Hydrogen induced stress cracking in supermartensitic stainless steels – Stress threshold for coarse grained HAZ

• Two typical supermartensitic stainless steels used in offshore pipelines were tested.
• The test objective was to establish hydrogen cracking stress thresholds.
• The tests were performed in weld HAZ subjected to varying thermal histories.
• The thermal history (PWHT/reheated) was found to have a significant effect.

The objective of the present work was to investigate the influence of the microstructure on the hydrogen cracking susceptibility of two typical pipeline supermartensitic stainless steels. The work has concentrated on the coarse-grained heat affected zone because this is the typical crack initiation point. The hydrogen cracking susceptibility was tested by notched tensile test samples containing a simulated coarse grained heat affected zone. The samples were tested under constant load immersed in 3.5% NaCl and subjected to a negative potential corresponding to that induced by the cathodic corrosion protection system. The load was increased by a small amount every second day in order to establish the threshold stress for hydrogen cracking during service in seawater with hydrogen being introduced on the steel surface by the cathodic protection system. A significant influence of the second heat cycle was observed. This effect was attributed to both a variation in yield strength and the influence of precipitates on hydrogen solubility and diffusion.