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

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The mechanism proposed in Fig. 9 is

adequate to explain the initiation of cracks

on polished testpieces or components without

the presence of geometric discontinuities.

However, in engineering components, there are

several stress concentrators, such as scratches,

notches, machining marks, corrosion pits, and

microconstituents such as grain boundaries,

triple points, and inclusions, that individually or

synergistically can reduce the initiation time.

Since the initiation depends essentially on

plastic deformation mechanisms, high-strength

materials normally present a higher resistance to

fatigue crack nucleation. In this sense, several

surface-hardening treatments are employed to

selectively reinforce the material, aiming to

retard crack initiation and therefore to increase

fatigue life.

The chemical composition and/or the micro-

structure of the surface can be modiﬁed by

thermochemical treatments, such as carburizing

or nitriding, or by cold deformation processes,

such as shot peening or surface rolling.

Mechanical parts that necessarily present stress

concentrators, such as crankshafts, gears, and

bolts, can be subjected to these treatments

to increase the fatigue limit of the material.

Figure 11 shows a micrograph of the transverse

section of a bolt, where the thread was cold

formed by surface rolling. As a consequence,

surface grains are ﬂattened due to the mechan-

ical deformation imposed. In this case, besides

increasing hardness and mechanical strength,

the process avoids the introduction of harmful

machining marks.

Surface treatments may also increase fatigue

life by the introduction of compressive residual

stresses on the surface of the material. As long as

the material remains in linear elastic conditions,

the principle of stress superposition can be

employed to describe the actual stress state in

materials containing residual stresses. There-

fore, the effective stress, S

, is given by the

sum of the applied stress, S, to the residual stress,

res

=S+S

res

(Eq 1)

Similarly, the effective minimum and maximum

stresses are deﬁned, respectively, as:

max

res

(Eq 2)

min

res

(Eq 3)

Consequently, the effective stress amplitude,

mean stress, and load ratio are given, respec-

tively, by:

max

min

max

res

)7(S

min

res

)

max

min

(Eq 4)

max

min

max

res

)+(S

min

res

)

max

min

res

(Eq 5)

min

max

min

res

max

res

(Eq 6)

Therefore, the presence of a residual-stress

ﬁeld does not affect the stress amplitude but

affects the mean stress and the load ratio.

A compressive residual stress reduces the mean

stress and the load ratio, increasing the number

∆S/2

Increasing

Fig. 10 Mean stress effect on S-N fatigue curves

Fig. 11

Optical micrograph of the transverse section of a

thread ﬁllet machined by surface rolling. The ma-

terial consists of duplex stainless steel

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of cycles for crack nucleation and vice versa.

In some situations, where high surface com-

pressive residual stresses are found, such as in

materials subjected to surface-hardening treat-

ments, a crack may initiate below the surface,

where the compressive residual-stress level is

lower. An example of subsurface crack nuclea-

tion is observed in Fig. 12 for a surface-rolled

ductile cast iron subjected to bending-rotating

fatigue.

Fatigue Crack Propagation. Basically, fati-

gue crack propagation can be divided into three

stages: stage I (short cracks), stage II (long

cracks), and stage III (ﬁnal fracture).

A fatigue crack, once initiated, propagates

along high shear-stress planes (45



), as sche-

matically represented in Fig. 13. This is known

as stage I or the short crack growth propagation

stage. The crack propagates until it is deceler-

ated by a microstructural barrier, such as a grain

boundary, inclusions, or pearlitic zones, that

cannot accommodate the initial crack growth

direction. Therefore, grain reﬁnement is capable

of increasing fatigue strength of the material due

to the insertion of a large quantity of micro-

structural barriers, that is, grain boundaries, that

must be overcome in stage I of propagation.

Surface mechanical treatments, such as shot

peening and surface rolling, contribute to the

increase in the number of microstructural

barriers per unit of length due to the ﬂattening of

the grains.

