Formation Of Austenite Research Articles

Influence of isothermal tempering on microstructures and hydrogen-environmentally embrittlement susceptibility of laser additively manufactured ultra-high strength AerMet100 steel

Good hydrogen-environmentally embrittlement (HEE) resistance is necessary for safe service of critical main load-bearing ultra-high strength steel (UHSS) components of advanced aircrafts in marine atmosphere environment. Controlling isothermal tempering time is one important heat treatment method to optimize HEE resistance of UHSS by designing microstructure. Influence of isothermal tempering time at 482 °C on microstructures and HEE susceptibilities of laser additively manufactured (LAM) AerMet100 steel was investigated by several material characterization methods and slow strain rate tensile (SSRT) tests in both of air and 3.5 wt% NaCl aqueous solution. Results mainly indicate that microstructure evolution behaviors of LAM AerMet100 steel successively include martensite and retained austenite decomposition, M2C carbide formation and coarsening, and film-like reverted austenite formation and thickening. Short-time tempering specimens of LAM AerMet100 steel has the highest strength-loss-index in all of the specimens due to the lowest austenite amount. The lower HEE susceptibility of LAM AerMet100 steel in long-time tempering condition is mainly corresponding to M2C carbide coarsening and film-like reverted austenite thickening. Furthermore, difference of hydrogen charging method strongly influences macro-fracture behaviors of the SSRT specimens, but micro-cracking modes of the steel in different conditions are predominantly martensite packet/plate boundary cracking. Under open-circuit potential condition, HEE cracking zone is prone to be origin from pitting corrosion sites, and the SSRT specimen in long-time tempering condition have a good ductility; in contrast, under cathodic polarization condition, HEE cracking zone is around the circumferential zones, and the tensile specimens are brittle fracture. These findings are important when considering achieving improved HEE resistance for LAM AerMet100 steel tempered at 482 °C of a longer isothermal tempering time at the strength level of interest.

Hot-rolled Al-added medium Mn steel (Fe-8Mn-2.85Al-1Si-0.2C): Microstructural evolution and tensile behavior

In this study, a low Al (<3%) added medium manganese steel has been developed through melting casting route in an open-air induction furnace. The cast steel ingot has been intercritical annealed at 720 °C after hot deformation, that includes hot forging and hot rolling treatment, respectively. Microstructure characterization coupled with X-ray diffraction (XRD) has been performed after each processing step to understand the extent of austenite formation and its subsequent transformation to ferrite/martensite. The processed steel possesses the ultimate tensile strength, yield strength and ductility of 1000±20 MPa,700±10 MPa and 18.5 ± 0.5, respectively. To understand the possible cause of enhanced strength and deformation mode rietveld analysis has been carried out. It reveals that developed (final processed) steel possesses ∼18% (volume%) of austenite that subsequently reduced to ∼6.139% after tensile testing, indicating significant transformation of austenite to martensite (continuous and enhanced TRIP effect). The morphological characteristic of martensite in developed steel has been investigated using transmission electron microscopy (TEM). It reveals lath morphology of martensite due to low carbon content. The TEM and 3D atom probe analysis reveals the precipitation of intergranular type kappa (k) carbide in intercritically annealed specimens due to probable segregation of C and Al. However, volume fraction of the same is low. The SFE of the steel has been found to be ∼16 mJ m− 2, calculated by the modified Olson and Cohen method that also concludes TRIP is the preferred deformation mechanism in the developed steel.

Austenite Nucleation and Growth as a Function of Starting Microstructure for a Fe–0.15C–5.56Mn–1.1Si–1.89Al Medium‐Mn Steel

The effects of starting microstructure and intercritical annealing temperature on the phase‐transformation kinetics and microstructural evolution of a prototype Fe–0.15C–5.56Mn–1.1Si–1.89Al medium‐Mn third‐generation advanced high‐strength steel are examined. The starting microstructures comprise 1) an as‐received cold‐rolled (CR) microstructure containing a significant fraction of ferrite and tempered martensite and 2) an austenitized and quenched martensite–ferrite (MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing are proposed. For the CR starting microstructure, austenite can nucleate at ferrite/cementite interfaces, and at ferrite grain boundaries. In the case of the MF starting microstructure, which contains thin films of interlath retained austenite (RA), austenite forms on the martensite lath boundaries or grows directly from the existing interlath RA. The studies are interpreted using the DICTRA module of Thermo‐Calc. At 665 °C, the austenite reversion kinetics of the MF samples are faster than the transformation kinetics in the CR structure. At higher temperatures where most of the cementite has dissolved, the austenite fraction in both starting microstructures is very similar.

