Performance of Dissimilar Metal Welds Under Cyclic Loading and Corrosive Environments
welds (DMWs).
Nickel-based dissimilar metal welds (DMWs) are critical components in many industrial applications, including aerospace, automotive, nuclear power, and marine engineering. These welds are often subjected to harsh conditions, including cyclic mechanical loading, which significantly impacts their fatigue life. Understanding the fatigue behavior of DMWs under cyclic loading is essential for predicting their long-term performance and ensuring the structural integrity of systems that utilize these welds (Mouginot, & Hänninen, 2013).
Dissimilar metal welds (DMWs) are commonplace in power plants. Atomic interdiffusion between the alloys often has a profound impact on the microstructure of the weld region, and hence on the mechanical properties. There is also the effect of heat in put during the welding process, which can lead to microstructural changes and the formation of undesirable phases. These effects, among others, have been observed to cause premature failure of steam containers at the joints (Maurya, Pandey, & Chhibber, 2021).
Cyclic mechanical loading refers to repeated application of stress or strain on a material over time, causing progressive and localized structural damage known as fatigue. In DMWs, fatigue failure is primarily attributed to the accumulation of microcracks that initiate at stress concentration points and propagate with each loading cycle. Nickel-based DMWs, due to their excellent high-temperature strength and corrosion resistance, are widely used in applications where they experience cyclic mechanical loads (Clark, 2015).
The fatigue life of these welds is largely dependent on the interaction between the base metals, filler materials, and the welding processes involved. Nickel-based alloys are often joined to steels or other metals with differing mechanical and thermal properties, creating a zone of mismatch at the weld interface. This mismatch in physical properties can lead to stress concentration and thermal fatigue, which are exacerbated by cyclic mechanical loading (Ramirez et al., 2020).
The microstructure of nickel-based DMWs plays a critical role in their fatigue performance. Cyclic loading affects the weld region, heat-affected zone (HAZ), and base metals differently due to the microstructural heterogeneity at the dissimilar weld interface. Nickel-based welds typically have high ductility and resistance to oxidation, but their fatigue life can be compromised by brittle phases that form at the weld interface, such as carbides or intermetallic compounds (Brayshaw, J., Cooper& Sherry, 2019).
Fatigue cracks in nickel-based DMWs initiate in regions of microstructural discontinuities, including grain boundaries and precipitate-rich areas (Savoie et al., 2021). Additionally, the presence of residual stresses from the welding process can act as a catalyst for crack initiation under cyclic mechanical loads. These residual stresses, coupled with thermal expansion mismatches between the dissimilar metals, make DMWs particularly susceptible to fatigue-related failures (Bourgeois, 2015).
In many industrial applications, DMWs experience combined thermal and mechanical cyclic loading, which compounds the fatigue damage. For instance, in power plants or aerospace engines, components made from nickel-based alloys may be exposed to high temperatures, causing thermal cycling along with mechanical loading. This combination leads to thermal fatigue, which significantly reduces the fatigue life of the welds (Zhao et al., 2023).
Thermal cycling causes expansion and contraction of the materials, further enhancing stress concentrations at the weld interfaces, particularly at the dissimilar metal junction. When combined with mechanical cyclic loading, this increases the rate of crack initiation and propagation, reducing the fatigue resistance of the DMWs. Studies show that nickel-based DMWs exhibit reduced fatigue strength under these combined loadings compared to cases of purely mechanical cyclic loading (Laha et al., 2017).
Defects introduced during the welding process can also significantly influence the fatigue life of nickel-based DMWs. Common weld defects include porosity, lack of fusion, incomplete penetration, and inclusions, all of which act as stress concentrators. These defects can accelerate crack initiation and growth under cyclic loading, leading to premature fatigue failure (Li et al., 2023).
According to Gao et al. (2019) weld defects in nickel-based DMWs, particularly at the interface of dissimilar metals, can reduce fatigue life by up to 30%. The study highlighted the need for improved welding techniques and quality control to minimize such defects and enhance the fatigue performance of DMWs in critical applications.
