Method of treating ferrous surfaces subjected to high friction strains

In a method of increasing the wear resistance and the corrosion resistance f opposed bearing surfaces of parts subjected to reciprocal friction, in particular when the product of the pressure distributed over the bearing surfaces by the relative speed of the latter exceeds 0.4 MPa.m/s, thermochemical diffusion of nitrogen is effected by nitriding or nitrocarburizing in a molten salt bath at a temperature of 570.degree. C..+-.15.degree. C. followed by an oxidizing or phosphating surface chemical reaction providing resistance to wet corrosion. The nitriding or nitrocarburizing molten salt bath is made up of alkaline carbonates and cyanates and further contains sulfur-containing substances in the following percentages by weight: PA1 30%<CNO.sup.- <45% PA1 15%<CO.sub.3.sup.2- <25% PA1 15%<Na.sup.+ <25% PA1 20%<K.sup.+ <30% PA1 1%<Li.sup.+ <6% PA1 1 ppm<S.sup.2- <100 ppm The time for which parts are immersed in the bath is between 15 minutes and 45 minutes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a method of increasing the wear and 
corrosion resistance of ferrous surfaces subjected to intense reciprocal 
friction. 
To be more specific, the invention concerns the treatment of opposed 
ferrous metal bearing surfaces subjected to intense reciprocal friction, 
especially if the product of the pressure distributed over the bearing 
surfaces by the relative sliding speed of the latter exceeds 0.4 MPa.m/s. 
2. Description of the Prior Art 
Some parts, such as washers, chasers, tools (wrenches, screwdrivers, 
pliers), lock mechanisms, knurling tools, pins, clips, chain links, etc., 
are subjected to high strains, especially pressure strains, and can be 
greatly deformed, for example, bending during mounting and flexing in 
operation. They must also have good corrosion resistance. Many of these 
parts are also thin. Methods of treating ferrous metal parts to increase 
their friction and corrosion resistance properties at one and the same 
time have already been described, in particular in FR-A-2 672 059, U.S. 
Pat. No. 5,346,560 and U.S. Pat. No. 5,389,161. 
FR-A-2 672 059 describes a method of treating ferrous metal parts to 
improve their friction and corrosion resistance properties involving 
nitriding and then oxidizing the parts, which are then coated with a 
polymer varnish. In one preferred embodiment the nitriding and oxidation 
are carried out in molten salt baths, the nitriding being carried out in a 
bath of molten salts based on alkaline cyanates and carbonates and the 
oxidation being carried out in a bath of molten salts based on alkali 
metal oxygenated salts, hydroxides, nitrates and carbonates. The nitriding 
bath advantageously further contains sulfur-containing substances. 
U.S. Pat. No. 5,346,560 describes a comparable technology, except that 
nitriding/oxidation is followed by impregnation with a hydrophobic wax 
having a high molecular weight. 
U.S. Pat. No. 5,389,161 describes nitriding the parts in a bath of 
sulfur-containing salts based on alkaline carbonates and cyanates, 
followed by phosphating. 
The methods mentioned hereinabove are very effective and are increasingly 
used in industrial practice. They do have a limitation, however, which is 
that their effectiveness is significantly reduced if the operating 
conditions of the parts become very severe, i.e. if the product of the 
pressure distributed over the rubbing bearing surfaces by the relative 
sliding speed of the latter exceeds a particular critical threshold, 
typically in the order of 0.4 MPa.m/s to 0.5 MPa.m/s. 
One object of the present invention is to remedy this drawback. 
The present invention meets this object by proposing a treatment method for 
simultaneously improving the wear resistance and the corrosion resistance 
of ferrous metal surfaces subjected to severe reciprocal friction whose 
effectiveness remains substantially constant when the parts are very 
highly strained. 
The method of the invention utilizes thermochemical diffusion of nitrogen 
by nitriding or nitrocarburizing in a molten salt bath followed by 
oxidizing or phosphating in a molten salt bath. It is characterized by a 
rigorous selection, specifically arrived at to achieve the stated object, 
in particular of a set of conditions concerning the thermochemical 
diffusion of nitrogen, including the concentrations of the various 
constituents of the molten salt bath and the treatment time. 
