FATIGUE MECHANISMS IN P/M COMPONENTS

Worcester Polytechnic InstituteApril 28-29, 2004

Diana Lados & Diran Apelian

Morris Boorky Powder Metallurgy Research Center

OUTLINE

I. Impact of Porosity on Fatigue and Fatigue Crack Growth Behavior of P/M Components (examples from the literature)

Relevance of open/closed porosity

Open/closed porosity measurement techniques

II. WPI project … Objectives and Experimental Plan

BACKGROUNDFactors controlling fatigue behavior …

1. Porosity/density:

Amount (% Porosity)Type (Open/Closed)Morphology (size/shape - due to initial powder

morphology and sintering conditions)Distribution (initial powder size)

2. Microstructural phases:

Amount (% Phase)Type (Martensite, Bainite, Pearlite, Ferrite,

+ Cementite, Ni-rich areas, etc.)Size (Fine/Coarse)

BACKGROUNDFatigue life vs. density/porosity …

Fatigue limit Porosity %

for both: P/M ironP/M steels

in: as-sinteredheat treated (quench and tempered)

BACKGROUNDFatigue life vs. density/porosity … sintered P/M iron

Water atomized

Reduced sponge

4 hrs @ 2050°F

30 min @ 2280°F

Sintering treatment affects fatigue life at intermediate porosity ranges

(water atomized iron powder, single or double pressed, and sintered @ 2050°F (30 min) and 2280°F (4 hrs))

Life (samples from reduced sponge powder) >Life (samples from water atomized powder)

(single, double, triple sequence sintering @ 2050°F or 2200°F)

BACKGROUNDFatigue life vs. density/porosity … sintered P/M steels

Large

Small

Base

Bimodal

20 min @

2050°F

30 min @

2340°F

ABC (atomized)

MH (sponge)Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (SE)

Fe-1.5Cu-4Ni-0.5Mo-0.5C (AE)and Fe-2Cu-0.8C (30 min @ 2050°F)

BACKGROUNDFatigue life vs. density/porosity … sintered P/M steels

Fe-2Cu-2.5Ni (60 min @ 2280°F)Fe-1.5Cu-0.6C (30 min @ 2050°F)

BACKGROUNDFatigue life vs. density/porosity … Q&T - P/M steels

S Q&T

Fe-1.5Cu-1.75Ni-0.5Mo-0.6C

Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (sponge)

Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (atomized)

Fe-1.5Cu-4Ni-0.5Mo-0.5C (atomized)

BACKGROUND∆Kth vs. density/porosity … sintered P/M iron and steels

Fe (atomized)

S Q&T

Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (sponge)

Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (atomized)

Fe-1.5Cu-4Ni-0.5Mo-0.5C (atomized)Fe-1.5Cu-1.75Ni-0.5Mo-0.6C Fe-1.75Ni-0.5Mo-0.5C (atomized)

BACKGROUNDFCGR and ∆KFT vs. density/porosity …

Fe-2Cu-2.5Ni (60 min @ 2280°F)Fe-1.5Cu-0.6C (30 min @ 2050°F)

BACKGROUND∆KFT vs. density/porosity …

Fe-Cu-Ni-Mo-C alloys

as-sintered Fe-0.8Cu-1.8Ni-0.5Mo-0.2Mn-0.4C

BACKGROUND∆KFT vs. density/porosity …

FL 4605

heat treatedas-sinteredFL = pre-alloyed

FLN = pre-alloyed + elemental Ni blend

FN = elemental blended

BACKGROUND∆KFT vs. density/porosity …

Homogeneous

Inhomogeneous

Fe-1.75Ni-0.5Mo-0.5C

as-sintered

BACKGROUNDTotal porosity vs. Open/Closed porosity …

Open porosity

Isolated porosity

Closed 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 !!)

Isolated porosity: pores present in the initial powder particles (not affected by compaction and sintering)

BACKGROUNDOpen porosity … the controlling parameter ?

P/M iron

Tension-

Plane bending

• Axial testing – volume properties

• Bending – surface properties

compression

BACKGROUNDOpen porosity … the controlling parameter ?

P/M iron

Region I: closed porosity (∆Kth ≈ ct.)

Region II: ∆Kth porosity

Region III: open porosity (∆Kth ≈ ct.and low)

Fe-1.75Ni-0.5Mo-0.5C

(no increase in threshold above ~7.5 g/cm3)

BACKGROUNDOpen porosity level … function of density only ?

