Hydraulic cylinder rods turn black

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Black nitride is a relatively recent alternative to the hard-chrome plated hydraulic cylinder rod. With reports of achieved service life three times that of conventional chrome, longer seal life and comparable cost, black nitride rods for hydraulic cylinders are an option that all hydraulic equipment users should be aware of.

Black nitriding is an atmospheric furnace treatment developed and patented in the early 1980's. It combines the high surface hardness and corrosion resistance of nitriding with additional corrosion resistance gained by oxidation. The process begins with the cleaning and super-polishing of the material to a surface roughness of 6 to 10 Ra. The steel bars or tubes are then fixed vertically, and lowered into an electrically heated pit furnace.

The furnace sequence involves nitrocarburizing the steel at temperatures up to 1150F in an ammonia atmosphere. The steel's surface is converted to iron nitride to a depth of typically 0.001". Atmospheric oxidizing is employed to produce a black, corrosion resistant surface film.

The process generates a thin (0.001") uniform and extremely hard (64 to 71 Rc) iron nitride layer infused with a film of iron oxide. Beneath the iron nitride layer is a nitrogen-enriched, hardened diffusion zone. The diffusion zone functions as a lightly hardened case with a hardness gradient ranging from around 55 Rc just below the iron nitride layer to approximately 40 Rc at a depth of 0.015". Testing has verified that black nitride bar machines and welds as well as hard chrome plated stock.

Available in standard diameters up to 5" (127 mm), black nitride hydraulic cylinder rods offer the following benefits over conventional hard chrome:

  • Superior corrosion and wear resistance
  • Better oil retention (longer seal life)
  • Dimensional uniformity
  • Dent resistant - without the need for induction hardening
  • No pitting, flaking or micro-cracking
  • Environmentally friendly process

One of my clients reports three times the service life in corrosive environments when compared with conventional hard-chrome plating. But on the other hand, another client trialled black nitride on salt trucks and found the rods started flaking after one year's service. I've also heard one report of long, small diameter black nitride rods snapping under load. So like a lot of other things hydraulic, it seems the performance of black nitride cylinder rods depends on the nature of the application.

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