Strength transgranular cracking

A special form of HIC that occurs in low-strength steels in the presence of a stress is stress-oriented hydrogen-induced cracking (SOHIC) [127,130,131]. It is characterized by the formation of a stacked array of hydrogen-induced cracks, as shown in Figs. 16 and 17. These small cracks are oriented parallel to the rolling direction, but the stack itself is perpendicular to the stress. The cracks become hnked by transgranular cracks to produce an overall crack perpendicular to the stress. 

When a is small, K Kt and therefore K will exceed the low grainboundary strength at low applied load so that is still far below K c, the critical stress intensity for transgranular crack growth. When a is large the opposite result follows from equation (5.93). 

In the case of hydrogen-induced SCC of high-strength steels, one could add strength, no matter how this is achieved. Environmental criteria for SCC are necessary but not sufficient SCC will not occur without a susceptible metallurgy. The only exceptions to this rule are transgranular cracking processes in pure metals, e g., iron in anhydrous ammonia [69] or copper in sodium nitrite solution [65,70]. 

Fractography and crack propagation studies can differentiate between trans- and intergranular fracture in polyphase acrylics. A two-phase acrylic which exhibited intergranular fracture had a lower tensile strength than ones which fractured only transgranularly. 

Transgranular stress corrosion cracks are known [7.49] from i) austenitic steels in acidic chloride solutions, ii) low-strength ferritic steels in acidic media, iii) ferritic steels in phosphate solutions, iv) carbon steel in water saturated with CO2 and CO, v) a-brass in ammonia solutions that do not cause surface films, vi) aluminium alloys in NaCl/K2Cr04 solutions and vii) magnesium alloys in diluted fluoride solutions. For further study of fracture surface appearance, see, e.g. Lees [7.49] and Scully [7.53]. 

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