Wear, 24 (1973) 323-335 Q Elsevier Sequoia S.A., Lausanne
J. M. GEORGES*.
323 - Printed
in The Netherlands
and M. BARON**
Lab~rat~~re de Te~hn~log~~des Surfaces, ~cole Centrale (Received
2, 1972; in revised form January
de Lyon, 69 130 &a&
A pin and disk study of carbonitrided steel revealed different types of wear. The best results were with carbonitrided material rubbing on carbonitrided material. The effect of load and the role of brittleness on wear has been studied. Differences in structure, hardness and work-hardening of the various surface layers as well as debris in the interface caused the difference in wear behaviour of the layers.
Apparent area of contact of the pin. Tangential speed of sliding. Temperature. Average radius of the rubbing mark on the disk. Perpendicular co-ordinate to the contact area plane. Original hardness of the pin. Hardness after wearing. Variation of hardness. Rate of wear of the pin. Loss of height of the pin. Loss of average height of the track on the disk. Covered distance by the pin on the disk. Number of revolutions. Rate of wear of the disk. Samples weight before wearing. Initial weight of the pin. Initial weight of the disk. Samples weight after wearing with particles. Pin weight after wearing with particles. Disk weight after wearing with particles. Weight of worn samples without particles. * Maitre de Conference associe, Ecole Centrale de Lyon, 69 130 Ecully (France). ** Ingenieur Societe Thermi-Lyon, Ingtnieur des Services Techniques, 195, rue de Gerland, (7e) (France).
42 Kw,, @ W
c-1 C,(Z) ci
ci A% u P H
J. M. GEORGES.
Weight of worn pin without particles. Weight of worn disk without particles. Weight of particles. Weight lost by samples after wearing. Particle transformation coefficient. Particle transformation coefficient as a function of the worn layer. Superficial layer of the pin. Superficial layer of the disk. Volume lost by the pin. Adhesive wear coefficient. Applied load. Hardness of the softer body.
Carbonitriding is a thermochemical surface treatment. Simultaneous diffusion ofcarbon and nitrogen into the surface forms various depths of martensite, according to the temperature used ‘.‘. Although it is an industrial process which superficially hardens parts to reduce mechanical wear, the mechanism of wear has not been studied. Also little information is available on the wear of materials, the properties of which vary with depth from the surface3.4. 1. APPARATUS
A “pin and disk” type wear machine described elsewhere5*6 was used. A conical pin (Figs. l(a) and l(b)) ru b s on a rotating cylindrical plate. The apparent area of contact (A) stays approximately constant throughout the test and is 1+ 0.1 mm’. The contact area moves on the plate with a speed (V) producing a circular track of 24 mm in diameter (Fig. l(b)). In all tests, the constant tangential sliding speed was 5 cm/s. The plate was placed in a cup, to gather particles. The temperature and humidity were maintained constant at (II= 21” & 1°C) and (40+ 5’(;,) respectively. Samples were made of ahoy steel AFNOR 14 NC 12 of chemical composition: c=0.12-0.170~,
Mn=0.35 to 0.600,
Ni=2.5 to 3.%,
Cr = 0.6 to 0.9”;;
Si = 0.1 to 0.90,,, S + P = 0.035Ob maximum
They were carbonitrided industrially. The temperature cycle is given in Fig. 2. The chemical composition of the furnace gases was: co 17.5”” N, 33.5”,,
co, 0.3”;, CH, 7.01,,
H, 33.6”); NH, %I”;,
The microstructure is shown in Fig. 3. Figure 4 shows the results of analyses for carbon and nitrogen and X-ray diffraction analyses for retained austenite on successive surface layers obtained by machining. Samples were polished with abrasive paper (400 grade) to an average roughness of 1 to 2 jlrn c.1.a. before test. Using a Leitz mi~rodurimeter with a load
Fig. 1. Wear specimens. Fig 2. Thermal
Fig. 3. Metallographic
(a), side view; (b), plan. cycle.
of 100 g, micro-hardness explorations were carried out either by sectioning techniques or following electrolytic polishing away of successive surface layers. Mechanically worked surfaces by rubbing and wearing undergo changes in mechanical properties. Meyer and Tabor’ have shown that workhardening increases hardness as measured by Brine11 or Vickers test. This property has been used to
.I. Cl. GI:ORGFS.
