The tempering embrittlement and its overaging phenomenon (counter^segregation) of alloy steel 2.25Cr-1Mo at tempering temperature have been studied. The non-equilibrium segregation law of phosphorus in alloy steel 2.25Cr-1Mo (0.15C

2.32Cr

0.95Mo

0.009P) tempering at 650 °C for 140 h was obtained with using the kinetics theory of non-equilibrium segregation r The test results showed that critical time t

c

of phosphorus non-equilibrium segregation in alloy steel 2.25Cr-1Mo at 650 ℃ was 20 h

at this time the atomic concentration of phosphorus at grain boundary increased to maximum value 2.79% from 0.016% at original status. According to kinetic calculation of test data

the obtained calculated curve conformed with the test results which verified the diflusion mechanism of non-equilibrium grain segregation for vacancy-complex model.