Study of welding parameters’ effects on residual stress and hardness in 316 stainless steel pipes: Experimental and analytical investigations

Abstract During service, residual stresses are generated as a result of plastic deformation induced by mechanical loads, thermal loads, or variations in the applied welding. Despite extensive research on welding processes, there remains a significant gap in understanding the combined effects of different welding parameters on the residual stresses and hardness of welded 316 stainless steel pipes. This study addresses this gap by systematically examining parameters of welding currents (90, 100, 110, 120, 140, and 160A), metal wire types (6010, 6013, and 7018), and wire diameters (3.2 and 2.25 mm), to provide a comprehensive analysis of their impact on mechanical properties. As part of the experimental work, arc welding, hardness tests, and X-ray diffraction tests were conducted. In different welding conditions, residual stresses increase as the hardness decreases, according to theoretical calculations based on X-ray diffraction analysis. It is observed that as residual stresses increase, hardness levels decrease, providing valuable insight into the relationship between these factors. In this systematic investigation, existing knowledge is extended.


Introduction
In some of manufacturing processes, the residual stresses result from exiting heat and mechanical forming process.Changes in geometrical shapes and properties have often produced residual stresses in materials (Akinlabi et al., 2018).To investigate the influence of various manual welding process parameters on the mechanical and microstructural properties of 316 stainless steel pipes, both experimental and theoretical studies have been carried out.Shielded metal arc welding, also known as stick welding, has involved melting and metals being joined by heating them using an electric arc formed between a rod-like metal and electrode.A welding cable has established a connection between the electrode holder and a terminal of the power source, and another cable has established a connection between the workpiece and the remaining terminal.Welding has involved three primary concerns: A solidified weld metal (W.M.) has consisted of base metals or base metals and filler metals, Heat-affected zone (HAZ) has been a region wherein the base metal has undergone heating at high temperatures but below the melting point, A base metal (B.M.) has been subjected to moderate heating or has remained unheated altogether (Handbook, 1993;Series, 1988).
In the realm of experimental investigation, Karadeniz et al. (2007) have underscored the significance of welding parameters on the depth of penetration in Erdemir 6842 steel, highlighting the direct correlation with welding current and the influence of arc voltage.The practical application of these findings is crucial for selecting optimal welding parameters to enhance penetration levels and overall weld quality in similar steel applications.Tseng and Hsu (2011) directed their study toward the effects of the activated tungsten inert gas welding process on 316 L stainless steel, exploring the impacts of various oxide fluxes on weld morphology, angular distortion, deltaferrite content, and hardness, thereby providing a comprehensive overview of weld quality determinants.The investigation by Ranjbarnodeh et al. (2015) on dissimilar TIG welds between low carbon steel and ferritic stainless steel has shed light on the importance of understanding residual stress origins to manage them effectively during welding, with special focus given to a thermomechanical model for prediction and control.
When it comes to modeling and theoretical insights, the relationship between welding parameters and residual stresses has been thoroughly explored.Wang et al. (1999) have documented how residual stresses can alter thermal expansion behavior, while Li (2016) utilized in-situ deflection and X-ray diffraction to investigate these stresses in oxide scale formation.Bastola et al. (2023) brought attention to the generation of residual stresses in metal additive manufacturing due to thermal energy and thermal cycling, emphasizing the complex interplay between thermal dynamics and material behavior.Kumar and Nagamani Jaya (2023) have added to this narrative by discussing the significant role of residual stresses in material reinforcement, indicating how these stresses induce compressive states that influence the mechanical behavior of reinforced materials.
The connection between welding parameters, residual stresses, and material hardness in 316 stainless steel pipes is a critical aspect that has been meticulously investigated in the present study.By utilizing shielded metal arc welding and varying welding currents and wire diameters, the study has examined how these parameters affect the residual stresses and hardness, using a combination of arc welding, hardness tests, and X-ray diffraction.The findings underscore the intricate relationship between welding conditions and the resultant mechanical properties of the welded components.
Furthermore, factors such as electrode type, welding technique, welder skill, and joint design are highlighted as determinants of weld quality.The implications of SMAW in producing high-quality welds are evident in its application in industries that demand impeccable welding standards, such as submarine hull construction and high-pressure pipeline fabrication.Cruz et al. (2020) have provided insights into the corrosion characteristics of SLM-316 L specimens under residual stresses, while Wu et al. (2020) leveraged machine learning algorithms to untangle the complex variables affecting residual stresses in different alloys.Chao et al. (2021) have examined the effect of heat treatments on residual stress in 316 L steel produced by selective laser melting, establishing a correlation with microstructural changes, which aids in optimizing structural designs.
In the quest for enhancing the microstructure and mechanical properties of 316 stainless steel, Guan et al. (2021) have implemented laser shock peening, revealing its efficacy in improving microdefects, microhardness, and introducing beneficial compressive residual stress.Conversely, Sander et al. (2021) found that rapid heating and cooling-induced residual stresses could lead to corrosion and pitting in 316 L stainless steel, although not affecting the passivity of the material.Finally, the study points to the essential nature of further research into the specific effects of activated welding parameters on SS 316, a material widely used across numerous industries, which informs subsequent practical applications and contributes to the broader knowledge base in welding science.

Preparation of welding samples
The samples of stainless steel 316 pipes were cut to exact dimensions 100 mm for each pipe diameter and thickness 8 mm, as shown in Figure 1, ground with a 45-degree angle with a grinder machine, and tested for fit or matching.Welding process was done according to AWS standard (American Welding Society Committee on Piping and Tubing, 1986), using arc welding.Wire types (6010, 6013, and 7018) and welding currents (90, 100, 110, 120, 140, and 160A) during the welding process.