Simultaneous enhancement of strength-ductility via multiple precipitates and austenite in a novel precipitation-hardened martensitic stainless steel

In this study, the advantages of the novel precipitation-hardened martensitic stainless steel (abbreviated as novel steel) in terms of an excellent strength–ductility balance are fully elucidated utilizing various methods and tests. The novel steel increases the yield and ultimate tensile strength from 1480 ± 20 MPa and 1500 ± 18 MPa to 1530 ± 15 MPa and 1650 ± 20 MPa, respectively, and simultaneously improves the elongation from 9.7 ± 0.5% to 17.8 ± 1.1% over that of conventional PH13–8Mo steel. Specifically, the novel steel is mainly strengthened and toughened by three factors, including fine grain strengthening, precipitation strengthening of multiple precipitates (Cu, NiAl) and the TRIP effect. The contribution of fine grain strengthening is quantitatively evaluated through the Hall–Petch relationship. Meanwhile, the value of strengthening contributions by multiple precipitates are quantitatively evaluated through the shearing or Orowan mechanism. Significantly, compared with conventional PH13–8Mo steel, the addition of Cu leads to the formation of multitudinous metastable austenite in the novel steel. The TRIP effect can be generated by metastable austenite upon applied loading conditions. An accessible pathway is found to exploit high-performance stainless steel.

Influence of Vanadium Micro-Alloying on the Microstructure of Structural High Strength Steels Welded Joints.

The inter-critically reheated grain coarsened heat affected zone (IC GC HAZ) has been reported as one of the most brittle section of high-strength low-alloy (HSLA) steels welds. The presence of micro-alloying elements in HSLA steels induces the formation of microstructural constituents, capable to improve the mechanical performance of welded joints. Following double welding thermal cycle, with second peak temperature in the range between Ac1 and Ac3, the IC GC HAZ undergoes a strong loss of toughness and fatigue resistance, mainly caused by the formation of residual austenite (RA). The present study aims to investigate the behavior of IC GC HAZ of a S355 steel grade, with the addition of different vanadium contents. The influence of vanadium micro-alloying on the microstructural variation, RA fraction formation and precipitation state of samples subjected to thermal cycles experienced during double-pass welding was reported. Double-pass welding thermal cycles were reproduced by heat treatment using a dilatometer at five different maximum temperatures of the secondary peak in the inter-critical area, from 720 °C to 790 °C. Although after the heat treatment it appears that the addition of V favors the formation of residual austenite, the amount of residual austenite formed is not significant for inducing detrimental effects (from the EBSD analysis the values are always less than 0.6%). Moreover, the precipitation state for the variant with 0.1 wt.% of V (high content) showed the presence of vanadium rich precipitates with size smaller than 60 nm of which, more than 50% are smaller than 15 nm.

Austenite Formation and Manganese Partitioning during Double Soaking of an Ultralow Carbon Medium‐Manganese Steel

Double soaking (DS) is a thermal processing route intended to produce austenite–martensite microstructures in steels containing austenite‐stabilizing additions and consists of intercritical annealing (primary soaking), followed by heating and brief isothermal holding at an increased temperature (secondary soaking), and quenching. Herein, experimental dilatometry during DS of a medium‐manganese (Mn) steel with nominally 7 wt% Mn and an ultralow residual carbon concentration, in combination with phase‐field simulations of austenite formation during secondary soaking, is presented. The feasibility of maintaining heterogeneous Mn distributions during DS is demonstrated and insight is provided on the effects of the secondary soaking temperature and prior Mn distribution on the ferrite‐to‐austenite phase transformation during the secondary soaking portion of the DS treatment.

