Austenite Research Papers - Academia.edu (original) (raw)

AISI 316 austenitic stainless steels have been plasma nitrided using a dc glow discharge unit in order to investigate the influence of gas composition on microstructure and corrosion behavior of treated samples. Corrosion properties of... more

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- Materials Engineering, Condensed Matter Physics, Microstructure, Corrosion

Plastic deformation of highly alloyed austenitic transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of "-martensite within the original austenite. The "-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes "-martensite embedded in the austenitic matrix via clustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in the DIFFaX routine; the faulted austenite and "-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction. research papers J. Appl. Cryst. (2011). 44, 779-787 Stefan Martin et al. Stacking fault model of "-martensite 785

- by Daniel Simek
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- Engineering, Mathematical Sciences, Physical sciences, Plastic deformation

Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their mi-crostructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.
The main advantages of additive manufacturing (AM) technologies
of metallic parts compared to conventional synthesis and shaping pro-
cesses lie in their ability to produce complex and/or customized parts
with a short lead time, albeit in relatively low numbers. These advan-
tages are exploited for example when patient-specific implants are
produced, when complex, structurally optimized parts lead to
performance-critical weight savings, or when AM is used for the repair
of expensive metallic jet engine parts. The alloys usually envisaged in
these applications are biocompatible, high temperature and lightweight
materials such as Ti-, Ni-, Al- and Mg-based alloys. Consequently, re-
views of alloys in AM have typically focussed on these materials [1–3],
with two notable exceptions which do discuss steels, however with a
different focus than the present review [4,5]. Yet, the most successful of
all alloy families since the dawn of the iron-age 3000 years ago, namely
steel, has received relatively little attention with respect to providing a holistic view of the interplay of alloy design, microstructure, properties
and AM processing.

- by Dierk Raabe
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- Microstructure, 3D printing, Additive Manufacturing, Solidification

The primary goal of this investigation was to create austempered ductile cast iron (ADI) with a fully ferritic microstructure without compromising its mechanical properties. This was achieved by applying a novel heat treatment process. This process consists of austempering and subsequent isothermal tempering below the inter critical (A 1 ) temperature of ductile cast iron. Ductile cast iron samples were initially austenitized at 927 • C (1700 • F) for 2 h and then austempered at three different temperatures 260 • C (500 • F), 316 • C (600 • F) and 385 • C (725 • F) and finally isothermally tempered at 484 • C (900 • F) for 2 h. This resulted in a fully ferritic microstructure. The effect of this tempering on the physical and mechanical properties of the material was examined and compared with conventionally processed ADI. Tests results show that when ADI is austempered at 260 • C (500 • F) and subsequently tempered at 484 • C (900 • F) it has significantly better mechanical properties than the samples initially austempered at other two temperatures (i.e. 316 • C (600 • F) and 385 • C (725 • F)).

- by Ronald Adi
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- Materials Engineering, Mechanical Engineering, Heat Treatment, Ferrite

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by Xray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure-austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)z+x, whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases-ferrite and Zr(Fe,Cr,Ni)2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)z+x and Zr2(Ni,Fe), whereas alloys with 83 and 92 wt pet Zr contain three phases-a-Zr, Zr2(Ni,Fe), and Zr(Fe,Cr,Ni)z+x. Fe3Zr-type and Zr3Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.

- by Sean McDeavitt
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- Materials Engineering, Mechanical Engineering, Materials Science, Microstructure

The importance of various form of heat treatment operations on medium carbon steel in order to forester the problem that may arise in making a wrong choice of these steel materials or faulty heat treatment operations which may give rise to serious disruption in terms of human safety, higher cost and untimely failure of the machine components is of great concern. The mechanical properties such as ductility, toughness, strength, hardness and tensile strength can easily be modified by heat treating the medium carbon steel to suit a particular design purpose. Tensile specimens were produced from medium carbon steel and were subjected to various forms of heat treatment processes like annealing, normalizing, hardening and tempering. The stiffness, ductility, ultimate tensile strength, yield strength and hardness of the heat treated samples were observed from their stress-strain curve. The value of the yield strength (σ y ) was observed to be higher for the tempered specimen possibly as a result of the grain re-arrangement, followed by the hardened, normalized and annealed specimens. The value of the ultimate tensile strength (σ u ) were also observed to be in the order; hardened> tempered>normalized>annealed.

