7/31/2019 2008400
1/4
J. Mater. Sci. Technol., Vol.25 No.5, 2009 607
Effect of Varying Carbon Content and Shot Peening upon
Fatigue Performance of Prealloyed Sintered Steels
R. Bidulsky1), M. Actis-Grande1), M. Kabatova2) and J.
Bidulska3)
1) Politecnico Torino, Alessandria Campus, V. T. Michel, 5,
Alessandria 15000, Italy2) Institute of Materials Research, SAS,
Watsonova 47, Kosice, 04011, Slovakia3) Technical University of
Kosice, Faculty of Metallurgy, Letna9, Kosice 04200, Slovakia
[Manuscript received July 21, 2008, in revised form January 8,
2009]
The aim of the work was to find out how the modification of
surface treatment and microstructures affect thefatigue
characteristics of the considered sintered materials. Two different
systems were prepared: as-sinteredand shot peened prealloyed
sintered (Astaloy CrL based) steels with addition of 0.5% and 0.7%
C. Sinteringwas carried out in laboratory tube furnace in an
atmosphere of pure gases 75%N2+25%H2. The sinteringtemperature was
1180C and sintering time was 60 min. Heating and cooling rates were
10C/min. Fatiguetests were carried out in symmetric plane bending
at stress ratio R=1 with frequency of about 24 Hz.The presented
experimental results showed that prealloyed water-atomised steels,
with surface modification,exhibit positive effects on the fatigue
failure resistance, and for that reason are suitable for
high-performanceapplications.
KEY WORDS: Sintered steels; Shot peening; Fatigue strength; S-N
curves
1. Introduction
In the present time a trend for PM (Powder Met-allurgy) steels
is an increasing use in highly stressedapplications such as gears,
where high fatigue per-formance is required[1]. Modification of
density andmicrostructure (pressing and sintering) can b e
suc-cessfully achieved for the fatigue limits of PM steelsat about
300 MPa in average[1], this means that sec-ondary operations are
generally necessary to reach thehigher fatigue properties. A
suitable modification offunctional surfaces may cause a sensitive
upgrade in
the properties of low alloyed steel. To achieve theproduction of
sintered parts with high-performanceapplications it is therefore
necessary to apply a fur-ther optimization of processing
conditions, accord-ing to Sonsino[2], such as service loading,
componentgeometry, and manufacturing. Shot peening is anindustrial
process often used to improve the compo-nent properties[35],
especially fatigue life and fatiguestrength.
In comparison with wrought steels[6,7], fatiguebehavior of
sintered steels is more complicatedand depends on some factors
related to sinteredmicrostructures[814]. Fatigue properties of
sintered
steels depend on plasticity and strength of microstruc-tures, as
well as porosity. Pores are generally inter-connected and this
implies that the sintering contacts,which actually bear the load in
the material (pores ofcourse cannot be load-bearing), are isolated
in thesecases, and description of the microstructure has tofocus on
the sintering contacts.
Relationship to strength is often expressed by ra-tio between
fatigue strength and tensile strength. Formany metals and their
alloys, the ratio between fa-tigue strength and tensile strength,
C/Rm, is closeto 0.38[9, 1517].
The aim of this work was to determine the ef-
fect of surface modification on the fatigue endurance.
Corresponding author. Prof.; Tel.: +39 0131 229232; Fax:+39 0131
229399; E-mail address:
[email protected];[email protected] (R. Bidulsky).
Fatigue strength was evaluated by Wohler curves forthe plane
bending fatigue tests on unnotched speci-mens of as-sintered and
shot peened alloys.
2. Experimental
Commercially pre-alloyed water-atomisedHoganas Fe-(Cr, Mo)
powder (Astaloy CrL contain-ing 1.5% Cr and 0.2% Mo) was used as
base material.The other commercial raw materials were CR 12graphite
powder and HW wax powder as lubricant.Powder mixtures were
homogenized in a Turbula
mixer. Two different specimen types, compacted at600 MPa to a
green density of 7.0 gcm3, wereprepared: dog-bone tensile (ISO
2740) and fatigue(ISO 3928) specimens. Formulation and
processingparameters of the tested alloys are presented in Table1.
