PERFORMANCE OF P/M COMPONENTS DURING DYNAMIC LOADING Worcester Polytechnic Institute October 22-23, 2003 Diana Lados & Diran Apelian M orris B oorky P owder M etallurgy R esearch C enter
OUTLINE ß Background (and examples of fatigue studies from the literature) Objectives ß ß Our Approach … Experimental Plan ß Critical experimental details
BACKGROUND General perspectives … Two design concepts Defect intolerant Defect tolerant
BACKGROUND Defect intolerant … Cyclic Strain range, De p Ferrous Stress, s Stress, s Monotonic Nonferrous Strain, e LCF (N < 10 5 ) HCF (N > 10 5 ) s -N curve e -N curve CSS curve (Basquin eq.) : (Coffin-Manson eq.) : (cyclic stress-strain eq.) : D B ( ) C ⋅ N ( ) A ⋅ N D e = s = p a ' ( ) ' n K D s = ⋅ D e p D e D s p ' ) c ' ) b ( ( 2 N 2 N = e ⋅ = s ⋅ f f 2 2
BACKGROUND Defect intolerant … contd. LCF – high plastic deformation/ low loading cycles LCF HCF HCF – quasielastic behavior/ very high loading cycles ( r low) 10 < N f tr < 1000 ( r high) PM steels (Ni-Mo)
BACKGROUND Defect tolerant … D K FT II III I Fatigue crack growth curve (LEFM) log(da/dN) da m ( ) C K = D (Paris eq.) : dN m ( ) da C K D = ( ) dN 1 R K K - ⋅ - D (Forman eq.) : FT D K th log D K
BACKGROUND High cycle fatigue (HCF) … P/M iron Region A: - nucleation of microcracks; Region B: -appearance of slip bands on the specimen surface; Region C: - characteristic S-N curves where final failure is caused by macrocracking; P/M iron
BACKGROUND High cycle fatigue (HCF) … P/M iron … contd. Region I: - mostly closed porosity; - cracking in the specimen interior; - transgranular path between isolated pores; Region II: - transition from closed to open porosity; - cracks nucleate @ specimen surface at isolated pores and pore clusters; - some broken sintering necks; Region III: - pores connected to each other (open) - biphasic material: matrix phase + pore phase; - simultaneous cracks @ specimen surface - broken surface is smooth in both fatigue and P/M iron ductile fast fracture regions (broken sintering necks).
BACKGROUND High cycle fatigue (HCF) … P/M iron … contd. Tension- compression Water atomized Reduced Plane sponge bending • Axial testing – volume properties Life (samples from reduced sponge powder) > • Bending – surface properties Life (samples from water atomized powder) • Fatigue limit (bending) < Fatigue limit (axial loading)
BACKGROUND High cycle fatigue (HCF) … P/M steels Fe-1.75Ni-1.5Cu-0.5Mo-0.6C (TM & S) Fe-2Cu-0.8C (P & F) Fatigue life increases with increasing density and pore/matrix interactions pore shape factors BUT density alone can not describe fatigue behavior of such PM materials
BACKGROUND High cycle fatigue (HCF) … P/M steels … contd. Fe-1.75Ni-1.5Cu-0.85Mo-0.6C Fe-1.75Ni-1.5Cu-0.5Mo-0.6C • no significant difference between binder-treated and diffusion alloyed • If Mo Fatigue life
BACKGROUND Low cycle fatigue (LCF) … P/M steels With decreasing density the differences between the strain life curves become smaller Increasing porosity reduces microstructural influence on fatigue life Fe-1.75Ni-0.5Mo
BACKGROUND Cyclic stress-strain … P/M iron I. Pure elastic response II. Microcracks opening (plastic strain- softening) III. Pronounced opening of microcracks overrides matrix hardening IV. Growth of macro-cracks / final failure Changes in hysteresis loop indicate hardening/softening of the material
BACKGROUND Cyclic stress-strain … P/M iron … contd. • r K’ P/M iron : cyclic softening • n’ unaffected by density • high density materials K’~K’ fully-dense
BACKGROUND Cyclic stress-strain … P/M steels Inhomogeneous Homogeneous P+F+M P Fe-1.75Ni-0.5Mo: low strain - softening & high strain - hardening Fe-2Cu-2.5Ni: work hardening Fe-1.5Cu-0.6C: softening Fe-0.8P (F&P) : cyclic hardening
BACKGROUND Fatigue crack growth studies, da/dN vs. D K … ß Near threshold PM/C & W similar behavior; ß Higher D K , PM inferior to C & W, cracks grow one order of magnitude faster; ß Pseudo fracture toughness D K c much lower in PM, 20-50 MPa m 1/2 (compared to 80-130 MPa m 1/2 for quenched and tempered steels).
