The Effect of Small Additions of Co, Al and Mn on the Secondary Hardening in Fe-Mo-C Martensite

Abstract

The effect of small additions of third elements (Co, Al and Mn) on the secondary hardening during aging in Fe-4%Mo-0.2%C martensite has been studied by means of hardness measurements, electric resistivity measurements, optical microscopy and transmission electron microscopy.
The results obtained are as follows:
(1) In the isothermal aging, the maximum hardness of the specimens which contained of Co, Al and Mn in Fe-4%Mo-0.2%C, was higher than that in plain Fe-4%Mo-0.2%C martensite. In these specimens, the time to attain the maximum hardness was shorter than in plain Fe-4%Mo-0.2%C martensite.
(2) Especially, in the specimens containing of 0.1%Al and 0.1%Mn, the increasing rate of hardness before reaching a hardness peak was larger than in plain Fe-4%Mo-0.2%C and Fe-4%Mo-0.1%Co-0.2%C martensites. Remarkable differences were also observed by electric resistivity measurements and transmission electron microscopy. This result coincided well with the hardness change.
(3) The addition of Al and Mn retards the anihilation of dislocations in matrix. This offers the nucleation sites of Mo₂C. The addition of Co increases the supper saturation of Mo. These may be considered as the reasons why Mo₂C precipitation were accelerated.