Steel sheets of low carbon steel and medium carbon steel were friction stir-processed separately at a constant weld pitch (ω/v) ratio of 6. The microstructures of PLCS and PMCS were observed dominated with bainite and martensite hard microconstituent phases, respectively. The average block width \(({d}_{{\text{b}}})\) of the bainite is slightly higher than that of martensitic phase. The carbide particle morphology in PLCS is the mixture of globular and small fibers, while it is irregular and elongated branched fibers type in PMCS. Also, PMCS consists of different morphologies of retained austenite (RA) such as blocky and film types. The dispersed particles in the matrix show \(\left( {001} \right)_{{M_{3} C}} // (\overline{1}10)_{B}\) and \(\left( {\overline{2}\overline{1}1} \right)_{{M_{23} C_{6} }} // (\overline{1}10)_{B}\) type orientation relationship which confirms the presence of M23X and M3C types of carbide. The lath thickness \(({d}_{{\text{L}}})\) of bainite is observed to be larger than that of martensites. The volume fraction of the retained austenite is observed to be around ~ 2% in the case of PLCS as compared to the volume fraction of ~ 5% in the case of PMCS. The experimental results while being compared with the predicted results of two different models, namely the Hall–Petch (H–P) and Langford–Cohen (L–C), are matched reasonably well in the case of PLCS, whereas a strong deviation is been observed in the case of PMCS.