SUMMARY OF THE INVENTION 
Thus the present invention provides a method of increasing the wear 
resistance and the corrosion resistance of opposed bearing surfaces of 
parts subjected to reciprocal friction, in particular when the product of 
the pressure distributed over the bearing surfaces by the relative speed 
of the latter exceeds 0.4 MPa.m/s, said method being suitable for ferrous 
metal parts made of iron, additional metallic elements and carbon, with a 
minimum concentration by weight of 2.5% of additional metal elements or 
0.45% by weight of carbon, wherein thermochemical diffusion of nitrogen to 
harden the bearing surfaces is effected by nitriding or nitrocarburizing 
in a molten salt bath at a temperature of 570.degree. C..+-.15.degree. C. 
followed by a reaction providing resistance to wet corrosion, and: 
(i) the nitriding or nitrocarburizing molten salt bath is made up of 
alkaline carbonates and cyanates and further contains sulfur-containing 
substances in the following percentages by weight: 
30%&lt;CNO.sup.- &lt;45% 
15%&lt;CO.sub.3.sup.2- &lt;25% 
15%&lt;Na.sup.+ &lt;25% 
20%&lt;K.sup.+ &lt;30% 
1%&lt;Li.sup.+ &lt;6% 
1 ppm&lt;S.sup.2- &lt;100 ppm 
(ii) the time for which said parts are immersed in said nitriding or 
nitrocarburizing molten salt bath is between 15 minutes and 45 minutes; 
and 
(iii) the reaction providing resistance to wet corrosion is a chemical 
surface reaction selected from the group comprising oxidizing reactions 
and phosphating reactions. 
The method applies to ferrous metal parts made of iron, additional metal 
elements, in particular Cr, Mo, V, Al, and carbon, with a minimum 
concentration by weight of 2.5% additional metal elements or 0.45% carbon. 
All of these conditions, namely the composition of the nitriding or 
nitrocarburizing bath, the time of immersion of the parts to be treated in 
the bath, and the composition of the parts to be treated, must be complied 
with if the stated object is to be achieved, as explained hereinafter, in 
particular in the examples. 
Table I below shows, for the nitriding nitrogen thermochemical diffusion 
step, the concentrations of the various constituents of the bath and the 
treatment time in accordance with the prior art (FR-A-2 672 059, U.S. Pat 
No. 5,346,560 and U.S. Pat No. 5,389,161) and in accordance with the 
present invention. 
TABLE I 
__________________________________________________________________________ 
Nitriding Bath Composition 
Sulfur 
Alkaline Carbonates and Cyanates 
compounds 
Treatment 
Method (% by weight) (ppm) Time 
of CNO.sup.- 
CO.sub.3.sup.2- 
Na .sup.+ 
K.sup.30 
Li.sup.30 
S.sup.2- 
(min) 
__________________________________________________________________________ 
FR-A-2672059 
35-65 
1-25 
25-42.6 
42.6-62.5 
11.3-17.1 
10-10000 A 
NS 
US-A-5,346,560 
35-65 
1-25 
25-42.6 
42.6-62.5 
11.3-17.1 
A, NS NS 
US-A-5,389,161 
NS NS NS NS NS 10, N 90 .+-. 15 
Present 30-45 
15-25 
15-25 
20-30 
1-6 1-100, N 
15-45 
Invention 
__________________________________________________________________________ 
A: advantageous 
N: necessary 
NS: not specified 
In accordance with the present invention, the thermochemical diffusion 
step, effected under the specific conditions stated hereinabove, is 
followed by a chemical reaction causing the formation on the surface of 
substances adapted to resist wet corrosion; this chemical reaction is 
either an oxidizing reaction or a phosphating reaction. 
In accordance with the present invention, said oxidizing reaction is 
carried out in a molten salt bath made up of alkaline hydroxides, nitrates 
and carbonates, together with a powerful oxidizing agent, i.e. an agent 
having a normal oxidation-reduction potential relative to the reference 
electrode less than or equal to -1 volt, for example alkaline bichromate, 
at a temperature between 350.degree. C. and 550.degree. C., and with an 
immersion time of the parts to be treated in said bath between 10 minutes 
and 30 minutes, and the composition of said molten salt bath, in terms of 
percentages by weight, is as follows: 
9%&lt;CO.sub.3.sup.2- &lt;17% 
25%&lt;NO.sub.3.sup.- &lt;30% 
15%&lt;OH.sup.- &lt;20% 
powerful oxidizing anion (e.g. bichromate)&lt;1%. 
