Canale L.C.F., Mesquita R.A., Totten G.E. Failure Analysis of Heat Treated Steel Components

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with shades of black and brown, as can be pro-

duced by heating the steel. The color brown

corresponds to the temper color, which is pro-

duced by heating steel to approximately 238



(460



F) in air and cooling to room temperature.

Chromium plating was observed on the outer

diameter surface of the gear. There were no

shear lips evident around the periphery of the

fracture surface. Fracture ridges emanated from

origins located on the inner diameter edge, and

the fracture surface exhibited a faceted appear-

ance indicative of a brittle intergranular failure.

Fig. 48 Typical lack of fusion area

Fig. 49 Direction of fracture propagation

Fig. 50

SEM fractographs documenting the topographic

features of the failed weldment. (a) Location A

showing overload features. (b) Location B showing overload

features. (c) Location C showing weld defect features. Original

magniﬁcation: 1200·

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An SEM was used to examine the fracture

surface and to document the mode of failure.

Figure 54 shows SEM fractographs made at a

typical fracture origin. The fracture topography

was intergranular, which is indicative of a brittle

mode of failure (Ref 3). No defects were ob-

served that could be associated with the cause

of the fracture.

A metallographic specimen was removed

at the location shown in Fig. 52. The specimen

was prepared using standard metallographic

techniques. Figure 55 shows the microstructure

(tempered martensite) and typical cracks that

were present. The cracks were intergranular,

and an oxide was present in the cracks. The

Fig. 51

Energy-dispersive spectrographs of failed weld and

base material. (a) Location A weld metal. (b) Loca-

tion B weld metal. (c) Location C weld metal

Fig. 52

As-received internal spur gear showing location of

cracks

Fig. 53

Macrographs of (a) magnetic particle indications of

cracks and (b) opened crack revealing the fracture

surface

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morphology of the cracks is typical of grinding

cracks (Ref 8).

A hydrogen analysis conducted on the inter-

nal gear yielded a value of 1.85 ppm, which is

not considered to be high enough to cause

hydrogen embrittlement.

Hardness measurements were made on the

internal gear to verify the heat treat condition.

The hardness of the gear near the observed

cracking was 44 HRC. This hardness value was

below the drawing hardness requirement of 50

to 53 HRC. The bulk hardness of the gear was

51 HRC.

The lower-than-expected hardness at the

crack initiation sites indicates that there was

some event that caused excessive local heating

of the microstructure. Because of the nature of

the part and typical manufacturing sequence, it

is likely that the grinding cracks occurred as a

result of aggressive grinding. Based on the

results of this investigation, it is concluded that

the internal spur gear failed due to aggressive

grinding.

Main Landing Gear Axle

During taxi, the MLG axle separated. The

axle had been installed for only 90 days, with a

total ﬂight time of 62 h. The axle was fabricated

from 300M high-strength, low-alloy steel.

Shown in Fig. 56 is the as-received failed

MLG axle. Also shown in Fig. 56 is the location

of the fracture, which occurred adjacent to a

chromium- and cadmium-plated area. Also

shown in Fig. 56 is a worn (discolored) area of

chromium plating.

The fracture surface was examined visually

and at up to 50 · magniﬁcation using a stereo-

microscope. Figure 57 is a macrograph of the

fracture surface. Shown in Fig. 58 is an SEM

photograph of the wear that had removed the

protective plating adjacent to the fracture sur-

face. Multiple origins were located around the

periphery of the axle. Faceted areas character-

istic of an intergranular, delayed mode of failure

had progressed through approximately 80% of

the thickness. Fatigue propagated from the ends

of the intergranular areas, with several areas of

the wall being totally penetrated by the fatigue.

The relatively large intergranular areas and the

irregular shape of the intergranular area are

indicative of stress corrosion.

An SEM was used to examine the fracture

surface and to document the mode of failure.

Shown in Fig. 59 are SEM fractographs doc-

umenting topographic features, that is, inter-

granular topography and fatigue striations, on

the fracture surface. From the ends of the inter-

granular area, fatigue propagated through the

wall thickness. Also documented in Fig. 59 are

the lack of plating at a fracture origin and cor-

rosion products, which were present at the

intergranular fracture origins. The lack of plat-

ing and the presence of corrosion products are

indicative of a stress-corrosion failure. Also

shown in Fig. 59 is an area where the chromium

was intact.

