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

Подождите немного. Документ загружается.

in, thus causing additional taper.) The top

portion of the hub of the pinion in Fig. 12(a) is

used only as a spacer and need not be heat

treated. A much shorter hub with a steel tubing

spacer (Fig. 12b) would solve the problem in

austenitization, making forging easier and, in

most cases, reducing total cost. The bevel pinion

shown in Fig. 13 presents a similar problem,

although the hub extension is necessary. Here,

steel or, preferably, a heat-resistant alloy cap

Fig. 9

Schematic of a gear that is difﬁcult to harden without

the distortion shown. Source: Ref 3

Fig. 10

Design solutions to the distortion problem shown in

Fig. 9. Source: Ref 3

Fig. 11

Distortion often encountered when quenching a

notch. Source: Ref 3

Fig. 8

Drive shaft pinion with fatigue fractures propagating

from the acute-angular edge of the helical gear.

Source: Ref 13

Fig. 7(a, b) Compressor transmission shaft with a fracture propagating from the acute-angled keyway. Source: Ref 13

12 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:49PM Plate # 0 pg 12

that will create mass can be put over the thin hub

before austenitization to retard the heating rate

during carburizing.

Because of the extremely rapid heating

rate, austenitization with high-frequency elec-

tric current can be likened to reverse rapid

quenching. Accordingly, design is of utmost

importance. As a general rule, steel exposed

to the ﬂux of the inductor will heat fastest on

corners, around holes (as shown in Fig. 14), and

through thin sections. The bottoms of keyways

and the roots of gear teeth and splines are aus-

tenitized last, often mainly by conduction from

adjacent areas.

As in quenching, however, induction tooling

can be designed to concentrate ﬂux by using

appropriate coil conﬁguration and laminated

core material. The different frequencies avail-

able provide not only for various depths of

heating but also for the sharpness of the heating

effect, because the induction-heated layer is

often much thinner than the hardened depth of

austenitized steel. The extent of conduction

is a function of the differential between the

surface temperature and that of the core. Thus,

preheating, either in the induction coil with a

suitable delay or in a furnace, can be employed

to reduce heat transfer inward.

Fig. 13 Redesign of a bevel pinion using electron beam welding that was impossible to heat treat in one piece. Source: Ref 11

Fig. 12

Two designs for gear-and-hub combinations. (a) Difﬁcult to heat treat without excessive taper in the bore. (b) A pr eferred

design. Source: Ref 11

Component Design / 13

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:49PM Plate # 0 pg 13

Finally, part design recommendations to

avoid distortion and cracking problems (Fig. 15)

include:

 Parts that exceed the following dimensions

often must be straightened or press quenched

to maintain dimensional stability: long and

thin parts, L=5d for water quenching and

L=8d for oil quenching (L is the length,

and d is the thickness or diameter); and parts

that possess large cross-sectional area (A)

and are thin (t), which are deﬁned as A=50t.

 Balance the areas of mass.

 Avoid sharp corners and reentrant angles.

 Avoid sharp corners between heavy and thin

sections.

 Avoid single internal or external keys, key-

ways, or splines.

 Provide adequate ﬁllet or radius at the base

of gear teeth, splines, and serrations.

 Do not have holes in direct line with the

sharp angles of cutouts.

 Avoid sharp corners at the bottom of small

openings, such as in drawing or piercing

dies, because spalling or ﬂaking is likely to

result at these points.

 Keep hubs of gears, cutters, and so on as near

the same thickness as possible, because

dishing is likely to occur.

 Order stock large enough to allow for

machining to remove decarburized surfaces

and surface imperfections, such as laps and

seams.

 Do not drill screw holes closer than 6.35 mm

(0.25 in.) from the edges of die blocks

or large parts, where possible. Cracking

may be avoided by using steel that may be

hardened by using lower quench severity,

or, if possible, pack the bolt hole to

reduce thermal stresses arising due to

quenching.

 Avoid blind holes, if possible.

 Design all parts with round corners and

ﬁllets wherever possible.

 Use air-hardening or high-carbon (oil- and

air-hardening) tool steel on unbalanced and

intricately shaped dies.

 Add extra holes, if possible, on heavy,

unbalanced sections to allow for faster and

more uniform cooling when quenched.

