Because of the local hardening, welded joints in ferritic steels are susceptible to hydrogen-induced stress corrosion cracking. The problem is severe in H2S media, and hardness limits are normally imposed to avoid failure, exemplified by the NACE MR0175 criteria. Such limits have been derived from field experience and laboratory tests under static loading. In practice, most plant undergoes load fluctuation, and the present study examined the effect of a small fluctuating load component on sulphide stress cracking (SSC) behaviour. Weld metals were produced in C–Mn steel using the submerged arc and shielded metal arc processes, to give deposit hardness from 180HV to 245HV. Precracked samples were tested in the NACE TM-01- 77 Method A environment, both statically and with load variations of 5% or 10% of the nominal value, at frequencies of 0.15 and 0.015Hz. Imposition of fluctuating load was found to promote SSC at 245HV, the threshold stress intensity was reduced by up to 60%.The effect of cyclic load was slightly increased at higher load variation and with reduced frequency of application.
Keywords: Welds, SSC, stress intensity, carbon-manganese steel, fluctuating load.