Haugen, Veronica (NTNU (Norwegian University of Science and Technology)) | Rogne, Bjørn Rune Søraas (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology) ,and SINTEF) | Østby, Erling (SINTEF)
Steels that are used in arctic climates are introduced to extreme environmental weather conditions, where, e.g., the temperature may fall as low as -60°C. This phenomenon will inevitably affect the material properties since structural steels undergo the ductile to brittle transition. Hence, the fracture toughness may be substantially reduced after welding. In order to establish reliable models to predict the behavior of steels under such extreme conditions, the knowledge of the local properties would represent a vital contribution. In the present investigation, local properties are measured in microstructures typically present in the intercritically reheated coarse grained HAZ (ICCGHAZ) of a 490 MPa forging. Focused ion beam (FIB) is used to fabricate pillars from weld thermally simulated ICCGHAZ with cooling rate corresponding to a cooling time between 800 and 500°C (Δt8/5) of 5 and 10 s, followed by nanomechanical testing by compression. The results achieved showed significant scatter in the stress-strain data, depending on the actual local microstructure being tested. Bulk martensite and upper bainite, as well as blocky martensite-austenite (MA) particles present at prior austenite grain boundaries, were examined.
The phenomenon of low toughness in certain regions in welding of structural steels is well known (Thaulow, 1985, 1987). These are typically located in the coarse grained heat affected zone (CGHAZ, Thaulow, 1985, 1987; Grong, 1986) and the intercritically reheated coarse grained HAZ (Akselsen, 1987, Davis, 1994). The embrittlement has been attributed to the formation of brittle microstructures. In the former case, a brittle coarse upper bainite is usually present in a certain critical volume fraction (Grong, 1986; Zhang, 2009). For the ICCGHAZ, the formation of hard, brittle martensite-austenite (MA) islands adds to the problem (Tanoue, 1980; Akselsen, 1987, 1988). The macroscopic behaviour of such microstructures have been studied by extensive impact and fracture toughness testing, which will bring forward some average of the properties of the microstructure present in the actual test sample.