Low cycle fatigue of a high strength metastable austenitic steel

The low cycle fatigue (LCF) behavior of a high strength, metastable austenitic steel called TRIP steel has been studied. High strain LCF experiments on cylindrical, well-polished specimens under diametral strain control were carried out. To study the effect of a mixed austenite-martensite matrix, LCF tests were also done on the TRIP steel after inducing significant amounts of martensite in the austenite matrix by means of a very high unidirectional prestrain. To establish the role played by the martensite transformation, tests were also run above the M_D. The amount of martensite induced was magnetically measured by means of a “permeameter” built specifically for this purpose. It was found that the LCF life of the TRIP steel, both at room temperature (in the presence of martensitic transformation) and at 200°C (in the absence of the transformation), was related to the plastic strain range, ε_PR, by the Manson-Coffin law. Either cyclic hardening or softening occurred at room temperature, depending primarily upon the plastic strain range used in cycling. Hardening was observed below 3 pct plastic strain range. For LCF tests at 200°C, cyclic softening was observed in all cases. The hardening and softening behavior has been found to depend on the martensitic transformation taking place in these steels during cycling. However, the LCF life correlated best to the percent reduction in area, independent of the extent of the martensite transformation.     

Finite-element method simulation of effects of microstructure, stress state, and interface strength on flow localization and constraint development in Nb/Cr2Nb in situ composites

The effects of volume fraction of particles, stress state, and interface strength on the yield strength, flow localization, plastic constraint, and damage development in Nb/Cr₂Nb in situ composites were investigated by the finite-element method (FEM). The microstructure of the in situ composite was represented in terms of a unit rectangular or square cell containing Cr₂Nb particles embedded within a solid-solution-alloy matrix. The hard particles were considered to be elastic and isotropic, while the matrix was elastic-plastic, obeying the Ramberg-Osgood constitutive relation. The FEM model was utilized to compute the composite strength, local hydrostatic stress, and plastic strain distributions as functions of volume fraction of particles, stress state, and interface strength. The results were used to elucidate the influence of volume fracture of particles, stress state, and interface property on the development of plastic constraint and damage in Nb/Cr₂Nb composites.     

On the influence of interactions between phases on the mechanical stability of retained austenite in transformation-induced plasticity multiphase steels

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M _s temperature induced by carbon enrichment.     

The influence of martensite shape,concentration, and phase transformation strain on the deformation behavior of stable dual-phase steels

A continuum model is developed to examine the influence of martensite shape, volume fraction, phase transformation strain, and thermal mismatch on the initial plastic state of the ferrite matrix following phase transformation and on the subsequent stress-strain behavior of the dual-phase steels upon loading. The theory is developed based on a relaxed constraint in the ductile matrix and an energy criterion to define its effective stress. In addition, it also assumes the martensite islands to possess a spheroidal shape and to be randomly oriented and homogenously dispersed in the ferrite matrix. It is found that for a typical water-quenched process from an intercritical temperature of 760 °C, the critical martensite volume fraction needed to induce plastic deformation in the ferrite matrix is very low, typically below 1 pct, regardless of the martensite shape. Thus, when the two-phase system is subjected to an external load, plastic deformation commences immediately, resulting in the widely observed “continuous yielding” behavior in dual-phase steels. The subsequent deformation of the dual-phase system is shown to be rather sensitive to the martensite shape, with the disc-shaped morphology giving rise to a superior overall response (over the spherical type). The stress-strain relations are also dependent upon the magnitude of the prior phase transformation strain. The strength coefficienth and the work-hardening exponentn of the smooth, parabolic-type stress-strain curves of the dual-phase system also increase with increasing martensite content for each selected inclusion shape. Comparison with an exact solution and with one set of experimental data indicates that the theory is generally within a reasonable range of accuracy. Formerly Visiting Professor, Department of Mechanical and Aerospace Engineering, Rutgers University     

Role of nitrogen in the cyclic deformation behavior of duplex stainless steels

The role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It can be attributed to the greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with increasing nitrogen content because austenite dominantly takes part in plastic deformation, being more strain hardened due to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change because ferrite, more dominantly accommodating plastic strain, rarely shows a change of strain-hardening behavior due to the similar nitrogen content in ferrite.     

