a study on friction and wear behaviour of carburized,
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A study on friction and wear behaviour of carburized,carbonitrided and borided AISI 1020 and 5115 steels
B. Selcuka,*, R. Ipekb, M.B. Karamsc
aEngineering Faculty, Department of Mechanical Engineering, Cumhuriyet University, 58140 Sivas, TurkeybHigh Vocational Training College, Cumhuriyet University, 58140 Sivas, Turkey
cEngineering Faculty, Department of Mechanical Engineering, Erciyes University, 38090 Kayseri, Turkey
Received 25 June 2001; received in revised form 4 March 2002; accepted 29 October 2002
Abstract
The friction and wear characteristics of AISI 1020 and 5115 steel surfaces improved by various thermochemical heat treatments such as
carburizing, carbonitriding andboronizingwere determined.Samples prepared fromthe testmaterialswere treatedat liquid andgases carburizing,
gases carbonitriding and solid boronizing mediums. The hardness distributions, microstructures and X-ray diffraction studies were performed.
The wear tests were carried out with pin-on-disc sample configurations and weight losses were determined as a function of sliding distance
and applied load. The friction behaviours of tested samples were also examined. Thus, the heat treating capacity of a simple steel such as AISI
1020 was determined and compared with other treated steel samples.
# 2002 Elsevier B.V. All rights reserved.
Keywords: Wear behaviour; AISI 1020 and 5115 steels; Carbonitriding; Boronizing
1. Introduction
Most of the engineering components subject to wear and
they should be selected as suitable for using purposes. For
effective use of steels in wear resistant applications, it is
essential to improve the surface properties of steels with the
most effective heat treatment.
AISI 1020 steel is a low carbon and cheap material widely
used in manufacturing of simple constructions and machine
elements. The surface properties of this steel are usually
improved by carburizing. On the other hand, AISI 5115
steel, which is an alloyed steel, is used for machine elements
such as cam shafts, gears and other transmission elements
after surface treated by carburizing or nitriding.
If the AISI 1020 steel surface can be further improved
by a thermochemical heat treatment such as carbonitriding
[1,2] or boronizing [36], it can be used as gear drives,
pump shafts, guide bars etc. [6]. The researches performed
on carbonitriding show that the tribological properties of
the low carbon steels can be improved by this treatment
[1,2,69]. The hardness of the steel including 0.200.25 C%
is increased from 720 to 940 HV by carbonitriding [10].
Boronizing is a thermochemical surface hardening process
in which boron atoms diffuse into a metal surface to form
hard boride layers [5]. The boronizing process is technically
well developed and widely used in industry to produceextremely hard and wear resistant surfaces. Since 1971,
development in the field of surface boronizing has made
great strides. There are a number of methods for carrying
out boronizing [11]. It should be noted that the trend in the
past to replace high-quality steels by inferior ones and to
imbue these with the corresponding properties through
boronizing has not become established practice. As with
other surface coating technologies, nowadays the typical
steels required for the application are chosen and are bor-
onized to provide them with extra protection. Large-scale
boronizing was first applied to drive gears for petrol-driven
engines in 1979. Meanwhile, similar helical gear airs have
long-since become standard fittings in vehicle and stationary
engines [6].
The aim of the present study is to characterize the friction
and wear behaviour of carburized, carbonitrided and boro-
nized low carbon and alloy steels (i.e. AISI 1020 and 5115
steels).
2. Experimental procedure
AISI 1020 and 5115 steels were selected as test material
because they are widely used in different area of machine
Journal of Materials Processing Technology 141 (2003) 189196
* Corresponding author.
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constructions with conventional surface treated such as car-
burizing and nitriding or induction hardening, respectively.
The test samples were machined from the above materials
in rectangular block shape with 20 mm 20mm 30 mm
dimensions and they were surface treated by various treat-
ment methods such as carburizing, carbonitriding and
boronizing. Treatment conditions are given in Table 1. Formetallographic examination, some specimens were treated
together with the test samples under same conditions. The
treated specimens were sectioned, mounted, polished and
etched with 2% nital echant for microstructure examination.
The surface phases and wear debris were also examined by
X-ray diffraction method.
The wear tests were carried out on a universal wear tester
with pin-on-disc configuration. The counterface materialwas boronized AISI 8640 steel in disc shape with 30 mm
Fig. 1. Hardness distribution of the test materials treated with various methods: (a) carburized (930 8C, 3 h) and carbonitrided (860 8C, 3.5 h) steels,
(b) boronized steels (950 8C, 1.5 h).
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inner, 75 mm outer diameters and 12 mm thickness. Its
maximum surface hardness is slightly above the hardest
sample hardness (i.e. 2910 HV). The frictional forces were
recorded and examined for frictional behaviours. Weight
loss of the all samples were determined as a function ofsliding distance and test loads. Weighing was performed
with an analytic balance which has sensitive of 0.1 mg.
Test durations were selected as 1030456090180
300420 and 540 s with the constant test speed of 3.6 m/s.
