At present, water is mostly used as the quenching agent in the heat treatment of square rectangular tubes. This is because in the quenching medium, due to the fast cooling rate of water, square rectangular tubes are prone to deformation during quenching. Therefore, when selecting the quenching method, it is necessary to make the steel pipe meet the quenching requirements and control the deformation within the allowable range.

According to the above analysis, it is ideal to first quench the steel pipe in a rotating manner, and at the same time perform internal and external cooling. For internal cooling, water is sprayed into the pipe by nozzles, and the flow rate of water is preferably not less than 10m/s. Spraying or immersion in the quenching tank can be used for external cooling, but there must also be a stirring nozzle in the quenching tank. In short, the quenching of steel pipes should meet the requirements of the cooling capacity, and the cooling should be as uniform as possible. This is the guiding ideology for selecting the quenching method.

The ideal quenching agent is to cool faster in the high-temperature zone to avoid the disintegration of austenite and improve the hardenability of the steel; in the low-temperature zone, the cooling rate is slower to reduce the structural stress during martensite transformation and reduce deformed or cracked. An ideal quenching agent does not exist, but these two factors should be considered when selecting a quenching agent.

The choice of quenching agent should be considered according to various factors such as the type of steel pipe, technical requirements, and steel composition. Taking oil pipes as an example, the heat treatment of high-grade (strength) steel pipes is a quenching and tempering process, that is, quenching and high-temperature tempering. Most of the steel types are medium-carbon alloy structural steel with low carbon content. The thickness (wall thickness) of the steel pipe is basically the same. Water may preferably be used as the quenching agent.

The advantage of water is that the cooling speed is fast, it can improve the hardenability of steel, and it can reduce the alloy content in steel and reduce production costs. In addition, water is used as a quenching agent, which is stable and economical in production. Also due to the fast cooling rate of water, the deformation of the steel pipe is large, and in the case of some defects on the surface of the steel pipe, it will expand into defects during quenching and be scrapped.

If the carbon content in the steel is high, in order to reduce deformation or cracking during quenching, rapid quenching oil or water-based quenching liquid with a slow cooling rate should be selected as the quenching agent.