Ti Maraging Steel Research Articles

Atomic-scale understanding of solute interaction effects on grain boundary segregation, precipitation, and fracture of ultrahigh-strength maraging steels

Understanding the fundamental mechanisms of embrittlement and de-embrittlement is crucial for the development of strategies toward advanced steels with improved performance. In this study, the solute interaction effects on grain boundary (GB) segregation, precipitation, and fracture of Fe–Ni–Ti–(Mo) maraging steels were systematically investigated through a combination of experimental and theoretical techniques, including scanning transmission electron microscopy, atom probe tomography, and first-principles calculations. Our results reveal that the Fe–Ni–Ti maraging steel suffers from serious intergranular embrittlement and the mechanism is related to the formation of coarse Ni3Ti precipitates and associated precipitate-free zones (PFZs) at GBs, which facilitate the crack initiation and growth along the GBs. Interestingly, adding Mo to the maraging steel effectively suppresses the intergranular embrittlement, thereby substantially improving the ductility. Atomistic analyses reveal that Mo de-embrittles the GBs by reducing the segregation of Ni and Ti, which substantially inhibits the formation of coarse Ni3Ti precipitates and PFZs at the GBs, thereby alleviating their harmful impact on the GB cracking. In addition, the Mo segregation enhances the GB cohesion, which may play a minor role in suppressing the GB fracture.

Effect of Heat Treatment on Tensile Properties and Microstructure of Co-Free, Low Ni-10 Mo-1.2 Ti Maraging Steel.

Production of high-quality maraging steel is dependent not only on the production technology but also on the alloying design and heat treatment. In this work, cobalt-free, low nickel, molybdenum-containing maraging steel was produced by melting the raw materials in a vacuum induction melting furnace and then refining with a shielding gas electroslag remelting unit. The critical transformation temperatures of the investigated steel samples were determined experimentally by differential scanning calorimetry (DSC) analysis and theoretically aiding Thermo-Calc software. Types and chemical composition plus volume fraction and starting precipitation temperature of suggested constituents calculated with the aid of Thermo-Calc software. The microstructures of forged steel specimens that were heat-treated under several conditions were evaluated by X-ray diffraction (XRD), optical microscopy (OP), scanning electron microscopy (SEM), and electron backscattering (EBSD), in addition to transmission electron microscopy (TEM). The mechanical properties of the investigated steel specimens were evaluated by measuring the tensile strength properties and micro-hardness, furthermore, estimating their fracture surface using scanning electron microscopy at lower magnification. The metallographic results show that the microstructure of steel in aged conditions includes high-alloyed martensite and nickel-rich phase, in addition to the low-alloyed-retained-austenite, intermetallic compounds, and lavas-phase (MoCr). Furthermore, TEM and EBSD studies emphasized that the produced steel has high dislocation density with nano-sized precipitate with an average size of ~19 ± 1 nm. Moreover, the metallographic results show that the mentioned microstructure enhances the tensile properties by precipitation strengthening and the TRIP phenomenon. The tensile strength results show that the n-value of investigated steel passes two stages and is comparable with the n-value of TRIP-steel. Steel characterized by 2100 MPa ultimate tensile strength and uniform elongation of more than 7% can be produced by the investigated production routine and optimum heat treatment conditions.

Effect of aging temperature on the heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel for cryogenic applications

The evolution of heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel was investigated at different aging temperatures. As the aging temperature increases, more reversed austenite forms with the recovery of martensite matrix. When aging temperature is up to 560 °C, more lath-like reversed austenite coalesces together and leads to the formation of martensite and austenite dual-phase microstructure. Fine η-Ni3(Ti, Al) particles initially precipitate at 440 °C and grow up with increased aging temperature. Notably, it was found that η-precipitates can be encompassed in reversed austenite at 500 °C. Thermodynamic calculations further verify the austenite reversion, and nanoprecipitation can occur independently of each other though there is competition of Ni element. However, it leads to the coarsening and dissolution of η-precipitates as the aging temperature further increases. The desirable heterogeneous microstructure of the aged martensite matrix, soft reversed austenite and stable η-precipitates at 500 °C contributes to a high yield strength (~ 1 GPa, 25 °C; ~ 1.4 GPa, -196 °C) and a reasonable cryogenic impact toughness (~ 60 J, − 196 °C), which is mainly ascribed to the precipitation strengthening of η-precipitates, transformation-induced plasticity (TRIP) toughening effect from the lath-like reversed austenite and the increased misorientation toward high-angle grain boundary.