BACKGROUND Fatigue crack growth studies, da/dN vs. D K … contd. 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 P)
SUMMARY OF THE LITERATURE REVIEW Many investigations on various P/M materials, but little ß knowledge on fatigue mechanisms and fracture; Pore/matrix interactions and how the presence of pores ß influences/changes the behavior of the matrix are not understood; ß Characteristic microstructural features as well as inhomogeneities need to be individually understood and further correlated to the pore structure (deconstruct/reconstruct); Fatigue life data corroborated with a fundamental ß understanding of the alloys behavior predictive abilities; ß There are no systematic studies to provide “knowledge based recipes” to optimize material characteristics and processing parameters for enhanced fatigue and fatigue crack growth
OBJECTIVES Study the effects of density/porosity on the fatigue ß initiation and propagation in P/M components; Investigate the porosity/microstructure interactions; ß ß Understand the effects of different microstructural phases on dynamic properties – mechanisms; Create guidelines for fatigue design corroborated with ß the fundamental understanding of the alloys behavior; ß Optimize the material characteristics and processing parameters for enhanced fatigue response.
EXPERIMENTAL APPROACH Materials selection … Chemical Graphite C Ni Mo Mn O composition additions [%] ~0.003 ~0.1 0.6 1.8 0.50-0.55 0.15-0.20 Molding grades particles ( 50-75 m m ) Admixed Pre-alloyed (QMP ATOMET 4001 (QMP ATOMET 4601 Mo pre-alloyed powder Ni-Mo pre-alloyed powder) admixed with Ni)
EXPERIMENTAL APPROACH Phases … Phase I (a): Mechanistic understanding of the effects of Phase I (a): pore amount/type on fatigue behavior; ß Find the relationship density-open/closed porosity ratios for our composition-processes; ß Pore/Microstructure (matrix) interactions; Phase I (b): Microstructure effects on fatigue response; Phase I (b): ß Microstructure 1 vs. Microstructure 2; Phase II: Is fatigue resistance a state function ??? Phase II: ß Effects of pore size/shape on fatigue.
EXPERIMENTAL APPROACH Phase I … Density –closed/open porosity relationship ß Produce samples of our composition in both pre-alloyed and admixed conditions; ß Adjust compaction (conventional press, warm compaction, powder forging, etc.) to get the full range of densities: 7.75 Density or <6.5 7.0 [g/cm 3 ] highest possible Open pores 100% 100% Porosity Open Closed Closed pores Set 1 Set 2 Set 3
EXPERIMENTAL APPROACH Phase I … Density levels selection Set 1 Set 2 Set 3 Density [g/cm 3 ] 6.8-6.9 7.2-7.25 7.75+ 70% open 30% open Pore porosity porosity Low level of amount/ & & closed porosity type 30% closed 70% closed porosity porosity Micro- structure
EXPERIMENTAL APPROACH Phase I … Compaction +Sintering ß Compaction: ‘ low densities (Set 1): normal compaction; ´ intermediate densities (Set 2): controlled temperature compaction (warm compaction 145 ° F ); ” high densities (Set 3): powder forging. ß Sintering: · temperature: T=2050 ° F ; 6 time: t=30 min; ÿ T and t invariant for phase I.
EXPERIMENTAL APPROACH Phase I … Heat treatment ß Post sintering heat treatment: ÿ austenitize @ 1600-1700 ° F for 30 min (similar austenitic grains) ÿ quench to 2 microstructures ( for both pre-alloyed and admixed ) : Martensite + Bainite Martensite + ~10% R.A. Martensite + Pearlite + ~10%R.A. Martensite + Bainite + Pearlite ÿ temper @ 350-450 ° F for 30 min-1 hr (similar matrix micro-hardness)
EXPERIMENTAL APPROACH Phase I … Effects of pores and microstructures ß Two microstructural considerations are relevant: Phase I(a): pores vs. matrix - How porosity interacts with the matrix and when microstructure becomes cause of failure - Two microstructures (M / M+X) will be analyzed at three porosity levels and the pore-to-matrix transition will be investigated for all the 12 cases (6 for pre-alloyed and 6 for admixed) Phase I(b): matrix 1 vs. matrix 2 - How different microstructures influence fatigue behavior - Two microstructures will be studied and their effects on fatigue initiation and propagation will be assessed for both pre-alloyed
EXPERIMENTAL APPROACH Phase I … Two microstructural considerations Low High density density A. Pore Pore/Matrix Matrix control control control Cooling Fatigue Microstructure 1 rate 1 behavior 1 ? ? B. Cooling Fatigue Microstructure 2 rate 2 behavior 2
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