A R C H I V E S o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)Volume 10

Issue 3/2010

5 – 10

1/3

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 5 - 1 0 5

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

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 5 - 1 0 6

count (average number of graphite nodules per unit area), NF was measured using a Leica QWin quantitative analyzer at 200 x.

Fig. 1. (a) Cantilever made of aluminum alloy (weight 580 g), (b) thin-walled ductile iron cantilever (weight 380g)

Fig. 2. (a) Rotors made of aluminum alloy and (b) thin-walled ductile iron casting

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.

3. Results and discussion 3.1. Austenitisation

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)

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).

(a)

(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)

118

119

120

121

122

123

124

125

126

127

128

0 5 10 15 20

Time, min.

dL/L

ox

10-4

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

a)

b)

Austenitic matrix

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 5 - 1 0 7

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)

b) time 15 min. (rib 1.9 mm)

c) time 20 min. (rib 1.9 mm)

d) time 20 min. (plate 22 mm)

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

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 of austempering 15 min.

c) time of austempering 90 min.

d) time of austempering 20 min.

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

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 5 - 1 0 8

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 Austempering time, min. HV 0.07, kG/mm2

Matrix

5 407 +/- 8.3 Ausferrite

15 411 +/- 18.7 Ausferrite

30 427.8 +/-14 Ausferrite

90 430.8 +/- 15 Ausferrite

393.8+/- 15* Ausferrite 120

285.8 +/-18* Ferrite

0

500

1000

1500

2000

2500

3000

20 40 60 80 100 120 140

2θ, oC

Inte

nsity

, cps

.

Austenite (111)

Ferrite (110)

Austenite (200) Ferrite

(200)

Austenite (220)Ferrite

(221)

Austenite(331)

Ferrite (220)

0

500

1000

1500

2000

2500

3000

20 40 60 80 100 120 140

2θ, oC

Inte

nsity

, cps

.

Austenite (111)

Ferrite (110)

Austenite (200) Ferrite

(200)

Austenite (220)Ferrite

(221)

Austenite(331)

Ferrite (220)

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

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].

0.0044

0.3548aC

γγ

−= (1)

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 (1.86-1.93%) 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.

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120tγ , min.

V, %

1.85

1.86

1.87

1.88

1.89

1.90

1.91

1.92

1.93

1.94

C, %

Fig. 8. Volume fraction of retained austenite Vγ and carbon

content in the retained austenite Cγ as a function of austempering time. • - Vγ, ° - Cγ

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 22/sm 444 15472001112

γRm

== (2)

TWDI 22/sm 500 877200630

γRm

== (3)

a) time 5 min

b) time 120 min

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 5 - 1 0 9

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7

Strain, %

Stre

ss, M

Pa

30 min.15 min.5 min.

0

100

200

300

400

500

600

700

0 2 4 6 8Strain, %

Stes

s, M

Pa

0

50

100

150

200

0 1 2 3Strain, %

Stre

ss, M

Pa

Fig. 9. Stress-strain curves of the samples taken from the rib of

the cantilevers made of austempered ductile iron (a), ductile cast iron (b) aluminum alloy (c) and a comparison of strength to weigh ratios versus elongation (points) with ASTM standards (lines) (d)

Aluminum alloy 22/sm 818 612750170

γRm

== (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.

4. Conclusions 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.

Acknowledgements This work was supported by grant KBN No. N N597 372135 References [1] K.L. Hayrynen, K.R. Brandenberg, Carbidic Austempered

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[2] D. Venugopolan, A kinetic model of the γ α+ Gr eutectoid transformation in spheroidal graphite cast iron, Metallurgical and Materials Transactions, vol.21A, (1990) 913-918.

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[5] K.F. Laneri, J. Desimoni, R.C. Mercader, R.W. Gregorutti, J.L. Satutti, Thermal dependence of austempering transformation kinetics of compacted graphite cast iron, Metallurgical and Materials Transactions A, vol. 32, (2001) 511-518.

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[8] N. Darwish, R. Elliot, Austempering of low manganese ductile irons, Part 3. Variation of mechanical properties with heat treatment conditions, Material Science and technology, vol. 9, (1993), 882-889.

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