The concept of low lattice misfit and high-density of nanoscale precipitates obtained through solution treatment was adopted to obtain ultrahigh strength maraging steel without compromising elongation. An “ultrahigh strength-high toughness” combination was successfully obtained in 19Ni3Mo1.5Ti maraging steel with ultimate strength of ~1858 MPa and static toughness of ~110 MJ m−3. Maraging steel had extremely high density (2.3 × 1024 m−3) of nanoscale precipitates with minimum lattice misfit of less than 1% at the solutionization temperature of 820 °C. Two kinds of nanoscale precipitates, namely, η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) contributed to ultrahigh strength. The size of nanoscale precipitates governed the movement of dislocations, cutting versus by-passing. Theoretical estimate of ordering and modulus contribution to strengthening suggested that ordering had a dominant influence on strength. The toughness was closely related to the characteristic evolution of nanoscale precipitates such that the high density of nanoscale precipitates contributed to increase of elastic deformation and low lattice misfit contributed to increase of uniform deformation. The nanoscale size and low lattice misfit of precipitates were the underlying reasons for the high-performance of maraging steel. Moreover, the combination of high-density of nanoscale precipitates and low lattice misfit is envisaged to facilitate the futuristic design and development of next generation of structural alloys.

Strengthening of cobalt-free 19Ni3Mo1.5Ti maraging steel through high-density and low lattice misfit nanoscale precipitates

It is known that mechanical properties of hot working die steel deteriorate during service due to dynamic recovery, recrystallization and plastic deformation of tempered martensite. In order to clarify the associated microstructure evolution and fracture mechanism, a systematic investigation was carried out on a widely used hot working die steel. Samples were tensile deformed at different temperatures ranging from 25 ℃ to 640 ℃ and examined using SEM and TEM. Results obtained have shown that mechanical properties as well as fracture mode change with increasing testing temperature. These phenomena have been elaborated together with microstructure evolution during high temperature deformation. Deformation mechanisms and roles of different carbides in crack formation and propagation have been discussed.

Microstructure evolution and fracture mechanism of H13 steel during high temperature tensile deformation

We elucidate here the impact of aging temperature on microstructural evolution and strengthening behavior on low lattice misfit cobalt-free maraging steel. The best combination of high-strength (1850 MPa) and high-toughness (125.4 MJ m−3) was obtained at the optimal aging temperature of 520 °C, without sacrificing ductility. Electron back scattered diffraction studies suggested that preferred orientations of {101}, fraction of high-angle grain boundary (HAGB) and total length of grain boundary per unit area (μm/μm2) were increased with increase of aging temperature, which was beneficial to both strengthening and toughening of maraging steel. The strengthening contribution from the precipitates was transformed from shearing mechanism to bypass mechanism when the aging temperature is greater than 520 °C. The aging tempered steel of 520 °C provided maximum strengthening increment of 1463 MPa through shearing mechanism, while granular reverted austenite at this temperature contributed to high toughness.

Aging phenomenon in low lattice-misfit cobalt-free maraging steel: Microstructural evolution and strengthening behavior

Additive manufacturing (AM) has been recently used to deposit metal powder on top of conventional metals. Of particular interest is hybrid additively manufactured steels which were found to be a suitable solution to benefit from features of each metal at different spots of a mechanical component. Due to its superior mechanical characteristics, maraging steel (MS1) has recently attracted tremendous attention for additive manufacturing applications mainly in aerospace, tool and die, and marine industries or to be 3D printed on top of other metals as a hybrid product using different techniques such as Direct Metal Laser Sintering (DMLS). In this paper a predictive finite element (FE) model and a combined analytical-numerical framework were developed to evaluate the mechanical performance of hybrid additively manufactured components and facilitate the prediction of hardness and fatigue life of these parts. The proposed tools were employed in two scopes: First to simulate the indentation hardness test of hybrid DMLS-MS1-H13 steels; and second to calculate fatigue crack nucleation life of maraging steel including defects (i.e. welding residual stresses). Parameters such as local and global displacements, changes in Young’s modulus, and hardness, high cycle fatigue life, welding temperature distribution, and residual stress were investigated. The hardness experiments were carried out to improve the reported data found in similar studies, which were used as the main resource to validate the proposed numerical framework. The capabilities of the presented frameworks enable this work to target existing ambiguities in additively manufactured mechanical components.

Numerical tools to investigate mechanical and fatigue properties of additively manufactured MS1-H13 hybrid steels

This paper presents an investigation on the strength and toughness of laser-welded T-250 maraging steel joint, aimed at elucidating the influence mechanism of aging temperature on microstructures and mechanical properties of the joints. The results showed that as the aging temperature increased, the ultimate tensile strength of welded joints increased, reaching a maximum of 1640.5 MPa at 520 °C, and then decreased. The static toughness of welded joints decreased at first and increased later with the aging temperature increasing. The minimum of 38.8 MJ m −3 for the static toughness was obtained at 560 °C. There were two types of reverted austenite respectively distributing in grain boundaries and in the matrix of martensite, due to the change of aging temperatures. This study underscores that the Ni 3 (Ti, Mo) precipitate and reverted austenite are the critical factors influencing the strength and toughness of welded joints. The Ni 3 (Ti, Mo) precipitate in the weld metal improves the strength of welded joints remarkably as its volume fraction increases. The reverted austenite in grain boundaries is harmful to the toughness of welded joints, while the reverted austenite in the matrix is beneficial to the toughness of welded joints because of its finely dispersive distribution and its ability to prevent crack propagation. Increasing the amount of reverted austenite in the matrix is an effective way to improve mechanical properties of laser-welded maraging steel joints.

Influence of aging temperature on strength and toughness of laser-welded T-250 maraging steel joint

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 Ni 3 (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 Ni 3 (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 Ni 3 (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.

Effect of post-weld heat treatments on strength and toughness behavior of T-250 maraging steel welded by laser beam

The laser-welded Fe-19Ni-3Mo-1.5Ti maraging steel joints with high strength and toughness were successfully obtained using the approach of the combination of Cu and aging temperature. This study underscores that the element of Cu has a significant effect on precipitates and reverted austenite, by influencing the element diffusion and phase transformation. For the strength, the element of Cu in the weld metal brings a lot of e-Cu precipitates during the aging period. The amount of e-Cu precipitates increases gradually with the increase of the aging temperature, which is beneficial to the strength. For the toughness, the addition of Cu changes the diffusion kinetic condition to promote the diffusion of Ni, resulting in the decrease of segregation of elements. The reverted austenite in grain boundaries which has an adverse effect on the toughness is correspondingly reduced. Meanwhile, Cu makes the thermodynamic condition different, with a lower critical driving force of phase transformation. This causes the increase of reverted austenite in the matrix, which is advantageous to the toughness. An excellent combination of strength and toughness of laser-welded maraging steel joint can be optimized by the appropriate content of Cu and aging temperature.

Zhiling Tian

Papers

Influence of aging temperature on strength and toughness of laser-welded T-250 maraging steel joint

Effect of post-weld heat treatments on strength and toughness behavior of T-250 maraging steel welded by laser beam

The role of copper in microstructures and mechanical properties of laser-welded Fe-19Ni-3Mo-1.5Ti maraging steel joint

Strength and toughness of t250 maraging steel joint hybrid-treated with laser welding and aging