Seminar: Metallography of casting alloys and metallurgical defects
Influence of surface defects and
metallurgical defects on the fatigue
strength of ductile iron
-1-
Seminar: Metallography of casting alloys and metallurgical defects
Characteristic surface of a fatigue fracture
Borderline between
the fast fracture area
and the fatigue
fracture area
Crack start point
-2-
Seminar: Metallography of casting alloys and metallurgical defects
Characteristic surface of fatigue fracture
Crack start point
Borderline between the fast fracture area and the fatigue fracture area
-3-
Seminar: Metallography of casting alloys and metallurgical defects
State of stress at different fatigue loadings
Push/pull
Bending
Torsion
Surface effect
-4-
Seminar: Metallography of casting alloys and metallurgical defects
Cast parts with different fatigue loadings
Push/pull
Piston rod
Bending
Axle arm
Torsion
Crankshaft
-5-
Seminar: Metallography of casting alloys and metallurgical defects
Damaging effects to the fatigue strength
Surface defects
• Roughness
• Pin holes
• Inclusions
• Blow holes
• Local depressions
(removed sand and slaginclusions)
Casting skin defects Microstructure defects
• 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
-6-
Seminar: Metallography of casting alloys and metallurgical defects
Fatigue fracture caused by an inclusion in the surface
zone of GJS 600-3 with casting skin
Inclusion
-7-
Seminar: Metallography of casting alloys and metallurgical defects
Fatigue fracture caused by Dross
in the surface zone of GJS 400-15
-8-
Seminar: Metallography of casting alloys and metallurgical defects
Different fatigue testing machines
Resonant
testing
machine
Rotary bending testing machine
Source: Walter + Bai AG
Source: Russenberger Prüfmaschinen AG
-9-
Seminar: Metallography of casting alloys and metallurgical defects
Samples of specimen for fatigue testing by ASTM E 466
4 point bending equipment
Round specimen
Geometrical rated
break point
Constant
test area
Flat specimen
- 10 -
Seminar: Metallography of casting alloys and metallurgical defects
Microstructure of GJS 400 in as
cast condition
25x
Microstructure of GJS 400 with
graphite flotation
16x
- 11 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of the surface and the casting skin on the bending
fatigue strength of ferritic nodular iron
350
Stress Amplitude [MPa]
300
250
200
150
100
50
0
1,E+04
1,E+05
1,E+06
1,E+07
Cycles [N]
GJS 400 without defects & machined surface
GJS 400 with graphite flotation & casting skin
GJS 400 without defects & casting skin
- 12 -
Seminar: Metallography of casting alloys and metallurgical defects
Microstructure of GJS 400 with
61 % Nodularity
50x
Microstructure of GJS 400 with
70 % Nodularity
50x
- 13 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of structural defects on the compression-tension
fatigue strength of ferritic nodular iron
350
Stress Amplitude [MPa]
300
250
200
150
100
50
0
1,E+04
1,E+05
1,E+06
1,E+07
Cycles [N]
GJS 400 without defects
GJS 400 with 61% Nodularity
GJS 400 with 70% Nodularity
- 14 -
Seminar: Metallography of casting alloys and metallurgical defects
Microstructure of GJS 400 with nonmetallic inclusions
100x
- 15 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of structural defects on the rotary bending fatigue
strength of ferritic nodular iron
350
Stress Amplitude [MPa]
300
250
200
150
100
50
0
1,E+04
1,E+05
1,E+06
1,E+07
Cycles [N]
GJS 400 without defects
GJS 400 with 70% Nodularity
GJS 400 with 61% Nodularity
GJS 400 with nonmetallic inclusions
- 16 -
Seminar: Metallography of casting alloys and metallurgical defects
Microstructure of GJS 700
with 100 % pearlite
100x
Microstructure of GJS 700 with
20 % ferrite
100x
- 17 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of structural defects on the compression-tension
fatigue strength of pearlitic nodular iron
350
Stress Amplitude [MPa]
300
250
200
150
100
50
0
1,E+04
1,E+05
1,E+06
1,E+07
Cycles [N]
GJS 700 with 100 % pearlite
GJS 700 with 20 % ferrite
- 18 -
Seminar: Metallography of casting alloys and metallurgical defects
Microstructure of GJS 700 with 1,25 % spiky graphite
200x
- 19 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of graphite defects on the rotary bending
fatigue strength of pearlitic nodular iron
450
Stress Amplitude [MPa]
400
350
300
250
200
150
100
50
0
1,E+04
1,E+05
1,E+06
1,E+07
Cycles [N]
GJS 700 without defects
GJS 700 with 1,25 % spiky graphite
- 20 -
Seminar: Metallography of casting alloys and metallurgical defects
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.
