The function of heat treatment of cold drawn seamless pipes is to improve the mechanical properties of steel pipes and precision steel pipes, eliminate residual stress and improve the machining performance of steel pipe metals. According to the different purposes of heat treatment, the heat treatment process of cold drawn seamless pipes can be divided into two categories: preliminary heat treatment and final heat treatment.
The purpose of preparatory heat treatment is to improve the processability of the steel pipe, eliminate internal stress and prepare a good metallographic structure for the final heat treatment. The heat treatment process includes annealing, normalizing, aging, quenching and tempering, etc. The purpose of the final heat treatment is to improve the mechanical properties such as hardness, wear resistance and strength of the steel pipe.
What problems can be caused by improper heat treatment of cold drawn seamless pipes?
1) Unqualified structure and properties of cold drawn seamless pipe: unqualified hardness is one of the most common heat treatment defects. The main manifestations are insufficient hardness, insufficient quenching cooling rate, surface decarburization, insufficient steel hardenability, excessive retained austenite after quenching, insufficient tempering and other factors.
2) The cold drawn seamless pipes specification is unqualified: outer diameter, ovality, and bending degree are out of tolerance
The change of steel pipe outer diameter often occurs in the quenching process, and the steel pipe outer diameter increases due to the volume change (caused by the change of the structure). The sizing process is often added after the tempering process.
Changes in the ovality of steel pipes: the ends of steel pipes are mainly large-diameter thin-walled pipes.
Steel pipe bending: caused by uneven heating and cooling of steel pipes, it can be solved by straightening. For steel pipes with special requirements, a warm straightening process (about 550 ℃) should be used.
3) Cracks on the surface of cold drawn seamless pipe: It is caused by too fast heating speed or cooling speed and excessive thermal stress.
In order to reduce the heat treatment crack of the steel pipe, on the one hand, the heating system and cooling system of the steel pipe should be formulated according to the steel type, and the appropriate quenching medium should be selected; on the other hand, the quenched steel pipe should be tempered or annealed as soon as possible to eliminate its stress.
4) Scratches or hard injuries on the surface of cold drawn seamless pipe: the relative sliding between the steel pipe and the workpiece, tool and roller table occurs.
5) The cold drawn seamless pipe is oxidized, de-C, overheated or overburned.
6) The surface of cold drawn seamless pipe heat treated by protective gas is oxidized: the heating furnace is not well sealed, and air enters the furnace.
The composition of the furnace gas is unstable, and the quality control of the heating of the billet (steel pipe) must be strengthened.