- by Tajudeen Ajiboye
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- Mechanical Engineering, Civil Engineering, Materials Science, Ductility

A model is developed to describe the endpoint of carbon partitioning between quenched martensite and retained austenite, in the absence of carbide formation. The model assumes a stationary a/g interface, and requires a uniform chemical potential for carbon, but not iron, in the two phases, leading to a metastable equilibrium condition identified here as "constrained paraequilibrium" or CPE. The model is explained with example calculations showing the characteristics of the constrained paraequilibrium condition, and applications are discussed with respect to new microstructures and processes, including a new "quenching and partitioning," or Q&P process, to create mixtures of carbon-depleted martensite, and carbon-enriched retained austenite. Important new implications with respect to fundamental elements of the bainite transformation are also discussed.  A new process concept, "quenching and partitioning" (Q&P) has been proposed recently for creating steel microstructures with retained austenite. The process involves quenching austenite below the martensitestart temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature. The process concept is reviewed here, along with the thermodynamic basis for the partitioning treatment, and a model for designing some of the relevant processing temperatures. These concepts are applied to silicon-containing steels that are currently being examined for low-carbon TRIP sheet steel applications, and medium-carbon bar steel applications, along with a silicon-containing ductile cast iron. Highlights of recent experimental studies on these materials are also presented, that indicate unique and attractive microstructure/property combinations may be obtained via Q&P. This work is being carried out through a collaborative arrangement sponsored by the NSF in the USA, CNPq in Brazil, and the EPSRC in the United Kingdom.

- by Bruno De Cooman
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- Materials Engineering, Mechanical Engineering, Carbon, Gibbs Free Energy

An investigation was carried out to examine the influence of austempering temperature on the microstructure and mechanical properties of a high-carbon (1.00%), high-silicon (3.00%) and high-manganese (2.00%) cast steel. Cylindrical tensile specimens and compact tension specimens were prepared from this cast steel according to ASTM standards and were given four different austempering heat treatments to produce different microstructures. The tensile properties and fracture toughness of these materials were studied at room temperature in ambient atmosphere. Test results indicate that maximum fracture toughness is obtained in this steel when the microstructure contains very high austenitic carbon (X C ). g g ᮊ

The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution during the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed.

Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (l phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations,

In an Fe-9 at.% Mn maraging alloy annealed at 450°C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5-15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.

The stacking fault and interfacial energies of three transformation-and twinning-induced plasticity steels (TRIP/TWIP) (Fe-22/25/ 28Mn-3Al-3Si wt.%) were determined by experimental and theoretical methods. Analysis of Shockley partial dislocation configurations in the three alloys using weak-beam dark-field transmission electron microscopy yielded stacking fault energy (SFE) values of 15 ± 3, 21 ± 3 and 39 ± 5 mJ m À2 for alloys with 22, 25 and 28 wt.% Mn, respectively. The experimental SFE includes a coherency strain energy of $1-4 mJ m À2 , determined by X-ray diffraction, which arises from the contraction in volume of the stacking fault upon the facecentered cubic (fcc) to hexagonal close-packed (hcp) phase transformation. The ideal SFE, computed as the difference between the experimental SFE and the coherency strain energy, is equal to14 ± 3, 19 ± 3 and 35 ± 5 mJ m À2 , respectively. These SFE values were used in conjunction with a thermodynamic model developed in the present work to calculate the free energy difference of the fcc and hcp phases and to determine a probable range for the fcc/hcp interfacial energy in the three Fe-Mn-(Al-Si) steels investigated. In addition, the interfacial energies of three Fe-18Mn-0.6C-0/1.5(Al/Si) TWIP and five Fe-16/18/20/22/25Mn binary alloys were also determined from experimental data in the literature. The interfacial energy ranged from 8 to 12 mJ m À2 in the TRIP/TWIP steels and from 15 to 33 mJ m À2 in the binary Fe-Mn alloys. The interfacial energy exhibits a strong dependence on the difference in Gibbs energy of the individual fcc and hcp phases. Accordingly, an empirical description of this parameter is proposed to improve the accuracy of thermodynamic SFE calculations.