Effects of N2 content in shielding gas on microstructure and toughness of cold metal transfer and pulse hybrid welded joint for duplex stainless steel

The influences of N2 content in shielding gas on microstructure and impact toughness of different zones in cold metal transfer and pulse (CMT-P) hybrid welded joint of duplex stainless steel (DSS) were systematically studied. The results showed that the N2-supplemented in shielding gas significantly facilitated austenite formation (weld root: 39.9%→41.2%, weld filler: 40.5%→43.7%, and heat affected zone (HAZ): 36.4%→39.6%). However, when the N2 content in the shielding gas exceeded 4%, there was no significant change in austenite content because of reaching the solubility limit of N atoms. In addition, γ2 precipitated both in the weld root and HAZ but not in the weld filler, which cannot be inhibited by N2 addition in the shielding gas. Besides, a great number of Cr2N and dislocations in addition to γ2 formed in the HAZ, and the content of Cr2N and dislocation significantly increased with the increase of N2 content in the shielding gas. Furthermore, the HAZ exhibited the lowest toughness in comparison with other zones. In addition, with the increase of N2 content in the shielding gas from 0% to 6%, the toughness increased first and then decreased, and reached the maximum when 4% N2 added to the shielding gas as (weld root: 122.0 J/cm2, weld filler: 135.0 J/cm2, and HAZ: 91.8 J/cm2). According to detailed microstructure analysis and toughness level, Ar+4% N2 was recommended as the shielding gas to join DSS by using CMT-P welding technique.

Study on Prior Austenite Grain Size and Softening Fraction in Governing Microstructure and Mechanical Behavior of Medium Mn Steel after Intercritical Annealing

The effects of the prior austenite grain (PAG) size and softening fraction on the microstructure and mechanical properties of 0.15C–7Mn steel after intercritical annealing (IA) are investigated. The hot‐forged alloy is subjected to double‐hit compression under different conditions to obtain different PAG sizes and softening fractions. Comparing the samples with the maximum (67 μm) and minimum (33 μm) PAG sizes, different grain morphology and size of reversed austenite, and carbide re‐dissolution degree are noted after IA. The ultimate tensile strength (UTS), total elongation (TE), and product of UTS and TE increase linearly with decreasing PAG size. The decrease in PAG size increases the nucleation rate and volume fraction of the retained austenite. The softening fraction affects the morphology of the microstructure and formation of carbide particles after IA, and the austenite formation kinetics. In particular, a smaller PAG size and higher softening fraction can yield austenite grains with more suitable mechanical stability and higher volume fraction, which is conducive to the transformation‐induced plasticity effect and the improvement of mechanical properties.

Effect of in-situ Ni interlayer on the microstructure and corrosion resistance of underwater wet 316L stainless steel laser cladding layer

Underwater wet laser cladding technology as a new online repair technology, has a wide application prospect in the field of marine engineering. In this work, the underwater wet laser cladding layers of 316L stainless steel with in-situ Ni interlayer on EH40 were successfully manufactured using the underwater wet laser cladding technology. The effect of the in-situ Ni interlayer on the elemental distribution, microhardness, properties and interfacial characteristics of the underwater wet cladding layer was investigated. The in-situ Ni interlayer formed a diffusion buffer between the substrate and cladding layer to mitigate the Fe, Cr and C element diffusion and increased the Ni content in the cladding layer. The addition of the in-situ Ni interlayer promoted the formation of austenite. The wear resistance of the underwater wet laser cladding layer with in-situ Ni interlayer was higher than that of the in-air laser cladding layer, and the main wear mechanism was abrasive wear. In addition, the corrosion resistance of the underwater wet laser cladding layer was improved effectively by applying the in-situ Ni interlayer, and the corrosion resistance basically reached the same level with the in-air laser cladding layer. Thus, the in-situ Ni interlayer has guiding significance for the underwater wet laser cladding technology of Fe-based materials.

