AN EBSD STUDY OF THE MICRO STRUCTURAL DEVELOPMENT DURING ANNEALING OF A FOLDED SUPER DUPLEX STAINLESS STEEL SHEET SAMPLE. H S Ubhi 1), J R Saithala 2) and H Jiang 1) 1) Oxford Instruments, High Wycombe. HP12 3SE. UK. 2) Fine Tubes Ltd, Plymouth, PL6 7LG UK Abstract: Super Duplex Stainless Steels (SDSS) show complex precipitation and transformation behavior during heat treatment processes, which affects both mechanical and corrosion properties. This report presents some data on the microstructures that develop after folding and subsequent precipitation during heat treatment of UNS S32750 SDSS sheet samples. The microstructural and textural changes have been followed using SEM/EBSD techniques. Upon folding both a texture and strain gradient form in the folded/bent region, subsequent heat treatment at 845 o C results in the ferrite phase to transforms to sigma, austenite and chi phases. Transformation was found to be accelerated by strain. Complete transformation of the ferrite phase occurred within half the annealing time required in the unstrained regions. The local mis-orientations in the ferrite and austenite phases reduced during annealing, however, the reduction in the austenite was not very high and a significant amount remained even after the longest annealing time. The texture components that developed during the folding process remained unchanged even after one hour annealing at 845 o C. The implication of these findings could have a bearing on the formation of sigma phase during welding of SDSS that may have residual stresses introduced during final processing. Key words: EBSD; super duplex stainless steel (SDSS); folded strain gradient, phase transformations. 1 Introduction The microstructural development during the heat treatment of duplex steels has been the subject of numerous studies [1,2,3], that have shown formation of deleterious intermetallic phases such as sigma and chi. However, there has been little reported on the kinetics of these phases due to strain. This study is an attempt to get some insight into this phenomenon. In a recent study [4], the strain or local mis-orientation profiles across the bent regions in folded sheets, were reported. The folding process introduced both, tensile and compressive gradients in addition to texture gradients [6]. The bent regions hence provide a suitable location to study the affects of varying strain and texture on phenomena such as recovery, recrystallisation and phase transformations in one specimen. The current work was therefore performed on folded super duplex stainless steel (SDSS) UNS S3270. Since the EBSD technique can easily distinguish the various phases during the transformation of SDSS, in addition to determining in local strain and texture, this technique has been used to follow the microstructural changes during heat treatment of the folded specimens.
2. Experimental SDSS sheet 1.5mm thick was folded perpendicular to the rolling direction to 90 o in a press brake. Specimens were then cut from the folded sheet and heat treated in air at 845 o C for 1, 3, 6, 10, 20 and 60 minutes. The respective heat treatment profiles are shown in Figure 1a.The specimens were then mounted in Bakelite and prepared by standard metallographic practice. Colloidal silica was used for the final finishing step. The polished specimens were examined in a FEGSEM and EBSD data acquired using an Oxford Instruments - Nordlys F+ detector with CHANNEL5 software. EBSD maps were collected from four regions A, C, N and T in each specimen as illustrated in Figure 1b i.e. one away from the fold (A) and 3 from the across the fold/bent section, i.e. compressed (C), neutral (N) and tensile(t). All data was acquired at 15kV acceleration voltage 17nA probe current with a step size of 0.2µm. a) b) Figure 1. a) Schematic diagram showing the 4 regions from where EBSD data was acquired b) Heat Treatment profiles of UNS S32750 material at 845C for various times. 3 Results and Discussion The microstructures in the parent sheet and the compressed region before and after heat treatments for 60 minutes are shown in the SEM back-scattered images in Figures 2 (a-c). The change in microstructure is evident, but information regarding phase distribution or fractions is difficult to discern from these images. a) b) c) Figure 2. SEM back-scattered images from a) parent as received sheet b) folded compressed region and c) folded compressed region after 60 minutes at 845 o C. However, these are easily discriminated by EBSD as illustrated in the series of phase EBSD maps in Figure 3 (a-c, d-f and g-h) from the as received, and specimens heat treated for 3 and 60
