Brittle Cracking of a Cylindrical Spiral Gear of Axle Gearbox

TECHNICAL ARTICLE—PEER-REVIEWED

Brittle Cracking of a Cylindrical Spiral Gear of Axle Gearbox

Yu-zhou Gao • Xiao-lei Xu • Zhi-wei Yu

Submitted: 10 April 2013 / in revised form: 11 October 2013 / Published online: 26 November 2013

� ASM International 2013

Abstract A cylindrical spiral gear of locomotive axle

gearbox failed after standing in site for 10 days. The failed

gear had been assembled with the spiral gear-shaft. Inspec-

tion in site found that cracking occurred on the spoke plate

between the shaft bore and the oil-pressed hole. The gear

teeth are required to be carburized, but the surfaces of the

shaft bore and the oil-pressed hole of gear are surface-coated

to prevent carburizing. The crack initiated from the surface

of oil-pressed hole and propagated along the radial of the oil-

pressed hole. Metallurgical examination reveals that anti-

carburizing measure was not conducted on the surface of oil-

pressed hole as the specification. A simplified evaluation of

the maximum hoop tensile on the wall of oil-pressed hole

subjected to internal pressure for swelling bore is far insuf-

ficient to lead to brittle fracture. Delayed fracture of gear, the

predominance of intergranular fracture micro-mechanism,

and the presence of several secondary micro-cracks in the

depth of case layer on the oil-pressed hole surface are a clear

indication of hydrogen embrittlement. Hydrogen absorption

occurred during gas carburizing process.

Keywords Gear � Intergranular brittle fracture �Hydrogen embrittlement � Carburizing

Background

Workers at the locomotive assemble workshop reported that

an abnormal sound was heard from the site of axle gearbox.

Site inspections found that a cylindrical spiral gear of

locomotive axle gearbox cracked (Fig. 1). Cracking took

place on the spoke plate between the shaft hole and the oil-

pressed hole of gear. The cracked gear had been assembled

with the spiral gear-shaft and placed in the site for 10 days.

The crack propagated continuously up to tooth when dis-

mounting the cracked gear. The cracked gear is made of

30CrMnTi steel. The gear teeth are required to be carburized

and the depth of the carburized layer is specified as

1.3–1.7 mm. The surface hardness and core hardness of

tooth flank are, respectively, specified as HRC 56–60 and

HRC 30–45. The cracked gear and the gear-shaft were

interference-fitted. The shaft bore and the oil-pressed hole of

gear are required to be surface-coated to prevent carburizing.

Experiments and Methods

The chemical composition of the failed gear material was

determined by spectroscopy chemical analysis. The micro-

structure at various locations was observed by optical

microscope (OPM). Microhardness profiles from the surface

to the interior in different regions were made using Vickers

system with a load of 1000 g to estimate the depth of the

carburized layer. The criterion for determining case depth

described in Chinese standard GB [1] is the depth of material

with hardness greater than HV550. The crack surfaces were

observed visually and using scanning electron microscopy

(SEM).

Observation Results

Visual Observation

The cracked gear is shown in Fig. 2. It can be seen that the

crack crosses the oil-pressed hole up to the tooth part.

Y. Gao � X. Xu (&) � Z. Yu

Department of Material Science and Engineering, Dalian

Maritime University, Dalian 116026, People’s Republic of China

e-mail: [email protected]

123

J Fail. Anal. and Preven. (2014) 14:68–75

DOI 10.1007/s11668-013-9766-2

Visual observation shows the appearance of crack on the

surface of shaft bore (Fig. 3). The crack is throughout the

whole internal surface of the shaft bore. Close-up view

shows that the crack is generally straight but with a small

deflection at the tooth part (Fig. 4). The crack is straight at

the upper portion of oil storage slot, but the zigzag-shaped

crack is present at the lower portion (Figs. 3, 4d). No

abnormal contact marks were found on the surface of shaft

bore.

