Method for the surface treatment of an iron or iron alloy article

A surface layer which is composed of the carbonitride of molybdenum is formed on an article made of iron or an iron alloy by heating the article in the presence of a material containing molybdenum and a treating agent. The treating agent may be composed of at least one of the cyandides and cyanates of alkali metals and alkaline earth metals. The layer adhering closely to the article can be formed efficiently at a temperature which is so low that virtually no thermal strain may develop in the article.

RELATED APPLICATIONS 
The subject matter of the application is related to that of application 
Ser. No. 23,862, filed Feb. 3, 1987, now U.S. Pat. No. 4,765,847 and to 
that of application Ser. No. 80,828, filed July 24, 1987. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for surface treatment which forms a 
layer of molybdenum (Mo) carbonitride on the surface of any articles made 
of iron or an iron alloy, such as dies, jigs, tools and machine parts. 
2. Description of the Prior Art 
Carbides of molybdenum (Mo), such as Mo.sub.2 C and (Mo,Fe).sub.6 C, has a 
hardness of more than Hv 1500 and have superior resistance to wear and 
seizure than that of the carbide of iron (Fe.sub.3 C) or the nitride of 
iron (Fe.sub.2.about.3 N). Molybdenum carbide has been made to exist in 
high speed steel in the form of (Mo,Fe).sub.6 C to improve wear resistance 
in addition to hardness. However, the carbide of Mo has a lower hardness 
and a poorer wear resistance than those of the carbide of V, Ti or the 
like, having a hardness of about Hv 3000 and, therefore, there have been 
only few practical uses thereof for a wear resistant coating layer. In 
addition, MoN is also poor in wear resistance compared with VN, TiN. 
Although MoS is an excellent solid lubricant, the seizure resistance of 
the carbide and nitride of Mo has not sufficiently been examined. The 
inventors of this invention have found that the carbonitride of Mo 
exhibits an excellent seizure resistance, and they conceived of forming a 
surface layer composed of the carbonitride of Mo on the surface of iron or 
an iron alloy article (hereinafter referred to as an article to be 
treated) thereby to improve the properties of the article to be treated. 
In a conventional method for coating the carbide of molybdenum, the iron 
alloy article is immersed in a molten salt bath composed of the chloride 
system to form a layer of the carbide of molybdenum on the surface of the 
article. 
According to the above method, however, the article is heated at a 
temperature which is higher than the A.sub.c1 transformation point of 
iron, which is about 700.degree. C. The heat is likely to develop (in the 
article) a stress which causes it to crack if it has a complicated shape. 
Moreover, it worsens the working environment, because treatment is done at 
high temperatures. 
To form a surface layer containing molybdenum, there have also been 
proposed methods which employ a temperature which is lower than about 
700.degree. C. They include CVD (chemical vapor deposition) and PVD 
(physical vapor deposition) employing halides of molybdenum. It is, 
however, difficult to form by any of those methods a layer having a 
uniform thickness and adhering closely to the surface of the article. They 
involve a complicated process which requires expensive facilities. 
Moreover, they require the presence of hydrogen or a reduced pressure 
which lowers the efficiency of the operation. 
SUMMARY OF THE INVENTION 
Under these circumstances, it is an object of this invention to provide a 
method which can form a layer of the carbonitride of molybdenum adhering 
closely to the surface of an article made of iron or an iron alloy, 
efficiently by employing a very simple apparatus and heating the article 
at a low temperature so that no thermal strain may develop therein. 
According to a first aspect of this invention, there is provided a method 
for the surface treatment of an article made of iron or an iron alloy 
which comprises preparing a material containing molybdenum and a treating 
agent comprising at least one of cyanides and cyanates of alkali metals 
and alkaline earth metals, and heating the article in the presence of the 
material and the treating agent at a temperature not more than 650.degree. 
C. so that molybdenum, nitrogen and carbon may be diffused through the 
surface of the article to form a surface layer composed of the 
carbonitride of molybdenum. 
