Beyond Strength-ductility Enhancement by Additive Manufacturing: Advanced Low-cycle Fatigue Performance by Synergy of WAAM Fabrication and Precision Heat Treatment

Modified 9Cr-1Mo steel has been a workhorse in industries that demand tough, high-temperature materials, especially nuclear power. This type of steel is chosen because it holds up well under extreme conditions—it’s truly resistant to stress corrosion and performs reliably at high temperatures, which makes it ideal for essential parts like reactor vessels and heat exchangers. However, even a material as strong as modified 9Cr-1Mo faces challenges when subjected to the intense, repetitive cycles of stress and temperature fluctuations found in these environments. Over time, this back-and-forth stress weakens the material, causing tiny cracks that eventually lead to failure. This process, called low-cycle fatigue (LCF), is a major issue for critical parts. So, extending the fatigue life of modified 9Cr-1Mo steel is essential if we want to ensure the safety and longevity of these components. Recently, wire-arc additive manufacturing (WAAM) has emerged as a promising way to make large metal parts layer-by-layer. WAAM has clear advantages over traditional methods—it’s fast, efficient, and uses less material. But there’s a catch: WAAM’s rapid heating and cooling process can create inconsistencies in the steel’s microstructure. These irregularities could affect how well the steel stands up to the repetitive stresses it encounters in real-world applications. Given that fatigue resistance is critical, there’s a need to figure out if the steel made by WAAM can perform as well as traditionally manufactured steel. To address this, Professor Shouwen Shi along Xiaomei Liu, Gaoyuan Xie, Xu Chen from the School of Chemical Engineering and Technology at Tianjin University took a closer look. In their recent study, published in the International Journal of Fatigue, they investigated whether specific heat treatments applied after the WAAM process could enhance the fatigue resistance of modified 9Cr-1Mo steel. By experimenting with various heat treatment protocols, they hoped to make the microstructure of WAAM-produced steel more uniform, reducing internal stresses that might otherwise lead to cracking. They aimed to pinpoint the most effective heat treatment method to make WAAM-produced modified 9Cr-1Mo steel as durable and fatigue-resistant as possible. Their goal was not just to explore WAAM as a potential alternative for producing modified 9Cr-1Mo steel but to show that it could be a reliable option for creating parts that need to endure intense cyclic loading over long periods. Their work could open the door for WAAM to be widely used in nuclear and other industries where parts need to withstand high stress and long service lives.

To explore whether WAAM and specific heat treatments could improve the fatigue resistance of modified 9Cr-1Mo steel, the researchers conducted a series of carefully controlled experiments, focusing on both the steel’s structure and mechanical performance. They began by producing samples through the WAAM process, using a specialized welding wire (ER90S-B9). Building each sample took 105 layers, creating slabs that measured roughly 300 mm by 273 mm by 29 mm. They closely controlled settings like wire feed rate, temperature, and current to keep the microstructure uniform since any inconsistencies would affect how well the steel holds up under stress. This approach created a distinct microstructure, with the steel showing finer martensitic laths than what’s typically seen in hot-rolled steel, an effect of the fast heating and cooling cycles in WAAM. With the WAAM samples prepared, the team tested three different heat treatments—labeled HT1, HT2, and HT3—to see how each would affect the steel’s microstructure and strength. Each treatment involved cycles of normalizing and tempering, but HT2 and HT3 had additional tempering steps to refine the structure further. This approach worked especially well with HT2, which notably reduced grain size without changing the martensite lath width. This finer grain structure was expected to make the steel more resistant to fatigue since stronger grain boundaries help prevent cracks from forming and spreading under repeated stress. Next, the researchers examined each sample’s tensile strength at 350°C, a temperature relevant to many industrial applications. Here, they observed that WAAM samples, overall, had a higher cyclic stress response than steel made through conventional processes. But the HT2-treated samples really stood out—demonstrating a 70% improvement in fatigue life at a strain amplitude of 0.5% compared to the baseline HT1-treated samples. This increase in fatigue life seemed to be connected to the effects of the HT2 treatment on the steel’s grain boundaries, making them tougher and more resistant to deformation and crack initiation. To better understand the steel’s behavior under cyclic loading, they ran low-cycle fatigue tests at different strain amplitudes. Predictably, all samples softened over time—a common trait for martensitic materials—as repeated stress gradually turns their lath structures into equiaxed subgrains, lowering dislocation density. However, the WAAM samples, especially those treated with HT2, held their fatigue life better than expected, performing on par with or even surpassing traditional hot-rolled steel. The refined and consistent structure from the HT2 treatment appeared to stabilize dislocation movements within the material, enhancing fatigue resistance by making cracks less likely to form, even under repeated strain. Moreover, the authors used scanning electron microscopy and electron backscatter diffraction to analyze how the different treatments influenced crack formation and spread. In HT2-treated samples, they saw that cracks usually started along slip bands and grain boundaries. However, the refined structure in these samples slowed down the cracks’ advance. The cracks encountered smaller, denser grains that acted as barriers, limiting their path and extending the steel’s fatigue life. Compared to samples treated with conventional methods, which saw cracks progressing more quickly through coarser grain boundaries, the HT2-treated WAAM samples demonstrated a significant advantage in slowing down crack growth, indicating the potential of this treatment approach for enhancing the steel’s durability under cyclic stress.

The research work of Professor Shouwen Shi and colleagues is exciting because it opens up a new way of thinking about how we make steel parts that need to handle intense, repetitive stress over long periods of time. By combining a specialized type of 3D printing, called WAAM, with carefully tuned heat treatments, the researchers found a way to produce steel that’s not only strong but also much better at resisting the cracks and wear that usually develop with repeated use. This combination could make a big difference in industries like nuclear power or aerospace, where even small improvements in durability can mean fewer part replacements, less downtime, and much lower maintenance costs. We think what’s especially promising here is that WAAM, along with a tailored heat treatment like HT2, might actually be able to replace more traditional manufacturing methods without losing any of the quality. Usually, making large and complex steel parts the old-fashioned way creates a lot of material waste and requires a significant amount of time. WAAM can streamline this whole process, and with the added heat treatment, it produces a structure in the steel that’s just as resilient—if not more so—than what you get with conventional steel. The new approach fits well with today’s focus on sustainable manufacturing because it cuts down on waste and costs while delivering high-quality, long-lasting materials. Beyond the immediate benefits, we believe the work of Professor Shouwen Shi et al could have a ripple effect across different materials and industries. By showing how to create a refined structure in steel that’s highly resistant to fatigue, the researchers have set a foundation for exploring similar techniques in other metals and applications. Over time, this work could help set new standards in manufacturing, showing how we can make materials that are tougher and last longer, ultimately raising the bar for what’s possible in high-stress environments.