Cooling rate effect on vacuum brazed joint properties for 2205 duplex stainless steels

Transcription

1 Materials Science and Engineering A354 (2003) 82/91 Cooling rate effect on vacuum brazed joint properties for 2205 duplex stainless steels L.H. Chiu *, W.C. Hsieh, C.H. Wu Department of Materials Engineering, Tatung University, 40 Chungshan North Road, 3rd Sec., Taipei 10451, Taiwan, People s Republic of China Received 30 April 2002; received in revised form 5 November 2002 Abstract The influence of cooling rates and fillers on the mechanical properties and corrosion resistance of vacuum brazed joints for two types of 2205 duplex stainless steels was investigated. The amount of a sigma phase precipitated in the duplex stainless steel (DSS) was increased at a slow cooling rate. When the cooling rate was increased to C s 1, the suppression of sigma-phase precipitation was observed. As for the mechanical property of the brazed joints, shear strengths of the vacuum brazed joints were obtained in the range 46/79, 110/135, and 173/250 MPa for filler material of BNi-7 powder, BNi-3 powder, and copper foil, respectively. Most of the brazed joints were ruptured at the brazed gap. However, the joints of GMTC 2205 DSS, using BNi-3 filler and Cu foil under the cooling rate of 0.1 and C s 1, were broken at parent metal. The ruptures at parent metal were caused by a large amount of brittle s-phase precipitation on the substrate. Besides, owing to the dissimilarity between the filler composition and DSS, galvanic corrosion behavior was observed when placing the brazed joints in a chloride solution. # 2003 Elsevier Science B.V. All rights reserved. Keywords: 2205 Duplex stainless steel; Vacuum brazed joint; Corrosion; Sigma phase 1. Introduction In the last decade, duplex stainless steels (DSSs) were shown to not only have better mechanical and corrosion resistance properties than austenitic stainless steel but also lower costs, especially when applied in chloridecontaining environments. DSSs are, therefore, considered as an upgraded austenitic stainless steel and have found increasing use in oil and gas production, and process systems [1 /3]. However, during the cooling stage the ferrite phase in DSS transformed into some harmful phases, such as the s-phase [4]. s-phase particles were precipitated at the interfaces of a/g (ferrite/austenite) and penetrated into the a-phase or grew along the a/g-interface in a shape of a butterfly. The preferential precipitation of s-phase from a-phase is well known owing to the richness of Cr and Mo in a- phase [5]. Harmful precipitates affect seriously the * Corresponding author. Tel.: / ; fax: / address: lhchiu@ttu.edu.tw (L.H. Chiu). mechanical and corrosion properties, and limit the applications of DSS. Potgieter [6] reported that the general and pitting corrosion resistance of SAF 2205 DSS under 10% H 2 SO 4 solution and 3.5% NaCl water solution was greatly degraded by the s-phase precipitation. Wilms et al. [7] explained that the depletion of Cr and Mo in the s-phase boundary was the reason that a high corrosion area was produced. It was shown by Tsai and Chen [8] that s-phase precipitation would increase the opportunity of DSS to stress corrosion. Traditional welding methods such as tungsten inert gas (TIG) arc welding, metal inert gas (MIG) arc welding and submerged arc welding (SAW) can be used to join DSS [9]. However, welding may introduce potential corrosion problems in the heat-affected zone resulting from the heating and cooling steps under unfavorable conditions. The variation in the a- and g- phase ratio and some harmful phases are also precipitated during the cooling of the weldment [3,9]. A processing method, such as vacuum brazing, without a temperature gradient existed is worthwhile to investigate. The processing parameters for vacuum brazing including the temperatures, degrees of vacuum, /02/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi: /s (02)

2 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 83 Table 1 Chemical compositions of the DSSs (wt.%) Material Fe C Si Mn Cr Ni Mo Cu N PRE GMTC Avesta and cooling rates, can be suitably controlled. When specimen for the vacuum brazing process is heated in a high-temperature furnace, the temperature distribution in the entire substrate and the brazed joint is homogeneous. Thus, the temperature gradient and the heataffected zone of the welded joint will not be presented at the brazed joint under various cooling schedules. The purpose of this study is to evaluate the properties of a vacuum brazed joint of the DSS under a number of cooling schedules. The cooling rates are chosen so as to alleviate the effects of harmful phase precipitations (such as s-phase) in the DSS. Different fillers were used to study the brazing processing conditions for the DSS. 2. Experimental 2.1. Materials A 2205 (UNS S31803) DSS round bar with a diameter of 65 mm, supplied by Gloria Material Taiwan Company (GMTC), and a 2205 DSS plate, 2 mm in thickness, obtained from Avesta Sheffield Co. (Avesta), were used as the specimen materials. The chemical analysis of these two materials is shown in Table 1. Two types of DSS specimens, GMTC 2205 and Avesta 2205, were cut into dimension of 55 mm/15 mm/2 mm. Three commercial fillers for high-temperature application in the brazing of stainless steel were used in this study. They are two nickel-based alloy powders, BNi-3 and BNi-7, and a commercial copper foil. The compositions and brazing temperature ranges of three fillers are shown in Table 2. All three fillers can be suitably immersed in the 2205 DSS solution treatment of the temperature range between 1000 and C Vacuum brazing A single-chamber vacuum furnace offering a high vacuum of 10 6 Torr, and a two-chamber vacuum furnace, offering 10 4 Torr were used in this experiment. The gas or oil quench process in the two-chamber furnace could be conducted using nitrogen or heat treatment oil to obtain different cooling schedules. A schematic view of the brazed joints for the lap test is shown in Fig. 1. A fixture made of graphite was used to clamp the specimens in the vacuum brazing process and the gap between the two base metals were controlled at 0.02 mm. The nickel-based filler powder was mixed with the Nicrobraze S-binder at a ratio of 3:1 to form a paste spreading on the entrance of the brazing joint uniformly, but the copper foil was directly inserted into the gap of brazed joint. The specimen and the filler were placed into the graphite fixture in vacuum furnace. The furnace was heated to brazing temperature at C for BNi-3 and BNi-7, and at C for copper foil for 10 min in 10 5 Torr vacuum. The fillers filled the joint gap by the capillary absorption at the brazing temperature. The specimens were cooled to room temperature Fig. 1. Schematic view of the brazed joints. Table 2 Chemical compositions and brazing temperature ranges of three fillers Filler Chemical compositions (wt.%) Brazing range (8C) Cr B Si Fe P Ni Cu BNi-3 / Balance / 1010/1177 BNi Balance / 927/1093 Cu foil / / / 0.1 / 0.2 Balance 1090/1150

3 84 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 using two cooling schedules at the rate of 0.1 and C s 1 in the single-chamber furnace. To schedule cooling rate of 0.25, 0.46, and C s 1, the specimens were loaded into the heating chamber of the two-chamber vacuum furnace for brazing and then cooled using nitrogen gas quench in the second chamber Structural analyses Upon preparing the Groesbeck solution (4 g NaOH/ 4 g KMnO 4 /100 g H 2 O) as the etching solution, the ground and polished specimens were dipped into the heated solution (90 8C) for 150 s. The microstructure and the s-phase were observed in an optical microscope. The area of the s-phase region was quantitatively analyzed to calculate the volume ratio using an image analyzer. An X-ray diffractometer (XRD) with Cu Ka radiation was used to investigate the phases presented in the DSS specimens after the brazing process. 3. Results and discussion 3.1. Microstructure studies and s-phase analysis The microstructures of the GMTC and Avesta 2205 DSSs etched in Groesbeck solution are shown in Fig. 2. The microstructure of GMTC 2205 was signified by the two-phase (a-ferrite and g-austenite) region having an approximate a/g fraction of 50:50 with grain size of 15/ 30 mm in width and approximately 200 /300 mm in length. The microstructure of Avesta 2205 DSS has shown the same a/g fraction of 50:50 as GMTC 2205 DSS. However, the grain size was much narrower with 3/5 mm in width and 10/25 mm in length. To study the effects of cooling rates on the two 2205 DSS microstructures, five cooling schedules ranging from to C s 1 were conducted. The microstructures of the GMTC 2205 DSS specimens cooled at different rates are shown in Fig. 3. From Fig. 3(a) /(c), it was obvious that specimens cooled from the nitrogen gas quench could avoid the dark phase (identified as s-phase) precipitated on the grain bound Lap shear test Single lap testing was conducted to evaluate the influence of the fillers on the brazing joint strengths of the vacuum brazed joints. The single lap length was kept at 6 mm and the length to thickness ratio was at 3:1. The lap test for the vacuum brazed joint was tested in a MTS 810 material test system at a constant cross-head speed of 6 mm min Corrosion test To evaluate the pitting corrosion resistance of GTMC 2205 and Avesta 2205 DSS, the ASTM G48 specification [10] was referenced. The specimens were prepared in dimensions of 20 mm/15 mm/2 mm for parent metal and 15 mm/10 mm/4 mm for the brazed joints. The vacuum brazed joint specimens were consisted of the parent metal and the brazed gap as twice the thickness of the parent metal. The surfaces of these specimens were ground and polished using 1 mm Al 2 O 3 powder, and then ultrasonically rinsed in acetone solution for 15 min. The specimens were immersed into a 6% FeCl 3 solution at C, the critical pitting temperature of 2205 DSS, for 24 h. The specimens before and after the corrosion tests were weighed by using an electronic balance. The corroded morphology of the brazing joint after the immersion test was observed using a Hitachi S2400 scanning electron microscope (SEM). Fig. 2. Microstructures of two as-received DSSs: (a) GMTC 2205 and (b) SAF 2205, etched in Groesbeck solution.

4 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 85 Fig. 3. Microstructures of GMTC 2205 DSS after C solution treatment and cooled at different rates: (a) C s 1, (b) C s 1, (c) C s 1, (d) 0.1 8C s 1, and (e) C s 1, etched in Groesbeck solution. aries. The dark phases were observed in the microstructures of GMTC 2205 DSS cooled at 0.1 8C s 1 as shown in Fig. 3(d). Furthermore, the dark phases were significantly presented in the microstructures of GMTC 2205 DSS cooled at the rate of Cs 1 as shown in Fig. 3(e). The slow cooling rate elevated dark phase precipitation on the grain boundary. The phase precipitation can be quantitatively analyzed using an image analyzer. The quantities of s-phase precipitation were calculated as 0/0.2, 0/0.5, 0/1, 7.89/1, and 17.59/2 vol.% for the specimens cooled at 0.85, 0.46, 0.25, 0.1, and C s 1, respectively. For GMTC 2205 DSS at the cooling rates of 0.1 and C s 1, we found that the s-phase precipitation

5 86 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 increased. It revealed that the decrease in cooling rates caused the increase of the grain boundary precipitation. Chen s results [5] showed that the s-phase precipitated as the 2205 DSS cooled from the solution temperature of 1020 /1080 8C with the cooling rate lower than C s 1. The amount of precipitation has increased as the cooling rate was further reduced. When the cooling rate was increased above C s 1, the harmful precipitation was suppressed. Microstructures of Avesta 2205 DSS specimens after brazing treatment at C following by different cooling rates and etched in Groesbeck solution, are shown in Fig. 4. When the specimens of Avesta 2205 DSS was heated to C for 10 min and then cooled Fig. 4. Microstructures of Avesta 2205 DSS after C solution treatment and cooled at different rates: (a) C s 1, (b) C s 1, (c) C s 1, (d) 0.1 8C s 1, and (e) C s 1, etched in Groesbeck solution.

6 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 87 at 0.1 and C s 1 to room temperature, we obtained the corresponding s-phase precipitations of 2.99/0.6 and 12.19/1.5 vol.%, respectively. The amount of s-phase precipitation of Avesta 2205 DSS followed the same trend as observed in the GMTC 2205 DDS, in which the s-phase precipitation was increased with decreasing cooling rates. The microstructures of the vacuum brazed joints of GMTC 2205 DSS brazed at C per 10 5 Torr using the BNi-7 filler under different cooling rates are shown in Fig. 5. InFig. 5(b) and (c), a nickel-based solid solution phase dispersed with some dark particles is shown between an island-like intermetallic phase (Ni 3 P) and the parent metal. The layers between the parent metal and filler in the brazed joint cooled at Cs 1 differed from joints cooled at 0.1 and C s 1. For the nitrogen gas quenched joint, the interface layer between the filler and parent material is flat as shown in Fig. 5(a). Owing to the longer holding time at high temperature in furnace cooling stage, the interface layer in the joint cooled at 0.1 8C s 1 showed more penetration and reaction with parent metal than joints cooled at and C s 1. The microstructures of the vacuum brazed joints using BNi-3 filler have exhibited similar behavior except that the Ni 3 B intermetallic compound phase was formed in the center region of the joint XRD analysis Fig. 6 shows the XRD patterns for GMTC 2205 and Avesta 2205 DSS specimens after solution annealing at C and cooling at given rates. The XRD patterns of the nitrogen gas quenched DSS specimens show the presence of a-ferrite and g-austenite phases, and the s- phase precipitation can be inhibited under rapid cooling schedules. However, the XRD patterns of specimens cooled at 0.1 and C s 1 were very similar and depicted the presence of a-ferrite, g-austenite and s- phases. The XRD patterns shown in Fig. 6 were corresponded to the microstructure results that the s- phase precipitated in the brazing condition with the slow cooling rate. The precipitation of the s-phase in DSS can be avoided by using vacuum brazing with the nitrogen gas quench. In Fig. 6(a), when the s-phase peaks are presented, the relative intensities of the a-phase peak decreases. It is, therefore, apparent that the s-phase was precipitated by the decomposition of the a-phase. The decrease of the a-phase peak in the XRD patterns agreed with the results of Jimenez et al. [11] Lap shear testing The single lap test was conducted to evaluate the influence of the fillers on the strengths of the vacuum Fig. 5. Microstructures of the vacuum brazed joint using the BNi-7 filler for GMTC2205 DSS plate brazed at C, cooled at different rates: (a) C s 1, (b) 0.1 8C s 1, and (c) C s 1.

7 88 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 Fig. 7. Strengths of the vacuum brazed joints at given conditions. Fig. 6. X-ray diffraction patterns of (a) GMTC 2205 and (b) Avesta 2205 DSS, solution treated at C and cooled at the given rates. brazed joints. The single lap length was 6 mm and the length to thickness ratio was 3. The gap between two base metals was controlled at 0.02 mm. The filler was placed outside the gap and heated to the brazing temperature as the filler metal produced a capillary flow to the clearance. The shear strengths of the vacuum brazed joints at given conditions are summarized in Fig. 7. It can be seen from Fig. 7 that the joint shear strengths were in the range 46/79, 110 /135, and 173/ 250 MPa for BNi-7 filler, the BNi-3 filler, and the copper foil, respectively. Nickel alloy brazing fillers are implemented for joining high-temperature components, particularly for aerospace and nuclear industry applications. Nickel base fillers containing boron, phosphorus, and silicon as melting point depressants are used extensively. These melting point depressants form intermetallic compound phases with boron and phosphorus that are detrimental to the mechanical properties of brazed joints. The high phosphorus content of BNi-7 (10% P) resulted in the formation of an intermetallic phase (Ni 3 P) in the joint during the brazing process. Chiu et al. [12] reported that the shear strengths of the brazed joints using the BNi-7 filler for the AISI 304 stainless steel were 33 and 45 MPa, brazing at 927 and 990 8C at a lap ratio of 5. Strength improvement can be achieved by raising the brazing temperature. Therefore, as shown in Fig. 7, the shear strengths of the 2205 DSS joints brazed at C using the BNi-7 filler were between 469/5 and 799/5 MPa. Note that the higher the brazing temperature, the stronger the diffusion and reaction are. Lugscheider et al. [13] used Ni /21.5Cr/11.6Si filler to join AISI 321 stainless steel with the brazing temperature of C and vacuum of 10 4 Torr with mm clearance. It was reported that with a lap ratio of 5, the shear strength reached approximately 110 MPa and the rupture area was in the base metal. Tensile stresses of AISI 321 joints at brazing temperature of C with the Ni /21.5Cr /11.6Si filler were lower than those joints brazed at C. At brazing temperature of C with the diffusion treatment, diffusion is not sufficient to eliminate silicide phases in the brazing gap. For the most part, joints brazed at the temperature of C show no silicide phases in the brazing gap. Consequently, the strengths for brazed parts using BNi-7 filler brazing at C were higher than those brazed at 990 8C as shown in Fig. 7. From the microstructures of joints of GMTC 2205 steel specimens brazed at C under a vacuum of 10 5 Torr using BNi-7 filler and cooled at 0.1 8C s 1, the metallurgical bonding can be seen in the interface as shown in Fig. 5. The strength of lap joints using the BNi-3 filler for Avesta 2205 DSS specimens brazed at C for 10 min fell between 116 and 135 MPa. The boron content in the filler reduced the melting point in the Ni/4.5Si/ 3B system. A brittle Ni 3 B intermetallic compound was formed during the brazing process. The joints were also fractured in the middle of the brazing gap. Additional quantity of boron (3.5 wt.%) in BNi-3 filler is less than

8 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 89 the phosphorus (10 wt.%) in BNi-7 filler. Therefore, the strengths of brazed joints using BNi-3 filler were higher than those joints using BNi-7 filler. From Fig. 7, it can also be seen that the shear strengths of the brazed joints using the copper foil were greater than those of joints using nickel-base powders. The shear strengths of the Avesta 2205 DSS brazed joints with a lap ratio of 3 using copper foil brazing at C were between 220 and 236 MPa, and no obvious intermetallic phases were presented in the brazing gap. The cooling rate effect on vacuum brazed GMTC 2205 DSS with coarse grain was investigated. When BNi-3 is used as the filler for GMTC 2205 DSS with brazing temperature at C for 10 min under 10 5 Torr vacuum, followed by cooling at 0.1 and C s 1 brazed joints were fractured in the parent metal instead of the brazing gap. As if the maximum recorded load was divided by the fractured area (15 mm /2 mm), the shear strengths of the brazed joints were 3999/20 and 3599/15 MPa for specimens cooled at 0.1 and C s 1, respectively. If the maximum load was divided by the overlap area (15 mm/6 mm), the shear strengths were 1339/7 and 1199/5 MPa. However, the lap strength of a GMTC 2205 DSS brazed joint cooled at C s 1 was 1119/5 MPa and the fracture location was at the brazed gap. From the previous analysis of s-phase precipitation, the GMTC 2205 DSS was very sensitive to s-phase precipitation. At the cooling rate of C s 1, the amount of s-phase precipitation was over 15 vol.%. When the amount of s- phase precipitation was too great, the shear strength of the GTMC 2205 DSS metal reduced. Thus, the joining area applied by the filler may carry a higher load than that sustained by the substrate area. This caused the lap specimen to fracture on the GTMC 2205 DSS plate as opposed to the brazed gap. However, the shear strength of the DSS plate was not affected by the harmful precipitated s-phase when cooled at C s 1, and the specimen fractured at the brazed gap. Therefore, by controlling the cooling rate in the vacuum brazing process, it is possible to avoid s-phase precipitation and reduce the degradation to the joint strength. Fig. 8. Weight loss of two solution treated specimens as a function of cooling rate immersed in a 6% FeCl 3 solution at C for 24 h. the amount of chromium, molybdenum, and nitrogen, where PRE/wt.% Cr/3.3 wt.% Mo/16 wt.% N [1]. The calculated PRE values of the two steels were 36.7 for GMTC 2205 DSS and 33.5 for Avesta 2205 DSS. When the amount of s-phase precipitation is modest, the corrosion resistance of two DSSs is affected mainly by the PRE value. Therefore, the results shown in Fig. 8 are corresponded to the PRE value of Table 1. In Fig. 8, the weight losses for the two 2205 DSS specimens cooled at 0.1 and C s 1 were much greater than those for the specimens cooled at C s 1. From the results of Wilms et al. [7], the corrosion of DSS became worse as the amount of s-phase precipitation increased. In this study, when the cooling rate from the solution temperature was decreased, the s-phase precipitation increased. GMTC DSS is more sensitive to s-phase precipitation than Avesta 2205 DSS. The weight losses of the immersed GMTC 2205 DSS plate were obviously higher than those of Avesta 2205 DSS. The change of cooling rates showed a strong impact to the DSS corrosion properties. Fig. 9 indicates the weight losses from brazed joints using BNi-3, BNi-7, and copper foil for GMTC Corrosion test After solution treatment, both GMTC DSS and Avesta DSS plates were cooled at rates ranging from 0.85 to C s 1. Specimens were then cut from the steels and immersed in a 6% FeCl 3 solution to study the corrosion behavior. The weight losses of the immersed specimens are shown in Fig. 8. Avesta 2205 DSS had higher weight loss (0.05 mg mm 2 ) than GMTC 2205 DSS (0.01 mg mm 2 ). Based on Table 1, the pitting resistance equivalent (PRE) value was calculated from Fig. 9. Weight loss of GMTC 2205 DSSs brazed joint immersed in a 6% FeCl 3 solution at C for 24 h.

9 90 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 DSS as a function of the cooling rate. As can be seen in Fig. 9, the weight losses from GMTC 2205 DSS brazed joints exhibited a similar trend as those shown in Fig. 8. The thickness of the brazed gap (0.02 mm) was thin in comparison to the total thickness (4 mm) of the brazed joint. When the cooling rate decreased, the weight loss from corrosion increased owing to the increasing s- phase precipitation in the DSS plate. However, the filler has an impact on the corrosion of brazed joints especially for specimens cooled at C s 1. Because of the galvanic effect between the filler and GMTC 2205 DSS plate, the weight loss (0.2 mg mm 2 ) from brazed joints cooled at C s 1 was much higher than that (0.01 mg mm 2 ) from the parent metal specimen. The weight loss amount of brazed joints using Avesta 2205 DSS was the same as the brazed joints using GMTC 2205 DSS. Fig. 10 shows the corroded morphologies of Avesta 2205 DSS brazed joints using three fillers after the immersion test. As can be seen in corroded joints using BNi-3 filler (Fig. 10(a)) and BNi-7 filler (Fig. 10(b)), deep channels and holes were observed in the interface layers between the parent metal and intermetallic compound phases in the middle region of the solidified filler. Some pinholes were also observed in the parent metal near the interface layers. For the brazed joints using copper foil, the copper layer was completely corroded. Severe brazed joint corrosion resulted from the composition gradient between the parent metal and the filler. Galvanic corrosion was depicted at the brazed joints in the Avesta 2205 DSS plates. In summary, galvanic corrosion has occurred in both brazed DSS joints using three fillers, BNi-3, BNi-7, and copper foil, in the chloride solution. As a galvanic cell was established, obvious corrosion occurred in the brazed joint, especially at the interface between the two steel plates. Hence, the corrosion resistance of the DSS also decreased. Fig. 10. The corroded morphologies of the brazed joints using (a) BNi-3, (b) BNi-7, and (c) Cu foil as filler after immersed in a 6% FeCl 3 solution at C for 24 h. phase. For the immersion test in the 6% FeCl 3 solution, the change of cooling rates showed a strong impact to the DSS corrosion properties. Besides, the vacuum brazed joints of DSS are corroded by the galvanic cell effect owing to the large composition differences between the fillers and DSS substrates. Acknowledgements 4. Conclusion After solution treatment, the slow cooling rates promote s-phase precipitation at the grain boundary for both 2205 DSSs. When the cooling rate was increased to C s 1, offering by a nitrogen gas quench, the suppression of s-phase precipitation was observed. The shear strengths of the vacuum brazed joints were obtained in the range 46/79, 110/135, and 173 /250 MPa for BNi-7 filler, BNi-3 filler, and copper foil, respectively. Most of the brazed joints were fractured at the brazed gap. However, the joints of GMTC 2205 DSS, using BNi-3 filler and Cu foil, cooled at 0.1 and C s 1, were broken at parent metal owing to the large amount of precipitation of brittle s- The authors are grateful for the support of the National Science Council, Republic of China, under Contract NSC E References [1] J.O. Nilsson, Mater. Sci. Technol. 8 (1992) 685. [2] J. Charles, Welding in the World 36 (1995) 43. [3] R.A. Walker, Mater. Sci. Technol. 4 (1988) 78. [4] P.H. Pumphrey, G.D.W. Smith, M. Prager, Mater. Sci. Technol. 6 (1990) 209. [5] T.H. Chen, J.R. Yang, Mater. Sci. Eng. A 311 (2001) 28. [6] J.H. Potgieter, Br. Corr. J. 27 (3) (1992) 219. [7] M.E. Wilms, V.J. Gadgil, J.M. Krougman, F.P. Ijsseling, Corr. Sci. 36 (1994) 871. [8] W.T. Tsai, M.S. Chen, Corr. Sci. 42 (2000) 545.

10 L.H. Chiu et al. / Materials Science and Engineering A354 (2003) 82/91 91 [9] K. Beklers, Welding in the World 36 (1995) 111. [10] ASTM Designation, G48-92, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution. [11] J.A. Jimenez, M. Carsi, O.A. Ruano, J. Mater. Sci. 35 (2000) 907. [12] L.H. Chiu, S.Y. Shi, H.M. Wu, in: Nano-structured and Amorphous Materials Symposium, Taipei, Taiwan, [13] E. Lugscheider, O. Knotek, K. Klohn, Welding J. 58 (10) (1978) 319s.