influence of surface defects and metallurgical defects on the fatigue strength of ductile iron

- 1 -

Seminar: Metallography of casting alloys and metallurgical defects

Influence of surface defects and metallurgical defects on the fatigue

strength of ductile iron

- 2 -


Characteristic surface of a fatigue fracture

Crack start point

Borderline between the fast fracture area and the fatigue fracture area

- 3 -


Characteristic surface of fatigue fracture

Crack start point

Borderline between the fast fracture area and the fatigue fracture area

- 4 -


State of stress at different fatigue loadings

Surface effect

Push/pull

Bending Torsion

- 5 -


Cast parts with different fatigue loadings

Piston rod

Push/pull Bending Torsion

Axle arm Crankshaft

- 6 -


Surface defects Casting skin defects Microstructure defects

• Roughness • Pin holes • Inclusions • Blow holes • Local depressions (removed sand and slag- inclusions)

• Graphite flotation • Ferritic skin • Lamellar graphite skin • Dross • Surface zone decarbonization • Surface zone oxidation • Inclusions of lustrous carbon

• Degeneration of graphite • Nonmetallic inclusions • Grain boundary carbides • Mikro / makro-porosity

Damaging effects to the fatigue strength

- 7 -


Fatigue fracture caused by an inclusion in the surface zone of GJS 600-3 with casting skin

Inclusion

- 8 -


Fatigue fracture caused by Dross in the surface zone of GJS 400-15

- 9 -


Rotary bending testing machine

Source: Walter + Bai AG Source: Russenberger Prüfmaschinen AG

Resonant testing

machine

Different fatigue testing machines

- 10 -


Flat specimen

Samples of specimen for fatigue testing by ASTM E 466

Round specimen

Geometrical rated break point

4 point bending equipment

Constant test area

- 11 -


Microstructure of GJS 400 in as cast condition

25x

Microstructure of GJS 400 with graphite flotation

16x

- 12 -


Influence of the surface and the casting skin on the bending fatigue strength of ferritic nodular iron

0

50

100

150

200

250

300

350

1,E+04 1,E+05 1,E+06 1,E+07

Cycles [N]

Str

es

s A

mp

litu

de

[M

Pa

]

GJS 400 without defects & machined surface GJS 400 without defects & casting skin

GJS 400 with graphite flotation & casting skin

- 13 -


Microstructure of GJS 400 with 61 % Nodularity

Microstructure of GJS 400 with 70 % Nodularity

50x 50x

- 14 -


Influence of structural defects on the compression-tension fatigue strength of ferritic nodular iron

0

50

100

150

200

250

300

350

1,E+04 1,E+05 1,E+06 1,E+07

Cycles [N]

Str

es

s A

mp

litu

de

[M

Pa

]

GJS 400 without defects GJS 400 with 61% Nodularity GJS 400 with 70% Nodularity

- 15 -


Microstructure of GJS 400 with nonmetallic inclusions

100x

- 16 -


Influence of structural defects on the rotary bending fatigue strength of ferritic nodular iron

0

50

100

150

200

250

300

350

1,E+04 1,E+05 1,E+06 1,E+07

Cycles [N]

Str

es

s A

mp

litu

de

[M

Pa

]

GJS 400 without defects GJS 400 with 61% Nodularity

GJS 400 with 70% Nodularity GJS 400 with nonmetallic inclusions

- 17 -


Microstructure of GJS 700with 100 % pearlite

100x

Microstructure of GJS 700 with 20 % ferrite

100x

- 18 -


Influence of structural defects on the compression-tension fatigue strength of pearlitic nodular iron

0

50

100

150

200

250

300

350

1,E+04 1,E+05 1,E+06 1,E+07

Cycles [N]

Str

es

s A

mp

litu

de

[M

Pa

]

GJS 700 with 100 % pearlite GJS 700 with 20 % ferrite

- 19 -


Microstructure of GJS 700 with 1,25 % spiky graphite

200x

- 20 -


0

50

100

150

200

250

300

350

400

450

1,E+04 1,E+05 1,E+06 1,E+07

Cycles [N]

Str

es

s A

mp

litu

de

[M

Pa

]

GJS 700 without defects GJS 700 with 1,25 % spiky graphite

Influence of graphite defects on the rotary bending fatigue strength of pearlitic nodular iron

- 21 -


Abstract 1

> The appearance of cracks at the casting skin normally are caused by single defects like sand, dross or slag with an Ø < 1 mm.

