Springs cracked in service six weeks after shipment, yet every QC check had passed. The deposit was in spec; the interface was not.
This case shows how senior QC finds what routine testing misses: cross-sectioning, microhardness, adhesion testing and tracing the fault chain from pre-treatment to baking.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Passing thickness and stopping there | Interface faults pass unnoticed | Cross-section parts in every dispute |
| 2. Polishing destroys the interface | Faults are polished away | Finish with 1 micrometer diamond |
| 3. Etching too long | Layer structure is wiped out | Etch in short steps and inspect |
| 4. Blaming the coating first | The real fault is underneath it | Section before blaming the deposit |
| 5. No microhardness baseline | Soft deposits fail late | Measure HV against the spec |
| 6. Baking skipped for small parts | Delayed hydrogen cracking | Bake per strength class and drawing |
| 7. One section, one conclusion | A local fault is misread as general | Section 3 or more parts from different positions |
| 8. Records without photos | No evidence for the analysis | Photograph every stage with a scale |
| 9. Root cause stops at the lab | The process keeps making faults | Close the loop with a process change |
Best Practices
- Cross-section at least three parts from every failure investigation
- Document every stage with scale-marked photos
- Measure microhardness against the coating specification
- Inspect the interface before blaming the deposit
- Confirm hydrogen risk and bake compliance for high-strength steel
- Close every failure with a process change and a verification run
Implementation Roadmap
| Step | Frequency |
|---|---|
| Section and photograph the failed part | Per failure |
| Measure microhardness and adhesion | Per failure |
| Review the failing lot's process logs | Per failure |
| Verify the fix across 3 production lots | After each change |
| Trend failure types by month | Monthly |
Field failure analysis flow
- Section - Mount, polish to 1 micrometer. (Poor polishing erases the fault.)
- Inspect - Interface, layers, cracks. (Surface checks alone mislead.)
- Measure - Microhardness and thickness. (Out-of-spec layers fail early.)
- Fix - Change process, verify 3 lots. (Analysis without a fix repeats.)
Hydrogen embrittlement shows weeks after plating. | The interface, not the surface, carries the truth.
Failure analysis reference data
Reference values for sectioning and failure analysis of plated parts.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Section finish | 1 micrometer diamond polish | Coarse finish hides cracks |
| Microhardness | Per spec, e.g. 500-900 HV for hard chrome | Soft deposits fail early |
| Hydrogen bake | 190-230 C, 4-24 h per strength class | Skipped bake risks delayed cracks |
| Adhesion test | Bend, file or thermal shock 150-250 C | Detects interface failure |
| Sections per failure | 3 or more parts | One section misreads the lot |
| Etch time | Short steps with inspection | Over-etch removes the layer |
| Thickness Cpk | 1.33 or higher at sectioned points | Links field life to the process |
Sectioning record card
| Stage | Parameter | Evidence |
|---|---|---|
| Mounting | Cold resin, vacuum | Photo |
| Polishing | 9 to 3 to 1 micrometer | Photo |
| Etching | Short steps | Photo |
| Measurement | Microhardness and thickness | Values |
| Conclusion | Fault and process step | Report |
Hydrogen bake decision table
| Tensile strength | Bake time at 190-230 C |
|---|---|
| Below 1000 MPa | 4 hours |
| 1000-1400 MPa | 8-16 hours |
| Above 1400 MPa | 24 hours per spec |
| Springs and high-carbon steel | Follow the drawing |
Case 1 - springs cracking six weeks after shipment
Scenario. High-carbon steel springs cracked in service six weeks after shipment even though thickness and salt spray passed every QC gate.
Action. Cross-sections showed cracks starting at the deposit-substrate interface, and the lot's process log revealed the bake step had been shortened to one hour.
Result. Bake time was restored to 8 hours started within 4 hours of plating, field claims stopped, and the adhesion test Cpk rose from 0.9 to 1.6.
Case 2 - chromium flaking on hydraulic rods in a Gulf state
Scenario. Hydraulic rods flaked after three months in a hot, humid Gulf environment, and the customer blamed the hard chrome deposit, which measured in spec.
Action. Sectioning showed the fault at the nickel underlayer interface, and humidity checks traced it to a rinse that ran dry during summer water restrictions.
Result. The rinse timer was interlocked with the line, and flaking claims dropped to zero over the following two quarters.
Frequently Asked Questions
Why do parts fail when QC passes?
Routine QC checks surfaces; many field failures start at the interface and appear under load or time.
When do I cross-section?
For any field failure, any adhesion complaint, and any dispute where surface checks disagree with performance.
How many parts should I section?
At least three from different positions in the failing lot before drawing any conclusion.
What finish do I need?
A 1 micrometer diamond final polish; coarse finishes wipe out cracks and layer detail.
How long should high-strength steel bake?
190-230 C for 4-24 hours depending on strength class, started within 4 hours of plating.
What is microhardness for?
It catches soft deposits that wear or crack early even when thickness is correct.
How do I test adhesion?
Use bend, file or thermal shock tests per ASTM B571, combined with a cross-section for proof.
What records prove the analysis?
Scale-marked photos at every stage plus the process log of the failing lot.
When is a failure closed?
When the root cause is traced to a process step, a fix is implemented, and three lots verify it.
What Would You Like to Solve?
Describe the failure mode, material, coating and process history of your failing part, and share what routine QC currently passes.