Abstract
The 304L stainless steel is widely used in nuclear power plant system medium transmission due to its good economic benefits and excellent performance. However, during service, it faces the risk of stress corrosion cracking (SCC). This paper uses characterization testing methods such as OM, EBSD, hardness testing, and SSRT to study the influence of light rolling deformation (10%) on the microstructure, mechanical properties, and stress corrosion performance of nuclear-grade 304L stainless steel. After 10% rolling deformation treatment, the dislocation density and small-angle grain boundaries increase significantly, and compressive stress/strain residuals exist within the material. The hardness of the material increases from 135HV to 198HV, and the SSRT test limit tensile strength in the simulated environment of primary circuit coolant is increased from 455 MPa to 528 MPa. After 10% rolling treatment of 304L stainless steel, it has reduced SCC susceptibility under the tested conditions.
1 Introduction
In recent years, frequent incidents of stress corrosion cracking (SCC) have occurred in austenitic stainless steel components serving in pressurized water reactor (PWR) safety systems (; ). Typical field cases include SCC indications detected in the safety injection pipeline of the Civaux-1 nuclear power plant in 2021 (), as well as cracks found in the pressurizer spray line of the Ohi-3 unit in 2020 (Terachi et al., 2023).
As a typical austenitic stainless steel, 304L stainless steel is extensively adopted for pipeline fabrication in PWR primary circuit safety systems. Rolling is an indispensable manufacturing process for such nuclear-grade pipes, and a variety of 304L rolled pipes with different diameters and wall thicknesses are currently widely applied in PWR safety systems ().
Nevertheless, existing studies (Peter L. et al., 2008; Lu et al., 2016; Meng et al., 2011; Lu et al., 2011; Lu et al., 2008a; Lu et al., 2008b) have demonstrated that rolling deformation can alter the initial stress and strain states of 304L stainless steel, further exerting a remarkable influence on the initiation and propagation of SCC cracks in high-temperature water environments.
L.W. Tsay et al. (2012) explored the effects of rolling temperature (25 °C and 150 °C) and sensitization treatment (650 °C/1 h) on the sulfide stress corrosion susceptibility of 304L, and verified the better corrosion resistance of 20% warm-rolled specimens in saturated H2S solution. investigated the microstructural evolution, tensile properties and SCC susceptibility of 304L stainless steel subjected to 20% cold rolling and sensitization treatment at 600 °C for 10 h. Yaguchi and Yonezawa (2014) proposed a potential intergranular SCC (IGSCC) mechanism for 304L austenitic stainless steel with 30% cold rolling reduction in simulated PWR primary water. Lu et al. (2010) measured the crack growth rate of 40% unidirectionally warm-rolled 304L (designed to avoid deformation-induced martensite) in oxygenated and deoxygenated high-purity water at 288 °C, and quantified the individual and coupled effects of rolling orientation and water chemistry on SCC behavior.
At present, most available studies focus on large rolling deformation levels, while systematic investigations on small deformation amplitudes remain insufficient. Against this background, the present work selects commercial 304L stainless steel pipes for PWR safety systems and performs a low rolling deformation of 10% thickness reduction. The microstructural evolution and SCC performance of deformed specimens are systematically analyzed in simulated PWR primary circuit water environment. The underlying microscopic mechanism governing the correlation between small rolling deformation and SCC behavior of 304L is clarified. This study compensates for the research gap regarding SCC performance of 304L stainless steel under low rolling deformation, and provides an important theoretical reference for mitigating the SCC risk of nuclear-grade 304L piping components.
2 Materials and methods
The chemical compositions of the nuclear-grade 304L stainless steel used in this study were provided in Table 1. Block samples with dimensions of 14 × 24 × 140 mm were cut from the PWR safety systems pipeline, and the block samples were subjected to rolling deformation treatment with a deformation amount of 10%. The unrolled sample (original state, ORI sample) and the 10% cold rolling deformation sample (cold working state, 10CW sample) were prepared for testing and characterization analysis, respectively.
