Showing papers on "Austenite published in 2005"

Welding Metallurgy and Weldability of Stainless Steels

Abstract: Preface 1 Introduction 2 Phase Diagrams 3 Alloying Elements and Constitution Diagrams 4 Martensitic Stainless Steels 5 Ferritic Stainless Steels 6 Austenitic Stainless Steels 7 Duplex Stainless Steels 8 Precipitation-Hardening Stainless Steels 9 Dissimilar Welding of Stainless Steels 10 Weldability Testing Appendix 1: Nominal Compositions of Stainless Steels Appendix 2: Etching Techniques for Stainless Steel Welds Author Index Subject Index

Effect of strain rate on the strain-induced γ→α'-martensite transformation and mechanical properties of austenitic stainless steels

Abstract: The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10−4 and 200 s−1. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.

The "quenching and partitioning" process: background and recent progress

Abstract: 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 martensite-start 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.

Microstructure and mechanical properties of friction stir welded SAF 2507 super duplex stainless steel

Abstract: The microstructure and mechanical properties of friction stir (FS) welded SAF 2507 super duplex stainless steel were examined. High-quality, full-penetration welds were successfully produced in the super duplex stainless steel by friction stir welding (FSW) using polycrystalline cubic boron nitride (PCBN) tool. The base material had a microstructure consisting of the ferrite matrix with austenite islands, but FSW refined grains of the ferrite and austenite phases in the stir zone through dynamic recrystallisation. Ferrite content was held between 50 and 60% throughout the weld. The smaller grain sizes of the ferrite and austenite phases caused increase in hardness and strength within the stir zone. Welded transverse tensile specimen failed near the border between the stir zone and TMAZ at the retreating side as the weld had roughly the same strengths as the base material.

Effect of Austenite Grain Size on the Morphology and Crystallography of Lath Martensite in Low Carbon Steels

Abstract: The crystallography, microstructure and mechanical property of as-quenched martensite of Fe-0.2C-Mn(-V) alloys of which the prior austenite grain sizes are 370-2 μm were studied. The prior austenite grain, whose size is larger than 28 μm, is divided by several packets. Those packets are subdivided by blocks containing sub-blocks, each of which corresponds to the Kurdjumov-Sachs variant. When the prior austenite grain size is about 2 μm, one packet tends to grow predominantly. Each packet is divided by blocks containing sub-blocks.

Analysis of white layers formed in hard turning of AISI 52100 steel

Abstract: The formation mechanisms and properties of white layers produced in machining of hardened steels are not clearly understood to date. In particular, detailed analysis of their structure and mechanical properties is lacking. This paper investigates the differences in structure and properties of white layers formed during machining of hardened AISI 52100 steel (62 HRC) at different cutting speeds. A combination of experimental techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), and nano-indentation are used to analyze the white layers formed. TEM results suggest that white layers produced at low-to-moderate cutting speeds are in large part due to grain refinement induced by severe plastic deformation, whereas white layer formation at high cutting speeds is mainly due to thermally-driven phase transformation. The white layers at all speeds are found to be comprised of very fine (nano-scale) grains compared to the bulk material. XRD-based residual stress and retained austenite measurements, and hardness data support these findings.

In-situ observations of lattice parameter fluctuations in austenite and transformation to bainite

Abstract: The isothermal transformation of high-carbon austenite-to-bainitic ferrite has been investigated with the in-situ technique of time-resolved X-ray diffraction using synchrotron radiation. The measurements indicate that prior to transformation, the austenite divided into regions with significantly different lattice parameters. It is possible that this is due to the development of carbon-rich and carbon-poor regions in the austenite, as a precursor to transformations including the bainite reaction. The lattice parameter became uniform as transformation progressed and the fraction of carbon-poor austenite decreased. The ferrite itself exhibited a large range of lattice parameters during the early stages of transformation, due to the trapping of carbon.

