Electron beam welding process for thick steel plate of turbine at Aachen Institute of Technology in Germany

Researchers from the Welding Research Institute of Aachen University of Technology in Germany reported on the development of a stable welding process for electron beam welding of thick plates used in the construction of offshore wind turbines. The relevant research results were published in Materials Science and Engineering Technology under the title "Development of a robust welding process for electric beam welding of thick plates for construction of offshore wind turbines".

As part of energy reform, the German government has decided to gradually phase out coal-fired power generation by 2030. Considering the need to gradually phase out nuclear energy simultaneously, in order to achieve this goal, it is necessary to further expand renewable energy, especially wind energy. The current trend is that the power of offshore wind turbines is increasing, with output power exceeding 10MW and blade lengths exceeding 100 meters. At present, the most commonly used foundation structure is single pile, which is a large steel pipe with a length of about 100 meters and a diameter of up to 10 meters. In the "HL Blech" project funded by the German Federal Ministry of Economic Affairs and Climate Action, new steel grades are being developed for this type of steel pipe. At present, a large portion of the annual production of Dillinger Steel Company, which is involved in the project, has been used to produce medium thick plates for offshore wind power foundations, indicating the economic significance of the project for the national steel industry. If the project is successful, S355 ML steel will be produced by the end of the project, which will greatly shorten the production time and thus reduce the cost of single pile construction. On the one hand, this will contribute to energy reform, and on the other hand, it will also help protect the single pile production and steelmaking bases in Germany and Europe.

The goal is to develop S355 ML steel that meets the EN10025-4 standard for welding single piles of wind turbines. These steels will be suitable for welding processes that improve production speed through higher deposition rates or higher power densities. The focus is on multi wire submerged arc welding and electron beam welding. Compared with the currently mature submerged arc welding process, both of these processes are expected to shorten production time. This will shorten production time.

To this end, it is necessary to determine the correlation between material composition, welding speed (input energy), and weld metal toughness during electron beam welding. By determining the appropriate material composition and corresponding welding parameters, the required toughness value can be achieved in the manufacturing process of wind turbine support structures.

In order to study and develop new electron beam optimized steels, it is necessary to characterize the welds and obtain positive effects in the steel production process. The characterization will be carried out on different materials with different alloy compositions (in accordance with EN10025-4 standard). For comparison purposes, three different heat input parameters should be studied. Therefore, three different parameters are required, each applicable to the welding of all steel grades to be studied. This article introduces the development process of welding parameters, and based on this, an optimized steel grade is developed in the subsequent process of the project.

Electron beam welding has high energy intensity and high welding depth, especially suitable for welding thick steel plates. For example, these steel plates can be used in offshore wind power generation systems. However, due to limitations in cooling conditions, it is usually not possible to achieve the required cold toughness for offshore applications. The goal of the researchers is to develop S355 ML steel that meets the EN10025 4 standard for welding single piles of wind turbines. For this purpose, three different energy input parameters were developed on three types of steel. The experiment is conducted on a steel plate with a wall thickness of 80 millimeters, and full penetration welding must be achieved. The top and bottom welds of the weld comply with DIN EN ISO 13919-1 standard. The unit length energy of the parameter ranges from 9.5 kJ mm-1 to 15.5 kJ mm-1. The average width of the resulting weld seam ranges from 5.5mm to 7.5mm, and burning of manganese alloy elements can be observed, especially at the top of the weld seam. In addition, through simulation estimation, the T8/5 time near the weld seam is between 11 seconds and 27 seconds.

Figure 1: Deep penetration welding process in left electron beam welding; Electron beam weld on right -80mm thick structural steel.

Figure 2: Upper left - Weld seam and hot adhesion zone; Right - measurement point for the width of the weld seam and heat attachment zone; Lower left - measurement point.

Figure 3: Development of an equivalent heat source.

Figure 4: Cross sections of the top and bottom of the weld seam; Left side - parameter 1, middle - parameter 2, right side - parameter 3.

Figure 5: Manganese content.

Figure 6: Top - Weld seam width; Bottom - width of the hot adhesion zone.

Figure 7: Top - Weld seam width (average); Bottom - width of the heat attachment zone (average).

Figure 8: Left temperature curve; Right T8/5 time.

Figure 9: Precision vacuum welding; Upper left - manganese content; Lower left - width of weld seam and heat affected zone; Middle - Cross section; Right - Weld beads at the top and bottom of the weld seam.

Table 1. Welding parameters.

The study determined three parameters for using electron beam welding to weld 80mm thick S355 ML steel plate. These parameters were developed and tested on four different alloy concepts, making them suitable for welding different materials with different alloy compositions. These parameters can be used to develop new types of steel for optimizing electron beam welding.

In summary, it can be said that the geometric shape of the weld seam complies with the DIN EN ISO 13919-1 standard.

The average width of the weld seam is about 7.5 millimeters (parameter 1), 6.5 millimeters (parameter 2), and 5.5 millimeters (parameter 3), so even if the electron beam deviates slightly, insufficient fusion can be avoided.

The welding speed for parameter 1 is 1.5 mm s-1, and the energy per unit length is 15.5 kJ mm-1; The welding speed of parameter 2 is 2 mm s-1, and the energy per unit length is 12.75 kJ mm-1; The welding speed for parameter 3 is 3 mm s-1, and the energy per unit length is 9.5 kJ mm-1.

The average width of the hot adhesion zone over the entire weld height varies from 2 millimeters to 3.5 millimeters, depending on the energy per unit length.

By measuring the alloying elements in the weld seam, the results show that during the welding process, as the alloy element with the lowest evaporation temperature, the manganese content will significantly decrease. The loss in the middle area of the weld seam can reach up to 10%, and the loss in the upper area of the weld seam can reach up to 30%, especially under slow parameters with high energy per unit length. Therefore, manganese can no longer be used as an alloying element to reduce the critical cooling rate. However, when developing new steel alloys, burning losses can be compensated by increasing the proportion of manganese in the base material.

By simulating an equivalent heat source, higher heating and cooling rates can be estimated. The cooling gradient of parameter 1 (15.5 kJ mm-1) is the slowest, with an average T8/5 time of 27 seconds; The average T8/5 time for parameter 2 (12.75 kJ mm-1) is 19 seconds; The T8/5 time for parameter 3 (9.5 kJ mm-1) is the shortest, at 11 seconds.

In addition, welding tests are conducted under low vacuum quality conditions, which is typical in welding applications with mobile or local vacuum. In this case, the formation and geometric shape of the weld seam, as well as the combustion of alloy elements, exhibit the same characteristics as the weld seam under high vacuum. Therefore, these parameters are also applicable to welding applications under high vacuum.

The developed welding parameters will be used to generate a large number of welds on the studied material to further determine its characteristics. The mechanical properties, microstructure properties, and local mechanical properties of these welds will be tested. From these studies, the positive effects of alloying elements and steelmaking processes were identified and incorporated into the development of new EB optimized steel grades.

Source: Yangtze River Delta Laser Alliance