Austenitic Structure Research Articles

High‐Nitrogen Nickel‐Free Stainless Steel: An Attractive Material with Potential for Biomedical Application

Metal materials such as stainless steel, cobalt‐based alloy, and NiTi alloy used in clinic contain a certain amount of nickel (Ni) for the structure stability. Nickel ions can gradually dissolve into the human body even with much slow corrosion rate, causing allergic, inflammation, local tissue proliferation, and other adverse reactions to some people. To avoid these adverse effects of nickel, a new high‐nitrogen nickel‐free stainless steel (HNNFSS) using nitrogen and manganese instead of nickel to stabilize the austenite structure is developed, which exhibits much better mechanical properties, good corrosion resistance, and biocompatibility. Compared with the conventional orthopedic metal implants, the adhesion strength of HNNFSS implant with the surrounding tissue in animal is found to be stronger with new bone formation, promoted by both alkaline phosphatase (ALP) and osteocalcin (OCN) expressions. The vascular stent made of HNNFSS shows lower restenosis rate compared with the traditional 316L stainless steel stent, which is attributed to the promotion of endothelial cell and inhibition of smooth muscle cells, and superior blood compatibility. In conclusion, results from large number of studies show that high‐nitrogen nickel‐free stainless steel as a new kind of high‐performance biomedical metallic material possesses great application potential.

Thermal structural ratcheting simulation—Evaluation of industrial-used constitutive models

This article presents an evaluation of the performance of conventional constitutive models in modelling the structural ratcheting of an austenitic stainless steel structure subjected to a thermomechanical loading. This work highlights the influence of monotonic and cyclic behaviour in ratcheting responses. Propositions on how to determine constitutive models and which ones are able to reproduce ratcheting responses are also presented. Models employing only a few kinematic hardening are found to be inadequate in predicting structural ratcheting. Results predicted by a simplified version of Chaboche model proposed here are found in good agreement as compared to experimental measurements.

Electrochemical performance of the interfacial region between an AISI 420 and a wire arc additive manufactured PH 13–8Mo martensitic stainless steel

In this study, the electrochemical response of the interfacial region between an AISI 420 martensitic stainless-steel (MSS) substrate and a wire arc additive manufactured PH 13-8Mo MSS part was investigated. The complicated thermal cycle through the deposition of the PH 13-8Mo part led to the formation of distinct zones, including far heat-affected zone (FHAZ), close heat-affected zone (CHAZ), partially melted zone (PMZ), and fusion zone (FZ) at the interfacial bonding region. The formation of an austenitic structure with coarse grains was observed in the PMZ and FZ adjacent to the substrate. The corrosion performance of the interface region was degraded significantly, ascribed to the formation of high density of fine chromium-rich carbides. The corrosion onset at the interface region was initiated at the carbide/matrix interface in the CHAZ and PMZ, where chromium depleted regions at the carbides’ periphery intensify the micro-galvanic coupling effect. The corrosion resistance of the as-printed PH 13-8Mo sample was remarkably higher than the 420 MSS and interface samples. The formation of a uniform lath martensitic structure with low energy levels at lath boundaries and the absence of metastable chromium-rich micro-constituents, such as carbides, due to the low carbon content of the PH 13-8Mo MSS alloy, contributed to its superior electrochemical performance.

Correlation between Precipitation and Recrystallisation during Stress Relaxation in Titanium Microalloyed Steel