Table II below indicates the composition of the oxidizing bath in 
accordance with the present invention and in accordance with the prior art 
(FR-A-2 672 059, U.S. Pat. No. 5,346,560 and U.S. Pat. No. 5,389,161). 
EP 637 637 describes a method of nitriding ferrous metal parts in which the 
parts are treated by immersion for an appropriate time in a bath of molten 
salts essentially comprising alkali metal carbonates and cyanates and 
containing a sulfur-containing substance, wherein, during their immersion 
in the bath, the parts are raised to a positive electrical potential 
relative to a counter-electrode dipping into the bath such that a high 
current flows through the bath from the parts to the 
TABLE II 
______________________________________ 
Composition of the oxidizing bath 
Method of (% by weight) 
______________________________________ 
FR-A-2 672 059 alkaline carbonates + nitrates: 
between 85% and 99.5% 
alkaline oxygenated salt + 
hydroxides: remainder to 100% 
US-A-5,346,560 oxidizing alkaline salts, nature and 
concentration unspecified 
Present 9% &lt; C0.sub.3.sup.2- &lt; 17% 
invention 25% &lt; NO.sub.3.sup.- &lt; 30% 
15% &lt; OH.sup.- &lt; 20% 
powerful oxidizing anion &lt; 1% 
______________________________________ 
counter-electrode. According to EP 637 637, the treatment time can be from 
10 minutes to 150 minutes, the temperature can be between 450.degree. C. 
and 650.degree. C. and the liquid active part of the bath can contain 30% 
to 40% CNO-anion, 15% to 25% CO.sub.3.sup.2- anion, 20% to 30% K.sup.+ 
cation, 15% to 25% Na.sup.+ cation, 0.5% to 5% Li.sup.+ cation, 0.5% to 5% 
Li.sup.+ cation and between 1 ppm and 6 ppm of S.sup.2-. 
According to EP 637 637 the current densities used on the parts to be 
treated are between 300 A/m.sup.2 and 800 A/m.sup.2, preferably between 
450 A/m.sup.2 and 500 Am.sup.2. 
Note that even if the composition of the nitriding bath of EP 637 637 is 
similar to that of the nitriding bath of the present invention, the two 
methods are fundamentally different. Firstly, in contradistinction to EP 
637 637, no current flows through the molten salt baths of the present 
invention. Secondly, the method in accordance with the present invention 
is in two steps, the thermochemical diffusion step being followed by an 
oxidizing or phosphating step, whereas EP 637 637 is critical of 
multi-step methods and claims a single-step method. 
In accordance with the present invention, the nitrogen thermochemical 
diffusion step by nitriding or nitrocarburizing mentioned above may be 
preceded by pre-nitriding carried out in a bath having a similar 
composition to that used for the nitriding or the nitrocarburizing. 
The pre-nitriding is carried out at a temperature from 520.degree. C. to 
550.degree. C. for a period from 60 minutes to 180 minutes and is followed 
by cooling to a temperature of approximately 370.degree. C. to 400.degree. 
C. (i.e. cooling by approximately 150.degree. C.). 
The embodiment of the invention including the pre-nitriding treatment 
reconciles a high hardness of the treated part in a thin surface zone with 
deep diffusion of sufficient nitrogen for the treated part to have better 
fatigue resistance that obtained without the pre-nitriding treatment. 
The thermochemical nitrogen diffusion step after pre-nitriding is 
advantageously of reduced duration, between 15 minutes and 30 minutes. 
When the above operations have been carried out, it is particularly 
advantageous to complete the treatment by application to the surface of a 
product adapted both to reduce the tendency to seizing and to facilitate 
accommodation (i.e. the ability of the parts to conform to each other 
during rubbing contact). 
The anti-seizing product can be a metal having a low Young's modulus such 
as Ag, Sn, Pb, Cd or a so-called "anti-friction" alloy such as Sn/Pb, 
Zn/Ni, etc. deposited in the form of a thin layer. 
It can instead be a polymer coating, a wax impregnation, a so-called 
"soluble" oil or a varnish, possibly charged with a solid lubricant such 
as graphite, molybdenum disulfide, PTFE. 
