Thin wall ductile and austempered iron castings

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Volume 10
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FOUNDRY ENGINEERING
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Thin wall ductile and austempered iron
castings
E. Fraś*, M. Górny
AGH University of Science and Technology, Reymonta 23, 30-059 Cracow, Poland
*Corresponding author. E-mail adress: edfras@agh.edu.pl
Received 30.04.2010; accepted in revised form 01.07.2010
Abstract
It has been shown that it is possible to produce thin wall castings made of ductile iron with wall thickness in the range of 1.2 to 2.9 mm
(without chills, cold laps and misruns). Thin wall ductile iron castings can be lighter (380 g) than their substitutes made of aluminium
alloys (580g). The kinetics of austenitising transformation was studied in unalloyed ductile iron. The advance of transformations during
austenitising was monitored by measurement the fraction of martensite and also by dilatometic studies. It has been shown that in thin wall
ductile iron castings austenitising at 880 oC for 20 minutes is adequate to obtain the austenite matrix at the end of the first stage of
austempering heat treatment cycle.
Keywords: thin wall castings, ductile iron, austempered iron, austenitising, austempering
1. Introduction
The ADI market has been continuously growing with a
rate estimated at 16% per year [1] There are numerous studies on
ADI, particularly on (a) the kinetics of austempered of cast iron
[1-6], (b) microstructural characterization, (c) mechanical
properties [7-9] (d) fatigue [10], (e) properties and machinability
[10] as well as other applications [11]. While the parameters for a
successful production of high quality ADI are well established,
the same cannot be said of thin wall austempered ductile iron
castings (TWADI). Thin wall ductile iron castings (TWDI) are
characterized by an extremely large nodule count and hence with
relatively small interparticle spacings. High nodule count,
homogeneous structure and high cooling rate of the thin wall
castings make it possible to eliminate the use of alloying elements
such as Ni and Cu for increasing austemperability. Accordingly,
thin wall ductile iron castings can be as an ideal material in
producing thin walled austempered ductile iron castings
(TWADI). Accordingly, the aim of this work is to determine the
heat treatment cycle for production TWADI castings, which are
lighter than substitutes made of aluminum alloys, but with
superior mechanical and wear properties.
2. Experimental
Figures 1 and 2 show the geometry of a cantilever and
rotor shaped castings made of an aluminum alloy, as well as their
thin wall counterparts made of ductile iron and austempered
ductile iron. The cantilever molds (chemically bonded 75-mesh
silica sand ) and rotor molds (Shaw process) were equippated with
Inmold system. The raw materials were Sorelmetal, steel scrap
and commercially pure silicon. The metal was preheated at
1500oC and then poured into the mold. The spheroidizer (Elmag
5800; 44-48 % Si, 5.5-6.2 % Mg, 0.8-1.2 % RE, 1.0 % Al) and
inoculant Foundrysil (73-78 % Si, 0.75-1.25 % Ca, 0.75-1.25 %
Ba, 0.75-1.25 % Al) were used. The chemical composition of the
cast iron was 3.59-3.68 % C; 3.01-3.10 % Si; 0.1-0.12 % Mn;
0.02 % P; 0.01 % S; and 0.023-0.027 % Mg. Metallographic
characterization was made on samples cut from cantilevers ribs
(1.9 mm) from the runners (20x 22x 320 mm) The average nodule
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5
count (average number of graphite nodules per unit area), NF was
measured using a Leica QWin quantitative analyzer at 200 x.
a)
are as follows: for the ribs λ= 43 μm, NF =2170 mm-2, and for the
plate λ= 202 μm, NF =157 mm-2.
From the dilatation curve it is possible to examine the end
of stage I transformation. Referring to Fig. 4 the end of stage I is
marked by a plateau in the curve (at about 1000 s).
b)
Fig. 1. (a) Cantilever made of aluminum alloy (weight 580 g),
(b) thin-walled ductile iron cantilever (weight 380g)
Austenitisation at 880oC for 20 minutes and austempering at
400oC for various times (5, 15, 30, 90 and 120 min) were used.
