Effect Of Strain-induced Martensitic Transformation Research Articles

The effect of strain-induced martensitic transformation (SIMT) on the heterogeneous deformation behavior among constituent phases in TRIP-assisted duplex stainless steel

Transformation-induced plasticity (TRIP) assisted duplex stainless steels (DSSs) possess a multi-phase microstructure during tensile deformation due to strain-induced martensitic transformation (SIMT). The characteristics of martensitic nucleation in the metastable austenite phase were characterized using the EBSD-based in-situ tensile test and crystal plasticity finite element method (CPFEM). In particular, the nucleation-controlled SIMT kinetic criteria involving nucleation at the intersection or/and boundary of fault-bands was integrated into the crystal plasticity model based on the determination of SIMT initiation in the in-situ tensile test. The accuracy of the model is verified by the evolution law of the volume fraction of martensite and the characteristics of phase transition positions. The initial nucleated martensite pinned austenite to develop the SIMT rapidly. With further deformation, the martensite gradually intersects with each other, forming a package to encapsulate the austenite, hindering the deformation of the austenite to inhibit the SIMT. The martensitic transformation inhibits the occurrence of local strain concentration, and the increase of volume fraction of martensite leads to a decrease in the coefficient of variation about strain distribution in the austenite+martensite system, indicating a more coordinated deformation of the austenite+martensite system. With a further transformation of martensite, the partition strain of the ferrite increases, and the deformation incongruity increases between the ferrite and the austenite+martensite system.

Explaining the drop of work-hardening rate and limitation of transformation-induced plasticity effect in metastable stainless steels during tensile deformation

While the occurrence of the transformation-induced plasticity effect in metastable austenitic stainless steels has been adequately explained, the reason behind the fall of work-hardening rate after the maximum value remains largely unknown. In the present work, via extensive work-hardening analysis and examination of the mechanical properties and strengthening mechanisms by the Hollomon analysis, Hall-Petch law, and rule-of-mixtures, it is revealed that the yielding of the deformation-induced martensite leads to the drop of work-hardening rate and limits the strengthening effect of strain-induced martensitic transformation and tensile ductility.

Effect of strain-induced martensite reverse transformation on microstructure evolution and electrochemical behaviour of 2304 lean duplex stainless steel

ABSTRACT The effect of strain-induced martensitic transformation (SIMT) and its reversion (SIMRT) on the microstructure evolution and localised corrosion of 2304 lean duplex stainless steel (LDSS) was investigated. The cold-rolled LDSS was subjected to reverse annealing at 600°C to 900°C for 1800 s. To evaluate the microstructure, scanning (SEM) and transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used as the main techniques. Subsequent electrochemical characterisation included cyclic polarisation and double loop-electrochemical potentiokinetic reactivation test. SIM formation was noticed after cold rolling, and a partial reverse transformation was seen after annealing at 600°C. At 600°C, SIMRT led to decrease in pitting resistance and increase in the degree of sensitisation (DOS). Increasing the annealing temperature favoured localised and intergranular corrosion resistance. The obtained results indicate that reversed austenitic and recrystallised grains have a beneficial effect on desensitisation and corrosion resistance of 2304 LDSS.

The effect of martensitic transformation on the cavitation erosion resistance of a TIG-deposited Fe-Cr-C-Al-Ti layer

This study shows the effect of strain-induced martensitic transformation (austenite (γ)–to–martensite (α′)) on the cavitation erosion resistance of a Fe-Cr-C-Al-Ti deposited layer with a high share of metastable austenite in the structure. For comparison, the cavitation erosion of a E308L-17 deposited layer and an AISI 316L stainless steel substrate was also evaluated. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction (XRD) was used to examine martensitic phase transformation. An optical microscope (OM), a scanning electron microscope (SEM), and a 3D optical profilometer were utilized to evaluate the eroded specimens' surfaces. The chemical composition of the Fe-Cr-C-Al-Ti deposited layer was analyzed using an energy-dispersive microanalysis system (EDS). The cavitation results revealed that the Fe-Cr-C-Al-Ti specimen exhibited a resistance to cavitation erosion approximately 4 and 10 times higher than the E308L-17 and AISI 316L specimens, respectively. As shown, effects of γ → α′ transformation were mainly responsible for the results.