In all cases the thickness of the layer of said product must be sufficient 
to have a significant effect, but not too thick to cause excessive creep 
due to the high pressure on the bearing surfaces. We have found that a 
thickness of the anti-seizing product layer between 2 .mu.m and 15 .mu.m 
is sufficient. 
For randomly lubricated bearing surfaces, the surface of the parts is 
advantageously sculpted, for example grooved of knurled, to provide traps 
for wear debris and a reserve of lubricant. 
We have analyzed metallographic sections in an attempt to explain the 
mechanisms by which the method of the present invention acts. Accordingly, 
we have carried out microhardness measurements on sectioned test pieces of 
steel with various compositions treated in various ways. The results, 
described in detail in the following examples, show that good tribological 
performance is obtained at very high P.times.V (pressure.times.relative 
velocity) values if: 
the thickness of the surface layer of nitrides is between 10 .mu.m and 20 
.mu.m, of which substantially the half in contact with the substrate is 
very compact while the other (surface) half is slightly porous; 
the hardness of the supporting steel is high at the surface and then falls 
off very quickly to reach the core hardness in a few tens of micrometers. 
Good results typically correspond to nitriding (or nitrocarburizing) 
carried out under conditions such that the equivalent hardened depth, 
measured from the hardened steel surface under an external layer of 
nitrides (defined as the depth at which the increase of hardness brought 
about by nitriding is 37% of the increase at the surface) is between a 
minimum of 20 .mu.m and a maximum of 120 .mu.m, the nil depth hardness 
extrapolated from the hardnesses at staggered depths being at least three 
times the core hardness. 
With the specified current densities, the method of EP 637 637 mentioned 
above does not achieve the same nitriding (or nitrocarburizing) effect as 
the present invention, in terms of morphology of the surface nitride layer 
and the supporting steel hardness gradient referred to hereinabove. 
Although theoretical considerations must not be regarded as implying any 
limitation on the scope of the invention, the following explanation could 
account for the particular tribological properties imparted to very highly 
strained steel parts by the method of the present invention. 
The fact that the pressure distributed over the bearing surfaces is high 
implies that localized pressures are also very high: hence the need for 
high mechanical specifications, in particular hardness, at the surface and 
in the underlying layer. 
The mechanical parts that the invention concerns are for the most part 
subject to misalignment and consequent edge bearing effects that amplify 
excess straining phenomen. This leads to the requirement for relatively 
high accommodation of the steel. However, in most cases, this property is 
incompatible with the high hardness mentioned above, since very hard 
layers are only slightly ductile, often fragile and subject to scaling. 
The highly negative hardness gradient that characterizes parts treated in 
accordance with the invention represents an acceptable compromise, since 
the very hard surface layer is thin: the properties of thin layers are 
known to be very different from those of solid materials. 
It is also probable that the method of the invention yields residual 
compression stresses in the surface layers that are favorable in the 
intended applications. 
Finally, note that the energy dissipated by friction, which is directly 
related to the P.times.V product and to the coefficient of friction, can 
be high: not only is the P.times.V product high (&gt;0.4 MPa.m/s), but the 
coefficient of friction is also high for most intended applications since 
the lubrication conditions are random, the parts even being required to 
function dry (without lubrication) in some cases. Good surface 
anti-seizing properties are therefore required; the presence of substances 
having solid lubrication properties can therefore only be favorable. 
DETAILED DESCRIPTION OF EXAMPLES OF THE INVENTION

The invention will now be described in more detail with reference to the 
following non-limiting examples in which, unless indicated otherwise, all 
proportions and percentages are by weight. 
EXAMPLE 1 
Batches of pin and disk type test pieces of steel with the following 
composition: C: 0.3%, Cr: 13%, the remainder being iron, heat treated by 
quenching followed by annealing, were nitrided under the following 
conditions: 
composition of the molten salt bath: 
CNO.sup.- =37% 
CO.sub.3.sup.2- =18% 
Na.sup.+ =17% 
K.sup.+ 24% 
Li.sup.+ =4% 
S.sup.2- =6ppm 
bath temperature: 565.degree. C.; 
immersion time of parts in the bath: 30 minutes. 
