The effects of W-contents (4.5, 6.0, 8.0 w/o) on tensile properties and impact toughness in Fe-18.5 Ni-W-1.2 Ti-0.1 Al with varying heat treatments were investigated. Heat treatment temperatures varied from 750℃ to 900℃ for solution treatments, and from 450℃ to 550℃ for aging treatments. The optimum condition of high yield strength (250 Ksi) and good toughness (25 Joule) was obtained in the alloy composition of Fe-18.5 Ni-4.5 W-1.2 Ti-0.1 Al after the solution treatment at 815℃ and aging at 500℃ .
It was found that tensile elongation behavior of the Fe-18.5 Ni-4.5 W-1.2 Ti-0.1 Al (W-250 alloy) was very sensitive to the heat treatment conditions. Under a constant low temperature aging (450℃/3hrs), the tensile elongation decreased from 8 to 2% as the solution treatment temperature increased from 750 to 900℃ . On the other hand, under a constant solution treatment (900℃/1 hr), the elongation increased from 2 to 8% as the aging temperature increased from 450℃ to 500℃ .
The effects of three different heat treatment conditions on tensile, impact, and low cycle fatigue behavior in the W-250 alloy were studied. The heat treatments applied were the solution treatment (815℃/1 hr/AC), the solution treatment plus maraging treatment (815℃/1 hr/AC+480℃/3 hrs/AC), and the solution treatment plus overaging treatment (815℃ /1 hr/AC+650℃ /3 hrs/AC).
This Co-free "W-250" maraging steel had comparable tensile properties with conventional Co-containing 250 grade maraging steel under the standard heat treatment. The overaging treatment (at 650℃ for 3 hrs, followed by air cooling) produced about 13% volume fraction of retained austenite in martensitic matrix. The yield strength of the overaged condition was lower than that of the solution treated condition. The retained austenite formed during the overaging treatment did not offer any advantage in impact energy over the fully martensitic structure of the solution treated condition.
The total strain controlled low cycle fatigue behavior of W-250 maraging steel were investigated for the above various heat treatment conditions. In incremental step test, cyclic softening occurred for solution treatment and maraging treatment, while cyclic hardening occurred for overaging. Degress of cyclic softening of this alloy were found to be significantly less than those of the conventional 18Ni (C-300) maraging steel. Under total strain controlled fatigue tests, maraged material exhibited the highest fatigue life, and solution treated material displayed the lowest life. Maraged material experienced cyclic softening, while the solution treated and overaged materials underwent initial cyclic hardening followed by cyclic softening to final fracture. It was found that fatigue crack propagation rates of the solution treated and the maraged material were about same. However, the rates were twice higher than that of the overaged condition. The great reduction of fatigue crack propagation in the overaged condition was due to the strain induced transformation of austenite to martensite during the test.
The precipitates formed during heat treatments were identified by transmission electron microscopy. The major strengthening precipitates formed during the aging treatment of W-250 steel were orthorhombic $Ni_3W$ and hexagonal $η-$Ni_3Ti$. The orientation relationships of the pricipitates and matrix were identified by electron diffraction spot pattern analysis. The orientation relationship of the pricipitates and matrix were identified by electron diffraction spot pattern analysis, the arientation relationship of $η-Ni_3Ti$ was ${110}_{α'}$//$(0001){η-Ni_3Ti}$ and $<1\bar{1}1>_α'$, //$[2\bar{1}\bar{1}0]{η-Ni_3Ti}$, while those of $Ni_3W$ was ${110}_α'$//(010) ${Ni_3W}$ and $<1\bar{1}1>_α'$, //[100] ${Ni_3W}$. It was found in this work of spot pattern analysis that a short range ordering of B2 cuperlattice existed in the BCC martensitic matrix under the aged condition. It was also observed that the solvus temperature of the pricipitates increased with increasing W-contents. $Fe_2W$ and η-$Ni_3Ti$ were present in unaged material after the low temperature solution treatments (750-800℃ /1 hr).
It was found that under the aged condition the stress corrosion rate of W-250 alloy decreased with decreasing solution treatment temperature from 900℃ to 800℃ in 3.5% NaCl solution. And the stress corrosion crack propagation rate of the aged material was about one-fifth of the 4340 martensitic steel, which was comparable with that of the conventional Co-containing maraging steel.
This study demonstrates that this newly developed Co-free W-bearing "W-250" maraging steel is a good candidate for replacing the expensive Co-containing conventional "C-250" grade maraging steel.
