JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 876-877 High temperature oxidation of the 18Mn18Cr0.5N austenitic steel Z. KUBE~,J. VESELA, Z. WEISS* Department of Metallurgy and Materials Science, University of West Bohemia, Americkb 42, 306 14 Plze5, Czech Republic The 18Mnl8Cr0.5N steel is a high strength non- magnetic austenitic steel for turbogenerator end rings. It has been used for large turbogenerators up to 1000 MW, in which Rp0.2 values of 1320 Nm -2 or greater are required for the end ring material [1, 2]. We report here on high temperature oxidation of this steel and on its compositional changes in the near-surface region when it is held at elevated temperatures in air. The samples for this study were cut from a block of cold formed material having the chemical compo- sition given in Table I. Polished samples were then annealed for time periods ranging from 1 to 16 h at T A B L E I Bulk composition of the 18Mn18Cr0.5N steel samples (wt %) C Si Mn Cr N P S 0.04 0.40 19.31 19.89 0.68 0.025 0.0007 1.0 L . O ~,.,C r - !... [ Cr Cl 5 I / iO,, ; 1 ' ~ ", ~ ................................................... "~" = t- o fo! ~, Mr 8 (a) • ~ .......................................... C.r._. (si /~'\, -~ i,/ \. _ \' ~ C ,/ -..._. C ,~ . ...... • ........ ~ ~-. : /C SI Sl . . . . . . . . . . . . . . . . . . '~ .--.~. ......O I I 5 10 15 Depth (gin) temperatures of 670 °C, 760 °C and 860 °C in air. In addition to metallographic investigations and X-ray diffraction (XRD) analysis, in-depth concentration profiles of the surface layers of the samples were analysed by glow discharge optical emission spectro- scopy (GD-OES) [3] using the LECO SDP-750 spectrometer [4]. For comparison, the same experi- ments were carried out with the 18Cr8Ni austenitic stainless steel. The results indicate complex changes in the near-surface layer, revealed by GD-OES depth profiles and metallographic investigations (Figs 1 and 2). In all the depth profiles presented, the scaling (full scale, wt %) for particular elements is contained in the figure caption. The scale layer (not visible in Figs lb and 2b) consists of Mn203 and (Mn, Fe)203 oxides, both confirmed by X-ray dif- fraction. Below the oxide film we found a layer strongly depleted in manganese (bright in Figs lb and 2b) which has a feritic structure, as revealed by X-ray diffraction. Below this region, a layer en- riched in carbon exists. Observed compositional 6 (a) 1.00 0.5 I I Cr 0 I 0 r'-. Fe i Cr./ ~ . . . . Cr_ ...... ,io , / s i ~ ~ ii Mn/ /?\, Fe l Cr ,/Fe ' Cf .-- --- - C_ ; ~ ' " ,,('_ Si __ Mn .... ............... 5 10 15 Depth (gm) Figurel 18Mn18Cr0.5N steel annealed at 760 °C for i h: (a) depth profile; (b) structure of the near-surface region. Fe 100% ; Mn 100% ; Cr 25% ; C 0.2% ; Si 1% ; O 40%. *Current address: LECO Corporation, Spectrosc. Appl. Lab, 300 Lakeview Ave, St. Joseph, M149085-2396, USA. Figure2 18Mnl8Cr0.5N steel annealed at 760 °C for 16 h: (a) depth profile; (b) structure of the near-surface region. Fe 100% ; Mn 100% ; Cr 25% ; C 0.2% ; Si 1% ; O 40%. 876 0261-8028 0 1995 Chapman & Hall