Stability Of Austenite Research Articles

An alternative quenching and partitioning (Q&P) process via spheroidization

A hot-rolled Fe-CMnSiMo alloy was subjected to quenching followed by a spheroidization treatment, which resulted in a microstructure consisting of Mn enriched cementite particles dispersed in the ferrite matrix. The as hot-rolled and spheroidized material was subjected to short-duration intercritical annealing (IA) treatment followed by the full quenching and Q&P treatments. The IA treatment was such that the austenite formed as a result of dissolution of cementite particles retains its higher Mn content. The influence of this chemical inhomogeneity on martensite transformation and retained austenite stability was investigated using a dilatometer and interrupted compression experiments, respectively. The results were compared with the microstructure obtained in the absence of chemical heterogeneity by subjecting the hot-rolled material to similar heat treatment directly without the quenching and spheroidization treatment. After the IA treatment, the Ms temperature of the spheroidized material was 36 °C higher than the Ms temperature of the as hot-rolled material. Nevertheless, a higher fraction of retained austenite was stabilized in the spheroidized material after the IA treatment followed by full quenching. The SEM, TEM and DICTRA modelling results showed that cementite particles dissolve during the IA treatment of the spheroidized material and the product austenite retains its higher Mn content. Interrupted compression test results show that the retained austenite in the spheroidized material has greater stability because of its higher Mn content and small size.

Microstructure and mechanical properties of Co-added and Al-added austenitic stainless steels

The effects of Co and Co + Al addition on the microstructure, deformation-induced processes, and mechanical properties of a wrought Fe–14Cr–9Ni–3Mn–0.3C austenitic stainless steel were investigated. Whereas Al contributes to a reduced density and has a noticeable impact on the microstructure formation, Co addition could be regarded as a strategy to replace Ni, for instance to avoid Ni allergy. During tensile deformation at 300 °C, dynamic strain aging occurred in all steels but the associated increase in the work hardening rate was most obvious for the steel containing Co + Al. As the tensile test temperature was decreased to −196 °C, the yield strength of all steels increased. Furthermore, planar glide features such as deformation-induced α′-martensitic transformation became more dominant. Co and Co + Al additions reduced the Mdγ→5%α′ temperature and the amount of deformation-induced α′-martensite. The extent to which deformation-induced α′-martensite formation was suppressed was much greater upon Co + Al addition than upon Co addition alone. Accordingly, the work hardening rate at a given tensile test temperature was highest for the reference steel and lowest for the Co + Al-added steel with the most stable austenite of all steels. The highest tensile elongation of steels was reached at temperatures below their respective Mdγ→5%α′ temperature. For the reference steel, the interval between the temperature associated with maximum elongation and Mdγ→5%α′ temperature was the smallest. For the other two steels, in particular the one containing both Co and Al, the preceding interval was noticeable. This observation is likely related to the non-uniform spatial distribution of alloying elements and local variations in deformation mechanisms.

A novel quenching-electroshocking-tempering process for toughness improvement by microstructure refining and austenite stability tailoring in aviation bearing steel

In this work, the microstructural evolution and mechanical properties of aviation M50 steel under a novel quenching-electroshocking-tempering (Q-E-T) process were investigated. Microstructural observations indicate that the carbon concentration and volume fraction of retained austenite (RA) in Q-E-T specimens are higher than those in conventional quenching‐tempering (Q‐T) specimens due to the significant carbon partition induced by the introduced electroshocking treatment (EST). Meanwhile, the diameter of packets in the Q-E-T specimens is refined from 1.71 μm to 1.34 μm, which is attributed to the segmentation of blocky as-quenched RA by bainite transformation during EST. The mechanical analysis shows that an excellent combination of ultimate tensile strength and impact toughness is achieved for M50 steel subjected to the Q-E-T process. Compared with the conventional Q-T process, the impact absorbing energy of M50 steel is increased by 24.1% without any loss of tensile strength. Additionally, the thermal and mechanical stabilities of RA in the Q-E-T specimens are both improved. This work provides a novel strategy for improving the impact toughness of M50 by refining the microstructure and appropriately tailoring the stability of RA via an efficient Q-E-T process.

