Acoustic Emission from Arctic Steels

• Main Author: Larsen, Andreas Vrenne
• Other Authors: Thaulow, Christian
• Format: Master Thesis
• Language: English
• Published: NTNU 2015
• Subjects: Produktutvikling og produksjon; Materialer; Arctic; Østby
• Online Access: http://hdl.handle.net/11250/2350253

As the search for hydrocarbons moves into the Arctic regions new materials are required to meet the new challenges due to the harsh environment. Arctic Materials 2, is a cooperation between SINTEF, NTNU, DNV GL, several material manufactures and Oil&Gas companies. Brittle fracture initiated in the Heat Affected Zone (HAZ), which is created by welding, in low temperatures is of interest. When a material is welded the microstructure changes due to heat transmitted from the welding, and the transition temperature is significantly reduced. The three main factors which primarily decide the toughness of the material after welding are the base material chemical composition, the maximum temperature from welding and the rate of cooling. This transformation of microstructure combined with low temperatures makes it vulnerable to brittle fracture, even though the base material is ductile. Brittle fracture in steel is linked to the microstructure in the material with respect to initiation, propagation and arrest of cracks. Traditional testing is often not accurate enough to cause any visible signs in the load displacement curves at microcracking. Acoustic Emission (AE) makes this possible. The main objective has been to investigate the relationship between Acoustic Emission (AE) amplitude and the arrested cleavage microcrack size based on a theoretical relationship presented by Lysak (1996) and further developed by Østby et aI. (2012). This relationship may provide quantitative data as input for development of the micromechanical based cleavage fracture models for steel. In this context the Multiple Barrier Model is used as a model to describe a cleavage fracture initiated at M-A particles (Lambert-Perlade et al., 2004) and (Martin-Meizoso et al., 1994). Fractographic investigation has been carried out with SEM and EDS. In addition, AE signals have been analyzed and linked to arrested microcrack. Furthermore, improvements to the procedure for post processing and analysis of the AE signals have been made. Only one arrested microcrack was found which could be connected to AE amplitude, but it deviated from the curve presented in Østby et al. (2012). The reason might be differing perceptions of how to measure and what to measure. Further on were large scatter in the fracture toughness observed. This might be linked to presence of upper bainite and autotempered martensite. The formation of M-A phases was reduced due to the low amount of carbon. No initiations observed in M-A particles were observed. The Multiple Barrier Model could not be linked to initiation in M-A particles, but different stages were seen at larger inclusions. Smooth surfaces observed on the fracture surface, were investigated, but not identified. Further testing and localization of arrested microcracks is necessary to confirm its relationship to AE amplitude.