What is the largest diameter super duplex bar?
Gnee Steel stocks super duplex stainless steel bars up to 16 inches/406.4 mm in diameter, but why not larger?
We believe this is the largest reasonable diameter that can be produced while maintaining consistent properties through the center of the bar.
Super duplex grades offer an unparalleled combination of high strength, excellent corrosion resistance, and cost competitiveness. However, it is well known that they are prone to forming deleterious microstructural phases within the steel if cooled too slowly during production or heated to too high a temperature during fabrication or use. Rapid cooling becomes increasingly difficult as size increases.
Structure of Metals
In simple terms, a metal bar is a crystalline material. When etched and viewed under a microscope, the internal structure of a metal consists of many individual grains of varying sizes and orientations. The nature, size, and type of these grains affect its physical and mechanical properties. The microstructure within a steel is a factor of its composition and thermal history. For this reason, steelmaking is often compared to baking-controlling the recipe, furnace time, and temperature.
Super duplex stainless steel consists of a mixture of ferrite and austenite grains that form when the metal is rapidly cooled after a high temperature soak (called solution annealing). Solution annealing ensures a consistent composition throughout the product, while rapid quenching effectively "freezes" this consistent composition. The mixture of austenite and ferrite grains gives it an excellent combination of properties. Super duplex stainless steel combines the corrosion resistance of austenitic stainless steel, the strength of ferritic stainless steel, the brittleness resistance of austenitic stainless steel, as well as good impact toughness and ease of fabrication.
If the cooling rate is slower than expected, other types of grains, so-called "phases", may form in their place. Phases such as sigma and chi phases are called "intermetallic phases" because they are compounds composed of two or more metals. They form in the range of 550-1000℃, but form fastest at around 850℃. Both sigma and chi phases are rich in chromium and molybdenum, which means that the surrounding areas are lacking in chromium, that is, the chromium content is low. Since chromium mainly enhances corrosion resistance, these areas around the sigma phase have much lower corrosion resistance. Furthermore, the presence of these hard, brittle phases results in a significant reduction in impact strength. Clearly, avoiding the formation of sigma and chi phases is essential to producing high-performance stainless steels!
So how does this phenomenon limit the maximum diameter of super duplex stainless steel bars? Even when quenching in water immediately after removal from the heat treatment furnace, heat cannot be removed from large bars quickly enough to avoid the formation of these negative phases. Fabricators meeting Norsok standards must control the transfer time from the furnace to the water bath, as well as the maximum quench water temperature. The bars can also be stirred during the quenching process to improve heat transfer and increase the cooling rate. However, above 16 inches/406 mm in diameter, the cooling rate is still too slow to ensure that sigma or twill grains will not form. This maximum diameter is sometimes referred to as the "dominant section," which is the maximum cross-sectional thickness allowed for a bar, forging, or casting. As a result, both mechanical and physical properties will be below the required levels. While a test certificate may show acceptable properties, this is usually related to test samples taken closer to the surface, as these samples cool more quickly.
Your options
Gnee Steel has therefore chosen not to supply larger diameter Super Duplex bars. If you need to produce larger components from Super Duplex, what are your options?
1/ Produce from plate.
Gnee Steel supplies Super Duplex plate up to 3 inches/76.2 mm thick. This allows us to supply larger diameter components, albeit limited by the maximum plate thickness.
2/ Custom Forgings
If a part cannot be machined from 3-inch/76.2 mm thick plate or 16-inch/406.4 mm diameter bar, a custom forging can be an alternative, as long as the dominant cross-section does not exceed the maximum allowable dimensions.
3/ Hot Isostatic Pressing (HIP)
Hot Isostatic Pressing (HIP) is not widely used due to its relative cost, lead time, and availability. In this process, parts are produced from powder in a high-pressure furnace. Because the powder mold ("pattern") used to make the part may contain a center hole and other features, the dominant cross-section can be much lower than if it were machined from a solid part. Therefore, subsequent heat treatment can achieve the required rapid cooling rates.
4/ Post-Machining Heat Treatment
For many parts, post-machining heat treatment is a viable option. Machining typically removes one-third to one-half of the starting weight from a solid bar. In addition, a center hole can significantly reduce the dominant cross-section. The only risk of post-heat treatment is the possible distortion during cooling. Therefore, heat treatment should be carried out after proofing in order to complete the final finishing stage.
