Origin of Microstructural Irreversibility in Ni-Ti Based Shape Memory Alloys during Thermal Cycling

Different microstructures of Ni-Ti- and Ni-Ti-Fe-based shape memory alloys were subjected to thermal cycling: dipping in liquid nitrogen, for approximately 5 minutes, and then bringing it back to room temperature or austenite (cubic: B2) ↔ martensite (monoclinic: B19′) reversible solid-state phase transformation. Direct electron backscattered diffraction (EBSD) observations could bring out aspects of microstructural irreversibilities: namely, changes in grain size, misorientation buildup, and presence of retained martensite. The average changes in grain size (Δd) differed by almost 2 to 4 times between different microstructures. The highest Δd was typically observed in structures having maximum clustering of fine (d < 5 μm) grains. The sample with highest Δd was also subjected to multiple thermal cycling. Although Δd scaled linearly with d after the first thermal cycle, the scatter increased during subsequent thermal cycles. Grain or orientations deviating from the linear behavior were clearly anisotropic crystallographically. With repeated thermal cycling, the patterns of changes in Δd, austenite misorientation, and retained martensite content were similar. A phenomenological model or hypothesis, based on 40 deg á 001 ñ \left\langle {001} \right\rangle orientation relationship between austenite and martensite phases, was proposed to address the observed patterns of microstructural irreversibility.     

The effect of austenite prestrain above the Md temperature on the martensitic transformation in Fe-Ni-Cr-C alloys

The effect of austenite prestrain above theM _d temperature on the structure and transformation kinetics of the martensitic transformation observed on cooling was determined for a series of Fe-Ni-Cr-C alloys. The alloys exhibited a shift in martensite morphology in the nondeformed state from twinned plate to lath while theM _s temperature, carbon content, and austenite grain size were constant. The transformation behavior was observed over the temperature range 0 to -196°C as a function of tensile prestrains performed above theM _d temperature. A range of prestrains from 5 pct to 45 pct was investigated. It is concluded that the response of a given alloy to austenite prestrain above theM _d temperature can be correlated with the morphology of the martensite observed in the nondeformed, as-quenched state. For the range of prestrains investigated, the transformation of austenite to lath martensite is much more susceptible to stabilization by austenite prestrain above theM _d temperature than is the transformation of austenite to plate martensite.     

Reverse transformation of ferrite and pearlite to austenite in an ultrafine-grained low-carbon steel fabricated by severe plastic deformation

Reverse transformation characteristics of a low-carbon steel consisting of ultrafine-grained (UFG) ferrite and severely deformed pearlite by severe plastic deformation were investigated and compared to those of the steel having coarse-grained (CG) ferrite and undeformed pearlite by austenitization and subsequent air cooling. Coarse-grained steel exhibited two serial transformation stages, i.e., pear-lite → austenite followed by ferrite → austenite. Contrarily, UFG steel transformed with the three serial stages, i.e., probably carbon-supersaturated ferrite → austenite, not-fully-dissolved pearlite → austenite, and ferrite → austenite transformations.     

Evaluation of the nucleation potential of intragranular acicular ferrite in steel weldments

An empirical model has been developed to evaluate the relative nucleation potential (p) of intragranular acicular ferrite (IAF) in steel weldments. The p value was found to be a function of austenite grain size (g) and the volume fraction of IAF (ν_IAF), or, alternatively, a function of the mean free path of inclusions and ν_IAF. The model indicates that the presence of inclusions promotes IAF by increasing the intragranular nucleation surface area, but excessive inclusions deteriorate the formation of IAF due to decreasing austenite grain size, increasing grain boundary associated inclusions, and impairing the intragranular nucleation potential. A transition from p < 1 to p > 1 is predicted to occur at the grain size of about 100 μm. Analyses of experimental data show that the minimum inclusion size to ensure a good nucleation potential is in the range of 0.25-0.8 μm.     

Influence of cooling rate on the microstructure and retained austenite in an intercritically annealed vanadium containing HSLA steel

The influence of cooling rate from the intercritical αγ region on the microstructure of a vanadium bearing HSLA steel was investigated by transmission electron microscopy. Oil quenching produced an essentially ferrite-martensite dual phase structure with ∼4 vol pct of fine particle and thin film retained austenite. In contrast, the slower air cooling resulted in a larger amount (∼10 vol pct) of retained austenite in addition to the ferrite and martensite phases. A major portion of the retained austenite in the air cooled specimen was of the blocky morphology (1 to 6 μm , with the remainder eing the submicron variety, similar to that of the oil quenched specimen. In conformance with the terminology of earlier studies, “retained” ferrite and “transformed” ferrite were observed in the air cooled steel while oil quenching completely suppressed the transformed ferrite. Retained ferrite, the cleaner of the two in terms of precipitate content, is the high temperature ferrite that coexists with the austenite at the intercritical temperature and which is retained on cooling. The transformed ferrite, on the other hand, forms from the decomposition of the austenite and contains banded carbonitrides (row precipitation) much like the initial microstructure of the HSLA steel. formerly known as B. V. N. Rao formerly with General Motors Research Laboratories, Warren, MI,     