According to a second aspect of this invention, there is provided a method 
for the surface treatment of an article made of iron or an iron alloy 
which comprises preparing a material containing molybdenum and a treating 
agent comprising at least one of cyanides and cyanates of alkali metals 
and alkaline earth metals and at least one of the chlorides, 
borofluorides, fluorides, oxides, bromides, iodides, carbonates, nitrates 
and borates of alkali metals and alkaline earth metals, and heating the 
article in the presence of the material and the treating agent at a 
temperature not more than 650.degree. C. so that molybdenum, nitrogen and 
carbon may be diffused through the surface of the article to form a 
surface layer composed of the carbonitride of molybdenum. 
The use of the specific treating agent enables the formation of an 
excellent surface layer composed of the carbonitride of molybdenum at a 
low temperature not exceeding 650.degree. C. The use of such a low 
temperature substantially prevents the development of any thermal strain 
in the iron or iron alloy of which the article is made, improves the ease 
of treatment and eliminates the consumption of a large amount of energy. 
As the layer is formed by diffusion, it has strong adhesion which cannot 
be achieved in any carbide or nitride layer formed by PVD not involving 
any diffusion. It also has a high degree of density and a practically 
satisfactory thickness. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the invention is shown by way of illustrative examples.

DETAILED DESCRIPTION OF THE INVENTION 
According to this invention, a layer which is composed of the carbonitride 
of molybdenum, is formed on the surface of an article made of iron or an 
iron alloy. The article may be of any material containing carbon, such as 
carbon or alloy steel, cast iron or a sintered iron alloy, or of any 
material not containing carbon, such as pure iron. The material may or may 
not contain nitrogen. 
The article is placed in a coexisting relationship with a material 
containing molybdenum and a treating agent and they are heated together so 
that molybdenum, nitrogen and carbon may be diffused through the surface 
of the article to form thereon a layer composed of the carbonitride of 
molybdenum. This layer is composed of the carbonitride consisting mainly 
of molybdenum. A diffusion layer, which is a solid solution of nitrogen 
and carbon in iron, is formed immediately under the carbonitride layer of 
molybdenum. 
The material containing molybdenum is used to supply molybdenum which is 
diffused through the surface of the article. In this connection, it is 
possible to use metals, alloys, or compounds of molybdenum. Examples of 
the metals include pure molybdenum and the alloys thereof, such as 
ferromolybdenum (Fe-Mo) and the like. Examples of the compounds include 
chlorides, bromides and oxides, such as MoCl.sub.5, MoBr.sub.3 and 
Na.sub.2 MoO.sub.4. One or more of these metals or compounds are employed. 
The use of an oxide of molybdenum, such as MoO.sub.3 or the like, is 
particularly preferred from a practical standpoint. 
The treating agent is used to supply nitrogen and carbon which are diffused 
through the surface of the article and also serves as a medium which 
assists the diffusion of molybdenum therethrough. It is composed of one or 
more of the cyanides and cyanates of alkali metals and alkaline earth 
metals (hereinafter referred to as the first treating agent). It is also 
possible to use a mixture of the first treating agent and one or more of 
the chlorides, fluorides, borofluorides, oxides, bromides, iodides, 
carbonates, nitrates and borates of alkali metals and alkaline earth 
metals (hereinafter referred to as the second treating agent). The first 
treating agent supplies the nitrogen and carbon which are diffused through 
the surface of the article. The second treating agent is employed to 
control the melting point, viscosity, evaporation, etc. the first treating 
agent and improve the stability of the treatment, if required. 
More specifically, the first treating agent may, for example, be NaCN, KCN, 
NaCNO or KCNO, or a mixture thereof. The second treating agent may, for 
example, be NaCl, KCl, CaCl.sub.2, LiCl, NaF, KF, LiF, KBF.sub.4, Na.sub.2 
CO.sub.3, Li.sub.2 CO.sub.3, K.sub.2 CO.sub.3, NaNO.sub.2, KNO.sub.3, 
LiBr, KI or Na.sub.2 O, or a mixture thereof. 
When the material containing molybdenum is mixed with the treating agent, 
it is preferable to employ 0.5 to 70% by weight of the material based on 
the weight of the treating agent. There is a tendency that the amount out 
of this range makes it difficult to continuously form a surface layer, and 
it is easier to continuausly form a layer as the amount of the material 
approaches the middle value of the range. 
