Abstract: A new type of R5 mooring chain steel was tempered at different temperature after being quenched at 950 ℃.Effects of tempering temperature on microstructure and mechanical properties of R5 mooring chain steel were investigated through microstructure observation,tensile and impact test and impact fracture analysis.The results show that the microstructure of R5 mooring chain steel after quenching and tempering at temperatures of 570-690 ℃ was tempering sorbite,but the size,morphology and distribution of carbides in the microstructure were different.When tempering temperature was in the range of 570-600 ℃,the flake and long strip carbides in the microstructure unevenly distributed on the grain boundaries of original austenite,martensite laths and in the inside of martensite laths,and the microstructure kept the direction of martensite.The flake and long strip carbides gradually became spherical and grew up when tempering temperature was in the range of 630-690 ℃.With the increase of tempering temperature the strength of R5 mooring chain steel decreased,but the elongation after fracture and impact absorption energy increased.When the tempering temperature was 630 ℃,the R5 mooring chain steel had optimum comprehensive mechanical properties,whose tensile strength reached 1 082 MPa,proof strength at plastic extension reached 973 MPa,elongation after fracture reached 17.0%,and impact absorption energy reached 66 J.