Hydraulic Cylinder Materials: Barrel Tubes and Piston Rods

The barrel: honed tube

Cylinder barrels are usually seamless or drawn tube in E355 (St52) class steel, supplied honed or skived-and-roller-burnished to a fine internal finish; SAE 1020/1045 DOM tube covers the same ground in inch practice. Key points:

• Internal tolerance is commonly around H8–H9, with surface roughness low enough (typically Ra ≈ 0.2–0.4 µm after burnishing) for seals to ride on an oil film rather than on machining marks.
• Weldability matters — ports, flanges and end caps are welded on — which is one reason medium-carbon E355-class steels beat higher-alloy choices here.
• Wall thickness follows pressure and buckling rules from the cylinder design codes, not the material grade alone.

The rod: chrome-plated bar

Piston rods are most often CK45 / SAE 1045, hard chrome plated:

• The steel core carries the tensile and buckling load; quenched-and-tempered or normalised condition per duty.
• Hard chrome (typically of the order of 20–30 µm) provides corrosion protection and a hard, low-friction running surface, finished to roughly Ra 0.1–0.3 µm.
• Where impact or stone damage threatens the rod, induction-hardened CK45 underneath the chrome resists denting that would otherwise cut seals.
• Heavier duty or highly loaded rods step up to 42CrMo4 (4140) quenched and tempered, chrome plated the same way.

Finish is a functional spec

Seal life is set less by the steel than by the surfaces: too rough abrades seals, too smooth can starve lubrication, and chrome defects (pits, cracks, thin edges at journals) become leak and rust paths. Salt-spray ratings and chrome thickness belong on a rod specification alongside the steel grade; on tubes, the honing/burnishing finish and ID tolerance are the buying specs. Neither part is exotic — cylinders succeed on consistent, ordinary materials executed to consistent finishes.

Rod steel variants worth knowing

Alongside CK45, 20MnV6-type microalloyed steels are widely used for rods: they offer higher and more consistent yield strength than 1045 in the as-rolled condition with good weldability for the rod-end joint — relevant because the rod eye or flange is welded before chroming. For corrosive environments (marine, food, offshore), rods move to hard-chromed stainless or, increasingly, to alternative coatings (thermal-sprayed or laser-clad layers) where hexavalent-chrome processes are restricted; the base-steel logic — strength, weldability, dent resistance — stays the same underneath the surface system.

What actually gets specified

A complete cylinder material call-out normally names, for the barrel: tube standard and grade, delivery condition (stress-relieved for honed tube), ID tolerance and roughness, and pressure-test requirement; for the rod: steel grade and condition, chrome thickness and hardness, roughness, and a corrosion test duration. Omitting the finish and coating items while specifying the steel in detail is the most common weakness in real purchase specifications.