Literature: Open porosity is assumed to gradually increase with densityIs this correct ? Or the “path to density” prevails over the “density” itself …

BACKGROUNDOpen porosity measurement techniques …

Open porosity penetrated by He

Isolated porosity

Closed porosity

Open porosity penetrated by oil

Open porosity unresolved by penetrating oil

Oil-impregnation (ASTM B328)

Gas-impregnation

More accurate measurements of open porosity due to increased pore penetration ability of gases compared to oils - RECOMMENDED

Calculate the interconnected porosity from the volume of oil that has impregnated the specimen

BACKGROUNDGas-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 (P1V1=P2V2)

pore free density pycnometric density bulk density

BACKGROUNDCalculation of open/closed porosity using He pycnometry …

freepore

bulktotal 1V

−ρρ

−= % Total porosity = 100 x Vtotal

cpycnometri

bulkopen 1V

ρρ

−= % Open porosity = 100 x Vopen

Vclosed = Vtotal – Vopen % Closed porosity = 100 x Vclosed

* The amount of isolated porosity was assumed insignificantly small

BACKGROUNDOpen/closed porosity results … P/M iron

0

5

10

15

20

25

5.50 6.00 6.50 7.00 7.50 8.00

Density (g/cm3)

Poro

sity

(%)

Danninger et al. [18]Ledoux and Prioul [1]Lados and Apelian (pycnometry)Lados and Apelian (oil)

Total porosity

Open porosity

Closed porosity

BACKGROUNDOpen/closed porosity results using He pycnometry …

… P/M steels in sintered conditionsSintered conditions

0

0.25

0.5

0.75

1

1.25

6.8 6.9 7 7.1 7.2 7.3 7.4

Density (g/cm3)

Clo

sed

poro

sity

(%)

A4601A4001

A4601A4001

BACKGROUND

Quenched and tempered conditions

0

0.25

0.5

0.75

1

1.25

6.8 6.9 7 7.1 7.2 7.3 7.4

Density (g/cm3)

Clo

sed

poro

sity

(%)

A4601A4001

Open/closed porosity results using He pycnometry … … P/M steels in quenched and tempered conditions

A4601 after oil quench

A4001 after gas quench

BACKGROUNDOpen/closed porosity results using He pycnometry … … P/M steels in high temperature/long time sintering

Sintered conditions

0

1

2

3

4

5

6

7

6.8 6.9 7 7.1 7.2 7.3 7.4

Density (g/cm3)

Clo

sed

poro

sity

(%)

A4601 @ 2350°F for 6 hrs

A4601 @ 2050°F for 30 min (pycnometry)

oil pycnometry

BACKGROUNDSources of errors in evaluating open porosity …

1. Open porosity measurement technique:Higher closed porosity level is measured using oil-impregnation (the segments of open porosity channels that are not reached by oil are incorrectly assumed to be closed porosity);It is more accurate at high level of closed porosity;Pycnometric measurements are more accurate;

2. Correlating open/closed porosity to density alone: Simple correlations open porosity level – density are incorrect;The path followed to reach the density is critical for correct interpretations of open/closed porosity;No closed porosity is achieved through compaction and regular sintering T and t.

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 different 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 APPROACHMaterials selection …

Pre-alloyed

(QMP ATOMET 4601 Ni-Mo pre-alloyed powder)

Admixed

(QMP ATOMET 4001Mo pre-alloyed powder admixed with Ni)

0.60.15-0.180.50-0.551.75-1.8[%]

Sintered CMnMoNiChemical

composition

Graphite Ni

Graphite

Molding grades particles (70-85 µm)

EXPERIMENTAL APPROACHPhases …

Phase I (a):Phase I (a): Mechanistic understanding of the effects of pore amount on fatigue behavior;

Pore/Microstructure (matrix) interactions;

Phase I (b):Phase I (b): Microstructure effects on fatigue response;

Microstructure 1 vs. Microstructure 2;

Phase II:Phase II: Is fatigue resistance a state function ???

Effects of pore size/shape/type on fatigue.

EXPERIMENTAL APPROACHPhase I … Two microstructural considerations

Low High density density

Pore Pore/Matrix Matrix control control control

A.

