May 1980

Document Type


Degree Name



Dept. of Materials Science and Engineering


Oregon Graduate Center


Hydrogen-assisted delayed brittle fracture of high strength, low alloy steels is highly dependent on microstructure. Several specific microstructural features were observed to significantly alter both the static and dynamic resistance to hydrogen-assisted cracking of a 4340 steel. Increasing grain size increased both the threshold stress intensity (K [subscript th]) and the Stage II crack growth rate. More homogeneous distribution of impurities such as P and S increased K [subscript th] but hardly affected Stage II crack growth rate. And a fine dispersion of retained austenite platelets greatly improved both K[subscript th] and Stage II crack growth rate. Sensitivity of K[subscript th] to hydrogen concentration was found to be greatest below approximately 2 ppm nascent hydrogen while above 2 ppm H K[subscript th] became relatively independent of hydrogen concentration. Although not observed at hydrogen levels below 2 ppm, Stage II crack growth rates did not show sensitivity to hydrogen concentration.





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