FATIGUE MECHANISMS IN P/M COMPONENTS Worcester Polytechnic Institute October 27-28, 2004 Diana Lados & Diran Apelian M orris B oorky P owder M etallurgy R esearch C enter
OUTLINE I. Impact of Porosity and Microstructure on Fatigue and Fatigue Crack Growth Behavior of P/M Components (examples from the literature) Effects of porosity � Effects of microstructure � II. WPI project … Objectives and Experimental Plan
BACKGROUND Factors controlling fatigue behavior … 1. Porosity: Amount (% Porosity) � Type (Open/Closed) � Morphology/Distribution (influenced by sintering � conditions and initial powder type/morphology) 2. Microstructure: Transition from pore-control (low density) to � microstructure-control (high density) Homogeneous vs. Heterogeneous � (pre-alloyed vs. admixed) Phase/Amount of phase (Martensite, Bainite, Pearlite, � Ni-rich areas, etc.)
BACKGROUND Fatigue life vs. density/porosity … pore amount Fatigue limit Porosity % for both: � P/M iron � P/M steels in: � as-sintered � heat treated (quench and tempered)
BACKGROUND Fatigue life vs. density/porosity … pore amount Fe-1.75Ni-1.5Cu-0.5Mo-0.6C Fe-2Cu-0.8C
BACKGROUND Fatigue life vs. density/porosity … pore amount Fe-1.5Cu-0.6C Fe-2Cu-2.5Ni (60 min @ 2280°F) (30 min @ 2050°F)
BACKGROUND Fatigue life vs. density/porosity … pore type The effect of porosity type (open vs. closed) for a given density has not been reported in the literature ?????
BACKGROUND Fatigue life vs. density/porosity … pore morphology 30 min @ 2340°F 2340ºF 2192ºF 20 min @ 2050°F Fe-1.5Mo-0.7C Pre-alloyed ABC (atomized) Fe- 1.75Ni-1.5Cu-0.5Mo-0.5C (SE) and Admixed MH (sponge) Fe- 4Ni-1.5Cu-0.5Mo-0.5C (AE)
BACKGROUND Fatigue life vs. density/porosity … pore morphology 2050ºF 2282ºF 2340ºF 2050ºF Fe-1.75Ni-1.5Cu- Fe-4Ni-1.5Cu- 0.5Mo-0.5C 0.5Mo-0.5C Fe-4Ni-1.5Cu-0.5Mo S based on sponge Fe powder (diffusion alloyed ) A based on atomized Fe powder
BACKGROUND Fatigue life vs. microstructure … pore-control vs. microstructure-control With increasing density the differences between the strain life curves become larger Decreasing porosity increases microstructural influence on fatigue life Fe-1.75Ni-0.5Mo
BACKGROUND Fatigue life vs. microstructure … pore-control vs. microstructure-control Higher density Enhanced resistance to fatigue crack growth Uniform shifts Density/porosity dominates FCGR over the microstructure of the matrix Fe-1.75Ni-0.5Mo-0.5C (homogeneous - divorced pearlite)
BACKGROUND Fatigue life vs. microstructure … homogeneous (pre-alloyed) vs. heterogeneous (admixed) Homogeneous structure better fatigue crack growth resistance than inhomogeneous structure (different as-sintered microstructures for the homogeneous (divorced pearlite) and inhomogeneous (ferrite, pearlite, and martensite) ∆ K, MPa m 1/2 Fe-1.75Ni-0.5Mo-0.5C
BACKGROUND Fatigue life vs. microstructure … homogeneous (pre-alloyed) vs. heterogeneous (admixed) Different as-sintered Pre-alloyed microstructures: Partially Pre-alloyed: martensite alloyed Partially alloyed: pearlite + Ni-rich ferrite, martensite, Ni-rich areas Admixed Admixed: pearlite + Ni-rich ferrite, martensite, Ni-rich areas Fe-4Ni-1.5Cu-0.5Mo-0.5C
BACKGROUND Fatigue life vs. microstructure … heterogeneous 1 (diffusion alloyed) vs. heterogeneous 2 (binder treated) Diffusion • no significant difference alloyed between diffusion alloyed and binder treated Binder treated Fe-1.75Ni-1.5Cu-0.5Mo-0.6C (divorced pearlite, martensite, and Ni-rich ferrite )
BACKGROUND Fatigue life vs. microstructure … microstructure 1 vs. microstructure 2 High cooling rate: finer pearlite + more martensite + bainite Slow cooling rate: pearlite + martensite + bainite Fe-4Ni-1.5Cu-1.5Mo-0.8C (diffusion alloyed )
OBJECTIVES Study the effects of density/porosity on the fatigue � initiation and propagation in P/M components; Investigate the porosity/microstructure interactions; � Understand the effects of microstructural homogeneity and � microstructural phases on dynamic properties – mechanisms; Create guidelines for fatigue design corroborated with the � fundamental understanding of the alloys behavior; Optimize the material characteristics and processing � parameters for enhanced fatigue response.