In situ observation of growth behaviour of acicular ferrite in low-carbon steel containing 13 ppm magnesium

SS400 steel with 13?ppm magnesium was prepared to study the effects of supercooling degree on the formation behaviour of acicular ferrite (AF) and microstructures of Mg-containing low-carbon steel. The inclusions characterised using an automated inclusion analyzer, ASPEX, were mostly MnS, MgAl₂O₄ and MgO with sizes of 1–2?μm. The growth behaviour of AF during the cooling process for austenite transformation to ferrite and continuous cooling temperature diagram was investigated using in situ observation with high-temperature confocal laser scanning microscopy. The initial formation temperature of AF decreased with increasing cooling rate. The temperature of AF transformation was in a narrow range of around 100–150?K. The probability of AF nucleation from inclusions and the average AF lath growth rate increased with increasing cooling rate due to the larger driving force and thermal strain energy around the inclusions during the cooling processes.     

Theory of modelling the isothermal austenite grain growth in a Si‐Mn TRIP steel

The development of a model to predict the isothermal austenite grain growth during soaking of a low carbon Si‐Mn TRIP steel is described. After reviewing the existing models for isothermal grain growth, a general model dⁿ=d₀ⁿ + K₁t exp(K₂/T) was selected and a procedure was delineated to calculate the values of the different constants of the equation starting with the real three‐dimensional austenite grain size. This paper also deals with an improved etching technique to reveal the austenite grain boundaries.     

Structure and properties of a microduplex maraging steel

A simple two-step thermal processing technique was devised to impart a microduplex structure in a high strength 250 grade commercial maraging steel. A martensite grain size of approximately 1μm was obtained with interspersed islands of retained austenite whose volume fraction and mechanical stability could be controlled by varying the thermal processing conditions. The microstructure and mechanical properties of the microduplex structure were compared to those of the alloy in the maraged, martensitic condition. Due to the presence of the austenite phase, the microduplex structure showed a much smaller temperature and strain rate dependence of deformation than the martensitic structure. A remarkable increase in uniform elongation was observed below theM _d temperature of retained austenite. The microduplex structure did not show any significant advantage in fracture toughness over the martensitic structure when compared at similar strength levels. By suitably adjusting austenitic stability a deformation-induced phase transformation (TRIP) of the retained austenite in the microduplex structure could be made to occur; however, the transformation did not lead to any evident increase in toughness. The microduplex structure exhibited a slight improvement in fracture toughness at high strain rate in contrast to the martensitic structure in which the rate effect significantly reduced the toughness.     

Spatially resolved X-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese steel arc welds

Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite; however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld.     

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

Changes in transformation behaviour and microstructure of a CrV-spring steel by thermomechanical treatment

The effect of austenite deformation on the transformation behaviour was investigated on a CrV-spring steel with the major attention put on the martensitic transformation. In the first part, a small review is given on the relation between the state of austenite after hot deformation and its influence on the formation of martensite. In the laboratory tests, the second part of the paper, a conventional heat treatment (CHT) was compared with two types of austenite conditioning by thermomechanical treatment (TMT): TMT_R - with deformation above the recrystallization temperature ?_R leading to a fully recrystallized austenite and TMT_N- with deformation below ?_R with a not recrystallized but possibly polygonized austenite. For the laboratory tests, the hot deformation simulator Wumsi was employed. After quenching In oil, the martensite after TMT consisted of associations of many fine fragments with a smaller number of large acicular martensitic units than observed after CHT. In both TMT-variants small ferritic areas (< 1 μ m) could be revealed. Different behaviour of martensite during tempering at low temperatures was observed after CHT and TMT. It can be explained by reduced inherent stresses generated during martensitic transformation after TMT, presumably as a result of a better ability of deformed austenite to withstand the accommodation strain during martensitic transformation. This may have considerable consequences for the toughness properties of tempered martensite.     

Development of thermo-mechanical treatments of a maraging steel for yield strengths above 3 GPa

Hot-rolling of austenite and cold-rolling of martensite were combined with aging treatments to obtain new microstructures in a maraging steel with a high Mo-content. Purpose of the investigation is to acquire an optimum combination of ultra high yield stress (σ_r>3000 MPa) and ductility (or toughness). The best results were obtained by treatments consisting of high amounts of plastic deformation between 1000°C and 600°C (ausforming) and aging of the martensite which had formed from the deformed austenite. Ausaging around 550°C induced pronounced intercrystalline embrittlement. Intercritical heat treatment at 700°C provide considerable ductility (uniform elongation) however, insufficient strength (<2000 MPa). The most favourable thermo-mechanical treatments are discussed in context with fabrication methods for the investigated steel.     