> Single defects in the cast skin are more significant than anomalies in the microstructure at the casting skin and in the cast wall.

> The blasting of the cast skin improve the bending fatigue strength up to 50 % compared to non-blasted cast surfaces.

> Blasted surfaces have a 25 % reduced bending fatigue strength compared to machined surfaces.

> The prevention of “big defects” is state of the art, but the producing of castings completely without failures are unrealistic. The design of the castings have to tolerate different micro structural and surface defects.

- 22 -


Microstructure Metallurgy / Moulding sand

Base material (graphite types VI) Optimized process

Non metallic inclusions Increasing level of residual Mg-content

Graphite type VI V III Decreasing of residual Mg-content

Carbidic inclusions Alloying with Mo

Graphite degeneration in the surface zone High S-level in the molding sand

Tested ADI modifications

- 23 -


0100

200300400500

600700800

9001000

0 100 200 300 400 500

Zeit in min

Tem

pera

tur

in °

C

ADI heat treatment of the samples

Austenitizing: 890°C / 210 min

Quenching: salt bath

Holding: 380°C / 150 min

Microstructure of base material afterheat treatment (500 : 1)

- 24 -


Non metallic inclusions caused by high Mg-contents

% C % Si % Mn % P % S % Mo % Ni % Cu % Mg

3,57 2,14 0,23 0,019 0,004 - 1,72 0,75 0,065

Nodularity 84,6 %

Particle density 155 1/mm²

Non metallic inclusions

0,41 %

- 25 -


Non metallic inclusions caused by high Mg-contents


3,62 2,11 0,21 0,020 0,003 - 1,71 0,83 0,075

Nodularity 78,7 %


Non metallic inclusions

0,74 %

- 26 -


Influence of Mg-content on the volume of non metallic inclusions

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,040 0,045 0,050 0,055 0,060 0,065 0,070 0,075 0,080

Mg-content in %

Reference

No

n m

etal

lic in

clu

sio

ns

(%)

- 27 -



3,63 2,20 0,27 0,018 0,006 - 1,84 0,82 0,019

Influence of the Mg-content on the graphite typ

Nodularity 72,6 %


- 28 -



3,56 2,21 0,21 0,006 0,007 - 1,84 0,84 0,011

Nodularity 48,1 %


Influence of the Mg-content on the graphite typ

- 29 -


Influence of the Mg-content on the nodularity and graphite type III

0

20

40

60

80

100

0 0,01 0,02 0,03 0,04 0,05

Mg-content (%)

Nodularity (%) Graphite type III (%)

Typ III (2)

Typ III (1) Reference

Per

cen

tag

e

- 30 -


Influence of Mo-content on carbide formation


3,52 2,29 0,26 0,023 0,004 0,33 1,85 0,76 0,041

Nodularity 85,2 %


Carbide 0,21 %

- 31 -



3,62 2,24 0,26 0,030 0,008 0,74 1,74 0,78 0,036

Nodularity 87,7 %


Carbide 0,38 %


- 32 -



0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8

Mo-content (%)

Car

bid

e co

nte

nt

(%)

- 33 -



3,62 2,13 0,25 0,019 0,008 - 1,76 0,81 0,040

Nodularity 86,8 %


Lamellar zone 1 mm

Lamellar graphite-zone in ductile iron

- 34 -


Mechanical values of all test variants

0

100

200

300

400

500

600

700

800

900

1000

Basematerial

Nonmetallic

(1)

Nonmetallic

(2)

Graphite typ III(1) (2)

Carbides (1)

Carbides (2)

Variants

Str

eng

th (

MP

a)

0

1

2

3

4

5

6

7

Fra

ctu

re e

lon

ga

tio

n (

%)

Rm [MPa] Rp0,2 [MPa] A [%]

- 35 -


Fatigue strength

0

50

100

150

200

250

300

350

Basematerial

Non metallic inclusions Graphite type III(1) (2)

Carbides (1)

Carbides (2)

Casting skin defect(shot blasted)

Variants

Str

ess

amp

litad

e (M

Pa)

Compression-tension fatigue strength [MPa] Rotary bending fatigue strength [MPa] Bending fatigue strength [MPa]

(1) (2) (as cast)

influence of surface defects and metallurgical defects on the fatigue strength of ductile iron

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