Reverted austenite with distinct characteristics in a new cobalt-free low lattice misfit precipitate-bearing 19Ni3Mo1.5Ti maraging steel

We elucidate here the morphological evolution of reverted austenite in a newly designed cobalt-free maraging steel and its significance in predicting the mechanical behavior. Three distinct types of reverted austenite, granular reverted austenite at grain boundaries (γG1), lamellar reverted austenite in the matrix (γL) and globular reverted austenite (γG2) were observed, depending on the aging temperature and time. At low temperatures (560 °C and 640 °C), only γG1 and γL were observed. While at relatively high temperature (700 °C), γG1 and γL decreased with the increase of holding time and were completely transformed to γG2 at equilibrium condition. The observation of three different morphologies of reverted austenite are envisaged to be a consequence of competition between the nucleation rate and growth rate of reverted austenite at different aging temperatures. The weak texture of maraging steel with high Schmid factor at high aging temperature implied that γL and γG2 are the reasons for superior toughness and ductility. The present study of nature of austenite is also of significant interest in next generation of advanced high strength steels because of its remarkable effect on ductility.

The Effect of Loading during Martensitic Transformation of a Maraging Steel after Prior Austenite Grain Size Refinement by Thermal Cycling

Abstract This work demonstrates the efficiency of microstructural tuning by external loading during the martensitic phase transformation of a 12Cr-9Ni-2Mo-0.7Al-0.35Ti maraging steel. Additional prior austenite grain size refinement is achieved by thermal cycling yielding industrially relevant grain sizes, for which not the full set of all 24 characteristic Kurdjumov-Sachs orientation relation variants of martensite is obtained within each grain. For the last thermal cycle three different cases are investigated: i) one specimen is annealed at a higher temperature before the last cooling, ii) one specimen is loaded with a tensile stress of 120 MPa and iii) one specimen with an equivalent 120 MPa Mises stress in torsion during the transformation. This study is one of the first combining the effects of a small grain size, a high dislocation density and external loadings. Orientation imaging microscopy data is presented, revealing the effect of loading type during transformation on the final texture and interface/boundary character.

On the origin and contribution of extended kinks and jogs and stacking fault ribbons to deformation behavior in an ultrahigh strength cobalt-free maraging steel with high density of low lattice misfit precipitates

We elucidate here the deformation mechanisms and underlying reasons that contributed to high ductility (10.2%) and high static toughness (112.5 MJ m−3) in an ultrahigh strength (1860 MPa) cobalt-free 19Ni3Mo1.5Ti maraging steel characterized by high density (2.3 × 1024 m−3) of η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) nanoscale precipitates with low lattice misfit of ＜1% with the martensite matrix. Multiple deformation processes occurred during plastic deformation. Lath-morphology of martensite was dramatically segmented with angles of 30°, 60° or 120° with large pile-up of dislocations at the segmented boundaries. This occurred because of the interactive ability of edge and screw dislocations along the martensite habit planes, which led to kinks and jogs. The low lattice misfit (0.6% ~ 0.9%) precipitates interacted with dislocations leaving stacking fault ribbons within precipitates that build a large long range of back stress producing a high strain-hardening response. Additionally, nanoscale twinning occurred. The above contributions to ductility are envisaged to be in addition to the significantly reduced elastic interaction between the low lattice misfit nanoscale precipitates and dislocations that reduces the ability for crack initiation at the particle-matrix interface.

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

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 ~1858MPa and static toughness of ~110MJm−3. Maraging steel had extremely high density (2.3 × 1024m−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.

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

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 ε-Cu precipitates during the aging period. The amount of ε-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.