Significance of Retained Austenite Stability on Yield Point Elongation Phenomenon in a Hot‐Rolled and Intercritically Annealed Medium‐Mn Steel

The effect of retained austenite stability on yield point elongation (YPE) phenomenon in a hot‐rolled and intercritically annealed medium‐Mn steel is elucidated here. Furthermore, the influencing factors of austenite reversion are studied. The results indicate that the shear phase transition is suppressed by the strain hardening of prior austenite grain introduced by low‐temperature rolling, resulting in refined martensite laths with high dislocation density. The nucleation and growth of intercritical austenite are accelerated by high‐density dislocation substructures. Different characteristics of retained austenite (RA) are obtained through tailoring the rolling temperature and austenite reversion transformation (ART) process. YPE phenomenon is a result of the equilibrium between work hardening (via martensitic transformation and dislocation multiplication) and softening (by stress relaxation and dislocation glide). In the initial deformation stage, large amounts of geometrically necessary dislocations (GND) are “stored” in the RA with higher stability, and the local stress can be effectively relaxed and transferred, leading to the occurrence of YPE. The optimum properties are obtained in R760‐A630‐0.3h samples with a combination of yield strength of 918 MPa, tensile strength of 1019 MPa, and total elongation of 25.3%, which is mainly attributed to the finer ferrite matrix and sustained transformation‐induced plasticity effect.

Metallurgical Factors Affecting the Tensile Properties of Weld Heat-Affected Zone in Medium Mn Transformation-Induced Plasticity Steel

The role of retained austenite in the mechanical property changes in the medium Mn transformation-induced plasticity (TRIP) steel heat-affected zone (HAZ) was elucidated throug a comparison with conventional TRIP steel. The two steel grades had similar microstructures consisting of ferritic phases with a high fraction of retained austenite in the as-received state; however, the difference in properties after welding was significant. The microstructures of both steels were dramatically changed to a martensitic matrix in the coarse-grained HAZ region. The medium Mn TRIP steel HAZ can have a higher retained austenite fraction than the conventional one; hence, an excellent combination of tensile strength and elongation can be obtained. The decisive difference between the two steels was determined in terms of austenite stability and the initial grain size.

Quenching and partitioning (Q&P) processing of a (C+N)-containing austenitic stainless steel

This paper presents a systematic study of quenching and partitioning (Q&P) processing of a (C + N)-containing austenitic stainless steel with a special focus laid on the influence of partitioning conditions on the mechanical properties and its comparison with C-containing Q&P-treated stainless steels. Similar to C-containing stainless steels, martensite phase in the studied steel exhibited pre-precipitation phenomena such as interstitial segregation to defects below 250 °C and precipitation reactions such as the formation of paraequilibrium M3C carbides (M representing Fe and substitutional elements) above 300 °C. The interstitial partitioning between martensite and austenite and the dependence of mechanical properties on the partitioning conditions resembled those of C-containing stainless steels. With increasing partitioning temperature, interstitial enrichment in the austenite and ductility were enhanced while tensile strength decreased. Similar observations were made upon prolonged partitioning at 250 °C. In contrast, the ductility of steels partitioned at 350 °C and 450 °C barely changed with partitioning time. For partitioning at 450 °C, this was because most of the interstitial partitioning at 450 °C took place within the first 3 min of partitioning. For partitioning at 350 °C, the interstitial enrichment of fine austenite regions was completed rapidly as well and the later interstitial partitioning into coarse austenitic regions gave rise to little increase in austenite stability due to their large size. The optimal product of ultimate tensile strength and total elongation, 47 GPa%, was obtained after partitioning at 450 °C for 3 min. The sufficiency of a short partitioning to obtain superior mechanical properties is of great practical significance in the context of growing global concerns about energy demand and climate change.