??B.Cooling Fatigue rate 2 behavior 2

Cooling Fatigue rate 1 behavior 1 Microstructure 1

Microstructure 2

EXPERIMENTAL APPROACHPhase I … Density levels selection

Micro-structure

Set 37.8+

Set 27.2-7.3

Set 1~6.8

Density [g/cm3]

Produce samples of our composition in both pre-alloyed and admixed conditions;

Adjust compaction (conventional press, warm compaction, powder forging, etc.) to get the full range of densities:

EXPERIMENTAL APPROACH

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.

Phase I … Compaction +Sintering

EXPERIMENTAL APPROACHPhase I … Heat treatment

Post sintering heat treatment:

austenitize @ 1700°F for 30 min (similar austenitic grains)

quench to 2 microstructures (for both pre-alloyed and admixed):

temper @ 400°F for 1 hr (similar matrix micro-hardness)

Martensite + R.A. +

(5%Ni reach areas)

35%Martensite + 60%Pearlite + R.A. + (~5%Ni reach areas)

EXPERIMENTAL APPROACHPhase I … Heat treatment … Microstructures A4001

Martensite + R.A. +

(5%Ni reach areas)

35%Martensite + 60%Pearlite + R.A. +

(~5%Ni reach areas)

EXPERIMENTAL APPROACHPhase I … Heat treatment … Microstructures A4601

Martensite + R.A.

35%Martensite + 60%Pearlite + R.A.

EXPERIMENTAL APPROACHFatigue testing … Specimens and equipment

Dog-bone specimens for pull-pull/push-pull

CT specimens for FCGR

[Courtesy of Westmoreland]-

+

σmin

σmax

σmean

σa

EXPERIMENTAL APPROACHFatigue testing …

-The experiments will be conducted by the WPI team in collaboration with FTA;

-1 sample for each of the 12 conditions (prealloyed+admixed, 3 density levels, 2 microstructures)

-The tests will be done at an outside testing facility in parallel with the fatigue crack growth work;

-3 failed samples at 4 life levels for each of the 12 conditions):

* 103-104

* 104-105

* 105-106

* 106-107

2. Fatigue crack growth tests (E647)

1. Pull-pull / Pull-push tests (E466)

EXPERIMENTAL APPROACHPhase II … Is Fatigue Limit a State Function ?

One density level is selected (~7.25 g/cm3, same as Set 2 in Phase I) and 4 ways of achieving are investigated in parallel (we choose the most attractive alloy from fatigue point of view [of the 12 combinations] and concentrate our attention on how pore size/shape will influence its behavior):

1. Compaction - coarser powder, 100-105 µm;

2. Normal compaction to *.* g/cm3, followed by a different temperature/time sinter (same closed porosity as Route 3);

3. Double press/Double sinter (same closed porosity as 2);

4. Surface densification (7.0 g/cm3 - core and 7.25 g/cm3 -outer shell).

Pore size/shape/type effects on the fatigue behavior

EXPERIMENTAL APPROACHPhase II … Various pore sizes/shapes

Pore morphology (size/shape)

4. Surface densified(core 7.0 g/cm3 and outer layer 7.25 g/cm3)

3.DP/DS

2. Press to *.*

g/cm3 and different sinter to

7.25 g/cm3

1. Coarser particles

100-105 µmMolding grades

particles70-85 µm

Case study

7.0 / 7.25

Fatigue crack growth work will be conducted for one selected microstructure and one density level

EXPERIMENTAL APPROACHOther experimental considerations …

Inclusion level is low shed light on pore and microstructure effects;

Low residual stress levels are critical to understand the true behavior of the materials and have a fair comparison basis:

- stress relief is done during tempering - additional stress relieving may be needed after machining.

FUTURE WORK …Finish the open/closed porosity study;

Do a parallel study high temperature/long time sintering vs. DP/DS to determine the processing parameters allowing the same amount of closed porosity;Analyze microstructural results and microhardness for the two heat treatments on both the homogeneous and non-homogeneous materials for each of the microstructures;

Perform static tensile tests to get YS, Young’s modulus, UTS for all cases;

Check the residual stress level and decide if an additional stress relieving is needed after the post-sintering heat treatment;

Prepare samples for the life study (200 dog-bone samples) and the fatigue crack growth work (16 compact tension specimens);

Machine all the samples;

Start fatigue work.