The conventional 18 Ni maraging steel possessing excellent strength and toughness contains about 8% Co.
Recently, low-cost Co-free maraging steel containing Mo and W have been developed by INCO and KAIST, respectively. The composition of the W containing steel was 18Ni-4.5W-1.4Ti-0.15Al-bal. Fe.
This paper shows comparison of microstructure and mechanical properties of newly developed W and Mo containing maraging steel after various heat treatment. Normal aging and overaging condition were applied in the heat treatment. Overaging condition was designed to simulate the condition of HAZ during welding cycle. Retained austenite were formed by austenite reversion during overaging treatment. Effects of retained austenite on mechanical properties were also investigated. The formation of retained austenite was studied by X-ray and dilatation. Precipitates was characterized by TEM.
The result showed that W and Mo containing alloys exhibited comparable tensile, impact, fatigue behavior under equal heat treatment condition. In total strain-controlled LCF, both alloys showed cyclic softening for solution treated and normal aged condition. Cyclic hardening was observed for overaged condition. Cyclic softening behavior was also reported in Co-containing regular maraging steel. LCF life was the longest for normal aged condition, and the shortest for solution treated condition among normal aging, overaging, and solution treatment.
Formation of retained austenite (13-14%) enhanced elongation while lowering yield strength.
TEM analysis showed that the precipitates formed in the normal aged W-containing steel was rod-shape orthorhombic $Ni_3W$ and plate-shape hexagonal $\eta-Ni_3$ Ti. The orientation relationship was as follows.
$\{011\}_{\alpha{'}}$ // $(010)_{Ni_3W}$, $<1\bar{1}1>_{\alpha{'}}$ // $[100]_{Ni_3W}$
$\{011\}_{\alpha{'}}$ // $(0001)_{\eta-Ni_3Ti}$, $<1\bar{1}1>_{\alpha{'}}$ // $[11\bar{2}0]_{\eta-Ni_3Ti}$
