Strain-induced Martensite Formation Research Articles

Effects of Heat Treatment and Build Orientation on the Evolution of ε and α' Martensite and Strength During Compressive Loading of Additively Manufactured 304L Stainless Steel

The effect of heat-treatment and build orientation on martensitic phase transformation in additively manufactured (AM) 304L stainless steel is studied and compared with conventionally produced wrought material. The relationships between observed martensitic transformations and material microstructures and their effects on mechanical strength are established through experimental observations. In situ high-energy X-ray powder diffraction measurements were performed to monitor the evolution of e and α' martensite during compressive loading of stainless steel. Electron backscatter diffraction (EBSD) was used to provide insight on initial grain morphology, crystallographic misorientation within grains, and crystallographic texture. Heat treatment alters the microstructure of AM samples creating different initial conditions. This difference in starting microstructure resulted in variability in martensitic transformation during compressive deformation. The rate of martensitic transformation decreased for samples treated with temperatures up to 1100oC, after which the AM microstructures recrystallized, resulting in increased rate of martensitic transformation for those samples treated at higher temperatures. It was also observed that aligning the axis of compression with the AM build direction resulted in a lower rate of strain-induced martensite formation as opposed to aligning the compression axis perpendicular to it. More favorable distribution of crystal orientations in the latter loading orientation promoted martensitic transformation. These and additional experimental observations from EBSD in terms of kernel average misorientation, mean grain orientation spread, and mean crystallite size reveal strong microstructural effects on strength of additively manufactured metallic materials.

Reversible dislocation movement, martensitic transformation and nano-twinning during elastic cyclic loading of a metastable high entropy alloy

The present study contends with the room temperature microstructural response of a non-equiatomic metastable high entropy alloy to the elastic cyclic deformation. The stress and strain-induced martensite formation and reversion are recognized as the main microstructural evolutions which are directly correlated with the reversibility of dislocation movement. Two different patterns of reversion for deformation driven epsilon martensite are identified. Full reversion of stress-induced epsilon martensite results in development of nano-twined matrix, the various aspects of which have been described through a dislocation-based model. The strain-induced martensite also goes through partial reversion leading to lath fragmentation which in-turn significantly influences the martensite stability. Interestingly, the presence of a well-developed dislocation substructure is characterized within the martensite bands, which seems to be phenomenal owing to the low imposed strain, low temperature and low stacking fault energy of the experimented material. The development of vein and wall/channel structures is justified through proposing a conceptual based model regarding the interaction of the stacking faults and subsequent generation of the perfect dislocations.

Influence of shot peening on residual stresses and tribological behavior of cast and austempered ductile iron

Austempered ductile irons exhibit an interesting combination of properties such as low cost, elevated strength, fatigue and wear resistance. In the present contribution, the influence of shot-peening on the dry-sliding wear behavior of as-cast and austempered ductile irons was evaluated. Specimens from a casting with an initial ferritic-pearlitic matrix were submitted to austempering at 320 °C for 60 min after which an ausferritic microstructure with 40 HRC and approximately 30% retained austenite was obtained. The wear and friction behaviors were assessed by applying ball-on-disc tests using a hard metal (WC) counter-body on as-cast and austempered materials before and after shot-peening and after removal of the rough surface layer created by the shot-peening process. The microstructure of the tested materials was evaluated by X-ray diffraction, optical and scanning electron microscopy. After shot-peening, the austempered samples experienced an elevated increase in surface hardness because of the formation of strain-induced martensite, but a reduction in wear resistance caused by the increase in surface roughness. By removing 20 μm from the surface of the shot-peened samples, it was possible to reduce roughness while preserving the hardened layer, which was found to improve wear resistance.

Kinetics of static recrystallization and strain induced martensite formation in low carbon austenitic steels using impulse excitation technique

The defect recovery and recrystallization as well as the strain induced martensite formation in heavily cold rolled 316L austenitic steel has been investigated using Impulse excitation technique. It has been observed that the amount of strain induced martensite is small even after 80% cold rolling and hence the martensite reversion do not alter the elastic modulus to any significant extent during annealing. On the other hand the stored energy of deformation as well as the subsequent annealing phenomenon like recovery and recrystallization has significant effect on the elastic properties of these alloys. The measured temperature variation of elastic moduli data during recovery and recrystallization has been analyzed using the empirical Kolmogorov-Johnson-Mehl-Avrami (KJMA) modelling framework to obtain the kinetic parameters in the steel.