05““-500. *, ‘\
01oLooi Fig. 4. Hardness.
‘--~______* . .
types of structure
study work-hardening of rubbing parts by comparing at every stage of wear the new hardness H(Z) with the initial hardness H,,(Z). It seemed interesting to evaluate these variations and specially to study the parameter: AH=
p and d stand for pin and disk respectively. Different methods have been used for wear measurement. The weight loss of samples was measured and the standard deviation in the course of 30 weighings was under 3.5/100 mg. The size loss of samples was measured according to the method of Rabinowicz’. The apparent area of contact of the pin on the disk was constant during the test and measurements of weight lost and height of pins correlated. The wear rate of the pin K, was evaluated by K, = AZ,,‘Ax, where AZ, is the loss of height (Z,) of the pin when contact area (A) covers the distance Ax, on the disk.
As, = 2nRN where R is the average radius of the wearing track on the disk, N is the number of revolutions. In a same manner, the rate of wear of the disk is defined as
K, = AZ,/Ax, whereAZ, is the average loss of height of the track on the disk when a geometrical point of its area has passed a distance Ax, from the pin. Wear debris was gathered after each test run by immersing the apparatus in ethyl alcohol and filtering. Slow sliding speeds do not allow the particles to fly off into the atmosphere; practically all the particles remain on the samples. Particle analysis by Coulter counter’ gave results as a function of sphere diameters, spheres having the same volume as the particles.
Knowing precisely the weight of samples before wearing. w, = &,+
where W,, is the weight of the pin before wearing, k&0 is the weight of the plate before wearing. Knowing the weight of samples after wearing when they are covered with particles emitted during rubbing, w, = &x+&i Knowing the weight of samples after wearing when they are clear of particles. w, = wp,+ The
weight of particle EJ= w,-
on the plate:
is compared to the loss of weight of parts through wearing. w=
The particle transformation
coefficient is then
c, = w/w 3. NATURE
OF THE PROBLEM
The study of cross sections of carbonitrided steel reveals the presence of three different zones (Fig. 4) superficial layers (a), a transition zone (b) and a core (c). Theoretically it is necessary to consider the specimens studied as being composed of an important number (Ci) of layers and for a complete study, it is necessary to study the behaviour of all layers of the pin on all layers of the plate which involves a great number of tests. For simplicity, we have considered the couples given in Table I. When studying the surface layer of the plate, Fig. 4, the test is interrupted when the depth of wear on the plate reaches 5 pm. Tests carried out are presented in Table 11. Tests represented by ( x ) show the influence of the treatment on contact surfaces. The study of surface layer/surface layer behaviour (A) shows the surface TABLE
Surface layer of the pin on surface layer of the plate. Core heart of the pin on surface layer of the plate. Core heart of the pin on core heart of the plate.
.I. bl. GEORGES.
layer behaviour in wearing from surface layers to the core of the sample. The influence of load (.) was considered in the case of surface layer/surface layer wear. 4. INFLUENCE
ON WEAR OF SPECIMENS
Weight losses of the pin and of the plate by a rubbing distance A.yd are given in Figs. 5(a), (b) and (c). S’ize 1asses of pins AZ,,, for every case, confirm the results (Fig. 6) taken between two “wear runs” between reference marks of Figs. 5(a), (b) and (c). Volumetric analyses of particles are given in Figs. 7(a), (b) and (c). Table III gives the experimental results. For couples of the same hardness, wear rates of the pin and of the disk are the same (core heart/core heart or surface layer/surface layer) (Table III). For the same test conditions. wear behaviour of similar pins differ according to the plate condition. The core heart/core heart type wear is by fractures producing big particles, while the core heart/surface layer contact is adhesive. Wear by fracture (Fig. 8) is characterized by the formation of big particles (bigger than 150 pm). Adhesive wear makes small junctions. Thus these
Fig. 5. Loss of weight AW of the pin or the disk as a function of the distance Ax,(P=60 daN). (a) Surface layer/surface layer contact; (b) core heart/core heart contact: (c) core heart/core heart contact.
