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AL- Taqani , Vol . 19 , No .2 , 2006

EFFECT OF SURFACE TREATMENT ON THE FATIGUE PERFORMANCE

OF AISI4340 STEEL+

Ahmed M. Mosa*

Abstract :

The effect of surface treatment on fatigue life of AISI4340 steel was studied. The

specimens were heat treated by oil quenching , then they were low tempered at 200C, medium

tempered at 320C and high tempered at 650C. The specimens were then tested. Best fatigue

performance was obtained from high tempered specimens. The surface of 650C tempered

specimens was then ground, polished, and chromium plated. It was found that best fatigue

performance was obtained from specimens that were oil quenched and tempered at 650C, then polished

AISI4340.

200C 320C 650C.

.

.

650C .

Introduction :

Fatigue is the progressive, localized and permanent structural change, that occurs in a

material subjected to repeated or fluctuating strains, at engineering stresses that have values

well below tensile strength of that material [ 1 ].

Fatigue may cause fracture after a sufficient number of fluctuations. The process of

fatigue consists of crack initiations, crack propagation to a critical size, then sudden fracture

of the remaining cross –

section [ 2 ].

The fatigue damage is caused by the simultaneous action of cyclic stress , tensile stress

and plastic deformation. The plastic deformation caused by repeated stress initiates the crack,

and the tensile stress leads to crack growth. So, the variations in mechanical properties,

chemical composition, microstructure and macro - structure, have reasonable effect on fatigue

resistance of the material [ 3 ] .

+Received on 8/12/2004 , Accepted on 22/6/2005

*Lecturer / Mosul Institute of Technology

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AL- Taqani , Vol . 19 , No .2 , 2006

Mosa studied the effect of surface finish of AISI 1005 steel specimens. It was concluded

that an improvement of 50% in fatigue life was obtained for ground specimens as compared

to turned specimens [ 4 ]. Ball states that high work – hardening capacity materials lead to

great improvement in wear and fatigue resistance [ 5 ]. Shephard et al, states that better

surface finish and deeper thermally stable compressive residual stresses, lead to superior

fatigue resistance [ 6 ]. Spice et al , studied the effect of vacuum carburizing , reheating to

refine grain size and gas-carburizing specimen. It was concluded that reheating gives the best

results [ 7 ].

Altenberger et al, found that deep rolling was quite effective in retarding the initiation of

fatigue crack for Ti-6Al-4V specimen [ 8 ]. Gean indicates that neither nugget porosity nor

weld size has any significant effects on fatigue properties of the weld [ 9 ].

Experimental Techniques

Ultra – high strength AISI4340, was studied for its wide use in aircraft and automotive

industries, such as : gears, shafts, connecting roads, crankpin….etc. Chemical composition

and mechanical properties are shown in Table 1 , for annealed, 200mm diameter bar asreceived from Al-Kindi company.

Table ( 1 ): Chemical Composition and Mechanical Properties of UHS 4340 Steel

Element ( wto% ) Mechanical properties

C Mn Si Cr Ni Mo

Yield

strength

Mpa

UTS

Mpaδ%

RA

%HB

0.39 0.6 0.3 0.8 1.4 0.4 427 745 22 50 220

Fatigue and tensile specimens were turned on copy machine lathe TOS-SN50B. Impact

specimens were machined on universal milling machine, Iwashita NK65.

Heat treatment procedure for the specimens, in volves electrical furnace Heraeus-KR170: heating to 870C , holding time 20 minutes, and quenching in mineral oil. Tempering

procedure was : low tempering at 200C for 80 minutes, medium tempering at 320C for 80minutes, and high tempering at 650C for 80 minutes. Normalizing was chosen as a

substitution for quenching and high tempering. Charpy impact tests were carried out on

impact testing machine Amster PW 30/15 K. Tensile tests were carried out on Tokyokoki

universal testing machine – RUF50. Rockwell hardness tests were carried out on universalhardness machine Wolpert-Diastar 2RC. Fatigue tests were carried out on Rotary-Bending

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AL- Taqani , Vol . 19 , No .2 , 2006

machine Schenck-WP 120 . All tests were conducted in Mechanical Testing Laboratory ,

Mosul Institute of Technology and in Mechanical Engineering Department, Mosul University.

High tempered specimens at 650C, gave the best results for fatigue performance as

shown in Table (2). Then the surface of high tempered specimens at 650C was treated as

follows: Group (1), grinding with silicon carbide emery paper grade 320 for 5 minutes. Group

(2), grinding, polishing by magnesia grade 4 micron for 5 minutes. Group (3) grinding then

Chromium electroplating. Then fatigue tests were conducted on these specimens. Best results

for fatigue performance were for polished specimens.

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AL- Taqani , Vol . 19 , No .2 , 2006

Table ( 2 ): Mechanical Properties of UHS 4340 Steel after Heat – Treatment

Heat-

treatment

Yield

strength

(Y.S)Mpa

UTS

Mpa

Elong

%

RA

%Hardness Y.S.

UTS

Impact

strength

Ak J/cm2

Endurance

limit

Mpa

Enduranc

e limit

UTS

NormalizedFrom 870C

861 1279 12 36HRC36

(HB363)0.67 62 345 0.27

Oil Q. 850C&

Tempered at

200C 1680 1980 11 39

HRC53

(HB520)0.85 25 550 0.28

Oil Q. 850C&

Tempered at

320C

1620 1760 12 44HRC49

(HB490)0.92 17.5 500 0.284

Oil Q. 850C

&tempered at

650C860 1020 20 60

HRC31

(HB290)0.84 125 465 0.447

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AL- Taqani , Vol . 19 , No .2 , 2006

Discussion :

1. Effect of Heat Treatment on Mechanical Properties of AISI4340 Steel . visualizing mechanical test results from heat-treated 4340 steel specimens, tabulated in

table ( 2 ), it can be noticed that specimens tempered at 650C have better ductility, impact

strength , and endurance limit to tensile strength ratio. Also ,when comparing normalized and650C tempered specimens ( Fig 1 and Table 2 ); although normalized specimens have better

hardness and tensile strength, endurance limit and impact strength are low. The cause can be

expressed as follows : 650C tempered specimens, since they have higher ductility, then they

have greater ability for continuous permanent shape variation at surface irregularities , leading

to redistribution of surface stresses, so decreasing strain-hardening would issue at the surface

as a consequence of shape variation , and this would delay cracks propagation , and hence

higher endurance limit.

