A thermal processing technique for TRIP steels

An investigation was made to determine the effect of thermal cycling between martensite and reverted austenite on the structure and mechanical properties of a Fe-24 pct Ni-4 pct Mo-0.3 pct C TRIP steel. Measurements of hardness, tensile properties, X-ray line broadening, and metallographic structure indicate that repetitive cycling both strengthens the austenite and raises theM _D from below room temperature to above room temperature. It is shown that cyclical thermal processing produces mechanical properties essentially comparable with thermomechanical processing in this particular TRIP steel.     

Relationship between austenite dislocation density introduced during thermal cycling and M s temperature in an Fe-17 wt pct Mn alloy

The reason why thermal cycling decreases the martensite start (M _s ) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitic transformation. The M _s temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strain energy (ΔG _sh ) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling. The ΔG _sh value increased from 19.3 to 28.8 MJ/m³ due to the increase in austenite dislocation density from 1.5 × 10¹² to 3.8 × 10¹³/m² with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the increase in austenite hardness with the number of thermal cycles.     

The effects of prior deformation and transformations on the microstructure of an iron-nickel alloy

The microstructure of an Fe-31.4 pet Ni-0.3 pet C alloy was examined via transmission electron microscopy as a function of thermomechanical treatment. The effects of prior deformation, rapid reversion to austenite and thermal cycling on the microstructure were investigated, and operative strengthening mechanisms under various conditions were correlated to observed structures. When midrib twinned, plate martensite of this alloy was deformed at room temperature, dislocation glide was the operating mode, and the midrib twins and plate like structure were completely dissolved after 80 pet cold rolling. The microstructure of reverted austenite without prior deformation was composed of sheared plates, but became finely equiaxed with prior deformation of the martensite. The superior strength of reverted austenite in comparison to annealed austenite was due to a grain size refinement and a higher dislocation density. However, the strengthening observed in reverted austenite with prior deformation in comparison to reverted austenite without prior deformation was due to a grain size effect alone. Repeated thermal cyclings increased the strength of reverted austenite. This was due to increases in the dislocation density since the grain structure was principally dictated by the first martensite transformationreversion cycle.     

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.     

The reversible martensite transformation in iron-platinum alloys near Fe3Pt

A series of iron-platinum alloys containing 25 or 27 at. pct platinum and with ordering of the γ-phase varying from substantial disorder to nearly complete order have been thermally cycled between 25°C and - 196°C. The kinetics of the γ⇌α transformations, the hysteresis revealed by electrical resistanceJtemperature plots, the thermoelastic growth and the reappearance of an identical microstructure after thermal cycling (the microstructural memory effect) were studied as a function of the ordering of the γ-phase. Thermoelastic growth does not appear to be affected by changes in the degree of order of partially ordered specimens but the microstructural memory was imperfect in the most highly ordered specimen examined. In agreement with earlier observations by Dunne and Wayman,³ the difference between the A_s and M_s temperatures and the hysteresis decrease markedly as the order is increased. It is shown by transmission electron microscopy that in all but the most highly-ordered specimens the α- γ transformation produces plates of austenite with a high density of dislocations. These plates are separated from the surrounding untransformed parent austenite by arrays of dislocation loops lying in the interfaces between untransformed parent austenite and the original martensite plates. All the dislocations have a Burgers vector direction which is the same as that of the usual slip dislocation in austenite. Such dislocations lying in a habit plane must be sessile. In the well-ordered specimen dislocation pairs, typical of glide dislocations in a crystal with long-range order, were formed in the austenite formed by the reverse transformation. These dislocations were segregated into roughly plate-like clusters, but the number of clusters in unit volume was appreciably less than the number of original plates of martensite. In this case, no arrays of sessile loops of dislocations mark the locations of the original martensite-austenite interface. It is deduced from the microscopic and kinetic results that the inherited nuclei responsible for the microstructural memory effect are located in localized volumes of highly dislocated austenite formed by the α- γ transformation. No unique dislocation configurations which could be associated with specific nuclei were found. The effects of ordering on the various kinetic effects and the microstructural memory are discussed in terms of the concept of inherited nuclei, the change of the flow stress of the γ-phase with ordering and temperature and the variation of T_o and the transformation driving-force with ordering. Formerly at Northwestern University     

