Austenitic TRIP Steel Research Articles

Influence of microstructure on hydrogen trapping and diffusion in a pre-deformed TRIP steel

Austenitic steels are known to exhibit a low hydrogen diffusion coefficient and hence a good resistance to hydrogen embrittlement. Therefore, it is an experimental challenge to investigate their hydrogen diffusion properties. In this study, the electrochemical permeation technique is used to determine the hydrogen diffusion coefficients in different pre-deformed states (φ = 0, 0.32, 0.39, 0.49) of the high-alloy austenitic TRIP steel X3CrMnNiMoN17-8-4 in a temperature range of 323 K–353 K. In combination with microstructural analysis, a correlation between phase transformation from γ-austenite to α′-martensite and dislocation density is shown. As a result of the lattice transformation from fcc to bcc, the diffusion rate of hydrogen is significantly increased (Dapp,φ = 0 = 3.6 × 10−12 cm2 s−1, Dapp, φ = 0.32 = 1.6 × 10−11 cm2 s−1at 323 K). With higher degrees of deformation, the dislocation density also increased in the martensite islands, resulting in a degressive growth of the diffusion coefficient (Dapp, φ = 0.39 = 5.3 × 10−11 cm2 s−1, Dapp, φ = 0.49 = 1.1 × 10−10 cm2 s−1at 323 K). Moreover, detailed calculations are performed to describe the way of hydrogen trapping and to give a possible mechanism of diffusion.

Ultrafine-grained CrMnNi steels: Lueders phenomenon and texture inheritance

The mechanical behaviour as well as the microstructure and texture development were investigated for ultrafine-grained austenitic TRIP steels with two different austenite stabilities. The ultrafine-grained microstructure of low-carbon, high-alloy CrMnNi steels was obtained after thermo-mechanical processing (TMCP) consisting of cold rolling process up to 90% of rolling degree and subsequent reversion annealing treatment at different temperatures and holding times. The mechanical properties were evaluated during tensile tests at room temperature complemented by infrared thermography, digital image correlation, ferromagnetic phase fraction measurements as well as nanoindentation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed before and after tensile tests. In addition, the texture evolution was followed from cold rolling via reversion annealing up to tensile deformation. The results showed that an ultrafine-grained austenitic microstructure with a mean grain size between 0.7 and 1 μm was realized due to the applied TMCP. The ufg microstructure led to a significant increase in yield strength to more than 900 MPa for both steel variants. In addition, a Lueders band phenomenon was observed for steel with the lowest austenite stability, which was accompanied by dislocation-lack yield point and stress-assisted martensitic phase transformation at about 400 MPa. Texture inheritance from fcc cold rolling texture via the bcc cold rolling texture into fcc reversion annealing texture and again bcc texture after tensile deformation was observed. This texture inheritance is an indicator for the diffusionless shear transformation in this type of steels during reversion annealing.

Modeling of the cyclic deformation behavior of austenitic TRIP-steels

The mechanical behavior of TRIP-steels (TRansformation Induced Plasticity) under monotonic loading conditions has been extensively studied both experimentally and by continuum mechanical modeling. The cyclic response received far less attention so far, although the mechanically induced martensitic phase transformation highly affects the cyclic deformation behavior. Especially in high alloy austenitic TRIP-steels a pronounced cyclic hardening is observed in cyclic deformation curves, which evolves proportionally to the volume fraction of deformation-induced martensite. In the present contribution a phenomenological material model is proposed, which is able to capture this effect. A rate independent two-surface plasticity approach is employed. The first surface is of J2-type and describes plastic yielding, whereas the second surface is of Drucker-Prager type and represents the mechanically induced phase transformation. The formulation of the plastic behavior incorporates nonlinear kinematic hardening. The model is calibrated based on a consistent set of uniaxial cyclic deformation experiments with constant strain amplitude under tension-compression. Results of the simulations match the cyclic deformation curves for a large range of strain amplitudes including the transformation induced cyclic hardening well. The qualitative and quantitative agreement between numerical and experimental data as well as the capabilities and limitations of the model are discussed in detail.

