Abstract: 40Cr13 steel is an important material for manufacturing plastic molds. The plastics release chlorides and acids during hot pressing, which accentuates the surface pitting of the mold steel and indeed degrades the surface quality of the plastics. The effects of solution temperature on the microstructure evolution and chloride ion corrosion resistance of 40Cr13 steel were investigated via X-ray diffraction, optical microscopy, scanning electron microscopy, hardness testing, electrochemical tests and immersion corrosion in this study. The results showed that the microstructure of 40Cr13 steel was composed of ferritic matrix and dispersive M₂₃C₆ carbides enriched by Cr after hot rolling and air cooling. The increased solution temperature resulted in coarsened prior austenite grains and the gradual dissolution of M₂₃C₆ carbides. In the case of solution treatments at 900 ℃ and 980 ℃, the matrix of 40Cr13 steel was dominated by martensite. By comparison, lath martensite and retained austenite were prevalent when solution treated at 1 050 ℃ and 1 130 ℃. The content of the retained austenite increased with the increase of solution temperature. The results of the electrochemical tests and immersion corrosion showed that the conduction of solution treatment improved the chloride ion corrosion resistance of 40Cr13 steel, as compared with the hot rolling sample. When the solution temperature was raised up to 1 050 ℃, the values of the corrosion weight loss and corrosion current density approached the minimums, whereas those of the corrosion potential, pitting potential, impedance arc diameter and |Z|_{0.01 Hz} reached the maximums. Meanwhile, the value of the high phase angle approached close to 85°. The reason for the best corrosion resistance demonstrated by the sample solution treated at 1 050 ℃ should be mainly ascribed to the limited amount of the M₂₃C₆ carbide which was largely dissolved into the matrix, the low content of the retained austenite and the moderate grain size of the prior austenite.