Carbon partitioning between ferritic and austenitic phases is essential for austenite stabilization in the most advanced steels such as those produced by the quenching and partitioning (Q&P) process. The atomistic analysis of the carbon partitioning in Q&P alloys is, however, difficult owing to the simultaneous occurrence of bainite transformation, which can also contribute to carbon enrichment into remaining austenite and hence overlap with the carbon partitioning from martensite into austenite. Therefore, we provide here a direct atomic-scale evidence of carbon partitioning from martensite into austenite without the presence of bainite transformation. Carbon partitioning is investigated by means of atom probe tomography and correlative transmission electron microscopy. A model steel (Fe–0.59 wt.% C (2.7 at.% C)–2.0 wt.% Si–2.9 wt.% Mn) with martensite finish temperature below room temperature was designed and used in order to clearly separate the carbon partitioning between martensite and austenite from the bainite transformation. The steel was austenitized at 900 C, then water-quenched and tempered at 400 C. Approximately 8 vol.% retained austenite existed in the as-quenched state. We confirmed by X-ray diffraction and dilatometry that austenite decomposition via bainite transformation did not
occur during tempering. No carbon enrichment in austenite was observed in the as-quenched specimen. On the other hand, clear carbon enrichment in austenite was observed in the 400 C tempered specimens with a carbon concentration inside the austenite of 5–8 at.%. The results hence quantitatively revealed carbon partitioning from martensite to austenite, excluding bainite transformation during the Q&P heat treatment.

- by francesco chichi
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- XRD, Austenite, Retained Austenite

We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered k-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L912 ordered precipitates (so called k-carbides) resulting in high strength (yield stress y1?0 GPa) and ductile (elongation to fracture y30%) steels. Novel insights into dislocation–particle interactions in a Fe–30.5Mn–8.0Al–1.2C (wt-%) steel strengthened by nanoscale k-carbides are discussed.

- by Dierk Raabe
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- Nanostructure, Mechanical twinning, Twinning, TWIP steels

The kinetics of diffusional transformations in steels depend on a large number of parameters, such as grain geometry, nucleation behavior, and growth kinetics. An essential step in the process is the lattice transformation, the kinetics of which can be expressed by the mobility of the interface between the matrix and the newly forming phase. To study the intrinsic mobility of the austenite-ferrite interface, i.e., the mobility of the interface in the absence of long-range diffusion, differential thermal analysis (DTA) experiments were performed on high-purity dilute Fe-Co and Fe-Cu alloys. The data obtained were analyzed using a three-dimensional geometry, based on a tetrakaidecahedron model for the austenite grain and a recent interface mobility model. The observed transformation behavior is described accurately, with the interface mobility being composition dependent.

- by Jilt Sietsma
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- Materials Engineering, Mechanical Engineering, Kinetics, Ferrite
- by Marcel Somers
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- Engineering, Materials, Diffusion, Kinetic

High-Chromium White Cast Iron is a material highly used in mining and drilling shafts for oil extraction, due to its high wear resistance. However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. This paper studies the effects of different cooling media after a destabilization treatment on the microstructure, hardening and abrasion resistance behaviors of a hypoeutectic high chromium white cast iron. The results show that although air cooling followed by immersion in CO 2 can effectively reduce the retained austenite, this is not enough to transform completely the retained austenite into martensite. The low retained aus-tenite percentages improve bulk hardness, but they decrease the abrasion resistance of the high chromium cast iron. The best combination of hardness and wear resistance was found in the samples cooled in air, due to the percentage of retained austenite and a moderate precipitation of chromium carbide.