TABLE 1
| Element | C | Si | Mn | Cr | Ni | N | P | S |
|---|---|---|---|---|---|---|---|---|
| Content | 0.025 | 0.42 | 1.02 | 18.32 | 8.86 | 0.069 | 0.028 | <0.010 |
Chemical compositions of steel investigated in this study (wt%).
Microstructure characterization was carried out using the Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), Electron Back-Scattered Diffraction (EBSD), Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD). Performance testing methods included hardness testing and stress corrosion cracking sensitivity testing. The hardness test was performed using a Qness Q10A + device, with a measurement range of HV0.001∼HV10 and a dwell time of 10 s. The slow strain rate test (SSRT) was used as the evaluation method for SCC sensitivity, with three parallel specimens for each reduction. The plate-shaped tensile sample for the SSRT test is shown in Figure 1, with a strain rate of 5 × 10−7/s. The test environment was set based on the primary water chemistry parameters during the power operation of nuclear power plants: the test temperature and pressure were set to 325 °C/15.5 MPa; ultrapure water was used to prepare the solution, which contained 1200ppm B3+ and 2.2 ppm Li+, added in the form of boric acid (H3BO3) and lithium hydroxide (LiOHH2O), respectively. During the entire test period, the dissolved hydrogen concentration in the solution was controlled at approximately 2.6 ppm, and the dissolved oxygen concentration was controlled below 10 ppb. SSRT was used to perform tensile tests in high-temperature water until the test piece fractured, and the stress-strain curve was obtained.
FIGURE 1
3 Results
3.1 Microstructure analysis
Figure 2 shows the microstructure of the ORI sample and the 10CW sample by SEM, both of which are austenitic. By comparison, it is found that the grain size becomes smaller after 10% rolling deformation, and grain deformation may cause dislocation slip to the grain boundaries, leading to changes in the grain boundaries (). Therefore, further characterization of the microstructure is conducted.
FIGURE 2
Figure 3 shows the characterization results by EBSD. The local orientation difference distribution in the Kernel Average Misorientation (KAM) graphs in Figure 3a and (b) is used to characterize the degree of local plastic deformation, indirectly reflecting the dislocation density and stress distribution. The KAM results indicate that the 10CW sample did not undergo grain recrystallization and recovery after 10% rolling deformation, resulting in a large local orientation difference and high dislocation density and stress level. Further characterization of the surface residual stress of the specimens was performed by XRD. As shown in Figure 4, the average surface residual stress of the ORI and 10CW specimens was 326.6 MPa and 656.0 MPa, respectively. Statistical analysis of the grain boundaries shows the results as shown in Figure 3c. After 10% rolling deformation, the proportion of Low-angle grain boundaries (LAGBs) in the sample significantly increased, and LAGBs are composed of dislocation walls, with a higher dislocation density around them. This further validates that 10% rolling deformation can increase the dislocation density.
FIGURE 3
FIGURE 4
3.2 SCC resistance analysis
Figure 5 shows the engineering stress-engineering strain curve of the sample after the SSRT test in the simulated high-temperature and high-pressure water of the primary circuit. The average ultimate tensile strength of the ORI sample and the 10CW sample is 455 MPa and 528 MPa, respectively. The 10CW sample has a higher tensile strength and cross-sectional contraction rate in the simulated PWR primary circuit water environment.
FIGURE 5
Figure 6 presents the electrochemical polarization curves of the specimens. It can be seen that the 10CW sample exhibits a higher corrosion potential, a lower corrosion current density, a wider passive region, and a higher breakdown potential, demonstrating superior corrosion resistance compared to the ORI sample.
