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

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Failure in Steel Forging

Md. Maniruzzaman and Richard D. Sisson, Jr.,

Worcester Polytechnic Institute

Stephen R. Crosby, The Stanely Works

Charlie Gure (deceased)

IN-PROCESS OR SERVICE FAILURES of

forgings may occur for a variety of reasons. The

starting material may be of insufﬁcient quality to

be adequately formed without cracking, or the

forging process may introduce various types of

discontinuities that cause failure during services.

For example, well-known forging-related dis-

continuities include:

 Laps

 Bursts

 Flakes

 Segregation

 Cavity shrinkage

 Centerline pipe

 Parting-line grain ﬂow

 Inclusions

Forging discontinuities are discussed in more

detail in the texts on forging (Ref 1–4).

This article describes six case studies of

failures with steel forgings (summarized in

Table 1). The case studies illustrate difﬁculties

encountered in either cold forging or hot forg-

ing in terms of preforge factors and/or dis-

continuities generated by the forging process.

Tables 2 and 3 summarize these factors for

cold and hot forging, respectively. Supporting

topics that are discussed in the case studies

include:

 Validity checks for buster and blocker

design

 Lubrication and wear

 Mechanical surface phenomenon

 Forging process design

 Forging tolerances

As case studies were being selected, each of

the aforementioned supporting topics was

reviewed for any impact that particular study

had on the case being examined. It is a well-

known fact that forging solutions have several

possible avenues to follow. There is no unique

theory in plasticity that leads to the solution.

Most of the work reported here was performed

using the minimum amount of energy to create

the particular product. Factors unrelated to the

deformation process, such as chemistry, micro-

structure, phase, grain size, segregation, and

prior strain history, are not addressed here.

Instead, factors directly related to the deforma-

tion process itself are presented in this abbre-

viated discussion.

Wear, plastic deformation processes, and

laws of friction are introduced as a group of

Table 1 Failure analysis of steel forgings and components

Case study Defect Solution

Crankshaft underﬁll Unable to ﬁll crankshaft ﬂanges with existing press

capacity

Introduce creep stages for last increment of

displacements

Tube bending Unable to control exterior wall thinning and interior wall

thickening

Introduce induction heating and cooling to limit the

heated axial tube length prior to making the bend

Spade bit Unable to achieve center web thickness at programmed

force and sufﬁcient ﬂow to wings

Adjust the die angle to create more shear stress, enabling

full ﬂow to the wings

Trim tear Forge material tore at trimline when forging was

trimmed immediately following ﬁnish forging

Introduce a delay time after forge and prior to trim,

allowing the forge material to cool and gain strength

Upset forging Cracking at circumferential bulge after upset Re-examine the strain and strain rate and process map

for stable ﬂow

Flow-through laps

and avoidance

Material foldover at tops of rib and ﬂange intersections

and cases of material ﬂow under previously ﬁlled

ﬂanges

Replace the input piece with a newly designed preform

piece, following the design procedures given in this

work

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Failure Analysis of Heat Treated Steel Components

L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 133-149

DOI: 10.1361/faht2008p133

www.asminternational.org

subjects that have been considered in the case

studies. Added factors that were evaluated in the

case studies are:

 Crankshaft underﬁll: induction coil inside

diameter and stock diameter, equivalent

current depth and subsequent time for con-

duction to reach a uniform stock tempera-

ture, total heating time for scale control,

transfer time to press, and forging force

applied

 Tube bending: precise heat input, control of

temperature, and heated axial length of tube

 Spade bit: direction of forging relative to

part shape and assessment of shear effect in

extended wings

 Trim tear: trimmer tool tolerances, part

temperature, and process time

 Upset forge: principal strains and equivalent

plastic strain

 Flow through: strains going from round or

ﬂattened piece to ﬁnish, and assessing the

need for a more generalized shape for input

to ﬁnish die. Preform design—streamline

shape (not the same for aluminum-, mag-

nesium-, steel-, titanium-, or nickel-base

alloys)

 Lubrication: select one that provides the

lowest coefﬁcient of friction and other

acceptable properties

 Forge process: total process for entire

manufacturing train, including heat treat-

ment and product testing

 Forge checking: ﬁxture check for critical

dimensions

 Forge tolerances: component to ﬁt the cus-

tomer’s assembly

 Simulation of process: verify that laws of

plasticity are met

Forging Process Design

Forging process design requires the applica-

tion of integrated engineering principles that

bring together factors such as:

