Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment
This paper describes the method of raising contact fatigue strength, designated for application in the manufacture of gears. Investigations were carried out on 41CrAlMo7 grade steel (closest equivalent: Nitralloy 135M) following quenching and tempering, nitriding and double shot-peening. Special...
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irk-123456789-1358172018-06-16T03:06:06Z Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment Nakonieczny, A. Monka, G. This paper describes the method of raising contact fatigue strength, designated for application in the manufacture of gears. Investigations were carried out on 41CrAlMo7 grade steel (closest equivalent: Nitralloy 135M) following quenching and tempering, nitriding and double shot-peening. Special emphasis should be placed on the combination of the nitriding and shot-peening technologies. Both nitriding and shot-peening have been significantly modified and differ from the classical processes. Nitriding was carried out in such a way as to prevent the formation on the surface of a compound layer, composed of nitrides ε and γ. Shot-peening was carried out as a double stage operation, employing a change of parameters. As the result of combining the modernized processes, a new technology was developed, allowing the attaining of a contact fatigue limit that is comparable to that obtained by carburizing or induction hardening. The paper describes the methodology and the experimental work, as well as the achieved values of contact fatigue strength. Other surface parameters are also given, such as roughness, hardness, state and distribution of residual stresses. Описано метод підвищення контактної втомної міцності сталі для виробництва механічних приводів. Досліджено сталь 41CrAlMo7 (еквівалент: Nitralloy 135M) після гартування та відпуску, азотування та подвійної дробиноструминної обробки. Особливу увагу приділено поєднанню азотування та дробиноструминної обробки. Азотували так, щоб попередити утворення на поверхні складного шару з нітридів ε та γ. Дробино-струминну обробку виконували у два етапи, змінюючи параметри. В результаті створено нову технологію, що дає можливість досягати межі контактної втоми, яку можна порівняти з отриманою за навуглецювання чи індукційного зміцнення. Описан метод повышения контактной усталостной прочности стали для производства механических приводов. Исследована сталь 41CrAlMo7 (эквивалент: Nitralloy 135M) после закалки и отпуска, азотирования и двойной дробеструйной обработки. Особенное внимание обращено на сочетание азотирования и дробеструйной обработки. Азотировали так, чтобы предупредить образование на поверхности сложного слоя из нитридов ε и γ. Дробеструйную обработку выполняли в два этапа, изменяя параметры. В результате создана новая технология, которая дает возможность достигать предела контактной усталости, которую можно сравнить с полученной при науглероживании или индукционном упрочнении. 2012 Article Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment / A. Nakonieczny, G. Monka // Фізико-хімічна механіка матеріалів. — 2012. — Т. 48, № 6. — С. 24-29. — Бібліогр.: 5 назв. — англ. 0430-6252 http://dspace.nbuv.gov.ua/handle/123456789/135817 en Фізико-хімічна механіка матеріалів Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
This paper describes the method of raising contact fatigue strength, designated for application
in the manufacture of gears. Investigations were carried out on 41CrAlMo7 grade
steel (closest equivalent: Nitralloy 135M) following quenching and tempering, nitriding
and double shot-peening. Special emphasis should be placed on the combination of the
nitriding and shot-peening technologies. Both nitriding and shot-peening have been significantly
modified and differ from the classical processes. Nitriding was carried out in such
a way as to prevent the formation on the surface of a compound layer, composed of nitrides
ε and γ. Shot-peening was carried out as a double stage operation, employing a change
of parameters. As the result of combining the modernized processes, a new technology
was developed, allowing the attaining of a contact fatigue limit that is comparable to that
obtained by carburizing or induction hardening. The paper describes the methodology and
the experimental work, as well as the achieved values of contact fatigue strength. Other
surface parameters are also given, such as roughness, hardness, state and distribution of
residual stresses. |
| format |
Article |
| author |
Nakonieczny, A. Monka, G. |
| spellingShingle |
Nakonieczny, A. Monka, G. Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment Фізико-хімічна механіка матеріалів |
| author_facet |
Nakonieczny, A. Monka, G. |
| author_sort |
Nakonieczny, A. |
| title |
Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment |
| title_short |
Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment |
| title_full |
Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment |
| title_fullStr |
Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment |
| title_full_unstemmed |
Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment |
| title_sort |
contact fatigue strength of 41cralmo7 grade steel by nitriding and shot-peening treatment |
| publisher |
Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| publishDate |
2012 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/135817 |
| citation_txt |
Contact fatigue strength of 41CrAlMo7 grade steel by nitriding and shot-peening treatment / A. Nakonieczny, G. Monka // Фізико-хімічна механіка матеріалів. — 2012. — Т. 48, № 6. — С. 24-29. — Бібліогр.: 5 назв. — англ. |
| series |
Фізико-хімічна механіка матеріалів |
| work_keys_str_mv |
AT nakoniecznya contactfatiguestrengthof41cralmo7gradesteelbynitridingandshotpeeningtreatment AT monkag contactfatiguestrengthof41cralmo7gradesteelbynitridingandshotpeeningtreatment |
| first_indexed |
2025-07-10T00:10:04Z |
| last_indexed |
2025-07-10T00:10:04Z |
| _version_ |
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| fulltext |
24
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2012. – ¹ 6. – Physicochemical Mechanics of Materials
CONTACT FATIGUE STRENGTH OF 41CrAlMo7 GRADE STEEL
BY NITRIDING AND SHOT-PEENING TREATMENT
A. NAKONIECZNY, G. MONKA
Institute of Precision Mechanics, Warsaw, Poland
This paper describes the method of raising contact fatigue strength, designated for appli-
cation in the manufacture of gears. Investigations were carried out on 41CrAlMo7 grade
steel (closest equivalent: Nitralloy 135M) following quenching and tempering, nitriding
and double shot-peening. Special emphasis should be placed on the combination of the
nitriding and shot-peening technologies. Both nitriding and shot-peening have been signi-
ficantly modified and differ from the classical processes. Nitriding was carried out in such
a way as to prevent the formation on the surface of a compound layer, composed of nitri-
des ε and γ. Shot-peening was carried out as a double stage operation, employing a change
of parameters. As the result of combining the modernized processes, a new technology
was developed, allowing the attaining of a contact fatigue limit that is comparable to that
obtained by carburizing or induction hardening. The paper describes the methodology and
the experimental work, as well as the achieved values of contact fatigue strength. Other
surface parameters are also given, such as roughness, hardness, state and distribution of
residual stresses.
Keywords: contact fatigue strength, hardness, roughness, residual stresses, double shot-
peening, nitriding and shot peening, thickness of nitrided layer.
The problems of mechanical properties of nitrided structural steels remain an open
issue despite many years of implementation of this process. This is connected both
with modification of nitriding, as well as with new possibilities of application, inclu-
ding nitriding, other surface treatments, such as, e.g. shot-peening of nitrided surfaces.
In this project, fatigue strength tests in static, dynamic and fatigue conditions were
conducted on 41CrAlMo7 grade steel specimens, subjected to quenching and tempe-
ring, controlled gas nitriding and finally strengthened by precision double stage shot-
peening. The steel nitriding process was carried out at an unconventionally low tempe-
rature (470°C), advantageous from the point of view of fatigue strength. The applica-
tion of double stage shot-peening, following precision nitriding constituted a novel,
original solution.
Experimental. Investigations of mechanical properties in static, dynamic and
fatigue conditions were carried out on specimens made of 41CrAlMo7 grade steel,
quenched and tempered to a hardness of 43…44 HRC.
The investigations of mechanical properties in the static tests for tensile and im-
pact strength were carried out on 41CrAlMo7 grade steel specimens in the following
conditions: quenched and tempered (QT); quenched, tempered and nitrided (QT+N);
quenched and tempered (QT), nitrided (N) and double shot-peened (2SP) – (QT+N+2SP).
Contact fatigue strength (TH) tests with measurements of and distribution of resi-
dual stresses were carried out on specimens in the following conditions: QT+N;
QT+N+2SP.
Corresponding author: A. NAKONIECZNY, e-mail: nakon@imp.edu.pl
25
Preparation of specimens for testing. The gas nitriding process. Nitriding was
carried out on specimens made of quenched and tempered 41CrAlMo7 grade structural
steel. The specimens prepared for this investigation were austenitized at 920°C, oil
quenched and tempered for 2 h at 490°C to a hardness of 43…44 HRC.
Nitriding was carried out in an industrial pit-type furnace, model Nx609, manu-
factured by Nitrex Metal Inc., with a retort of 600 mm dia × 900 mm height, equipped
with computer control, an ammonia dissociating equipment, a rapid cooling system, as
well as a neutralizer of effluent gases. The nitriding process was carried out at 470°C
for 16 h, in an atmosphere comprising NH3 + NH3 diss., with a nitriding potential ran-
ging from 4.99 to 1.74.