 Do not machine knife blades to a sharp cut-

ting edge before hardening.

 Avoid deep scratches and tool marks. The

insertion of identiﬁcation marks on the

hardened component is recommended, pre-

ferably after hardening, with tools having

well-rounded edges and minimum defor-

mation (shallow penetration depth) and at

positions far away from the high-stress-

concentration zones (reentrant angles,

bends, and so on).

 On long, delicate parallels, shafts, and so on,

rough out and have pieces annealed to

remove stresses before ﬁnish machining.

 Always use the grade or composition of steel

most suitable for the work that the part has to

perform.

 Design symmetry is also an important vari-

able to minimize distortion.

 A general rule for solving such quench dis-

tortion problems is that the “short side is the

hot side,” which means that the inside of the

bowed metal was quenched more slowly

than the opposite side.

Steel Grade and Condition. Although steel

cracking is most often due to nonuniform

heating and cooling, material problems may

be encountered. Some typical material prob-

lems include the compositional tolerances:

“dirty” steels, those containing greater than

0.05% S (such as SAE 1141 and SAE 1144),

are more prone to cracking. It is well known

that cracking propensity increases with carbon

content. Therefore, the carbon content of the

steel is one of the determining factors for

Fig. 14

Section through a hole in a part following rapid heating in an induction coil, showing distortion that leads to cracking.

Source: Ref 11

14 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 14

quenchant selection. As a rule of thumb, plain

carbon steels with less than 0.35% C rarely

crack on hardening, even under severe quench-

ing conditions. The carbon content of steel

should never be greater than necessary for

the speciﬁc application of the part. As a general

rule, steels with more than 0.35% C will

require oil quenching to avoid cracking. This

means that higher-cost, higher-alloy steels are

required for adequate response to the slower

oil quench when carbon content exceeds

0.35%.

However, when strength alone (and/or

hardness), without the toughness of a quenched

and tempered microstructure, fulﬁlls the mini-

mum engineering requirements, the use of

Fig. 15 Part design recommendations for minimal internal stresses. Source: Ref 3

Component Design / 15

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 15

cold-ﬁnished bars made with extra-heavy draft

or elevated-temperature drawing should receive

consideration. In fact, why heat treat when it is

unnecessary? Thus, SAE 1050, SAE 1140, or

SAE 1144 steels should be particularly attractive

to ﬁrms with no heat treating facilities or with no

commercial heat treater nearby. By adjustment

of their composition and the degree to which

they are cold (or warm) worked, these steels can

be made to have good machining characteristics

(Ref 6).

On the other hand, it is well known that

regions containing high concentrations of coarse

carbide microstructure as a result of improper

forging may become the initiation point for

subsequent quench cracking, particularly with

parts of complex shape. It is important to

provide a sufﬁcient forging for microstructure to

become ﬁne and uniform. Because part manu-

facture, such as gear production, often requires

machining, the condition of the steel that is

going to be machined is critically important.

Some workers have recommended normalized

and subcritical-annealed steels as the ideal

condition. The subcritical annealing process

reduces the carbon content and alloy carbide

content in the austenite, allowing the production

of more lath martensite in the microstructure,

which provides higher fracture toughness and

higher impact toughness.

Steel hardenability is determined by its

chemistry. The quench conditions required to

obtain the desired properties are a function

of the hardenability. Therefore, if the steel

chemistry is incorrect, the selected quench pro-

cess conditions may, if too severe, lead to

cracking. Unfortunately, this problem is not

uncommon.

Techniques for Controlling Distortion

In applying one or more of the effective

methods of minimizing distortion, cost is usually

the major consideration. Therefore, in planning

manufacturing operations, it behooves the pru-

dent processor to evaluate the costs of mini-

mizing distortion against the alternatives (Ref

14). In almost any instance, there are at least

three alternatives:

 Change to another heat treating process

 Make allowances for stock removal in ﬁn-

ishing operations to correct the distortion

 Incorporate straightening operations as re-

quired

In considering the alternatives that relate to

minimization of distortion, it is assumed that the

grade of material is ﬁxed, and no deviations are

allowed in this area. There are often instances,

especially for parts of complex design, where a

change in steel composition will permit a less

drastic quench and thereby reduce distortion.

Such changes are usually to steels with higher

hardenability.

