G. Saindrenan

A study of the recrystallization of an IF steel by kinetics models

The primary recrystallization of a 70, 80 and 90% cold rolled interstitial-free (IF) steel was studied using optical micrographs (OM) and microhardness tests. Both the JMAK model and a new recrystallization model were used to analyze the recrystallization kinetics. The experimental data plotted according to the JMAK model could be represented by straight lines with JMAK exponents n falling in the range from 1.3 to 2.6. The experimental data also showed good agreement with the new model, which involves three microstructural parameters. It can be used to quantitatively study recrystallization kinetics. For this purpose, the mobility (M) of the boundaries is obtained using a non-linear curve fit method. Trend analysis shows that M increases with annealing temperature and rolling reduction. Furthermore, the dependence of the mobility on initial strain can be interpreted in terms of solute drag theory.

In-situ SEM studies of grain boundary migration during recrystallization of cold-rolled nickel

The grain boundary migration (GBM) rate during primary recrystallization depends on the driving force P exerted on the boundary and on the boundary mobility M. M is composed of two terms: an intrinsic mobility of the boundary depending on its structure and a chemical mobility due to the action exerted on the boundary by impurities. The purpose of the present work is to investigate quantitatively the law V = f(P) during primary recrystallization of nickel.

Diffusion of sulfur from the grain boundaries to the surface in polycrystalline nickel

Segregation of an impurity on the free surface of a metal is a commonly used phenomenon to obtain bulk diffusion data from surface coverage kinetic measurements. During annealing of nickel, sulfur segregates to grain boundaries as well as onto the free surface as the two phenomena act simultaneously. Yet a driving force exists to convert segregation into surface segregation since the free enthalpy of surface segregation ({minus}140 kJ mol{sup {minus}1}) is more negative than the free enthalpy grain boundary segregation ({minus}98 kJ mol{sup {minus}1}). As a result, as long as the equilibrium value of surface coverage is not reached, the intergranular sulfur can diffuse to the surface along the grain boundaries path. In this work the authors use this property to deduce grain boundary diffusion constants of sulfur from surface enrichment kinetics.

Application of percolation theory to surface segregation during recovery

Abstract Surface segregation of sulphur on heavily cold rolled nickel is examined during recovery using in-situ Auger Electron Spectroscopy. An unexpected incubation period immediately followed by a very quick segregation step is observed. This unexpected behaviour is discussed using a model, which takes the different stages of recovery and especially the role of dislocation walls into account. In as-cold rolled nickel, dislocations appear as cells with thick walls. During recovery, dislocation wall refinement makes the diffusion of sulphur along dislocation walls much faster. The percolation of this structure is responsible for sudden acceleration of surface segregation. The main conclusion of this study is that superficial segregation kinetics remains very slow until a network of subgrain boundaries acting as diffusion short passes is established and used by the sulphur to reach the surface quickly.

Effects of Metastable Diffusion Short-Circuits on Surface Segregation

The role of defects introduced by both quenching and plastic deformation on the acceleration of surface segregation kinetics in the nickel-sulphur system is discussed. An estimation of the diffusion coefficient of sulphur in as-quenched single-crystal of nickel is given.Regarding cold-worked structures, the annihilation of the vacancies produces very quick segregation of sulphur atoms on the dislocation network at the beginning of the annealing treatment. However, the diffusion acceleration becomes truly significant when the pipes inside sub-grain boundaries resulting from the deformation are rearranged properly to form a percolated network.These results demonstrate that the micro-structural evolution of diffusion short circuits should be taken into account when describing complex diffusion kinetics.

Use of AES to determine low solubilities of impurities: Case of MnS in austenitic stainless steel

The solubility of manganese sulfide in a commercial 304 austenitic stainless steel was studied using AES to determine surface sulfur segregation. The segregation mechanism at stainless steel surfaces were extensively studied, but thermodynamic data such as segregation free energy, or manganese sulfide dissolution heat have not been established, although these data are of great interest because the MnS dissolution parameters give the amount of free sulfur in solid solution. The only solubility data for manganese sulfide in austenitic steel was given to Turkdogan in 1955 and concerned MnS in iron-manganese alloys. In the present study, the Auger spectra are used to calculate the free energy of dissolution of manganese sulfide as well as the sulfur segregation parameters in austenitic stainless steel.