- by OSCAR FABIAN HIGUERA COBOS
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- Hardness, Wear Testing, Chromium Carbides, High Chromium White Irons

The influence of initial microstructure on discontinuous dynamic recrystallization (DDRX) has been investigated by using high purity and ultra high purity austenitic stainless steels with various initial grain sizes. After uniaxial compression tests at constant strain rates and various temperatures, the steady state microstructure or the state corresponding to the maximum strain (e = 1) attained in the test was analyzed by scanning electron microscopy aided with automated electron back scattering diffraction. Recrystallized grain size d rec and twin boundary fraction f TB measurements were carried out. The mechanical behavior was also investigated by comparing experimental stress-strain curves with various initial grain sizes. DDRX kinetics was described by the classical Avrami equation. It was concluded that larger initial grain sizes promoted a delay in the DDRX onset in the two alloys. It was also observed that the softening process progressed faster for smaller initial grain sizes. The effect of initial grain size is larger in the HP material and becomes more pronounced at low temperature.

Cryoprocessing, a supplementary process to conventional heat treatment process, is the process of deep-freezing materials at cryogenic temperatures to enhance the mechanical and physical properties of materials being treated. The execution of cryoprocessing on cutting tool materials increases wear resistance, hardness, and dimensional stability and reduces tool consumption and down time for the machine tool set up, thus leading to cost reductions. The effects of cryoprocessing on tool steels and carbides, metallurgical aspects including reduced amount of retained austenite, precipitation of η-carbides, phase change in carbides, improvement in wear resistance, and applications are reviewed for manufacturing industry. Although it has been confirmed that cryogenic processing can improve the service life of tools, the degree of improvement experienced and the underlying mechanism remains ambiguous. The steps involved in cryoprocessing are critical enough to account for the significant incongruity in posttreated performance.

- by Simranpreet Gill
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- Engineering, Heat Treatment, Mathematical Sciences, The

A modeling approach to account for the effect of deformation on the austenite to ferrite transformation is described. In this approach the stored energy of deformation resulting from the formation of a dislocation substructure is considered to result in an elevation of the driving force for transformation. A scheme which accounts for the competition between softening and transformation processes is also described. Model data illustrating the effects of discrete dislocations and cell structures, with and without concurrent recovery are presented. Comparison of modeled and experimental transformation curves indicates that deformation affects the transformation principally via a reduction in the undercooling required for nucleation rather than via an acceleration of the growth kinetics.

- by Jilt Sietsma
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- Engineering, Kinetics, Ferrite, Plastic deformation
- by Dagoberto Santos
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- Materials Engineering, Mechanical Engineering, Materials Science, Diagrams

Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size < 10 nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.

- by Dierk Raabe
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- Phase Transformations, Segregation, Nanocomposites, Transmission Electron Microscopy

A study has been made of the conditions which lead to intergranular brittle fracture in 4340-type steels at an ultra high yield strength level '(200 ksi, 380 MPa) in both an ambient environment and gaseous hydrogen. By means of Charpy impact tests on commercial and high purity steels, and by Auger electron spectroscopy of fracture surfaces, it is concluded that one-step temper embrittlement (OSTE or "500~ embrittlement"), and low K intergranular cracking in gaseous hydrogen are primarily the result of segregation of P to prior austenite grain boundaries. Segregation of N may also contribute to OSTE. Most, if not all, segregation apparently occurs during austenitization, rather than during tempering. Elimination of impurity effects by use of a high purity NiCrMoC steel results in an increase in Kth for hydrogen-induced cracking by about a factor of five (to the range 130 to 140 MNm-3/2). These observations are discussed in terms of our understanding of the mechanisms of OSTE and hydrogen-assisted cracking.

