The precipitation sequence of Ni3Ti in Co-free maraging steel

TEM, microdiffraction, and X-ray microanalysis were used to study the precipitation processes in Co-free maraging steel. Austenite crystals were found to nucleate in the martensite matrix as the first step in the precipitation sequence of Ni₃Ti. The austenite reversion is a result of Ni segregation. Ni₃Ti nucleates in the austenite. The shape and orientation of Ni₃Ti is determined by the austenite precursor. The same sequence occurs for heterogeneous nucleation on dislocations and grain boundaries. At the later stages of growth Mo is incorporated in the Ni₃Ti lattice.     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

Magnetic properties of maraging steel in relation to deformation and structural phase transformations

Magnetic properties in annealed and cold rolled conditions have been investigated for maraging steel grade 18%Ni-2400. The austenite content, coercive field, saturation magnetisation and remanence were determined after ageing for 1 h in the temperature range from 400 to 800°C. The results show that the degree of deformation imparted to martensite influences both the amount of reverted austenite and the magnetic properties obtained following ageing. Transmission electron microscopy was carried out in order to investigate the structural changes taking place during reversion of austenite.     

Influence of Manganese and Nickel on the α´ Martensite Transformation Temperatures of High Alloyed Cr‐Mn‐Ni Steels

The martensite start temperature (M_s), the martensite austenite re‐transformation start temperature (A_s) and the re‐transformation finish temperature (A_f) of six high alloyed Cr‐Mn‐Ni steels with varying Ni and Mn contents in the wrought and as‐cast state were studied. The aim of this investigation is the development of the relationships between the M_s, A_s, A_f, T₀ temperatures and the chemical composition of a new type of Cr‐Mn‐Ni steels. The investigations show that the M_s, A_s and A_f temperatures decrease with increasing nickel and manganese contents. The A_f temperature depends on the amount of martensite. Regression equations for the transformation temperatures are given. The experimental results are based on dilatometer tests and microstructure investigations.     

冷轧变形对Custom 465钢组织和性能的影响

 研究了冷轧变形对Ti强化马氏体时效不锈钢Custom 465时效析出、逆转变奥氏体相变和力学性能的影响。结果表明：①在510 ℃时效过程中,杆状Ni3Ti金属间化合物持续析出、长大,致使快速发生硬化;②510 ℃时效过程中,发生逆转变奥氏体相变,冷轧变形使逆转变奥氏体相变滞后;③析出Ni3Ti金属间化合物的同时,马氏体基体发生部分回复。     

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Solid-state transformation kinetics of a 350 grade commercial maraging steel were investigated using a nonisothermal dilatometric technique. Two solid-state reactions—namely, precipitation of intermetallic phases from supersaturated martensite and reversion of martensite to austenite—were identified. Determination was made of the temperatures at which the rates of these reactions reached a maximum at different heating rates. The kinetics of the individual reactions in terms of activation energy were analyzed by simplified procedures based on the Kissinger equation. An estimated activation energy of 145 ± 4 kJ/mol for the precipitation of intermetallic phases was in good agreement with reported results based on the isothermal hardness measurement technique. Martensite to austenite reversion was associated with an activation energy of 224 ± 4 kJ/mol, which is very close to the activation energy for diffusion of substitutional elements in ferrite. Results were supplemented with microstructural analysis.     

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Solid-state transformation kinetics of a 350 grade commercial maraging steel were investigated using a nonisothermal dilatometric technique. Two solid-state reactions—namely, precipitation of intermetallic phases from supersaturated martensite and reversion of martensite to austenite—were identified. Determination was made of the temperatures at which the rates of these reactions reached a maximum at different heating rates. The kinetics of the individual reactions in terms of activation energy were analyzed by simplified procedures based on the Kissinger equation. An estimated activation energy of 145 ± 4 kJ/mol for the precipitation of intermetallic phases was in good agreement with reported results based on the isothermal hardness measurement technique. Martensite to austenite reversion was associated with an activation energy of 224 ± 4 kJ/mol, which is very close to the activation energy for diffusion of substitutional elements in ferrite. Results were supplemented with microstructural analysis.     

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M₂C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.     

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo,and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M₂₃C₆ carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.     

Precipitation reactions and strengthening behavior in 18 Wt Pct nickel maraging steels

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. Formerly with the University of Illinois, Formerly with the University of Illinois     

The strength and fracture toughness of 18 Ni (350) maraging steel

The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni₃Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe₂Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.     

Shape memory behavior of FeNiCoTi single and polycrystals

We present experimental and theoretical evidence of thermoelastic martensites in Fe29Ni18Co4Ti alloys. In this class of alloys, the high strength in the austenite domains limits the slip deformation as verified with transmission electron microscopy. The restriction of slip permits a higher degree of recoverability of the transformation. Using both single crystals with [123] orientation and polycrystals, the appearance of martensite plates upon deformation, and their reversion back to austenite upon heating (the shape memory effect), is revealed with in-situ optical microscopy. Theoretical results for the transformation strains and the detwinning of martensite are presented, which demonstrate convincingly the potential of these classes of alloys. Electrical resistance measurements identified the stress and temperature levels at the onset of forward and reverse transformations in isothermal deformation and thermal cycling experiments, respectively. The return of the electrical resistance to its reference value, upon austenite to martensite followed by martensite to austenite transformation, verified the recovery in the transformation strains measured in the experiments.     