FIGURE 6
Figure 7 shows the SEM microscopic morphology of the fracture surfaces of the ORI sample and the 10CW sample after the SSRT test conducted in the simulated PWR primary circuit water. As shown in Figure 7a, at the fracture edge in contact with the simulated PWR primary circuit water of the ORI sample, a brittle cleavage fracture surface was found. In the middle area away from the fracture surface in contact with the simulated PWR primary circuit water, a mainly ductile crater-like structure was observed. Through Figure 7b, it was found that the fracture edge of the 10CW sample also had discontinuous brittle cleavage fracture surfaces. In the middle area away from the fracture surface in contact with the simulated PWR primary circuit water, the fracture was mainly in a ductile crater-like form, indicating a ductile fracture characteristic. This indicates that the cold rolling with a light deformation amount (deformation amount of 10%) did not significantly change the mode of stress corrosion cracking.
FIGURE 7
Observation of the surface of the tensile fracture samples of ORI and 10CW showed that, as shown in Figure 8, compared with the dense cracking points on the surface of the ORI sample, the cracking points on the surface of the 10CW sample were relatively fewer. The residual compressive stress on the light cold-deformed surface tended to close the initiating cracks, which corresponded to the microscopic characterization of the grains by EBSD. In the 10CW sample, there were more dislocations, subgrains, and LAGBs, which were related to the residual compressive stress in the 10CW sample (; Yun Bi et al., 2003). Rolling caused compressive plastic deformation in 304L stainless steel, and the compressive stress was introduced into the material during the light deformation process. After the deformation, the material did not fully recover but maintained residual compressive strain internally, and the residual compressive stress/strain was concentrated in the defects such as lattice distortions formed by dislocations within the grains. The residual compressive stress/strain in the sample tended to close the crack initiation process in the material, making the difficulty of SCC crack initiation greater than that of the ORI sample, and the required tensile stress for SCC to occur would also be higher (; Shimada et al., 2002). In summary, the SCC sensitivity of the 10CW sample is lower than that of the unrolled ORI sample.
FIGURE 8
Figure 9 shows the high-magnification surface morphology characteristics of the area near the crack tip after the SSRT test. The surface of the specimen forms a typical double-layer oxide film structure: the outer layer is a loose and irregular coarse granular Fe3O4, and the inner layer is a dense NiCr2O4 spinel film.
FIGURE 9
4 Discussion
The results indicate that the 10% cold-rolled sample did not experience substantial recrystallization or recovery. The resulting non-equilibrium microstructure exhibits severe local lattice distortion, with a high-density dislocation network formed in the 10CW matrix.
Grain boundary analysis further confirmed a significant increase in the fraction of LAGBs. As LAGBs consist of dense dislocation walls, their abundance reflects high stored distortion energy in the material. This high concentration of dislocation defects and subgrain structures directly impacts the SCC performance. (Zhang et al., 2023).
Combined with microstructural analysis, the high-magnitude surface residual compressive stress introduced by 10% cold rolling deformation is identified as the key factor inhibiting SCC. The residual compressive stress counteracts the applied tensile stress, keeping the surface stress field in a compressive state, while promoting the stable formation and repair of the passive film (). Furthermore, the LAGBs introduced by cold rolling optimize the grain boundary structure, reducing intergranular corrosion susceptibility (Valerie, 2004; Zhang et al., 2015).
Although the high dislocation density and LAGBs by cold rolling may act as preferential sites for localized corrosion, the inhibiting effects of residual compressive stress and grain boundary optimization dominate at the light deformation level of 10%, ultimately resulting in significantly lower SCC susceptibility of the 10CW specimen compared to the original annealed ORI sample.
SSRT results reveal that 10% cold rolling deformation remarkably improves the ultimate tensile strength of the material in simulated PWR primary circuit water. This demonstrates that the microstructures induced by 10% cold rolling effectively strengthen the load-bearing capacity of the matrix.
Fractographic analysis shows that both the ORI and 10CW specimens present a mixed fracture mode, characterized by brittle cleavage at the edge and ductile dimples in the central region. It indicates that moderate 10% cold rolling deformation cannot alter the fundamental fracture mechanism of the material under the above environmental conditions (; ).