Sintering was carried out in laboratory tube fur-nace in an
atmosphere of pure gases 75%N2+25%H2.The sintering temperature was
1180C and sinteringtime was 60 min. Heating and cooling rates
were10C/min. The surface modifications were carriedout on KP-1 fa.
G. Fischer laboratory testing ap-paratus. Parameters of testing
apparatus were steelgranulate S11 with dzD=0.6 mm, angle between
shot
stream and peened surface 90 deg., shot velocityv7000=70.98 ms1.
Specimens were tested in static
tensile test on a ZWICK 1387 machine at an extensionrate of 0.1
mm/min. Fatigue tests were carried out insymmetric plane bending at
stress ratio R=1, usingan SCHENCK PWON testing apparatus. The
maxi-mum number of cycles was 107. Batches of 15 speci-mens were
tested. The profile surface roughness, Ra,was measured by using a
tangent profilometer, Hom-mel Tester T1000. Light and scanning
microscopywere employed for microstructural evaluations.
3. Results and Discussion
The profile unevenness was expressed by the arith-metical mean
deviation of the profile surface rough-ness Ra. The profile surface
roughness is the average
7/31/2019 2008400
3/4
J. Mater. Sci. Technol., Vol.25 No.5, 2009 609
Fig. 4 The representative microstructures of Fe1.5Cr0.2Mo0.7C
materials
where, mcrsmaximum compressive residual stress,0.2yield
strength, srssurface residual stress, Rcoefficient, [R=0.997]
Based on these equations, mcrs and srs valueswere calculated and
given in Table 4.
Table 4 The calculated residual stresses of the in-vestigated
specimens
Material State mcrs/MPa srs/MPaFe1.5Cr0.2Mo0.5C shot-peened
406.52 412.28Fe1.5Cr0.2Mo0.7C shot-peened 445.22 437.53
If we consider the residual stresses produced byvarious
treatments, stress relaxation is a function ofthe eventual movement
of dislocation, and of the sta-bility of the metallurgical
structure. Any modifica-tion to the microstructure leads to a
modification ofthe distribution of residual stresses and according
to
literature [24] the parameters of shot peening affectsthe
surface residual stress as well as the surface cov-erage besides
properties of target materials. Residualstress relaxation should be
taken into account in fa-tigue life analysis. A quantitative
criterion which isable to take into consideration the residual
stress in-duced by shot peening on fatigue strength has not yetbeen
developed[25,26].
The relative increase of the surface stiffness inthe
investigated materials are presented due to thedensification
reduces the strain in the entire cross-section at a given load
level, representing by residualstress; and the elimination of the
detrimental effect of
pores open to the surface
[27]
. For high-dense sinteredsteels[10] the presence of isolated
pores, which act asstress concentrators is dominant; the degree of
the lo-cal strain increases depending on pore geometry, onthe
distance between the pores and on the local stressdirections and
their interactions. The original powderparticle surfaces may be
contaminated by impuritiesor inclusions[10,28]. Their presence can
give rise todeterioration of interfaces between the adjacent
par-ticles, thus nucleating the first microcracks, as wellas
providing easy paths for crack growth and crackpropagation.
The role of microstructure constituents is impor-tant as it has
been reported in literature [29, 30], theamount of martensite
basically controls the fatiguestrength of PM steels. As underlined
by Bergman[30],a pearlitic Fe-Cu-C steel has a bending fatigue
limitof 220 MPa at sintered density 7.1 g/cm3, while
Fig. 5 Fe1.5Cr0.2Mo0.5C, detail on a surface somethingclose to a
fully densified area affected by shotpeening
Fig. 6 Fe1.5Cr0.2Mo0.5C, initiation area and beginningof fatigue
crack propagation of as-sintered sample
martensitic PM steels have been reported to reachbending fatigue
limits of 380 MPa at the same den-sity. Consequently, a lot can be
gained by optimizingthe microstructure of PM steels. Results of
fatiguestrength are in agreement with literature data for
PMmaterials[9, 15, 2933], information on the fatigue prop-erties of
Fe-(Cr, Mo) steels with higher carbon contentis however limited.
