Note: Descriptions are shown in the official language in which they were submitted.
~3Q3~
Field_of the Invention
This invention relates ~enerall~ to the metal science
and therm~l treatment of metals and alloys. More specifi-
cally, the invention relates to processes for chemical and
thermal treatment of steel ~workpieces to obtain coatings
by diffusive precipitation.
The proce~ according to the inventiorl can find appli-
cation for obtaining coatin~s capable of imitating, in
terms of their physical and chemical properties,~uch pre-
cious metals a9 gold and platinum with the aim of rsducing
consumption of these metals or using ~uch coatings as al-
ternative materials for parts and components in instrument
making (precision friction pairs, electrical contacts, ter-
minals, variable resistor wires), in medicine (for making
dentures and surgical toDls), in electrical engineering
(electrical contacts), in horology (fabricating watcb ca-
9ing9, bracelets and watch Parts), in jewelr~ practice
(fDr making decorations and di~hware), and in the chemical
industry (for protecting parts ~rom corrosion or making
them more resistant to heat).
The process according to the invention can also be
used duri~g fabrication of reflector screens of various
designation9 and for application Df coatings to protect
parts from ~ea-watcr corrosion.
Backgrou~d of the Tnvention
Widely used nowdays are various techniques for obtailing,
normall~ galvanically, precious metal coatings (gold-plating
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and palladizi~g). However, galvanizi~g most o~ten ~ails
to ensure reliable coatings on parts of shaped con~igura-
tion. Thus, coatings obtained by the known methods are in-
herentl~ disadvantaæeous because of low bonding with the
base metal, non-uniform thickness especially at the corners
o~ workpieces, low hardness, and susceptibili-ty to wear.
Also, application o~ thase known processes necessitates
workpiece sur~ace pretreatment, such as mechanical cleani~g
and pickling.
To improve the physical and chemical properties of
coatinæs, new shop processes have made way to industrial
use lately, one such process involving chemical a~d thermal
treatment o~ metal parts by di~usive precipitation.
~ usion coatings are sur~ace layers characterized
by low porosity and high bondi~g with the b~se matal. Di~u-
sion coatine processes can provide surface layers o~
various chemical compositions to guarante~ such advantageou~
properties as high resistance to wear, su~ficient hardness,
tolerance to corrosive atmosphere, and high mechanical
strenæth.
There is known a process for chemical ~nd thermal
treatment o~ steel workpieces accompanied by the formation
of a coating thereon obtained by di~u~ive precipitation
or deposition onto the ba~e metal o~ a substance from a
melt of a low-melting-point metal (cf. USSR Inventor's
Certificate ~o. 582,329; IPC C 23 C 9/10).
Sodium is used a~ the lo~-melting-point metal o~ the
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melt, whereas platinum is employed a~ the substancs which
forms a coating in the course of diffusive precipitation.
Preferred conditions of the diffusive precipitation
temperature - 630 to 6700C; duration - 5 to 6 hr~.
Therefore, -the aforedescribed process resides in the
use of a fu~ible or low-melting-point metal in nhich there
are introduced other metal elements to precipitate onto
the base metal and thus L orm a diffusion coating. An i90-
thermal proce~s of mas~ transfer takes place in ~hich the
gub9tance i9 dissolved in the form of metal elements in
the melt of fusible metal to be transferred and adsorbed
on the surface of the workpiece being coated to bond there-
with, and the element(s) are interacted with the metal of
the workpiece through diffusion. As a result, a coating i9
formed on the base metal composed of the elements taking
part in the diffusive precipitation.
However, this process fails to provide coatings of
predetermined stoichiometric composition with substantially
uniform coating layer thickness.
In addition, the use of the known processe~ of chemical
and thermal treatment by diffusive precipitation with preci-
ous metals snd alternative imitation metals is disadvantage-
ous due to high losses of such metals during the processes.
Summary of the Invention
It is therefore an ob~ect of the present invention to
provide a process for chemical and thermal treatment of
steel workpieces to obtain on the base metal thereof a
coating of a predetermined 9toichiometric composition.
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Another o~ject i5 provide a process for chemical
and thermal treatment of steel workpieces to obtain on
the base metal thereof a coating of predetermined thickness
su~ficient for meetin~ the demands of designated service
conditions.