- by Charles McMahon and +1
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- Materials Engineering, Mechanical Engineering, High Pressure, Fracture Mechanics
- by PIL RYUNG CHA
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- Materials Engineering, Mechanical Engineering, Microstructure, Kinetics

The combination of different surface treatments for improving the erosion resistance of an AISI 304 stainless steel was studied. Six kinds of sample conditions were tested in a slurry composed of distilled water and SiC particles: High temperature gas nitriding (HTGN), low temperature plasma nitriding (expanded austenite), high temperature gas nitriding followed by a PVD-TiN coating, low temperature plasma nitriding followed by a PVD-TiN coating as well as PVD-TiN coated and uncoated samples in the solubilized condition. The erosion tests were performed during 6 h in a jet-like device with a normal angle of incidence and an impact velocity of 8.0 m/s. Wear rates were assessed by accumulated mass loss measurements and through analysis of scanning electron microscopy images of the worn surfaces. The results were related to the microstructure and hardness of the surface to establish a ranking of the different surface treatments. After the first few minutes of testing cutting of the surface occurred in the solubilized, in the HTGN and in the low temperature plasma nitrided AISI 304 samples, whereas TiN coated samples did not show any cutting marks, although some indentation marks could be observed. The TiN coated samples showed wear resistances one order of magnitude greater than the solubilized, HTGN and low plasma nitrided samples.

Investigations were carried out to study the effects of heat treatment on the mechanical properties of rolled medium carbon steel. The steel was heated to the austenizing temperature of 830 0 C and water quenched; It was reheated to the ferrite -austenite two phase region at a temperature of 745 0 C below the effective Ac 3 point. The steel was then rapidly quenched in water and tempered at 480 0 C to provide an alloy containing strong, tough, lath martensite (fibres) in a ductile soft ferrite matrix. The result shows that the steel developed has excellent combination of tensile strength, impact strength and ductility which is very attractive for structural use.

- by Oladele Isiaka
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- Mechanical Engineering, Ferrite, Austenite

The effect of austempering on the microstructure and toughness of nodular cast iron (designated as CuNiMoSG) alloyed with molybdenum, copper, nickel, and manganese has been studied. Light microscopy (LM), scanning electron microscopy (SEM), and X-ray diffraction technique were performed for microstructural characterization, whereas impact energy test was applied for toughness measurement. Specimens were austenitised at 860 °C, then austempered for various times at 320 and 400 °C, followed by ice-water quenching. Austempering at 320 °C produces a microstructure consisting of a mixture of acicular bainitic ferrite and the stable carbon-enriched austenite. In this microstructure ε-carbides are also identified after austempering up to 5 h. Fracture mode is changed from ductile to brittle with the prolonged time of austempering at 320 °C. The highest impact energy (115 kJ) corresponds not only to ductile fracture, but also to the maximum value of the volume fraction of retained austenite. Only martensitic structure was observed during austempering at 400 °C, inducing brittle fracture and significantly low-impact energy (10–12 kJ).

- by Olivera A Eric
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- Engineering, Fracture, Scanning Electron Microscopy, Copper

The potential for suppressing unacceptable austenite grain growth during carburizing by Nb microalloying additions in the range of 0.02 to 0.11 wt% to a Ti-modified SAE 8620 carburizing steel were evaluated. Alloys, were designed based on fundamental equilibrium thermodynamic analyses, as part of an extensive study on the effects of alloy composition, thermomechanical history, and pseudo-carburizing conditions on austenite grain coarsening behavior. Laboratory samples were produced to simulate both conventional hot rolling and controlled rolling practices designed to produce different initial precipitate distributions. Pseudocarburizing heat treatments, i.e. without a carburizing atmosphere, were performed in the temperature range of 950 to 1 100°C for holding times of 30 to 360 min. Precipitate distributions, including size, number density, morphology, distribution, and chemical composition in selected samples from the as-rolled and pseudo-carburized conditions were evaluated with transmission electron microscopy on extraction replicas. Results showed that increasing Nb additions to the Ti-modified SAE 8620 steel restrained austenite grain coarsening, and increased the grain coarsening time, especially at temperatures below 1 050°C. The Nb-free (Ti-modified) steel yielded either severely duplex grain structures or pseudo-normal grain growth (with very large mean grain diameter). However, holding a Ti-Nb-modified steel (e.g. 0.06 Nb wt%) at 950°C for 6 h or at 1 000°C for 4 h. produced fine and uniform austenite grain structures (with a mean grain diameter less than 20 mm). The finer grain sizes observed in the Ti-Nb-modified steels were due to the presence of Nbrich precipitates that hinder austenite grain coarsening, and precipitate distributions and grain growth behaviors are also influenced by the steel rolling history. The results indicate that Nb can successfully be used to suppress grain growth in carburizing steels. Fig. 14. Microstructure map based on austenite grain growth modes for the CR 0.02Nb steel. Solid and opened circles represent the experimental data.