Effect of thermocycling-induced stress on properties of orthodontic NiTi wires

Background/purposeIn orthodontic applications, NiTi wires are under continuous bending stress and exposed to fluctuations in temperature over long durations. The sensitivity of NiTi to temperature can have a considerable influence on its mechanical properties. This study investigated the effects of deflected NiTi wire, presented in stress-induced (detwinned) martensite microstructure, combined with thermal cycle on the microstructure and mechanical properties.Materials and methodsWe tested four types of as-received orthodontic NiTi: (1) Nitinol Classic (3 M Unitek), (2) Sentalloy (Tomy), (3) 27 °C CuNiTi (Ormco) and (4) 40 °C CuNiTi (Ormco). Each group of specimens was subjected to three different testing conditions: (1) temperature fluctuations (5000 cycles) between 5 and 55 °C, (2) continuous three-point bending force and (3) combination of thermal cycling and bending stress.ResultsThe specimens that underwent thermocycling as well as loading exhibited a substantial narrowing in stress hysteresis, which may be attributed to crystallinity lower than that of as-received NiTi wires. Reduced crystallinity can manifest in a number of imperfections, such as dislocations and internal stress, as well as a less-organized structure. Micro X-ray diffraction (XRD) analysis revealed the existence of martensite phase in Sentalloy wires subject to thermal and stress conditions. Under loading conditions, stress-induced martensite of NiTi wires exposed to temperature fluctuations of 5–55 °C also induced cyclic changes in bending stress. In a simulated intra-oral environment, the stability of austenite↔martensite transformation decreased.ConclusionThis study determined that bending stress in conjunction with repeated temperature fluctuations can greatly affect the microstructure and mechanical properties of NiTi wires.

Role of Al element in tailoring the austenite mechanical stability and tensile properties of medium Mn steels

Al is generally employed as a key alloying element to tailor the austenite mechanical stability and mechanical properties in medium Mn TRIP steels. However, the effect of Al on the austenite stability has been limitedly studied, leaving contradictory results without a self-consistent explanation. This study addressed this issue through a systematic study of medium Mn TRIP steels with compositions of 0.2C–8Mn-0/1.5/3Al (wt.%), while providing new insights into the factors influencing austenite stability and mechanical properties. It was found that the variation of the Mn distribution in austenite from non-equilibrium to near-equilibrium state and the weakened strain incompatibility between phases due to sufficient recovery of tempered martensite can be achieved with the addition of Al, which contributed to slowed strain-induced martensite transformation (SIMT) and improved austenite stability in the experimental steels annealed at low temperatures in the two-phases region, resulting in a decrease in ultimate tensile strength (UTS) and an increase in total elongation (TE). Furthermore, fully recovered tempered martensite with the hardness much lower than that of austenite was obtained in the 3Al steel annealed at a high temperature in the two-phase region, which led to a significant reduction in yield strength and thus low UTS. Meanwhile, heterogeneous strain distribution was induced by the remarkable gap in hardness between the two phases, which can be responsible for the relatively high austenite mechanical stability. The varying strain states and variation of Mn distribution in austenite can be achieved for the medium Mn TRIP steels with different Al contents, contributing to a wide range of UTS&TE combination with high strength and elongation product (≥40 GPa·%). And the elucidation of the effect of different strain states also opens a new window to adjust the austenite stability and mechanical properties of medium Mn TRIP steels.

Improving mechanical properties and corrosion resistance of 0.5 wt.% C TRIP steel by adjusting retained austenite stability and microstructural constituents