The Role of Retained Austenite on Tensile Properties of Steels with Bainitic Microstructures

Abstract: In high-carbon, silicon-rich steels it is possible to obtain a very fine bainitic microstructure by transformation at low temperatures (200– 300 � C). This microstructure consists of slender ferrite plates, with thicknesses of several tens of nm, in a matrix of retained austenite. Whereas strength is mainly provided by to the fine scale of the ferrite plates (stronger phase), ductility is mostly controlled by the retained austenite (softer phase). Further improvement in ductility is achieved by strain induced transformation of austenite to martensite, the so called TRIP effect. In order to take full advantage of this effect, the mechanical stability of the austenite, i.e., its capability to transform to martensite under strain, must not be too low nor excessively high. Two main aspects of the mechanical stability of the retained austenite, morphology and chemical composition, have been studied to determine the role that these play on the ductility behaviour of the bainitic steels studied. It is suggested that the chemical composition has the strongest effect on the ductility of these new high strength alloys.

Antibacterial properties, corrosion resistance and mechanical properties of Cu-modified SUS 304 stainless steel

Abstract: This investigation studies the effects of Cu content and ageing treatment on the microstructural, mechanical, corrosion and antibacterial properties of SUS 304 austenitic stainless steel. Cu was added respectively to SUS 304 stainless steels in proportions of 1.5, 2.5, 3.5, 4.5 and 5.5 wt.%. A vacuum arc remelting furnace was used to remelt SUS 304 stainless steel with various added Cu contents. These ingot alloys underwent hot rolling and various heat treatments, and were then cut into test specimens. A series of microstructural investigation, tensile tests, corrosion tests and antibacterial tests were conducted to study the properties of Cu-containing SUS 304 austenitic stainless steel. Microstructural observations reveal that the amount of retained δ-ferrite in the as-cast SUS 304 steel decreases as the Cu content increases. After hot rolling, the retained δ-ferrite disappears and α′-martensite forms in the austenitic matrix. The results of the tensile tests reveal that the ultimate tensile strength (UTS) declines as the Cu content increases below 2.5 wt.%. However, the ultimate tensile strength increases with the Cu content above 2.5 wt.%. X-ray diffraction analysis reveals that adding Cu suppresses the formation of strain-induced martensite (α′-martensite). The corrosion test indicates that the pitting potential declines as the Cu content in SUS 304 steels increases. The results of the antibacterial test reveal that adding a proper amount of Cu (such as 2 wt.%) gives SUS 304 stainless steel an excellent antibacterial property.

Phase transitions in Ni2+xMn1-xGa with a high Ni excess

Abstract: Ferromagnetic shape memory alloys Ni$_{2+x}$Mn$_{1-x}$Ga were studied in the range of compositions $0.16 \le x \le 0.36$. Experimental phase diagram, constructed from differential scanning calorimetry, transport and magnetic measurements, exhibits distinctive feature in a compositional interval $0.18 \le x \le 0.27$, where martensitic and magnetic transitions merge in a first-order magnetostructural phase transition ferromagnetic martensite $\leftrightarrow$ paramagnetic austenite. Observed in this interval of compositions a non-monotonous behavior of the magnetostructural phase transition temperature was ascribed to the difference in the exchange interactions of martensitic and austenitic phase and to the competition between increasing number of valence electron and progressive dilution of the magnetic subsystem which occur in the presence of a strong magnetoelastic interaction. Based on the experimental phase diagram, the difference between Curie temperature of martensite $T_C^M$ and Curie temperature of austenite $T_C^A$ was estimated. Influence of volume magnetostriction was considered in theoretical modeling in order to account for the existence of the magnetostructural phase transition over a wide range of compositions.

Microstructural evolution in 13Cr-8Ni-2.5Mo-2Al martensitic precipitation-hardened stainless steel

Abstract: The microstructure of 13Cr–8Ni–2.5Mo–2Al martensitic precipitation-hardened (PH) stainless steel has been investigated using transmission electron microscopy, three-dimensional atom probe and small-angle X-ray scattering. A high number density (∼10 23–25 m −3 ) of ultra-fine (1–6 nm) β-NiAl precipitates are formed during aging at 450–620 °C, which are spherical in shape and dispersed uniformly with perfect coherency with the matrix. As the annealing temperature increases, the size and concentration of the precipitates increase concurrently while the number density decreases. The Mo and Cr segregation to the precipitate–matrix interface has been detected and is suggested to suppress precipitate coarsening. In the sample aged for 500 h at 450 °C, the matrix decomposes into Cr-rich (α′) and Cr-poor (α) regions. The decrease in the strength at higher temperature (above 550 °C) is attributed to the formation of larger carbides and reverted austenite.

