Table of Contents
Failure Case 1: Cracked Transmission Gear
Application Context
Automotive transmission gear, 42 teeth, module 2.0
What Occurred
15% of gears developed cracks during heat treatment (carburizing followed by oil quenching)
Root Cause Analysis
Sharp fillet radius specification (R0.2mm vs R0.5mm minimum recommended)
- PM microstructure contains distributed porosity acting as stress concentrators
- Sharp geometry combined with thermal stress and phase transformation stress
- Result: Crack initiation during quench
Corrective Solution
- Increased fillet radius to R0.8mm
- Modified to slower oil quench rate
- Implemented gradual quench temperature reduction
Results After Correction
Crack rate reduced from 15% to 0.2%
Estimated Cost Impact
Approximately $180,000 (scrap material + production delay + tooling modification)
Lesson Learned
PM Design Rule: Minimum fillet radius R0.5mm for standard parts; R0.8mm or greater for heat-treated components
Failure Case 2: Delaminated Lock Cylinder
Application Context
Commercial lock cylinder body, multi-level compacted part
What Occurred
8% of parts exhibited horizontal splitting (delamination) during sintering
Root Cause Analysis
Excessive compaction lubricant (1.5% vs 0.8% standard specification)
- Engineer increased lubricant attempting to reduce ejection force
- Created lubricant-rich layer between compaction levels
- During sintering, lubricant burned out leaving weakly bonded interface
Corrective Solution
- Reduced internal lubricant to 0.8%
- Implemented die wall lubrication spray system
- Increased press ejection capacity
Results After Correction
Delamination rate reduced to <0.1%
Estimated Cost Impact
Approximately $45,000 (tooling rework + scrapped production)
Lesson Learned
Excessive lubricant (>1.2%) creates weak planes. Use die wall lubrication rather than increasing powder lubricant content.
Failure Case 3: Warped Heat Sink Component
Application Context
Copper-infiltrated heat sink for 200W power module
What Occurred
Parts exhibited significant warpage during copper infiltration process at 1150 deg C
- Specification: +/-0.05mm flatness
- Actual measured: +/-0.4mm deviation
Root Cause Analysis
Asymmetric fin design (fins located on one side only)
- Copper infiltration occurred unevenly due to gravity and capillary effects
- One side became significantly heavier than other
- Part sagged under own weight at elevated temperature
Corrective Solution
- Redesigned with symmetric fin pattern (fins on both sides)
- Added temporary support ribs for sintering (removed post-process)
- Implemented graphite support plate fixture
Results After Correction
Flatness achieved: +/-0.06mm (within specification)
Estimated Cost Impact
Approximately $75,000 (tooling redesign + scrapped parts + project delay)
Lesson Learned
Design for geometric symmetry in high-temperature PM processes. Asymmetric mass distribution causes warping.
Failure Case 4: Insufficient Press-Fit Strength
Application Context
Automotive rocker arm hub pressed onto shaft assembly
What Occurred
Hub assemblies failed pull-off testing requirement (80 kN required, only 45 kN achieved)
Root Cause Analysis
Incorrect material selection (FC-0205 specified vs FN-0408 needed)
- Material selected primarily on cost basis ($1.20 vs $1.80 per part)
- FC-0205 yield strength: 280 MPa (as-sintered)
- Required material yield strength: >450 MPa
- Hub yielded during press assembly, reducing retention
Corrective Solution
Changed specification to FN-0408 material
Results After Correction
Pull-off force achieved: 95 kN (exceeded requirement)
Estimated Cost Impact
Approximately $220,000 (redesign + new tooling + testing program + production delay)
Lesson Learned
Never compromise material specification to save cost without thorough strength analysis. Under-specification leads to higher total cost through failures.
Failure Case 5: Leaking Hydraulic Valve Body
Application Context
Hydraulic valve body for 200 bar (2900 psi) operating pressure
What Occurred
18% of parts failed pressure leak testing (>5 cc/min leakage rate)
Root Cause Analysis
Inadequate sintered density (6.5 g/cm3 vs 7.2 g/cm3 required)
- Standard sintering process used without densification step
- Interconnected porosity provided leakage path for pressurized fluid
- No leak-sealing treatment applied
Corrective Solution
- Implemented copper infiltration process
- Increased final density to 7.4 g/cm3
- Added 100% pressure leak testing inspection
Results After Correction
Zero leakage failures in production
Estimated Cost Impact
Approximately $320,000 (scrapped parts + customer returns + warranty costs)
Lesson Learned
Leak-tight pressure applications require either:
- High sintered density (>7.2 g/cm3), or
- Infiltration treatment (copper), or
- Resin impregnation sealing
Failure Case 6: Premature Sprocket Tooth Wear
Application Context
Industrial chain sprocket, 20 teeth, continuous duty application
What Occurred
Sprocket teeth failing after only 200 hours service (specification: 5,000 hours minimum)
Root Cause Analysis
No heat treatment specified
- Designer assumed "high-strength PM alloy" designation (FN-0408) provided adequate hardness
- As-sintered hardness: 75 HRB (inadequate for wear resistance)
- Teeth wore rapidly leading to stress concentration and eventual fracture
Corrective Solution
- Added carburizing heat treatment process (surface hardness 58-62 HRC)
- Increased tooth fillet radius from R1.5mm to R2.0mm
Results After Correction
Service life achieved: 12,000 hours (2.4x original specification)
Estimated Cost Impact
Approximately $95,000 (warranty replacements + reputation impact + redesign)
Lesson Learned
Wear-critical and high-contact-stress applications require heat treatment specification. Material grade alone typically insufficient.