O~e more object is to provide a process for chemical
and thermal treatment of steel workpieces to obtain on the
base metal thereof a coating which ~ould imitate in physical
and chemical propertie 5 precious metals, that is a coating
possesing high resistance to corrosion a~d having luster
and coloration imitating gold or platinum.
~ hese and other attending objects and advantages are
attained by that in a proces~ ~or chemical and thermal trsat-
ment o~ steel workpieces to form a coating thereon by di~usi-
ve precipitation on the base metal o~ the steel workpiece
o~ a substance ~rom a low-melting-point metal melt according
to the invention, the diffusive precipitation process i9
carried out at a temperature of between 720 and 820C ~or
a duration su~ficient to obtain a coati~g layer o~ required
thickness, t~e melt pre~erably containing sodium or lithium
as the low-melti~g-point metal, the substance beiDg precipi-
tated having the form o~ an intermetallic compound.
The use o~ molten sodium or lithium make 9 it possible
to obviate the need for the workpiece sur~ace pretreatment
b~ pickling, since melts of the above metals act to remove
sur~ace oxides and enable to clean ~Norkpieces and parts
of complex configuration, as well as i~terior cavities
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53e:~39
and grooved portions thereof due to the fact that such
melts have high wetting power and ~luidity. Also, the
use of baths o~ such molten metals makes a subsequent mecha-
nical cleanin~ of the workpiece sur~ace super~luous, as
the remainder o~ the melt penetrated into slits and cle~ts
of tAe workpiece can be evacuated by washing the workpiece
in water. In addition, the loss o~ the substance used for
precipitating a coati~g by di~fusion is negligeable, because
such a substance dissolves in sodium or lithium melts is
small quantities, whereas the chemical and thermal treatment
process follows a pattern whereby the amount of the substance
dissolYed in the melt i9 substantially equal to the amount
thereof diposited on the workpiece surface being coated.
Introduced to the melt as a substance being precipitated
is an intermetallic compound o~ stoichiometric composition
having predetermined physical and chemical properties, such
as compounds capable of imitating precious metals, to enable
to obtain coatings composed o~ the compound introduced into
the melt, that is coatings of predetermined composition,
since dissolution in the melt, transfer and precipitation
o~ the compound elements takes place according to the
stoichiometric proportion corresponding to the composition
of the compound introduced, whereby the composition o~ the
coating layer obtained tends to be uniform throughout the
thickness thereof.
The process is conducted at a temperature within a
range o~ between 720 and 820C. Such temperature conditions
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53 ~3 9
provide ~or all the basic physical and chemical processes
necessary for ensuring chemical and thermal treatment to
take place, particularly, sufficiently vigorous dissolution
of various intermetallic compounds in the melt, diffusive
transfer of the elements being precipitated from the melt
toward the base metal of the workpiece, and formation of
coatings having a thickness suf~icient to meet the demands
imposed by intended service conditions of the workpiece. The
thickness of the coating depends on the duration of diffusive
precipitation in turn determined by the physical and chemical
parameters of the process.
For effecting the process the amount of the intermetallic
compound to be introduced into the melt is preferably
determined by:
G1 ~ 0.03 G2 ~ S ~- ~ , where
G1 = weight of the intermetallic compound, in g;
G2 = weight o~ the low-melting-point metall, in g;
S = surface area of the workpiece being coated, in cm2;
= required thickness of the coatinæ layer, in cm; and
= density of the intermetallic compound, in g/cm3.
In order to obtain coatings imitating in color platinu~
and having su~f~cient resistance to corrosion, tolerance
to oxidation at high temperatures, and high mechanical
characteristics (hardness and wear resistance), it is
advisable for the di~fusive precipitation to be conducted
at a temperature of from 72~ to 78~C for a duration bet-
ween 6 and 8 hrs in a molten bath containing lithium and
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an intermetallic compound of nickel and aluminum.
For obtaining coating~ imitating gold the diffusiveprecipitation is preferably carried out at a temperature
between 780 and 820C for 6 to 8 hrs in a melt containing
~odium and an intermetallic compound of palladium and
indium.
~ olten sodium likewise provides necessary conditions
for obtaining a coating of required properties (sufficient
solubility of the compound, deposition of the compound
elements on the base metal of the steel workpiece, no
vi~ible steel dissolution at a phase combination: intermetal-
lic compound of palladium and indium -- sodium -- steel).
Preferred proce3s parameters of 730 to 820C and the
precipitation time of 6 to 8 hrs ensure the deposition of a
coating having a thickness practicable for a wide range of
applications.
