Journal of Failure Analysis and Prevention Volume 12 Issue 4 2012 [Doi 10.1007%2Fs11668-012-9584-y]...

8/12/2019 Journal of Failure Analysis and Prevention Volume 12 Issue 4 2012 [Doi 10.1007%2Fs11668-012-9584-y] Zhi-Wei Y

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T E C H N I C A L A R T I C L E P E E R - R E V I E W E D

Analysis of a Cracked Diesel Engine Camshaft

Zhi-wei Yu

Xiao-lei Xu

Submitted: 7 December 2011 / in revised form: 8 May 2012 / Published online: 6 June 2012

ASM International 2012

Abstract A truck diesel engine camshaft was found

cracked following a straightening operation after thecamshaft was carburized. The camshaft is made of

16MnCrS5 steel and was required to be surface-carburized.

The cracking occurred just at the transition fillet root of a

cam at a near middle position on the camshaft. This loca-

tion bore the maximum tensile stress in the straightening

process, a process involving three-point bending. The crack

surfaces once exposed exhibit cleavage morphology,

indicative of brittle fracture as the failure mechanism.

Microstructural observation revealed an intergranular

network of carbides and intergranular microcracks present

in the carburized layer and a banded structure consisting of

ferrite and pearlite in the core. These metallurgical defects

decreased the deformation capacity of the carburized

camshaft, in fact creating a metallurgical stress concen-

tration that promoted the cracking of camshaft.

Keywords Camshaft Stress concentration

Instantaneous brittle fracture

Intergranular network carbides Banded structure

Introduction

A truck diesel camshaft was found cracked following

straightening operation. The cracked camshaft is made

from 16MnCrS5 steel (C: 0.140.19, Si: B0.40, Mn: 1.00

1.30, P B 0.035, S: 0.0200.035, Cr: 0.801.10, Fe: bal-

ance; Table1). The main steps involved in the fabrication

of the camshaft are: (i) normalizing the blank at 950 C for

4 h, (ii) coarse-machining, (iii) carburizing at 945 C for8 h, followed by air cooling, and (iv) straightening. The

carburized layer is specified to a minimum depth of

2.0 mm. The secondary carbides are not allowed to be

present in the case layer. The grade of banded structure of

the blank is specified to be below the first level, as defined

by a Chinese standard [1].

This paper describes the metallurgical investigation of

the cracked camshaft, and addresses cause.

Investigation Methods

The chemical composition of the camshaft material was

determined by spectroscopy chemical analysis. The

microstructure at various locations was observed by scan-

ning electron microscopy (SEM) and optical microscopy

(OPM). The fractured surfaces were analyzed using SEM.

Observation Results and Discussion

Visual Examinations

The cracked camshaft is shown in Fig. 1. Visual inspection

indicates that the crack is situated at a fillet root of a cam in

the middle of the camshaft (marked by arrow in Fig. 1). A

close-up view revealed five discrete short cracks extended

circumferentially along the fillet root (Fig. 2).

Fracture surfaces were exposed by pulling the crack

apart, as shown in Fig. 3. Two surfaces were observed: a

light yellow area representing the surface of the preexisting

crack and a sliver area representing the freshly opened

fracture surface. Original crack surface is radial and

Z. Yu X. Xu (&)

Electromechanics and Material Engineering College, Dalian

Maritime University, Dalian 116026, Peoples Republic of China

e-mail: [email protected]; [email protected]

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extends to a depth of 2.0 mm (marked in Fig. 3). A small

faceted features are present on the entire surface, a con-

dition which is associated with brittle fracture [2]. SEM

observations indicate that the fracture surface is uniform

(Fig.4a) and radiating crack propagation traces were seen

(Fig.4b). This feature confirms that the cracks propagated

from the fillet surface toward the core. Cleavage mor-

phology is revealed on both the original crack surface

(Fig.4c) and the freshly opened fracture surface (Fig. 4d),

once again an indication of a brittle crack propagation andfracture mechanism. The fractographic features suggest

that the cracking of the camshaft was by a mechanism of

instantaneous brittle fracture.

In the vicinity of the main cracks, multiple dispersive

microcracks paralleling the main crack were found on the

fillet surface (Fig. 5a). The edges of the microcracks mat-

ched each other (Fig. 5b). The indication is that the cracks

at the fillet were produced during the straightening

operation.