The advance of austenitising was monitored by measurement the
fraction of martensite and dilatometry equpment (DT – 1000)
while austempering was monitored by XRD means in a Bruker
diffractometer. The tensile properties were measured using
specimens cut from the cantilever ribs in the as-cast and
austempered conditions.
x 10
-4
(b)
Fig. 3. Microstructures of the specimen taken from thin wall
casting with wall thickness of 1.9 mm: a) and from plate (22 mm)
128
127
126
125
124
dL/Lo
Fig. 2. (a) Rotors made of aluminum alloy and (b) thin-walled ductile
iron casting
(a)
123
122
121
Austenitic matrix
120
3. Results and discussion
119
118
3.1. Austenitisation
0
10
15
20
Time, min.
Figure 3 shows the exhibited microstructures found in the
thin walled sections, in the runner and in the plate (22 mm)
locations. From the metallographic determinations and from Eq.
(1) it is found that in the thin walled regions the interparticle
spacing is smaller than in the runner locations and in plate (22
mm). Results of interparticle spacing (λ) and nodule count (NF)
6
5
Fig. 4. Dilatometry curve of austenitisation process
Metalographic examinations were confirmed by dilatometry
study. Notice from figure 5 that after 5 minutes at the
austenitising temperature, the matrix consists of matensite and
ferrite, while after 15 minutes a fully martensitic structure is
developed. Moreover, in the plate locations, the exhibited
ARCHIVES of FOUNDRY ENGINEERING Volume 10, Issue 3/2010, 5-10
microstructure after 20 minutes at 880oC is also fully martensitic
(Fig. 4d) suggesting that the nodule count in this region does not
have a significant effect on the austenitising kinetics. Summing up
austenitisation for 20 minutes was considered to be enough for
full austenitisation heat treatment at 880 oC.
a) time 5 min. (rib 1.9 mm)
3.2. Austempering
Figures 5a-c show the austempered matrix microstructures
obtained in ribs (wall thickness 1.9 mm) after austenitising at
880oC for 20 minutes and austempering at 400oC for 5-90
minutes.
a) time of austempering 5 min.
b) time 15 min. (rib 1.9 mm)
b) time of austempering 15 min.
c) time 20 min. (rib 1.9 mm)
c) time of austempering 90 min.
d) time 20 min. (plate 22 mm)
d) time of austempering 20 min.
Fig. 5. Microstructure of cast iron taken from the rib of cantilever
(wall thickness 1.9 mm), and from the plate (22 mm) austenitized
at 880 oC and water quenched; etched with 3 % Nital
Fig. 6. Microstructure of heat treated cast iron, taken from the
cantilever rib of (a-c) (wall thickness 1.9 mm), and from the plate
(22 mm) (d); austenitising 880oC /20 min and austempering at
400oC
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In particular, notice that as time increases from 5 to 90 minutes,
there are no significant changes in the exhibited microstructures.
Table 1.
Results of microhardness measurements
Microhardness
Matrix
HV 0.07, kG/mm2
5
407 +/- 8.3
Ausferrite
15
411 +/- 18.7
Ausferrite
30
427.8 +/-14
Ausferrite
90
430.8 +/- 15
Ausferrite
393.8+/- 15*
Ausferrite
285.8 +/-18*
Ferrite
120
3000
a γ − 0.3548
(1)
0.0044
where: aγ is the lattice parameter of retained austenite, nm and Cγ
is carbon content in retained austenite in wt.%.
From Fig. 7 is it evident that the carbon content (1.861.93%) corresponds to a range, which ensures thermodynamic and
mechanical stability [5]. XRD tests confirm that after 5 minutes of
austempering time at temperature 400 oC field a stable ausferrite
structure. After austenitising at 880 oC for 20 min and
austempering at 400 oC for 5 min the ausferrite is present in the
rotor.
35
2000
1000
1.92
0
40
60
80
100
120
1.93
25
500
20
1.94
30
Austenite
Austenite
(331)
(220) Ferrite
Austenite
Ferrite
(221)
Austenite (200) Ferrite
(220)
(200)
(111)
1500
V ,%
Intensity, cps.
a) time 5 min
Ferrite
(110)
2500
Cγ =
1.91
20
1.90
15
1.89
1.88
10
140
1.87
2θ, o C
5
1.86
0
3000
b) time 120 min
Intensity, cps.
Austenite
Austenite
(331)
(220) Ferrite
Austenite
Ferrite
(221)
Austenite (200) Ferrite
(220)
(200)
(111)
1000
0
20
40
60
80
100
120
40
60
80
100
120
Fig. 8. Volume fraction of retained austenite Vγ and carbon
content in the retained austenite Cγ as a function of austempering
time. • - Vγ, ° - Cγ
2000
1500
20
tγ , min.
