Stainless steel is one of the world’s most recognized and widely used materials. Though known for its anti-corrosive properties, it is susceptible to sensitization, a type of corrosion, during welding fabrication processes. Knowing how to prevent stainless steel sensitization during those processes can help ensure the material retains its anti-corrosive properties.
Sensitization relates to several factors, such as the selection of base material, the material used as filler, heat, environmental conditions during welding, and post-weld treatment of the material. Restricting material prone to corrosion, in combination with after-weld treatment, is essential in avoiding sensitization.
What Makes Stainless Steel Unique?
Stainless steel is used in various applications ranging from surgical and medical instruments, household utensils, and durable goods to heavy industry, architecture, aerospace, shipbuilding, and automotive manufacturing. Stainless steel is the common name for several different types of steel – austenitic, ferritic, martensitic, and duplex – known primarily for their anti-corrosive properties. Developed to perform in several corrosive environments, stainless steel is an iron-based alloy composed of nickel, manganese, nitrogen, molybdenum, carbon, and, most importantly, chromium.
Chromium is the necessary element that makes steel stainless. All stainless steels require a minimum of 10.5% chromium when the material is cast. The chromium content improves the corrosion resistance by forming a thin layer of chromium oxide film on the steel’s surface. When placed under the right conditions, the film can also add self-repairing properties to the material. Combining chromium with other elements forms different crystal structures, giving the various steels the properties needed for specific machining, welding, and forming applications.
What is Sensitization?
Sensitization is the loss of alloy integrity. It occurs when certain alloys in stainless steel are held to a temperature of 425 – 815°C, characterized as a sensitizing temperature, which causes the alloys to become susceptible to intergranular corrosion. Specifically, it refers to the precipitation of carbides along the grain boundaries of the material. This precipitation happens due to the carbides being insoluble at such temperatures. To precipitate, the carbides must absorb the chromium from the surrounding metal, creating a chromium-depleted zone along the grain boundaries of the material. Essentially, the higher the degree of carbide precipitation, the more significant the amount of intergranular corrosion that happens.
Effects of Sensitization
In corrosive environments, as the grain boundaries interface with these sensitized areas, they become reactive to the environment, promoting inter-granular corrosion that can result in stress corrosion, cracking, and ultimately failure in some stainless steels. Intergranular corrosion is recognized by localized attacks adjacent to the grain boundaries with relatively little corrosion of the grains. As the adjacent area around the grains corrodes, the grain crumbles and falls out.
How to Prevent Sensitization
Several methods have been adapted to prevent or minimize sensitization. The first is a process referred to as solution-annealing. This post-weld heat treatment allows chromium diffusion into the grain boundary zone. The alloy is heated to a temperature above 1065C, then water or air cools rapidly.
Another technique to prevent stainless steel sensitization incorporates elements such as niobium or titanium that act as stabilizers in stainless steels. Such elements have a greater affinity for carbon than chromium does. Forming carbide with these elements can reduce the carbon available in the alloy, allowing for the formation of chromium carbides to preserve the material’s corrosion resistance.
Finally, another method is reducing the carbon content of the stainless steel to 0.03%. With a lower amount of carbon, insufficient carbon is available for carbide formation. Today, many stainless grades have low-carbon versions, such as 304L and 316L. Lower carbon can avoid sensitization but should only be used in applications with lower strength property requirements.
Contact the Experts at Atlantic Stainless
Atlantic Stainless is a proud distributor of the highest quality stainless steel products and services. Our expert sales team has combined experience of over 150 years, allowing us to assist you with any questions you may have. Contact us today to learn more about what we can do for you!
