- Email: [email protected]
Oxide scale on stainless steels and its effect on sticking during hot-rolling
Journal Pre-proof Oxide Scale on Stainless Steels and its Effect on Sticking during Hot-Rolling Seung-Rok Kim, Soyeon Lee, Hyung-Gu Kang, Jin-Woo Park
PII:
S0010-938X(19)31272-7
DOI:
https://doi.org/10.1016/j.corsci.2019.108357
Reference:
CS 108357
To appear in:
Corrosion Science
Received Date:
20 June 2019
Revised Date:
16 October 2019
Accepted Date:
19 November 2019
Please cite this article as: Kim S-Rok, Lee S, Kang H-Gu, Park J-Woo, Oxide Scale on Stainless Steels and its Effect on Sticking during Hot-Rolling, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108357
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Oxide Scale on Stainless Steels and its Effect on Sticking during Hot-Rolling
Seung-Rok Kima‡, Soyeon Leea‡, Hyung-Gu Kangb and Jin-Woo Parka,* Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea b
ro of
a
Stainless Steel Research Group, Technical Research Laboratory, POSCO, Pohang 37859,
-p
Republic of Korea
Fax: +82-221235834
Jo
ur
na
lP
‡These authors contributed equally.
re
*Corresponding author’s contact: E-mail: [email protected], Phone: +82-221235834,
1
ro of
Graphical abstract
re
-p
Highlights
Oxide scales of ferritic and austenitic STS show various composition and microstructure according to Cr content.
lP
Dense Cr2O3 layer continuously formed along the interface in high Cr STS facilitates sticking failure.
ur
na
Composite structure of Fe3O4, FeCr2O4 and FexNiy intermetallic compounds prevent sticking failure.
ABSTRACT
Jo
The failure of oxide scales on ferritic stainless steels (STSs) has been known to cause an unexpected and severe sticking problem during the hot-rolling process. The oxide scale fragments formed during hot-rolling are stuck on the surfaces of the working roll, deteriorating the surfaces of both the roller and the rolled materials. In this study, we investigate the effects of the compositions and microstructures of thermally grown oxide scales on the degree of 2
sticking during hot-rolling in three different kinds of commercial ferritic STSs and one austenitic STS. The STS samples are oxidized at around 1280 °C for 2 h, and the oxide scales formed on the STSs are identified by X-ray diffraction (XRD). Additionally, the cross-sectional microstructure and chemical compositions of the oxide scales on the STSs are examined by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM), respectively. The oxide scales adjacent to the interfaces with the bulk STS are analyzed by high-resolution
ro of
transmission electron microscopy (HR-TEM). The Gleeble test, which is a high-temperature (1250 °C) tensile test, is conducted to analyze the effects of the stress induced in the oxide scales during hot-rolling on the fracture of the scales. According to the microstructural analysis and mechanical test results, a brittle Cr2O3 layer, where cracks are easily formed and propagate
-p
through the scale thickness under the stress induced by hot-rolling, is formed along the
re
interfaces of STSs with high Cr contents. This effect increases the probability of sticking failure in high-Cr STSs. In contrast, scales with a composite-like structure of Fe- and Cr-oxides are
lP
formed on the surface of STSs containing low and medium levels of Cr. According to the Gleeble test results, the composite-like oxide scales show higher mechanical resistance to
na
tensile stress than the Cr2O3 layers in the high-Cr STSs. In addition, our microstructural analysis results reveal that FexNiy intermetallic compounds are observed as a form of a
ur
composite-like structure inside the oxide scales in the austenitic STS and that this structure also
Jo
delays crack propagation through the oxide scales and reduces the degree of sticking. Keywords: sticking; scale; ferritic stainless steels; hot-rolling, Gleeble test;
1. Introduction
3
Stainless steels (STSs) are one of the most important classes of alloys, as they are extensively used for automobiles, ships, buildings and space vehicles, providing safety and convenience to human life[1] because of their mechanical and chemical robustness[2]. The most important property of STS is corrosion and rusting resistance to harsh environments[2, 3]. To properly protect STS from corrosion, the Cr content should be more than 11 wt%[2]. Based on Fe-Cr alloy systems, STS contains many additional alloying elements for enhancing
ro of
its mechanical properties and machinability as well as for improving its corrosion resistance at high temperature[2-4].
There are three types of STS, ferritic, austenitic, and martensitic STS, according to the alloying elements and their effect on microstructure and crystallographic orientation[2, 3, 5].
-p
Specifically, ferritic STSs have a body-centered cubic structure that is the same as the
re
crystallographic structure of Fe at room temperature[3], and its Cr content ranges from 11 to 30 wt%, with only a small amount of other elements, such as C, N and Ni [3], Ni is added to
lP
stabilize austenitic STSs having a face-centered cubic structure, which is the same as that of Fe at temperature ranges between 900 and 1400 °C. Martensitic STSs are similar to quenched Fe-
na
C alloys having a body-centered tetragonal structure, and they generally contain low levels of Cr and medium levels of C.
ur
Austenitic STSs show excellent formability even at cryogenic temperatures[3] and has good corrosion resistance[6] and mechanical stability at high temperature[4]. However, the
Jo
price of Ni is constantly fluctuating, and the demands for STS with low and stable cost has prompted the development of ferritic STS without Ni[7]. Furthermore, to enhance the corrosion resistance, highly purified and microalloyed ferritic STSs containing high Cr contents (>18-19 wt%) have been developed as a comparable counterpart to the austenitic STSs even in corrosive
4
environments[7]. For these reasons, to date, many researchers have focused on the corrosion[4, 8, 9] and mechanical properties[6, 10, 11] of both ferritic and austenitic STSs. During hot-rolling of STS in ambient air, Cr oxide scale with adherent, self-healing and corrosion-resistant properties is generally formed on the surface of the STS[2, 12]. At the same time, sticking failure frequently occurs, and surface defects are formed on the STS and rollers, particularly in the case of hot-rolling of high-Cr ferritic STSs [5, 13, 14]. During sticking,
ro of
which is an unexpected phenomenon occurring in the hot-rolling process of STS, fragments are stuck on surfaces of a working roll, deteriorating the surfaces of both the roller and the rolled materials [5, 15, 16]. Sticking usually appears at a rear part of the roughening rolls and at a front part of the finishing rolls, leaving numerous dents and scratches on the surfaces,
-p
which finally induces serious problems with the surface quality and productivity of the STS
re
and the lifetime of a roller [14, 17].
Because the surface quality is of great importance for the final STS product [5], sticking
lP
failure issues have been constantly addressed by many engineers, and researchers have made efforts to find the mechanism of sticking failure in terms of the manufacturing process
na
conditions and alloying compositions of the STS [5, 16, 18]. Regarding the manufacturing processes, researchers have found that a chilled working roll enhanced the sticking resistance,
ur
while severe sticking occurred in a high-temperature range [5]. On the other hand, in terms of the alloying compositions of the STS, it was reported that increasing the Cr content or lowering
Jo
the C/N content ratio causes a more serious sticking phenomenon during the hot-rolling process [14, 18, 19]. In addition, Si increases the formation and growth of Cr oxide scales, thereby increasing the sticking problem [16]. Additionally, the addition of Zr, Cu and Ni has been found to alleviate sticking failure, as they play an important role as a lubricant [7]. Therefore, the alloying compositions seem to determine the oxide scale microstructure that controls the degree 5
of sticking [7]. Nonetheless, thus far, there has been no clear understanding of the effect of alloy composition on the microstructure and property of the oxide scales formed on STS during hot-rolling and their influence on sticking failure. Hence, the fundamental understanding of sticking failure in terms of the alloy composition and microstructure of the oxide scales will provide insights into designing STSs less affected by sticking failure. In this study, we investigated the relationship between the microstructures of oxide scales
ro of
and the degree of sticking during the hot-rolling process in four kinds of commercial STS. In the three ferritic STSs, including high-Cr ferritic STS, and one austenitic STS, sticking failure was investigated and compared in terms of the microstructure and mechanical properties of the oxide scales as a function of the Cr content and presence of Ni. The microstructures of the
-p
oxide scales were investigated via optical microscopy (OM), scanning transmission electron
re
microscopy (STEM), high-resolution X-ray diffraction (HR-XRD), electron probe microanalysis (EPMA), field-effect scanning electron microscopy (FE-SEM), and high-
lP
resolution transmission electron microscopy (HR-TEM), which provided us with information on the three-dimensional distribution of compositions and compounds within the oxide scales.
na
After confirming the composition and growth behavior of the oxide scale in each STS, a tensile test at high temperature (Gleeble test), where the STS could be subject to similar stress
ur
conditions to those during the hot-rolling process, was performed. By examining the fractured surfaces after the Gleeble tests, the different mechanisms of sticking failure in the four model
Jo
steels and their relationship with the alloying contents could be explained. 2. Experimental procedures 2.1 The chemical compositions of STSs
6
The chemical compositions (wt.%) of the four model STS alloys are summarized in Table 1. The STS4A, STS4B and STS4C are low-, medium-, and high-Cr ferritic STSs without Ni, respectively. STS3 is austenite STS with 8 wt% Ni content. All the alloys have low C and Ti contents. Other elements such as Si, Mn, W, Mo, Nb, Ti, C, P, and S are used for various purposes including oxidizing, corrosion-resisting or formability. Here, since the other elements were contained less than 1 wt% without large variation between samples, we limited the
ro of
characteristic elements to Fe, Cr and Ni for analyzing the sticking behavior of STS in this study. The STSs with thickness about 200 mm were oxidized at about 1000 to 1200 °C for 2 h at atmospheric pressure and simultaneously were pressed by finishing rolls in a commercial product plant, resulting final thickness of STSs to about 30 mm. During the process, descaling
-p
of the outer oxide scales on the STSs was frequently performed by a high-pressure water
re
hydraulic descaler at every interval of each step. According to the product statistic from the visual detection method, sticking failures were most frequently observed in STS4C among the
lP
four types of STS, implying that the sticking behavior of STSs having high Cr contents during hot-rolling can be significantly different from those of other STSs.
na
2.2 Structural analysis of the oxide scale
ur
Chemical and microstructural analyses of the oxide scale remaining after descaling were performed using OM, HR-XRD (SmartLab, Rigaku), EPMA (JXA-8500F, JEOL), FE-SEM
Jo
(JEOL-7800F, JEOL), STEM (JEM-F200, JEOL), and HR-TEM (JEM-F200, JEOL). The STS specimens were sectioned, mounted on epoxy and polished to facilitate handling, and finally etched using methanol, nitrile acid, and hydrochloric acid for decontamination. For HR-TEM analysis, samples were prepared with a focused ion beam (FIB, crossbeam 540, ZEISS). The thicknesses of the oxide scales were observed by OM, the phases in the outer most regions of 7
the oxide scales were confirmed by HR-XRD, and the interfaces between the oxide scales and the bulk STS were analyzed by EPMA, FE-SEM and HR-TEM, respectively. The phase of oxide scale was analyzed by the peak information obtained from HR-XRD theta-2 theta scan mode. The HR-XRD was equipped with incident beam monochromator where the Cu Kα1 (λ=1.541 Å) generated at 45 kV/200 mA was used as a radiation source with a receiving 5° Soller slit. A scan speed was 2°/min with 0.02° step. To obtain the electronic
ro of
microscopic images, FE-SEM is equipped with EPMA with electronic beam conditions of 15 kV/50 nA acceleration voltage. The EPMA image condition was set to 5000 times magnification, 65536 pixels (256 by 256) resolution, and the 12 msec dwell time.
