Abstract: Effect of induction hardening on the microstructure, hardness, and hardened layer thickness was analyzed of a high-end CNC machine tool guide rail formed by HT300. The causes of cracks during the quenching process were also analyzed. The results show that after intermediate frequency induction hardening, the surface hardness of the machine tool integrated guide rail of cast iron HT300 can reach over 700 HV (60 HRC), and the depth of the harded layer can reach 5.5 mm. After induction hardening heat treatment, the surface layer of the cast iron guide rail mainly forms cryptocrystalline martensite, the transition layer mainly consists of martensite+pearlite structure, and the core is mainly composed of pearlite structure. Crack analysis shows that the graphite in cast iron is mainly E-type graphite, and the phosphorus eutectic structure present in the cast iron dissolves and precipitates during quenching, reducing the surface strength of the cast iron. After deep layer induction hardening, the tensile stress formed on the surface layer exceeds the strength of the cast iron, which is the main reason for the cracking of the cast iron during quenching. The cracking source is located on the surface, and the cracking location is mostly in the phosphorus eutectic region.

Key words: machine tool guide rail, induction hardening, HT300 cast iron, phosphorus eutectic, microstructure