The hardness of the axle was determined to be

53.9 HRC. This met the hardness requirement of

53 to 55 HRC.

A chemical analysis by atomic absorption

veriﬁed that the MLG axle was fabricated from

300M steel.

A metallographic specimen was prepared

through a typical fracture origin on the axle. The

specimen was prepared using standard metallo-

graphic techniques. As determined from the

metallographic specimen, the thickness of the

chromium plating was approximately 0.06 mm

Fig. 54

SEM fractographs showing the topographic features

of a typical fracture origin. (a) Fracture surface. Ori-

ginal magniﬁcation: 20·. (b) Location A showing intergranular

fracture. Original magniﬁcation: 3000·

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(0.0024 in.). Shown in Fig. 60 is the micro-

structure of the axle at a typical fracture origin.

The chromium plating ended approximately

1.6 mm (0.062 in.) from the origin. The hard-

ness of the light, discolored area at the

chromium runout was 58.4 HRC, which exceeds

the hardness requirement of 53 to 55 HRC. Past

the chromium runout, the part was supposed to

be cadmium plated. However, past the chro-

mium runout to the origin, no plating was pre-

sent. The wear documented in Fig. 60 had

removed the protective cadmium plating from

this location. This allowed corrosion to occur.

Shown in Fig. 61 is a secondary branch crack

characteristic of stress corrosion. This micro-

graph also shows the chromium plating that had

been penetrated. Documented in Fig. 62 is the

appearance of the crack (unetched) after further

polishing. Note the worn (missing) chromium.

A hydrogen analysis conducted on the axle

yielded hydrogen content of 4.2 ppm, which is

relatively high and considered high enough

to cause hydrogen embrittlement (Ref 4). This

hydrogen is in the form of atomic hydrogen and

can be generated in corrosion reactions and then

absorbed by steel.

Fig. 56 As-received failed axle

Fig. 55

Micrographs showing the appearance of the cracks and microstructure. (a) Surface cracking evident. Original magniﬁcation:

50·. (b) Surface cracking and MnS inclusions. Original magniﬁcation: 100·. (c) Extended surface crack. Original magni-

ﬁcation: 100 · . (d) Intergranular crack along prior-austenite grain boundaries. Original magniﬁcation: 200·

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Based on the results observed, it was deter-

mined that the MLG axle failed due to stress-

corrosion cracking. The protective cadmium

plating had been worn away, allowing corrosion

to occur at locations where the cadmium was no

longer present.

Nondestructive Testing and Failure

Analysis of Fin Attach Bolts after

Full-Scale Fatigue Testing

During the inspection, after the ﬁrst lifetime

of fatigue testing, one aft ﬁn attach bolt was

found to have broken. Five additional bolts were

found to have crack indications. The location of

the bolts and the nomenclature used for identi-

ﬁcation of these bolts are shown in Fig. 63. The

failed bolt (bolt A) and the additional ﬁve bolts,

identiﬁed as B, C, G, H, and I, were submitted to

the laboratory to determine if ultrasonic testing

would be a suitable ﬁeld inspection technique

for this application and to examine the failed bolt

and cracked bolts to determine the failure

modes.

The speciﬁcation requirements for these bolts

are for a cadmium-plated, forged hex head

metric bolt that has a close tolerance shank.

The material can be any of four material

speciﬁcations: BS S147, S148, S149, or S158.

The compositions of these steels are shown

in Table 2. These steels are heat treated to

a minimum tensile strength of 1100 MPa

(160 ksi) minimum. The threads and the head-

to-shank ﬁllet radius are rolled after heat treat-

ment. Machining of the ﬁllet radius is not

permitted. The speciﬁed dimensions are shown

in Fig. 64 and Table 3.

The failed bolt and the additional bolts were

examined using nondestructive testing, visual

examination, scanning electron microscopy,

metallography, and analytical chemistry. The

hardness of the parts was also measured.

Nondestructive testing was performed on

the submitted bolts to determine if ultrasonic

inspection is a suitable ﬁeld inspection method.

Magnetic particle inspection was used as a

conﬁrmation of the ultrasonic inspection

method.

Ultrasonic reference standards were fabri-

cated from AN-8 and AN-6 pan stock bolts.

