Chromium-base coating for wear-resistant steel and method of preparing same

The invention concerns an abrasion resistent coating for steel and a method which makes it possible to obtain such a coating. The coating comprises two surface layers. The first surface layer, the outer layer, is formed of the phase (Cr, Fe).sub.2 B and possibly the phase M.sub.23 C.sub.6, while the second surface layer, the inner one, is formed of the phase (Fe, Cr).sub.2 B. In order to obtain this coating, a boriding is effected on steels at a temperature less than or equal to 950.degree. C. for a period of time greater than or equal to four hours, followed by a chromizing at a temperature less than or equal to 950.degree. C. for a period of time greater than or equal to 10 hours. Application to all steels in order to increase their resistance to wear is possible.

The field of the present invention is chromium-base coatings for steel of 
high resistance to wear by abrasion, and processes employing vapor-phase 
chromizing. 
Various surface treatments have been proposed in order to increase the 
hardness, resistance to wear by abrasion, corrosion resistance, and the 
like of parts of ferrous metal or steel, and, more generally, in order to 
improve the mechanical properties of surface layers. These principal 
treatments are, in particular, nitriding, boriding and chromizing. 
French Pat. Nos. 2,018,609 and 2,450,286 describe boriding processes in 
which a metal part is subjected to a boriding activator at a temperature 
between 850.degree. C. and 1100.degree. C. for a sufficient period of 
time. In this way there is obtained a layer of iron borides which improves 
the wear-resistance of the treated steel. 
The vapor chromizing of steels is well-known to those skilled in the art. 
Thus French Pat. No. 1,012,401 and its Patents of Addition 60,539 and 
60,686 describe treatments which lead to the formation of chromium-base 
diffusion alloys on the surface of the steel. 
The transport of chromium onto the surface of the material to be treated is 
effected by means of halides, which are the only chromium compounds which 
are in vapor state at the diffusion temperatures. 
The deposition of chromium on the alloy to be treated is effected in 
accordance with two main reactions: 
either (1) by exchange between chromium halide and iron in accordance with 
a reaction which, in the case of the chloride, is: 
##EQU1## 
or (2) by reduction of the chromium halide in a hydrogen medium in 
accordance with a reaction which, in the case of the chlorides, may be 
written: 
##EQU2## 
The application of these methods of treatment to the surface of mild steels 
leads to obtaining surface layers having a high chromium content, which 
have the advantage of being rust proof, but do not have high degrees of 
hardness. 
In the case of steels with a carbon content of more than 0.15%, and 
provided that the treatment is carried out at a temperature at which the 
steel is in austenitic state, the layer formed by chromizing consists of 
chromium carbides. These carbides are the carbide M.sub.23 C.sub.6 in the 
layers closest to the surface and the carbide M.sub.7 C.sub.3 in the 
subjacent layers. M designates here a metal which, in addition to chromium 
(Cr), may be iron (Fe), molybdenum (Mo), or vanadium (V). It is known that 
the hardnesses of M.sub.23 C.sub.6 and M.sub.7 C.sub.3 are about 1200 
HV.sub.0.01 and 2100 HV.sub.0.01 respectively. 
In the following text, the percents are always in masse. 
More recently, variants in treatment have been proposed in order to 
increase the resistance to wear of the steels. 
Thus French Pat. No. 2,439,824 proposes a method by which a steel 
containing at least 0.2% carbon is first subjected to ionic nitriding 
followed by a conventional vapor-phase chromizing at a temperature between 
850.degree. C. and 1100.degree. C. The nitriding requires ionic 
bombardment of the specimen at a temperature of between 450.degree. and 
570.degree. C. and a pressure of 2.5 to 8 millibars. It is stated that 
such a treatment makes it possible to effect practically complete 
elimination of the carbide M.sub.7 C.sub.3 which gives rise to formation 
of cracks in or scaling of the steel. In this way a single surface layer 
of the carbide M.sub.23 C.sub.6, of more than 30 .mu.m, would be formed. 
The nitriding treatment which forms the object of French Pat. No. 2,454,471 
gives rise to limited enrichments in nitrogen (1 to 2%). After chromizing, 
this prior treatment will, in accordance with that French patent, make it 
possible to obtain layers of carbides of at least 20 .mu.m while retaining 
a sufficiently fine austenitic grain size and the absence of deep 
decarburization. 
