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Fatigue crack growth in a low‐alloy steel in gaseous environments
Author(s) -
Popp Wolfgang,
Kaesche Helmut
Publication year - 1990
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.199000385
Subject(s) - hydrogen , saturation (graph theory) , materials science , alloy , oxygen , water vapor , fracture mechanics , metallurgy , growth rate , carbide , adsorption , composite material , crack closure , dislocation , alloy steel , chemistry , geometry , mathematics , organic chemistry , combinatorics
The growth of fatigue cracks in notched and pre‐fatigued DCB‐specimen of low‐alloy steel 90 MnV 8 with 0.89% C, 2.06 Mn, 0.32 Cr, tempered to average strength, was studied in UHV and in nitrogen, hydrogen, oxygen and water vapour, respectively. Conditions were chosen such that linear‐elastic fracture mechanics were applicable, and that static‐load crack extension (i.e. H‐induced dry SCC) was excluded. The gas pressure was varied between 10 −9 and 1 000 hPa, at temperatures of 298, 358 and 423 K. Hydrogen, oxygen and water vapour all increase the crack growth rate. While the influence of oxygen levelled off at ca. 0.1 hPa, both hydrogen and water vapour caused a monotonous increase of the crack growth rate over the whole range of pressures investigated. Oxygen is assumed to accelerate crack growth by surface adsorption (or surface oxidation), enhancing the production of dislocations at the strained crack tip, thus enhancing the rate of plastic deformation in the plastic zone preceeding the crack, with a saturation of the effect due to saturation of either adsorption or oxidation. Both hydrogen and water vapour are assumed to act via the ingress of atomic hydrogen into the plastic zone essentially by dislocation transport, thus enhancing the local concentration of hydrogen stripped off dislocations at carbide/matrix‐interfaces. Crack extension occurs by coagulation of microcracks thus formed. The concept can be developed quantitatively.