Magnetic measurements of the reverse martensite to austenite transformation in a rolled austenitic stainless steel

The reverse (-) transformation in 304 stainless steel (SS) has been studied by magnetic measurements. Specimens rolled 15 to 55% reduction in thickness were annealed at various temperatures and times. After annealing at temperatures between 300–;600°C for 5 min the saturation magnetization values increased when compared to the saturation magnetization values after rolling. Specimens rolled to 40 to 55% reduction after annealing at 500°C showed the highest saturation magnetization. Saturation magnetization sharply decreases at annealing temperature above 625°C which indicates the start of reverse (-) transformation. The decrease in saturation magnetization is rapid for annealing time from 5 to 40 min, whereas, the decrease in saturation magnetization is relatively low for annealing time above 40 min. The hardness values after reverse (-) transformation at temperatures between 300–;600°C is slightly greater attributed to the increase in martensite and above this temperature the hardness dropped substantially as a result of recovery and recrystallization. The results show that there is a decrease in coercive force at temperatures between 300–;500°C and may be due to an increase in martensite phase. A further decrease in coercive force at temperature between 500 and 625°C may be attributed to the sweeping out of some dislocation from the martensite phase. This is followed by a sharp increase in coercive force at temperature up to 800°C and is attributed to a shape magnetic anisotropy effect. At temperatures between 800–;900°C a rapid decreased in coercive force occurs. At temperatures between 900–;1100°C the decrease in coercive force is not so sharp dominant. The decrease in coercive force above 800°C corresponds to softening of the stainless steel due to recrystallization. From the changes in the values of saturation magnetization the A _s temperature is estimated to be between 625–;650°C, and the A _f to be between 900–;950°C.     

Magnetic structure of the modulated martensite phase in Ni-Mn-Ga ferromagnetic shape memory alloys

The magnetic structure of the seven layered (7 M) modulated martensite phases in Mn-rich Ni-Mn-Ga alloys was studied using Mössbauer spectroscopy. The Mössbauer results clearly demonstrate that in contrast to the non-modulated tetragonal structure two new magnetic sublattices exists for the 7 M orthorhombic martensite phase. Based on the unit cell symmetry and atomic coordination, the additional magnetic sublattices have been assigned to the Ni site. The variation in the magnetic properties of the martensite phases has been related to the underlying magnetic structure.     

Effects of martensite formation and austenite reversion on grain refining of AISI 304 stainless steel

The austenite–martensite transformation followed by annealing for austenite reversion in AISI 304 stainless steel has been investigated in order to study the effect of this thermo-mechanical process on grain refinement. In particular the effect of cold reduction, annealing temperature and annealing times have been analysed. After getting ultrafine grains the effect of the grain size on the hardness and on the tensile properties has been evaluated, showing a Petch-Hall dependency in the fully analysed range (down to 0.8 m grain size).     

Relation between microstructures of martensite and prior austenite in 12 wt% Cr ferritic steel

Tempered martensitic 9–12 wt% Cr ferritic steels are used as heat resistant materials in power plant, where service under conditions of high temperature and pressure for several decades is required, and an adequate resistance to creep is one of the key requirements. In this type of steels, failure has been found to occur preferentially at prior austenite grain boundaries if the prior austenite grains are coarse. It appears that the prior austenite grain boundaries can act as a site of especial weakness in the tempered martensitic microstructure. It would therefore be useful to investigate whether the properties of prior austenite grain boundaries could be modified by some appropriate thermomechanical processing method. One approach to this is to attempt to increase the fraction of annealing twins in the austenite phase and to investigate whether this has an effect on the properties of the martensite after transformation and tempering. In this study, thermomechanical treatments involving hot-rolling have been applied and the fraction of austenite twins produced determined using electron backscatter diffraction analysis. The treatment giving the highest fraction of austenite twins was identified and the effect of the increase in twin fraction on the characteristics of the martensite was investigated. It was found that the fraction of coincidence site lattice boundaries in martensite along prior austenite grain boundaries increased with increasing fraction of prior austenite twin boundaries.     

