Abstract: The approximate analysis method is used to make clear the damage mechanisms during cross wedge rolling. Hot tension and compression of 25CrMo4 specimens were implemented under three temperatures (1040， 1100 and 1160℃) and two strain rates (10 and 100s-1) on thermalmechanical simulator. The true stressstrain curve and microstructure was obtained and compared under tension and compression condition. Some conclusions can be drawn: The recovery and recrystallization leads to the softening mechanism during compression， and the plastic damage leads to the softening mechanism during tension. Plastic damage undergoes three phases: nucleation， growth and coalescence， whose type involves grain boundary damage and inclusion damage. Streamline is some place where refined grain， grain boundary damage and inclusion damage are apt to take place. The inclusion damage consists of that at grain boundary and inside grain. The axial tension stress is the main damage in highspeed railway axle during cross wedge rolling.

Key words: cross wedge rolling, highspeed railway axle steel, 25CrMo4, hot compression, hot tension, plastic damage