Rigid fluid lines carrying high-pressure (3,000 psi or greater) hydraulic fluid should be made from which material?

• A. Annealed steel
• B. Copper
• C. Annealed or 1/4-hard corrosion-resistant steel
• D. Aluminum

Answer: (C)

High-pressure rigid hydraulic lines must resist internal pressure without yielding or fatigue, and they must endure chemical exposure from the hydraulic fluid. That requires a material that combines sufficient strength and hardness with good corrosion resistance.

Annealed steel is very ductile and easy to form, which is helpful for shaping lines, but it yields under high pressure and can deform over time, compromising integrity. Copper, while reasonably corrosion-resistant, does not provide the necessary strength or fatigue life for several thousand psi and tends to work-harden during bending, making it unreliable for high-pressure lines. Aluminum is lighter but generally doesn’t offer the required yield strength or fatigue resistance for rigid lines at this pressure level and can present compatibility issues with fittings and fluids.

Choosing steel that is corrosion-resistant and either annealed or in a 1/4-hard state gives the needed balance: annealed for ductility in shaping and 1/4-hard for higher strength, while still resisting fluid-induced corrosion. This combination provides a line that maintains its shape under high pressure, resists fatigue, and stays durable in the hydraulic environment.