Failure Case 7: Out-of-Tolerance Bearing Bore
Application Context
Self-lubricating bronze bearing, Dia. 50mm bore specification
What Occurred
Bore diameter exhibited excessive variation: +/-0.15mm (specification: +/-0.03mm)
- Bearings loose on shaft assembly
- Caused vibration and noise in application
Root Cause Analysis
No sizing operation included
- Engineer assumed as-sintered tolerance would meet specification
- Sintering shrinkage naturally varies 0.5-1.5% depending on density and atmosphere
- No secondary operation to control final dimension
Corrective Solution
Added sizing operation (re-strike in precision die) to manufacturing process
Results After Correction
Tolerance achieved: +/-0.02mm (better than specification)
Estimated Cost Impact
Approximately $60,000 (100% machining of initial production run to salvage parts)
Lesson Learned
Tolerances tighter than +/-0.05mm typically require sizing or machining operations. As-sintered tolerance: +/-0.1-0.3mm typical.
Failure Case 8: Corroded Stainless Steel Components
Application Context
Food packaging machine guide rails specified as 304 stainless steel
What Occurred
Parts exhibited corrosion after only 3 months of daily washing operations
Root Cause Analysis
Inadequate sintering atmosphere
- Parts sintered in nitrogen atmosphere (insufficient reducing power)
- Surface chromium depletion occurred during sintering
- Protective passive film did not form properly
- Corrosion initiated at surface
Corrective Solution
- Changed to high-purity hydrogen atmosphere (dew point <-40 deg C)
- Implemented passivation treatment (nitric acid per ASTM A967)
- Added electropolishing for improved surface finish
Results After Correction
Zero corrosion observed in 24-month field evaluation
Estimated Cost Impact
Approximately $140,000 (product recall + replacement + additional qualification)
Lesson Learned
Stainless steel PM requires:
- High-purity reducing atmosphere (hydrogen or vacuum)
- Passivation chemical treatment
- High sintering temperature (1280-1350 deg C)
Failure Case 9: Seized Planetary Gearbox
Application Context
Power tool planetary gearbox with three planet gears
What Occurred
Gearbox seized during initial customer use (complete jam)
Root Cause Analysis
Tolerance stack-up error in assembly
- Three PM planet gears (+/-0.1mm tolerance each)
- PM ring gear (+/-0.1mm tolerance)
- Worst-case stack-up: +/-0.4mm total variation
- Insufficient clearance specification
- Interference condition in worst-case tolerance combination
Corrective Solution
- Applied sizing operation to planet gears (improved to +/-0.03mm)
- Machined ring gear (improved to +/-0.02mm)
- Increased design clearance by 0.2mm
Results After Correction
Zero seizure incidents in 500,000+ units produced
Estimated Cost Impact
Approximately $280,000 (tooling modifications + scrapped inventory + customer refunds)
Lesson Learned
Account for tolerance accumulation in assemblies. PM part tolerances (+/-0.1mm typical) multiply in stacked assemblies.
Failure Case 10: Brittle Aerospace Bracket
Application Context
Aerospace structural bracket, 17-4PH stainless steel
What Occurred
Bracket fractured during assembly bolt torquing operation
- Brittle fracture with no plastic deformation
- Unexpected failure in controlled assembly environment
Root Cause Analysis
Incorrect heat treatment condition
- Specification called for H1150 condition (target 40 HRC)
- Parts actually processed to H900 condition (44 HRC)
- Communication error with heat treatment supplier
- Higher hardness resulted in significantly reduced ductility
Corrective Solution
- Re-heat treated parts to proper H1150 condition
- Achieved 38 HRC with 12% elongation (vs 3% in H900)
- Implemented stricter heat treatment verification procedures
Results After Correction
All structural testing requirements met
Estimated Cost Impact
Approximately $420,000 (qualification program restart + tooling + project delay)
Lesson Learned
Heat treatment specifications must be precise and verified. Higher hardness does not always mean better performance - ductility often more critical for structural applications.
Cost Summary Table
| Failure Case | Root Cause Category | Estimated Cost Impact |
|---|---|---|
| 1. Cracked gear | Sharp geometry | $180,000 |
| 2. Delamination | Excess lubricant | $45,000 |
| 3. Warped heat sink | Asymmetric design | $75,000 |
| 4. Weak press-fit | Wrong material | $220,000 |
| 5. Leaking valve | Inadequate density | $320,000 |
| 6. Worn sprocket | No heat treatment | $95,000 |
| 7. Over-tolerance bore | No sizing operation | $60,000 |
| 8. Corroded stainless | Poor atmosphere | $140,000 |
| 9. Seized gearbox | Tolerance stack-up | $280,000 |
| 10. Brittle bracket | Wrong heat treatment | $420,000 |
| Total | ~$2.1M |
Common Design Rules Summary
Geometry Guidelines
- Fillet radius: R0.5mm minimum (R0.8mm for heat-treated parts)
- Wall thickness: 2-10mm optimal range for PM
- Draft angles: 1-3° to facilitate ejection
- Symmetric design: Prevents warping during high-temperature processing
Material Selection
- Match material grade to strength requirements (don't under-specify)
- Specify heat treatment for wear-critical and high-stress applications
- Stainless steel requires high-purity sintering atmosphere
Tolerance Management
- As-sintered: +/-0.1-0.3mm typical capability
- With sizing: +/-0.03-0.05mm achievable
- Account for tolerance stack-up in assemblies
Processing Requirements
- Leak-tight applications: Density >7.2 g/cm3 or infiltration
- Lubricant content: 0.6-1.0% (avoid excess)
- Heat treatment: Follow specifications precisely
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