The influences of complementary tempering process on residual austenitic,residual stress and surface rigidity in transformation layer were studied in order to improve the machining quality and worklife of WEDM mould.

Furthermore,the best grain refinement heat treatment process for this material is twice sub-temperature quenching+high-temperature tempering process.

It is learned that the theory of removing sclerosis of residual austenite after transformation and theory of carbide deposition has different effect during different time in high-temperature tempering.

By means of three times procssing at high temperature tempering,the coarse nonequilibrium structure of 85Cr 2Mn 2Mo steel recrystallizes absolutly ,the austenite grain was effectively refined and structure heredity was cut.

The high temperature tempering can eliminate the residual stress caused by quenching, this can avoids the deformation in the following electric machining; after ultralow-temperature disposal process, the wear resistance of die increases 2 to 3 times, so its using time is consumedly prolonged.

In this paper,the relationships of structures and properties of 10Cr9MolVNbNsteel austenization normalizing at different temperature and time and dustenizationnormalizing and high temperature tempering following aging at 580 - 660℃ for zero to5000h were studied by means of transmission electron microscope (TEM) and scanningelectron microscope (SEM).