Therma 4622Nb^®: where smart Niobium (Nb) alloying makes the difference for high temperature performance of a new ferritic stainless steel

The stainless steel industry has been quite dynamic on the development of innovative and disruptive solutions over the last two decades. Just as a reference, the global production of stainless grades was around 20 million tonnes in the year 2000 and the expectation is that more than 60 million tonnes will be produced between January and December 2024.

Stainless steel coils with Niobium alloying technology to be used in high temperature applications

During these growth years, many emerging players developed their own capacities to produce conventional and commodity grades, bringing the consumption of stainless steels up. At the same moment, the market leaders continued to invest in R&D and kept on the vanguard of technology and innovation, adding new stainless steel alloys to their portfolio.

The implementation of these high-performance alloys enabled the development of important sectors, like the automotive and energy generation industries. For example, internal combustion engine vehicles that have exhaust systems operating in extreme temperatures (>1000°C) can be quite efficient in the catalytic conversion process, reducing the impact of exhaust gases in the environment, while power-plants operating with higher temperatures can also be more thermal efficient, using less fuel to produce energy and therefore generating less carbon emissions.

Within this context, CBMM and Outokumpu have been working together for many years on the development of new alloys and applications stainless steels containing Niobium (Nb). The latest example of this partnership was the development of the Therma 4622Nb^® which is a high-chromium ferritic stainless steel with smart Nb alloying, enabling its application in extreme temperature environments and providing a competitive alternative to Ni-alloyed austenitic grades and Mo-alloyed acid proof grades.

Usually, austenitic stainless steels tend to have higher mechanical properties (absolute values) in high temperatures when compared to ferritic stainless steels, however Niobium (Nb) technology can be a quite effective way to enhance the performance of ferritic stainless steels in such environments. The smart Nb alloy design promotes the formation of some Nb-rich intermetallic phases that will precipitate in the grain boundaries, creating a pinning effect and avoiding creep issues, losses on mechanical properties and other detrimental effects to the application. The chart below brings the SAG Test results for 1.5 mm samples:

Bar chart comparing deflection of Therma 4828, Core 4509, and Therma 4622Nb stainless steels at temperatures of 850°C, 900°C, and 950°C.

In such high temperatures, the mechanical integrity of the components is crucial for a safe operation, therefore the alloy design with Nb and few other elements will have an important role of strengthening due to solid solution, brining an additional performance to this alloy and providing an important alternative to conventional grades, such as AISI 304/304L.

Thinking about the application opportunities for the Therma 4622Nb^® one of the main effects that Niobium (Nb) additions can bring to ferritic stainless steels is the possibility of controlling the crystallographic texture during production process, promoting the formation of preferential fibres that will enhance the deep drawability of this material and that’s how Therma 4622Nb^® can be formed using typical processes including folding, bending, and drawing.

Manufacturing process where the steel is rolled to get in the thickness of the coil plates still at the manufacturing stage.

When operating in high temperatures, ferritic stainless steels can face problems connected to the sensitization effect, leading to intergranular corrosion. Sensitization will happen when chromium carbides (Fe, Cr)23C6 precipitate in the regions adjacent to the grain boundaries (in temperatures between and 450°C 850°C), resulting in depletion of chromium which can directly affect the passivation of the steel and therefore its corrosion resistance. Nb-technology will help mitigating this problem, as Nb will react with C and N forming a different kind of carbides or nitrides, Nb (C,N), and leaving Cr free to passivate the material and open the possibility for Therma 4622Nb^® be easily welded with conventional methods.

In short, this new ferritic stainless steel offers excellent high-temperature properties, cost stability, excellent creep performance and corrosion resistance compared to equivalent nickel-alloyed austenitic grades. This outstanding performance opens the door to the application in extreme temperature exhaust systems, heat exchangers, furnaces, annealing boxes, air-heaters and many more energy related components.

CBMM and Outokumpu will continue to work together for the development of new Nb-bearing stainless steels that will support the transition to a more sustainable future.

Therma 4622Nb®: where smart Niobium (Nb) alloying makes the difference for high temperature performance of a new ferritic stainless steel

Therma 4622Nb^®: where smart Niobium (Nb) alloying makes the difference for high temperature performance of a new ferritic stainless steel