Showing papers on "Austenite published in 2007"

Creep-resistant, Al2O3-forming austenitic stainless steels.

Abstract: A family of inexpensive, Al2O3-forming, high-creep strength austenitic stainless steels has been developed. The alloys are based on Fe-20Ni-14Cr-2.5Al weight percent, with strengthening achieved through nanodispersions of NbC. These alloys offer the potential to substantially increase the operating temperatures of structural components and can be used under the aggressive oxidizing conditions encountered in energy-conversion systems. Protective Al2O3 scale formation was achieved with smaller amounts of aluminum in austenitic alloys than previously used, provided that the titanium and vanadium alloying additions frequently used for strengthening were eliminated. The smaller amounts of aluminum permitted stabilization of the austenitic matrix structure and made it possible to obtain excellent creep resistance. Creep-rupture lifetime exceeding 2000 hours at 750 degrees C and 100 megapascals in air, and resistance to oxidation in air with 10% water vapor at 650 degrees and 800 degrees C, were demonstrated.

The influence of aluminum on hot deformation behavior and tensile properties of high-Mn TWIP steels

Abstract: The influence of aluminum (0–3 wt.%) on the high-temperature flow stress and recrystallization kinetics of two austenitic 25 wt.% Mn-bearing TWIP steels were investigated and compared with the behavior of a low-carbon steel. In addition, tensile properties were determined over the temperature range from −80 to 200 °C. It was observed that the hot deformation resistance is slightly higher for the 25Mn3Al than for the 25Mn steel, but in both steels significantly higher than for the low-carbon steel. The static recrystallization kinetics is significantly retarded in both steels compared to the rate in the low-carbon steel. The activation energies of hot deformation and static recrystallization are higher than those for the low-carbon steel. In contrast to the high temperature behavior, below RT, the 25Mn steel possessed a higher tensile strength and a higher work hardening rate than the 25Mn3Al steel due to strain-induced martensite formation. With increasing temperature up to 200 °C, the deformation mode changed gradually to mechanical twinning. In the 25Mn3Al steel, the elongation increased with decreasing temperature as a result of enhanced mechanical twinning.

Effect of structure evolution induced by ultrasonic peening on the corrosion behavior of AISI-321 stainless steel

Abstract: A nanocrystalline surface layer was produced on an AISI-321 stainless steel by severe plastic deformation via ultrasonic peening (UP). The microstructural evolution of the surface layer was characterized by means of X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The volume fraction of strain-induced α-martensite as a function of the effective strain ( e ¯ ) was evaluated quantitatively using XRD and magnetic measurements. Considering the e ¯ magnitudes and the TEM data obtained, it is concluded that a grain refinement of austenitic structure passes ahead of the α-martensite formation, particularly in the top surface layer. The nanocrystalline austenitic grain structure (mean grain size ∼ 15 nm) was observed at e ¯ = 0.45 , while the startup of the strain-induced martensitic transformation was revealed at the strain extent of 0.62. The nanostructured surface layer formed after straining to e ¯ = 0.8 already contains mainly the martensite nanograins characterized by an average size of about 10 nm. Grain size increased gradually up to 60 nm within the layer containing both austenite and martensite phases at a depth of about 30 μm from the treated surface. Both the microhardness behavior of the stainless steel surface and its corrosion performance in 3.5% NaCl solution can be enhanced by the UP. They are shown to be in correlation with: (i) the grain refinement process and (ii) the increase in the volume fraction of strain-induced α-martensite.

Microstructure and texture of a lightly deformed TRIP-assisted steel characterized by means of the EBSD technique

Abstract: The microstructural and textural changes after a tensile strain of 10% were observed by orientation contrast measurements in a TRIP-assisted steel. On the undeformed samples it was shown that the electron back scatter diffraction (EBSD) technique could be used successfully for determining the volume fraction of the microstructural constituents bainite, ferrite and austenite, whereas after deformation only the BCC and FCC phases could be separated. The results show that the tensile strain of 10% gave rise to a drop in residual austenite content from 10 to 4%, which was also confirmed by magnetic measurements. The texture data showed only minor orientation rotations after 10% tensile strain for the BCC ferrite and bainite grains, whereas the residual austenite did show a significant texture change. By meticulously monitoring the local intra-granular misorientations it was concluded that the BCC phases (ferrite and bainite) took up the larger part of the nominal strain whereas the residual austenite primarily responded to the mechanical load by a partial (stress-induced) martensite transformation. Hence, the texture change observed in the residual austenite could be attributed to the orientation selective character of the phase transformation.

