The effect of austempering time on mechanical properties of a ductile iron

The tensile fracture characteristics of austempered ductile irons with dual matrix structures and different ausferrite volume fractions have been studied for an unalloyed ductile cast iron containing (in wt.%) 3.50 C, 2.63 Si, 0.318 Mn, and 0.047 Mg. Specimens were intercritically austenitized (partially austenitized) in two phase region (a + c) at various temperatures for 20 min and then quenched into a salt bath held at austempering temperature of 365°C for various times and then air cooled to room temperature to obtain various ausferrite volume fractions. Conventionally austempered specimens with fully ausferritic matrix and unalloyed as-cast specimens having fully ferritic structures were also tested for comparison. In dual matrix structures, results showed that the volume fraction of proeutectoid ferrite, new (epitaxial) ferrite, and ausferrite [bainitic ferrite + high-carbon austenite (stabilized or transformed austenite)] can be controlled to influence the strength and ductility. Generally, microvoids nucleation is initiated at the interface between the graphite nodules and the surrounding ferritic structure and at the grain boundary junctions in the fully ferritic microstructure. Debonding of the graphite nodules from the surrounding matrix structure was evident. The continuity of the ausferritic structure along the intercellular boundaries plays an important role in determining the fracture behavior of austempered ductile iron with different ausferrite volume fractions. The different fracture mechanisms correspond to the different levels of ausferrite volume fractions. With increasing continuity of the ausferritic structure, fracture pattern changed from ductile to moderate ductile nature. On the other hand, in the conventionally austempered samples with a fully ausferritic structure, the fracture mode was a mixture of quasi-cleavage and a dimple pattern. Microvoid coalescence was the dominant form of fracture in all structures.

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“…These results are consistent with the published literature ( Ref 3,4,8,[15][16][17][18][19][20][21][22][23][24][25][26].…”

Section: Heat Treatments and Microstructuressupporting

confidence: 96%

The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

Kılıçlı

Erdoğan

2009

J. of Materi Eng and Perform

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“…The ductile cast iron with this desired ausferrite microstructure displays optimum mechanical properties. [1][2][3][4][5][6][7][8][9][10][11][12]14,15,19 The sequence of microstructural changes as a function of time observed during austempering in the present study and show the features described above (Figs. 5 and 6).…”

Section: Heat Treatments and Microstructuresmentioning

confidence: 91%

“…Several previous studies [1][2][3][4][5][6][7][8][9][10][11] have suggested that the ductility increases as the volume fraction of high carbon austenite increases. However, the analysis of the present tensile results shows that not only the amount, the distribution and stability of the high carbon austenite but also the amount, nature and distribution of the ferrite phase are important.…”

Section: 2% Proof and Tensile Strengthmentioning

confidence: 99%

“…In recent years, considerable interest has been shown in the property improvements obtained in ductile iron by austempering heat treatments and the effect of heat treating variables such as temperature and time of austenitising and austempering on mechanical properties are well established. [1][2][3][4][5][6][7][8][9][10][11] In the newly developed ADI with a dual matrix structure (DMS), the structure consists of proeutectoid ferrite and ausferrite or martensite. [12][13][14][15][16][17][18][19][20][21] This microstructure is obtained by austempering or quenching from a temperature at which both ferrite and austenite are present, rapidly enough for a significant fraction of the austenite to transform into ausferrite or martensite.…”

Section: Introductionmentioning

confidence: 99%

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Tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

