Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy Sebastien Jegou, Laurent Barrallier, Regis Kubler To cite this version: Sebastien Jegou, Laurent Barrallier, Regis Kubler. Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy. Acta Materialia, Elsevier, 2010, 58 (7), pp.2666-2676. 10.1016/j.actamat.2009.12.053. hal-01333592 HAL Id: hal-01333592 https://hal.archives-ouvertes.fr/hal-01333592 Submitted on 24 Jun 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy S. Jegou a,b,*, L. Barrallier a, R. Kubler a a MecaSurf Laboratory, Arts & Metiers ParisTech, Aix-en-Provence, France b Aubert & Duval, Eramet Group Abstract Phase transformations during nitriding of a ternary carbon iron-based alloy Fe–3%Cr–0.345%C were studied, aiming for a better understanding of residual stresses generation and evolution. The relationship between the precipitation of Cr7C3 carbides and CrN nitrides, the induced volume change and the mechanical properties were investigated at three distinct depths of the diffusion zone. The relaxation of residual stresses arose through phase transformations according to the diffusion of nitrogen but also of carbon. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nitriding; Iron alloys; Residual stresses; Phase transformations; Precipitation 1. Introduction Understanding the generation of residual stress implies identifying misfits (or eigenstrains) between different In integrated design, a residual stress state must be opti- regions or phases within one part. In the case of nitriding, mized in order to enhance the mechanical properties of a knowledge of residual stress development is lacking owing part, such as fatigue life, wear or corrosion resistance, and to the complexity of the phenomenon. The precipitation of also to control residual macroscopic deformations to avoid nitrides due to additional nitrogen atoms implies a volume additional machining. Nitriding is a well-established increase in the surface (volumetric eigenstrain) balanced thermo-chemical treatment of iron alloys based on nitrogen with the untransformed core material, resulting in diffusion [1]. The objectives lie in taking advantage of high compressive residual stress generation. Induced volume hardening and internal stresses due to second phase change finds its origins in coupled effects between lattice strengthening [2]. This study deals with gas nitriding result- distortions, volume changes accompanying precipitation, ing in the formation of a two sub-layer surface: the thermal effects and chemical phase evolution [6–8]. A phe- compound layer composed of iron nitrides e-Fe2–3N and/ nomenological approach of macroscopic residual stresses 0 or c -Fe4N, increasing the tribological properties and corro- was recently made by Vives Diaz et al. [9] in the case of bin- sion resistance [3], followed by the diffusion layer, where ary nitrided iron-based alloys according to the different mainly sub-microscopic MN nitrides (M = Cr, V, Mo, ...) regions of a nitrided surface exhibiting different kinds of precipitate, ensuring fatigue and wear resistance [4]. precipitation of nitrides (semi- and incoherent, discontinu- Nitriding is an example where residual stresses arise ous). This approach is supported by the characterization of through the generation of chemically induced misfits [5]. the nanometric precipitation by means of atom probe tomography [10]. The dependence of the residual stress gra- * dient on nitriding conditions (time, temperature, nitriding Corresponding author. Address: MecaSurf Laboratory, Arts & potential) have been were fully characterized [11]. Stress Metiers Paris Tech, Aix-en-Provence, France. E-mail addresses: [email protected], jegou_sebastien@yahoo. relaxation was investigated during the treatment by fr (S. Jegou). in situ X-ray diffraction (XRD) analysis over the first 10 lm below the surface, concluding with stress relaxation 2. Experimental procedure by the superposition of thermally induced and precipita- tion-induced residual stresses, cooling having no influence 2.1. Material and nitriding process on the diffusion zone [12,13]. No residual plastic straining takes place through the diffusion surface after nitriding Specimen elaboration and nitriding treatments were car- [14]. The influence of post-treatment loading on diffusion ried out by Aubert & Duval. Specimens were from an iron- was also investigated and concluded with a thinner diffu- based alloy with 0.35 wt.% C and 3 wt.% Cr, quenched and sion layer with higher compressive residual stresses [15]. annealed at 590 °C. They were then nitrided for different In order to predict residual stress, some attempts were times (10, 50 and 100 h) and temperatures (500, 520 and made to model nitriding based on diffusion of nitrogen 550 °C) for a given nitrogen potential. All specimens were considering the precipitation of MN nitrides and linked ground and polished for the purpose of optical microscopy to a mechanical model. But all these models, which are and chemical analyses. Composition profiles were carried often available for binary iron-based alloys, only determine out by electron probe microanalysis analysis (EPMA) a global stress state and need some assumptions based on using a Cameca Electron Microprobe SX100 with acceler- either the diffusion of carbon (fraction of cementite) or ating voltage 10 kV and current 100 nA. Profiles were phenomenological stress-relaxation equations based on a obtained across the sections with single measurement thermal creep phenomenon [16–19]. points every 15 lm. For better quantification of carbon, In the case of carbon iron-based alloys, incoherent glow discharge optical emission spectrometry (GDOES) nitrides are formed by the transformation of initial car- was also performed along the depth of the specimens. bides [20,21]. Diffusion of the released carbon is usually reported during nitriding, with decarburization of the 2.2. Phase characterization using XRD close surface and carbon enrichment at the nitrogen diffu- sion front [15,22]. Carbon generally drives cementite pre- X-ray phase analyses were carried out using a cipitation at grain boundaries parallel to the nitrided Siemens D500 diffractometer with Cr Ka radiation (k = surface through the diffusion layer [22,23]. However, 0.228975 nm). Scanning conditions were in the range any metallurgical relations between these observations 40° 6 2h 6 140° with a step size of 0.032° in 2h and step and residual stress evolution have not been totally time 300 s. The JCPDS database was used for phase iden- exploited, limiting understanding to a complex interaction tification [29]. between nitrogen and carbon atoms and the correlation of experimental observations between the decarburization 2.3. Microstructural characterization of the surface with the decreasing residual stress at the beginning of the diffusion layer [7,18]. Moreover, even Optical microscopy investigations were performed prior if the coherent precipitation zone exhibits similar micro- to any analysis to ensure the quality of nitriding. Specimens structure evolution across the depth in terms of nitride were etched with 3% nital (3 ml HNO3, 97 ml ethanol). size and volume fraction [20,24,25], the gradient of resid- They were observed using a Leitz Aristomet optical micro- ual stresses can still not be explained, since no discontin- scope and a JEOL JMS 6400 scanning electron microscope. uous precipitation zone is observed as in the case of iron- Transmission electron microscopy (TEM) samples were based alloys. machined using focused ion beam (FIB) sample prepara- Thus, a nitrided layer is not homogeneous, and the tion [30]. Films were directly prepared on sample cross sec- microstructure has to be characterized in term of volume tions at the desired depths and thinned down to 50 nm. change evolution according to nitrogen and also carbon TEM observations were carried out using a JEOL JEM diffusion. Carbon is a key point not considered enough 2010F, and sample preparations with a Philips FIB 200 and too often neglected in residual stress generation and TEM instrument. evolution. Competition with residual stress relaxation Image analysis was performed using ImageJ software. through a controlled creep phenomenon is not yet fixed. The sizes of the precipitates were determined using the Indeed, carbide transformation during nitriding has not equivalent calculated diameters, and the standard devia- been considered in the estimation of volume change. tions obtained were used to determine the confidence This work deals with phase transformations and interval. induced volume changes according to nitrogen and carbon diffusion during the nitriding process. It aims for a better 2.4. Hardness measurements understanding of residual stress development and predic- tion of volume heterogeneities for a multi-scale numerical Hardness-depth profiles were obtained using a Leica model,