In most instances, however, immediate

changes in workpiece composition are not fea-

sible. Pros and cons of the three most likely

alternatives (listed previously) are discussed

separately in the paragraphs that follow.

Consider Change to Another Process. In

this area, there are sometimes two or three pos-

sibilities, such as changing from a through-

hardening steel to a case-hardening type or

changing to one that does not require rapid

cooling, such as nitriding. One of the most likely

changes that is often made in this area is to the

use of localized heating, such as induction. For

example, a shaftlike member requires hardening

only in certain bearing areas. This can be

accomplished easily by induction and, in addi-

tion to eliminating distortion, is often more

economical for other reasons. Parts such as ring

gears represent other examples where a change

to induction hardening resulted in keeping dis-

tortion within acceptable limits.

Increase Stock Allowance. In many in-

stances, allowance for stock removal in the

ﬁnishing operation (usually grinding) is the most

economical approach. Under these conditions,

some study is usually necessary to determine the

magnitude of distortion caused by heat treating

and thereby how much stock allowance is

required for cleanup. Frequently, it is necessary

to take reasonable precautions (perhaps some

special procedures) in heat treating and then

take further steps by increasing stock removal

in ﬁnishing.

Mechanical straightening, either during

processing or applied to heat treated parts, offers

a third approach for solving distortion problems.

Straightening is sometimes used as the sole

technique for correcting distortion, but more

often it is used in conjunction with systematic

stock removal.

Heat Treating Practices for Minimizing

Distortion. Positioning in the furnace may

have a marked inﬂuence on total distortion,

especially for parts having a relatively large

length-to-cross section ratio. For example, for

long, shaftlike parts (solid or tubular), the

16 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 16

poorest loading technique would be to pile them

horizontally and more or less indiscriminately

on the furnace hearth. Under these conditions,

distortion begins immediately and continues as

the parts heat up and lose strength. Parts at the

bottom of the pile will naturally distort the most,

because they are subject to the greatest stress

during heating.

In contrast to the poor technique described

previously, the best technique for such parts

would be to hang them (preferably with spaces

between each part) in a vertical furnace for

heating. As a rule, some further improvement

can be achieved by heating in molten salt as

opposed to a gaseous atmosphere. This is due to

the fact that some support is supplied by the

buoyancy effect of molten salt.

One possible disadvantage (relating to dis-

tortion) of heating in molten salt is the heating

rate. Parts are heated four or ﬁve times as fast in

a medium of molten salt compared to heating in

a gaseous atmosphere. Rapid heating sometimes

increases distortion, especially when various

section thicknesses are involved.

Position during quenching may also have a

marked effect on the total amount of distortion.

Parts that are hung vertically in the furnace to

minimize distortion should likewise be hung

vertically in the quenching tank; that is, deep

tanks are preferred for this type of work. In

most instances, minimum distortion of speciﬁc

workpieces is achieved when the quenching

medium is not agitated. To obtain full hardness,

however, agitation usually must be used. When

minimum distortion is required, if agitation is

used, the quenching medium should be agitated

with agitating force at the bottom of the tank.

Although this speciﬁc system is used for water,

the principle applies to any quenching medium.

The quenching medium should never be agitated

from the side in such a system when minimum

distortion is important.

Choice of quenching medium can affect

distortion. Typical quenching media listed in

approximate order of decreasing cooling power,

are as follows:

 Water

 Brine solutions (aqueous)

 Caustic solutions (aqueous)

 Polymer solutions

 Oils

 Molten salts

 Molten metals

 Gases, including still or moving

 Fog or mists

 Air

The higher rate of heat extraction (quenching

power) is obtained by agitated brine. Minimum

distortion would be obtained by vertical heating,

then cooling (quenching) by hanging in still air.

Of course, this technique would not usually be

practical, largely because the parts would have

to be made from air-hardening steels. There are

several factors that inﬂuence choice of quench-

ing medium, but hardenability of the steel is

usually the key factor. Cooling rate thus has a

marked effect on the amount of distortion.

Consequently, the quenching speed should

never be faster than is required to attain the

required critical cooling rate, when distortion is

an important consideration.

Special quenching techniques may be

needed. Vertical heating and quenching one part

at a time could be construed as a special tech-

nique. However, commonly recognized special

techniques include:

 Martempering

 Press quenching

 Cold-die quenching

All of these methods can be used effectively to

reduce distortion, but as a rule, they are all

relatively costly and will greatly increase the

total manufacturing cost.

