Abstract: 4Cr5Mo2V steel with different hardness (41.2, 45.0, 49.3 HRC) was obtained by quenching and different tempering heat treatment processes. The effect of tempering hardness on the erosion resistance of the steel in molten aluminum was studied by making the samples rotate dynamically in the molten aluminum alloy. The results show that with increasing tempering hardness, the erosion mass loss of the test steel for the same erosion time decreased. With the extension of erosion time, fine pitting pits appeared on the face-aluminum surface of the test steel first, and then partly peeling and partly severe corrosion appeared, and finally comprehensive corrosion occurred. The intermetallic compound of the interface layer consisted of a dense η-Fe₂Al₅ phase inner layer and a porous α_H-Fe₂Al₈Si phase outer layer. With increasing tempering hardness, the thickness of the inner layer of back-aluminum surface increased gradually, and the proportion of the outer layer decreased gradually, the chromium content in the matrix increased, resulting in the improvement of erosion resistance of the test steel in the molten aluminum alloy.