fatigue properties of machinable austempered ductile iron

7/27/2019 Fatigue Properties of Machinable Austempered Ductile Iron

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METAL 2007 22.-24.5.2007, Hradec nad Moravic

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FATIGUE PROPERTIES OF MACHINABLE AUSTEMPERED

DUCTILE IRON

Eng.M.Tadayon saidi (1,4), Eng.N.Baghersaee (2) , Prof.Dr.N.Varahram (3),Prof.M.Hanumantha Rao

(1),Dr.G.V.S.Nageswara Rao

(1

1) Metallurgical & Materials Dep. National Institute of Technology,Warangal, AP,India.

2) Material selection & Testing and Conformity Assessment Supervisor, RWTUV Iran

Joint Venture Co.,Member of TUV NORD Group.3) Associate professor-Dept.of Met.Science & Eng.-Sharif University of Technology-

Tehran,Iran.

4)Materials Engineering Dep,AZAD University,Karaj Branch,Iran.

Abstract:

MADI is new engineering material with very desirable properties such as higher strength at

the same hardness compared to regular ductile iron, significantly better fatigue performance

than regular ductile iron or austempered ductile iron, better machinability than regular

austempered ductile iron and similar to as cast ductile iron.

In this study,fatigue properties of Machinable Austempering Ductile was investigated using

rotary bend fatigue testing device.

Chemical composition of sample was selected on the base of previous work which results in

good mechanical properties and low hardness after austempering process.

The results of this study ,indicates that for an austempered ductile iron with 753 (N/mm2)

Ultimate Tensile Strength,541 (N/mm2) Yield Strength with 14% elongation and hardness of

240 (BHN),fatigue limit is the range of 367 Mpa .

Key word:

MADI (Machinable Austempered Ductile Iron), Fatigue, Endurance Limits1

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- E.mail: [email protected]


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1. Introduction:

The development and commercialization of Austempered Ductile Iron (ADI) has provided the

design engineer with a new group of cast ferrous materials which offer the exceptional

combination of mechanical properties equivalent to cast and forged steels and production

costs similar to those of conventional Ductile Iron. In addition to this attractive performance:cost ratio, ADI also provides the designer with a wide range of properties, all produced by

varying the heat treatment of the same castings, ranging from 10-15% elongation with 125 ksi

(870 MPa) tensile strength, to 250 ksi (1750 MPa) tensile strength with 1-3% elongation.

Although initially hindered by lack of information on properties and successful applications,

ADI has become an established alternative in many applications that were previously the

exclusive domain of steel castings, forgings, weldments, powdered metals and aluminum

forgings and castings.[1,2]

But ADI not as easily machined as pearlitic or ferritic ductile iron and this is one of

disadvantages of ADI as compared to regular ductile iron and following fail may be acquire

prior to heat treatment before final finishing (final machining ):

Dimensional changes after heat treatment.Increased total cost of production .[3,4]Improved machinability of ADI ,caused new family of ADI that was named MADI is, thus,

many advantages. MADI is new engineering material with very desirable properties such as

higher strength at the same hardness compared to regular ductile iron, significantly better

fatigue performance than regular ductile iron or austempered ductile iron, better machinability

than regular austempered ductile iron and similar machinablity to as cast ductile iron.[5,6]

ADI has fatigue properties equal or superior to those of forged steels. When subjected to

surface treatments such as rolling, peening or machining after heat treatment, the fatigue

strength of ADI is increased significantly. Previous study also indicate that ADI is moderately

notch sensitive in fatigue, with a notch sensitivity ratio (ratio of notched to un-notched

endurance limits) ranging from 1.2 to 1.6 for the notch geometry tested. Conventional ferriticand pearlitic Ductile Irons have a notch sensitivity of about 1.6 and steels with fatigue

strengths similar to ADI exhibit notch sensitivity ratios as high as 2.2-2.4. To avoid problems

caused by notch sensitivity, components with sharp corners should be redesigned to provide

generous fillets and radii. When required, fillet rolling or shot peening can be employed to

further increase resistance to fatigue failure.[

Studies show an important relationship betweenaustempering temperature and the endurance

limit of shot peened ADI. The dramatic rise in endurance limit in ADIs austempered above

600F (315C) is related to the increased response to peening resulting from the higher austenite

contents characteristic of higher austempering temperatures.

