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

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requirement of 46 to 48 HRC for 4330V-

modiﬁed steel heat treated to the 1515 to

1655 MPa (220 to 240 ksi) tensile strength

range. The hardness of the nut was 45 HRC,

which was above the hardness requirement of

40 to 43 HRC. The high hardness value did

not appear to be detrimental.

A hydrogen analysis conducted on the hinge

bolt yielded a value of 1.5 ppm, which is con-

sidered low to cause hydrogen embrittlement.

The diameter of the bolt at the fracture was

measured to be 10.6 mm (0.419 in.), meeting

the drawing requirement of 10.7+0.25 mm

(0.422+0.010 in.). The radius at the transition

where the failure occurred was 1.5 mm

(0.060 in.), meeting the drawing requirement of

1.6+0.25 mm (0.063+0.010 in.).

A spectrographic analysis veriﬁed that the

bolt was 4330V-modiﬁed steel.

A tensile test was conducted on a production

bolt. The bolt failed in the threaded area. The

failing load was determined to be 17,600 kg

(38,820 lb). No requirement was available.

A metallographic section was removed

through the origin location shown in Fig. 29.

The specimen was prepared using standard

metallographic techniques. Figure 31 shows

micrographs taken of the fracture origin. The

tempered martensitic microstructure was nor-

mal. No untempered martensite was present.

Corrosion products were observed on the frac-

ture surface at the origin (Fig. 32). However,

there was no evidence of pitting or other corro-

sion processes that would have produced the

corrosion products. No aluminum IVD was

present on the surface at the origin, because the

IVD had been removed prior to measuring

residual stresses.

Based on the results of this investigation, it is

concluded that the inboard ﬂap hinge bolt failed

in a delayed mode of failure, which was most

likely stress-corrosion cracking. No material

anomalies were observed that would account for

the failure of the inboard ﬂap hinge bolt.

Fig. 26 Typical fracture features. (a) Overall view (500 mm). (b) Intergranular fracture apparent at fracture origin (7 mm)

Fig. 27

Metallographic examination of primary crack. Frac-

ture path followed prior-austenite grain boundaries.

No precipitates in grain boundaries (50 mm) were observed.

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Failure Analysis of a Nose Landing

Gear Piston Axle

This investigation determined the mode and

initiating cause of failure of a nose landing gear

(NLG) piston axle. Failure of the piston axle

occurred on the aircraft during taxiing after the

aircraft had accumulated approximately 60 h in

service. When failure occurred, the nose wheel

completely separated from the piston strut. The

NLG piston assembly was fabricated from 300M

steel and heat treated to a 1930 to 2070 MPa (280

to 300 ksi) ultimate strength range. The axle

was then shot peened and low-embrittlement

cadmium plated. White paint was applied to the

inside and outside surfaces of the axle, with

exception of the two land surfaces for the wheel

bearing cans. The failed NLG piston assembly

was removed from the wheel and submitted for

examination. Figure 33 shows the as-received

piston assembly and the failed axle.

A visual examination of the failed axle

revealed that the fracture surface followed

a circumferential path and contained a large

discolored region (Fig. 34). Except for the

discolored region, the fracture was typically

gray colored and contained chevron markings,

as shown in Fig. 35, indicating the failure orig-

inated at the discolored region. It was deter-

mined that the discolored region, from which the

failure appeared to originate, was located at the

bottom of the axle, at approximately the 6:30

o’clock position. The sketch in Fig. 36 shows the

approximate failure location. Failure of the axle

initiated approximately 145 to 149 mm (5.70 to

5.85 in.) from the threaded outboard end.

An optical examination with a stereomicro-

scope at up to 30· magniﬁcation conﬁrmed that

the failure of the axle originated at the discolored

region. Chevron marks were evident that indi-

cated the crack propagated away from the ends

of the discolored region, terminating at an area

diametrically across from it, as shown in

Fig. 35(b). The discolored region extended

along the circumference of the axle for a cord

distance of 22.15 mm (0.872 in.) and a maxi-

mum depth from the inside diameter surface

of 3.38 mm (0.133 in.). Actual wall thickness

at that location was 3.81 mm (0.150 in.). The

plane of the discolored region was approxi-

mately 25



from being perpendicular to the

longitudinal axis of the axle, along machining

marks. These dimensions were shown in Fig. 33

Fig. 28 As-received ﬂap hinge bolt and nut

Fig. 29 Fracture surface of the failed ﬂap hinge bolt

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and 36. When examined, the discolored region

showed a color variance ranging from a gold

color to reddish brown. Also, small patches of

blue were evident. This observed color variation

is a characteristic typically observed on steel

surfaces that have been exposed to elevated

temperature, such as during heat treatment

(Ref 1). The machining marks indicate a surface

much rougher than is normal.

