Austempered Ductile Iron (ADI) Alternative Material for ... · PDF fileAUSTEMPERED DUCTILE IRON (ADI) ALTERNATIVE MATERIAL FOR HIGH-PERFORMANCE APPLICATIONS G. Artola IK4-Azterlan,

AUSTEMPERED DUCTILE IRON (ADI) ALTERNATIVE MATERIALFOR HIGH-PERFORMANCE APPLICATIONS

G. ArtolaIK4-Azterlan, Durango, Bizkaia, Spain

Tecnun (University of Navarra), Donostia, Gipuzkoa, Spain

I. Gallastegi and J. IzagaIK4-Azterlan, Durango, Bizkaia, Spain

M. BarrenaAdilan Group, Iurreta, Bizkaia, Spain

A. RimmerADI Treatments Ltd., West Bromwich, UK

Copyright � 2016 American Foundry Society

DOI 10.1007/s40962-016-0085-8

Abstract

Austempered ductile iron (ADI) grades are standardized,

and the requirements of current international standards (EN

1564-12/ASTM A897-15) are given terms of conventional

mechanical properties, such as hardness and tensile

strength. Nevertheless, these properties do not show the real

potential of the ADI grades. In order to promote the use of

ADI parts in place of other materials, this work proposes a

comparison between GJS-900-8 and GJS-1200-3 grades,

both in terms of conventional and advanced mechanical

properties, employing stress intensity factors and critical

CTODs (Crack Tip Opening Displacement). This study is

completed with mechanical fatigue testing, so that it can be

shown that the service life of ADI parts is comparable to that

given by other heavier and more expensive options.

Keywords: austempered ductile iron, ADI, fracture

mechanics, fatigue, alternative materials

Introduction

The critical defect size for the transition between plastic

yielding (plain stress) and brittle fracture (plain strain) is

proportional to the square of the toughness to yield strength

ratio (KIC/Rp0.2)2. This ratio tends to be lower for high-

strength materials in comparison with low-strength mate-

rials. High-strength materials can thus show brittle

behavior in the presence of smaller defects, and both

fracture mechanics and fatigue become more relevant.

This fact must be taken into account when alternative,

higher-strength materials, are employed to substitute any

mechanical part by a lightweight solution. In this situation,

conventional mechanical testing by itself is not anymore

adequate for structural integrity calculations. It must be

combined with toughness measurements in order to prop-

erly assess the mechanical design of the part.

Austempered ductile iron (ADI) is an excellent alternative for

this type of substitution, and in this paper, fracture and fatigue

behaviors of two representative grades, GJS-900-8 and GJS-

1200-3 according to EN 1564-2012, are investigated.

Fracture response of ADI is frequently associated with

V-Notch Charpy impact testing,1 as it is considered a

useful brittleness-related ‘‘go/no-go’’ check for quality

control in certain ductile iron grades. Nevertheless, notched

bar impact testing does not properly describe the toughness

of cast irons2,3 and absorbed energy is not useful for design

purposes. Thus, testing methods that are specific to fracture

mechanics are compulsory in this case.

International Journal of Metalcasting/Volume 11, Issue 1, 2017 131

Experimental Procedure

All the materials involved in the experimental procedure

were processed in industrial facilities and under production

conditions. The cast specimens for the study were obtained

from 22 one-inch thickness type Y2 keel-blocks from the

same heat. It was first checked that the as-cast materials

were capable of reaching GJS-900-8 and GJS-1200-3

grades before proceeding to the austempering heat

treatment.

After confirming that the keel blocks were in good condi-

tion to manufacture ADI, they were machined to obtain

two sets of specimen pre-shapes, with a minimum

machining clearance of 2 mm.

Each set contained the following pre-shapes:

• One Ø 20 9 140 mm cylinder for a tensile testing

specimen.

• Two 12.5 9 12.5 9 165 mm prisms for impact

testing specimens.

• Three 15 9 30 9 160 mm prisms for fracture

toughness testing specimens.

• Four Ø 25 9 180 mm cylinders for fatigue testing

specimens.

Austempering of both sets was performed under controlled

atmosphere with a carbon potential of 0.8 during austeni-

tization and inert gas protection during the transfer to the

isothermal bath. The samples were treated together with

production parts in two different batches, which were

selected according to the heat treaters expertise in order to

achieve each targeted grade.

After heat treatment, pre-shapes were finish machined to

standard testing specimens and it was verified that the two

sets corresponded to GJS-900-8 and GJS-1200-3 grades,

respectively.