An evaluation of fracture properties of type-304 austenitic stainless steel at high deformation rate using the small punch test

Among stainless steel, type-304 austenitic stainless steel is the most widely used because of high corrosion-resistance as well as favourable mechanical properties due to strain-induced martensitic transformation (SIMT). In order to utilize the application of type-304 austenitic stainless steel in dynamic loading condition, the fracture behaviours with the effect of the local distribution of SIMT in the bending deformation using a relatively small specimen need to be investigated. The effect of SIMT as well as thermal softening phenomenon at a high deformation rate has been examined insufficiently and mechanism for rate-sensitivity of fracture-mechanical characteristic is still unclear. In present study, experiment and FE simulations are performed for the SP test at different deflection rates for type-304 austenitic stainless steel. In order to describe the damage and fracture process of materials, a modified version of the Johnson–Cook damage model is applied. After confirming the validity of computation, the rate-sensitivity of fracture-mechanical characteristics of the steel is examined at room temperature. In addition, the effects of temperature and SIMT on the force-deflection curve and fracture properties are discussed to clarify the mechanism for an improvement of impact fracture-mechanical characteristics of the steel.

Ductile Fracture Behaviour of Hot Isostatically Pressed Inconel 690 Superalloy

Herein we assess the differences in Charpy impact behavior between Hot Isostatically Pressed and forged Inconel 690 alloy over the temperature range of 300 °C to − 196 °C. The impact toughness of forged 690 exhibited a relatively small temperature dependence, with a maximum difference of ca. 40 J measured between 300 °C and − 196 °C, whereas the HIP’d alloy exhibited a difference of approximately double that of the forged alloy over the same temperature range. We have conducted Charpy impact testing, tensile testing, and metallographic analyses on the as-received materials as well as fractography of the failed Charpy specimens in order to understand the mechanisms that cause the observed differences in material fracture properties. The work supports a recent series of studies which assess differences in fundamental fracture behavior between Hot Isostatically Pressed and forged austenitic stainless steel materials of equivalent grades, and the results obtained in this study are compared to those of the previous stainless steel investigations to paint a more general picture of the comparisons between HIP vs forged material fracture behavior. Inconel 690 was selected in this study since previous studies were unable to completely omit the effects of strain-induced martensitic transformation at the tip of the Chary V-notch from the fracture mechanism; Inconel 690 is unable to undergo strain-induced martensitic transformation due to the alloy’s high nickel content, thereby providing a sister study with the omission of any martensitic transformation effects on ductile fracture behavior.

A Computational Investigation on Small Punch Test for Evaluating Fracture Toughness of TRIP Steel at Higher Deformation Rate

It is considered that fracture toughness of TRIP steel can be evaluated by means of the small punch (SP) test. However, a mechanism of improving fracture toughness in the SP test for TRIP steel under the impact condition might not be clarified through merely experimental works because the effect of strain-induced martensitic transformation (SIMT) coupled with temperature in TRIP steel becomes quite complicated at high deformation rate. Therefore, a computational simulation on the SP test for an evaluation of the mechanism is indispensable. Since the specimen in the SP test will be fractured at a certain level of deformation, it is necessary to express the fracture by incorporating damage model into constitutive equation of the material in simulation of the SP test. In the present study, a finite element analysis is performed for the SP test with a specimen made of type-304 austenitic stainless steel by an inclusion of damage parameter and its evolution equation. After showing the validity of the computation, the effects of deformation rate, temperature, and SIMT on the force-deflection curve and deformation behavior are examined. Then, the mechanism of rate-sensitive fracture toughness in TRIP steel is challenged to be clarified.

Effects of Austempering Temperature on Strength, Ductility and Toughness of Low-C High-Al/Si Carbide-Free Bainitic Steel

The microstructure and the mechanical properties of a low-carbon Al/Si-alloyed carbide-free bainitic steel austempered at temperatures between 300 and 350 °C have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction analysis, and mechanical property tests. The results show that an excellent combination of tensile strength, ductility, and impact toughness is obtained at the austempering temperature of 320 °C—but neither at the lowest temperature of 300 °C nor at the highest temperature of 350 °C. These results are correlated with the features of the constituent phases of the microstructure, especially the amount and size of retained austenite, which are largely dependent on the austempering temperature. The observations are also due to the inconsistent effects of strain-induced martensitic transformation on strength, ductility, and toughness. The results of the present study suggest that an optimized isothermal temperature may also exist for any other bainitic steel, at which, if austempering treatment is carried out, an improved combination of tensile and impact properties can be obtained.