The hardness measurements were made at

several locations around the circumference of

the axle adjacent to the fracture surface. The

hardness measured 53 to 55 HRC, which con-

formed to the drawing requirement of 53 to 55

HRC for 300M steel heat treated to a 1930 to

2070 MPa (280 to 300 ksi) condition.

A section containing the discolored region

was removed from the axle and examined with

the SEM. The locations where SEM examin-

ations were performed are shown in Fig. 35(a).

Prior to SEM examination, the discolored region

was cleaned with acetone and replication in an

attempt to remove the scale from the fracture

surface. After several attempts, only a small

amount of the scale could be eliminated, indi-

cating the scale was ﬁrmly attached to the frac-

ture surface. The SEM fractographs adjacent to

the inside diameter surface (area 1) were pri-

marily intergranular and appeared to be covered

with scale. At areas beyond the inside diameter

surface, the SEM fractographs revealed a mixed

intergranular with transgranular features and

patches of scale. These are shown in Fig. 37.

Along the periphery of the discolored region,

fatigue striations could be observed at a higher

magniﬁcation, as shown in Fig. 38. The depth of

Fig. 30

SEM examination of the ﬂap hinge bolt. (a) Overall view of fracture surface (1.25 mm). (b) Intergranular fracture evident at

origin (10 mm)

Fig. 31

Micrographs of the fracture origin. (a) Section 1-1, Fig. 29. Fracture is intergranular at prior-austenite grain boundaries

(50 mm). (b) Location A showing evidence of corrosion product at fracture facets (25 mm)

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the fatigue growth region was approximately

0.05 mm (0.002 in.). Rapid fracture extended

beyond the fatigue region. The SEM fractograph

in Fig. 39 shows dimpled features that char-

acterize a ductile mode of rapid fracture. When

examining the discolored region with SEM at

low magniﬁcation, there was no evidence of

shear lip along the inner diameter surface, which

indicates that the fracture did not initiate sub-

surface, as would be the case for delayed failure

resulting from hydrogen embrittlement.

In an attempt to determine the composition

of the observed scale or to identify any con-

taminant that may be associated with the fracture

surface, an energy-dispersive x-ray (EDX) ana-

lysis was performed on the discolored region.

The EDX analysis revealed no other element

except the ones common to the base metal

composition.

Fig. 32

Micrograph showing the microstructure at the frac-

ture origin. Microstructure consists of quenched and

tempered martensite (25 mm). IVD, ion vapor deposited

Fig. 33

As-received nose landing gear piston assembly and

the failed axle

Fig. 34 Discolored region of the fracture

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To examine the microstructure of the dis-

colored region, a section was removed (section

A-A, Fig. 35b) for metallographic examination.

The microstructure was tempered martensite,

which is typical for 300M steel heat treated to

the 1655 to 2070 MPa (280 to 300 ksi) ultimate

strength range (Fig. 40). A secondary crack was

observed starting at the inner diameter surface

(Fig. 40b), at a location showing large surface

irregularities. In an attempt to investigate this

secondary crack, the crack was opened and the

fracture surface examined with a light micro-

scope and the SEM. The fracture depth was

relatively small (0.25 mm, or 0.01 in.), which

made it difﬁcult to determine with the light

microscope if a high-temperature scale was

present. The SEM examination revealed inter-

granular and ductile transgranular features

(Fig. 41). These features suggest a secondary

heat treat crack.

Fig. 35

Fracture surface of the failed axle. (a) Black arrows show locations for SEM examination. (b) White arrows show fracture

direction and location of metallographic sections.

Fig. 36 Axle wall thickness, discolored region dimensions, and fracture location

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In order to verify the material composition,

one section of the piston axle was chemically

analyzed, using the atomic absorption spectro-

scopy method. The results of the analysis are

shown in Table 1. These values met the require-

ments of AMS 6419 for 300M steel.

