Macroscopic and Microscopic Processes in the Delayed Fracture Crack Growth of High Strength Steels
1978 Volume 64 Issue 12 Pages 1737-1746
Details
1978 Volume 64 Issue 12 Pages 1737-1746
A detailed study was made of the macroscopic and microscopic fracture behaviors in slow crack growth process for a JIS SNCM 8 steel (AISI 4340) and for an SNCM 23 steel (AISI 4320) immersed in distilled water as a function of applied stress intensity factor KI at various temperatures. The crack growth rate changed following after the change of temperature during growth process without delay, but an incubation time was observed with decreasing KI depending on the change in stress intensity factor ΔKI. It was found that the crack growth occurred as discontinuous steps for all test conditions. The frequency of microscopic crack jumps was strongly dependent on testing temperature while virtually independent of KI, and the crack jump distance was not influenced by temperature but might be related to microscopic nature of crack ing mechanics. Furthermore, it was indicated that the time intervals between crack jumps were controlled by a thermally activated process with apparent activation energy of about 9000 cal/mol. These experimental data may be considered to make proof of hydrogen embrittlement mechanism in slow crack growth process with the assumption that the critical hydrogen concentration required for crack growth is inversely proportional to KI.