Inconsistent effects of mechanical stability of retained austenite on ductility and toughness of transformation-induced plasticity steels

The tensile and impact properties of two low-carbon bainitic transformation-induced plasticity (TRIP) steels, one being Al-free and the other being Al-containing, have been investigated. The two steels are featured by the existence of different sizes and shapes of retained austenite showing different mechanical stabilities. Compared with the Al-containing steel, the Al-free steel shows significantly improved tensile strength, uniform strain and total elongation, but exhibits much decreased impact toughness. These results demonstrate conflicting effects of strain-induced martensitic transformation of retained austenite on tensile ductility and impact toughness of TRIP steels. It is indicated that a wide distribution of size and shape of retained austenite is favorable for sustained high strain hardening rate and increased uniform tensile strain; however, thin films of retained austenite, which is mechanically more stable, tends to enhance impact toughness.

繰返し予ひずみを受けたSUS304の高サイクル疲労挙動に及ぼす加工誘起マルテンサイト変態の影響

繰返し予ひずみを受けたSUS304の高サイクル疲労挙動に及ぼす加工誘起マルテンサイト変態の影響

Effects of Strain-Induced Martensitic Transformation on the Solid Particle Erosion Behavior of Fe–Cr–C–Ni/Mn Austenitic Alloys

The effects of Ni and Mn concentrations and also the impact velocity on the solid particle erosion behavior of Fe–12Cr–0.4C–xNi/Mn (x = 5 and 10) alloys were investigated with respect to strain-induced martensitic transformation. The critical strain energy (CSE), which is defined as the energy required to initiate the martensitic transformation increased with increasing Ni and Mn concentrations. As the impact velocity decreased, the solid particle erosion resistance of the low CSE alloy improved compared to that of the high CSE alloy under the given ranges of impingement angles and impact velocities. This result was most likely due to an increase in the volume fraction of martensite that formed during the solid particle erosion test in the low CSE alloy when the impact velocity was decreased.

Effects of Strain-Induced Martensitic Transformation on Fatigue Behavior of Type 304 Stainless Steel and Phase Transformation Analysis by EBSD

The effects of the strain-induced martensitic transformation on the fatigue behavior of type 304 stainless steel were studied. Rotating bending fatigue tests have been performed in laboratory air and in 3%NaCl solution using specimens subjected to tensile-prestrains at ambient temperature and -25°C. Martensitic transformation occurred more remarkably at -25°C than at ambient temperature. In laboratory air, the fatigue strengths of the prestrained specimens increased with increasing prestrain and the specimens subjected to the same prestrains at each temperature exhibited similar fatigue strength in spite of the different amount of martensitic phase. This indicated that the increase in fatigue strength of the prestrained specimens was primarily attributed to work hardening. In 3%NaCl solution, the fatigue strengths of the prestrained specimens at -25°C decreased significantly compared with those in laboratory air. EBSD analysis revealed that the strain-induced martensitic transformation took place within slip bands and corrosion pits were observed at the crack initiation site in the large prestrained specimens. Based on these results, it was confirmed that the strain-induced martensitic transformation exerted large influence on the fatigue behavior in 3%NaCl solution.

Effect of quantity of martensitic transformation on fatigue behavior in type 304 stainless steel

The effects of strain-induced martensitic transformation on the fatigue behavior of type 304 stainless steel were studied. Rotating bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using specimens subjected to tensile-prestrains at ambient temperature and -25°C. Martensitic transformation developed more extensively at -25°C than at ambient temperature. In laboratory air, the fatigue strengths of the prestrained specimens increased with increasing prestrain and the specimens subjected to the same prestrains at each temperature exhibited similar fatigue strength in spite of the different amount of martensitic phase. This suggests that the increase in fatigue strength of the prestrained specimens is primarily attributed to work hardening. In 3%NaCl solution, the fatigue strengths of the prestrained specimens at -25°C decreased significantly compared to those in laboratory air. EBSD analysis revealed that the strain-induced martensitic transformation took place within slip bands but the crack initiated in austenitic phase in laboratory air. On the other hand, in 3%NaCl solution, corrosion pits generated at slip bands, from which crack initiation took place. Based on these results, it was confirmed that the strain-induced martensitic transformation exerted a large influence on the fatigue behavior in 3%NaCl solution.

Effect of strain-induced martensitic transformation on fatigue behavior of type 304 stainless steel

The present paper describes the effect of prestrain on fatigue behavior in type 304 stainless steel. Rotating bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using specimens subjected to the tensile-prestrains of 15%, 30% and 60%. A particular attention was paid to the strain-induced martensitic transformation during stress cycling. The fatigue strength of the prestrained specimens increased with increasing prestrain in laboratory air, but decreased significantly in 3%NaCl solution compared with in laboratory air. The strain-induced martensitic transformation occurred in the prestrained specimens, and martensite phase increased with increasing prestrain and with stress cycling at the fatigue limit stress in the 30% and 60% prestrained specimens. The coaxing effect took place remarkably in the unprestrained specimen, but decreased with increasing prestrain. The increase in fatigue strength of the prestrained specimens in laboratory air and the coaxing effect were attributed to both work hardening and the strain-induced martensitic transformation, where the contributions of the former and the latter decreased and increased with increasing prestrain, respectively. Since corrosion pits were observed at the crack initiation site in the large prestrained specimens, the decrease in fatigue strength in 3%NaCl solution was due to larger environmental susceptibility of martensite phase.