The test loads were arranged from 50 to 355 N with incre-
ments of 50 N.
3. Result and discussion
3.1. Properties of the layers
The present thermochemical surface heat treatments are
widely used to improve surface properties of ferrous mate-
rial for increasing wear, corrosion and fatigue performance
of the materials. To improve friction and wear properties,
generally, the hardness of the surface should be increased.
The hardness distributions of the test materials are plotted in
Fig. 1. It was not possible to accurately measure the micro-
hardness of the dark and bright needles in the boronizedsamples because of the fineness of the structure. It is clear
that the boride layer has the highest hardness with the
shallowest case depth. The microhardness data indicated
that there is a gradual decrease in hardness from the surface
of the boride layer towards the interface. It is fact that,
generally, the treatment durations and temperatures increase
the case depth of material treated depending on diffusion
process. On the other hand, the surface hardness depends on
compounds produced with iron or alloying elements in
diffusion zone. The low grade boron Fe2B phase which
can be confirmed by the following X-ray diffraction analysis
(Fig. 2) is especially desirable for industrial applications.The Fe2B phase observed in boride layer is the most hard
among the others, i.e. martensite, iron carbide or iron nitride
[e-Fe (N, C), g-Fe4 (N, C)]. Therefore the hardest layer is
Table 1
Heat treatment conditions applied to test steel
Material Surface treatment Temperature (8C) Durations (h) Quenching
temperature (8C)
Tempering
temperature (8C)
Hardening medium
AISI 1020 Carburizing 930, liquid 1.534 840 200, 1 h Oil at 50 8C, 30 min
Carbonitriding 860, gas (12 m3/h
endogas 0.8 m
3
/h ammonia)
3.5 840 200, 1 h Oil at 50 8C, 30 min
Boronizing 930950 1.53
AISI 5115 Carburizing 930, liquid 1.534 840 200, 1 h Oil at 50 8C, 30 min
Carbonitriding 860, gas (12 m3/h
endogas 0.8 m3/h ammonia)
3.5 840 200, 1 h Oil at 50 8C, 30 min
Boronizing 930950 1.53
AISI 8640 Boronizing (counterface) 950 1 850, 1 h Air
Fig. 2. X-ray diffraction pattern of boride layer of 5115 steel (Cu Ka-ray) and Fe2B phase recorded by X-ray diffraction method in wear debris (Cu Ka-ray).
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obtained on boronized surface (i.e. 2700 HV for 1020 and
2861 HV for 5115 steels).
The microstructures of the treated layer can be seen from
Fig. 3. The boride layer with 6080 mm depth exhibits
characteristic saw tooth morphologies similar to each other
obtained on the surface of AISI 1020 and 5115. Although the
carburized 1020 and 5115 steels have typical martensite
structure, carbonitrided steels have carbonitride compounds
caused by nitrogen in the nitrocarburizing atmosphere. In the
austenitic carbonitrided layer, the nitrogen austenite phase
decomposes in a discontinuous manner on a fine scale to
ferrite and Fe4N, with the precipitation occurring on both
grain boundaries [11].
Boronizing of steels is greatly influenced by the alloying
elements present, such as C, Cr, Mn etc. Carbon, the most
important alloying element, is not incorporated in tooth
boride layer. As a result, it is pushed along in front of the
layer into the substrate. The boron cementite, Fe3 (C, B) can
Fig. 3. Microstructure of treated layers: (a) boronized layer at 950 8C, for 1.5 h; (b) carbonitrided layer at 860 8C, for 3.5 h.
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replace upto 80% of the carbon by boron at 1000 8C but only
60% of it at 700 8C. Chromium replaces iron to form (Fe,
Cr)B and (Fe, Cr)2B. The incorporation of chromium into
the phases serves to increase the hardness considerably.
The higher the chromium content of matrix, the more the
boron prefers to diffuse along the grain boundaries. If the
material include manganese, the manganese concentrationsin the boride layer gradually increases and its greatest nearer
the matrix [36]. The test samples have low carbon. There-
fore, the properties of the boride layer are not affected
considerably from carbon content. However, AISI 5115
steel have l.l% Cr and 1.25% Mn. Therefore, the layer
thickness of this steel is shallower than that of 1020 steel.
The hardness produced on the 5115 steel is 820 HV with
core hardness of 460 HV after carbonitriding. On the other
hand, carburized 5115 steel have 795 HV hardness value
with core hardness of 375 HV. After carbonitriding, the fine-
bainite formation increases significantly the matrix hardness
and chromium provides nitrides increasing the hardness.
Therefore, carburized 5115 steel shows the lower hardness
value than that of carbonitrided one. But, carburized and
carbonitrided 5115 steels have approximately same hardness
distribution from 200 to 510 mm case depths.
The hardness of carbonitrided 1020 steel is 743 HV which
is above the hardness value of carburized 1020 steels. It is
observed that carbonitriding is a more effective process
than carburizing for low carbon and low alloyed steel.
