observation of fatigue crack paths in nodular cast...

Materials Engineering, Vol. 16, 2009, No. 3

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OBSERVATION OF FATIGUE CRACK PATHS

IN NODULAR CAST IRON AND ADI MICROSTRUCTURES

Lukáš Bubenko

1, Radomila Konečná

1, Gianni Nicoletto

2

Received 11st June 2009; accepted in revised form 2nd July 2009

Abstract

When speaking about quality of construction materials, fatigue crack propagation resistance is one of the

most important considered properties. That is essentially influenced by character of matrix. Here presented

contribution deals with the fatigue crack propagation mode through the matrix of as-cast nodular cast iron (NCI)

and austempered ductile iron (ADI), whereas influence of microstructure has been considered and discussed.

Experimental materials used in presented contribution were pearlitc-ferritic NCI and heat treated ADI 800.

Pearlitic-ferritic NCI was used as the base for ADI production. Experiments were performed on mini round

compact tension (RCT) specimens using an Amsler vibrophore. Fatigue crack paths in both materials were

investigated and compared. Light microscopy was used to analyze the microstructure, crack initiation and

propagation within broken specimens. In both tested materials fatigue cracks always initiated at graphite-matrix

interface, while graphite nodules remained generally unbroken, eventually only surface of nodules was damaged.

Though, comparing two materials with different microstructures, the diversity of fatigue crack propagation

modes at high ∆K and low ∆K was observed.

Keywords: nodular cast iron; austempered ductile iron; fatigue crack; initiation; propagation.

1. Introduction

Cast irons have several manufacturing and engineering advantages compared with steels. These include a 20 - 40 % lower manufacturing cost, better vibration damping, and lower volume shrinkage during solidification [1]. Austempered ductile iron presents further excellent combination of properties, such as high strength, good ductility, toughness, fatigue strength and wear resistance those are unavailable in other grades of cast iron [2]. Furthermore, the density of ADI is lower than cast steel and so ADI has the advantage of higher specific strength compared to steel. As a result, ADI is considered a very promising engineering material, and an economical substitute for wrought or forged steel in several structural applications in the automotive industry (crankshafts, transmission gears, connecting rods), defense (canon shells, aircraft landing gears, etc.), earth-moving machinery, railroads, etc [3, 4].

Pearlitic cast iron with nodular graphite usually forms the basis for the production of ADI materials. The attractive properties of ADI are related to its unique microstructure that consists of acicular ferrite and high carbon austenite. For production a single-step austempering process is conventionally

used. This process consists of austenitizing the casting in the temperature range of 871 - 982 °C for sufficient time to get a fully austenitic matrix, and then quenching it to an intermediate temperature (austempering temperature) range of 260 - 400 °C to avoid formation of pearlite. The casting is maintained at this austempering temperature for 2 - 4 h depending on the section size [5]. Large amount of silicon present in ductile iron suppresses the precipitation of carbides during the austempering reaction and retains substantial amount of stable high carbon austenite.

Previous investigations have demonstrated that ADIs austempered at 400 °C have better fatigue crack initiation and growth resistance than those austempered at 250 °C at similar stress ranges. Lower austempering temperature results in a higher hardness, but lower toughness material and the higher austempering temperature generally improved toughness and damage tolerance, but with lower strength and hardness [3].

Small amount of alloying elements (depending on the maximum wall-thickness of the component), such as nickel, molybdenum and copper are generally added to ADI to avoid formation of pearlite during the austempering process [6, 7].

1 L. Bubenko, Ing.; R. Konečná, prof. Ing. PhD. - Department of Materials Engineering, Faculty of Mechanical Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovak Republic. 2 G. Nicoletto, prof. Ing. - Department of Industrial Engineering, University of Parma, Viale G.P. Usberti, 181/A, 431 00 Parma, Italy. *Corresponding author, e-mail address: [email protected]


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Considerable work has been done to understand the microstructural characteristics of ADI and their effect on the mechanical properties. However, direct reports on observations of the microprocess of fracture are rare. Further studies are needed to clarify the micromechanism of fracture in ADI, especially the initiation and propagation of microcracks and their interaction with the microstructure [2].

2. Material and experimental procedures

Experimental materials used for fatigue crack propagation tests were the pearlitic-ferritic NCI and heat treated ADI 800, both cast in the form of cylindrical bars of 25 mm diameter and 200 mm long. Pearlitic-ferritic NCI was used as the basic material for ADI production, and the following progress of heat treatment was applied: 785°C/30 min - 820°C/135 min - 385°C/90 min. Production process of pearlitic-ferritic NCI was not supplied, though. Chemical composition of original cast iron is shown in Tab. 1 and mechanical properties of heat treated ADI 800 in Tab. 2.

