DESENSITIZATION OF AUSTENITIC AND DUPLEX STAINLESS STEELS BY LASER SURFACE MELTING Paper 806

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1 DESENSITIZATION OF AUSTENITIC AND DUPLEX STAINLESS STEELS BY LASER SURFACE MELTING Paper 806 Weng Kin Chan 1, Chi Tat Kwok 1, Kin Ho Lo 1, Zhichao Cheng 1 1 Department of Electromechanical Engineering, University of Macau, Taipa, Macau Abstract Sensitization is one of the corrosion mechanisms which cause widespread problems in stainless steels, particularly in welded assemblies and improperly heat-treated work pieces heated at 500 to 800 C. As the Cr-rich carbides and sigma phase form, they deplete their neighboring regions of Cr, thereby lowering the Cr contents of the surface oxides over these regions. Once the contents of Cr in the regions adjacent to the carbides drop below 12 wt%, the oxide layers lose their protectiveness. Then the stainless steels will suffer from intergranular corrosion attack. Laser surface melting (LSM) can be easily achieved by melting the surface with a laser beam followed by rapid solidification for homogenizing chemical compositions, desensitization of improperly heated surface (i.e. redisolving the chromium carbides) and even removing surface cracks. In the present study, LSM of austenitic stainless steels (AISI 304, 321 and 347) and duplex stainless steel (AISI 2205) was attempted for desensitization by a 2.5 kw Nd:YAG laser and the susceptibility of the sensitized stainless steels before and after LSM to intergranular corrosion was also evaluated. The degree of sensitisation (DOS) of the stainless steels was determined by the double loop electrochemical potentio-kinetic reactivation method in solution of 0.5 M H 2 SO 4 and 0.01 M KSCN at 25 o C by a potentiostat. The corrosion morphology and surface hardness were also investigated as well. Desensitization of stainless steels was successfully achieved by LSM and their intergranular corrosion resistance was found to be significantly enhanced as reflected by the decrease in DOS due to the low chromium depletion or possible solute segregation at the boundaries. Keywords: sensitization, stainless steels, intergranular corrosion, laser surface melting Introduction In the recent years, laser surface treating has been widely used for remanufacturing the engineering components with the merits of refined microstructure, strong metallurgical bond, low dilution ratio, minimum distortion and high processing speed. It brings great economic & social benefits including lower energy consumption, reduction of pollution, save precious materials and high value added to the components [1]. Laser surface melting (LSM) is an easy and economical process for surface modification. It is possible to produce a homogeneous surface with fine grain structure which is beneficial to pitting corrosion resistance of some stainless steels [2]. Austenitic stainless steels (ASSs) have excellent resistance to uniform corrosion. However, they are prone to localized corrosion such as intergranular corrosion, intergranular stress corrosion cracking, crevice corrosion and pitting corrosion. Intergranular corrosion (IGC) and intergranular stress corrosion cracking are caused by sensitization which refers to the formation of chromiumrich carbides along the grain boundaries and the concurrent depletion of chromium in the immediate vicinity. If the local chromium content drops below 12 wt%, then the chromium depleted zones become prone to local corrosion [3]. LSM can be achieved by melting the surface with the laser beam followed by rapid solidification for desensitization of improperly heated surface and removing surface cracks. LSM is reported to be an effective route for improving the IGC resistance of 304 and 316 [3,4]. Duplex stainless steels (DSSs) are constituted by a two-phase microstructure with a balance between austenite and ferrite. These steels are usually selected for applications requiring mechanical strength in medium to severe corrosive environments, such as chemical and petrochemical processes, paper production, chemical tankers, desalination plants and even civil engineering and architectural purposes [5]. The corrosion property of DSSs has been extensively studied [6-8]. However, the effect of LSM on IGC behavior of other grades such as DSSs and stablised ASSs is scarcely reported. In present study, the microstructure and IGC behavior of various sensitised ASSs and DSS before and after LSM were studied. In addition, their corrosion morphologies after double loop electrochemical potentiokinetic reactivation tests were investigated. Experimental details LSM of austenitic stainless steels 304 (Fe-18.4%Cr-8.7 Ni-1.6%Mn-0.08C), 321 (Fe-18%Cr-11%Ni-0.15%Ti- 0.08%C), 347 (Fe-17%Cr-10%Ni-0.8%Nb+Ta-0.08%C),