a) d) g) b) e) h) c) f) i) 20µmm Figure 3. EBSD phase maps from regions C, N and T a-c) as received sheet, d-f) 3minutes and g- i) 60 minutes at 845 o C minutes at 845 o C respectively. The variation in the rates of phases and fraction changes during heat-treatment are shown in Figure 4 (a-c) and 5 (a b). It is clear from these sets of data that, while the initial ferrite fraction of approximately 50% transforms to austenite, sigma and chi phases, the rate and amount of the sigma phase in the strained regions is higher and only a small amount of chi forms. In the unstrained regions the time for completion of transformation is about 6 minutes while it is about 3 minutes in the strained regions. The ratio of transformed austenite to sigma is approximately 1:1, in the unstrained regions, whereas it is about 3:7 in the compressed regions. The variation of the mean mis-orientation in the ferrite, austenite as sigma phases during heat treatment are shown in the EBSD maps and plots in Figures 6 and 7 respectively. These data show that while the mean misorientation in the ferrite in the strained regions reduces with annealing time, it does not reduce significantly in austenite and remains at the same low levels for the sigma phase. The strain in the austenite may be due to the formation of the sigma phase as well as slow recovery and recrystallisation in the newly formed austenite from ferrite. The changes in texture at the bend from folding and subsequent heating are shown in Figures 8 a- f) and g-p) in the as folded sheet and after heat-treatment for 3 minutes at 845 o C. It is clear that the texture in the as received sheet is reinforced in the tensile region, i.e. strengthening of the {111} and {110} fibre components in the fcc and bcc phases and the rotated cube and weak {110} fibre develop in the bcc and fcc phases in the compressed region respectively. During annealing these texture components remain unchanged and the sigma phase develops a weak texture. Detailed interpretation of texture development is a subject of further work.
a) b) c) Figure 4. a-c) Plots of % phase vs region for austenite, ferrite and sigma respectively. a) b) Figure 5 (a-b). Plots of % phase vs time at 845 o C for the region with no strain and compression respectively. 4 Conclusions During annealing transformation of the ferrite phase to austenite and sigma phases was found to be accelerated by strain. Complete transformation of the ferrite phase in the strained regions required half the time that required in the unstrained regions. The fraction of sigma phase in the strained regions was also higher than that found in the unstrained regions. Local mis-orientations did not indicate completely strain free grains in the austenite phase. The local mis-orientations in the sigma phase were low. The texture components that developed during the folding process in the austenite were unchanged even after one hour annealing at 845 o C and is the subject to further study. The corrosion behavior of these specimens is also being investigated. 5 References. 1. Michael Pohl, Oliver Storz, Thomas Glogowski, Effect of intermetallic precipitations on the properties of duplex stainless steel. Materials Characterization [J] 2007, 58, 65 71. 2. E. M. L. E. M. Jackson, P. E. de Visser, and L. A. Cornish~Distinguishing Between Chi and Sigma Phases induplex Stainless Steels Using Potentiostatic Etching. Materials Characterization [J] 1993, 31:185-190. 3) R.A. Perren, T. Suter, C. Solenthaler, G. Gullo, P.J. Uggowitzer, H. Bohni, M.O. Speidel Corrosion resistance of super duplex stainless, steels in chloride ion containing environments:investigations by means of a new microelectrochemical method II. Influence of precipitates Corrosion Science [J] 2001, 43, 727-745. 4) H S Ubhi and, H Jiang, Study of Recovery and Recrystallisation in Folded bcc, fcc and hcp Sheet Samples, Rex & GG, [C] 2010 to be published.
a) d) g) b) e) h) c) f) i) 20µm Figure 6. EBSD mean mis-orientations maps from regions C, N and T a-c) as received sheet, d-f) 3minutes and h-i) 60 minutes at 845oC a) b) c) Figure 7. a-c) Plots of mean mis-orientation vs region for austenite, ferrite and sigma phases.
a) g) b) h) i) c) j) d) l) m) e) n) f) o) p) Figure 8. Series of pole figures from the austenite ferrite and sigma phases from a-f) as folded sheet and g-p) after 3 minutes at 845 o C anneal.