The fracture toughness behaviour of a 18 Ni (300 grade) maraging steel in various solution treated and aged conditions

This study aimed to evaluate the aging effects on mechanical properties of 18Ni (300) maraging steel processed by additive manufacturing powder bed fusion and subjected to different aging conditions. The aging temperatures were 470 °C, 510 °C, and 530 °C, with exposure times ranging from 0.33 to 96 h, followed by air cooling. Despite the layer-by-layer material consolidation mechanism, weak preferred orientation was found in the as-built condition; moreover, only martensite and austenite were found in the X-ray diffraction results. This alloy showed a steep increase in hardness, compressive strength, and tensile strength, while the work-hardening ability decreased with aging. Despite using thin single-edge bending samples (5 mm), the plane-strain fracture toughness (K_IC) size requirement was met, and valid K_IC results were 49 ~ 64 $\mathit{MPa}\sqrt{m}$. This result could help design new additive manufacturing applications of high-strength steel, such as thin-walled, lattice structures, and reduced cross-section parts projected to minimize weight in different industries.

This paper elucidates here the strength and toughness behavior of T-250 maraging steel welded by laser beam under different approaches of three post-weld heat treatments, i.e. aging (A), solutionizing+aging (SA) and homogenizing+solutionizing+aging (HSA). The microstructures of the weld metals with A and SA processes both comprised of finely dispersive Ni₃(Ti, Mo) precipitates, small martensite lath and reverted austenite along the grain boundary. However, in the weld metal with HSA process, it exhibited the same Ni₃(Ti, Mo) precipitate with the large martensite lath and the absence of reverted austenite. The ultimate tensile strength and static toughness of the welded joint with HSA process were 1350.6 MPa and 63.8 MJ m⁻³, respectively. The static toughness has been remarkably improved from 71% to 91% of the applied parent metal compared with that of the welded joint with A process. The present study underscores that the Ni₃(Ti, Mo) precipitate and martensite are significant to ensure the high strength of welded joints. Due to its inconsistent deformation with the matrix of martensite, the reverted austenite has a notable influence on the toughness of welded joints. It shows that the post-weld heat treatments of HSA process can influence the mechanical behavior of welded joints by eliminating the reverted austenite.