Development of a mathematical equation describing the strain hardening behaviour of metastable AISI 301 austenitic stainless steel

The strain hardening behaviour of AISI 301 metastable austenite steel was analysed by evaluating tensile data against the empirical mathematical equations of Hollomon, Ludwik and Ludwigson. It was found that these equations were inadequate to model this TRIP steel with low stacking fault energy (SFE). It was found that the fraction of strain-induced martensite could be expressed as a sigmoidal function of the applied strain. The log-log plots of true stress and true plastic strain from 5% to εUTS performed with uniaxial isothermal tests at 30 °C were thereafter adequately fitted with a sigmoidal curve. The instantaneous strain hardening exponent was determined as the slope of the above-mentioned sigmoidal curve at a specific strain value. The strain hardening exponent and the rate of strain hardening dσ/dε) increases with deformation due to formation of strain-induced martensite to a maximum and thereafter decreases as the volume fraction of strain-induced martensite approximates saturation. The variation of the instantaneous strain hardening exponent as a function of plastic strain and the strength coefficient, K, at 30 °C was deduced. A high value of K, 1526MPa, was determined. A correlation between the extent of martensitic transformation and the value of the instantaneous strain hardening exponent was observed. This work is part of the project that seeks to develop a constitutive model describing the flow stress during plastic deformation as a function of both plastic strain and the resulting martensitic transformation at different temperatures and strain rates and which accounts for the isotropic hardening process.

Role of surface oxide layers in the hydrogen embrittlement of austenitic stainless steels: A TOF-SIMS study

Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values.For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances.A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance.

Relationship between hydrogen embrittlement and Md30 temperature: Prediction of low-nickel austenitic stainless steel's resistance

Hydrogen embrittlement (HE) of several low-nickel austenitic stainless steels (AISI 300 series) was studied with special attention to the impact of strain induced α′-martensite. The susceptibility of the steels to HE is judged with respect to the relative reduction of area (RRA): The HE susceptibility is lower for larger RRA-values. Strain-induced martensite formation was evaluated within in the framework of the Olson-Cohen model, revealing a linear relationship between RRA and the probability β of martensite nucleus formation in the steels. In order to widen the scope of data evaluation to literature data, the consideration of a parameter alternative to β is required. It is demonstrated that among other parameters the Md30 temperature (Nohara), which assesses the stability against martensitic transformation, can serve as an indicator to predict HE of AISI 300 series steels. Regarding the Md30 temperature (Nohara), a trend-line with respect to the RRA-values is found. Thereby, the RRA-values of low-nickel austenitic stainless steels group into three distinct regimes; (1) for Md30 > −80 °C, where RRA-values decrease with increasing Md30 temperature, (2) at Md30 ≈ −80 °C, where RRA-values show a large variation (‘threshold band’), and (3) for Md30 < −80 °C, showing constant RRA-values of nearly 100%. Some RRA data points that deviate from the trend line can be explained by the special microstructure of the investigated samples.

Research on Forming Temperature of Metastable Austenitic Stainless Steel Head Based on Strain-Induced Martensitic Transformation

The formation of strain-induced martensite (SIM) is found in metastable austenitic stainless steel (m-ASS) during cold forming, and the presence of SIM may cause reductions in toughness, ductility, and corrosion resistance of m-ASS. These mechanical properties can be restored and improved by proper heat treatment after forming, however, which obviously raises the manufacturing costs. One low-cost way to reduce the SIM amount during m-ASS forming is to maintain the forming temperature at an appropriate level. This paper intends to investigate an approach to determine the optimum forming temperature at which the strain-induced martensitic transformation (SIM-Tr) of m-ASS head during forming can be restrained within a limited intensity. First, static tensile tests were conducted on S30408 conventional cylindrical tensile specimens under different temperatures varying from 20 °C to 180 °C, and then the effect of deformation temperature on SIM was evaluated. Second, according to the stacking fault energy (SFE) calculation method, m-ASS's chemical composition was taken into further consideration to investigate its effect on SIM. Finally, a formula was established based on SIM and chemical composition for optimization of forming temperature. In addition, the results obtained by this formula were compared with those of the experiment by S30408 ASS head stamping tests, and the satisfactory matching is found for the proposed forming temperatures and predicted ferrite number (FN) values (readings of the Ferritescope measurement, as a representation of the amount of martensite in this study). Furthermore, an enhancement in the cryogenic impact properties and a fewer quantity of delta-ferrite in the microstructure of m-ASS heads are observed when warm stamping is performed as compared with the cold stamped head.

