Decomposition of expanded austenite in AISI ... - UNESP Ilha Solteira

Decomposition of expanded austenite in AISI 316L stainless steel nitrided at 723K F. A. P. Fernandes1, L. C. Casteletti1, G. E. Totten2 and J. Gallego*3 Expanded austenite (cN), which can be produced during plasma nitriding of austenitic stainless steels, provides high levels of strength, toughness and corrosion resistance by comparison with traditional nitride layers. However, expanded austenite properties can be lost due to decomposition caused its thermodynamic metastability. In the present work, austenitic stainless AISI 316L steel was plasma nitrided at 723 K for 5 h at 500 Pa and microstructurally characterised by X-ray diffraction (XRD), and optical and transmission electron microscopy (TEM) 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 cN layer and some areas were observed with a singular lamellar morphology very similar to the pearlite colonies found in carbon steels. Selected area electron diffraction (SAED) analysis suggests that these areas are composed of bcc ferrite and cubic chromium nitrides produced after a localised decomposition of the expanded austenite layer. Amorphous expanded austenite was observed in some areas of the investigated samples. The occurrence of cN decomposition was associated with microsegregation of ferrite stabilisers (Cr, Mo) and depletion of an austenite stabiliser (Ni) in localised regions of the expanded austenite layer. Keywords: Austenitic stainless steel, Plasma nitriding, Expanded austenite, Decomposition, Nitride formation, Amorphous regions, Transmission electron microscopy, EDS microanalysis

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 (cN)1–5 which has higher strength, toughness and corrosion resistance compared to the traditional nitride white layers. However, expanded austenite layer properties are negatively affected by decomposition resulting from thermodynamic metastability.6,7 The aim of the present work is to investigate microstructural changes in expanded austenite produced by nitriding at 723 K.

Experimental procedure Discs of approximately 3 mm thickness were machined from a commercial round AISI 316L stainless steel bar 1

Department of Materials Engineering, Saõ Carlos School of Engineering, University of Saõ Paulo, Saõ Paulo, SP, Brazil Department of Mechanical and Materials Engineering, Portland State University, Portland, OR, USA 3 Department of Mechanical Engineering, Univ Estadual Paulista – UNESP at Ilha Solteira, Ilha Solteira, SP, Brazil 2

*Corresponding author, email [email protected]

ß 2012 IHTSE Partnership Published by Maney on behalf of the Partnership DOI 10.1179/1749514812Z.00000000025

(chemical composition in Table 1). Polished surfaces for nitriding were prepared after wet grinding with sandpaper (up to 1500 grit) and final mechanical polishing with 1?0 mm alumina. Sputtering in a 500 Pa argon atmosphere for 0?5 h at 673 K was followed by direct current plasma nitriding at 723 K for 5 h at the same pressure. The nitriding atmosphere was composed of 80 vol.-%H2 and 20 vol.-%N2. Small pieces were cut from the nitrided layers for preparation of conventional cross-sectional metallographic samples, which were examined using a Zeiss Axiotech optical microscope. After bakelite mounting and the preparation described above, 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 using a Rigaku Geirgerflex difractometer (Bragg-Brentano configuration) at Cu Ka1 radiation (154?05 pm) and a graphite monochrome device. The diffracted intensity was recorded between 30u and 100u, at a sweep speed of 2u min21. X-ray diffraction (XRD) patterns were compared with crystallographic information files available from the Inorganic Crystal Structure Database.8 A plan-view section of the nitrided layer, where the treated surface of the thin foil is normal to incident electron beam, was inspected by transmission electron microscopy (TEM). The substrate side of nitrided samples was carefully ground to 150 mm when 3?0 mm

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Decomposition of expanded austenite at 723K

1 a optical cross-section of plasma nitrided AISI 316L at 723 K showing 15 mm thick homogeneous expanded austenite layer and b XRD patterns of substrate and nitrided layer

electron beam was adjusted to give a detecting dead time between 30 and 40%. Crystallographic parameters were determined from selected area electron diffraction patterns (SAEDs), using a standard calibrated camera length.