Here the effect of retained austenite stability and martensite-austenite island/fresh martensite on mechanical properties, fracture behavior, and corrosion resistance of 0.5 wt.% C TRIP steel was studied. The volume fraction of retained austenite is distinctly high at ∼ 30% in 0.5 wt.% C TRIP steel compared to the conventional TRIP steels (10–20%) while the morphologies of retained austenite are mainly film-like, mixed film-like and small blocky (0.3–0.7 μm), and large blocky (0.7–1.5 μm), respectively, in the steels after bainitic holding at 360 °C, 400 °C and 450 °C. In addition, the volume fraction of bainite decreases from 53 vol.% to 44 vol.% and 32 vol.%, and that of martensite-austenite island/fresh martensite content increases from 2 vol.% to 9 vol.% and 21 vol.% with increasing temperatures. The two-stage mechanical stability of retained austenite with large volume fraction and mixed film-like and blocky morphologies ensures stable and continuous TRIP effect. As a result, after bainitic holding at 400 °C for 30 min, the superiorly comprehensive mechanical properties are obtained for the steel, showing ultrahigh tensile strength of 1220 MPa and retaining good ductility of 44%. Moreover, the effects of multiphase on strength and ductility were discussed and the presences of martensite-austenite island/fresh martensite and blocky retained austenite lead to premature crack initiation. Crack initiation and propagation mainly happen along the interface between deformation-induced martensite and the ferritic matrix and that between M-A island/FM and the ferritic matrix. The decreasing carbon content in RA and the occurrence as well as increase of M-A island/FM can deteriorate corrosion resistance of 0.5 wt.% C TRIP steels.

TEM Characterization of Retained Austenite Stabilization on TRIP Steel

TEM Characterization of Retained Austenite Stabilization on TRIP Steel

Investigation of heat treatment strategies for improvement of production capacity of coin stamping dies

This paper presents a study on the production capacity of stamping dies for minting coins under different heat treatment processes, particularly austenite temperatures, using a vacuum furnace. In the study, the specimen material was made from Bohler K340 steel. The first heat treatment strategy included heating the specimens to a stable austenite temperature at 1080°C, then holding for 120 minutes and cooling rapidly with gas quenching media with a pressure of 5 bar. Tempered treatment was performed at 500°C. Another strategy was similar to the first one, except the heating temperature at 1060°C and double tempering at 575°C. After the heat treatment process, microstructure observation and hardness testing were carried out. Finally, the production of the die capacity testing on the coining machine was conducted. The results of the specimen surface photographs of the two heat treatment strategies show a significant difference in the microstructure, similarly with the hardness value obtained from single tempering is 61.98 HRC and from double tempering is 57.94 HRC. The production capacity test result indicates that the average capacity of first and second dies are 101,250 coins and 171,150 coins, respectively. It means that the second dies exceed the minimum standard production capacity of dies as determined by the Quality Control department, which is 150,000 coins.

Dynamic Ferrite Formation and Evolution above the Ae3 Temperature during Plate Rolling Simulation of an API X80 Steel

Thermo-mechanically controlled rolling is a technique used to produce steel strips and plates. One of the steels widely used in the production of heavy plates for application in oil and gas pipelines is API X80. The hot rolling process of this family of steels consists of applying deformation passes at high temperatures, mainly above Ae3, inside the austenite phase field. It has been shown that during deformation, the phenomenon of dynamic transformation (DT) of austenite into ferrite leads to lower hot deformation resistance within the stable austenite region. In this investigation, hot torsion simulations of an industrial rolling process under continuous cooling conditions were used to monitor the formation of ferrite by DT. Stress–strain flow curves and equivalent mean flow stresses followed by sample characterization via optical and electron microscopy showed the inevitable formation of ferrite above the Ae3. The employed 10-pass deformation schedule was divided into 5 roughing and 5 finishing passes, thereby promoting an increased volume fraction of ferrite and decreased critical strain for the onset of DT and dynamic recrystallization (DRX). A microstructural analysis confirmed the formation of ferrite from the first roughing strain until the last finishing pass. The volume fraction of DT ferrite increased due to strain accumulation, an increased number of deformation passes and as the temperature approached the Ae3, leading to a characteristic torsion texture at the end of the simulation.

