Editor’s Note: This is the third in a multi-part series on materials used in fluid power hose fittings. We will cover carbon steel, stainless steel, brass and plastic.
By Josh Cosford, Contributing Editor
In this third installment of our four-part series on fitting materials, I discuss stainless steel. Stainless steel offers a level of corrosion resistance and reliability above and beyond carbon steel while offering strength advantages over brass. As you’d imagine, the extra benefits of stainless steel come at a cost, and that cost is cost. But let’s delve into why stainless could be worth using in your hydraulic or pneumatic machine.
When people think of products designed to resist corrosion, what’s likely to come to mind is rusting adaptors that were exposed to water, salt, and caustic chemicals. To be sure, you definitely want to mitigate the result of such harmful exposure, and that’s where stainless steel fittings make the top of the list for engineers and technicians to select. Continuous exposure to ambient conditions will surely cause carbon steel fittings to rust, corrode and pit over time, but you can expect reliable performance from their stainless counterparts.
However, what’s often forgotten is the potential for harm is much worse than some flakey rust on your elbows, tees and branches. A corroded fitting may simply leak or break entirely, causing expensive loss of fluid or pump dry running at worst. But when the internal surfaces of your fittings begin to corrode, you will have no idea it’s occurring and for how long, but you can bet that iron oxide particles circulating through your hydraulic or pneumatic system will eventually cause damage to expensive pumps, valves and actuators.
It’s clear that stainless steel provides a benefit to fluid power systems exposed to moisture and chemicals, whether internally or externally, so your next step is to choose the alloy appropriate for your application. Stainless steel starts with an iron and carbon base, like most steels, which also uses various elements like manganese and silicon. The primary driver of SS magic is chromium, which must be at least 10.5% concentration by weight. Chromium provides the steel with a surface layer that prevents oxidation, and even despite dents and scratches, it will “self-heal” to ensure an oxidation-resistant surface.
The most common specific alloy used for fluid power is 316, which uses 10-14% nickel to improve corrosion resistance and toughness. As well, 316 alloy uses molybdenum to improve resistance to pitting and crevicing. The 316L variant is a lower carbon recipe that offers superior weldability, making it perfect for larger or custom fittings not produced from machining or forging.
For pneumatic applications where pressure and corrosion demands are more subdued, 303 or 304 alloys may be more suitable. 304 stainless uses a higher content of chromium and carbon but lower nickel, while 303 drops the chromium slightly in favor of increased carbon, sulfur and phosphorus. Both alloys are similar, although 303 comes with increased machinability with a slightly reduced corrosion resistance. Extreme applications may call for alloys such as 17-4 or 2205, although they tend to have specific requirements for aerospace or offshore applications.
316 will usually cost more to manufacture than 303/304, but the performance is commensurate with cost. If your application has a low pressure and corrosion resistance requirement, select 304 alloy. If the same application is produced through a machine tool, then 303 will be more suitable. When in doubt, 316 should handle all but the most extreme fluid power fitting applications.
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