Hard Chrome Plating
Also: industrial chrome, engineering chrome, hard chromium plating, functional chrome, AMS 2460, QQ-C-320 Class 2, chrome plating
Hard chrome plating deposits 25-250 µm of chromium at 850-1,050 HV on steel shafts, rods, molds and bores: the classic wear and salvage coating, ground to size after plating.
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Hard chrome is electrodeposited chromium from a chromic acid bath, plated thick (25-250 µm, 0.001-0.010", and up to 0.5 mm for salvage) for wear resistance rather than appearance. The deposit is 850-1,050 HV (about 66-70 HRC), has a low coefficient of friction against steel, sheds most fluids and resists many acids. Hydraulic rods, piston rings, print rolls, mold cavities, gun barrels and worn shafts are the classic uses.
AMS 2460 (which replaced QQ-C-320 for aerospace) and ASTM B177 cover engineering chrome. Two features of the process drive the engineering. First, throwing power is poor: chrome builds heavily on edges and corners and barely plates into bores unless a conforming anode is placed inside, so parts are masked, edges radiused and the deposit ground to size afterwards. Second, the deposit is micro-cracked and under tensile stress, which lowers the fatigue strength of the base part; high-strength steel is shot peened before plating and baked after (AMS 2460 requires both above a stated strength).
The bath is hexavalent chromium, which is regulated and slowly being replaced: high-velocity oxygen fuel (HVOF) tungsten carbide spray, electroless nickel and trivalent chrome alternatives are taking the aerospace and hydraulics business. Job shops still run hard chrome for salvage and general wear work, and the finish after grinding is as good as any coating gets.
At a glance
| Typical tolerances | Deposit thickness 25-250 µm (0.001-0.010") for wear, 250-500 µm (0.010-0.020") or more for salvage; as-plated variation is 2:1 or worse between edges and centres, so the part is plated 25-75 µm oversize and ground to a final tolerance of ±0.005-0.013 mm (±0.0002-0.0005"). Thin flash chrome (2-5 µm) for molds and gauges is plated to size without grinding. Bores and internal surfaces plate only with conforming anodes. |
|---|---|
| Size limits | Rods and rolls up to 6-10 m (20-33 ft) long and several tonnes at hydraulics and roll shops; job shops commonly take 2-3 m and a few hundred kilograms. Internal diameters are practical above about 25 mm (1") with an internal anode. Small parts are rack plated individually, never barrelled. |
| Surface finish | Matte to satin silver-grey as plated, with nodules on edges; ground and polished to 0.1-0.4 µm Ra (4-16 µin) and a bright mirror when required. Hardness 850-1,050 HV (66-70 HRC), micro-cracked at 300-1,000 cracks per centimetre, friction coefficient around 0.15-0.2 against steel. Softens above about 400 °C (750 °F). |
| Lead time | 1-2 weeks including pre-grind, plating and finish grinding; 2-3 weeks for large rolls or aerospace parts with peening, baking and inspection. Simple flash chrome on molds in 3-5 days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon, alloy and tool steels, and cast iron are the normal substrates. Stainless steel plates after a reverse-etch or nickel strike. Aluminum takes hard chrome over a zincate and nickel strike, mostly on cylinder bores and molds. Copper alloys plate easily. Titanium and magnesium need specialist pre-treatment. Steel above about 1,240 MPa (180 ksi) requires peening before and baking after plating.
What drives the cost
- Thickness and surface area: chrome deposits slowly, about 25 µm per hour at best
- Pre-plate and post-plate grinding, which usually cost more than the plating
- Masking and conforming anodes for bores, keyways and selective areas
- Shot peening before and hydrogen embrittlement bake after, on high-strength steel
- Part size: long rods and heavy rolls need dedicated tanks and handling
- Inspection: thickness, adhesion, porosity, and AMS 2460 or ASTM B177 conformance reports
- Environmental surcharges on hexavalent chromium processing
When to use it
- Hydraulic and pneumatic cylinder rods, piston rings, plungers and valve stems that see sliding wear
- Restoring worn shafts, bores and journals to size, then grinding to the original drawing
- Molds, dies and extrusion tooling that need a hard, release-friendly surface
- Print, embossing and calender rolls that need a hard, polishable, corrosion-resistant face
- Gun barrels, gauges and precision surfaces that need a thin, very hard flash
When not to
- Complex shapes with recesses and internal passages: the deposit will not reach them evenly; use electroless nickel
- Fatigue-critical parts without a peen-and-bake plan; hard chrome reduces fatigue strength
- Thin decorative finishes on consumer goods: decorative chrome over nickel is a different, cheaper process
- Parts that see thermal cycling above 400 °C (750 °F) or impact loads that crack a hard layer
- Aluminum parts needing a simple hard surface: hardcoat anodize is cheaper and needs no strike
Design tips
- Specify AMS 2460 (or ASTM B177) with the thickness after finishing, the surface finish and the areas to be plated; call out "chrome plate and grind to size".
- Radius outside edges to at least 0.8 mm (0.03") and chamfer the ends of plated diameters; chrome builds up at sharp edges and chips.
- Leave 25-75 µm (0.001-0.003") of grinding stock in the pre-plate dimension, and pre-grind to a good finish so the chrome is uniform.
- State the steel strength; above about 1,240 MPa (180 ksi) require shot peening before and a 190 °C (375 °F) bake after, per the spec.
- Avoid plated keyways, cross holes and blind bores where possible; mask them and plate only the bearing lengths.
- For bores, tell the plater the length-to-diameter ratio so they can plan a conforming anode.
- If the part will be welded or repaired later, remember chrome must be stripped first.
Frequently asked questions
- How hard is hard chrome plating?
- About 850-1,050 HV, or 66-70 HRC equivalent, harder than any hardened steel it is applied to. It keeps that hardness to roughly 400 °C (750 °F) and then softens.
- How thick is hard chrome plating?
- Typically 25-250 µm (0.001-0.010"), plated oversize and ground back to the drawing. Salvage jobs go to 0.5 mm (0.020") and beyond; flash chrome on molds and gauges is 2-5 µm and is not ground.
- Hard chrome vs decorative chrome?
- Decorative chrome is a 0.25-0.5 µm flash over bright nickel for appearance; the nickel does the corrosion work. Hard chrome is plated directly on the steel, hundreds of times thicker, for wear. They come from different shops.
- Does hard chrome plating reduce fatigue strength?
- Yes, typically by 20-40% at high cycles on smooth steel specimens, because the deposit is micro-cracked and in tension. Shot peening before plating and baking after restores most of it, and aerospace specs require both on high-strength steel.