Once it was confirmed that the materials were as required,

fracture and fatigue specimens were machined. B(E) spec-

imens with a 12.5 9 25 mm section and a straight notch

were employed for toughness testing.

S–N fatigue curves were built by regression to two load

conditions, defined depending on the nominal Rp0.2 indi-

cated in EN 1564-2012 for each grade being tested. The

average stress rm was kept constant and equal to half the

nominal Rp0.2, and two different stress amplitudes of

magnitude 0.5�Rp0.2 and Rp0.2 were applied.

All tests followed applicable European EN standards,

except for fracture testing and uniaxial fatigue that were

performed after BS7448-1:1991 and ASTM E466-15.

Results and Discussion

As-Cast Condition Verification

The material in as-cast condition fulfilled the requirements

for a proper austempering treatment both in terms of

chemical composition and microstructure, as shown in

Tables 1, 2 and Figures 1, 2.

The amounts of Ni, Mo and Cu fit industrial experience

recommendations to improve hardenability. The nodule

count resulted in over 200 nodules per square millimeter.

The as-cast material was also subjected to conventional

mechanical testing as shown in Table 3, in order to reflect

the improvement that is achieved by means of the

austempering process.

Notched and unnotched impact test specimens were used to

emphasize the influence of the notch.

Austempered Condition Verification

Microstructure and mechanical testing confirmed that the

heat treatment of both specimen sets was successful as

GJS-900-8 and GJS-1200-3 were obtained. Mechanical

testing results in Tables 4 and 5 back-up this statement.

The obtained microstructures shown in Figures 3 and 4 are

composed of the expected ausferritic matrix, with less

austenite and sharper ferrite needles explaining the strength

increase for GJS-1200-3 grade.

It is remarkable how impact testing results do not differ

much between the two grades, despite the significant ten-

sile resistance increase from GJS-900-8 to GJS-1200-3.

It is also noticeable that elongation values are comparable

for both grades, since even the high-strength ADI speci-

mens reached an elongation close to 10 %.

Fracture and Fatigue Characterization

The above-mentioned ductility is also evident in the crack

tip opening displacement (CTOD) tests. As shown in Fig-

ure 5, the ratio Fmax/FQ, where Fmax is the maximum test

load and FQ is the point where the curve deviates from

elastic response, is greater than 1.1.

Table 1. Chemical Composition (%)

C Si Mn Mg Ni Mo Cu

3.69 2.28 0.19 0.037 2.45 0.20 0.79

132 International Journal of Metalcasting/Volume 11, Issue 1, 2017

This situation corresponds to elastic–plastic fracture

mechanics, and thus, the studied ADI specimens should not

be described as brittle, from a linear elastic fracture

mechanics point of view. KIC cannot be reported and both

KQ and dc are used instead in Table 6.

The values in the extra column for (KQ/Rp0.2)2 are pro-

portional to the critical defect size that would mean a

change from ductile failure to fracture mechanics driven

failure. The immediate conclusion is that despite GJS-

Table 2. As-Cast Microstructural Features of theMaterial

Nodularity(%)

Nodule count (nod./mm2) Ferrite/pearlite ratio

[90 200 20/80

Figure 1. Unetched as-cast microstructure.

Figure 2. Etched as-cast microstructure.

Table 3. As-Cast Mechanical Properties

Hardness (HB10/3000 W) 273

Rp0.2 (MPa) Rm (MPa) A (%)

Tensile testing 625 852 4.7

KV (J) Unnotched (J)

Impact testing 5 5 5 39 34 37

Table 4. Mechanical Properties of the GJS-900-8 Samples

Hardness

(HB10/3000 W)

314

Rp0.2 (MPa) Rm (MPa) A (%)

Tensile testing 622 963 10.4

KV (J) Unnotched (J)

Impact testing 8 9 9 105 99 106

Table 5. Mechanical Properties of the GJS-1200-3 Samples

Hardness

(HB10/3000 W)

397

Rp0.2 (MPa) Rm (MPa) A (%)

Tensile testing 1035 1260 9.8

KV (J) Unnotched (J)

Impact testing 7 7 8 104 102 94

Figure 3. Microstructure of the GJS-900-8 set.

International Journal of Metalcasting/Volume 11, Issue 1, 2017 133

1200-3 samples offer a yield strength 66 % higher than the

GJS-900-8 samples, the increase in toughness is only 27 %

and the threshold defect size for failure mechanism tran-

sition is reduced 40 %.