When the stress-intensity factor, K, increases

as a consequence of crack growth or higher

applied loads, slips start to occur in different

planes close to the crack tip, initiating stage II of

propagation. While stage I of propagation is

orientated 45



in relation to the applied load,

propagation in stage II is perpendicular to load

direction, as depicted in Fig. 13. An important

characteristic of stage II propagation is the

presence of ripples on the fracture surface,

known as striations, which are only visible with

the aid of a scanning electron microscope. Not

all engineering materials exhibit striations. They

are clearly seen in pure metals and many ductile

alloys, such as aluminum alloys. In steels, they

are frequently observed in cold-worked alloys.

Figure 14 shows examples of fatigue striations

in an interstitial-free steel and in aluminum

alloys. The most accepted mechanism for the

formation of striations on the fatigue fracture

surface of ductile metals (Ref 9) consists of

successive blunting and resharpening of the

crack tip, as represented in Fig. 15.

Finally, stage III is related to the unstable

crack growth as K

max

approaches K

. At this

stage, crack growth is controlled by static modes

of failure and is very sensitive to the micro-

structure, load ratio, and stress state (plane-

stress or plane-strain loading).

Macroscopically, the fatigue fracture surface

can be divided into two distinct regions, as

shown by Fig. 16. The ﬁrst region corresponds to

the stable fatigue crack growth and presents

a smooth aspect due to the friction between

the crack-wake faces. Sometimes, concentric

marks, known as beach marks, can be seen on

the fatigue fracture surface as a result of suc-

cessive arrests or decrease in the fatigue crack

growth rate due to a temporary load drop or to an

overload that introduces a compressive residual-

stress ﬁeld ahead of the crack tip.

The other region corresponds to the ﬁnal

fracture and presents a ﬁbrous and irregular

aspect. In this region, the fracture can be either

brittle or ductile, depending on the mechanical

properties of the material, dimensions of the

part, and loading conditions. The exact fraction

of area of each region will depend on the applied

load level. High applied loads will result in a

small stable fatigue crack propagation area,

as depicted in Fig. 16(a). On the other hand,

Fig. 12

Probable subsurface crack nucleation site in a sur-

face-rolled ductile cast iron testpiece tested under

bending-rotating conditions

Stage I

Stage II

Surface

Fig. 13 Stages I and II of fatigue crack propagation

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if lower loads are applied, the fatigue crack

will have to grow longer before the applied

stress-intensity factor, K, reaches the fracture

toughness value of the material, resulting in a

smaller area of fast fracture (Fig. 16b).

Ratcheting marks are another macroscopic

feature that can be observed in fatigue fracture

surfaces. These marks originate when multiple

Fig. 14

Fatigue striations in (a) interstitial-free steel and (b)

aluminum alloy AA2024-T42. (c) Fatigue fracture

surface of a cast aluminum alloy where a fatigue crack was

nucleated from a casting defect, presenting solidiﬁcation den-

drites on the surface. Arrow at top right indicates fatigue striations.

(a)

(b)

(c)

(d)

(e)

Fig. 15

Proposed mechanisms of striation formation in stage

II of propagation. (a) No load. (b) Tensile load. (c)

Maximum tensile load. (d) Load reversion. (e) Compressive load.

Source: Ref 9

Fig. 16

Fatigue fracture surface. (a) High applied load.

(b) Low applied load

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cracks, nucleated at different points, join to-

gether, creating steps on the fracture surface.

Therefore, counting the number of ratchet marks

is a good indicator of the number of nucleation

sites. Figure 17 presents in detail some ratchet

marks found on the fracture surface of a large

SAE 1045 rotating shaft, fractured by fatigue.

Similar to the initiation phase, many factors

can affect long fatigue crack propagation rates.

Among them, special attention should be given

to the effects of load ratio and the presence of

residual stresses.

Increasing the load ratio has a tendency to

increase the long crack growth rates in all

regions of the fatigue crack growth rate versus

applied stress-intensity factor range curve, or

simply, da/dN versus applied DK curve. Gen-

erally, the effect of increasing load ratio is less

signiﬁcant in the Paris regime than in near-

threshold and near-failure regions (Fig. 18).