 Relationship between the important sub-

system of a deformation system (Fig. 1)

Table 2 Factors in analysis of cold forging failures

Preforge factors

1. Raw material—chemistry, microstructure, mechanical properties, size, surface ﬁnish, and cleanliness

2. Shape sequencing—general nature of shape to be created; strain, strain rate, and load requirements

3. Forging—equilibrium forging temperature, strain and strain rate, workpiece volume control, forge equipment, loading and transfer

devices, lubrication, parts collection, inspection, and annealing

4. Trimming

Causes of defects during cold forging

1. Cracking—Three factors combine to produce cracks: stress from thermal expansion and contraction, hydrogen, and a susceptible

microstructure.

2. Product underﬁll—poor ﬂow, sufﬁcient volume, and proper distribution

3. Unbalanced forces—laps/lap ﬁllin, nonhomogeneous strain, strain rate, nonuniform microstructure, and work hardening

a. Seams—external and internal—on or within a metal surface, an unwelded fold or lap that appears as a crack usually resulting from

a discontinuity

b. Inclusions—raw material; internal and external substance that is foreign and insoluble to the matrix; particles of a foreign material

in metallic matrix. Particles are usually compounds, such as oxides, sulﬁdes, or silicates but may be of any substance that is foreign

and insoluble to the matrix.

c. Tears—occur when the equivalent plastic strain exceeds the capability of the material

d. Entrapped scale—forged in contamination consisting primarily of oxides but can include other products left on metals

4. Strain hardening—increase in hardness and strength of metals caused by plastic deformation at temperatures below the recrystallization

range; also known as work hardening

5. Flow through/push through—condition at which excessive material is provided in the preform in error, such that as elements of the

shape are completely ﬁlled, such as ﬂanges or rails, the central material continues to displace outward underneath the ﬁlled ﬂanges

6. Porosity/voids—small openings, interstices, or channels within a consolidated solid mass or agglomerate usually larger than atomic

or molecular dimensions

7. Segregation—In the casting process, the solidifying front moves away from the surface of the casting as a plane front, and

lower-melting-point constituents in the solidifying alloy are driven toward the center. This is called normal segregation.

8. Internal shearing—This effect can occur when material displacements cause excessive sliding of adjacent volumes of material.

9. Surface impurities—any foreign substance deposited on the part unintentionally

10. Grain size structure—The number of grains per unit volume and the phase of the material dictate the forging response.

11. Flakes, blisters—These ﬂaws typically result from the raw material or other processing steps but may show up when materials are

forged.

12. Residual stresses/distortion—Most materials (especially steels) will have residual stresses after cold forging; distortion occurs when

the stresses are not symmetrical.

13. Lubrication—dies and workpiece—viscosity and ﬂow, hydrodynamics of lubrication, friction, heat generation and power losses,

coefﬁcients of friction

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 Interdependence of forging process para-

meters (Fig. 2)

 Forging process design task overview

(Fig. 3)

 Relationship between process and machine

variables (Fig. 4)

 Characteristics of forging machines

(Table 4)

 Workability modeling (process maps show-

ing zones of stable ﬂow) of workpiece

behavior to achieve stable deformation at

a speciﬁed rate and proper evolution of

microstructures and properties

Forging Tolerances

The need for veriﬁcation of the nominal

dimensions and application of forging toler-

ances is important for quality assurance. Toler-

ances are required on forged products to allow

for practical variations in die preparation, tem-

perature effects during forging, equipment, and

distortion during and after heat treatment. Forg-

ing tolerance review is a basic requirement to

ensure that the part meets the multitude of design

features and tolerances. A listing of the more

important forging tolerances includes:

 Dimensional—length, width, center-to-

center, and external-internal

 Die wear—generally approximately

0.102 mm (0.004 in.)/surface

 Die closure—thickness of approximately

0.813 to 6.35 mm (0.032 to 0.250 in.) as a

function of plan form area at trimline

Table 3 Factors in analysis of hot forging failures

Preforge factors

1. Raw material—chemistry, microstructure, mechanical properties, size, surface ﬁnish, and cleanliness

2. Shape sequencing—shape nature, temperature, strain and strain rate, upset tooling, fuller (roll), open-die tooling

3. Hot forging—temperature, strain and strain rate, forge center cell, loading and transfer device, lubrication, parts collection, and inspection