Effects of Cu addition on formability and surface delamination phenomenon in high-strength high-Mn steels

The formability of austenitic high-Mn steels is a critical issue in automotive applications under non-uniformly-deformed environments caused by dynamic strain aging. Among austenite stabilizing alloying elements in those steels, Cu has been known as an effective element to enhance tensile properties via controlling the stacking fault energy and stability of austenite. The effects of Cu addition on formability, however, have not been sufficiently reported yet. In this study, the Cu addition effects on formability and surface characteristics in the austenitic high-Mn TRIP steels were analyzed in consideration of inhomogeneous microstructures containing the segregation of Mn and Cu. To reveal determining factors, various mechanical parameters such as total elongation, post elongation, strain hardening rate, normal anisotropy, and planar anisotropy were correlated to the hole-expansion and cup-drawing test results. With respect to microstructural parameters, roles of (Mn,Cu)-segregation bands and resultant Cu-rich FCC precipitates on the formability and surface delamination were also discussed.

Martensite formation during tensile deformation of high-alloy TRIP steel after quenching and partitioning route investigated by digital image correlation

High-alloy austenitic TRIP steel was manufactured using quenching and partitioning route (Q&P) for further enhancement in strength which leads to a microstructure consisting of thermally induced martensite, carbides and stabilized austenite. The austenite is stabilized during partitioning due to diffusion of carbon and nitrogen from supersaturated martensite into the austenite. For evaluation of deformation behavior micro tensile specimens were manufactured and deformed in situ inside scanning electron microscope. The evolution of the microstructure was characterized by X-ray diffraction before and after deformation. During in situ deformation two austenite orientations (〈001〉 and 〈101〉 regarding load axis) were investigated and occurring strain localizations were observed by means of digital image correlation. The strain localizations could be correlated with formation of ε- and twinned α’-martensite. Using high resolution EBSD (HR-EBSD) the transformation behavior during tensile deformation is discussed. Due to the inhomogeneous internal stress distribution and the increased defect density in the austenite, preferred martensite formation is to be expected despite the increased chemical stability. This observation is attributed to the so-called constraining effect.

Cu addition effects on TRIP to TWIP transition and tensile property improvement of ultra-high-strength austenitic high-Mn steels

Austenitic high-Mn steels have been nominated as desirable ultra-high-strength cold-rolled steels whose mechanical properties are greatly improved by powerful deformation mechanisms of transformation- and twinning-induced plasticity (TRIP and TWIP). In this study, an austenitic high-Mn TRIP steel was suggested to achieve a good strength-ductility balance, and 1–2 wt.% Cu was added as an element for increasing stacking fault energy (SFE) as well as an austenite stabilizer to exploit a transition from TRIP to TWIP. The non-Cu-added steel showed the highest yield and tensile strengths (502 MPa and 1137 MPa, respectively) and the lowest elongation (34.6%) with a serrated flow. Yield and tensile strengths decreased with increasing Cu content, while the elongation was the highest in the 1%-Cu-added steel. TRIP and TWIP mechanisms showed good agreements with calculated SFEs in consideration of (Mn,Cu)-segregated bands. In the non-Cu-added steel, the TRIP occurred step by step as localized deformation bands passed through the specimen gage section to activate the serrated flow, which were reduced (or improved) by the transition from TRIP to TWIP with increasing Cu content. In the 1%-Cu-added steel, overall tensile properties were improved (yield strength; 461 MPa, tensile strength; 1093 MPa, elongation; 65.1%) as both TRIP and TWIP were well homogenized to produce synergic effects.

Influence of Temperature and Strain Rate during Thermomechanical Treatment of a Metastable Austenitic TRIP Steel Compacted by SPS/FAST

High‐alloy Fe–19Cr–3Mn–4Ni–0.5Si–0.17N–0.17C TRIP/TWIP steel samples are processed by SPS/FAST (Spark Plasma Sintering/Field‐Assisted Sintering Technology) and subsequently thermo‐mechanically treated by Quenching‐Deformation‐Partitioning (QDP). Because a martensite start temperature (Ms) does not exist for this material, it is not possible to form as‐quenched α’‐martensite during the QDP treatment. Therefore, α’‐martensite is formed by strain‐induced transformation. To investigate the influence of the compressive deformation step of the QDP treatment (referred to as pre‐deformation) and the combined α’‐martensite formation on the microstructure and the mechanical properties, the deformation temperature is varied between −60 °C and 20 °C for two different strain rates (0.0004 s−1 and 1 s−1). The results show that a reduction in pre‐deformation temperature and a low strain rate increase the volume fraction of strain‐induced α’‐martensite during pre‐deformation. Furthermore, the compressive yield strength increases. It is obvious that the austenitic‐martensitic QDP‐treated steel could be assigned to the 3rd generation of Advanced High Strength Steels (AHSS). The steel exhibits compressive offset yield strengths of between 1400 MPa and 1700 MPa as a function of the QDP conditions and the α’‐martensite content which is formed during pre‐deformation.

