The paper deals with an analysis of preheating and heat treatment after welding and their influence on the final hardness level in case of welded jonits for common materials applied for manufacture of pressure vessel plates. During the welding process, the higher values of hardness and simultaneously decreasing of plasticity was observed and measured in the heat affected zone. Therefore, there was necessary to preheat the base material, or to apply the heat treatment after welding, or to suggest any combination of both mentioned ways. First of all, the groups of tested plates with required parameters were chosen for the application of the submerged arc welding method (according to ÄSN EN ISO 4063), and preparation of samples for hardness measurement (ÄSN EN ISO 6507) as the next step. The main goal of this experiment was to determine the hardness level for two fine grained steel grades after welding with the given parameters, as well as to adjust or to eliminiate the heat treatment process (preheating, post-heating), if they are not necessary.