Stress-Induced Phase Transformation and Its Correlation with Corrosion Properties of Dual-Phase High Carbon Steel

It is well known that stress-induced phase transformation in dual-phase steel leads to the degradation of bulk corrosion resistance properties. Predicting this behaviour in high carbon steel is imperative for designing this grade of steel for more advanced applications. Dual-phase high carbon steel consists of a martensitic structure with metastable retained austenite which can be transformed to martensite when the required energy is attained, and its usage has increased in the past decade. In this study, insight into the influence of deformed microstructures on corrosion behaviour of dual-phase high carbon steel was investigated. The generation of strain-induced martensite formation (SIMF) by residual stress through plastic deformation, misorientation and substructure formation was comprehensively conducted by EBSD and SEM. Tafel and EIS methods were used to determine corrosion intensity and the effect of corrosion behaviour on hardness properties. As a result of the static compression load, the retained austenite transformed into martensite, which lowered its corrosion rate by 5.79% and increased the dislocation density and the length of high-angle grain boundaries. This study demonstrates that balancing the fraction of the martensite phase in structure and dislocation density, including the length of high-angle grain boundaries, will result in an increase in the corrosion rate in parallel with the applied compression load.

Comparative study on the forming and reversion of strain-induced martensite in two duplex stainless steels: Developing a model for VSM analysis of powders or fine chips

Abstract An original feature of this work is the proposal of two equations to fit the volume fraction of ferromagnetic (α + α′) phases that can be applied in the measurement of magnetic saturation of comminuted duplex and superduplex stainless steels (for example, powders or filed chips). Duplex stainless steels contain similar volume fractions of austenite (γ) and ferrite (α) in their microstructure. Two steels exemplify this class, namely the most widely used duplex UNS S31803 and superduplex UNS S32520 stainless steels. The phenomena of work hardening, formation, and reversion of strain-induced martensite (α′) in austenite were compared in both stainless steels. Samples were work-hardened and annealed under identical conditions, and their behavior was evaluated mainly through X-ray diffraction and magnetic measurements. Notably, the volume fraction of strain-induced α′ in duplex stainless steel was as high as 32%, which indicated that this steel had a greater tendency to form α′ than superduplex stainless steel, for which the corresponding value equaled 15%. Annealing at 650 °C for 2 h promoted the reversion of strain-induced α′ into γ, decreasing the volume fraction of the former phase from 32 to 2% (duplex) and from 15 to 6% (superduplex).

Cavitation erosion resistance of a non-standard cast cobalt alloy: Influence of solubilizing and cold working treatments

The wear behavior of a non-standard cast cobalt chromium alloy (Co30Cr19Fe), during vibratory cavitation test, was investigated. Low stacking fault energies (SFE) cobalt alloys deform by complex plastic deformation mechanisms, which enhance the cavitation erosion (CE) resistance, since the onset of localized stress, leading to fatigue failure and material removal, is delayed. The purpose of this work is to characterize the main operating deformation mechanisms during cavitation erosion testing of the non-standard Co30Cr19Fe alloy. As cast, solution treated, and 15% and 30% cold rolled specimens were tested. The as cast microstructure consisted of ~2 mm fcc alpha grains and hcp epsilon-martensite. The 1473 K solubilization treatment led to primary recrystallization and formation of 250 µm new grains. The 15% cold worked microstructure consisted of heavily deformed fcc phase containing deformation twins and epsilon-martensite. Ultrasonic cavitation testing was carried out, during 40 h, according to ASTM G32-09. The solubilized specimens presented the worst behavior, whereas, the 30% cold worked specimens were the most CE resistant. The CE results are discussed based on the microstructural parameters: amount of alpha fcc and epsilon-hcp phases, grain sizes, relative amounts of twinning, slip lines and strain induced martensite formation.