-p
2.3 Gleeble test
re
The Gleeble test (Gleeble 3800, Imdea materials), which is a thermomechanical tensile test, was conducted in this work to simulate the stress conditions experienced by the oxide scales
lP
during hot-rolling, upon which the oxide scales are simultaneously under compression vertically and under tension horizontally and the bare bulk STS is exposed to the roller material.
na
Schematic descriptions of samples prepared for the Gleeble test are shown in Fig. 1a. As shown in Fig. 1b, the samples were anchored in the heat sink by using the holes at the two sides and
ur
were heated by the direct resistance heating system. The thermocouple provided temperature signals for accurate feedback control of specimen temperature. Under the ambient atmosphere,
Jo
the temperature was set to 950 °C, and the strain rate was fixed at 1 s-1. Fig. 1c shows the samples prepared for microstructure analysis by chopping them into a few-millimeter unit. Normal samples were chopped from region C1 in Fig. 1a, and Gleeble samples were chopped from region C2. The compound types on the exposed surface of these Gleeble samples were
8
compared with those on the surface of the normal samples, which provides direct information regarding the sticking failure. 3. Results and discussion 3.1 Structural analysis of the oxide scale OM images of the oxide scales are shown in Fig. 2. We measured the thicknesses of the
ro of
oxide scales at several different points and averaged the values. As seen in Fig. 2a to c, the thicknesses of the oxide scales decrease with increasing Cr contents, as the highly oxidative Cr element preferentially forms a Cr-oxide layer on the surface of the STS, which restricts further oxidation and growth of oxide scales on the STS [20, 21]. As shown in Fig. 2d, the
-p
thickness of the oxide scale on STS3 is similar to that on STS4B, as these two STSs have the
re
same Cr content of 18 wt.%, implying that the Cr content is a determining factor in the formation and growth of the oxide scales [20]. The randomly distributed bright particles in
lP
STS3, as shown in Fig. 2d, seem to be precipitated Fe-Ni intermetallic compounds that can be formed during the growth of the oxide scale inside the furnace in regions with high local Ni
na
concentrations [22, 23].
The major phases in the outermost oxide scale regions of the STSs could be identified by
ur
the peaks at various positions obtained from HR-XRD, as shown in Fig. 3. According to the
Jo
normalized peak intensity in Fig. 3, the oxide scales on STS3 are mostly (Cr, Fe)-Ni-oxide, and the oxide scales on the surface of the ferritic STSs (STS4A, STS4B and STS4C in Table 1) are Fe-oxides. (Cr, Fe)-Ni-oxide is a Ni-oxide compound in which Ni was substituted with either Cr or Fe. The major phases in STS4A, STS4B and STS4C are Fe2O3, Fe3O4, Fe, and FeCr2O4, while those in STS3 are (Cr, Fe)-Ni-oxide and FexNiy, including the intermetallic compounds 9
Fe3Ni2, FeNi3 and FeNi. These compositions are consistent with those of the major oxide scales on STSs previously reported by others [24-27]. Notably, the Fe peak exists in both STS4B and STS4C, while no Fe peak was observed in STS4A, where the oxide scale has the highest thickness among the samples in Table 1 (Fig. 1). The Fe peaks observed in STS4B and STS4C indicate the detection of the substrate when the penetration depth of the X-ray is deeper than the thickness of the oxide scales [14, 24, 28]. The types and compositions of the scales are
ro of
confirmed by EPMA elemental mapping analysis, as shown in Fig. 4. As described in the inset image in the top left of Fig. 4, the elemental distributions were examined by EPMA from the inner oxide scale/bulk interface region (red dashed line box).
-p
Therefore, the EPMA images in Fig. 4 showed oxide scale within about 10 μm from the bulk interfaces, which are partial portion from the whole oxide scales according to the OM images
re
in Fig. 2. The mass concentrations (%) of Fe, Cr, O, and Ni were identified by the different colors and degrees of contrast in Fig. 4. In sample STS4A, Cr was distributed as percolated
lP
precipitates, while Fe and O were uniformly distributed throughout the scale. As the Cr content increases from sample STS4A to STS4C, a layer of an Fe-deficient and Cr-rich phase was
na
formed along the interfaces between the oxide scale and bulk; this phase seems to be either an FeCr2O4 or a Cr2O3 phase, according to EPMA analysis in previous reports [26, 29]. In STS4B,
ur
the percolated precipitates and a thin layer of a Cr-rich phase coexist in the scale.
Jo
According to the EPMA mapping of the Ni-containing STS3 sample, O-deficient and Nirich phases are uniformly distributed throughout the scale as small particles, which seem to correspond to the shiny particles observed in the OM images in Fig. 2. According to Fig. 4, Crand Fe-rich particles are also uniformly distributed throughout the scale of STS3. Here, the average portion of the elements are in order of Fe, O, Cr and Ni which takes 35.9, 14.6, 15 and 10
6.1%, respectively, obtained from the raw matrix data of the EPMA images. Based on phase identification by XRD in Fig. 3, the Ni-rich particles seem to be FexNiy intermetallic compounds. Additionally, based on the distribution of O in STS3 shown in Fig. 4 and the XRD results in Fig. 3, the Cr2O3, FexNiy and Fe-oxide particles appear to coexist, forming a particulate composite-like structure. With the fact that the Fe diffuse faster than Cr and Ni, and Fe and Cr have higher oxidation potential at the interface between bulk and oxide scale, so-
ro of
called reactive zone [30, 31], Fe diffuses outward and form Fe-oxide, and Cr is oxidized near the reactive zone majorly forming Cr2O3 [32]. As the oxidation proceed, the abundant Fe from the bulk constantly diffuses out while Cr contents at the reactive zone begin to decrease due to the formation of Cr2O3 [30, 31],
resulting high local concentration of Ni at the reactive zone
-p
up to 50~60 wt% [30, 33] where various Fe-Ni intermetallic with solid solution can be formed
re
thermodynamically [34].
As shown in Fig. 5, approximately 5 μm-thick oxide scales from the interface with the bulk
lP
of STS4A were analyzed by cross-sectional FE-SEM images, regular TEM images, and associated fast Fourier transform (FFT) patterns. High-magnification TEM images of selected
na
areas in regions 1 and 2 in Fig. 5a are displayed in Fig. 5b. According to Fig. 5b, the scales in STS4A have a composite-like structure where dark gray particles (S2 in Fig. 5b) are embedded
ur
in light gray regions (S1 and S3 in Fig. 5b). The FFT patterns in Fig. 5c identified S1, S2 and S3 as Fe3O4, FeCr2O4, and Fe3O4, respectively. The matrix of the oxide scale (S1 and S3) was
Jo
Fe3O4, and the inclusion (S2) was FeCr2O4. Thus, in the case of STS4A, the Cr-rich phase dispersed throughout the scale (Fig. 4) is FeCr2O4 inclusions embedded in the Fe3O4 matrix. This incompletely mixed structure between FeCr2O4 and Fe3O4, both of which have AB2O4 spinel structures, can be explained by thermodynamic calculations and additional magnetic and ionic effects occurring in the two spinels containing Fe ions [35], which are beyond the scope 11
of this study. As shown in Fig. 6, approximately 5 μm-thick oxide scales from the bulk of STS4C were analyzed by cross-sectional STEM, regular TEM images, and selected area diffraction patterns (SADPs). Within the oxide scale in STS4C (Fig. 6a), two regions (R1 and R2 in Fig. 6b) were selected to verify the compositions determined by EPMA in Fig. 4. According to the SADP analysis in Fig. 6c, both R1 and R2 were identified as Cr2O3, which is consistent with the
ro of
EPMA elemental mapping results in Fig. 4. Based on the TEM results, it was clearly verified that the oxide scales that formed along the interface with the bulk of STS4C are a layer of Cr2O3, while the oxide scale in STS4A is a composite of Fe3O4 and FeCr2O4.
