Stainless Steel Tubes
An article recently published on MachineDesign.com, titled “Comparing Stainless Steel and Other Metals,” is an excellent overview of what stainless steel is as well as its types, properties, and manufacturing challenges. The article, written by Norm Ellis, focuses on the use of stainless steel in the aerospace industry. It contains a table that compares austenitic, ferritic, martensitic, precipitation hardening, and duplex type stainless steels, their percentages of chromium and nickel, and their UTS, yield strength, elongation, and hardness properties. Although steel is typically 66% heavier than aluminum, it is because of steel’s specific strength that it is of interest to aircraft engineers. (Specific strength is the ratio of strength to density.)
According to the American Iron and Steel Institute, stainless steel is steel that contains more than 10% chromium, either with or without other alloying elements. Stainless steel resists corrosion, maintains its strength at high temperatures, and is easily maintained or sanitized. For these reasons, stainless steel is often used in the food, medical, transportation, and aerospace industries.
One of the most common grades of stainless steel is Type 316, the type Lytron uses for the tubes of its standard stainless steel heat exchangers and some tubed cold plates. Type 316 is an austenitic (chromium-nickel stainless class) stainless steel containing 2%–3% molybdenum. The inclusion of molybdenum gives 316 greater resistance to various forms of deterioration, while nickel and manganese keep the metal in an austenitic microstructure. Austenitic stainless steels generally have the best corrosion resistance of all stainless steels because they contain at least 16% chromium. These alloys are also weldable, which is important in the manufacturing of heat exchangers and cold plates.
When selecting metals for liquid cooling components or systems, there are a large number of considerations. Strength and corrosion resistance are just two of the considerations. In most aerospace liquid cooling applications, weight is one of the other major considerations. Vacuum-brazed aluminum heat exchangers are typically the cooling technology selected because they are lightweight as well as strong. In applications where weight is much less of a factor, the strength and cleanliness of stainless steel might make it worth sticking with stainless.
John, Engineer