Influence of the thermal cycle on microstructure formation during direct laser deposition of bainite-martensitic steel

The article presents the formation of structure and mechanical properties of bainite-martensitic steel in the process of exposure to a laser source. By varying the inter-pass temperature, the thermal cycles take place in a different temperature range, at which the formation of martensite and metastable austenite occurs. The formation of residual metastable austenite has a significant impact on the mechanical properties and performance of the product. During continuous growth, the process of phase transitions takes place in the high-temperature region. In the high-temperature region and slow cooling, bainite ferrite forms. The ferritic structure leads to brittle fracture at negative temperatures, which is unacceptable in cold-resistant steels. When specimens are deposition with a pause, the formation of the structure occurs in the low-temperature region, forming mainly bainite-martensitic structures. The resulting properties are quite high, but anisotropy is also preserved.

Influence of annealing time on the microstructure and properties of additively manufactured X2CrNiMoN25–7–4 duplex stainless steel: Experiment and simulation

Duplex stainless steels (DSS) manufactured additively using the powder bed fusion – laser beam/metal (PBF-LB/M) process exhibit a predominantly ferritic microstructure due to the process inherent ultrahigh cooling rates. In this microstructural constitution, the high potential of a balanced two-phase ferritic-austenitic microstructure typical for DSS is not exploited, which is why these steels are subsequently heat-treated. The present study focuses precisely on this heat treatment considering PBF-LB/M-processed X2CrNiMoN25–7–4 DSS solution annealed at different times and aims to explain the influence of the as-built microstructure on the phase formation during solution annealing as a function of heat-treatment time and how different annealing times affect the mechanical and corrosive properties. The as-built microstructure was investigated using electron backscattered diffraction (EBSD), X-ray diffraction (XRD), and atom probe tomography (APT) and the subsequently solution annealed ones using EBSD, XRD, secondary electron microscopy (SEM), energy dispersive (EDS) and wavelength dispersive X-ray spectrometry (WDS). Additionally, the results were compared to DICTRA® simulations. The corrosive properties were examined by potentiodynamic measurements in 0.5 molar sulfuric acid, whereas the mechanical properties were characterized by tensile testing. The ferritic microstructure in the as-built condition is characterized by a supersaturation with nitrogen, accompanied by the formation of nanoscale Cr2N. During heat treatment, these precipitates serve as nucleation sites for austenite formation, so that a balanced, ferritic-austenitic microstructure is already achieved after 3 min. Compared to longer annealing times (5 and 60 min), the highest strength and corrosion resistance is obtained with an annealing time of 3 min, as longer times result in grain coarsening.

Electrochemical behavior of a spray formed Cu-Al-Ni-Mn shape memory alloy

The electrochemical properties of a Cu-Al-Ni-Mn shape memory alloy (SMA) processed by spray-forming were evaluated by electrochemical impedance spectroscopy in an aqueous solution of 3.5% w/v NaCl. The SMA studied was hot rolled and heat-treated at 250 °C and 550 °C. The XRD analysis showed the austenite formation after heat treatment at 550 °C. EDS analysis identified an enrichment of aluminum and manganese on the surface of SMA after immersion in a saline solution, since they are the most active elements, whose oxides have the most negative formation free energies. The impedance of the untreated and heat-treated at 550 °C alloy increased two magnitude orders as the immersion time increases up to 286 h due to the formation of protective corrosion products. The SMA heat-treated at 250 °C showed the lowest corrosion resistance after 286 h of immersion in a saline solution.

Formation of deformation induced martensite in fatigued 316L steels manufactured by selective laser melting (SLM)

Destabilization of originally fully austenitic structure and formation of DIM (deformation induced martensite) were studied in SLMed 316L steels fabricated using different manufacturing systems and fatigued at room temperature in as-built state in two laboratories (TU Kaiserslautern and IPM Brno). Several microscopic techniques were adopted to reveal the distribution and morphology of DIM in the volume of fatigued steels. Results of microstructure investigations are confronted with the ferritescope measurements taken at different locations of fatigued specimens. Some factors which may impact the DIM formation in fatigued SLMed 316L steel are discussed.