Detailed Description of the Invention
~ he invention ~ill now be described in greater detail
with reference to various preferred modes of carrying it out.
A wor~piece to be coated is placed into a reaction ves-
sel such as an ampoule fabricated from an inert material
(one that fails to dissolve in the molten metal used for pre-
c~pitating the coating). rhereafter, in a neutral gaseous
atmosphere, ~uch a3 argon, the ampoule is filled with a
melt of a lo~-melting-point alkali me-tal, such as sodium
or lithium, and a substance to be precipitated onto the
base metal in the form of an intermetallic compound.
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303~
The ampoule is then sealed in the argon atmosphere
by welding or ~oint stopp~ and placed in a furnace, such
as a muffle electric furnace, for the process of diffusive
precipitation to be carried out therein at a temperature
of from 72~ to 820C for a duration o~ time necessary to
obtain a la~er of coating of required thickness.
The proporti on o~ the intermetallic compourLd to be
present in the melt is determined by:
G1 = 3 G2 + S~ , where
G1 = wei~ht of the lntermetallic compound, in g;
G2 ~ weight of the low-melting-point metal, in g;
S = surface area of the workpiece, in cm2;
= required thickness of the coating layer, in cm; and
= density of the intermetallic compound, in g/cm2.
~ he first term of the above equation takes account of
the amount o~ compound necessary for saturation oY the
molten sodium or lithium, and for a wide range of compounds
it corresponds to 0.03~ G2, that is saturation concentration
is ensured by a value of close to 3 mass per cent.
~ he second term of the equation takes account oY the
amount of compound necessary for dipositing a required
thickness of -the coating layer, and is determined by the
size of the ~orkpiece to be coated, the thickness of the
coating layer and the density o~ the compound, that is
this term establishes a general connection between the
geometrical dimensions and the mass o~ substance through
its densit~.
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~ 3 9
A~ter holding the ampoule at a temperature providi~g
~or diffusive precipitation, and subsequent to cooling,
it is opened for the workpiece having a coating deposited
thereon to bs extracted -therefrom and washed in the running
water.
In order to obtain a coating which would imitate
platinum in terms of color and physical-chemical properties,
the workpiece is subjected -to diffusive precipitation at
a temperature between 720 and 780C ~or 6 to 8 hrs in a
molten metal composition comprising lithium a~d an i~ter-
metallic compound of nickel and aluminum, the amount of the
compound being determined accordiDg to the aforecited equa-
tion.
~ or obtaining a coatiDg which would imitate gold in
terms of color and physical-chemical properties, the di~fu-
sive precipitation is carried out at a temperature between
780 and 820C within 6 to 8 hrs in a melt composed of
molten sodium and an intermetallic compound of palladium
and indium.
Described hereinbelow are ~arious specific examples
of the best mode ~or carrying out the process according
to the in~ention.
Example 1
Placed in an ampoule is 8 tea-spoonfulL of steel o~
the following compositio~ (in mass per cent): C ~ 0.08;
~n = 1 - 2; Cr = 17 - 19; ~i = 9 - 11; ~i = 0.7; ~e = the
balance, and a sample of intermet~llic compound comprising
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53~39
a mixture of palladium and indium in the ratio of 56 to 44
mass per cent, respecti~ely, for the ampoule to be filled
with molten sodium.
~ he amount G1 = 5.17 g of the intermetallic compound
is determined by the above equation, where
G2 = 150 g;
S = 37 cm2;
= 0.0018 cm; a~d
~ = 10 g/cm3.
Subsequent to seali~g in argon a-tmosphere, the ampoule
is placed into a muf~le electric furnace where it is held
~or 6 hrs at a temperature of 780C. Thereafter, the ampoule
is ope~ed for the workpiece to be retrieved therefrom and
washed in the runnin~ water.
As a result of the chemical and thermal treatment, a
coating i~ ~ormed o~ the workpiece which is similar in
corrosion resista~t properties and luster to gold, this
coating layer having a thickness of 18 mkm, a mîcrohardness
o~ between 2,100 and 2,400 MPa, and a gold-pink coloration.
100 hrs o~ testing in acidic and alkaline media evide~-
ced no coating surface corrosion.