Although the macroscopic fracture modes of 10CW and ORI samples are similar, there are significant differences in the microscopic mechanism of crack initiation. The density of microcracks on the surface of 10CW sample is significantly lower than that of ORI sample. This is due to the residual compressive stress effect introduced by 10% rolling deformation, which is mainly stored in the form of lattice distortion and dislocations. This internal residual compressive stress field has a physical effect of actively closing microcrack gaps, effectively hindering the penetration path of the corrosive medium and suppressing the initial initiation of cracks.
In conclusion, 10% rolling deformation forms a high-density dislocation network, increases LAGBs, and introduces residual compressive strain, thereby enhancing the mechanical strength of the matrix, reducing Fe diffusion, and increasing the critical energy barrier for SCC crack initiation. Therefore, the 10% cold-deformed sample exhibits lower SCC sensitivity in the simulated PWR primary circuit water environment.
5 Conclusion
This paper employed SEM, EBSD, TEM and SSRT as characterization and testing methods, using the original state (ORI) 304L stainless steel as the reference, to investigate the influence of 10% cold rolling deformation (10CW) on its microstructure and SCC sensitivity in the simulated PWR primary circuit water environment. The main conclusions are as follows.
After 10% rolling deformation treatment, the austenite grains become finer, the dislocation density significantly increases, the proportion of LAGBs increases significantly, and residual compressive stress is introduced. The SSRT test shows that the ultimate tensile strength of the 10CW sample is higher than that of the ORI sample, and the matrix bearing capacity is enhanced. Both samples exhibit a mixed fracture mode of edge cleavage and central ribbed fracture, indicating that the 10% rolling deformation has not changed its basic SCC fracture characteristics.
The residual compressive stress introduced by 10% rolling deformation can increase the critical energy barrier for SCC crack initiation, inhibit the formation of microcracks, reduce the number of surface crack sources, and at the same time, 10% rolling deformation hinders the diffusion of Fe elements, reduces the coverage of the surface loose Fe3O4 oxide layer, maintains the barrier integrity of the inner layer NiCr2O4 spinel film, avoids its cracking and peeling under stress, and reduces the SCC sensitivity.
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
BS: Conceptualization, Formal Analysis, Methodology, Visualization, Writing – original draft. HL: Conceptualization, Validation, Writing – review and editing. BY: Supervision, Writing – review and editing. SZ: Formal Analysis, Methodology, Writing – review and editing. CL: Conceptualization, Formal Analysis, Methodology, Visualization, Writing – review and editing.
Funding
The author(s) declared that financial support was not received for this work and/or its publication.
Conflict of interest
Authors BS and HL were employed by CGN Cangnan Nuclear Power Co., Ltd. Authors BY, SZ and CL were employed by Suzhou Nuclear Power Research Institute Co., Ltd.
The handling editor HL declared a past co-authorship/collaboration with the author(s) CL at the time of review.
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The author(s) declared that generative AI was not used in the creation of this manuscript.
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Summary
Keywords
light rolling deformation, microstructure, nuclear-grade 304L, stainless steel, stress corrosion
Citation
Shan B, Li H, Yang B, Zhang S and Li C (2026) Effects of light rolling deformation on the microstructure and stress corrosion properties of nuclear-grade 304L. Front. Mater. 13:1839882. doi: 10.3389/fmats.2026.1839882
Received
26 March 2026
Revised
30 April 2026
Accepted
11 June 2026
Published
13 July 2026
Volume
13 - 2026
Edited by
Hong Luo, University of Science and Technology Beijing, China
Reviewed by
Ahmad Nurul Muttaqin, Politeknik Negeri Ujung Pandang, Indonesia
Yue Pan, Hainan University, China
Updates
Copyright
© 2026 Shan, Li, Yang, Zhang and Li.
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*Correspondence: Chengtao Li, lct1980@163.com; Bin Yang, youngbinnn@163.com; Shugang Zhang, 1761954273@qq.com
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.