Thermodynamic Simulation of Solidification of Ti-Containing Steels with Consideration for Possibility of Peritectic Transformation and Second Phase Precipitation

An algorithm is proposed for predicting the phase composition of titanium-containing steels after solidification. The approach is based on thermodynamic calculations and provides for crystallization through the formation of ferrite and austenite, as well as a peritectic reaction. The algorithm takes into account the possibility of precipitation of TiN, TiS, MnS and TiC0.5S0.5 from the liquid phase upon crystallization. Two possible behaviors of ferrite upon crystallization are considered: frozen and fast diffusion of elements in the metal sublattice of this phase. Calculations illustrating the operation of the proposed algorithm have been performed.

Regularities of structure and texture development in austenitic steel during cold rolling and heat treatment

The paper studies the regularities of the structure and texture formation in austenitic steel AISI 304 under cold rolling and subsequent heat treatment. Regularities of structure and texture formation depending on the rolling type and deformation degree were established. The ratios of main texture components, the volume fractions and the distortion of the martensite's crystalline lattice were analyzed. It is shown that mechanisms of the austenite martensite phase transformation in the course of cold rolling may change depending on various deformation schemes, as well as mechanisms of the reverse phase transformation variate under subsequent annealing. Herewith depending on the rolling type and deformation degree, the crystallographic texture formation of the nucleating austenite under the heat treatment can have single component or multi-component character and follows different orientation relations. The more scattered martensite texture after deformation results in the formation of fine-dispersed austenite, which hardens the material keeping a sufficient level of plasticity. The work was carried out with the financial support of&nbsp;the Ministry of Science and Higher Education of Russian&nbsp;Federation (Agreement No. 075-15-2021-1352).

Analyzing the effect of inoculant addition on the solidification of ductile cast irons using thermal analysis

PurposeThe final properties of ductile iron are decided by the inoculant processing while pouring the melt. The shape and size of nodules generated during solidification are of paramount importance in solidification of ductile cast iron. The purpose of this study is to examine the effect of different inoculant addition on the solidification of ductile cast iron melt through thermal analysis.Design/methodology/approachThermal analysis has recently grown as a tool for modeling the solidification behavior of ductile cast irons. Iron properties will be predicted by analyzing the cooling curve patterns of the melts and predicting the related effectiveness of inoculant processing. In this study, thermal analysis is used to evaluate the need for inoculation.FindingsThe amount and type of inoculation will affect the amount of undercooling during the solidification of ductile cast iron. It is found that the addition of 0.1 to 0.4 Wt.% inoculant lowers the austenite dendrite formation starting temperature while increasing the eutectic freezing temperature. Microstructure analysis revealed that the addition of inoculation increases the nodule count from 103 to 242 nodules. The beneficial effects of inoculation are sustained by an improved graphitization factor, which shows the formation of graphite nodules in the second phase of the eutectic reaction.Originality/valueThe inoculation treatment has improved metallurgical occurrences such as carbide to graphite conversion, graphite microstructure control, graphite nodule count at the start of solidification and the last stage of solidification, which determines the soundness of casting. The foundry industry can follow these steps for monitoring the solidification of ductile iron castings.

Effect of Cu on the Microstructure and Mechanical Properties of a Low-Carbon Martensitic Stainless Steel.

Reversed austenite is of vital importance in low-carbon martensitic stainless steel because it improves impact toughness. However, a proper amount of reversed austenite is obtained by tempering at a critical temperature, which reduces the strength of the steel. Therefore, how to improve strength-toughness matching is an important problem. Copper (Cu) is an effective strengthening element in steels. However, there is little in-depth discussion on the role of Cu on the microstructure and mechanical properties of low-carbon martensite steel. In this work, the effect of different Cu content on the reversed austenite formation, tensile strength, and impact toughness of a low-carbon martensitic stainless steel (0Cr13Ni4Mo) was systematically investigated through use of a transmission electron microscope (TEM), transmission Kikuchi diffraction (TKD), atom probe tomography (APT), and other characterization methods and mechanical property tests. The results showed that the addition of Cu decreased the phase transition temperatures of martensite and austenite and increased the volume fraction of the reversed austenite. APT results indicated that Cu-rich clusters first formed with alloying elements such as ferrum (Fe) and nickel (Ni) and then grew to be precipitates through rejection of the alloying elements. The Ni atoms diffused towards the interface between the precipitates and the martensite matrix, which provided heterogeneous nucleation sites for the reversed austenite. Cu precipitations strengthened tensile strength during tempering. However, it generated temper brittleness in the steel at a tempering temperature of 450 °C, resulting in the impact energy of the 3Cu-steel being only 7 J. A good combination with higher tensile strength (863 MPa) and ductility (192 J) was obtained when tempering at 600 °C in the presence of Cu-rich precipitates and a sufficient volume fraction of the reversed austenite. The results provide guidance for the design of steels with reversed austenite and Cu and promote the development of high-strength and high-toughness steels.