This study investigated the correlation between strain-induced precipitation (SIP) and static recrystallisation (SRX) in Ti microalloyed steel during stress relaxation after controlled compression. The final compression temperature strongly influenced the order of SIP and SRX and thus the evolution of the austenite structure. Precipitation-time-temperature (PTT) curve obtained for the experimental steel exhibited an inverted “S” shape. A recrystallisation kinetics model revealed that SRX, which occurs preferentially above 940 °C, resulted in delayed subsequent SIP, thus causing deviation in the PTT curve from the typical ‘C’ shape. Below 940 °C, the fastest nose temperature for precipitation was located at 900 °C, and the precipitate was constituted by TiC particles with a NaCl-type FCC structure. The dynamic competition between SIP and SRX processes were evaluated by comparing the relative magnitude of the recrystallisation driving force and precipitation pinning force during stress relaxation, combined with the evolution of precipitate and austenitic structure. The results indicated that the plateau period occurred because of the precipitation pinning effect inhibited recrystallisation-induced austenite softening. However, the non-uniform distribution of SIP restricted the mobility of the boundaries to a portion of the austenite grains, resulting in abnormal grain growth during the plateau period.

High Manganese TWIP Steel with Increased Corrosion Resistance

The paper describes the development of austenitic steel with the TWIP effect, which is alloyed with chromium to increase corrosion resistance. The experimental heat of this steel was cast in an experimental melting furnace and subsequently subjected to hot and cold rolling. After cold rolling, the appropriate recrystallization annealing temperature was applied to obtain the optimal austenitic grain size. X-ray diffraction proved that the steel contains a fully austenitic structure. After recrystallization annealing, the sheets achieved a TS of more than 950 MPa with an elongation of 40%. The corrosion resistance of this steel is increased with the addition of chromium.

Effect of heat treatments on metastable pitting of 316L stainless steel fabricated by selective laser melting

Effect of heat treatments on metastable pitting of 316L stainless steel fabricated by selective laser melting (SLM) was studied by scanning electron microscopy, potentiodynamic polarization, electrochemical noise (EN) and transient analysis techniques. The results show that the selective laser melted (SLMed) 316L stainless steel was composed of honeycomb cells with different orientations, and a small number of lack-of-fusion (LOF) pores were randomly distributed. After heat treatment, the honeycomb-like cellular structure transformed into austenite structure, and the non-metallic inclusions coarsened and grew. Non-metallic inclusions can be divided into two types according to the composition difference, one was manganese silicate enriched by Mn, Si and O and the other was MnS coexisted with the manganese silicate. The potentiodynamic polarization results show that the pitting potential of SLMed 316L stainless steel decreased after heat treatment. During the EN tests, three kinds of current transient were found, which corresponded to the metastable pitting initiated by the manganese silicate oxide inclusions, MnS coexisted with the manganese silicate and pore defects, respectively. Statistical analysis results of the transient signals show that the heat treatment reduced the nucleation rate of metastable pitting, and increased the transient amplitude, pit radius and pit stability product. The decreased pitting performance after heat treatment can be attributed to the low pit growth resistance.

Influence of Austenitisation Time and Temperature on Grain Size and Martensite Start of 51CrV4 Spring Steel

51CrV4 spring steel is a martensitic steel grade that is heat treated by quenching and tempering. Therefore, austenitisation is an important step that influences steel properties. The main goal of austenitisation is to obtain a single-phase austenite structure that will transform into martensite. We studied the influence of austenitisation parameters on grain growth and martensite transformation temperatures. The samples were quenched from different austenitisation temperatures (800–1040 °C) and were held for 5, 10 and 30 min. The martensite start transformation temperatures (MS) were determined from dilatometric curves, and the hardness was measured using the Vickers method. The microstructure of the samples and the size of the prior austenite grains were characterised using optical microscopy. The increase in the size of the prior austenite crystal grains increases the MS temperature. However, this trend is visible up to 960 °C, where the results start to deviate. High temperatures, 960 °C and above, cause both grain growth and increased carbide dissolution along with chemical homogenization of the steel. The added influence of strong solute diffusion caused a big deviation in the results. The stability of carbides during austenitisation were evaluated with scanning electron microscopy (SEM) and thermodynamic calculations of equilibrium phases using the Thermo-Calc program. MC-type vanadium carbides are stable up to 956 °C under equilibrium conditions, but the SEM results show that they were present in the microstructure even after annealing at 1040 °C. This means that crystal growth is slowed down, which is positive, and that the austenite contains less carbon, so the hardness is lower.