4. Trimming—trimmer unit and capacity, ﬂash removal, temperature trace of product ﬂashline

Causes of defects during hot forging

1. Cracking—occurs when the imposed equivalent plastic strain exceeds the material capability at the temperature of operation—surface

(hot tears), cooling (centerline cracking)

2. Product underﬁll—underachieved thickness goal, inadequate material displacements, poor 3-D ﬂow, inability of input shape to

subsequent follow-on dies to satisfy local volume requirements, control of centroid path of newly created shapes

3. Unbalanced forces—laps/lap ﬁllin, nonhomogeneous strain, strain rate, nonuniform and continuous microstructure

a. Seams—external and internal—on or within a metal surface, an unwelded fold or lap that appears as a crack usually resulting from a

discontinuity

b. Inclusions—raw material; internal and external substance that is foreign and insoluble to the matrix; particles of a foreign material

in metallic matrix. The particles are usually compounds, such as oxides, sulﬁdes, or silicates but may be of any substance that is

foreign and insoluble to the matrix.

c. Hot tears—occur when the equivalent plastic strain exceeds the capability of the material at the temperature of operation

d. Entrapped scale—forged-in contamination consisting primarily of oxides but can include other products left on metals

4. Flow through/push through—condition at which excessive material is provided in the preform in error, such that as elements of the

shape are completely ﬁlled, such as ﬂanges or rails, the central material continues to displace outward underneath the ﬁlled ﬂanges

5. Porosity and voids—small openings, interstices, or channels within a consolidated solid mass or agglomerate usually larger than atomic

or molecular dimensions

6. Segregation—In the casting process, the solidifying front moves away from the surface of the casting as a plane front, and

lower-melting-point constituents in the solidifying alloy are driven toward the center. This is called normal segregation.

7. Internal shearing—This effect can occur when material displacements cause excessive sliding of adjacent volumes of material.

8. Surface impurities—any foreign substance deposited on the part

9. Grain size structure—The number of grains per unit volume and the phase of the material dictate the forging response.

10. Flakes/blisters—These ﬂaws typically result from the raw material or other processing steps but may show up when materials are

forged.

11. Residual stresses/distortion—Most steel forgings will have inherently residual stresses and distortion due to cold straightening

or following quenching.

12. Lubrication—dies and workpiece—viscosity and ﬂow, hydrodynamics of lubrication, friction, heat generation and power losses,

coefﬁcients of friction

Equipment

Material

system

Control

system

Constitutive

Equation

Workability

Control

System

Fig. 1

Relationship between important subsystems of a de-

formation system. Source: Ref 5

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 Match—alignment of the top and bottom

dies

 Radii—strong inﬂuence on material dis-

placements

 Flash extension

 Straightness—taken as a separate feature

and then assessing its effect on the remainder

of other tolerances

Data on

Billet material

Ram velocity Strain rate

Billet/Forging

Geometry,

Volume and thickness

Die temperature,

cooling

Interface

lubrication

Contact time

under pressure

Temperature

distribution in

forging

•Metal flow

• Forging load

• Forging energy

Friction

Conditions and

coefficient

Flow stress/

forgeability

Data on

Billet material

Ram velocity Strain rate

Billet/Forging

Geometry,

Volume and thickness

Die temperature,

cooling

Interface

lubrication

Contact time

under pressure

Temperature

distribution in

forging

•Metal flow

• Forging load

• Forging energy

Friction

Conditions and

coefficient

Flow stress/

forgeability

Fig. 2 Interdependence of forging process parameters

Fig. 3 Forging process design task overview

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 Draft angle

 Datum plane location for three-plane (x, y , x)

setup

 Alternative machined tooling points

 Finish allowance between forging and

machined part

Tolerance review is conducted in various

ways, and in the past, numerous forgings have

been rejected and held for material review until

some decision could be reached regarding their

disposition for rejection or alternative repair.