On removal from the nitriding bath, the test pieces were phosphated in 
accordance with the teaching of U.S. Pat. No. 5,389,161 (Example 1) and 
then coated with soluble oil. 
Friction tests were then carried out on a laboratory simulator, with a pin 
rubbing on a disk with a reciprocating rectilinear movement under the 
following conditions: 
travel: 8 mm, 
distributed pressure: 70 MPa, 
sliding speed: 0.006 m/s, 
P.times.V=0.42 MPa.m/s, 
surroundings: dry in air, 
test duration: 8 hours. 
The test result was characterized by the cumulative wear of the pin and the 
disk and by the surface states of the rubbing bearing surfaces. 
The results obtained were as follows: 
cumulative wear of pin+disk: 0.1 mm, 
state of surfaces at end of test: polished. 
With regard to the corrosion resistance of the treated parts, the results 
obtained were compatible with those stated in U.S. Pat. No. 5,389,161, 
i.e. several hundred hours resistance to salt spray. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 320, 
nil depth hardness (HV100): 1 300, 
equivalent hardened depth: 30 .mu.m. 
Note that the equivalent hardened depth, measured from the hardened steel 
surface under an external layer of nitrides, was between 20 .mu.m and 120 
.mu.m and that the nil depth hardness extrapolated from the hardness at 
staggered depths was at least three times the core hardness, which 
conforms to the favorable configuration previously mentioned in the 
description. 
EXAMPLE 2 (Comparative) 
Cumulative pin and disk wear tests were carried out on test pieces of the 
same composition as Example 1 but without any treatment, i.e. without any 
conditioning of the surface. The tests were ended prematurely (i.e. after 
a few minutes, at most 30 minutes); seizing was observed, with significant 
deterioration of the surface state and high wear (1 mm to 2 mm). 
EXAMPLE 3 
Test pieces with the same composition as in Example 1 were treated as in 
Example 1, except that only the disk was treated. 
Performance was degraded compared to that with both parts treated; it 
remained acceptable, however: 
cumulative wear of pin+disk: 0.3 mm; 
surface state at end of test: slight scoring. 
EXAMPLE 4 
Batches of pin and disk type test pieces of steel having the following 
composition: C: 0.08%, Cr: 17%, the rest being iron, heat treated by 
quenching followed by annealing, were nitrided and phosphated and then 
tested under the same conditions as in Example 1. 
The results were comparable with those of Example 1 in terms of friction 
performance and resistance to corrosion (salt spray). 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 350, 
nil depth hardness (HV100): 1 350, 
equivalent hardened depth: 25 .mu.m. 
Note that the equivalent hardened depth, measured from the hardened steel 
surface under an external layer of nitrides, was between 20 .mu.m and 120 
.mu.m and that the nil depth hardness extrapolated from the hardness at 
staggered depths was at least three times the core hardness, which 
conforms to the favorable configuration previously mentioned in the 
description. 
EXAMPLE 5 
Batches of pin and disk type test pieces of steel having the following 
composition: C: 0.4%, Cr: 5%, Mo: 1.3%, V: 0.4%, the remainder being iron, 
heat treated by quenching followed by annealing, were nitrided under the 
same conditions as in Example 1. 
All the parts were then phosphated, followed by impregnation with soluble 
oil as described in U.S. Pat. No. 5,389,161 (Example 1). 
The batches of treated test pieces were tested as in Example 1. The 
cumulative wear and surface state results are summarized in Table III 
below. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 400, 
nil depth hardness (HV100): 1 400, 
equivalent hardened depth: 40 .mu.m. 
Note that the equivalent hardened depth, measured from the hardened steel 
surface under an external layer of nitrides, was between 20 .mu.m and 120 
.mu.m and that the nil depth hardness extrapolated from the hardness at 
staggered depths was at least three times the core hardness, which 
conforms to the favorable configuration previously mentioned in the 
description. 
EXAMPLE 6 (Comparative) 
Batches of test pieces identical to those of Example 5 were nitrided as in 
Example 5, except that the treatment time was increased to four hours. 
They were then phosphated as in Example 5. 
The batches of treated test pieces were tested as in Example 1. The 
cumulative wear and surface state results are indicated in Table III 
below. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 400, 
nil depth hardness (HV100): 1 000, 
equivalent hardened depth: 170 .mu.m. 
