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Effect of surface grinding on chloride induced SCC of 304L
Materials Science & Engineering A 658 (2016) 50–59
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of surface grinding on chloride induced SCC of 304L Nian Zhou a,b,n, Rachel Pettersson b,c, Ru Lin Peng d, Mikael Schönning e a
Department of Material Science, Dalarna University, SE-79188 Falun, Sweden KTH, SE-10044 Stockholm, Sweden Jernkontoret, SE-11187 Stockholm, Sweden d Department of Management and Engineering, Linköping University, SE-58183 Linköping, Sweden e Corrosion Department, Avesta Research Centre – Outokumpu Stainless AB, SE-774 22 Avesta, Sweden b c
art ic l e i nf o
a b s t r a c t
Article history: Received 21 December 2015 Accepted 25 January 2016 Available online 29 January 2016
The effect of surface grinding on the stress corrosion cracking (SCC) behavior of 304L austenitic stainless steel in boiling magnesium chloride has been investigated. SCC tests were conducted both without external loading and with varied levels of four-point bend loading for as-delivered material and for specimens which had been ground parallel or perpendicular to the loading direction. Residual stresses due to the grinding operation were measured using the X-ray diffraction technique. In addition, surface stress measurements under applied load were performed before exposure to evaluate the deviation between actual applied loading and calculated values according to ASTM G39. Micro-cracks initiated by a high level of tensile residual stress in the surface layer were observed for all the ground specimens but not those in the as-delivered condition. Grinding along the loading direction increased the susceptibility to chloride induced SCC; while grinding perpendicular to the loading direction improved SCC resistance. Surface tensile residual stresses were largely relieved after the initiation of cracks. & 2016 Elsevier B.V. All rights reserved.
Keywords: Stress corrosion cracking Residual stress Austenitic stainless steel 304L Grinding
1. Introduction Standard austenitic stainless steels are very widely used for industrial applications. However, they are highly susceptible to chloride stress corrosion cracking (SCC) which can lead to catastrophic failures. One review in 1983 showed that 37% of almost one thousand failure cases of the austenitic stainless steel 304 in the chemical industry were attributed to stress corrosion cracking [1]. Depending on the microstructure of the material and the nature of the environment, SCC may be intergranular or transgranular [2]. For example, Ghosh and Kain observed cracking to be transgranular for solution annealed, cold worked and surface machined 304L stainless steel in a chloride environment [3]. Jin et al. [4] showed grain boundary engineering could shift the fracture from intergranular to transgranular stress corrosion cracking of 304 stainless steel plate. It is well recognized that the surface conditions including geometrical, physical and mechanical properties of machined components will largely affect their functional performance, such as corrosion resistance. Ghosh et al. [5] demonstrated a higher SCC susceptibility of surface machined 304L in 5 N H2SO4 þ0.5 N NaCl n Corresponding author at: Department of Material Science, Dalarna University, SE-79188 Falun, Sweden. E-mail address: [email protected] (N. Zhou).
http://dx.doi.org/10.1016/j.msea.2016.01.078 0921-5093/& 2016 Elsevier B.V. All rights reserved.
environment than solution annealed material. A higher surface roughness and poor surface finish have been reported to initiate pits, which have been suggested as precursors to cracks [6]. The presence of strain induced martensite on the surface from machining resulted in higher SCC susceptibility of 304L austenitic stainless steel [7]. Cracking in different patterns along and transverse to the milling direction has been shown by Lyon et al. [8]. In general it is considered that SCC occurs as a result of the interaction of three factors: a corrosive environment, a susceptible alloy and the presence of tensile stresses [2]. However, microstructural effects and stress localization may also play a significant role. Karlsen et al. [9] demonstrated that strain heterogeneity due to the low stacking fault energy of austenitic stainless steels promoted strain localization during surface mechanical treatment, which in turn promoted crack initiation. The improvement of SCC resistance of 304 austenitic stainless steel by laser peening has been demonstrated by Lu et al. [10]; they attributed this to the generation of high-level compressive residual stress and grain refinement. Cold rolling texture was found to influence corrosion behavior of 304 stainless steel; the presence of close pack crystallographic planes parallel to the sample surface was reported to improve the corrosion properties [11]. Grain boundary misorientation ahead of the crack tip in 316 stainless steel has been detected by the Transmission Kikuchi diffraction (TKD) technique [12]. Research has shown that there is a connection between intergranular crack propagation and the misorientation of the grain
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boundaries along which the cracks propagate. [13,14]. A duplex structure of stainless steel has generally been found to improve ClSCC resistance compared to austenitic grades, although this may depend on the actual test environment [15]. There is a range of loading methods that can be used to assess stress corrosion cracking susceptibility. Four-point bend loading is widely used for strip material in the elastic deformation regime. The specimen holder design and elastic stress calculation are described in ASTM G39 [16] and ISO 7539 [17]. However, the actual stress in the specimen surface may deviate appreciably from the calculated values if residual stresses are present or local plasticity occurs. In addition, stress relaxation during exposure should be taken into account since differences in stress relaxation with different loading methods and for different materials have been observed to cause different corrosion behavior in the same environment [18]. When fabricating stainless steels, grinding is an important and widely used surface finishing process. Grinding induces considerable plastic deformation and generates thermal energy, both of which lead to changes in the residual stress state in the material. It has been shown that the sum of externally applied stresses and residual stresses can lead to crack initiation and propagation and also demonstrated that an induced high-level of compressive residual stress from laser peening can be released during the U-bend process [10]. However, little research has been performed to define the role of pure residual stresses in crack initiation and growth. The aim of this work is to contribute to the understanding of the role of residual stresses by studying the chloride induced stress corrosion behavior of as-delivered and surface ground 304L austenitic stainless steel. Specimens have been exposed both without external loading and under different four-point bend loads.
2. Material and methods 2.1. Material The material investigated in this study was 304L austenitic stainless steel with 2B surface finish supplied by Outokumpu Stainless AB as test coupons 400 mm 150 mm 2 mm in dimensions. The as-delivered material had been solution annealed (1100 °C, forced air cooling and water quenched), pickled and roll leveled. The chemical composition and main measured mechanical properties perpendicular to the rolling direction at room temperature are given in Tables 1 and 2, respectively. Fig. 1 shows the microstructure of the material from EBSD mapping. Ferrite measurement according to ASTM E1245 [15] showed 1.7% of ferrite to be present in the material. For these measurements the crosssection of the as-delivered material was etched in 40% NaOH solution, using 2.5 V for 3 s, then measurements were made on 10 fields throughout the thickness at 1000 magnification. 2.2. Grinding operations The grinding operations were conducted on a Chevalier FSG2A618 grinding machine; the set-up is described in detail in [19]. A Kemper RADIX Go grinding wheel (150 mm in diameter, 50 mm in width), which is an expanding roller made of 20 mm thick rubber, Table 1 Chemical composition (wt%) of 304L austenitic stainless steel. C
Si
Mn
P
S
Cr
Ni
Nb
Cu
Co
N
0.019
0.32
1.55
0.029
0.001
18.22
8.11
0.011
0.31
0.16
0.071
51
Table 2 Mechanical properties of 304L austenitic stainless steel measured perpendicular to the rolling direction at room temperature. Rp0.2(MPa)
Rm(MPa)
Elongation (%)
Hardness (HB)
230
642
54
170
was used. Grinding belts (50 mm in width, 473 mm in length) with conventional aluminum oxide grit were mounted on the grinding wheel and the test coupon was mounted on the grinding table. The grinding operations were performed along the rolling direction of the material. A fixed grinding speed vs=23 m/s, a fixed feed rate vw =8 m/s and a fixed motor power of 600 W were used. Grinding was first performed for 5 min using 60# grit size abrasive to remove the as-delivered material surface, and then followed by 2.5 min grinding with a new 60# grit size abrasive to get the final surface finish. No grinding lubrication was used during the operations. A piezo-electric transducer-based dynamometer, Kistler 9275B, was mounted under the working table to measure the normal force during the grinding operations. The measured normal force was 100 N in this study. 2.3. Material characterization SEM (Scanning electron microscopy, FEG-SEM Zeiss Ultra 55) was used to investigate the surface topography and ECCI (electron channeling contrast imaging, Hitachi FEG-SEM SU-70) to investigate the near surface microstructure evolution. Cross-sections of selected specimens after exposure were investigated from both longitudinal (LD) and transversal (TD) directions and some fracture surfaces were examined. The in-depth profiles of residual stresses parallel ( σ∥) and perpendicular ( σ⊥ ) to the rolling directions were determined by X-ray diffraction for both as-delivered and ground samples. Cr– Kα radiation was used and residual stresses were determined from the measured diffraction peak at 2θ 128° for the {220} lattice planes. The method is described in detail in [20] and the set-up used in the current work is similar to that used in the authors' previous work on duplex stainless steel 2304 [21]. In-situ surface stress measurements were also made on specimens subjected to loading in the same four-point bending fixtures which were used for the stress corrosion cracking testing. The loading was increased in steps to levels of 10, 200, 300 and 500 MPa, calculated according to ASTM G39 [16]. At each loading level, the specimen was kept one hour for stress relaxation, then X-ray diffraction was used to measure the actual surface stresses parallel to the loading direction. It should be noted that the two higher loads are above the measured proof stress of the as delivered material and thus strictly outside the range for which the four-point loading formula [16] is valid. After the measurement, all specimens which were kept in the holder at 500 MPa were heated in a furnace at 155 °C for 24 h, then allowed to cool to room temperature in the furnace. Surface residual stresses were measured again to investigate the stress relaxation. Surface stresses were also measured after corrosion tests. 2.4. Stress corrosion tests Chloride induced stress corrosion cracking susceptibility was tested without external loading and under four-point bend loading. As illustrated in Fig. 2, three types of specimens were tested. The as-delivered specimens, cut with the long axis parallel to the rolling direction, are denoted AD. Grinding was performed parallel to the rolling direction, and specimens were cut either parallel to the rolling/grinding direction, denoted ground-RD or transverse to
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Fig. 1. Cross-section microstructure and grain orientation of 304L austenitic stainless steel as-delivered.
Fig. 2. Schematic illustration of the orientation and designation of the different specimens tested.
rolling/grinding direction, denoted ground-TD. The test environment was boiling magnesium chloride solution according to ASTM G36 [22] Prior to testing all the specimen edges were ground down using 800# grinding paper to avoid sharp edges and specimens were then allowed to passivate in air for at least 24 h before exposure. The specimens were exposed in a flask connected to a water cooled condenser, and the temperature was maintained at 155 °C 71 °C with the help of a thermometer in the test solution. The first set of tests was without external loading to investigate to role of residual stresses. All three types of specimens (dimension 45 mm 10 mm 2 mm) were exposed for 20 h, then removed and checked for macro-cracks. This was done in a stereomicroscope (Nikon SMZ-2T with ColorView Soft camera and CellA ColorView Soft image software) at 10–63 magnification. If no macro-cracks were observed, the specimens were then put back for another 20 h exposure. In the second series of tests, a four-point bend load was applied to specimens of dimensions 65 mm 10 mm 2 mm according to ASTM G39 [16]. The loading direction was along the longitudinal direction. After application of the load, each specimen was kept one hour in air to allow possible stress relaxation before exposure. In this case the initial test period was 24 h.
3. Results 3.1. Pre-corrosion characterization 3.1.1. Surface topography SEM images of as-delivered and ground surface topography are shown in Fig. 3. The as-delivered surface in Fig. 3(a) is typical for a 2B surface and shows that the pickling process during production slightly etched the grain boundaries. For the ground specimen, as illustrated in Fig. 3(b), surface defects such as deep grooving, smearing, adhesive chips and indentations were found. The generation of surface defects during grinding has been described in detail in previous work [19] and such defects are also observed when grinding duplex stainless steels [21].
3.1.2. Residual stresses The in-depth residual stress profiles parallel ( σ∥) and perpendicular ( σ⊥ ) to the rolling directions as well as the full width at half maximum (FWHM) profiles of both as-delivered and ground specimens are presented in Fig. 4. Fig. 4(a) shows a low level of residual stress in the as-delivered material. Both σ∥ and σ⊥ are close to zero from the surface layer to the subsurface region. However, the grinding operations generated tensile σ∥ but compressive σ⊥ in the surface layer as seen in Fig. 4(b). The tensile σ∥ was highest in the ground surface, up to 361 746 MPa, and dropped rapidly to compression within a depth of around 15 mm. The compressive σ⊥ showed a relative low value of 54 720 MPa in the surface layer, but increased rapidly to reach a peak of almost 250 MPa in the subsurface region, and then dropped gradually to zero in the bulk material. The full width of half maximum (FWHM) revealed broadening of the diffraction peak, which may be considered to indicate plastic deformation [23]. FWHM showed similar values from surface to subsurface for as-delivered material. However, for the ground specimen, a decrease of FWHM with increasing depth was observed, which reveals a gradient of plastic deformation over 40 mm in thickness under the ground surface. 3.1.3. Surface stresses under external loading Surface stresses measured in the as-delivered, ground-RD and ground-TD specimens subjected to four point bending are presented in Fig. 5. For all three specimens, four loading steps of 10, 200, 300 and 500 MPa were applied. For the as-delivered material, an additional specimen with one more loading step of 110 MPa was measured to check the accuracy and repeatability. For the asdelivered material the actual surface stresses are close to the calculated loading in the elastic regime. Above the proof stress of the material, which is around 230 MPa, the slope of the curve decreases significantly, so that a calculated applied load of 500 MPa gives an actual stress of only 250 MPa. It should be noted, as mentioned earlier, that this loading level is outside the valid regime for four-point loading according to ASTM G39. The results were very different for the ground specimens (Fig. 5 (b)). Both of the curves show linear trends over the whole loading range, indicating that work hardening has significantly increased the local proof stress. The actual surface stress of the ground-TD specimen is around two thirds of the calculated load. For the ground-RD sample, the slope is a little higher compared to the TD direction. It is seen that as the applied stress adds on to the residual tensile stress in the rolling and grinding direction, the actual stress is 750 MPa when a calculated loading of 500 MPa is applied. Fig. 6 presents the measured surface stress in the loading direction at 500 MPa applied loading and after heat treatment at 155 °C for 24 h. The results show that stress relaxation is of the order of around 50 MPa. 3.2. Corrosion behavior without external loading 3.2.1. Surface and in-depth morphology after exposure Typical SEM images of the surface morphology after corrosion
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Fig. 3. Surface topography of (a) as-delivered material, (b) ground specimen.
Fig. 4. In-depth residual stresses and full width at half maximum profiles of (a) as-delivered material, (b) ground specimen. Positive values denote tensile stresses and negative compressive stresses.
testing are shown in Fig. 7. Pitting was observed on all specimens. The pits were small and the density was low. Micro-cracking took place in all ground specimens even in the absence of any external loading (Fig. 7(b)). Micro-cracks on the ground surfaces exhibited extensive branching and were primarily oriented perpendicular to the grinding marks i.e. perpendicular to the direction with the highest tensile residual stress. However, in the case of as-delivered specimens, there is no evidence of micro-cracks. 3.2.2. Cross-section investigation after exposure By using the ECCI technique, diffraction contrast images can be obtained to analyze deformation and damage of the crystalline material [24]. Images at different magnifications showing typical cross-section microstructures of the ground specimens after exposure without external loading are presented in Fig. 8. It can be clearly seen that the grinding operation generated a heavily
Fig. 6. Stress relaxation in four-point bend specimens after heat treatment at 155 °C for 24 h for as-delivered and ground specimens.
Fig. 5. Measured surface stresses in the loading direction for four-point bending of (a) as-delivered material, (b) ground specimens.
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Fig. 7. Surface morphology after exposure without external loading: (a) as-delivered specimen, (b) ground-RD specimen.
deformed surface layer comprised of fragmented grains and dislocation sub-cells only a few microns in thickness. Below this layer, densely populated slip bands with multiple orientations were observed. Both the ground-RD and ground-TD specimens show similar features from the cross-section investigations. As illustrated in Fig. 8(a), micro-cracks in the absence of external loading appeared mainly perpendicular to the grinding marks i.e. perpendicular to the direction of highest residual tensile stress. The cracks initiated from the ground surface and ranged in length from less than 1 mm up to more than 10 mm. The micro-cracks were thus largely within the highly deformed surface layer. On the other hand, in the section parallel to the grinding marks (Fig. 8(b)), only small rather blunt points of attack were seen; these had not developed into cracks. The results agree well with the surface morphology investigation. Detailed microstructural characterization at higher magnification revealed that both intergranular microcracks (Fig. 8(c)) and transgranular micro-cracks (Fig. 8(d)) were present. Some branching occurred (Fig. 8(d)) and cracks could run parallel to and across deformation slip bands.
3.3. Corrosion behavior with four-point bend loading 3.3.1. Macro-crack examination Macro-cracks developed in some specimens exposed with external loading and could result in sample fracture. Macro-cracks are defined in this study as those that can be observed by stereo microscopy with highest magnification 63. Table 3 gives a summary of the macro-crack examination from the corrosion test under different four-point bend loading values as well as without external loading. No macro-cracking occurred during exposure without external loading regardless of surface conditions. However for specimens under four-point bending, one out of three asdelivered specimens loaded to 50 MPa was cracked through thickness after exposure in boiling MgCl2 for 24 h. With increasing the applied loading up to 110, 200 and 300 MPa respectively, all the exposed as-delivered specimens cracked. For the ground-RD specimens, large cracks were observed on both specimens with 50 MPa or 300 MPa loading. In the case of ground-TD specimens no macro-cracks were seen at applied load levels of 50 MPa or 110 MPa.
Fig. 8. Cross-section microstructures after exposure without external loading: (a) ground-RD specimen sectioned parallel to the rolling/grinding direction, (b) ground-RD specimen sectioned perpendicular to rolling/grinding, (c) ground-RD specimen showing intergranular micro-crack, (d) ground-RD specimen showing transgranular microcrack and multi-branched micro-crack.
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Table 3 Extent of micro- and macroscopic cracking after exposure with four-point bend loading. The measured surface stresses from Figs. 4 to 5 are also included for comparison. Specimen
Applied four-point bend loading (MPa)
As-delivered Without external loading Ground-RD Ground-TD As-delivered 50 110 200 300 Ground-RD 50 300 Ground-TD 50 110
Measured (or interpolated) surface stress (MPa)
Exposure time No. of specimens tested
No. of specimens with macro-cracks
Presence of microcracks
5 360 54 (20), (50) 100 126, 166 177, 202 (400) 607 (35) (70)
20 h þ 20 h
0 0 0 1 2 2 2 2 2 0 0
No Yes Yes No No No No Yes Yes Yes Yes
24 h
3.3.2. Surface morphology after exposure Initial examination of the cracked surfaces was performed by stereo microscopy. When cracking occurred, there were usually multiple cracks, with one major crack extending nearly through the specimen. The cracks tended to run parallel to each other, and were all perpendicular to the loading direction. Detailed characterization of all the three types of specimens after testing at 50 MPa four-point bend loading is shown by the SEM micrographs in Fig. 9. The macro-cracks were wide and exhibited multiple branching (Fig. 9(a) and (c)). Pitting was observed in varying degrees for all specimens. Even in the absence of macrocracks, large pits were found, as exemplified by the as-delivered specimen shown in Fig. 9(b). Pits were also seen on the ground surfaces and showed some tendency to follow the micro-cracks. Cracks were sometimes seen to extend from pits, indicating that pits may be precursors to cracks. Similar to the corrosion tests without external loading, micro-cracks with extensive branching were also found on the exposed surfaces of all ground specimens regardless of loading or the occurrence of macro-cracks. Microcracks were primarily oriented perpendicular to the grinding
2 3 3 3 2 2 2 2 2 2 2
marks. This applied even for the ground-TD specimens, which means that the residual stress effect outweighed the effect of the applied load and the possible notch effect from the grinding marks. 3.3.3. Cross-section investigation after exposure Cross-sections parallel to the loading direction were examined for all the cracked specimens after exposure. Fig. 10 presents SEM images of two specimens as examples: the AD and ground-RD specimens with 50 MPa applied load. These show that the macrocrack path was mainly branched transgranular, in good agreement with the fracture surface investigation (see below). Pits were evident on the surface and macro-cracks were always associated with such pits, i.e. always initiated and propagated from the pits. Micro-cracks were also observed to initiate from the surface and have penetration depths of up to 10 mm, but these were not normally associated with pits (Fig. 10(b)). No micro-cracking was observed in AD cross-section (Fig. 10(a)). Cross-sections both parallel and perpendicular to the loading directions of all the ground specimens after exposure under
Fig. 9. Surface morphology after exposure with 50 MPa four-point bend loading: (a) as-delivered cracked specimen, (b) as-delivered uncracked specimen, (c) ground-RD cracked specimen, (d) ground-TD uncracked specimen. The loading direction is horizontal in all cases.
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Fig. 10. Cross-section microstructures showing stress corrosion cracking after exposure with 50 MPa four-point bend loading: (a) as-delivered specimen sectioned in the longitudinal direction, (b) ground-RD specimen sectioned in the longitudinal direction.
Fig. 11. Cross-section microstructures after exposure with 50 MPa four-point bend loading showing: (a) ground-RD specimen parallel to rolling/grinding direction, (b) ground-TD specimen perpendicular to rolling/grinding direction, (c) surface pit and micro-cracks in ground-RD specimen, (d) surface pit and micro-cracks in ground-TD specimen.
Fig. 12. Fracture surfaces after exposure: (a) as-delivered specimen under 50 MPa loading, (b) ground-RD specimen under 50 MPa loading.
different applied loads were examined by ECCI. Images of two specimens are illustrated in Fig. 11 as examples. A high density of pits and micro-cracks was observed for all the specimens, in
agreement with the surface morphology results. For the groundRD specimens, 50 MPa four-point bend loading significantly increased the density and size of the pits. The penetration depth of
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micro-cracks also obviously increased, especially for cracks perpendicular to the loading/grinding direction (Fig. 11a), when comparing with the same specimen exposed without external loading. For the ground-TD specimen, the same load also significantly increased the penetration depth and density of microcracks perpendicular to the loading direction i.e. parallel to the grinding direction (Fig. 11(b)). However the change was small for micro-cracks parallel to the loading direction. Higher magnification characterization revealed that multiple cracks often originated from a single pit (Fig. 11 (c) and (d)), which also agrees with the exposed surface observation. Both transgranular and intergranular micro-cracks were observed to be associated with pits (Fig. 11 (c)) and these could run parallel to or across the deformation slip bands (Fig. 11(d)). 3.3.4. Fracture surface investigation Fig. 12 presents SEM images showing fracture surfaces of both as-delivered and ground-RD specimens which cracked at 50 MPa applied stress. The fracture appearance of both specimens shows typical cleavage fracture which is predominantly transgranular, although there are also some local indications of intergranular cracking. Some indications of crack branching are seen for both of the specimens. 3.4. Stress relaxation after exposure Surface stresses of ground specimens before and after exposure under different loading conditions were measured to correlate the SCC behavior and residual stresses; the results are presented in Fig. 13. The grinding operations generated tensile residual stresses up to more than 350 MPa parallel to the grinding direction and compressive residual stresses in the perpendicular direction. For the ground-RD specimen (Fig. 13(a)), after 40 h exposure without applying any external loading, surface tensile stresses parallel to the grinding marks significantly reduced to below 100 MPa; while perpendicular to the grinding marks, compressive stresses increased slightly. These effects can be directly related to the formation of micro-cracks which relax the surface tensile stress. After exposure at 50 MPa applied load, surface tensile residual stresses reduced even more, to below 50 MPa, this is attributable to the formation of both micro- and macro-cracks. The change in the perpendicular direction was again small. For 300 MPa loading, surface stress reduced to zero in the loading direction. In the case of ground-TD specimen (Fig. 13(b), the surface stress relaxation due to exposure without external loading was similar to the ground-RD specimen. The influence of applied loading up to 110 MPa on surface stress relaxation was relatively small, and it
57
may be recalled that this specimen exhibited no macro-cracking. In-depth residual stress profile of a ground-RD specimen before and after exposure in the absence of external loading is presented in Fig. 14. Residual stresses were measured both parallel and perpendicular to rolling/grinding directions of the specimen. As shown in the figure, the tensile component reduced dramatically after exposure from surface to sub-surface until reaching zero at the depth where nearly all the micro-cracks stopped. The change of compressive component showed small, in-depth residual stresses showed similar values before and after exposure. In-depth surface morphology (Fig. 14) was also characterized of this specimen with different distances from the original surface. The penetration depths of micro-cracks were very uneven. Some of them disappeared after 2 mm of surface has been polished away, while some penetrated to a depth more than 10 mm from surface. Very few micro-cracks can be observed at 13 mm electrolytic polishing depth. The results agree with the cross-section observation.
4. Discussion 4.1. Residual stress distribution Residual stresses for the as-delivered material are close to zero, while grinding operations generated massive residual stresses in the surface and sub-surface layers of the steel. During grinding, thermally and mechanically induced residual stresses may be produced simultaneously; the relative significance varies from one process to another [25]. Heat is generated from the interaction between the abrasive grit and the workpiece material during grinding operations. Due to the low thermal conductivity of 304L stainless steel, a temperature gradient is formed from surface to the bulk. During the cooling period, contraction of the surface layer is hindered by the bulk thus resulting in surface tension and subsurface compression [25,26]. However, in the present case tensile residual stresses parallel to the grinding direction and compressive residual stresses perpendicular to the grinding direction were observed. This measured anisotropic surface residual stress can be attributed to the anisotropic plastic deformation of the ground surface layer, and indicates that residual stresses induced by mechanical effects dominate over thermal effects in this study. The surface layer experienced compressive plastic deformation in the grinding direction and tensile deformation in the transverse direction. After the grinding zone moved away, the constraint by the material beneath surface resulted in tensile σ∥ but compressive σ⊥ in the surface layer [20,27]. Such anisotropic residual stress distributions have also been observed in previous
Fig. 13. Measured surface residual stress of ground specimens parallel and perpendicular to rolling and grinding directions before and after exposure: (a) ground-RD specimen, (b) ground-TD specimen.
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Fig. 14. In-depth residual stress parallel and perpendicular to the rolling/grinding directions of ground-RD specimen before and after exposure without external loading and corresponding micrographs showing the appearance of the microcracks at each depth.
