Technical trends

Controlled shot peening is a

cold working process in which

the surface of a metal compo-

nent is bombarded with many

thousand small, spherical media com-

monly referred to as shot.

The result of the cold working process

is the compression of the outer surface

and an increase in the fatigue properties

of the metal component. As each piece of

shot strikes the metal component a small

indentation results on the surface. For the

indentation to be created, the surface ten-

sion must give. Below each indentation

created the metal grains have been com-

pressed. As the compressed metal tries to

restore the surface indention to the origi-

nal shape, a hemisphere of cold-worked

metal - highly stressed in compression -

is produced. The overlapping indentations

create a uniform layer of residual

compressive stress on the surface of the

component.

The compressive stresses induced by

controlled shot peening squeeze the grain

boundaries of the shot-peened area

together. It is well known that fatigue

cracks will not initiate in nor will they

propagate into or through an area of a

metal component that is in a net com-

pressed state. Since nearly all fatigue fail-

ures originate at or near the surface of a

metal component, a uniform compressive

layer on the surface of the component

will significantly increase its life. It's

worth noting that the magnitude of resid-

ual compressive stress induced by con-

trolled shot peening is at least as great as

half the tensile strength of the material

being shot peened.

Most long-term fatigue failures can be

traced back to one root cause, tensile

stress. Tensile stresses can result from

externally applied loads or from residual

stresses from manufacturing processes

such as machining, grinding or welding.

Tensile stresses attempt to pull, stretch or

tear the surface apart and most often lead

to crack initiation. By compressing the

surface of a metal component through

controlled shot peening, the initiation of

fatigue cracks is significantly delayed and

in many cases eliminated.

Pressed and sintered ferrous powder

materials are in increasingly higher

demand as the powder metallurgy

industry has grown into applications

involving more highly stressed compo-

nents. The following example illustrates

the improvements in a specific powder

metallurgy material that was measured

before and after controlled shot peening.

Ancorsteel 1000 B with 2% copper and

0.9% graphite had an endurance limit of

240 MPa when tested without controlled

shot peening. The test specimens were

then shot peened and tested. The con-

trolled shot peened specimens had an

endurance limit of 280 MPa, an increase

of 16%1.

Additional testing has shown that by

optimising the controlled shot peening

parameters, the endurance limit of sin-

tered steel powder metal alloys can be

raised by 22 per cent and the fatigue life

increased by a factor of ten2. Automotive

components such as gears, sprockets and

connecting rods are excellent candidates

for powder metallurgy and controlled

shot peening. Unpublished test results

from a major manufacturer of powder

metallurgy components for the automo-

tive industry reveal that the controlled

shot peened engine component's fatigue

strength increased 31 per cent over the un-

peened component. While controlled

shot peening is most effective on higher

hardness and higher density powder met-

allurgy components, lower hardness and

lower density components also exhibit

substantial benefits from controlled shot

peening.

PM gears, like their wrought steel

counterparts, respond well to the intro-

duction of compressive stresses by shot

peening. It is important that the density

of the powder metallurgy component be

high enough to retain the induced resid-

ual stress. Simply put, this means that :

"The higher the density of the

material/component the greater the

'Shotgun' methodcan improve PMsteel fatigue lifeShot peening is a mature process widely usedin the mechanical engineering industry toimprove the fatigue surface characteristics ofmetal components. A team from MIC France hasdemonstrated that endurance limits of sinteredsteel PM alloys can be raised by more than 20per cent and fatigue life by a factor of 10…

48 MPR July/August 2004 0026-0657/04 ©2004 Elsevier Ltd. All rights reserved.

Figure 1. Tests show that shot peening has more impact in terms ofraising the strength of PM parts than in wrought equivalents.

amount of residual compressive stress

that will be retained."

Previous information indicated that

densities below 7.4 g/cm3 did not respond

well because the pores become fracture

initiation points [3]. However, recent test-

ing of test specimens with densities of 7.0

g/cm3 has resulted in marked improve-

ments in hardness and in crush.

As with wrought metals, the same doc-

trine holds true for PM parts: "The hard-

er the material at the surface, the higher

the residual stress induced by shot peen-

ing and therefore the higher the resistance

to failure." However, to achieve the high-

est residual stress at the greatest depth

beneath the surface, the part must be

peened with a shot that is as least as hard

as the surface being peened [4].

If the surface of a gear is induction

hardened, carburised or carbonitrided,

the most effective peening must be per-

formed with hard shot in the 58-62 HRC

range. Standard hardness shot in the

45-52 HRC range will deform itself on

contact rather than indent the surface of

a very hard part and will produce about

half the residual compressive stress than

when using 58-62 HRC shot. Figure 2

shows a comparison of a powdered metal

surface shot peened with different shot

hardnesses.