Near the threshold stress-intensity factor,

, the effects of R ratio are mainly attributed

to crack closure effects, where crack faces come

in contact at an applied K

that is higher than the

minimum applied stress-intensity factor, K

min

Several different mechanisms may contribute

to premature crack closure. One of them consists

of plasticity-induced closure, represented in

Fig. 19(a). As the crack grows, the material that

has been previously permanently deformed

within the plastic zone now forms an envelope of

plastic zones in the wake of the crack front. This

leads to displacements normal to the crack sur-

faces as the restraint is relieved. This is no pro-

blem while the crack is open; however, as the

load decreases, the crack surfaces touch before

the minimum load is reached, shielding the

crack. This type of premature contact can also

occur due to the crack-wake roughness and

irregularities (Fig. 19b) or by the presence of

corrosion subproducts, such as oxides (Fig. 19c).

As observed in Fig. 20, the effect of closure

produces a reduction in the effective DK range

because of the increase in the effective K

min

reducing the driving force for fatigue crack

growth. The effect is more signiﬁcant near the

threshold region because the crack tip opening

displacements are smaller and the crack faces

are closer to each other. Additionally, for the

same applied DK, higher R ratios increase the

applied values of K

max

and K

min

, increasing

eff

For most materials, the Paris regime is con-

sidered closure-free and K

max

-independent, and

Fig. 17

Ratcheting marks, indicated by the arrows, in an SAE

1045 shaft fractured by fatigue

∆ K

Increasing

da/dN

Near threshold

Final failure

Paris´ regime

Fig. 18

Schematic representation of the R ratio effect on

fatigue crack growth curves. The near-threshold,

Paris regime, and ﬁnal failure regions are also indicated on the

curves.

Plastic deformation

envelope

(a)

(b)

(c)

Premature contact points

Oxides

Plastic zone

Crack tip

Fig. 19

Crack closure mechanisms induced by (a) plasticity,

(b) roughness, and (c) oxide

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the crack growth rates are generally very similar

for tests conducted under different R ratios. Near

the ﬁnal failure, the effects of R ratio are related

to the higher monotonic fracture component as

max

approaches K

. Therefore, for the same

applied DK, K

max

values are higher for tests

conducted under higher applied R ratios, and

consequently, da/dN values are higher.

The effects of residual stress on fatigue crack

growth are related to alterations in the R ratio

and in the applied DK. In other terms, the resi-

dual stresses affect the two parameters that

control the crack driving force, that is, K

max

and

eff

. When a crack is introduced in a plate

subjected to a residual-stress ﬁeld, a residual

stress-intensity factor, K

, arises that can either

decrease or increase the crack driving force

parameters.

The superposition principle can also be

applied in terms of the stress-intensity factor,

provided that the material remains linearly

elastic. In this sense, K

can be added to K

max

and

min

max

(Eq 7)

min

(Eq 8)

As a result, R

and DK

are deﬁned as follows. If

min

40, then:

min

max

min

max

(Eq 9)

max

min

= K

max

ðÞ7 K

min

ðÞ

max

min

=DK

(Eq 10)

If K

min

j0, then:

=0 (Eq 11)

max

(Eq 12)

It is important to note that these equations

assume that the part of the fatigue cycle during

which the crack is closed at its tip (i.e., K

50)

makes no contribution to crack growth.

Distortion

Distortion is the least serious mode of failure,

but it can lead a part to failure or a structure to

collapse. It is easy to recognize but very difﬁcult

to prevent. This is due to the fact that distortion

does not involve the part itself but its use and

design. There are four reasons for distortion:

yielding, buckling, creep, and residual stresses.

Yielding. When a load is put on a part, and it

causes the part to be permanently distorted, it is

unable to perform the intended function and

therefore must be considered failed. In a well-

designed part, the stresses never exceed the yield

point, and the part deforms only elastically; that

is, when the load is released, the part returns to

its original dimensions.