Strong and Ductile Martensitic Steels for Automotive Applications

The limits of strength and ductility of a medium‐carbon silicon chromium spring steel are investigated for the case of conventional heat treatment including austenitization, quenching and tempering. The effect of phosphorus and austenite deformation prior to quenching was studied by measuring mechanical properties after quenching and tempering and by microstructural investigation. Strong influence of phosphorus on the ductility is observed for the quenched and tempered martensite without prior austenite deformation. The minimum in ductility found after tempering at 350°C is explained by the formation of cementite and grain boundary segregation of phosphorus. Two thermomechanical treatments were tested involving different austenite conditions produced by variation of the deformation temperature. The deformed conditions, recrystallized or work‐hardened, exhibit higher ductility at all tempering temperatures tested. A combined thermomechanical treatment is proposed that provides the highest ductility after tempering at 300°C independent of the phosphorus content. All thermomechanical treatments described in this study refine or eliminate carbide films at prior austenite grain boundaries. It was found possible to increase the tensile strength and the fatigue limit by deformation of austenite prior to quenching while maintaining or increasing the ductility level.     

Texture development in dual-phase cold-rolled 18 pct Ni maraging steel

Austenite and martensite textures were studied in 18 pct Ni 350-maraging steel as a function of various degrees of cold rolling. The austenite phase in the samples was produced by repeated thermal cycling between ambient and 800 °C. The austenite phase thus formed was mechanically unstable and transformed to the martensite phase after 30 pct cold rolling. The texture developed as a result of cold rolling, and its effect upon microstructure and hardness has been studied.     

Effect of Cr and N on the Martensitic Transformation in Fe‐8%Mn Alloys

The influence of Cr and N on the transformation temperatures of a Fe‐8%Mn alloy has been investigated by means of equilibrium thermodynamics and dilatometry. The addition of Cr and N resulted in the presence of ferrite or α'‐martensite at room temperature, with the microstructure transforming to a single phase austenitic microstructure with increasing temperature. Only high amounts of Cr or N in excess of 0.2% prevented the transformation to a single phase austenitic microstructure. The addition of alloying elements resulted in a decrease of the martensite start temperature M_s. The effect on the austenite start temperature A_s was smaller. The effect of thermal cycling resulted in a stabilization of the transformation temperatures. More cycles were required to reach stable phase transformation temperatures when N was added to Fe‐Mn‐Cr alloys.     

Phase transformation-induced grain refinement in rapidly solidified ultra-high-carbon steels

Superplastic forming offers a promising approach for reducing the cost of high-performance metal components with complex shapes. Severe thermomechanical deformation is one method for producing the fine grain structure needed to permit superplastic forming economically. Our approach to generating fine-grained microstructures is by cyclic heat treatment of rapidly solidified material. First, a metastable structure is produced by rapid quenching of the liquid metal. Then, solid-state phase transformations at modest temperatures are employed to refine this structure. In the ultra-high-carbon steels (UHCS) studied, the brittle as-cast structure of martensite and austenite was transformed, after cyclic heat treatment, to a ductile mixture of 1-μm ferrite and 0.25-μm carbide. Varying the heat-treat temperatures by 100 °C within the transformation range had little effect on the scale of the microstructure. Higher C resulted in coarser carbide spheroids, addition of Al refined the microstructure, and the finest mean carbide size was obtained with an intermediate level (5 pct) of Cr. Refinement of the martensite plates retained austenite via cyclic tempering and austenitization was found to be the key step in the overall mechanism for phase transformation-induced grain refinement in rapidly solidified UHCS.     