Dissimilar TIG Welding of HSLA to Austenitic High-Mn TRIP Steels

Abstract The microstructure and mechanical properties of dissimilar butt-joints of high-strength low-alloy steel to austenitic high Mn TRIP steels produced by manual TIG welding with AISI 309L filler metal were investigated. The heat input was varied by changing the welding current between 40 and 70 A. A fully austenitic microstructure with nearly equiaxed grains and without any iron/manganese carbide is observed in the high Mn TRIP base metal, whereas polygonal ferrite and pearlite colonies oriented along the rolling direction are found in the high-strength low-alloy steel base metal. Coarse and refined heat-affected zones (HAZ) were observed in the high-strength low-alloy steel, whereas no significant HAZ was detected at the TRIP side. Bainite and/or martensite grew in the coarse HAZ of the high-strength low-alloy steel steel joined with highest current. The weld seam was formed by austenitic dendrites with interdendritic ferrite and martensite. The best welding condition was achieved with an intermediate current of 50 A, where the hardness transition was smooth, fracture occurred in the HSLA base metal and the joint ductility was higher.

Mechanical Stability and Deformation-Induced Transformation of Retained Austenite in TRIP Steels at Low Temperatures

The tensile properties and the stability of retained austenite in TRIP steels with different volume fraction of retained austenite have been studied at low temperature. The steels showed a good valance of strength and ductility at 193 K. Their work-hardening rates were decreased linearly and kept a high value in the high strain regime at 193 K. The retained austenite was mostly transformed into martensite less than 10% strain at 193 K.

Dissimilar Friction Stir Welding of HSLA Steel to Austenitic High-Mn TRIP Steel

The microstructure and mechanical properties of dissimilar butt-joints between a high-strength low alloyed (HSLA) grade and an austenitic high Mn TRIP steel were investigated. The tool rotation and the tool offset toward the TRIP steel were varied between 300–500 rpm and 1–2 mm, respectively. Tool advancing speed amounted to 100 mm/min. Maximum tension stress was observed for the butt-joint welded with 300 rpm and 2 mm offset. The lowest increase in hardness within the stirred zone also occurred for this FSW condition, indicating that this tool rotation is more promising for welding dissimilar joints of commercial HSLA and high Mn TRIP steels. The weld microstructure consisted mainly of a stirred zone, and neither significant HAZ nor TMAZ are observed. However, two main lobular regions are observed, one at the bottom and another one at the top side of the welds. Besides, the HSLA develops a multiphase microstructure consisting of bainite, martensite and retained austenite phases, whereas no e/a martensite is found in the stirred zone of the austenitic high-Mn TRIP steel.

Observation austenite memory and significant enhancement of tensile properties during cyclic reverse martensite transformation in a Fe-Ni-C TRIP steel

In this study, the influence of reverse martensite transformation (reverse transformation) on microstructure development and mechanical properties of Fe-24Ni-0.3C metastable austenitic TRIP steel was investigated. Microstructural characterization by electron backscatter diffraction (EBSD) system proved that large amount of low angle boundaries appeared after 1-cycle of reverse transformation (γ→α→γ). It is also found that the 1-cycle reversely transformed austenite and original austenite exhibited similar shape, size and orientations indicating that austenite memory appeared during reverse transformation. By increasing the number of reverse transformation cycle, fraction of low angle boundaries significantly increased. Uniaxial tensile test exhibited that yield and ultimate tensile strengths significantly improved even by 1-cycle reverse transformation comparing to the starting material. In addition, further continuation of reverse transformation up to 5- or 7-cycle causes gradual increase in yield and ultimate tensile strengths as well. The significant improvement in yield strength should be originated from increasing the dislocation density that are introduced during reverse transformation.

Control of retained austenite morphology through double bainitic transformation

The feasibility controlling the morphology of retained austenite in TRIP steel was investigated utilizing double bainitic transformation which is two-step isothermal heat treatment. The change in morphology of the retained austenite from a blocky to film type improved the tensile properties owing to higher mechanical stability of film-like retained austenite.

