Combining induced compression and ferritic nitrocarburizing for an effective proactive and reactive surface engineering treatment for die casting tools J.V. Skoff Badger Metal Tech, Inc

Introduction Mechanical Ambient Compressive Stress FPP or MPCMP & Thermal Diffusion Compressive Stress Solder Protection

History of Peening and FNC Peening 11 th Century – Toledo Blades Automotive Leaf Springs – 20 th Century Today – Aerospace, Automotive, Military, Nuclear, Industry FNC or Nitriding Over 100 years old – Adolph Frye – Affinity Fe & N Evolution to today’s many types Gas, 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 Strip N(.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 - Controls Correct 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 steel Electron probe micro analysis Nitrogen concentration near 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 component highest 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, Untreated Peening – F Nitriding - N Courtesy Journal of Solid Mechanics and Materials Engineering – Keio University Japan – Vol 2 No. 2008

Chemical Composition Comparison Similar chemical compositions With exception of Ni, Cu, Al Die steels have higher amts Cr, 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) NFNN

Higher concentration of Nitrogen Higher 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 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 test FN was higher than N – implies porous compound layer effect on N NP (polished removal of compound layer) 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 stress AISI 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 400x FNC to a stoned surface FNC to MPCMP compound layer Diffusion density Dievar coupon – Rc denser 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