Stainless maraging steel is a special type of maraging steel having a yield strength level exceeding about 1380MPa with high toughness as well as effective corrosion resistance. A new stainless maraging steel, Fe-10Cr-9.5Ni-W-0.25Ti-0.3Al, has been developed in this work. Since the alloy is strengthened by precipitates formed in martensitic matrix during aging, the precipitation kinetics upon aging treatments are important in determining an optimum heat treatment condition for obtaining good mechanical properties of the alloy. To distinguish the role of each precipitation hardening element, W(2,4wt. %) and 0.25wt. % Ti were added individually or simultaneously to the Fe-10Cr-9.5Ni-0.3Al base alloy. Hardness and electrical resistivity were measured at various aging temperatures from 300 to 600℃ and time up to 15 days. Microstructural examinations before and after the heat treatment were also made by optical, X-ray and TEM analyses.
The addition of titanium to the Fe-10Cr-9.5Ni-0.3Al alloy increased $A_s$ temperature from 609 to 657℃, while the $A_s$ temperature did not greatly vary with the addition of tungsten. The microstructures at room temperature were fully martensitic although both titanium and tungsten addition decreased the $M_s$ temperature. The electrical resistivity measurement revealed that at least two or more kinds of precipitates were formed by aging the Fe-10Cr-9.5Ni-W-0.25Ti-0.3Al alloy. Many precipitates could be formed in the alloys upon aging treatments. TEM analysis showed that the major strengthening precipitate was $\eta-Ni_3Ti$ in its beginning followed by the formation of $Ni_3W$ at the later stage.
The hardness change with aging time showed no incubation time, which was also confirmed by electrical resistivity measurement. The calculated apparent activation energy was 185kJ/mole, at the 50% fraction formed in the alloy containing tungsten. The precipitate growth exponent, n, in the equation $y=1-\exp(-kt)^a$ were in the range of 0.35∼0.6, and this suggested that a pipe diffusion mechanism prevailed in precipitation process. The increased content of tungsten was found to increase the rate of aging kinetics. When the Fe-10Cr-9.5Ni-0.25Ti-0.3Al alloy was aged at 450℃ for 100 hrs, some retained austenite was observed. When tungsten content was lowered, the amount of retained austenite was reduced. This study shows that the maximum strengthening precipitates in the stainless maraging steel are $\eta-Ni_3Ti$ combined with $Ni_3W$ precipitate. Optimum aging treatment for producing the maximumum amount of $\eta-Ni_3Ti$ or $Ni_3W$ precipitates is important for maximizing the yield strength in these alloys.
