Showing papers on "Austenite published in 2013"

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

Abstract: Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the...

Development of 3rd generation AHSS with medium Mn content alloying compositions

Abstract: In this paper, four different steel compositions, centered on Mn as the main alloying element, are designated as candidates for Third Generation AHSS grades. The design of these steels is based on controlling the deformation behavior of the retained austenite. Thus, heat treatment process parameters are determined in order to obtain different amounts and morphologies of retained austenite. The evolution of the microstructure, during processing as well as deformation, is characterized by using optical, electron microscopy techniques and mechanical tests. The effect of alloy composition and processing parameters on the deformation mechanisms of these steels is discussed.

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

Abstract: 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.

Quenching and Partitioning Steel Heat Treatment

Abstract: Quenching and partitioning (QP property advancements continue to be made through research on this emerging technology. Early investigations [1] also proposed a corresponding thermodynamic model for Q&P steel and its heat treatment, which is now referred to as constrained carbon equilibrium [3]. Since first proposed in 2003, Q&P steel has gained interest for its potential to enhance properties of strength and ductility with compositions similar to transformationinduced plasticity (TRIP) steel and has been proposed as a third-generation automotive steel (Fig. 1) [4]. Many researchers [5–17] have investigated the relationship between properties and microstructures of Q&P steels subjected to various heat treatments and showed that the ultrahigh strength of Q&P steel results from martensite laths, while its good ductility is attributed to TRIP-assisted behavior of retained austenite during deformation. De Moor et al. [14] examined the stability of retained austenite and showed that the TRIP effect occurs in Q&P steels, thereby effectively contributing to the significant strain hardening. Santofimia et al. [15, 16] and Takahama et al. [17] analyzed microstructural evolution during annealing Editor’s Note The following is a preview chapter from the upcoming volume Steel Heat Treating Fundamentals and Processes, Volume 4A, ASM Handbook, Jon Dossett and George Totten, editors. The volume is scheduled for publication later this year.

Effect of Grain Size on Thermal and Mechanical Stability of Austenite in Metastable Austenitic Stainless Steel

Abstract: 1) Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku Fukuoka, 819-0395 Japan. 2) International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395 Japan. 3) Formerly Kyushu University. Now at Nippon Steel & Sumitomo Metal Corporation, 1-8 Fuso-cho, Amagasaki, Hyougo, 660-0891 Japan. 4) Department of Materials Science and Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395 Japan.

Microstructure, tensile and toughness properties after quenching and partitioning treatments of a medium-carbon steel

Abstract: The effect of different microstructures on the tensile and toughness properties of a low alloy medium carbon steel (0.28C–1.4Si–0.67Mn–1.49Cr–0.56Mo wt%) was investigated, comparing the properties obtained after the application of selected quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments. After Q&T the strength–toughness combination was the lowest, whereas the best combination was achieved by Q&P, as a result of the carbon depletion of the martensite and the high stabilization of the austenite. Nonetheless, the presence of islands of martensite/austenite (MA) constituents after Q&P treatments prevented the achievement of toughness levels comparable to the ones currently obtainable with other steels and heat treatments.

The influence of grain size on the strain-induced martensite formation in tensile straining of an austenitic 15Cr–9Mn–Ni–Cu stainless steel

Abstract: In order to improve understanding on the behavior of ultrafine-grained austenitic stainless steels during deformation, the influence of the austenite grain size and microstructure on the strain-induced martensite transformation was investigated in an austenitic 15Cr–9Mn–Ni–Cu (Type 204Cu) stainless steel. By different reversion treatments of the 60% cold-rolled sheet, varying grain sizes from ultrafine (0.5 μm), micron-scale (1.5 μm), fine (4 μm) to coarse (18 μm) were obtained. Some microstructures also contained a mixture of ultrafine or micron-scale and coarse initially cold-worked austenite grains. Samples were tested in tensile loading and deformation structures were analyzed after 2%, 10% and 20% engineering strains by means of martensite content measurements, scanning electron microscope together with a electron backscatter diffraction device and transmission electron microscope. The results showed that the martensite nucleation sites and the rate of transformation vary. In ultrafine grains strain-induced α′-martensite nucleates at grain boundaries and twins, whereas in coarser grains as well as in coarse-grained retained austenite, α′-martensite formation occurs at shear bands, sometimes via e-martensite. The transformation rate of strain-induced α′-martensite decreases with decreasing grain size to 1.5 μm. However, the rate is fastest in the microstructure containing a mixture of ultrafine and retained cold-worked austenite grains. There the ultrafine grains transform quite readily to martensite similarly as the coarse retained austenite grains, where the previous cold-worked microstructure is still partly remaining.