Microstructure Refinement of 301 Stainless Steel via Thermomechanical Processing

The current study applied thermomechanical processing (TMP) on 301 austenitic stainless steel to produce an ultrafine-grained austenitic structure, examining the dual effects of deformation at subzero temperature and TMP cycles on the strain-induced α′-martensitic transformation and austenite reversion occurring upon subsequent annealing. Three TMP schemes were adopted: (1) one cycle using a strain of 0.30, (2) two cycles using a strain of 0.20, and (3) three cycles using a strain of 0.15. Each cycle consisted of tensile deformation at −50 °C followed by annealing at 850 °C for 5 min. Compared to other schemes, the use of three cycles of the 0.15 strain scheme resulted in a significant formation of the martensitic phase to about 99 vol.%. Consequently, the austenite reversion occurred strongly, providing a mixture of the austenitic structure of reverted ultra-fine grains and retained coarse grains with an average grain size of 1.9 µm. The development of a mixed austenitic structure was found to lower the austenite stability and thus enhance the α′-martensitic transformation upon deformation in subsequent cycles. Moderate growth of high-angle grain boundaries occurred in the austenitic phase for all schemes, reaching a maximum of 64% in cycle 3 of the 0.15 strain scheme. The tensile behavior during subzero deformation was generally characterized by an initial strain hardening by slip (stage I), followed by a remarkable increase in strain hardening rate due to the strain-induced α′-martensitic transformation (stage II). Further straining promoted breakage of the α′-martensite banded lath structure for forming dislocation cell-type martensite, which was marked by a decline in strain hardening rate (stage III). Accordingly, the latter hardening stage had a lesser hardness enhancement of deformed samples with an increasing number of cycles. Nevertheless, the yield strength for samples processed by the 0.15 strain scheme improved from 450 MPa in cycle 1 to 515 MPa in cycle 3.

Formation of core-shell austenite as promoted by alloying solutes in hot-rolled medium Mn steel

Recently, there has been a new trend to design chemically heterogeneous austenite by adjusting the various thermal treatment processes in medium Mn steel. In this work, we found that the single or synergistic addition of ferrite stabilizers (e.g., Al) accelerated the heterogeneous distribution of Mn and the formation of a core-shell austenitic structure compared with the addition of an austenite stabilizer (e.g., Cu) under the conventional quenched-annealing process. In addition, the Mn-enriched austenite shell played a significant role in suppressing the martensitic transformation of the Mn-depleted austenite core upon cooling. The sharp Mn gradient distribution corresponded to the heterogeneous kinetics of the deformation-induced martensitic transformation, resulting in the gradual transformation-induced plasticity (TRIP) effect.

On the role of carbonitrides in formation of austenite grains during continuous hot rolling of pipe steel

A study of the formation of austenite grain size during rolling of a thick strip on a continuous mill by simulating multi-stage rolling on a Gleeble complex has been performed. It is shown that at low degrees of deformation in the stands of the finishing group of a rolling mill, the formation of a different-grained austenite structure can occur due to selective grain growth without the formation of recrystallization nuclei, despite the precipitation of carbonitrides of microalloying elements. The value of the difference between the deformation strengthening of neighboring grains, sufficient for the grain boundary to overcome the retardation caused by the carbonitride particles, has been determined. It is possible to avoid this negative phenomenon by increasing the degree of deformation in each pass and ensuring the conditions for the formation of recrystallization nuclei. An example of a similar phenomenon observed when rolling beryllium is given.The research was carried out with the financial support of the Ministry of Education and Science of Russia as part of the implementation of the program of the world-class Scientific Center in the direction of "Advanced Digital Technologies" of SPbPU (agreement dated April 20, 2022 No. 075-15-2022-311).