These bolts were machined so that the head

thickness and lengths were similar to the test

articles. Notches were cut into the bolts at the

head radius and at the midpoint length of the grip

Fig. 58

Overall view at origin. (a) Worn cadmium plating at

origin. (b) SEM photograph of fracture surface

Fig. 57

Macrograph of fracture surface of the failed axle.

Typical fracture origins are shown at arrows.

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on the shank. The dimensions of the reference

standards and notches are noted in Fig. 65 and

Table 4. While every attempt was made to make

the manufactured notches at the head-to-shank

ﬁllet radii and the shank identical, it was not

always possible because of manufacturing

Fig. 59

SEM fractographs documenting the fracture features found at the origin. (a) Fatigue striations emanating from the origin

(200 mm). (b) Intergranular fracture at origin (location A, 50 mm). (c) Intergranular fracture and corrosion products found at

the origin (13 mm). (d) Intergranular fracture found at interface between chromium plating and steel (50 mm)

Fig. 60

Lack of plating at fracture origin. (a) Overall view of microstructure through fracture origin (500 mm). (b) Lack of plating at

fracture origin (50 mm)

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tolerances. The ultrasonic inspection was con-

ducted with an ultrasonic transducer (20 MHz;

3.18 mm, or 0.125 in., in diameter). The delay

line was removed for this application. The

instrument settings are noted in Table 5. The

sensitivity of the instrument was adjusted so that

an 80% signal response was obtained from the

notch under the bolt head of the reference stan-

dard (4.17 mm, or 0.164 in., long; 0.686 mm,

or 0.027 in., deep) and a notch on the shank of

the machined reference standard (4.57 mm, or

0.180 in., long; 4.57 mm, or 0.18 in., deep). Sig-

nal responses for the reference standards are

shown in Fig. 66. Each of the submitted bolts

was inspected with an additional 6 dB of gain to

increase the sensitivity of the inspection. Signal

responses for each of the bolts are shown in

Fig. 67. The ultrasonic inspection showed that a

crack indication was present at the shank-to-

head ﬁllet radius of bolt C. No crack indications

were found in the other bolts.

The submitted bolts were inspected using

standard magnetic particle inspection techni-

ques. The results indicate that bolts C, G, and I

had circumferential crack indications at the

shank-to-head ﬁllet radius. Bolt I had a cir-

cumferential crack indication around the shank

of the bolt. The magnetic particle indications are

shown in Fig. 68.

After nondestructive testing, bolts C, G, and B

were inspected visually. The as-received bolts

are shown in Fig. 69. After inspection, the cracks

Fig. 61

Secondary branch cracking observed. (a) Overall

etched microstructure. (b) Location E, showing

cracking on prior-austentite grain boundaries. (c) Location F,

showing voids in chromium plating and associated cracking

Fig. 62

Unetched micrograph showing branch cracking and void in chromium plating. (a) Overall view of secondary crack

(500 mm). (b) Location G showing void in chromium plating and associated cracking (50 mm). (c) Location H showing

cracking along prior-austenite grain boundaries (50 mm)

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indicated by magnetic particle inspection were

exposed. This was accomplished by centering

the threaded portion of the bolts in the chuck of a

lathe and drilling a small pilot hole. A larger drill

bit was then used to drill the center of the bolt,

leaving a wall thickness of approximately

0.76 mm (0.030 in.). Using a hacksaw, a small

saw cut was made opposite to the crack. The

head of the bolt was held in a vise, and the shank

of the drill bit was inserted into the hole of the

bolt to support the bolt shank. Using the drill bit,

the bolt was bent away from the crack indi-

cation, keeping the crack faces in tension. The

exposed crack was then analyzed.

Bolt B showed no crack indications, using

ultrasonic inspection or magnetic particle

inspection. This bolt showed signiﬁcant cir-

cumferential scoring around the periphery of the

Fig. 63 Schematic of bolt location and nomenclature used

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shank, at approximately the midpoint of the

shank. Minor deformation at the corners of the

bolt head was detected. Because no crack in-

dication was seen by nondestructive testing

(NDT), no additional work was performed on

this bolt.