Effect of titanium on the as-cast microstructure of a 16%chromium white iron

Abstract: This research work studies the effect of systematic titanium additions (up to 2 wt.%) to a 16%Cr, 2.5%C white cast iron. The study was undertaken in six laboratory made alloys with different titanium amounts. Alloys were melted in an open induction furnace by using high purity raw materials. Such additions caused small hard titanium carbide particles to precipitate within the proeutectic austenite therefore promoting a strengthening of matrix; such particles also contributed to increase bulk hardness of the overall alloy. A structure refinement, as measured by the secondary dendrite arm spacing, was also observed as the titanium amount was increased. Titanium carbide precipitation caused a small decrease in the eutectic carbide volume fraction. The fracture toughness remained constant since the strengthening of matrix was compensated with a decrease in the volume of eutectic carbides. According to this study, titanium can be used as an alloying element to increase the hardness and perhaps wear resistance without affecting fracture toughness in high-chromium cast alloys. The results are discussed in terms of the precipitation nature of such small hard titanium particles.

Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment temperature

Abstract: Nitriding treatments of austenitic stainless steels can be performed only at relatively low temperatures in order to avoid a decrease of corrosion resistance due to chromium nitride formation. These conditions promote the formation of the so-called S phase, which shows high hardness and good corrosion resistance. In the present paper, the influence of the treatment temperature of glow-discharge nitriding process on the microstructural and mechanical characteristics of AISI 316L steel samples was evaluated. Glow-discharge nitriding treatments were performed at temperatures in the range 673–773 K for 5 h at 10 3 Pa. The modified surface layer of the nitrided samples consists mainly of the S phase and, according to metallographic technique analysis, it seems to be essentially a modification of the austenite matrix. All the nitrided sample types show a peculiar surface morphology due to both plasma etching during nitriding and the presence of slip steps and relieves at grain boundaries, the latter features presumably due to the formation of the nitrided layer. X-ray diffraction analysis shows that for the samples nitrided at temperatures up to 723 K, besides the S phase, small chromium nitride precipitates are present at the surface, while using higher treatment temperatures both chromium (CrN) and iron (γ'-Fe 4 N) nitrides precipitate along the grain boundaries and in the middle of the grains, and their amount increases as treatment temperature increases. High hardness values (from ∼1450 to ∼1550 HK 0.01 , depending on nitriding conditions) are observed in the modified layer with a steep decrease to matrix values. Preliminary corrosion resistance tests, carried out in 5% NaCl aerated solution with the potentiodynamic method, show that with the used treatment parameters a substantial improvement of corrosion resistance can be achieved when glow-discharge nitriding treatments are performed at temperatures in the range 703–723 K.

Transformation-induced plasticity in ferrous alloys

Abstract: We study the mechanical behavior of a class of multiphase carbon steels where metastable austenite at room temperature is found in grains dispersed in a ferrite-based matrix. During mechanical loading, the austenite undergoes a displacive phase change and transforms into martensite. This transformation is accommodated by plastic deformations in the surrounding matrix. Experimental results show that the presence of austenite typically enhances the ductility and strength of the steel. We use a recently developed model (Turteltaub and Suiker, 2005) to analyze in detail the contribution of the martensitic transformation to the overall stress-strain response of a specimen containing a single island of austenite embedded in a ferrite-based matrix. Results show that the performance of the material depends strongly on the lattice orientation of the austenite with respect to the loading direction. More importantly, we identify cases in which the presence of austenite can in fact be detrimental in terms of strength, which is relevant information in order to improve the behavior of this class of steels.

