Single-phase Austenitic Steels Research Articles

Effects of Tensile Testing Temperature on Mechanical Properties and Deformation Behavior in Medium Mn Steels

In single-phase austenitic steels, the optimum deformation temperature in the tensile test to obtain high tensile strength-elongation balance (TS×El) and work hardening rate (dσ/dε) depends on control of the stability of austenite. In order to clarify the effects of the deformation temperature in complex phase steels containing austenite, in this study, the effects of the tensile testing temperature on mechanical properties and deformation behavior were investigated in detail using steel A and steel B with a chemical composition of 0.15C-0.5Si-5.0Mn (wt%). Steels A and B consisted of ferrite and retained austenite, but contained different volume fractions of retained austenite, namely, 29 % and 17 % as a result of annealing at 660 °C and 620 °C for 2 h, respectively. The stability of the retained austenite of steel B was higher than that of steel A. In steel A, TS×El and dσ/dε achieved their maximum values at 20 °C, decreased from 20 to 100 °C, and then remained almost unchanged at more than 150 °C. On the other hand, in steel B, TS×El and dσ/dε achieved their maximum values at -40 °C, decreased from -40 to 50 °C and remained almost unchanged at more than 100 °C. These results can be explained by the stability of retained austenite and the transformation rate from retained austenite to martensite. It should be noted that control of the stability of retained austenite and the transformation rate from retained austenite to martensite led to an adjustment of the optimum deformation temperature to achieve the high TS×El and dσ/dε in medium Mn steels, in the same manner as in single-phase austenitic steels.

Micromechanics and microstructure evolution during in situ uniaxial tensile loading of TRIP-assisted duplex stainless steels

Transformation-induced plasticity (TRIP) assisted duplex stainless steels, with three different stabilities of the austenite phase, were investigated by synchrotron x-ray diffraction characterization during in situ uniaxial tensile loading. The micromechanics and the deformation-induced martensitic transformation (DIMT) in the bulk of the steels were investigated in situ. Furthermore, scanning electron microscopy supplemented the in situ analysis by providing information about the microstructure of annealed and deformed specimens. The dependence of deformation structure on austenite stability is similar to that of single-phase austenitic steels with shear bands and bcc-martensite (α′) generally observed, and blocky α′ is only frequent when the austenite stability is low. These microstructural features, i.e. defect structure and deformation-induced martensite, are correlated with the micro- and macro-mechanics of the steels with elastoplastic load transfer from the weaker phases to the stronger α′, in particular this occurs close to the point of maximum rate of α′ formation. A clear strain-hardening effect from α′ is seen in the most unstable austenite leading to a pronounced TRIP effect.

Deformation mechanisms in an austenitic single-phase duplex microstructured steel with nanotwinned grains

A novel type of duplex microstructure is generated in a single-phase austenitic steel (AISI 316L; X2CrNiMo19-12), consisting of plastically compliant recrystallized austenitic grains as the matrix containing coarse non-recrystallized grains with a nanotwinned austenitic (nt-γ) structure as strengthening inclusions. This novel type of single-phase yet duplex microstructured steel exhibits an excellent combination of strength and ductility. We study the plastic co-deformation mechanisms between the nanotwinned and the recrystallized grains under tension using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). At tensile strains below 5%, the nt-γ grains nearly deform homogeneously in conjunction with the surrounding statically recrystallized (SRX) grains without generating notable strain localization near their interfaces. The anisotropic plastic deformation of the nt-γ grains with predominant shear parallel to the twin boundaries results in a higher dislocation density in the neighboring SRX grains. As the strain exceeds 12%, localized deformation occurs within the nt-γ grains in the form of shear banding. A strain gradient is developed in the surrounding SRX grains as a function of distance from the nt-γ/SRX interface. Deformation twinning is observed in the SRX grains near the nt-γ grains, while away from nt-γ grains dislocation slip dominates the deformation. The strengthening effect of the strong and ductile nt-γ grains may offer a novel approach to strengthen austenitic steels and related alloys by generating a nanotwinned/recrystallized duplex microstructure.