Temperature dependence of tensile deformation behavior and strain hardening of lean duplex stainless steels

Two lean duplex stainless steels with different manganese contents (4 and 8 wt.%) were deformed in the tensile mode in the temperature range of 25–300 °C. The deformation-induced martensitic transformation was characterized as the main austenite deformation mechanism of 4Mn steel at room temperature. In contrast, the strain hardening behavior of 8Mn steel was regulated by the cooperation of twinning induced plasticity and transformation induced plasticity. This was justified considering the lower austenite fraction and austenite stability in 4Mn compared to the steel containing a higher manganese content. Mechanical properties such as ultimate tensile strength and uniform elongation in both steels were significantly changed as the deformation temperature was increased. Differences in chemical composition and deformation temperature led to changing deformation mode due to strain partitioning between austenite and ferrite, austenite stability, and stacking fault energy. The most important factors influencing the mechanical properties are austenite stability and stacking fault energy at elevated temperatures. It was observed that as the austenite stability increases, martensitic transformation occurs directly from austenite grain and annealing twin boundaries but does not significantly improve strength and ductility. The strain partitioning between austenite and ferrite could provide a proper condition for strain-induced martensite transformation and deformation twinning, thereby enhanced mechanical properties at elevated temperatures.

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·%.

Investigations on Dry Sliding Wear Behavior of a Wire Arc Additively Manufactured Nickel-based Superalloy

ABSTRACT Wire Arc Additive Manufacturing (WAAM) is a low-cost method of fabricating large components from advanced materials such as nickel-based superalloys. Nickel-based superalloys have varied industrial applications and suit the WAAM method due to their excellent weldability when used in conjunction with metal inert gas welding equipment used in the current study. All WAAM specimens were subjected to a standard dry sliding wear test to examine wear behavior. During the wear study, load, sliding velocity, and sliding distance were all considered for both wrought and WAAM alloy. The lowest wear was observed at a lower load and higher sliding velocity, whereas the highest wear was observed at a higher load and lower sliding velocity. Wear mechanisms and wear debris corresponding to lowest and highest wear conditions were also being investigated. The transition from lowest to highest wear rate in both alloys was attributed to the transition from abrasive to adhesive and delamination wear mechanisms. Microstructure analysis revealed that the nickel-based WAAMed alloy had an equiaxed dendritic structure, whereas the nickel-based wrought alloy had a cellulose structure. Both specimens had a stable austenite structure in the Ni-Fe-Cr phase, as determined by X-ray diffraction analysis. The X-ray diffraction line profile analysis technique by the Williamson-Hall relationship revealed that WAAM alloys have higher micro-strain, higher dislocation density, and smaller crystallite size when compared to wrought alloys. The experimental results showed a 23% increase in hardness and a considerable improvement in wear resistance in the WAAMed alloy compared to the wrought alloy.

Tensile deformation behavior related with strain-induced martensitic transformation in a duplex Fe-Mn-Al-C low-density steel

The tensile deformation behavior of a duplex Fe-Mn-Al-C low-density steel with a structure of banded ferrite and austenite has been studied in the present work. The steel exhibits a good strength-ductility match and a typical three-stage deformation behavior, under the joint contributions of transformation-induced plasticity (TRIP) effect, twinning-induced plasticity (TWIP) effect and dislocation slipping. Even though the steel has a stacking fault energy of 53 mJ∙m −2 that is suitable for the TWIP effect, it just worked at the final deformation stage. The TRIP effect played an important role during the majority of deformation process except at an initial stage, and the austenite was dramatically consumed at the later stage. This is mainly determined by an unnormal stability of austenite. For estimating the austenite stability, the relationship between the critical size of austenite grain ( D crit ) and critical temperature for strain-induced transformation ( M s σ ) was established. The calculated D crit of 12 μm reflects that the strain-induced martensitic transformation dominates the TRIP effect in the duplex steel. • Joint effect of TRIP and TWIP effects contribute to the excellent ductility. • This duplex low-density steel shows a three-stage deformation behavior. • A quantitative estimation method of austenite stability is proposed.

"Analysis of Welding Parameters on TIG with 316 Stainless Steel Alloy"

Among other applications, austenitic stainless steel is frequently used in heat exchangers, furnace and in aircraft engine parts. It has chromium and nickel in it. Austenitic stability over a wide temperature range and strong corrosion resistance are both facilitated by nickel and chromium respectively. It is typically applied to process streams that contain chloride or halide. The impact of parameters on the weld joint for 316 austenitic stainless steel welding with tungsten inert gas was investigated in this work. Welding settings influence the weld’s mechanical qualities. Determining the ideal welding circumstances for obtaining the highest tensile strength was the aim of optimization.