In accordance with French Pat. No. 2,460,340, a nitriding treatment 
followed by chromizing can only be applied to steels having at least 0.8% 
carbon (0.8 to 2%). This double treatment is said to lead to the formation 
of "mixed layers probably formed of chromonitrides." These layers are said 
to avoid wear by abrasion while eliminating the fragility of the sharp 
edges of the parts treated in this manner. Furthermore, these parts are 
said to have good resistance to dry and wet corrosion. 
More extensive metallurgical and tribological studies have been carried out 
on these different coatings and have made it possible to clarify, in the 
case of chromizing, the morphology, composition and resistance to wear of 
the layers obtained. Formed of the phases M.sub.23 C.sub.6 and M.sub.7 
C.sub.3, the layers may also contain Cr.sub.2 N nitrides when the halogen 
introduced into the cement is ammonium chloride NH.sub.4 Cl. These 
coatings have rather substantial resistance to wear but the morphology and 
the internal stresses of the carbide M.sub.7 C.sub.3 limit their wear 
resistant capacity. 
In the case of sequential treatments of nitriding followed by chromizing, 
it has been shown that the manner of nitriding (ionic or gaseous), or even 
the presence of carbonitrides, had no substantial influence on the nature 
of the component phases of the layers obtained. The predominant criterion 
has proven to be the initial thickness of the carbide layer. In general, 
the coatings obtained are distributed in two main sublayers. One, on the 
surface, is composed of the major phase Cr.sub.2 (C,N) and of the carbide 
(M.sub.23 C.sub.6). The other, the subjacent layer, is composed, to the 
extent of about one-half the thickness of the coating, of the carbide 
M.sub.7 C.sub.3. However this carbide in this case still retains a 
columnar structure, which is detrimental to good resistance to wear. 
Summarizing, therefore, the layers obtained by chromizing treatment, or 
nitriding followed by chromizing, in accordance with the prior art 
described in the aforementioned French patents, comprise a sublayer of 
chromium carbide M.sub.7 C.sub.3. In its state of crystallization this 
carbide reduces the wear resistance of the chromized layers. 
An object of the present invention is to provide a new coating and a method 
of obtaining it in which the formation of the carbide M.sub.7 C.sub.3, 
with its basaltic crystallization, is avoided. 
An object of the invention, therefore, is a coating for steel of high 
resistance to wear by abrasion, characterized by the fact that it 
comprises at least two surface layers, the first of which, located on the 
surface, essentially comprises the phase (Cr,Fe).sub.2 B while the other 
layer, the inner one, primarily comprises the phase (Fe,Cr).sub.2 B, 
wherein Cr, Fe, B designate chromium, iron, boron, respectively.

In the formula (Cr,Fe).sub.2 B, it is to be understood that the iron 
partially replaces the chromium in its network and vice versa in the 
formula (Fe,Cr).sub.2 B. In both cases, the substitution is effected in a 
percentage of less than 18%. 
The phase (Cr,Fe).sub.2 B may contain another phase of the type M.sub.23 
C.sub.6 in which M represents primarily chromium which also may be 
substituted by iron, vanadium, molybdenum, nickel or manganese. 
The thickness of the first layer may advantageously be at least 12 .mu.m 
and the thickness of the second at least 18 .mu.m. 
This coating is preferentially applied to a steel having a carbon content 
of at least 0.15%. 
The invention also concerns a process for obtaining a coating on steel 
which is characterized by the fact that in the first step the steel in 
austenitic state is borided at a temperature of less than or substantially 
equal to 950.degree. C. for a period of time greater than or substantially 
equal to 4 hours, and by the fact that in a second step chromizing is 
effected at a temperature lower than or substantially equal to 980.degree. 
C. for a period of time greater than or substantially equal to 10 hours, 
which steps may be followed by a thermal hardening and tempering 
treatment. 
The boriding can be effected in the presence of a boriding agent of the 
type B.sub.4 C+Na.sub.2 B.sub.4 O.sub.7 in a vinyl binder in order to 
produce a layer of iron borides FeB and Fe.sub.2 B of a thickness of at 
least 30 .mu.m. 
The layer of iron borides has a thickness close to 40 .mu.m. 
The chromizing may be effected in vapor phase in the presence of a cement 
comprising a 60:40 iron chromium powder, an anti-sintering agent (Al.sub.2 
O.sub.3), a carrier (NH.sub.4 Cl) and a hydrogen flow rate of about 300 
liters per hour. 
More particularly, the boriding is effected at 950.degree. C. for four 
hours and the chromizing at 950.degree. C. for 15 hours. 