Investigation of changes in the structure of heat-resistant iron-containing chromium-nickel alloy as a result of long-term operation

Pyrolysis furnaces operate in harsh temperature and power conditions. Radiant coils are the most vulnerable part in their design. The coil pipe operates in two modes: operational - at a constant pressure and temperature not exceeding 1050 °С, and cleaning mode, which is characterized by increased pressure and higher temperature (about 1100 °С) compared to the operating one, which leadsto a thermal force effect. Such operating conditions of coil pipes lead to changes in the structure and properties of the material. In this regard, chromium-nickel steels are widely used for the manufacture of coils, and for operation at higher temperatures (1000-1050 °C), the content of chromium and nickel is increased, obtaining iron-containing chromium-nickel alloys, to ensure a stable austenitic structure. Identification of the dominant damaging mechanism makes it possible to determine the remaining resource and replace the coils in time to prevent possible emergencies. It is obvious that structural changes in the pipe metal will have a decisive influence on the performance of the structure as a whole. This paper presents the results of a study of the influence of some operational factors on the state of the metal of pipes of a standard furnace coil made of heat-resistant iron-containing chromium-nickel alloy. A change in the matrix microstructure in the form of partial homogenization of austenite was revealed. The fact of the occurrence of carbide reactions has been established. The presence of an oxidation zone mainly along the boundaries of dendrites was revealed, and the main types of oxides were determined. The dominant damaging mechanism of radiant tubes during operation has been determined.

Effect of Thermal Treatment on Corrosion Behavior of AISI 316L Stainless Steel Manufactured by Laser Powder Bed Fusion.

The effect of post-processing heat treatment on the corrosion behavior of AISI 316L stainless steel manufactured by laser powder bed fusion (L-PBF) is investigated in this work. Produced stainless steel was heat treated in a broad temperature range (from 200 °C to 1100 °C) in order to evaluate the electrochemical behavior and morphology of corrosion. The electrochemical behavior was investigated by potentiodynamic and galvanostatic polarization in a neutral and acidic (pH 1.8) 3.5% NaCl solution. The microstructure modification after heat treatment and the morphology of attack of corroded samples were evaluated by optical and scanning electron microscopy. The fine cellular/columnar microstructure typically observed for additive-manufactured stainless steel evolves into a fine equiaxed austenitic structure after thermal treatment at high temperatures (above 800 °C). The post-processing thermal treatment does not negatively affect the electrochemical behavior of additive-manufactured stainless steel even after prolonged heat treatment at 1100 °C for 8 h and 24 h. This indicates that the excellent barrier properties of the native oxide film are retained after heat treatment.

Low temperature mechanical behavior of fine- and ultrafine-grained 304 austenitic stainless steel fabricated by cryogenic-rolling and annealing

Fine-grained (FG) austenite structure, and ultrafine-grained (UFG) structure consisting of austenite matrix and residual strain-induced α'-martensite (SIMα′) were fabricated by combining cryogenic-rolling with annealing for 304 austenitic stainless steel (ASS). Tensile and Charpy V-notch impact tests at 20–298 K were carried out to investigate the low temperature mechanical behavior of FG- and UFG-ASS. We found that FG-ASS showed a better match among strength, plasticity and toughness at 20–298 K. The prevailing cleavage was observed in the fracture surface of UFG-ASS at 77 K and 20 K, resulting from the inhibited SIMα′ transformation and residual SIMα′. Single serration of both FG- and UFG-ASS can be divided into Stage I–IV at 20 K, in which Stage II exhibited two mini work-hardening types of three-stage and continuous-reduction. Stress cliff fall, abrupt increase of strain and instantaneous strain rate were found simultaneously during Stage III. The critical temperature to occur serrated flow increased and the serrated flow intensified with refining grain size, accompanying with that more SIMα′ and more stacking faults were detected in FG- and UFG-ASS, respectively. This paper indicated that there was a strong grain size-dependent interaction between serrated flow and SIMα′ transformation for metastable ASS, which was also in favor of understanding the grain size-dependent serrated flow of other metastable alloys.