Hall–Petch Behavior in Ultra-Fine-Grained AISI 301LN Stainless Steel

Abstract: An ultra-fine-grained AISI 301LN austenitic stainless steel has been achieved by heavy cold rolling, to induce the formation of martensite, and subsequent annealing at 800 °C, 900 °C, and 1000 °C, from 1 to 100 seconds. The microstructural evolution was analyzed using transmission electron microscopy and the yield strength determined by tension testing. Ultra-fine austenite grains, as small as ∼0.54 μm, were obtained in samples annealed at 800 °C for 1 second. For these samples, tensile tests revealed a very high yield strength of ∼700 MPa, which is twice the typical yield strength of conventional fully annealed AISI 301LN stainless steels. An analysis of the relationship between yield strength and grain size in these submicron-grained stainless steels indicates a classical Hall–Petch behavior. Furthermore, when the yield dependence on annealing temperature is considered, the results show that the Hall–Petch relation is due to an interplay between fine-grained austenite, solid solution strengthening, precipitate hardening, and strain hardening.

Alumina-Forming Austenitic Stainless Steels Strengthened by Laves Phase and MC Carbide Precipitates

Abstract: Creep strengthening of Al-modified austenitic stainless steels by MC carbides or Fe2Nb Laves phase was explored. Fe-20Cr-15Ni-(0–8)Al and Fe-15Cr-20Ni-5Al base alloys (at. pct) with small additions of Nb, Mo, W, Ti, V, C, and B were cast, thermally-processed, and aged. On exposure from 650 °C to 800 °C in air and in air with 10 pct water vapor, the alloys exhibited continuous protective Al2O3 scale formation at an Al level of only 5 at. pct (2.4 wt pct). Matrices of the Fe-20Cr-15Ni-5Al base alloys consisted of γ (fcc) + α (bcc) dual phase due to the strong α-Fe stabilizing effect of the Al addition and exhibited poor creep resistance. However, adjustment of composition to the Fe-15Cr-20Ni-5Al base resulted in alloys that were single-phase γ-Fe and still capable of alumina scale formation. Alloys that relied solely on Fe2Nb Laves phase precipitates for strengthening exhibited relatively low creep resistance, while alloys that also contained MC carbide precipitates exhibited creep resistance comparable to that of commercially available heat-resistant austenitic stainless steels. Phase equilibria studies indicated that NbC precipitates in combination with Fe2Nb were of limited benefit to creep resistance due to the solution limit of NbC within the γ-Fe matrix of the alloys studied. However, when combined with other MC-type strengtheners, such as V4C3 or TiC, higher levels of creep resistance were obtained.

Magnetocaloric effect in Heusler alloys Ni50Mn34In16 and Ni50Mn34Sn16

Abstract: We present results of detailed ac susceptibility, magnetization and specific heat measurements in Heusler alloys Ni50Mn34In16 and Ni50Mn34Sn16 These alloys undergo a paramagnetic to ferromagnetic transition around 305 K, which is followed by a martensitic transition in the temperature regime around 220 K Inside the martensite phase both the alloys show signatures of field-induced transition from martensite to austenite phase Both field- and temperature-induced martensite–austenite transitions are relatively sharp in Ni50Mn34In16 We estimate the isothermal magnetic entropy change and adiabatic temperature change across the various phase transitions in these alloys and investigate the possible influence of these transitions on the estimated magnetocaloric effect The sharp martensitic transition in Ni50Mn34In16 gives rise to a comparatively large inverse magnetocaloric effect across this transition On the other hand the magnitudes of the conventional magnetocaloric effect associated with the paramagnetic to ferromagnetic transition are quite comparable in these alloys

Variant Selection of Reversed Austenite in Lath Martensite

Abstract: The microstructure of partially reversed lath martensite in 13%Cr–6%Ni steel was examined by electron backscatter diffraction, and the crystallographic character of the reversed austenite is discussed in relation to the mechanism of ‘austenite memory’. Most of the reversed austenite grains had the same orientation as the original austenite matrix before martensitic transformation. However, some austenite grains had a different orientation in a twin relationship to the other major austenite grains, although all the reversed austenite grains retained a Kurdjumov–Sachs relationship to the martensite matrix. On the basis of the crystallographic relationships among the habit plane, the close packed direction of austenite and the martensite lath boundary, we suggest that the austenite variants are theoretically limited to two kinds within one packet and five kinds within one original austenite grain. In addition, we found that internal stress introduced by martensitic transformation plays an important role in determining the austenite variant: internal stress operates so that reversed austenite selects the same variant as that present in the original austenite matrix before martensitic transformation. This phenomenon is understood as the austenite memory.