2 and duplex stainless steel 2205 (Fe-22.3%Cr-5.6Ni- 2.9%Mo-1.5%Mn-1.6Cu -0.03C) in form of plate with the dimensions of 20x20x5 mm 3 were attempted using a 2.5- kw CW Nd:YAG laser. In order to sensitize the stainless steels, the ASSs were aged at 600 o C for 40 hour while the DSS were aged at 800 o C for 40 hour in a furnace. LSM of the sensitized ASSs and DSS were carried out by the laser with power of 1 kw, a beam size of 4 mm in diameter and a scanning speed of 25 mm/s, argon flowing at 20 l/min was used as the shielding gas. The surface was achieved by overlapping the melt tracks with degree of overlapping of 30%. The sensitized specimens attained by aging were designated as XXX-S and the aged specimens followed by LSM were designated as LM-XXX-S (XXX represents the stainless steel). The microstructures the laser-melted specimens were studied by optical microscopy and X-ray diffractometry (XRD). To investigate IGC behavior before and after LSM, the specimens were embedded in cold curing epoxy resin, exposing a surface area of 1 cm 2. The degree of sensitisation (DOS) was determined by the double loop electrochemical potentio-kinetic reactivation (DLEPR) method at 25 o C by a potentiostat (VoltaLab 10) according to ISO Standard [9]. The test solution comprised of 0.5 M H 2 SO 4 and 0.01 M KSCN was prepared from reagent grade chemicals in distilled water. All potentials were measured with respect to a saturated calomel electrode (SCE, 0.244V versus SHE at 25 o C) as the reference electrode. Two parallel graphite rods served as the counter electrode for current measurement. Scanning was started from open circuit potential and reversed from potential of +300 mv (SCE) at a scan rate of 6 V/h. The DOS was evaluated by measuring the ratio of (I r / I a ) 100%, where I r is the peak reactivation current density and I a is the peak activation current density. In addition, the corrosion morphologies of the laser-melted specimens after DLEPR test were studied by scanning electron microscopy (SEM). Results and discussion 1. Microstructural analysis The transverse cross-sectional views and microstructures at a higher magnification of molten pools and the sensitized zones of the various sensitized stainless steels after LSM are shown in Figs. 1 to 4. The surface layers of all specimens were free of cracks and pores. From the top of the surface, the laser-melted specimens consisted of the melted zone (MZ) and the substrate (sensitized zone). Ditches at the grain boundaries were clearly observed in the sensitized zone of LM-304-S as shown in Fig. 1(c) while sigma phase (σ) was found at the γ δ boundaries as shown in Fig. 4(c). LSM caused melting and rapid solidification and melt zones of about 0.3 mm were obtained. Refined dendritic microstructure and homogenized compositions were achieved after LSM. According to the XRD spectra as shown in Fig. 5, lasermelted 304, 321 and 347 contained austenite as the major phase and δ-ferrite as the minor phase. On the other hand, laser-melted 2205 was mainly composed of δ, with γ as the minor phase. In addition, the secondary phases such as the carbides in the aged ASSs and sigma phase in aged DSS are completely removed after LSM. In LM-304-S, the skeletal network of residual δ-ferrite is present in the austenitic matrix as shown in Fig. 1 Austenitic dendrites in different orientations are observed in LM- 321-S and LM-347-S as shown in Fig. 2 and 3. After LSM, the grain size of the austenite in LM-304-S and LM-347-S was refined due to rapid solidification. On the contrary, the grain size in LM-2205 was increased and its microstructure is shown in Fig. 4. The Widmanstatten structure of semi-continuous dendritic austenite was present in the columnar grain boundaries of ferrite. When the molten pool of various stainless steels solidifies, the possible phase transformation sequence upon cooling may be represented as: liquid liquid + δ δ δ + γ The degree of completion of the transformation and hence the final phase structure depend on the composition of the stainless steels and the processing parameters [10]. The ratios of the chromium equivalent (Cr eq ) and the nickel equivalent (Ni eq ) of various stainless steels are shown in Table 1. As the Cr eq /Ni eq ratio increased, the ferriteforming tendency of the stainless steels increased. The highest δ-ferrite content was observed in 2205, which had the highest Cr eq /Ni eq ratio. In addition, the solid-state transformation of δ to γ is considered to be diffusional. Thus, the high solidification rate typical in LSM would also suppress δ to γ transformation, resulting in high δ- ferrite content for However, the disturbance of the δ/γ balance in the melt zone might change the mechanical properties and corrosion resistance of DSS [11]. Stainless steels Cr * # eq /Ni eq ratio Phase present in Phase present in sensitized specimens laser-melted specimens γ, Cr 7 C 3, γ, δ Cr 23 C γ, TiC γ, δ γ, NbC γ, δ γ, δ, σ, Cr 23 C 6 γ, δ * Cr eq = [Cr] + [Mo] + 1.5[Si] + 0.5[Nb] # Ni eq = [Ni] + 30[C] + 30[N] + 0.5[Mn] Table 1 Cr eq /Ni eq ratio of and phase present in various stainless steels at sensitized and laser-melted conditions.