The process of the invention is applied to a low-alloy steel of type 35 CD 
4. 
As indicated above, the main advantage of the invention is that for the 
first time the formation of the carbide M.sub.7 C.sub.3, both in the 
surface layer and in the deeper layers, can be avoided. Another advantage 
resides in the fact that the invention makes it possible to provide 
coatings which have a phase on their surface which contains chromium 
boride. It has not been previously possible to obtain such a coating. 
Thus, for the first time a coating containing chromium boride, the 
tribological properties of which are well known, can be made available 
without substantially modifying the chromizing treatment. The process of 
the invention can be applied to all types of steel, regardless of their 
carbon content. 
In order to produce the coating which is the object of the present 
invention, one proceeds in the following or an equivalent manner. 
A boriding treatment is first carried out with the steels, which may be 
alloyed or non-alloyed, and whose carbon content may, if necessary, be 
less then 0.10%. This treatment, independently of the technique employed 
(powder cementation, salt baths, EKABOR, slurry coating, ionic, etc.), 
should produce a compact layer of iron boride of at least 15 .mu.m 
thickness. 
After the boriding of a steel part, a layer of compounds having a base of 
the phases Fe.sub.2 B and/or FeB is created on the surface of said part. 
A barrier layer is thus formed which, during the chromizing, will make it 
possible to develop sublayers having a base of chromium boride (Cr.sub.2 
B) and iron boride enriched in chromium (Fe,Cr).sub.2 B. Depending on its 
thickness, the initial iron-boride layer limits to a greater or lesser 
extent the flow of carbon which can, after having migrated through it, 
combine with the chromim deposited during the course of chromization. If 
the initial iron-boride layer is of sufficient thickness, then only the 
carbide M.sub.23 C.sub.6 can be formed, along with the boride Cr.sub.2 B. 
After the thermochemical treatments, the heat treatments necessary for the 
acquisition of the internal characteristics of the substrate can be 
carried out. It is preferable to proceed with a new austenitizing after 
the chromizing, avoiding, if possible, water-quenching. 
In order that the invention may be duly understood, an example is given 
below in order to demonstrate and emphasize importance of the thickness of 
the initial layer of compact iron boride. The steel used is a low alloy 
steel of type 35 CD 4 which is widely used in industrial production. In 
this example, three samples of the steel are borided, the first at 
890.degree. C. for 11/2 hours, the second at 890.degree. C. for 4 hours 
and the third at 950.degree. C. for 4 hours. 
The method of treatment is carried out in conventional manner, but under 
the aforementioned conditions, by "slurry coating" in the presence of 
B.sub.4 C+Na.sub.2 B.sub.4 O.sub.7 in a vinyl binder. After this 
treatment, the three samples have a compact layer of iron borides FeB and 
Fe.sub.2 B with thicknesses of 4, 15 and 40 .mu.m respectively. 
These three samples are then subjected to a conventional chromizing, but 
under the following conditions: 
Cement: 
ferrochromium powder: 60-40 
antisintering agent: Al.sub.2 O.sub.3 
carrier: NH.sub.4 Cl 
rate of flow off hydrogen: 300 liters/hour 
Rate of isothermal rise: 150.degree. C./hour 
Thermal arrest: 950.degree. C. 
Time at thermal arrest: 15 hours 
The nature of the cements does not form an object of the invention since 
the formation of these cements involve known techniques. However, the 
order of the treatments used and the temperatures reached during the 
treatments produce the results which lead to the invention. 
After the chromizing treatment, the steel substrates are austenitized at 
850.degree. C., oil quenched, and then tempered at 250.degree. C. for two 
hours. 
Examination of the phases of the three samples by x-ray diffractometric 
analyses and analysis by Castaing electronic microprobe in correlation 
with the equilibrium diagrams, shows the following: 
Sample 1--An initial layer of iron borides of 4 .mu.m. 
The layer of iron boride has been consumed and has given rise to the 
formation of a coating of chromium carbides M.sub.23 C.sub.6 and M.sub.7 
C.sub.3 without any particularly substantial improvement in the state of 
crystallization of said latter carbide. 
Sample 2--An initial layer of iron borides of 15 .mu.m. 
In this case the M.sub.7 C.sub.3 carbide layer has practically disappeared, 
its thickness being less than 1 .mu.m. The M.sub.23 C.sub.6 carbide, in 
the form of an independent sublayer, remains. However, a chromium boride 
Cr.sub.2 B appears here with enrichment of the iron boride (Fe.sub.2 B) 
with chromium. 
