In order to improve the surface hardness and wear resistance of square rectangular tubes, surface treatment can be carried out, that is, flame surface quenching, high and medium frequency surface quenching, and chemical heat treatment. Generally speaking, high-frequency and medium-frequency surface quenching are mostly, and the heating temperature is 850-950°C. Considering its poor thermal conductivity, the heating rate should not be too fast, otherwise melting and quenching crack defects will occur. High-frequency quenching requires that the matrix structure of the rectangular tube after normalizing is mainly pearlite. Water spray or polyvinyl alcohol aqueous solution is used for cooling, the tempering temperature is in the range of 200-400°C, and the hardness is 40-50HRC, which can ensure the hardness and wear resistance of the square tube surface.

In addition, the square tube heat treatment also has the following three characteristics:

(1) Dimensional stability For high-precision square tubes, the precision required is high, so dimensional stability must be maintained. Due to the straightening in the air, the cooling rate is slow, so it has a stabilizing effect on austenite, which will increase the number of retained austenite square tubes in the structure, so cold treatment must be carried out;

(2) Reducing Quenching Deformation Since the rectangular tube is slender, it is easy to deform during hardening, so its deformation must be strictly controlled. Heat treatment is a very critical process. During the quenching and cooling process, the plasticity of supercooled austenite is used to carry out Timely straightening is the key step to ensure the increase of its qualification rate. For this reason, it should be quenched in a hot bath or cooled in oil for a certain period of time to propose thermal straightening. At the same time, hanging heating should be carried out during heating to reduce the deformation of quenching. For high-precision guide rails, gas nitriding or ion nitriding is performed to reduce deformation;

(3) The high-hardness square tube mainly bears the contact fatigue load, so it must have high hardness, so it should be quenched, surface quenched or chemical heat treatment, etc., followed by low-temperature tempering treatment.