中厚板中间坯冷却过程中晶粒长大及控制方法

为控制中厚板中间坯长时间待温导致的晶粒长大,研究了中间强制水冷却对奥氏体组织的影响.通过对Q345B钢和含Nb-Ti钢采用1050℃变形后快冷至1050~950℃预定温度保温的热模拟方法,确定了中间坯冷却过程中的晶粒尺寸变化规律,提出了中厚板冷却过程中晶粒长大的控制方法,建立了Q345B钢和含Nb-Ti钢在中间冷却过程中的晶粒长大模型.在中间冷却过程中,Q345B钢晶粒稳定性较差,而含Nb-Ti钢晶粒稳定性良好,归因于以铌为主的析出相对奥氏体晶界的钉扎作用.中间坯的强制冷却可控制奥氏体晶粒长大,63mm厚中间坯强制冷却可有效减小平均晶粒尺寸约20μm.在实际生产中,经中间强制冷却后16 mm厚度Q345B钢板的冲击韧性提高25%~70%.     

Effect of austenitisation temperature on austenite transformations in 0·7%C Cr–Mo PM steel

Abstract

The effect of austenitisation temperature on austenite transformations on 0·7%C Astaloy CrL steel was studied by dilatometry. The steel has a good hardenability, forming martensite at most of the austenitisation temperatures and cooling rates investigated. Only on cooling from 1073 K, austenite transforms into bainite completely at 3 K s^?1 and partially at 12·5 K s^?1. The effect of austenitisation temperature on the prior austenitic grain size is quite poor because of the pinning effect of pores. The martensite start temperature M_s increases slightly with the austenitisation temperature up to 1173 K and decreases at 1523 K. This trend is due to the presence of nanometric carbides (Cr₂₃C₆), which were detected at TEM. They dissolve almost completely in austenite at 1523 K only, increasing the stability of austenite against the martensitic transformation. The effect of temperature in the range from 1073 K up to 1523 K is poor. As a consequence, the microstructural characteristics of hardened steels are very similar.     

On the Significance of Nature of Strain-Induced Martensite on Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d < 100 nm), ultrafine (d ≈ 100 to 500 nm), and submicron (d ≈ 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)–high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocation-cell–type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell–type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell–type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperature-time annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell–type structure in lieu of lath-type facilitates NG/UFG structure with a high strength–high ductility combination. Interestingly, the yield strength follows the Hall–Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.     

Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates

The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling) and 40 kJ/cm (slow cooling) were used to generate single-pass thermal cycles with peak temperatures in the range of 750 °C to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start temperatures (T _s ) of 550 °C to 560 °C while cooling from a peak temperature (T _p ) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The T _s value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation to lath martensite. In HSLA-100 steel, the T _s value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the final microstructure and is harder than the CGHAZ of HSLA-100 steel.     

Effect of TiN particles and microstructure on fracture toughness in simulated heat-affected zones of a structural steel

Thermally stable TiN particles can effectively pin austenite grain boundaries in weld heat-affected zones (HAZs), thereby improving toughness, but can also act as cleavage initiators. The HAZs simulated in a GLEEBLE 1500 TCS using two peak temperatures (T _p ) and three cooling times (Δ∼ _8/5) have determined the effects of matrix microstructure and TiN particle distribution on the fracture toughness (crack tip opening displacement (CTOD)) of three steels microalloyed with 0.006, 0.045, and 0.1 wt pct Ti. Coarse TiN (0.5 to 6 μm) particles are identified in steels with the two higher levels of Ti, and fine Ti(C, N) (35 to 500 nm) particles were present in all three steels. Large prior austenite grain size caused by higher T _p decreased fracture toughness considerably in steels containing coarse TiN particles but had little effect in their absence. Fracture toughness was largely independent of matrix microstructure in the presence of coarse particles. Cleavage fracture initiation was observed to occur at coarse TiN particles in the samples with a large prior austenite grain size. Alloy thermodynamics have been used to rationalize the influence of Ti content on TiN formation and its size.     

Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates

The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling) and 40 kJ/cm (slow cooling) were used to generate single-pass thermal cycles with peak temperatures in the range of 750 °C to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start temperatures (T _s ) of 550 °C to 560 °C while cooling from a peak temperature (T _p ) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The T _s value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation to lath martensite. In HSLA-100 steel, the T _s value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the final microstructure and is harder than the CGHAZ of HSLA-100 steel.     

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.