Stabilization of retained austenite due to partial martensitic transformations

The stabilization effect of retained austenite has been studied using FeNiC alloys with M_s temperatures below 0°C via a two-step cooling procedure, i.e. the samples were first cooled to a temperature (T_a) below M_s temperature and then heated to room temperature (RT), after being held at RT for a while, the samples were recooled to low temperatures (23 or 82 K) and then heated to RT. It was found that, during the second step of cooling, the martensitic transformation occurred at a temperature of M_s′ which was lower than T_a. With increasing the amount of martensite formed during the first cooling, the difference in the martensitic transformation starting temperatures, ΔM_s = M_s − M_s′, increased. The mechanism of the stabilization of retained austenite during the second step of cooling is proposed to be mainly due to the inhibition effect produced by the previously formed martensite. The aging processes, which retard the growth of the previously formed martensite plates and reduce the number of the available nucleation sites, are the necessary conditions for the above mechanism to operate. By simplifying the internal resisting stress acting on the retained austenite due to the existence of martensite phase as a hydrostatic compressive stress, which increases with increasing the amount of martensite, the change in ΔM_s is discussed from a thermodynamic point of view.     

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part II. effect of testing temperature on stress-strain behavior and deformation-induced austenite transformation

Stress-strain behavior and deformation-induced transformation of retained austenite were studied for intercritically annealed and isothermally transformed Si-Mn and Ni steels as a function of testing temperature between −80 °C and 120 °C. Rapid decrease of retained austenite at small strains dominates at low-temperature testing and in microstructures containing martensite. The austenite transformation in microstructures without martensite shifts to larger strains with increasing testing temperature. The accompanying increase of strain-hardening rates at larger strains deters the onset of necking and improves ductility. The benefits of the austenite transformation lead to a peak in ductility between 20 °C and 70 °C in the Si-Mn steel and at 70 °C in the Ni steel. The peaks are dependent on the nature of the dispersed microconstituents produced in the ferrite during isothermal transformation. Higher testing temperatures enhance the mechanical stability of the austenite and result in lower ductility.     

Stabilization and two-way shape memory effect in Cu-Al-Ni single crystals

The two-way shape memory effect (TWME) induced by stabilization of the martensite phase during aging has been studied in Cu-13.4 Al-4.0 Ni (mass pct) single crystals. The influence of the degree of long-range order on the transformation has been determined by using different heat treatments. The transformation temperatures are strongly influenced by the degree of order in the austenite: annealing from above or below the second neighbor L2₁ ordering temperature changes the M _s by more than 100 °C. It has been established that the diffusion in the austenite as well as in the martensite phase is considerably slower in this alloy than in other Cu-based ones, due to the presence of Ni. The obtained TWME has a similar efficiency as when other more complex thermomechanical trainings are made. In this alloy, the TWME by stabilization is not complete, in contrast to that in Cu-Zn-Al single crystals.     

Austenite ferromagnetism and martensite morphology

The Curie temperature of the austenite, the martensite-start temperature, and martensite morphology have been determined in a series of nil-carbon Fe?Ni and Fe?Ni?Co alloys. For these alloys, austenite ferromagnetism aboveM _s is a necessary, but not sufficient, condition for the formation of lenticular rather than packet martensite. In contrast to Fe?Ni alloys where lenticular martensite only forms below ≈O°C, some of the Fe?Ni?Co alloys transform to this structure at temperatures up to ≈200°C. The results support the hypothesis that the resistance of austenite to plastic deformation affects the habit plane and thus morphology of the martensite which forms.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Characterization of Reverted Austenite during Prolonged Ageing of Maraging Steel CORRAX

Microstructure and mechanical properties were studied in CORRAX maraging steel during prolonged ageing up to 300 h at 798 K. Strengthening of maraging steel was caused by the formation of an intermetallic phase enriched in Ni and Al which exhibits an ordered B2 (CsCl) superlattice structure. Precipitation hardening was accompanied by an increase in micro‐hardness with peak hardness after about 12 h of ageing. After 300 h of ageing, the micro‐hardness value is still high, corresponding to 94% of the peak hardness. The reverse transformation of martensite to austenite does not take place during prolonged ageing as shown by X‐ray and electron backscatter diffraction analyses. The experimentally determined amount of austenite (1‐2 vol.%) is in good agreement with the calculated value (about 2.5 vol.%).     

Austenite Formation in a Cold-Rolled Semi-austenitic Stainless Steel

The progress of the martensite (α′) to austenite (γ) phase transformation has been thoroughly investigated at different temperatures during the continuous heating of a cold-rolled precipitation hardening metastable stainless steel at a heating rate of 0.1 K/s. Heat-treated samples have been characterized using different experimental complementary techniques: high-resolution dilatometry, magnetization, and thermoelectric power (TEP) measurements, micro-hardness-Vickers testing, optical/scanning electron microscopy, and tensile testing. The two-step transformation behavior observed is thought to be related to the presence of a pronounced chemical banding in the initial microstructure. This banding has been characterized using electron probe microanalysis. Unexpectedly, dilatometry measurements seem unable to locate the end of the transformation accurately, as this technique does not detect the second step of this transformation (last 20 pct of it). It is shown that once the starting (A _S) and finishing (A _F) transformation temperatures have been estimated by magnetization measurements, the evolution of the volume fractions of austenite and martensite can be evaluated by TEP or micro-hardness measurement quite reliably as compared to magnetization measurements. The mechanical response of the material after being heated to temperatures close to A _S, A _F, and (A _F ? A _S)/2 is also discussed.