For heat treatment, methods, such as immersion in a molten salt bath, 
electrolysis in a molten salt, application of a paste and the like, may be 
employed. 
According to the immersion method, the treating agent is melted to form a 
molten salt bath and the material containing molybdenum and the article to 
be treated are immersed in the molten salt bath. When the material 
containing molybdenum is immersed in the molten treating agent, molybdenum 
is dissolved therein. The material which is immersed may, for example, be 
in the form of a powder having a particle size preferably under 200 mesh, 
or a thin plate. Alternatively, it may be a bar or plate serving as an 
anode so that the anodic dissolution of molybdenum may take place in the 
molten salt bath. Molybdenum is dissolved at a speed depending on the kind 
and size of the material which is employed. It is, therefore, necessary to 
hold the molten salt bath at or about a predetermined treating temperature 
for an appropriate length of time before immersion therein of the article 
to be treated. 
The anodic dissolution of molybdenum proceeds quickly and thereby improves 
the efficiency of the treatment. It also has the advantage that no 
undissolved material collects in the bottom of the bath. A vessel which 
holds the molten salt bath, or another conductive material may be used as 
a cathode. The anodic dissolution proceeds at a high speed when the anode 
has a high current density. It is, however, sufficient to employ a 
relatively low current density insofar as no electrolysis is essentailly 
required for dissolving molybdenum. It is appropriate to employ a current 
density from 0.1 to 0.8 A/cm.sup.2. 
The molybdenum which has been dissolved, as well as the nitrogen and carbon 
which have been supplied by the treating agent, are diffused through the 
surface by the article to form a layer which is composed of the 
carbonitride of molybdenum. The vessel which holds the molten salt bath 
may be made of, e.g., graphite, titanium or steel. It is most preferable 
to use a carbonaceous vessel graphite or the like. In this case, a large 
amount of Mo can be diffused in the carbonitride layer as will later be 
described in Examples. 
According to the electrolysis method, the material containing molybdenum is 
immersed in a molten salt bath of the treating agent so that molybdenum 
may be dissolved therein, and the article to be treated is immersed 
therein as a cathode, while a vessel which holds the molten salt bath or a 
separate conductive material is used as an anode. Molybdenum can be 
dissolved in a way which is similar to either of the ways which have 
hereinabove been described in connection with the immersion method. 
Alternatively, the meatrial containing molybdenum can be used as the 
anode, while the article to be treated serves as the cathode. This method 
has the advantage that the anodic dissolution of molybdenum and the 
formation of a surface layer can be accomplished simultaneously. In any 
event, the cathode may have a current density of 2 A/cm.sup.2 or below. A 
range of from 0.05 to 1.0 A/cm.sup.2 is practically appropriate. 
Both of the above methods can be carried out either in an atmosphere 
exposed to the open air, or in the presence of a protective gas, such as 
nitrogen or argon. 
According to the paste method, a paste is prepared from a mixed powder of 
the treating agent and the material contain molybdenum, or from a powder 
obtained by crushing a solidified product of a molten treating agent in 
which molybdenum has been dissolved, and the article to be treated is 
coated with the paste and heated. 
The paste can be prepared by adding to the powder an aqueous solution of 
dextrin, glycerin, water glass, ethylene glycol, alcohol, etc., as a 
binder. The paste is applied to the surface of the article to form a layer 
usually having a thickness of at least 1 mm. Then, the article is usually 
placed in a container and is heated in a heating furnace. It is usually 
sufficient to heat the article in an atmosphere exposed to the open air. 
If a non-oxidizing atmosphere is employed, however, it is advantageously 
possible to apply a paste layer having a smaller thickness. This method 
has the advantage of enabling the formation of a surface layer on only 
that part or parts of the article to which the paste has been applied. The 
powder from which the paste is prepared may have a particle size which 
enables it to pass through, say, a sieve of 100 mesh. The use of a 
somewhat coarser or finer powder may, however, not present any substantial 
problem. 
According to this invention, it is important to employ a heating 
temperature not exceeding 650.degree. C. in order to ensure that 
substantially no strain develops in the substrate, i.e. the iron or iron 
alloy of which the article to be treated is made. It is, however, 
desirable to employ a temperature which is not lower than 450.degree. C. 
