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Shot peening effect on 904 L welds corrosion resistance
Journal of Constructional Steel Research 115 (2015) 276–282
Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Shot peening effect on 904 L welds corrosion resistance Barbara Nasiłowska a,⁎, Zdzisław Bogdanowicz a, Michał Wojucki b a b
Military University of Technology, Institute of Machine Building, Kaliskiego 2, 00-908 Warsaw, Poland Institute of Precision Mechanics, Duchnicka 3, 01-796 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 14 August 2015 Accepted 28 August 2015 Available online 8 September 2015 Keywords: Welding laser beam Welding TIG method 904 L-SS Shot peening Corrosion resistance
a b s t r a c t Here we investigated the effect of shot peening on the corrosion resistance of 904 L austenitic stainless steel joints made by CO2 laser beam and GTAW welding. Corrosion tests on the fusion zone in welds made by each of these welding methods were performed on shot peened and non-peened specimens. The samples were placed in a salt chamber and inspected after 24, 48, 72, 120, 240, 480, 720 and 1000 h. Pitting corrosion on non-peened welds made by GTAW welding was visible in the heat-affected zone from the face of the weld and reached a depth of 40 μm. Shot peening of welded joints decreased corrosion by 75%. In the case of shot peened welds made by laser beam welding, pitting corrosion had not occurred after 1000 h of salt mist exposure. © 2015 Elsevier Ltd. All rights reserved.
1 . Introduction The corrosion resistance of 904 L stainless steel is highly affected by the content of the alloying elements, the type of applied heat treatment and the condition of the surface layer [1,2]. The occurrence of pitting corrosion is a common reason for austenitic steel degradation in aqueous environments (e.g., in solutions containing halide and/or chloride ions) (Fig. 1) [1,2]. A characteristic feature of pitting corrosion is the presence of pits formed as a result of an interaction between a passive film (which plays a role of a cathode) and locally depassivated regions (“anodes”) on the surface of the material [3,4]. The role of an anode can be played by the internal inhomogeneities of metals (i.e., non-metallic inclusions, separations, deformations) and by external inhomogeneities, such as edges, scratches, dents, scale residues and other residues [3,4]. Chemical composition also has a crucial influence on the pitting corrosion resistance of stainless steel. Gooch et al. [5] proposed a quantitative estimation (pitting resistance equivalent — PRE index) that allows for an evaluation of a contribution of alloying elements to the corrosion behaviour of the material: PRE ¼ % Cr þ 3:3ð%MoÞ þ xð%NÞ
ð1Þ
where x = 16 for duplex (austenitic–ferritic) stainless steels and x = 30 for austenitic stainless steels. ⁎ Corresponding author. E-mail addresses: [email protected] (B. Nasiłowska), [email protected] (Z. Bogdanowicz), [email protected] (M. Wojucki).
http://dx.doi.org/10.1016/j.jcsr.2015.08.041 0143-974X/© 2015 Elsevier Ltd. All rights reserved.
The higher the PRE index value the better the resistance to pitting corrosion. In the case of 904 L stainless steel, the PRE index value is within the range of 32.2 to 39.9 [6,7]. It is worth noting that these PRE values are below the threshold of full corrosion resistance, which for steel has been established as 40–60. Compressive stresses introduced into the layers during shot peening treatment lead to an increase of microhardness (coming from a high dislocation density and finer grain size) in this area [8,9]. Such findings are in line with those reported for 304 steel [8], 18CrNiMo7-6 steel [10], 316 steel [11] and S30432 steel [12–14], nitrided after initial shot peening. It was found that the application of shot peening, prior to the nitriding process, resulted in an almost doubling of surface microhardness. Therefore, an additional protection is needed to improve the resistance of 904 L stainless steel components to aqueous environments [15]. Compressive stress to the surface layer has been shown to inhibit the development of pitting corrosion and is therefore a potential protective approach. One way of controlling the stress rate at the surface layer of a material is to apply shot peening. This process is based on the partial transfer of speeding bead peening pressure kinetic energy to the treated material, resulting in compressive stress. An important objective of shot peening is to change the condition of the surface layer of the treated material by impacting it with a shot to create plastic deformation. Analyses have shown significant improvement in stress distribution performance (hardness, fatigue strength, yield strength, impact strength and elongation decrease, as well as an increase in the resistance to tribological wear) after shot peening of austenitic stainless steel [16–18]. However, to our knowledge, the impact of
B. Nasiłowska et al. / Journal of Constructional Steel Research 115 (2015) 276–282
Fig. 1. Pitting corrosion development scheme [3,4].
Table 1 Sample numbers, welding method and type of surface treatment. Sample No./numer
Welding method
Type of treatment
521/1.1 ÷ 3 521/2.1 ÷ 3 521/3.1 ÷ 3 521/4.1 ÷ 3
GTAW Laser GTAW Laser
Non peened
277
Pneumatic shot peening of the surface layer of 904 L steel was carried out by the Institute of Precision Mechanics (IMP) in Warsaw. The process involved a stream of spring steel shot (640 HV) with a diameter of 0.8 mm at 0.5 MPa. The exposure duration was 6 min and the sample was 100% covered. The intensity of shot peening was fC = 0.25 mm, as determined using the Almen strip (“A” type, with a thickness of 1.3 mm, Grade II). Prior to the commencement of the test, the samples were acclimatised for 24 h at a temperature of 296 ± 2 K and 50 ± 5% relative humidity. The study of corrosion resistance was conducted in accordance with ISO 9227:2012 “Corrosion tests in artificial atmospheres — Salt spray tests”, in the HERAEUS VÖTSCH S 1000-type salt chamber. The elements were tested in a chamber with 5% NaCl solution at an operating temperature of 307 to 309 K. Corrosion damage assessment was carried out in accordance with PN-EN ISO 10289: 2002 “Methods for corrosion testing of metallic and other inorganic coatings on metallic substrates — Rating of test specimens and manufactured articles subjected to corrosion tests” and allowed for the determination of a protection rating (RP) and for the identification of corrosion changes to the surface layer of the specimen. 3 . Material
Peened
shot peening on pitting corrosion of 904 L steel welds made by GTAW and laser welding has yet to be investigated.
2 . Research methodology Corrosion resistance tests were carried out on flat elements (45 × 12 × 5 mm) made of 904 L austenitic stainless steel, either GTAW- (tungsten arc welding) or laser beam-welded; which came from the same delivery batch. To compare the effect of shot peening on the corrosion resistance of welded joints, some elements were subjected to an additional treatment of shot peening, according to Table 1. TIG welding, using a tungsten electrode with a 2.5 mm diameter MTC, of MT-904 L and the G/W 20 25 5 CuL (20% Cr, 25% Ni, 4.5% Mo, 1.5% Cu) binder was made perpendicular to the direction of rolling, with two stitches. The root of the weld was then removed and prewelding conducted. The elements used in the tests were made at the Chemical Equipment Construction Plant of the Azoty Group in Tarnów (Zakład Budowy Aparatury Chemicznej), in accordance with the production technology used in manufacturing chemical equipment. Laser welding was performed at the Kielce University of Technology, using a Triumph 1005 CO2 laser with a lens focal distance of 260 mm, with a spot (0.4 mm diameter) on the sample surface, with a P of 4.5 kW and a welding speed of v = 1.4 m/min, in a helium enclosure.
The research specimens were of 904 L austenitic stainless steel (Cr: 19–21, Ni: 24–26, Mo: 4–5, Cu: 1.2–2.0, Mn: ≤ 2.0, P: ≤ 0.030, S: ≤ 0.010, Si: ≤ 0.7, N: ≤ 0.15, C: ≤ 0.02, wt. %), either GTAW- (Fig. 2a) or laser-welded (Fig. 2b). Selected GTAW- and laser-welded samples were shot peened (Table 1). In the GTAW-weld (made with three stitches) samples, a flat front of melt solidification occurred with staged grain boundary melting. Longer heat stress from the heat source and hence slower solidification affected the growth of dendritic side branches. The lower solidification point of the grain boundaries caused a flow in of the binder to the base material (BM) along the fusion line. In the case of the single stitch weld made by laser welding, transcrystallisation was observed with the formation of dendrites over the entire length from the fusion zone to the front line of heat dissipation. 4 . Welded joints before and after shot peening 4.1 . Structure Upon examination of the BM structure (Fig. 3a, b), the zones of the laser- (Fig. 3c, d) and GTAW-welding (Fig. 3e, f) of the 904 L steel after shot peening indicated the deformation of austenite grains caused by a cold work of the surface layer to a depth of 150 to 200 μm. 4.2 . Microhardness Microhardness measurements were carried out in accordance with the 0.1 HV Vickers hardness method, both before and after
Fig. 2. Structure of a weld made by GTAW welding (a) and laser beam welding with beam parameters: P = 4.5 kW and v = 1.4 m/min (b).
278
B. Nasiłowska et al. / Journal of Constructional Steel Research 115 (2015) 276–282
Fig. 3. Microstructure of the surface layer of the base material after pneumatic shot peening (a, b) of the joints welded by either laser (c, d) or GTAW (e, f). Images from Quanta FEG 3D scanning electron microscope (FEI Company) with a FSD (a, c, e) and EBSD (b, d, f) detector. The dashed lines show the depth of the strengthened surface layer of elements after shot peening and W markings indicate the microstructure of the weld.
Fig. 4. Distribution of microhardness in the surface layer of base material and the laser beam welded elements.
Fig. 5. The distribution of microhardness into the surface layer of the base material and GTAW welded components.
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279
Fig. 6. Internal stress in the weld (W), heat affected zone (HAZ) and base material (BM) in the x-axis (a) and y-axis (b) before and after shot peening.