Tab. 1

Chemical composition of base material (in wt. %)

Material C Si Mn Cu Ni Mo

PER 3.61 2.47 0.23 0.67 0.04 0.27

Tab. 2

Mechanical properties of heat treated cast iron

Material Rm [MPa] RP 0,2 [MPa] A5 [%]

ADI 800 888 ± 12.4 621 ± 13.6 14.7 ± 1.9

Experiments were performed using the AMSLER 421 vibrophore, the device that works on the principals of electromagnetic resonance, and is usually used for push-pull fatigue tests. Machine was considered due to its high loading frequency and relatively cheap operation and service. With respect to the small amount of used material, mini RCT specimens with extra small dimensions (only 22 mm of diameter) were prepared, as shown in Fig. 1.

Fig. 1. Mini RCT specimen

Specimens were furnished with straight through notch, and denominated as PER and ADI regarding the type of represented material. Specimens together with suitable grips were prepared according to the ASTM E399 standard. Due to the load limit of the test machine length of notch and thickness of specimens were adapted, however (see dimensions in red of Fig. 2).

Fig. 2. Geometry of mini RCT specimen with

adapted dimensions

Experiments were performed to generate long stable cracks in mini RCT specimens of ADI and PER materials to investigate the fracture paths through the respective microstructures at different levels of ∆K using light metallographic microscopy. The stress intensity factor range ∆K for the RCT geometry is given by (1),

∆K = ∆F (πa)1/2

Y/wt (1)

where a is the crack length, w and t are specimen width and thickness, respectively, ∆F = (Fmax - Fmin) is the load range and Y = f(a/w) is the geometry factor for the RCT specimen.

Fatigue tests were initially performed at relatively high load (i.e. high ∆K) to initiate crack from the starter notch. As the fatigue crack propagated, manual reduction of the applied load reduced ∆K to near threshold values to obtain crack arrest.

The structural analysis was carried out applying metallographic techniques and digital image analysis software on polished sections according to the standard EN STN 42 0461.

3. Results and discussion

The microstructures of tested materials are shown in Fig. 3. Specimen PER was characterized by a pearlitic-ferritic matrix according to STN. The ferrite content ranges from 10 to 20 % (Fe 15), mostly around regularly distributed graphite nodules. Those were observed as fully globular, with size range from 30 to 60 µm (VI6).


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a) PER

b) ADI

Fig. 3. Microstructure of specimens PER and ADI,

etched with 3 % Nital

The ADI specimen presents a matrix formed by coarse laths of acicular ferrite, characteristic for upper bainite, and fractions of retained austenite, with some residual ferrite around graphite elements.

The character of matrix and distribution of graphite nodules as well as the globularity has a significant influence to fatigue properties in general. Previous studies have shown that the fatigue strength of ADI increases with increasing nodularity and nodule count [8]. Also pores and microshrinkages, observed in tested material, have significant influence on initiation and propagation of fatigue cracks, as presented in [3, 8, 9].

In both PER and ADI specimens, fatigue crack paths started to grow from the prepared notch due to the stress concentration, subsequently propagating in the direction perpendicular to the direction of cyclic loading (Fig. 4). However, a considerable difference of crack propagation within two analyzed materials is shown by comparison of macroviews. In case of specimen PER (Fig. 4a), first part of crack is straight, but second part markedly changes its direction, while fatigue crack in specimen ADI grows straight, just the final part of crack is irregular (Fig. 4b).

The macroviews demonstrate that the mechanisms of crack propagation are very different at high ∆K (i.e. straight growth with minor influence of microstructure) and at low ∆K (i.e. meandering growth with strong microstructural influence).

a) PER

b) ADI

Fig. 4. Macrocrack, magn. 7.5 x

Taking a closer view to cracks, other differences are visible. In pearlitic-ferrititc NCI, the crack at high ∆K propagated mainly through the matrix with local secondary cracks and considerable changes in direction, possibly due to the contribution of nodules below the surface (Fig.5b). At low ∆K (Fig.5a), the crack path changes characteristics and is controlled by the presence of graphite nodules. The crack propagates always at the graphite-matrix interface (Fig. 6a) as an indication of the weak nodule-matrix bond, while graphite nodules remained unbroken or merely surface of nodules was damaged. That was characteristic for both materials. Between neighboring graphite nodules crack propagated mainly through the pearlite, what is obvious considering mostly pearlitic matrix, creating usually the path with shortest distance between these two nodules (Fig. 6a). Pearlite itself provides favorable conditions for crack propagation, because it is a lamellar composite of soft, ductile ferrite and hard, brittle cementite.

