The precipitation behavior in double tempering between 100 ℃ and 600 ℃ of 4~6Cr-2~4W-2～4Mo-1.5~2.0V and Cr-W-Mo-V+0.05~0. 10N serious semi-high speed steel (HSS) quenched at 1050 ℃ has been studied

in which the morphology and composition of carbides are analyzed by Transmission and scanning electron microscopy

energy dispersive spectroscopy and electron energy loss spectroscopy

and the volume fraction of retained austenite are measured by X-ray diffraction method. The results indicated that the secondary hardening effect of steel was reinforced with addition of nitrogen

the precipitation of carbonitrides in tempering between 425 ℃ and 475 ℃ were attributed to nitrogen

and large amount of compound carbonitrides deposited and formed by tempering carbides at 550 ℃ touched to surface of nitride. The amount of retained austenite in semi-HSS + N tempered below 400 ℃ was 15%～17%

and decreased obviously with tempering temperature increasing and was less than 3% in steel tempered at 525 ℃.