2. Effect of Surface Hardness on Endurance Limit.From Fig 2, it can be observed that higher hardness values increase endurance limit for

tempered AISI4340 steel specimens for high-cycle regime ( >104 ). But endurance limit was

impaired for low-cycle regime (<104), and high repeated stresses, because ductility is the

more important factor. This can be explained as lower ductility and toughness, with higher

hardness values, will lead to smaller strain-hardening values and redistribution of surface

stresses, but higher local stresses will encourage crack propagation at low cycle regime[8].

3. Effect of Mechanical Properties on Endurance Limit.Fig.3 , shows that endurance limit for AISI4340 steel specimen , is directly

proportional to hardness, and tensile strength , but inversely proportional to impact strengthfor tempered specimens.

Stresses are concentrated at internal and external discontinuities. The values of thesestresses are high, and are hard to be estimated. If the value of these stresses is higher than

cohesive resistance of the steel particles, then microscopic crack may be initiated, and spread

at a specific speed depending on repeated stress value and number of cycles [10]. Also,

thermal stresses may be induced in steel due to mechanical machining and heat treatment. The

stress values depend on : drastic quenching, tempering temperature, and type of machining.

Fatigue failure is caused by combination of internal stresses effect with stresses caused byexternal loads [5]. The effect of surface roughness and internal stresses is readily seen from

Fig.(4).Polished AISI4340 steel specimens have greater endurance limit, because rough surfaces are

stress concentration areas, thus leading to decreasing endurance limit. The ground, chromium

plated specimens have lower endurance limits due to higher tensile internal stresses caused byelectroplating [6].

Conclusions :

1. Endurance limit is directly proportional to hardness and tensile strength, but inversely

proportional to impact strength.

2. 650C tempered AISI4340 steel specimens have optimized results of ductility ,

toughness , impact strength , and endurance limit to tensile strength ratio.

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AL- Taqani , Vol . 19 , No .2 , 2006

3. Polished AISI4340 steel specimens have greater endurance limit compared to groundand chromium plated specimens.

Fig (1): S-N Curve for AISI 4340 Steel at Various Heat Treatment

Fig (2): S-N Curve for AISI 4340 Steel at Various Brinell Hardness

0

100

200

300

400

500

600

700

800

1 10 100 1000

N,cycle

S t r e s s S , M P

a

Normalized

Tempered 200CTempere 320C

Tempered 650C

*104

0

100

200

300

400

500

600

700

800

900

1000

1 10 100 1000

N, cycle

S t r e

s s S , M P

a

HB 520

HB 290

HB 363

HB 490

*103

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AL- Taqani , Vol . 19 , No .2 , 2006

0

200

400

600

800

1000

1200

1 10 100 1000

N,cycle

S t r e s s S , M P

a

Ground

Ground polished

Chromium plated

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

14001500

1600

1700

1800

1900

2000

0 100 200 300 400 500 600

Endurance Limit, MPa

I . S , J

o u l / c m 2 - U . S , M P

a - B r i n e l l

H a r d n e s s

ultimat strength (U.S.)

Brinell hardness

impact strenght *4(I.S.)

Fig(3):Endurance Limit Relationships for AISI4340 steel

Fig(4): Effect of Surface Roughness on Endurance Limit on

*104

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Reference

1- Briggs A., Acoustic Microscopy Clarendon Press , Oxford pp139 1992 .2- Wilks T.R and Cavallaro G.P, Fatigue of Surface - Hardening Gears Technomic

Publishing Co., USA , pp331-346 1995 .3- Cartz L., Design Consideration and Materials Selction ASM International , vol20 ,

pp1329. 1997

4-. Mossa A.M “ The Influence of Surface Finish on The Fatigue Life of Low Carbons Steel

”Mosul University AL-Rafidain Engineering. Vol.1, No.1 pp42-48, 1999

5- Ball A. On The Importance of Work Hardening in the Design of Wear Resistance

Materials , Wear,91 pp201-207 1993 .6-. Shepard. M.S, Prevey. P.S., and Jayaraman. N “ Effects of Surface Treatment on Fretting

Fatigue Performance of Ti-6Al-4V ”, National Turbine Engineer HCF Conference ,April 14-16, Monterey,CA 2003 .

7- Spice. J.J, and Matlock .D.K., “Optimized Carburized Steel Fatigue Performance as

Assessed With Gear and Modified Burger Fatigue Tests”, SAE Technical Power Series

No. 2002-01-1003 , 2002 .

8- Altenberger I. .Nalia, R.K, and Noster V. “On the Fatigue Behavior and Associated of

Residual Stresses in Deep-Rolled and Laser Shock Peened Ti,6AL-4V Alloys at

Ambient and Elevated Temperatures” Material Science Research int-vol.1 2001 .

9-A. Gear, West Gat S.A., and Kucza J.C. “Static and Fatigue Behavior of Spot-Welding

5182 -0 Al Alloy Sheet ” Cleveland College Engineering , March 1999

10-W.D.Callister, Material Science and Engineering edition John Wiley Inc, 5th pp209-213

2001 .

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