Whenever the handling of parts individually

is involved, the cost of heat treating increases

rapidly. For example, tubes made from 52100

steel, 1.2 m (4 ft) long, 63.5 mm (2.5 in.) in

diameter with a 3.175 mm (0.12 in.) wall thick-

ness were required to be hardened to a minimum

of 60 HRC with a maximum total indicator

reading of 0.75 mm (0.030 in.). No salt bath of

sufﬁcient depth was available for quenching;

thus, martempering could not be considered.

Several different procedures were tried, but the

one that was ultimately successful consisted of

heating vertically (hanging) and quenching in

unagitated oil, one tube at a time. This procedure

cost more than six times the estimated cost for

bundling the tubes, then mass heating and

quenching. Therefore, the cost of such a practice

would normally be considered prohibitively

expensive. Straightening during processing

may have solved the problem, but it also is very

expensive.

Martempering can, under many conditions,

be used to effectively reduce distortion and

can still be applied to mass production. This

Component Design / 17

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 17

depends largely on workpiece shape and size.

However, for long, shaftlike or otherwise

unwieldy workpieces, martempering requires

handling each workpiece individually and is

thus a relatively expensive operation, particu-

larly when simultaneous straightening is incor-

porated.

Press quenching is probably the oldest special

quenching technique and is still used to a con-

siderable extent. The greatest use of press

quenching is for gears that cannot be heat treated

with sufﬁcient dimensional accuracy by mass

quenching in baskets.

Selection of press quenching should be done

with the full knowledge that it is very expensive.

First, the presses are expensive machine tools.

Second, the dies are expensive, and the dies must

be tailored to the speciﬁc workpiece. Also, press

quenching is slow, tedious, and thus expensive.

Minimum distortion is achieved by press

quenching, but heat treating cost is high.

Dry die quenching is another tedious and

expensive process and should be considered

only for highly specialized applications.

Examples of Failures due

to Heat Treatment

Different types of errors and failures produced

in heat treatment (Ref 15) include:

 Heating errors: heating too fast causing

stresses in outer zones; heating non-

uniformly or locally overheating; heating at

too high a temperature or for too long a time

(distortion, cracking, residual stresses, de-

carburization, alloy depletion)

 Temperature errors: overheating (scaling,

burning, internal oxidation, hot shortness,

grain coarsening, aging, phases precipi-

tation)

 Heat treating errors: improper thermal

cycle; too high a temperature; too low a

temperature; improper heating rate; impro-

per cooling rate; improper soaking from

timing errors or nonuniform quenchant;

improper atmosphere control, which is cri-

tical in carburizing and nitriding; delay

between quench and temper; improper aging

treatment or postweld heat treatment (im-

proper, unacceptable, or mixed structures and

microstructural features; temper embrittle-

ment; sigma-phase embrittlement; 475



embrittlement; sensitization; carburization;

metal dusting; sulﬁdation; nitridation; dis-

bonding of chromium-molybdenum steels

overlayed by austenitic stainless steels)

Inﬂuence of design, steel grade, and condition

are illustrated in the following examples.

Example 1, in Fig. 16 (Ref 16), shows two

AISI W1 carbon steel concrete roughers that

failed after a few minutes of service. Cracking

occurred at the change in section size due

to bending stresses. Although the section

change has a smooth, ﬁlleted surface, it is still

a very effective stress concentrator. Sub-

sequent design changes involved a tapered

change in section at the cracked location and

later at the start of the wrench above the

cracked region.

Example 2 illustrates that holes placed too

close to the edges of components are a common

source of failure during heat treatment or in

service. Figures 17(a) and (b) (Ref 16) show an

AISI O1 tool steel die that cracked during oil

quenching. The die face contained numerous

ﬁne cracks. The left side of the die broke off

during quenching. Figure 17(b) shows both sides

of the fracture. Temper color (arrow), typical of

the 205



C (400



F) temper used in this case, is

apparent. This indicates the depth of the crack

produced during quenching that was open dur-

ing tempering. Coarse machining marks and

deep stamp marks were also present. Sharp,

unﬁlleted corners may also promote quench

cracking.

