Beitrag zum Verständnis der Schwingungsrisskorrosion austenitischer Stähle

Translated title of the contribution: A contribution towards the understanding of corrosion fatigue in austenitic steels

Clemens Vichytil

Research output: ThesisDoctoral Thesis

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Abstract

In this work the corrosion fatigue behaviour of austenitic CrNiMo and CrMnN stainless steels was investigated in high chloride (43 wt. % CaCl2) solution at elevated temperatures (120°C). The aim was to understand failure mechanisms and develop new concepts for a better behaviour under cyclic loading in corrosive environments. Influence of Cl- contents, temperature, alloying element concentration was investigated. Alloys of both alloying families with varying alloying element content were tested in solution annealed, 14% and 27% cold worked condition in inert and corrosive media. Investigations were split in electrochemical experiments and mechanical testing. During electrochemical experiments cyclic polarization scans were performed and the repassivation behaviour at open circuit potential investigated. By systematic testing (different Cl- concentration and varying temperature) it could be shown, that little amounts of Nickel (ca. 7 wt. %) significantly increase the corrosion resistance of CrMnN steels. Nevertheless, CrNiMo based steels are better corrosion resistant by far. Under combined cyclic mechanical and corrosive loading fatigue experiments and crack propagation rate experiments were conducted. In inert environment CrMnN steels are superior compared to CrNiMo steels due to their higher mechanical strength. In highly corrosive environments CrMnN steels fails mainly due to stress corrosion cracking and active dissolution after depassivation of the surface. With increasing degree of cold work, the susceptibility to stress corrosion cracking is increased, which results in a loss of fatigue strength in corrosive environment. CrNiMo alloys indicate an overall satisfying behaviour under cyclic loading in extreme corrosive environments, but even at these steels the susceptibility to stress corrosion cracking is increased with decreasing alloying element content and enhanced mechanical properties by cold working. In CrNiMo allyos fatigue cracks propagate slower as in inert environment due to oxide induced crack closure effects. It could be shown that both positive properties of the alloying concepts can be united, if material and environment are precisely adjusted on each other. An empirical model was developed, which allows drawing a conclusion on the behaviour of a material under mechanical loading in corrosive environment, by simply recording a cyclic polarization curve and measuring the repassivation behaviour of an arbitrary material-media combination. Application limits can be efficiently determined and long lasting as well as extensive corrosion fatigue testing and stress corrosion cracking experiments are reduced to a minimum.
Translated title of the contributionA contribution towards the understanding of corrosion fatigue in austenitic steels
Original languageGerman
QualificationDr.mont.
Supervisors/Advisors
  • Pippan, Reinhard, Assessor B (internal)
  • Mori, Gregor Karl, Assessor A (internal)
Publication statusPublished - 2012

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embargoed until 23-08-2017

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