Cast Iron Casing Cracking Due to Chunky Graphite Formation

TECHNICAL ARTICLE—PEER-REVIEWED

Cast Iron Casing Cracking Due to Chunky Graphite Formation

Jakub Kaczorowski • Karol Jozwiak •

Marco Innocenti

Submitted: 3 July 2012 / in revised form: 4 March 2013 / Published online: 15 May 2013

� ASM International 2013

Abstract Quality inspection of heavy turbomachinery

casings made of ductile cast iron has revealed anomalies in

the form of characteristic relief pattern on roughly

machined surfaces. Detailed microstructural observations

determined that this anomaly is related to degeneration of

cast iron microstructure where undesired chunky graphite

was formed instead of evenly dispersed graphite nodules.

Mechanical tests showed that this microstructural alteration

led to deterioration of mechanical properties, especially

plasticity. Regularity was found as this problem affected

mainly thick sections of casts, especially where the cooling

rate was limited. This article discusses the consequences of

chunky graphite formation, detection methods, acceptance

criteria, and preventive actions.

Keywords Chunky graphite � Degenerated

nodular cast iron � LCF cracking � Preventive actions

Introduction

Chunky graphite, also called degenerated graphite, is

considered as microstructural anomaly in nodular cast

irons. It is defined as large interconnected graphite cell,

possibly with nodules at intercellular boundaries [1]. Its

appearance is not identified in international standards

related to cast irons such as ASTM A247 [2] or ISO 945

[3]. However, this microstructural anomaly is widely dis-

cussed in technical studies.

The chunky graphite occurrence is the most commonly

associated with low cooling rates. Typically, degenerated

graphite is found inside thick sections of casts ([100 mm)

or in ductile cast iron volumes with restricted cooling rates

(risers).

The material having degenerated graphite is well known

to have lower tensile properties, especially UTS and

elongation, as well as low cycle fatigue (LCF) resistance

[4]. Rapid deterioration of tensile parameters starts with

chunky graphite occupying as little as 10% volume after

which the tensile parameters stay at the same reduced level.

There are a number of studies that report factors con-

tributing to formation of degenerated graphite. Among

them are metal chemistry, magnesium treatment and

inoculation, pouring temperature, and rate of solidification.

The solidification rate proportionally affects the formation

of chunky graphite meaning that higher rates favor for-

mation of sound nodular graphite [5]. Therefore, the most

common practice in foundries dealing with heavy castings

is to accelerate cooling rates with the use of additional

chillers in thick sections. A molten batch chemistry and

inoculation agent has been also reported to influence the

graphite form formation. Additions of Ce and Sb has been

found to promote nodular graphitization [6, 7].

In order to prevent the occurrence of degenerated graph-

ite, typical quality control is focused on metallographic

evaluation of coupons attached to the casting or a separate

casting meant for control purposes. However, this approach

is inadequate to detect degenerated graphite as normally the

cooling rate of the coupons does not reflect the deep seated

alloy. Therefore, a combination of cast design modeling,

J. Kaczorowski (&)

General Electric Company Polska Sp. z o.o., Warsaw, Poland

e-mail: [email protected]

K. Jozwiak

Warsaw Institute of Aviation, Warsaw, Poland

M. Innocenti

General Electric Oil & Gas Nuovo Pignone S.r.l., Florence, Italy

123

J Fail. Anal. and Preven. (2013) 13:445–450

DOI 10.1007/s11668-013-9693-2

coupon testing, finished part visual inspection coupled with

portable metallography should be considered.

An alternative to classic inspection is the application of

nondestructive testing for the microstructure’s verification.

In this respect, combination of ultrasound inspection (UT)

with virtual prototyping has been utilized [8] or magnetic

adaptive testing [9].

Case Study

A turbomachinery casing made of nodular cast iron has

been reported damaged after 40k h of operating time. The

damage was associated with radial cracks passing through

technical holes. Visualization of the casing together with

exemplary crack as seen after dye penetrant application is

presented in Fig. 1. During the reviewing of the foundry

drawings an overlap was observed between the location of

cracking and the location of risers.

The crack-affected fragment of the casing was cut-off

using a band saw. The freshly cut surfaces showed char-

acteristic dark discoloration around the crack (Fig. 2a). In

contrast, surfaces distanced from the crack showed shiny

sound appearance.