3 Substrate Figure 2 Optical micrographs of LM-321-S crosssectional view, microstructure of molten pool. Substrate (c) Figure 1 Optical micrographs of LM-304-S crosssectional view, microstructure of molten pool, (c) microstructure of sensitized zone. Figure 3 Optical micrographs of LM-347-S crosssectional view, microstructure of molten pool. Substrate Sensitized zone

4 α γ 50 µm (c) γ σ Figure 4 Optical micrographs of LM-2205-S crosssectional view, microstructure of molten pool, (c) microstructure of sensitized zone. 2. Intergranular corrosion behavior Fig. 6 shows the DLEPR curves of various stainless steels attained by different conditions: sensitized, and sensitized and followed by LSM. For all specimens, corrosion attack in forward scan takes place at the locations anodic to the austenitc matrix, that is, at the Cr depleted zones adjacent to the grain boundaries, interdentritic regions, δ-ferrite or σ phase (due to segregation effects). On the other hand, corrosion attack in reverse scan arises from incomplete passivation during a forward scan reflecting the degree of Cr depletion resulting from chromium carbide precipitation or σ phase at the grain boundaries. Table 2 shows the data of I r, I a and DOS of various specimens extracted from the curves. For laser-melted 304 and 321, they possess much smaller value of DOS than that of the aged specimens. Among the specimens, the DOS of 304-S is the highest (36.6%) but it is much reduced after LSM (1.2%). For 347 and 2205, the difference in DOS is not significant before and after LSM because they are unsensitized and have low susceptibility to IGC. From the values of DOS, the IGC resistance ranking of the sensitized stainless steels is: 347-S > 2205-S > 321-S > 304-S After LSM, the IGC resistance ranking becomes: LM-347-S > LM-321-S > LM-2205-S > LM-304-S Figure 5 XRD spectra of various sensitized and lasermelted stainless steels 304, 321, (c) 347 and (d) Specimens Table 2 DOS of various specimens after DLEPR test I r (ma/cm 2 ) I a (ma/cm 2 ) I r / I a DOS (%) Interpretation * 304-S Sensitized LM-304-S S Slightly sensitized Slightly sensitized LM-321-S Unsensitized 347-S Unsensitized LM-347-S Unsensitized 2205-S Unsensitized LM-2205-S Unsensitized * Conforming to ISO 12732:2006 [9].

