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Journal ArticleDOI

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

TL;DR: In this article, the effect of aging temperature on microstructures and mechanical properties of laser-welded T-250 maraging steel joints was investigated, and it was shown that the Ni 3 (Ti, Mo) precipitate and reverted austenite are the critical factors influencing the strength and toughness of welded joints.
Abstract: 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.
Citations
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Journal ArticleDOI
TL;DR: In this paper, 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.
Abstract: 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.

44 citations

Journal ArticleDOI
TL;DR: In this article, a systematic investigation was carried out on a widely used hot working die steel and the results obtained have shown that mechanical properties as well as fracture mode change with increasing testing temperature.
Abstract: 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.

41 citations

Journal ArticleDOI
TL;DR: In this paper, the impact of aging temperature on microstructural evolution and strengthening behavior on low lattice misfit cobalt-free maraging steel was elucidated, and the best combination of high strength (1850 MPa) and high-toughness (125.4 MJm−3) was obtained at the optimal aging temperature of 520°C, without sacrificing ductility.
Abstract: 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.

34 citations

Journal ArticleDOI
Dong Pan1, Yuguang Zhao1, Xiaofeng Xu1, Yitong Wang1, Wenqiang Jiang1, Xueying Chong1 
TL;DR: In this article, a high-energy and instantaneous electropulsing is devised to improve the ductility of T250 steel, and the substructure evolution and aging behavior of T 250 steel was systematically investigated by Transmission Electron Microscope (TEM) characterization, APT, Finite Element Modelling (FEM) simulation and Electron Back-Scattered Diffraction (EBSD) technology.

25 citations

Journal ArticleDOI
TL;DR: In this article, the dependence of both the strength and ductility in Cu-bearing nano-precipitated steels on compositions, tempering and aging was investigated, where Fe-2Cu (wt%, Cu-steel) steel by Cu-rich precipitates (CRPs), Fe- 2Cu-5Ni-3Mn-1.5Al steel (Cu/Ni steel) by co-preceipitates of CRPs+NiAl, and Fe-1Mo (Mo-steel), Fe.
Abstract: We investigated the dependence of both the strength and ductility in Cu-bearing nano-precipitated steels on compositions, tempering and aging, where Fe-2Cu (wt%, Cu-steel) steel by Cu-rich precipitates (CRPs), Fe-2Cu-5Ni-3Mn-1.5Al steel (Cu/Ni steel) by co-precipitates of CRPs+NiAl, and Fe-2Cu-5Ni-3Mn-1.5Al-1Mo (Mo-steel) steel by co-precipitates of CRPs+NiAl+Mo2C precipitates were paid special attentions on. The strength and ductility of Cu-steel were sensitive to tempering temperatures rather than aging time. Tempering and aging in Cu/Ni steels remarkably increased the strength with seriously sacrificing of ductility because larger-sized blocky CRPs segregated along the grain boundaries where Cu partitioning along grain boundaries was partially dominate, resulting in brittle fracture in almost all of aged Cu/Ni steels. Mo-steel, however, showed superior strength and ductility than Cu/Ni steel because Mo addition reduced the possibility of blocky CRPs by decreasing Cu partitioning along grain boundaries, and promoted the formation of hierarchical co-precipitates where large-sized coarsening co-precipitates contributed to the ductility and small-sized secondary co-precipitates had significant strengthening, thus resulting in the improvement of strengthening-ductility.

24 citations

References
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Journal ArticleDOI
TL;DR: Precipitation hardening has long been used to increase the strength of commercial alloys, such as quenched and tempered steels and the duralumin type aluminium alloys as discussed by the authors.
Abstract: Precipitation hardening has long been used to increase the strength of commercial alloys, such as quenched and tempered steels and the duralumin type aluminium alloys. The theoretical treatments of precipitation hardening are briefly considered. The equations for strengthening by ‘hard’ indeformable particles and by ‘soft’ deformable particles are presented, and the implications are discussed. These lead to the concept of an optimum particle size for a given system, but the optimum can vary from system to system depending upon the particle characteristics. A broad comparison is made between the increments in strength that occur due to precipitation in commercial alloys and the predictions of the theories; an important contribution to these increments in strength is shown to derive from variations in the volume fraction of precipitated particles that can be employed in the various systems.