Characteristics of nano/ultrafine-grained austenitic TRIP steel fabricated by accumulative roll bonding and subsequent annealing

In this paper, microstructure and mechanical properties of nano/ultrafine-grained (UFG) austenitic Fe-24Ni-0.3C steel fabricated by accumulative roll bonding (ARB) and subsequent annealing were studied. The results indicated that after 6cycles of the ARB process, elongated UFG austenite with mean grain size of about 300nm was obtained. Annealing treatment of 6-cycle ARB processed austenite at temperatures between 773 and 873K resulted in formation of partly and fully recrystallized austenites with mean grain sizes between 750nm to 2.5μm. Discontinuous yielding accommodated with Lüders band deformation was observed in the stress-strain curves of the ARB processed specimens. Microstructural observation indicated that the Lüders band propagation corresponding to the propagation of the region where deformation-induced martensitic transformation occurred. It was found that deformation-induced martensitic transformation provided extra strain hardening ability, resulting in significant enhancement of uniform elongation in UFG metastable austenitic steel having ultrahigh strength and considerable uniform elongation.

Investigation of Phase Transformations in High-Alloy Austenitic TRIP Steel Under High Pressure (up to 18 GPa) by In Situ Synchrotron X-ray Diffraction and Scanning Electron Microscopy

In order to clarify the difference between the deformation-induced e-martensite (e 1) and the pressure-induced e-iron (e 2), high-pressure quasi-hydrostatic experiments were performed on a low-carbon, high-alloy metastable austenitic steel. In situ synchrotron X-ray diffraction measurements as well as post-mortem investigations of the microstructure by electron backscatter diffraction were carried out to study the microstructural transformations. Three processes were observed during compression experiments: first, the formation of deformation-induced hexagonal e 1-martensite, as well as small nuclei of deformation-induced bcc α′-martensite (α 1′) within the fcc γ-matrix due to non-hydrostaticity in the experiments; second, the onset of the phase transformation from the metastable fcc γ-austenite into the hexagonal pressure-induced e 2-iron phase occurred at around 6 GPa; third, during decompression, the hexagonal pressure-induced e 2-iron transformed partially into bcc α′-martensite (α 2′). Completely different characteristics with regard to habitus as well as to orientation relationships were observed between the pressure-induced phases (e 2-iron phase and α 2′-martensite) and the deformation-induced martensites (e 1- and α 1′-martensite).

Deformation Bands in High‐Alloy Austenitic 16Cr6Mn6Ni TRIP Steel: Phase Transformation and Its Consequences on Strain Hardening at Room Temperature

In this study, the formation of deformation bands during plastic deformation in austenite is analysed. The movement of (partial) dislocations on neighboring lattice planes formed pronounced deformation bands, which were identified as ϵ‐martensite if the stacking faults are predominantly aligned on every second {111} lattice plane. In these deformation bands, α′‐martensite nuclei are formed, severely fragmenting the mean free path of dislocations. The strain hardening is found to depend directly on the nucleation rate of α′‐martensite. The α′‐martensite nuclei represented effective barriers for (partial) dislocation movement in the deformation bands. On the basis of three different grain sizes, the strain‐hardening capability of the martensitic γ → ϵ → α′ phase transformation is analysed. Although the triggering stress for nucleation of α′‐martensite is influenced by grain size, the absolute value of the strain hardening due to α′‐martensite formation is the same. This is related to the fact that the continuous fragmentation of the dislocation mean free path is in the same order of magnitude for all grain sizes. Using a basic approach that treated the continuous reduction of the dislocation mean free path as a Hall–Petch‐like effect, comparable with strain hardening in TWIP steels, the contribution of the effect to the increasing strength in austenitic TRIP steels is estimated.

Fracture Mechanics Characterization of Sintered 30 Vol.-% Al<sub>2</sub>O<sub>3</sub>/TRIP Steel Composites Using SENB Miniature Samples

Ceramic particle reinforced metal matrix composites (PRMMCs) combine the strength and brittleness of ceramics with the toughness of a metallic matrix. In order to use these materials in construction and operational design their fracture mechanical behavior must be evaluated. In this study, a 30 vol.-% Al2O3 reinforced austenitic TRIP steel processed by powder metallurgical technique was investigated using precracked miniature SENB-specimens in 3-point-bending. An elastic-plastic analysis by means of the J-integral method in combination with optical crack observation showed the materials ability of stable crack growth, i. e. R-curve behavior. In addition to the mechanical tests microstructural studies were performed, whereby particle debonding and fracture as well as martensitic phase transformation and crack bridging within the matrix were identified as fracture energy dissipating mechanisms.

