By: Josh Cosford, Contributing Editor
Editor’s Note: This is the first in a multi-part series on materials used in fluid power hose fittings. We will cover carbon steel, stainless steel, brass and plastic.
Fittings and adapters used in fluid power are manufactured using various materials selected by designers to suit the specific needs of the application. The criteria for selecting construction material are considerations for the fluid medium, the ambient environment and the pressure capacity required.
Keep in mind that air is a fluid, so fittings used to transfer air under pressure are more diverse than those required for high-pressure hydraulics. The environment plays a part in material construction, as fittings must withstand whatever pitfalls the world targets their exterior. And without the capacity for strength under pressure, no fitting has the right to claim citizenship within the fluid power borders. This article is the first in a series where we discuss the various construction materials used in fluid power, and I will discuss carbon steel, followed by brass, stainless and plastic.
The go-to hydraulic fitting material is forged carbon steel, which has the strength to withstand even the highest-pressure systems. Forging provides a denser, stronger material with a better grain structure, which is crucial for handling the high pressures and stresses in hydraulic systems. Cast steel is inexpensive and easier to produce in complex shapes but has less uniform grain structure and can have porosity issues, making it less suitable for high-pressure hydraulic applications.
Steel fittings are typically closed-die forged, which uses two dies that close together to squeeze the steel into its blank shape. That shape may include part number designations, size callouts or even company logos. Once forged, the blanks are trimmed of flash, which is the excess material extruded through the die openings and not useful to the shape of the fitting.
Oftentimes, the steel forgings are heat treated to improve the properties of the steel. They may be normalized, quenched, tempered and/or case hardened, each of which instills extra properties of strength and hardness that improve the pressure capacity and durability of the fitting.
The second-last stage of steel fitting production is CNC machining, which turns the blanks into semi-finished pieces, including the threads, drillings and final shapes such as the tapered face of the JIC fitting. Some production facilities use purpose-built threading machines, which use tooling and techniques specific to the threads and shapes applied to the same machine operation run in the thousands.
Rarely do carbon steel fittings leave the factory without some form of surface coating to help protect the fitting from damaging environmental conditions. The most common forms use zinc, either in the form of zinc or zinc-nickel, and these fittings may be yellow, gold or clear in color. Be aware that yellow zinc fittings may appear at first glance like brass, but the faint rainbow iridescence of zinc is a giveaway compared to the homogeneous hue of brass.
Phosphate coatings are less like plating and more like surface treatment and provide a superior base for painting. However, because steel enjoys myriad possible treatments, manufacturers may offer electroless nickel, black oxide or even hot-dip galvanized treatments. If your application requires specific technology, reach out to your local fitting manufacturer to see what they offer.
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