Bolt C showed a highly polished shank sur-

face, with a circumferential scoring line in the

same approximate location as bolt B. Some

rounding of the head corners was evident. No

identifying mark or part numbers were stamped

on the head of the bolt. After exposing the crack,

Table 2 Allowable chemistries of the submitted bolts

Element

Allowable composition range of British Speciﬁcations, wt%

BS S147 BS S148 BS S149 BS S158

Max Min Max Min Max Min Max Min

C 0.38 0.43 0.36 0.41 0.38 0.43 0.22 0.29

Si 0.20 0.35 0.15 0.35 0.20 0.35 0.15 0.35

Mn 0.75 1.00 0.60 0.90 0.65 0.85 0.50 0.80

P ... 0.025 ... 0.025 ... 0.025 ... 0.020

S ... 0.020 ... 0.025 ... 0.020 ... 0.015

Cr 0.40 0.60 0.50 0.80 0.70 0.90 0.90 1.20

Mo 0.20 0.30 ... ... 0.20 0.30 0.15 0.25

Ni 0.60 0.70 1.10 1.50 1.65 2.0 ... 0.30

Fig. 64 Schematic of bolt dimensions

Table 3 Allowable bolt dimensions

Size code

Thread

A tolerance B min

JDiameter Pitch Max Min Max Min Min +0.00.3 +0.00.2

03 M3 0.5 5.5 4.88 0.4 0.2 2.990 2.965 6.08 6.0 2.0 0.4 ...

04 M4 0.7 7.0 6.38 0.5 0.2 3.990 3.965 7.74 7.6 2.5 0.4 1.0

05 M5 0.8 8.0 7.38 0.5 0.2 4.990 4.965 8.87 8.5 3.0 0.5 1.2

06 M6 1.0 10.0 9.28 0.5 0.2 5.990 5.965 10.95 9.5 3.5 0.7 1.6

08 M8 1.25 13.0 12.28 0.5 0.2 7.987 7.962 14.26 12.4 4.5 0.7 2.0

10 M10 1.5 17.0 16.08 0.6 0.3 9.987 9.962 18.90 14.8 5.0 0.8 2.0

12 M12 1.5 19.0 18.17 0.6 0.3 11.984 11.959 21.10 15.5 6.0 0.9 2.0

14 M14 1.5 22.0 21.17 0.6 0.3 13.984 13.959 24.49 17.5 7.0 1.0 2.0

16 M16 1.5 24.0 23.17 0.6 0.3 15.984 15.959 26.75 19.5 8.0 1.1 2.0

Note: All dimensions in millimeters

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the resulting fracture surface was examined

(Fig. 70). The fracture surface of crack C had the

characteristic ratchet marks or shear ridges that

indicate multiple initiation sites along the outer

edge of the bolt. The shape and appearance of

the fracture surface was indicative of fatigue.

Bolt G showed circumferential scoring

(Fig. 69), and some rounding of the head corners

was evident. After the crack was exposed,

the resulting fracture surface was examined

Fig. 65

Sketch of reference standards used during ultrasonic

inspection

Table 4 Dimensions of reference standards

used for ultrasonic testing

Specimen

identiﬁcation

mm in. mm in.

HL1 12.4 0.487 5.66 0.223

HL2 10.1 0.399 2.39 0.094

HL3 7.70 0.303 1.37 0.054

HL4 5.99 0.236 0.81 0.032

HS1 9.47 0.373 3.96 0.156

HS2 7.82 0.308 1.98 0.078

HS3 6.50 0.256 1.17 0.046

HS4 4.16 0.164 0.69 0.027

HS5 2.46 0.097 0.15 0.006

SL1 12.3 0.485 5.87 0.231

SL2 11.2 0.440 3.40 0.134

SL3 8.13 0.320 1.52 0.060

SL4 5.82 0.229 0.81 0.032

SS1 8.97 0.353 3.18 0.125

SS2 7.65 0.301 2.24 0.088

SS3 5.16 0.203 0.76 0.030

SS4 4.57 0.180 0.46 0.018

SS5 2.82 0.111 0.18 0.007

Table 5 Ultrasonic inspection techniques

equipment settings

Tune 0

Reject Off

Gain 53 dB

Frequency 25 MHz

Video ﬁlter FW

Damping Minimum

Delay 0

Range 8.3

Velocity 1

Repetition rate 3

Fig. 66 Ultrasonic signal responses from the manufactured reference standards

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