To enhance the fatigue life of nickel-based DMWs under cyclic mechanical loading, several strategies have been proposed. Post-weld heat treatment (PWHT) is one common method used to reduce residual stresses and improve the microstructural uniformity at the weld interface. PWHT has been shown to mitigate the formation of brittle phases and enhance the fatigue resistance of DMWs (Shankar et al., 2020).
In addition to PWHT, advanced welding techniques such as laser welding and friction stir welding (FSW) have been employed to reduce the occurrence of weld defects and improve the mechanical properties of the weld zone. These techniques offer more precise control over the heat input, minimizing residual stresses and enhancing the fatigue performance of nickel-based DMWs.
The impact of cyclic mechanical loading on the fatigue life of nickel-based DMWs is a critical consideration in industries where these welds are exposed to repeated stress cycles. The fatigue life of these welds is influenced by microstructural factors, residual stresses, weld defects, and the synergy between mechanical and thermal loadings. By addressing these factors through improved welding techniques and post-weld treatments, the fatigue resistance of nickel-based DMWs can be significantly enhanced, ensuring their long-term reliability in demanding applications (Bourgeois, 2015).
2.2 To investigate the effects of corrosive environments on the degradation mechanisms of DMWs, including stress corrosion cracking (SCC) and galvanic corrosion.
Dissimilar metal welds (DMWs) are critical components in various industrial sectors, such as aerospace, automotive, nuclear power plants, and marine engineering, due to their ability to join metals with different properties. However, the integrity of DMWs is often compromised by corrosive environments, leading to the initiation of degradation mechanisms such as stress corrosion cracking (SCC) and galvanic corrosion. These degradation phenomena are particularly challenging in industries where DMWs are subjected to cyclic loads, temperature fluctuations, and aggressive chemical environments. Understanding the effects of corrosive environments on the degradation mechanisms of DMWs is essential for ensuring their long-term reliability and performance (Okonkwo et al., 2019).
Corrosive environments, including aqueous media, high-temperature atmospheres, and acidic or alkaline conditions, accelerate the degradation of materials used in DMWs. The complex nature of DMWs, arising from the metallurgical differences between the joined metals, creates microstructural heterogeneities that can act as initiation points for corrosion. In addition, the formation of intermetallic compounds at the weld interface further contributes to the susceptibility of DMWs to corrosion (Liang, 2009).
The degradation of DMWs in corrosive environments is influenced by several factors, including the nature of the base metals, the composition of the filler material, the welding process, and post-weld heat treatments. The interplay of these factors determines the resistance of DMWs to corrosion and their long-term stability (Ahonen, 2015).
Stress corrosion cracking (SCC) is a major degradation mechanism in DMWs, characterized by the propagation of cracks under the combined influence of tensile stress and a corrosive environment. SCC is often observed in environments containing chlorides, sulfides, or caustic solutions, which can penetrate the protective oxide layer on metal surfaces and initiate cracks (Maurya, Pandey, & Chhibber, 2021) .
In DMWs, SCC is particularly problematic due to the mismatch in mechanical properties between the dissimilar metals. The differing thermal expansion coefficients and yield strengths of the base metals create localized stresses at the weld interface, promoting crack initiation. Additionally, the presence of residual stresses from the welding process exacerbates the likelihood of SCC. Research has shown that high-alloy steels and nickel-based alloys commonly used in DMWs are susceptible to SCC, especially in chloride-rich environments (Benedetti, Orempuller, Russo, Fontanari, & Rossi, 2023).
Studies on SCC in DMWs have highlighted the role of microstructural features such as grain boundary sensitization and the presence of secondary phases in promoting crack initiation and propagation. For instance, sensitized stainless steels in DMWs are vulnerable to intergranular SCC due to the formation of chromium-depleted zones at the grain boundaries. Similarly, nickel-based welds are prone to transgranular SCC under the influence of tensile stresses and aggressive corrosive media (Lahiri, 2017).
Galvanic corrosion is another significant degradation mechanism in DMWs, occurring when two dissimilar metals are electrically coupled in the presence of an electrolyte. The electrochemical potential difference between the metals drives the corrosion of the anodic metal, while the cathodic metal is protected. In DMWs, the difference in electrochemical potentials between the base metals and the filler material can accelerate the degradation of the less noble metal (Al-Elyani, 2015).