Ni–Co–Mo–Ti Maraging Steel Hysteretic Loops Calculations

In this article, a modification is suggested for an original Hamad model to become completely adequate to bulk ferromagnetic materials such as maraging steel. A verification of hysteretic loops modeling was carried out, showing a good agreement between simulated and experimental curves of Ni–Co–Mo–Ti maraging steel hysteretic loops. Predicted hysteresis loss energy was estimated as a result of this model. It is showed that the dipping in liquid nitrogen and solution treating affect hysteresis energy loss of Ni–Co–Mo–Ti maraging steel.

Transformation hardening and kinetics for stress assisted and temperature driven martensitic transformation in steels

In the present study a new phase fraction based transformation hardening and further a stress and temperature dependent transformation kinetics following the postulates in internal variable thermodynamics is presented. The generally formulated kinetics equation is applied to describe the progress of phase transformation in a Cr–Ni–Mo–Al–Ti maraging steel and represents a good tool to predict the continuous change of the slope of isovolume fractions.

Effect of Cu on the evolution of precipitation in an Fe–Cr–Ni–Al–Ti maraging steel

The evolution of precipitates in an Fe–Cr–Ni–Al–Ti stainless maraging steel alloyed with Cu was investigated during aging at 525 °C. Atom probe tomography was used to reveal the development of precipitates and to determine their chemical composition. Two types of precipitates were observed to form during the aging process. Based on their chemical composition these are assumed to be NiAl B2 and Ni 3(Ti,Al) ( η-phase). The two phases of precipitates were found to develop independently of each other and the addition of Cu was found to accelerate precipitation. However, the effect of Cu on the nucleation of these phases is different: on the one hand, in the case of NiAl, Cu is incorporated and thus reduces the activation energy by reducing the lattice misfit; on the other hand, Cu acts as a nucleation site for the precipitation of Ni 3(Ti,Al) by forming independent Cu clusters.

Influence of temperature and aging time on hardness and magnetic properties of the maraging steel grade 300

The magnetic properties and hardness of a Ni–Co–Mo–Ti maraging steel 300 grade were measured as function of aging temperature. The austenite and martensite phase quantifications in the different heat treatment conditions were carried out by X-ray diffraction using direct comparison method. The behavior of the hardening, magnetization saturation and coercive force against aging temperature and time were explained taking into account the variation of austenite volume fraction with aging time and temperature.

Effect of aging and deformation on the microstructure and properties of Fe−Ni−Ti maraging steel

The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550 °C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni3Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.

Magnetic properties of a Ni–Co–Mo–Ti maraging 350 steel

The magnetic properties of a Ni–Co–Mo–Ti maraging steel 350 aged at different temperatures were measured. The effects of the aging time and cold deformation before aging were studied. It is found that the mechanical and magnetic hardening effects are not correlated in the maraging 350 steel. The behaviour of the saturation magnetisation and the coercive force against aging time at 440, 500, 560 and 650 °C are discussed taking into account previous results on the same material.

A magnetic study of the maraging 350 steel

In this work the magnetic properties of a Ni–Co–Ti maraging steel grade 350 were measured in many processing conditions. The influence of the aging temperature is analyzed. Thermomagnetic analyses were also carried out to investigate martensite–austenite transformation.

Back stress evolution and iso-volume fraction lines in a Cr–Ni–Mo–Al–Ti maraging steel in the process of martensitic transformation

The quantitative identification of the back stress evolution is carried out for a Cr–Ni–Mo–Al–Ti maraging steel in the process of martensitic transformation under thermomechanical loading conditions. The evolution depends on the direction of the applied stress: tensile, compressive or shear. The iso-volume fraction lines (the iso- ξ lines) are determined between the transformation-start and -finish lines on the applied stress–temperature plane. The iso- ξ lines are composed of a tensile-branch and a compressive-branch, each of which is parallel to the martensite-start line. The interaction point of the tensile- and compressive-branches of the iso- ξ line shifts to the direction of the back stress evolution. The almost hold stress-independent martensite-finish line, identified in a previous paper (Mater. Sci. Eng. A308 (2001) 25) is proved to be attributed to the back stress evolution.

