The great explor atory potential in deep waters has led companies in the oil sector to search for the necessary technological improvement to make the offshore exploration and production feasible. In parallel to searching for new technologies, the reduction in risks of structural failure has become the sector s constant practice, considering that such failures mean high costs due to partial use of equipments, extemporary maintenance, production stops, material losses and, mainly, damage to the ecosystem and loss of lives. A study has been made concerning the influence of the fraction of tempered martensite on the kinetics of fatigue crack growth in a grade R4 structural steel, largely used for fabricating offshore mooring chains by means of flash welding. CT specimens were machined from the base material as well as the welded joints and subjected to quenching from different austenizing temperatures and tempering at a given temperature. Aiming at the comparison with the material in industrial condition, CT specimens were also machined from chain links after flash welding, quenching and tempering under industrial conditions. After that, in a total of five microstructural conditions, the specimens were cyclically loaded in order to obtain the fatigue crack growth curves (a versus N and da/dN versus DK curves). The results show that the fatigue life of the material has increased when decreasing the fraction of tempered martensite, the industrial condition is associated with the smallest fatigue resistance and the fatigue life depends on the position where the fatigue specimens were taken form the chain like.