For gear applications, shot peened ADI has single tooth bending fatigue and contact fatigue

superior to as cast and conventionally heat treated Ductile Irons, and cast and through

hardened steels. They also show that peened ADI is competitive with gas nitrided and case

carburized steels.

To this point we have discussed fatigue strength in terms that assume infinite life below a

certain load. In fact, as the number of loading cycles are increased all materials undergo

changes in their ability to withstand further loading.

Experimental procedure :In order to achieve optimum fatigue properties of Machinable Austempering Ductile Iron and

compare to ADI fatigue properties,Effect of different alloying element such as copper, nickel,

molybdenum and manganese-which increase hardenability of ADI-on the mechanical


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properties such as unique tensile properties, elongation and hardness were investigated as a

baseline.

Y-blocks mold according to ASTM A536-84 , and plunging spheroisation method followed

by inoculation Fe-Si were used.Molten metal was poured into green sand mould and then left

for cooling to room temperature.

Heat treatment cycle was selected on the base of previous work which results in goodmechanical properties and low hardness after austempering process as followed:

The specimens were austenite at 850OC for 1 hour .

The specimens were austempered at 390OC for 1 hour.

The optimum composition of alloying elements according to table 1 to achieve optimum

tensile strength and hardness selected as base metal composition and fatigue properties

deliberated.

Following Standards cover the requirements and works to be carried out:

1. Preparation of specimens according to ASTM E3.

2. Micro-etching according to ASTM E 407.

3. Optical microscopic evaluation according to ASTM E883.

4. Graphite microstructure according to ASTM A 247.5. Hardness test according to ASTM E 10.

6. Tensile test according to DIN EN 1563 & ASTM A370.(fig.3)

7. Preparation of fatigue specimens according to ASTM E468.

Result and discussion:ADI is a group of material whose mechanical properties can be varied over a wide range by a

suitable cycle of heat treatment.

Compared to competitive materials, the machinability of Ductile Iron has been one of its

major advantages. When the substantial increases in strength and wear resistance offered by

ADI are considered, it would be logical to assume that ADI could present machiningproblems. However, cost savings in machining are frequently mentioned as reasons for

converting to ADI. The reasons for this surprising combination of mechanical properties and

machinability are two-fold. First, the machinability of the softer grades of ADI is equal or

superior to that of steels with equivalent strength, and second, the predictable growth

characteristics of ADI during austempering allow, in many cases, for it to be machined

complete in the soft as-cast or annealed state before heat treatment. This allows for faster

machine feeds and speeds and greatly increased tool life.(fig1)

Fig.1. Relative Machinability of Several Ferrous Materials


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The effect of austempering time & alloying element added at various amount on the

machinability of ADI were investigated.According to previous study, the lowest cutting force

was obtained when machining the as-cast specimens. the best results among the as-cast

specimens according to the cutting force and surface roughness belonge to 0.7% Ni &

0.7%Cu alloyed specimens.[4,5,6]

According our study the best mechanical property achieved by adding alloying element suchas chemical composition in table 1.

Table 1. Chemical composition of sample.

Sample % C % Si % Mn % Ni % Cu % Mo

A 3.24 3.7 0.35 0.97 0.6 0.25

Increasing austempering time did not have a considerable influence on the cutting force and

surface roughness, but it is slightly influence on the mechanical property. Therefore when it is

considered that 60 min austempering time led to the best result.Optimum composition of alloying element & heat treatment cycle according to tensile

strength, elongation & hardness selected as base metal composition and fatigue properties

deliberated.

