The heat treatment process for stainless steel pipes generally includes three processes: heating, heat preservation and cooling, and sometimes only two processes: heating and cooling. These processes are interconnected and cannot be interrupted.

Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties. There are four basic processes for overall heat treatment of stainless steel pipes: annealing, normalizing, quenching and tempering.


Annealing:
1. Complete annealing and isothermal annealing
Complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for castings, forgings, and hot-rolled profiles of various carbon steels and alloy steels with hypoeutectoid components, and sometimes for welded structures. It is generally used as the final heat treatment of some unimportant workpieces or as a pre-heat treatment of some workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel used to make cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching.
3. Stress relief annealing
Stress relief annealing is also called low-temperature annealing (or high-temperature tempering). This annealing is mainly used to eliminate residual stresses in castings, forgings, welded parts, hot-rolled parts, cold-drawn parts, etc. If these stresses are not eliminated, they will cause deformation or cracks in the steel parts after a certain period or in the subsequent cutting process.

The meaning of annealing: heat the workpiece to an appropriate temperature, use different holding times according to the material and workpiece size, and then cool it slowly (slow cooling rate). The purpose is to make the internal structure of the metal reach or approach a state of equilibrium, obtain good process performance and use performance, or prepare the organization for further quenching.



Normalizing:
The meaning of normalizing: heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the obtained organization is finer. It is often used to improve the cutting performance of the material and is sometimes used as a final heat treatment for some parts with low requirements.



Quenching
1. Saltwater quenching
The workpiece quenched in salt water makes it easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that cannot be hardened, but it is easy to cause the workpiece to deform seriously or even crack.
2. Oil quenching
Using oil as a quenching medium is only suitable for quenching some alloy steels or small-sized carbon steel workpieces with relatively large stability of supercooled austenite.
The significance of quenching: After the workpiece is heated and kept warm, it is quickly cooled in a quenching medium such as water, oil, other inorganic salts, organic aqueous solutions, etc. After quenching, the steel becomes hard, but also brittle.

Tempering:
To reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at a suitable temperature above room temperature and below 710℃ and then cooled. This process is called tempering.
The significance of tempering:
1. Reduce brittleness, and eliminate or reduce internal stress. After quenching, steel parts have great internal stress and brittleness. If they are not tempered in time, they will often deform or even crack.
2. Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. To meet the requirements of different performances of various workpieces, the hardness can be adjusted by appropriate tempering, the brittleness can be reduced, and the required toughness and plasticity can be obtained.
3. Stabilize the size of the workpiece
4. For some alloy steels that are difficult to soften by annealing, high-temperature tempering is often used after quenching (or normalizing) to make the carbides in the steel appropriately aggregate and reduce the hardness to facilitate cutting.


Annealing, normalizing, quenching, and tempering are the "four fires" in the overall heat treatment, among which quenching and tempering are closely related and are often used together. They are indispensable. The "four fires" have evolved into different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high-temperature tempering is called quenching and tempering. After some alloys are quenched to form a supersaturated solid solution, they are placed at room temperature or a slightly higher appropriate temperature for a long time to improve the hardness, strength, or electrical and magnetic properties of the alloy. Such a heat treatment process is called aging treatment. The method of combining pressure processing deformation with heat treatment effectively and closely to obtain a good strength and toughness combination for the workpiece is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only prevents the workpiece from oxidation and decarburization, keeps the surface of the workpiece smooth after treatment, and improves the performance of the workpiece, but also allows the infiltration agent to be passed for chemical heat treatment.

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