Example 3, Fig. 18 (Ref 16) shows a 76 by 87

by 64 mm (3 by 3

by 2.5 in.) AISI O1 tool

steel die that cracked during oil quenching. The

cracking pattern (emphasized using magnetic

particles) that emanates from the sharp corners is

visible. A few cracks are also associated with the

holes that are rather close to the edges. Temper

color was observed on the crack surfaces, indi-

cating that the cracks were present before tem-

pering.

Example 4, in Fig. 19 (Ref 16), shows an-

other example of a quench crack initiated by a

sharp corner. This ﬁxture was also made of AISI

O1 tool steel that was oil quenched. In this case,

the corner was ﬁlleted, but there was a nick in the

corner where cracking began. The shape of this

ﬁxture is also poor for steel that must be oil

quenched. The thinner outer regions cool more

rapidly, forming martensite ﬁrst, while the more

massive central region cools at a slower rate. An

air-hardenable steel would be a better choice for

this part.

18 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 18

Example 5 provides another case of a poor

design for liquid quenching, as shown in Fig. 20

(Ref 16). This 76 mm diameter by 76 mm long

(3 in. by 3 in.) threaded part made of AISI W2

carbon tool steel cracked in half at an undercut at

the base of the threads. Figure 20 shows the two

broken halves along with a cold-etched disk

taken from the hollow portion of the part. The

hardened outer case can be seen in the fracture

detail and in the cold-etched disk. Similar

parts, without the undercut, were successfully

hardened.

Example 6, in Fig. 21 (Ref 16), is a punch

made of AISI S7 tool steel that cracked during

quenching because of rough machining marks (a

common cause of quench cracking). Because of

the section size, the punch was oil quenched to

540



C (1005



F), then air cooled. The cracking

pattern has been emphasized with magnetic

particles. Temper color was observed on the

crack walls.

Example 7, Fig. 22(a,b) (Ref 16) show a

classic example of a failure due to improper

Fig. 16

AISI W1 (0.85% C) tool steel concrete roughers that

failed after short service (2 min for S, 7 min for S11).

Failures occur at the change of sections. Source: Ref 16

Fig. 17

(a) Front view of an AISI O1 tool steel die that

cracked during oil quenching. The die face contains

holes that are close to the edge for safe quenching. (b) Side view of

broken die halves showing the mating fracture surfaces and

temper color (arrow) on the crack surfaces. Source: Ref 16

Component Design / 19

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 19

electrical discharge machining (EDM) techni-

que. Die cavities are often machined by EDM.

The technique has many advantages, but failures

have been frequently observed due to failure to

remove the as-cast surface region associated

with the as-quenched martensitic layer. Cavity

surfaces must be stoned or ground, then tem-

pered to prevent such failures. Figure 22(a)

shows four 3.2 mm (0.125 in.) diameter EDM

holes in an AISI A4 tool steel primer cup plate.

The holes were ﬁnished by jig-bore grinding,

during which spalling was observed at many

of the holes (see upper-right hole). The surface

was swabbed with 10% aqueous nitric acid to

reveal regions affected by EDM. Figure 22(b)

shows the microstructure of these regions.

An as-cast region was present at the extreme

edge (approximately 35.5 HRC). Beneath this

layer was a region of as-quenched martensite

(approximately 63.5 HRC). Next was a back-

tempered region (approximately 56 HRC) and

then the base-unaffected interior (59 to 61

HRC). The brittle nature of the outer layers and

the associated residual-stress pattern caused the

spalling.

In many EDM-related failures, the as-cast

layer is not observed because of the technique

used or because of subsequent machining. In

these failures, however, an outer layer of brittle

as-quenched (white etching) martensite is pre-

sent. Such a failure is shown in Fig. 23; this

failure occurred in a plastic mold die made from

AISI S7 tool steel. The crack followed the lower

recessed contour of the larger-diameter gear

teeth and extended to a depth of approximately

1.6 mm. Etching of the surface revealed an as-

quenched martensite surface layer (thin, white

layer), while the internal structure was grossly

Fig. 18

AISI O1 tool steel that cracked during oil quenching.

Note the cracks emanating from the sharp corners.

