[0001] The present invention relates to a method for masking carburizing and nitriding,
and in particular to a method for masking carburizing and nitriding by means of a
powder for prevention of carburizing and nitriding so that a portion of a metallic
part such as steel etc. is hardened by carburizing and nitriding while the toughness
of the other portion remained in non-carburized or non-nitrided states is maintained.
[0002] In metallic machine parts such as cams, shafts, pistons, pins or various gears and
cutting tools used for automobiles, ships etc., the parts as a whole require toughness,
while the parts undergoing friction require high-level wear resistance. As a method
of obtaining such machine parts having both toughness and wear resistance, there is
a method in which a tough steel material is used and only the portion thereof requiring
wear resistance is hardened by subjecting it to carburizing and nitriding treatment,
and in this case, the other portion not to be hardened is prevented by masking from
being carburized and nitrided, thus keeping its toughness.
[0003] Although this type of conventionally used masking material includes copper plating
or tin plating, the plating operation for masking is complicated and cumbersome, so
coating-type masking materials for forming a gas barrier film were developed recently
and came to be distributed rapidly. That is, the coating-type masking materials are
those containing chemical powder (e.g. borax and borosilicic acid or tin powder) having
a carburizing and nitriding preventing action added to a small amount of resin and
solvent, and this coating is applied onto a specific portion of a steel material before
thermal treatment for carburizing and nitriding. Then, the material is introduced
into a furnace charged with a carburizing and nitriding agent or in an atmosphere
of carburizing and nitriding gas and then heated at 300 to 1000°C whereby the resin
in the coating is thermally decomposed and disappears, while the carburizing and nitriding
preventing components in the coating are baked on the surface of the steel material
to form a carburizing and nitriding preventing film by which the material is prevented
from contacting with the carburizing and nitriding components, thus preventing said
coated portion from being carburised and nitrided. In this case, if an uneven coating,
pinhole defects etc. are present on the carburizing and nitriding preventing film,
the object of preventing carburizing and nitriding cannot be achieved, so it is essential
to form a defect-free and uniform carburizing and nitriding preventing film.
[0004] However, this coating has a less amount of incorporated resin components working
as vehicle components (because a larger amount leads to significant discharge of its
decomposed gas, and the baking of the carburizing and nitriding preventing components
is thus prevented), so its flow-out is inadequate, and it is necessary that the coating
should be diluted with solvent and applied repeatedly in order to form a uniform film.
Accordingly, this requires careful operation and much labor using brush etc.
[0005] As a method for masking of carburizing and nitriding in which reliable carburizing
and nitriding effects can be obtained regardless of whether the surface of a substrate
treated is flat, curved or uneven, the present inventors disclosed, in Japanese Patent
Application No. 91436/96, a method wherein a powder for prevention of carburizing
and nitriding containing as essential components a boron-based inorganic compound
having a carburizing or nitriding preventing action and thermally fusible resin to
be thermally decomposed under carburizing and nitriding conditions is fused and bonded
onto the portion of a treated metal to be prevented from being carburized and nitrided.
By this method for carburizing and nitriding preventing treatment, a thick film excellent
in carburizing and nitriding preventing effects could be formed uniformly using the
simple means regardless of the shape and surface conditions of a base material to
be treated.
[0006] In the above method for masking of carburizing and nitriding by use of the carburizing
and nitriding preventing agent comprising the substance having a carburizing and nitriding
preventing action and thermally fusible resin in the carburizing and nitriding treatment,
the above carburizing and nitriding preventing agent should accurately and completely
be applied without any gap to the portion whose carburizing and nitriding are intended
to be prevented. For such application, only the portion to be subjected to carburizing
and nitriding treatment should be heated accurately and adequately at a temperature
not lower than the fusing temperature of the thermally fusible resin.
[0007] The object of the present invention is to provide a method for accurately carburizing
and nitriding a portion to be subjected to carburizing and nitriding treatment by
permitting a carburizing and nitriding preventing agent to be fused and bonded accurately
to only the portion to be masked from being carburised and nitrided.
[0008] The above problem has been solved by a method for masking from carburizing and nitriding
which is characterized in that only the portion to be prevented from carburizing or
nitriding is heated selectively by a laser beam.
[0009] The present invention relates to a method for masking a portion of a metal material
to be carburized or nitrided, which comprises coating a powder for prevention of carburizing
or nitriding over a region including at least said portion to be prevented from being
carburized and nitrided, heating only said portion by laser beam, thereby fusing and
bonding the powder to only the portion to be prevented from being carburized and nitrided.
[0010] FIG. 1 shows a side view of one embodiment for carrying out the carburizing and nitriding
preventing treatment of the present invention.
[0011] The present invention relates to a method for masking a portion of a metal material
to be carburized or nitrided, which comprises coating the powder for prevention of
carburizing or nitriding over a region including at least said portion to be prevented
from being carburized and nitrided, heating only said portion by laser beam, thereby
fusing and bonding the powder to only the portion to be prevented from being carburized
and nitrided.
[0012] In particular, the present invention relates to the method for masking carburizing
and nitriding in which a powder comprising as essential components the substance having
a carburizing or nitriding preventing action and the thermally fusible resin to be
thermally decomposed under carburizing or nitriding conditions is used as the powder
for prevention of carburizing and nitriding.
