Hydrogen Induced Delayed Cracking of Type III Steel Specimens

The influence of loading mode on the apparent yield stress, which is the external stress required to cause the local macroscopic plastic deformation, and the possibility of hydrogen induced delayed cracking of Type III specimens are investigated. Results showed that the apparent torsional yield stress of the severly charged Type III specimen did not decrease and hydrogen induced delayed cracking and fracture would not occur on the original crack or notch plane. For a combined Type I + III specimen, the apparent torsional yield stress could be decreased only when K_I was so large that hydrogen induced delayed plastic deformation could occur under the action of K_I itself. For the charged Type III cracked or notched specimens of ultra high strength steel, the hydrogen induced delayed cracking could occur along the planes inclined at an angle of 135° to the original crack or notch plane after a large enough torque has been sustained for a sufficiently long period of time. This delayed failure was a typical intergranular fracture. A flat shear fracture would be obtained on the original crack or notch plane if the charged Type III specimen was twisted to fracture immediately. For the uncharged or outgased Type III specimen, no delayed cracking or failure occurred even if a maximum torque had sustained for a long time. The interactive energy between shear stress field of the Type III crack and the strain field of an interstitial hydrogen atom is calculated. The result shows that the interactive energy has a minimum value on the planes inclined at an angle of 135° to the original crack or notch plane. Hence, atomic hydrogen will diffuse to and be enriched in these planes and hydrogen induced delayed cracking and fracture will occur along these planes when the concentration of hydrogen in these planes reaches a critical value.