In summation, the results of this investigation

indicated that the failure of the NLG piston axle

was introduced from a pre-existing defect. This

defect was present on the axle prior to the ﬁnal

assembly of the part to the aircraft. The defect

had a brittle intergranular fracture surface

feature and a discoloration characteristic of a

thermal or quench crack. These phenomena

demonstrated that the crack occurred on the

piston axle during or prior to the heat treatment

Fig. 37

SEM fractographs showing brittle intergranular structure in discolored region of the fracture surface. (a) Intergranular fracture

partially covered with scale at the area adjacent to the inner diameter surface (10 mm). (b) Fracture surface away from the

inner diameter surface showing intergranular and transgranular features (10 mm)

Fig. 38

SEM fractographs showing fatigue growth at regions close to the outer diameter surface. (a) At boundary of discolored region

(5 mm). (b) Outside the boundary of the discolored region (5 mm)

Fig. 39

Rapid fracture at location outside the discolored

region (5 mm)

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process. Because of the machining marks and

the initiation of cracking at these asperities, it

is possible that cracking occurred during the

machining operation, in a fashion similar to the

formation of grinding cracks.

Based on the results of this examination, it is

concluded that the failure of the NLG piston

axle was due to the pre-existing crack on the

axle. This crack was created prior to or during

heat treatment of the part.

Multiple-Leg Aircraft-Handling Sling

A new multiple-leg aircraft-handling sling

that had just received proof testing was found to

have a cracked clevis. The failed adapter was

machined from 4330V-modiﬁed steel that had

been heat treated to a 1240 to 1380 MPa (180 to

200 ksi) tensile strength range.

Figure 42 shows the adapter part of the mul-

tiple-leg aircraft-handling sling as received for

examination. Also shown in Fig. 42 is the loca-

tion and appearance of the crack that was at a

clevis. The crack penetrated completely through

the wall of the clevis.

The fracture surface was opened, and the

fracture surface was examined visually and at up

to 50 · magniﬁcation using a stereomicroscope.

Figure 43 shows a macrograph of the fracture

surface. Fracture ridges indicated that origins

were present on the outside surface of the clevis.

The fracture surface appeared faceted, which is

characteristic of an intergranular, delayed mode

of failure.

Fig. 40

Microstructure and secondary cracking at the discolored region. (a) Normal tempered martensite typical of 300M (25 mm).

(b) Secondary cracking apparent on inner diameter surface (100 mm)

Fig. 41

SEM fractographs of opened secondary crack. (a) Origin of secondary crack (33 mm). (b) Intergranular fracture apparent

(10 mm)

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An SEM was used to examine the fracture

surface and the side of the clevis at the origin.

Figure 44 shows SEM photographs document-

ing topographic features of the fracture surface

and corrosion pitting on the side of the clevis

at the fracture origin. The pits were coated with

primer, which indicated they were present

prior to painting. Also documented in Fig. 44

are intergranular topography and corrosion pro-

ducts at the fracture origins. These features are

all characteristic of a stress-corrosion failure

(Ref 4).

A laboratory overload fracture produced in

the clevis was examined in the SEM. Figure 45

shows an SEM fractograph documenting dim-

ples that are characteristic of an overload mode

of failure.

Hardness measurements were made on the

adapter to verify the heat treat condition. The

hardness of the adapter was 41.4 HRC, and this

met the hardness requirement of 40 to 43 HRC.

The hydrogen content of the adapter was

determined to be 1 ppm, which is considered

low in relation to producing a hydrogen em-

brittlement failure.

A metallographic specimen was prepared

through a fracture origin. The specimen was

prepared using standard metallographic techni-

ques. Figure 46 shows the microstructure, which

was normal (tempered martensite).

Also shown in Fig. 46 is a corrosion pit that

was coated with paint, indicating the pit was

present prior to painting.

Based on the results of this investigation,

it was concluded that the adapter failed due to

stress-corrosion cracking. Corrosion pits at the

fracture origin were present prior to painting.

Failure Analysis of an Aircraft Hoist

Sling during Static Test

An aircraft hoist sling was successfully tested

to an ultimate load of 136,000 kg (300,000 lb).