The severity of galvanic corrosion in DMWs is influenced by factors such as the surface area ratio between the dissimilar metals, the conductivity of the electrolyte, and the presence of passivating layers on the metals. For example, in seawater environments, the combination of stainless steel and carbon steel in a DMW can lead to severe galvanic corrosion of the carbon steel, compromising the structural integrity of the weld (Rathore et al., 2024).
Recent research has focused on developing strategies to mitigate galvanic corrosion in DMWs, such as the application of protective coatings, the use of corrosion inhibitors, and the design of welds with reduced galvanic coupling. In addition, the selection of compatible base metals and filler materials with similar electrochemical properties has been shown to reduce the risk of galvanic corrosion in DMWs (Rosseel et al., 2022).
In many cases, SCC and galvanic corrosion act synergistically, leading to accelerated degradation of DMWs. The localized anodic dissolution caused by galvanic corrosion can create stress concentration sites that promote crack initiation in SCC. Similarly, the propagation of SCC cracks can expose fresh metal surfaces, exacerbating galvanic corrosion. This interaction is particularly detrimental in harsh environments, such as marine and chemical processing industries, where both SCC and galvanic corrosion are prevalent (Raj, & Mudali, 2006).
Corrosive environments pose a significant challenge to the longevity and performance of DMWs by promoting degradation mechanisms such as stress corrosion cracking and galvanic corrosion. The complex interplay of metallurgical factors, environmental conditions, and mechanical stresses makes DMWs highly susceptible to these forms of degradation. To mitigate these effects, ongoing research is focusing on improving the design and material selection for DMWs, optimizing welding processes, and developing corrosion-resistant coatings and inhibitors. Understanding the mechanisms of SCC and galvanic corrosion, and their interaction, is crucial for developing effective strategies to enhance the durability of DMWs in corrosive environments (French, 2002).
Dissimilar Metal Welds (DMWs) are frequently used in critical industries such as aerospace, nuclear power plants, marine engineering, and automotive sectors due to the need for joining different materials with distinct properties, such as stainless steel and nickel alloys. However, DMWs are particularly vulnerable to degradation when exposed to corrosive environments, especially in applications where components are subjected to aggressive chemical and mechanical stressors. Stress Corrosion Cracking (SCC) is a deterioration mechanism that occurs when tensile stress, whether residual or applied, combines with a corrosive environment, leading to the initiation and propagation of cracks. SCC is particularly dangerous in DMWs because it is often not detectable until significant damage has occurred, sometimes leading to catastrophic failures.
SCC in DMWs can be driven by the interaction of localized tensile stresses (arising from welding or operational conditions) and corrosive agents such as chloride ions, caustic environments, or high-temperature water in nuclear reactors. These environmental stressors weaken the grain boundaries of welds, especially in regions with phase transitions or microstructural heterogeneity, such as the heat-affected zones (HAZ) near the weld interface Das, Kumar, Sahu, & Gollapudi, 2022).
2.3 What are the strategies for optimizing welding practices, material selection, and maintenance approaches to improve the long-term performance of DMWs in industrial applications?
Dissimilar metal welds (DMWs) are a critical aspect of numerous industrial applications, including aerospace, nuclear power plants, marine engineering, and automotive industries. The joining of different metals, which often possess varying mechanical properties and thermal expansion rates, poses significant challenges in terms of weld integrity and long-term performance. Failure of DMWs can lead to catastrophic consequences in safety-critical industries, making it essential to optimize welding practices, material selection, and maintenance strategies to enhance the reliability and longevity of these joints (Tasalloti Kashani, 2017).
Welding processes play a pivotal role in the integrity of DMWs. Different welding techniques have been developed and refined to mitigate the risks of cracking, thermal stress, and metallurgical incompatibilities. Techniques like gas tungsten arc welding (GTAW), laser beam welding (LBW), and friction welding are widely applied in DMW fabrication due to their ability to produce high-quality joints with minimal thermal distortion (DuPont et al., 2013).
