FATIGUE MECHANISMS IN P/M COMPONENTS Worcester Polytechnic - - PowerPoint PPT Presentation

FATIGUE MECHANISMS IN P/M COMPONENTS

Worcester Polytechnic Institute April 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

BACKGROUND

Factors 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)

BACKGROUND

Fatigue life vs. density/porosity …

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 … 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)

BACKGROUND

Fatigue 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)

BACKGROUND

Fatigue 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)

BACKGROUND

Fatigue 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)

BACKGROUND

FCGR 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 FL 4605

heat treated as-sintered

FL = 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

BACKGROUND

Total 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)

BACKGROUND

Open porosity … the controlling parameter ? P/M iron

Tension- Plane bending

• Axial testing – volume properties
• Bending – surface properties

compression

BACKGROUND

Open 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)

BACKGROUND

Open porosity level … function of density only ? Literature: Open porosity is assumed to gradually increase with density

Is this correct ? Or the “path to density” prevails over the “density” itself …

BACKGROUND

Open 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

• pen porosity due to increased

pore penetration ability of gases compared to oils - RECOMMENDED Calculate the interconnected porosity from the volume of

• il that has impregnated the

specimen

BACKGROUND

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 (P1V1=P2V2) pore free density pycnometric density bulk density

BACKGROUND

Calculation of open/closed porosity using He pycnometry …

free pore bulk total

1 V

−

ρ ρ − = % Total porosity = 100 x Vtotal

c pycnometri bulk

• pen

1 V ρ ρ − =

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

* The amount of isolated porosity was assumed insignificantly small

BACKGROUND

Open/closed porosity results … P/M iron

5 10 15 20 25 5.50 6.00 6.50 7.00 7.50 8.00 Density (g/cm3) Porosity (%)

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

Total porosity Open porosity Closed porosity

BACKGROUND

Open/closed porosity results using He pycnometry … … P/M steels in sintered conditions

Sintered conditions 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) Closed porosity (%)

A4601 A4001

A4601 A4001

BACKGROUND

Quenched and tempered conditions 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) Closed porosity (%)

A4601 A4001

Open/closed porosity results using He pycnometry … … P/M steels in quenched and tempered conditions A4601 after oil quench A4001 after gas quench

BACKGROUND

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

Sintered conditions 1 2 3 4 5 6 7 6.8 6.9 7 7.1 7.2 7.3 7.4 Density (g/cm3) Closed porosity (%)

A4601 @ 2350°F for 6 hrs A4601 @ 2050°F for 30 min

(pycnometry)

• il

pycnometry

BACKGROUND

Sources 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 APPROACH

Materials selection … Pre-alloyed (QMP ATOMET 4601 Ni-Mo pre-alloyed powder) Admixed (QMP ATOMET 4001 Mo pre-alloyed powder admixed with Ni) 0.6 0.15-0.18 0.50-0.55 1.75-1.8

[%] Sintered C Mn Mo Ni Chemical composition

Graphite Ni Graphite

Molding grades particles (70-85 µm)

EXPERIMENTAL APPROACH

Phases … 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 APPROACH

Phase 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 APPROACH

Phase I … Density levels selection Micro- structure Set 3 7.8+ Set 2 7.2-7.3 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 APPROACH

Phase 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 APPROACH

Phase I … Heat treatment … Microstructures A4001 Martensite + R.A. + (5%Ni reach areas) 35%Martensite + 60%Pearlite + R.A. + (~5%Ni reach areas)

EXPERIMENTAL APPROACH

Phase I … Heat treatment … Microstructures A4601 Martensite + R.A. 35%Martensite + 60%Pearlite + R.A.

EXPERIMENTAL APPROACH

Fatigue testing … Specimens and equipment Dog-bone specimens for pull-pull/push-pull CT specimens for FCGR

[Courtesy of Westmoreland]

• +

σmin σmax σmean σa

EXPERIMENTAL APPROACH

Fatigue 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
• utside 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 APPROACH

Phase 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 -

• uter shell).

Pore size/shape/type effects on the fatigue behavior

EXPERIMENTAL APPROACH

Phase II … Various pore sizes/shapes

Pore morphology (size/shape)

• 4. Surface

densified (core 7.0 g/cm3 and

• uter 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 µm

Molding grades particles

70-85 µm

Case study

7.0 / 7.25

• Fatigue crack growth work will be conducted for one

selected microstructure and one density level

EXPERIMENTAL APPROACH

Other 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.