Table 1 Chemical composition (wt-%) of 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.

Results and discussion Table 2 Lattice expansion D reflection of XRD results

calculated

for

each

cN reflection

2h angle/u

Lattice parameter/A˚

D expansion/%

{111} {200} {311}

41.2 46.0 83.4

3.79 3.94 3.84

5.3 9.5 6.7

The microstructure of the AISI 316L sample was composed of equiaxed austenitic grains with 40 mm average size and typical annealing twins, as shown in Fig. 1a. Optical microscopy indicates that plasma nitriding at 723 K results in a homogenous and uniform layer without significant nitride formation. Small, but perceptible, changes in layer thickness were noted, 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. 1b. The substrate presents diffraction peaks at 43?7u, 50?8u, 74?7u, 90?7u and 95?9u 2h scattering angles, which are consistent respectively with {111}, {200}, {220}, {311} and {222} reflections of c austenite. Small intensity of {220}c reflection was attributed to texturing by rolling. After plasma treatment at 723 K some of these c reflections, such as {111} and {200}, were observed with low relative intensity, while others reflections are not well defined. Diffraction peaks were observed at

diameter discs were punched with an appropriate device. A transparent thick lacquer protective layer was applied to the nitrided surface to prevent degradation during TEM sample preparation. Thin foils were obtained by jet electropolishing of one side where a 5 vol.-% perchloric acid–95 vol.-% acetic acid solution (by volume) at room temperature was used with a polishing potential/current of 40 V/40 mA respectively. The TEM observations were performed using a Philips CM120 microscope operated at 120 kV (l53?35 pm) and equipped with an energy dispersive X-ray spectroscopy (EDS) microanalysis device. EDS counting was performed during 100 s live time where the spot size of the

a selected spots near (002); b selected spots near (131) 2 Dark field TEM images showing two sets of stacking faults bundles in same area of cN thin foil sample

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3 Thin foil TEM images showing a fine chromium nitride precipitation and b amorphous region (A) near decomposed cN area (D)

41?2u, 46?0u and 83?4u 2h scattering angles after nitriding at 723 K respectively associated with {111}cN, {200}cN and {311}cN expanded austenite reflections. The results in Table 2 confirm the increase in the cN lattice parameter and its anomalous behaviour relating to untreated fcc austenite (ao5359?11 pm) due to anisotropic expansion.1 Atomic nitrogen in solid solution occupies an octahedral interstitial site in the fcc lattice of austenite. Its presence promotes a decrease in 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 cN reflection in the XRD pattern, according to Warren’s theory.9 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 cN is slightly higher (14%)

4 a TEM image showing regions of expanded austenite (P1) and cN decomposition (P2), and b, c EDS microanalysis spectra of P1 and P2 positions


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lower c stabiliser. This chemical behaviour is probably related to microsegregation events, which may have occurred during manufacture of the rolled bar used as substrate, or during nitriding treatment of the specimens.

than those obtained by XRD analysis which is due to the lower level of compressive residual stress in thin foil samples.10,11 The TEM investigation showed the microstructure of the expanded austenite layer to be complex. In some regions, as shown in Fig. 3a, small rounded particles (10–15 nm) were readily observable in the thin foils observed under dark field contrast. The ring type SAED pattern indicates that there is a larger number of diffracting particles with some preferential orientation (texture) related to the substrate (not investigated). Indexing diffracted rings have shown that these particles are crystalline and compatible with cubic chromium nitride (CrN) whose volume fraction is considerably smaller than the detection limit of the XRD technique. Amorphous expanded austenite was also observed in scattered regions of the nitrided layer, as shown in Fig. 3b. This kind of structure presents typically an image with monotone contrast and this is crystallographically confirmed by the presence of broad and diffuse halos in a ring type SAED pattern. The localised loss of crystallinity can be attributed to the effect of nitrogen implantation combined with the low diffusivity of substitutional elements such as chromium.12 Some regions of the expanded austenite layer presented a peculiar lamellar microconstituent as shown in Figs. 3b and 4a. SAED analyses showed that the lamellar regions were composed of bcc ferrite and cubic CrN. Formation of the aggregate can be considered a result of expanded austenite decomposition,11,13 considered harmful to both wear and corrosion resistance of the nitrided layer. EDS microanalyses confirm that chemical composition of regions with and without decomposition are significantly different, as shown in Table 3. Regions of normal expanded austenite cN as P1 in Fig. 4a usually have lower chromium and molybdenum content (both bcc ferrite a-iron stabilisers) and higher nickel (a fcc austenite c-iron stabiliser).14 Nevertheless, decomposed cN regions such as P2, contain a very high concentration of a stabilisers and