EXPERIMENTAL APPROACH Materials selection … Molding grades particles ( 70-85 µ m ) Chemical Sintered Ni Mo Mn composition C [%] 0.65 1.75-1.8 0.50-0.55 0.15-0.18 Ni Graphite Graphite Admixed (hybrid) Pre-alloyed (QMP ATOMET 4001 (QMP ATOMET 4601 Mo pre-alloyed powder Ni-Mo pre-alloyed powder) admixed with Ni)
EXPERIMENTAL APPROACH Project phases … Phase I (a): Phase I (a): Effects of porosity amount on fatigue behavior; Phase I (b): Microstructural effects on fatigue response; Phase I (b): Phase II: Effects of porosity type & morphology (size/shape) Phase II: on fatigue. Is fatigue resistance a state function ???
BACKGROUND Total porosity … Open porosity Total Porosity: Open + Closed + Isolated Open porosity: continuous pore channels intersecting the surface of the specimen (and each other) Closed porosity: closed gaps between powder particles resulting from compaction and/or sintering (not accessible to the surface BUT can be connected to each other !!) Closed Isolated porosity: pores present in the initial porosity Isolated powder particles (not affected by compaction porosity and sintering)
EXPERIMENTAL APPROACH Phase I … Porosity considerations 3 “total porosity” levels (total porosity is � Porosity measured from geometry/weight data) amount amount Pores of the same type (open) and similar � is studied morphology for both pre-alloyed and admixed Set 1 Set 2 Set 3 Density [g/cm 3 ] ~6.9 ~7.2 7.83 Micro- structure
EXPERIMENTAL APPROACH Open porosity measurement techniques … Open porosity Open porosity Closed unresolved by penetrated by He porosity penetrating oil Open porosity penetrated by oil Isolated porosity Gas-impregnation Oil-impregnation (ASTM B328) More accurate measurements of Calculate the interconnected open porosity due to increased porosity from the volume of oil that has impregnated the pore penetration ability of gases compared to oils - RECOMMENDED specimen - OVERESTIMATIONS
EXPERIMENTAL APPROACH Gas-impregnation measurement techniques … pycnometry Gas displacement pycnometer: a sample of known weight (a solid, a powder, or a porous material) is placed in one of the chambers and the change in pressure needed to balance the two chambers is used to calculate the volume of the sample (P 1 V 1 =P 2 V 2 ) bulk density pore free density pycnometric density
EXPERIMENTAL APPROACH Calculation of open/closed porosity using He pycnometry … ρ = − V 1 bulk % Total porosity = 100 x V total total ρ − pore free ρ = − V 1 bulk % Open porosity = 100 x V open open ρ pycnometri c V closed = V total – V open % Closed porosity = 100 x V closed * The amount of isolated porosity was assumed insignificantly small
EXPERIMENTAL APPROACH Phase I … Porosity/density considerations 3 “total porosity” levels (total porosity is � Porosity measured from geometry/weight data) amount amount Pores of the same type (open) and similar � is studied shape factor for both pre-alloyed and admixed Set 1 Set 2 Set 3 Density [g/cm 3 ] ~6.9 ~7.2 7.83 Micro- structure
EXPERIMENTAL APPROACH Phase I … Compaction + Sintering Compaction: � low densities (Set 1): normal compaction; � intermediate densities (Set 2): controlled temperature ↔ compaction (warm compaction 145 ° F ); high densities (Set 3): powder forging. � Sintering: � � temperature: T=2050 ° F ; � time: t=30 min; � T and t invariant for phase I.
EXPERIMENTAL APPROACH Open/closed porosity results using He pycnometry … Sintered conditions 1.25 1 Closed porosity (%) 0.75 A4601 A4001 0.5 Admixed Pre-alloyed 0.25 0 6.8 6.9 7 7.1 7.2 7.3 7.4 Density (g/cm 3 )
EXPERIMENTAL APPROACH Phase I … Microstructural considerations Low density High density I. Pore control Pore/Matrix control Matrix control Powder Fatigue Pre-alloyed type (1) behavior (1) (homogeneous) ? ? II. Powder Admixed Fatigue type (2) behavior (2) (heterogeneous) Cooling Fatigue Microstructure 1 rate (H) behavior (H) ? ? III. Cooling Fatigue Microstructure 2 rate (L) behavior (L)
EXPERIMENTAL APPROACH Phase I … Heat treatment Post sintering heat treatment: � � austenitize @ 1700 ° F for 30 min (similar austenitic grains) � No-oil quench to 2 microstructures ( for both pre-alloyed and admixed ) : 40%Martensite + 60%Pearlite + Martensite + (~5% Ni reach areas) (5% Ni reach areas) � temper @ 400 ° F for 1 hr (similar matrix micro-hardness)
EXPERIMENTAL APPROACH Phase I … Heat treatment 1 … High bar He quench 6 bar He quench (~9.9 ºF/sec) 10 bar He quench (cooling rate close to oil quench <20 ºF/sec) 7 bar He quench (~11.7 ºF/sec) Pre-alloyed Admixed 100% Martensite 90-95% Martensite (5% Ni reach areas)
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