Phase Transformation In Stainless Steel Research Articles

Modelling the Evolution of Phases during Laser Beam Welding of Stainless Steel with Low Transformation Temperature Combining Dilatometry Study and FEM

In this study, the evolution of volume fractions during laser beam welding (LBW) of stainless steel, with a specific focus on incorporating the low transformation temperature (LTT) effect using the dilatometer, has been proposed. The LTT effect refers to the phase transformations that occur at lower temperatures and lead to the formation of a martensitic microstructure, which will significantly influence the residual stresses and distortion of the welded joints. In this research, the LTT conditions are achieved by varying the Cr and Ni content in the weld seam by varying the weld parameter, including laser power, welding speed and filler wire speed. The dilatometer analysis technique is employed to simulate the thermal conditions encountered during LBW. By subjecting the stainless steel samples to controlled heating and cooling cycles, the kinetics of the volume fractions can be measured using the lever rule and empirical method (KOP and Lee). The phase transformation simulation model is computed by integrating the thermal and metallurgical effects to predict the volume fractions in LBW joints and has been validated using dilatometer results. This provides valuable insight into the relationship between welding parameters and phase transformations in stainless steel with the LTT effect during laser beam welding. Using this relationship, the weld quality can be improved by reducing the residual stresses and distortion.

Pulsed fusion grade deuterium plasma induced structural phase transformation in stainless steel using medium energy plasma focus device

Samples of ferrite -austenite stainless steel (SS) have been exposed to fusion grade plasma and radiations using a medium energy plasma focus device, ‘MEPF-12’. Three different polished samples of SS (10 mm diameter ×2 mm thick) were irradiated using single and multiple ( five and ten) plasma focus discharges. Each plasma focus discharge comprised energetic and high intensity deuterium ions, energetic deuterium plasma streams and 2.45 MeV neutrons produced via D(d, 3He)n fusion reactions. At sample position, typical values of deuterium ion fluence and neutron fluence were measured to be ∼1018ions/m2 and ∼1010 neutrons/m2, respectively, in each plasma focus discharge. Post irradiations, the samples have been characterized using different techniques for study of their surface modification characteristics, specifically, crystallinity, surface morphologies and surface roughness. The XRD analysis of the SS sample exposed to single or more plasma focus discharges revealed structural phase transformation from initial mixed ferrite [α(BCC)] -austenite [γ(FCC)] phase to austenite phase. Effects of deuterium ion concentration and transient rise in surface temperature on structural phase transformation have been discussed in detail. Surface morphologies studied using SEM has revealed erosion, melting and recrystallization of surface layer along with various other defects like pores, blisters, open bubbles and so on. Measurements using surface profilometer confirmed increase in surface roughness of the samples on irradiation. Detail discussion on the variation in the surface damage features with increasing number of plasma focus discharges used for irradiating samples and its comparison with virgin sample have been included in this report.

Influence of Strain State on the Kinetics of Martensitic Transformation Induced Plasticity (TRIP) in AISI 304 Stainless Steel

The influence of strain state on the kinetics of martensitic phase transformation in stainless steel 304 is investigated. The methodology involves stretching various specimens with different widths over a hemispherical punch. The specimens are stretched to various depths by controlling the punch traveling in order to achieve different strain levels. The volume fraction of strain induced martensite is then measured using ferrite scope and plotted versus strain for different strain states. The results show that the martensitic transformation curve has the lowest value for plane strain condition among strain states used for determination of forming limit diagram. There are a close proximity between the two sets of the experimental data and those of the modified Olson‐Cohen and Santacreu's models.