Kılıçlı¹,

Erdoğan²

2006

Materials Science and Technology

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In the present study, an unalloyed ductile iron containing Fe-3 . 50C-2 . 63Si-0 . 318Mn-0 . 047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region azc at various temperatures of 795, 805, 815 and 830uC for 20 min and then quenched into salt bath held at austempering temperature of 365uC for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferritezhigh carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in azc temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900uC for comparison. The new ferrite was absent in these samples. The volume fraction of proeutectoid ferrite, new ferrite and ausferrite can be controlled to determine the strength and ductility. Austempered specimens in azc temperature range exhibited much greater ductility than conventionally austempered ones. The tensile strength increased while ductility decreased with increasing AFVF. On the other hand, the ductility increased with increasing proeutectoid ferrite and new ferrite volume fractions at the expense of strength. The specimen with ,47 . 2%AFVF exhibited the best combination of high strength and ductility. The strength and ductility of this material is much higher than that of ferritic grades. Its strength is at the same level as while ductility almost more than four times higher than that of pearlitic grades. Meanwhile, the specimen with ,75%AFVF exhibited the best combination of high strength and ductility compared with those of pearlitic grades. The strength of this material is much higher and its ductility is almost more than two times higher than that of pearlitic grades yet slightly lower than that of ferritic grades. This material also meets the requirements for the strength of quenched and tempered grades and its ductility is higher than that of this grade.

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“…The wear performance of a ductile iron is significantly influenced from the microstructure. [1][2][3][4][5][6][7] In as-cast condition, pearlitic microstructure, consisting of very hard lamellar carbide in a soft ductile matrix of ferrite, exhibits superior wear resistance than ferritic microstructure. Further improvement of wear resistance can be achieved by applying austempering heat treatment, which results in bainitic microstructure.…”

Section: Introductionmentioning

confidence: 99%

High Temperature Tensile and Abrasive Wear Characteristics of As-cast Ductile Irons

et al. 2003

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In this study, tensile and abrasive wear performances of two different quality as-cast ductile irons have been examined at various temperatures between 25 and 600°C. Tensile tests were carried out with a strain rate of 3ϫ10 Ϫ4 /s. Wear tests were performed under a compression stress of 5.7 N/mm 2 , by rubbing samples on 125 mm Al 2 O 3 abrasive grains. In the temperature range of 100-300°C, where serrated flow is observed during tensile testing, tensile strength values of both ductile irons were invariable. Above 400°C, further increase of temperature caused dramatic decrease in tensile strength. As a general trend, abrasive wear resistances of the both ductile irons increased with increasing tensile strength. Exceptionally, maximum resistance to abrasive wear is obtained from both ductile irons at 100°C, probably due to dynamic strain aging.KEY WORDS: abrasive wear; ductile iron; high temperature; strain aging; tensile characteristics. Results and DiscussionTensile test results of the investigated ductile irons are presented in Fig. 3 as "Stress-Strain" curves. Serrated flow, which is the common feature of dynamic strain aging, is apparent in the temperature range between 100-300°C and much more pronounced for the material DI 1 than the material DI 2.Previously, Lui and Chao 9) reported serrated flow during low strain rate (3ϫ10 Ϫ4 /s) tensile testing of a ferritic ductile iron in the temperature interval between 150 and 350°C. Shieh et al. 10) presented serrated flow on the "StressStrain" curves of bainitic austempered ductile irons at testing temperatures lower than 280°C, which was attributed to the presence of acicular ferrite in the microstructure. Since higher volume fraction of ferrite contribute to the distinct dynamic strain aging, serrated flow is more apparent on the "Stress-Strain" curves of the material DI 1 than that of the material DI 2.Dynamic strain aging, which leads to inhomogeneous deformation characterised by serrated flow, is mainly relevant to the diffusion of interstitial atoms such as carbon or nitrogen to dislocations in motion. When the temperature and the strain rate are such that, the speed of interstitial atoms is more than that of dislocations, dislocations are pinned by interstitial atoms. Serrations occur due to rapid dislocation multiplication during deformation. In the process of dislocation multiplication stress increases, but once the dislocations are released the stress drops to sustain their movement ISIJ International, Vol. 43 (2003) until interstitial atoms diffuse and pin these mobile dislocations again. [11][12][13] The effect of test temperature on the strength and ductility of the investigated ductile irons is plotted in Figs. 4 and 5, respectively. The material DI 2 exhibited higher tensile strength and lower elongation at fracture values than the material DI 1 at the entire temperature range. The tensile strengths of both ductile irons were maintained almost constant up to 400°C and then dropped sharply at higher temperatures. It is reported that, 10) in the t...

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The effect of austempering time on mechanical properties of a ductile iron

Cited by 14 publications

References 15 publications

The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

Tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

High Temperature Tensile and Abrasive Wear Characteristics of As-cast Ductile Irons

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