TY - GEN A1 - Mente, Tobias A1 - Böllinghaus, Thomas ED - Böllinghaus, Thomas ED - Lippold, J. C. ED - Cross, C. E. T1 - Numerical investigations on hydrogen-assisted cracking in duplex stainless steel microstructures N2 - Duplex stainless steels (DSS) are used in various industrial applications, e.g. in offshore constructions as well as in chemical industry. DSS reach higher strength than commercial austenitic stainless steels at still acceptable ductility. Additionally, they exhibit an improved corrosion resistance against pitting corrosion and corrosion cracking in harsh environments. Nevertheless, at specific conditions, as for instance arc welding, cathodic protection or exposure to sour service environments, such materials can take up hydrogen which may cause significant property degradation particularly in terms of ductility losses which, in turn, may entail hydrogen-assisted cracking (HAC). The cracking mechanism in DSS is different from steels having only a single phase, because hydrogen diffusion, stress-strain distribution and crack propagation are different in the austenite or ferrite phase. Therefore, the mechanism of HAC initiation and propagation as well as hydrogen trapping in DSS have not been fully clarified up to the present, as for most of the two-phase microstructures. At this point the numerical simulation can bridge the gap to a better insight in the cracking mechanism regarding the stress-strain distribution as well as hydrogen distribution between the phases, both austenite and ferrite, of the DSS. For that purpose, a two dimensional numerical mesoscale model was created representing the microstructure of the duplex stainless steel 1.4462, consisting of approximately equal portions of austenite and ferrite. Hydrogen assisted cracking was simulated considering stresses and strains as well as hydrogen concentration in both phases. Regarding the mechanical properties of austenite and ferrite different statements can be found in the literature, dependent on chemical composition and thermal treatment. Thus, various stress-strain curves were applied for austenite and ferrite simulating the HAC process in the DSS microstructure. By using the element elimination technique crack critical areas can be identified in both phases of the DSS regarding the local hydrogen concentration and the local mechanical load. The results clearly show different cracking behavior with varying mechanical properties of austenite and ferrite. Comparison of the results of the numerical simulation to those of experimental investigations on DSS will improve understanding of the HAC process in two phase microstructures. KW - duplex stainless steel 1.4462 (2205) KW - numerical simulation KW - hydrogen assisted cracking KW - diffusion PY - 2016 UR - http://link.springer.com/chapter/10.1007%2F978-3-319-28434-7_16 UR - http://www.springer.com/us/book/9783319284323 SN - 978-3-319-28432-3 SN - 978-3-319-28434-7 U6 - https://doi.org/10.1007/978-3-319-28434-7_16 SP - Part V, 329 EP - 359 PB - Springer International Publishing CY - Switzerland ET - 1 AN - OPUS4-35591 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - GEN A1 - Böllinghaus, Thomas A1 - Steppan, Enrico A1 - Mente, Tobias ED - Böllinghaus, Thomas ED - Lippold, J. C. ED - Cross, C. E. T1 - Hydrogen assisted cracking of a subsea-flowline N2 - Since the mid-nineties, supermartensitic stainless steels (SMSS) have increasingly been applied to welded subsea-pipeline systems in the North Sea oil and gas fields, especially to flowlines at mild sour service conditions. However, in 2001 cracking and leaks occurred during installation and service start-up of two SMSS flowlines in the Norwegian Tune gas condensate field, welded with a new developed matching filler wire. Brittle transgranular cracking started especially at inter-run lack of fusion and propagated brittle, predominantly through the weld metal. The present paper provides a brief overview of the original failure case and respective sequence of events leading to complete replacement of the SMSS by carbon steel flowlines in 2002. Then, detailed investigations of a circumferential weld sample of the failed Tune flowline are highlighted, targeted at comparison of the failure appearance to previous investigations of this filler material type and to search for possible explanations for the brittle fracture at the crack initiation area. SEM investigations of the fracture surface revealed