-p
As shown in Fig. 7, approximately 6 μm-thick oxide scales from the bulk of STS3 were analyzed by cross-sectional STEM, regular TEM images, and SADPs. According to Fig. 7a,
re
the oxide scales were mainly composed of two different regions: the light gray region, as in Y1
lP
and Y3 in Fig. 7b, and the dark gray region, as in Y2. These two regions were entangled. As shown in Fig. 7c, SADPs were obtained from Y1, Y2 and Y3. According to the SADP analysis
na
in Fig. 7c, Y1 and Y3 corresponded to FexNiy compounds, including FeNi and Fe3Ni2, while Y2 was identified as Cr2O3. Based on the analysis results in Fig. 4 and Fig. 7, the FexNiy compounds entangled with Cr2O3 are embedded in the Fe-oxide layer. The inclusion of Ni in
ur
Fig. 4 was identified as Fe and Ni intermetallic compounds, FexNiy, having various
Jo
stoichiometries. As mentioned above, these FexNiy intermetallic compounds can be precipitated from the bulk STS during the growth of the oxide scale inside the furnace in regions with a high local Ni concentration [22, 23]. 3.2 Gleeble test
12
After the Gleeble test, the morphology and the elemental distributions of the top fractured surfaces (white dashed line box in the inset image in Fig. 8) were analyzed by FE-SEM and EPMA, respectively, as shown in Fig. 8. In this experiment, each mass concentration (%) of Fe, Cr, O, and Ni was identified by the different colors and degrees of contrast as in Fig. 4. Odeficient and Fe-rich regions were considered cracks, indicating that the bulk STS was exposed after the Gleeble test. Following the Gleeble test, the ferritic STSs had many delaminated wide
ro of
cracks, while the austenitic STS3 showed continuous domains with small cracks. For quantitative analysis of the elements on the exposed surface areas, we selected lines (the red arrows) to profile the normalized intensities of the height and O, Fe, Cr, and Ni contents, as shown in Fig. 9. The sticking failure occurs at the interfaces between the bare bulk
-p
STS and the roller materials [19]. Hence, as the O-deficient and Fe-rich regions increase in
re
size, the possibility of sticking failure increases. Comparing Fig. 9a to c based on the abovementioned criteria revealed a larger number of and wider cracks as the Cr content
lP
increased from STS4A to STS4C, which is consistent with the previously observed sticking failure results, where the sticking failure is most severe in STS4C.
na
The Gleeble test results can be explained by the brittle fracture of the Cr2O3 layer formed along the interface of the high-Cr STS (STS4C) [36]. Additionally, comparing Fig. 9b and 9d,
ur
which have the same Cr content, larger cracks were observed in STS4B than in STS3, which indicates that STS4B has a higher potential for sticking failure. Although the Cr contents of
Jo
STS4B and STS3 were similar, the entangled intermetallic FexNiy compounds and Cr2O3 embedded in the Fe-oxide matrix in STS3 seem to be much more resistant to cracking than the mixed structure of the composite-like scale and Cr-rich layer of STS4B (Fig. 4), thereby ultimately reducing the occurrence of sticking failure. The major phases at the outermost oxide scale regions of the STSs after the Gleeble test 13
were examined by HR-XRD, as shown in Fig. 10. According to the normalized peak intensities in Fig. 10, the kinds of phases were similar to those obtained before the Gleeble test (Fig. 3). However, compared to the intensities of the other peaks, the Fe peak intensities in STS4A, STS4B and STS4C were highly increased, and the relative increase enlarged in the order of STS4C, STS4B and STS4A. This finding can be explained by the previous results in Fig. 9, where large cracks and exposed areas of the bulk STS were frequently observed in the order of
ro of
STS4C, STS4B and STS4A. Thus, we can conclude that a higher Cr content within the bulk STS results in Cr-rich layer formation along the interface, which leads to brittle cracking and a higher possibility of sticking failure during hot-rolling [14]. Among the ferritic STSs, STS4A and STS4B showed higher sticking resistance than that of STS4C due to their composite-like
-p
structure of oxide scales consisting of a Fe3O4 matrix embedded with FeCr2O4. STS3 also
re
showed high resistance to cracking due to the composite-like structure of FexNiy, Cr-rich, and Fe-oxide phases. This composite structure led to even better resistance to cracking than STS4B,
lP
which has a similar level of Cr content.
As shown in Fig. 11a, the oxide scales of ferritic STS with low Cr content have a composite
na
structure where FeCr2O4 inclusions are dispersed and embedded in the Fe3O4 matrix. However, as the Cr content within the bulk increases, the composite-like structure disappeared, and a
ur
dense Cr2O3 layer was formed along the interface between the bulk and oxide scale. In the
Jo
austenitic STS, FexNiy intermetallic compounds, which are composed of Fe-Ni compounds of various stoichiometries, and Cr2O3 are embedded in an Fe-oxide layer making composite structure. Comparing the structures and bulk exposure frequency during the Gleeble test among the ferritic STS revealed that larger cracks are more frequently observed in STSs having a Cr2O3 layer along the interface than in STSs with a composite structure of Fe3O4 and FeCr2O4, 14
as the former is more brittle than the latter, as schematically described in Fig. 11b. Furthermore, the austenitic STS forms uniformly distributed micro-sized cracks on the top of the oxide scales during the Gleeble test, where the FexNiy compounds in the intermixed structure seem to block severe cracks or prevent delamination of the oxide scales from the bulk, which reduced the occurrence of sticking failure. 4. Conclusions
ro of
In this study, we investigated the chemical compositions and microstructures of the oxide scales in ferritic and austenitic STSs, where the ferritic STSs were classified as a function of their Cr content in the bulk. By examining the variations in chemical composition on the top
-p
surface of the oxide scales before and after the Gleeble test, we determined the correlation between 1) Cr content and 2) the presence of Ni in the bulk and sticking failure. According to
re
the microstructure and composition analysis, scales with a composite-like structure of Fe- and
lP
Cr-oxides are formed on the surface of STSs containing low and medium levels of Cr, while Cr2O3 layer structure was formed on the interface of STSs containing high Cr level. As the
na
composite-like oxide scales in STSs with low Cr contents and the distributed FexNiy intermetallic compounds in STS3 show higher mechanical resistance to tensile stress than the
ur
scales with Cr2O3 layer structure in STS with high Cr content, cracks are expected to easily formed and propagate through a brittle Cr2O3 layer in oxide scale under the stress induced by
Jo
hot-rolling. Consequently, among ferritic STSs, STSs with uniformly distributed and inclusionembedded microstructures are desirable, as their microstructures are much more resistant to cracking and delamination, which ultimately reduces the occurrence of sticking failure. Finally, to experimentally figure out the influence of brittleness in the layered structured Cr2O3 on the sticking problem presented in this study, the mechanical properties of each scale 15
components and real-time tracking of crack initiation and propagation path through the oxide scale should be further investigated, which is our on-going work.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ro of
Acknowledgements
We acknowledge the Technical Research Laboratory and POSCO for their financial support
-p
of this work.
Received: ((will be filled in by the editorial staff))
re
Revised: ((will be filled in by the editorial staff))
lP
Published online: ((will be filled in by the editorial staff))
na
References
[1] R.A. Lula, Stainless steel, American Society for Metals, Metals Park, OH, 1985.
ur
[2] K.H. Lo, C.H. Shek, J.K.L. Lai, Recent developments in stainless steels, Materials Science and Engineering: R: Reports, 65 (2009) pp. 39-104. [3] J.R. Davis, Stainless steels, ASM international, 1994.
Jo
[4] S.H.C. Park, Y.S. Sato, H. Kokawa, K. Okamoto, S. Hirano, M. Inagaki, Corrosion resistance of friction stir welded 304 stainless steel, Scripta Materialia, 51 (2004) pp. 101-105. [5] C.-Y. Son, C.K. Kim, D.J. Ha, S. Lee, J.S. Lee, K.T. Kim, Y.D. Lee, Mechanisms of sticking phenomenon occurring during hot rolling of two ferritic stainless steels, Metallurgical and materials transactions A, 38 (2007) pp. 2776. [6] Y. Xiong, Y. Yue, Y. Lu, T. He, M. Fan, F. Ren, W. Cao, Cryorolling impacts on microstructure and mechanical properties of AISI 316 LN austenitic stainless steel, Materials Science and Engineering: A, 709 (2018) pp. 270276. [7] C. Zhang, Z.Y. Liu, Y. Xu, G.D. Wang, The sticking behavior of an ultra purified ferritic stainless steel during hot strip rolling, Journal of Materials Processing Technology, 212 (2012) pp. 2183-2192. 16
[8] J.W. Lee, C.J. Wang, J.G. Duh, NaCl-induced accelerated oxidation of 304 stainless steel and Fe-Mn-Al alloy at 900 degrees C, Journal of Materials Science, 38 (2003) pp. 3619-3628. [9] D.B. Wei, J.X. Huang, A.W. Zhang, Z.Y. Jiang, A.K. Tieu, F. Wu, X. Shi, S.H. Jiao, Deformation of oxide scale and surface roughness transfer during hot rolling of stainless steel 304L, International Journal of Surface Science and Engineering, 3 (2009) pp. 459-470. [10] Z.C. Zhu, Y. He, X.J. Zhang, H.Y. Liu, X. Li, Effect of interface oxides on shear properties of hot-rolled stainless steel clad plate, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 669 (2016) pp. 344-349. [11] P.Q. La, Q. Meng, L. Yao, M.X. Zhou, Y.P. Wei, Effects of al element on microstructure and mechanical properties of hot-rolled 316L stainless steel, Acta Metallurgica Sinica, 49 (2013) pp. 739-744. [12] D. Geneve, D. Rouxel, P. Pigeat, B. Weber, M. Confente, Surface composition modification of high-carbon low-alloy steels oxidized at high temperature in air, Applied Surface Science, 254 (2008) pp. 5348-5358.
ro of
[13] L. Hao, Z.Y. Jiang, X.W. Cheng, J.W. Zhao, D.B. Wei, L.Z. Jiang, S.Z. Luo, M. Luo, L. Ma, Effect of Extreme Pressure Additives on the Deformation Behavior of Oxide Scale during the Hot Rolling of Ferritic Stainless Steel Strips, Tribology Transactions, 58 (2015) pp. 947-954. [14] L. Hao, Z.Y. Jiang, D.B. Wei, D.Y. Gong, X.W. Cheng, J.W. Zhao, S.Z. Luo, L.Z. Jiang, Experimental and Numerical Study on the Effect of ZDDP Films on Sticking During Hot Rolling of Ferritic Stainless Steel Strip, Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science, 47A (2016) pp. 5195-5202.
-p
[15] D.J. Ha, H.K. Sung, S. Lee, J.S. Lee, Y.D. Lee, Analysis and prevention of sticking occurring during hot rolling of ferritic stainless steel, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 507 (2009) pp. 66-73.
re
[16] D.J. Ha, J.S. Lee, N.J. Kim, S. Lee, Effects of Alloying Elements on High-Temperature Oxidation and Sticking Occurring During Hot Rolling of Modified Ferritic STS430J1L Stainless Steels, Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science, 43A (2012) pp. 74-86. [17] J.S. Lee, C.Y. Son, C.K. Kim, D.J. Ha, S.H. Lee, K.T. Kim, Y.D. Lee, Sticking Mechanisms Occurring during Hot Rolling of Ferritic Stainless Steels, in: Advanced Materials Research, Trans Tech Publ, 2007, pp. 3-6.
lP
[18] K.S. Tan, M. Krzyzanowski, J.H. Beynon, Effect of steel composition on failure of oxide scales in tension under hot rolling conditions, Steel Research, 72 (2001) pp. 250-258. [19] S. Chandra-ambhorn, K. Ngamkham, N. Jiratthanakul, Effects of Process Parameters on Mechanical Adhesion of Thermal Oxide Scales on Hot-Rolled Low Carbon Steels, Oxidation of Metals, 80 (2013) pp. 61-72.
na
[20] D. Genève, D. Rouxel, B. Weber, M. Confente, Segregation across the metal/oxide interface occurring during oxidation at high temperatures of diluted iron based alloys, Materials Science and Engineering: A, 435-436 (2006) pp. 1-11. [21] W. Jin, J.Y. Choi, Y.Y. Lee, Nucleation and growth process of sticking particles in ferritic stainless steel, Isij International, 40 (2000) pp. 789-793.
ur
[22] P. Marshall, Austenitic stainless steels: microstructure and mechanical properties, Springer Science & Business Media, 1984.
Jo
[23] G.I. Silman, Compilative Fe – Ni phase diagram with author’s correction, Metal Science and Heat Treatment, 54 (2012) pp. 105-112. [24] X.W. Cheng, Z.Y. Jiang, D.B. Wei, L. Hao, J.W. Zhao, L.Z. Jiang, Oxide scale characterization of ferritic stainless steel and its deformation and friction in hot rolling, Tribology International, 84 (2015) pp. 61-70. [25] Y. Behnamian, A. Mostafaei, A. Kohandehghan, B.S. Amirkhiz, D. Serate, Y. Sun, S. Liu, E. Aghaie, Y. Zeng, M. Chmielus, W. Zheng, D. Guzonas, W. Chen, J.L. Luo, A comparative study of oxide scales grown on stainless steel and nickel-based superalloys in ultra-high temperature supercritical water at 800°C, Corrosion Science, 106 (2016) pp. 188-207. [26] C.-W. Yang, J.-H. Kim, R.E. Triambulo, Y.-H. Kang, J.-S. Lee, J.-W. Park, The mechanical property of the oxide scale on Fe–Cr alloy steels, Journal of Alloys and Compounds, 549 (2013) pp. 6-10. 17
[27] E. Fedorova, M. Braccini, V. Parry, C. Pascal, M. Mantel, F. Roussel-Dherbey, D. Oquab, Y. Wouters, D. Monceau, Comparison of damaging behavior of oxide scales grown on austenitic stainless steels using tensile test and cyclic thermogravimetry, Corrosion Science, 103 (2016) pp. 145-156. [28] X.W. Cheng, Z.Y. Jiang, J.W. Zhao, D.B. Wei, L. Hao, J.G. Peng, M. Luo, L. Mac, S.Z. Luo, L.Z. Jiang, Investigation of oxide scale on ferritic stainless steel B445J1M and its tribological effect in hot rolling, Wear, 338 (2015) pp. 178-188. [29] C.-W. Yang, S.-M. Cho, Y.-H. Kang, J.-S. Lee, J.-W. Park, The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels, Materials Science and Engineering: A, 556 (2012) pp. 246-252. [30] T. Jonsson, S. Karlsson, H. Hooshyar, M. Sattari, J. Liske, J.E. Svensson, L.G. Johansson, Oxidation After Breakdown of the Chromium-Rich Scale on Stainless Steels at High Temperature: Internal Oxidation, Oxidation of Metals, 85 (2016) pp. 509-536. [31] A. Col, V. Parry, C. Pascal, Oxidation of a Fe–18Cr–8Ni austenitic stainless steel at 850°C in O2: Microstructure evolution during breakaway oxidation, Corrosion Science, 114 (2017) pp. 17-27.
ro of
[32] B. Diawara, Y.-A. Beh, P. Marcus, Nucleation and Growth of Oxide Layers on Stainless Steels (FeCr) Using a Virtual Oxide Layer Model, The Journal of Physical Chemistry C, 114 (2010) pp. 19299-19307. [33] T. Jonsson, H. Larsson, S. Karlsson, H. Hooshyar, M. Sattari, J. Liske, J.E. Svensson, L.G. Johansson, HighTemperature Oxidation of FeCr(Ni) Alloys: The Behaviour After Breakaway, Oxidation of Metals, 87 (2017) pp. 333-341.
-p
[34] L.J. Swartzendruber, V.P. Itkin, C.B. Alcock, The Fe-Ni (iron-nickel) system, Journal of phase equilibria, 12 (1991) pp. 288-312. [35] S.E. Ziemniak, R.A. Castelli, Immiscibility in the Fe3O4–FeCr2O4 spinel binary, Journal of Physics and Chemistry of Solids, 64 (2003) pp. 2081-2091.
Jo
ur
na
lP
re
[36] M. Nakamichi, H. Kawamura, T. Sawai, Y. Ooka, M. Saito, Evaluation of the mechanical behaviour of Cr2O3 coating film under tensile load, Fusion Engineering and Design, 29 (1995) pp. 465-469.
18
ro of -p
re
Fig. 1. (a) Schematic descriptions of the samples after the Gleeble test, (b) digital image of the test setup and (c) scheme of prepared samples that are chopped from regions C1 and C2 in (a),
Jo
ur
na
lP
representing normal and Gleeble samples, respectively.
19
ro of -p re
lP
Fig. 2. OM images of oxide scales in each STS: (a) STS4A, (b) STS4B, (c) STS4C and (d)
Jo
ur
na
STS3; the average thickness of each oxide scale is given in each figure.
20
ro of -p re lP na
Jo
ur
Fig. 3. HR-XRD comparisons between different types of STS.
21
ro of -p re lP na
ur
Fig. 4. Cross-sectional FE-SEM images and EPMA compositional distributions of STS4A,
Jo
STS4B, STS4C and STS3. The inset schematic image is the measured interface position between the oxide scales and bulk STS (red dashed line box).
22
ro of -p re lP
na
Fig. 5. (a) Cross-sectional FE-SEM image of STS4A, (b) enlarged HR-TEM images of regions
Jo
ur
1 and 2 in (a) and (c) FFT patterns of regions S1, S2 and S3 in (b).
23
ro of -p re lP
Fig. 6. (a) Cross-sectional STEM image of STS4C, (b) enlarged HR-TEM images acquired
Jo
ur
na
from the red box in (a) and (c) SADPs of regions R1 and R2 in (b).
24
ro of -p re
lP
Fig. 7. (a) Cross-sectional STEM image of STS3, (b) HR-TEM images obtained from the red
Jo
ur
na
box region in (a) and (c) SADPs of regions Y1, Y2 and Y3 in (b).
25
ro of -p re lP na
ur
Fig. 8. Top view FE-SEM images and EPMA compositional distributions of STS4A, STS4B, STS4C and STS3 after the Gleeble test. The inset schematic image is the measured top surface
Jo
near the fractured position (white dashed line box).
26
ro of -p re lP na ur Jo Fig. 9. Line profile along the red arrow in the top-view EPMA mappings after the Gleeble test: (a) STS4A, (b) STS4B, (c) STS4C and (d) STS3. 27
ro of -p re lP na
Jo
ur
Fig. 10. HR-XRD comparisons between different types of STS after the Gleeble test.
28
ro of
-p
Fig. 11. Schematic descriptions of (a) the components in the four different STSs and (b) types
Jo
ur
na
lP
re
of cracking of the oxide scales during the Gleeble test in each STS.
29
Fig. 1. (a) Schematic descriptions of the samples after the Gleeble test, (b) digital image of the test setup and (c) scheme of prepared samples that are chopped from regions C1 and C2 in (a), representing normal and Gleeble samples, respectively. Fig. 2. OM images of oxide scales in each STS: (a) STS4A, (b) STS4B, (c) STS4C and (d) STS3; the average thickness of each oxide scale is given in each figure. Fig. 3. HR-XRD comparisons between different types of STS.
ro of
Fig. 4. Cross-sectional FE-SEM images and EPMA compositional distributions of STS4A, STS4B, STS4C and STS3. The inset schematic image is the measured interface position
-p
between the oxide scales and bulk STS (red dashed line box).
Fig. 5. (a) Cross-sectional FE-SEM image of STS4A, (b) enlarged HR-TEM images of regions
re
1 and 2 in (a) and (c) FFT patterns of regions S1, S2 and S3 in (b).
lP
Fig. 6. (a) Cross-sectional STEM image of STS4C, (b) enlarged HR-TEM images acquired from the red box in (a) and (c) SADPs of regions R1 and R2 in (b).
na
Fig. 7. (a) Cross-sectional STEM image of STS3, (b) HR-TEM images obtained from the red box region in (a) and (c) SADPs of regions Y1, Y2 and Y3 in (b).
ur
Fig. 8. Top view FE-SEM images and EPMA compositional distributions of STS4A, STS4B, STS4C and STS3 after the Gleeble test. The inset schematic image is the measured top surface
Jo
near the fractured position (white dashed line box). Fig. 9. Line profile along the red arrow in the top-view EPMA mappings after the Gleeble test: (a) STS4A, (b) STS4B, (c) STS4C and (d) STS3. Fig. 10. HR-XRD comparisons between different types of STS after the Gleeble test. 30
Fig. 11. Schematic descriptions of (a) the components in the four different STSs and (b) types
Jo
ur
na
lP
re
-p
ro of
of cracking of the oxide scales during the Gleeble test in each STS.
31
Table 1. Names of various STSs and corresponding chemical compositions (wt%). Other elements such as Si, Mn, W, Mo, Nb, P, S are included less than 1 wt%. Sample names
Fe Balance Balance Balance Balance
Cr 11 18 21 18
Ni 0 0 0 8
C ≤0.01 ≤0.01 ≤0.01 ≤0.05
Ti 0.05 0.06 ≤0.4 0.04
Jo
ur
na
lP
re
-p
ro of
STS4A STS4B STS4C STS3
32
PII:
S0010-938X(19)31272-7
DOI:
https://doi.org/10.1016/j.corsci.2019.108357
Reference:
CS 108357
To appear in:
Corrosion Science
Received Date:
20 June 2019
Revised Date:
16 October 2019
Accepted Date:
19 November 2019
Please cite this article as: Kim S-Rok, Lee S, Kang H-Gu, Park J-Woo, Oxide Scale on Stainless Steels and its Effect on Sticking during Hot-Rolling, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108357
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Oxide Scale on Stainless Steels and its Effect on Sticking during Hot-Rolling
Seung-Rok Kima‡, Soyeon Leea‡, Hyung-Gu Kangb and Jin-Woo Parka,* Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea b
ro of
a
Stainless Steel Research Group, Technical Research Laboratory, POSCO, Pohang 37859,
-p
Republic of Korea
Fax: +82-221235834
Jo
ur
na
lP
‡These authors contributed equally.
re
*Corresponding author’s contact: E-mail: [email protected], Phone: +82-221235834,
1
ro of
Graphical abstract
re
-p
Highlights
Oxide scales of ferritic and austenitic STS show various composition and microstructure according to Cr content.
lP
Dense Cr2O3 layer continuously formed along the interface in high Cr STS facilitates sticking failure.
ur
na
Composite structure of Fe3O4, FeCr2O4 and FexNiy intermetallic compounds prevent sticking failure.
ABSTRACT
Jo
The failure of oxide scales on ferritic stainless steels (STSs) has been known to cause an unexpected and severe sticking problem during the hot-rolling process. The oxide scale fragments formed during hot-rolling are stuck on the surfaces of the working roll, deteriorating the surfaces of both the roller and the rolled materials. In this study, we investigate the effects of the compositions and microstructures of thermally grown oxide scales on the degree of 2
sticking during hot-rolling in three different kinds of commercial ferritic STSs and one austenitic STS. The STS samples are oxidized at around 1280 °C for 2 h, and the oxide scales formed on the STSs are identified by X-ray diffraction (XRD). Additionally, the cross-sectional microstructure and chemical compositions of the oxide scales on the STSs are examined by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM), respectively. The oxide scales adjacent to the interfaces with the bulk STS are analyzed by high-resolution
ro of
transmission electron microscopy (HR-TEM). The Gleeble test, which is a high-temperature (1250 °C) tensile test, is conducted to analyze the effects of the stress induced in the oxide scales during hot-rolling on the fracture of the scales. According to the microstructural analysis and mechanical test results, a brittle Cr2O3 layer, where cracks are easily formed and propagate
-p
through the scale thickness under the stress induced by hot-rolling, is formed along the
re
interfaces of STSs with high Cr contents. This effect increases the probability of sticking failure in high-Cr STSs. In contrast, scales with a composite-like structure of Fe- and Cr-oxides are
lP
formed on the surface of STSs containing low and medium levels of Cr. According to the Gleeble test results, the composite-like oxide scales show higher mechanical resistance to
na
tensile stress than the Cr2O3 layers in the high-Cr STSs. In addition, our microstructural analysis results reveal that FexNiy intermetallic compounds are observed as a form of a
ur
composite-like structure inside the oxide scales in the austenitic STS and that this structure also
Jo
delays crack propagation through the oxide scales and reduces the degree of sticking. Keywords: sticking; scale; ferritic stainless steels; hot-rolling, Gleeble test;
1. Introduction
3
Stainless steels (STSs) are one of the most important classes of alloys, as they are extensively used for automobiles, ships, buildings and space vehicles, providing safety and convenience to human life[1] because of their mechanical and chemical robustness[2]. The most important property of STS is corrosion and rusting resistance to harsh environments[2, 3]. To properly protect STS from corrosion, the Cr content should be more than 11 wt%[2]. Based on Fe-Cr alloy systems, STS contains many additional alloying elements for enhancing
ro of
its mechanical properties and machinability as well as for improving its corrosion resistance at high temperature[2-4].
There are three types of STS, ferritic, austenitic, and martensitic STS, according to the alloying elements and their effect on microstructure and crystallographic orientation[2, 3, 5].
-p
Specifically, ferritic STSs have a body-centered cubic structure that is the same as the
re
crystallographic structure of Fe at room temperature[3], and its Cr content ranges from 11 to 30 wt%, with only a small amount of other elements, such as C, N and Ni [3], Ni is added to
lP
stabilize austenitic STSs having a face-centered cubic structure, which is the same as that of Fe at temperature ranges between 900 and 1400 °C. Martensitic STSs are similar to quenched Fe-
na
C alloys having a body-centered tetragonal structure, and they generally contain low levels of Cr and medium levels of C.
ur
Austenitic STSs show excellent formability even at cryogenic temperatures[3] and has good corrosion resistance[6] and mechanical stability at high temperature[4]. However, the
Jo
price of Ni is constantly fluctuating, and the demands for STS with low and stable cost has prompted the development of ferritic STS without Ni[7]. Furthermore, to enhance the corrosion resistance, highly purified and microalloyed ferritic STSs containing high Cr contents (>18-19 wt%) have been developed as a comparable counterpart to the austenitic STSs even in corrosive
4
environments[7]. For these reasons, to date, many researchers have focused on the corrosion[4, 8, 9] and mechanical properties[6, 10, 11] of both ferritic and austenitic STSs. During hot-rolling of STS in ambient air, Cr oxide scale with adherent, self-healing and corrosion-resistant properties is generally formed on the surface of the STS[2, 12]. At the same time, sticking failure frequently occurs, and surface defects are formed on the STS and rollers, particularly in the case of hot-rolling of high-Cr ferritic STSs [5, 13, 14]. During sticking,
ro of
which is an unexpected phenomenon occurring in the hot-rolling process of STS, fragments are stuck on surfaces of a working roll, deteriorating the surfaces of both the roller and the rolled materials [5, 15, 16]. Sticking usually appears at a rear part of the roughening rolls and at a front part of the finishing rolls, leaving numerous dents and scratches on the surfaces,
-p
which finally induces serious problems with the surface quality and productivity of the STS
re
and the lifetime of a roller [14, 17].
Because the surface quality is of great importance for the final STS product [5], sticking
lP
failure issues have been constantly addressed by many engineers, and researchers have made efforts to find the mechanism of sticking failure in terms of the manufacturing process
na
conditions and alloying compositions of the STS [5, 16, 18]. Regarding the manufacturing processes, researchers have found that a chilled working roll enhanced the sticking resistance,
ur
while severe sticking occurred in a high-temperature range [5]. On the other hand, in terms of the alloying compositions of the STS, it was reported that increasing the Cr content or lowering
Jo
the C/N content ratio causes a more serious sticking phenomenon during the hot-rolling process [14, 18, 19]. In addition, Si increases the formation and growth of Cr oxide scales, thereby increasing the sticking problem [16]. Additionally, the addition of Zr, Cu and Ni has been found to alleviate sticking failure, as they play an important role as a lubricant [7]. Therefore, the alloying compositions seem to determine the oxide scale microstructure that controls the degree 5
of sticking [7]. Nonetheless, thus far, there has been no clear understanding of the effect of alloy composition on the microstructure and property of the oxide scales formed on STS during hot-rolling and their influence on sticking failure. Hence, the fundamental understanding of sticking failure in terms of the alloy composition and microstructure of the oxide scales will provide insights into designing STSs less affected by sticking failure. In this study, we investigated the relationship between the microstructures of oxide scales
ro of
and the degree of sticking during the hot-rolling process in four kinds of commercial STS. In the three ferritic STSs, including high-Cr ferritic STS, and one austenitic STS, sticking failure was investigated and compared in terms of the microstructure and mechanical properties of the oxide scales as a function of the Cr content and presence of Ni. The microstructures of the
-p
oxide scales were investigated via optical microscopy (OM), scanning transmission electron
re
microscopy (STEM), high-resolution X-ray diffraction (HR-XRD), electron probe microanalysis (EPMA), field-effect scanning electron microscopy (FE-SEM), and high-
lP
resolution transmission electron microscopy (HR-TEM), which provided us with information on the three-dimensional distribution of compositions and compounds within the oxide scales.
na
After confirming the composition and growth behavior of the oxide scale in each STS, a tensile test at high temperature (Gleeble test), where the STS could be subject to similar stress
ur
conditions to those during the hot-rolling process, was performed. By examining the fractured surfaces after the Gleeble tests, the different mechanisms of sticking failure in the four model
Jo
steels and their relationship with the alloying contents could be explained. 2. Experimental procedures 2.1 The chemical compositions of STSs
6
The chemical compositions (wt.%) of the four model STS alloys are summarized in Table 1. The STS4A, STS4B and STS4C are low-, medium-, and high-Cr ferritic STSs without Ni, respectively. STS3 is austenite STS with 8 wt% Ni content. All the alloys have low C and Ti contents. Other elements such as Si, Mn, W, Mo, Nb, Ti, C, P, and S are used for various purposes including oxidizing, corrosion-resisting or formability. Here, since the other elements were contained less than 1 wt% without large variation between samples, we limited the
ro of
characteristic elements to Fe, Cr and Ni for analyzing the sticking behavior of STS in this study. The STSs with thickness about 200 mm were oxidized at about 1000 to 1200 °C for 2 h at atmospheric pressure and simultaneously were pressed by finishing rolls in a commercial product plant, resulting final thickness of STSs to about 30 mm. During the process, descaling
-p
of the outer oxide scales on the STSs was frequently performed by a high-pressure water
re
hydraulic descaler at every interval of each step. According to the product statistic from the visual detection method, sticking failures were most frequently observed in STS4C among the
lP
four types of STS, implying that the sticking behavior of STSs having high Cr contents during hot-rolling can be significantly different from those of other STSs.
na
2.2 Structural analysis of the oxide scale
ur
Chemical and microstructural analyses of the oxide scale remaining after descaling were performed using OM, HR-XRD (SmartLab, Rigaku), EPMA (JXA-8500F, JEOL), FE-SEM
Jo
(JEOL-7800F, JEOL), STEM (JEM-F200, JEOL), and HR-TEM (JEM-F200, JEOL). The STS specimens were sectioned, mounted on epoxy and polished to facilitate handling, and finally etched using methanol, nitrile acid, and hydrochloric acid for decontamination. For HR-TEM analysis, samples were prepared with a focused ion beam (FIB, crossbeam 540, ZEISS). The thicknesses of the oxide scales were observed by OM, the phases in the outer most regions of 7
the oxide scales were confirmed by HR-XRD, and the interfaces between the oxide scales and the bulk STS were analyzed by EPMA, FE-SEM and HR-TEM, respectively. The phase of oxide scale was analyzed by the peak information obtained from HR-XRD theta-2 theta scan mode. The HR-XRD was equipped with incident beam monochromator where the Cu Kα1 (λ=1.541 Å) generated at 45 kV/200 mA was used as a radiation source with a receiving 5° Soller slit. A scan speed was 2°/min with 0.02° step. To obtain the electronic
ro of
microscopic images, FE-SEM is equipped with EPMA with electronic beam conditions of 15 kV/50 nA acceleration voltage. The EPMA image condition was set to 5000 times magnification, 65536 pixels (256 by 256) resolution, and the 12 msec dwell time.
-p
2.3 Gleeble test
re
The Gleeble test (Gleeble 3800, Imdea materials), which is a thermomechanical tensile test, was conducted in this work to simulate the stress conditions experienced by the oxide scales
lP
during hot-rolling, upon which the oxide scales are simultaneously under compression vertically and under tension horizontally and the bare bulk STS is exposed to the roller material.
na
Schematic descriptions of samples prepared for the Gleeble test are shown in Fig. 1a. As shown in Fig. 1b, the samples were anchored in the heat sink by using the holes at the two sides and
ur
were heated by the direct resistance heating system. The thermocouple provided temperature signals for accurate feedback control of specimen temperature. Under the ambient atmosphere,
Jo
the temperature was set to 950 °C, and the strain rate was fixed at 1 s-1. Fig. 1c shows the samples prepared for microstructure analysis by chopping them into a few-millimeter unit. Normal samples were chopped from region C1 in Fig. 1a, and Gleeble samples were chopped from region C2. The compound types on the exposed surface of these Gleeble samples were
8
compared with those on the surface of the normal samples, which provides direct information regarding the sticking failure. 3. Results and discussion 3.1 Structural analysis of the oxide scale OM images of the oxide scales are shown in Fig. 2. We measured the thicknesses of the
ro of
oxide scales at several different points and averaged the values. As seen in Fig. 2a to c, the thicknesses of the oxide scales decrease with increasing Cr contents, as the highly oxidative Cr element preferentially forms a Cr-oxide layer on the surface of the STS, which restricts further oxidation and growth of oxide scales on the STS [20, 21]. As shown in Fig. 2d, the
-p
thickness of the oxide scale on STS3 is similar to that on STS4B, as these two STSs have the
re
same Cr content of 18 wt.%, implying that the Cr content is a determining factor in the formation and growth of the oxide scales [20]. The randomly distributed bright particles in
lP
STS3, as shown in Fig. 2d, seem to be precipitated Fe-Ni intermetallic compounds that can be formed during the growth of the oxide scale inside the furnace in regions with high local Ni
na
concentrations [22, 23].
The major phases in the outermost oxide scale regions of the STSs could be identified by
ur
the peaks at various positions obtained from HR-XRD, as shown in Fig. 3. According to the
Jo
normalized peak intensity in Fig. 3, the oxide scales on STS3 are mostly (Cr, Fe)-Ni-oxide, and the oxide scales on the surface of the ferritic STSs (STS4A, STS4B and STS4C in Table 1) are Fe-oxides. (Cr, Fe)-Ni-oxide is a Ni-oxide compound in which Ni was substituted with either Cr or Fe. The major phases in STS4A, STS4B and STS4C are Fe2O3, Fe3O4, Fe, and FeCr2O4, while those in STS3 are (Cr, Fe)-Ni-oxide and FexNiy, including the intermetallic compounds 9
Fe3Ni2, FeNi3 and FeNi. These compositions are consistent with those of the major oxide scales on STSs previously reported by others [24-27]. Notably, the Fe peak exists in both STS4B and STS4C, while no Fe peak was observed in STS4A, where the oxide scale has the highest thickness among the samples in Table 1 (Fig. 1). The Fe peaks observed in STS4B and STS4C indicate the detection of the substrate when the penetration depth of the X-ray is deeper than the thickness of the oxide scales [14, 24, 28]. The types and compositions of the scales are
ro of
confirmed by EPMA elemental mapping analysis, as shown in Fig. 4. As described in the inset image in the top left of Fig. 4, the elemental distributions were examined by EPMA from the inner oxide scale/bulk interface region (red dashed line box).
-p
Therefore, the EPMA images in Fig. 4 showed oxide scale within about 10 μm from the bulk interfaces, which are partial portion from the whole oxide scales according to the OM images
re
in Fig. 2. The mass concentrations (%) of Fe, Cr, O, and Ni were identified by the different colors and degrees of contrast in Fig. 4. In sample STS4A, Cr was distributed as percolated
lP
precipitates, while Fe and O were uniformly distributed throughout the scale. As the Cr content increases from sample STS4A to STS4C, a layer of an Fe-deficient and Cr-rich phase was
na
formed along the interfaces between the oxide scale and bulk; this phase seems to be either an FeCr2O4 or a Cr2O3 phase, according to EPMA analysis in previous reports [26, 29]. In STS4B,
ur
the percolated precipitates and a thin layer of a Cr-rich phase coexist in the scale.
Jo
According to the EPMA mapping of the Ni-containing STS3 sample, O-deficient and Nirich phases are uniformly distributed throughout the scale as small particles, which seem to correspond to the shiny particles observed in the OM images in Fig. 2. According to Fig. 4, Crand Fe-rich particles are also uniformly distributed throughout the scale of STS3. Here, the average portion of the elements are in order of Fe, O, Cr and Ni which takes 35.9, 14.6, 15 and 10
6.1%, respectively, obtained from the raw matrix data of the EPMA images. Based on phase identification by XRD in Fig. 3, the Ni-rich particles seem to be FexNiy intermetallic compounds. Additionally, based on the distribution of O in STS3 shown in Fig. 4 and the XRD results in Fig. 3, the Cr2O3, FexNiy and Fe-oxide particles appear to coexist, forming a particulate composite-like structure. With the fact that the Fe diffuse faster than Cr and Ni, and Fe and Cr have higher oxidation potential at the interface between bulk and oxide scale, so-
ro of
called reactive zone [30, 31], Fe diffuses outward and form Fe-oxide, and Cr is oxidized near the reactive zone majorly forming Cr2O3 [32]. As the oxidation proceed, the abundant Fe from the bulk constantly diffuses out while Cr contents at the reactive zone begin to decrease due to the formation of Cr2O3 [30, 31],
resulting high local concentration of Ni at the reactive zone
-p
up to 50~60 wt% [30, 33] where various Fe-Ni intermetallic with solid solution can be formed
re
thermodynamically [34].
As shown in Fig. 5, approximately 5 μm-thick oxide scales from the interface with the bulk
lP
of STS4A were analyzed by cross-sectional FE-SEM images, regular TEM images, and associated fast Fourier transform (FFT) patterns. High-magnification TEM images of selected
na
areas in regions 1 and 2 in Fig. 5a are displayed in Fig. 5b. According to Fig. 5b, the scales in STS4A have a composite-like structure where dark gray particles (S2 in Fig. 5b) are embedded
ur
in light gray regions (S1 and S3 in Fig. 5b). The FFT patterns in Fig. 5c identified S1, S2 and S3 as Fe3O4, FeCr2O4, and Fe3O4, respectively. The matrix of the oxide scale (S1 and S3) was
Jo
Fe3O4, and the inclusion (S2) was FeCr2O4. Thus, in the case of STS4A, the Cr-rich phase dispersed throughout the scale (Fig. 4) is FeCr2O4 inclusions embedded in the Fe3O4 matrix. This incompletely mixed structure between FeCr2O4 and Fe3O4, both of which have AB2O4 spinel structures, can be explained by thermodynamic calculations and additional magnetic and ionic effects occurring in the two spinels containing Fe ions [35], which are beyond the scope 11
of this study. As shown in Fig. 6, approximately 5 μm-thick oxide scales from the bulk of STS4C were analyzed by cross-sectional STEM, regular TEM images, and selected area diffraction patterns (SADPs). Within the oxide scale in STS4C (Fig. 6a), two regions (R1 and R2 in Fig. 6b) were selected to verify the compositions determined by EPMA in Fig. 4. According to the SADP analysis in Fig. 6c, both R1 and R2 were identified as Cr2O3, which is consistent with the
ro of
EPMA elemental mapping results in Fig. 4. Based on the TEM results, it was clearly verified that the oxide scales that formed along the interface with the bulk of STS4C are a layer of Cr2O3, while the oxide scale in STS4A is a composite of Fe3O4 and FeCr2O4.
-p
As shown in Fig. 7, approximately 6 μm-thick oxide scales from the bulk of STS3 were analyzed by cross-sectional STEM, regular TEM images, and SADPs. According to Fig. 7a,
re
the oxide scales were mainly composed of two different regions: the light gray region, as in Y1
lP
and Y3 in Fig. 7b, and the dark gray region, as in Y2. These two regions were entangled. As shown in Fig. 7c, SADPs were obtained from Y1, Y2 and Y3. According to the SADP analysis
na
in Fig. 7c, Y1 and Y3 corresponded to FexNiy compounds, including FeNi and Fe3Ni2, while Y2 was identified as Cr2O3. Based on the analysis results in Fig. 4 and Fig. 7, the FexNiy compounds entangled with Cr2O3 are embedded in the Fe-oxide layer. The inclusion of Ni in
ur
Fig. 4 was identified as Fe and Ni intermetallic compounds, FexNiy, having various
Jo
stoichiometries. As mentioned above, these FexNiy intermetallic compounds can be precipitated from the bulk STS during the growth of the oxide scale inside the furnace in regions with a high local Ni concentration [22, 23]. 3.2 Gleeble test
12
After the Gleeble test, the morphology and the elemental distributions of the top fractured surfaces (white dashed line box in the inset image in Fig. 8) were analyzed by FE-SEM and EPMA, respectively, as shown in Fig. 8. In this experiment, each mass concentration (%) of Fe, Cr, O, and Ni was identified by the different colors and degrees of contrast as in Fig. 4. Odeficient and Fe-rich regions were considered cracks, indicating that the bulk STS was exposed after the Gleeble test. Following the Gleeble test, the ferritic STSs had many delaminated wide
ro of
cracks, while the austenitic STS3 showed continuous domains with small cracks. For quantitative analysis of the elements on the exposed surface areas, we selected lines (the red arrows) to profile the normalized intensities of the height and O, Fe, Cr, and Ni contents, as shown in Fig. 9. The sticking failure occurs at the interfaces between the bare bulk
-p
STS and the roller materials [19]. Hence, as the O-deficient and Fe-rich regions increase in
re
size, the possibility of sticking failure increases. Comparing Fig. 9a to c based on the abovementioned criteria revealed a larger number of and wider cracks as the Cr content
lP
increased from STS4A to STS4C, which is consistent with the previously observed sticking failure results, where the sticking failure is most severe in STS4C.
na
The Gleeble test results can be explained by the brittle fracture of the Cr2O3 layer formed along the interface of the high-Cr STS (STS4C) [36]. Additionally, comparing Fig. 9b and 9d,
ur
which have the same Cr content, larger cracks were observed in STS4B than in STS3, which indicates that STS4B has a higher potential for sticking failure. Although the Cr contents of
Jo
STS4B and STS3 were similar, the entangled intermetallic FexNiy compounds and Cr2O3 embedded in the Fe-oxide matrix in STS3 seem to be much more resistant to cracking than the mixed structure of the composite-like scale and Cr-rich layer of STS4B (Fig. 4), thereby ultimately reducing the occurrence of sticking failure. The major phases at the outermost oxide scale regions of the STSs after the Gleeble test 13
were examined by HR-XRD, as shown in Fig. 10. According to the normalized peak intensities in Fig. 10, the kinds of phases were similar to those obtained before the Gleeble test (Fig. 3). However, compared to the intensities of the other peaks, the Fe peak intensities in STS4A, STS4B and STS4C were highly increased, and the relative increase enlarged in the order of STS4C, STS4B and STS4A. This finding can be explained by the previous results in Fig. 9, where large cracks and exposed areas of the bulk STS were frequently observed in the order of
ro of
STS4C, STS4B and STS4A. Thus, we can conclude that a higher Cr content within the bulk STS results in Cr-rich layer formation along the interface, which leads to brittle cracking and a higher possibility of sticking failure during hot-rolling [14]. Among the ferritic STSs, STS4A and STS4B showed higher sticking resistance than that of STS4C due to their composite-like
-p
structure of oxide scales consisting of a Fe3O4 matrix embedded with FeCr2O4. STS3 also
re
showed high resistance to cracking due to the composite-like structure of FexNiy, Cr-rich, and Fe-oxide phases. This composite structure led to even better resistance to cracking than STS4B,
lP
which has a similar level of Cr content.
As shown in Fig. 11a, the oxide scales of ferritic STS with low Cr content have a composite
na
structure where FeCr2O4 inclusions are dispersed and embedded in the Fe3O4 matrix. However, as the Cr content within the bulk increases, the composite-like structure disappeared, and a
ur
dense Cr2O3 layer was formed along the interface between the bulk and oxide scale. In the
Jo
austenitic STS, FexNiy intermetallic compounds, which are composed of Fe-Ni compounds of various stoichiometries, and Cr2O3 are embedded in an Fe-oxide layer making composite structure. Comparing the structures and bulk exposure frequency during the Gleeble test among the ferritic STS revealed that larger cracks are more frequently observed in STSs having a Cr2O3 layer along the interface than in STSs with a composite structure of Fe3O4 and FeCr2O4, 14
as the former is more brittle than the latter, as schematically described in Fig. 11b. Furthermore, the austenitic STS forms uniformly distributed micro-sized cracks on the top of the oxide scales during the Gleeble test, where the FexNiy compounds in the intermixed structure seem to block severe cracks or prevent delamination of the oxide scales from the bulk, which reduced the occurrence of sticking failure. 4. Conclusions
ro of
In this study, we investigated the chemical compositions and microstructures of the oxide scales in ferritic and austenitic STSs, where the ferritic STSs were classified as a function of their Cr content in the bulk. By examining the variations in chemical composition on the top
-p
surface of the oxide scales before and after the Gleeble test, we determined the correlation between 1) Cr content and 2) the presence of Ni in the bulk and sticking failure. According to
re
the microstructure and composition analysis, scales with a composite-like structure of Fe- and
lP
Cr-oxides are formed on the surface of STSs containing low and medium levels of Cr, while Cr2O3 layer structure was formed on the interface of STSs containing high Cr level. As the
na
composite-like oxide scales in STSs with low Cr contents and the distributed FexNiy intermetallic compounds in STS3 show higher mechanical resistance to tensile stress than the
ur
scales with Cr2O3 layer structure in STS with high Cr content, cracks are expected to easily formed and propagate through a brittle Cr2O3 layer in oxide scale under the stress induced by
Jo
hot-rolling. Consequently, among ferritic STSs, STSs with uniformly distributed and inclusionembedded microstructures are desirable, as their microstructures are much more resistant to cracking and delamination, which ultimately reduces the occurrence of sticking failure. Finally, to experimentally figure out the influence of brittleness in the layered structured Cr2O3 on the sticking problem presented in this study, the mechanical properties of each scale 15
components and real-time tracking of crack initiation and propagation path through the oxide scale should be further investigated, which is our on-going work.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ro of
Acknowledgements
We acknowledge the Technical Research Laboratory and POSCO for their financial support
-p
of this work.
Received: ((will be filled in by the editorial staff))
re
Revised: ((will be filled in by the editorial staff))
lP
Published online: ((will be filled in by the editorial staff))
na
References
[1] R.A. Lula, Stainless steel, American Society for Metals, Metals Park, OH, 1985.
ur
[2] K.H. Lo, C.H. Shek, J.K.L. Lai, Recent developments in stainless steels, Materials Science and Engineering: R: Reports, 65 (2009) pp. 39-104. [3] J.R. Davis, Stainless steels, ASM international, 1994.
Jo
[4] S.H.C. Park, Y.S. Sato, H. Kokawa, K. Okamoto, S. Hirano, M. Inagaki, Corrosion resistance of friction stir welded 304 stainless steel, Scripta Materialia, 51 (2004) pp. 101-105. [5] C.-Y. Son, C.K. Kim, D.J. Ha, S. Lee, J.S. Lee, K.T. Kim, Y.D. Lee, Mechanisms of sticking phenomenon occurring during hot rolling of two ferritic stainless steels, Metallurgical and materials transactions A, 38 (2007) pp. 2776. [6] Y. Xiong, Y. Yue, Y. Lu, T. He, M. Fan, F. Ren, W. Cao, Cryorolling impacts on microstructure and mechanical properties of AISI 316 LN austenitic stainless steel, Materials Science and Engineering: A, 709 (2018) pp. 270276. [7] C. Zhang, Z.Y. Liu, Y. Xu, G.D. Wang, The sticking behavior of an ultra purified ferritic stainless steel during hot strip rolling, Journal of Materials Processing Technology, 212 (2012) pp. 2183-2192. 16
[8] J.W. Lee, C.J. Wang, J.G. Duh, NaCl-induced accelerated oxidation of 304 stainless steel and Fe-Mn-Al alloy at 900 degrees C, Journal of Materials Science, 38 (2003) pp. 3619-3628. [9] D.B. Wei, J.X. Huang, A.W. Zhang, Z.Y. Jiang, A.K. Tieu, F. Wu, X. Shi, S.H. Jiao, Deformation of oxide scale and surface roughness transfer during hot rolling of stainless steel 304L, International Journal of Surface Science and Engineering, 3 (2009) pp. 459-470. [10] Z.C. Zhu, Y. He, X.J. Zhang, H.Y. Liu, X. Li, Effect of interface oxides on shear properties of hot-rolled stainless steel clad plate, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 669 (2016) pp. 344-349. [11] P.Q. La, Q. Meng, L. Yao, M.X. Zhou, Y.P. Wei, Effects of al element on microstructure and mechanical properties of hot-rolled 316L stainless steel, Acta Metallurgica Sinica, 49 (2013) pp. 739-744. [12] D. Geneve, D. Rouxel, P. Pigeat, B. Weber, M. Confente, Surface composition modification of high-carbon low-alloy steels oxidized at high temperature in air, Applied Surface Science, 254 (2008) pp. 5348-5358.
ro of
[13] L. Hao, Z.Y. Jiang, X.W. Cheng, J.W. Zhao, D.B. Wei, L.Z. Jiang, S.Z. Luo, M. Luo, L. Ma, Effect of Extreme Pressure Additives on the Deformation Behavior of Oxide Scale during the Hot Rolling of Ferritic Stainless Steel Strips, Tribology Transactions, 58 (2015) pp. 947-954. [14] L. Hao, Z.Y. Jiang, D.B. Wei, D.Y. Gong, X.W. Cheng, J.W. Zhao, S.Z. Luo, L.Z. Jiang, Experimental and Numerical Study on the Effect of ZDDP Films on Sticking During Hot Rolling of Ferritic Stainless Steel Strip, Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science, 47A (2016) pp. 5195-5202.
-p
[15] D.J. Ha, H.K. Sung, S. Lee, J.S. Lee, Y.D. Lee, Analysis and prevention of sticking occurring during hot rolling of ferritic stainless steel, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 507 (2009) pp. 66-73.
re
[16] D.J. Ha, J.S. Lee, N.J. Kim, S. Lee, Effects of Alloying Elements on High-Temperature Oxidation and Sticking Occurring During Hot Rolling of Modified Ferritic STS430J1L Stainless Steels, Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science, 43A (2012) pp. 74-86. [17] J.S. Lee, C.Y. Son, C.K. Kim, D.J. Ha, S.H. Lee, K.T. Kim, Y.D. Lee, Sticking Mechanisms Occurring during Hot Rolling of Ferritic Stainless Steels, in: Advanced Materials Research, Trans Tech Publ, 2007, pp. 3-6.
lP
[18] K.S. Tan, M. Krzyzanowski, J.H. Beynon, Effect of steel composition on failure of oxide scales in tension under hot rolling conditions, Steel Research, 72 (2001) pp. 250-258. [19] S. Chandra-ambhorn, K. Ngamkham, N. Jiratthanakul, Effects of Process Parameters on Mechanical Adhesion of Thermal Oxide Scales on Hot-Rolled Low Carbon Steels, Oxidation of Metals, 80 (2013) pp. 61-72.
na
[20] D. Genève, D. Rouxel, B. Weber, M. Confente, Segregation across the metal/oxide interface occurring during oxidation at high temperatures of diluted iron based alloys, Materials Science and Engineering: A, 435-436 (2006) pp. 1-11. [21] W. Jin, J.Y. Choi, Y.Y. Lee, Nucleation and growth process of sticking particles in ferritic stainless steel, Isij International, 40 (2000) pp. 789-793.
ur
[22] P. Marshall, Austenitic stainless steels: microstructure and mechanical properties, Springer Science & Business Media, 1984.
Jo
[23] G.I. Silman, Compilative Fe – Ni phase diagram with author’s correction, Metal Science and Heat Treatment, 54 (2012) pp. 105-112. [24] X.W. Cheng, Z.Y. Jiang, D.B. Wei, L. Hao, J.W. Zhao, L.Z. Jiang, Oxide scale characterization of ferritic stainless steel and its deformation and friction in hot rolling, Tribology International, 84 (2015) pp. 61-70. [25] Y. Behnamian, A. Mostafaei, A. Kohandehghan, B.S. Amirkhiz, D. Serate, Y. Sun, S. Liu, E. Aghaie, Y. Zeng, M. Chmielus, W. Zheng, D. Guzonas, W. Chen, J.L. Luo, A comparative study of oxide scales grown on stainless steel and nickel-based superalloys in ultra-high temperature supercritical water at 800°C, Corrosion Science, 106 (2016) pp. 188-207. [26] C.-W. Yang, J.-H. Kim, R.E. Triambulo, Y.-H. Kang, J.-S. Lee, J.-W. Park, The mechanical property of the oxide scale on Fe–Cr alloy steels, Journal of Alloys and Compounds, 549 (2013) pp. 6-10. 17
[27] E. Fedorova, M. Braccini, V. Parry, C. Pascal, M. Mantel, F. Roussel-Dherbey, D. Oquab, Y. Wouters, D. Monceau, Comparison of damaging behavior of oxide scales grown on austenitic stainless steels using tensile test and cyclic thermogravimetry, Corrosion Science, 103 (2016) pp. 145-156. [28] X.W. Cheng, Z.Y. Jiang, J.W. Zhao, D.B. Wei, L. Hao, J.G. Peng, M. Luo, L. Mac, S.Z. Luo, L.Z. Jiang, Investigation of oxide scale on ferritic stainless steel B445J1M and its tribological effect in hot rolling, Wear, 338 (2015) pp. 178-188. [29] C.-W. Yang, S.-M. Cho, Y.-H. Kang, J.-S. Lee, J.-W. Park, The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels, Materials Science and Engineering: A, 556 (2012) pp. 246-252. [30] T. Jonsson, S. Karlsson, H. Hooshyar, M. Sattari, J. Liske, J.E. Svensson, L.G. Johansson, Oxidation After Breakdown of the Chromium-Rich Scale on Stainless Steels at High Temperature: Internal Oxidation, Oxidation of Metals, 85 (2016) pp. 509-536. [31] A. Col, V. Parry, C. Pascal, Oxidation of a Fe–18Cr–8Ni austenitic stainless steel at 850°C in O2: Microstructure evolution during breakaway oxidation, Corrosion Science, 114 (2017) pp. 17-27.
ro of
[32] B. Diawara, Y.-A. Beh, P. Marcus, Nucleation and Growth of Oxide Layers on Stainless Steels (FeCr) Using a Virtual Oxide Layer Model, The Journal of Physical Chemistry C, 114 (2010) pp. 19299-19307. [33] T. Jonsson, H. Larsson, S. Karlsson, H. Hooshyar, M. Sattari, J. Liske, J.E. Svensson, L.G. Johansson, HighTemperature Oxidation of FeCr(Ni) Alloys: The Behaviour After Breakaway, Oxidation of Metals, 87 (2017) pp. 333-341.
-p
[34] L.J. Swartzendruber, V.P. Itkin, C.B. Alcock, The Fe-Ni (iron-nickel) system, Journal of phase equilibria, 12 (1991) pp. 288-312. [35] S.E. Ziemniak, R.A. Castelli, Immiscibility in the Fe3O4–FeCr2O4 spinel binary, Journal of Physics and Chemistry of Solids, 64 (2003) pp. 2081-2091.
Jo
ur
na
lP
re
[36] M. Nakamichi, H. Kawamura, T. Sawai, Y. Ooka, M. Saito, Evaluation of the mechanical behaviour of Cr2O3 coating film under tensile load, Fusion Engineering and Design, 29 (1995) pp. 465-469.
18
ro of -p
re
Fig. 1. (a) Schematic descriptions of the samples after the Gleeble test, (b) digital image of the test setup and (c) scheme of prepared samples that are chopped from regions C1 and C2 in (a),
Jo
ur
na
lP
representing normal and Gleeble samples, respectively.
19
ro of -p re
lP
Fig. 2. OM images of oxide scales in each STS: (a) STS4A, (b) STS4B, (c) STS4C and (d)
Jo
ur
na
STS3; the average thickness of each oxide scale is given in each figure.
20
ro of -p re lP na
Jo
ur
Fig. 3. HR-XRD comparisons between different types of STS.
21
ro of -p re lP na
ur
Fig. 4. Cross-sectional FE-SEM images and EPMA compositional distributions of STS4A,
Jo
STS4B, STS4C and STS3. The inset schematic image is the measured interface position between the oxide scales and bulk STS (red dashed line box).
22
ro of -p re lP
na
Fig. 5. (a) Cross-sectional FE-SEM image of STS4A, (b) enlarged HR-TEM images of regions
Jo
ur
1 and 2 in (a) and (c) FFT patterns of regions S1, S2 and S3 in (b).
23
ro of -p re lP
Fig. 6. (a) Cross-sectional STEM image of STS4C, (b) enlarged HR-TEM images acquired
Jo
ur
na
from the red box in (a) and (c) SADPs of regions R1 and R2 in (b).
24
ro of -p re
lP
Fig. 7. (a) Cross-sectional STEM image of STS3, (b) HR-TEM images obtained from the red
Jo
ur
na
box region in (a) and (c) SADPs of regions Y1, Y2 and Y3 in (b).
25
ro of -p re lP na
ur
Fig. 8. Top view FE-SEM images and EPMA compositional distributions of STS4A, STS4B, STS4C and STS3 after the Gleeble test. The inset schematic image is the measured top surface
Jo
near the fractured position (white dashed line box).
26
ro of -p re lP na ur Jo Fig. 9. Line profile along the red arrow in the top-view EPMA mappings after the Gleeble test: (a) STS4A, (b) STS4B, (c) STS4C and (d) STS3. 27
ro of -p re lP na
Jo
ur
Fig. 10. HR-XRD comparisons between different types of STS after the Gleeble test.
28
ro of
-p
Fig. 11. Schematic descriptions of (a) the components in the four different STSs and (b) types
Jo
ur
na
lP
re
of cracking of the oxide scales during the Gleeble test in each STS.
29
Fig. 1. (a) Schematic descriptions of the samples after the Gleeble test, (b) digital image of the test setup and (c) scheme of prepared samples that are chopped from regions C1 and C2 in (a), representing normal and Gleeble samples, respectively. Fig. 2. OM images of oxide scales in each STS: (a) STS4A, (b) STS4B, (c) STS4C and (d) STS3; the average thickness of each oxide scale is given in each figure. Fig. 3. HR-XRD comparisons between different types of STS.
ro of
Fig. 4. Cross-sectional FE-SEM images and EPMA compositional distributions of STS4A, STS4B, STS4C and STS3. The inset schematic image is the measured interface position
-p
between the oxide scales and bulk STS (red dashed line box).
Fig. 5. (a) Cross-sectional FE-SEM image of STS4A, (b) enlarged HR-TEM images of regions
re
1 and 2 in (a) and (c) FFT patterns of regions S1, S2 and S3 in (b).
lP
Fig. 6. (a) Cross-sectional STEM image of STS4C, (b) enlarged HR-TEM images acquired from the red box in (a) and (c) SADPs of regions R1 and R2 in (b).
na
Fig. 7. (a) Cross-sectional STEM image of STS3, (b) HR-TEM images obtained from the red box region in (a) and (c) SADPs of regions Y1, Y2 and Y3 in (b).
ur
Fig. 8. Top view FE-SEM images and EPMA compositional distributions of STS4A, STS4B, STS4C and STS3 after the Gleeble test. The inset schematic image is the measured top surface
Jo
near the fractured position (white dashed line box). Fig. 9. Line profile along the red arrow in the top-view EPMA mappings after the Gleeble test: (a) STS4A, (b) STS4B, (c) STS4C and (d) STS3. Fig. 10. HR-XRD comparisons between different types of STS after the Gleeble test. 30
Fig. 11. Schematic descriptions of (a) the components in the four different STSs and (b) types
Jo
ur
na
lP
re
-p
ro of
of cracking of the oxide scales during the Gleeble test in each STS.
31
Table 1. Names of various STSs and corresponding chemical compositions (wt%). Other elements such as Si, Mn, W, Mo, Nb, P, S are included less than 1 wt%. Sample names
Fe Balance Balance Balance Balance
Cr 11 18 21 18
Ni 0 0 0 8
C ≤0.01 ≤0.01 ≤0.01 ≤0.05
Ti 0.05 0.06 ≤0.4 0.04
Jo
ur
na
lP
re
-p
ro of
STS4A STS4B STS4C STS3
32