1.1 A brief introduction to the production line of the steel pipe
The welding equipment is mainly composed of a welding power supply (GINNCOLN welding machines and US control systems), control boxes, wire feeding devices, welding head devices, tracking devices and adjustment mechanisms. The welding power supply adopts a digital power supply, which can be welded with inner and outer wires and above. The welding part of the unit is automated, and the laser welding seam automatic tracking system is used to realize the automatic positioning and monitoring of the inner and outer welding heads.
1.2 Information on manufacturing steel pipes
A comparative test was carried out on the production of steel pipes using the newly designed welding torch and the traditional welding torch. The basic information about manufacturing the steel pipe is shown in Table 1.
Table 1 Basic information on the production of steel pipes
| Steel grade |
Specification/mm |
Welding methods |
Wire quantity |
Execution standard |
| |
Diameter |
Wall thickness |
| L245M |
1422 |
14 |
Internal and external submerged arc welding |
1 wire each
|
API Spec 5L-2012
|
1.3 Two kinds of welding torches
In the case of using automatic tracking systems of internal and external welding seams, the two kinds of welding torches are replaced with the same material contact tip, and each kind of welding torch is continuously produced on the same steel pipe unit for 50 hours. The steel pipes are produced according to the basic information in Table 1. Each welding torch produces 60 steel tubes, with a length of about 12 meters.
1.3.1 A comparison of welding deviation between internal and external welding seams
The first group of tests: Select the first 10 steel pipes welded by the new welding torch and the traditional welding torch. The contact tips of the first 10 steel pipes are free of wear. In accordance with the requirements of API Spec 5L-2012, 3 samples of the welding deviation of the inner and outer welding seams were taken on each steel pipe, and the average value of the measurement results was recorded. 10 sets of welding deviation test data were recorded for each welding torch. See Table 2 for the internal and external weld deviation of the traditional welding torch and the new design welding torch, and the comparison is shown in Figure 3.
Table 2 Welding deviation of the traditional welding torch and new welding torch for inside and outside welding seam
| Items |
Minimum values |
Maximum values |
Average values |
Standard deviation |
| Traditional welding torch |
0.60 |
1.70 |
1.10 |
0.35 |
| New welding torch |
0.20 |
1.20 |
0.81 |
0.36 |
Figure 3 A comparison of welding deviation between the new welding torch and the traditional welding torch
The second group of tests: the steel pipes processed by each welding torch were sampled according to the welding time used by the contact tip. According to the requirements of API Spee 51-2012, a welding deviation test was randomly selected on the steel pipes welded by the two welding torches every even number of hours. Take the sample and record the average value. After the contact tips on the two welding torches have been used for 20 hours, the variation of welding deviation with the use time of the contact tips is shown in Figure 4. The good and bad welding deviations are shown in Figures 5 and 6.
Figure 4 The welding variation with the use time of contact tip
Figure 5 Good welding deviation
Figure 6 Poor welding deviation
1.3.2 Influence of welding torch on the impact of welding seams
Through a set of experiments, the influence of line energy on weld impact energy is verified. Adjust the welding line capacity during welding. Take impact samples on the 3 groups of welded steel pipes, and perform the test according to the API Spec 5I-2012 test method. The sample size is 10mm x 10mm x 55mm, and the test results are shown in Table 3.
Table 3 A comparison of weld impact energy of samples with different input energy
| Items |
Line energy /(kcm-1) |
Impact energy |
| 1 |
2 |
3 |
Average values |
| 1 |
30 |
100 |
105 |
90 |
98 |
| 2 |
27 |
130 |
125 |
120 |
125 |
| 3 |
24 |
160 |
155 |
165 |
160 |
Please note that the test temperature is -10°C.
Through the test, it is found that the linear energy has a great influence on the impact energy of the weld, mainly because when the welding material and the base metal sample are the same; when the welding linear energy is great, the cooling rate of the welding seam structure is slow and the crystallization time is relatively long. The grain grows. By guaranteeing the welding penetration and the overlapping of the inner and outer welding seams, when the line energy is reduced, the cooling speed of the welding seam is fast; the austenite grains formed in the welding seam have no time to grow, and the grains can be fine. The value of welding seam’s toughness mainly depends on the size of grains in the welding seam. When the proportion of refined grains is high, the welding seam’s toughness is high and vice versa. By controlling the heat input, the welding seam can obtain a fine and uniform structure, and the impact toughness of the welding seam will increase.
In the welding process, the wear and tear of the traditional contact tip increases and the conductive area decreases when it is used for about 12 hours, the temperature of the welding wire rises and the penetration depth decreases. To ensure penetration, the contact tip needs to be replaced in time; Increase the current and power consumption, and the welding line energy will be increased. According to Table 3, it is found that the impact toughness of the welding seam will decrease. At this time, the contact tip needs to be replaced.
For steel pipes welded with a new welding torch, the impact samples are taken according to API Spec 5L-2012, and the steel pipes processed by the corresponding contact tip are used continuously for 5, 10, 15, 20, 25 to 50 hours for weld seam impact tests. The service life is about 20 hours, because the device can fix the position of the welding wire, which can extend the service life of the contact tip to about 55 hours. The change of welding seam impact value with time when welding with a new welding torch is shown in Figure 7.
Figure 7
It can be seen from Figure 7 that the welding seam impact energy changes with time when the steel is welded with the new welding torch. The weld impact test data is stable, and the welding seam impact energy does not fluctuate with the increase in welding torch use time.