The effect of severe marforming on shape memory characteristics of a Ti-rich NiTi alloy processed using equal channel angular extrusion

A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Grain Refinement by Cyclic Displacive Forward/Reverse Transformation in Fe-High-Ni Alloys

A novel method for grain refinement of martensite structures was proposed, in which transformation strain is accumulated by cyclic displacive forward and reverse transformations. This method can refine martensite structures in an Fe-18Ni alloy because a high density of austenite dislocations is introduced by a displacive reverse transformation in addition to an inheritance of dislocations in body-centered cubic martensite into austenite during cyclic transformation. The addition of a small amount of carbon accelerates structure refinement significantly, which results in the formation of ultra-fine-grained structures after ten cycles.

     

Improvement in the Shape Memory Response of Ti<Subscript>50.5</Subscript>Ni<Subscript>24.5</Subscript>Pd<Subscript>25</Subscript> High-Temperature Shape Memory Alloy with Scandium Microalloying

A Ti_50.5Ni_24.5Pd₂₅ high-temperature shape memory alloy (HTSMA) is microalloyed with 0.5 at. pct scandium (Sc) to enhance its shape-memory characteristics, in particular, dimensional stability under repeated thermomechanical cycles. For both Ti_50.5Ni_24.5Pd₂₅ and the Sc-alloyed material, differential scanning calorimetry is conducted for multiple cycles to characterize cyclic stability of the transformation temperatures. The microstructure is evaluated using electron microscopy, X-ray diffractometry, and wavelength dispersive spectroscopy. Isobaric thermal cycling experiments are used to determine transformation temperatures, dimensional stability, and work output as a function of stress. The Sc-doped alloy displays more stable shape memory response with smaller irrecoverable strain and narrower thermal hysteresis than the baseline ternary alloy. This improvement in performance is attributed to the solid solution hardening effect of Sc.     

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part I. derivation of flow curve equations

A semi-mechanistic model for predicting the flow behavior of a typical commercial dual-phase steel containing 20 vol pct of ‘as quenched’ martensite and varying amounts of retained austenite has been developed in this paper. Assuming that up to 20 vol pct of austenite with different degrees of mechanical stability can be retained as a result of certain thermomechanical treatments in a steel of appropriate low carbon low alloy chemistry, expressions for composite flow stress and strain have been derived. The model takes into account the work hardening of the individual microconstituents(viz., ferrite-@#@ α, retained austenite- γ _r, and martensite -α′) and the extra hardening of ferrite caused by accommodation dislocations surrounding the ‘as quenched’ as well as the strain-induced(γ _r→ α′) martensite. Load transfer between the phases has been accounted for using an intermediate law of mixtures which also considers the relative hardness of the soft and the hard phases. From the derived expressions, the flow behavior of dual phase steels can be predicted if the properties of the individual microconstituents are known. Versatility of the model for application to other commercial steels containing a metastable phase is discussed.     

Frictional Heat-Induced Phase Transformation on Train Wheel Surface

By combining thermomechanical coupling finite element analysis with the characteristics of phase transformation [continuous cooling transformation （CCT） curve], the thermal fatigue behavior of train wheel steel under high speed and heavy load conditions was analyzed. The influence of different materials on the formation of the phase transformation zone of the wheel tread was discussed. The result showed that the peak temperature of wheel/track friction zone could be higher than the austenitizing temperature for braking. The depth of the austenitized region could reach a point of 0.9 mm beneath the wheel tread surface. The supercooled austenite is transformed to a hard and brittle martensite layer during the following rapid cooling process, which may lead to cracking and then spalling on the wheel tread surface. The decrease in carbon contents of the train wheel steel helps inhibit the formation of martensite by increasing the austenitizing temperature of the train wheel steel. When the carbon contents decrease from 0.7% to 0.4%, the Ac3 of the wheel steel is increased by 45 ℃, and the thickness of the martensite layer is de creased by 30 %, which is helpful in reducing the thermal cycling fatigue of the train wheel tread such as spalling.     