Shot-peening of specimens. Shot-peening of the specimens surface was carried
out by a pneumatic method, employing a special shot-peening stand, the PEEN-IMP,
for which patent No PL204718 was obtained in 2009. This equipment enables conti-
nuous control of the shot impingement energy within a wide range and reduces the
amount of shot needed to assure a continuous and uninterrupted flux of the air-shot
mixture down to 0.25 kg. Shot grain size and process parameters (air pressure and
exposure time) were experimentally determined in such a way as to assure that the Ra
roughness parameter is not significantly altered after shot-peening in comparison with
the initial measurement or that after nitriding. During shot peening, the specimen was
rotated while the shot was impinged perpendicularly to the specimen axis. Surfaces
which were not subjected to shot-peening, were masked.
Shot-peening process parameters for 41CrAlMo7 grade steel specimens, were the
following in two stages. First stage: shot: tungsten carbide of 0.9 mm diameter; air pres-
sure during operation p = 4 bar; exposure time t = 15 min; distance from a nozzle to a
peened surface, l = 400…420 mm; specimens rotated during shot-peening; nozzle dia-
meter 6.4 mm; shot-peening intensity, determined by A-type Almen strips, fA = 0.22 mm;
coverage of shot-peened surface 98%. Second stage: shot: tungsten carbide of 0.65 mm
diameter; air pressure during operation p = 4 bar; exposure time t = 18 min; distance
from a nozzle to a peened surface l = 400…420 mm; specimens rotated during shot-
peening; nozzle diameter 6.4 mm; shot-peening intensity, determined by A-type Almen
strips, fA = 0.23 mm; shot-peening intensity, determined by AI-type Almen strips after
the 1st and 2nd stage, fA = 0.26 mm; coverage of shot-peened surface 98%.
Methodology. The static tensile strength test. Investigations of strength properties
in the static tensile test at ambient temperature were carried out in accordance with
experimental procedure PB/2-1/LB-4, employing a hydraulic tensile machine, manu-
factured by Instron, model 8801, with a dynamometer range of 0 to 100 kN.
Contact fatigue strength (TH) test was carried out with the aid of the ULP equip-
ment. In it the cylindrical specimen of 8 mm dia and 40 mm length cooperates with two
pressure rollers (driving and driven) with a diameter of 150 mm and contact radius r =
= 10 mm, both made of LH15 (equiv. 52100) steel and a hardness of 60 HRC. The
specimen is loaded by means of a system of levers and an appropriate deflection of a
calibrated spring. The design of the equipment, as well as specimen dimensions allow
up to 5 measurements to be made on it. During the test, the specimens and rollers move
by rotation without slippage. A diagrammatic presentation of the specimen – roller
contact system is shown in Fig. 1.
The ULP-2 apparatus is equipped with an electronic safety system which causes
shutting off of the driving power source and unloading of the specimen – roller system
when vibrations caused by the spalling of the roller or specimen surface begin to rise.
The tests were carried out with a frequency f = 300 Hz and the assumed base NG =
= 2⋅107 cycles, in accordance with the specification [3] and procedure described in the
machine service manual.
26
Measurements of residual stresses were carried out by the X-ray with the aid of a
Rigaku diffractometer, model PSF-3M, employing the sin2ψ method and radiation
from a chromium source.
Fig. 1. Schematic of contact between
a specimen and rollers in contact fatigue
strength testing: 1 – specimen being tested;
2 – suspension of specimen; 3 – loading
roller (driven); 4 – loading roller (driving);
A, B – loading mechanism;
C, D – housing of loading rollers.
Measurements of surface roughness.
Measurements of the surface roughness
of specimens on measurement parts were
carried out with the aid of the Hommel
Tester Type T20, employing a LV50
measuring head, along elementary dis-
tances of 5 mm.
Metallurgical evaluations compri-
sed: surface hardness of specimens,
employing loads of 0.5, 1, 5 and 10 kg;
HV 0.5 microhardness profiles across the nitrided case; thickness of the nitride com-
pound layer; effective case depth at 400 HV.
Hardness measurements were carried out on a Zwick type 32.12.002 semi auto-
matic hardness tester, as well on a Vickers hardness tester manufactured by Frank com-
pany and with a serial number of 620. The range of test loads used in surface hardness
measurements and microhardness profiles allowed a characterization of specimen har-
dening, significant from the point of view of contact fatigue strength and wear resis-
tance.
The microstructure of nitrided cases was evaluated with the aid of a Zeiss Neo-
phot 2 optical microscope, employing magnifications of ×200 and ×500 on mounts
with sections perpendicular to the specimen surface, after etching them with Nital. The
optical microscope was also used to measure the thickness of the superficial compound
layer. The effective case depth at 400 HV, significant from the point of view of fatigue
strength, was determined from microhardness profiles across the section.
