A creep technique was applied on a Gleeble-1500 thermal simulator for monitoring the aging precipitation in ultra-low carbon steels containing various coppers. The aging hardening curve was obtained by the hardness testing. With the aid of an optical microscope and TEM, the microstructure and the aging precipitates were detected. The results indicate that when the precipitation occurs during the creep a plateau will appear on the creep curve; the left-hand and right-hand endings of the plateau correspond to the precipitation start （Ps） and finish （Pf） times, respectively. The Pf obtained from the creep curve coincides with the peak hardness time （tp） at the aging hardening curve. A precipitation-time-temperature （PTT） diagram of the steel can be obtained.
The structure variation of deformed austenite during the relaxation stage after deformation at various temperatures in an Nb-B ultra low carbon bainitic steel and Fe-Ni alloy was studied by the thermo-simulation. Optical microscope and TEM were applied to analyze the microstructure after RPC （Relaxation-precipitation-controlling phase transformation technique） and the evolution of dislocation configuration. The particle tracking autoradiography （PTA） technique, revealing the distribution of boron, was employed to show the change of boron segregation after different relaxation times. The results indicate that during the relaxation stage the recovery occurs in the deformed austenite, the dislocations rearrange and subgrains form. During the subsequent cooling the boron will segregate at the boundaries of subgrains.
Xuemin WangBing CaoChengjia ShangXueyi LiuXinlai He
