Austenite Particles Research Articles

X-Ray Residual Stress and Strain-Induced Transformation of Retained Austenite in TRIP-Aided Dual-Phase Steels.

The effects of strain-induced transformation (SIT) on X-ray residual stress of retained austenite islands in TRIP-aided dual-phase steels were examined under uniaxial tension, and were discussed using a micromechanics theory. If the retained austenite particles are stable against the SIT and strain hardening, high tensile residual stress of 100 to 200 MPa occurs in the particles. The stress corresponds to that estimated from the micromechanics theory. In such a case, the retained austenite particles effectively increase the internal stress of the ferrite matrix similar to bainite and martensite particles. When the retained austenite particles easily transform to martensite during straining, the residual stress is decreased by plastic relaxation resulting from expansion and shear strains on transformation, although the strain-induced martensite increases the flow stress of the steels.

TRIP型複合組織鋼のバウシンガー効果に及ぼす変形温度の影響

The influence of deformation temperature on the Bauschinger effect (BE) of a high-strength TRIP-aided dual-phase (TDP) steel containing retained austenite particles of 10 vol% was investigated in a temperature range between 27 and 200°C. The BE of the TDP steel depended significantly on the deformation temperature, although such a temperature dependence did not appear in a conventional ferrite-martensite dual-phase (DP) steel. Large BE, i.e., remarkable transitional softening and subsequent permanent softening proposed by Orowan, appeared at 100 and 200°C, at which the retained austenite particles were more resistant to the strain-induced martensite transformation (SIT) than those at 27°C. The Bauschinger stress obtained at 200°C was as high as that of the DP steel. The retained austenite particles as a hard phase increased the Bauschinger stress. The SIT, however, was expected to lower the Bauschinger stress by the following reason. Expansion and shear strains resulting from the SIT reduced both internal stresses in the retained austenite islands and in the ferrite matrix.

Effects of tensile-testing temperature on deformation and transformation behavior of retained austenite in a 0.14C-1.2Si-1.5Mn steel with ferrite-bainite-austenite structure

A 0.14% C-1.21% Si-1.57% Mn steel was processed by intercritical annealing followed by isothermal transformation to produce a structure of 75% ferrite, 13% bainite and 12% retained austenite. The role of retained austenite was studied by directly observing the deformation and transformation behavior of the retained austenite. The stability of the retained austenite was changed by varying tensile-testing temperature from − 80 °C to 120 °C. With increasing temperature, the stability of the retained austenite with deformation increased. Furthermore, the austenite stability increased with a decrease in the austenite particle size. For samples tested at both 20 °C and 120 °C, retained austenite was observed to remain in the deformed structure at high strain. The deformed retained austenite improved ductility by suppressing void formation at the interface between retained austenite and either ferrite or bainite. In contrast, for samples deformed at − 80 °C, void formation preferentially occurred by interface decohesion of martensite-ferrite or martensite-bainite. The results of this study indicate that the ductility in low-carbon steel with high contents of retained austenite can be altered by control of the austenite stability.

Observation of deformation and transformation behavior of retained austenite in a 0.14C-1.2Si-1.5Mn steel with ferrite-bainite-austenite structure

A 0.14wt.%C-1.21wt.%Si-1.57wt.%Mn steel was intercritically annealed at 770°C for 5 min and then isothermally held at 400°C for 4 min followed by oil quenching to room temperature. The as-annealed microstructure consisted of 75% ferrite, 13% bainite, and 12% retained austenite. The steel was used to establish a microscopic method to identify retained austenite, and to observe the deformation and transformation behavior of the retained austenite with tensile deformation. Submicron-sized articles of retained austenite could be distinguished from ferrite, bainite and martensite in light microscopy when etched with 5% nital followed by etching in a 10% sodium metabisulphite solution. The retained austenite particles were observed in three types of locations: type I at ferrite grain boundaries always in contact with bainite; type II at ferrite grain boundaries out of contact with bainite; and type III within ferrite grains. It was also observed that the larger retained-austenite particles transformed more easily to martensite during deformation at room temperature. Even after deformation to a tensile strain greater than 32%, about 4% of the retained austenite remained, and the retained-austenite particles were observed in the deformed structure.