In a good design, the part operates in the

elastic range, that is, below yielding point;

beyond this, the part will be permanently

deformed, and greater loads will cause the part

to actually break. This point is considered to be a

very basic point to design and applies when the

load on a part is applied in a quasi-static way,

such as the load on a building structure or the

stress in the legs of a desk. A ductile failure is

Time

max

min

∆K

eff

Time

max

∆K

=∆K

eff

(b)

min

(a)

Fig. 20 Load ratio effect on DK

eff

in a fatigue cycle. (a) K

min

. (b) K

min

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one where there is a great deal of distortion of the

failed part. Commonly, a ductile part fails when

it distorts and can no longer carry the needed

load. However, some ductile parts break into

two pieces and can be identiﬁed because there is

a great deal of distortion around the fracture

face, similar to what would happen if too much is

placed load on a low-carbon steel bolt.

Buckling. The failure of an engineering

component is not always caused by materials

fracture. In many occasions, the component

distortion may be sufﬁcient to put it out of

function. The distortion can be elastic or plastic.

The elastic distortions are temporary; however,

they may be sufﬁcient to cause interference on

the mobile parts. The plastic distortion is per-

manent and can be a result of an overload or

creep deformation. The overload causes per-

manent plastic deformation when the material

yield limit is overcome. This may happen in the

presence of stress concentrators, high tempera-

ture, inadequate heat treatment, or incorrect

materials selection for the component applica-

tion. Compressive overloads may lead the

material to overcome the buckling strength

limit, such as the one shown in Fig. 21 for an

aluminum part. The buckling strength is essen-

tially a design problem (not metallurgical), and

the load depends on the dimensions of the part

and the Young’s modulus of the material (the

only materials factors involved).

Creep is a time-dependent phenomenon that

causes a part failure if it is under both quasi-

static load and temperatures higher than 0.3 T

(absolute melting temperature). Creep strain

may produce sufﬁciently large deformation or

distortion that a part can no longer perform its

intended function. The two general types of

creep processes are grain-boundary sliding and

voids at grain boundaries (cavitation creep).

The creep processes are easily identiﬁed by

the local ductility and large numbers of inter-

granular cracks that will depend on the tem-

perature and strain rate imposed. In general, a

high strain rate combined with high temperature

results in ductile fracture, followed by a large

elongation and neck formation. Additionally,

the grains near the fracture surface tend to be

elongated. On the other side, the combination of

low strain rate and high temperature results in

intergranular brittle fracture, with low elonga-

tion or necking. Intergranular fracture in such

conditions normally initiates by grain-boundary

sliding from triple points or at grain-boundary

intersections with second-phase particles, caus-

ing cavities on the material microstructure, as

presented in Fig. 22.

Once the crack nucleates, it propagates by

grain boundaries, and given that some sig-

niﬁcant plastic deformation may take place,

the fracture surface tends to exhibit grains of

equiaxial shape. Therefore, to increase creep

strength, the material is normally heat treated to

increase the grain size, reducing the ratio

between the grain surface area and volume. In

turbines that work at very high temperatures, the

creep mechanism must be considered. In this

case, the component may be produced from

monocrystals that signiﬁcantly increase the

creep resistance.

Most creep curves show three distinct stages

(Fig. 23). After the elastic strain, there is a region

of increasing plastic ﬂow at decreasing rate (ﬁrst

stage), followed by a region of approximately

constant strain rate (secondary stage), and ﬁnally

a region of intense increase in the strain rate,

which rapidly extends to fracture (third stage).

Fig. 21 Aluminum part that suffered buckling

Cavities

(b)

(a)

Fig. 22

Intergranular crack formation at high temperature by

grain-boundary sliding at (a) triple points and

(b) inclusions

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Residual stresses can play a signiﬁcant role

in explaining or preventing failure of a compo-

nent. One example of residual stresses prevent-

ing failure is the use of shot peening processes

that increase the fatigue life of a component by

inducing surface compressive stresses.

