How A Proper Alloy Design Can Enhance The Weldability Of Ferritic Stainless Steel

Stainless Steel Industry

It is not any different for the stainless steel industry, where so much has changed in the last two decades, proving the point on how dynamic this industry is. The crude stainless production was around 19.5 million tonnes in the year 2000, with Europe being responsible for 40% of this volume, Asia for another 40%, North America had 10% share, and China only 2.5%. However, in 2021 we could see that the crude stainless production grew to around 59 million tonnes, representing more than a 200% growth, with China now representing 56% of the global production, followed by the rest of Asia representing 20%, Europe with 12% share and North America with 4%.

Over these last 20 years many new grades were developed and brought to the market in the widest range of applications, and new industries were created, such as hydrogen production. Other industries are facing challenges for the future, such as conventional energy generation with fossil sources. However, regardless of advances, a few processes are still crucial for the proper application of stainless steels, and welding is one of them.

The American Welding Society (AWS) has an interesting definition of welding; “A materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material.” By reading this definition, it is possible to see that welding is in fact an energy intensive process, where thermodynamics meets metallurgy and mechanics, therefore, controlling the welding quality is a challenge that must be overcome to keep proper performance of materials in this new world of applications.

Challenges With Alloy Design

Alloy design could help the industry overcome the quality challenge on the weldability of stainless steels, which can impact not only the performance of components, but also the sustainability of applications, by using less energy and reducing scrap levels. Even though the majority of stainless steel production today still is for Austenitic Grades (around 75%), the focus of this article will be given to ferritic stainless steel alloys, which represent around 20% of global production. These grades are gaining momentum and finding more space among the end users of stainless steel, due to the nickel price instability.

The main ferritic stainless steel grade that is used in the industry today is the non-stabilized AISI 430. This material is very versatile, and is being used in white goods, capital goods, infrastructure, architecture and many other segments, however as this grade is not stabilized, the majority of its applications do not require welding. In order to comply with the quality requirements, other ferritic grades had to be developed to facilitate the weldability performance and allow application in all other industry segments, like mobility, energy generation and capital goods.

During this development process, two main aspects needed to be covered on the alloying level in order to improve the performance of these materials; ferritic phase stabilization, and sensitization control. Additionally, small adjustments had to be done on the process level, such as controlling the welding energies, controlling the welding atmosphere with proper protection gases, development of proper welding wires and development of new processes, such as laser welding or pulsed GMAW welding.

Considering the phase stabilization aspect mentioned above, direct action must be taken on steel production, by controlling chemical composition and avoiding unwanted phase transformations after the welding process. An example is the formation of martensite in the grain boundaries, which can embrittle the material and compromise the application. When the material is not properly stabilized and the temperature rises, a partial transformation happens in the heat affect zone (HAZ) and welded area where ferrite transforms into ferrite and austenite. Then when the heat input is taken out, and a fast cool down starts, martensite is formed at the grain boundaries.

To avoid this unwanted phenomenon, a proper balance between gamma-genic (austenite stabilizers) elements and alpha-genic (ferrite stabilizers) elements is required. An important methodology of checking this balance is by using the Kaltenhauser Ferrite Factor (KFF), which predicts the amount of martensite and ferrite in a given chemical composition. The KFF summarizes two main elements as equivalents, Creq for the ferrite stabilizers and Nieq for the austenite stabilizers, and if this factor is above 13.5, the ferritic phase will be stabilized, and the formula is the following KFF = %Cr + 6%Si + 8%Ti + 4%Mo + 2%AI + 4%Nb -2%Mn – 4%Ni -40 (%C + %N).1

Strategies, Solutions, and Sustainability

By looking at the formula mentioned above it is possible to predict the first step towards a strategy of alloy design, which would be reducing the carbon and nitrogen contents, since they are strong austenite stabilizers. However, reducing these elements requires time, energy and technological advances, which in many cases, steel makers do not have available, such as a Vacuum Oxygen Decarburization (VOD). Adding elements such as niobium (Nb) and titanium (Ti) at the same moment as controlling other elements, is a clever solution to avoid this phase transformation.

The addition of these elements will also support with the sensitization performance, which is the second issue mentioned previously. Sensitization is the precipitation of chromium carbides in the grain boundaries when the material is exposed to higher temperatures, usually between the range of 450°C and 850°C. This will lead to the formation of chromium depleted zones at the grain boundaries, leading to intergranular corrosion (IGC) which can compromise the application.

Controlled Niobium (Nb) additions can be quite beneficial, since this element is more avid to react with carbon and nitrogen than chromium, partially or completely replacing the carbides and nitrides of chromium by carbonitrides of Nb (C, N). This will allow chromium to be once again free to react and form the passive layer, keeping the corrosion resistance of the welded area. From a sustainability perspective, avoiding corrosion failure will also increase the lifespan of components and therefore avoid high scrap rates, and unnecessary material production for replacement, which reduces emissions.

These adjustments in chemical composition brought to life some important ferritic alloys over the decades, such as the AISI 409, AISI 430Nb, the EN 1.4509 (441), the UNS S43932 (439), the AISI 444, the SUS445J1 which have niobium, titanium, moly and other elements in their alloy design. They are currently used with success in mobility, energy generation, infrastructure, capital goods, food industry and many other segments that require weldability performance.

The future of the stainless steel industry is foreseeing many applications that could bring the world to a more sustainable reality, such as renewable power generation, electrification, smart cities infrastructure. and more. However, more than 50% of stainless steel demand is still price sensitive, meaning that if prohibitive price increases happen, end users can sometimes use other solutions such as aluminum, coated carbon steels, and other alloys. Once again, ferritic stainless steels are in the spotlight and will have the opportunity to capture more space into the universe of stainless steel and other alloys, therefore having a proper alloy design for good weldability will be an important enabler of this trend.

References

1. Kaltenhauser, R. H. 1971. Improving the engineering properties of ferritic stainless steel. Metals Engineering Quarterly, 41-47
2. Gonçalves CN, Carvalho GMA, Siqueira JS, Renzetti RA. Influence of Nb content on sensitization and pitting corrosion ofwelded AISI 430 ferritic stainless steel. Soldagem & Inspeção. 2019;24:e2421. https://doi.org/10.1590/0104-9224/SI24.21