Control of Reflective Properties of Flake Metal Products

AbstractLaboratory scale atomization/milling trials have established that the surface characteristics and the reflective properties of flake metal products depend critically on the shape of the powders before milling. For both pure iron and austenitic stainless steel flake particles, superior reflective properties were recorded for flake with smooth surfaces and rounded outlines produced by milling spherical powders, compared with flake having rough surfaces and jagged outlines obtained by milling irregularly shaped atomised powders. The importance of controlling flake morphology and reflective properties is demonstrated by the outstanding quality of the fingerprints which could be revealed on light or dark backgrounds of various textures using the pure iron and stainless steel flake products. PM/0598

Effects of Second Phase Morphology on RetainedAustenite Morphology and Tensile Properties in a TRIP-aided Dual-phase Steel Sheet.

The relationship between second phase morphology and retained austenite morphology and the influences of these two kinds of morphology on tensile properties of a 0.17C-1.41Si-2.00Mn (mass%) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400°C.A large amount of fine retained austenite was obtained when the second phase morphology was "a network structure" or "an isolated fine and acicular one." The retained austenite particles were nearly isolated in the ferrite matrix away from bainite islands and were moderately stable. On the other hand, "an isolated coarse structure" of second phase resulted in a small amount of more stable retained austenite film along bainite lath boundary.The influence of second phase morphology on the flow curve significantly differed from that of a conventional ferrite-martensite dual-phase steel. Isolated retained austenite particles lowered the flow stress, and resultantly reduced the effects of second phase morphology (i.e., network effect or fine grain size effect) on flow stress. However, the isolated retained austenite particles enhanced effectively the ductility, particularly at 50-100°C, due to the moderate strain induced transformation. On the other hand, retained austenite films along bainite lath boundary scarcely influenced on tensile properties of the steel. These results were discussed on the basis of a continum theory.

Effect of amount of second phase on texture and anisotropy of superplasticity indicators of sheets of microduplex steel 03Kh26N6T

1. Differences in the rolling texture in steel 03Kh26N6T produced from different melts are not associated with the austenite content prior to rolling, but are governed primarily by the amount of solid inclusions of titanium carbonitrides. 2. When austenite is present in steel prior to cold rolling, the metallographic texture is reduced after heating for SPD, and the more austenite present prior to rolling, the more equiaxial the shape of its particles. 3. The level and anisotropy of the superplasticity characteristic of steel 03Kh26N6T sheets are determined mostly by the extent to which the austenite particles that form on heating for SPD are nonequiaxial.

An atom probe FIM study of interphase precipitation in a model alloy steel

Interphase precipitation of vanadium carbide, particularly when associated with the growth of pearlite, has been studied in a model alloy steel. The precipitates, characteristically dispersed non-randomly in rows or sheets in the proeutectoid and pearlitic ferrite regions, provide an important strengthening increment in a wide variety of commercial microalloyed steels. The precipitates form at the γ/α transformation interface during austenite decomposition, but this has proved difficult to examine in detail in partially transformed commercial steels because the parent austenite phase and the transformation interface are destroyed by the martensite reaction on further cooling to room temperature. In the present work this problem is avoided by using model alloys based on a high-Mn high-C steel that retains the parent austenite at room temperature, thus allowing examination of the interface by TEM and the untransformed austenite by atom probe FIM techniques. Weak beam TEM has failed to reveal any evidence of vanadium carbide precipitation in the austenite ahead of the interface. However, atom probe FIM has revealed evidence of pre-precipitation clusters on ultrafine particles in the untransformed austenite.

Effects of Volume Fraction and Stability of Retained Austenite on Ductility of TRIP-aided Dual-phase Steels.

The effects of silicon and manganese contents on volume fraction and stability of retained austenite particles in 0.2C-(1.0-2.5)Si-(1.0-2.5)Mn (mass%) TRIP-aided dual-phase steels were investigated. In addition, the relationships between above retained austenite parameters and ductility at room and moderate temperatures were discussed through studies on strain-induced transformation behavior of retained austenite.As increasing the silicon and manganese contents except for 2.5 mass% manganese steel, the initial volume fraction of retained austenite increased with accompanied by reducing carbon concentration in retained austenite. It was found that the ductilities of these steels became maximum at a given temperature between 23 and 175°C, i.e., a peak temperature. The peak temperature was concluded to agree well with the temperature at which the strain-induced transformation of retained austenite was suppressed moderately for each steel. Moreover, the peak temperature Tp (°C) was related to estimated martensite-start temperature Ms (°C) of the retained austenite as Tp=3.04Ms+187. Strength-ductility balance, i.e., the product of tensile strength and total elongation, at the peak temperature linearly increased with an increase in the initial volume fraction of retained austenite.