Unfortunately, there are also processes or

processing errors that can induce excessive

tensile residual stresses in locations that may

promote failure of a component. The internal

state of stress is caused by thermal and/or

mechanical processing of the parts. Common

examples of these are bending, rolling, or

forging a part. Thermal residual stresses are

primarily due to differential expansion when a

metal is heated or cooled. Two control factors

are thermal treatment (heating or cooling)

and restraint. Both the thermal treatment and

restraint of the component must be present to

generate residual stresses. Residual stresses

can result in visible distortion of a component.

However, in the case of residual stresses, the

distortion can also be useful in estimating the

magnitude or direction of these stresses.

Wear-Assisted Failure

Wear may be deﬁned as damage to a solid

surface caused by the removal or displacement

of material by the mechanical action of a con-

tacting solid, liquid, or gas. It may cause sig-

niﬁcant surface damage, and the damage is

usually thought of as gradual deterioration.

While the terminology of wear is unresolved,

the following categories are commonly used:

adhesive wear, abrasive wear, erosive wear,

fretting, cavitation, rolling, contact fatigue, and

corrosive wear.

Adhesive wear has been commonly identiﬁed

by the terms galling or seizing. It is caused by

the material transference from one surface to

another during their relative movement due to a

solid-state welding process. Figure 24 shows a

schematic representation of this process. High

contact pressure among the surface roughness

results in local plastic deformation and points

of microwelding. The movement between the

surfaces causes the rupture of the junctions,

resulting in a rough peak in one surface and a

valley on the other. Eventually, the tip of a peak

may break, and an abrasive particle is formed.

Abrasive wear, or abrasion, is caused by the

displacement of material from a solid surface

due to hard particles or protuberances sliding

along the surface. The particles may be found

free between two surfaces or attached to one of

them, and the wear level depends on the relative

hardness between the particle and the surface

(Fig. 25). The abrasion may also happen due to

the protuberances or sharp asperities on one of

the surfaces in contact. The process of abrasive

erosion may be considered as abrasive wear.

Erosion, or erosive wear, is the loss of

material from a solid surface due to relative

motion in contact with a ﬂuid that contains solid

particles. In this case, the particle is found to be

dispersed in a ﬂuid or gas means, and it reaches

the surface under relatively high velocity

(Fig. 25d). Figure 26 shows the microstructure

of the transversal section of an H11 tool steel

that has been subject to abrasive erosion.

Fatigue wear can be characterized by the

formation of cracks superﬁcially and/or sub-

superﬁcially and the removal of posterior

material due to cyclic loading of solid surfaces.

The sliding contact and/or rolling between solids

Time

Stage I

Stage II

Stage III

∆ε

∆t

∆ε

∆t

= creep rate

Fracture X

Fig. 23

Schematic strain-time curve at constant load and

temperature showing the three stages of creep

Adhesion

Particle

Fig. 24

Transference mechanism of a material from one

surface to another and the formation of an abrasive

particle in the process of adhesive wear

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or the repetitive impact of solids and/or liquids

in a surface are responsible for the superﬁcial

fatigue. When two surfaces of this nature inter-

act due to load application, the area effectively

in contact may be very small, resulting in high

compressive and shear stresses that may lead to

crack nucleation. If only rolling is present, the

maximum shear stress takes place just below the

surface, giving rise to cracks that propagate

parallel to the surface and emerge at the surface,

causing part of the material to separate from the

component, as shown in Fig. 27.

However, pure rolling is not found in in-

service conditions. Normally, there is some

sliding between the two surfaces, which alters

the stress ﬁeld due to an increase in the shear

component, displacing the resulting stress closer

to the surface. The cracks start to nucleate on

the component surface, propagating at a very

shallow angle, as shown in Fig. 28.

Fretting fatigue is considered a phenomenon

where the damage is introduced by a conjunction

of events consisting of adhesion, oscillatory

movement of very low amplitude, oxidation, and

abrasion. The small oscillatory movements may

cause points of adhesion on the surface that

eventually break, forming oxidized particles that

(b)

(d)

(a)

(c)

Fig. 25

Abrasive wear. (a) Free particle between two sur-

faces. (b) Particle attached to one of the surfaces.