The casing fragment containing the crack was subjected

to force-opening by application of compression force on

the back cut area. The appearance of the force opened piece

containing the fracture surface, lab fracture surface and the

back-cut surface is given if Fig. 2b. From this image, it is

evident that the primary crack although being stained with

corrosion products shows characteristic two separate

propagation fronts. Based on the beach-mark geometry of

the propagation fronts it is possible to identify two crack

initiation areas, both located in the entry of the technical

hole. From the same image, one can read out that the

primary fracture surface is located within the dark discol-

ored area (clearly visible on the back-cut surface).

One of the mirror fractures has been subjected to

detailed fractographic analysis using scanning electron

microscope (SEM). The fracture topography observed at

two magnifications is presented in Fig. 3. The fracture

surface was dimpled and filled with two distinct graphite

forms i.e. irregular dispersed form occupying most of the

surface and typical nodular shape rarely occurring. This

type of irregular dispersed graphite is consistent with

chunky graphite.

After fractographic studies, the casing piece containing

the fracture surface was cut transversally to the fracture

surface along a line between the crack propagation path

and the origin. After mounting, grinding, polishing and

etching with Nital the metallographic section was submit-

ted to observations using optical microscope. As shown in

Fig. 4, the crack propagated trans-granularly through the

matrix. The microstructure itself has ferritic matrix with

graphite taking the dispersed shape consistent with chunky

graphite. Only rarely the expected nodular graphite could

be observed. For comparison purpose, an additional

metallographic section was prepared from the volume

having sound appearance on the cut surface. The repre-

sentative microstructures are presented in Fig. 5. The

microstructure in the sound area is typical for properly cast

nodular iron and consists of nodular graphite distributed

regularly in the ferrite matrix.

The cast iron material from the degenerated area and

sound material was sampled to carry out tensile tests and

hardness tests. For the purpose of the tests, two parallel

samples (A and B) were obtained from degenerated and

sound microstructures. The tensile test was carried out at

room temperature and at 450 �C. The results are presented

in Table 1.

Fig. 1 Cast iron casing affected by crack passing through the technical hole. Image to the right taken after application of dye penetrant

446 J Fail. Anal. and Preven. (2013) 13:445–450

123

The tensile test results show that the two microstructures

representing the same piece of casting behave differently

when submitted to tensile stress at a given temperature.

The difference is preserved for both test temperatures. At a

given temperature, the yield strength is virtually the same

for both microstructures. The difference lies in the ultimate

tensile strength where the degenerated material shows 60

units lower strength (drop by 15%) at room temperature.

Brinell hardness followed the tensile strength, i.e., was

significantly lower for the degenerated material. The

Fig. 2 (a) Fresh cut surface showing dark discoloration in the volume surrounding the crack, (b) fracture surface appearance after back-cut and

forced crack opening

Fig. 3 Primary fracture surface morphology at two magnifications as

seen in SEM

Fig. 4 Microstructure of the casing near the primary fracture surface,

two magnifications, etched with Nital

J Fail. Anal. and Preven. (2013) 13:445–450 447

123

largest difference between the sound and altered micro-

structures was, however, seen in terms of plastic properties

of the material. Elongation and reduction of area drop

significantly to the level of 3% for the degenerated mate-

rial. Such low plasticity makes this microstructure highly

unwanted as it would fail catastrophically without any

warning.

After the tensile test, the ruptured specimens were

submitted to fractographic analysis using SEM. Figure 6

shows the morphologies of ruptured specimens represent-

ing chunky graphite and sound material. The specimen

representing the degenerated material ruptured leaving

similar fractographic features as the primary service frac-

ture observed in the casing.

Owing to the nature of turbomachinery operation, its

components, especially thick sections such as casings are

often exposed to temperature gradients and, therefore,

thermal stresses. Repeatability of thermal cycles may lead

to thermal fatigue which is considered as LCF. A parallel

test campaign has been carried out to characterize the LCF

behaviors of sound and degenerated cast irons. The results

are presented Fig. 7 for two temperatures (room and

350 �C). From the LCF graph, similar conclusions can be

drawn as those from the tensile data. Material with

degenerated graphite shows drop in LCF life at both the

testing temperatures.

Failure Analysis Conclusions

The failure investigation coupled with service history of

the component and stress analysis of the crack-affected

area led to the conclusion that the failure is related to LCF

generated by thermal stresses of the component. Even

though the stresses were within the acceptable range, the

material was not capable of operating at the designed

conditions because of local drop in material properties

related to formation of chunky graphite.

