Presentazione orale al European Congress on Advanced Material and Processes (Euromat 2005), Praga (R. Ceca), 5/10/2005 - 8/10/200

FIRRAO D.

GEROSA R.

GHIDINI A.

MATTEIS P.

PINASCO M.R.

RIVOLTA B.

SILVA G.

STAGNO E.

English

FIRRAO, Donato

MATTEIS, PAOLO

PINASCO M. R.

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

05 August 2020POLITECNICO DI TORINORepository ISTITUZIONALERelation between toughness, infinite fatigue life and microstructure in large blooms for automotive plastic molds. /FIRRAO D.; MATTEIS P.; PINASCO M.R.; STAGNO E.; GEROSA R.; RIVOLTA B.; SILVA G.; GHIDINI A.. - (2005).OriginalRelation between toughness, infinite fatigue life and microstructure in large blooms for automotive plasticmolds.Publisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/1667876 since:RELATION BETWEEN TOUGHNESS, INFINITE FATIGUE LIFE AND MICROSTRUCTURE IN LARGE BLOOMS FOR AUTOMOTIVE PLASTIC MOLDS.D. Firrao, P. MatteisDip. di Sc. dei Materiali e Ing. Chimica - Politecnico di TorinoM.R. Pinasco, E. StagnoDip. Chimica e Chimica Industriale - Università di GenovaR. Gerosa, B. Rivolta, G. SilvaDip. Meccanica - Politecnico di MilanoA. GhidiniLucchini Sidermeccanica S.p.A.Overall views of a bumper mold.Stressesapplied stresses: injection pressure thermal gradients notch effectswear by reinforced resins flow fatigue: millions of piecesstresses raised by: cracks (improper weld bed depositions), abnormal operations (incomplete extraction).different microstructures expected at increasing depths after quenchany microstructure could be found at mold faceExperience-based design, no usual defect-allowance calculation procedure Reported macroscopically brittle in-service failuresPlastic molds machined from 1x1x3 m forged and pre-hardened steel blooms Applicationsautomotive components (bumpers, dashboards, …)Steel mill operationsingot casting (ESR refining is not possible)forging to 1x1 m sectionsdehydrogenizationoil quenchingtempering (one or more stages)Mold machining shop operationschip-removal and/or electrical-discharge machining to the mold shape, grinding with or without polishing in selected areaslocal surface treatments eventual corrections using weld bed depositionsCommercial warehouse operationsremoval of rough and decarburized surfaces (up to 10-20 mm) sawing to requested dimensionsC Cr Mn Ni Mo Si S P1.273840CrMnNiMo8-6-40.350.451.82.11.31.60.91.20.150.250.20.4<0.03 <0.03Examined bloom 0.42 2.0 1.5 1.1 0.21 0,37 0.002 0.006Steel compositionUsual Production cycleForgingcomparable ingot and bloom sectionsome repeated forging steps total reduction ratio much lower than in rolling (and not comparable)Usual Production cycle (cont.)Heat treating in airStep Temperature Durationhydrogen removal a few daysaustenitizing 840-880°C 1-2 daysoil quench - -tempering to 330-300 HB(two stages)550-600°C 1-2 days (each stage)SamplingBlanks38 mm thick KIC specimensHardness & metallographyRound tensile specs.Mold blankResidualCharpy specs.Forged & heat-treated surfacesDepthSampling (cont.): fatigue specimens38 mm KIC specimens559142 SubsurfaceClose-to-coreBWL/2Rotating bending fatigue specimens (L )55 mm depth 105 mm450 mm 650 mm - coreMetallography – microstructures vs. depth (Nital etch)Fine and ultra-fine pearlite, upper bainitemodified by temperingFine pearlite, upper bainite modified bytemperingTempered martensite, retained austenitetransformed during tempering.Lower bainite modified by tempering,retained austenite transformed duringtemperingHardness decreases from surface to core.Tensile properties are adequate at the surface and decrease markedly at core (esp. YS).Fracture toughness values are rather low.Charpy KV impact absorbed energy50% FATT: Core 150°C - Surface 270°CMechanical properties: hardness, tension, fracture toughness310320330340350360370Hardness (HV100)3035404550550 300 600 900 1200Depth (mm)KIc (MPa√m)KIcnot valid600700800900100011001200Tensile properties (MPa)UTSYSSubsurface position (156 mm depth)Metallography: microstructures at chosen positions (Nital etch)Close-to-core position (552 mm depth)Lower bainite modified by temperingRetained austenite with finely scattered dark carbides due to its transformation during temperingPearlite is completely absent.Tempered martensiteMetallography: subsurface microstructure – detail (Nital etch)Fine and ultra-fine pearlite Upper bainite, modified and subjected to carbide coarsening during the tempering stages(lightly attached upper bainite)Metallography: close-to-core microstructure – detail (Nital etch)Rotating Bending Fatigue• All samples have been machined from thetwo halves of two broken KIc samples, onenear the surface (B14) and the other next tothe core (B9).BWL/2Dmin (nom)= 6 mmRotating Bending Fatigue• Samples from two halves were tested in theoriginal condition (B9 and B14);• Samples from the other two halves weretested after air quenching and doubletempering (B9T and B14T). Austenitizationwas carried out at about 860°C for 45minutes, the first tempering at 590°C for 3hours and the second at 550°C for 3 hours.For each of the two broken KIc samples, two different conditions have been investigated:Rotating Bending FatigueD values were calculated according to thestaircase method (UNI-3964); the maximumnumber of cycles was assumed at 4.2.106(frequency = 50 Hz). Here follows an example.Test n. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15[MPa]540530520510500 X X490 O X O X480 O O X470 X O O460 O O450 O440o = test passedx = test failedRotating Bending Fatigue Limits-Results-• The material near the surface has a better fatiguebehaviour than the one next to core• Re-heat treatment highly improves the fatigue limit (25%)Depth [mm] σD (50%) [MPa]B9 625 493B9T 625 618B14 181 559B14T 181 700Fractography (As-received specs.)Fatigue fracture surfaceSubsurfaceClose-to-coreOverload fracture surfaceSubsurfaceClose-to-coreInitiation at inclusions was rareAs-receivedSubsurfaceClose-to-coreFractography: fatigue areasRe-heat-treatedAs-receivedSubsurfaceClose-to-coreFractography: overload areasRe-heat-treatedConclusionsMixed microstructures occur throughout a pre-hardened steelbloom for dies apt to large plastic components fabrication.The fracture toughness is exceptionally low for a Q&T steel.At the tested depths, KIc values were 38 MPa m ca. close tothe bloom surface and 43 MPa m ca. near the core.The low toughness is attributed to the slack quench, due to thelarge molds dimensions (1x1x3 m).Endurance limits were about 560 MPa for the steel close to thesurface and 495 MPa for the steel near the core. They scalewith the steel tensile strength, not with its fracture toughness.Endurance limits for samples individually re-heat-treatedincreased 25%, keeping the differences due to the location.This presentation was titledRELATION BETWEEN TOUGHNESS, INFINITE FATIGUE LIFE AND MICROSTRUCTURE IN LARGE BLOOMS FOR AUTOMOTIVE PLASTIC MOLDSThe authors were ItalianD. Firrao, P. Matteis, Politecnico di TorinoM.R. Pinasco, E. Stagno, Università di GenovaR. Gerosa, B. Rivolta, G. Silva, Politecnico di MilanoA. Ghidini, Lucchini Sidermeccanica S.p.A...JUST IN CASE YOU WERE LATEFOR INFORMATIONpaolo.matteis@polito.itTel.: +39-011-5644711Sorry, do not ask for me; sometimes I have difficulty in finding myselfTHANK YOU FOR YOUR ATTENTION

Relation between toughness, infinite fatigue life and microstructure in large blooms for automotive plastic molds.

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Relation between toughness, infinite fatigue life and microstructure in large blooms for automotive plastic molds.

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