February 1980

Document Type


Degree Name



Dept. of Materials Science


Oregon Graduate Center


Hydrogen assisted cracking in low alloy ultrahigh strength AISI 4340 steel heat treated to above 200 ksi yield strength was slightly dependent on microstructural features such as grain size, carbide type, amount of retained austenite and grain boundary composition. The susceptibility of the steel to hydrogen was similar for both internal as well as external hydrogen induced embrittlement. Increasing the prior austenite grain size slightly improved the threshold stress intensity but did not have any effect on region II crack growth rates. The change from epsilon carbide to cementite increased the region II crack growth rates. The presence of retained austenite slightly improved the threshold stress intensity for hydrogen assisted cracking. However, its thermal and mechanical instability actually increased the region II crack growth rates. Acoustic emission analysis confirmed the discontinuous nature of hydrogen assisted cracking. These discontinuous steps were found to be dependent upon the prior austenitic grain size. From acoustic emission results and based on the fact that the region II crack growth rates were independent of grain size, it was possible to propose a mechanistic model for hydrogen assisted cracking which combined the models proposed by Troiano and Petch and Stables.





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