AISI 4140 - Induction Hardening

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					                                                                Materials and Design 24 (2003) 25–30

 The effects of induction hardening on wear properties of AISI 4140 steel
                          in dry sliding conditions
                                                     Y. Totik1, R. Sadeler, H. Altun, M. Gavgali
                                       Department of Mechanical Engineering, Ataturk University, 25240, Erzurum, Turkey

                                                           Received 2 July 2002; accepted 11 October 2002


   Wear behaviour of induction hardened AISI 4140 steel was evaluated under dry sliding conditions. Specimens were induction
hardened at 1000 Hz for 6, 10, 14, 18, 27 s, respectively, in the inductor which was a three-turn coil with a coupling distance of
2.8 mm. Normalised and induction hardened specimens were fully characterised before and after the wear testing using hardness,
profilometer, scanning electron microscopy and X-ray diffraction. The wear tests using a pin-on-disc machine showed that the
induction hardening treatments improved the wear behaviour of AISI 4140 steel specimens compared to normalised AISI 4140
steel as a result of residual stresses and hardened surfaces. The wear coefficients in normalised specimens are greater than that in
the induction hardened samples. The lowest coefficient of the friction was obtained in specimens induction-hardened at 875 8C
for 27 s.
   2002 Elsevier Science Ltd. All rights reserved.

Keywords: Wear; Induction hardening; Dry sliding; AISI 4140 steel; Pin-on-disc

1. Introduction                                                                        depends of hardening depth and the magnitude and
                                                                                       distribution of residual compressive stress in the surface
   Many mechanical parts, such as shafts, gears, springs,                              layer w4–6x.
etc. are subjected to surface treatments, before the                                      By most recent estimates, improved attention to fric-
delivering, in order to improve wear behaviour. The                                    tion and wear would save developed countries up to
effectiveness of these treatments depends both on sur-                                 1.6% of their gross national product, or over $100
face materials properties modification and on the intro-                               billion annually in the USA alone w7x. The magnitude
duction of residual stresses. Among these treatments,                                  of the financial loss associated with friction and wear
induction hardening is one of the most widely employed                                 arises from the fact that entire mechanical systems, be
to improve component durability w1–3x. It determines                                   they automobiles, are frequently scrapped whenever only
in the workpiece a tough core with tensile residual                                    a few of their parts are badly worn. In the case of an
stresses and a hard surface layer with compressive                                     automobile, the energy consumed in its manufacture is
stresses, which have proved to be very effective in                                    equivalent to that consumed in 100 000 miles of opera-
extending the component fatigue life and wear                                          tion w8x.
resistance.                                                                               The object of the present work is to study the role of
   Induction surface hardened low alloyed medium car-                                  the induction hardening on the wear behaviour of the
bon steels are widely used for critical automotive and                                 AISI 4140 steel specimens when a harder material, such
machine applications which require high wear resistance.                               as WC-%6Co, is used as a counterpart.
Wear resistance behaviour of induction hardened parts                                  2. Experimental procedure
  *Corresponding author. Tel.: q90-442-2312381; fax: q90-442-                          2.1. Specimens preparation and hardness measurement
  E-mail address: (Y. Totik).                                       Since the aim of the induction hardening is to improve
  1                                                                 the surface hardness, the carbon content in steel to be

0261-3069/03/$ - see front matter                2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 1 - 3 0 6 9 Ž 0 2 . 0 0 0 9 9 - 7
26                                             Y. Totik et al. / Materials and Design 24 (2003) 25–30

Table 1
Chemical composition of AISI 4140 steel

                     C                 Si                 Mn              P               S             Cr         Mo             Ni
AISI 4140            0.3954            0.225              0.785           0.0187          0.0070        0.847      0.166          0.0795

Table 2
Induction hardening parameters

Specimens                 Frequency                  Heating time                  Temperature           Cooling           Microstructure
                          (Hz)                       (s)                           (8C)
1                         –                          –                             –                     –                 Normalised
2                         1000                        6                            815                   Water             Normalised
3                         1000                       10                            830                   Water             Normalised
4                         1000                       14                            845                   Water             Normalised
5                         1000                       18                            860                   Water             Normalised
6                         1000                       27                            875                   Water             Normalised