Hardness test
The Rockwell test is conducted at an ambient temperature of 10°C to 35°C and is used to determine the depth of penetration of a 1/16-inch steel ball into the specimen.Initially, a minor load of 10 kg is applied for a duration of 10 s.Subsequently, a major load of 100 kg is employed to deepen the indentation, which is maintained for 15 s to ensure consistent results.Detailed information about the measured hardness can be found in ASTM E18 (ASTM, 2003)

X-ray diffraction and residual stresses calculations
Williamson-Hall (W-H) plots, which are also known as Warren-Averbach plots, are used to analyze broadening of diffraction peaks in X-ray or neutron diffraction data.It provides information about the material's microstructural parameters, such as the size of crystallites and microstrain.
The W-H plot is constructed by plotting the integral breadth (β) of the diffraction peak as a function of the scattering vector (4πsin(θ)/λ), where θ is the diffraction angle and λ is the wavelength of the incident radiation.Integral breadth represents the angular range over which the peak is observed and is related to the full width at half maximum (FWHM).In the W-H plot, the integral breadth is plotted against the scattering vector squared (s^2) on a linear scale.The plot typically exhibits a linear relationship.A linear relationship is represented by a slope equal to the inverse of crystallite size and an intercept equal to microstrain according to the Williamson-Hall equation.The Williamson-Hall equation is given by (Prabhu et al., 2014): Where: β is the integral breadth of the diffraction peak.θ is the diffraction angle.K is a shape factor related to the sample and instrument geometry.λ is the wavelength of the incident radiation.D is the crystallite size.ε is the microstrain.

Results and discussions
Residual stresses can be caused by the thermal expansion behavior of a uniform material under a gradient temperature (Ericsson, 1986;Kruth et al., 2010).Different thermal expansion coefficients in a multiphase material (Uebing et al., 2021) and different densities change due to phase transitions in the metal, leading to residual stresses.So that external and internal oxidation may cause growth stresses (Bertali et al., 2016;Jasim, 2007;Mohammad et al., 2022;Ochał et al., 2021;Tewari et al., 2010).An X-ray diffractometer was used to perform X-ray diffraction (XRD) with Cu Kα radiation at 1.5406 Å wavelength.The dimensions for the sample inspection are (50 mm long, 50 mm wide, and 8 mm thick), angles of incidence ranged from 10° to 90°.Results of XRD analyses are based on scattering angles, wavelengths, and incident angles of X-ray beams on crystalline specimen surfaces.Figure 2 illustrates the X-ray diffraction (XRD) patterns of eight samples that were exposed to varying welding parameters at welding magnitude (W.M.).Specifically, the analysis includes the following configurations: Sample A with wire 6010 at a diameter of 3.2 mm and a current of 90A, Sample B with the same wire and diameter at a current of 100A, Sample A-wire 6010, d=3.2mm, current=90A B-wire 6010, d=3.2mm, current=100A C-wire 6010, d=3.2mm, current=110A H-wire 7018, d=3.2mm, current=160A It is crucial to acknowledge that the correlation between hardness and residual stresses exist in a welded joint is complex and can vary depending on the specific welding conditions and material properties.Experimental measurements, such as hardness testing and nondestructive evaluation techniques, are typically conducted to assess the presence and distribution of residual stresses present in welded joints, according to the resultant for different welding current, welding wire types, and wire diameters, Figure 6 shows that when the hardness increase the residual stresses decrease.Welding currents ranging from 90A to 160A require an understanding of residual stress measurements.X-ray diffraction analysis was used to measure residual stress.A consistent relationship was observed between residual stresses and hardness in welded components.Stress distribution and structural implications are provided by these measurements.
Temperature-induced precipitation-hardened overaging can dissolve precipitates or coarsen their microstructure over time.This phenomenon can reduce the material's strength but may also increase its hardness.Welded joints also experience residual stresses that can be reduced as they age.There are several factors that influence the relationship between residual stress and hardness.To understand the behavior of residual stresses and hardness in welded joints, it is crucial to consider the specific circumstances and conduct appropriate testing or analysis.A-wire 6010, d=3.2mm, current=90A B-wire 6010, d=3.2mm, current=100A C-wire 6010, d=3.2mm, current=110A E-wire 6013, d=2.25mm, current=110A D-wire 6013, d=2.25mm, current=90A F-wire 6013, d=2.25mm, current=120A H-wire 7018, d=3.2mm, current=160A (Continued)

Conclusions
This study presents novel findings on the impact of welding parameters on the hardness distribution of 316 stainless steel weldments.Our results indicate that a welding current of 100A results in notable changes in hardness, albeit less pronounced than at 110A.We have discovered that the hardness in the weld metal (W.M.) can reach 230 HR at 90A and 100A currents, contrasting with the minor changes observed at 110A.Significantly, at 140A and 160A, hardness levels escalate to 242 HR and 260 HR, respectively, underscoring the strong influence of welding current.With wire type 6013 and a 90A current, there is a marked hardness increase, peaking at 302 HR.Additionally, using wire type 6010, an extensive hardness distribution becomes evident, especially at 90A, revealing a hardness upsurge in the W.M. up to 230 HR.Contrarily, the combination of 90A current with wire type 7018 either yields unsatisfactory results or proves to be infeasible, emphasizing the challenges of this specific parameter set.The study elucidates a clear inverse correlation between residual stress and hardness, offering critical insights for optimizing the welding process to tailor the mechanical properties of welded components.
Figure 1.Flow chart of stainless steel 316 pipe welding.

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Figure (Continued)

Figure 3 .
Figure 3.The distribution of hardness in welding joints made with wire 6010.

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Figure 4.The distribution of hardness in welding joints made with wire 6013.

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Figure 5.The distribution of hardness in welding joints made with wire 7018.

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Figure 6.An analysis of the relationship between residual stress and hardness in welded joints.