If any temperature that is lower than 450.degree. C. is employed, the 
surface layer can only be formed slowly. In practice, therefore, it is 
advisable to select a temperature of 500.degree. C. to 650.degree. C., 
which falls within the range of temperatures usually employed for the high 
temperature tempering of die steels or the tempering of structural steels. 
With a longer treatment time, a thicker surface layer will result. Also, as 
the time is longer, the surface layer has a higher content of molybdenum. 
Therefore, the length of time to be selected for the treatment depends on 
the desired thickness of the surface layer to be formed or its desired 
content of molybdenum. It is usually in the range of from 1 to 50 hours. 
Referring to the thickness of the surface layer, it is practically advisble 
that it have a total thickness of, say, 1 to 30 microns. A surface layer 
having a greater thickness may cause a reduction in toughness of the 
substrate and a spalling of the layer. 
The inventors of this invention are not yet certain about the mechanism 
through which this invention enables the formation of a surface layer 
composed of the carbonitride of molybdenum. The following is, therefore, 
an assumption based on the results of their analysis by X-ray diffraction 
and an X-ray microanalyzer and their study of the relationship existing 
between the length of time spent for the treatment and the thickness of 
the layer thereby formed. In the following description, the letters "m", 
"n", "o" and "p" appearing as suffixes represent different numerals. 
Nitrogen (N) and carbon (C) are diffused into the surface of the article 
made of iron or an iron alloy and react with iron (Fe) to form a layer of 
nitride which can be represented as Fe.sub.m (C,N).sub.n. This nitride 
contains any carbon (C) or nitrogen (N) that the article may originally 
contain. A solid solution of nitrogen and carbon in iron which can be 
represented as Fe-N-C is formed immediately under the nitride layer. These 
reactions gradually proceed from the surface of the article to its 
interior. 
The diffusion of nitrogen and carbon is immediately followed by the 
diffusion of, e.g., molybdenum (Mo) into the nitride layer, and these two 
kinds of diffusion proceed together. The latter diffusion is a reaction 
which causes Mo to replace Fe in Fe.sub.m (C,N).sub.n and thereby convert 
the nitride to (Mo,Fe).sub.o (C, N).sub.p. This reaction also gradually 
proceeds from the surface of the article to its interior. This layer of 
(Mo,Fe).sub.o (C,N).sub.p has an outer surface portion toward which it 
appears to contain a large amount of molybdenum, and an inner surface 
portion contacting the substrate toward which it appears to contain a 
large amount of iron. Therefore, it may sometimes be more appropriate to 
express it as a layer of Mo.sub.o (C, N).sub.p, insofar as its outer 
surface portion contains only a very small amount of iron. 
Moreover, it is possible that other reactions may also take place to form a 
compound of Mo and N, or Mo, N and C on the surface of the substrate. The 
thickness of the (Mo,Fe).sub.o (C,N).sub.p layer, the thickness of the 
layer formed by a solid solution of iron, nitrogen and carbon, the ratio 
of their thicknesses and their chemical compositions depend on the 
material of a substrate, the treating temperature and time, and the kind 
and the mixing ratio of the substrances in the treating agent, etc. 
The inventors of this invention have previously proposed a method which 
treats the surface of an article made of an iron alloy to form thereon a 
layer composed of the nitride or carbonitride of molybdenum (Japanese 
Patent Application No. 288885/1985). This method essentially consists of 
two stages of treatment. The article is first subjected to nitriding 
treatment so that a nitrided layer composed of a compound of iron and 
nitrogen, or iron, carbon and nitrogen, may be formed on the surface of 
the article. Then, the article is placed in a coexisting relationship with 
a material containing molybdenum and a treating agent which is composed of 
one or more of the chlorides, fluorides, borofluorides, oxides, bromides, 
iodides, carbonates, nitrates and borates of alkali metals and alkaline 
earth metals or one or both of an ammonium halide and a metal halide, and 
they are heated together at a temperature not exceeding 700.degree. C., so 
that molybdenum may be diffused into the nitrided layer to form on the 
article a surface layer composed of the nitride or carbonitride of 
molybenum. 