The four holes, which are close to the edge, also contribute to

cracking. Source: Ref 16

Fig. 19

Fixture made from AISI O1 tool that cracked during

oil quenching. The design is poor for liquid

quenching. Source: Ref 16

(a) (b)

Fig. 20

Threaded part made from AISI W2 carbon tool steel that cracked during quenching at an undercut at the base of the threads.

Source: Ref 16

20 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 20

overaustenitized (note the retained austenite,

white, and coarse plate martensite). Both factors

led to cracking. If the EDM surface layer was not

present, poor service life would have resulted

anyway due to poor microstructural condition.

Example 8, in Fig. 24(a,b) (Ref 16), shows a

41 mm (1.6 in.) square 1.4 kg (3 lb) AISI S5

tool steel sledgehammer head that cracked dur-

ing quenching. A disk cut from the head was

macroetched, revealing a heavily decarburized

surface (Fig. 24b). Such a condition promotes

quench cracking, particularly in liquid-quench-

ing grades such as S5 (oil quenched), due to

differential surface stresses. A deep stamp mark

also helped promote cracking.

Example 9 presents a situation with stamp

marks, such as that shown in Fig. 25 (Ref 16)

that commonly promote quench cracks. This

was present on an air-quenched die made from

AISI S7 tool steel. In this case, the die was

not tempered, another prime cause of quench

cracking.

Example 10, Fig. 26 (Ref 17) shows SAE

4140 grade steel seamless tubing that

failed because of quench cracks. During pro-

duction of hydraulic cylinder housings being

fabricated from this steel seamless tubing,

magnetic particle inspection indicated the pre-

sence of circumferential and longitudinal cracks

in a large number of cylinders. Figure 26(a) is a

cross section of the tube showing extensive

cracking revealed by dye-penetrant inspection.

Figure 26(b) is a scanning electron microscope

(SEM) micrograph showing intergranular frac-

ture at a crack origin. Figure 26(c) is an SEM

micrograph illustrating the brittle mode of

failure associated with the fracture. Figure 26(d)

is a micrograph showing the typical con-

centrations of nonmetallic stringers in the tube

material, and Fig. 26(e) is a micrograph show-

ing a quench crack with a heavy oxide. Although

the steel met the compositional requirements

of SAE 4140, the sulfur level was 0.022%

and would account for the formation of the sul-

ﬁde stringers observed. The combination of the

clustered, stringer-type inclusions and the

quenching conditions was too severe for this

component geometry. The result was a high

incidence of quench cracks that rendered the

parts useless.

Example 11 presents the case of six wrist

pins designed especially for a high-performance

six-cylinder automotive engine (Ref 18) that

failed after 4800 km (3000 mi) of normal

operation. The wrist pins were made of low-

carbon steel carburized on both the outer and

inner diameters. Two failed wrist pins were

submitted for examination. Sample 1 had frac-

tured into three pieces (Fig. 27). Sample 2 had

not fractured but exhibited circumferential

cracks on the surface of the central zone. Some

of the cracks had progressed for most of the 360



of the pin. Both samples showed some evidence

of scoring on the outer diameter.

The fractured faces of sample 1 were battered

but showed a fairly smooth annular ring around

both the outer and inner diameters, with a ductile

and ﬁbrous core. The condition of the fractured

faces did not permit the deﬁnite establishment of

a fatigue failure. Figure 28 shows the dimen-

sions of a pin. The machining on the inside

diameter surface (indicated by “B”) was rela-

tively rough. The inner diameter had a raised

central section with a small ﬁllet on either side

(indicated by “A”).

The core (Fig. 29) had a banded micro-

structure of ferrite and pearlite and contained

some MnS inclusions. The case (Fig. 30) showed

a tempered martensite matrix with a nearly

continuous grain-boundary network of cemen-

tite. Other cracks started on the surface caused

by the cementite network (Fig. 31). All cracks

progressed inward from the carburized surface

and circumferentially around the pin. Hardness

tests were performed on the cross section of

the pin, and the depth of case penetration to a

value of 50 HRC was measured. Results indi-

cated a case depth of 0.89 to 1.0 mm (0.035

Fig. 21

Punch made of AISI S7 tool steel that cracked during

quenching because of rough machining marks

(a common cause of quench cracking). Source: Ref 16

Component Design / 21

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_1-42.pdf/Chap_01/ 18/8/2008 2:50PM Plate # 0 pg 21