Austenite Formation in the Oxidized Layer of Ultra-High-Strength 13Ni15Co10Mo Maraging Steel

Maraging steels are precipitation hardening alloys that can achieve an ultra-high yield strength (~3 GPa), however associated with low toughness. During exposure to high temperatures, an oxidation process occurs on the surface of these steels, generally, the oxides formed are hematite and/or magnetite. The aim of this study was to investigate oxidation on a maraging 13Ni15Co10Mo at annealing temperature of 900 °C. The bulk microstructure was investigated by several complementary techniques and the oxidized surface was characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). The results showed that the bulk microstructure, at annealed condition, consists of a lath martensite with a hardness of round 400 HV. The most external and oxidized surface contains the oxides hematite, magnetite and kamiokite. Finally, the presence of austenite was detected in the first 2 μm below the surface. Chemical microanalysis indicated that the austenite is stable at room temperature in this region due a composition gradient that makes this region rich in nickel and cobalt. The composition gradient is due atom diffusion during oxides formation. Austenite near to the surface is very convenient as it could avoid crack initiation and propagation, improving toughness.

Accelerated design of chromium carbide overlays via design of experiment and machine learning

Chromium carbide overlays with MC-type primary carbides show a significantly lower cracking susceptibility during welding. However, optimizing the composition and improving the wear-resistant properties is difficult by traditional one-variable-at-a-time methods. Here we propose a method using design of experiments and machine learning to accelerate this process. A closed-loop design process was developed by using existing data to predict future experiments and suggest new ones. The influence of Nb, V, and Ti on the overlay hardness was established in an active database. XRD analysis shows that Nb would promote the formation of austenite in the matrix while Ti would foster the formation of martensite. The efficacy of this method is demonstrated by predicting the simultaneous effects of the carbide formers on overlay hardness, which highlights the possibilities of using machine learning in the alloy design of chromium carbide overlays.

Effect of repeated laser scanning on the microstructure evolution of carbon-bearing martensitic steel manufactured by laser powder bed fusion: Quenching-partitioning drives carbon-stabilized austenite formation

Carbon-bearing martensitic steels have gained interest for their use in aerospace and defense applications due to their ultra-high strength and reasonable ductility. In addition, additive manufacturing (AM) is an effective manufacturing process for a high degree of control over geometry. Microstructure evolution during additive manufacturing should be importantly considered because it is closely related to the formation of defects, mechanical properties, and conditions of post-heat treatments. This study investigated the microstructure evolution of carbon-bearing martensitic steel during laser powder bed fusion (LPBF) to identify the effect of rapid solidification, solid-state transformation, and in-situ heat treatments on the formation of the as-built microstructure. As a result of investigation of a reconstructed prior austenite map by the Kurdjumov-Sachs orientation relationship, it was confirmed that the austenite is generated mainly at the columnar, martensite block, and lath boundaries. A Scheil-Gulliver model represented that carbon and molybdenum are segregated to the columnar boundary, and this could be the main reason why austenite showed high fraction at room temperature. The carbon concentration in the austenite is much higher than that of the martensite interior, which is significantly caused by carbon partitioning derived by repeated laser scanning. A constrained carbon equilibrium model was used to estimate the final fraction of austenite, and the results showed that the characteristics of the as-built microstructure can be reasonably estimated. The microstructural evolution mechanism introduced here is expected to allow successful LPBF of carbon-bearing martensitic steels.