This prior method and the method of this invention are similar to each 
other in that they can both form a surface layer composed of the 
carbonitride of molybdenum by employing a salt bath or paste process at a 
temperature which is sufficiently low to prevent substantially the 
development of any thermal strain in the substrate. This invention can, 
however, be significantly distinguished from the prior method in a number 
of other respects including the following: 
(A) Properties of the product of treatment: 
The products of the two methods under comparison greatly differ from each 
other in toughness, though they do not make any substantial difference in 
surface hardness, or wear or seizure resistance. 
Referring to nitriding treatment in general, it is usual practice to avoid 
the formation of a layer of any compound on the surface of the substrate 
so that it may not lower its toughness. The prior method, however, makes 
it essential to form a layer of a compound having a large thickness. This 
necessarily results in the formation of a layer of a solid solution or 
iron and nitrogen which also has a large thickness. The presence of a 
large amount of nitrogen in solid solution is obvious from the results of 
analysis by an X-ray microanalyzer which will be referred to in further 
detail in the description of examples. The presence of these layers have 
an adverse effect on the toughness of the substrate. 
On the other hand, according to the present invention, the amount of a 
solid solution of nitrogen in the substrate is extremely small and the 
thickness of a layer of a solid solution of iron, nitrogen and carbon is 
small, as will be obvious from the description of examples. Therefore, it 
apparently has a higher degree of toughness than any article treated by 
the prior method. 
(B) Efficiency: 
The method of this invention, which can form a surface layer by a single 
stage of treatment, is more efficient than the prior method which requires 
two different stages of treatment. Moreover, the method of this invention 
requires less facility, since it involves only a single stage of 
treatment. 
The inventors of this invention engaged in concentrative investigations and 
a large number of practical experiments to obviate the problems of the 
prior method. As a result, they have found the method of this invention 
which can form a surface layer of the nitride or carbonitride by a single 
stage of treatment. The layer is substantially equal to that but more 
excellent in toughness than that obtained by two stages of treatment. As a 
nitride or carbonitride- forming element, there can be used vanadium (V), 
chromium (Cr), titanium (Ti), tungsten (W), molybdenum (Mo) or the like. 
These elements have free energy for nitride formation which is large in 
minus. In the case of the prior method based on two stages treatment, it 
was possible to form a surface layer of the nitrides or carbonitrides of 
all these elements. In the case of the method of this invention based on 
only a single stage of treatment, it was possible to form the nitrides or 
carbonitrides of V, Cr, and Mo, but it was difficult to form a surface 
layer composed of the nitrides or carbonitrides of Ti, W and Ta, in spite 
of the result of various studies and considerations thereabout. 
Therefore, the surface layer forming reaction according to this invention 
is not explainable based on free energy for nitride formation. 
The invention will now be described more specifically with reference to a 
variety of examples. 
EXAMPLE 1 
A graphite vessel holding a mixture consisting of 53% by weight of NaCNO, 
12% by weight of KCl and 35% by weight of CaCl.sub.2 was heated in an 
electric furnace in an atmospheric environment, whereby a molten salt bath 
at a temperature of 570.degree. C. was prepared from those substances. A 
powder of pure molybdenum having a particle size under 100 mesh was added 
to the molten salt bath until it occupied 15% by weight of the molten salt 
bath. A sample of the material to be treated was immersed in the molten 
salt bath and after they had been held therein for a period of 8 hours, it 
was taken out and cooled by air. The sample was a round bar of high speed 
tool steel (JIS-SKH 51) having a diameter of 6 mm and a length of 20 mm. 
The sample was ground to expose a cross-sectional surface after any 
unnecessarily adhering bath material had been washed away, and the 
cross-sectional structure of the surface layer which had been formed 
thereon was examined through a microscope. 
FIG. 1 is a microphotograph of 400 magnifications showing the 
cross-sectional structure of the sample. The formed layer was a layer 
having a smooth surface and composed of an inner layer having a thickness 
of about 5 .mu.m and an outer layer having a thickness of about 3 .mu.m. 