Evaluation of Deformation and Fracture Behavior in 304L Austenitic Steel Harmonic Structures through Nanoindentation

Investigating the mechanical properties of harmonic structures during various stages of deformation, particularly after fracture, is critical. Herein, nanoindentation is used to evaluate the local deformation and fracture behavior of SUS304L steel harmonic structures. Electron backscattering diffraction is employed to observe the strain distribution in the fracture‐deformed samples, where high kernel average misorientation is evident near the shell–core boundary region. Comparison of the deformed sample reveals that the nanohardness of the shell and core regions significantly increases after deformation. This phenomenon indicates the capability of strain hardening. Furthermore, plastic inhomogeneity is observed before the fracture occurs. Strain‐induced α′‐martensite is observed in the fractured area, especially in the core region near the shell–core boundary, because of the high strain. High nanohardness is evident due to the high dislocation density and formation of strain‐induced α′‐martensite. The resulting high stress concentration can lead to void formation and crack initiation originating from the region near the shell–core boundary.

Hot Working Behavior in Multiphase Steel with Ti and V.

This study investigated the effect of hot working conditions on changes in yield stress and the softening degree in the newly developed multiphase steel with Ti and V microadditions. The research was performed on the GLEEBLE 3800 thermomechanical simulator. In order to determine the σ-ε curves, continuous compression tests were carried out. The samples were plastically deformed at temperatures from 900 °C to 1100 °C at the rate of 0.1 s−1, 1 s−1 and 10 s−1. The activation energy of the plastic deformation was 375 kJ·mol−1. The analysis of the shape and course of the curves indicated that the decrease in strain hardening was mainly the result of the continuous dynamic recrystallization process. Two-stage compression with isothermal holding of the samples was also carried out between the two stages of deformation lasting from 1 s to 50 s. The structure of primary austenite was generated using the ARPGE software. The different size of austenite grain is the result of various thermally activated processes—when increasing the strain rate from 0.1 s−1 to 10 s−1, the average grain size of the primary austenite decreases from approx. 16 µm to approx. 6 µm. The time t0.5 needed to form 50% of the austenite fraction recrystallized at 1100 °C is approx. 4 s and extends to approx. 10 s with the reduction in the plastic deformation temperature to 900 °C. The time of complete austenite recrystallization tR, which varies from approx. 50 s to approx. 90 s in the tested temperature range, lengthens even more. The obtained results make it possible to develop thermomechanical treatment technology for the production of forgings from the tested multiphase steel.

Preliminary investigation on underwater wet welding of Inconel 625 alloy: microstructure, mechanical properties and corrosion resistance

This paper aimed to investigate the feasibility of underwater wet welding for Inconel 625 alloy using Ni-based filler. The microstructure, elemental distribution, and properties of underwater wet welded Inconel 625 alloy joints were analyzed. The grain boundary type and grain growth orientation across the fusion boundary were particularly investigated. A continuous and smooth weld with no visible defects was obtained. The nickel based welds was fully austenite structure with extensive grain boundary migration. The heat-affected zone experienced obvious grain growth. A compositional transition zone with a width of 50 μm was observed across the fusion boundary. Apparent epitaxial growth across the fusion boundaries was found and the grain boundaries roughly perpendicular to the fusion boundaries are was mainly high angle grain boundaries. The density of low Σ CSL grain boundary in Inconel 625 base metal was higher than that in Ni-based weld metal. The ultimate tensile strength of underwater wet welded joints reached up to 83% that of Inconel 625 alloy. The impact toughness of HAZ was deteriorated because of the segregation of carbide and Laves phase in grain boundary. The corrosion resistance of Ni-based welds maintained at a high level based on the corrosion current density, although it was inferior to that of Inconel 625 alloy. The expected research results could enrich the underwater welding/repair technology of Ni-based alloy structures.