The Fe-(Cr, Mo) steels exhibit anexceptionally high fatigue
strength, in the range from230 MPa to 250 MPa, which is a suitable
value forPM steels in general, and in particular at a densitylevel
of 6.97.0 g/cm3. Kabatova et al.[34] and Po-
lasik et al.[35] reported that the microstructural con-stituents
controlled subcritical fatigue crack growth:easiest along prior
particle boundaries and obstructedby high-strength regions. Thus
fractographic featuresincluding ductile failure through sinter
necks, cleav-age and interparticle fracture Figs. 5 and 6 are
shownin as generally observed in PM steels.
Finally, the modification of functional surfaces ofFe-(Cr, Mo)
steels increased fatigue failure resistanceby means of stronger
segments on the specimens sur-faces due to an almost fully
densified surface layerobtained by shot peening. This caused a
decreasein the crack propagation. Moreover since
compressivestresses are introduced into the surface and
subsurfacelayers by shot peening, fatigue cracks do not
easilyinitiate or propagate through an area under compres-sion, in
accordance with literature [21, 25, 26, 36 38].
7/31/2019 2008400
4/4
610 J. Mater. Sci. Technol., Vol.25 No.5, 2009
4. Conclusions
(1) The result showed an increase in fatiguestrength of about
34% due to surface hardening byshot peening processes. This was due
to the densi-fication of surface layer and the elimination of
thedetrimental effect of pores open to the surface.
(2) The gain in fatigue performance in Fe-(Cr, Mo)with 0.7C is
primarily due to a shift from mainly
pearlitic to mainly bainitic microstructure.(3) The influence of
surface modification on the
cyclic properties of PM steels still requires further
in-vestigations; the information on the fatigue propertiesof
Fe-(Cr, Mo) steels with higher carbon contents israther limited to
the present.
Acknowledgements
Authors thanks research project CNR-SAS andproject VEGA
2/6209/26. R. Bidulsky thanks the Po-litecnico di Torino, the
Regione Piemonte, and the CRTFoundation for co-funding by the
fellowship for visiting
professor.
REFERENCES
[1 ] Technical Trends: Met. Powder Rep., 2004, 59(5), 26.[2 ]
C.M. Sonsino: Powder Metall., 1990, 33(3), 235.[3 ] V. Bacova and
D. Draganovska: Mater. Sci., 2004,
40(1), 125.[4 ] M. Mihalikova, M. Bursak, J. Michel and K.
Ko-
valova: J. Met. Mater. Min., 2005, 15(2), 53.[5 ] D. Kniewald,
J. Brezinova and A. Guzanova: Com-
munications, 2004, 6(2), 37.[6 ] F. Novy, P. Palcek and M.
Chalup ova: Kovove Mater.,
2005, 43(6), 447.[7 ] F. Novy, M. Cincala, P. Kopas and O.
Bokuvka:
Mater. Sci. Eng., 2007, 462A, 189.[8 ] G.F. Bocchini: Rev.
Powder Metall. Phys. Ceram.,
1985, 2(4), 313.[9 ] R. Bidusky and M. Actis-Grande: High Temp.
Mater.
Process., 2008, 27(4), 249.[10] E. Dudrova, M. Kabatova and R.
Bidulsky: in Proc.
RoPM 2005, ed. R.L. Orban, Cluj-Napoca, TU 2005,Vol. 1, 101.
[11] V. Simkulet and M. Selecka: Powder Metall. Prog.,2006,
6(4), 156.