- 21 -
Seminar: Metallography of casting alloys and metallurgical defects
Tested ADI modifications
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
- 22 -
Seminar: Metallography of casting alloys and metallurgical defects
Temperatur in °C
ADI heat treatment of the samples
1000
900
800
700
600
500
400
300
200
100
0
0
100
200
300
400
Austenitizing:
890°C / 210 min
Quenching:
salt bath
Holding:
380°C / 150 min
500
Zeit in min
Microstructure of base material after
heat treatment (500 : 1)
- 23 -
Seminar: Metallography of casting alloys and metallurgical defects
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 %
- 24 -
Seminar: Metallography of casting alloys and metallurgical defects
Non metallic inclusions caused by high Mg-contents
%C
% Si
% Mn
%P
%S
% Mo
% Ni
% Cu
% Mg
3,62
2,11
0,21
0,020
0,003
-
1,71
0,83
0,075
Nodularity
78,7 %
Particle density
121 1/mm²
Non metallic
inclusions
0,74 %
- 25 -
Seminar: Metallography of casting alloys and metallurgical defects
Non metallic inclusions (%)
Influence of Mg-content on the volume of
non metallic inclusions
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0,040
Reference
0,045 0,050
0,055
0,060 0,065
0,070 0,075
Mg-content in %
- 26 -
0,080
Seminar: Metallography of casting alloys and metallurgical defects
Influence of the Mg-content on the graphite typ
%C
% Si
% Mn
%P
%S
% Mo
% Ni
% Cu
% Mg
3,63
2,20
0,27
0,018
0,006
-
1,84
0,82
0,019
Nodularity
72,6 %
Particle density
188 1/mm²
- 27 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of the Mg-content on the graphite typ
%C
% Si
% Mn
%P
%S
% Mo
% Ni
% Cu
% Mg
3,56
2,21
0,21
0,006
0,007
-
1,84
0,84
0,011
Nodularity
48,1 %
Particle density
212 1/mm²
- 28 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of the Mg-content
on the nodularity and graphite type III
100
Percentage
80
60
Typ III (1)
Reference
40
20
Typ III (2)
0
0
0,01
0,02
0,03
0,04
Mg-content (%)
Nodularity (%)
Graphite type III (%)
- 29 -
0,05
Seminar: Metallography of casting alloys and metallurgical defects
Influence of Mo-content on carbide formation
%C
% Si
% Mn
%P
%S
% Mo
% Ni
% Cu
% Mg
3,52
2,29
0,26
0,023
0,004
0,33
1,85
0,76
0,041
Nodularity
85,2 %
Particle density
230 1/mm²
Carbide
0,21 %
- 30 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of Mo-content on carbide formation
%C
% Si
% Mn
%P
%S
% Mo
% Ni
% Cu
% Mg
3,62
2,24
0,26
0,030
0,008
0,74
1,74
0,78
0,036
Nodularity
87,7 %
Particle density
257 1/mm²
Carbide
0,38 %
- 31 -
Seminar: Metallography of casting alloys and metallurgical defects
Influence of Mo-content on carbide formation
Carbide content (%)
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Mo-content (%)
- 32 -
0,8
Seminar: Metallography of casting alloys and metallurgical defects
Lamellar graphite-zone in ductile iron
%C
% Si
% Mn
%P
%S
% Mo
% Ni
% Cu
% Mg
3,62
2,13
0,25
0,019
0,008
-
1,76
0,81
0,040
Nodularity
86,8 %
Particle density
149 1/mm²
Lamellar zone
1 mm
- 33 -
Seminar: Metallography of casting alloys and metallurgical defects
Mechanical values of all test variants
1000
7
900
Strength (MPa)
800
700
5
600
4
500
3
400
300
2
200
1
100
0
0
Base
material
Non
metallic
(1)
Non
metallic
(2)
Graphite typ III
(1)
(2)
Carbides
(1)
Variants
Rm [MPa]
Rp0,2 [MPa]
A [%]
- 34 -
Carbides
(2)
Fracture elongation (%)
6
Seminar: Metallography of casting alloys and metallurgical defects
Fatigue strength
350
Stress amplitade (MPa)
300
250
200
150
100
50
0
Base
material
Non metallic inclusions
(1)
(2)
Graphite type III
(2)
(1)
Carbides
(1)
Carbides
(2)
Casting skin defect
(as cast)
(shot blasted)
Variants
Compression-tension fatigue strength [MPa]
Rotary bending fatigue strength [MPa]
Bending fatigue strength [MPa]
- 35 -