strengthening of the BM surface, in the heat affected zone (HAZ) and in the weld area (W) of the welded samples (laser- and GTAW-welded). Dynamic surface treatment of the welded joints resulted in a strengthening of the surface layer because of the surface cold work. Both the laser- and GTAW-welded samples showed a significant increase in microhardness (Figs. 4 and 5). As a result of plastic roll forming in the 904 L steel production process, the microhardness of the BM prior to shot peening was ~290 HV (20 microns into the surface), which increased to ~ 400 HV after shot peening. Thermal cycling in the laser welding process resulted in a release and re-growth of grains in the form of dendrites. This release and re-growth resulted in a return of the microhardness of the HAZ W to the normative value (190 HV). The shot peening treatment resulted in an increase of microhardness to ~400 HV in the HAZ and W areas at a depth of ~ 20 μm from the surface of the sample (Fig. 4). The fusion zone of the laser beam welded joint was strengthened at a depth of ~300 μm. The thermal cycle and segregation of the alloying elements included in the binder of the GTAW-weld result in an increase in the microhardness (at 20 μm depth and within the axis of the weld) from 154 HV before shot peening to the normative value (190 HV) at the 195 μm level. The microhardness of the HAZ of the non-peened components at a depth of 20 μm was ~190 HV. The shot peening process resulted in an increase in microhardness to ~420 HV in the HAZ and ~ 380 HV in the W area at a ~ 20 μm depth (Fig. 4). The zone of the GTAW-welded joint was strengthened to a depth of ~250 μm (Fig. 5).
4.3 . Internal stress In the analysed areas (W, HAZ, BM), the compressive stress on the surface of the laser- and GTAW-welded joints increased after shot peening (Fig. 6). Deeper metal layers do not allow for free and complete spreading of the plastic deformation arising from the shot peening process, which results in the formation of compressive stress [7]. The compressive stresses in the W after shot peening varied from −364 to −661 MPa, depending on the performance method and measurement directions. Before shot peening, tensile stress process were observed in the HAZ and W. However, compressive stress always occurred in the BM after welding and this value increased after the shot peening process. The greatest compressive internal stress (~800 MPa) occurred in the HAZ and BM in laser-welded joints after shot peening. In the HAZ of GTAW joints the stresses after shot peening (higher thermal effect) were −295 (x) and −81 MPa (W). 4.4 . Corrosion resistance tests Comparative studies were carried out regarding the impact of the corrosive environment on shot peened and non-peened GTAW- and laser-welded samples. The tested pieces were placed in a salt spray chamber and subjected to periodic review after 24, 48, 72, 120, 240, 480, 720 and 1000 h. The test results indicated that, in the HAZ of non-peened samples, pitting and general corrosion were initiated,
Fig. 7. The surfaces of samples welded with GTAW and laser beam before and after being placed in the salt chamber.
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Table 2 Results of corrosion resistance in salt spray for welded joints. Sample No./numer
Welding method
Protection rate RP
521/1.1÷3 521/2.1÷3 521/3.1÷3 521/4.1÷3
GTAW — Non peened Laser — Non peened GTAW — Peened Laser — Peened
RP9 RP9 RP9 RP10
which were visible when using a scanning electron microscope at ×250 magnification (Fig. 1a). The surface roughness values of the samples prior to shot peening were: Ra = 3.9 μm and RY5 = 18.53 μm. After shot peening the surface roughness values were: Ra = 3.98 μm and RY5 = 18.77 μm. The photographs in Fig. 7 show the impact of the salt mist (5% NaCl solution) atmosphere on GTAW- and laser-welded joints before and after (1000 h) exposure.
The results of the corrosion resistance tests for welded joints are given in Table 2. In the case of GTAW-welded peened and non-peened samples and non-peened laser-welded joints, the corrosion protection rating as determined by external visual inspection was RP9, according to PN-EN ISO 10289: 2002. This indicates that the proportion of the sample surface covered by corrosion was ≤0.1%. In contrast, on the surface of the laser-welded shot peened samples, the protection rating was RP10, which indicates complete absence of corrosion. The corrosive environment produced certain changes in the surface layer of the welds, with a thin layer of general corrosion being visible under a scanning electron microscope (Fig. 8). The largest corrosion centres were observed in non-peened joints welded using GTAW method, after 1000 h of exposure to salt mist. The said changes are shown in Fig. 8 in the form of topography images of the fusion zone in welds made by GTAW and laser beam welding, peened and not peened, after 720 and 1000 hours of exposure to corrosive environment. The fusion line separating the face of
Fig. 8. The topography of peened and non-peened surfaces of joints welded using GTAW and laser beam methods, exposed to the impact of corrosive environment: BM — base material, W — weld.
B. Nasiłowska et al. / Journal of Constructional Steel Research 115 (2015) 276–282
281
Fig. 9. Cross-section of GTAW welded joints. Arrows indicate corrosion.
weld reinforcement from the base material is marked in the photographs. Structural analyses of the cross-section of welded joints revealed that the surface cold work by shot peening resulted in the inhibition of corrosion and surface damage. The strain on the surface layer that occurs as a result of shot peening increases microhardness and the density of dislocations [8]. Accumulation of stress causes the activation of the surface and thus the susceptibility to environmental impacts. The strain (surface hardening) shifts the potential in a negative direction, reducing the surface selfpassivation. Due to the influence of several thermal cycles (longer action of high temperature and slower solidification) during GTAW welding, the degradation of the material in the salt chamber occurred mainly from the side of the weld face (Fig. 9). Pitting corrosion initiated in the HAZ, where the largest corrosion microcells occurred. After 1000 h, this corrosion had penetrated between 20 and 40 μm into the surface (Fig. 9 and 10). Analysis of the salt mist treated samples after 720 and 1000 h indicated that the laser-welded plus shot peened group were most resistant to corrosion. In laser-welded non-peened samples after 1000 h of salt mist exposure, the pitting corrosion occurred along the fusion line from the root of the weld (where the largest external inhomogeneities [in the form of weld reinforcement] were observed) (Fig. 10). Analysis of the metallographic cross-section and topography of welded joint surfaces showed that the corrosion initiated mainly in
the HAZ and reached the following values after 1000 h: GTAWwelded, peened = ~ 5 to 10 μm into the surface from the weld face; GTAW-welded, not peened = ~20 to 40 μm into the surface from the weld face; laser-welded, not peened ~30 μm into the surface from the side of the weld root. Typical NaCl crystals were observed near the HAZ on the surfaces of the GTAW-welded samples, which covered a significant part of the surface, leading to more intense corrosion (Fig. 11a). In case of the laser-welded samples, in the HAZ, the Ni\\NaCl crystals were observed in the form of branches covering a smaller part of the surface (Fig. 11d). The formation of various forms of crystals in the HAZ of GTAW- and laser-welded joints was probably caused by changes in the chemical composition (Fig. 11). 5. Summary and conclusions 904 L austenitic stainless steel is considered as having a high resistance to corrosion. Here we found no visible changes to the surface of the BM samples after exposure to a salt mist corrosive environment of 5% NaCl solution at 307 to 309 K. The rate of corrosion protection according to the PN-EN ISO 10289: 2002 was RP10 (completely resistant). GTAW- and laser-welding reduced the protection factor to RP = 9 (high resistance), indicating that the surface of the sample covered by corrosion was b 0.1%. Shot peening of welded joints contributed significantly to the improvement of corrosion resistance. For all welded joints subjected to shot peening, the inhibition of corrosion processes were
Fig. 10. Cross-section of laser beam welded joints. Arrows indicate corrosion.
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Fig. 11. Increase in NaCl crystals in the HAZ of GTAW-welded samples (a) and laser-welded samples (d) and changes in the chemical composition of the three principal alloying elements (i.e., nickel, chromium and molybdenum) (c, f) on a line passing through the welded joint (b, e).
observed after 1000 h of exposure to corrosive environment. Whereas in the case of laser-welds after shot peening, the corrosive environment did not affect the samples. The HAZ was the principal area of corrosion in the GTAW-welded joints and this occurred from the weld face side. After 1000 h of exposure to the corrosive environment, the corrosion pitting in that area in non-peened samples reached a depth of 40 μm. In the peened elements, the pitting depth after the same period did not exceed 10 μm (Tab. 9). In the non-peened laser-welded joints, corrosion occurred after 1000 h of exposure to the corrosive environment. The corrosion was observed from the weld root side, along the fusion line, and reached a comparable depth. In the same test conditions after shot peening of laser-welded joints, pitting corrosion did not occur, which should be regarded as a highly promising result. This indicates that elements of 904 L steel should be subjected to shot peening and laser-welding to achieve best results in terms of corrosion resistance. Moreover, corrosion pitting in non-peened GTAW- and laser-welded samples, despite its small size, could also have a significant impact on the development of microcracks, which can then contribute to structural damage. Various types of NaCl crystallisation occurred at the fusion zones of sample surfaces. Typical uniform NaCl crystals appeared in the HAZ of non-peened GTAW samples. Whereas branched NaCl\\Ni crystals occurred on the HAZ surface of the laser-welded samples. Formation of differently structured crystals impacted the corrosion processes. These differently structured crystals resulted from variations in the chemical composition of the welded joints (Fig. 11 c, f). In conclusion, in corrosive environment tests of 904 L steel welded joints, the laser-welded plus shot peened samples were most resistant. The process of shot peening lead to the formation of a passive layer that blocked the direct impact of corrosion centres and prevented the development of corrosion. Analysis of weld metallographic sections revealed that the joints made by GTAW without shot peening were most susceptible to uniform and pitting corrosion. Based on our findings, we propose that the joints of 904 L steel should be made by laser-welding and be shot peened to achieve total resistance to the impact of salt mist corrosive environments.