Even though this has typically been from review

of the part drawings, another useful way to

assess a completed forging is a ﬁxture check. A

uniquely designed ﬁxture in conjunction with a

dimensional inspector sets the forging into ﬁxed

tooling point locations and proceeds with the

check “go” or “no-go,” determining whether

the part will or will not serve its function in the

assembly. In many cases, special ﬁxtures and

gages can conﬁrm the accuracy of dimensions

that are critical to the function of the component

dimension. Large forgings are good candidates

for ﬁxture checking.

Wear and Lubrication

Surface interactions of two materials are

inﬂuenced by small regions where contact is

made at the atomic level. The real area of contact

is determined by elastic and plastic deformation

under consideration of loading. Lubrication

reduces friction by introducing a viscous and

low-shear-strength layer at junctions. Surface

interactions can lead to wear, or the removal of

material as a result of mechanical action.

Wear types include:

 Adhesion wear: particle transfers (pulled

off) from one and adheres to the other

 Abrasive wear: a hard, rough surface plows

grooves into the softer one

Fig. 4 Forging equipment characteristics; relationship between process and machin e variables

Table 4 Characteristics of forging presses

Equipment type Deformation rate Temperature loss Consistency Production rate

Hydraulic press Slow High Very good Low

Mechanical press Slow to medium Moderate Good Moderate to high

Screw press Moderate to high Moderate Fair to good Moderate to high

Hammer High Low Fair to good Moderate

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 Corrosive wear: mechanical action removes

a protective layer from a surface and exposes

it to corrosive attack

 Surface fatigue wear: spalling occurs after

the formation of surface or subsurface cracks

 Volume wear: proportional to the load and

distance traveled and inversely proportional

to the material hardness

During the early 1950s, the importance of

proper lubrication was recognized on the shop

ﬂoor. If an inexperienced oiler inadequately

applied lubricant (in spray or paste form), then

forging problems could occur even for an

acceptable preliminary workpiece (preform).

Alternatively, a questionable preform for the

ﬁrst closed impression die would be proven

acceptable if an experienced oiler knew where

the lubricant should be applied over the die

impression and also when the die impression

needed additional heavy lubricant in given die

locations that appeared difﬁcult to ﬁll.

These anecdotes vanished quickly as more

science replaced art in forging. Presently, there

are numerous ways that lubricants are used in the

forging industry. Wrapping the workpiece dur-

ing heating is an approach to prevent the for-

mation of scale in the case of steel or thin metal

sheets or cloths with impregnated graphite, in

addition to the automatic spraying of lubricants.

Lubricants play an important part in forging by

minimizing the load required for maximizing

material ﬂow, protecting the die surface ﬁnish

(critical for a speciﬁc lubricant), and assisting

the entire forging process.

Lubricant performance factors include:

 Adequate lubricity

 Stability in gas-ﬁred and electric furnaces

 Protect stock against atmospheric conta-

minants

 Provide good surface ﬁnish

 Act as a release agent

 No buildup in die cavity

 Ease of application and removal

 Conform to Environmental Protection

Agency (EPA) and Occupational Safety and

Health Administration (OSHA) require-

ments

 Acceptable cost

 Compatible with die materials

Graphite products for forging lubrication

are:

 GPC—for hot and warm forging

 Die lubricants—GP series

 GP 100—low dilution ratios and spray

application

 LS—oil and water

 Precoat workpiece—contains graphite as a

lubricant pigment

Adhesion colloids are reliable for high

pressure and temperature. Types include:

 Colloidal—dispersions

 Delta forge lubricants—for hammer, press,

and upsetters

 Deltaglaze—protective lubricants for billets

applicable to steel

Case Studies

Case Study 1: Crankshaft Underﬁll. There

are several large steel forging components, such

as ship crankshafts and airplane landing gear,

being manufactured successfully in the United

States and throughout the world today (2008).

Crankshaft forgings in the weight range of 2268

to 4536 kg (5000 to 10,000 lb) are products

made by a forging process creating a pair of

ﬂanges and a pinion shaft diameter at one time.

The inboard and outboard ﬂanges along with the

pinion diameter become integral parts of the

main shaft diameter.

The forging operation creates one set of

ﬂanges by means of a working stroke in line with

the major shaft diameter, while a 90



off-set load

forges the pinion shaft between the ﬂanges.

