Abstract: 35CrMnSi low alloy steel bars with different diameters were quenched by oil cooling at 900 ℃ and tempered at 230 ℃. The quenching critical diameter of the 35CrMnSi low alloy steel was explored by analyzing the relationship between the diameter of the steel bar and the hardness of the cross section and the microstructure of the center of the cross section. The results show that the correlation between quenched martensite volume and hardness is no longer consistent with the corresponding relationship provided by SAE J406 standard due to the presence of bainite. The martensite content of Ø60 mm steel rod is only about 40%, but the hardness of the center of the axis is close to the hardness corresponding to 90% martensite according to SAE J406 standard. The strength and toughness of the center of the axis are proved to meet the requirements of the strength and toughness of ultra-high strength steel, it can be determined that the quenching critical diameter of the 35CrMnSi low alloy high-strength steel is not less than Ø60 mm. The microstructure at the center of the axis of the Ø90 mm steel bar is granular bainite and upper bainite, which have limited influence on the hardness, and the hardness of 450 HV10 (46 HRC) is much higher than that of 50% martensite of SAE J406 standard, which also proves that quenching critical diameter of the 35CrMnSi steel predicted by the quenched martensite content of SAE J406 standard is much lower than the actual value.

Key words: 35CrMnSi steel, quenching critical diameter, hardness, martensite, bainite