Abstract: By means of metallography, scanning electron microscopy and electron-beam microprobe, the white layer and the melted layer formed on the surface of a vacuum heated 18-4-1 high speed steel after oil quenching are studied. The results show that both layers are arisen from the instantaneous carbon pick-up during the oil quench after vacuum heating. The microstructure of the white layer consists of a large amount of retained austenite with some martensite and granular M6C type carbide. The necessary conditions for the carbon pick-up are vacuum high temperature heating and the use of oil as a quenchant.

Key words: High speed steel, Melted layer, Vacuum heated oil quenching, White layer