Characterization of carbon ion implantation induced graded microstructure and phase transformation in stainless steel

Austenitic stainless steel 316L is ion implanted by carbon with implantation fluences of 1.2×1017ions-cm−2, 2.4×1017ions-cm−2, and 4.8×1017ions-cm−2. The ion implantation induced graded microstructure and phase transformation in stainless steel is investigated by X-ray diffraction, X-ray photoelectron spectroscopy and high resolution transmission electron microscopy. The corrosion resistance is evaluated by potentiodynamic test. It is found that the initial phase is austenite with a small amount of ferrite. After low fluence carbon ion implantation, an amorphous layer and ferrite phase enriched region underneath are formed. Nanophase particles precipitate from the amorphous layer due to energy minimization and irradiation at larger ion implantation fluence. The morphology of the precipitated nanophase particles changes from circular to dumbbell-like with increasing implantation fluence. The corrosion resistance of stainless steel is enhanced by the formation of amorphous layer and graphitic solid state carbon after carbon ion implantation.

Development of SQUID-Based System for Nondestructive Evaluation

This article describes the development of a superconducting quantum interference device (SQUID)-based system for nondestructive evaluation. The setup incorporates an in-house developed thin-film-based Nb SQUID with readout flux locked loop electronics and consists of a liquid helium cryostat with adjustable stand-off distance, a precision XY- thetas scanner for studying both flat and cylindrical samples, and a data acquisition system. The system has been used for the detection of artificially engineered subsurface defects in aluminum plates and to track magnetic-to-nonmagnetic phase transformation in stainless steel [grade 316L(N)] weldment specimens subjected to low cycle fatigue deformation.

Thermal Induced Processes in Laminar System of Stainless Steel – Beryllium

The paper reports on investigation of the laminar system ‘stainless steel 12Cr18Ni10Ti – Be’ at thermal treatment. There have been determined sequences of phase transformations along with relative amount of iron-containing phases in the samples subjected to thermal beryllization. It has been revealed that thermal beryllization of stainless steel thin foils results in γ→α transformation and formation of the beryllides NiBe and FeBe2. It has also been revealed that direct γ→α- and reverse α→γ-transformations are accompanied by, correspondingly, formation and decomposition of the beryllide NiBe. It is shown that distribution of the formed phases within sample bulk is defined by local concentration of beryllium. Based on obtained experimental data there is proposed a physical model of phase transformations in stainless steel at thermal beryllization.

Deforming Texture Effect on Phase Transformation in Stainless Steels

Deforming Texture Effect on Phase Transformation in Stainless Steels

Possibility as a magnetic recording media of austenitic stainless steel using stress-induced phase transformation

A new magnetic recording media which uses the phase transformation of austenitic stainless steel, is proposed. The transformation from austenite to martensite occurs due to tribological stresses, such as indentation and sliding, has the potential to induce phase transformation which in turn generates a locally ferro-magnetized surface. Surface observations were made using magnetic force sensing systems having a sensing mechanism similar to that of magnetic force microscope (MFM). Two-dimensional distribution of the magnetic force measured by the sensing systems showed many possible applications in magnetic recording, and magnetic field freezing, etc. Scratching experiments with a load of 0.4 mN verified the possibility of the material's application as a recording media with spatial resolution of sub-micrometer. Memorizing phenomena of magnetic field in a stainless steel plate was also observed. After a uniform tensile stress generated the phase transformation of a stainless steel thin plate in a magnetic field, the magnetic force distribution on the steel surface was measured. The distribution was similar to that of the magnetic field which was induced by an exciting coil. The results indicated that information about the external magnetic field distribution was frozen in the stainless steel test specimen. By using this phenomenon, an analogue-type recording system capable of memorizing a two-dimensional distribution of a magnetic field at any arbitrary point in time by an external force trigger is proposed.

Mössbauer study of martensitic transformation in Xe ion-irradiated type 304 austenitic stainless steel

Foil specimens of type 304 stainless steel have been irradiated with Xe+ ions in the range of 100–400 keV and 1×1020−1×1021 ions/m2 to elucidate the dynamics of the ion-induced martensitic phase transformation in stainless steel. It has been clearly shown by depth selective conversion electron Mossbauer spectroscopy (DCEMS) that the ion-induced martensitic phase in type 304 stainless steel has grown from the surface to a depth dependent both on the ion energy and the fluence of the Xe+ ions. Especially, we observed by means of DCEMS that the extension of the martensitic phase into the interior of stainless steel has been induced with increasing ion energy. It is concluded from these results that the depth distribution of the ion-induced martensitic phase is stress-induced by the formation of the highly pressurized Xe+ inclusion in type 304 stainless steel.