                                                                              important process parameters that control both the hard-
                                                                              ness value and hardness depth. These parameters can be
                                                                              easily controlled during the induction hardening. In the
                                                                              tests, the specimens were induction hardened for differ-
                                                                              ent times at a frequency with 1000 Hz, and later
                                                                              quenched in water. The inductor was a three-turn coil
                                                                              with a coupling distance of 2.8 mm. Table 2 shows the
Fig. 1. Schematic representation of the pin-on disc test configuration.       selected induction hardening parameters.
                                                                                 The hardness of the AISI 4140 steel with normalised
Table 3                                                                       and induction hardening were measured as Vickers
The test conditions from pin-on-disc tribo test                               hardness using a PC Controlled Buehler–Omnimet tes-
                                                                              ter. A standard microhardness tester, equipped with a
Parameters                                                 Experimental
                                                           conditions         Vickers indenter and a 500-g indention load was used
                                                                              for surface and subsurface hardness measurement.
Applied load (N)                                           10
Velocity (msy1)                                            0.08
Scar diameter (mm)                                         10                 2.2. Wear tests
Environment                                                Air
Temperature (8C)                                           ;20
Duration (s)                                               900                   The wear tests of the AISI 4140 steels with normali-
Roughness (Ra, mm)                                         0.16               sed and induction hardening used in this work were
Test ball diameter (mm)                                    5 mm
                                                                              performed in Teer POD2-Pin-on-disc tribometer, using
                                                                              the test configuration shown in Fig. 1. WC-%6Co ball
induction hardened should be between 0.3 and 0.5%.                            was used with a diameter of 5 mm as counterpart. After
Thus, the AISI 4140 (42CrMo4) steel used in the tests                         samples were cleaned by ultrasound in acetone for 5
was normalised prior to the induction hardening. The                          min, a pair of pins and disc were inserted into the
investigation was performed on cylinder specimens                             machine. A particular load was applied and the rotation
made of AISI 4140 (42CrMo4) steel, which is especially                        disc started. Experiments were carried out at a linear
adaptable for machining and where surface hardness is                         speed of 0.08 msy1 and at a load of 10 N. The
desirable. The chemical composition is indicated in                           atmosphere used was air with a relative humidity of
Table 1.                                                                      ;45% and temperature of ;20 8C. Experimental data
   Specimens were prepared normalised (hardness of                            were recorded continuously during the wear tests: coef-
307 HV). The cylinders (length of 120 mm) were                                ficient of friction and time. The wear friction test
machined to a diameter of 20 mm prior to induction                            conditions are given in Table 3.
hardening.                                                                       After 2000 cycles of rotation in the case of 0.08
   Induction hardening is a process where steel is hard-                      msy1 speed, which resulted in a distance of 50 m, the
ened by means of induction heating and later quenching                        experiment was stopped, wear specimens were removed
in water. The hardening temperature and time are most                         and the worm surfaces were examined using profilo-
                                             Y. Totik et al. / Materials and Design 24 (2003) 25–30                                            27

Fig. 2. (a) SEM image of the wear track after 3000 cycles; (b) the surface profile, the wear track and their superposition used in calculating the
wear volume.

                                                                            2.3. Microstructural analysis

                                                                               Microstructures of the AISI 4140 steels with normal-
                                                                            ised and induction hardening were characterised using a
                                                                            Nikkon Epithot 105 optic microscope, a Jeol 6400
                                                                            Scanning Electron Microscope (SEM), a Rigaku Dmax
                                                                            2200 X-Ray diffractometer with a CuKa radiation

                                                                            3. Results and discussions

                                                                               Surface hardness, as well as subsurface hardness,
Fig. 3. Microhardness distribution for induction hardening treatment.       distributions were obtained with induction hardening is
                                                                            shown in Fig. 3. The main factor that controls the
                                                                            hardness was the heating time as the power output was
metry. The arithmetic mean roughness value (Ra) of the                      the same at all the processes at induction hardened
surfaces was evaluated by using a Mitutuyo profilometer.                    specimens with a frequency of 1000 Hz. The hardness
   The wear coefficient (k) was calculated using the                        values increased from surface of the specimen toward
equation for wear which takes the form ksVyNL. Where                        its centre, depending on the increase of the induction
V is wear volume, N is the load and L is the sliding                        temperature and time, as shown in Fig. 3. The hardness
distance. To calculate the wear volume, the profiles were                   depths increased more considerably at longer induction
recorded by a Mitutuyo profilometer before and after                        times, as the times supplied were those requested for
induction hardening. Then, from the superimposed pro-                       carbide to be dissolved.
files, the wear volume was calculated, as shown in Fig.                        Because the wear resistance of a material is related
2.                                                                          to its microstructure and because changes in microstruc-

                Fig. 4. Microstructures of the AISI 4140 steel with (a) normalised and (b) induction hardening 27 s at 875 8C.
28                                           Y. Totik et al. / Materials and Design 24 (2003) 25–30

                            Fig. 5. XRD patterns of the AISI 4140 steels with normalised and induction hardening.