The effect of filler metal on microstructure and mechanical performance of GTAW inconel 625 at room and elevated temperature

The aim of this study was evaluating the influence of ER321 and ERNiCr-3 filler metals on the mechanical performance of the Inconel 625 gas tungsten arc welded joints at room and elevated temperature and correlating with the microstructure. Optical and scanning electron microscopy was conducted to characterize microstructure features at the fusion and the interfaces. Microhardness measurements, tensile test, Charpy impact test, and creep rupture test were applied to the weldments to investigate the effect of filler metal on mechanical properties of the joints. Creep rupture test was performed at 600 °C under load stresses of 150, 200 and 300 MPa. At high stress levels (300 MPa), nucleation and coalescence of voids accelerates and a ductile fracture with dimple morphology for both filler metals was seen. At lower stress level of 150 MPa, the creep deformation becomes dominant for both weldments and the voids which are mainly micro-creep voids get smaller in size and are located near the precipitates. A 100% increase in stress level (from 150 MPa to 300 MPa), reduces the creep rupture life of the ERNiCr-3 and ER321 welds by about 11 and 30 times, respectively. The results show that stringers of δ-Ferrite in inter-dendritic region of austenite appear in ER321 weldments along with some precipitates which are highly enriched in Ti and C. The formation of the unmixed zone with a different concentration gradient of the elements is observed at the Inconel 625/ER321 stainless steel interface. XRD pattern reveals the formation of (Fe2Mo) Laves phase in ER321 weld metal where the amount of Fe in the weld increases and Mo solubility decreases. ERNiCr-3 weldments show fully austenitic structure and some NbC precipitates. EDS analysis of ERNiCr-3 weld metal confirms high amount of Mo and Nb at the inter-dendritic region, whereas the dendritic core is depleted from Mo and Nb. By applying different filler metals, different fracture morphology was revealed under different loads. A non-uniform hardness profile and the minimum microhardness value of 120 HV were seen in the wide unmixed zone of ER321 weld metal due to its heterogeneous structure and also due to lack of weld metal dilution. It was shown that ER321filler metal does not give an appreciable tensile and impact properties comparing to ERNiCr-3 weldments. The tensile elongation was greater for ERNiCr-3 weldments (13.1%) than ER321 joints (10.2%) andthe average impact toughness of ERNiCr-3 weldments was about two times higher than ER321.

Microstructure and mechanical properties of Nb-alloyed austenitic CrNi steel fabricated by wire-feed electron beam additive manufacturing

We investigated the phase composition, microstructure, mechanical properties, and fracture mechanisms of Nb-alloyed austenitic stainless steel, produced by electron beam additive manufacturing (EBAM), after different regimes of the post-EBAM solid-solution treatments. As-build EBAM-produced steel has a predominantly austenitic structure, contains vermicular δ-ferrite (Vδ = 5–7%) and Nb-enriched precipitates (Laves phase Fe2Nb, FeNb and NbC). Niobium-alloying assists the stabilization of austenitic phase but does not suppress the columnar grain growth during EBAM-fabrication of austenitic stainless steels. Solid-solution treatments provide a change in morphology of δ-ferrite, a decrease in the ferrite volume fraction and the dissolution of intermetallic phases (Fe2Nb, FeNb), which all affect the tensile properties. Post-EBAM treatments contribute a decrease in the yield strength of the steel and improve the elongation-to-failure. All EBAM-produced specimens have good mechanical properties, which are comparable with those for steels produced by conventional methods. Irrespective of treatment regime, EBAM-fabricated specimens exhibit ductile transgranular fracture. For the EBAM-produced Nb-alloyed steel, the phase composition is described from the point of view solidification mode, and the effects of the precipitate hardening and solid-solution strengthening on the mechanical properties and fracture mechanisms were discussed.

Evaluating the Elemental Composition, Transformation Behavior, Crystalline Structure, and Mechanical Properties of Three 0.016-Inch by 0.022-Inch Nickel-Titanium Archwires: An In Vitro Study.