Approach and mechanism of toughness enhancement for a high carbon Q-P-T steel

ABSTRACTBased on the previous study of Cr-containing Fe-0.63C-1.52Mn-1.49Si-0.62Cr-0.036Nb high carbon steel, although normalization process as a pretreatment of quenching–partitioning–tempering (Q-P-T) was performed in this high carbon steel to raise the ductility, its impact toughness only is 7.4 J/cm2. Microstructural characterization reveals that strain-induced twin-type martensite transformed from considerable chunky retained austenite destroys toughness. Therefore, a novel Cr-free Fe-0.67C-1.52Mn-1.49Si-0.038Nb (wt.%) high carbon steel was designed and normalization process as pretreatment of Q-P-T process was still employed so that the volume fraction of retained austenite evidently decreases from 21% to 10%. This high carbon steel not only keeps high tensile strength (1660 MPa) and elongation (29.0%), but also exhibits high impact toughness of 33.2 J/cm2. The high impact toughness is attributed to the elimination of chunky retained austenite accompanying dispersive and fine retained austenite with high mechanical stability, which effectively reduces the formation of brittle strain-induced twin-type martensite during deformation.

Influence of Strain Induced Martensite Formation of Austenitic Stainless Steel on Wear Properties

In this study, the wear characteristics and behaviors of both AISI304 and AISI316 austenitic stainless steels were evaluated under the different rotation speeds, using block-on-ring wear testing machine. From the microstructure analysis, at the first, the transition of strain induced-martensite was observed in wear-affected region of both AISI304 and AISI316. It is considered that the interfacial friction stress between test block and ring may trigger the phase transformation of meta-stable γ-fcc to e/α′ martensite. Moreover, it is also observed that the amount of martensite increases with increasing rotation speed for both AISI304 and AISI316. The thickness of transformed martensite was deeper in AISI304, compared to AISI316 at all rotation speed. It can be explained by that the existence of molybdenum in AISI316 increases stacking fault energy, which can retard nucleation and growth of strain induced-martensite. Furthermore, reverse transformation was also detected at subsurface layer of both AISI304 and AISI316.

Formation of strain-induced martensite in selective laser melting austenitic stainless steel

The austenitic stability of selective laser melted austenitic stainless steel was investigated by the tensile tests in the 80–300 K temperature range. The selective laser melting process greatly improved the yield strength, but severely suppressed the stain-induced α′ martensitic transformation compared with the conventional austenitic stainless steel. The high density of low-angle grain boundaries and fine cellular microstructures were observed in the selective laser melted austenitic stainless steel, which suppressed dislocation slip and deformation twinning during deformation, resulting in the reduction in the nucleation sites of α′ martensite and the enhancement of the austenitic stability.

Magnetic properties of a 17.6 Mn-TRIP steel: Study of strain-induced martensite formation, austenite reversion, and athermal α′-formation

Abstract Strain-induced martensite (SIM) formation was evaluated upon cold-rolling of a 17.6 wt.%Mn-TRIP steel by means of magnetic measurements, X-ray diffraction, and high-resolution electron backscatter diffraction (EBSD). α′-martensite formation was observed to be dependent on the presence of prior e-martensite. Upon deformation, the coercivity of the ferromagnetic α′-martensite is characterized by strong magnetic shape anisotropy. Austenite (γ) reversion was evaluated by means of in situ magnetic measurements during continuous annealing. The experimental results were compared to thermodynamic simulations. It turned out that γ-reversion was not completed in the regime where a γ-single phase field is expected, which suggests the splitting of α′ → γ transformation into two stages. The Curie temperature of remaining α′-martensite was determined as being ∼620 °C. Magnetic properties presented an annealing time-dependence within the temperature range of 500–600 °C, suggesting long-range diffusional α′ → γ transformation. With the aid of electron channeling contrast image technique (ECCI), we noticed that the formation of γ-nanograins in the early stages of reversion is sufficient to induce strong magnetic shape anisotropy in this steel. After full austenitization at 800 °C, further in situ magnetic measurements were also used to track the magnetic response of the material upon controlled cooling. Athermal formation of α′-martensite within the prior athermal e-phase was clearly observed for temperatures lower than 100 °C. Using thermodynamic modeling we also calculated the start temperature for e-formation (Mse). Results showed that e-martensite is indeed expected to form before α′, which was confirmed in all cases by means of EBSD.

Magnetic measurement of strain-induced martensite formation in Fe–30Ni steel

ABSTRACTThis investigation focuses on deformation-induced plasticity in Invar-type steel alloys. The effect is being studied in an austenitic model alloy, containing 30 wt-% of nickel. Its temperature dependent mechanical properties are being presented. Furthermore, the martensitic phase content has been determined by magnetic means in an alloy with two ferromagnetic phases for the first time. The results show that the α′-martensite formation within the austenitic phase with primarily wavy glide mechanism allows an increase in ductility of around 10% at the Msσ temperature of −5°C. This is the point of maximum uniform elongation. Near the Ms temperature, a microstructure of 70 vol.-% deformation-induced α′-martensite can be achieved.

