DECOMPOSITION OF EXPANDED AUSTENITE IN AISI 316L STAINLESS STEEL NITRIDED AT 450°°°°C

Frederico Augusto Pires Fernandes1, Luiz Carlos Casteletti2, George Edward Totten3 and Juno Gallego4*

1Doctoral Student, Department of Materials Engineering, São Carlos School of Engineering, University of São Paulo, Brazil. 2Full Professor, Department of Materials Engineering, São Carlos School of Engineering, University of São Paulo, Brazil. 3Visiting Professor, Department of Mechanical and Materials Engineering, Portland State University, USA 4Associate Professor, Department of Mechanical Engineering, Univ Estadual Paulista – UNESP at Ilha Solteira, Brazil. *Corresponding author: gallego@dem.feis.unesp.br

ABSTRACT Abstract: Expanded Austenite (γN) can be produced during plasma nitriding of austenitic stainless steels and it provides high levels of strength, toughness and corrosion resistance compared to traditional nitride layers. However, expanded austenite properties are significantly harmed by decomposition caused by thermodynamic metaestability. In the present work, austenitic stainless AISI 316L steel was plasma nitrided at 450°C for 5 hours at 5mbar and microstructurally characterized by X-ray diffraction (XRD), optical and transmission electron (TEM) microscopy which confirmed the presence of FCC expanded austenite with a lattice parameter up to 9.5% larger than untreated austenite. TEM analyses of thin foils showed that fine nitrides were formed in the γN layer and some areas were observed with a singular lamellar morphology very similar to the pearlite colonies found in carbon-steels. SAED (selected area electron diffraction) analysis suggests that these areas are composed of BCC ferrite and cubic chromium nitrides produced after a localized decomposition of the expanded austenite layer. The occurrence of γN decomposition is associated with a microsegregation of ferrite stabilizers (Cr, Mo) and depletion of an austenite stabilizer (Ni) in regions of the layer in which wear and corrosion properties were lost. INTRODUCTION

AISI 316L austenitic stainless steel provides excellent corrosion resistance due to higher amounts of chromium, nickel and molybdenum combined with lower carbon content. However, its hardness and wear resistance are relatively poor, limiting some applications. Low temperature nitriding can improve both hardness and wear resistance with the formation of expanded austenite (γN)1-3 which has higher strength, toughness and corrosion resistance compared to the traditional nitride white layers. However, expanded austenite layer properties are significantly harmed by decomposition caused by its thermodynamic metaestability4. The aim of the present work is to investigate microstructural changes in expanded austenite produced by nitriding at 450°C.

EXPERIMENTAL PROCEDURE

Disks of approximately 3mm thickness were machined from a commercial round AISI 316L stainless steel bar whose chemical composition is given in Table 1. Polished surfaces for nitriding were prepared after wet-grinding with sandpaper up to 1500 grit and final mechanical polishing with 1.0µm alumina. Sputtering in a 500Pa argon atmosphere was performed during 0.5h at 400°C followed by direct current plasma nitriding at 450°C for 5h at the same pressure. The nitriding atmosphere was composed of 80% H2 and 20% N2 by volume.

Tab. 1: Chemical composition (wt%) of the AISI 316L stainless steel

C Mn Si Cr Ni Mo N Cu Fe

0.019 1.47 0.40 16.26 10.50 2.02 0.067 0.47 Bal.

Small pieces were cut from the nitrided layers for preparation of conventional cross-sectional metallographic samples for observation under Zeiss® Axiotech optical microscope. After Bakelite mounting and the metallographic preparation procedure previously described, the samples were etched in a fresh solution containing 75 ml hydrochloric acid and 25 ml nitric acid.

The phase constituents in the substrate and plasma nitrided layer were analyzed with a Rigaku® Geirgerflex difractometer (Bragg-Brentano configuration) using Cu Kα1 radiation (1.5405Å) and graphite monochromator. The diffracted intensity was recorded between 30° to 100° range, swept with speed of 2°/min-1. Diffracted reflections obtained from X-Ray Diffraction (XRD) patterns were compared with Crystallographic Information Files (CIF) available from the Inorganic Crystal Structure Database (ICSD).

Transmission electron microscopy (TEM) was applied to investigate a plan-view section of nitrided layer where the treated surface of the thin foil is normal to incident electron beam. The substrate side of nitrided samples were carefully ground up to 150µm thick when 3.0mm diameter disks were punched with an appropriate device. A transparent thick lacquer protective layer was deposited on the nitrided surface to prevent its degradation during TEM sample preparation. Thin foils were obtained by one-side jet electro-polishing where a 5% perchloric acid-95% acetic acid solution (by volume) at room temperature was used with a polishing potential/current of 40V/40 mA respectively. The TEM observations were performed in a Philips® CM120 microscope equipped with an EDS microanalysis device operated at 120kV (λ = 0.0335Å). Crystallographic parameters were determined from Selected Area Electron Diffraction Patterns (SAED), using a standard calibrated camera length (λL = 36.4Å.mm). RESULTS AND DISCUSSION The microstructure of the AISI 316L sample was composed by 40µm equiaxed austenitic grains as shown in Fig. 1(a). Optical microscopy indicates that plasma nitriding performed at 450°C results in a homogenous and uniform layer without significant nitride formation. Small, but perceptible, changes in layer thickness were noted which are probably due to anisotropic nitrogen diffusion into austenite grains with different crystallographic orientations.

