Increase in efficiency with electro-gas arc welding of higher-strength shipbuilding steel for application under standard and low temperature conditions (AVIF-Nr. A 248)

Increase in efficiency with electro-gas arc welding of higher-strength shipbuilding steel for application under standard and low temperature conditions (AViF-Nr. A248)

Responsible research centres: Rostock University (Prof. Wanner), RWTH Aachen (Prof. Reisgen)

Time period: 01.01.2008 - 30.06.2010

Final report: CMT 17/2011


Project summary:

 

Shipbuilding requires production-oriented, cost-efficient and stress-resistant structures due to welding. Therefore a new efficient welding process has been developing especially in pre-assembly since the 80´s. The use of HSLA steel also increased. Due to the constant increasing requirements in the different types of ships like the hugeness of containerships or the operations in arctic regions they need more secure welding processes for those applications. In addition to that it is important to achieve testable statements for assessing the performance to avoid cracks as a result of the different types of strains during the operations.

The immediate use of the more powerful electrogas high performance arc welding for standard strength steels is not possible concerning HSLA steels. Welding of these HSLA steels without taking special measures would not be successful. A consequence will be the loss of strength and toughness in the heat affected zone (HAZ) owing to the high energy input per unit length and the grain growth. Due to it the use of this special welding process was not realized in German shipbuilding.

The overall aim is an increase in productivity of the final assembly of vertical joint design (type of joint) for the shipbuilding exterior body shells designed with HSLA steels in thicknesses from 15 to 30 mm with the use of electrogas arc welding. Under the conditions of real production facts, selected HSLA steels for normal and low temperatures are used.

The process is for vertical-up welded joints. The main aim of this investigation is to reduce the energy input per unit length. Reaching a value which generates high strength and toughness by using HSLA steels and ensure a safe and stable welding process is essential. Under this conditions the welding speed should be enhance in comparison to the common variants of electrogas arc welding.

Due to that experimental studies on appropriate samples are executed by optimising the welding parameters through special flux cored welding wires and gases as well as joint design preparation and wire feeder. On the other hand a targeted variation of the welding process technology (welding current and power source) is investigated regarding to a positive influence on the mechanical-technological properties of the welded joint. The impulse arc technique known from the gas metal arc welding (GMAW welding) has been used on this investigation. Also the possibility of a rotating arc will be researched. Besides the general welding requirements there is a need to focus on the shipbuilding joint designs. The results of the research will be verified by investigating the welded joint material behaviour with tensile and fatigue tests, metallographic structural analysis, Charpy impact tests (toughness analysis), non-destructive tests, welding seam measurement and economic efficiency calculation.

The foundation of this EG-welding project is determined by the requirements given by the Germanischen Lloyd. These essential requirements will be fulfilled. The resistance of the weld seam is verified by tensile testing, the samples are cracked in the base metal. The Charpy impact test shows that all values are significant higher than the minimum requirements (34 J): An A36 (test temperature = room temperature, thickness = 13 mm) reaches 86 J in the weld metal, 71 J in the fusion line and 110 J in the HAZ. A D36 (test temperature = 0°C, thickness = 30 mm) reaches 75 J in the weld metal, 173 J in the fusion line and 206 J in the HAZ. The hardness testing indicates a clear profile without any prohibited hardness steps.

Furthermore the single-side EG-welding technique is tested and does not show any process problems with the root by using weld pool backups. These pool backups are trapezoidal and also used in MSG-welding processes.

The input of energy per unit length firstly is 67 kJ/cm by a plate thickness of 13 mm and secondly the energy input is 116 kJ/cm at a plate thickness of 30 mm. In the first process the welding speed reaches 5.0 m/h and in the second one it is 7.0 m/h. The mechanical-technological quality values of the toughness testing form the A36 are only slightly a bit lower however with a value of 56 J it is over the minimum requirements
(34 J) from the Germanischen Lloyd. The test temperature has been room temperature and the results of the Charpy Impact Tests are: 98 J in the weld metal and 143 J in the HAZ. The tensile testing has been passed by only cracking in the base metal. The results of the D36 (used as base metal) are not showing any essential differences compared to the welding with double-sided water-cooled copper-shoes. The Charpy impact testing values are 120 J in the weld metal, 161 J in the fusion line and 239 J in the HAZ.
The test temperature has been 0°C. The results are higher than the minimum requirements. Furthermore the hardness line does not show any irregularity like significant hardness steps.

The required mechanical-technological properties by the classification societies were safely achieved through the EG-welding tests of the studied HSLA steels. The advantage of 6-7 times higher welding speed of the electrogas arc welding then the GMAW-welding could be demonstrated.

 

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