Fig. 6. Loss of height heart/surface layer;-(c)
of the pin AZ, as a function core heart/core heart.
of Ax,. (a)
Fig. 7. Volume distribution (%V) of gathered particles as a function of the diameter of equivalent sphere (@)(P = 60 daN) average spectra. (a) Surface layer/surface layer; (b) core heart/surface layer; (c) core heart/core heart.
III Initial hardness (Vickers) HPO
Surface layer/ surface layer
Core heart/ surface layer
Core heart/ core heart
K, x lo-’
K, x lo-’
Diameter of particles families (pm)
850 3oowxoo 450
J. M. GFORGES.
Fig. 8. Evidence
in the tracks
of the rubbing
surface of u pin (core heartrcorc
phenomena cause different wear rates (proportion rate of the pin obtained by a surface layer/surface during test. This phenomena was further studied. 5. WEAR
OF THE SURFACE
10 to 3000). The smallest wear layer contact is not constant
The wear of all the layers of carbonitriding of the pin rubbing against the first layer of carbonitriding of the disk under a load of 60 N was studied by measuring the rate of wear of the pin K, = A&/Ax, and work hardening AH(Z) (Fig. 9). Summarized statistical particle analyses were carried out on samples from different levels of wear of the pin: Z= Z= Z= Z=
25 150 250 600
,um corresponds pm corresponds ,nm corresponds pm corresponds
to wear between the depths 0 pm and 25 pm, to wear between the depths 50 pm and 150 pm, to wear .between the depths 150 pm and 250 pm, and to wear between the depths 400 pm and 600 pm.
They are given in Fig. 10. During tests, wear and work hardening of the disks were negligible. Results of various measurements made on the pin as a function of Z are plotted in Fig. 11. During the various wear tests, the superficial hardness of the pin varies. This nonhomogeneous hardness on the rubbing area does not correspond to the original hardness H,(Z). There are important differences of behaviour between the first 25 pm of the surface, between 25 pm and 150 pm, and the rest of the layer, making three layers. The average hardness after rubbing is greater than the original hardness due to work hardening forming austenite or bainite of greater hardness (10 to 14) by mechanical structural transformation. On the contrary, the layer consisting essentially of martensite became softer, probably due to thermal effects during rubbing as seen by
WEAR OF CARBONlTRIDED
Fig. 9. Variation in hardness with depth 2, from the surface of the pin, H0 original H, superficial hardness of the pin after wearing, and AH = H - Ho curve.
25~ --15op -.250/1 ____-. 6DOP
Fig. 10. Particle size distribution equivalent sphere (4s) for different Z=600 pm.
comparing AH(Z) and C,(Z) graphs (Fig. 11). It may be expected that the weight of debris is equal to the loss of weight of the samples. WI= W
but experiment proved that in all cases C, > 1, C, reaching a maximum for a depth of layer of 75 pm, where the only component is martensite. During rubbing, heat is evolved in the contact area. Martensite is very hard thus the small contact areas have high flash temperatures, resulting in oxidation and a softening of the structure. Initial interface phenomena are very complicated. An asperity of the pin in contact with the disk, mechanically work-hardens to a maximum hardness then breaks off in the interface to provide particles harder than the pin and to cause abrasive wear8. These particles, in turn, work harden the pin and form other