Preventive Actions

Observed ductile iron deterioration interferes with design

criteria used during the part design phase. Typically, two

approaches are seen to overcome this issue. It is either by

avoiding the occurrence of degenerated graphite by thor-

ough casting simulations supported by scrupulous quality

inspection or by accepting degenerated graphite in those

Table 1 Tensile test results at

two temperatures for samples

obtained from degenerated and

sound microstructures

Specimen

Temperature,

�C

Stress at offset yield

(0.2%), MPa

Peak stress,

MPa

Elongation,

%

Reduction of

area, %

Hardness,

HB

Sound A Room 261 397 21 19 151

Sound B Room 262 400 22 23

Chunky A Room 261 349 3.9 3.3 135

Chunky B Room 272 333 2.8 3.1

Sound A 450 194 270 33 28 n.a.

Sound B 450 196 261 32 26 n.a.

Chunky A 450 199 245 2.8 3.4 n.a.

Chunky B 450 200 246 1.4 0.5 n.a.

Fig. 5 Microstructure of the casing near the primary fracture surface,

two magnifications, etched with Nital


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volumes of castings that are considered as nonstressed in

the final component. The first approach is dedicated to

foundry engineering (design of casting operation) and the

quality acceptance criteria set by the customer. The second

solution is dedicated to the part-design team collaborating

with casting-design team to agree on such combinations of

casting operation so that occurrence of degenerated

graphite is minimized in high stress areas.

The above preventive actions are considered as

long -term solution where enough time is available for

proper design and collaboration. If a problem with micro-

structure occurs in the current production, then an

immediate action is required. Typically, a characteristic

darkening is discovered during rough machining. In this

case, the part is inspected with the use of portable micro-

scope and if chunky graphite is confirmed, then a map of

affected volume is drawn and shared with the design team

who will compare the expected stresses with the properties

of the degenerated material. If these allowance criteria are

met, then the parts are allowed for use; otherwise, they are

scrapped.

The following could serve as baseline for corrective

actions when occurrence of degenerated graphite is highly

possible:

• Implementation of onsite inspection of critical areas

with the use of naked eye. This could be based on

characteristic darkening visible on freshly machined

surfaces. Once darkening is observed, then metallo-

graphic inspection using portable microscope could be

used for final acceptance. Acceptance criteria must be

agreed in respect of the materials and implemented by

design teams, but typically up to 10% of volume

occupied by degenerated graphite is accepted

• Review of mechanical test sampling locations to insure

that they are taken from critical areas and/or from

representative coupons/appendixes. Coupons due to

small size (75 mm) may not be representative. Trepans

from thick sections are considered in place of designed

holes.

• Review of casting process to provide adequate cooling

rate in critical areas by 3D modeling with various

scenarios. This action is considered as long-term

program.

• Review of casting process to allow for the formation of

proper microstructure (inoculation, additives, etc.).

This action is considered as a long-term program

References

1. Davis, J.: Cast Irons, p. 297. ASM International, Materials Park

(1996)

2. Standard Test Method for Evaluating the Microstructure of

Graphite in Iron Castings (2010)

3. Microstructure of cast irons. Part 1: Graphite classification by

visual analysis, BS (2008)

Fig. 6 SEM micrographs of ruptured tensile specimens representing

(a) chunky graphite and (b) nodular graphite

Fig. 7 Low cycle fatigue behavior of segregated and sound nodular

cast iron

J Fail. Anal. and Preven. (2013) 13:445–450 449

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4. Farrell, T.: The influence of ASTM type V graphite form on

ductile iron low cycle fatigue. AFS Trans. 91, 61–64 (1983)

5. Ignaszak, Z.: Study on data base of modelling concerning casting

phenomena in cast-iron-mould simulation system. Key Eng.

Mater. 457, 305–311 (2011)

6. Tsumura, O.: Effects of rare earth elements and antimony on

morphology of spheroidal graphite in heavy-walled ductile cast

iron. Jpn. Cast. 67, 540 (1995)

7. Larranaga, P.: Effect of Sb and Ce on the formation of chunky

graphite during solidification of heavy-section castings of near-

eutectic spheroidal graphite irons. Metall. Mater. Trans. 3, 654 (2009)

8. Ignaszak, Z.: Specific structures in heavy ductile iron castings and

its identification. Inzynieria Materialowa 140, 716 (2006)

9. Vertesy, G.: Nondestructive characterization of ductile cast iron by

magnetic adaptive testing. J. Magn. Magn. Mater. 322, 3117

(2010)


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Cast Iron Casing Cracking Due to Chunky Graphite Formation