Ferrite
(110)
500
140
2θ, o C
Fig. 7. Diffraction pattern of ribs austenitised at 880oC /20 min
and austempering at 400oC
Hence 5 to 10 minutes can be enough for austempering.
Moreover, in the samples taken from the plate (22mm)
approximately 60 % martensite is still present. Apparently, under
similar heat treatments at these locations there is a significant
effect of nodule count (2170 mm-2 for ribs and 157 mm-2 for
plate) on the austempering kinetics. Microhardness measurements
8
1.85
0
2500
C ,%
Austempering
time, min.
on ribs (Table 1) show that average hardness slightly increases
(from 407 to 431 HV) with austempering time (from 5 to 90
minutes).
In addition, Fig. 6 shows XRD diffraction patterns for samples
austenitized at 880oC for 20 minutes and austempered at 400oC in
a salt bath for 5, 15, 30, 90 and 120 minutes prior to air cooling.
Notice that there are only diffraction peaks corresponding to
austenite and ferrite. Figure 7 gives the measured volume
fractions of retained austenite (Vγ) and the carbon content in the
retained austenite estimated from the equation [7].
Fig 8.a-c show the stress-strain curves corresponding to flat
samples taken from cantilever ribs made of austempered ductile
iron (TWADI) and ductile iron (TWDI), as well as from the
aluminum alloy. Notice the magnitude of the tensile properties of
TWADI (Rm = 1112 MPa), and yield strength (Rp,0.2 = 1007 MPa,
Fig.9a) and of TWDI (Rm = 630 MPa) and Rp,0.2 = 310 MPa.
Moreover the elongation of the TWDI is 7 to 8% and TWADI is
4-6 % versus only 1.9 for the aluminum alloy. Fig. 9d show
tensile strength to weight ratios In castings made of Al alloy,
TWDI and TWADI. The tensile strength to weight ratios for
TWADI, TWDI and the aluminum alloy are given below
TWADI
TWDI
Rm 1112
=
= 154 444 m 2 /s 2
γ
7200
Rm
630
=
= 87 500 m 2 /s 2
γ
7200
(2)
(3)
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a)
1200
1000
30 min.
15 min.
5 min.
800
Stress, MPa
Aluminum alloy
400
200
0
1
2
3
4
5
6
7
Strain, %
700
b)
4. Conclusions
Stess, MPa
600
500
400
1.
Thin wall ductile iron is excellent base material for heat
treatments as it does not require expensive alloying elements
nor long heat treatment times.
2.
Extremely high nodule counts in thin wall ductile iron and
short difussional lenghts for alloying elements lead to
reduced austempering times.
3.
It has been shown that in thin wall ductile iron castings
austenitizing at 880 oC for 20 minutes and austempering at
400 oC for 5 minutes is adequate to obtain an ausferrite
structure.
4.
It is possibile to produce austempered ductile iron castings of
cantilevers with a rib-wall thickness of 1.9 mm and high
mechanical properties (Rm = 1112 MPa, and 6% elongation)
using only 50 min as a total heat treatment time.
5.
Thin wall austempered ductile iron castings can be lighter
than their substitutes made of aluminium alloys, but with
superiour mechanical properties. Also, they are less
expensive by far and hence they should persuade constructors
to design such castings while producers to produce them.
300
200
100
0
0
2
4
6
8
Strain, %
c)
200
Stress, MPa
(4)
In particular, notice that TWADI has superior tensile strength to
weight ratios compared with any other casting.
It is worth also mentioning that somewhat similar properties have
been reported in the work by Ganes et al, [12] for simple plate
shaped castings. However, in this work, the Si content was kept at
2.45 % and the mold material was a sodium silicate sand with a
mixture of 50% insulating LDASC and 50% silica sand in order
to keep the nodule count in the 500-700 mm-2 range. Moreover,
the total heat treatment time was 227 minutes while it took only
25 to 50 min in the present work.
600
0
Rm 170
=
= 61 818 m 2 /s 2
γ
2750
150
100
50
0
0
1
Strain, %
2
3
d)
Acknowledgements
This work was supported by grant KBN No. N N597
372135
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