Microstructural evolution at the initial stages of continuous annealing of cold rolled dual-phase steel

Abstract: The performance of cold rolled dual-phase (DP) steels depends on their microstructure, which results from the thermomechanical processing conditions, involving hot rolling, cold rolling, and continuous annealing. The knowledge on the influence of each annealing stage on the microstructure formation is essential for manufacturing high-quality DP steels. In the present work, the effects of some intercritical annealing parameters (heating rate, soaking temperature, soaking time, and quench temperature) on the microstructure and mechanical properties of a cold rolled DP steel (0.08% C–1.91% Mn) were studied. The microstructure of specimens quenched after each annealing stage, simulated on a Gleeble, was analyzed using optical, scanning, and transmission electron microscopy. The tensile properties, determined for specimens submitted to complete annealing cycles, are influenced by the volume fractions of martensite, bainite, martensite/austenite (MA) constituent, and carbides, which depend on annealing processing parameters. The results obtained showed that the yield strength (YS) increase and the ultimate tensile strength (UTS) decrease with the increasing intercritical temperature. This can be explained by the increased formation of granular bainite associated with the increased volume fraction of austenite formed at the higher temperatures. The experimental data also showed that, for the annealing cycles carried out, UTS values in excess of 600 MPa could be obtained with the steel investigated.

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract: Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion de...

Mechanical Properties of Low-Temperature Bainite

Abstract: The mechanical properties of a bainitic microstructure with slender ferrite plates (20-65 nm in thickness) in a matrix of carbon-enriched retained austenite were characterized. The microstructure is generated by isothermal transformation at temperatures in the range 200-300°C. A yield strength as high as 1.5 GPa and an ultimate tensile strength between 1.77 to 2.2 GPa was achieved, depending on the transformation temperature. Furthermore, the high strength is frequently accompanied by ductility (£ 30%) and respectable levels of fracture toughness (

52nd Hatfield Memorial Lecture Large chunks of very strong steel

Abstract: Most new materials are introduced by selectively comparing their properties against those of steels. Steels set this standard because iron and its alloys have so much potential that new concepts are discovered and implemented with notorious regularity. In this 52nd Hatfield Memorial Lecture, I describe a remarkably beautiful microstructure consisting of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes. The crystals are generated by the partial transformation of austenite, resulting in an extraordinary combination of strength, hardness and toughness. All this is in bulk steel without the use of expensive alloying elements. We now have a strong alloy of iron, which can be used for making items that are large in all three dimensions, which can be made without the need for mechanical processing or rapid cooling and is cheap to produce and apply.

Low temperature plasma nitrocarburising of AISI 316 austenitic stainless steel

Abstract: Plasma nitrocarburising was carried out at a low temperature to improve the surface hardness of AISI 316 austenitic stainless steel without degradation of its corrosion resistance. It was found that nitrogen and carbon atoms can simultaneously be dissolved into the austenite lattice during the nitrocarburising process, forming a nitrogen and carbon supersaturated solid solution, which has a face centre tetragonal (fct) structure (i.e. the S phase). GDS analysis showed that the maximum nitrogen and carbon concentrations in the nitrocarburised S-phase layer occur at different depths from the surface. The nitrocarburised layers have not only high hardness like the nitrided layer, but also high thickness and gradually reduced hardness profile like the carburised layer.

Effects of austenite reversion during overageing on the mechanical properties of 18 Ni (350) maraging steel

Abstract: The sequence of austenite reversion during overageing in 18 Ni (350) maraging steel was examined and its effects on the mechanical properties were evaluated. Austenite with different morphological features were identified at different stages of overageing. The reverted austenite caused decrease in the yield strength (YS) and ultimate tensile strength (UTS) and increase in the tensile ductility. Though the presence of austenite appeared to be beneficial to impact toughness in the initial stages of overageing, severe embrittlement was noticed in samples subjected to prolonged ageing. The observed deterioration in toughness with continued overageing was associated with the coarsening of intermetallic precipitates formed during the early stages of ageing. Electron probe micro analysis (EPMA) carried out on fracture surface identified the embrittling species as precipitates rich in titanium and nickel.