In case of ADI, both at high and low ∆K (Fig. 5c, d), the fatigue crack path connects neighboring graphite nodules and nodules detach from matrix. Also local microdefects should influence crack propagation, as in pearlitic-ferritic NCI. An


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example of crack branching and reconnection is observed at low ∆K (Fig. 5c). Since crack branching in ADI is due to crack-nodule interaction, it occurs when a favorable nodule is below the observation plane in accordance to [3, 10].

a) PER

b) ADI

Fig. 6. Crack initiation and propagation through the

graphite-matrix interface, etched with 3 % Nital

As reported before, the matrix of ADI is the combined phase structure of acicular ferrite and retained austenite. Crack always initiated at graphite-matrix interface, as shown in Fig. 6b, and propagated along the least-energy paths which are often the interfaces between ferrite laths and austenite. According to Greno et al. [10], graphite-matrix interfaces are irregular, with sharp corners that in some cases constitute imminent microcracks that emanate from the nodules. The vicinity of the front of the main crack increases the values of ∆K in preferential points of crack initiation. This causes the propagation of microcracks from the nodules toward the main crack, in an inverse direction of propagation to the general crack growth, until joining the main crack. In some occasions, it is feasible that when these microcracks reach certain length, the system adopts the necessary geometry to cause a load shedding effect on the main crack, reducing its applied ∆K, and eventually stopping its propagation. In that case, the general crack path continues in some other point around the nodule, where favorable conditions exist for the initiation of a new crack that keeps the propagation process, repeating the process when encountering another nodule. This mechanism justifies the presence of crack branching, observed in the encounter of the main crack with nodules (Fig. 5c).

Initiation of secondary microcracks was observed close to the main crack. When these small cracks simultaneously propagate besides the main crack, the available elastic energy for the propagation of the main crack is obviously reduced, mainly because of the creation of a larger cracked surface, thus reducing the general rate of crack propagation and in some cases causing locally the sudden arrest of the crack, as shown in Fig. 7a.

a) PER - low ∆K b) PER - high ∆K

c) ADI - low ∆K d) ADI - high ∆K

Fig. 5. Magnified views of fatigue cracks, no etched, Nomarski


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The orientation of the ferrite laths in the matrix can influence the crack path too. The crack often propagate along ferrite-austenite interfaces which lie approximately normal to the applied load direction, bud cut through the bainitic ferrite laths which lie parallel to the applied load direction. On the other hand, as several bainitic ferrite laths usually have the same orientation, they form so the cluster of laths. However, the orientation of each single cluster is generally various. Due to described microstructure, deflection of crack was observed, when fatigue crack propagated following the orientation of clusters of bainitic ferrite.

a) main crack with secondary microcrack

b) crack path between graphite nodules

Fig. 7 ADI, etched with 3 % Nital

In general, the crack path in ADI between two graphite nodules is not the path with the shortest distance, but is the link-up of the localized least-energy path (Fig. 7b). And so, due to the combination of described mechanisms of fatigue crack propagation, ADI is tougher and more fatigue resistant than other ductile irons.

4. Conclusions

Fatigue crack paths in pearlitic-ferritic NCI and ADI 800 were observed to understand the influence of the microstructure. The following conclusions can be reached:

• The mechanisms of fatigue crack propagation for both as-cast and austempered ductile iron involved decohesion of graphite nodules, microcracking from nodules, link-up of selected microcracks with main cracks, and crack propagation by connecting the graphite nodules. The fatigue crack propagation path depended strongly on the location of the next graphite nodule ahead of the crack tip, but in general was perpendicular to the loading direction. However, in pearlitic-ferritic NCI fatigue crack connected graphite nodules only at low ∆K. At high ∆K crack propagation was influenced by the heterogeneous matrix microstructure.

• The crack path in ADI between two graphite nodules is not the path with the shortest distance, but is the link-up of the localized least-energy path, usually ferrite-austenite interface. Direction of crack propagation is also influenced by ferrite-laths, eventually clusters of ferrite laths orient-tation, relative to the loading direction. Due to this mechanism ADI is tougher and more fatigue resistant, compared with other ductile irons.

• Graphite nodules in both nodular cast irons remained unbroken eventually just surface of nodules was damaged during the fatigue crack propagation.

Acknowledgements

This work was done as a part of KEGA grant

No.3/6110/08 and MATMEC net-lab project funded

by Emilia Romagna region.

References

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[3] Stokes, B., Gao, N., Reed, P. A. S. Effect of graphite nodules on crack growth behaviour of austempered ductile iron. Materials Science and Engineering A 445 – 446 (2007) 374-385.


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[4] The application of high – strength cast irons (ADI – austempered ductile iron) in high – performance diesel engines – part I. Foundry Trade Journal 248-250, 2006. http://www.aditreatments.com/pdf%5CADI%20in%20large%20diesel%20engines%20part%201.pdf, obtained on 14th October 2007.

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