The cross-sectional structure of this sample was analyzed by an X-ray 
microanalyzer. The results are shown in FIG. 2. Nitrogen and carbon, as 
well as molybdenum and iron, were found in the surface layer. More 
molybdenum and nitrogen were found in the outer layer than in the inner 
layer, while more iron and carbon were found in the inner layer. Only a 
very small amount of a solid solution of nitrogen was found in the 
substrate immediately under the surface layer. The analysis of the layer 
through its outer surface indicated the presence of about 70% of 
molybdenum. The analysis of the layer by X-ray diffraction showed 
diffraction patterns corresponding to those of MoN(.delta.) and 
(Mo,Fe).sub.6 C. Accordingly, it was evident that the inner layer was a 
layer of the carbonitride of molybdenum and iron expressed as 
(Mo,Fe).sub.m (C,N).sub.n, while the outer layer was a layer of the 
carbonitride of molybdenum including a very small amount of solid solution 
of Fe, expressed as (Mo,Fe)(C,N). 
EXAMPLE 2 
A graphite vessel holding a mixture consisting of 57% by weight of NaCNO, 
13% by weight of NaCN, 9% by weight of NaCl and 21% by weight of 
CaCl.sub.2 was heated in an electric furnace in an atmospheric 
environment, whereby a molten salt bath at a temperature of 570.degree. C. 
was prepared from those substances. A powder of MoO.sub.3 having a 
particle size under 325 mesh was added to the vessel until it occupied 15% 
by weight of the molten salt bath. A sample in the form of a round bar of 
JIS SKH51 high speed tool steel having a diameter of 8 mm and a length of 
20 mm was immersed in the molten salt bath. After eight hours, it was 
taken out and cooled by air. 
FIG. 3 is a microphotograph of 400 magnifications showing the 
cross-sectional structure of the sample. The surface layer which has been 
formed thereon was a double layer, the inner layer of which was extremely 
thinner than the outer layer thereof. The thickness of the inner layer was 
about 2 .mu.m and the thickness of the outer layer was about 12 .mu.m. The 
analysis of the layer by X-ray diffraction showed diffraction patterns 
corresponding to those of MoN(.delta.) and (Mo,Fe).sub.6 C. From the 
result of the analysis by an X-ray microanalyzer shown in FIG. 4, it was 
confirmed that the outer layer was composed of the carbonitride of 
molybdenum and iron expressed as (Mo,Fe)(C,N). The inner layer was 
considered to be iron carbonitride expressed as Fe.sub.m (C,N), although 
it was difficult to be so defined because of the extremely thin layer 
thereof. 
EXAMPLE 3 
A graphite vessel holding a mixture consisting of 57% by weight of NaCNO, 
13% by weight of NaCN, 9% by weight of NaCl and 21% by weight of 
CaCl.sub.2 (i.e. of the same composition with the mixture employed in 
EXAMPLE 2) was heated in an electric furnace in an atmospheric 
environment, whereby a molten salt bath at a temperature of 610.degree. C. 
was prepared from those substances. In addition, a powder of MoO.sub.3 
having a particle size under 325 mesh was added to the vessel until it 
occupied 15% by weight of the molten salt bath. A sample in the form of a 
round bar of industrial pure iron, having a diameter of 7 mm and a length 
or 20 mm, was immersed in the molten salt bath. After eight hours, it was 
taken out and cooled by air. The cross-sectional structure of the sample 
was shown in FIG. 5. The layer formed on its surface was a double layer 
composed of an inner layer having a thickness of about 12 .mu.m and an 
outer layer having a thickness of about 5 .mu.m. The surface layer was 
analyzed by an X-ray microanalyzer and the result was shown in FIG. 6. The 
analysis of the layer by X-ray diffraction showed diffraction patterns 
corresponding to those of MoN(.delta.) and Fe.sub.3 C. Therefore, it was 
evident that the outer layer was the carbonitride of molybdenum and iron 
expressed as (Mo,Fe).sub.m (C,N).sub.n and the inner layer was iron 
carbonitride, including a very small amout of solid solution of 
molybdenum, expressed as Fe.sub.o (C,N).sub.p. 
