Régis Kubler

Evolution of residual stress in the diffusion zone of a model Fe-Cr-C alloy during nitriding*

Abstract Development and evolution of compressive residual stress during nitriding treatment are studied. A model carbon iron-based alloy Fe-3wt.%Cr-0.35wt.%C was nitrided in gas at 550 °C for different times. Microstructural investigation indicated an influence of the transformation and the associated carbon diffusion on the in-situ relaxation of residual stress induced by nitride precipitation. The transformation of initially present carbides into nitrides and the associated carbon accumulation ahead of the nitriding front is particularly taken into account. The distribution of residual stress in the nitrided case was predicted with a self-consistent mechanical model, using the volume changes associated with the phase transformations. To this end the nitrogen and carbon concentration distributions were converted into the equilibrium phase fractions of carbides and nitrides. An excellent correlation was obtained between experimental (X-ray diffraction analysis) and, thus, calculated independent residual stress distributions in the ferrite matrix.

External Reference Samples for Residual Stress Analysis by X-Ray Diffraction

The GFAC (French Association for residual stress analysis) decided in 2007 to work on external reference samples for residual stress analysis by X-ray diffraction as defined in the XPA 09-285 and EN 15305-2009 standards. Seven materials are studied: ferritic steel, martensitic steel, aluminium alloy, titanium alloy, 2 types of Nickel-Chromium alloy and tungsten thin layers deposited on silicon wafers. The purpose of this external round robin campaign is threefold: (i) to give possibilities for each laboratory involved in the campaign test to obtain external reference samples for each material tested, (ii) to validate a common procedure for qualification of external samples and (iii) to commercialise validated external reference samples through the GFAC association. A common approach of X-Ray diffraction parameters, samples geometry and standard procedure has been chosen and adopted by each laboratory involved in these tests. No indication in terms of residual stress calculation method is given; the choice of the method (centroid, middle point, maximum of peak, fitting…) is the choice of the laboratory according to their X-ray diffraction set-ups, softwares and experience. Once all samples are analysed, values given by each laboratory are compared and analysed.

On Residual Stresses Development during Nitriding of Steel: Thermochemical and Time Dependence

This work deals with the development of residual stresses during nitriding of steels. The main features of a chemico-thermo-echanical model of nitriding are presented. A micro-macroapproach is applied based on volume change computation in agreements with thermochemical modifications. Results are correlated with the characterization of a ternary Fe-C-3w.%Cr alloy nitrided at 550°C for various time. Residual stress-depth analyses are carried out by X-rays diffraction. Residual stress generation is deeply dependant on chemical and thermodynamical evolutions during the treatment, taking advantage on microstructural effects.

Stress Analysis in UMo-Al Fuel Using X-Ray Diffraction

UMo-Al specimens are analyzed using X-ray diffraction techniques. One specimen was partially irradiated using a heavy ion beam 127I. Another specimen was thermally annealed 2h at 400°C. Those treatments result in the formation of an interaction layer between UMo particles and Al matrix. UMo, Al and UAl3 phases are identified in the treated specimen using X-ray diffraction. Only aluminium phase exhibits a crystallographic fiber texture, the other phases having an isotropic crystallographic texture. X-ray stress analyses are performed. After irradiation, stress analyses show that UMo phase is in a compressive stress state whereas the stress level in the formed UAl3 in the interaction layer is not that high.

Experimental Analysis of Shot Peening on Carburized or Carbonitrided Parts

This experimental study focuses on the influence of shot peening on parts initially treated by carburizing or carbonitriding. Experimental investigations have been carried out: optical observations of the microstructures, in-depth hardness measurement, X-ray diffraction analysis of residual stresses. A comparison is made between the carburizing anad the carbonitriding treatments.

Residual stress development during nitriding of steels

Nitriding is a well-established thermochemical surface treatment of carbon micro-alloyed steels aiming enhancing surface properties such as fatigue, wear and corrosion resistances. The idea is taking benefits from the high hardening level due to a fine nitride precipitation and also the compressive residual stress state. Due to some complex interactions of phenomena during nitriding, the last has not been completely explained yet. When interest is focused on stress depth gradient and time evolution, difficulties find origins in pronounced heterogeneities whether it is the chemical gradient due to nitrogen diffusion, the resulting gradient of microstructure or the gradient of volumetric misfits. Relaxation of residual stresses is so usually described using a thermally controlled creep phenomenon due to the couple of diffusion and stress, but depend on phenomenological descriptions. A key point is also disregard that is the diffusion of carbon and its redistribution across the nitrided surface during the treatment. Based on experimental characterizations of model carbon iron-based alloys, the role of phase transformations, especially carbides, is explored in order to give better understandings of the residual stress development during nitriding.

Phase transormations involving residual stresses during gaseous nitriding of steel

Nitriding of steels is an important treatment in duplex hardening methods. This treatment is known in the art but some advances still need to be done in terms of residual stress understanding. Although they originate from volume micro-loading accompanying the precipitation of nitrides, questions about their in-depth distribution of a nitrided layer during the treatment are still a challenge. A chemico-thermo-micromechanical model has been developed on the volume change computation of secondary phase transformation. Supported by some experimental observations (TEM, X-ray analysis, Electron Probe Micro Analysis (EPMA), ...), this model gives some better understanding about nitriding. Residual stresses are mainly due to the precipitation of semi-coherent MN nitrides (M=Cr,V,Mo,. . . ). Moreover carbon, often not enough considered in the literature, is of importance in the treatment as it involves a second kind of precipitation that is the transformation of carbides M23C6 and M7C3 into incoherent nitrides. The volume change of this transformation is a critical entry data of the mechanical modelling.