Growth of small particles of iron-cobalt alloys prepared by gas-evaporation technique

The crystal structure and habit of small particles of iron-cobalt alloys prepared by a gas-evaporation technique have been investigated by means of electron microscopy and X-ray diffraction. Most of the particles consist of the single phase of ferrite or austenite, though the nominal composition of starting materials is around 87 at% Co in which the two phases exist at room temperature. This result suggests that the two-phase region in the phase diagram is narrower for the small particles than for the bulk system. The morphology of the ferrite or the austenite particles is similar to that of the pure-iron or the pure-cobalt ones, respectively, but considerably modified due to the impurity effect. On rare occasions, the particles consisting of the two phases are observed with the crystal relation of (110) fer|(200) aus. They show the external shape in which the two halves of a football are stuck together. It is inferred that they grow through the coalescence process between the ferrite and the austenite particles.

Intragranular Nucleation of Austenite

The mechanism of intragranular nucleation of austenite in a duplex stainless steel (Fe-23.1 Cr-6.1 Ni-3.1 Mo-O.165 N-0.017 C, wt.%) weld metal and heat-affected zone (HAZ) has been examined. In the weld metal the acicular austenite is found to nucleate intragranularly on inclusions and subsequent plates form sympathetically resulting in a fine interlocked microstructure. Austenite plates adopt the Kurdjumov-Sachs orientation relationship with the ferrite matrix and grow with diffusion-controlled mechanism as evident from partitioning of substitutional alloying elements. At least one set of fine intrinsic dislocations on the austenite/ferrite interphase interfaces is observed suggesting that the boundaries are semi-coherent. The high cooling rates involved in the HAZ result in a supersaturated ferrite matrix where precipitation of intragranular austenite occurs as a result of reheating associated with subsequent passes. Austenite particles in the HAZ nucleate preferentially away from gain boundary austenite allotriomorphs indicating that intragranular precipitation is favoured by the supersaturated matrix.

Solidification sequences in stainless steel dissimilar alloy welds

A series of laser and gas tungsten arc welds traversing stainless steels of different chemical compositions has been studied, to elucidate the role of austenite or ferrite nucleation and cooling rate on solidification behaviour. It has been found that a steel with a high CrEQ/NiEQ ratio can be induced to solidify as metastable austenite by initiating the weld in a steel with a lower CrEQ/NiEQ ratio in which the thermodynamically stable solidification mode is austenitic. The austenite dendrites are then found to continue growth across the weld junction into the undiluted regions of the high CrEQ/NiEQ ratio material. By providing austenite particles in this way, nucleation is rendered unnecessary and it is found that solidification to metastable austenite can be induced at cooling rates significantly lower than previously encountered. The results of these and other experiments in which the welding speed was changed during the experiment are interpreted to yield new information about the mechanisms of solidification and transformation in stainless steels during welding under conditions consistent with high cooling rates.MST/1240

Solidification sequences in stainless steel dissimilar alloy welds

AbstractA series of laser and gas tungsten arc welds traversing stainless steels of different chemical compositions has been studied, to elucidate the role of austenite or ferrite nucleation and cooling rate on solidification behaviour. It has been found that a steel with a high CrEQ/NiEQ ratio can be induced to solidify as metastable austenite by initiating the weld in a steel with a lower CrEQ/NiEQ ratio in which the thermodynamically stable solidification mode is austenitic. The austenite dendrites are then found to continue growth across the weld junction into the undiluted regions of the high CrEQ/NiEQ ratio material. By providing austenite particles in this way, nucleation is rendered unnecessary and it is found that solidification to metastable austenite can be induced at cooling rates significantly lower than previously encountered. The results of these and other experiments in which the welding speed was changed during the experiment are interpreted to yield new information about the mechanisms o...

