Stress analysis of the working dies WORLD MONEY FAIR BERLIN 29/1/2015

Stress analysis of the working dies WORLD MONEY FAIR BERLIN 29/1/2015

Aims The knowledge of the stress state at each step of the production workings of the working dies in order to ensure a neutral stress when in press. Introduction of new processes and production steps might be monitored in order to avoid unwanted residual stresses: Mechanical workings Galvanic process (hard chromium) PVD coating (TiN, CrN, multylayer) CVD coating Laser engraving Colorful coating 2

Residual stress Is a stress present in a solid material after the original cause of the stress has been removed Compressive residual stress acts by pushing the material together Tensile residual stress pulls the material apart The stress state is a characteristic parameter of the material state (as microstructure and texture) and it defines the material proprieties Causes: 3 Casting (thermal residual stress) Reshaping (inhomogeneous plastic deformation) Mechanical (working residual stress) Joining (brazing, welding) Coating

Stress effects Residual stress is a key parameter for components and constructions of every size: parts of engine; aircrafts; bridges; ships; micro electrical devices On working dies residual stress has detrimental effects: Working lifetime Friction increase Monitoring of residual stress can give information on dies status and increase production performance 4 Applied stress Residual stress Resulting load

Samples 5 A B C1 C2 D E F G H Description Steel cut Steel turned Steel hobbed Experimental: samples Steel hobbed after annealing Working die turned Working die thermal treated (quenching+ tempering) before striking Working die thermal treated (quenching+ tempering) after striking Working die thermal treated (quenching+ tempering) and coated (PVD) before striking Working die thermal treated (quenching+ tempering) and coated (PVD) after striking Denomination: 1 Eurocent Common Face - Steel: Bohler K340 Coating: CrN; T deposition 350 C; thickness ~2 µm SI unit for stress is the Mega Pascal (MPa)

Residual stress measurements ASTM has standard test methods for the hole-drilling method and for X-ray diffraction measurements 6

X-Ray introduction X-Ray has been discovered in 1895 by German physicist Wilhelm Conrad Röntgen λ from 0.01 to 10 nm Diffrazione Bragg Law nλ= 2d sin Θ [1913] 1920-1930 Forerunners studies on stress measurements by X-Ray 7

Residual stress measurement by X-Ray X-ray diffraction is a non-destructive technique able to measure residual stress on crystalline and polycrystalline materials Residual stress causes crystal lattice distortion and a measurement of interplanar spacing of the crystal lattice determinates the residual stress magnitude Sampling depth: from few microns to some tens Crystalline structure Deformation Rotating the sample respect the measurement system, stress tensor can be obtained 8

Experimental: measurement conditions I X-ray measurements have been performed with the methodology sin2(psi) in the dies interested region Radiation: Mo K-alpha (λ = 0.7093 Å) Angolar step: 0.04 Sampling time: 10 sec/point Analyzed region: 10x2 mm Standard: UNI EN 15305: 2008 2Ɵ range: 118-126 corresponding [550] Ferrite peak 9 Instrument scheme

XRD instrument used Geometry «theta-theta» with a high resolution goniometer based on torque direct drive (patent) Flexible optics and structure allow diffraction, reflectivity and fluorescence measurements Primary optics Goebel mirror With Molybdenum anode for in depth measurement Euler Cradle allows a triax movement of the sample 10 Test accredited by UNI EN 15305:2008, Residual stress analysis by X-ray diffraction

Experimental: measurement conditions II Atomic Force Microscopy Analyses have been performed in order to evaluate the surface roughness of coated and uncoated samples Analytical conditions: Instrument: P47H NT-MDT Area analyzed: 80 x 80 microns Lateral resolution: 10nm Mode: Semicontact Samples analyzed: E; G The AFM principle is based on the interaction force of a tip (some nanometer apex size) with the atoms of the surface of the sample 11

CrN coating SIMS profile Intensity (counts) 4 10 3 10 Na + Al + K + V + Cr + Fe + CrO + 10 4 10 3 Intensity (counts) 2 10 10 2 1 10 10 1 0 10 10 0 12 0 1000 2000 Depth (nm)

Results (I): Stress measurements Sample C2 Sample D 13 XRD scan in correspondence of [550] Ferrite peak. Peak shift in function of φ gives the stress value

Results (II): Stress measurements Sample E Sample F 14 In thermal treated (quenching+ tempering) samples a peak bordering is observed. The size of crystalline grain is decreased after thermal treatment by carbides precipitation inside the phase Fe-bcc.

Results (III): X-ray diffraction patterns Sample D Sample E 15 Complete XRD spectra confirms carbides precipitation. Bordering of high angles peaks [Fe-bcc] New peaks at low angles [carbides]

Results (IV): Morphology measurements Surface morphology comparison between PVD coated and uncoated samples. A qualitative change of the surface is observable Anyway increase of average roughness and peak to peak values is limited Sample E Average roughness: Sample G 237.9 nm Sample E 267.7 nm Sample G Peak to Peak: Sample G 0.54 µm Sample E 0.71 µm 16

Results (V): Stress values Samples Description Residual Stress [Mpa] A Steel cut + 175.71 B Steel turned +743.12 C1 Steel hobbed Less than -900 C2 Steel hobbed after annealing +22.62 D Working die turned +143.89 E F G H Working die thermal treated (quenching+ tempering) before striking Working die thermal treated (quenching+ tempering) after striking Working die thermal treated (quenching+ tempering) and coated (PVD) before striking Working die thermal treated (quenching+ tempering) and coated (PVD) after striking +15.24-462.56 +54.20-782.00 17

Conclusions Samples A; B; D (after mechanical treatments) present tensile residual stresses Samples after thermal treatments present an important reduction of residual stress (C2, E) Samples after hobbing (plastic deformation) present significant compressive residual stress values (C1) Negligible difference in residual stress among coated and uncoated samples (G, E) Samples after striking (F, H) present compressive residual stresses Carbides precipitations are observed in thermal treated (quenching+ tempering) samples Limited differences in morphologies due to coating application 18

Future developments New portable XRD equipment for residual stress analyses Low power source (4W) Cr (Kα) anode Linear strip detector Psi angle from +45 to -45 Positioning by laser pointer 19

IPZS S.p.A. Italian Mint Via Gino Capponi 47/49 00179 ROME www.ipzs.it Fondazione Bruno Kessler Centro Materiali e Microsistemi (FBK-CMM) via Sommarive 18, Trento 38123, Italy, www.fbk.eu

Steel cut

Steel turned

Working dies thermal treated