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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.
Gas tungsten arc welds made on wrought magnesium AZ31 plate have
been characterized for corrosion in saline solution (3.5% NaCl). Microstructural
changes induced by the welding process resulted in different
environmental behaviour of each zone (BM-base metal, HAZ-heat affected
zone and FZ-fusion zone). The faster kinetics of corrosion in FZ and especially
HAZ are attributed to (a) the coarse microstructure, consisting of
large grains, and (b) very small amounts of ß-phase in the grain boundaries.
Also, hardness traverses have shown that these zones are weaker
than the base metal. Based on the significant effect of grain size on strength
in magnesium alloys, the weld metal grains have been systematically refined
using controlled oscillation during welding, and by adding a grain
refiner to the weld pool. Detailed microstructure analyses have been carried
out and the relationships between corrosion behaviour, mechanical
properties and microstructure (grain refinement and second-phases formation)
are highlighted.
Engineering design of spot-welded components as well as computational analyses of the local stress in the weld zone require relevant fatigue test data. Those test data, however, are dependent on the applied specimens, on the specimen restraint, on the load introduction into the weld zone as well as on the definition of the specimen failure. The discussion of the advantages and drawbacks as well as of the influence of these factors shows that a deliberate selection of specimens is necessary for different test purposes. For the comparability of the test results it is essential in addition to specify comparable test conditions, e.g., specimen fixturing, load introduction and definition of the specimen failure. On account of the variety of variants, a standardization of suitable specimens and respective test conditions hence constitutes an approach to the determination of relevant and comparable test results.
Hot cracks as stress corrosion cracking initiation sites in laser welded corrosion resistant alloys
(2005)
Although hot cracks at weld surfaces of corrosion resistant alloys (CRAs) might provide access to respective aggressive media and thus, might provide the conditions for a local potential and pH drop, the interaction between hot cracking and corrosion phenomena has not been investigated up to the present. This particularly concerns the initiation of stress corrosion cracking inside such crevices.
As a first approach to clarify the influence of hot cracks on corrosion resistance, considering particularly stress corrosion cracking, hot crack afflicted laser welds of three different CRAs have been subjected to a series of different corrosion tests.
Besides the fact that the Drop Evaporation (DE) Test represents a more realistic procedure than standard immersion tests it turned out that hot cracks can obviously represent an initiation site of stress corrosion cracking in the austenitic stainless steel AISI 309 (German No. 1.4828). The susceptibility of such hot crack afflicted laser welds to stress corrosion cracking (SCC) significantly depends on temperature and chloride concentration of the test environment In comparison, the high nitrogen austenitic stainless steel AISI S 34565 (German No. 1.4565) did not exhibit any stress corrosion cracking associated with hot cracking. This material exhibited pitting corrosion and stress corrosion only in the base material. The Alloy 50 (German No. 2.4850), a Nickel based material, was susceptible to pitting corrosion, but showed no tendency towards stress corrosion cracking at all.
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.