1. Introduction
Various materials have been used in machinery and components related to the marine engineering sector, including marine structures, ships, and offshore platforms. Enhancing the performance of marine machinery and components through improvements in marine materials is essential from both technical and economic perspectives. The application of metal materials in the marine environment presents various degradation challenges, necessitating the development and implementation of appropriate technologies. Among these challenges, corrosion remains a significant concern, and anti-corrosion materials are commonly employed to mitigate its effects. In addition to corrosion-resistant materials, various protective measures, such as rustproofing, heat treatment, and surface treatment, have been applied to enhance corrosion resistance (Çakir et al., 2021; Chandler, 1985; Kim et al., 2020; Wang et al., 2021; Xu et al., 2023).
Residual stress in materials varies depending on the treatment method applied. Residual stress in mechanical parts or structures refers to the internal stress that remains in solid materials after the original cause of stress has been removed. It may be introduced during material manufacturing or processing, influencing the material’s performance and properties. Therefore, the accurate measurement of residual stress is is critical for material quality evaluation, process optimization, and service life assessment. Residual stress can be categorized as either macroscopic or microscopic. Both types may be present in a component simultaneously. Macroscopic residual stress extends over a significantly wider range than the particle size of the material (Kandil et al., 2001). Microscopic residual stress, which arises due to microstructural variations within a material, often results from the presence of different phases or components in the material (Kandil et al., 2001). Various mechanisms, including welding, heat treatment, and plastic deformation, can generate such residual stress.
When residual stress is present in not only various parts used in ships, automobiles and aircraft, but various designed structurals such as pipes, pressure tanks, power plants, and cryogenic equipments, it can lead to structural damage(Azevado and Neto, 2004; Gautam et al., 2021; Nam et al., 2021; Ray et al., 2000). In some cases, severe plastic deformation induced by residual stress may result in distortion, ultimately leading to fracture (Azevado and Neto, 2004; Gautam et al., 2021; Kandil et al., 2001; Nam et al., 2021; Ray et al., 2000; Youtsos, 2006). Furthermore, several studies have suggested that residual stresses caused by external loads and internal stresses during the operation of machines and devices affect the durability of these components (Azevado and Neto, 2004; Gautam et al., 2021; Lei et al., 2000; Ray et al., 2000; Youtsos, 2006).
The fracture of a material refers to the process by which the material becomes damaged or collapses due to physical factors such as internal stress and external loads. Fracture significantly influences the properties and performance of the material and is a crucial factor in evaluating its safety and reliability.
There are various causes of material fracture, with the formation of tensile stress being a primary factor. To enhance the fatigue life and corrosion resistance of metal materials subjected to cyclic loads, surface modification techniques such as shot peening are commonly applied. Shot peening significantly improves fatigue life by projecting shot balls onto the material, inducing plastic deformation and generating compressive residual stress beneath the surface (Kim et al., 2013; Kobayashi et al., 1998; Leguinagoicoa et al., 2022; Lee et al., 2021; Trung et al., 2017; Voorwald et al., 2009; Wang et al., 1998).
To accurately measure physical properties, the devices and technologies used must have minimal influence on the measured properties. To address this challenge, various residual stress measurement methods have been developed (ASTM Standards, 2013; Gautam et al., 2021; Guo et al., 2021; Lee et al., 2006; Schajer, 2013; Suzuki, 2017). Numerous techniques are available for measuring residual stress, broadly categorized into destructive and nondestructive techniques (Dive and Lakade, 2021; Hauk, 1997; Kandil et al., 2001; Rossini, 2012; Ruud, 1982; Schajer, 2013; Wikipedia, 2024; Withers et al., 2008). Each of these techniques possesses unique characteristics, and destructive or non-destructive methods are actively employed in practical applications based on specific requirements.
This study aims to apply surface treatment methods to mitigate material degradation in advance and to measure the residual stress generated as a result. In this study, shot peening was applied among various surface treatment technologies used to improve the service life of devices or components. And then shot peening was applied to three types of stainless steel commonly used in industrial applications. The residual stress induced by shot peening in these stainless steel samples was measured using the hole drilling method (HDM), X-ray diffraction method (XRD), and instrumented indentation technique (IIT). The results obtained from these methods were compared and analyzed.
2. Materials and Experimental Methods
2.1 Materials
In this study, three types of stainless steel (STS303, STS316L, and STS410) were used to measure residual stress. Table 1 presents their mechanical properties.
Specimens were prepared in dimensions of 50 × 50 × 10 mm, and shot peening was applied as a surface treatment. The shot peening coverage was set to 200%, with an arc height of 0.36 mmA. Table 2 provides the applied shot peening conditions (Boeing Company, 2018; SAE Standards, 2018). Shot peening was employed to induce compressive residual stress in the depth direction. The distribution of the induced residual stress was measured using three different measurement methods, and the results were compared and analyzed.
Table 3 summarizes the characteristics of HDM, XRD and IIT, which are the three residual stress measurement methods applied in this study.
