heat treatment tempering:

1. Tempering in order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at a proper temperature higher than room temperature but lower than 710 ℃, and then cooled. This process is called tempering.

2. Purpose of tempering:

① , reduce the internal stress and brittleness, the quenched parts have great stress and brittleness, if not tempered in time, it will often produce deformation or even cracking.

② Adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness, strength, plasticity and toughness can be adjusted by tempering.

③ . stabilize the workpiece size. Through tempering, the metallographic structure can be stabilized to ensure that no deformation will occur in the later use.

④ Improve the cutting performance of some alloy steels.

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