Spark Plasma Sintering and Strength Behavior under Compressive Loading of Mg-PSZ/Al<sub>2</sub>O<sub>3</sub>-TRIP-Steel Composites

Composite materials, which consist of a metastable austenitic TRIP-steel matrix (CrMnNi TRIPsteel; TRansformation Induced Plasticity) reinforced by alumina particles (25 vol.% ceramic, designated as AT 25/75) and reinforced by alumina and MgO partially stabilized zirconia particles (Mg-PSZ) (35 vol.% ceramic, designated as AT 25/75 + MgPSZ) were synthesized through spark plasma sintering (SPS). In the AT 25/75 + MgPSZ, the steel particles were mainly surrounded by alumina. Hence, mostly steel/alumina and alumina/MgPSZ interfaces existed. The mechanical behavior of the as-sintered samples was characterized by compression tests at room temperature and 40 °C and in a range of strain rates between 103s-1and 103s1. The influence of the ceramic content, strain rate and temperature on TRIP-effect of the steel matrix was investigated. Due to the increasing ceramic volume fraction, AT 25/75 + MgPSZ exhibits the highest compressive yield strength under all loading conditions and no strain rate sensitivity. This composite showed no measurable TRIP-effect, due to the low fracture strain. The deformation-induced α’martensite within the steel particles in pure steel and AT 25/75 primary depends on the testing temperature and the strain rate. This is attributed to an increase of stacking fault energy with rising temperature. High strain rates cause adiabatic heating, counteracting the martensitic transformation.

Influence of Powder Particle Size on the Compaction Behavior and Mechanical Properties of a High-Alloy Austenitic CrMnNi TRIP Steel During Spark Plasma Sintering

In this study, varying powder particle size fractions (<25, 25 to 45, 45 to 63 µm) of a TRIP steel powder were compacted by spark plasma sintering (SPS). Densification initiated at a slightly lower temperature with decreasing particle size due to increasing green density. With decreasing powder particle size fraction, the as-sintered materials exhibited smaller grain sizes. Compression tests revealed a slight decrease of the compressive yield strength with increasing particle size and, accordingly, larger grain size. A few large deformation bands formed in bigger grains, while many thin deformation bands were formed in smaller grains. α′-Martensite nuclei formed successively inside the deformation bands, reducing the mean free path of (partial) dislocation slip. Due to the size of the deformation bands, α′-martensite formation started at lower strains with increasing particle size. When α′-martensite formation was initiated, work hardening was influenced more by α′-martensite formation than by the grain size of the steel matrix. Hence, work hardening increased with increasing particle size.

TRIP steel retained austenite transformation under cyclic V-bending deformation

To investigate the transformation behavior of TRIP steel retained austenite under cyclic load, cyclic V-bending deformation of low carbon Si-Mn TRIP600 was studied by experiment and finite element in this paper. The results showed that, under cyclic V-bending deformation, retained austenite in TRIP steel transformed into martensite gradually with the increasing of bending times, and for the symmetrical characteristic, upper surface and lower surface presented the same transformation tendency. From the first to the fourth V-bending deformation, retained austenite volume fraction decreased nearly linearly and then attained saturation step by step. Compressive stress state was helpful for martensite transformation than tension stress state with V-bending deformation, and strain magnitude was the determining factor for retaining austenite martensitic transformation. With the increasing of bending times effective stress increased and the relationship between maximum effective stress and bending times was nearly linear. Effective stress and effective strain distribution were non-uniform, the maximum effective stress and effective strain were present in the center of the samples. The relationships between retained austenite and V-bending times, and retained austenite with effective strain were set up as Eqs.(1)–(5). The relationship was typical quadric function, decreased linearly for the initial deformation and attained saturation finally.

Biaxial in-phase and out-of-phase cyclic deformation and fatigue behavior of an austenitic TRIP steel

The fatigue behavior of a metastable austenitic cast steel was studied under biaxial in-phase and out-of-phase cyclic deformation by using cruciform specimens. Different phase shifts φ between the principal strains in the range of φ=0° (equibiaxial push–pull) and φ=180° (pure shear) were investigated. Deformation induced martensitic phase transformation was observed after an incubation period and caused pronounced cyclic hardening depending on the plastic strain amplitude. No significant correlation between the phase shift φ and the martensite formation rate or the cyclic hardening rate was found. In tests with φ=180° (shear) a slower martensite formation rate and a much higher number of cycles to failure were observed than in other biaxial and uniaxial tests. In out-of-phase tests (φ=22.5°,45°,90°,135°) similar fatigue lives were observed as in φ=0° (equibiaxial) tests. The uniaxial Basquin–Manson-Coffin relationship is conservative for biaxial loading of the TRIP steel. Investigations of Surface cracks after fatigue failure lead to the assumption of different crack types under φ=0° and φ=180° loading according to [1] which cause the differences in the fatigue lives.