These operations are generally performed fol-

lowing one local heating of the starting bar

diameter for forging a set of crankshaft throws,

including the two ﬂanges and an offset pin dia-

meter. The forging process is repeated until all

of the ﬂanges plus the offset pinion diameter are

created along the major diameter of the crank-

shaft. The nature of the ready-for-assembly ﬁn-

ish forging design for the incrementally forged

crankshafts includes locations where material is

provided for machining along with selected

as-forged surfaces.

During the forging of the ﬂanges, there had

been cases of small amounts of underﬁll at the

ﬂange extremities, as shown in Fig. 5. That

extent of underﬁll has caused the entire com-

ponent to be rejected.

A test run was planned to measure material

displacements while the ﬂanges were being

forged at the prior selected process variables of

strain, strain rate, temperature of workpiece and

dies, and forging force exerted. The conclusion

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reached was that since the workpiece material

temperature at the end of the press working

stroke was still in the hot working range, an

extended length of time with the maximum force

applied would be helpful to displace the rela-

tively small amount of missing material into the

remote regions of the ﬂange dies. The thought

was that allowing material to creep would aid in

the ﬁnal ﬁlling of the die cavities.

Creep is an example of viscous ﬂow and is

deﬁned as continuing ﬂow at constant stress. At

characteristic stresses, the creep strain reaches a

steady state in which the rate of straining is

constant. This is called the steady-state creep

rate,

e (or d

e=dt). In hot working, the relation-

ship between temperature (T), stress (s), and

strain rate (

e) in the steady-state condition is best

expressed as:

e=A(sinh as)

exp (7Q=RT)

where A, a, and n

are temperature-independent

constants, Q is the activation energy, R is the

universal gas constant, and T is the temperature

in Kelvin.

At low stresses characteristic of creep (as5

0.8), this equation reduces to:

e=A

exp (7Q=RT)

which describes the relationship among three

variables under creep conditions.

Evaluation of experimental data of the acti-

vation energy, Q, indicates that some metals in

hot working soften the recovery process of

repolygonization, and others soften by dynamic

recrystallization. Thus, there is a distinct corre-

spondence between hot working and viscous

creep deformation.

In this case study, several time elements (all

less than 60 s) were established in subsequent

trials where all process variables were moni-

tored (including a lower-than-press-capacity

force), and ﬂange ﬁll results were measured.

Finally, the proper combination of the important

process variables, of which temperatures played

an important part, enabled the consistent ﬁlling

of the ﬂange extremities, as shown in Fig. 6.

Case Study 2: Tube Bending. Bending

large-diameter tube (310 stainless steel) created

a90



bend in the diameter range of 635 to

762 mm (25 to 30 in.) and at a nominal wall

thickness of 12.7 mm (0.50 in.) This offered a

major challenge to manufacture. The challenge

was to create a 90



angle bend without excess

thinning at the outer wall and excess thickening

at the inner wall.

Earlier efforts had centered on using gas

heaters around the circumference of the pipe

Fig. 6

Both crankshaft ﬂanges ﬁlled. Pinion shaft diameter

located between ﬂanges of single “throw”

Fig. 5

(a) Crankshaft ﬂanges not ﬁlled. Main shaft diameter shown between ﬂanges of adjacent “throws”. (b) Crankshaft ﬂange with

left side ﬁlled and right side not ﬁlled. Pinion shaft diameter located between ﬂanges of single “throw”

Failure in Steel Forging / 139

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and, in some cases, heating inside the tube.

However, success was limited since the axial

length of the heated zone was excessively long

to permit control of the incremental bending

strain and strain rate.

As developments continued, a European

company (Cojafax) had designed and manu-

factured a large bending machine to handle a

762 mm (30 in.) diameter stainless steel as the

input and to impart a 90



bend with controlled

thinning/thickening of the tube walls. An

external axially thin induction coil supplied the

heat and enabled control of the heated zone

while a 90



bend was being made.

The approach in the manufacturing process

was to exert an axial force in line with the

straight tube axis and then push the tube for a

low strain rate through the induction coil until

the heat input was sufﬁcient to cause material

displacements through the heated zone. In some

cases, cooling rings were added to control the

axial heated length, as shown in Fig. 7. This

process was used for a number of trials in an

attempt to achieve the goal of a 90



bend on a

762 mm (30 in.) diameter tube with 12.7 mm

(0.50 in.) initial wall thickness.