Direct Observations of Sigma Phase Formation in Duplex Stainless Steels Using In-Situ Synchrotron X-Ray Diffraction

Abstract: The formation and growth of sigma (σ) phase in 2205 duplex stainless steel (DSS) was observed and measured in real time using synchrotron radiation during 10 hour isothermal heat treatments at temperatures between 700 °C and 850 °C. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In-situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. The data were further analyzed using a modified Johnson–Mehl–Avrami–Kolmogrov (JMAK) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMAK exponent, n, at low fractions of sigma was found to be approximately 7.0; however, toward the end of the transformation, n decreased to values of approximately 0.75. The change in the JMAK exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMAK equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high-temperature value to room temperature.

Deformation induced martensite in an AISI 301LN stainless steel: characterization and influence on pitting corrosion resistance

Abstract: In austenitic stainless steels, plastic deformation can induce martensite formation. The induced martensite is related to the austenite (g) instability at temperatures close or below room temperature. The metastability of austenite stainless steels increases with the decreasing of stacking fault energy (SFE). In this work, the deformation induced martensite was analyzed by X ray diffraction, electron back scatter diffraction (EBSD), magnetic methods and atomic force microscope (AFM) in samples of a low SFE austenitic stainless steel, AISI 301LN and compared with a medium SFE stainless steel, AISI 316L. Both techniques, X ray diffraction and EBSD, presented similar quantities for the a’-martensite. Texture results indicate that the crystallographic orientation of the formed a’-martensite is {001} and {103} . The morphology of a’-martensite was analyzed by AFM. Corrosion tests showed that deformation reduces pitting corrosion and generalized corrosion resistance in both steels.

Effect of friction welding parameters on mechanical and metallurgical properties of ferritic stainless steel

Abstract: Friction welding is one of the most economical and highly productive methods in joining similar and dissimilar metals. It is widely used in the automotive and aerospace industrial applications. Ferritic stainless steel (AISI430) is normally difficult to weld by fusion methods, due to the associated problems such as grain growth and retained austenite content. Such problems can be alleviated by the friction joining process. The present study utilized a continuous drive friction welding machine to join cylindrical specimens of ferritic stainless steel of similar composition and shape (equal diameter and length). The processing parameters such as friction pressure, friction time, upsetting pressure and upsetting time were changed in order to understand the role of parameters on the strength related aspects of friction processed joints. The joints were subjected to mechanical testing methods such as the uni-axial tension test, and charpy ‘v’ notch impact tests. The micro hardness variation across the joint zone was determined. Micro structural studies were also carried out. The characteristics such as tensile strength, toughness and microstructural aspects exhibited by friction processed joints were compared to parent materials.

Design of Structure, Composition and Heat Treatment Process for High Strength Steel

Abstract: For steel with combination of high strength (~2000MPa) and toughness, along with low cost, the designed structure should be low-temperature tempered, fine lath martensite with high density of dislocation, coated by film of austenite with considerable thickness and distributed with fine e (η) or (and) complex carbide. Correspondently, the steel should contain less than 0.5 (wt%) of carbon, certain amount of alloying elements for lowering Ms, such as Ni, Mo and (or) Mn, carbide forming element, e.g. Nb, as well as Si or (and) Al, the element depressing the formation of cementite, the brittle phase in high strength steel. The heat treatment process is suggested as: austenitizing at a temperature slightly above Ac3, followed by quenching at Ms-Mf, partitioning either at quenching temperature or at slightly above Ms for a few minutes, cooling down to room temperature and tempering at low temperature about half an hour.

Transformation induced plasticity assisted steels: stress or strain affected martensitic transformation?

Abstract: Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.