This study discusses the microstructural evolution and examination of α'-martensite in fusion zones and heat-affected zones under multi-pass dissimilar stainless steels welding. The morphology, quantity, grain size, and chemical composition of the α'-martensite and δ-ferrite were analyzed using optical microscopy (OM), an image analyzer (IA), a ferritscope (FS), field emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS), respectively. Recrystallization induced grain refinement of α'-martensite in over-lapping heat-affected zones (HAZ1 > 2 and HAZ2 > 3) after the second and third pass of welding. The Creq, Nieq, and K-factor values were calculated in an elemental analysis according to the Kaltenhauser equation. The higher Nieq (8.0–9.26) and lower K-factor (7.73–9.50) were examined for α'-martensite in the second pass fusion zone (FZ-2) as opposed to δ-ferrite. Comparatively, the a-ferrite indicated higher Creq (22.30–22.91) and K-factor (15.72–16.68) values.