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KieBling R.,University of Stuttgart | Roos E.,MPA Stuttgart
Welding and Cutting | Year: 2012

Spot-welded joints pass through a process-dependent and location-dependent time-temperature cycle during the welding operation. For the more precise characterisation of the resulting different structural zones in the heat-affected zone (HAZ), Fig. 1, with their different mechanical properties, comprehensive tests were performed with the thermomechanical testing system called Gleeble2000. Within the framework of these investigations, the various structural regions of a heat-affected zone were produced and tested experimentally in a targeted way. The obtained knowledge about the mechanical behaviour of the individual structural zones can be referred to, in particular, for an improved failure assessment and can be used for a detailed simulation of the failure behaviour of spotwelded joints.

Michler T.,Adam Opel AG | Naumann J.,BMW AG | Hock M.,Linde Group | Berreth K.,MPA | And 2 more authors.
Materials Science and Engineering A | Year: 2015

Austenitic stainless steels with three different nickel contents were cold worked to various forming degrees at various temperatures to obtain a wide variety of cold worked microstructures. Dislocation density analyses using the Williamson-Hall method provide first indications that strain hardening using technically relevant cold forming parameters increases the susceptibility of austenitic stainless steels to hydrogen environment embrittlement mainly by creating a microstructure with a "critical" amount of dislocations. Although an effect of prior-existing martensite may not be totally excluded, this effect seems to be minor compared to the effect of dislocation substructure. Macroscopic residual stresses have no significant influence on the susceptibility of cold worked austenitic stainless steels to hydrogen environment embrittlement in tensile tests. © 2015 Elsevier B.V.

Sawada K.,Japan National Institute of Materials Science | Bauer M.,MPA Stuttgart | Kauffmann F.,MPA Stuttgart | Mayr P.,University of Graz | Klenk A.,MPA Stuttgart
Materials Science and Engineering A | Year: 2010

Creep tests were performed at 600 °C to investigate the microstructural changes in welded joints of 9% Cr-tempered martensite ferritic steel (grade E911), focusing on the influence of the multiaxiality of stress on the microstructure of a fine-grained heat-affected zone (FGHAZ). A higher creep cavity density was observed in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. This difference in the creep cavity density was related to a high degree of stress multiaxiality in the central part of the specimen. It was observed that VX particles were stable during long-term creep in both the base metal and the FGHAZ. M23C6 carbides coarsened faster in the FGHAZ than in the base metal; however, the effect of stress multiaxiality on the coarsening could not be detected. A Z-phase formed during creep, and the number density of the Z-phase particles was higher in the center of the specimen than in the surface region. The subgrain sizes increased and the dislocation densities decreased during long-term creep. However, the evolution of the dislocation substructure was not significantly different in the inner part of the specimen compared to the surface regions. © 2009 Elsevier B.V. All rights reserved.

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