Search results for "Austenite"
showing 10 items of 32 documents
Influence of preoxidation on the corrosion of steels in liquid lead-bismuth eutectic
2004
Abstract Oxidation studies of martensitic/ferritic and austenitic steels have been conducted in static lead bismuth eutectic (LBE). Samples were preoxidized in an air/H2O gas mixture prior to immersion in LBE. Preoxidation films grown on HT-9 at 800°C for 48 h had a bilayer structure, an outer Fe-rich layer, and an inner Cr-rich layer. Glancing angle x-ray diffraction data found that two distinct structures were present in this oxide: Fe1+xCr2−xO4 spinel (cubic, face-centered cubic [fcc]) and (Cr,Fe)2O3 (rhombohedral, corundum). Magnetite formation (Fe3O4) was ruled out. Immersion in LBE resulted in the growth of an Fe-rich film on top of the preoxidation layer. It was concluded that the gr…
Kinetics of Sigma Phase Precipitation in Niobium-Stabilized Austenitic Stainless Steel and Effect on the Mechanical Properties
2014
Stabilized austenitic stainless steels are widely used in nuclear and oil industries. The 316 Nb steel grade presented in this study holds a small amount of delta ferrite in the austenitic matrix which tends to transform into sigma phase during prolonged exposures in the temperature range of 600-1000°C. Sigma phase is promoted by ferritic elements such as chromium, molybdenum, niobium and silicon. Time-Temperature-Transformation (TTT) diagram of the δ-ferrite evolution is established thanks to DSC experiments and quantitative metallographic analysis. It is observed that the highest sigma phase formation rate occurs between 800 and 900°C, and that the transformation of ferrite begins after a…
Corrosion Resistance of Alloys in IGCC Environment
2001
IGCC recovery boiler materials are subject to severe corrosion in environments composed of a gas mixture (CO-H 2 -H 2 O-CO 2 -H 2 S) and vanadium-containing fly-ash resulting from the combustion of oil residues. This paper presents a study of the corrosion behaviour of two Fe-Ni-Cr austenitic alloys in a typical IGCC environment at 400°C. The alloy composition influences the vanadium incorporated in the corrosion scales during the test, which affects the scale growth process. The valence state (determined by micro-XANES) and the amount of the incorporated vanadium is modified by a subsequent air flow at 400°C.
The influence of thin hard coatings on frictional behaviour in the orthogonal cutting process
2000
New knowledge about the tribological response deriving from the interaction of the substrate/coating-chip system, with special attention to the orthogonal cutting process when chatter-free end turning using natural contact tools, is developed. In order to evaluate the frictional behaviour of this process under modified contact conditions, experimental investigations including the contact temperature, the contact loads, friction and the frictional heat flux per unit area were carried out. In contrast to the most obvious approach, the coefficient of sliding friction versus the cutting speed, the contact temperature, the normal pressure and the interface control factor is considered. A number …
Microstructural Characterization of Thermo-Mechanical Treated TRIP Steels
2007
The increasing demand for the reduction of automobiles CO2 emissions for environmental preservation leads the automotive industries towards the mechanical components weight reduction. Sheet steels with multiphase microstructures exhibit favourable combinations of strength and ductility. The so called TRIP steels have a metastable microstructure that consists of a continuous ferrite matrix containing a dispersion of hard second phases martensite and bainite. These steels also contain retained austenite, at room temperature, that represents the source of the TRansformation Induced Plasticity effect. When the material is subjected to deformation step, the retained austenite transforms itself i…
Microstructural evolution of wear-resistant FeCrB and FeCrNiCoB coating alloys during high-energy mechanical attrition
2008
Mechanical milling/attrition provides a convenient scope of simulating the microstructural changes encountered by wear-resistant coating alloys subjected to deformation under high frequency and high-intensity impact loading or accelerated wear condition. In the present study, the microstructural evolution of two commercial coating materials, FeCrB (Armacor M) and FeCrNiCoB (Armacor C), in the course of low- and high-intensity mechanical attrition, was monitored by X-ray diffraction and high-resolution transmission electron microscopy. While low-intensity milling leads to marginal grain refinement but no change in phase-aggregate in FeCrB, similar mechanical attrition causes boride precipita…
Influence of friction on the local mechanical and electrochemical behaviour of duplex stainless steels.
2006
International audience; The electrochemical behaviour of ferritic and austenitic phases in duplex stainless steel (UNS S32304) and the modifications induced by straining during sliding were studied by potentiodynamic polarisation curves determined at the microscale in a 1MNaCl (pH 3) solution, using an electrochemical microcell. The mechanical properties and stress state of each phase were determined by microhardness and X-ray microdiffraction measurements, before and after straining. The results show that sliding generates elastic straining of the ferrite and plastic deformation of the austenite. The electrochemical behaviour of these phases is dramatically altered, inducing a reduction of…
Understanding sigma-phase precipitation in a stabilized austenitic stainless steel (316Nb) through complementary CALPHAD-based and experimental inves…
2014
Abstract Sigma-phase precipitation in a 316Nb “stabilized” austenitic stainless steel was studied through complementary CALPHAD-based and dedicated experimental investigations. Thermokinetic calculations performed using Thermo-Calc (with the DICTRA module) and MatCalc software showed that the sigma phase (σ) precipitated directly at γ-austenite grain boundaries (GB) via a common solid-state reaction when carbon and nitrogen contents fell below a critical threshold. Residual δ ferrite was found to be more susceptible to σ-phase precipitation; this type of precipitation occurred via two mechanisms that depended on the concentration profiles of δ-ferrite stabilizing elements induced by previou…
Study of the mechanical effects on passivity breakdown by local probe techniques
2006
This paper aims at demonstrating that a relationship exists between surface stress and pitting corrosion. The surface stress field generated by polishing was first calculated using a thermo-mechanical model and a finite element code. Pitting corrosion tests performed at the microscale along the austenite/ferrite interface using the electrochemical microcell technique were then analyzed considering the microstructure and the residual surface stress field. Mechano-chemical criteria are proposed leading to an enhancement of pitting corrosion of duplex steels.
A corrosion study of austenitic and martensitic steels under boiler conditions by means of 57Fe conversion electron mössbauer spectroscopy
1978
Abstract The growth of protective oxide layers on a martensitic FeCr (1.4122) and two austenitic FeCrNi steel samples (1.4550 and Incoloy 800) was studied by means of the 57Fe Conversion Electron Mossbauer Spectroscopy (CEMS). Scanning electron microscopy was applied to characterize the topographical nature of the oxide layers. Following a technical procedure of running-in steam generators, the steel specimens were oxidized in oxygen-free water at 295°C, 142 bar and a pH value between 9 and 10 for periods up to 70 h. In the case of the steels 1.4122 and 1.4550 the only oxide phase detected was non-stoichiometric magnetite, whereas nickel ferrite of nonstoichiometric composition was found…