- by Khaled AlOgab
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- Engineering, Microstructure, Soil Cementation, Grain Growth

The pseudo-elastic behavior of Shape memory alloy (SMA) truss and cantilever beam are investigated. Brinson's one-dimensional material model, which uses the twinned and detwinned martensite fractions separately as internal variables, is applied in the algorithm to establish the SMA stress-strain characteristics. This material model also incorporates different young's modulus for austenitic and martensite phase to represent the true SMA characteristics. In this model, a cosine function was used to express the evolution of the stress induced martensite fractions during the forward and reverse martensite phase transformation. A finite element formulation for the SMA truss member considering the geometric nonlinearity is proposed and the results are compared with the corresponding linear analysis. As a step forward, a finite element formulation for an SMA cantilever beam with an applied end moment is proposed. The load displacement characteristic for both the loading and unloading phases are considered to check the full pseudo-elastic hysteretic loop. In the numerical investigation, the stress-strain variation along the beam depth is also examined during the loading and unloading process to investigate the forward and reverse martensite phase transformation phenomena. Newton-Raphson's iterative method is applied to get convergence to the equilibrium for each loading steps. During a complete loading-unloading process, the temperature is kept constant as the model is essentially an isothermal model. Numerical simulation is performed considering two different temperatures to demonstrate the effect of temperature on the hysteretic loop.

The thermal diffusivity of the metastable undercooled austenite is relevant for the quantitative analysis of the carbon and low-alloy steel quench. The standard laser-flash method requires prior thermal equilibrium between the sample and the furnace, which may not be possible to achieve without allowing the metastable phase to transform. Nevertheless, depending upon the steel's hardenability, the thermal transient due to a laser pulse may be much shorter than a cooling transient sufficiently steep to prevent the transformation of the austenite. In one such case, flash measurements were performed during continuous sample cooling and the thermal diffusivity of the metastable austenite was determined by using an extension of the standard analytical model. The adopted analytical model and data reduction procedure are described and the limitations and uncertainties of this method are discussed, also with the aid of a non-linear numerical simulation. The measured thermal diffusivity of the undercooled low-alloy austenite decreases linearly from 5.4•10-6 m 2 s-1 at 1133 K to 4.3•10-6 m 2 s-1 at 755 K; this trend is in broad agreement with one previous set of measurements upon a low-alloy undercooled austenite and with a large number of previous standard measurements upon stable (high-alloy) austenitic stainless steels.

This research uses a continuum mechanics approach, combined with metallurgical analysis, to understand microscopically the effect of the phases existing within the austenite microstructure. The approach used is a classical continuum mechanics at a microscopic level to understand the stress distribution among the constituents within the austenite grains. Further, by employing simple failure theories, the research predicts the preferential cracking path numerically.

Nonisothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti, and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of this study is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). Electron microscopy investigations of the as-received steel proved the presence of Ti-rich, Nb-rich, and Mo-rich precipitates. The steel has then been subjected to austenitizing heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. Thermal cycles have a strong effect on the final austenite grain size. Using a mean field approach, a model is proposed for the dissolution of Nb-rich precipitates. This model has been coupled to a Zener-type austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the nonisothermal heating path simulating selected thermal profiles of the HAZ.

Austenitic stainless steel sheets are important technological materials due to their diverse applications ranging from chemical transportation, shipbuilding, automotive and aircraft industry. During deformation at room temperature or at work the sheet materials undergo phase, structural and mechanical changes. As the deformation increases, these particular changes may become critical for occurrence of local weak points and consequent breakage.
In order to determine the phase, structural and mechanical changes of sheet materials, experimental arrangements allowing deformation greater than 30% are increasingly used. Such a method of greater use is the hydraulic bulding. As the biaxial tensile deformation increases, the indicator diagrams display apparent serrated flow and a change in the magnetic permeability. The slow rate strain transformation under biaxial tensile strength is revealed by means of microstructural, XRD analysis and hardness tests. The correlation between the mechanical and structural response of the different zones of the deformed sheet material is revealed.