5 (d) Figure 6 DLEPR curves for sensitized 316L before and after LSM 3. Corrosion morphology (c) Corrosion morphologies of the sensitized and laser-melted specimens after DLEPR tests were observed by SEM as shown in Fig. 7. From Fig. 7(i)-(c)(i), corroded 304-S reveals ditch structure (more grains completely surrounded by ditches), corroded 321-S reveals dual structure (some ditches at grain boundaries), and corroded 347-S reveals step structure (some step between grains but no ditches at grain boundaries). This indicates that IGC attack is the most serious for 304-S at the grain boundaries [Fig. 7(i)]. For 2205-S, the boundaries between the σ phase and γ/δ was attacked as shown in From 7(d)(i). On the other hand, the laser-melted ASSs exhibited less corrosion attack at the interdentritic boundaries [Fig. 7(ii)-(c)(ii)] with enhanced IGC resistance which is attributed to low chromium depletion or possible solute segregation at the boundaries [12]. For the laser-melted 2205, small and shallow pits were observed at the more active ferritic phase [Fig. 7(d)(ii)]. From the corrosion morphology, it might be concluded that a more homogenous microstructure and dissolution of carbide phases in the solid solution of the laser-melted ASSs, were the beneficial factors for enhancing the IGC resistance. However, the disturbance of the δ/γ balance in the melt zone of DSS 2205 might not be advantageous to IGC resistance due to the galvanic effect of δ and γ phases. Conclusions LSM was attempted for desensitization of austenitic and duplex stainless steels. After LSM, the sensitized austenitic stainless steels 304, 321 and 347 were essentially austenitic with the presence of a small amount of ferrite while carbide phases formed by aging were completely dissolved. For laser-melted duplex stainless

6 steel 2205, ferrite became the major phase and the δ/γ phase balance was disturbed but σ phase was removed. The intergranular corrosion resistance of 304 and 321 was found to be considerably improved as reflected by the reduced values of DOS due to the more homogenous microstructure and dissolution of carbide phases in the solid solution. While of the effect of LSM on the DOS of 347 and 2205 was not significant because they were unsensitized under the present aging conditions. (i) (ii) corrosion resistance, in Proceedings of 6th International Symposium on Environmentally Conscious Design and Inverse Manufacturing (EcoDesign 2009), Sapporo, Japan, paper GT3-1E-6. [2] Kwok, C.T., Man, H.C., Cheng, F.T. (1998) Cavitation erosion and pitting corrosion of laser surface melted stainless steels, Surface and Coatings Technology 99, [3] Parvathavarthini, N., Mulki, S., Dayal, R.K., Samajdar, I., Mani, K.V., Raj, B. (2009) Sensitization control in AISI 316L(N) austenitic stainless steel: Defining the role of the nature of grain boundary, Corrosion Science 51, [4] Anthony, T.R., Cline, H.E. (1978) Surface normalization of sensitized stainless steel by laser surface melting, J. Appl. Phys. 49, (ii) (i) [5] Ota rola, T., Hollner, S., Bonnefois, B., Anglada, M., Coudreuse, L., Mateo, A. (2005) Embrittlement of a superduplex stainless steel in the range of oc, Engineering Failure Analysis 12, [6] Moura V.S., Lima L.D. (2008) Influence of microstructure on the corrosion resistance of the duplex stainless steel UNS S31803, Mater Charact. 59, (c)(i) (c)(ii) (d)(ii) (d)(i) γ [7] Invernizzi A.J., Sivieri E. (2008) Corrosion behaviour of Duplex stainless steels in organic acid aqueous solutions, Mater Sci Eng A 485, [8] Merello R, botana FJ. (2003) Influence of chemical composition on the pitting corrosion resistance of nonstandard low-ni high-mn N duplex stainless steels. Corrosion Science 45, [9] International Standard, ISO (2006), Corrosion of metals and alloys - Electrochemical potentiokinetic reactivation measurement using the double loop method (based on Cihal's method). σ Figure 7 Corrosion morphologies of 304, 321, (c) 347, (d) 2205 at (i) sensitized and (ii) sensitized and lasermelted conditions after DLEPR test Acknowledgments The work described in this paper was fully supported by research grant from the Science and Technology Development Fund (FDCT) of Macau SAR (Grant no. 019/2009/A1). References [1] Wong, P.K., Chan, W.K., Kwok, C.T., Lo, K.H. (2009) Applications of laser remanufacturing for wear and [10] Noble, D.N. (1990) Selection of wrought duplex stainless steels, ASM Handbook, Vol. 6, 10th ed., Welding, Brazing, and Soldering, ASM International, Materials Park, OH, USA, [11] Kwok, C.T., Fong, S.L., Cheng, F.T., Man H.C. (2006) Pitting and galvanic corrosion behavior of laserwelded stainless steels, Journal of Materials Processing Technology 176, [12] Kaul, R., Mahajan, S., Kain, V., Ganesh, P., Chandra, K., Nath, A.K., Prasad, R.C. (2008) Laser surface treatment for enhancing intergranular corrosion resistance of AISI type 304 stainless steel, Corrosion 64,