The KQ values obtained in the test plan resemble the KIC

results presented by other authors4–6 and are higher than

those employed in certain studies of ADI application as

alternative to steel.7

Nevertheless, none of the checked references describes the

crack tip ductility attained on 12.5 mm thick proportional

specimens.

Regarding fatigue behavior, the comparison between GJS-

900-8 and GJS-1200-3 samples must be done in terms of

load-carrying capacity. GJS-1200-3 offers a higher abso-

lute fatigue limit, the use of this stronger grade instead of

GJS-900-8 only makes sense if it benefits from higher yield

strength. When stresses are normalized by the yield

strength, it turns out that the S–N curve for GJS-900-8 is

above GSJ-1200-3. Figure 6 reflects this fact.

Figure 7 is a representative micrograph of the fracture

surfaces that were observed in all fatigue tests. The fatigue

cracks grew semi-circumferentially with herringbone

marks pointing to the center of the semi-circumference,

what allows finding the crack nucleation point.

In all cases, fatigue crack nucleation spots were found to

coincide with micro-shrinkage porosity sites located on theFigure 4. Microstructure of the GJS-1200-3 set.

0

5000

10000

15000

20000

25000

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

LOA

D (N

)

COD (mm)

GJS-1200-3GJS-900-8

Figure 5. Average toughness test curves for the studiedmaterials.

Table 6. Average Results of the BS-7448-1 Tests

Grade KQ

(MPa�m1/2)dc(mm)

(KQ/Rp0.2)2

(mm)

GJS-900-8 50.3 0.05 6.5

GJS-1200-3 63.7 0.05 3.9

40%

50%

60%

70%

80%

90%

100%

110%

10000 100000 1000000 10000000

Δσ(%

Rp0

.2)

Number of cycles to failure

GJS-1200-3(σm=425MPa)

GJS-900-8(σm=300MPa)

Figure 6. S–N test curves.

Figure 7. Shrinkage porosity which acted as fatiguenucleation point.

134 International Journal of Metalcasting/Volume 11, Issue 1, 2017

skin of the specimens. Figure 8 shows a detail of the crack

initiation point for the specimen of Figure 7.

This means that the inherent fatigue resistance of a defect-

free ADI is higher than in Figure 6, and therefore, it would

last longer in service.

Conclusions

GJS-900-8 and GJS-1200-3 grades develop plastic crack

tip blunting for the studied thickness. The obtained

toughness results agree with the literature data.

The obtained fatigue curves point to service lives that are

higher than the reference values given in the European

Standard.

Avoiding the exclusive use of yield strength-based failure

criteria in the design is a major concern, since the range of

applications where ADI grades become an alternative

material to current solutions is extended when design cal-

culations take into account fracture and fatigue criteria.

Acknowledgments

The authors most sincerely thank to Furesa S. Coop.,member of ADILAN Group, and to ADI TreatmentsLtd. for their invaluable support in this work.

REFERENCES

1. M.F. Hafiz, Impact propeties and fracture in

austempered SG-cast iron. AFS Tran. 2009, 415–422

(2009)

2. K.R.W. Wallin, Equivalent charpy-V impact criteria

for nodular cast iron. Int. J. Metalcast. 8(2), 81–86

(2014)

3. A. Iglesias, I. Gallastegi, G. Artola, M. Muro et al.,

71st World Foundry Congress (2014)

4. S.K. Putatunda, Development of austempered ductile

cast iron (ADI) with simultaneous high yield strength

and fracture toughness by a novel two-step

austempering process. Mater. Sci. Eng. A 315, 70–80

(2001)

5. H.E. Elsayed, M.M. Megahed, A.A. Sadek, K.M.

Aboulela, Fracture toughness characterization of

austempered ductile iron produced using both

conventional and two-step austempering processes.

Mater. Des. 30, 1866–1877 (2009)

6. A. Basso, J. Sikora, Review on production processes

and mechanical properties of dual phase austempered

ductile iron. Int. J. Metalcast. 200, 7–14 (2012)

7. M. Kuna, M. Springman, M. Madler, P. Hubner, G.

Pusch, Fracture mechanics based design of a railway

wheel made of austempered ductile iron. Eng. Fract.

Mech. 72, 241–253 (2005)

Figure 8. Detail of the shrinkage porosity which acted asfatigue nucleation point in Figure 7.

International Journal of Metalcasting/Volume 11, Issue 1, 2017 135