However, shortly after relieving the load,

the weld on the aft right-hand ﬁtting failed.

The ﬁtting was fabricated from welded 17-4PH

stainless steel plates.

Figure 47 shows the as-received failed portion

of the test ﬁxture. The failure occurred in a

consumable electrode weld that traversed

around the length of the fracture, which was

approximately 69 cm (27 in.). A dark, dis-

colored area was present in the fractured weld

over a length of approximately 122 mm (4.8 in.)

(location C).

Lack of fusion was a general condition

of the weld. Figure 48 shows a typical area

where lack of fusion was present in the weld.

Table 1 Chemical analysis of nose landing

gear piston axle

Chemical composition, %

Si Mn Ni Cr Mo V Fe

1.42 0.72 1.79 0.68 0.49 0.008 bal

Fig. 42

As-received failed multiple-leg aircraft-handling

sling

Fig. 43

Appearance of the fracture surface of the multiple-

leg aircraft-handling sling

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Figure 49 documents the direction of fracture,

as evidenced by the convergence of river pat-

terns, at a typical location. The side of the

fracture containing location C had been placed

in tension due to bending. Portions of the frac-

ture at locations A, B, and C shown in Fig. 47

were excised and examined on an SEM. Fig-

ure 50 shows SEM fractographs documenting

topographic features of these portions of the

fracture surface. The areas at A and B showed

evidence of overload, that is, dimples. At loca-

tion C in the dark, discolored area, the fracture

topography was different than at A and B. The

topography at location C was indicative of a

heated, oxidized surface.

An energy-dispersive spectrographic (EDS)

analysis conducted on the fractures at A and B

basically detected iron, chromium, nickel, and

copper. Nickel was not detected at location C.

The EDS spectra are shown in Fig. 51. The base

metal also showed the presence of iron, chro-

mium, nickel, and copper, which are present in

17-4PH stainless steel.

In conclusion, a different consumable weld-

ing electrode was used at location C (no nickel

present) than at locations A and B, where nickel

was present. It is recommended that 17-4PH

ﬁller metal be used when welding 17-4PH steel.

Fig. 44

SEM fractographs of the service fracture. (a) Overall view. (b) Intergranular topography at origin. Original magniﬁcation:

1500·. (c) Pits on side of clevis at origin. Original magniﬁcation: 400·. (d) Pit and corrosion products at origin. Original

magniﬁcation: 1000·

Fig. 45

Dimpled rupture indicating overload failure in a

laboratory-produced failure. Original magniﬁcation:

5000·

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This was apparently done at locations A and B

but not at C, where no nickel was present. Nickel

gives ductility to the weld and helps to prevent

cracking during cooling from the welding tem-

perature (Ref 7). It appears that at location C,

where no nickel was present, a crack was

formed, and the fracture surfaces at C became

discolored and pebbled in appearance at an

elevated temperature. Lack of fusion due to in-

sufﬁcient heating was a general condition in the

weld, and this also contributed to the failure. In

summary, the weld, when placed under tension,

failed due to a crack and lack of fusion.

Failure Analysis of an Internal Spur Gear

This investigation analyzed cracks that were

present in an internal spur output gear. The

internal gear is part of the planetary gear system

for a canopy. The gear was made from 4340 steel

heat treated to the 1790 to 1930 MPa (260 to

280 ksi) tensile strength range. The part was

rejected after magnetic particle inspection due to

multiple crack indications along the inside sur-

faces next to the gear spline. The typical manu-

facturing sequence is forge, machine, heat treat,

grind, and plate.

Figure 52 shows the internal gear as received

for examination. Also shown in Fig. 52 is the

appearance of typical cracks, which were loca-

ted on the inside of the internal gear on areas

next to the gear spline. The cracks were located

completely around the circumference on both

sides of the gear. A crack was opened, and the

fracture surface was examined visually and at up

to 30 · magniﬁcation using a stereomicroscope.

Figure 53 shows macrographs of the fracture

surface. The fracture surface was discolored

Fig. 46

Microstructure consisting of tempered martensite at

fracture origin. Original magniﬁcation: 200· . Cor-

rosion pit coated with paint is evident in the micrograph.

Fig. 47

As-received portion of the failed aft right-hand ﬁtting

for the hoist sling

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