M(2)C PRECIPITATES IN ISOTHERMAL TEMPERING OF HIGH CO-NI SECONDARY HARDENING STEEL

The effects of isothermal tempering on the coarsening behavior of hexa gonal M(2)C precipitates and the secondary hardening reaction in ultra high-strength AerMet 100 steel were investigated. The tempering temper atures were 468 degrees C, 482 degrees C, and 510 degrees C, and the t empering time spanned the range from 1 ro 400 hours. Experimental stud ies of the coarsening behavior of the carbides were made by utilizing transmission electron microscopy (TEM), scanning electron microscopy ( SEM), and X-ray diffractometry (XRD). The hardness at the secondary ha rdening peak was about HRc 55. The average length and diameter of M(2) C carbides were 4 to 8 nm and 1.5 to 2.5 nm, respectively, al all thre e tempering temperatures; hence, the aspect ratio was almost 3, an equ ilibrium value in this case. The size of the M(2)C carbides increased monotonically with time, but the growth kinetics did not exactly follo w the classical coarsening behavior. The amount of precipitated austen ite increased with tempering time and temperature. M(2)C precipitates were still relatively fine even after 200 hours of tempering. This fea ture seemed to be closely related to the high hardness maintained afte r prolonged tampering.