Influence of recovery on dynamic segregation of sulphur to surface of cold rolled nickel

Abstract It is now well established that the kinetics of impurity segregation is greatly increased by the defect annihilation process occurring during the return to equilibrium of a material initially in a metastable state (quenched, irradiated, cold worked). During the return to equilibrium of a cold worked material, i.e. during a recrystallisation anneal, both the recovery and recrystallisation stages contribute to the acceleration of the segregation kinetics. In this work, an attempt is made to study the contribution of each stage separately, with particular emphasis on the recovery mechanisms of cold worked pure nickel and the way these mechanisms affect sulphur segregation taking place in this material. For this purpose, recrystallisation and segregation anneals at 455°C were carried out on lightly (0.25 true strain) and heavily (0.8 true strain) cold rolled nickel. It was found that sulphur segregation in the lightly deformed material took place entirely during the incubation time preceding recrystallisation, i.e. during the recovery stage. The process is thought to involve dislocation pipe diffusion for which a diffusion coefficient of 6.4×10-9 m2 s-1 was estimated. In the heavily deformed material, segregation was concomitant with recrystallisation and the mechanism thought to be most probable is that involving impurity drag.

Influence of Stress on Intergranular Brittleness of a Martensitic Stainless Steel

Phosphorus intergranular segregation can strongly decrease grain boundary cohesion in steels. Martensitic stainless steel 17-4 PH was tempered 4 hours at 600°C and aged for long times at 320°C (up to 15700 h). Tempering treatment at 600°C leads to phosphorus intergranula segregation, although there is no sign of intergranular brittleness because of the low hardness of the steel. Ageing treatment at 320°C makes the steel much harder ( α’ p ecipitation) and it is then possible to obtain intergranular ruptures. It was observed that the s hift of the ductile-to-brittle transition temperature due to ageing at 320°C was more important if the ageing treatment was carried out under a tensile stress of 500 MPa, even though the hardness l evel i unchanged by the stress. This shift was attributed to an increase of phosphorus concentr ation in the grain boundaries that lie perpendicularly to the tensile stress axis, which corres ponds to the longest side of the Charpy specimen. The interpretation of this effect is based on the following assumption: the segregation free energy of phosphorus is increased for grain boundaries normal to the tensile axis. Tempering treatment at 600°C leads to phosphorus segregation in all the grain boundaries. But, during ageing at 320°C, the application of a tensile stress make the phosphorus diffuse from grain boundaries that lie parallel to the tensile axis toward grain boundari es that lie normal to the tensile axis. This assumption was confirmed by intergranular segregation m easurements made on specimens that were heat treated under a flexion load. Introduction Structure materials often undergo stresses during service. This i s the case of the martensitic stainless steel 17-4 PH used in nuclear power plants which is likel y to remain in service for very long times at temperatures about 300°C (10 4 – 10 hours). It has been shown that intergranular segregation of phosphorus decreases the grain boundary cohesion of 17-4 PH steel, which leads to a dramatic increase of its ductile-to-brittle transition tempera ture (DBTT) [1–3]. The stress applied to the steel during service can reach several hundreds MPa and, up to now, its effect on phosphorus segregation and mechanical properties of steels has not been extensi v ly studied. This paper presents some preliminary results concerning the influence of st ress on intergranular phosphorus segregation and consequently on the DBTT of the 17-4 PH steel. Material Microstructure. 17-4 PH is a martensitic stainless steel. Its chemical c omposition is given in Table 1. The microstructure of the steel is illustrated in Fig. 1: the forme r austenitic grains ( γ grains) are divided in lath packets ( α grains). The steel is usually annealed at 1050°C, which leads to a γ grain size of 17 μm. Table 1: Chemical composition of 17-4 PH [% wt]. C Si Mn Ni Cr Mo Cu Co Nb N S P 0,031 0,31 0,81 4,82 15,61 0,03 3,12 0,03 0,21 0,03 0,02 0,016 Defect and Diffusion Forum Online: 2003-02-14 ISSN: 1662-9507, Vols. 216-217, pp 275-284 doi:10.4028/www.scientific.net/DDF.216-217.275

A‐TIG welding of UR 52N+ superduplex stainless steel

Summary The improved performance due to the use of flux in automatic welding of a superduplex stainless steel with the A‐TIG process is presented. The flux used during welding enables molten pool depth to be increased in a single pass, with complete penetration of a plate 5.6 mm thick and without edge preparation. Welding energy is located in the range of values recommended for this type of steel. The beads obtained were observed using optical micrography and were subjected to mechanical and corrosion tests, as well as ferrite content measurement testing. In addition, the influence of flux grain measurement and the nature of welding gas on flux effectiveness were studied. These tests reveal that the use of flux not only enables superduplex stainless steels 5.6 mm thick to be welded in a single pass but it also improves mechanical properties, corrosion characteristics and austenite content of welded joints, while maintaining an austenite volume fraction identical to that of the parent metal.

Ségrégation interfaciale : application aux matériaux industriels

La segregation interfaciale est un mecanisme qui est a l’origine d’endommagements bien connus dans les materiaux metalliques. Dans cet article, on montre que, pour des metaux industriels contenant notamment des precipites de l’element segregeant, il est necessaire de corriger les equations classiques de segregation pour tenir compte de la variation de la concentration volumique libre d’impuretes avec la temperature. Le cas de la segregation du soufre dans l’Invar est pris comme exemple.