Internal hydrogen-induced subcritical crack growth in stainless steels
Huang, Jia-Hong
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https://hdl.handle.net/2142/20665
Description
Title
Internal hydrogen-induced subcritical crack growth in stainless steels
Author(s)
Huang, Jia-Hong
Issue Date
1989
Doctoral Committee Chair(s)
Altstetter, Carl J.
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Language
eng
Abstract
Three types of stainless steel--austenitic, ferritic and duplex--were cathodically precharged with hydrogen at high temperature in a molten salt electrolyte. Constant strain rate tests and sustained load tests were performed in air at temperatures from 0 to 50$\sp\circ$C with hydrogen contents up to 41 wt. ppm. The electrical potential drop method with optical calibration was used to monitor the crack growth position, continuously.
The log(da/dt) vs. K curves had definite thresholds for subcritical crack growth (SCG), but the stages II and III were not always clearly delineated. In the unstable austenitic steel the threshold decreased with increasing hydrogen content or increasing temperature, but beyond about 15 wt. ppm the stage II became less distinct. In the stable stainless steel, SCG was observed on a specimen containing 41 wt. ppm hydrogen. In the duplex alloy, the second stage of cracking was highly K-dependent and the crack growth behavior was more sensitive to bulk hydrogen content than to temperature. In the ferritic alloy, SCG at 25$\sp\circ$C was observed on a specimen precharged with 2.2 wt. ppm hydrogen.
Fractographic features were correlated to stress intensity, hydrogen content and temperature. In the unstable austenitic steel, more interface fracture occurred at low temperature and high hydrogen content, while more microvoid coalescence (MVC) occurred at low hydrogen content. In the duplex alloy, a flat fracture surface with narrow tear ridges was observed for specimens containing above 8 wt. ppm hydrogen. In the ferritic alloy, specimens tested in air fractured by MVC in contrast to those tested in 108 kPa hydrogen, which showed intergranular and transgranular facets on fracture surfaces.
The interpretation of the phenomena is based on: the different hydrogen diffusivity and solubility in ferrite and austenite, stress-induced phase transformation, and outgassing from the crack tip. Comparing the SCG behavior of internal hydrogen with that of external hydrogen, it is found that external hydrogen is more damaging than internal hydrogen. This is probably because the critical hydrogen concentration for SCG must be reached at a location that is very near the crack tip.
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