It should be noted that both curves do

not cross the neutral axis due to residual

compression created from the carbonitrid-

ing process prior to shot peening. Figure 2

shows significantly more residual com-

pression when using a harder shot media.

Using the "softer" shot resulted in negligi-

ble fatigue life increases. This was most

likely due to the fact that little additional

residual compression was imparted than

was already present from the carbonitrid-

ing process [2]. Figure 3 shows additional

fatigue results of the sintering process

with carbonitriding and shot peening

(with 58-62 HRC Shot).

Figure 3 shows a series of fatigue life

curves and the effect of post sintering

treatments on sintered steel Fe-2%, Cu-

2.5%, Ni- under constant and variable

loading. The specimens were tested in

fully reversed bending and consisted

of machined bars with a radius acting

as the stress concentration (stress factor,

K = 1.49).

A description of the curves is as follows:

A-1: As Sintered, Constant Loading

A-2: As Sintered, Variable Loading

B-1: Sintered & Shot Peened, Constant

Loading

B-2: Sintered & Shot Peened, Variable

Loading

C-1: Sintered, Carbonitrided & Shot

Peened, Constant Loading

C-2: Sintered & Shot Peened, Variable

Loading

As part of a project sponsored by

the German Federal Ministry of

Education and Research, powder metal

alloys were tested for suitability in

metal-powder.net July/August 2004 MPR 49

Residual stress plot from carbonitrided &shot peened powered metal

(Fe - 1.5%; density = 7.4 g/cm3)

15.0

0

-15.0

-30.0

-45.0

-60.0

-75.00.050 0.1 0.15 0.2

Depth (mm)

Res

idua

l str

ess

(Kg/

mm

2 )

45-52 HRc shot 58-62 HRC shot

Figure 2. A comparison of the effect on a PM component’s surface of different shot.

Technical trends

30

45

60.0

151.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09

A-1 A-2

App

lied

stre

ss -

kg/

mm

2

B-1 B-2 C-1 C-2

Fatigue life curvesFe-2%, Cu-2.5%, Ni (7.6 g/cm3)

Number of cycles

Figure 3. The differeing effects of post-sintering treatment.

THIS article was taken from a paper

entitled Shot peening applications on

PM components given by the author,

Jean Yves Thieuleux of MIC France,

at EuroPM 2003 held in Valencia

under the auspices of the European

Powder Metallurgy Association

The author

DEVELOPED in conjunction with

ENSAM Technical University in

France, Peenstresssm makes it possible

to determine the optimum shot peen-

ing parameters for a given set of mate-

rials, hardnesses and geometries. The

computer produces a residual stress

model that can be compared to the

applied stress [9]. The computer pro-

gramme presently contains a fairly

extensive list of materials, and can be

modified or increased as needed.

A software guide

50 MPR July/August 2004 metal-powder.net

Technical trends

gearing applications. A MSP4.0 Mo-

0.1Nb powdered metal gear was tested

against a reference-machined 20Mn Cr5

case hardened steel. Tooth root load car-

rying capacity tests produced the follow-

ing fatigue strength results with wrought

as 100 per cent or baseline.

Baseline: Un-peened 20Mn Cr5 (wrought

steel): 100 per cent;

Un-peened MSP4.0 Mo- 0.1Nb (powder

metal): 82 per cent;

Shot Peened MSP4.0 Mo- 0.1Nb (powder

metal): 109 per cent;

The test proved that the un-peened

powdered metal had fatigue strength

18 per cent less than the wrought

steel gear material. The shot peened

powdered metal's fatigue strength was

9 per cent higher than the reference

wrought steel [5].

The following case study is an excel-

lent illustration of the value of shot peen-

ing high density PM parts with hardened

shot. In this study, gears of Fe-Mo alloy

at a density of 7.5 g/cm3 were case hard-

ened to 60 HRC. The shot hardness was

63 HRC and the specified intensity was

0.016" A. The following results are the

endurance limit (at 3 million cycles).

Bending fatigue tests consisted of single

tooth loading [6].

Sintered & Case Hardened (baseline):

900 MPa

Sintered, Case Hardened & Ground:

770 MPa

Sintered, Case Hardened & Shot Peened:

1030 MPa

Shot peening improved the baseline

condition by 130 MPa (14.6 per cent).

This is typical of the benefits of the resid-

ual compressive stress from shot peening.

The residual compressive stress lowers the

applied tensile stresses created from the

bending fatigue test.

It is worth noting that the endurance

limit of the gear tooth roots that were

ground decreased 130 MPa (~ 14 per cent)

from the baseline condition. It is general-

ly believed that a smoother surface will

respond better under fatigue conditions as

potential crack initiation sites are consid-

ered eliminated. What is sometimes

neglected is the fact that grinding under

many circumstances can introduce resid-

ual tensile stresses if not properly con-

trolled. Residual tensile stresses will act to

accelerate a fatigue failure as they are

additive to applied tensile stresses. It is

believed this is what contributed to the

decrease in fatigue life.