Fracture was prepared by carefully pulling the crack

surfaces apart to facilitate examination and identifica-

tion of the fracture mechanism. The fracture surface

shows typical chevron marks originating from the ini-

tiating sites (Fig. 5), which shows macroscopically

embrittlement [2]. The fracture surface reveals three

Fig. 2 Cracked gear

Fig. 3 Crack morphology on the surface of shaft hole

Fig. 4 Close-up observation on crack morphology: (a) upper end of

gear, (b) lower end of gear, (c) tooth flank, (d) surface of shaft hole

Fig. 1 Cracked gear and gear-shaft assembled (offered by

manufacturer)

J Fail. Anal. and Preven. (2014) 14:68–75 69

123

distinguishable zones from the crack propagation

marks, which are marked as I, II, and III. From macro

observation, it can be determined that the crack initi-

ated from the surface of the oil-pressed hole and

propagated along the radial of the oil-pressed hole to

form zone I and zone II, then propagating along the

radial of the gear to form zone III. It is noted that the

fracture surface at two sides of the oil-pressed hole

exhibits a silvery gray layer of 1–2 mm (Fig. 5b).

SEM Observation

The fracture surface in the vicinity of the oil-pressed hole and

the shaft bore was emphatically observed by SEM. At two

sides of the thick oil-pressed hole (Fig. 6) and the left side of

thin oil-pressed hole (Fig. 7a, b), particularly in a depth of

around 1 mm, the nature of the fracture is almost totally

intergranular, but cleavage or quasi-cleavage morphology is

revealed at the right side of thin oil-pressed hole (Fig. 7c, d).

The secondary cracks and ductile tearing [3] were found on

intergranular fracture surface (Figs. 6b, d, 7b). The fracture

surface both in the tightened and the un-tightened thread

regions of thick oil-pressed hole reveals similar fracto-

graphic features. It is suggested that the characteristic of

intergranular fracture in the thread region is not related to the

tightening operation. The fracture surface close to the shaft

bore (region F1 in Fig. 5b) and away from the oil-pressed

hole (regions F2, F3, and F4 in Fig. 5b) shows quasi-cleav-

age coexisting a small amount of shallow dimples or

cleavage morphology (Fig. 8).

Microstructure Examination

The cross-sectional metallographic specimen through gear

tooth and the metallographic specimen of the fracture

surface smoothed were prepared. From the macro mor-

phology of the metallographic specimens, it can be seen

that the outline of tooth had been case hardened to a depth

of approximately 1.5 mm (Fig. 9a). The macro-case layer

was revealed on the surface of the oil-pressed hole

(Fig. 9b), suggesting that operating of anti-carburizing was

not conducted on the surface of oil-pressed hole. The case

depth on the surface of thin oil-pressed hole, especially at

the right side, is thinner than that of thick oil-pressed hole.

Absence of case layer on the surface of shaft bore suggests

that the operating of anti-carburizing was conducted on the

surface of shaft bore.

The microstructure of the carburized layer is mainly

composed of the fine needle-like martensite and the

retained austenite without intergranular network-like car-

bides and oxides (Fig. 10a). The microstructure of the

transition region between the case and the core is com-

posed of the martensite and the lower-bainite (Fig. 10b).

The core is composed of the lath-martensite and upper-

bainite (Fig. 10c).

The examination on the chemical composition of the

failed gear materials indicates that the failed gear was

fabricated from 30CrMnTi steel (Table 1).

Hardness Examination

The surface and core hardness values of tooth (average

value of three readings) are, respectively, HRC 60.0 and

HRC 32.8. These hardness values are corresponding to the

specified.

The microhardness profiles in pitch, tip, and groove

regions of tooth, and representative regions of the oil-

pressed hole from the surface to the interior were measured

(Fig. 11). It can be seen that the case depth of tooth part is

Fig. 5 Macro-fracture surface: (a) general view, (b) close-up view on

the vicinity of oil-pressed hole

70 J Fail. Anal. and Preven. (2014) 14:68–75

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Fig. 6 SEM observation on fracture surface in the vicinity of thick oil hole: (a) low magnified view (left side), (b) boxed zone in (a) showing

intergranular fracture, (c) low magnified view (right side), (d) boxed zone in (c) showing intergranular fracture

Fig. 7 SEM observation on fracture surface in the vicinity of thin oil hole: (a) low magnified view (left side), (b) boxed zone in (a) showing

intergranular fracture, (c) low magnified view (right side), (d) boxed zone in (c) showing cleavage morphology


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generally 1.50–1.65 mm (Fig. 11a), which corresponds to

the specified (1.30–1.70 mm). For the thick oil-pressed

hole, the case depth in the thread tip is about 1.6 mm

(Fig. 11b), which corresponds to that of tooth. For the thin

oil-pressed hole, the case depth at left side is thicker (about

0.8 mm), but the case depth at the right side is thinner

(about 0.27 mm) (Fig. 11c). No case depth was evaluated

on the surface of shaft bore. The examination results of

case depth are in accordance with the metallographic

observation.

