There are three types of online heat treatment processes for the weld seam of the second-hand welded pipe production line: annealing, normalizing and quenching + tempering. The manufacturer only needs to select the corresponding process according to the pipe products and customer needs.

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Annealing (A)

The function of the online annealing of the weld of the second-hand welded pipe production line is to reduce the hardness of the weld and the heat-affected zone, improve the plasticity, refine the grain, and eliminate the residual stress.

The annealing process often used is: heating the weld and heat-affected area to 640~680°C, then air cooling to about 350°C, and then adding spray water or bathing to cool to close to the indoor temperature.

Normalizing (N)

The purpose of normalizing is to eliminate the coarse grains in the weld area of ​​the second-hand welded pipe production line, refine and uniform the weld structure, and improve the mechanical properties of the weld.

The process route is: heating the weld area to 30~50°C above the Ac3 line, that is, 920°C~950°C, air cooling to 350°C, and then forced cooling with coolant.

Remarks: Heat treatment Ac3 is a temperature code, A represents the critical point, and c means heating. The heat treatment Ac3 is different for steel grades with different carbon content and different alloying element compositions. In the iron-carbon phase diagram, for better use, we named the temperature of 230°C (the Curie point of cementite) as A0, and 727°C as A1 (the PSK line of the iron-carbon phase diagram), 770 °C (the Curie point of ferrite, magnetic transition point) is A2, 727~912°C is A3 (GS line of iron-carbon phase diagram), and 1394~1495°C is A4 (NJ above the iron-carbon phase diagram) String)

Quenching + Tempering (Q T)

The function is to reduce the hardness of the weld area, improve the plasticity and toughness, reduce the internal stress, and obtain good comprehensive mechanical properties.

The heat treatment process is: heating the weld area to the normalizing temperature, spraying water for quenching, then heating to the tempering temperature (540 ~ 650°C), air cooling to 350°C, and then adding water for forced cooling.

The latter two heat treatment processes are mostly used for high-grade steel-grade oil and gas pipes and high-grade welded pipes according to process requirements.

Air cooling length

After the welding seam of the second-hand welded pipe production line is heated by medium frequency induction heating, the air cooling speed should not be too fast, otherwise it will affect the structure and performance of the welding seam area. From the perspective of heat treatment, longer holding time is conducive to the homogenization of alloying elements and the homogenization of the weld structure, and the comprehensive mechanical properties of the weld area, such as plasticity and toughness, are also better. However, due to process layout and plant land economy constraints, it is impossible to leave the air cooling section very long. This requires an analysis of the factors that affect the air cooling effect and seeks a balance between the two to ensure the welding after heat treatment. The seam has good comprehensive mechanical properties and can use the site more economically. At present, the air-cooling length of large-scale welded pipe units is mostly between 50 and 60m. From the perspective of production practice, it can basically meet the needs of normal production of welded pipes and on-line heat treatment of welds.

Another way to clarify the air-cooling length L is to determine the air-cooling length based on the type of heat treatment, welding rate, and air-cooling speed, etc., according to the following formula.

In formula calculation:

L——The air cooling length of the weld after heat treatment, m;

v——Unit welding speed, M/min;

△T——The difference between the heat treatment heating temperature and the final cooling temperature of the weld, °C;

Vc——cooling rate, °C/s.

The above is an explanation of the online heat treatment process and air cooling length of the weld of the second-hand welded pipe production line, as well as the calculation formula for the air-cooling length of the second-hand welded pipe production line. I hope it will be helpful to you.

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