brittle areas only in the direction towards the top side of the weld while the major part of the investigated surface exhibited ductile fracture. As an approach to clarify, if the fracture was a consequence of hydrogen assisted cracking, five small sized specimens have been cut out of the original sample. Cracking has been introduced parallel to the original fracture surface in these specimens at respective saw cuts and bending. The results show that brittle transgranular cracking appeared only in the specimen cooled down to very low temperatures by liquid nitrogen and in the sample charged with hydrogen to an average concentration of about 15 ml/100 g. However, a fracture similar to the original surface was observed only in the hydrogenized specimen. As a further result, very similar fracture surfaces of supermartensitic stainless steel weld metals had been observed on specimens subjected to hydrogen assisted cold cracking (HACC) as well as to hydrogen assisted stress corrosion cracking (HASCC). In total, the results indicate that brittle fracture starting at the inter-run lack of fusion were not initiated by high notch tip deformation rates, but rather influenced by hydrogen, probably taken up during welding. KW - supermartensitic stainless steel KW - hydrogen assisted cracking KW - fracture topography PY - 2016 UR - http://link.springer.com/chapter/10.1007/978-3-319-28434-7_17 UR - http://www.springer.com/us/book/9783319284323 SN - 978-3-319-28432-3 SN - 978-3-319-28434-7 U6 - https://doi.org/10.1007/978-3-319-28434-7_17 SP - Part V, 361 EP - 379 PB - Springer International Publishing CY - Switzerland ET - 1 AN - OPUS4-35592 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - GEN A1 - Böllinghaus, Thomas A1 - Mente, Tobias A1 - Wongpanya, Pornwasa A1 - Viyanit, Ekkarut A1 - Steppan, Enrico ED - Böllinghaus, Thomas ED - Lippold, J. C. ED - Cross, C. E. T1 - Numerical modelling of hydrogen assisted cracking in steel welds N2 - Hydrogen assisted stress corrosion and cold cracking represent still a major topic regarding the safety of welded steel components against failure in many industrial branches. Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or at cathodic protection. Additionally, understanding and avoidance of hydrogen entry into weld microstructures from gaseous pressurized environments becomes increasingly important for renewable energy components. There are two types of metallurgical mechanisms associated with hydrogen assisted cracking, i.e. the cracking as well as hydrogen transport and trapping mechanisms. For numerical modelling, it has to be considered that both types are not independent of each other, that the mechanisms are not yet completely clarified and that validation of such models strongly depends on implementation of the correct hydrogen related materials properties. However, quite significant achievements have been made in modelling of hydrogen assisted cracking by indirect coupling of thermal, stress-strain as well as hydrogen uptake and diffusion analyses. After a brief introduction into the subject and by revisiting various proposed cracking mechanisms, the present contribution focuses on recent developments of a numerical model based on a comparison of actual hydrogen concentrations and mechanical loads with respective hydrogen dependent material properties as crack initiation and propagation criteria. The basic procedure for numerical simulation of crack initiation and propagation is outlined and it is shown how such numerical simulations can be validated experimentally. Furthermore, it is highlighted how such a procedure has been extended to a comprehensive model for life time prediction of welded steel pipeline components and experimentally verified. Finally, it is outlined how the model can be extended to simulate cracking in heterogeneous steel microstructures on the different scales. KW - hydrogen assisted cracking KW - numerical simulation KW - supermartensitic stainless steel KW - high strength low alloyed structural steel KW - duplex stainless steel PY - 2016 UR - http://link.springer.com/chapter/10.1007%2F978-3-319-28434-7_18 UR - http://www.springer.com/us/book/9783319284323 SN - 978-3-319-28432-3 SN - 978-3-319-28434-7 U6 - https://doi.org/10.1007/978-3-319-28434-7_18 SP - Part VI, 383 EP - 439 PB - Springer International Publishing CY - Switzerland ET - 1 AN - OPUS4-35593 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -