59

Journal of Al Azhar University Engineering Sector

Vol. 5, No. 14, January 2010, 59- 65

EFFECT OF SECOND STEP AUSTEMPERING TEMPERATUREAND TIME ON THE IMPACT PROPERTIES OF TWO-STEP

AUSTEMPERED SG-CAST IRON

Mahmoud HafizAl-Azhar University, Faculty of Engineering, Department of Mechanical Engineering,

Nasr city, Cairo, Egypt

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ABSTRACTThis study investigates the effect of a novel two-step austempering temperature on the impactproperties of austempered spheroidal graphite (SG) cast iron. The study shows that the impactproperties of a two-step austempered SG-cast iron are significantly influenced by the secondstep austempering temperature and time. At a given second step austempering time,decreasing the second step temperature enhanced the resistance of the present material to thecrack propagation as well as the impact toughness. The maximum energy absorbed during thecrack propagation and the fracture processes are obtained at shorter second step austemperingtime. It has also been found that the maximum crack initiation energy in the steppedaustempering is achieved at different second step austempering time. It occurs at 22 min forthe 360°C treatment and at 82 min for the 320°C. However, the crack initiation energy of theformer is higher than that of the latter.

KEYWORDS : Spheroidal Graphite Cast Iron, Two Step Austempering, ImpactProperties

1. INTRODUCTIONTwo-step austempering heat treatment has attracted considerable interest in recent years.Several studies [1-11] have shown that this novel austempering process can be used to

EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEPAUSTEMPERED SG-CAST IRON

enhance the mechanical properties of austempered SG-cast iron. In two-step austemperingtreatment the austempering process is performed at two different austempering temperatures.This can be achieved in two heat treatment routes namely; high–low and low–highaustempering temperature regimes. The high–low stepped heat treatment (HLAT) relies onaustempering at high temperature in the first step for a time sufficient to complete stage Ireaction in the eutectic cell (i.e. to produce the maximum retained austenite content). Thenimmediately, the austempering temperature is decreased to the second step. This in turnincreases the driving force for the stage I reaction and prevents the stage II reaction in theeutectic cell [2]. Contrarily, the low–high temperature regime (LHAT) is based in the highsupercooling to lower austempering temperature and a very short hold at this temperaturefollowed by austempering at higher temperature, which assists in rapid diffusion of carbon tosaturate austenite [7].

The influence of two-step austempering process on the tensile properties and strain hardeningbehaviour is reported [4,6,10, 11,14]. The effect of two-step austempering on the fracturetoughness of austempered SG-cast iron is clarified,[5,12,13]. The effect of this novelaustempering process on the abrasion resistance has been also studied [6,9]. However, therehas been a limited reports covering the area of impact properties [3,4] . There are rather lackof investigation on the effect of second step temperature and time on the impact properties oftwo-stepped austempered SG-cast iron.

2. EXPERIMENTAL PROCEDUREAn spheroidal graphite cast iron Y-block (Fe-3.68C-2.42Si-0.008Mn-0.034Cr-0.021Ni-0.01S- 0.035P-0.05Mg, mass%), 40 mm x 90 mm x 210 mm in parallel section, produced in acommercial foundry using a high frequency electric melting furnace has been used for theexperiments. Charpy V-notched (CVN) impact test specimens (10mm×10mm×55mm) weremachined from the test according to ASTM standards [15].

The specimens were processed by stepped austempering process of high-low austemperingtreatment (HLAT). The test specimens were austenitized at 910°C for 60 min, and thenquickly quenched into molten salt bath maintained at 400°C for 8 min. immediately after that,the specimens were transferred to another salt baths which were maintained at 360 and 320°Cfor 22, 52, 82 and 172 min, respectively and finally air cooled.

Two-step austempered SG-cast iron Charpy specimens were tested at room temperature in acomputer aided instrumented (CAI) impact testing machine. Using CAI system facilitatesrecording of the total fracture energy as well as its two parts, namely; crack initiation energy(Ei) and crack propagation energy Consequently, much information concerning the details ofdynamic fracture behaviour of a material can be studied. At least three specimens were testedfor each heat treatment.

Microstructural features of the matrix structure were studied using optical microscope afteretching the samples with 2%nital.

3. RESULTS AND DISCUSSIONSFig. 1 shows the optical microstructures of specimens processed by the two-step process. Theaustempered microstructure shows a matrix of dark needle-shaped ferrite plus bright etchingaustenite with graphite nodules dispersed in the matrix. Two-step austempering process

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EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEPAUSTEMPERED SG-CAST IRON

resulted in a mixed microstructure consisting of both upper and lower ausferrite. Thecharacteristics of the two depended on the second step austempering temperature and time.This can be observed in Fig.1.

The effect of the second step austempering temperature and time on crack initiation energyhas been presented in Fig.2. Clearly, each treatment shows a maximum value depends on thesecond step austempering time. The maximum crack initiation energy in the steppedaustempering is achieved at different second step austempering time. It occurs at 22 min forthe 360°C treatment and at 82 min for the 320°C. These times might be related to themaximum in the austenite carbon content which has been contributed to its mechanicalstability [2]. There is a perceptible decrease in the crack initiation energy accompaniedextending the second step austempering time beyond the time needed for maximum crackinitiation energy.

The influence of the second step austempering temperature and time on the crack propagationenergy is presented in Fig. 3. Obviously, specimens subjected to short second step time (22min) exhibit greater resistance for the crack propagation. The resistance of two-stepaustempered SG-cast iron for the crack propagation energy is influenced significantly by thesecond step temperature. For all the investigated second step austempering time, the crackpropagation resistance of the second step austempering of 320°C is largely superior to thecorresponding values of 360°C austempering temperature.

The total fracture energy (Ei+Ep) has long been generally accepted among materialsengineers as a measure of the toughness of a material. Fig.4 shows the variation of the impacttoughness of the two-step austempered SG-cast iron as a function of the second stepaustempering temperature and time. Clearly, at a given second step austempering time, theimpact toughness values of specimens austempered at 320°C are distinctly higher than thoseof 360°C. Impact toughness is thus seen to decrease with increasing the second stepaustempering temperature.

The impact toughness decreases with increasing the second step austempering time.However, beyond 22 min, the difference between the specimens austempered at a second steptemperature of 320°C and those at 360°C is rather big. At the end of the first step treatmentat 400°C (8 min) there is an untransformed austenite in the microstructure and areas of lowercarbon austenite.

During the early stage of the second step austempering these areas will transform to highcarbon austenite and ferrite. Consequently, on cooling to room temperature thesetransformed areas will be retained as austenite. Extending the second step austempering timemay results in a decrease in the retained austenite content, due to the stage II reaction.

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EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEPAUSTEMPERED SG-CAST IRON

Fig. 1 Microstructures of two-step austempered SG-cast iron.

125 µm125 µm

125 µm125 µm

125 µm 125 µm

125 µm125 µm

320 360SEECOND STEP AUSTEMPERING TEMPERATURE (C)

172

82

SEC

ON

DST

EP A

UST

EMPE

RIN

G T

IME

(min

)

52

22

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EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEPAUSTEMPERED SG-CAST IRON

0

1

2

3

4

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6

7

8

9

22 52 82 172

400-320400-360

CR

AC

K IN

ITIA

TIO

N E

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(J)

SECOND STEP TIME (min)

Fig. 2. Effect Of The Second Step Austempering Temperature And Time On The CrackInitiation Energy For Two-Step Austempering Process.

0

1

2

3

4

5

6

7

22 52 82 172

400-320400-360

CR

AC

K P

RO

PAG

ATI

ON

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ERG

Y (J

)

SECOND STEP TIME (min)

Fig. 3. Crack Propagation Energy As A Function Of The Second StepAustempering Temperature And Time.

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EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEPAUSTEMPERED SG-CAST IRON

The ductility index defined as the ratio of the crack propagation energy to the crack initiationenergy is plotted in Fig.5. The effect of the second step austempering temperature and time isclearly seen in the ductility index. Shorter second step austempering time is required for thepresent SG-cast iron to reach the optimum ductility index values. The second stepaustempering temperature significantly affects the ductility index of the present material. Themore intensively interwoven ferrite needles serves as crack propagation barriers. The lesssharp needles reduce the stress concentration effect. These two microstructural features havecontributed to the high ductility index of the lower second step temperature (320°C).

FRA

CTU

RE

ENER

GY

(J)

SECOND STEP TIME (min)0

2

4

6

8

10

12

14

16

22 52 82 172

400-320400-360

Fig. 4 Variation Of Impact Toughness With Second StepAustempering Temperature And Time.

QU

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TY IN

DEX

0

0.2

0.4

0.6

0.8

1

22 52 82 172

400-320

400-360

SECOND STEP TIME (min)

Fig. 5. The Ductility Index As A Function Of The SecondStep Austempering And Time.

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EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEPAUSTEMPERED SG-CAST IRON

4. CONCLUSIONS1. The second austempering time for optimum crack initiation resistance is about 22 min

for austempering temperature of 360°C. The time is higher at lower temperature.2. The lower the second step austempering temperature the higher the crack propagation

energy, the impact toughness and the ductility index, at the given austempering time.3. Shorter second austempering time is in the favour of the crack initiation resistance of

the present SG-cast iron at second step temperature of 360°C. However, it is alsosuitable to achieve higher resistance to crack propagation as well as ductility indexand toughness.

REFERENCES1. T. S. Shih, C. K. Lin and H. Z. Twan, “Mechanical properties of various-section

ADIs”, AFS Trans. 105 (1997), 367-376.2. H. Bayyati and R. Eliott, “ Stepped austempering heat treatment of a 0.67%Mn-Mo-

Cu ductile iron”, Materials Science and Technology,13 (1997) 117-124.3. A. Nazarboland and R. Eliott, “Influence of heat treatment parameters on stepped

austempering of 0.37%Mn-Mo-Cu ductile iron”, Materials Science and Technology,13 (1997) 223-233.

4. H. Bayyati and R. Eliott,“Role of austenite in promoting ductility in an austemperedductile iron”, Materials Science and Technology,13 (1997) 319-328.

5. C. Hsu and T. Chuang, “Influence of stepped austempering process on the fracturetoughness of austempered ductile iron”, Metallurgical and Materials Trans. A, 32A(2001) 2509-2513.

6. M. N. Ahmadabadi, H.M. Ghasemi, M. Osia, “Effects of successive austemperingon the tribological behavior of ductile cast iron”, Wear 231 (1999.) 293–300

7. S. K. Putatunda, “Development of austempered ductile cast iron (ADI) withsimultaneous high yield strength and fracture toughness by a novel two-stepaustempering process”, Materials Science and Engineering A315 (2001) 70–80

8. J. Yang, S. K. Putatunda, “Improvement in strength and toughness of austemperedductile cast iron by a novel two-step austempering process”, Materials and Design25 (2004) 219–230

9. G. Francucci, J. Sikora, R. Dommarco, “Abrasion resistance of ductile ironaustempered by the two-step process”, Materials Science and Engineering A 485(2008) 46–54

10. J. Yang, S. K. Putatunda, “Influence of a novel two-step austempering process onthe strain-hardening behavior of austempered ductile cast iron (ADI)”, MaterialsScience and Engineering A 382 (2004) 265–279

11. E. V. Pereloma and C. S. Anderson, “Microstructure and properties of austemperedductile iron subjected to single and two step processing, Materials Science andTechnology, 22 (2006) 1112-1118.

12. K. S. Ravishankar, K. R. Udupa and P. P. Rao, “Development of austemperedductile iron for high tensile and fracture toughness by two step austemperingprocess”, Proceedings of the 68th world foundry congress (WRC), Chennai, India,Feb.7-10 (2008) 35-40

13. A. H. Elsayed, M.M. Megahed, A.A. Sade, K.M. Abouelela , “Fracture toughnesscharacterization of austempered ductile iron produced using both conventional andtwo-step austempering processes”, Materials and Design 30 (2009) 1866–1877

14. S. Daber, P.P. Rao, “Formation of strain-induced martensite in austempered ductileiron”, J Mater Sci 43 (2008):357–367

15. ASTM, “Standard Test Methods for Notched Bar Impact Testing of MetallicMaterials , ASTM Designation: E 23, American Society for Testing and Materials,West Conshohocken, PA, Vol. 03. (2001).

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