Self-restraint TEKKEN test to investigate cold crack sensitivity: From metallographic examination of the cracks, surface residual stress analysis to acoustic emission analysis to detect the time of crack initiation
Source: BAM
The great potential of high-strength structural steels with a yield strength ≥ 690 MPa offers economic and design advantages in the joining of steel structures in terms of energy and resource efficiency. These steels are primarily joined using GMA welding processes, although developments in modern inverter technology and control technology enable the use of controlled arc variants, such as the modified spray arc (mod. SA). Welding with modified spray arc offers the possibility to join narrower weld joints, e.g., with reduced weld opening angle of 30°, with less heat input. The special microstructure and high strength of these steels increase the demands on welding processing. In particular, it is important to avoid hydrogen-assisted cold cracks, which are caused by complex interactions of local factors such as crack-critical microstructure, increased hydrogen concentration and mechanical stress or strain.
In the present work, these three influences of hydrogen-assisted cracking were investigated on a high-strength structural steel S960QL. A special self-stressing cold crack test, the TEKKEN test, was used to investigate the cold crack sensitivity. With this, statements can be made about the behavior of the joint for a specific seam geometry, i.e. by modifying the test geometry with regard to the seam opening angle (60° and 30°), and under very high shrinkage restraint. In addition, the effectiveness of a heat treatment of the specimens from the welding heat was investigated, with which a significant hydrogen reduction can be achieved in order to avoid hydrogen-assisted cold cracking. The evaluation or the assessment of the cold-crack sensitivity of the high-strength structural steel S960QL was finally carried out considering the microstructure, the hydrogen concentration in the weld metal, residual stress formation and the fracture surface topography of the cracks that occurred. As a result, the weld metal produced at reduced weld opening angle show slightly higher hydrogen concentrations on average. Furthermore, increased micro- and macro-crack formation occurred in these weld metal samples. All samples without dehydrogenation heat treatment shows cracks in the root notch due to hydrogen-assisted cracking. These could be effectively avoided by reheating immediately after welding.
Hydrogen-Assisted Cracking in GMA Welding of High-Strength Structural Steel — A New Look into This Issue at Narrow Groove
Thomas Schaupp, Nina Schröder, Dirk Schröpfer, Thomas Kannengießer
published in Metals, Vol. 11, issue 6, page 904, 2021
BAM division Testing Devices and division Equipment and Weld Mechanics