824 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated the reversion mechanism from deformation induced martensite (α') to austenite (γ) by means of magnetic analysis and transmission electron microscopy.
Abstract: Reversion mechanism from deformation induced martensite (α') to austenite (γ) has been investigated in two metastable austenitic stainless steels, 15.6%Cr-9.8%Ni (the 16Cr-10Ni) and 17.6%Cr08.8%Ni (the 18Cr-9Ni) steels, by means of magnetic analysis and transmission electron microscopy. Metastable γ almost completely transforms to lath α' by 90% cold rolling, and the α' again reverts to γ during annealing at temperatures above 700 K. Deformation induced α' in the 16Cr-10Ni steel undergoes a martensitic shear reversion during heating to 923 K annealing, while that in the 18Cr-9Ni steel does a diffusional nucleation-growth reversion on 923 K annealing. Grain refining processes are greatly influenced depending on the reversion mechanism. Martensitically reversed γ has a high density of dislocations immediately after the reversion and the γ grains are refined through recovery and recrystallization process just like that taking place in a deformed γ. On the other hand, diffusionally reversed γ is characterized by the nucleation of equiaxed γ grains within the α' matrix and the γ grains gradually grow during annealing.The reversion mechanism significantly depends on the chemical compositions of steels and annealing temperature. An increase in the Ni/Cr ratio causes an increase in the Gibbs free energy change between fcc and bcc structure, leading to a fall-down of austenitizing temperature for the martensitic shear reversion. The critical driving force required for the complete martensitic shear reversion is about -500 J/mol. To obtain the critical driving force in the 18Cr-9Ni steel, it should be heated to a high temperature above 1 023 K. However, the diffusional reversion can easily occur because the martensitic shear reversion temperature is too high in the 18Cr-9Ni steel. The 16Cr-10Ni steel also undergoes the diffusional reversion when it was annealed at low temperatures below the martensitic shear reversion, 923 K.

274 citations

Journal ArticleDOI
TL;DR: In this article, the Johnson-Mehl-Avarami equation was used to describe the kinetics of precipitate nucleation in maraging steels C-250 and T-250, and the crystallography, structure and composition of the strengthening precipitates were studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns.
Abstract: The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni3Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni3Ti (initially) and Fe2Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis.

262 citations

Journal ArticleDOI
TL;DR: In this paper, the evolution of microstructure in a 350 grade commercial maraging steel has been examined, and the formation of austenite of different morphologies identified in detail.
Abstract: Evolution of microstructure in a 350 grade commercial maraging steel has been examined. In the earlier stages of aging, the strengthening phases are formed by the heterogeneous precipitation, and these phases have been identified as intermetallic compounds of the Ni3 (Ti, Mo) and Fe2Mo types. The kinetics of precipitation are studied in terms of the activation energy by carrying out isothermal hardness measurements of aged material. The mechanical properties in the peak-aged and overaged conditions were evaluated and the flow behavior examined. The overaging behavior of the steel has been studied and the formation of austenite of different morphologies identified. The crystallography of the austenite has been examined in detail. From the microstructural examination of peak-aged and deformed samples, it could be inferred that the dislocation-precipitate interaction is by precipitate shearing. Increased work hardening of the material in the overaged condition was suggestive of looping of precipitates by dislocations.

168 citations

Journal ArticleDOI
TL;DR: In this paper, a series of studies of phase transformations in maraging steels were conducted using APFIM and transmission electron microscopy (TEM) and thermochemical calculations.
Abstract: This article introduces a series of studies of phase transformations in maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Hardness measurements, transmission electron microscopy (TEM), and thermochemical calculations were also used. The composition and morphology of precipitates in the commercial-grade C-300 steel were compared for different aging times at 510 °C to investigate the aging sequence. Both Ni3Ti and Fe7Mo6 were found to contribute to age hardening. The decomposition starts with the formation of small Mo-enriched Ni3Ti particles at very short aging times. The Fe7Mo6 phase forms at a later stage of aging. The matrix concentrations of both Ti and Mo were measured and were found to be low after standard aging conditions. The observation of the Fe7Mo6 μ phase is supported by thermochemical calculations. Austenite reversion has been found at the aging temperature, and its composition approaches the predicted equilibrium composition after 8 hours of aging.

113 citations

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