The significance of deformation mechanisms on the fracture behavior of phase reversion-induced nanostructured austenitic stainless steel

We describe here the relationship between grain structure, deformation mechanism and fracture characteristics in an austenitic stainless steel. This was accomplished using the novel concept of phase reversion that enabled a wide range of grain size from nanograined/ultrafine grained (NG/UFG) to coarse-grained (CG) regime to be obtained in a single material through change in temperature-time annealing sequence. In the NG/UFG structure, a marked increase in abundance of stacking faults (SFs) and twin density with strain was observed that led to a decrease in the average spacing between adjacent SFs, thus converting stacking faults into twins. Twinning in NG/UFG structure involved partial dislocations and their interaction with the grain boundaries, including SF overlapping and the coordinated nucleation of partial dislocations from the grain boundaries. The plastic zone in the NG/UFG structure resembled a network knitted by the intersecting twins and SFs. With SFE ~30 mJ/m2, the minimum stress for twin nucleation was ~250 MPa for the experiment steel and the corresponding optimal grain size (dop) wa ~120 nm. In contrast, in the CG structure, strain induced martensite formation was the deformation mechanism. The difference in the deformation mechanism led to a clear distinction in the fracture behavior from striated fracture in high strength-high ductility NG/UFG alloy to microvoid coalescence in the low strength-high ductility CG counterpart. The underlying reason for the change in fracture behavior was consistent with change in deformation mechanism from nanoscale twinning in NG/UFG alloy to strain-induced martensite in the CG alloy, which is related to change in the stability of austenite with grain size. An analysis of critical shear stress required to initiate twinning partial dislocations in comparison to that required to nucleate shear bands is presented. The appearance of striated fracture in the NG/UFG alloy suggests a quasi-static step wise crack growth process.

Austenite stability and mechanical properties of nanocrystalline Fe–Mn alloy fabricated by spark plasma sintering with variable Mn content

The effects of Mn content on the austenite stability and mechanical properties of nanocrystalline Fe–(4–10)%Mn alloys were investigated by means of micrograph observation, X-ray analysis, and mechanical testing. The Fe–Mn alloy samples with nanosized crystallite were successfully fabricated by spark plasma sintering because of an extremely short densification time, not only to reach the theoretical density value but also to prevent grain growth. The increased Mn content reduced the crystallite size and increased the volume fraction of austenite. In particular, the volume fractions of austenite in the nanocrystalline Fe–Mn samples were more than approximately four times higher than the equilibrium values. The progress of the compressive deformation increased the volume fraction of strain-induced martensite from the austenite decomposition. The kinetics of the strain-induced martensite formation were evaluated using the empirical equation considering the austenite stability factor. Both larger Mn content and smaller crystallite resulted in the increase in the austenite stability, contributing to not only a higher volume fraction of austenite after sintering, but also an improvement in its mechanical properties during the deformation.

Effects of concurrent strain induced martensite formation on tensile and texture properties of 304L stainless steel of varying grain size distribution

Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS.

Effects of plastic straining on the corrosion resistance of TRIP-aided lean duplex stainless steels

In the present paper, the effects of plastic deformation on the corrosion resistance of strip cast TRIP (transformation-induced plasticity)-aided stainless steel (19Cr-DSS) and conventional lean duplex stainless steel (LDX2101) were studied by observations of deformed microstructure and electrochemical experiments. The electrochemical results indicate that the corrosion susceptibility of experimental steels in 3.5% NaCl solution is increased with increasing deformation amounts. The Mott–Schottky analysis revealed that the passive films formed on both 19Cr-DSS and LDX2101 exhibit the n-type and p-type semiconducting behaviors regardless of strains. With the increase in true strain, the acceptor and donor densities are both continuously increased. The total density of acceptor and donor in 19Cr-DSS is increased more rapidly with increasing strain than that of LDX2101, indicating that the corrosion resistance of 19Cr-DSS is more sensitive to plastic deformation compared to that of LDX2101. For the TRIP-aided 19Cr-DSS, the deterioration of corrosion resistance after plastic deformation may have been caused by the decrease in low-Σ CSL boundaries especially the twin (Σ3) boundaries, the increase in dislocations, and the formation of strain-induced martensite during plastic deformation.