Crush and impact Improvement

There is still very little data available

on crush and impact improvement, but

whereas shot peening has little or no

effect on wrought metals, it appears to

have a remarkably significant effect on

both the crush and impact resistance of

powder metal gears. There are unpub-

lished tests results that indicate impact

strength improvements of 70 per cent are

attainable by shot peening PM gears after

heat-treating. Tests showed that un-

peened gear teeth broke at 400 mm in the

drop test. Similar peened gears required a

drop from 530 mm before breaking. One

extensive crush test to evaluate the effect

of shot peening on spur gears showed an

average improvement of 44 per cent as

compared to the un-peened control

group. A separate test by yet another

powder metallurgy component manufac-

turer on an automotive sprocket with a

density of 7.0 g/cm3 resulted in a 6.6 per

cent increase in hardness and a 23.3 per

cent increase in crush over the un-peened

control group. These numbers are

Figure 4. Even components for lighter electrical hand tools can benefitfrom the shot peening process.

Figure 5. PM is moving into areas of high-stressed compo-nents. Shot peening can help significantly.

page 52

52 MPR July/August 2004 metal-powder.net

Technical trends

impressive given the lower density of the

sprocket.

Powder Forging is an ideal PM process

for shot peening. Powder forging produces

the same density as both wrought billet

forgings and sand castings (7.82 - 7.84

g/cm3) but actually has a higher ultimate

tensile strength of 930-1900 MPa than the

other two processes.

The results of the Ford Motor

Company test work concluded that con-

trolled shot peening of a C-0.5%, Cu-2%,

powder metal rod with a 7.82 g/cm3 den-

sity increased the fatigue strength by 27

per cent, over the un-peened rods at 90

per cent reliability levels. The fatigue test

specimens resulted in an increased of

fatigue strength from 262 MPa for the

non-peened specimens to a maximum of

407 MPa for the optimised peened speci-

men, a 55 per cent improvement [7].

Chemically Assisted Surface

Enhancement (CASE) is another surface

improvement method that has significance

in relation to high-density gears. This is a

two-stage process developed by the Metal

Improvement Company. The first stage

requires that gear teeth, including the

pressure faces, be shot peened in a normal

manner.

Mirror finish

This is followed by the second stage: a

fast micro honing of the surface to almost

a mirror finish but with a negative Rsk

that retains lubricant. This is an ideal sur-

face to promote increased micro-pitting

fatigue life of gear teeth [8].

Overall controlled shot peening has

shown itself to be an economic and

efficient process that the powder metal-

lurgy industry can use to increase the

fatigue failure resistance of various mate-

rials and components such as gears and

connecting rods.

Through sample testing and applica-

tion of the information gained, compo-

nents designed for PM manufacture com-

bined with controlled shot peening can be

lighter, smaller and much less costly to

produce.

Taking advantage of the residual com-

pressive stress induced by controlled shot

peening now allows conventional wrought

steel components to be replaced by

PM/shot-peened components and suffer

no sacrifice in performance, yet substan-

tially reduce cost. Further research in this

area may lead eventually to the publica-

tion of a design specification for PM/shot

peened components.

1. O'Brian: Impact and Fatigue

Characterisation of selected Ferrous PM

Materials, Annual Powder Metallurgy

Conference, Dallas, TX. May 1987.

2. Sonsino C M, Schlieper G, Huppmann

Wi : "How to improve fatigue properties

of sintered steels by combined mechani-

cal and thermal surface treatments."

Modem Developments in Powder

Metallurgy. Volumes 15-17. 1985.

3. Pennington J N: "Exploring powder

forging.' Modern Metals. February 1996.

4. Eckersley J S: "Gearing Up for Higher

Loads." Impact: Review of Shot Peening

Technology, Metal Improvement

Company.

5. Link, Kotthoff; Suitability of High

Density Powder Metal Gears for Gear

Applications; Gear Technology,

January/February 2001.

6. Strehl R: "Tooth root fatigue of a

high density gear." WZL TH Aachen.

7. Chernenkoff R A, Mocarski S, and

Yeager D A: "Increased Fatigue Strength

of Powder-Forged Connecting Rods by

Optimised Shot Peening" SAE Internat-

ional Congress and Exposition 1995

8. Eckersley J S: "The C.A.S.E. for

Superfinishing." Impact: Review of Shot

Peening Technology. Metal Improvement

Company.

9. Le Guernic Y: "Peenstresssm Software

Selects Shot Peening Parameters."

Impact: Review of Shot Peening

Technology, Metal Improvement

Company.

References

Figure 6. Manufacturers can retain the versatility of PM, whilecoming very close to wrought strength.

Figure 7. Shot peening can help win the race for a PM part tobe an equipment manufacturer’s preferred choice.

page 50