Hard-Chrome Score on a High-Strength Steel Strut Piston
During a C-check, a score 0.002 in deep and 0.75 in long is found in the chromium-plated bore of a 4340 steel landing-gear shock strut piston. The repair manual permits blending out surface defects provided the blend is a smooth taper of not less than 20:1 (length to depth), the part is subsequently re-plated and baked, and no more than 0.010 in of base metal is removed. Chromium thickness is 0.003–0.005 in. The plating shop can turn the part around in five days. The aircraft is on a tight schedule, and a supervisor suggests polishing the score out with a flap wheel and returning the aircraft to service.
1. Calculate the minimum blend dimensions required by the 20:1 rule and state how much material the repair removes.
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Blend geometry. A 20:1 taper means the taper must run at least 20 times the depth on each side of the defect so the load path is not interrupted by a step.
- Taper length per side = 20 × 0.002 in = 0.040 in
- Total blended length = 0.75 + 0.040 + 0.040 = 0.83 in
- Depth removed at the deepest point = 0.002 in, i.e. the full depth of the score plus a margin to reach sound material
Base metal question. The chromium layer is 0.003–0.005 in, so a 0.002 in score may not have penetrated to the steel. However, blending removes the surrounding chromium and exposes base metal in a shallow dish across roughly 0.83 in of length. That is within the 0.010 in base metal allowance, but the exposed area must be re-plated - there is no 'blend and polish' option that leaves an acceptable surface.
Judgement: measure the actual depth, do not assume it. Verify with a dial indicator or replica before cutting, and re-verify the finished depth against the 0.010 in limit.
2. Evaluate the supervisor's proposal to polish the score out and release the aircraft.
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Why it must be rejected:
- Seal path. The bore is a sealing surface. A local polish leaves a dished area, and the edge of the dish acts as a step that cuts the dynamic seal as it passes. The result is fluid loss and a strut that will not hold extension.
- Stress concentration. A free-hand polish leaves a scratch or an abrupt shoulder. On a fatigue-critical, high-strength steel component, an uncontrolled scratch is a crack initiator. The 20:1 taper exists specifically to control this.
- Surface integrity. A flap wheel smears and cold-works the surface and generates local heat. On 4340 this can produce untempered martensite and residual tensile stress - a hidden defect under a shiny surface.
- Corrosion protection lost. Chromium protects the steel and provides the wear surface. A blended patch with no chromium will corrode, and corrosion pits become crack starters.
- Not approved data. Free-hand polishing to an undefined depth is not a manual-approved repair. Signing it off would be a false certification.
Acceptable route: blend to the 20:1 taper, measure, re-plate, bake, inspect, and release on approved data - accepting the five-day delay. Schedule pressure is not a technical justification.
3. The part is re-plated. Explain the additional process controls and inspections required before it can be returned to service.
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Hydrogen embrittlement control. Chromium plating generates hydrogen at the cathode, and high-strength 4340 absorbs it. Without a bake, the part can fail in a brittle manner days or weeks after entry into service, at loads well below its normal capability.
- Bake to the plating specification - typically within a defined short interval after plating, at a controlled temperature (of the order of 190–230 °C) for the specified duration.
- Control the temperature strictly: too low and the hydrogen is not removed, too high and the steel is overtempered and loses strength. Record the oven chart.
- Do not grind, shot-peen or bend the part between plating and baking.
Dimensional and metallurgical checks:
- Chromium thickness within 0.003–0.005 in, measured on the finished surface.
- Bore diameter and roundness within tolerance; base metal removal within the 0.010 in allowance.
- Hardness check to confirm no overheating during blending or baking.
Inspection:
- Magnetic particle inspection of the blended and plated area after finishing to detect cracks from the blending operation.
- Visual inspection for pits, nodules, blistering and poor adhesion in the new chromium.
Release: this is a critical component; carry out a duplicate inspection, confirm the plating and baking certificates are traceable to the part, and record the blend dimensions, thickness readings and inspection results on the paperwork before signing the certificate of release to service.