References [1] P. Sharma, H. Roy, Pitting corrosion failure of an AISI stainless steel pointer rod, Eng. Fail. Anal. 44 (2014) 400–407. [2] http://www.all-met.pl/faq/ (types of corrosion for advanced). [3] D. Nakhaie, M.H. Moayed, Pitting corrosion of cold rolled solution treated 17–4 PH stainless steel, Corros. Sci. 80 (2014) 290–298. [4] http://www.substech.com/2015. [5] E. Tasak, A. Ziewiec, Spawalność materiałów konstrukcyjnych, tom.1- Spawalność stali, Kraków (2009) 426–427. [6] J.F. Lancaster, Metallurgy of welding, Woodhead Publishing Limited, Cambridge, 2006. [7] http://www.bssa.org.uk/2015. [8] L. Shen, L. Wang, Y. Wang, Ch. Wang, Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening, Surf. Coat. Technol. 204 (2010) 3222–3227. [9] A. Nakonieczny, Dynamiczna powierzchniowa obróbka plastyczna — kulowanie (shot peeling), IMP, Warsaw, 2002. [10] P. Fu, C.H. Jiang, Residual stress relaxation and micro-structural development of the surface layer of 18CrNiMo7-6 steel after shot peening during isothermal annealing, Mater. Des. 56 (2014) 1034–1038. [11] M. Kumagai, K. Akita, Y. Itano, M. Imafuku, S. Ohya, X-ray diffraction study on microstructures of shot/laser-peened AISI316 stainless steel, J. Nucl. Mater. 443 (2013) 107–111. [12] K. Zhan, C.H. Jiang, V. Ji, Effect of prestress state on surface layer characteristic of S30432 austenitic stainless steel in shot peening process, Mater. Des. 42 (2012) 89–93. [13] K. Zhan, C.H. Jiang, V. Ji, Surface mechanical properties of S30432 austenitic steel after shot peening, Appl. Surf. Sci. 258 (2012) 9559–9563. [14] K. Zhan, C.H. Jiang, V. Ji, Uniformity of residual stress distribution on the surface of S30432 austenitic stainless steel by different shot peening processes, Mater. Lett. 99 (2013) 61–64. [15] D.D. Nage, V.S. Raja, Effect of nitrogen addition on the stress corrosion cracking behaviour of 904 L stainless steel welds in 288 °C deaerated water, Corros. Sci. Eng., India (2006) 2317–2331. [16] K. Murugaratnam, S. Utili, N. Petrinic, A combined DEM–FEM numerical method for shot peening parameter optimization, Adv. Eng. Softw. 79 (2015) 13–26. [17] O. Takakuwa, H. Soyama, Suppression of hydrogen-assisted fatigue crack growth in austenitic stainless steel by cavitation peening, Int. J. Hydrog. Energy 37 (2012) 5268–5276. [18] A. Stratulat, J.A. Duff, T.J. Marrow, Grain boundary structure and intergranular stress corrosion crack initiation in high temperature water of a thermally sensitised austenitic stainless steel, observed in situ, Corros. Sci. 85 (2014) 428–435.
Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Shot peening effect on 904 L welds corrosion resistance Barbara Nasiłowska a,⁎, Zdzisław Bogdanowicz a, Michał Wojucki b a b
Military University of Technology, Institute of Machine Building, Kaliskiego 2, 00-908 Warsaw, Poland Institute of Precision Mechanics, Duchnicka 3, 01-796 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 14 August 2015 Accepted 28 August 2015 Available online 8 September 2015 Keywords: Welding laser beam Welding TIG method 904 L-SS Shot peening Corrosion resistance
a b s t r a c t Here we investigated the effect of shot peening on the corrosion resistance of 904 L austenitic stainless steel joints made by CO2 laser beam and GTAW welding. Corrosion tests on the fusion zone in welds made by each of these welding methods were performed on shot peened and non-peened specimens. The samples were placed in a salt chamber and inspected after 24, 48, 72, 120, 240, 480, 720 and 1000 h. Pitting corrosion on non-peened welds made by GTAW welding was visible in the heat-affected zone from the face of the weld and reached a depth of 40 μm. Shot peening of welded joints decreased corrosion by 75%. In the case of shot peened welds made by laser beam welding, pitting corrosion had not occurred after 1000 h of salt mist exposure. © 2015 Elsevier Ltd. All rights reserved.
1 . Introduction The corrosion resistance of 904 L stainless steel is highly affected by the content of the alloying elements, the type of applied heat treatment and the condition of the surface layer [1,2]. The occurrence of pitting corrosion is a common reason for austenitic steel degradation in aqueous environments (e.g., in solutions containing halide and/or chloride ions) (Fig. 1) [1,2]. A characteristic feature of pitting corrosion is the presence of pits formed as a result of an interaction between a passive film (which plays a role of a cathode) and locally depassivated regions (“anodes”) on the surface of the material [3,4]. The role of an anode can be played by the internal inhomogeneities of metals (i.e., non-metallic inclusions, separations, deformations) and by external inhomogeneities, such as edges, scratches, dents, scale residues and other residues [3,4]. Chemical composition also has a crucial influence on the pitting corrosion resistance of stainless steel. Gooch et al. [5] proposed a quantitative estimation (pitting resistance equivalent — PRE index) that allows for an evaluation of a contribution of alloying elements to the corrosion behaviour of the material: PRE ¼ % Cr þ 3:3ð%MoÞ þ xð%NÞ
ð1Þ
where x = 16 for duplex (austenitic–ferritic) stainless steels and x = 30 for austenitic stainless steels. ⁎ Corresponding author. E-mail addresses: [email protected] (B. Nasiłowska), [email protected] (Z. Bogdanowicz), [email protected] (M. Wojucki).
http://dx.doi.org/10.1016/j.jcsr.2015.08.041 0143-974X/© 2015 Elsevier Ltd. All rights reserved.
The higher the PRE index value the better the resistance to pitting corrosion. In the case of 904 L stainless steel, the PRE index value is within the range of 32.2 to 39.9 [6,7]. It is worth noting that these PRE values are below the threshold of full corrosion resistance, which for steel has been established as 40–60. Compressive stresses introduced into the layers during shot peening treatment lead to an increase of microhardness (coming from a high dislocation density and finer grain size) in this area [8,9]. Such findings are in line with those reported for 304 steel [8], 18CrNiMo7-6 steel [10], 316 steel [11] and S30432 steel [12–14], nitrided after initial shot peening. It was found that the application of shot peening, prior to the nitriding process, resulted in an almost doubling of surface microhardness. Therefore, an additional protection is needed to improve the resistance of 904 L stainless steel components to aqueous environments [15]. Compressive stress to the surface layer has been shown to inhibit the development of pitting corrosion and is therefore a potential protective approach. One way of controlling the stress rate at the surface layer of a material is to apply shot peening. This process is based on the partial transfer of speeding bead peening pressure kinetic energy to the treated material, resulting in compressive stress. An important objective of shot peening is to change the condition of the surface layer of the treated material by impacting it with a shot to create plastic deformation. Analyses have shown significant improvement in stress distribution performance (hardness, fatigue strength, yield strength, impact strength and elongation decrease, as well as an increase in the resistance to tribological wear) after shot peening of austenitic stainless steel [16–18]. However, to our knowledge, the impact of
B. Nasiłowska et al. / Journal of Constructional Steel Research 115 (2015) 276–282
Fig. 1. Pitting corrosion development scheme [3,4].
Table 1 Sample numbers, welding method and type of surface treatment. Sample No./numer
Welding method
Type of treatment
521/1.1 ÷ 3 521/2.1 ÷ 3 521/3.1 ÷ 3 521/4.1 ÷ 3
GTAW Laser GTAW Laser
Non peened
277
Pneumatic shot peening of the surface layer of 904 L steel was carried out by the Institute of Precision Mechanics (IMP) in Warsaw. The process involved a stream of spring steel shot (640 HV) with a diameter of 0.8 mm at 0.5 MPa. The exposure duration was 6 min and the sample was 100% covered. The intensity of shot peening was fC = 0.25 mm, as determined using the Almen strip (“A” type, with a thickness of 1.3 mm, Grade II). Prior to the commencement of the test, the samples were acclimatised for 24 h at a temperature of 296 ± 2 K and 50 ± 5% relative humidity. The study of corrosion resistance was conducted in accordance with ISO 9227:2012 “Corrosion tests in artificial atmospheres — Salt spray tests”, in the HERAEUS VÖTSCH S 1000-type salt chamber. The elements were tested in a chamber with 5% NaCl solution at an operating temperature of 307 to 309 K. Corrosion damage assessment was carried out in accordance with PN-EN ISO 10289: 2002 “Methods for corrosion testing of metallic and other inorganic coatings on metallic substrates — Rating of test specimens and manufactured articles subjected to corrosion tests” and allowed for the determination of a protection rating (RP) and for the identification of corrosion changes to the surface layer of the specimen. 3 . Material
Peened
shot peening on pitting corrosion of 904 L steel welds made by GTAW and laser welding has yet to be investigated.
2 . Research methodology Corrosion resistance tests were carried out on flat elements (45 × 12 × 5 mm) made of 904 L austenitic stainless steel, either GTAW- (tungsten arc welding) or laser beam-welded; which came from the same delivery batch. To compare the effect of shot peening on the corrosion resistance of welded joints, some elements were subjected to an additional treatment of shot peening, according to Table 1. TIG welding, using a tungsten electrode with a 2.5 mm diameter MTC, of MT-904 L and the G/W 20 25 5 CuL (20% Cr, 25% Ni, 4.5% Mo, 1.5% Cu) binder was made perpendicular to the direction of rolling, with two stitches. The root of the weld was then removed and prewelding conducted. The elements used in the tests were made at the Chemical Equipment Construction Plant of the Azoty Group in Tarnów (Zakład Budowy Aparatury Chemicznej), in accordance with the production technology used in manufacturing chemical equipment. Laser welding was performed at the Kielce University of Technology, using a Triumph 1005 CO2 laser with a lens focal distance of 260 mm, with a spot (0.4 mm diameter) on the sample surface, with a P of 4.5 kW and a welding speed of v = 1.4 m/min, in a helium enclosure.
The research specimens were of 904 L austenitic stainless steel (Cr: 19–21, Ni: 24–26, Mo: 4–5, Cu: 1.2–2.0, Mn: ≤ 2.0, P: ≤ 0.030, S: ≤ 0.010, Si: ≤ 0.7, N: ≤ 0.15, C: ≤ 0.02, wt. %), either GTAW- (Fig. 2a) or laser-welded (Fig. 2b). Selected GTAW- and laser-welded samples were shot peened (Table 1). In the GTAW-weld (made with three stitches) samples, a flat front of melt solidification occurred with staged grain boundary melting. Longer heat stress from the heat source and hence slower solidification affected the growth of dendritic side branches. The lower solidification point of the grain boundaries caused a flow in of the binder to the base material (BM) along the fusion line. In the case of the single stitch weld made by laser welding, transcrystallisation was observed with the formation of dendrites over the entire length from the fusion zone to the front line of heat dissipation. 4 . Welded joints before and after shot peening 4.1 . Structure Upon examination of the BM structure (Fig. 3a, b), the zones of the laser- (Fig. 3c, d) and GTAW-welding (Fig. 3e, f) of the 904 L steel after shot peening indicated the deformation of austenite grains caused by a cold work of the surface layer to a depth of 150 to 200 μm. 4.2 . Microhardness Microhardness measurements were carried out in accordance with the 0.1 HV Vickers hardness method, both before and after
Fig. 2. Structure of a weld made by GTAW welding (a) and laser beam welding with beam parameters: P = 4.5 kW and v = 1.4 m/min (b).