TRIP steels: the features of chemical composition and structure, prospects of application (overview)

This article provides a brief overview of steels prone to transformation induced plasticity (TRIP), which belong to the class of advanced high-strength steels (AHSS). The necessary set of properties in these steels is formed due to the partial preservation of supercooled austenite in the structure. The article considers the mechanism of TRIP transformation, which depends on the value of the temperature of the beginning of the martensitic transformation. It is shown that the amount and stability of supercooled austenite can be influenced by varying the temperature and time parameters of heat treatment. In addition to heat treatment, the qualitative and quantitative parameters of metastable austenite are significantly influenced by the alloying of TRIP steels, and the alloying elements themselves can be divided into several main groups (ferrite-stabilizing, increasing the stability of supercooled austenite and micro-alloying). In the final part of the article, the prospect of using TRIP steels in the aerospace industry is considered, where reducing the metal consumption of parts is a priority. It is also worth considering that the production of parts using additive technologies is widespread in the aerospace industry. Therefore, the use of TRIP steels as a material in additive manufacturing leads to the formation of a new concept for creating parts with a unique set of properties, primarily with high structural strength, light weight and the possibility of self-adaptation to extreme external exposures. The research was carried out under contract No. 13.2251.21.0107 on the topic "Investigation on 3D-printing of Advanced High Strength Steels with TRIP effect for realization of self-adapting aerospace structural elements", funded by the Ministry of Science and Education of the Russian Federation within the joint program for science and technology cooperation and implementation of joint call for Russian-Italian projects for 2021-2023.

Influence of Mn content on rolling performance of lean duplex stainless steel

ABSTRACT Cold rolling test as well as room temperature compression was carried out to investigated the influence of Mn content on the cold deformation properties of lean duplex stainless steel. The cold deformation mechanism was also discussed by observation of cold-rolled samples with cracks and analysis of the work hardening curve. The results demonstrated that the morphology and size of grains were both influenced by the Mn content. The quenched martensite content varied with Mn content, indicating that Mn plays critical role in both the difficulty of stress-induced martensite transformation of austenite grains in LDSS and the transformation process from austenite to martensite during solution treatment. LDSS with a lower Mn content exhibits lower austenite stability. After quenching, plenty of the austenite was transformed into martensite which performs much higher hardness than ferrite. With the increase in hardness difference between phases, the plasticity of the alloys decreased caused by each phase bore more uneven deformation.

Mechanical properties and wear resistance of direct energy deposited Fe–12Mn–5Cr–1Ni-0.4C steel deposited on spheroidal graphite cast iron

This study investigates the wear resistance of an Fe–12Mn–5Cr–1Ni-0.4C (high-manganese steel) produced by direct energy deposition (DED) and spheroidal graphite cast iron. The high-manganese steel was deposited on the spheroidal graphite cast iron substrate. Near the deposit/substrate interface, the graphite phases of the cast iron are completely solutionized owing to the low carbon content of the high-manganese steel. In addition, the steel was deposited with a dense microstructure without interfacial cracks owing to the strong austenite stabilization effect. The wear behavior for various wear loads and sliding speeds was studied using ball-on-disk tribology tests with counterpart balls (KS STB2 bearing steel). The results show that the wear rates of the deposited high-manganese steel and the spheroidal graphite cast iron depended more on the wear load than on the sliding speed. At a low wear load of 20 N or less, the spheroidal graphite cast iron showed a higher wear rate than the deposited high-manganese steel owing to the self-lubricating effect of graphite. When the wear load condition was 50 N, the high-manganese steel had a higher wear resistance than the cast iron owing to the work-hardening effect. Therefore, it is expected that high-manganese steels, produced by DED, with high wear resistance under a high wear load will be effective for repairing spheroidal graphite cast iron products.