Controlled heat input is essential to prevent thermal mismatch and residual stresses, which can lead to cracking and degradation. Research has shown that preheating and post-weld heat treatment (PWHT) can significantly reduce residual stress by relieving thermal gradients between the base metals. Additionally, the use of narrow gap welding techniques has been identified as a way to control dilution and limit the mixing of dissimilar metals, reducing the formation of brittle intermetallic phases (Lautre, 2024).
Filler material selection is another strategy to optimize weld performance. Utilizing nickel-based filler metals like Inconel 82/182 has proven effective in mitigating thermal expansion mismatches and reducing the likelihood of stress corrosion cracking (SCC). This approach balances the metallurgical properties of the base metals, enhancing the overall durability of the weld joint (Issler et al., 2004).
The selection of materials for DMWs directly impacts their long-term performance, especially in environments exposed to thermal cycling and corrosive conditions. DMWs often join materials such as carbon steel, stainless steel, and nickel alloys, where differences in corrosion resistance and mechanical behavior must be carefully managed (David et al., 2013).
Nickel-based alloys, particularly those with high chromium content, are frequently chosen for their superior corrosion resistance, mechanical strength, and ability to handle thermal expansion differences. These alloys can reduce the risk of galvanic corrosion when paired with dissimilar metals, especially in high-temperature and high-pressure environments (Mayr et al., 2019).
Recent advancements in materials science have led to the development of advanced high-strength steels (AHSS) and high-entropy alloys (HEAs), which offer enhanced mechanical properties and better resistance to stress and thermal fatigue. These materials are gaining attention for their potential use in DMWs, particularly in applications where conventional materials might fail due to aggressive operating conditions (Mayr et al., 2019).
To mitigate the challenges posed by metallurgical incompatibility, the use of transition layers or buttering layers has been explored. These layers act as a buffer between dissimilar metals, minimizing the formation of brittle phases and reducing the thermal expansion mismatch. Studies have shown that materials like nickel-based alloys used as buttering layers can significantly improve the mechanical and corrosion properties of DMWs (Mayr et al., 2019).
Maintenance is crucial in preserving the long-term performance of DMWs, particularly in industries where they are subjected to cyclic loads, corrosive environments, and high temperatures. The implementation of condition-based monitoring systems, such as non-destructive testing (NDT) techniques, allows for early detection of defects such as cracks, corrosion, and stress corrosion cracking (SCC) (Mayr et al., 2019).
Phased array ultrasonic testing (PAUT) and radiographic testing (RT) have proven effective in identifying subsurface defects and microstructural changes in DMWs. Additionally, eddy current testing (ECT) and magnetic particle inspection (MPI) are widely employed for detecting surface flaws and corrosion in welds. By integrating these NDT techniques into regular maintenance schedules, industries can predict failures before they occur, enabling timely interventions that extend the service life of DMWs (Raj, & Mudali, 2006).
Corrosion protection methods, including coatings and cathodic protection, are essential for mitigating the effects of corrosive environments. Protective coatings, such as thermal-sprayed ceramic coatings, provide a physical barrier that prevents the interaction of corrosive agents with the metal surfaces, while cathodic protection reduces the electrochemical potential of the weld, inhibiting galvanic corrosion. Stress-relief techniques, such as shot peening and laser shock peening, have also been explored as maintenance strategies to enhance the mechanical resilience of DMWs. These techniques introduce compressive stresses into the weld region, reducing the likelihood of fatigue cracking and improving the overall fatigue life of the joint (Tasalloti Kashani, 2017).
Optimizing the long-term performance of DMWs requires a comprehensive approach that integrates advanced welding practices, judicious material selection, and proactive maintenance strategies. Through the use of controlled heat input, suitable filler materials, and innovative techniques like transition layers, the metallurgical and mechanical challenges of DMWs can be effectively mitigated. Additionally, adopting advanced materials such as nickel-based alloys and implementing condition-based maintenance strategies like NDT and corrosion protection can significantly extend the service life of DMWs in industrial applications. As industries continue to demand higher performance from their materials and structures, further research into these optimization strategies will be critical for ensuring the reliability and safety of DMWs in the long term (David et al., 2013).