During these trials, process data were gener-

ated to show the variables that were used, such as

machine force, bending moment, axial tube

length inside the induction coil, tube axial speed,

and amount of heat supplied, all of which were

used to control the shift of the tube neutral axis

and thus the thinning and thickening of the tube

walls.

Optimizing the process variables for a 90



bend on the 762 mm (30 in.) stainless steel tube

resulted in minimal thinning of the outer wall, as

shown in Fig. 8. The thinning of the outer wall of

several tubes met the initial program goal of

18%.

Case Study 3: Spade Bit. The cutting end of

a proposed wood boring spade bit (AISI 1000

series) consists of a central web connected to

angular extensions from each side of the web, as

shown in Fig. 9. The forging process for the

spade bit forge was designed so that the ﬁnish

shape could be cold forged in a continuous

line, starting out with wire, straighten, clean,

lubricate, room-temperature forge, trip ﬂash,

and followed by heat treatment. The inherent

tooling design presented an opportunity to run a

series of numerical experiments using computer

Fig. 7 Closeup of tube bending assembly. (a) Induction coil and water ring. (b) Induction coil and partially heated tube and water ring

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simulations varying the forging direction, so that

the direct compressive force applied by the dies

to the central web of the workpiece would also

then have components of shear acting to cause

material displacements in the extensions.

Several die rotations were attempted to create

the longest wing extensions with a minimal

central web and the lowest forging force. The

fundamental idea behind this approach was that

steels in shear are weaker than steels in direct

compression. Strain-limiting criteria for cold

forging low-alloy steels are shown in Fig. 10.

Initially, analyses were made to determine the

strains to be encountered when forging a round

bar to a ﬂat central section with two attached

wings off at different angles. Shear stresses were

Fig. 9 (a) Spade bit drawing. (b) Photograph of each half

Fig. 8

Bent product tube with 90



bend and minimal

thinning at outer tube wall of ~18% of starting tube

wall thickness

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determined by Mohr’s three-dimensional (3-D)

analysis to quantify the stresses and strains

throughout the central web and extended wings.

Forming simulation software Antares (from

UES-Software, now defunct) was used for the

deformation analysis, followed by a shop trial.

Strains in the wings as well as in the center

section were calculated along with the com-

pressive force and stress on the center section,

before/after which simulations veriﬁed the ana-

lysis. Following a few shop trials supported by

simulations, an optimized process was deter-

mined that led to the minimum amount of energy

to be used to forge the spade bit center and

wings.

Case Study 4: Trim Tear. During the forging

sequence of a typical mechanic tool product

(AISI 4000 series), the process was running

satisfactorily, except that tearing occurred at the

ﬂash trimline and then propagated into the forg-

ing proper. Trimming of the ﬂash around the

perimeter of a ﬁnish forging has traditionally

been a very dependable operation by maintain-

ing the proper clearance between the punch and

the trim tool blades, so that the deformation zone

between the punch, forging with ﬂash, and the

trimmer sheared the ﬂash with no bending of the

ﬂash extension.

Excessive clearance encourages material

bending displacements, which lead to shearing

in the zone. Figure 11 depicts a schematic of the

effect of punch-to-die clearance on character-

istics of edges of holes produced by piercing a

low-carbon steel. In any event, the ﬂash shearing

is not clean and adjacent to the draft wall of the

forging. In some cases, the shear mechanism

initiates a crack on the edge of the forging and

ﬂash. The case that is being reported here is one

where cracks initiated and propagated into the

forging proper. Micrographs of crack formation

at the ﬂash edge are shown in Fig. 12.

During the search for a single or multiple

solutions, several conventional avenues of

attack were followed:

 Trimmer tool setup for proper clearance at

the trimline, seating of the forging inside the

trip plates, and proper contact of the work-

piece in the trim plate nest should be assured.

 Raw material condition did show some evi-

dence of banding and inclusions.

 Temperature in the workpiece may be too

high due to high speed of production and

resulting lower strength at trim.

 Measured clearance between punch and die

is reduced.

Fig. 10 Strain-limiting criteria for cold forging low-alloy steels

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