Liquid Phase Sintering of AISI 316L Stainless Steel

Abstract The sintering behaviour of AISI 316L powders alloyed with 20%Cu has been studied as a function of various sintering parameters. Copper additions are known to improve compressibility, sinterability, and dimensional control of the final component, as well as improving the corrosion resistance of the sintered material. Specimens have been sintered in an industrial furnace using various N2–H2 atmospheres and sintering times. Microstructural and structural examination of the specimens revealed that: (i) copper additions can reduce nitride precipitation during sintering, thus preventing sensitisation; (ii) copper diffusion into the austenitic particles is promoted by hydrogen rich (i.e. reducing) atmospheres; and (iii) the chosen copper addition of 20% is excessive. The results confirm that copper can have a beneficial effect on the sintering process and in improving the final properties of components; however, its content must be optimised to obtain a homogeneous sintered material. Atmosphere composit...

Changes in precipitation and corrosion characteristics of 18/10Ti austenitic stainless steel exposed to prolonged annealing at 750–850 °C

This investigation deals with the analysis of intergranular acicular precipitations by X‐ray diffraction techniques and SEM. Results obtained during analysing acicular precipitate in 18/10Ti steel, isothermally annealed at 750–850 °C, can be summarised as follows:Intergranular needles are constituted by a complex phosphide of M2P type whose chemical composition can be written as (Ti,Cr,Fe,Ni)2P. Crystal growth direction of intergranular acicular M2P particles in austenite is 〈100〉γ. The following orientation relationship was determined between the complex phosphide of M2P type and austenite: . Corrosion testing in boiling 65% HNO3 revealed that there was a relationship between the occurrence of acicular precipitate and a reduction in corrosion rate to a level below 0.5 g m−2h−1.

On the bainitic structure formed in a modified A710 steel

Summary This study shows that the microstructure of quenched plates of modified A710 steel is characterized by laths of ferrite, which have a very similar crystallographic orientation within each packet, and interlath particles of martensite and/or austenite. This structure is thus consistent with the multiphase structures termed granular bainite (13). The volume fraction of the second-phase particles is about 0.15, about half austenite and half martensite. During aging at 604°C, a small amount of austenite transforms to cementite. It is suggested that the nature of the second-phase particles may have a profound influence on the toughness of these and similar steels.

An acoustic emission study of martensitic transformation of retained austenite in intercritically annealed HSLA steels

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.

Decomposition of deformed ferrite in a duplex stainless steel

The decomposition of deformed ferrite in a duplex stainless steel (U44L) has been examined using transmission electron microscopy. During annealing, austenite precipitation was extremely rapid and pinned the developing ferritic subgrain structure. Hence, the ferrite recrystallized via a continuous mechanism. Deformation of the ferrite, and to a lesser extent the austenite during growth/coarsening of the austenite, led to a ‘secondary recovery’ process. Subgrain coalescence, within the ‘primary’ ferrite subgrain structure and in the austenite particles, was observed after long term anneals: this process took place by the dissolution of the sub-boundaries formed during the secondary recovery process. In addition to the formation of austenite, M23C6, σ, and martensite were also observed.MST/311

Decomposition of deformed ferrite in a duplex stainless steel

The decomposition of deformed ferrite in a duplex stainless steel (U44L) has been examined using transmission electron microscopy. During annealing, austenite precipitation was extremely rapid and pinned the developing ferritic subgrain structure. Hence, the ferrite recrystallized via a continuous mechanism. Deformation of the ferrite, and to a lesser extent the austenite during growth/coarsening of the austenite, led to a ‘secondary recovery’ process. Subgrain coalescence, within the ‘primary’ ferrite subgrain structure and in the austenite particles, was observed after long term anneals: this process took place by the dissolution of the sub-boundaries formed during the secondary recovery process. In addition to the formation of austenite, M23C6, σ, and martensite were also observed.MST/311

The chemical composition of precipitated austenite in 9Ni steel

Analytical scanning transmission electron microscopy and a novel Mossbauer spectrometry technique were used to measure the chemical composition of austenite particles which precipitate during intercritical tempering of 9Ni steel. Both techniques showed an enrichment of Ni, Mn, Cr, and Si in the austenite. A straightforward analysis involving data on both austenite composition and austenite formation kinetics suggests that the growth of austenite particles is controlled by a 3-dimensional diffusion process. The segregation of solutes to the austenite accounts for much of its stability against the martensitic transformation at low temperatures. Composition inhomogeneities develop in austenite particles after long temperings; the central regions of the particles are lean in solutes and are first to undergo the martensitic transformation. However, changes in solute concentrations of the austenite during long temperings seem too small to account for the large changes in austenite stability. It appears that some of the stability of precipitated austenite must be microstructural in origin.