Because 1020 steel provides the hardnesses of 616 HV
with carburizing. On the other hand, the surface hardness
of the carbonitrided 1020 steel is slightly lower than that of
carbonitrided 5115 steel (i.e. 77 HV). However, it is higher
127 HV value than carburized itself. It can be seen from
Fig. 1a that the hardness distribution of carbonitrided 1020
and carburized 5115 steels are friendly to each other with
some differences. This is a good result for practical applica-
tions and gives a benefit from economical point of view.
Although carburizing develops deeper case with lower sur-
face hardness, the case, sufficient to ensure good tribologicalproperties with higher surface hardness and shallower
depth, is produced by carbonitriding. The details of the
layer properties of carburized and carbonitrided present
steels are given elsewhere [1].
The surface parameter, Ra, measured on the samples is
close to etcher other and Ra is changed between 0.34 and
0.52 before wear test. After wear test, it is increased to
0.612.23 but untreated samples are not evaluated. Any
strong evidence is not found to relate between the surface
treatment methods and the surface roughness for the steel
type used in this study.
3.2. Friction and wear behaviours
Friction behaviours of the test material are plotted in
Fig. 4. It is known that the friction force or coefficient are
increased rapidly in running in period of friction. However,
this period is very short and severe wear are occurred at this
period. After this period, friction force decreases to nearly
constant value with small changing. Friction coefficients
obtained under the same conditions are arranged at 0.45
0.62 for carburized, 0.360.57 for carbonitrided and 0.36
0.62 for boronized AISI 1020 steels, and 0.350.70 for
carburized, 0.320.54 for carbonitrided and 0.120.24 for
Fig. 4. Coefficient of friction recorded as frictional force during the tests (sliding distance: 600 m, test speed: 3.6 m/s).
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Fig. 5. Variation of the weight loss with sliding distance (load: 100 N).
Fig. 6. Relationship between weight loss and test load (sliding distance: 1500 m).
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Fig. 7. Optical micrographs of worn surfaces frictioned for 300 s at 109 N loads: (a) boronized AISI 5115 steel (400), (b) boronized AISI 1020 steel
(400), (c) carbonitrided AISI 1020 steel (500).
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boronized AISI 5115 steels. As it can be understood that the
lowest friction coefficient is occurred with boronized 5115
steel. This steel is followed by carbonitrided 5115, carboni-
trided 1020, boronized or carburized 1020 and carbunized
5115 steels. These are evident that the frictional coefficient
depends systematically on surface hardness. However, it can
be mentioned as a general rule that the coefficient dependsstrongly on surface roughness and applied load [10,12].
It can beseen from Figs.5 and 6 that weight loss of the test
steels were increased with test time and load. Untreated
samples show the same character with treated samples by
sliding distance of 900 m, except for carbonitrided 1020, but
the weight loss of the samples are lower than that of
untreated ones. After sliding distance of 900 m, wear speed
is decreased for above steels. The weight loss of carboni-
trided 1020 and 5115 steels are lower than those of carbur-
ized ones and higher than those of boronized ones. This
situation is valid for weight loss with test load. Boronized
samples have a wear resistance of 510 times higher than
that of carburized samples (Figs. 5 and 6). Boronized steels
are extremely resistant to abrasion and adhesion on account
of their great hardness. The boride layers have low welding
tendency [6]. This property is of great consequence for
adhesive wear and explains why boronized samples show
higher wear resistance. The clear superiority of boride layers
over other treatments in terms of resistance to wear, parti-
cularly at high temperatures [3,6], can be seen from present
results.
The X-ray diffraction result is the same in Fig. 2 and any
oxide particles were not observed in wear debris. This is an
evident for boronized layer resistant to high friction tem-
peratures. On the other hand, unfortunately, boronized layersare cracked by heavy loads due to the its higher hardness
(Fig. 7a). This flaked layer particles change the wear mode
from adhesive to abrasive. It can be seen from Fig. 7b, there
are some wear track parallel to the rolling direction. This
wear tracks are shallower on the carbonitrided samples and
some oxide particles adhering to the surface can also be seen
on this layer.
4. Conclusions
The conclusions derived from the present study can be
summarized as follows:
1. Carbonitriding treatment is an effective surface harden-
ing method for low carbon and low alloy cold working
steels. Therefore, this treatment is more useful than
carburizing treatment for this steel subject to wear.
However, effective case depth, which is equal to depth of
500550 HV hardness values, is deeper with carburized
steel than carbonitrided one.
2. Carbonitrided low carbon steels may be used for the
constructions in which carburized alloyed steels areused. Thus, an economical benefit can be provided.
3. Boronizing is the most effective thermochemical treat-
ment for all ferro materials. But, the boronized layers
are shallow and brittle. Therefore, boronizing provides
excellent wear resistance under only light loads. This
treatment can be applied to all steel but particularly to
low alloyed steels.
4. Boronized low carbon steels can be used instead of
carburized or carbonitrided low alloyed steels to be
worked under light load frictions.
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