Failure Causes Analysis

According to the assembly recorder, oil pressure of

swelling bore was 130 MPa when press-fitting gear and

gear-shaft (the specified pressure: B210 MPa). The oil-

pressed hole can be approximately considered to be a thick-

walled cylinder subjected to an internal pressure. The

stresses generated by the internal pressure P can be cal-

culated on the basis of a thick-walled cylinder [4]. The

estimated hoop tensile stress rh and the radial compressive

stress rr on the hole wall can be defined by following

expressions according to the deformation theory of elas-

ticity, using cylindrical polar coordinate system [4, 5]:

rh ¼b2 þ a2

b2 � a2P; rr ¼ �P

Fig. 8 SEM observation on fracture surfaces in the vicinity of shaft

hole and away from the two sides of oil-pressed hole: (a) showing

quasi-cleavage morphology (A3), (b) showing cleavage morphology

(B3)

Fig. 9 Macro structure showing carburized layer: (a) tooth, (b)

fracture surface smoothed


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where P is operational pressure; a and b are inner and outer

radius of cylinder, respectively.

In this case the hoop tensile tends to produce radial

crack. But the radial compressive stress rr can not lead to

the radial crack, whose role need not be regarded. Based on

the geometry of the failed gear, values of a and b for the

thick oil-pressed hole at location E (marked in Fig. 5b) are,

respectively, 8 and 14 mm, which is the weakest location

in the construction when swelling bore by pressing-oil.

Values of a and b for the thin oil-pressed hole at location B

(marked in Fig. 5b) are, respectively, 3 and 4 mm (wall

thickness at location B being assumed as 1 mm). The hoop

tensile stress rh on the walls of thick and thin oil-pressed

holes can be calculated by substituting the values of

respective a and b, and operational oil pressure of 130 MPa

in the expressions above. Hence, the operating hoop stress

are, respectively, rh (thick) = 256 MPa and rh

(thin) = 464 MPa. The yield strength and the ultimate

strength of 30CrMnTi material can be obtained from lit-

erature [6], with reference to 30CrMnTi toughened with a

similar microstructure and approximate core hardness

(around HRC 32). From Ref. [6], yield strength of

800 MPa and ultimate strength of 1000 MPa can be

obtained. It can be clearly informed that the estimated

maximum hoop tensile stress on the wall of the oil-pressed

hole during pressing-oil operation for the swelling bore is

much lower than the ultimate strength. Brittle fracture

occurs when the maximum stress in the material reaches a

critical value. It is suggested that the maximum hoop ten-

sile stress can not lead to instantaneous brittle cracking

even at the weakest location of oil-pressed hole when being

subjected to internal pressure of 130 MPa. Additionally,

there is no obvious difference in microstructure, fracture

morphology and hardness profile between the tightened and

the un-tightened thread portions. It is suggested that the

cracking of thick oil-pressed hole is not related to the

tightening thread. It means that both operation of tighten-

ing the thread of oil-pressed hole and operation of pressing-

oil for swelling bore can not lead to initiating instanta-

neously micro-cracks on the surface of oil-pressed hole. It

is suggested that in order to have brittle fracture a micro-

structural embrittlement is required.

From the observation and examination above, the

chemical composition of the gear material and core hard-

ness are within the specification. The surface hardness andFig. 10 Microstructure observation: (a) carburizing layer, (b) tran-

sition zone, (c) matrix

Table 1 Chemical composition of gear material (wt.%)

C Si Mn P S Cr Ti Cu Fe

Analysis 0.26 0.28 0.87 0.014 0.012 1.08 0.05 0.015 Bal.

30CrMnTiA 0.24–0.32 0.17–0.37 0.80–1.10 B0.035 B0.035 1.00–1.30 0.04–0.10 B0.30 Bal.