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Fig. 3. Microstructure of the surface layer of the base material after pneumatic shot peening (a, b) of the joints welded by either laser (c, d) or GTAW (e, f). Images from Quanta FEG 3D scanning electron microscope (FEI Company) with a FSD (a, c, e) and EBSD (b, d, f) detector. The dashed lines show the depth of the strengthened surface layer of elements after shot peening and W markings indicate the microstructure of the weld.
Fig. 4. Distribution of microhardness in the surface layer of base material and the laser beam welded elements.
Fig. 5. The distribution of microhardness into the surface layer of the base material and GTAW welded components.
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Fig. 6. Internal stress in the weld (W), heat affected zone (HAZ) and base material (BM) in the x-axis (a) and y-axis (b) before and after shot peening.
strengthening of the BM surface, in the heat affected zone (HAZ) and in the weld area (W) of the welded samples (laser- and GTAW-welded). Dynamic surface treatment of the welded joints resulted in a strengthening of the surface layer because of the surface cold work. Both the laser- and GTAW-welded samples showed a significant increase in microhardness (Figs. 4 and 5). As a result of plastic roll forming in the 904 L steel production process, the microhardness of the BM prior to shot peening was ~290 HV (20 microns into the surface), which increased to ~ 400 HV after shot peening. Thermal cycling in the laser welding process resulted in a release and re-growth of grains in the form of dendrites. This release and re-growth resulted in a return of the microhardness of the HAZ W to the normative value (190 HV). The shot peening treatment resulted in an increase of microhardness to ~400 HV in the HAZ and W areas at a depth of ~ 20 μm from the surface of the sample (Fig. 4). The fusion zone of the laser beam welded joint was strengthened at a depth of ~300 μm. The thermal cycle and segregation of the alloying elements included in the binder of the GTAW-weld result in an increase in the microhardness (at 20 μm depth and within the axis of the weld) from 154 HV before shot peening to the normative value (190 HV) at the 195 μm level. The microhardness of the HAZ of the non-peened components at a depth of 20 μm was ~190 HV. The shot peening process resulted in an increase in microhardness to ~420 HV in the HAZ and ~ 380 HV in the W area at a ~ 20 μm depth (Fig. 4). The zone of the GTAW-welded joint was strengthened to a depth of ~250 μm (Fig. 5).
4.3 . Internal stress In the analysed areas (W, HAZ, BM), the compressive stress on the surface of the laser- and GTAW-welded joints increased after shot peening (Fig. 6). Deeper metal layers do not allow for free and complete spreading of the plastic deformation arising from the shot peening process, which results in the formation of compressive stress [7]. The compressive stresses in the W after shot peening varied from −364 to −661 MPa, depending on the performance method and measurement directions. Before shot peening, tensile stress process were observed in the HAZ and W. However, compressive stress always occurred in the BM after welding and this value increased after the shot peening process. The greatest compressive internal stress (~800 MPa) occurred in the HAZ and BM in laser-welded joints after shot peening. In the HAZ of GTAW joints the stresses after shot peening (higher thermal effect) were −295 (x) and −81 MPa (W). 4.4 . Corrosion resistance tests Comparative studies were carried out regarding the impact of the corrosive environment on shot peened and non-peened GTAW- and laser-welded samples. The tested pieces were placed in a salt spray chamber and subjected to periodic review after 24, 48, 72, 120, 240, 480, 720 and 1000 h. The test results indicated that, in the HAZ of non-peened samples, pitting and general corrosion were initiated,
Fig. 7. The surfaces of samples welded with GTAW and laser beam before and after being placed in the salt chamber.
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Table 2 Results of corrosion resistance in salt spray for welded joints. Sample No./numer
Welding method
Protection rate RP
521/1.1÷3 521/2.1÷3 521/3.1÷3 521/4.1÷3
GTAW — Non peened Laser — Non peened GTAW — Peened Laser — Peened
RP9 RP9 RP9 RP10
which were visible when using a scanning electron microscope at ×250 magnification (Fig. 1a). The surface roughness values of the samples prior to shot peening were: Ra = 3.9 μm and RY5 = 18.53 μm. After shot peening the surface roughness values were: Ra = 3.98 μm and RY5 = 18.77 μm. The photographs in Fig. 7 show the impact of the salt mist (5% NaCl solution) atmosphere on GTAW- and laser-welded joints before and after (1000 h) exposure.
The results of the corrosion resistance tests for welded joints are given in Table 2. In the case of GTAW-welded peened and non-peened samples and non-peened laser-welded joints, the corrosion protection rating as determined by external visual inspection was RP9, according to PN-EN ISO 10289: 2002. This indicates that the proportion of the sample surface covered by corrosion was ≤0.1%. In contrast, on the surface of the laser-welded shot peened samples, the protection rating was RP10, which indicates complete absence of corrosion. The corrosive environment produced certain changes in the surface layer of the welds, with a thin layer of general corrosion being visible under a scanning electron microscope (Fig. 8). The largest corrosion centres were observed in non-peened joints welded using GTAW method, after 1000 h of exposure to salt mist. The said changes are shown in Fig. 8 in the form of topography images of the fusion zone in welds made by GTAW and laser beam welding, peened and not peened, after 720 and 1000 hours of exposure to corrosive environment. The fusion line separating the face of
Fig. 8. The topography of peened and non-peened surfaces of joints welded using GTAW and laser beam methods, exposed to the impact of corrosive environment: BM — base material, W — weld.
