At present, the rectangular tube heat treatment is mostly water as a quenching agent, because in the quenching medium, however, due to the faster cooling speed of water, the rectangular tube is prone to deformation during quenching, and the steel tube is seriously deformed because it can not be scrapped for the next process. Therefore, when selecting the quenching method, it is necessary to make the steel pipe meet the quenching requirements, and to control the deformation within the allowable range.

According to the above analysis, first of all, it is ideal for the steel pipe to be quenched in a rotating manner, and at the same time for internal and external cooling. Water is sprayed into the pipe with a nozzle for internal cooling, and the flow rate of water is not less than 10m/s. External cooling can be sprayed or immersed in the quenching tank, but the quenching tank must also have a stirring nozzle. In short, the quenching of the steel pipe should be the cooling capacity to meet the requirements, and the cooling should be as uniform as possible, which is the guiding idea 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 steel; The cooling rate is slower in the low temperature zone to reduce the organizational stress during martensitic transformation and reduce deformation or cracking. The ideal quenching agent does not exist, but the selection of quenching agent should consider these two factors.

The choice of quenching agent should be considered according to the variety of steel pipe, technical requirements and steel composition and other factors. Taking petroleum pipes as an example, the heat treatment of high-steel grade (strength) steel pipes is a tempering process, that is, quenching and high temperature tempering. Steel types are mostly selected with low carbon content of medium carbon alloy structural steel, the thickness of the steel pipe (wall thickness) is basically the same, the difference in the stress distribution state of the quenching is small, and the risk of cracking is relatively low, therefore, water can be preferred as the quenching agent.

The advantage of water is that the cooling speed is fast, the hardenability of the steel can be improved, and the alloy content can be reduced on the steel and the production cost can be reduced. In addition, water is used as quenching agent, which is stable and economical in production. Also due to the cooling speed of water, the steel pipe deformation is large, and in the case of some defects on the surface of the steel pipe, the quenching will expand into defects 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 slower cooling rate should be selected as the quenching agent.