Microstructure Examination

In order to verify whether any metallurgical anomalies

promoted the cracking of the camshaft, the microstructure

in the vicinity of the cracks was examined using OPM and

SEM. A representative OPM photograph is shown in

Fig.6. It can be seen that an equilibrium carburization

structure is formed. The outermost surface is a hypereu-

tectoid structure, consisting of fine pearlite and secondary

carbides, and sub-surface is eutectoid structure, consisting

of fine pearlite. The transition region is a hypoeutectoid

structure, consisting of ferrite and pearlite. A banded

structure consisting of ferrite and pearlite was observed in

core. The depth of carburized layer is 2.0 mm, the mini-

mum intended depth. The banded structure is rated as

3 heavy as defined by Chinese standard [1]. This rating

exceeds to the specification (B1 grade). SEM observation

revealed the presence of a continuous network secondary

carbides formation along the prior austenite grain bound-

aries to a depth of around 0.1 mm (Fig. 7). However, the

specification for this product is that the secondary carbides

were not to be present in the case layer at all. Microcracks

were observed to be to a depth of about 0.03 mm. It is

evident that the microcracks initiated and propagated along

the intergranular carbides (Fig.7). That is, the carbides

Table 1 Chemical composition of the camshaft material (wt.%)

Elements C Si Mn P S Cr Fe

Analyzed 0.17 0.27 1.12 0.010 0.017 0.88 Bal.

16MnCr5S

steel

0.14

0.19

B0.40 1.00

1.30

B0.035 0.020

0.035

0.80

1.10

Bal.

Fig. 1 Failed camshaft

Fig. 2 Close-up view showing crack morphology on the camshaft

surface

Fig. 3 Mating crack surfaces, as exposed: (a) right side of Fig. 2 and

(b) left side of Fig. 2

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Fig. 4 SEM examination of the crack surface: (a) general view, (b) radiating propagation traces, (c) cleavage morphology on the original crack

surface, and (d) cleavage morphology on the freshly opened fracture surface

Fig. 5 Microcracks on the fillet surface: (a) general view and (b) morphology of the edges of a microcrack

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surrounding the austenite grains promote the formation of

microcracks [3].

Hardness value of core was determined to be HB 152

(average value of five readings), in accordance with the

specification HB 140187.

Analysis of the Cause of the Cracking

Based on the observation and examinations above, it is

determined that the chemical composition of the camshaft

material and core hardness is within the specification.

However, the intergranular network secondary carbides

that appear in the case carburized layer and the banded

structure in the core are not what were specified.

The transition fillet of the cam acts a mechanical stress

concentrator [3], inherent in design and construction of thecamshaft. This location bears the maximum axial tensile

stress during the straightening process. Occurrence of a

banded structure in the core suggests that the intermittence

of flow lines can occur at fillet radius where a stress con-

centration condition already exists. The presence of

intergranular carbides in the case layer also further exac-

erbates the condition of stress concentration by providing a

convenient location for crack initiation and an easy

Fig. 6 Microstructure at the carburized surface of the camshaft;

OPM

Fig. 7 SEM examination of the microstructure of carburized layer

showing intergranular microcracks and intergranular network car-

bides: (a) low magnification and (b) high magnification

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pathway for propagation [4, 5]. In other words, the pres-

ence of an intergranular network of carbides in the case-

hardened layer worsens the deformation capacity of the

case. A mechanical stress concentration and the combina-

tion with an adverse metallurgical condition contributed to

the cracking of camshaft. It was incorrect heat treatment

procedures involving normalizing and carburizing pro-

cesses that led to the adverse microstructure that wasresponsible for the cracking of the camshaft when it was

straightened.

Conclusions

1. Circumferential cracking occurred on a diesel engine

camshaft in the form of instantaneous crack initiation

and propagation in a brittle mode. The cracks originated

at the fillet of a cam at the middle of camshaft. The

cracked region itself is a location of stress concentra-

tion and would have experienced tensile stress resultingfrom where the camshaft was being straightened.

2. A banded structure consisting of ferrite and pearlite is

presented in core and intergranular network carbides

are present in the carburized layer. These metallurgical

defects decreased the deformation capacity serving as a

metallurgical notch at a location of mechanical stress

concentration in the camshaft. This combination of

condition led to the camshaft having cracked.

Recommendations

Suitable normalizing process should be performed on the

blank before carburizing to eliminate the banded structureof the camshaft material. The carbon content of the case

must be controlled to a level at which carbide will not be

precipitated at grain boundaries. Select an appropriate

carburizing process to decrease the extent to which the

carburized camshaft would need to be straightened.

References

1. Steel-Determination of Microstructure. China Standard, GB13299-

91; 1991 (in Chinese)

2. Banuta, M., Tarquini, I., Gauvin, B.: Brittle fracture of a lifting stud

during assembly operations. J. Fail. Anal. Prev. 9, 208212 (2009)3. Diego, G., Serrano, M., Lancha, A.M.: Failure analysis of a

multiplier from a Kaplan Turbine. Eng. Fail. Anal. 7, 2734 (2000)

4. Asi, O.: Failure analysis of a crankshaft made from ductile cast

iron. Eng. Fail. Anal. 13, 12601267 (2006)

5. Krishnadev, M.R., Jain, S.C.: Improved productivity through

failure analysis: case studies in precision forging of aerospace

components. Eng. Fail. Anal. 14, 10531064 (2007)

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Journal of Failure Analysis and Prevention Volume 12 Issue 4 2012 [Doi 10.1007%2Fs11668-012-9584-y]...

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