In HSS welded structures, HFMI allows fatigue strength improvement factors by inducing compressive residual stresses, cold-worked surface region and modifying the weld toe shape. According to the literature, the factors are proposed based on failures at CAL and also checked with a limited dataset at VAL, the latter being more realistic of service loadings. Therefore, this paper investigates the behaviour of HFMI welds at CAL and VAL for R = -0.43. Studies are also performed at the microstructural level for locating the crack initiation and hardness. Fatigue damage sums are evaluated and discussed with the recommended values in the literature.