Influence of strain-history on deformation-induced austenite transformation in 304 stainless steel sheet

In the present investigation experiments were carried out to find strain history effects on deformation-induced austenite transformation in a metastable stainless steel sheet. The aim of this work was to obtain information on a final amount of martensite formed during γ → α’ transformation under various strain paths. All tests were performed at room temperature and at 0°C. Relationships of volume fraction α’ martensite vs true plastic strain X_M = f(ε) are presented and analysed.     

Correlation of Tensile Properties and Strain Hardening Behavior with Martensite Volume Fraction in Dual-Phase Steels

In the present study, tensile properties, strain hardening and fracture behavior of dual-phase (DP) steels were correlated with martensite volume fraction (V _M ). A series of DP steels with different amounts of V _M (28–50 %) were produced by cold rolling and subsequent intercritical annealing of a ferrite-pearlite starting structure. Hardness and tensile tests results of DP steels showed that variation of hardness, uniform elongation and total elongation with V _M was linear and obeyed the rule of mixtures, whereas yield strength and ultimate tensile strength exhibited a nonlinear variation with V _M . Analysis of strain hardening behavior of DP steels by the Hollomon analysis showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (n _I ) increased with increasing V _M , while the strain hardening exponent of second stage (n _II ) as well as transition strain between the deformation stages decreased.     

Transformation behavior of TRIP steels

True-stress (σ), true-strain (ε) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs ε) of materials in both conditions have a sigmoidal shape at temperatures above M_s ^σ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below M_s ^σ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below M_s ^σ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above M_s ^σ. Comparison of σ-ε andf-ε curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above M_s ^σ, but there is also a dynamic “transformation softening” contribution which becomes dominant below M_s ^σ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above M_s ^σ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below M_s ^σ.     

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T _M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T _M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T _M temperature.     

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.     

Crystallography and Morphology of Carbides in a Low-Cycle Fatigued 1Cr-1Mo-0.25V Steel

The carbide precipitation in 1Cr-1Mo-0.25V steel subjected to low-cycle fatigue (LCF) deformation at room and elevated temperatures was investigated by means of transmission electron microscopy. Based on the electron diffraction analyses, three types of carbides, M₃C-type cementite, M₂C, and MC, were identified in normalized and subsequently tempered specimen. The cyclic deformation at high temperature led to the following changes in morphology and composition of carbides: the spheroidization of cementite, the enhanced precipitation of H-carbide, the formation of M₂C and M₂₃C₆ at lath or prior-austenite grain boundaries, and the enrichment of Mo in most of carbides. Particular attention has been paid to the crystallographic orientation relationship (OR) between the cementite and the ferrite (α) matrix. The combined analyses based on the simulation of diffraction patterns and the trace analyses of habit plane on stereographic projection have shown that most cementite was related to the α matrix in accordance with Bagaryatskii OR, but in some cases, the Isaichev OR also was observed in the lath interior after LCF deformation at elevated temperature. In addition, M₂C obeyed the Burgers–Jack OR, and MC was related to the α by the Baker–Nutting OR.     

Effect of Grain Size and Testing Temperature on Low-Cycle Fatigue Behavior and Plastic Deformation Mode of Ti-2Al-2.5Zr

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on Ti-2Al-2.5Zr samples with different grain sizes (5 and 40 μm) at room temperature (RT) and low temperature (77 K). The results show that the coarse-grained samples of 40 μm exhibit a higher ductility and LCF life than the fine-grained ones at RT. Meanwhile, the fine-grained samples of 5 μm displayed improved ductility and LCF life at 77 K compared with those at RT. Microstructural observations using optical microscopy (OM), scanning electron microscopy, and transmission electron microscopy (TEM) revealed that a transition occurred in the plastic deformation mode, from twinning with slip to slip alone, as the grain size decreased from 40 to 5 μm at RT. Conversely, in the fine-grained samples fatigued at 77 K, twinning was activated and became one of the dominant plastic deformation modes. The improvement in the LCF life of the coarse-grained samples at RT and the fine-grained ones at 77 K could be attributed to the activation of deformation twinning. In addition, the cyclic stress response curves showed that cyclic stress saturation was exhibited in the fine-grained samples at all strain ranges. An initial cyclic hardening followed by cyclic softening was displayed ahead of the cyclic stress saturation in coarse-grained samples at high strain amplitudes. When the testing temperature decreased to 77 K, cyclic stress hardening prior to cyclic stress saturation also appeared in the fine-grained samples. The relation among the grain size, testing temperature, plastic deformation modes, and LCF life in Ti-2Al-2.5Zr was subsequently discussed.     