Low cycle fatigue behaviour of dual-phase steel with different volume fractions of martensite

Completely reversed low cycle fatigue tests were carried out at a constant strain rate of 10^?2s^?1 on 3 mm thick sheet specimens of a dual-phase steel treated to give 1.5 to 28% martensite without changing the carbon content. Hysteresis loop shape, stress/strain response and plastic strain energy as a function of applied cycles are analysed for different microstructures. Strain/life and plastic strain energy per cycle ($\text{Δ}\text{W}\text{?}{}_{\mathrm{<mtext>p</mtext>}}$)/life (2N_f) plots are discussed in terms of microstructure. It is shown that during cycling the shape of the hysteresis loop continuously changes at lower volume fractions of martensite whereas it remains more or less constant for microstructures with a higher percentage of martensite. A Coffin-Manson type of plot between log ($\text{Δ}\text{W}\text{?}{}_{\mathrm{<mtext>p</mtext>}}$) and log (2N_f) is found to be applicable to test results of dual-phase steel with a wide range of martensite contents and is thus more versatile than the plot between log (Δ?_p/2) and log (2N_f) for predicting the fatigue life.     

Effect of martensite to austenite reversion on the formation of nano/submicron grained AISI 301 stainless steel

The martensite to austenite reversion behavior of 90% cold rolled AISI 301 stainless steel was investigated in order to refine the grain size. Cold rolled specimens were annealed at 600–900 °C, and subsequently characterized by scanning electron microscopy, X-ray diffraction, Feritscope, and hardness measurements. The effects of annealing parameters on the formation of fully-austenitic nano/submicron grained structure and the mechanisms involved were studied. It was found that annealing at 800 °C for 10 s exhibited the smallest average austenite grain size of 240 ± 60 nm with an almost fully-austenitic structure.     

Quantitative phase analysis by X-ray diffraction of martensite and austenite in strongly oriented orthodontic stainless steel wires

A method is described for measuring the volume fractions and textures of martensite and austenite in strongly textured stainless steel orthodontic wires using a conventional X-ray diffractometer. These wires display a classic fibre texture with the 111 of the FCC austenite phase and the 110 of the BCC martensite phase aligned parallel to the wire axis. The samples analysed consisted of wire cross-sections bundled together and chemically polished in an epoxy disc. In this form the dominant lines in the XRD patterns are the austenite (111) and the martensite (110). On the basis of X-ray diffraction results from these two lines only, procedures are described for, (a) correcting the X-ray intensity data for both the finite size and irregular cross-sectional shape of the specimens in relation to the X-ray beam footprint, (b) separately measuring the texture of the austenite and martensite phases and, (c) correcting the 111 and 110 integrated intensities for texture. These procedures are illustrated using X-ray data from four different orthodontic wires. The factors limiting the accuracy of the phase analysis are discussed.     

Analysis of martensite–austenite constituent and its effect on toughness in submerged arc welded joint of low carbon bainitic steel

Martensite–austenite (M–A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M–A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M–A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M–A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M–A constituent and carbon concentration on the slender M–A constituent is higher than that on the massive M–A constituent. Meanwhile, the distribution profile of silicon on the M–A constituent shows an obvious inhomogeneity. Most of M–A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M–A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.     

Reverse transformation mechanism of martensite to austenite and amount of retained austenite after reverse transformation in Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel

Abstract

The reverse transformation mechanism of martensite to austenite and the volume fraction of retained austenite have been studied in an Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel by means of dilatometry, transmission electron microscopy and X-ray diffraction. Below a heating rate of 10 K s^-1, the reverse transformation of α' to γ occurs by diffusion, whereas it occurs by a diffusionless shear mechanism above 10 K s^-1. After reversion treatment at low temperatures, filmlike retained austenite is observed along α' lath boundaries, while reversion treatment at high temperatures produces granular retained austenite inside the α' laths in addition to filmlike retained austenite. The volume fraction of retained austenite at room temperature increases with increasing reversion treatment temperature, exhibiting a maximum at ~625^° C, above which it decreases with increasing reversion temperature.     