Sample 3--An initial layer of iron borides of 40 .mu.m. 
The coating system in this sample is entirely different. The initial 
iron-boride thickness does not permit the carbon to reach the surface in 
sufficient content with respect to the transport kinetics of the chromium. 
Therefore, the formation of the carbide layer M.sub.7 C.sub.3 has been 
eliminated and the course of diffusion in the system Fe, Cr, B, C at the 
chromization isotherm T is modified. 
A layer, with a thickness of about 16 .mu.m, consisting of the phases 
Cr.sub.2 B and Cr.sub.23 C.sub.6 in which iron is substituted (Fe less 
than or equal to 18%) is formed. Beneath this layer there is the phase 
consisting of the iron boride enriched in chromium by diffusion 
(Fe,Cr).sub.2 B. 
The hardness characteristics of these different phases are as follows: 
______________________________________ 
Chromium carbide 
M.sub.23 C.sub.6 
1600 .+-. 300 HV.sub.0.02 
Chromium carbide 
M.sub.7 C.sub.3 
2100 .+-. 250 HV.sub.0.02 
Chromium boride Cr.sub.2 B 1400 .+-. 200 HV.sub.0.02 
with 
chromium carbide 
M.sub.23 C.sub.6 
Iron boride enriched 
(Fe, Cr).sub.2 B 
2400 .+-. 400 HV.sub.0.02 
in chromium 
______________________________________ 
These three samples were subjected to a standard wear test on a tribometer 
in pin-disk configuration, in accordance with the following test 
parameters: 
samples 1, 2 and 3 in the form of flat disks of 35 CD 4 steel 
cylindrical pin with flat end of a diameter of 1.5 mm of 35 CD 4 steel, 
heat treated for a hardness of 310 HV.sub.0.5, 
normal force IN, namely an apparent normal static stress of 0.56 MPa, 
circumferential speed: 500 rpm 
linear speed of contact: 41 m/minute 
laboratory temperature: 20.degree. C. 
dry friction 
stressing distance: 50 km. 
The results obtained are set forth in the following table, as well as the 
results obtained with a reference sample which was treated by conventional 
chromatizing. It should be noted that the pins, which were not subjected 
to any specific treatment, exhibit extensive wear. Thus, it is essentially 
the wear of the disks which is to be observed. 
______________________________________ 
Average wear in cubic 
milimeters/100 km 
TYPE OF LAYER DISK PIN CUMULATIVE 
______________________________________ 
Reference sample 
1.20 1.80 3.00 
Sample 1 0.80 0.80 1.60 
Sample 2 0.74 1 1.74 
Sample 3 0.12 3.3 3.42 
______________________________________ 
It should be noted that the layer configuration for which the Cr.sub.2 B 
phase was constituted (Sample 3) gives results for resistance to wear 
which are particularly interesting when compared to those obtained after 
direct chromizing (Reference sample). 
In the case of the 15 .mu.m layer with initial boriding (Sample 2), the 
test results presented here are misleading, since in fact the wear in this 
case was limited to the M.sub.7 C.sub.3 layer, it not having been possible 
to reach the Cr.sub.2 B layer. 
Therefore, in the case of Sample 3, the results speak for themselves, the 
wear of the disk being practically negligible. The coating of the 
invention can therefore be applied to any metal part whose wear is to be 
negligible as compared with another part the wear of which is substantial, 
such as a gun barrel and the banding of a shell, respectively, for 
example. 
As to the morphology of the layers, the following points should be made: On 
the surface, the morphology of the layers is typically that of the 
M.sub.23 C.sub.6 phase. But in contrast to the layers obtained by direct 
chromizing, the crystals of chromium carbonitride Cr.sub.2 (C,N) are very 
rare in the case of the boron-chromizing. 
In cross-section the coating of Sample 1 is formed of two sublayers 
M.sub.23 C.sub.6 and M.sub.7 C.sub.3, which are of the same appearance as 
the chromized layers. On the other hand, the coating of sample 2 is 
composed of three sublayers M.sub.23 C.sub.6, M.sub.7 C.sub.3 and 
(Cr,Fe).sub.2 B, below which is found the general morphology of the iron 
boride layer which was not completely consumed during the exchanges. 
As to Sample 3, one no longer observes sublayers as such, the coating being 
present in the form of a poly-phase band of (Cr,Fe).sub.2 B and M.sub.23 
C.sub.6 below which there is the layer of initial iron boride which was 
not consumed during the exchanges.