[12] M. Kabatova, E. Dudrova and A.S. Wronski: PowderMetall.,
2006, 49(4), 363.
[13] H. Danninger, U. Sonntag and B. Kuhnert: Pract.
Metallogr., 2002, 39(8), 414.
[14] G. Rosenberg: Metalurgija, 2001, 40(1), 13.[15] J.R. Moon:
Powder Metall. Prog., 2002, 2(2), 63.[16] D. Rodzinak and M.
Slesar: Powder Metall. Inter.,
1980, 12(3), 127.[17] R.C OBrien: Metal Powder Rep., 1988, 43,
744.[18] J. Bidulska, T. Kvackaj and V. Bodak: Hutnicke Listy,
2002, 57(1-3), 11.[19] J. Bidulska, T. Kvackaj, V. Bodak and R.
Rakuciak:
Acta Metallurgica Slovaca, 2002, 8, 211.[20] A. Guzanova and J.
Brezinova: J. Met. Mater. Min.,
2008, 18(2), 7.[21] S. Tekeli: Mater. Lett., 2002, 57(3),
604.[22] L. Ceniga: J. Therm. Stresses, 2008, 31(8), 728.[23] L.
Ceniga: J. Therm. Stresses, 2008, 31(9), 862.[24] Y.K. Gao, M. Yao
and J.K. Li: Metall. Mater. Trans.
A, 2002, 33A, 1775.[25] A.C. Batista, A.M. Dias, J.L. Lebrun,
J.C. Le Flour
and G. Inglebert: Fatigue Fract. Eng. Mater. Struct.,2000, 23,
217.
[26] M. Guagliano and L. Vergani: Eng. Fract. Mech.,2004, 71,
501.
[27] K. Kanno, Y. Takeda, A. Bergmark, L. Alzati, B.Lindqvist,
Y. Ueda, K. Kanda and A. Zandonati: inProc. PM World Congress 2004,
eds. H. Danninger
and R. Ratzi, EPMA, Shrewsbury, 2004, 3, 178.[28] H. Karlsson,
L. Nyborg and O. Bergman: in Proc. PM
World Congress 2004, Vienna, eds. H. Danninger andR. Ratzi,
EPMA, Shrewsbury, 2004, 3, 24.
[29] U. Engstrom, K. Lipp and C.M. Sonsino: in Proc. Ad-vances
in PM and Particulate Materials, ed. S. Kas-sam, MPIF, Princeton,
2001, 10, 1.
[30] O. Bergman and A. Bergmark: in Proc. Advancesin PM and
Particulate Materials, Las Vegas, eds. R.Lawcock and M. Wright,
MPIF, Princeton, 2003, 7,270.
[31] S.L. Sigl and P. Delarbre: in Proc. Advances in PMand
Particulate Materials, Las Vegas, eds. R. Lawcockand M. Wright,
MPIF, Princeton, 2003, 7/54.
[32] H. Danninger and C. Gierl: Int. J. Mater. Prod.Technol.,
2007, 28(3-4), 338.[33] R. Bidulsky, D. Rodzinak, A. Guzanova and
J. Brezi-
nova: Acta Metallurgica Slovaca, 2006, 12(1), 54.[34] M.
Kabatova, E. Dudrova, A.S. Wronski and R.
Bidulsky: Powder Metall. Prog., 2007, 7(1), 20.[35] G.
Piotrowski, X. Deng N. Chawla and K.S.
Narasimhan: in Proc. PM World Congress 2004,Vienna, eds. H.
Danninger and R. Ratzi, EPMA,Shrewsbury, 2004, 3, 152.
[36] P.S. Song and C.C. Wen: Eng. Fract. Mech., 1999,63,
295.
[37] Y.K. Gao, X.B. Li, Q.X. Yang and M. Yao: Mater.Lett., 2007,
61(1), 466.
[38] F. Novy, M. Chalupova, M. Cincala and O. Bokuvka:
Acta Mech. Slov., 2007, 11(4), 189.
