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Hydrogen might cause severe degradation of supermartensitic stainless steels, if they are activated during exposure to sour environments. Consistent and comprehensive data for hydrogen transport in these materials are thus required to support, in particular, modelling of hydrogen assisted cracking as a part of life time assessment of welded steel components. In addition to previously published diffusion coefficients and subsurface concentrations of a supermartensitic stainless steel further data dependent on heat treatment are provided by this contribution. Furthermore, a higher alloyed material has been investigated in the state as received and also in the quenched condition, in order to approach the influences of chemical composition on hydrogen transport in supermartensitic stainless steels. With respect to welding it turned out that the diffusion coefficient and the subsurface concentration are markedly dependent on heat treatment of the materials.
Up to the present, the thermomechanical loads during welding and subsequent cooling under design-specific shrinkage restraint resulting from the interaction between the materials and the structure have often not been taken into consideration approperiately for weldability assessment of components. As compared to previous investigations using small specimens this report presents component weld tests under varying intensities of restraint. Online records of the reaction forces and moments demonstrate in which way the intensity of restraint affects the reaction stresses and moments and their meaning for the evaluation of the load level in welded components.
Eine Reihe nichtrostender austenitischer Cr-Ni-Stähle in der Nähe der eutektischen Rinne können beim Laserschweißen sowohl primär ferritisch als auch primär austenitisch erstarren. Speziell konzipierte Heißrißprüfungen zeigen eine Rißanfälligkeit der im Unterschied zum WIG-Schweißen mit großen Streckenenergien beim Laserschweißen mit kleinen Streckenenergien primär austenitisch erstarrenden Gefügebereiche. Die Ursache dafür ist, daß im Unterschied zu einer langsamen primär ferritischen Erstarrung bei einer schnellen Erstarrung über metastabile primär austenitische Zustände Restschmelzen entstehen können, die gegenüber dem Grundwerkstoff erhebliche temperaturabhängige Zähigkeitsverluste aufweisen. Es wird gezeigt, wie sich durch die Wahl auf den Werkstoff gezielt abgestimmter Schweißparameter die Heißrißsicherheit beim Laserschweißen weitestgehend sicherstellen läßt. Darüber hinaus werden Perspektiven vorgestellt, wie die Heißrißsicherheit bei Hybridschweißen durch die Auswahl vom Zusatzwerkstoffen und durch eine geeignete Prozeßführung zu erreichen ist.
Hydrogen assisted steel corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, such as offshore platforms and pipelines. Measurement of local and crack critical hydrogen concentrations in a specific weld is difficult and not very precise. Numerical simulations of hydrogen uptake and diffusion developed within the last twenty years have to be regarded as much more effective to determine local hydrogen concentrations in crack critical regions of welds. As an overview of existing theories of the state of the art in modelling of hydrogen cracking in steels, the present contribution shows that the development of a comprehensive numerical model for both types of hydrogen assisted cracking in welds, hydrogen assisted stress corrosion cracking and cold cracking, is still a major research task. As a first step towards that direction, a basic procedure for numerical simulation of crack initiation and propagation has been developed and experimentally verified. The approach can be extended to a comprehensive quantitative model for life time prediction and risk based inspection of welded steel components. (Steel specimen used was S355 along with other materials such as AISI 321, Incoloy 800, Inconel 690 and 600.)
Düring the past decades, weldability studies have predominantly been concentrated on the material and the welding process. In order to achieve a closer insight into the structural effects, component weldability tests have been performed by online monitoring of the reaction forces and moments. As a particular item, the effects of different strengths of the base and filier material on the reaction force and stress build up transverse to the welding direction have been studied at a specific structural restraint. As a most relevant result for practical welding it is demonstrated that the final reaction force and also the reaction stress level in the multilayer butt welds decreases with overmatch of the weld metal.
Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or cathodic protection. Thus, hydrogen assisted stress corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, as for instance offshore platforms and pipelines. In order to support conclusive testing and life time evaluation of welded steel components, a numerical model for hydrogen assisted cracking has been developed, particularly taking into consideration the geometrical effects of crack propagation on the respective hydrogen distribution alongside and ahead of the crack. Numerical calculations were based on finite element analysis of the hydrogen and stress-strain distribution by using a commercially available program. The model has been verified experimentally by slow strain rate experiments of supermartensitic stainless steels which are intended to be used more extensively as materials for welded flowlines in North Sea oil and gas production. As first results of such simulations the influence of the subsurface concentration provided by different H2S saturation levels in the NACE TM 0177-96 standard solution on crack propagation and the effect of crack shape on the hydrogen distribution profile are presented in this contribution.