MeV-ion-beam induced localized enhancement of magnetization in stainless steel foils

MeV ions were implanted into 301 and 304 stainless steel foils to investigate the localized modification of magnetic properties associated with the phase transformation. The intensity of the magnetization was found to increase by MeV Au + implantation because of the localized martensitic transformation. The increase in magnetization for 301 stainless steel was greater than that for 304 stainless steel because 301 stainless steel more easily transformed austenite into martensite. The beam-induced localized phase transformation in stainless steel can be applied to permanent magnetic recording.

Irradiation effect of rare-gas ions in type 304 austenitic stainless steel

Ar, Kr and Xe rare-gas ions were irradiated in foil specimens of type 304 stainless steel. An irradiation effect in the surface region of the foil specimens has been investigated by means of glancing angle X-ray diffraction (GXRD), Rutherford backscattering (RBS) and particle-induced X-ray emission (PIXE) in order to observe the retained rare-gas atoms and elucidate their correlation with the ion-induced martensitic phase transformation of γ phase → α phase. It has been seen that the amounts of Ar, Kr and Xe retained in stainless steel as estimated from PIXE spectra were affected by the sputtering under ion irradiation. It has been shown by GXRD measurements that the retained rare-gas atoms agglomerated and formed a highly-pressurized solid phase inclusion. It is clear from GXRD measurements that the amount of the phase transformation at saturation increased linearly with increasing ion range of the implanted rare-gas atoms irrespective of the ion species. We speculate from this linearity that the depth distribution of the ion-induced martensitic phase may overlap with that of the rare-gas inclusion in stainless steel, and the formation of the rare-gas inclusion has stress-induced the phase transformation of γ → α. However, before saturation of the ion-induced phase transformation, the degree of the phase transformation in stainless steel as estimated from the amount of retained rare-gas atoms clearly differed for the different ion species.

Local Control of Magnetic Property in Stainless Steel Surface by Ion and Laser Beams

MeV ion and Ar-ion laser beams were impinged onto 304 stainless steel foils to investigate the localized modification of the magnetic property associated with the phase transformation. The intensity of the magnetization was found to increase by MeV Au+ implantation because of the localized martensitic transformation, whereas that of the Ar-ion laser-irradiated areas was found to locally decrease at a laser power of 350 mW and almost extinguish at 550 mW with a beam spot size of 13.2 µm (at 1/e intensity) because of the localized retransformation into austenite. These results indicate that the beam-induced localized phase transformation in stainless steel can be applied to permanent magnetic recording.

Effects of X-ray irradiation on hydrogen-induced phase transformations in stainless steel

Effects of X-ray irradiation on hydrogen-induced phase transformations in stainless steel

The influence of phase transformation in stainless steel during static and fatigue loading

A comparative study of the mechanisms of plastic deformation under static tensile and dynamic zero-to-peak axial loadings (R = 0) has been carried out for an annealed austenitic stainless steel Type 321S12. For the tensile test the high value of the strain-hardening exponent and the high nominal tensile strength/yield strength ratio are associated with the strengthening effects involved in an austenite-martensite transformation. To induce this transformation a minimum value of strain displacement is necessary and such strains are first attained at the surface layers and only occur in the core grains for the regime of plastic instability beyond the level of the nominal tensile strength. The S/N curve reveals three distinct regimes of fatigue and the fatigue limit is ∼ 55% higher than the macro-yield strength of the alloy. Explanations given of the fatigue-crack process in the various regimes of the S/N curve are based on a comparison of the tensile and fatigue loading conditions.

Phase transformation of stainless steel during fatigue

Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous.