Materials Transactions, Vol. 52, No. 8 (2011) pp. 1701 to 1704 #2011 The Japan Institute of Metals
RAPID PUBLICATION
Enhancing Corrosion Resistance of 304 Stainless Steel GMA Welds with Electromagnetic Interaction Francisco F. Curiel1 , Rafael Garcı´a1 , Victor H. Lo´pez1; * and Jorge Gonza´lez-Sa´nchez2 1
Instituto de Investigaciones Metalu´rgicas, Universidad Michoacana de San Nicola´s de Hidalgo, A.P. 888, CP 58000, Morelia, Michoaca´n, Me´xico 2 Centro de Investigacio´n en Corrosio´n, Universidad Auto´noma de Campeche, Av. Agustı´n Melgar s/n, Col. Buenavista, CP 24039, Campeche, Campeche, Me´xico Plates of AISI 304 stainless steel were gas metal arc welded under the effect of an electromagnetic interaction. Samples of the heat affected zone were subjected to the double loop electrochemical potentiokinetic reactivation technique to evaluate the degree of sensitization (DOS). The as-received plates had a significant DOS whereas samples welded under electromagnetic interaction presented lower DOS as compared with samples plainly welded. The reduction in the DOS suggests that the electromagnetic interaction enables Cr redistribution in the austenitic grains during welding reducing Cr depleted zones. [doi:10.2320/matertrans.M2011087] (Received March 29, 2011; Accepted May 24, 2011; Published July 25, 2011) Keywords: magnetic field, welding, sensitization, heat affected zone, stainless steels
1.
Introduction
Austenitic stainless steels (ASS’s) are strategic materials that meet the requirements in mechanical properties and corrosion behaviour demanded by the petrochemical, fertiliser and energy generation industries. Fabrication of components usually involves cold deformation and fusion welding. Both processes may have a profound impact on the performance of the components in service as the microstructural features that these processes induce, in particular for 304 SS, accelerate sensitisation. Sensitisation is a phenomenon in which of Cr-rich carbides, Cr23 C6 , precipitate and make the ASS’s susceptible to localised attack in the form of pitting and intergranular corrosion in chloride containing electrolytes due to formation of Cr depleted zones near the grain boundaries.1,2) Metallurgical methods such as reduction of carbon content, addition of Mo, localised heat treatments and grain boundary engineering have been used to overcome sensitisation in the heat affected zone (HAZ) of welded components.2–5) Large magnetic fields have been used to modify solid phase transformations in ASS’s because they act as external driving force for nucleation of martensite at preferential sites and change its kinetics of transformation, possibly due to alteration of the diffusion processes.6,7) This work investigates the effect of the application of an axial magnetic, in the order of mT, during fusion welding of cold rolled plates of AISI 304 stainless steel on the electrochemical behavior of the HAZ in terms of degree of sensitization (DOS). It is thought that the interaction between the inherent magnetic field generated by the welding DC current and an external magnetic field will induce vibration of the atoms in the HAZ which in turn will promote diffusion of chromium in short distances.
*Corresponding
author, E-mail:
[email protected]
Fig. 1 Schematic of the experimental set up.
2.
Experimental Procedure
Plates of cold rolled 304 ASS (0.058C, 18.56Cr, 8.08Ni, 0.352Si, 1.512Mn mass%) with a thickness of 6 mm were machined to form a butt single V groove joint with a groove angle of 60 and a root face of 1 mm. The gas metal arc welding (GMAW) process was used to join plates in the as-received condition without and with the application of an external magnetic field of 3:2 mT. The magnetic field was induced parallel to the electrode, as shown in Fig. 1, by feeding electric current into a coil placed around the joint. Welding parameters were; DCEP, 190 A, 27 V, travel speed of 3.6 mm s1 and a mixture of 98%Ar + 2%O2 shielding gas flowing at 30 L min1 . An ER309L filler wire was fed into the joint at 180 mm s1 . The welding thermal cycles in the HAZ were recorded by placing k-type thermocouples as shown in Fig. 2. These curves were used to establish the width of the HAZ prone to sensitisation during continuous cooling. The site chosen for electrochemical testing was at 8 mm away from the mid axis of the weld bead. The zone determined as susceptible to undergo sensitisation during welding matches very well with that found by others.8,9)
F. F. Curiel, R. Garcı´a, V. H. Lo´pez and J. Gonza´lez-Sa´nchez
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Fig. 4 Typical curves obtained from the DL-EPR tests (BM refers to the base material in the as-received condition). Fig. 2 Thermal cycles recorded in the HAZ during welding.
α´
Ferrite 20 μm Fig. 3 Characteristic microstructure of the as-received AISI 304 SS.
Samples of 2 mm in thickness were cut, parallel to the weld bead, at the chosen position, mounted in resin with a Cu wire attached at the back of the sample for connection to the potenciostat. The exposed surface of the samples was ground to 12.6 mm as preparation for the electrochemical tests. The DOS in ASS’s is commonly assessed by the Double Loop Electrochemical Potentiokinetic Reactivation test (DL-EPR), which is a quantitative and reliable method with excellent reproducibility.10,11) A potentiostat/galvanostat was used with a conventional three electrodes electrochemical cell; a saturated calomel electrode (SCE) as reference electrode, a graphite bar as auxiliary electrode and the ASS samples as working electrode. The DL-EPR tests were conducted in 0.5M H2 SO4 + 0.01M KSCN solution at room temperature.10) The size of the samples was 12 6:3 mm2 and 76:4% of this area was exposed to the solution during the experiments. The samples were polarised anodicaly using a scan rate of 100 mV min1 in a potential range from 420 mV to return potential of 300 mV. The ratio of the reactivation current (Ir ) to the activation current (Ia ) peaks, Ir =Ia , was taken as the DOS.10) The potential values reported in this work are referred to the SCE. 3.
Results and Discussion
The microstructure of the ASS in the as-received condition is comprised of austenitic grains with ferrite bands,
0 -martensite laths as well as coherent and incoherent twin boundaries as shown in the optical micrograph in Fig. 3 (etched with Kalling’s 2). The presence of 0 martensite was expected as the plates were cold rolled and it is well documented that strain induces 0 -martensite formation.12–15) Figure 4 shows the potential versus current density curves obtained from DL-EPR tests of samples welded with and without electromagnetic interaction as well as the reference base material in the as-received condition. The sample welded with magnetic field presented the lowest reactivation current density peak, Ir , which is dictated by the content of Cr.2) The data from the curves in Fig. 4 were used to estimate the DOS. According to Dayal et al.2) an ASS is sensitised when Ir =Ia > 0:05. The material in the as-received condition presented an Ir =Ia ratio of 0.129, meaning that it is already in a sensitised state. This can be attributed to prior cold working that induced martensite transformation and deformed substructures like planar dislocation arrays and deformation twinning that act as discontinuities in the passive film and as energetically favourable nucleation sites for carbide precipitation.16,17) The highest DOS, 0.154, was measured for the sample welded without the application of the axial magnetic field. The larger DOS for this sample as compared to the asreceived indicates that the welding thermal cycle worsened the metallurgical condition, in terms of DOS, of the steel at the HAZ. The DOS of the sample welded with electromagnetic interaction was four times lower than the sample plainly welded. The samples subjected to the DL-EPR test were observed in the scanning electron microscope and the features of the surfaces are shown in Fig. 5. The images reveal that the 304 ASS in the as-received condition presented considerable damage at preferential sites such as pre-existent carbides and martensite within the grains. Welding in absence of magnetic field, Fig. 5(b), makes the plates more susceptible to corrosion in the HAZ. Conversely, when welding is performed with electromagnetic interaction by applying an external magnetic field, the extent of intergranular and transgranular corrosion in the HAZ was reduced. The qualitative trend observed on the surfaces of the electrochemically tested samples agrees with the DOS measurements.
Enhancing Corrosion Resistance of 304 Stainless Steel GMA Welds with Electromagnetic Interaction
(a)
10 μm
(b)
10 μm
(c)
10 μm Fig. 5 SEM images of the samples after polarisation in 0.5M H2 SO4 + 0.01M KSCN solution in (a) as-received condition, welded (b) without and (c) with electromagnetic interaction.