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the case depth of tooth correspond to the specified. How-

ever, the anti-carburizing measure was not performed on

the surface of oil-pressed hole. Unfortunately, the crack

just initiated from the surface of oil-pressed hole. It is

deduced that the failure of the gear is closely related to the

presence of carburized layer on the surface of oil-pressed

hole. From observation on the fracture exposed, the inter-

granular fracture morphology only appears in the case-

hardened layer of oil-pressed hole, while, the core reveals

cleavage morphology. And the thicker case depth exhibits

more obvious intergranular topography and higher inter-

granular fracture depth on the fracture surface (such as at

the two sides of thick oil-pressed hole and the left side of

thin oil-pressed hole). At the right side of thin oil-pressed

hole with a thin case-hardened layer, the characteristic of

intergranular fracture was not found. It implies that the

intergranular fracture may be representative of the char-

acteristic of the case-hardened layer other than the gear

material. It can be excluded that the defects of original

material and unsuitable integrated heat treatment (such as

tempered brittle) led to the intergranular brittle fracture.

Appearance of the intergranular fracture has relation to the

carburizing process. Intergranular brittle fracture may

occur if the prior austenite grain boundaries are embrittled.

In general, such an embrittlement is associated with high

phosphorus and carbon concentration in the base steel or

with hydrogen absorption from the carburizing atmosphere

[7, 8]. Another microstructural feature to consider when

dealing with the brittle fracture of carburized parts is the

possible formation of surface intergranular oxidation [9].

However, in present case, no intergranular internal oxida-

tion occurred on the case-hardened layer. In addition, grain

boundary carbides were not found in the carburized layer.

Phosphorous may segregate at the austenite grain bound-

aries during the carburizing process and reduce grain

boundary cohesion [10]. Chemical analysis of the steel,

however, reveals a low phosphorous content (in agreement

with the standard specification) and serious phosphorus

segregation on the intergranular fracture surface was not

probed by EDS. In order to determine if impurity (phos-

phorus) segregation is associated with the grain boundary

fracture noted on the carburized layer, an Auger analysis

should be performed [11]. The delayed brittle fracture

behavior in present case suggests the intergranular

embrittlement fracture should be more closely related to

the hydrogen embrittlement.

The carburizing media mainly contains acetone

(CH3COCH3) as carburizing agent and methanol (CH3OH)

as diluents. In gas carburizing process, the following

reaction would occur:

CH3COCH3 ! COþ 2Cþ 3H2;

CH3OH! COþ 2H2

Therefore, the carburized parts are in the atmosphere of

hydrogen and high temperature. Hydrogen absorption may

occur during gas carburizing. The authors studied the

influence of hydrogen on the critical fracture stress of

steels and they found that the brittle fracture stress is reduced

Fig. 11 Microhardness profile of various regions: (a) tooth, (b) thick

oil-pressed hole, (c) thin oil-pressed hole


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by 200–300 MPa when 1 ppm of hydrogen is absorbed in the

steel [8, 12]. Moreover, in view of the construction of oil-

pressed hole, it is difficult for the hydrogen atoms segregated

to tensile stress zone of subsurface to escape in the process of

heating after carburizing (tempering process). Under the

action of interference-fitting stress, the absorbed hydrogen

atoms would enrich into the grain boundary of the surface of

oil-pressed hole. As the hydrogen content at the boundary

increases, the interfacial strength decreases, leading to

initiation of intergranular cracks. A further support to the

hypothesis of the hydrogen embrittlement is given by the

presence of several secondary cracks on the fracture surface.

Conclusions

(1) The chemical composition and the core hardness of

the failed gear are within the range of the specifi-

cation. The surface hardness and the case depth of

gear tooth correspond to the specification.

(2) Anti-carburizing measure was not performed on the

surface of oil-pressed hole as specification, leading

to the presence of a case layer of 0.3–1.6 mm.

(3) The observed micro-mechanism on the fracture

surface at the carburized layer of oil-pressed hole

was predominantly intergranular.

(4) A simplified evaluation of the maximum hoop

tensile on the wall of oil-pressed hole subjected to

internal pressure for swelling bore is far insufficient

to lead to brittle fracture. The delayed fracture of the

gear and the predominance of intergranular fracture

micro-mechanism at the carburized layer are a most

likely indication of the hydrogen embrittlement.

Hydrogen absorption may occur during gas carbu-

rizing process.

Recommendations

Anti-carburizing measurement on the oil-pressed hole

should be strictly performed to avoid producing brittle case

layer on the surface of oil-pressed hole and leading to

brittle cracking of oil-pressed hole under the action of

interference-fitting stress when press-fitting gear.

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Brittle Cracking of a Cylindrical Spiral Gear of Axle Gearbox

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