EXAMPLE 4 
A graphite vessel holding a mixture consisting of 53% by weight of NaCNO, 
12% by weight of KCl and 35% by weight of CaCl.sub.2 (i.e. of the same 
composition as the mixture employed in EXAMPLE 1) was heated in an 
electric furnace in an atmospheric environment, whereby a molten salt bath 
at a temperature of 570.degree. C. was prepared. A plate of pure 
molybdenum having a length of 60 mm, a width of 30 mm and a thickness of 4 
mm was placed in the center of the molten salt bath. An electric current 
was passed through the bath between the molybdenum plate serving as an 
anode and the graphite vessel serving as a cathode for about 16 hours in 
such a way that the anode might have a current density of 0.6 A/cm.sup.2. 
The resulting weight loss of the molybdenum sheet indicated that as a 
result of anodic dissolution, the bath contained about 5% of molybdenum. A 
sample in the form of a round bar of JIS SKH51 high speed tool steel 
having a diameter of 6 mm and a length of 20 mm was immersed in the molten 
salt bath and after 24 hours, it was taken out and cooled by air. 
The sample was cut to expose a cross-sectional surface and the 
cross-sectional structure of the surface layer which had been formed 
thereon was examined by an optical microscope. It was a double layer 
composed of an inner layer having a thickness of about 10 .mu.m and an 
outer layer having a thickness of about 2 .mu.m in the same case as in 
EXAMPLE 2. The cross-sectional structure thereof was analyzed by an X-ray 
microanalyzer. As a result, iron, nitrogen and carbon, as well as about 
50% of molybdenum, were found in the surface layer as a whole, and more 
molybdenum and nitrogen were found in the outer layer than in the inner 
layer, while more iron and carbon were found in the inner layer. The 
analysis of the layer by X-ray diffraction gave diffraction patterns 
corresponding to those of MoN(.delta.) and (Mo,Fe).sub.6 C. 
EXAMPLE 5 
A stainless steel vessel holding a mixture consisting of 51% by weight of 
NaCNO, 21% by weight of NaCl and 28% by weight of Na.sub.2 CO.sub.3 was 
heated in an electric furnace in an atmospheric environment, whereby a 
molten salt bath at a temperature of 650.degree. C. was prepared from 
those substances. A powder of pure molybdenum having a particle size under 
100 mesh was added to the vessel until it occupied 15% by weight of the 
molten salt bath. A sample in the form of a round bar of industrial pure 
iron having a diameter of 7 mm and a length of 20 mm was immersed in the 
bath. Electrolysis was conducted by passing an electric current through 
the bath between the iron bar serving as a cathode and the stainless steel 
vessel serving as an anode for a period of eight hours in such a way that 
the cathode might have a current density of 0.05 A/cm.sup.2. Then, the 
sample was taken out of the bath and cooled by air. 
The sample was cut and its cross-sectional structure was examined through 
an optical microscope. In the same case as in EXAMPLE 2, the surface layer 
formed on the sample was a double layer composed of an inner and an outer 
layer. From the results of analysis by an X-ray microanalyzer, about 30% 
of molybdenum and nitrogen were found in the outer layer, and more iron 
and carbon in the inner layer. These results were all comparable to what 
had been obtained from the other examples of this invention. 
EXAMPLE 6 
A mixture consisting of 45% by weight of NaCNO, 10% by weight of KCl, 25% 
by weight of CaCl.sub.2 and 20% by weight of a powder of pure molybdenum 
was heated to a temperature of 650.degree. C. and the molten mixture was 
carefully stirred to form a uniform bath. One part by weight of graphite 
and one part by weight of alumina powder were added to four parts by 
weight of the bath. They were carefully mixed to prepare a treating agent. 
The treating agent was cooled and pulverized. Ethyl alcohol was added to 
the pulverized treating agent to form a slurry thereof. The slurry was 
applied to the surface of a sample of JIS S45C carbon steel to form a 
layer having a thickness of about 5 mm. After the slurry had been dried, 
the sample was heated at 570.degree. C. for eight hours in a nitrogen 
atmosphere and was, then, cooled. 
After the remaining treating agent had been removed from the sample, the 
surface layer which had been formed thereon was analyzed by X-ray 
diffraction and by an X-ray microanalyzer. It was a double layer including 
an inner layer of iron carbonitride expressed as Fe.sub.m (C, N).sub.n and 
an outer layer of the carbonitride of molybdenum and iron expressed as 
(Mo,Fe)(C, N.). It was comparable to the layer which had been obtained in 
EXAMPLE 3. 
EXAMPLE 7 
A heat resistant vessel holding a mixture consisting of 53% by weight of 
NaCNO, 12% by weight of KCl and 35% by weight of CaCl.sub.2 (i.e. of the 
same composition with the mixture employed in EXAMPLE 1) was heated in an 
electric furnance in an atmospheric environment, whereby a molten salt 
bath at a temperature of 570.degree. C. was prepared from those 
substances. A powder of pure molybdenum having a particle size under 100 
mesh was added to the vessel until it occupied 15% by weight of the molten 
salt bath. A sample in the form of a round bar of JIS SKH51 steel having a 
diameter of 6.5 mm and a length of 40 mm, which had been hardened and 
tempered under standard conditions, was immersed in the bath and after 
eight hours, it was taken out and cooled by air. After the remaining bath 
material had been washed away, the surface layer which had been formed on 
the sample was subjected to analysis by X-ray diffraction. It gave 
diffraction patterns corresponding to those of MoN(.delta.) and 
(Mo,Fe).sub.6 C. 
The sample (hereinafter referred to as Sample No. 1) was subjected to a dry 
friction test by a Falex lubricant testing machine employing a piece of 
gas carburized JIS-SCM415 chromium molybdenum steel as a counter material. 
The test was continued for a period of four minutes at a load of 200 kg, a 
rotating speed of 300 rpm and a sliding speed of 0.1 m/sec. For the sake 
of comparison, a similar test was conducted on each of a sample of 
JIS-SKH51 steel as hardened and tempered (Sample No. S1) and a sample of 
SKH51 steel as nitrided (Sample No. S2) 
Sample No. S1 showed a wear of about 17 mg/cm.sup.2. It showed a 
coefficient of friction which was as high as 0.280 when measured 30 
seconds after the test had been started. Sample No. S2 showed a wear of 
about 15 mg/cm.sup.2 and its coefficient of friction was as high as 0.265 
when measured 30 seconds after the test had been started. On the other 
hand, Sample No. 1 embodying this invention showed a wear which was as 
small as about 6 mg/cm.sup.2 and its coefficient of friction was as low as 
0.110 when measured 30 seconds after the test had been started. 
A similar friction test was also conducted on each of a sample of JIS-SKH51 
steel which had been coated with a layer of vanadium carbide (VC) having a 
thickness of about three microns by 1.5 hours of immersion in a molten 
salt bath having a temperature of 1020.degree. C. and a sample of the same 
steel which had been coated with a layer of titanium carbonitride 
expressed as Ti(C,N) and having a thickness of eight microns by four hours 
of CVD at 850.degree. C. The wear of each of these samples and its 
coefficient of friction were both substantially equal to those of Sample 
No. 1. Therefore, it is obvious that the surface layer which can be formed 
in accordance with the method of this invention is comparable in wear and 
seizure resistance to any surface layer formed by immersion in a high 
temperature molten salt bath or by CVD. 
EXAMPLE 8 
A heat resistant steel vessel holding a mixture consisting of 60% by weight 
of NaCN and 40% by weight of KCN was heated in an electric furnace in an 
atmospheric environment, whereby a molten salt bath at a temperature of 
600.degree. C. was prepared from those substances. A powder of MoO.sub.3 
having a particle size under 250 mesh was added to the vessel until it 
occupied 15% by weight of the molten salt bath. A sample in the form of a 
round bar of JIS SKH51 steel having a diameter of 8 mm and a length of 20 
mm was immersed in the bath and after 2 hours, it was taken out and cooled 
by air. 
After the remaining treating agent had been removed from the sample, it was 
subjected to analysis by X-ray diffraction and by an X-ray microanalyzer. 
The surface layer which had been formed thereon was a layer of the 
carobnitride of molybdenum and iron consisting mainly of a mixture of 
MoN(.delta.) and (Mo,Fe).sub.6 C.