When something so disruptive happens that hits daily life in unpredictable ways, adaptability becomes a key element, especially when it comes to business. We have seen many people adopting smart working, as face-to-face meetings and business trips have been replaced by frequent video calls. Business hasn’t stopped; it has merely adapted to the current circumstances.
While the length of the journey is uncertain, it’s very important and wise to use this period for self-improvement and to develop our skills. While the physical space available to us has been reduced with the lockdown of many cities, time seems to have increased. We should use it in the best possible way.
One idea might be to improve our knowledge in certain areas. An example from our industry might be how to undertake effective preliminary material selection in high-temperature applications.
Facing High-Temperature Corrosion Challenges with the Right Material
How much do you think you know about high-temperature corrosion, and what does “high temperature” really mean? As you may know, the Sandvik portfolio has a wide range of tubes designed exactly for this purpose. In order to help you avoid getting lost in a list of grades and international standards, we are here to guide and support you.
High-temperature conditions can be found in a wide variety of industries, including: heat treatment, metals, petrochemical, renewable energies and measurement systems. The very first step is to understand if your steel is required to work at a temperature equal to or higher than 932°F (500°C). This is the so-called crossover temperature, when gases in the atmosphere surrounding the steel start diffusing into it through the protective oxide layer while simultaneously bonding with the element, generating different corrosion mechanisms.
It’s known that stainless steels are characterized by a protective chromium oxide layer. So when oxygen, carbon and nitrogen diffuse into this protective “film,” they bond with the chromium element to form oxides, carbides and nitrides. In the case of oxidation, there is an over-formation of chromium oxide. The layer will not stick to the surface anymore and shows up as scaling (Fig. 4).
In the other cases, the precipitation of carbides and nitrides depletes the chromium from the protective oxide layer. This is the main characteristic of a stainless steel and will affect its main properties. On the other hand, sulfur is an element that will easily bond with nickel, forming nickel sulfides. All these compounds are detrimental to material quality and affect its corrosion properties.
The second important characteristic to consider is the creep or hot deformation. Beyond 500°C, proof strength and tensile strength diminish, so it’s important to think about how the material will withstand creep (strength and rupture). This deformation is not immediate and has a certain kinetic. Tests up to 10,000 and 100,000 hours are conducted to predict performance so that the most appropriate steel can be selected. There are some materials that perform better at these higher temperatures. It’s like the way we have had to change to maintain a decent life during this pandemic.
A comparison of the creep-rupture strength curves for some grades can be seen in Figure 8.
The third variable to consider to select the best steel for your application is structural stability. High-temperature materials are designed to work at high temperatures, but there are certain ranges that can maintain that level of performance for long periods. After a certain number of hours into the temperature window (the time frame that can vary from material to material), the precipitation of fragile phases – like sigma and gamma-prime phases – begins. These tend not to affect corrosion resistance much, but they prevent the material from performing mechanically as expected. In these cases, it is very important to consider the time variable or the kinetic impact of this unwanted reaction.
Because precipitation in the sigma phase (or other detrimental phases) doesn’t happen overnight, it can be very useful to analyze the TTT (time-temperature-transformation) diagrams of the grade. Figure 10 is an example. The TTT diagram shows 1% sigma-phase formation curves. The figures at the measuring points refer to sigma-phase percentages by volume.
Reading them is easier than it looks. The curves show the temperature and time when there is a 1% precipitation in the sigma phase. As long as the material selected works on the left of the curve, it is safe from this problem. At this point a question arises: What is the most important property in the high-temperature world?
The easiest answer is that there isn’t one. High-temperature applications require a continual trade-off between these three properties: oxidation/corrosion, creep and structure stability, and great control of the process conditions.
The high-temperature world is like the year 2020 – one that started in a complicated way, but life will go on when a solution is found. If you select the right grade, the life of the tube and your equipment will be prolonged. It’s always very important to maintain dialogue and collaboration between customers and suppliers. It is impossible to select a material without knowing both the application and the steel grades that are available.
Selecting the most suitable material for an industrial application is essential, and a greater understanding about high-temperature tube specifications can help your business to be cost-effective. It may also be time to consider new applications or to discuss corrosion issues.
Sandvik has a wide range of online supporting content that can help improve your understanding of materials performance. In the technical center on our website, you will find useful links to pressure calculation tables, podcasts, data sheets, corrosion knowledge and sustainable business initiatives.
If you’re working from home, grab a coffee and let Sandvik help you learn more about how to choose the right materials for your requirements.
All photos/graphics provided by Sandvik AM