How a proper alloy design can enhance the weldability of ferritic Stainless Steels
The development of technology and societies have been walking side-by-side in the last century, with one being a strong enabler of the other. As soon as a disruption happens in technology, societies thrive in it and our lives become easier and safer. It is nice to look back on the last 20 years and see how many things changed in this period. 20 years ago technologies such as smartphones, artificial intelligence and additive manufacturing were still on its first stages or did not even exist.
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 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.
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
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.
To read the entire June 2022 issue of the Stainless Steel World Magazine.
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