FIRRAO D.

GEROSA R.

GHIDINI A.

IENCO M.G.

MATTEIS P.

PINASCO M.R.

RIVOLTA B.

SCAVINO G.

SILVA G.

SILVESTRI A.

STAGNO E.

UBERTALLI G.

English

IENCO M. G.

PINASCO M. R.

MATTEIS, PAOLO

SCAVINO, Giorgio

UBERTALLI, Graziano

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

05 August 2020POLITECNICO DI TORINORepository ISTITUZIONALERelationships between tensile and fracture mechanics properties and fatigue properties of large plastic mold steel /FIRRAO D.; MATTEIS P.; SCAVINO G.; UBERTALLI G.; IENCO M.G.; PINASCO M.R.; STAGNO E.; GEROSA R.;RIVOLTA B.; SILVESTRI A.; SILVA G.; GHIDINI A.. - (2006). ((Intervento presentato al convegno TMS 2006 AnnualMeeting (TMS2006) tenutosi a San Antonio, Texas, USA nel 12/3/2006 - 16/3/2006.OriginalRelationships between tensile and fracture mechanics properties and fatigue properties of large plasticmold steelPublisher: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: 11583/1667886 since:2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USARelationships between Tensile and Fracture Mechanics Properties and Fatigue Properties of Large Plastic Mold SteelD. Firrao1, P. Matteis1, G. Scavino1, G. Ubertalli1, M. G. Ienco2, M. R. Pinasco2, E. Stagno2, R. Gerosa3, B. Rivolta3, A. Silvestri3, G. Silva3, A. Ghidini41Politecnico di Torino 2Università di Genova3Politecnico di Milano 4Lucchini Sidermeccanica2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAOverall views of a bumper mould.2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USASummary• Production cycle and critical issues of large plastic moulds• Sampling pattern and  re-heat-treatments• As-received microstructures• Mechanical properties and fatigue behaviour of as-received and re-heat-treated steel• Fracture surfaces• Conclusions2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAStresses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 1x1x2 m forged and pre-hardened steel blooms Applicationsautomotive components (bumpers, dashboards, …)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USASteel compositionSteel mill operationsingot casting (ESR refining is not possible)forging to 1x1 m sectionsdehydrogenizationoil quenchingtempering (one or more stages)C Cr Mn Ni Mo Si S P1.273840CrMnNiMo8-6-40.35-0.451.8-2.11.3-1.60.9-1.20.15-0.250.2 -0.4<0.03 <0.03Examined bloom 0.42 2.0 1.5 1.1 0.21 0,37 0.002 0.006Usual Production cycle (I)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA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 dimensionsUsual Production cycle (II)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAForging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(one or more stages)550-600°C 1-2 days (each stage)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAExperimental (I): sampling of the original bloom12x18 mm section blanks38 mm thick KIC specimens (LT)Mould blankResidualForged & heat-treated surfacesDepth[mm]As-receivedIndividually re-heat-treatedSlab2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA38 mm as-received KIC specs.BlanksRound tensile specs. (L)Metallographic samplesRe-heat-treated Charpy-V  specs. (LT)38 mm re-heat-treated KIC specs.Experimental (II): sampling pattern & re-heat-treatments Rotating bending fatigue specimens (L)Charpy-V  specs. (LT)Re-heat-treatments: 860°C ¾h / N2 or air / 590°C 3h / 550°C 3has-received or re-heat-treatedRound tensile specs. (L)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA55 mm depth 105 mm450 mm 650 mm - coreAs-received microstructures vs. depth (Nital etch)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAHardness, tensile and fracture toughness tests250275300325350375400Hardness [HV100]440 mm145 mm (I)145 mm (II)RHT6507708901010113012500 200 400 600 800 1000 1200Depth [mm]Tensile properties [MPa]YSUTSYS, RHTUTS, RHT2035506580951100 200 400 600 800 1000 1200Depth (mm)KIc [MPa√m]As-receivedRHT0,050,10,150,20,250,3Hardening exponentRHT250275300325350375400Hardness [HV100]440 mm145 mm (I)145 mm (II)RHT6507708901010113012500 200 400 600 800 1000 1200Depth [mm]Tensile properties [MPa]YSUTSYS, RHTUTS, RHT2035506580951100 200 400 600 800 1000 1200Depth (mm)KIc [MPa√m]As-receivedRHT0,050,10,150,20,250,3Hardening exponentRHT2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USACharpy-V tests & transition curvesAs received steelTransition curves 175 °C tests02040600 100 200 300Temperature [°C]KV [J]coresurfaceRHT0 100 200 300 400 500 600 700Depth [mm]175 °C2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USASurvivalProbabilityStress [MPa] As-received Re-heat-treatedCore(~560 mm)Surface (~140 mm)Core(~560 mm)Surface (~140 mm)10% 518 581 638 70690% 469 537 577 69450% 493 19 559 17 608 24 700 5test n. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 X 0[MPa]500 X X 2 0490 O X O X 2 2480 O O X 1 2470 X O O 1 2460 O O 0 2450 O 0 1Rotating bending fatigue tests – 4.2 Mcycles endurance limit Staircase method (example below: core as-received specimens) 25% increase2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USA40 mm depthintergranular123 mm depthintergranular & cleavage667 mm depthquasi-cleavage & ductile areasFractography (I): Charpy-V test - brittle areas (as received specs.)2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAFractography (II): KIc tests – as received specs.60 mm depth – intergranular & cleavage395 mm depthcleavage & ductile areas2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAFatigueprecrackBrittle propagationFractography (III): KIc tests – re-heat-treated specs.2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAFractography (IV): fatigue tests – fatigue areasAs-receivedSurface (~140 mm)Core (~560 mm)Re-heat-treated2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USASurface (~140 mm)intergranularCore (~560 mm)cleavage & ductileRe-heat-treated (originally ~560 mm)intergranular (partially ductile)Fractography (V): fatigue tests – overload areas2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAFractography (VI): remarksMacroscopically brittle (overload) fracture mechanisms • Charpy-V, KIc and fatigue test specimens with similar microstructures show similar microscopic fracture mechanisms.• Core and intermediate depth as-received microstructures show cleavage or quasi-cleavage fracture with some ductile areas.• Both as-received (low depth) and re-heat-treated tempered martensite microstructures show mainly intergranular fracture.Toughness of tempered martensite microstructures• Only the re-heat-treated samples show ductile regions at the crack tip of the KIc specs. (and thus higher toughness).• Differences in the tempered martensite carbide distribution, not observable by the O.M., must be supposed.2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAConclusions (I) Mixed microstructures occur throughout the examined bloom. The bloom fracture toughness is exceptionally low (about 40MPa√m) for a Q&T steel, considering the achieved UTS. The plain-strain fracture prevalently occurs by decohesion,coherently with the fact that, at room temperature, this steel is in itsbrittle temperature range. The low toughness must be attributed to the microstructures causedby the heat treatment, and in turn to the large dimensions of theblooms and of the moulds. The much higher toughness of the re-heat-treated samples must beattributed to microstructural differences on a sub-micron scale.2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USAConclusions (II) The rotating bending fatigue endurance limits scale with the tensilestrength, rather than with the fracture toughness. The endurance limits of the re-heat-treated samples is 25% higher,keeping the differences due to the original location. The low fracture toughness is a critical property; the lower fatigueendurance limit allows for a critical crack to develop more rapidlythan in a fully Q&T condition.2006 Annual Meeting; March 12-16, 2006 — San Antonio, TX, USARelationships between Tensile and FractureMechanics Properties and FatigueProperties of Large Plastic Mold SteelD. Firrao1, P. Matteis1, G. Scavino1, G. Ubertalli1, M. G. Ienco2, M. R. Pinasco2, E. Stagno2, R. Gerosa3, B. Rivolta3, A. Silvestri3, G. Silva3, A. Ghidini41Politecnico di Torino 2Università di Genova3Politecnico di Milano 4Lucchini SidermeccanicaThank you for your attention!

Relationships between tensile and fracture mechanics properties and fatigue properties of large plastic mold steel

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Relationships between tensile and fracture mechanics properties and fatigue properties of large plastic mold steel

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