- by Emil Yankov
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- Mechanical Engineering, Microstructure, Sheet Metal Forming, Structural Materials

The effect of thermomechanical processing conditions on Nb clustering and precipitation in both austenite and ferrite in a Nb-Ti microalloyed steel was studied using electron microscopy and atom probe tomography. A decrease in the deformation temperature increased the Nb-rich precipitation in austenite and decreased the extent of precipitation in ferrite. Microstructural mechanisms that explain this variation are discussed.

- by Julie Cairney and +1
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- Materials Engineering, Mechanical Engineering, Electron Microscopy, Clustering

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- by Victor Champagne, Jr.
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- Mechanical Engineering, Materials Science, Chemistry, Microstructure

In an Fe–9 at.% Mn maraging alloy annealed at 450 C reversed allotriomorphic austenite nanolayers appear on former Mn decorated
lath martensite boundaries. The austenite films are 5–15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron
microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation)
in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics
of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite
stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation
at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.

The results of cyclic tensile tests on superelastic NiTi shape memory alloy (SMA) wires are presented and discussed. The tests were carried out within a large experimental test programme for the MANSIDE Project, with the scope of verifying the suitability of SMA superelastic wires as kernel components for seismic protection devices. The mechanical behaviour is described by means of four fundamental quantities, namely: secant stiness, energy loss per cycle, equivalent damping and residual strain. The sensitivity to temperature and strain rate, as well as the in uence of strain amplitude and the e ects due to repeated cyclic deformation, are analysed in detail. The experimental results show that the characteristics of the superelastic wires are well suited for seismic applications, as both the recentring and the energy dissipating features of the devices can be easily obtained. Moreover, the in uence of the investigated parameters, within their usual range of variation in seismic protection devices, is compatible with the use of superelastic wires for practical applications.

- by Mauro Dolce
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- Mechanical Engineering, Civil Engineering, Titanium, Strain Rate

The effect of deoxidation products of Ce 2 O 3 , ZrO 2 and MgO particles on solidification microstructure has been studied in Fe-10mass%Ni, Fe-0.20mass%C-0.02mass%P and Fe-0.50mass%C-1mass%Mn alloys. The degree of the equiaxed crystallization is explained by the lattice misfit parameter between g (or d)-Fe and oxide. The single-phase solidification microstructure of Fe-10mass%Ni and Fe-0.50mass%C-1mass%Mn alloys is well related to austenite grain boundaries under the inhibition of grain growth by pinning. The correspondence between solidification structure and initial austenite grain has been studied in two-phases solidification of Fe-0.15 (or 0.30)mass%C-1mass%Mn-1mass%Ni alloy. The g-grain size decreases with decreasing the lattice misfit parameter between g-Fe and oxide and increases with decreasing the Zener pinning force. The number of g-grains to that of primary d-grains per unit area in a cross section increases with decreasing the aforementioned lattice misfit parameter, indicating that more than one nucleation event per d-grain occurs at d-ferrite grain boundary during d to g transformation.

- by geoff scamans
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- Engineering, Materials Science, Microstructure, Inclusion

- by Geoff Scamans
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- Engineering, Microstructure, Inclusion, Grain Growth

Laser heating caused a melting layer to form on the H13 steel, which usually has bad thermal conductivity and diffusivity. Therefore, the modified Ashby-Eastering heat-transfer equation was used to provide the temperature field for laser surface hardening in the melt. When the laser hardened H13 steel through surface melting, the basic microstructure of the dendrites was surrounded by an extremely fine lamellar structure in the melt layer. It is clear that the contours of the melting point isotherms and the critical phase transition temperature of H13 in the quenched and as-received conditions were comparable in the temperature distribution field under different laser energy densities. When the laser moves on, the phase transition temperature of H13 is raised and it becomes higher than the A 1 temperature because the heating rate during laser processing is usually N10 4 8C/s. The larger the grain size or the more heterogeneous the structure, the higher the temperature and the longer the duration required for transforming the steel into austenite. D