Impact strength testing by the Charpy method was carried out in accordance with
the PB/2-10/LB-4 procedure (edition 1 of 15.05.2002), employing an impact hammer
manufactured by Alpha, with the pendulum initial energy of 150 J.
Results of investigations and discussion. The results pertaining to strength pro-
perties, obtained in the static tensile tests QT, QT+N, as well as after QT+A+2SP have
been put together in Table 1.
In the case of specimens which were QT and QT+N, as well as QT+A+2SP, ten-
sile strength σuTS was similar, with a rising tendency on the part of the shot-peened
specimens. It was significant that the process of fracture of the nitrided case on
specimens after two-stage shot-peening became noticeable only after exceeding the σy
yield strength, while in the case of only nitrided ones, it was noticeable before reaching
the yield strength point.
Table also shows results of impact strength measurements by the Charpy method
on notched specimens, shaped like the letter U, after QT, QT+N, as well as after
QT+A+2SP. The impact strength results obtained after QT+A+2SP exhibit a drop in
27
impact strength by 26%, relative to the impact strength of only QT specimens. The
impact strength of specimens which were QT+A+2SP is slightly higher (35.4 J/cm2)
than that of specimens which were only QT+N. The latter was found to be 35.1 J/cm2.
The results of investigations of contact fatigue strength of the three groups of
specimens are given in the table. The contact fatigue limit TH for nitrided specimens
was determined at a level of 4130 MPa, while that for specimens which were nitrided
and shot-peened at 4453 MPa.
Results of investigations of mechanical properties of specimens
made of 41CrAlMo7 grade steel, following three modes of treatment
Treatment
Property
Quench and
temper (QT)
(43…44 HRC)
Quench and
temper + nitriding
(QT+N)
Quench and temper +
+ nitriding + two-stage
shot-peening (QT+N+2SP)
σy, MPa 1274.0 1251.0 1227.0
σuTS, MPa 1394.0 1329.0 1336.0
Impact strength
KCU, J/cm2 47.8 35.1 35.4
Contact fatigue
strength TH, MPa – 4130.0 4453.0
Roughness Ra
parameter, µm 0.235 0.392 0.523
Residual
compressive stresses
at surface σ, MPa
–526.2 –834.4 –1598.8
Surface hardness HV1 440.0 1281.0 1372.0
Fig. 2 shows distribution of residual stresses after the above described processes.
After quenching and tempering only, the mean value of residual compressive stresses
was –526.2 MPa, and after gas nitriding, the value of stresses increased, attaining
–834.4 MPa. Following the two-stage shot-peening process, there was a marked in-
crease in residual compressive stresses, reaching –1598.8 MPa. As a result of this shot-
peening operation, a decisively advantageous state of residual stresses was achieved in
the specimens down to a depth of 0.18 mm.
Fig. 2. Diagrams of average value of distribution of residual stresses, σRS,
of two 41CrAlMo7 steel samples after QT+N ( ) and QT+N+2SP ( )
in dependence of distance from surface, d.
28
Metallurgical evaluation. In the course of hardness testing by the Vickers me-
thod, employing test loads of 0.5, 1, 5 and 10 kg, the following surface hardness values
were obtained on the steel specimens:
– on quenched and tempered specimens: 448HV0.5, 440HV1, 442HV5 and
440HV10;
– on quenched, tempered and nitrided specimens: 1245.2HV0.5, 1291HV1,
1171HV5 and 1054.4 HV10;
– on quenched, tempered, nitrided and two-stage shot-peened specimens:
1351.2HV0.5, 1362.4HV1, 1248HV5 and 1142.8HV10.
The double shot-peening of quenched, tempered and nitrided specimens caused an
increase of surface hardness by approx. 90 HV units.
The hardness of the nitrided layer, measured at depth 50 µm from the surface, was
1098 HV0.5 and the core hardness was of 443 HV0.5. The effective hardened case
depth at 600, 500 and 400 HV0.5 was accordingly: 0.125 mm, 0.165 mm and 0.31 mm.
Microstructures obtained on polished and etched sections of metallographic specimens
(cylindrical after contact fatigue strength testing) are shown in Fig. 3.
Fig. 3. Microstructure of steel after quenching, tempering and nitriding (a)
and after quenching, tempering, nitriding and two-stage shot-peening (b).
