Considering the current global challenges such as global warming and climate change, more and more stringent rules and regulations are being imposed on the use of fossil fuels to reduce the global carbon footprint. The idea majors on developing alternative renewable sources of energy while at the same time improving energy efficiency. In particular, advanced ultra-supercritical thermal power units have exhibited great potential for reducing carbon dioxide emissions and improving energy efficiency. They use dissimilar metal welds between nickel-based alloys and 9-12%Cr martensitic heat-resistant steels which are applied in the high and low temperature parts， respectively. Therefore, it is necessary to understand the creep strength behaviors of dissimilar metal welds to enhance the durability of these thermal units.
Recently, Yu Zhang, Dr. Kejian Li, Dr. Zhipeng Cai and Jiluan Pan from Tsinghua University investigated the creep rupture behaviors and rupture mechanisms of the dissimilar metal weld between Inconel 617B and modified 9%Cr martensitic heat-resistant steel. The main objective was to determine the crack modes of dissimilar metal welds at different temperature-stress combinations to provide a comprehensive understanding of their microstructure characteristics and creep failure behaviors. The work is published in the journal, Materials Science and Engineering: A.
Creep tests were generally carried out at a temperature range of 600-620
°C and stress range of 130-240MPa. Consequently, different methods such as optical microscopy were used to examine the creep rupture behaviors and microstructure characteristics under different creep conditions.
The authors observed that the rupture conditions varied under different creep conditions. For instance, at higher temperatures the crept dissimilar metal welds ruptured along with the interface between weld metal and 9%Cr steel while at lower temperatures, they ruptured in 9%Cr base metal under high-stress level and inter-critical heat-affected zone of 9%Cr steel or interface between weld metal and 9%Cr steel under low-stress level. The rupture behavior in the 9%Cr base metal was mainly controlled by plastic deformation in which the growth of the formed dimples eventually led to transgranular fracture. On the other hand, the creep fracture in the inter-critical heat-affected zone of 9%Cr was as a result of the type IV crack. This type of crack was caused by matrix softening and lack of sufficient precipitation at the grain boundaries which led to brittle fracture.
Furthermore, it was worth noting that the creep rupture at the interface was initiated and propagated by the oxide notch. This further resulted in the interfacial failure which was also associated with stress concentration resulting from a mismatch of the thermal expansion coefficients and the difference in creep strength. Unlike previous studies, type I carbides were not observed in the interfacial region, therefore, raising questions on its alleged contribution to interfacial failure. Additionally, the Inconel weld metal exhibited higher oxidation resistance as compared to 9%Cr steel due to higher chromium content.
Based on the results, we can conclude that the research team successfully explored the creep rupture properties of the dissimilar metal welds between Inconel 617B and modified 9%Cr martensitic steel. The research team believes their approach could be extended to other types of materials and alloys and thus the study will play a significant role in developing advanced thermal power units.
Zhang, Y., Li, K., Cai, Z., & Pan, J. (2019). Creep rupture properties of dissimilar metal weld between Inconel 617B and modified 9%Cr martensitic steel. Materials Science and Engineering: A, 764, 138185.