work when grinding duplex stainless steel 2304 [21]. 4.2. Corrosion behavior without external loading The micro-cracks and their pattern on the ground surfaces after exposure are one of the most interesting observations in this study. Extensive stress corrosion micro-cracks were observed on the ground surfaces even in the absence of any external loading, while the as-delivered material showed no such cracking. Similar crack patterns have also been observed by Lyon et al. on milled 316 surfaces [8]. It has also been shown in other work that surface failure often occurs due to surface tensile residual stresses generated in metallic materials [2]. Compressive residual stress on the surface can delay crack initiation, and compressive residual stresses at depth can slow down the growth of cracks from the surface [10]. In the present study, high levels of tensile residual stresses up to 350 MPa were measured parallel to the grinding direction in the surface layer, with compressive stresses in the perpendicular direction. Consequently the micro-cracks tended to initiate normal to the grinding marks. Both the in-depth surface morphology characterization and the cross-section investigation showed the micro-cracks perpendicular to the grinding direction were more numerous and longer than those parallel to the grinding marks. This strongly indicates that the tensile residual stresses in the surface layer make austenitic stainless steel susceptible to SCC in the presence of chlorides even without any external loading, whereas compressive residual stress can retard the initiation of cracks. The majority of the micro-cracks arrested after penetrating 10–15 mm through the highly deformed ground surface layer. This correlates well to the position at which the residual stresses shift from tension to compression. A threshold stress is required to initiate and maintain the propagation of SCC [28]. Results of surface residual stress measurement after exposure showed that the formation of micro-cracks caused a significant release of the surface residual stress. 4.3. Corrosion behavior with four-point bend loading The in-situ X-ray diffraction measurements on as-delivered specimens and ground specimens demonstrated that the actual stress in the surface of the specimens deviated from the values calculated according to ASTM G39 [16] and were strongly affected by the surface preparation. The repeated measurement on the asdelivered material indicated good accuracy and repeatability of the results. The actual loading corresponded reasonably well with the calculated values from ASTM G39 in the elastic regime, although
the measurements showed an actual stress of 126/166 MPa when 200 MPa was applied. Above the proof stress the slope of the curve dropped dramatically, so a calculated applied stress of 300 MPa gave an actual value below 200 MPa. This serves to underline the point that four point bending should never be used in the plastic regime. The in-situ stress measurements under four point loading showed a different trend for ground specimens. The surface layer was highly deformed by the grinding operation, which was clearly shown from the cross-section ECC images with fragmented grain structures and deformation slip bands in the surface layer. Due to strain hardening in this layer, the measured surface stress appeared to increase nearly linear with increasing load even up to 500 MPa. The actual surface stress in such specimens with residual stresses and a strength gradient near the surface depends on interactions between the applied and residual stresses as well as the strength gradient in the specimen [29]. For the ground-RD specimen, the surface tensile residual stress along the loading direction resulted in a high surface stress, for example 750 MPa for loading at 500 MPa level. On the other hand, the ground-TD specimen with a low surface residual stress parallel to the applied stress showed initially similar surface stress as the as-delivered specimen but owing to its higher surface strength, the linear behavior remained up to loading of 500 MPa. A detailed analysis of interactions between the stresses and the effect of the strength gradient is an avenue worth further investigation. Stress relaxation was found to be low for all three types of specimens under 500 MPa four-point bend loading after annealing at 155 °C for 24 h: around 50 MPa. The main stress relaxation was due to the formation of micro- or macro-cracks. The crack path was mainly transgranular in this study, although there were some local areas of intergranular cracking. The macrocracks followed a general path which was normal to the loading stress. The SCC susceptibility increased considerably with increasing four-point bend loading and the data in Table 3 indicated that there is a threshold stress of around 50 MPa for macroscopic cracking. Pits were observed on all the exposed specimens to varying degrees. When characterizing pitting and cracking behavior, there is always a question as to whether pits formed after cracking, or whether cracking initiated from pits. Many works have demonstrated that pits can act as precursors to cracking [8,30,6]. The explanation is usually that the cause is a combination of stress concentration and a more aggressive environment in the pits [2]. In the present study, all macro-cracks were observed to be associated with pits, although there were also pits without any related cracks. This indicates that pits form before macro-cracks and act as precursors to macro-cracking. However, the behavior of the microcracks was different. Extensive micro-cracks appeared on the specimens that had been ground and could be both with and without associated pits. This indicates that tensile residual stress in the surface layer was the main driving force for the initiation of micro-cracks and pits tended to grow from these micro-cracks. In the case of ground-RD specimens, these pits appeared to grow together along the micro-cracks in the perpendicular direction to the grinding marks and led to the macro-crack initiation. Moreover, the micro-cracks in both directions of all ground specimens with four-point bend loading were longer that those in absence of external loading, especially in the direction perpendicular to grinding marks, indicating that applied tensile stresses promote micro-crack propagation. The macro-cracking showed that the ground-RD specimens were more susceptible to SCC than the as-delivered specimens. Surface finish, surface deformation and microstructure can all influence the SCC behavior; however, the detrimental role of residual tensile stress from grinding in combination with an applied
N. Zhou et al. / Materials Science & Engineering A 658 (2016) 50–59
stress is dominant here. In contrast, the improvement of SCC resistance of ground-TD specimens was also clear; although the induced compressive residual stresses were entirely released during loading and were then converted into a state of tension, much higher level of compressive residual stresses beneath the ground surface compensated for the surface tension and effectively retarded the appearance of macro-cracks. The results suggest the idea that grinding transverse to the main stress direction in a construction could possibly be used to decrease the cracking risk.
5. Conclusions The effect of surface grinding on chloride induced stress corrosion cracking of austenitic stainless steel 304L has been investigated. The following conclusions can be drawn:
Grinding generated tensile residual stresses parallel to the
grinding direction ( σ∥) but compressive residual stresses perpendicular to the grinding direction ( σ⊥ ) in the surface layer. Beneath the ground surface, a layer of compressive stresses in both directions was formed with much higher values in the perpendicular direction. The residual stress distribution observed in this study indicated that anisotropic mechanical effects dominated over isotropic thermal effects from grinding. Stress measurements on four-point bend loaded specimens demonstrated that the actual loading may deviate appreciably from that calculated according to the formula in ASTM G39. For annealed material with low residual stresses the formula gave a reasonable estimate of the applied stress up to the proof stress. For surface ground material there was a linear relationship up to much higher loads but the absolute value was strongly affected by residual stresses from grinding. Surface tensile residual stresses were found to be the main factor causing the initiation of micro-cracks on ground surfaces during exposure with or without an externally applied load. Crack arrest occurred when micro-cracks grew into a region 10– 15 mm below the surface with low or no tensile residual stresses. Macroscopic cracking leading to final failure only occurred under an applied load and there appeared to be a threshold at load of around 50 MPa. A residual tensile stress in the grinding direction increased the susceptibility to chloride-induced SCC while grinding perpendicular to the applied loading direction retarded cracking. It is therefore suggested that grinding transverse to the main stress direction in a construction could possibly be used to decrease the cracking risk. Pits tended to initiate at micro-cracks on the surface, and macro-cracks were always associated with pits. This indicates that pits formed before macro-cracks and acted as precursors to macroscopic crack development.
Acknowledgment This work was performed within the Swedish Steel Industry Graduate School with financial support from Outokumpu Stainless Research Foundation, Region Dalarna, Region Gävleborg, Länsstyrelsen Gävleborg, Jernkontoret, Sandviken Kummun and Högskolan Dalarna. The assistance with part of the sample preparation work by Ulf Modin at Högskolan Dalarna is also appreciated. The authors are grateful to Timo Pittulainen at Outokumpu Stainless AB for providing the test materials and the material data. References [1] R. Pettersson and E. Johansson, Stress corrosion resistance of duplex grades, in
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Duplex stainless steel world conference, Beaune, 2010. [2] D.A. Jones, Principles and Prevention of Corrosion, Pearson Education, US, 1996. [3] S. Ghosh, V. Kain, Effect of surface machining and cold working on the ambient temperature chloride stress corrosion cracking susceptibility of AISI 304L stainless steel, Mater. Sci. Eng. A 527 (2010) 679–683. [4] W. Jin, S. Yang, H. Kokawa, Z. Wang, Y.S. Sato, Improvement of intergranular stress corrosion crack susceptibility of austenite stainless steel through grain boundary engineering, J. Mater. Sci. Technol. 23 (2007) 785–789. [5] S. Ghosh, V.P.S. Rana, V. Kain, V. Mittal, S. Baveja, Role of residual stresses induced by industrial fabrication on stress corrosion cracking susceptibility of austenitic stainless steel, Mater. Des. 32 (2011) 3823–3831. [6] A. Turnbull, K. Mingard, J. Lord, B. Roebuck, D. Tice, K. Mottershead, N. Fairweather, A. Bradbury, Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure, Corros. Sci. 53 (2011) 3398–3415. [7] S. Ghosh, V. Kain, Microstructural changes in AISI 304L stainless steel due to surface machining: effect on its susceptibility to chloride stress corrosion cracking, J. Nucl. Mater. 403 (2010) 62–67. [8] K. Lyon, T. Marrow, S. Lyon, Influence of milling on the development of stress corrosion cracks in austenitic stainless steel, J. Mater. Process. Technol. 218 (2015) 32–37. [9] W. Karlsen, G. Diego, B. Devrient, Localized deformation as a key precursor to initiate of intergranular stress corrosion cracking of austenitic stainless steels employed in nuclear power plants, J. Nucl. Mater. 406 (2010) 138–151. [10] J. Lu, K. Luo, X. Cheng, J. Hu, F. Dai, H. Qi, L. Zhang, J. Zhong, Q. Wang, Y. Zhang, Effects of laser peening on stress corrosion cracking (SCC) of ANSI 304 austenitic stainless steel, Corrosion Science 60 (2012) 145–152. [11] B. Kumar, R. Singh, B. Mahato, P. De, N. Bandyopadhyay, D. Bhattacharya, Effect of texture on corrosion behavior of AISI 304L stainless steel, Mater. Charact. 54 (2005) 141–147. [12] M. Meisnar, A. Vilalta-Clemente, A. Gholinia, M. Moody, A.J. Wilkinson, N. Huin, S. Lozano-Perez, Using transmission Kikuchi diffraction to study intergranular stress corrosion cracking in type 316 stainless steels, Micron 75 (2015) 1–10. [13] S.-l Baik, M. Olszta, S. Bruemmer, D.N. Seidman, Grain-boundary structure and segregation behavior in a nickel-base stainless ally, Scr. Mater. 66 (10) (2012) 809–812. [14] S.M. Bruemmer, M.J. Olszta, M.B. Toloczko, L.E. Thomas, Linking grain boundary microstructure to stress corrosion cracking of cold rolled alloy 690 in PWRprimary water, Corrosion (2012) 9512–9527. [15] E. Johansson, T. Prosek, Stress corrosion cracking properties of UNS S32101- a new duplex stainless steel with low nickel content, in: Proceedings of the Corrosin 2007 Conference & Expo, 2007. [16] ASTM, G39-99, Standard Practice for Preparation and Use of Bent-beam Stresscorrosion Test Specimens, 2011. [17] ISO, 7539, Corrosion of Metals and Alloys – Stress Corrosion Testing, 1995. [18] U. Kivisäkk, G. Chai, Influence of test method and relaxation on the result from stress corrosion cracking tests of stainless steels in dilute neutral chlorides, Corrosion 59 (2003) 828–835. [19] N. Zhou, R.L. Peng, R. Pettersson, Surface characterization of austenitic stainless steel 304L after different grinding operations, J. Mater. Process. Technol. (2015), under review. [20] I. Noyan, J. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer-Verlag New York Inc, New York City, United States, 1987. [21] N. Zhou, R. Lin Peng, R. Pettersson, Surface integrity of 2304 duplex stainless steel after different grinding operations, J. Mater. Process. Technol. 229 (2016) 294–304. [22] ASTM, G36-94, Standard Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals Aned Alloys in a Boiling Magnesium Chloride Solution, Reapproved 2006. [23] V. Roy, B. Nahak, M.Z.K. Yusufzai, M. Vashista, Assessment of plastic deformation upon grinding using X-ray diffraction profiles, Int. J. Eng. Res. 2 (2) (2013) 119–124. [24] S. Johansson, J. Moverare, R. Peng, Recent applications of scanning electron microscopy, Pract. Metallogr. 50 (2013) 810–820. [25] J.P. Davim, Surface Intergrity in Machining, Springer London Ltd, London, 2010. [26] N.B. Fredj, H. Sidhom, C. Braham, Ground surface improvement of the austenitic stainless steel AISI 304 using cryogenic cooling, Surf. Coat. Technol. 200 (2006) 4846–4860. [27] G. Guo, Z. Liu, X. Cai, Q. An, M. Chen, Investigation of surface integrity in conventional grinding of Ti–6Al–4V, Adv. Mater. Res. 126–128 (2010) 899–904. [28] P. Marcus, Corrosion Mechanisms in Theory and Practice, CRC Press Inc, Boca Raton, 2012. [29] R. Lin Peng, J. Gibmeier, G. Chai, S. Johansson, Load Partitioning in a Duplex Stainless Steel With Surface Strength Gradient and Residual Stresses, in: JCPDS-International centre for diffraction data, 2009. [30] G. Hinds, L. Wickström, K. Mingard, A. Turnbull, Impact of surface condition on sulphide stress corrosion cracking of 316L stainless steel, Corros. Sci. 71 (2013) 43–52.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of surface grinding on chloride induced SCC of 304L Nian Zhou a,b,n, Rachel Pettersson b,c, Ru Lin Peng d, Mikael Schönning e a
Department of Material Science, Dalarna University, SE-79188 Falun, Sweden KTH, SE-10044 Stockholm, Sweden Jernkontoret, SE-11187 Stockholm, Sweden d Department of Management and Engineering, Linköping University, SE-58183 Linköping, Sweden e Corrosion Department, Avesta Research Centre – Outokumpu Stainless AB, SE-774 22 Avesta, Sweden b c
art ic l e i nf o
a b s t r a c t
Article history: Received 21 December 2015 Accepted 25 January 2016 Available online 29 January 2016
The effect of surface grinding on the stress corrosion cracking (SCC) behavior of 304L austenitic stainless steel in boiling magnesium chloride has been investigated. SCC tests were conducted both without external loading and with varied levels of four-point bend loading for as-delivered material and for specimens which had been ground parallel or perpendicular to the loading direction. Residual stresses due to the grinding operation were measured using the X-ray diffraction technique. In addition, surface stress measurements under applied load were performed before exposure to evaluate the deviation between actual applied loading and calculated values according to ASTM G39. Micro-cracks initiated by a high level of tensile residual stress in the surface layer were observed for all the ground specimens but not those in the as-delivered condition. Grinding along the loading direction increased the susceptibility to chloride induced SCC; while grinding perpendicular to the loading direction improved SCC resistance. Surface tensile residual stresses were largely relieved after the initiation of cracks. & 2016 Elsevier B.V. All rights reserved.
Keywords: Stress corrosion cracking Residual stress Austenitic stainless steel 304L Grinding
1. Introduction Standard austenitic stainless steels are very widely used for industrial applications. However, they are highly susceptible to chloride stress corrosion cracking (SCC) which can lead to catastrophic failures. One review in 1983 showed that 37% of almost one thousand failure cases of the austenitic stainless steel 304 in the chemical industry were attributed to stress corrosion cracking [1]. Depending on the microstructure of the material and the nature of the environment, SCC may be intergranular or transgranular [2]. For example, Ghosh and Kain observed cracking to be transgranular for solution annealed, cold worked and surface machined 304L stainless steel in a chloride environment [3]. Jin et al. [4] showed grain boundary engineering could shift the fracture from intergranular to transgranular stress corrosion cracking of 304 stainless steel plate. It is well recognized that the surface conditions including geometrical, physical and mechanical properties of machined components will largely affect their functional performance, such as corrosion resistance. Ghosh et al. [5] demonstrated a higher SCC susceptibility of surface machined 304L in 5 N H2SO4 þ0.5 N NaCl n Corresponding author at: Department of Material Science, Dalarna University, SE-79188 Falun, Sweden. E-mail address: [email protected] (N. Zhou).
http://dx.doi.org/10.1016/j.msea.2016.01.078 0921-5093/& 2016 Elsevier B.V. All rights reserved.
environment than solution annealed material. A higher surface roughness and poor surface finish have been reported to initiate pits, which have been suggested as precursors to cracks [6]. The presence of strain induced martensite on the surface from machining resulted in higher SCC susceptibility of 304L austenitic stainless steel [7]. Cracking in different patterns along and transverse to the milling direction has been shown by Lyon et al. [8]. In general it is considered that SCC occurs as a result of the interaction of three factors: a corrosive environment, a susceptible alloy and the presence of tensile stresses [2]. However, microstructural effects and stress localization may also play a significant role. Karlsen et al. [9] demonstrated that strain heterogeneity due to the low stacking fault energy of austenitic stainless steels promoted strain localization during surface mechanical treatment, which in turn promoted crack initiation. The improvement of SCC resistance of 304 austenitic stainless steel by laser peening has been demonstrated by Lu et al. [10]; they attributed this to the generation of high-level compressive residual stress and grain refinement. Cold rolling texture was found to influence corrosion behavior of 304 stainless steel; the presence of close pack crystallographic planes parallel to the sample surface was reported to improve the corrosion properties [11]. Grain boundary misorientation ahead of the crack tip in 316 stainless steel has been detected by the Transmission Kikuchi diffraction (TKD) technique [12]. Research has shown that there is a connection between intergranular crack propagation and the misorientation of the grain
N. Zhou et al. / Materials Science & Engineering A 658 (2016) 50–59
boundaries along which the cracks propagate. [13,14]. A duplex structure of stainless steel has generally been found to improve ClSCC resistance compared to austenitic grades, although this may depend on the actual test environment [15]. There is a range of loading methods that can be used to assess stress corrosion cracking susceptibility. Four-point bend loading is widely used for strip material in the elastic deformation regime. The specimen holder design and elastic stress calculation are described in ASTM G39 [16] and ISO 7539 [17]. However, the actual stress in the specimen surface may deviate appreciably from the calculated values if residual stresses are present or local plasticity occurs. In addition, stress relaxation during exposure should be taken into account since differences in stress relaxation with different loading methods and for different materials have been observed to cause different corrosion behavior in the same environment [18]. When fabricating stainless steels, grinding is an important and widely used surface finishing process. Grinding induces considerable plastic deformation and generates thermal energy, both of which lead to changes in the residual stress state in the material. It has been shown that the sum of externally applied stresses and residual stresses can lead to crack initiation and propagation and also demonstrated that an induced high-level of compressive residual stress from laser peening can be released during the U-bend process [10]. However, little research has been performed to define the role of pure residual stresses in crack initiation and growth. The aim of this work is to contribute to the understanding of the role of residual stresses by studying the chloride induced stress corrosion behavior of as-delivered and surface ground 304L austenitic stainless steel. Specimens have been exposed both without external loading and under different four-point bend loads.
2. Material and methods 2.1. Material The material investigated in this study was 304L austenitic stainless steel with 2B surface finish supplied by Outokumpu Stainless AB as test coupons 400 mm 150 mm 2 mm in dimensions. The as-delivered material had been solution annealed (1100 °C, forced air cooling and water quenched), pickled and roll leveled. The chemical composition and main measured mechanical properties perpendicular to the rolling direction at room temperature are given in Tables 1 and 2, respectively. Fig. 1 shows the microstructure of the material from EBSD mapping. Ferrite measurement according to ASTM E1245 [15] showed 1.7% of ferrite to be present in the material. For these measurements the crosssection of the as-delivered material was etched in 40% NaOH solution, using 2.5 V for 3 s, then measurements were made on 10 fields throughout the thickness at 1000 magnification. 2.2. Grinding operations The grinding operations were conducted on a Chevalier FSG2A618 grinding machine; the set-up is described in detail in [19]. A Kemper RADIX Go grinding wheel (150 mm in diameter, 50 mm in width), which is an expanding roller made of 20 mm thick rubber, Table 1 Chemical composition (wt%) of 304L austenitic stainless steel. C
Si
Mn
P
S
Cr
Ni
Nb
Cu
Co
N
0.019
0.32
1.55
0.029
0.001
18.22
8.11
0.011
0.31
0.16
0.071
51
Table 2 Mechanical properties of 304L austenitic stainless steel measured perpendicular to the rolling direction at room temperature. Rp0.2(MPa)
Rm(MPa)
Elongation (%)
Hardness (HB)
230
642
54
170
was used. Grinding belts (50 mm in width, 473 mm in length) with conventional aluminum oxide grit were mounted on the grinding wheel and the test coupon was mounted on the grinding table. The grinding operations were performed along the rolling direction of the material. A fixed grinding speed vs=23 m/s, a fixed feed rate vw =8 m/s and a fixed motor power of 600 W were used. Grinding was first performed for 5 min using 60# grit size abrasive to remove the as-delivered material surface, and then followed by 2.5 min grinding with a new 60# grit size abrasive to get the final surface finish. No grinding lubrication was used during the operations. A piezo-electric transducer-based dynamometer, Kistler 9275B, was mounted under the working table to measure the normal force during the grinding operations. The measured normal force was 100 N in this study. 2.3. Material characterization SEM (Scanning electron microscopy, FEG-SEM Zeiss Ultra 55) was used to investigate the surface topography and ECCI (electron channeling contrast imaging, Hitachi FEG-SEM SU-70) to investigate the near surface microstructure evolution. Cross-sections of selected specimens after exposure were investigated from both longitudinal (LD) and transversal (TD) directions and some fracture surfaces were examined. The in-depth profiles of residual stresses parallel ( σ∥) and perpendicular ( σ⊥ ) to the rolling directions were determined by X-ray diffraction for both as-delivered and ground samples. Cr– Kα radiation was used and residual stresses were determined from the measured diffraction peak at 2θ 128° for the {220} lattice planes. The method is described in detail in [20] and the set-up used in the current work is similar to that used in the authors' previous work on duplex stainless steel 2304 [21]. In-situ surface stress measurements were also made on specimens subjected to loading in the same four-point bending fixtures which were used for the stress corrosion cracking testing. The loading was increased in steps to levels of 10, 200, 300 and 500 MPa, calculated according to ASTM G39 [16]. At each loading level, the specimen was kept one hour for stress relaxation, then X-ray diffraction was used to measure the actual surface stresses parallel to the loading direction. It should be noted that the two higher loads are above the measured proof stress of the as delivered material and thus strictly outside the range for which the four-point loading formula [16] is valid. After the measurement, all specimens which were kept in the holder at 500 MPa were heated in a furnace at 155 °C for 24 h, then allowed to cool to room temperature in the furnace. Surface residual stresses were measured again to investigate the stress relaxation. Surface stresses were also measured after corrosion tests. 2.4. Stress corrosion tests Chloride induced stress corrosion cracking susceptibility was tested without external loading and under four-point bend loading. As illustrated in Fig. 2, three types of specimens were tested. The as-delivered specimens, cut with the long axis parallel to the rolling direction, are denoted AD. Grinding was performed parallel to the rolling direction, and specimens were cut either parallel to the rolling/grinding direction, denoted ground-RD or transverse to
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Fig. 1. Cross-section microstructure and grain orientation of 304L austenitic stainless steel as-delivered.
Fig. 2. Schematic illustration of the orientation and designation of the different specimens tested.
rolling/grinding direction, denoted ground-TD. The test environment was boiling magnesium chloride solution according to ASTM G36 [22] Prior to testing all the specimen edges were ground down using 800# grinding paper to avoid sharp edges and specimens were then allowed to passivate in air for at least 24 h before exposure. The specimens were exposed in a flask connected to a water cooled condenser, and the temperature was maintained at 155 °C 71 °C with the help of a thermometer in the test solution. The first set of tests was without external loading to investigate to role of residual stresses. All three types of specimens (dimension 45 mm 10 mm 2 mm) were exposed for 20 h, then removed and checked for macro-cracks. This was done in a stereomicroscope (Nikon SMZ-2T with ColorView Soft camera and CellA ColorView Soft image software) at 10–63 magnification. If no macro-cracks were observed, the specimens were then put back for another 20 h exposure. In the second series of tests, a four-point bend load was applied to specimens of dimensions 65 mm 10 mm 2 mm according to ASTM G39 [16]. The loading direction was along the longitudinal direction. After application of the load, each specimen was kept one hour in air to allow possible stress relaxation before exposure. In this case the initial test period was 24 h.
3. Results 3.1. Pre-corrosion characterization 3.1.1. Surface topography SEM images of as-delivered and ground surface topography are shown in Fig. 3. The as-delivered surface in Fig. 3(a) is typical for a 2B surface and shows that the pickling process during production slightly etched the grain boundaries. For the ground specimen, as illustrated in Fig. 3(b), surface defects such as deep grooving, smearing, adhesive chips and indentations were found. The generation of surface defects during grinding has been described in detail in previous work [19] and such defects are also observed when grinding duplex stainless steels [21].
3.1.2. Residual stresses The in-depth residual stress profiles parallel ( σ∥) and perpendicular ( σ⊥ ) to the rolling directions as well as the full width at half maximum (FWHM) profiles of both as-delivered and ground specimens are presented in Fig. 4. Fig. 4(a) shows a low level of residual stress in the as-delivered material. Both σ∥ and σ⊥ are close to zero from the surface layer to the subsurface region. However, the grinding operations generated tensile σ∥ but compressive σ⊥ in the surface layer as seen in Fig. 4(b). The tensile σ∥ was highest in the ground surface, up to 361 746 MPa, and dropped rapidly to compression within a depth of around 15 mm. The compressive σ⊥ showed a relative low value of 54 720 MPa in the surface layer, but increased rapidly to reach a peak of almost 250 MPa in the subsurface region, and then dropped gradually to zero in the bulk material. The full width of half maximum (FWHM) revealed broadening of the diffraction peak, which may be considered to indicate plastic deformation [23]. FWHM showed similar values from surface to subsurface for as-delivered material. However, for the ground specimen, a decrease of FWHM with increasing depth was observed, which reveals a gradient of plastic deformation over 40 mm in thickness under the ground surface. 3.1.3. Surface stresses under external loading Surface stresses measured in the as-delivered, ground-RD and ground-TD specimens subjected to four point bending are presented in Fig. 5. For all three specimens, four loading steps of 10, 200, 300 and 500 MPa were applied. For the as-delivered material, an additional specimen with one more loading step of 110 MPa was measured to check the accuracy and repeatability. For the asdelivered material the actual surface stresses are close to the calculated loading in the elastic regime. Above the proof stress of the material, which is around 230 MPa, the slope of the curve decreases significantly, so that a calculated applied load of 500 MPa gives an actual stress of only 250 MPa. It should be noted, as mentioned earlier, that this loading level is outside the valid regime for four-point loading according to ASTM G39. The results were very different for the ground specimens (Fig. 5 (b)). Both of the curves show linear trends over the whole loading range, indicating that work hardening has significantly increased the local proof stress. The actual surface stress of the ground-TD specimen is around two thirds of the calculated load. For the ground-RD sample, the slope is a little higher compared to the TD direction. It is seen that as the applied stress adds on to the residual tensile stress in the rolling and grinding direction, the actual stress is 750 MPa when a calculated loading of 500 MPa is applied. Fig. 6 presents the measured surface stress in the loading direction at 500 MPa applied loading and after heat treatment at 155 °C for 24 h. The results show that stress relaxation is of the order of around 50 MPa. 3.2. Corrosion behavior without external loading 3.2.1. Surface and in-depth morphology after exposure Typical SEM images of the surface morphology after corrosion
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Fig. 3. Surface topography of (a) as-delivered material, (b) ground specimen.
Fig. 4. In-depth residual stresses and full width at half maximum profiles of (a) as-delivered material, (b) ground specimen. Positive values denote tensile stresses and negative compressive stresses.
testing are shown in Fig. 7. Pitting was observed on all specimens. The pits were small and the density was low. Micro-cracking took place in all ground specimens even in the absence of any external loading (Fig. 7(b)). Micro-cracks on the ground surfaces exhibited extensive branching and were primarily oriented perpendicular to the grinding marks i.e. perpendicular to the direction with the highest tensile residual stress. However, in the case of as-delivered specimens, there is no evidence of micro-cracks. 3.2.2. Cross-section investigation after exposure By using the ECCI technique, diffraction contrast images can be obtained to analyze deformation and damage of the crystalline material [24]. Images at different magnifications showing typical cross-section microstructures of the ground specimens after exposure without external loading are presented in Fig. 8. It can be clearly seen that the grinding operation generated a heavily
Fig. 6. Stress relaxation in four-point bend specimens after heat treatment at 155 °C for 24 h for as-delivered and ground specimens.
Fig. 5. Measured surface stresses in the loading direction for four-point bending of (a) as-delivered material, (b) ground specimens.
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Fig. 7. Surface morphology after exposure without external loading: (a) as-delivered specimen, (b) ground-RD specimen.
deformed surface layer comprised of fragmented grains and dislocation sub-cells only a few microns in thickness. Below this layer, densely populated slip bands with multiple orientations were observed. Both the ground-RD and ground-TD specimens show similar features from the cross-section investigations. As illustrated in Fig. 8(a), micro-cracks in the absence of external loading appeared mainly perpendicular to the grinding marks i.e. perpendicular to the direction of highest residual tensile stress. The cracks initiated from the ground surface and ranged in length from less than 1 mm up to more than 10 mm. The micro-cracks were thus largely within the highly deformed surface layer. On the other hand, in the section parallel to the grinding marks (Fig. 8(b)), only small rather blunt points of attack were seen; these had not developed into cracks. The results agree well with the surface morphology investigation. Detailed microstructural characterization at higher magnification revealed that both intergranular microcracks (Fig. 8(c)) and transgranular micro-cracks (Fig. 8(d)) were present. Some branching occurred (Fig. 8(d)) and cracks could run parallel to and across deformation slip bands.
3.3. Corrosion behavior with four-point bend loading 3.3.1. Macro-crack examination Macro-cracks developed in some specimens exposed with external loading and could result in sample fracture. Macro-cracks are defined in this study as those that can be observed by stereo microscopy with highest magnification 63. Table 3 gives a summary of the macro-crack examination from the corrosion test under different four-point bend loading values as well as without external loading. No macro-cracking occurred during exposure without external loading regardless of surface conditions. However for specimens under four-point bending, one out of three asdelivered specimens loaded to 50 MPa was cracked through thickness after exposure in boiling MgCl2 for 24 h. With increasing the applied loading up to 110, 200 and 300 MPa respectively, all the exposed as-delivered specimens cracked. For the ground-RD specimens, large cracks were observed on both specimens with 50 MPa or 300 MPa loading. In the case of ground-TD specimens no macro-cracks were seen at applied load levels of 50 MPa or 110 MPa.
Fig. 8. Cross-section microstructures after exposure without external loading: (a) ground-RD specimen sectioned parallel to the rolling/grinding direction, (b) ground-RD specimen sectioned perpendicular to rolling/grinding, (c) ground-RD specimen showing intergranular micro-crack, (d) ground-RD specimen showing transgranular microcrack and multi-branched micro-crack.
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Table 3 Extent of micro- and macroscopic cracking after exposure with four-point bend loading. The measured surface stresses from Figs. 4 to 5 are also included for comparison. Specimen
Applied four-point bend loading (MPa)
As-delivered Without external loading Ground-RD Ground-TD As-delivered 50 110 200 300 Ground-RD 50 300 Ground-TD 50 110
Measured (or interpolated) surface stress (MPa)
Exposure time No. of specimens tested
No. of specimens with macro-cracks
Presence of microcracks
5 360 54 (20), (50) 100 126, 166 177, 202 (400) 607 (35) (70)
20 h þ 20 h
0 0 0 1 2 2 2 2 2 0 0
No Yes Yes No No No No Yes Yes Yes Yes
24 h
3.3.2. Surface morphology after exposure Initial examination of the cracked surfaces was performed by stereo microscopy. When cracking occurred, there were usually multiple cracks, with one major crack extending nearly through the specimen. The cracks tended to run parallel to each other, and were all perpendicular to the loading direction. Detailed characterization of all the three types of specimens after testing at 50 MPa four-point bend loading is shown by the SEM micrographs in Fig. 9. The macro-cracks were wide and exhibited multiple branching (Fig. 9(a) and (c)). Pitting was observed in varying degrees for all specimens. Even in the absence of macrocracks, large pits were found, as exemplified by the as-delivered specimen shown in Fig. 9(b). Pits were also seen on the ground surfaces and showed some tendency to follow the micro-cracks. Cracks were sometimes seen to extend from pits, indicating that pits may be precursors to cracks. Similar to the corrosion tests without external loading, micro-cracks with extensive branching were also found on the exposed surfaces of all ground specimens regardless of loading or the occurrence of macro-cracks. Microcracks were primarily oriented perpendicular to the grinding
2 3 3 3 2 2 2 2 2 2 2
marks. This applied even for the ground-TD specimens, which means that the residual stress effect outweighed the effect of the applied load and the possible notch effect from the grinding marks. 3.3.3. Cross-section investigation after exposure Cross-sections parallel to the loading direction were examined for all the cracked specimens after exposure. Fig. 10 presents SEM images of two specimens as examples: the AD and ground-RD specimens with 50 MPa applied load. These show that the macrocrack path was mainly branched transgranular, in good agreement with the fracture surface investigation (see below). Pits were evident on the surface and macro-cracks were always associated with such pits, i.e. always initiated and propagated from the pits. Micro-cracks were also observed to initiate from the surface and have penetration depths of up to 10 mm, but these were not normally associated with pits (Fig. 10(b)). No micro-cracking was observed in AD cross-section (Fig. 10(a)). Cross-sections both parallel and perpendicular to the loading directions of all the ground specimens after exposure under
Fig. 9. Surface morphology after exposure with 50 MPa four-point bend loading: (a) as-delivered cracked specimen, (b) as-delivered uncracked specimen, (c) ground-RD cracked specimen, (d) ground-TD uncracked specimen. The loading direction is horizontal in all cases.
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Fig. 10. Cross-section microstructures showing stress corrosion cracking after exposure with 50 MPa four-point bend loading: (a) as-delivered specimen sectioned in the longitudinal direction, (b) ground-RD specimen sectioned in the longitudinal direction.
Fig. 11. Cross-section microstructures after exposure with 50 MPa four-point bend loading showing: (a) ground-RD specimen parallel to rolling/grinding direction, (b) ground-TD specimen perpendicular to rolling/grinding direction, (c) surface pit and micro-cracks in ground-RD specimen, (d) surface pit and micro-cracks in ground-TD specimen.
Fig. 12. Fracture surfaces after exposure: (a) as-delivered specimen under 50 MPa loading, (b) ground-RD specimen under 50 MPa loading.
different applied loads were examined by ECCI. Images of two specimens are illustrated in Fig. 11 as examples. A high density of pits and micro-cracks was observed for all the specimens, in
agreement with the surface morphology results. For the groundRD specimens, 50 MPa four-point bend loading significantly increased the density and size of the pits. The penetration depth of
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micro-cracks also obviously increased, especially for cracks perpendicular to the loading/grinding direction (Fig. 11a), when comparing with the same specimen exposed without external loading. For the ground-TD specimen, the same load also significantly increased the penetration depth and density of microcracks perpendicular to the loading direction i.e. parallel to the grinding direction (Fig. 11(b)). However the change was small for micro-cracks parallel to the loading direction. Higher magnification characterization revealed that multiple cracks often originated from a single pit (Fig. 11 (c) and (d)), which also agrees with the exposed surface observation. Both transgranular and intergranular micro-cracks were observed to be associated with pits (Fig. 11 (c)) and these could run parallel to or across the deformation slip bands (Fig. 11(d)). 3.3.4. Fracture surface investigation Fig. 12 presents SEM images showing fracture surfaces of both as-delivered and ground-RD specimens which cracked at 50 MPa applied stress. The fracture appearance of both specimens shows typical cleavage fracture which is predominantly transgranular, although there are also some local indications of intergranular cracking. Some indications of crack branching are seen for both of the specimens. 3.4. Stress relaxation after exposure Surface stresses of ground specimens before and after exposure under different loading conditions were measured to correlate the SCC behavior and residual stresses; the results are presented in Fig. 13. The grinding operations generated tensile residual stresses up to more than 350 MPa parallel to the grinding direction and compressive residual stresses in the perpendicular direction. For the ground-RD specimen (Fig. 13(a)), after 40 h exposure without applying any external loading, surface tensile stresses parallel to the grinding marks significantly reduced to below 100 MPa; while perpendicular to the grinding marks, compressive stresses increased slightly. These effects can be directly related to the formation of micro-cracks which relax the surface tensile stress. After exposure at 50 MPa applied load, surface tensile residual stresses reduced even more, to below 50 MPa, this is attributable to the formation of both micro- and macro-cracks. The change in the perpendicular direction was again small. For 300 MPa loading, surface stress reduced to zero in the loading direction. In the case of ground-TD specimen (Fig. 13(b), the surface stress relaxation due to exposure without external loading was similar to the ground-RD specimen. The influence of applied loading up to 110 MPa on surface stress relaxation was relatively small, and it
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may be recalled that this specimen exhibited no macro-cracking. In-depth residual stress profile of a ground-RD specimen before and after exposure in the absence of external loading is presented in Fig. 14. Residual stresses were measured both parallel and perpendicular to rolling/grinding directions of the specimen. As shown in the figure, the tensile component reduced dramatically after exposure from surface to sub-surface until reaching zero at the depth where nearly all the micro-cracks stopped. The change of compressive component showed small, in-depth residual stresses showed similar values before and after exposure. In-depth surface morphology (Fig. 14) was also characterized of this specimen with different distances from the original surface. The penetration depths of micro-cracks were very uneven. Some of them disappeared after 2 mm of surface has been polished away, while some penetrated to a depth more than 10 mm from surface. Very few micro-cracks can be observed at 13 mm electrolytic polishing depth. The results agree with the cross-section observation.