Note that the equivalent hardened depth, measured from the hardened steel 
surface under an external layer of nitrides, was not between 20 .mu.m and 
120 .mu.m and that the nil hardness depth extrapolated from the hardnesses 
at staggered depths was not at least three times the core hardness. Thus 
these test pieces did not have all of the metallurgical characteristics 
conforming to the favorable configuration mentioned previously in the 
description. 
EXAMPLE 7 (Comparative) 
Batches of test pieces identical to those of Example 5 were nitrided under 
the following conditions: 
composition of the molten salt bath: 
CNO.sup.- =55% 
CO.sub.3.sup.2- =10% 
Na.sup.+ =20% 
K.sup.+ =13% 
Li.sup.+ =2% 
S.sup.2- =1 000 ppm 
bath temperature: 565.degree. C.; 
immersion time of parts in the bath: 90 minutes. 
They were then phosphated as in Example 5. The batches of treated test 
pieces were tested as in Example 1. The cumulative wear and surface state 
results are indicated in Table III below. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 400, 
nil depth hardness (HV100): 1 150, 
equivalent hardened depth: 140 .mu.m. 
As in Comparative Example 6 above, these test pieces did not have all of 
the metallurgical characteristics conforming to the favorable 
configuration mentioned previously in the description. 
TABLE III 
______________________________________ 
Cumulative 
Example Wear (mm) Surface State 
______________________________________ 
5 0.09 polished 
6 0.8 scaling 
7 0.6 scoring 
______________________________________ 
The results obtained in Example 5 confirm the high level of performance 
that can be expected of parts treated in accordance with the present 
invention. 
The results obtained in Comparative Examples 6 and 7 show that performance 
deteriorates when the claimed specifications of the present invention are 
not complied with. 
EXAMPLE 8 
Batches of pin and disk type test pieces of steel having the following 
composition: C: 0.4%, Cr: 5%, Mo: 1.3%, V: 0.4%, the remainder being iron, 
heat treated by quenching followed by annealing, were subjected to 
pre-nitriding by immersion for two hours in a nitriding bath having the 
same composition as in Example 1 at a temperature of 530.degree. C. The 
parts were then cooled to 380.degree. C. The parts were then nitrided in a 
nitriding bath having the same composition as in Example 1 at 570.degree. 
C. for 30 minutes. 
The treated parts were then tested as in Example 1. The friction test 
results obtained were as follows: 
cumulative wear: 0.11 mm, 
surface states: good. 
EXAMPLE 9 
Batches of pin and disk type test pieces of steel having the following 
composition: C: 0.3%, Cr: 13%, the remainder being iron, heat treated by 
quenching followed by annealing, were nitrided as in Example 1. 
On removal from the nitriding bath they were, in accordance with the 
invention, immersed for 15 minutes in an oxidizing bath at 450.degree. C., 
the bath having the following composition by weight of anions: 
CO.sub.3.sup.2- =15% 
NO.sub.3.sup.- =27% 
OH.sup.- =18% 
Cr.sub.2 O.sub.7.sup.2- =0.25% 
The parts were then impregnated with polyethylene wax as described in U.S. 
Pat. No. 5,346,560 (Example 1). 
The results of friction tests carried out under the same conditions as in 
Example 1 above were as follows: 
cumulative wear of pin+disk: 0.12 mm, 
surface states at end of test: good. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 350, 
nil depth hardness (HV100): 1 350, 
equivalent hardened depth: 25 .mu.m. 
EXAMPLE 10 
Test pieces identical to those of Example 9 were treated as in Example 9 
except that the polyethylene wax treatment was replaced by coating with 
fluoro-ethylene-propylene (FEP) to a thickness of 10 .mu.m, in accordance 
with the teaching of FR-A-2 672 059. 
The results for exactly the same disk and pin treatment are indicated in 
Table IV below. 
EXAMPLE 11 
Test pieces identical to those of Example 9 were treated as in the Example 
9 except that the polyethylene wax treatment was replaced by coating with 
a layer of polymer varnish charged with PTFE in accordance with the 
teaching of FR-A-672 059. 
The results for exactly the same disk and pin treatment are indicated in 
Table IV below. 
