Transgranular steels

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Microstructural examinations disclosed highly branched, predominantly transgranular cracks originating on the internal surface. Cracks of this form are typical of SCC in austenitic stainless steels. 

Cracking (SCC) HjO liquid phase. transgranular except chlorides and steel under some or static— is not susceptible to... 

The fracture mode of stress-corrosion cracks in austenitic stainless steels can be transgranular, intergranular or a mixture of both. One of the earliest environments found to cause problems was solutions containing chlorides or other halides and the data due to Copson (Fig. 8.30) is very informative. The test solution for that data was magnesium chloride at 154°C the alloys contained 18-20alloy with a composition of approximately 18Cr-8Ni has the least resistance to cracking in this environment. 

The behaviour of austenitic stainless steels in caustic solutions has received less attention than cracking in chloride environments. Transgranular cracking has been reported for low-carbon (< 0.05%) steels in caustic solutions, whereas higher carbon content alloys cracked intergranularly. Wilson and Aspen showed that resistance to cracking was not decreased by sensitisation heat treatments. Type 316 stainless steel has been shown to be more susceptible to cracking in caustic than type 304. ... 

Fig. 8.35 Reduction in area versus applied potential for 316 stainless steel, in 5 ppm chloride content water at 250°C. Horizontal arrows indicate RA% for tests in argon gas open-annealed, filled-sensitised. Vertical downward pointing arrows on the annealed curve indicate cracking-non-cracking boundaries. Full downward pointing arrow on the sensitised curve indicates commencement of cracking. Open upward pointing arrow on the sensitised curve indicates transition from transgranular to intergranular cracking...

Metallographic examinations of a cross-section through the failed clamp material confirmed it was an austenitic stainless steel and that extensive branched transgranular cracks were present throughout the material, as illustrated in Figure 7.49. 

Conclusions. Examination of the failed pipe clamp revealed the presence of brittle fracture surfaces as well as an extensive array of branched transgranular cracks. The identification of the clamp material indicated it was a forged 304 stainless steel component... 

Craig has discussed hydrogen effects in aluminum alloys and notes that the phenomenon is not too different from that in steels. It is possible to find intergranular or transgranular cracking or blistering. Blisters tend to form as a collection of nearsurface voids that coalesce to produce a large blister. 

Branched cracks, transgranular except for caustic embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). 

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