Cyclic-Loading Induced Lattice-Strain Asymmetry in Loading and Transverse Directions

Cyclic-loading effects on a nickel-based superalloy are investigated with in-situ neutron-diffraction measurements. The temperature evolution subjected to cyclic loading is estimated based on the lattice-strain evolution. The calculated thermoelastic responses are compared with the measured bulk temperature evolution. Two transitions in the temperature-evolution are observed. The first transition, observed with the neutron-measurement results, is associated with the cyclic hardening/softening-structural transformation. The second transition is observed at a larger number of fatigue cycles. It has a distinct origin and is related to the start of irreversible structural transformations during fatigue. A lattice-strain asymmetry behavior is observed. The lattice-strain asymmetry is quantified as a grain-orientation-dependent transverse/loading parameter. This strain-asymmetry evolution reveals the irreversible plastic deformation subjected to fatigue. The irreversible fatigue phenomena might relate to the formation of the microcracks. At elevated temperatures, the cyclic hardening/softening transition starts at lower fatigue cycles as compared to room temperature. A comparison between the room-temperature and the elevated-temperature fatigue experiments is performed. The asymmetry-parameter evolutions show the same irreversible trends at both the room and elevated temperatures.     

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.     

Thermal Stability in Nanocrystalline Fe-30?Wt?Pct Ni Alloy Induced by Surface Mechanical Attrition Treatment

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline surface layer is produced in Fe-30 wt pct Ni alloy, accompanying the formation of the strain-induced martensite. The thermal stability of nanocrystalline martensite and parent phase austenite in Fe-30 wt pct Ni alloy is studied by X-ray diffraction (XRD) and transmission electron microscope (TEM). The grain growth kinetics parameters, time exponent, n, and activation energy, Q, for both martensite and austenite, are determined, respectively. The TEM observations indicate that abnormal grain growth occurs during annealing at high temperatures.     

Effect of Mn Addition on Microstructural Modification and Cracking Behavior of Ferritic Light-Weight Steels

In the present study, effects of Mn addition on cracking phenomenon occurring during cold rolling of ferritic light-weight steels were clarified in relation to microstructural modification involving κ-carbide, austenite, and martensite. Four steels were fabricated by varying Mn contents of 3 to 12 wt pct, and edge areas of steel sheets containing 6 to 9 wt pct Mn were cracked during the cold rolling. The steels were basically composed of ferrite and austenite in a band shape, but a considerable amount of κ-carbide or martensite existed in the steels containing 3 to 6 wt pct Mn. Microstructural observation of the deformed region of fractured tensile specimens revealed that cracks which were initiated at ferrite/martensite interfacial κ-carbides readily propagated along ferrite/martensite interfaces or into martensite areas in the steel containing 6 wt pct Mn, thereby leading to the center or edge cracking during the cold rolling. In the steel containing 9 wt pct Mn, edge cracks were found in the final stage of cold rolling because of the formation of martensite by the strain-induced austenite to martensite transformation, whereas they were hardly formed in the steel containing 12 wt pct Mn. To prevent or minimize the cracking, it was recommended that the formation of martensite during the cooling from the hot rolling temperature or during the cold rolling should be suppressed, which could be achieved by the enhancement of thermal or mechanical stability of austenite with decreasing austenite grain size or increasing contents of austenite stabilizers.     

Influence of alloying by nitrogen on the strength and austenite stability of X18H10 steel

The strengthening and change in austenite stability when steel of X18H10 type alloyed by nitrogen are investigated, for applications where corrosion-resistant structural steel must operate satisfactorily at both high and cryogenic temperatures. Alloying by nitrogen may be regarded as a promising means of increasing the strength and stability of austenitic stainless steel. Preliminary cold or hot working increases the likelihood of martensite formation under load and consequently limits the working temperature range of the steel. High-strength nonmagnetic nitrogen steel based on X18AH10 steel with up to 0.22% N may be used for undeformable components at cryogenic temperatures. Without nitrogen, deformational martensite is always formed in such steel at temperatures below ?70°C. High strength, plasticity, and ductility may only be ensured in such steel by means of the TRIP effect or reduction in the grain size. Nitrogen effectively strengthens the solid solution in the high-temperature state. The use of combined high-temperature deformation and moderate-temperature deformation permits additional strengthening of the steel during thermomechanical treatment, including strain aging, which is effective where thermostable steel is required.