The paper presents rezults of microstructure (LM, TEM) investigation and hardness measurments of welded joints in martensitic precipitation hardened stainless steel containing copper, subjected to heat treatment. For the aging temperature up to 540◦C even for the very long times, the microstructure of the welded joints is similar to this one at lower temerature aging. After aging at 620◦C a distinct change of the microstructure was observed. Non-equilibrium solidification conditions of the weld metal, segregation and the diffusion of copper and the elements stablilizing the austenite cause the occurrence of the reverse transformation of the martensite into austenite as fast as just 1 hour at 620◦C. TEM investigations revealed the differences in dispersion of hardening copper precipitates after aging at temperature 620◦C for 1 and 4 hours.