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

Introduction

Mechanical AmbientCompressive Stress

FPP or MPCMP

&Thermal Diffusion

Compressive StressSolder Protection

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

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

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

MPCMP (FPP) Curves

formulated curves of compression developed for the MPCMP

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)

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)

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

FNC Curve - Typical

FNC alone provides some shallow compressive values

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

Same Recipe different Results

Benefits of FNC

• Reduce soldering - barrier • Increase fatigue strength

(nitrogen and carbon)

• Wear resistance (not caused by cavitation)

• Corrosion and oxidation resistance

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

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

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

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)

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

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

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

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

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

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

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

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

MPCMP + FNC - Example

Tooling initially treated with MPCMP (ML) and then FNC (TL) – BMT recipe

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