Influence of martensite morphology on the work-hardening behavior of high strength ferrite–martensite dual-phase steel

This study concerns influence of martensite morphology on the work-hardening behavior of high-strength ferrite–martensite dual-phase (DP) steel. A low-carbon microalloyed steel was subjected to intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA) to develop different martensite morphologies, i.e., fine and fibrous, blocky and banded, and island types, respectively. Analyses of work-hardening behavior of the DP microstructures by differential Crussard–Jaoul technique have demonstrated three stages of work-hardening for IQ and IA samples, whereas the SQ sample revealed only two stages. Similar analyses by modified Crussard–Jaoul technique showed only two stages of work-hardening for all the samples. Among different treatments, IQ route has yielded the best combination of strength and ductility due to its superior work-hardening behavior. The influence of martensite morphology on nucleation and growth of microvoids/microcracks has been correlated with the observed tensile ductility.     

TiC as heterogeneous nuclei of the (Fe, Mn)3C and austenite intergrowth eutectic in austenite steel matrix wear resistant composite

By calculation of thermodynamics, analysis of crystal structure and study of transmission electron microscope (TEM), scanning electron microscope (SEM) and electron probe microanalyzer (EPMA), it has been discovered that TiC is formed preferentially between austenite dendrites at the end of the solidification to act as heterogeneous nuclei for the crystallization of the (Fe, Mn)₃C (cementite) and γ₂-Fe (austenite) intergrowth eutectic in the austenite steel matrix wear resistant composite.     

Characterization of the carbides and the martensite phase in powder-metallurgy high-speed steel

A microstructural characterization of the powder-metallurgy high-speed-steel S390 Microclean was performed based on an elemental distribution of the carbide phase as well as crystallographic analyses. The results showed that there were two types of carbides present: vanadium-rich carbides, which were not chemically homogeneous and exhibited a tungsten-enriched or tungsten-depleted central area; and chemically homogeneous tungsten-rich M₆C-type carbides. Despite the possibility of chemical inhomogenities, the crystallographic orientation of each of the carbides was shown to be uniform. Using electron backscatter diffraction the vanadium-rich carbides were determined to be either cubic VC or hexagonal V₆C₅, while the tungsten-rich carbides were M₆C. The electron backscatter diffraction results were also verified using X-ray diffraction. Several electron backscatter diffraction pattern maps were acquired in order to define the fraction of each carbide phase as well as the amount of martensite phase. The fraction of martensite was estimated using band-contrast images, while the fraction of carbides was calculated using the crystallographic data.     

Accelerated spheroidization of hypoeutectoid steel by the decomposition of supercooled austenite

A thermal cycling treatment was used to produce a spheroidized structure of hypoeutectoid steel from direct decomposition of supercooled austenite. Scanning electron microscopy and quantitative metallography were employed to study the changing microstructure during the thermal cycling treatment. A conventional spheroidizing annealing was also investigated for comparison. It has been shown that the thermal cycling treatment results in a structure of cementite spheroidites homogeneously distributed in a ferrite matrix within a very short processing time, and the coarsening of cementite particles is controlled by the coupled diffusion of both carbon and iron atoms.     

Influence of martensite carbon content on the cyclic properties of dual-phase steel

A series of Fe---Mn---C alloys was quenched from 760° and 820°C. This treatment produced dual-phase microstructures in which the carbon contents of the martensite and ferrite phases were held constant while the percent martensite varied. The monotomic and cyclic properties of these steels were determined and the major influence on mechanical properties was found to be the percentage of martensite; both monotonic and cyclic stress levels increase linearly with martensite content. Carbon content of the phases also appears to play a role, particularly for cyclic properties. At constant martensite contents higher carbon levels result in better fatigue properties. Thus dual-phase steels with a higher alloy content (therefore able to be more slowly cooled resulting in higher carbon bearing martensite) may be preferable for cyclic applications.