J. M GEORGES.
I I. Comparative
results of surface layer/surface
layer tests P=60 daN
particles. There is a wide scatter in hardness of the rubbing surfaces. Two identical pins were rubbed with a constant load (60 N) on two identical plates. In one case, the test was interrupted every 100 rev and in the other every 500 rev. At each stop, the surface was cleaned to gather particles. In the second case the abrasive effect of the particles was more evident and the average rate of wear was greater ( x 2 or 3). Statistical analysis of particles proved to be difficult. Particles are not spherical but ellipsoidal and may be from the pin, or from the disk, or from destruction of other particles during rubbing. Considering the loss of weight of samples, particles coming from the plate may be neglected. The analysis however shows particle sizes of 5 pm, 12 pm and 25 pm diameter from the three layers respectively (Fig. 11). It has not been possible to determine the metallographic structure of particles but the smallest particles of 2 pm or less occur when the layer is essentially martensite, which confirms the hypothesis of the importance of hardness and oxidation. 6. INFLUENCE
When the load P varies on bodies in contact, the types of wear changer6. The rate of wear K, for adhesive wear is directly proportional to the load I’, according to Archardi7. !!!LcrP Ax,
WEAR OF CARBONITRIDED
where AV, is thelost volumeof the pin when it has run a distance Ax&,ctthe coefficient of adhesive wear which only depends on the nature of the couple of materials, P the applied load, and H the Meyer hardness of the softest body. If the apparent area of the pin A stays constant
Thus, if H is constant, K, is proportional to P. When the contact pressure increases18, the coefficient is no longer constant, and wear by fracture occurs. Many parameters such as layers with different mechanical properties, work hardening of the layers, the presence of residual strains and the different me~llographic structures make this phenomenon complicated. To study the wear rate of the pin, K, which is a function of load, tests of surface layer/surface layer with different loads (20,60,250,400 N) were carried out. Figure 12 shows the variations of K, as a function of P for different depths Z. Five characteristic depths of the pin were observed: two (25 pm and 150 pm) correspond to the surface layer; one (150 pm) to the structure of surface layers, one (300 pm) to transitional layers, one (600 pm) to the sub-surface metal. The rates of wear obtained corresponded to the wear observed on the pin after mechanical deterioration under normal test conditions. Figure 13 shows the changes in superficial hardness of the pin after rubbing; the original hardness curve (H,Z) is a continuous line. The scatter in hardness measurement increases with load. When load is low (20 N) the surface hardness follows closely the original hardness of the pin H,(Z). For higher loads this is not so, and work hardening occurs which may be about
Fig. 12. Wear coefficient (Kp) of the pin as a function of load for different depths 2,.
J. M GFORGFS.
Fig. 13. Superficial hardness of the pin as a function a:Ps20 N: j:P=60 N: 6:P=250 N; ~:I’=400 N.
of Z, for different loads. Original hardness H,(Z):
AH=H-H,,-300 HV. With a 400 N load, fractures appear on rubbing surfaces and no further increase in hardness is possible. Because of the work hardening increasing hardness and the scatter in measurement, Archard’s law is not applicable. 7. CONCLUSIONS
Study of the wear behaviour of carbonitrided steel shows that various surface layers and variation of load produce different types of wear and different wear rates. When the average contact pressure is 60 N/mm’, surface layer/surface layer contact gives the lowest wear. Wear causes changes in the mechanical properties of the surface layers revealed by microhardness testing. Depending upon metallographic structure of the layers. either softening or work hardening occurs. The greater the work hardening. the higher the wear rate. When load is higher (> 20 N) retained austenite can transform”. ls to reduce the wear coefficient. Phenomena such as the creation of abrasive particles, increases the scatter in results. Surface hardness, though playing an essential part, is not the only parameter requiring consideration. The brittle nature and depth of the surface layer which limit the maximum acceptable contact pressure must also be considered. ACKNOWLEDGMENTS
We thank the Partiot and Thermi Lyon Societies, for financing this study and allowing publication of the results, Messieurs Leclerc, Villars. Chatelus for helpful discussion, and Messieurs Gourmand, Allemant of Peugeot Automobile Society for the chemical analysis of samples.
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