The welding process of induction heat treatment is applied in the construction of wind tower tube
The wind tower tube is a medium thick plate with high strength and low alloy steel. As a large welding engineering structure, the welding area is the weakest part of the tower structure. Meanwhile, the tower is under a harsh environment such as low temperature and strong wind for a long time, especially the offshore wind power tower, and the welding area is prone to crack. Generally, it is necessary to conduct heat treatment on key welding parts after welding to improve their low-temperature fracture toughness and ensure the safe operation of the tower. However, there are many welding parts in the tower, a long construction period of heat treatment and high cost, and the hot shadow area is easy to be hardened during welding, which is highly sensitive to hydrogen and prone to hydrogen-induced delayed cracks. Especially when the welding joint is under great stress, it is more likely to produce all kinds of cracks, so in addition to welding must strictly consider the reasonable match of the strength and toughness of the welding material and the base material, select the suitable and efficient welding process, and optimize the welding parameters, the selection of heat treatment equipment and the process is also crucial. The traditional heat treatment process using ceramic resistance heater and asbestos insulation, the disadvantage is that the temperature control error is big, thick wall bar inside and outside the uniformity of temperature difference, weld high hardness, low efficiency, poor safety (resistance) exposed a lot of use on the human body and environment harmful material such as asbestos, ceramics are the layout of time-consuming, a lot of consumer goods, as shown in figure 1.
Based on the above reasons, the high frequency induction heat treatment process was developed to conduct heat treatment on the weld of the wind tower tube, which successfully solved the problem of post-welding heat treatment of datang Huangdao 5MW wind power tower tube, met the welding quality control requirements of wind tower tube, and ensured the overall quality of tower tube.
1. Wind tower tube material and induction heat treatment principle
A. Wind tower materials
The base material of the wind tower tube of the Datang Huangdao 5MW wind power project is Q345D, the plate thickness is 36mm, and the flange material is Q345E-Z35, 70mm. The diameter of the tower barrel is 4260mm. Its mechanical properties and chemical composition are in accordance with GB/T1591-2008. It is required that the weld joint at -40℃ should have an impact absorption energy AKV≥34J. The main parameters of part of the flange and tower tube requiring heat treatment are shown in Table 1. The welding groove adopts a V-shaped groove, carbon arc air gouging root, CO2 gas flow rate of 15 ~ 20L/min, submerged arc welding flux SJ101, and other welding parameters are shown in Table 2.
Table 1 Main parameters of flange and tower barrel:
Table 2 Welding process of flange and tower barrel:
B. Principle and model of induction heat treatment
The alternating current in the coil produces the alternating magnetic field, which generates the induced current in the workpiece and heats the workpiece by the induced current.In the process of induction heating, the whole section of the workpiece has the induction heating current, and the internal temperature gradient is small. The hottest region is below the surface of the workpiece, and the heat is rapidly conducted inside the metal, as shown in Figure 2.
FIG. 3 shows the induction heat treatment of the weld of the tower barrel. Before the heat treatment, the flexible heating electromagnetic induction coil is distributed on both sides of the weld of the tower barrel. The thermoelectric magnetic induction coil is less than the thickness of the wall of the tower barrel, and the insulation position, heating position, and current band are covered successively.
FIG. 2 Principle of induction heat treatment
FIG. 3 Welding seam induction heat treatment of tower drum
2. Heat treatment technology and result determination
A. Heat treatment process
Intermediate frequency induction heating equipment of ProHeat35 was used for heat treatment. Flexible heating electromagnetic induction coil with internal water supply; Rock wool is used for heat treatment weld insulation; There are 8 K-type thermocouple wires to measure the temperature, 4 internal installations, and 4 external installations to monitor the temperature of the inner and outer walls of the tower tube respectively, and the positions are located at the positions of 12, 3, 6 and 9 points of the tower tube respectively, as shown in FIG. 4.
Figure 4 Thermocouple position inside
Preheating before welding, 80 ~ 125℃; Post-welding heat treatment process: welding temperature above 300℃, temperature rise rate 90 ~ 104℃/h, cooling rate 100 ~ 130℃/h; The temperature rising speed can not be controlled below 300℃. Constant temperature and heat preservation temperature 630℃±15℃, heat preservation for 2.5h, post-welding heat treatment should be completed within 12h after welding. The process curve of post-welding heat treatment is shown in FIG. 5.
FIG. 5 Welding seam heat treatment process
B. Test results and analysis
(1) Temperature difference between inner and outer walls
FIG. 6 shows the inner and outer wall temperatures during induction heat treatment. As can be seen from Figure 6, the measured data of the inner and outer walls at the 8 temperature measuring points indicate that the temperature difference between the inner and outer walls is small, which can be basically controlled within 20℃, and is more uniform than ceramic heating or resistance heating of the inner and outer walls.
FIG. 6 Inner and outer wall temperature
(2) Weld hardness after heat treatment
Hardness test according to ASTM E92, the test results are shown in Figure 7.Hardness test shows that the hardness of heat-affected zone after heat treatment is slightly higher than 20HV, and the hardness of weld zone is > 200HV. According to the requirements, the hardness value of Q345 steel after heat treatment should be less than 320HV.
FIG. 7 Weld hardness after heat treatment
(3) Bending performance test
The bending properties of the welding process evaluation sample with the same heat treatment process were tested. The results showed that no crack was found in the surface bending test and the results were qualified.
(4) Process evaluation
The Charpy V notch test was carried out for the welding procedure evaluation samples with the same heat treatment process at -40℃ according to BS EN10045.1.Impact testing machine model JBN-500-086.See Table 3 for the impact test results. The impact results show that the impact absorption energy in the heat-affected zone is greater than that in the weld zone. The impact toughness of weld metal and heat-affected zone in the -40℃ impact test is much higher than the standard value.
Table 3 Charpy V-notch impact test results：
The induction heat treatment process has the advantages of uniform heating, high efficiency, obvious power saving effect, fast heating speed, and low cost, which meet the requirements of the post-welding heat treatment process.
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