EXAMPLE 12 
Test pieces identical to those of Example 9 were treated as in Example 9 
except that the polyethylene wax treatment was replaced by coating with a 
8 .mu.m thick layer of polymer varnish charged with MoS.sub.2. 
The results for exactly the same disk and pin treatment are indicated in 
table IV below. 
TABLE IV 
______________________________________ 
Cumulative 
Example Wear (mm) Surface State 
______________________________________ 
10 0.1 very good 
11 0.9 very good 
12 0.14 good 
______________________________________ 
EXAMPLE 13 
Batches of shaft and bearing shell test pieces in steel having the 
following composition: C: 0.4%, Cr: 5%, Mo: 1.3%, V: 0.4%, the remainder 
being iron, were treated as in Example 12 above. 
The treated test pieces were then tested by means of oscillating bearing 
tests under the following conditions: 
shaft diameter: 35 mm, 
shaft/bearing clearance: 0.1 mm, 
alternating rotation, 
frequency: 0.65 Hz, 
cycle: 15 seconds on, 60 seconds off, 
distributed pressure: 50 MPa, 
P.times.V: 0.4 MPa.m/s, 
surroundings: air, 
lubrication: by wiping parts before assembly with an oily rag, followed by 
addition of further lubricant. 
The test result was characterized by the time after which a temperature 
sensor in the bearing in line with the contact area and 2 mm from the 
surface indicated a rapid rise in temperature. 
Metallographic sections of the test pieces confirmed that the hardness 
gradient conformed to the favorable configuration mentioned in the 
description and in Example 1 above. 
When both parts were treated the duration of the test before a rapid rise 
in the temperature of the bearing was 320 hours. 
When only the bearing shell was treated, the duration of the test before 
the rapid rise in temperature of the bearing was 270 hours. 
This example confirms that it is preferable to treat both parts of the 
rubbing pair, but that performance is nevertheless acceptable when only 
one part is treated. 
By way of comparison, tests carried out with shafts and bearing shells that 
had not been treated led to seizing after less than 30 minutes. 
EXAMPLE 14 (Comparative) 
Test pieces identical to those of Example 13 above were treated and tested 
as in Example 13 except that the composition of the nitriding bath was as 
follows (not in accordance with the invention): 
CNO.sup.- =55% 
CO.sub.3.sup.2- =10% 
Na.sup.+ =20% 
K.sup.+ =13% 
Li.sup.+ =2% 
S.sup.2- =1 000 ppm 
The rapid rise in temperature occurred after 45 hours. 
EXAMPLE 15 (Comparative) 
Test pieces identical to those of Example 13 above were treated and tested 
as in Example 13 except that the nitriding time was four hours (not in 
accordance with the invention). 
The rapid rise in temperature occurred after 40 hours. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 250, 
nil depth hardness (HV100): 450, 
equivalent hardened depth: 350 .mu.m. 
The above measurements show that these test pieces did not have all of the 
metallurgical characteristics conforming to the favorable configuration 
mentioned previously in the description. 
EXAMPLE 16 (Comparative) 
Batches of shaft and bearing shell test pieces of steel having the 
following composition: C: 0.2%, Mo: 1.5%, V: 0.5%, the remainder being 
iron, i.e. a composition not in accordance with the invention, were 
treated and tested as in Example 13 above. 
The rapid rise in temperature occurred after 40 hours. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 280, 
nil depth hardness (HV100): 500, 
equivalent hardened depth: 400 .mu.m. 
The above measurements show that these test pieces did not have all of the 
metallurgical characteristics conforming to the favorable configuration 
mentioned previously in the description. The tribological performance was 
relatively poor. 
EXAMPLE 17 (Comparative) 
Batches of shaft and bearing shell test pieces in non-alloy steel having 
the following composition: C: 0.38%, the remaining being iron, quenched 
and then annealed, i.e. having a composition not in accordance with the 
invention, were treated and tested as in Example 13 above. 
The rapid rise in temperature occurred after 50 hours. 
Microhardness measurements on sectioned treated test pieces gave the 
following results: 
core hardness (HV100): 300, 
nil depth hardness (HV100): 500, 
equivalent hardened depth: 400 .mu.m. 
The above measurements show that these test pieces did not have all of the 
metallurgical characteristics conforming to the favorable configuration 
previously mentioned in the description. The tribological performance was 
relatively poor.