Effect of back stress evolution due to martensitic transformation on iso-volume fraction lines in a Cr–Ni–Mo–Al–Ti maraging steel

The effect of the back stress evolution on the iso-martensite volume fraction lines (the iso- ξ lines) is experimentally investigated in a Cr–Ni–Mo–Al–Ti maraging steel in the process of martensitic transformation by identifying the iso- ξ lines inside the transformation zone bounded by the martensite-start and -finish lines. Each iso- ξ line is composed of a tensile-branch and a compressive-branch, which are parallel to the tensile- and compressive-branches of the martensite-start line, respectively. When the back stress evolves due to martensitic transformation, the iso- ξ line translates as a whole into the direction of the back stress evolution with its slope unchanged. The ordinate of the intersection point of both branches stands for the value of the back stress. The back stress evolution identified in the present tests well supplements the evolution response determined in a quite different way in the unloading tests described in [Mater. Sci. Eng. A (2002) in press].

Mechanical properties of a Cr–Ni–Mo–Al–Ti maraging steel in the process of martensitic transformation

Mechanical properties of a Cr–Ni–Mo–Al–Ti maraging steel are studied experimentally in the process of martensitic transformation. The transformation-start and -finish temperatures are determined from the dilatometric curves under a tensile, compressive or shear hold stress. An anomalous temperature-dependence of the yield stress is investigated. By checking the closing of the dilatometric loop in a full heating–annealing–cooling thermal cycle under the applied hold stress, a back stress is confirmed to exist in both the axial and radial directions in the thin-walled tubular specimen, and its initial value is identified. The annealing condition is proved to have a marked effect on the initial value of the back stress. The back stress is shown to evolve in the progress of transformation. The evolution of the TRIP strain is evaluated under the hold stress in both the axial and shear direction. Especially when the hold stress is removed in the process of transformation, a backflow, due to the Magee effect, is observed.

Grain growth in 18Ni 1800 MPa maraging steel

The grain-growth behaviour in 18Ni8Co5MoO.4Ti maraging steel was investigated in the temperature range 1123–1323 K. Grain sizes were estimated by measuring the diameter of the equivalent area of the individual grains directly on the optical microscope using a calibrated digital eyepiece. Grain-boundary migrations and substructure analyses were done using an electron microscope. These studies indicate that the overall grain growth in the steel follows the relationship ΔD=ktn where ΔD is the increase in the grain size. However, during the initial stage, a “time lag” for the grain growth to start is observed which is attributed to the presence of highly dislocated austenitic matrix on annealing. Evidence of abnormal grain growth is also seen after annealing at 1173 K for 480 min and at 1123 K for 300 min. The growth exponents for the normal and abnormal grain growth were found to be 0.40–0.44 and 0.90–2.0, respectively. One significant deviation observed in the study wasn decreasing from 0.44-0.40 at higher temperatures during the normal growth. This has been critically discussed in the light of the unique transformation characteristics of the steel. The activation energy for growth was calculated to be 60.0–62.5 kcal mol−1, indicating the overall growth is controlled by self diffusion in γ-iron.

Effect of heat treatment on strain rate sensitivity in tensile properties of 13Ni-15Co-10Mo-0.2Ti maraging steel.

Tensile properties and resistivity changes during isothermal aging have been investigated mainly on a 13Ni-15Co-10Mo-0.2Ti maraging steel aged in the range from 400° to 575°C. A crosshead speed has been varied from 1mm/min to 0.005mm/mim, in order to investigate the effect of strain rate on the tensile properties.The embrittlement caused by the decrease in strain rate is found in the tensile properties of the maraging steel containing fine and coherent precipitates. The ductility of the overaged 13Ni-15Co-10Mo-0.2Ti maraging steel is also decreased at a lower strain rate. When this steel is averaged after cold rolling to 40%, the ductility parallel to the rolling direction is not decreased by the decrease in strain rate. Tensile properties insensitive to strain rate are obtainable after aging for about 10-100min in the range from 525° to 575°C.Investigation carried out on 14Ni-10Co-10Mo and 14Ni-15Co-7Mo maraging steels is included in order to consider the effect of chemical composition on the strain rate sensitivity. The maraging steels containing fine and coherent precipitates are also embrittled by the decrease in strain rate. The 14Ni-15Co-7Mo maraging steel continues to precipitate coherent precipitates for longer times at a certain aging temperature than does the 14Ni-10Co-10Mo one.