Carbide precipitation is a temperature-dependant process with slow kinetics that can occur during welding. Cold deformation before exposure to heat during welding has a combined effect on the DOS of ASS’s. At temperatures higher than 600 C sensitisation was enhanced leading to carbide formation and thereby chromium depletion near dislocations within austenite grains as well as along grain boundaries.18) Also, it was reported that the deleterious effects of different intermetallic compounds on pitting corrosion are associated with the formation of Cr-depleted zones adjacent to Cr-rich precipitates.19) On this basis, it is reasonable to assume in this study that carbide nuclei are already present along with Cr-rich precipitates in the base material before welding owing to prior cold deformation as the DL-EPR tests revealed a significant damage in the asreceived 304 ASS. Besides, the presence of martensite in cold worked stainless steels may induce sensitisation at temperatures as low as 350 C, so that precipitation of Cr23 C6 occurs within the grain interiors as result of an increase in dislocation density.2) Thus, all these account for the trans-
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granular and intergranular corrosion developed during polarisation in 0.5M H2 SO4 + 0.01M KSCN solution in the as received 304 ASS as well as in the welded samples with and without electromagnetic interaction. It is evident that the axial magnetic field applied during welding provided a beneficial effect on the resistance to localised corrosion in the HAZ. Formation of Cr23 C6 or M23 C6 in a 304 SS is thermodynamically favourable between 450 to 800 C20) and obviously the electromagnetic interaction generated during welding did not dissolved these compounds in the HAZ. However, the reduced DOS and damage on the surface suggests that the vibration caused by the electromagnetic interaction promoted Cr diffusion. Also, dislocation pipes, dislocation density and 0 -martensite formed owing to cold working enabled a faster path for Cr diffusion. As a result, Cr concentration was sufficiently raised to form the passive film in the surface of the welded samples providing a better resistance to localised corrosion. Further experiments and characterisation by transmission electron microscopy is under way to clarify the desensitisation mechanism. 4.
Conclusions
The application of an axial magnetic field of low intensity during GMA welding of cold deformed AISI 304 stainless steel seems to be a viable alternative to prevent sensitization of the HAZ. It is thought that precipitation of chromium carbides and further growth of pre-existent carbides within grains and along grain boundaries promoted by cold deformation were arrested or at least minimised. Because the DL-EPR test measures the DOS based on the peak reactivation current, Ir , and it is dictated by the content of Cr,2) the findings of this study suggest that the interaction between the external magnetic field applied during welding and the magnetic field induced by the welding current promoted Cr redistribution in the austenitic matrix, healing Cr depleted zones and providing thus a more continuous and corrosion resistant passive film. Acknowledgement FFC thanks CONACyT for the scholarship provided. REFERENCES 1) V. Kain, P. Sengupta, P. K. De and S. Banerjee: Metall. Mater. Trans. A 36A (2005) 1075–1084. 2) R. K. Dayal, N. Parvathavarthini and B. Raj: Int. Metall. Rev. 50 (2005) 129–155. 3) A. J. Sedriks: Corrosion of Stainless Steels, (Wiley-Interscience, New York, 1996) pp. 245–251. 4) R. Kaul, N. Parvathavarthini, P. Ganesh, S. V. Mulki, I. Samajdar, R. K. Dayal and L. M. Kukreja: Weld. J. (2009) 233s–241s. 5) H. Kokawa: J. Mater. Sci. 40 (2005) 927–932. 6) S. Nakamichi, S. Tsurekawa, Y. Morizono, T. Watanabe, M. Nishida and A. Chiba: J. Mater. Sci. 40 (2005) 3191–3198. 7) D. S. Martı´n, K. W. P. Aarts, P. E. J. Rivera, N. H. V. Dijk, E. Bruck and S. V. D. Zwaag: J. Magn. Magn. Mater. 320 (2008) 1722–1728. 8) H. Kokawa, M. Shimada, M. Michiuchi, Z. J. Wang and Y. S. Sato: Acta Mater. 55 (2007) 5401–5407. 9) P. d. Lima-Neto, J. P. Farias, L. F. G. Herculano, H. C. d. Miranda, W. S. Araujo, J. B. Jorcin and N. Pe´be´re: Corros. Sci. 50 (2008) 1149–
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