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

Abstract: 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.

Medium-Alloy Manganese-Rich Transformation-Induced Plasticity Steels

Abstract: The manganese concentration of steels which rely on transformation-induced plasticity is generally less than 2 wt pct. Recent work has highlighted the potential for strong and ductile alloys containing some 6 wt pct of manganese, but with aluminum additions in order to permit heat treatments which are amenable to rapid production. However, large concentrations of aluminum also cause difficulties during continuous casting. Alloy design calculations have been carried out in an effort to balance these conflicting requirements, while maintaining the amount of retained austenite and transformation kinetics. The results indicate that it is possible by adjusting the carbon and manganese concentrations to reduce the aluminum concentration, without compromising the mechanical properties or transformation kinetics. The deformation-induced transformation of retained austenite is explained quantitatively, for a range of alloys, in terms of a driving force which takes into account the very fine state of the retained austenite.

Texture, misorientation and mechanical anisotropy in a deformed dual phase stainless steel weld joint

Abstract: The deformation behavior of a duplex stainless steel weld joint is studied. A detailed texture analysis shows that the increase of the deformation percentage leads to a reinforcement of all possible {hkl}〈111〉 and {hkl}〈100〉 components in austenite, and of the {hkl}〈110〉 components in ferrite for both the base metal (BM) and the heat affected zone (HAZ). In the weld metal (WM), a strong scatter of the crystallographic orientation relationship (OR) initially found in the solidification microstructure is recorded after deformation. The analysis of the Kernel Average Misorientation (KAM) distribution shows that the deformation is more concentrated in the base metal than in the other parts of the weld joint. The final mechanical behavior studied through microhardness measurement and micromechanical calculations, allows us to separate the contribution of both microstructural and textural evolution to the overall strain hardening of the weld joint.

Microstructure development and mechanical properties of quenching and partitioning (Q&P) steel and an incorporation of hot-dipping galvanization during Q&P process

Abstract: The “quenching and partitioning” (Q&P) process has recently been substantiated to be a unique technological route for the production of high strength steels with significant amounts of retained austenite, and thus to provide better combination of strength and ductility. In this work, intercritically annealed specimens followed by Q&P treatment have been applied to low-carbon steel with chemical composition typical for conventional TRIP-assisted steels. Microstructure of the steel treated by the Q&P process was characterized by means of optical microscope, SEM, TEM and XRD. The study suggests that microstructure is mainly composed of ferrite, lath martensite, martensite–austenite islands, retained austenite and a small amount of bainite formed during partitioning. The fraction of bainite formed during partitioning is proportional to quenching temperature. The mechanical property of specimen treated by the Q&P process exhibits an improved combination of strength and ductility than that of the Q&T process. Two schemes of hot-dipping galvanization processes were designed. The results indicate that both hot-dip galvanizing schemes present a limited reduction in tensile strength and a slight enhancement of ductility. The scheme of galvanizing and partitioning after the quenching progress shows a better combination of strength and ductility.

Giant magnetocaloric effect in isostructural MnNiGe-CoNiGe system by establishing a Curie-temperature window

Abstract: An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.

Effect of alloying composition on low-cycle fatigue properties and microstructure of Fe-30Mn-(6-x)Si-xAl TRIP/TWIP alloys

Abstract: The change in low-cycle fatigue (LCF) properties and deformation microstructure due to the alteration of aluminum and silicon contents was studied in relation with the tensile properties in Fe–30Mn–(6−x)Si–xAl (x=0, 1, 2, 3, 4, 5, 6 wt%) alloys, which are high-Mn austenitic TRIP/TWIP alloys. Austenite to e-martensite transformation took place during LCF deformation in the TRIP alloys with x≤2 while mechanical twinning was not observed by electron-backscattering diffraction (EBSD) analysis in the TWIP alloys with x>2 after LCF deformation. The fatigue resistance of the alloys was shown to be correlated with the tensile proof strength and the hardening rate. Superior fatigue life of 8×103 cycles at a total strain range Δe=2% was found in the Fe–30Mn–4Si–2Al TRIP alloy with a low fraction of e-martensite, high tensile proof strength and low hardening rate at both tensile and fatigue deformations. On the other hand, a considerable decrease in the fatigue properties was observed in the alloys with decreasing proof strength and increasing hardening rate. Proof strength provided by the solid solution of Al and Si, represents the hampering of plastic deformation, and the hardening rate reflects the strain reversibility affected by the stacking fault energy (SFE) through the rate of austenite to martensite transformation in the TRIP alloys and the substructure formation in the TWIP alloys.