Fatigue deformation-induced response in a superduplex stainless steel

The cyclic deformation behavior of SAF 2507 superduplex stainless steel (SDSS) was studied under constant plastic-strain amplitudes. The cyclic hardening/softening curves show initial hardening, followed by softening and, finally, saturation behavior. Two regimes can be differentiated in the cyclic stress-strain curve (CSSC) of SDSS. The transition point at which the cyclic strain-hardening rate changes is identified to be ɛ _p/2=7 × 10⁻³. Transmission electron microscopy (TEM) results on dislocation structures suggested that there is a close relationship between the CSSC, hardening/softening curves, and the dislocation substructure evolution. In the low-plastic-strain-amplitude regime of the CSSC, the dislocation activity in the austenite grains is found to be higher than that in the ferrite grains. At higher plastic strain amplitudes, low-energy dislocation structures are found in the ferrite grains, while clusters and bundles of dislocations can be observed in the austenite grains. Strain localization due to formation of these structures resulted in a decrease in the cyclic strain-hardening rate within the high-plastic-strain-amplitude regime. Dislocation substructure evolution is also used to explain the shape of the hardening/softening curve.     

High-temperature low-cycle fatigue behavior of a NIMONIC PE-16 superalloy—Correlation with deformation and fracture

Low-cycle fatigue (LCF) responses of NIMONIC PE-16 for various prior microstructures and strain amplitudes have been evaluated and the fatigue behavior has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed at 923 K on samples possessing three different prior microstructures: alloy A in solution-annealed condition (free of γ^′ and carbides), alloy B with double aging treatment (spherical γ^′ of 18-nm diameter and M₂₃C₆), and alloy C with another double aging treatment (γ^′ of size 35 nm, MC and M₂₃C₆). All three microstructures exhibited an intial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudevs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behavior in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testign. Fresh precipitation of fine γ^′ was confirmed by the development of “mottled contrast” in alloy C. Evidence for the shearing of the ordered γ^′ precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to the reduction in the size of the γ^′ and consequent softening. Coarser γ^′ precipitates were associated with Orowan loops. The observed fatigue behavior has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples.     

The shape memory effect and superelasticity in two-phase polycrystalline α/β brasses

The shape memory effect (SME), superelasticity (SE), and cyclic deformation behavior of two-phase α/β brasses have been investigated at various temperatures, using tensile tests andin situ optical microscopic observations. The morphology and characteristics of the (thermoelastic) martensitic transformation and the mechanism of the SME are similar to those for single-phase β-brass, but the amount of irrecoverable strain is larger in the two-phase alloys due to plastic deformation of the α particles. After unloading and heating, the slipbands in the discrete a particles remain, whereas the martensite almost disappears; thus, the higher the volume fraction of α particles, the larger the amount of irrecoverable strain. The deformation behavior of alloy A at temperatures above the martensite start (M_s) temperature (with 26 pct α phase) is dominated by deformation of the α phase, so complete SE cannot be obtained after cyclic deformation, both at room temperature and at -40 °C. While in alloy B (containing 15 pct α phase), the deformation behavior is dominated by the formation of stress-induced martensite (SIM). The α particles are deformed before SIM formation on loading at room temperature, but on the contrary, SIM forms before the α particles are deformed on loading at -40 °C (>M_s). Complete SE can be obtained in alloy B after cyclic deformation at room temperature to a given strain but does not occur at -40 °C because the a particles are deformed along with the growth of pre-existing SIM under larger strain during cycling at this temperature.     

