Stainless Steel Components in High-Temperature Applications: Controlling Emissions with Niobium

Caio Pisano
on August 31, 2021
Car exhaust - how niobium can help to lower emissions

One of the hottest topics of the modern industry is the reduction of emissions and how all players can work together in the supply chain to lessen our impacts on the environment. If you are reading this article, you have probably already been exposed to some important actions of this strategy, such as the electrification of vehicles, smart cities infrastructure, de-materialization of constructions, green energy generation, green hydrogen initiatives, solid oxide fuel cells and many others.

Many technologies are being developed while many others are being optimized to support the achievement of this challenging goal. For this reason, this article aims to explore two different business segments that have one important similarity; both have operations in high temperatures, and the performance of the materials applied in these types of equipment is crucial for their proper operation.

The first of the segments above mentioned is Mobility. It is clear that sooner or later, most of the automotive fleet will be converted to battery electric vehicles (BEV) or hybrid electric vehicles (HEV). However, until then, many vehicles will continue to use ‘traditional’ internal combustion engines (ICE).

With that in mind, it’s important to highlight that it is possible to optimize the exhaust systems. One important aspect of this optimization is to work with higher temperatures in the hot endings as well as bringing the catalytic converter closer to the engine since the catalytic process will be more efficient and NOx emissions can be controlled.

Stainless steel exhaust system

The second market segment mentioned is Energy Generation, which is one of the most important and controversial segments due to the constant demand for energy for economies and societies to thrive. However, most of this generated energy comes from fossil fuels, like coal and natural gas.

Today, new technologies are consolidating themselves as feasible alternatives to conventional power generation (solar, wind, waves, hydrogen and others). However, until they are operational and efficient, fossil fuels will still be needed. Therefore, working to increase the efficiency of the process and control the emissions is crucial for our long-term goals. As in any thermal process, there are quite a few losses during the conversion of energy into electricity, and many publications have shown that working with higher temperatures and higher pressures for the saturated steam will affect in a very positive way the efficiency of the process.

These higher temperatures are known in the industry as advanced ultra-super critical (A-USC), and their range goes above 700°C. Working in such high temperatures can be complex for the materials, and some important issues could bring the equipment to failure. Creep and oxidation are the most important issues to be considered in such conditions. Therefore, the correct selection of materials is crucial for the safety and efficient performance of the heat exchangers and boilers. As well as in Mobility applications, stainless steels are the material class of choice for the Energy industry.

Schematics of a Ultra Supercritical (USC) Boiler

Schematics of a Ultra Supercritical (USC) Boiler. Source: Citation: Choi, S.M.; Park, J.S.; Sohn, H.-S.; Kim, S.H.; Cho, H.H. Thermal Characteristics of Tube Bundles in Ultra-Supercritical Boilers. Energies 2016, 9, 779.

Whenever exposed to extremely high temperatures, these stainless steels are also submitted to important load cases that can lead to failures, such as thermal fatigue, thermal oxidation and, as mentioned above, creep, which is the metallurgical issue that this article aims to address.

In a very simple way, the creep phenomenon is a time-dependent deformation that will affect metals, usually submitted to high temperatures and constant stresses. Depending on the temperature to which the components and materials are exposed, the stress needed to bring the material to failure may also vary, meaning that for higher temperatures, the condition is more extreme.

There are quite a few creep mechanisms, and they will be strongly dependent on the boundary conditions of the application (materials, temperature, load cases etc.), including the Grain Boundary Sliding deformation, which is the focus of this article and one of the main mechanisms for the temperature ranges that are involved in both applications (between 500°C and 1050°C).

Adding niobium (Nb) to the chemical composition of stainless steels can be crucial for producing good performance at these high temperatures, since Nb will precipitate along the grain boundaries and, in some cases, inside grains. These precipitates are in the form of Nb (C, N) or even in intermetallic phases, such as Laves Phase. They will pin these grains, making the sliding mechanism more difficult to take place. The temperature of dissolution of these precipitates will also be the bottleneck for the application, as once they dissolve, the sliding becomes facilitated.

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Many ongoing developments aim to increase the application temperature of these materials, by working in the chemical composition, microstructure, mechanical properties and processability. An important blocking point is the time and cost of validation. For example, the creep performance validation is a procedure that can take up to 100,000 hours per temperature to be representative for many applications, meaning more than 11 years of testing. So, working with a collaborative approach where all players of the manufacturing chain are involved can be more efficient and can bring fast results.

I like to consider this methodology as a Circular Approach since all players in the manufacturing chain are involved in such activities, from the raw material supplier to the OEM or energy plant developer. By following this mindset, costs can be shared, and emissions can be reduced, which mean that we can work to successfully achieve a more sustainable future for the planet.

If you liked the ideas presented in this article, join me in the webinar that will be held on the 11th of October, in which I will present and explore more the concepts behind these topics!

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