Effect of Strain-Induced Martensitic Transformation on Coaxing Effect of Austenitic Stainless Steels

The effects of martensitic transformation on the coaxing behavior were studied in austenitic stainless steels. The materials used were austenitic stainless steels, type 304 and 316. Conventional fatigue tests and stress-incremental fatigue tests were performed using specimens subjected to several tensile prestrains from 5% to 60%. Under conventional tests, the fatigue strengths of both steels increased with increasing prestrain. Under stress-incremental tests, 304 steel showed a marked coaxing effect, where the failure stress significantly increased irrespective of prestrain level. On the other hand, the coaxing effect in 316 steel decreased with increasing prestrain up to 15%, where the failure stresses were nearly the same. Above this prestrain level, the coaxing effect increased with increasing prestrain. In 304 steel, the coaxing effect is primarily dominated by work hardening at low prestrains, while the effect of strain-induced martensitic transformation increases with increasing prestrain. The coaxing effect in 316 steel is dominated by both work hardening and strain aging at low prestrains, but strain-induced martensitic transformation could play a significant role at high prestrains.

The effect of strain-induced martensitic transformation on mechanical properties of TRIP steel

In this paper, the effect of strain-induced martensitic transformation on mechanical properties is studied based on a developed TI model coupled with mixed hardening law of four phases and anisotropic yield function. Compared with H220BD and DDQ, TRIP steel shows excellent mechanical properties. Stress–strain curve and martensitic volume fraction during deformation process present the developed model is suitable to describe the deformation behaviors of TRIP steel. Single phase mechanical properties indicate high necking strain of martensite results in the improvement of the formability of TRIP steel. Hardening rate and strain hardening exponent with TRIP effect illuminate the mechanical properties are improved by martensitic transformation. The gradient of mechanical parameters describes the influence of martensitic transformation on mechanical properties of TRIP steel further.

K-0336 動的陽解法を用いたTRIP鋼の衝撃変形挙動の有限要素解析(G01-4 変形解析)(G01 計算力学部門一般講演)

Recently, the low alloy and high strength steel which improved TRIP steel is developed. Its application for the impact absorbing parts of an automobile is considered. However, the effect of strain-induced martensitic transformation on the impact energy absorbing characteristics of TRIP steel is still unclear. Here, the impact energy absorbing characteristics of TRIP steel and the influence of strain-induced martensitic transformation on the impact deformation behavior are evaluated by dynamic-explicit elasto-viscoplastic FEM.

低温材料の研究 ＩＩ ＳＵＳ３０４，３１６ステンレス鋼溶接部の極低温における機械的性質と加工による強磁性化

SUS 304 and 316 austenitic stainless steel plates were welded by filler wires of Y308 and Y316 using TIG method, and the strength and the toughness of the weldments were evaluated at cryogenic temperatures (77K and 4.2K). The change in magnetic properties due to the strain-induced martensite (α′) were also examined.The yield strength of 304 (parent metals and weld metals) are greater than those of 316 at room temperature, but at 77K and 4.2K, the yield strength of 316 overcomes those of 304. The elongation of both weld metals are smaller than those of their parent metals at room temperature, but become equal to those of their parent metals at 77K and 4.2K.The tensile tests of the notched specimens of weld metals show that the notch yield strength ratios of 304/Y308 and 316/Y316 weld metals keep 2.4 and 1.9 at 4.2K respectively, and both fracture modes correspond to the ductile manner, although the notch sensitivities decrease with decreasing temperature.The relations of the volume pct of γ, e′ and α′ to the amount of strain at 77K and 4.2K were obtained. The amount of martensite (α′) formed in the fractured specimens at 4.2K were 95% for 304/Y308 weld metal and 70% for 316/Y316 weld metal. An austenitic phase of 316/Y316 weld metal is more stable than that of 304/Y308 weld metal.It was found that the weld metals showed similar mechanical and magnetic properties to their parent metals at 77K and 4.2K because of the dominant effect of strain-induced martensitic transformation on those properties.