Summary The investigation of nitrided AISI 316L samples indicates that fine chromium nitride cannot be identified by XRD analysis, due to its low volume fraction, but can be identified by TEM. Other evidence of expanded austenite decomposition was found on the nitrided surface. Scattered regions of the expanded austenite layer were amorphous. The formation of these decomposed areas is related to enrichment in both chromium and molybdenum (ferrite stabilisers) and to depletion of nickel (austenite stabiliser) from of the expanded austenite.

Acknowledgements The authors would like to thank Brazilian research agencies CNPq (LCC and JG) and CAPES (FAPF) for grants received which supported this work.

References 1. H. Dong: Int. Mater. Rev., 2010, 55, 65–98. 2. J. P. Rivie`re, C. Templier, A. Decle´my, O. Redjdal, Y. Chumlyakov and G. Abrasonis: Surf. Coat. Technol., 2007, 201, 8210–8214. 3. T. Christiansen and M. A. J. Somers: Scr. Mater., 2004, 50, 35–37. 4. J. P. Rivie`re, P. Meheust, J. P. Villain, C. Templier, M. Cahoreau, G. Abrasonis and L. Pranevicius: Surf. Coat. Technol., 2002, 158– 159, 99–104. 5. D. Manova, J. W. Gerlach, F. Scholze, S. Ma¨ndl and H. Neumann: Surf. Coat. Technol., 2010, 204, 2919–2922. 6. D. L. Williamson, O. Ozturk, R. Wei and P. J. Wilbur: Surf. Coat. Technol., 1994, 65, 15–23. 7. T. Christiansen and M. A. J. Somers: Metall. Mater. Trans. A, 2006, 37A, 675–682. 8. Inorganic Crystal Structure Database (ICSD): ‘Crystallographic cards’, 2011, http://www.fiz-karlsruhe.de.w10001.dotlib.com.br/ icsd_web.html (accessed 12 September 2011). 9. B. E. Warren: ‘X-ray diffraction’; 1990, New York, Dover. 10. E. I. Meletis, V. Singh and J. C. Jiang: J. Mater. Sci. Lett., 2002, 21, 1171–1174. 11. D. R. G. Mitchell, D. J. Attard, G. A. Collins and K. T. Short: Surf. Coat. Technol., 2003, 165, 107–118. 12. G. A. Collins, R. Hutchings, K. T. Short, J. Tendys, X. Li and M. Samandi: Surf. Coat. Technol., 1995, 74–75, 417–424. 13. L. Wang, J. C. Sun and X. L. Xu: Surf. Coat. Technol., 2001, 145, 31–37. 14. R. W. K. Honeycombe and H. K. D. H. Bhadeshia: ‘Steels: microstructure and properties’, 3rd edn, 71–74; 2006, Oxford, Butterworth-Heinemann.

Table 3 Typical range of chemical composition (wt-%) obtained by EDS from different regions of expanded austenite cN layer in AISI 316L nitrided steel

106

cN

Cr/wt-%

Ni/wt-%

Mo/wt-%

normal decomposed

18.1–24.3 41.5–53.7

9.7–16.1 3.7–5.8

0.5–3.0 3.0–5.9


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