The XRD patterns obtained from untreated and nitrided AISI 316L steel are shown in Fig. 1(b). The substrate presents diffraction peaks at 43.7°, 50.8°, 74.7°, 90.7° and 95.9° 2θ scattering angles which are consistent respectively with {111}, {200}, {220}, {311} and {222} reflections of γ austenite. Small intensity of {220}γ reflection was attributed to the texture introduced by rolling. After plasma treatment, some of these γ reflections, such as {111} and {200}, were observed with low relative intensity while others reflections are completely missing. Diffraction peaks were observed on 41.2°, 46.0° and 83.4° 2θ scattering angles after nitriding at 450°C which are respectively associated with {111}γN , {200}γN and {311}γN expanded austenite reflections. The results in Tab. 2 confirm the increase of the γN lattice parameter and its anomalous behavior relating to untreated FCC austenite (ao = 3.59Å) due to anisotropic expansion1.

Fig.1: Optical cross section of plasma nitrided AISI 316L at 450°C showing 15µm thick homogeneous expanded austenite layer in (a); XRD patterns of substrate and nitrided layer in (b).

Tab. 2: Lattice expansion ∆ calculated for each reflection of XRD results.

γγγγN reflection 2θθθθ angle [°°°°] Lattice parameter [Å] ∆∆∆∆ expansion [%]

{111} 41.2 3.79 5.3

{200} 46.0 3.94 9.5

{311} 83.4 3.84 6.7 Atomic nitrogen in solid solution occupies an octahedral interstitial site in the FCC lattice of austenite. Its presence promotes a decrease of the stacking fault energy. Therefore, higher stacking fault densities and compressive residual stresses can be expected in the expanded austenite. The presence of planar defects and their elastic strain fields have resulted in different shifts for each γN reflection in the XRD pattern according to Warren’s theory5. Thin foil observations made by TEM have confirmed a massive formation of stacking faults in the expanded austenite layer as shown in Fig. 2. Calculations performed from the SAED patterns indicate that lattice expansion of γN is slightly higher (14%) than those obtained by XRD analysis which is due to the lower level of compressive residual stress in thin foil samples6.

Fig.2: Dark field TEM micrographs showing two sets of stacking faults bundles in same area of γN thin foil sample. Selected spots near (002) in (a) and (131) in (b). The investigation performed by TEM showed that the microstructure of the expanded austenite layer is complex. In some regions, small rounded particles (100 – 150Å) were found as shown in Fig. 3(a). The ring-type SAED pattern indicates that there are a larger number of diffracting particles with a preferential orientation (texture) related to the substrate which was not investigated here. Indexing diffracted rings have shown that these particles posses a crystalline structure compatible with cubic chromium nitride (CrN) whose volume fraction is considerably smaller than the detection limit of the XRD technique. The amorphization of expanded austenite was also observed in sparse regions of the nitrided layer as depicted in Fig. 3(b). This kind of structure is easily confirmed by broad and diffuse halos in a ring-type SAED pattern. The localized loss of crystallinity can be attributed to the effect of nitrogen implantation processing combined with the low diffusivity of substitutional elements such as chromium7.

Fig.3: Thin foil TEM micrographs showing fine chromium nitride precipitation in (a) and an amorphous region (A) near decomposed γN area (D) in (b). Some regions of the expanded austenite layer presented a peculiar lamellar microconstituent as shown in Fig.3(b) and Fig.4(a). SAED Analyses showed that the lamellae were composed of BCC ferrite and CrN. Formation of the aggregate can be considered a result of expanded austenite decomposition8 the presence of which can harm both wear and corrosion resistance of the nitrided layer. EDS microanalysis

confirm that chemical composition of regions with and without decomposition are significantly different. Regions of γN as P1 in Fig. 4(a) usually possess lower Cr and Mo (α stabilizers) and higher contents of Ni (γ stabilizer). Nevertheless, decomposed γN regions as P2 contain a very high content of α stabilizers and a low amount of γ stabilizer probably due to microsegregation events.

Fig.4: TEM micrograph showing in (a) regions of expanded austenite (P1) and γN decomposition (P2). EDS microanalysis spectra of P1 and P2 positions are presented respectively in (b) and (c). CONCLUSIONS The investigation of nitrided AISI 316L samples indicates that fine chromium nitride was formed which cannot be identified by XRD analysis due to a low volume fraction but TEM is able to identify its presence. Some sparse regions of the expanded austenite layer presented amorphization. Evidence of expanded austenite decomposition was found on the nitrided surface. The formation of these decomposed areas is related to enrichment of chromium/molybdenum (ferrite stabilizers) and depletion of nickel (austenite stabilizer) of the expanded austenite. ACKNOWLEDGEMENTS

The authors would like to thank Brazilian research agencies CNPq (L.C.C. and J.G.) and CAPES (F.A.P.F.) for grants received which supported this work. REFERENCES [1] Dong, H. International Materials Reviews, v. 55, 2010, pp. 65-98. [2] Rivière, J.P. et al. Surface & Coatings Technology, v. 201, 2007, pp. 8210-8214. [3] Christiansen, T. and Somers, M.A.J. Scripta Materialia, v. 50, 2004, pp. 35-37. [4] Williamson, D.L. et al. Surface & Coatings Technology, v. 65, 1994, pp. 15-23. [5] Warren, B. E. X-Ray Diffraction. Dover, New York, 1990, 381p. [6] Meletis, E. I., Singh, V. and Jiang, J. C. Journal of Materials Science Letters, v. 21, 2002, pp. 1171-1174. [7] Collins, G. A. et al. Surface & Coatings Technology, v. 74-75, 1995, pp. 417-424. [8] Mitchell, D.R.G. et al. Surface & Coatings Technology, v. 165, 2003, pp. 107-118.