BackgroundNickel-titanium (NiTi) archwires are considered the most attractive wires during the first stage of orthodontic treatment because of their unique properties throughout several generations. This study aimed to evaluate three different NiTi wires in terms of their elemental composition, transformation behavior, crystalline structure, and mechanical properties.Materials and methodsThe study used three different groups of NiTi archwires with dimensions of 0.016 x 0.022-inch (American Orthodontics®, Sheboygan, WI, USA). The first group included six superelastic NiTi archwires (NT3-SE®), with normal force and a stable structure that was not affected by temperature changes. The second group included six heat-activated NiTi archwires activated at 25°C (Thermal Ti-D®), with moderate force and a sensitive structure to thermal changes, especially at room temperature. The third group included six heat-activated NiTi archwires activated at 35°C (Thermal Ti-Lite®), with light force and a sensitive structure to thermal changes, especially at body temperature. X-ray fluorescence (XRF) was performed to determine wire element composition, whereas differential scanning calorimetry (DSC) was performed to determine the austenite finish temperature (Af). The X-ray diffraction (XRD) analysis was used to identify the crystalline structure at room temperature, and a three-point bending test was carried out under constant temperature (37°C) with respect to the instructions of ISO15841/DIS to evaluate the mechanical properties of these wires.ResultsThe XRF analysis revealed that the superelastic NiTi archwires (NT3-SE) were composed of NiTi and chrome, whereas the heat-activated wires (Thermal Ti-D and Thermal Ti-Lite) were composed of NiTi and copper. The DSC showed the Af was at +16.84°C for the superelastic type (NT3-SE), +23°C for the heat-activated at 25°C (Thermal Ti-D), and +33.99°C for the heat-activated at 35°C (Thermal Ti-Lite). The XRD analysis identified the crystalline structure at room temperature for the superelastic type (NT3-SE) as austenite, while for thermal types (Thermal Ti-D and Thermal Ti-Lite) were a compound structure of austenite and martensite phase. Finally, the bending test showed that the highest forces were delivered from the superelastic type (NT3-SE), followed by heat-activated at 25°C (Thermal Ti-D), while the lowest forces were delivered from heat-activated at 35°C (Thermal Ti-Lite). There was no significant difference between the superelastic type (NT3-SE) and thermally activated type at 25°C (Thermal Ti-D), while there was a significant difference between the two previous types and the thermally activated type at 35°C (Thermal Ti-Lite) for all studied unloading points.ConclusionsThe thermal types of archwires (Thermal Ti-D and Thermal Ti-Lite) had lower unloading values in comparison with the superelastic type (NT3-SE). The elemental composition was different between the superelastic wires and the thermal ones. The superelastic wires were also different from the other two types in terms of crystalline structure. The three types of archwires had an activation degree located in the range of oral cavity variations.

Effect of retained austenite on the microstructure and mechanical properties of cold-rolled medium-manganese Q&P steel

ABSTRACT The effect of retained austenite stability on microstructure evolution and mechanical properties of a new cold-rolled medium-manganese steel (0.12C, 7.69Mn, 1.45Si, 2.76Al wt-%) under intercritical annealing Q&P process at different partitioning temperatures was investigated. The evolution of retained austenite was characterized and statistically analysed using SEM, TEM, EBSD, and XRD techniques. The results show that the stability of retained austenite depends to a large extent on the applied heat treatment and is mainly attributed to the retained austenite content and morphology, whose carbon content is not the main influence. More retained austenite content and the formation of heterogeneous austenite structures (granular, massive, lamellar) are observed at a partitioning temperature of 150°C. The retained austenite of different morphologies has different stabilities and can sustain the TRIP effect at different strain stages, obtaining the best comprehensive mechanical properties with 34.5% elongation and the product of tensile strength and elongation is up to 43.5 GPa·%.