Kovacs and Hayrymen showed that the microstructure of steel & ADI are not the same. They

stated that the structure of ADI is an ausferritic structure composed of ferrite + carbon

enriched austenite.[7,8] Microstructure of MADI specimens shown on figure 2.

2- A) Microstructure of sample at 100x.


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2-B) Microstructure of sample at 500x

Fig.2. Microstructure of sample.

As shown in figure in 3 ADI has a fatigue property equal or superior to those of forge steel.

Fig3. Comparison of fatigue of ADI and different grade of forge steel.

Fatigue property of the sample shown in table 3 and figure (4-B).Comparison of figure (4-A)

with (4-B) reveals several interesting fact. grade 1050/700/7 of ADI have 370 (Mpa) fatigue

strength with endurance ratio of 0.35 ,and MADI have 367(Mpa) ) fatigue strength with

endurance ratio of 0.48 . According to previous study ,unlike conventional ductile iron ,the un

notch fatigue limit of ADI does not followed the tensile properties, demonstrates a maxima at

a condition of lower tensile strength and maximum stabilize austenite content in metal matrix.

These relationships result in an endurance ratio that is 0.5 for lower strength ADI & decreaseto 0.3 as the tensile strength increase to its maximum.[6,9,10]


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Above mention means that MADI with 14% elongation have endurance ratio similar to lower

strength ADI & its fatigue strength to be same as grade 1050 with better machinability

characteristic.

Table 3. Fatigue property of sample.

Force (gr) Number of cycle Stress (Mpa) Result

5250 54610 617 Fracture

4750 249710 590 Fracture

4000 348010 432 Fracture

3900 1853830 426 Fracture

3500 10000000 367 ---

- Rotating bending fatigue test

- Number of cycle per minute is 2800.

- Diameter of sample is 4 mm.

4-A ) Rotating Bending Fatigue of ADI Grade 1050.

4-B)Rotating Bending Fatigue of MADI

Fig.4 Comparison of Fatigue curve.


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Conclusion:Following conclusion could be summarized based on the results of present works:

Machinable austempered ductile Iron with 750 (Mpa) ultimate tensile strength have 367 (

Mpa) endurance limit.

MADI have better Machinable characteristic and fatigue property as compare to ADI gradewith the same mechanical properties.

References:1. Alan P. Druschitz and David C. Fitzgerald , MADI: Introduction a new ,Machinable

Austempered Ductile Iron, 2003 SAE Word Congress,Detroit,Michigan,March3-6,2003.

2. R.Keough ,Ductile iron data for design engineers, August 1998.

3. K.L.Hayrynen ,J.R.Keough,Austempered Ductile Iron-State of the Industry in 2003,Keith

Millis Symposium on Ductile Iron 2003.

4. Evaluation of Machinability of Austempered Ductile Iron in Terms of Cutting

Forces and Surface Quality Journal of Material Processing Technology 173(2006)260-

268.5. F.Zanardi,Fatigue Properties and Machinability of ADI, 2nd New Development in

Metallurgical Process Technology,2005.

6. M.Cemal Cakir,The Effect of Austempering Temperature and Time onto the Machinability

of Austempered ductile Iron,Material Since and Engineering A 407(2005)147-153.

7. B.Kovas,Heat Treating of Austempered Ductile Iron AFS Trans.91-75(1991),281-286.

8. Fritz Klocke and Carsten Klopper,Machinability characteristic of Austempered Ductile Iron , 2002 Word Conference on ADI.

9. F.Zanardi ,Machinable ADI in Italy, AFS Library copy:20050990 A.pdf, copyright 2005

American Foundry Society.

10. P.K.Rastogi,A Comparative Evaluation of Mechanical Properties and Machinability of

Austempered Ductile Iron(ADI) and Microalloyed Steel,SAE PAPER SERIES,International

Congress and Exposition,Detroit,Michigan,1991.

fatigue properties of machinable austempered ductile iron

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