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10⁻⁴s⁻¹ to 400 s⁻¹. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

The influence of Mo2C morphology and distribution on the fatigue crack initiation and propagation behavior of Fe-C-Mo dual-phase steels

Dual-phase microstructures consisting of ferrite with carbides (Mo₂C) surrounding equiaxed martensite packets have been developed in two alloys, Fe-O. 2C-4Mo and Fe-O. 2C-2Mo. These alloys were chosen because of the presence of two distinct carbide morphologies: (1) a needle-shaped interphase carbide structure, and (2) a fibrous carbide structure. Isothermal transformations were used to control the carbide morphology and distribution in the ferritic regions of the dual-phase microstructures. In the present research the effects of changes in carbide structure on low cycle fatigue (LCF) and fatigue crack growth (FCG) behavior were studied. Crack initiation was observed at prior austenite grain boundaries in the fibrous microstructure, and along intrusion/extrusion defects in the interphase needle microstructures for LCF tests. TEM studies revealed a carbide free region at prior austenite grain boundaries where initiation occurs for the fibrous case. The cyclic stress/strain response of the ferritic portions of the microstructure is determined by the ability of the carbides to homogenize the strain found there. This affects the stress/strain distribution in the composite ferrite-martensite microstructure by changing the hardness ratio of the two phases and subsequently alters the fatigue crack growth behavior and the macroscopic cyclic stress/strain response. Strain localization was also found to affect the roughness induced closure found for fatigue crack growth tests for low R tests (R = 0.1).     

Microstructure and Properties of Mo Microalloyed Cold Rolled DP1O00 Steels

Two kinds of ultra-high strength cold rolled dual phase steels have been developed by designing C-Si-Mn-Cr and C-Si-Mn-Cr-Mo alloy systems. Tensile strength and elongation of both steels exceed 1 100 MPa and 10%, respectively. The microstructures of both steels consist of massive martensite and ferrite. And the massive martensite of Mo-free steel disperses in the ferrite with volume fraction of 64%. However, the massive martensite of Mo-containing steel is connected or closed by small martensite islands each other, and martensite volume fraction is 69%.As to Mo-free steel, the yield strength, yield ratio, and work hardening exponent n are 548 MPa, 0.49, and 0.26,respectively. As for Mo-containing one, the yield strength, yield ratio, and n value are 746 MPa, 0. 66, and 0.33,respectively. Besides, the ferrite of Mo-free steel is deformed at the initial stage of plastic deformation. However,for Mo-containing one, Mo solution strengthened ferrite and small overaged martensite islands are deformed preferentially at small strain, which causes the yield strength to reach approximately 200 MPa higher than that of Mo-free steel.     

1300 MPa级Nb微合金化DH钢的组织性能

设计了不同相构成的超高强DH钢,抗拉强度均大于1300 MPa,组织由铁素体、马氏体、残留奥氏体和极少量碳化物构成。对比了不同相构成对超高强DH钢力学性能和应变硬化行为等的影响,并深入研究了残留奥氏体在超高强度DH钢中的作用机制。结果表明:随着马氏体和残留奥氏体体积分数的增大,铁素体体积分数的减小,实验钢屈服和抗拉强度同时升高,而延伸率呈先增大后减小趋势。软韧相铁素体体积分数的减小和硬相马氏体体积分数的增大导致屈服强度和抗拉强度增加。相对于回火马氏体,淬火马氏体对强度的提升更显著,在拉伸过程中转变的残留奥氏体的量是引起延伸率变化的主要原因,组织中显著的带状组织会造成颈缩后延伸率的明显降低。通过对应变硬化行为的分析表明,随着真应变的增大,应变硬化率呈减小的趋势,在真应变大于2%后的大范围内,对于应变硬化率,DH1>DH2>DH3,主要与铁素体体积分数有关;在真应变大于5.73%后,DH2钢的应变硬化率高于DH1钢和DH3钢,主要与DH2钢中更显著的TRIP效应有关。除了残留奥氏体体积分数,残留奥氏体中的碳含量对TRIP效应同样有显著的影响。较高比例的硬相马氏体组织结合适当比例的软韧相铁素体和残留奥氏体有助于DH2钢获得最良好的强塑积13.17 GPa·%,其中屈服强度达880 MPa,抗拉强度达1497 MPa,均匀延伸率为6.71%,总伸长率为8.8%,颈缩后延伸率为2.09%,屈强比0.59。