ture may take place during the wear process, it seems                       transient period, the friction coefficient reaches a maxi-
that in wear research emphasis should be placed on                          mum value. This value is the result of the formation of
microstructure w9–11x. The microstructures of AISI 4140                     large quantities of debris generated during wear. This
steel normalised and induction hardened for 27 s contain                    debris had formed from fragmentation under high con-
ferrite, perlite and metallic carbides, as shown in Fig. 4.                 tact pressure at the normalised steel surface. The highest
In addition, the induction hardening also caused an                         friction coefficient was found as 0.76 for normalised
increase at the amounts of the carbides and the marten-                     sample and this value is maintained until the end of the
site in steel.                                                              test. In the case of low sliding speed (0.08 msy1) the
   The X-ray patterns of the phases existing in the                         coefficient of friction has a relatively constant value
structure are presented in Fig. 5. It was found that the                    during the test and lies between 0.76 and 0.41. It is
density of the phases with increasing induction time                        obvious that increasing the induction time caused an
increased significantly. Increasing the density of the                      decrease in the coefficient of friction, attaining the
phases is expected to improve the wear behaviour of                         minimum value at the highest time of 27 s. The
AISI 4140 steel.                                                            reduction of the friction coefficient with increasing the
   Fig. 6 reveals the coefficient of friction for the AISI                  induction time is expected to be a result of increasing
4140 steels with normalised and induction hardening.                        the hardness and particularly compressive residual
Results on the frictional behaviour of the mating pair                      stresses.
substrate-ball are also included for comparison purposes.                      Fig. 7 shows the wear coefficients (rates) for the
For the normalised samples, as seen in Fig. 6, the steady                   induction times under 10 N load and at sliding speed of
state is achieved after approximately 1000 cycles. In the                   0.08 msy1. It can be seen that changing the induction

 Fig. 6. The variations in the coefficients of the friction with the revolution for the AISI 4140 steels with normalised and induction hardening.
                                           Y. Totik et al. / Materials and Design 24 (2003) 25–30                                   29

                                                                         es both the width of the wear track and the amount of
                                                                         the debris removed. The wear phenomenon occurred
                                                                         uniformly at the induction hardened specimens (Fig.
                                                                         8c,d). It can be seen that the worn surfaces are compar-
                                                                         atively smooth with little plowing and some very small

                                                                         4. Conclusions

                                                                           The following conclusions are established for AISI
                                                                         4140 steel in induction hardening conditions:
        Fig. 7. Wear coefficients vs. the induction times.               ● It was observed that the induction hardening treatment
                                                                           improved the wear behaviour by depending on the
time also causes change in the contact conditions con-                     process parameters selected in this work. This was
firmed by the coefficient of friction. It can be seen that                 attributed to process parameters of the induction
induction treatments cause a slight reduction in wear                      hardening influencing the residual stress-state of hard-
coefficients as compared to normalised specimens.                          ened parts, to a great extent due to different temper-
   Studies have been made of the relationship between                      ature distributions in the specimens during heating.
typical microstructures and wear behaviour of materials                    These compressive residual stresses were expected to
during sliding wear. But micromechanisms of the wear                       delay the formations of the microcracks generated as
of various microstructures are still not clear because the                 a result of wear loading on the wear surface. On the
dynamic changes taking place during sliding have not                       other hand, the microstructure was altered with the
been satisfactorily considered w12x. Surface topography                    induction hardening treatment, and it was observed
analysis of the worn surface was performed in a SEM                        that the martensites in the structure dispersed finely
to determine the influence of the induction hardening                      with the increasing of the induction time.
on the wear properties. Worn surfaces of the AISI 4140                   ● The induction hardening caused a decrease in the
steels with normalised and induction hardening are                         coefficient of the friction after attaining the steady
shown in Fig. 8. A severe adhesive wear was observed                       state, exceeding the transient periods depending on
on the normalised specimens as revealed in Fig. 8a,b.                      induction time. The lowest coefficient of the friction
Consequently, the amount of the debris removed from                        was obtained in specimens induction hardened at 875
the normalised specimens toward the sides of the wear                      8C for 27 s.
track was much more than that in induction hardening                     ● The wear coefficients in normalised specimens are
specimens. Besides, the increasing induction time reduc-                   greater than that in the induction hardened specimens.

        Fig. 8. SEM images of the wear traces of AISI 4140 steel (a) and (b) normalised; (c) and (d) induction hardened for 27 s.
30                                             Y. Totik et al. / Materials and Design 24 (2003) 25–30

  On the other hand, the wear coefficients are decreased                       w3x Melander M. Theoretical and experimental study of station-
  with increasing of the induction time.                                           ary and progressive induction hardening. J Heat Treating
                                                                                   1985;2:145 –165.
● The increasing induction time reduces both the width                         w4x Xu D-H, Kuang Z-B. A study on the distribution of residual
  of the wear track and the amount of the debris                                   stress due to surface induction hardening. J Eng Materials Tech
  removed and the wear phenomenon occurred uni-                                    (Trans ASME) 1996;118:571 –575.
  formly at the induction hardened specimens. Besides,                         w5x Semiatin SL, Stutz DE. Induction Heat Treatment of Steel
  it was found that the density of the phases increased                            ASM. Carnes Publication Services, 1986.
                                                                               w6x Kristoffersen H, Vomacka P. Influence of process parameters
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  Increasing the density of the phases is expected to                              2001;22:637 –644.
  improve the wear behaviour.                                                  w7x Jost HP. Tribology: origin and future. Wear 1990;136:1.
                                                                               w8x Ludema KC. Friction, Wear and Lubrication: A Textbook in
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                                                                                   ASME, 1977. p. 41 –46.
     w1x Thelning KE. Steel and its Heat Treatments. London: Butter-          w11x Wang Y, Lei T. Wear behaviour steel 1080 with different
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