Laser beam welding is precise, quick and highly automatable. Nevertheless, disadvantageous hardness profiles can result and promote cracking. By an ultrasonic post treatment, crystal defects, internal stress and grain structure can be altered to achieve uniform hardness. In the investigations round bars with 30~mm diameter made from stainless steel grade 1.4301 are welded by laser in a rotational process. Ultrasonic excitation is applied utilizing a longitudinal mode of the system. The weld pool is positioned in the node or the antinode of the amplitude distribution. The excitation amplitude varies at 0/2/4~µm and the treatment durations at 0/5/10~min. The welds are evaluated by metallographic cross sections and hardness measurements. The results indicate the effects of acoustic residual softening and hardening. With standard deviations of about 2~\%, the weld hardness is decreased by 3~\% with nodal excitation and increased by 4~\% with antinodal excitation. The difference between weld and base material hardness is not reduced since the base material is hardened at all ultrasonic parameters used.