E~ample 2
As distinct from Example 1, an ampoule ~ith a workpiece
in the ~orm of a steel ri~g of the following composition
(in mass per cent): C = 0.24 - 0.32; Fe = the balance, was
subjected to thermal treatment at a temperature of 800C
for 7 hrs.
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The initial data for determining the value o~ G1 ~ 1.16 g
of the întermetallic compound (palladium and indium) is
found from the above equation, where
G2 = 3 ~i
S = 9.4 cm2;
= 0.0025 cm; and
= 10 g/cm3.
As a result of the chemical and therrr~al treatment,
a coating layer is formed on the steel ring in corrosion
resistance and luster imitating gold and having a thickness
of 25 mkm, microhardness of between 1~850 and 29025 ~Pa, and
gold-pink in color. Corrosion resistance property is sub-
stantially the same as described with reference to the coa-
ting obtained according to Example 1
Example 3
Placed into an ampoule is a watch case of a steel of
the following composi-tion (in mass per cent): C = 0.07 -
- 0.13; Fe = the balance, and a sample of an intermetallic
compound of palladium and indium proportioned 56 to 44 mass
per cent, respectively, the ampoule beinæ then ~illed with
molten sodium.
The amount of the intermetallic compound is determined
according to the above equation, where
G2 - 70 æ;
S = 28 cm2;
= 0.0035 cm; and
= 10 ~/cm3.
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As a result, G1 = 3.08 g.
~ he ampoule with the 3~0redescribed content i9 held
in a muffle electric furnace for ~ hrs at a temperature of
820C.
~ he resultant chemical reaction and heat treatment
produce a coatin~ layer o~ 35 mkm in thickness and having
a microhardness of between 1t800 and 2,1~0 ~Pa, The color
o~ the coating thu~ obtained and corrosion resistance the-
reo~ are similar to those indicated with reference to
Example 1.
Example 4
A tea-spoonfull of steel is placed into an ampoule,
the steel having the followinæ composition (in mass per cent):
C = 0.24 - 0.32; Fe = the balance, and a sample of an
intermetallic compound o~ nickel and aluminum (G1 = 3-35 g)1
the ampoule being then ~illed with molten lithium. The
amount of G1 of the intermetallic compound is determined
accordin~ to the equation mentioned before, where
G2 = 82 g,
S = 37 cm ;
= 0.004 cm; and
= 6 g/cm .
Subsequent to sealing of the ampoule in an atmosphere
o~ argon, it is held in a furnace for 6 hrs at a temperature
o~ 720C. Therea~ter, the ampoule is opened and the ste~l
is removed there~rom to be ~ashed in the running water.
~he chemical and thermal treatment provides for the
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formation of a coating layer in corrosion resistance
propert~ and luster similar to platinum, a thickness
of 40 mkm, and microhardness of between 4,10~ and 4,200 MPa.
The color of the thus obtained coating is light-grey, while
resistance to corrosive acidic a~d alkaline media i9
comparable to a chrome-nickel steel.
Example 5
Placed into an ampoule is a watch case of a steel
having the following composition (in mass per cent):
C = 0.07 - 0.13; Fe _ the balance, and a sample of an inter-
metallic compound of nickel and aluminum (G1 = 2~9 g),
the ampoule being then filled with molten lithium. The
amount of G1 is determined from the equation applicable
to the previous examples, where
G2 ~ 40 g,
S = 28 cm ;
= 0.010 cm; a~d
= ~ g/cm~
~ he ampoule is held for 7 hrs at a temperature of 750C.
As a result of chemical and thermal treatment, a coa-
ting layer is formed approximating in corrosion resistant
properties and luster to platinum and having a thickness
of 130 mkm, and microhardness of between 4,500 and 4,800 MPa.
The corrosive resistance and color of the thus obtained
coating are similar to those produced by the process descri-
bed in Example 4.
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Example 6
The process is conducted substantially as described
in Example 5, the difference being in that an intermetallic
compound of nickel and aluminum in the amount of G1 = 3.~8 g
is used determined from the same equation, where
G2 = 4~ g~
S = 28 cm ;
= 0.013 cm; and
d = 6 g/cm3.
The ampoule is heat~treat~d for 8 hrs at a temperature
o~ 780C.
A coating is formed on the base metal of the workpiece
in corrosion resistance and luster similar to platinum ~ d
having a thickness of 130 mkm and microhardness of be~ween
4,500 and 4,800 MPa, the color and resistance to corrosion
being substantially similar to the coating described with
reference to Example 4.
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