HYDROGEN EMBRITTLEMENT IN HIGH STRENGTH CARBON STEEL

High strength carbon steels (HSLA) are limited to a relatively low yield strength/hardness in subsea use, due to hydrogen embrittlement.

The hydrogen source is normally the sacrificial anodes. The lowered potential causes formation of atomic hydrogen on the steel surface, and some of this hydrogen in absorbed in the steel (HISC).Cracking has been observed in high-strength alloys such as supermartensitic stainless steel and superduplex stainless steels, and extensive testing has been performed in order to establish limits for maximum stresses applied to these materials so that this can be avoided.

HISC has also been observed in high-strength carbon steel, such as subsea bolts, and some investigation has been performed on this.  The sensitivity for hydrogen cracking is proportional with the hardness of the material, and will become more critical for carbon steel as higher strength steels are being used. 

Questions:

• Which is the most reliable test method? This may be based on earlier thesis work, and published literature.
• Which criterium is essential? Threshold stress, global stress, KIC?
• Ultimate question: Is it possible to extend the maximum hardness permissible in subsea use for carbon steels? This would be of great interest for the offshore industry as higher strength steels could be used in subsea structures, cutting weight and possibly cost.

Proposed work:

- Literature study of hydrogen embrittlement, and HISC in particular

- Heat treatment of high strength steel to different hardness levels

- Testing of samples in electrolyte (salt water) at a potential comparable to cathodic protection. 

Samples must be stressed by for instance constant load or slow strain rate testing in tensile testing machine.