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
‫ﺍﻟﻤﺴﺘﺨﻠﺹ‬
‫ ﺘﻡ ﺘﻘﺴﻴﺔ ﺍﻟﻌﻴﻨﺎﺕ ﺒﺎﻟﺯﻴﺕ ﺜﻡ ﺍﺠﺭﺍﺀ ﺍﻟﻤﻌﺎﻟﺠﺔ‬.AISI4340 ‫ﺘﻤﺕ ﺩﺭﺍﺴﺔ ﺘﺄﺜﻴﺭ ﻤﻌﺎﻟﺠﺔ ﺍﻟﺴﻁﺢ ﺒﻌﻤﺭ ﺍﻟﻜﻼل ﻟﻠﺼﻠﺏ‬
‫ ﻭﺠـﺩ ﺒﻌـﺩ ﺃﺠـﺭﺍﺀ‬.650C ‫ ﻭﺍﻟﻤﺭﺍﺠﻌﺔ ﺍﻟﻤﺭﺘﻔﻌﺔ ﻋﻨـﺩ‬320C ‫ ﻭﺍﻟﻤﺭﺍﺠﻌﺔ ﺍﻟﻤﺘﻭﺴﻁﺔ ﻋﻨﺩ‬، 200C ‫ﺍﻟﻤﻨﺨﻔﻀﺔ ﻋﻨﺩ‬
‫ ﺘﻤﺕ ﻤﻌﺎﻟﺠﺔ ﺴـﻁﺢ ﺍﻟﻌﻴﻨـﺎﺕ ﺫﺍﺕ ﺍﻟﻤﺭﺍﺠﻌـﺔ‬.‫ﺍﻻﺨﺘﺒﺎﺭﺍﺕ ﺍﻥ ﺍﻓﻀل ﺍﺩﺍﺀ ﻟﻠﻜﻼل ﻜﺎﻥ ﻟﻠﻌﻴﻨﺎﺕ ﺫﺍﺕ ﺍﻟﻤﺭﺍﺠﻌﺔ ﺍﻟﻤﺭﺘﻔﻌﺔ‬
‫ ﻭﺠﺩ ﺍﻥ ﺍﻓﻀل ﺍﺩﺍﺀ ﻟﻠﻜﻼل ﻜـﺎﻥ ﻟﻠﻌﻴﻨـﺎﺕ ﺫﺍﺕ ﺍﻟﻤﺭﺍﺠﻌـﺔ‬.‫ ﻭﺍﻟﻁﻼﺀ ﺍﻟﻜﻬﺭﺒﺎﺌﻲ ﺒﺎﻟﻜﺭﻭﻡ‬، ‫ﺍﻟﻤﺭﺘﻔﻌﺔ ﺒﺎﻟﺘﺠﻠﻴﺦ ﻭﺍﻟﺘﻠﻤﻴﻊ‬
.‫ ﻭﺍﻟﺘﻲ ﺘﻡ ﺘﻠﻤﻴﻌﻬﺎ‬650C ‫ﺍﻟﻤﺭﺘﻔﻌﺔ ﻋﻨﺩ‬
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 as
received from Al-Kindi company.
Table ( 1 ): Chemical Composition and Mechanical Properties of UHS 4340 Steel
Element ( wto% )
C
Mn
Si
Cr
Ni
Mo
0.39
0.6
0.3
0.8
1.4
0.4
Mechanical properties
Yield
RA
UTS
δ%
HB
strength
Mpa
%
Mpa
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 80
minutes, 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 universal
hardness 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
Y.S.
UTS
Impact
strength
Ak J/cm2
Endurance
limit
Mpa
Enduranc
e limit
UTS
HRC36
(HB363)
0.67
62
345
0.27
39
HRC53
(HB520)
0.85
25
550
0.28
12
44
HRC49
(HB490)
0.92
17.5
500
0.284
20
60
HRC31
(HB290)
0.84
125
465
0.447
Heattreatment
Yield
strength
(Y.S)Mpa
UTS
Mpa
Elong
%
RA
%
Hardness
Normalized
From 870C
861
1279
12
36
1680
1980
11
1620
1760
860
1020
Oil Q. 850C&
Tempered at
200C
Oil Q. 850C&
Tempered at
320C
Oil Q. 850C
&tempered at
650C
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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 and
650C 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 strength
for tempered specimens.
Stresses are concentrated at internal and external discontinuities. The values of these
stresses 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 by
external 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 by
electroplating [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 ground
and chromium plated specimens.
800
Normalized
Tempered 200C
Tempere 320C
Tempered 650C
700
Stress S,M Pa
600
500
400
300
200
100
0
1
10
100
1000
4
N,cycle *10
Fig (1): S-N Curve for AISI 4340 Steel at Various Heat Treatment
1000
HB 520
900
Stress S,M Pa
HB 290
800
HB 363
700
HB 490
600
500
400
300
200
100
0
1
10
100
N, cycle
*103
Fig (2): S-N Curve for AISI 4340 Steel at Various Brinell Hardness
١٢٠
1000
I.S,Joul/cm2 - U.S,M Pa - Brinell Hardness
AL- Taqani , Vol . 19 , No .2 , 2006
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
ultimat strength (U.S.)
Brinell hardness
impact strenght *4(I.S.)
0
100
200
300
400
500
600
Endurance Limit, MPa
Fig(3):Endurance Limit Relationships for AISI4340 steel
1200
Ground
Ground polished
Stress S,M Pa
1000
Chromium plated
800
600
400
200
0
1
10
100
N,cycle
*104
Fig(4): Effect of Surface Roughness on Endurance Limit on
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1000
AL- Taqani , Vol . 19 , No .2 , 2006
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