Fig. 26

Fractography showing an H11 tool steel that has

suffered abrasive erosion

Fig. 27

Schematic representation of contact fatigue under

pure rolling between two surfaces

Fig. 28

Damage by contact fatigue in rolling combined with

sliding conditions in gears produced from a quen-

ched and tempered AISI 8620 carburized steel. (a) Transversal

section. (b) Frontal view from a formed cavity

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act as abrasives on the surface, since the small-

amplitude movements avoid their dispersion

apart from the source point. Figure 29 presents a

micrograph from a plasma nitrided Cr-Mo-V

steel, where a microcrack formed in the fretting

region.

More than one mechanism can be responsible

for the wear observed on a particular part. The

most critical function provided by lubricants is

to minimize friction and wear to extend equip-

ment service life. Gear failures can be traced to

mechanical problems or lubricant failure.

Lubricant-related failures are usually traced to

contamination, oil ﬁlm collapse, additive

depletion, and use of improper lubricant for the

application. The most common failures are due

to particle contamination of the lubricant. Dust

particles are highly abrasive and can penetrate

through the oil ﬁlm, causing plowing wear or

ridging on metal surfaces. Water contamination

can cause rust on working surfaces of gears and

eventually destroy metal integrity. To prevent

premature failure, gear selection requires careful

consideration of the following: gear tooth geo-

metry, tooth action, tooth pressures, construc-

tion materials and surface characteristics,

lubricant characteristics, and operating environ-

ment.

Environmentally Assisted Failure

Corrosion is chemically induced damage

to a material that results in deterioration of

the material and its properties. Corrosion

can seldom be totally prevented, but it can be

minimized or controlled by proper choice of

material, design, coatings, and occasionally

by changing the environment. Various types of

metallic and nonmetallic coatings are regularly

used to protect metal parts from corrosion.

Corrosion may result in failure of the com-

ponent. Several factors should be considered

during a failure analysis to determine the effect

corrosion played in a failure, such as type of

corrosion, corrosion rate, the extent of the cor-

rosion, and the interaction between corrosion

and other failure mechanisms.

Uniform, pitting crevice, galvanic, and stress-

corrosion cracking are the most common types

of corrosion. Uniform corrosion is characterized

by corrosive attack proceeding evenly over the

entire surface area or a large fraction of the total

area. General thinning takes place until failure.

On the basis of tonnage wasted, this is the most

important form of corrosion.

Stress-corrosion cracking necessitates a

tensile stress, which may be caused by residual

stresses, and a speciﬁc environment to cause

progressive fracture of a metal. Aluminum

and stainless steel are well known for stress-

corrosion cracking problems. However, all

metals are susceptible to stress-corrosion crac-

king in the right environment.

Pitting corrosion is a localized form of cor-

rosion by which cavities or holes are produced

in the material. Pitting is considered to be

more dangerous than uniform corrosion damage

because it is more difﬁcult to detect, predict, and

design against. Corrosion products often cover

the pits. A small, narrow pit with minimal

overall metal loss can lead to the failure of

an entire engineering system. Pitting corrosion,

which, for example, is almost a common

denominator of all types of localized corrosion

attack, may assume different shapes.

Crevice corrosion is a localized form of

corrosion usually associated with a stagnant

solution on the microenvironmental level. Such

stagnant microenvironments tend to occur in

crevices (shielded areas) such as those formed

under gaskets, washers, insulation material,

fastener heads, surface deposits, disbonded

coatings, threads, lap joints, and clamps. Crevice

corrosion is initiated by changes in local chem-

istry within the crevice.

Galvanic corrosion (also called dissimilar-

metal corrosion or, wrongly, electrolysis) refers

to corrosion damage induced when two dis-

similar materials are coupled in a corrosive

electrolyte. It occurs when two (or more) dis-

similar metals are brought into electrical contact

under water. When a galvanic couple forms, one

of the metals in the couple becomes the anode

Fig. 29 Fretting fatigue at the surface of a Cr-Mo-V steel

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and corrodes faster than it would all by itself,

while the other becomes the cathode and cor-

rodes slower than it would alone.

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