- by Mayank Madia
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- Engineering, Heat Transfer, Phase transition, Grain size

This paper examines the effects of heat treatment processes on the mechanical properties of as-cast Al-4% Ti alloy for structural applications. Heat treatment processes, namely, annealing, normalizing, quenching, and tempering, are carried out on the alloy samples. The mechanical tests of the heat treated samples are carried out and the results obtained are related to their optical microscopy morphologies. The results show that the heat treatment processes have no significant effect on the tensile strength of the as-cast Al-4% Ti alloy but produce significant effect on the rigidity and strain characteristic of the alloy. With respect to the strain characteristics, significant improvement in the ductility of the samples is recorded in the tempered sample. Thus, for application requiring strength and ductility such as in aerospace industries, this tempered heat treated alloy could be used. In addition, the quenched sample shows significant improvement in hardness.

- by ishaq aremu and +1
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- Engineering, Materials Engineering, Mechanical Engineering, Organic Chemistry

The aim of the study is to determine the effect of nitrogen content on precipitation reactions in nitrogen alloyed austenitic stainless steels during isothermal annealing up to 500 h at 450 °C and up to 72 h at 750 °C. The studied steels were P/M austenitic stainless steels with the nominal composition of 24Cr-22Ni-7.3Mo-3.5Mn-0,3…0,9N. The studied steels were manufactured by HIPing (Hot Isostatic Pressing) from gas atomized powders. Nitrogen content of the steels was varied by nitrogen alloying of the atomized melt and by further solid state nitriding of the gas atomized powders in a fluidized bed fumace when nitrogen contents exceeding nitrogen solubility in the liquid state were needed. The effect of nitrogen content on precipitation was studied with TEM as well as by tensile testing, Charpy-V impact testing and localized corrosion testing (pitting and crevice corrosion). Precipitating phases in the studied steels were either intermetallic phases (sigma and Laves) and/or chromium nitrides (Cr 2 N) depending on the nitrogen content of the steel.

- by Hannu Hänninen
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- Materials Science, Powder Metallurgy, Microstructure, Metallurgy

This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

- by Roland Oltra
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- Materials Engineering, Mechanical Engineering, Materials Science, Heat Treatment

We propose a numerical model for the dynamics of the austenitic-martensitic transformation which includes the effect of surface energy. Our numerical model uses a nonconforming finite element method to approximate the deformation and a numerical viscosity to stabilize the scheme. We present numerical experiments which demonstrate that the addition of a strain-gradient surface energy results in a more planar austenitic-martensitic interfacial layer.

- by Petr Kloucek
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- Applied Mathematics, Microstructure, Finite element method, Computer Model

To determine the full-field high-temperature thermal deformation of the structural materials used in high-speed aerospace flight vehicles, a novel non-contact high-temperature deformation measurement system is established by combining transient aerodynamic heating simulation device with the reliability-guided digital image correlation (RG-DIC). The test planar sample with size varying from several mm 2 to several hundreds mm 2 can be heated from room temperature to 1100 1C rapidly and accurately using the infrared radiator of the transient aerodynamic heating simulation system. The digital images of the test sample surface at various temperatures are recorded using an ordinary optical imaging system. To cope with the possible local decorrelated regions caused by black-body radiation within the deformed images at the temperatures over 450 1C, the RG-DIC technique is used to extract full-field in-plane thermal deformation from the recorded images. In validation test, the thermal deformation fields and the values of coefficient of thermal expansion (CTEs) of a chromiumnickel austenite stainless steel sample from room temperature to 550 1C is measured and compared with the well-established handbook value, confirming the effectiveness and accuracy of the proposed technique. The experimental results reveal that the present system using an ordinary optical imaging system, is able to accurately measure full-field thermal deformation of metals and alloys at temperatures not exceeding 600 1C.

- by Bing Pan
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- Digital Image Correlation, Optical Imaging, Measurement systems, Reliability