In the course of metallographic evaluations of all specimens, no evidence was
observed of a compound nitride layer, which can be regarded as beneficial from the
point of view of contact fatigue strength. The microstructure of the specimen which
was subjected to shot-peening exhibits a zone of plastic deformation at the surface.
CONCLUSIONS
The results of measuring mechanical properties in the static, dynamic and fatigue
systems are good. Such good properties on this steel have not been attained until now
and, in consequence, allow the following conclusions to be drawn: In the nitriding
process carried out on 41CrAlMo7 grade steel, a nitrided layer of approx. 0.3 mm was
generated, free of the surface compound layer. As a result of two-stage shot-peening of
specimens made of that steel, residual compressive stresses were achieved at the surface,
with a mean value of –1599 MPa. This resulted in a beneficial distribution of residual
compressive stresses in specimens down to a depth of 0.18 mm. The two-stage shot-
peening of specimens prior quenched and tempered and nitrided caused a rise in sur-
face hardness by approx. 90 HV units. Two-stage shot-peening of specimens caused a
34% increase in the Ra surface roughness parameter, relative to that of specimens after
nitriding. Strength properties of 41CrAlMo7 grade steel, determined by a static tensile
test (σy and σuTS parameters) for specimens which were nitrided only and for those
two-stage shot-peened following nitriding ,were similar, with a rising tendency in the
case of the shot-peened pieces. It was important that the process of surface decohesion
of shot-peened specimens became apparent only after exceeding the σy yield strength,
29
while in the case of nitrided only specimens it occurred before reaching that point. The
impact strength of the steel after two-stage shot-peening was found to be slightly
higher than that of specimens after nitriding only. The value of contact fatigue limit
(TH) for shot-peened specimens rose by approx. 8 in comparison with that for nitrided
only specimens. The results achieved in this study are of major significance to the
development of a new method of treatment of gears and other components made of
41CrAlMo7 grade steel, with a view to increasing their reliability and service life.
РЕЗЮМЕ. Описано метод підвищення контактної втомної міцності сталі для вироб-
ництва механічних приводів. Досліджено сталь 41CrAlMo7 (еквівалент: Nitralloy 135M)
після гартування та відпуску, азотування та подвійної дробиноструминної обробки. Осо-
бливу увагу приділено поєднанню азотування та дробиноструминної обробки. Азотували
так, щоб попередити утворення на поверхні складного шару з нітридів ε та γ. Дробино-
струминну обробку виконували у два етапи, змінюючи параметри. В результаті створено
нову технологію, що дає можливість досягати межі контактної втоми, яку можна порівня-
ти з отриманою за навуглецювання чи індукційного зміцнення.
РЕЗЮМЕ. Описан метод повышения контактной усталостной прочности стали для
производства механических приводов. Исследована сталь 41CrAlMo7 (эквивалент: Nitra-
lloy 135M) после закалки и отпуска, азотирования и двойной дробеструйной обработки.
Особенное внимание обращено на сочетание азотирования и дробеструйной обработки.
Азотировали так, чтобы предупредить образование на поверхности сложного слоя из
нитридов ε и γ. Дробеструйную обработку выполняли в два этапа, изменяя параметры. В
результате создана новая технология, которая дает возможность достигать предела кон-
тактной усталости, которую можно сравнить с полученной при науглероживании или ин-
дукционном упрочнении.
1. PN-EN10002-1+AC1. Metale. Próba rozciągania. Metale; badania w temperaturze otocze-
nia. (Polish Standard. Metals. Tensile test. Metals; testing at ambient temperature). – 2004.
2. PN-EN-10045-1. Próba udarności sposobem Charpy’ego. Metoda badania. (Polish Standard.
Impact testing by the Charpy metod). – 1994.
3. PN-83/H-04324. Metale. Badanie na zmęczenie stykowe. (Polish Standard. Contact fatigue
testing).
4. Opracowanie i wdrożenie technologii kulowania kół zębatych / A. Nakonieczny i inni //
Development and implemenattion of shot-peening of gears. Project report. Sprawozdanie z
pracy IMP n0. 103.08.0158. – Warszawa, 1992.
5. Controlled gas nitriding of 40 HM (AISI 4140) and 38 HMJ (41CrAlMo7) – equiv. Nitralloy
135M steel grades with formation of nitrided cases with and without the surface compound
layer composed of iron nitrided / J. Michalski, J. Tacikowski, P. Wach, J. Ratajski // Proble-
my Esploatacji. – 2006. – № 2. – P. 43–51.
Received 24.10.2012
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