J.V. Skoff Badger Metal Tech, Inc
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Transcript of J.V. Skoff Badger Metal Tech, Inc
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Combining induced compression and ferritic nitrocarburizing for an effective
proactive and reactive surface engineering treatment for die casting
tools
J.V. SkoffBadger Metal Tech, Inc
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Introduction
Mechanical AmbientCompressive Stress
FPP or MPCMP
&Thermal Diffusion
Compressive StressSolder Protection
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History of Peening and FNC
Peening11th Century – Toledo Blades
Automotive Leaf Springs – 20th CenturyToday – Aerospace, Automotive, Military, Nuclear, Industry
FNC or NitridingOver 100 years old – Adolph Frye – Affinity Fe & N
Evolution to today’s many typesGas, Heat, Time
Today – Automotive, Military, Industry
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Physics of shot peening
when media hits the surface there is a kinetic transfer of energy and a plastic deformation flow at the surface
just below the surface is a cold work compressive zone
the plastic flow at impact can also close minor cracks in softer metals such as hot work steels
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Compressive Curve - Typical
Example of non MPCMP compressive curve showing the high compression just below the surface which allows for polishing for cosmetic casting applications
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MPCMP (FPP) Curves
formulated curves of compression developed for the MPCMP
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Shot Peening controls (not blasting)
1. Media shape and size
2. Classification monitoring
3. Intensity – Almen StripN(.031”), A(.051”), C(.094”)
4. Saturation (no more compression)
5. Coverage (area)
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Benefits of MPCMP• INCREASE FATIGUE STRENGTH • CLOSE MINOR CRACKS (plastic flow)• RETARDS THE PROPAGATION OF EXISTING CRACKS• FURTHER INCREASE FATIGUE STRENGTH OF FNC
(when done before or after)• VERIFY SURFACE HARDNESS INTEGRITY • INCREASE EFFECTIVENESS OF WELDED AREAS• COUNTER EDM EFFECT
• POSSIBLE BUFFERING OF CAVITATION EFFECT (texturing)• REDUCE TENDENCY TO SOLDER (texturing)• REDUCE LAMINAR FILL (texturing)• ENHANCE FLOW (texturing)• BETTER POWDER COAT PAINT ADHESION (texturing)
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Chemistry of FNC
• Gases – Nitrogen, Carbon source• NH3, N, CO, or CO2• Heat – Atmosphere, Vacuum, Fluidized
Bed• Time – Below last temper temperature• Compound layer and diffusion zone
Micro hardness readings
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FNC Curve - Typical
FNC alone provides some shallow compressive values
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FNC - ControlsCorrect recipe for the steel
• Cleanliness – prevent contamination• Gases – type and %• Temperature – control• Time – enough but not too much• Pressure volume changes of gas
Too thick of diffusion zone leads to chipping, flaking, and cracking of the
FNC
micro hardness, diffusion zone, and compound layer
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Same Recipe different Results
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Benefits of FNC
• Reduce soldering - barrier • Increase fatigue strength
(nitrogen and carbon)
• Wear resistance (not caused by cavitation)
• Corrosion and oxidation resistance
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Characteristics of Each
MPCMP• Line of sight• Can weld or polish• Cannot repair existing cavitation breakout
FNC• No line of sight• Compound layer – should not be polished/stoned• Welding similar to nitrided surface
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Combining MPCMP (F) & FNC (N)
MPCMP then FNC
• Compressive value present after heat
• Proto X-ray diffraction coupons
• Keio U in Japan – AISI 4135 steelElectron probe micro analysis
Nitrogenconcentrationnear surface
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Compression Curves both sequences
First MPCMP then FNC (heat) compressive value and depth characteristics:•crystalline structure•dense compound layer•dislocation density•stratification pattern
First FNC then MPCMP – offers higher compressive characteristics:porous componenthighest fatigue strength
FPP removes porous compound layer component
the compound layer is brittle and will chip off if too thick
H-13 specimen 46-48Rc Oil
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2008 Study at Keio University Japan
• AISI 4135 Steel• Round Notch Charpy• Rotational Bending Fatigue Test• Fine Particle Peening (FPP)• Nitriding (N)• Various Combinations • FPP prior to N ref (FN)• FN - Higher fatigue strength than N alone• Highest fatigue strength with N prior to FPP (NF)
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Keio Specimens
7 specimens – FN, NF, F, N, FNP, NP, UntreatedPeening – FNitriding - N
Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008
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Chemical Composition Comparison
Similar chemical compositionsWith exception of Ni, Cu, Al
Die steels have higher amtsCr, Mo, V
Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008
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2008 Study Keio – The Why
a) compound layer N shows cracks and pores b) compound layer FN without cracks and pores (2% Nital etch)
FE-SEM view of surface shows stratification pattern and higher dislocation densities at the surface of the FN specimen enabling more nitrogen diffusion near the surface of (b)
N FNN FN
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Higher concentration of NitrogenHigher fatigue strength than N alone
hardness value of FN at surface higher than N. Depth of hardened layer .4mm (.016”)
more diffused N at the surface of FN. Higher dislocation densities and altered grain structure
Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008
Electron probe micro analysis
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Residual stress distribution
both N and FN generate residual compressive stress for increased fatigue strength
the stress value for FN an N and were similar (heat relaxes stress)
Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008
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Fatigue test results
Rotational bending fatigue testFN was higher than N – impliesporous compound layer effect on N
NP (polished removal of compoundlayer) Was higher than N
FN similar to FNP
Removal of porous compound layer after Nitriding by FPP (NF) shows the highest
fatigue strength.
Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008
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Distribution comparison FN and NF
H-13 compressive residual stressAISI 4135 compressive residual stress
FN
NF
Steels with greater hardness are able to generate higher compressive stress levels since yield stress is higher
Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008
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Profile Comparisons Dievar
400x
1000x 1000x
400x 400x
FNC to a stoned surface
FNC to MPCMP
compound layer
Diffusion density
Dievar coupon – 47-49 Rcdenser diffusion zone indicates higher concentration of nitrogen
STONED POLISHED
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MPCMP + FNC - Example
Tooling initially treated with MPCMP (ML) and then FNC (TL) – BMT recipe
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Summary & Conclusions
• Combining MPCMP and FNC have benefits over FNC or N alone
• MPCMP followed by FNC removes porosity and cracks in compound layer, increases fatigue strength and hardness, and increases N diffusion compared to N or FNC alone without disturbing the compound layer (preferred) (NEW or USED)
• FNC followed by MPCMP offers the greatest increase in fatigue strength, however, removes some of the compound layer which provides solder protection (alternate)
• MPCMP has advantages over FPP for HPDC