The molybdenum–iron–boron–chromium claddings with different Mo/B atomic ratios were produced on Q235 steel using argon arc welding. The microstructure and crystalline phases were studied by optical microscopy, scanning electron microscopy and XRD. In addition, the formation mechanism of hard phase was investigated by thermodynamic calculations and phase diagrams. The results showed that the claddings were composed of the Mo2FeB2, M3B2 (M: Mo, Fe and Cr) hard phases and σ-FeMoCr solid solution. In addition, calculated results revealed that the M3B2, MB and σ-FeMoCr were successively precipitated from the melting pool. Moreover, the maximum microhardness value of the cladding was about 1600 HV0.5. Wearing test indicated that claddings of lower Mo/B ratios have better wear resistance.