The effects of Ti-content (0.65-1.48 Ti) on tensile properties and impact toughness in Fe-18.5 Ni-4.5 W- Ti- 0.05 Al with varying heat treatments were investigated. Heat treatment temperatures varied from 750℃ to 950℃ for solution treatments and from 375℃ to 650℃ for aging treatments. The optimum condition of high yield strength (250 ksi) and good toughness was obtained in the alloy composition of Fe- 18.5 Ni- 4.5 W-1.35~1.4 Ti -0.05 Al after the solution treatment at 900℃ and aging at 500℃ for 3 hrs. It was found that impact energy and tensile properties were sensitivie to the Ti content, under a standard aging treatment (500℃/3hrs); the tensile strength increased from 1400 MPa to 1900 MPa and the impact energy decreased from 54 J to 12 J as the Ti content increased from 0.65 to 1.48 Ti. Overaging condition was designed to simulate the condition of HAZ during welding cycles. The retained austenite formed during the overaging treatment did not offer any advantage in impact energy compared with maraged state of the same level with fully martensite structure. As aging temperature increased from 550℃ to 650℃ with varying aging time, the amounts of retained austenite were studied by X-ray diffraction analysis. By varying aging temperatures from 375℃ to 550℃, the aging kinetics were studied by hardness measurements. At low temperature aging treatment, the exponent of Johnson - Mehl equation(n) was about 0.6. By the value of time to reach the maximum hardness, apparent activation energy estimated was about 295 KJ/mole. This value was about the same as the activation energy for volume diffusion of W in bcc-iron, 293KJ/mole reported. The precipitates formed during aging treatment were identified by transmission electron microscopy. The major strengthening precipitates formed during the aging treatment of Co-free W-bearing maraging steel was hexagonal n-$Ni_3Ti$ and orthorhombic $Ni_3W$. The orientation relationships of the precipitates and matrix were identified by electron diffraction spot pattern analysis. The orientation relationships of n-$Ni_3Ti$ was $(110)_α$//$(0001)_n$ and $<1\overline{1}1>_α$ // $[2\overline{1} \overline{1}0]_n$, while those of $Ni_3W$ was $(110)_α$// (010) and $<1\overline{1}1>_α$// [100]
