The effect of two different heat inputs, 1.2 and 0.8 kJ/ mg, on the microstructure associated with a welded high hardness armor (HHA) steel was investigated by ballistic tests. A novel way of comparing the ballistic performance between fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM) of the HHA joint plate was applied by using results of the limit velocity V50. These results of V50 were combined with those of ballistic absorbed impact energy, microhardness, and Charpy and tensile strength revealing that the higher ballistic performance was attained for the lower heat input. Indeed, the lower heat input was associated with a superior performance of the HAZ, by reaching a V50 projectile limit velocity of 668 m/s, as compared to V50 of 622 m/s for higher heat input as well as to both FZ and BM, with 556 and 567 m/s, respectively. Another relevant result, which is for the first time disclosed, refers to the comparative lower microhardness of the HAZ (445 HV) vs. BM (503 HV), in spite of the HAZ superior ballistic performance. This apparent contradiction is attributed to the HAZ bainitic microstructure with a relatively greater toughness, which was found more determinant for the ballistic resistance than the harder microstructure of the BM tempered martensite.
Keywords: V50 ballistic resistance; ballistic armor; fusion zone (FZ) and heat affected zone (HAZ); low alloy high strength steel; microstructure; shielded metal arc welding (SMAW).