2.2 Experimental Methods
Residual stress measurement using the HDM was conducted with an RS-200 hole drilling machine (MM, USA), a residual stress measuring instrument. Fig. 1 illustrates the hole drilling measurement equipment used in the study. A P3 Indicator (MM, USA) was utilized as the strain data acquisition (DAQ) system, and a CEA-06-062UL- 120 (MM, USA) strain gauge was employed. In accordance with ASTM Standards (2013, 2020), the nonuniform stress method was adopted, wherein strain gauges were attached at positions where residual stress was expected. Hole drilling was performed in 20 steps, with each step increasing by 50 µm in depth. A φ1.9 drill was used to maintain the hole diameter close to 2 mm. Measurements were conducted while drilling to a depth of 1,000 µm to enhance the reliability of the measurement method. Calculations were performed under the assumption that residual stress within a specified range is relieved when hole drilling reaches a certain depth. The integral method was applied as the analysis technique for nonuniform stress.
For residual stress measurement using XRD, measurements were conducted through electropolishing with a micro-X360s (Pulstec, Japan), a residual stress measuring instrument. Fig. 2 illustrates the XRD measurement equipment used in the study. Electropolishing was performed using an EP-3 (Pulstec, Japan). Measurements were conducted in accordance with ASTM Standards (2021), European Standard (2009), International Standard (2017), and SAE Standards (2003) and were analyzed using Bragg’s Law, which describes the relationship between interatomic planar distance and the incident angle inside the crystal when X-rays are introduced into the metal crystal. The X-ray exposure time was maintained at 45 s with a collimator size of φ1.0. Measurements were taken in seven steps up to a depth of 400 µm, with electropolishing performed at 50 µm increments from the surface. The collected data were analyzed using the Cosα method.
As for residual stress measurement by IIT, AIS3000 (Frontics, Korea) was used as residual stress measuring equipment. Fig. 3 shows the IIT measuring equipment used. In accordance with the Korea Standard (2018), continuous measurements were performed on the indentation depth according to the indentation load. In this instance, for residual stress measurement, a stress free specimen was prepared and subjected to heat treatment after electric discharge machining to create a reference. It was compared with the standard specimen.
Using the aforementioned measurement methods, the residual stress of shot-peened specimens was assessed at specified depths from the surface with the prepared measuring equipment.
3. Results and Discussion
3.1 Comparison of Residual Stress for Each Material by the Same Measurement Method
In Figs. 4–6, the residual stress results observed in three types of stainless steel (STS303, STS316L, and STS410) are comparatively presented for each measurement technique.
The residual stress measurements obtained through XRD are presented in Fig. 4. Here, the residual stress was recorded at every measurement point while electropolishing was performed at intervals of 50 µm, progressing in the depth direction from the surface. The residual stress approached zero at a depth of 400 µm, indicating that the compressive residual stress induced by shot peening was effectively distributed to this depth. Both STS303 and STS410 displayed comparable trends in their residual stress distribution. However, STS316L revealed a peak in compressive residual stress at approximately 100 µm of surface depth. The residual stress values measured from the top surface of the specimens were recorded as −855 MPa for STS303, −588 MPa for STS410, and −443 MPa for STS316L.
For both STS303 and STS410, the maximum compressive residual stress occurred at the top surface, with a gradual decrease observed as depth increased. In contrast, STS316L exhibited a higher level of compressive residual stress at around 100 µm compared to the surface, followed by a gradual decline in compressive stress with increasing depth. This phenomenon may be explained by the relatively lower tensile and yield strengths of STS316L in comparison to the other two materials, leading to greater compressive residual stress beneath the surface than at the topmost surface layer by shot peening. Moreover, the three specimens presented distinct residual stress distributions that were influenced by variations in surface roughness. Future studies should investigate the effects of surface roughness on residual stress profiles.
Fig. 5 comparatively illustrates the residual stress measurements obtained through the IIT method. For all three specimens used in this case, the residual stress on the top surface could not be easily measured due to the divergence caused by excessive stress. The residual stress distributions of STS303 and STS316L exhibited comparable patterns; however, STS410 displayed significant deviations in stress measurements near its surface. Notably, STS303 recorded an initial residual stress of −759 MPa at a depth of approximately 47 µm, whereas STS316L exhibited a residual stress of −546 MPa at the same depth. Both STS303 and STS316L demonstrated a gradual decrease in compressive residual stress, suggesting similar distribution characteristics. In contrast, STS410 revealed an initial tensile residual stress of +734 MPa at a depth of 47 µm, followed by a maximum compressive residual stress of −996 MPa at a depth of 270 µm. The compressive residual stress in STS410 also exhibited a tendency to decrease progressively with increasing depth. Importantly, STS410 displayed a distribution of tensile residual stress beneath the surface. This phenomenon can possibly be attributed to the indentation stress applied by the indenter of the IIT, which was relatively greater than the compressive stress generated by shot peening on the surface. This disparity may be due to the higher tensile and yield strengths, as well as the increased hardness, of STS410 compared to STS303 and STS316L.