4. Discussion 4.1. Residual stress distribution Residual stresses for the as-delivered material are close to zero, while grinding operations generated massive residual stresses in the surface and sub-surface layers of the steel. During grinding, thermally and mechanically induced residual stresses may be produced simultaneously; the relative significance varies from one process to another [25]. Heat is generated from the interaction between the abrasive grit and the workpiece material during grinding operations. Due to the low thermal conductivity of 304L stainless steel, a temperature gradient is formed from surface to the bulk. During the cooling period, contraction of the surface layer is hindered by the bulk thus resulting in surface tension and subsurface compression [25,26]. However, in the present case tensile residual stresses parallel to the grinding direction and compressive residual stresses perpendicular to the grinding direction were observed. This measured anisotropic surface residual stress can be attributed to the anisotropic plastic deformation of the ground surface layer, and indicates that residual stresses induced by mechanical effects dominate over thermal effects in this study. The surface layer experienced compressive plastic deformation in the grinding direction and tensile deformation in the transverse direction. After the grinding zone moved away, the constraint by the material beneath surface resulted in tensile σ∥ but compressive σ⊥ in the surface layer [20,27]. Such anisotropic residual stress distributions have also been observed in previous
Fig. 13. Measured surface residual stress of ground specimens parallel and perpendicular to rolling and grinding directions before and after exposure: (a) ground-RD specimen, (b) ground-TD specimen.
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Fig. 14. In-depth residual stress parallel and perpendicular to the rolling/grinding directions of ground-RD specimen before and after exposure without external loading and corresponding micrographs showing the appearance of the microcracks at each depth.
work when grinding duplex stainless steel 2304 [21]. 4.2. Corrosion behavior without external loading The micro-cracks and their pattern on the ground surfaces after exposure are one of the most interesting observations in this study. Extensive stress corrosion micro-cracks were observed on the ground surfaces even in the absence of any external loading, while the as-delivered material showed no such cracking. Similar crack patterns have also been observed by Lyon et al. on milled 316 surfaces [8]. It has also been shown in other work that surface failure often occurs due to surface tensile residual stresses generated in metallic materials [2]. Compressive residual stress on the surface can delay crack initiation, and compressive residual stresses at depth can slow down the growth of cracks from the surface [10]. In the present study, high levels of tensile residual stresses up to 350 MPa were measured parallel to the grinding direction in the surface layer, with compressive stresses in the perpendicular direction. Consequently the micro-cracks tended to initiate normal to the grinding marks. Both the in-depth surface morphology characterization and the cross-section investigation showed the micro-cracks perpendicular to the grinding direction were more numerous and longer than those parallel to the grinding marks. This strongly indicates that the tensile residual stresses in the surface layer make austenitic stainless steel susceptible to SCC in the presence of chlorides even without any external loading, whereas compressive residual stress can retard the initiation of cracks. The majority of the micro-cracks arrested after penetrating 10–15 mm through the highly deformed ground surface layer. This correlates well to the position at which the residual stresses shift from tension to compression. A threshold stress is required to initiate and maintain the propagation of SCC [28]. Results of surface residual stress measurement after exposure showed that the formation of micro-cracks caused a significant release of the surface residual stress. 4.3. Corrosion behavior with four-point bend loading The in-situ X-ray diffraction measurements on as-delivered specimens and ground specimens demonstrated that the actual stress in the surface of the specimens deviated from the values calculated according to ASTM G39 [16] and were strongly affected by the surface preparation. The repeated measurement on the asdelivered material indicated good accuracy and repeatability of the results. The actual loading corresponded reasonably well with the calculated values from ASTM G39 in the elastic regime, although
the measurements showed an actual stress of 126/166 MPa when 200 MPa was applied. Above the proof stress the slope of the curve dropped dramatically, so a calculated applied stress of 300 MPa gave an actual value below 200 MPa. This serves to underline the point that four point bending should never be used in the plastic regime. The in-situ stress measurements under four point loading showed a different trend for ground specimens. The surface layer was highly deformed by the grinding operation, which was clearly shown from the cross-section ECC images with fragmented grain structures and deformation slip bands in the surface layer. Due to strain hardening in this layer, the measured surface stress appeared to increase nearly linear with increasing load even up to 500 MPa. The actual surface stress in such specimens with residual stresses and a strength gradient near the surface depends on interactions between the applied and residual stresses as well as the strength gradient in the specimen [29]. For the ground-RD specimen, the surface tensile residual stress along the loading direction resulted in a high surface stress, for example 750 MPa for loading at 500 MPa level. On the other hand, the ground-TD specimen with a low surface residual stress parallel to the applied stress showed initially similar surface stress as the as-delivered specimen but owing to its higher surface strength, the linear behavior remained up to loading of 500 MPa. A detailed analysis of interactions between the stresses and the effect of the strength gradient is an avenue worth further investigation. Stress relaxation was found to be low for all three types of specimens under 500 MPa four-point bend loading after annealing at 155 °C for 24 h: around 50 MPa. The main stress relaxation was due to the formation of micro- or macro-cracks. The crack path was mainly transgranular in this study, although there were some local areas of intergranular cracking. The macrocracks followed a general path which was normal to the loading stress. The SCC susceptibility increased considerably with increasing four-point bend loading and the data in Table 3 indicated that there is a threshold stress of around 50 MPa for macroscopic cracking. Pits were observed on all the exposed specimens to varying degrees. When characterizing pitting and cracking behavior, there is always a question as to whether pits formed after cracking, or whether cracking initiated from pits. Many works have demonstrated that pits can act as precursors to cracking [8,30,6]. The explanation is usually that the cause is a combination of stress concentration and a more aggressive environment in the pits [2]. In the present study, all macro-cracks were observed to be associated with pits, although there were also pits without any related cracks. This indicates that pits form before macro-cracks and act as precursors to macro-cracking. However, the behavior of the microcracks was different. Extensive micro-cracks appeared on the specimens that had been ground and could be both with and without associated pits. This indicates that tensile residual stress in the surface layer was the main driving force for the initiation of micro-cracks and pits tended to grow from these micro-cracks. In the case of ground-RD specimens, these pits appeared to grow together along the micro-cracks in the perpendicular direction to the grinding marks and led to the macro-crack initiation. Moreover, the micro-cracks in both directions of all ground specimens with four-point bend loading were longer that those in absence of external loading, especially in the direction perpendicular to grinding marks, indicating that applied tensile stresses promote micro-crack propagation. The macro-cracking showed that the ground-RD specimens were more susceptible to SCC than the as-delivered specimens. Surface finish, surface deformation and microstructure can all influence the SCC behavior; however, the detrimental role of residual tensile stress from grinding in combination with an applied
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stress is dominant here. In contrast, the improvement of SCC resistance of ground-TD specimens was also clear; although the induced compressive residual stresses were entirely released during loading and were then converted into a state of tension, much higher level of compressive residual stresses beneath the ground surface compensated for the surface tension and effectively retarded the appearance of macro-cracks. The results suggest the idea that grinding transverse to the main stress direction in a construction could possibly be used to decrease the cracking risk.
5. Conclusions The effect of surface grinding on chloride induced stress corrosion cracking of austenitic stainless steel 304L has been investigated. The following conclusions can be drawn:
Grinding generated tensile residual stresses parallel to the
grinding direction ( σ∥) but compressive residual stresses perpendicular to the grinding direction ( σ⊥ ) in the surface layer. Beneath the ground surface, a layer of compressive stresses in both directions was formed with much higher values in the perpendicular direction. The residual stress distribution observed in this study indicated that anisotropic mechanical effects dominated over isotropic thermal effects from grinding. Stress measurements on four-point bend loaded specimens demonstrated that the actual loading may deviate appreciably from that calculated according to the formula in ASTM G39. For annealed material with low residual stresses the formula gave a reasonable estimate of the applied stress up to the proof stress. For surface ground material there was a linear relationship up to much higher loads but the absolute value was strongly affected by residual stresses from grinding. Surface tensile residual stresses were found to be the main factor causing the initiation of micro-cracks on ground surfaces during exposure with or without an externally applied load. Crack arrest occurred when micro-cracks grew into a region 10– 15 mm below the surface with low or no tensile residual stresses. Macroscopic cracking leading to final failure only occurred under an applied load and there appeared to be a threshold at load of around 50 MPa. A residual tensile stress in the grinding direction increased the susceptibility to chloride-induced SCC while grinding perpendicular to the applied loading direction retarded cracking. It is therefore suggested that grinding transverse to the main stress direction in a construction could possibly be used to decrease the cracking risk. Pits tended to initiate at micro-cracks on the surface, and macro-cracks were always associated with pits. This indicates that pits formed before macro-cracks and acted as precursors to macroscopic crack development.
Acknowledgment This work was performed within the Swedish Steel Industry Graduate School with financial support from Outokumpu Stainless Research Foundation, Region Dalarna, Region Gävleborg, Länsstyrelsen Gävleborg, Jernkontoret, Sandviken Kummun and Högskolan Dalarna. The assistance with part of the sample preparation work by Ulf Modin at Högskolan Dalarna is also appreciated. The authors are grateful to Timo Pittulainen at Outokumpu Stainless AB for providing the test materials and the material data. References [1] R. Pettersson and E. Johansson, Stress corrosion resistance of duplex grades, in
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