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Fig. 9. Cross-section of GTAW welded joints. Arrows indicate corrosion.
weld reinforcement from the base material is marked in the photographs. Structural analyses of the cross-section of welded joints revealed that the surface cold work by shot peening resulted in the inhibition of corrosion and surface damage. The strain on the surface layer that occurs as a result of shot peening increases microhardness and the density of dislocations [8]. Accumulation of stress causes the activation of the surface and thus the susceptibility to environmental impacts. The strain (surface hardening) shifts the potential in a negative direction, reducing the surface selfpassivation. Due to the influence of several thermal cycles (longer action of high temperature and slower solidification) during GTAW welding, the degradation of the material in the salt chamber occurred mainly from the side of the weld face (Fig. 9). Pitting corrosion initiated in the HAZ, where the largest corrosion microcells occurred. After 1000 h, this corrosion had penetrated between 20 and 40 μm into the surface (Fig. 9 and 10). Analysis of the salt mist treated samples after 720 and 1000 h indicated that the laser-welded plus shot peened group were most resistant to corrosion. In laser-welded non-peened samples after 1000 h of salt mist exposure, the pitting corrosion occurred along the fusion line from the root of the weld (where the largest external inhomogeneities [in the form of weld reinforcement] were observed) (Fig. 10). Analysis of the metallographic cross-section and topography of welded joint surfaces showed that the corrosion initiated mainly in
the HAZ and reached the following values after 1000 h: GTAWwelded, peened = ~ 5 to 10 μm into the surface from the weld face; GTAW-welded, not peened = ~20 to 40 μm into the surface from the weld face; laser-welded, not peened ~30 μm into the surface from the side of the weld root. Typical NaCl crystals were observed near the HAZ on the surfaces of the GTAW-welded samples, which covered a significant part of the surface, leading to more intense corrosion (Fig. 11a). In case of the laser-welded samples, in the HAZ, the Ni\\NaCl crystals were observed in the form of branches covering a smaller part of the surface (Fig. 11d). The formation of various forms of crystals in the HAZ of GTAW- and laser-welded joints was probably caused by changes in the chemical composition (Fig. 11). 5. Summary and conclusions 904 L austenitic stainless steel is considered as having a high resistance to corrosion. Here we found no visible changes to the surface of the BM samples after exposure to a salt mist corrosive environment of 5% NaCl solution at 307 to 309 K. The rate of corrosion protection according to the PN-EN ISO 10289: 2002 was RP10 (completely resistant). GTAW- and laser-welding reduced the protection factor to RP = 9 (high resistance), indicating that the surface of the sample covered by corrosion was b 0.1%. Shot peening of welded joints contributed significantly to the improvement of corrosion resistance. For all welded joints subjected to shot peening, the inhibition of corrosion processes were
Fig. 10. Cross-section of laser beam welded joints. Arrows indicate corrosion.
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Fig. 11. Increase in NaCl crystals in the HAZ of GTAW-welded samples (a) and laser-welded samples (d) and changes in the chemical composition of the three principal alloying elements (i.e., nickel, chromium and molybdenum) (c, f) on a line passing through the welded joint (b, e).
observed after 1000 h of exposure to corrosive environment. Whereas in the case of laser-welds after shot peening, the corrosive environment did not affect the samples. The HAZ was the principal area of corrosion in the GTAW-welded joints and this occurred from the weld face side. After 1000 h of exposure to the corrosive environment, the corrosion pitting in that area in non-peened samples reached a depth of 40 μm. In the peened elements, the pitting depth after the same period did not exceed 10 μm (Tab. 9). In the non-peened laser-welded joints, corrosion occurred after 1000 h of exposure to the corrosive environment. The corrosion was observed from the weld root side, along the fusion line, and reached a comparable depth. In the same test conditions after shot peening of laser-welded joints, pitting corrosion did not occur, which should be regarded as a highly promising result. This indicates that elements of 904 L steel should be subjected to shot peening and laser-welding to achieve best results in terms of corrosion resistance. Moreover, corrosion pitting in non-peened GTAW- and laser-welded samples, despite its small size, could also have a significant impact on the development of microcracks, which can then contribute to structural damage. Various types of NaCl crystallisation occurred at the fusion zones of sample surfaces. Typical uniform NaCl crystals appeared in the HAZ of non-peened GTAW samples. Whereas branched NaCl\\Ni crystals occurred on the HAZ surface of the laser-welded samples. Formation of differently structured crystals impacted the corrosion processes. These differently structured crystals resulted from variations in the chemical composition of the welded joints (Fig. 11 c, f). In conclusion, in corrosive environment tests of 904 L steel welded joints, the laser-welded plus shot peened samples were most resistant. The process of shot peening lead to the formation of a passive layer that blocked the direct impact of corrosion centres and prevented the development of corrosion. Analysis of weld metallographic sections revealed that the joints made by GTAW without shot peening were most susceptible to uniform and pitting corrosion. Based on our findings, we propose that the joints of 904 L steel should be made by laser-welding and be shot peened to achieve total resistance to the impact of salt mist corrosive environments.
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