Selecting the best alloy for a tubing application can be a daunting task, considering the wide variety of stainless steel and metal alloys available. Understanding the different alloy series can aid in material selection. Austenitic, ferritic, martensitic, precipitation hardening (PH) and duplex are the main series of stainless steel alloys.
The austenitic 300 series of stainless steel alloys contain nickel additions as well as chromium, which improves ductility and retains an austenitic crystal structure in the alloy even at room temperature. The molybdenum additions make the chromium oxide film denser, which enhances passivity and improves pitting and crevice corrosion resistance in chloride and marine environments.
Ferritic and martensitic 400 series stainless steels do not contain the nickel additions, so an austenitic structure is not retained at room temperature. The absence of nickel reduces material cost. Austenitic and ferritic stainless steels have low carbon levels that are not hardenable by heat treating. Ferritic stainless steels are easy to form, with sufficient corrosion resistance for dry applications.
Martensitic grades have higher carbon levels, so these alloys can be quenched and tempered to high hardness, strength and toughness levels. PH steels can provide corrosion resistance similar to austenitic steels but with higher tensile strength. Duplex stainless steel grades contain both ferrite and austenite. They have superior chloride stress corrosion cracking (SCC) and pitting resistance with about twice the strength of austenitic grades.
Identifying the environmental, thermal and mechanical performance requirements is the first step in selection, in order to determine what material properties are needed for reliable service throughout the tubing components lifetime. From the material property constraints, a group of candidate tubing materials can be formed.
High-temperature oxidation and scaling in power generation, refineries, furnaces, jet engines and chemical process plants is another performance challenge. Several stainless grades with high nickel and chromium additions have been developed for improved oxidation and scaling resistance at elevated temperatures, such as 309, 310, 321, 314, 330 and 446. Inconel® and Nimonic® superalloys are valuable for tubing within a jet engine, turbocharger or power generation gas turbine.
Courtesy of Eagle Stainless Tube & Fabrication, Inc.