René Le Gall

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.

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.

EBSD Phase Identification and Modeling of Precipitate Formation in HP Alloys

Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used as reformer tubes in petrochemical industries. Their composition includes Nb and Ti as strong carbide formers. In the ascast condition, alloys exhibit an austenite matrix with intergranular MC, M23C6 and/or M7C3 eutectic carbides. During exposure at high temperature, phase transformations occur: chromium carbides of M7C3 type transform into the more stable M23C6 type, intragranular M23C6 carbides precipitate, and a silicide, the G-phase (Ni16Nb6Si7), forms due to the instability of MC carbides (NbC). Thermodynamic simulation is of great help for understanding precipitate formation and transformations. Thermo-Calc and Dictra are used to simulate the precipitation of carbides in the austenite matrix during service. However, from an experimental point of view, M23C6 and M7C3 are not easy to distinguish in bulk alloys. Indeed, backscattered scanning electron microscopy does not bring any contrast between the two phases, and energy dispersive spectroscopy (EDS) analysis does not lead to carbon content and consequently to the distinction between M23C6 and M7C3. With transmission electron microscopy (TEM), sample preparation is difficult and the observed area is extremely small. In the present work, HP alloys are investigated by electron backscatter diffraction (EBSD) coupled to EDS. Carbides are identified on the basis of crystal structure, in the bulk, within their microstructural context, and the experimental procedure is both simpler and cheaper than TEM. Precipitates (M23C6, M7C3) could be identified by orientation mapping and single spot analysis.

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

Embrittlement of Steels by Liquid Zinc: Crack Propagation after Grain Boundary Wetting

This study characterizes the mechanical behavior of an advanced multiphase high strength steel by means of high temperature tensile testing. The results show a drastic reduction of the maximum tensile elongation from around 700 °C up to 950°C. Scanning electron microscopy investigations show that the temperature range for embrittlement is correlated with the total wetting of steel grain boundaries. Under external strain, crack propagates along the grain boundaries according to a mechanism that leads to the presence of nanometer-thick films of Zn at the crack tip, as shown by fine X-ray spectroscopy analyses. The effective temperature range for embrittlement is discussed. Mechanisms of i) external stress-free wetting, and ii) atomic-scale crack propagation, are today under discussion in the light of the literature, regarding in particular recent experimental results and theory about grain boundary wetting, intergranular penetration, and the correlation between surface energy and crack propagation rate.

Finite Element Simulation and Optimisation of Ageing Precipitation in Nickel Base Superalloys with a Low Gamma-Prime (g’) Volume Fraction

A finite element approach is used to simulate the precipitation of Ni3(Al,Ti) intermetallics in nickel-based superalloys containing a low volume fraction of spherical g’ precipitates, in which precipitation occurs following nucleation and growth mechanisms. Classical differential equations of nucleation and growth are implemented in the software Comsol (formerly Femlab), to compute the number of precipitates per unit volume and their mean size. Another originality of the model is the use of thermodynamic quantities coming from phase diagram computations (Thermo-Calc), like the temperature variation of the equilibrium g’ volume fraction, and the evolution of the concentration of g’ forming elements (Al, Ti) in the matrix with the volume fraction of precipitates. Once adjusted to experiment in the case of isothermal ageings, the model can be used to simulate precipitation during complex thermal histories. Finally, automatic heat treatment optimisation procedures are proposed and tested, which can reduce heat treatment times by a factor of more than five.

Intergranular Fatigue Cracking Enhanced by Impurity Segregation

It was previously reported that fatigue life of some alloys can be dramatically reduced if the grain boundaries contain a high level of impurity segregation before fatigue tests. In this paper the susceptibility of single phase brass samples (90Cu10Zn) to this form of damage is studied. After cold drawing of as cast brass bars, fatigue samples were heat treated at 800°C during 30min to promote recrystallization and impurity segregation at grain boundary. The samples were then tested under high frequency bending fatigue test at 200°C. After cracking, fracture surfaces were studied using both scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The SEM micrographs showed that the fractures were mostly intergranular. Chemical composition of intergranular cracks surface were analyzed using EPMA at low accelerating voltage. A high concentration of sulfur was found on most of grain boundary facets. The internal stress in alloys after fatigue was qualitatively estimated using electron backscattering diffraction in scanning electron microscopy. A high level of local misorientation was found near most grain boundaries. The mechanism of intergranular cracks formation during fatigue is discussed taking into account both the segregation of sulfur at grain boundaries and accumulation of plastic strain at grain boundaries

Direct Measurement of Impurity Segregation Concentration at Grain Boundaries for Polycrystalline Materials using EBSD and 3D Reconstruction of SEM images of Etch Grooves

Impurity segregation at grain boundaries in polycrystalline alloys is known to have a tremendous impact on the material properties such as mobility, cohesion.. But, direct measurement of grain boundary chemistry is quite complex and there are quite few results concerning polycrystals. In this paper we present an indirect method to measure segregation- misorientation dependence on polycrystalline Ni-S alloys using both EBSD and 3D reconstruction of etch grooves. Samples of Ni-S alloy (7.2 ppm at) have been annealed at different temperatures to get equilibrium segregation at grain boundaries. Then they have been etched near the transpassive potential to form etch grooves, whose geometry depends on the sulfur segregation level. Grain boundaries misorientation statistics and first results about the segregation-misorientation function are given here.

Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

ABSTRACT Fe–35Ni–25Cr–0.4C alloys with different compositions are aged between 750 and 1150°C up to ∼10,000 h. As-cast microstructure contains interdendritic carbides of type M7C3 (‘Cr7C3’) and MC (‘NbC’). At service temperatures, M7C3 transform into M23C6 (‘Cr23C6’) within hours. Then, a hardening precipitation of secondary intragranular M23C6 occurs over hundreds of hours, the nose of the ‘temperature-time-hardening’ curve being around 1000°C. G phase forms after long aging; its solvus temperature and formation kinetics depend on silicon content. Z phase is observed after long aging at 950°C or above. G and Z phases form at the expense of MC. Very long aging causes nitridation under air, with first a transformation of M23C6 into chromium-rich M2X carbonitrides (X = C,N), then of MC into chromium-rich MX carbonitrides.

Recent Developments in the Study of Grain Boundary Segregation by Wavelength Dispersive X-Ray Spectroscopy (WDS)

It was recently shown [1] that EMPA-WDS (Electron Probe MicroAnalysis by Wavelength Dispersive X-ray Spectroscopy) can be used to detect and to accurately quantify monolayer surface and grain boundary segregation. This paper presents the last developments of this application. It focuses on the measurement of sulphur grain boundary segregation in nickel on fractured surfaces. A special attention was paid to the quantification of the sulphur coverage, taking into account the non-normal incidence of the electron beam on a fracture surface. Sulphur grain boundary segregation kinetics was measured at 750°C in nickel to document the quantitative possibilities of the technique.