Ways of Numerical Prediction of Austenitic Grain Size in Heat-Affected Zone of Welds

In the present time there is a clear effort to achieve the most exact mathematical description of the behaviour of “Hi-tech” materials when exposed to temperature and stress loading. Besides the common numerically predicted values such as temperature, deformation and stress fields, or as the case may be structure changes during phase transformations, demands for prediction of the austenitic grain size in HAZ of welds become more and more frequent. That is why the present submission deals with the analysis of the determination of the grain size and grain growth kinetics of HR3C single-phase austenitic steel using the Monte Carlo Potts method. The procedure of obtaining the input data for numerical simulations will be demonstrated on HR3C steel, including the determination of grain growth kinetics and definition of all the parameters needed for a computational model. Results from the numerical simulation in Sysweld program will be then compared against the real experiment for a multi-layered weld made on HR3C tube.

Thermo-mechanical fatigue behaviour of a duplex stainless steel

In this paper the thermo-mechanical fatigue (TMF) behaviour of the duplex stainless steel X2CrNiMoN22-5-3 is presented. TMF tests were carried out in the temperature range 100–350 °C on the duplex steel and on single-phase ferritic and austenitic steels comparable to the two phases of the duplex steel. The cyclic stress response curves of the duplex steel are showing pronounced primary and secondary hardening, where the former is in agreement with the cyclic hardening regime of the single-phase austenitic steel and the latter with that of the ferritic steel. For reference, additional isothermal low cycle fatigue (LCF) tests were performed at two strain rates at minimum and maximum temperature of the TMF temperature range, proving dynamic strain ageing effects occurring at 350 °C.

High Temperature Corrosion of Chromium–Manganese Steels in Sulfur Dioxide

This work was aimed at explaining the corrosion mechanism of commercial Cr–Mn steels at 1073, 1173 and 1273 K in the atmospheres containing oxygen and sulfur. Three steels were selected for the investigations, two single-phase austenitic steels (Cr17Mn17 and Cr13Mn19SiCa) and a two-phase austenitic-ferritic steel Cr15Mn19. On all studied steels triplex scales were formed. The inner very thin, fine-grained part of the scale contained manganese, chromium and iron sulfides and oxides, the intermediate layer was built mainly of the MnCr2O4 spinel while MnO was the predominant constituent of the outer scale layer. According to the gravimetric measurements, after an initial incubation period, the oxidation of steel follows a parabolic rate law. Thermodynamic and kinetic aspects of the formation of oxide-sulfide and oxide layers were discussed. Oxidation was accompanied by depletion of the subscale region of the metallic core in manganese, which is the austenite former. Consequently austenite transformed into ferrite.

The effect of microstructure

Abstract The influence of microstructural parameters on low cycle fatigue damage in some single phase and biphasic stainless steels has been studied. In a single phase austenitic steel, the fatigue life increase with decreasing grain size is attributed to the positive effect of microstructural barriers in the bulk. In a metastable austenitic steel, the grain size of the initial microstructure has a combined influence on the volume fraction of martensite formed during cyclic loading and on the fatigue life. In this case, the beneficial effect of grain refinement is indirect because all cracks grow in the martensite. Possible damage patterns in a cluster of grains with different properties are discussed using the example of a duplex austenitic-ferritic steel. It is demonstrated that the propagation behaviour of short cracks related to the resistance of microstructural barriers in the bulk has a decisive influence on the fatigue life. The macroscopic fatigue resistance is thus controlled by damage events at the microstructure size scale.

Metastable pitting in 25 Cr duplex stainless steel

Evidence of metastable pitting in conventional austenitic stainless steels at room temperature is now common. Because of the small currents associated with individual events, most studies have been carried out on micro electrodes (typically 50 mm diameter). Here in contrast, with high chromium duplex stainless steels (DSS), metastable pitting is clearly observed on larger electrodes in tests at elevated temperatures in neutral NaCl solutions. Below the Critical Pitting Temperature (CPT), large metastable current peaks are observed across a range of low potentials, with less activity at higher potentials. Above the CPT, stable pits initiate at the same low potentials where metastability occurs below the CPT, usually preceded by one or two metastable events. There are evident differences between the growth of metastable pits—and the transition from metastable to stable pitting—in high-chromium DSS from that in single phase austenitic steels. A proposed model to explain the metastability in these highly alloyed duplex steels is described.