Furthermore, during the preparation of the stress-free specimen, no treatment was conducted to remove internal stress. Consequently, the unprocessed raw material was employed to produce the stress-free specimen. Thus, polishing was not performed to maintain sufficiently constant surface roughness across the surface, indicating that the impact of surface roughness was also partially included. Therefore, obtaining reliable measurements posed significant challenges due to the pronounced variations in residual stress when contrasted with the two other materials. This issue necessitates further investigation in future research, as it appears to be intricately linked to the fabrication process of the stress-free specimen.
The residual stress measurements acquired through the HDM are comparatively presented in Fig. 6. In Fig. 6(a), the stress in the horizontal direction is illustrated relative to the orientation of the strain gauge attachment, whereas in Fig. 6(b), the stress is depicted in the vertical direction. In these graphs, the term X-stress refers to the residual stress measured along the vertical axis of the strain gauge during the HDM assessment, whereas Y-stress corresponds to the residual stress measured along the horizontal axis. Remarkably, the data revealed negligible differences in residual stress between the X and Y axes, indicating that shot peening was uniformly distributed across the specimens.
Moreover, the data collected from the top surface of the specimen prior to drilling represent the zero-point measurements. These initial values reflect the condition of the material when no hole has been created on the surface. Consequently, the value derived from the integral method, which originates from a depth of 17 µm, complicates the acquisition of reliable data from the top surface. Overall, the distributions of residual stress in both the X-axis and Y-axis directions, as measured by HDM, were not significantly different. Remarkably, compressive residual stress was nearly diminished, and stress relaxation was observed at a depth of approximately 200 µm. All three materials under investigation displayed similar trends.
An analysis of the residual stress distribution conducted through three distinct measurement techniques revealed that the influence of compressive residual stress induced by shot peening penetrated to a depth of roughly 400 µm across the three stainless steel variants employed in this study. Overall, each measurement method exhibited unique characteristics with respect to the accuracy of the final measurements. The precision of these measurements is contingent upon several factors, including the mechanical properties, measurement conditions, and surface roughness of the target material.
Thus, future studies should account for these variables when interpreting measurement results. Furthermore, the residual stress distributions as determined by XRD and the HDM indicated minor variations in depth; nevertheless, the overall patterns of residual stress distributions were closely aligned. In contrast, the IIT displayed notable discrepancies in residual stress values relative to the previously mentioned methods. As discussed earlier, this divergence can be attributed to the necessity for the IIT method to utilize a stress-free specimen for accurate measurement and comparison. In this regard, the fabrication of a specific stress-free specimen is necessary along with precision correction.
3.2 Comparison of Residual Stress Distributions According to the Measurement Methods for Each Material
Based on the findings discussed in Section 3.1, the residual stress distributions obtained from each measurement method for each material (STS303, STS316L, and STS410) are comparatively illustrated in Figs. 7–9, respectively. The assessment of residual stress for each material revealed certain errors, influenced by the measurement method employed (Section 3.1). In particular, the IIT measurements indicate that compressive residual stress consistently persisted in the depth direction across all three specimen types. This consistent observation may be attributed to the comparable energy values released following the indentation process, along with the effects of the previously mentioned stress-free reference specimen.
The presence of measurement errors across these methods suggests that each technique offers its own set of advantages and limitations (Guo et al., 2021; Schajer, 2013). While variations in residual stress distribution are apparent among the different materials, the stress distribution graphs indicate similar trends.
This observation implies that the discrepancies in residual stress distribution, which are dependent on the measurement method, stem from a variety of factors, including the mechanical properties of the materials, inherent material characteristics, and potential measurement errors. Therefore, future research should consider these variables for further investigations.
4. Conclusions
(1) For the XRD method, measuring residual stress beyond 400 µm in the depth direction was challenging due to difficulties in electropolishing. Overall, similar residual stress distributions were observed for STS303 and STS410. However, for STS316L, a somewhat dispersed residual stress distribution was noted.
(2) In the case of the IIT, measurements on the top surface were impossible due to the overestimation of surface stress for all three types of specimens. STS303 and STS316L exhibited maximum compressive residual stress beneath the top surface and showed similar distributions thereafter. However, STS410 displayed tensile residual stress beneath the top surface, followed by maximum compressive residual stress at approximately 270 µm from the surface. Its compressive residual stress tended to decrease with increasing depth. However, obtaining reliable measurements was difficult, as the residual stress distribution exhibited sharp variations compared to the other two materials. This issue requires further investigation, as it also appears to be related to the fabrication of the stress-free specimen.
(3) For the HDM, no significant differences were observed in residual stress values, and similar trends were obtained for all three materials. However, measuring residual stress on the top surface was challenging due to difficulties in drilling zero-point adjustment.
(4) The residual stress distributions obtained using the three measurement methods contained some errors depending on the technique employed. This suggests that each method has its advantages and limitations. Future studies should consider various factors, such as measurement conditions, applied materials, mechanical properties, and material characteristics.
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