A series of five bead on plate autogenous tungsten-inert-gas (TIG) welds were performed on U.S. Navy HSLA-100 steel. Power variations in these welds was achieved by altering the welding speed, voltage and current and were as follows (in kJ/mm); 0.7, 1.1, 1.2, 1.6 and 2.2. No evidence was found of either weld metal or underbead HAZ cracking in any of the welds illustrating the advantage of low carbon steel for both weld wire and base plate. Microhardness traverses across both the weld metals and HAZs gave a maximum. Vickers diamond pyramid hardness of 345 HV in the coarse grain HAZ next to the fusion line in the lowest power weld; for the highest power weld this was somewhat lower at 328 HV. These are well below 375 which is usually considered to be the lowest Vickers Hardness value for which severe hydrogen induced cold cracking is observed in this type of steel. Optical, scanning and transmission electron microscopy studies of the coarse grain HAZ microstructure in the regions of maximum hardness was correlated with the continuous cooling transformation diagram for this steel and good agreement between observed and predicted microstructures was obtained.