[0013] According to the present invention, the powder for prevention of carburizing and
nitriding is applied by a coating method such as an electrostatic coating to the whole
of a material treated or to a region including at least the portion to be prevented
from being carburized and nitrided, and then only said portion is heated by laser
irradiation to a temperature higher than the fusing temperature, and said powder is
fused and bonded to just said portion . Hence, regardless of the shape and surface
conditions of the base material, said powder can be allowed to adhere easily and accurately
to only the portion to be prevented from being carburized and nitrided. The heating
temperature should be set higher than the fusing temperature of the fusible components
in the powder, and usually the powder is heated at about 120 to 350°C. The powder
on the site not irradiated with the laser, i.e. the site not requiring carburizing
and nitriding preventing treatment, is neither fused and bonded onto the material
treated and can thus be removed by e.g. air spraying after laser irradiation.
[0014] The heating means are desirably those capable of heating only the target portion
reliably and accurately regardless of whether the shape of the material treated has
a curved surface or unevenness or whether the site thereof to be prevented from being
carburized and nitrided has a complex or fine pattern. As such heating means, a laser
beam heating method is used in the present invention.
[0015] After the material to be treated is coated with a powder for prevention of carburizing
and nitriding, this heating means is used to strictly heat only the portion to be
prevented from being carburized and nitrided whereby the powder for prevention of
carburizing and nitriding can be allowed to adhere to only the heated site easily
and reliably. After the powder is fused and bonded onto the given portion in this
manner, the material is subjected to carburizing and nitriding treatment where a temperature
of 500 to 1000°C is usually used, so before this temperature is reached, the resin
in said powder is thermally decomposed and disappears, and only the substance having
a carburizing and nitriding preventing action remains to form a carburizing and nitriding
preventing film, thus preventing the portion from being carburized and nitrided.
[0016] The laser beam heating method used in the present invention is carried out preferably
using a scanning type beam irradiation apparatus equipped with a scanning head for
scanning depending on a pattern memorized in a computer to emit laser rays to only
a portion requiring heating.
[0017] For a pattern to be irradiated with the laser beam, the laser emitting head may be
moved by the computer, or while the emitting head is fixed, the material treated may
be moved, and in either case, it is preferable to trace the pattern by automatic control.
[0018] The width of the emitted laser beam is selected depending on the size and accuracy
of the portion to be subjected to carburizing and nitriding preventing treatment.
To strictly heat only the portion to be prevented from being carburized and nitrided,
the irradiation apparatus is preferably that capable of adjusting beam width to 1.0
mm, preferably 0.1 mm. For laser output, the size of the laser is selected such that
it can rapidly heat the desired portion of the material treated.
[0019] The computer-controlled laser generating apparatus satisfying these requirements
may be those commercially available, including e.g. scanning type laser maker apparatus
ML-4140 C (manufactured by Miyachi Technos K.K.).
[0020] The specific method of permitting the powder for prevention of carburizing and nitriding
adhere to the material treated is not particularly limited, and it is possible to
suitably modify and employ conventional methods such as fluidization dip method, spray
method, electrostatic adhesion method etc.
[0021] FIG. 1 shows a preferable example of an automatic apparatus capable of laser beam
heating and applying the powder for prevention of carburizing and nitriding. The method
for prevention of carburizing and nitriding according to the present invention will
be described by reference to this drawing.
[0022] Test specimen (6) requiring partial prevention of carburizing and nitriding is transferred
by 6-axis robot (3) to powder coating booth (2) where the powder for prevention of
carburizing and nitriding is allowed to adhere by electrostatic coating to the whole
of the test specimen. This test specimen is transferred by the 6-axis robot to the
laser irradiation position in YAG laser marker (4), and only the portion requiring
prevention of carburizing and nitriding is irradiated with the laser whereby the powder
for prevention of carburizing and nitriding is fused and bonded to only that portion
of the test specimen. For laser irradiation, the X- and Y-axes for laser irradiation
can be controlled by laser maker controller (5), and the Z-axis can be controlled
by the 6-axis robot. Thereafter, the robot transfers the test specimen to the coating
booth, and the powder not fusion-bonded is scattered by air blow.
[0023] The substance having a carburizing and nitriding preventing action, which constitutes
the powder for prevention of carburizing and nitriding in the present invention, is
preferably a boron-based inorganic compound or boron oxide-based amorphous substance.
Such a boron-based inorganic compound or boron oxide-based amorphous substance is
softened under heating conditions carburizing and nitriding treatment (usually 300
to 1000°C) and almost simultaneously with combustion by thermal decomposition of the
aforementioned fusible resin, forms a close carburizing and nitriding preventing film,
thereby demonstrating carburizing and nitriding preventing functions. As such compounds,
boron-based inorganic compounds are selected and these can be baked on the surface
of a base material at a temperature of 450°C or more to form a close carburizing and
nitriding preventing film thereon.
[0024] The boron-based inorganic compounds include e.g. borax, boron oxide, borosilicic
acid, phenyl boric acid etc. Boron oxide is particularly preferable.
[0025] The boron oxide-based amorphous substance used in the present invention is an amorphous
substance comprising boron oxide B
2O
3 and optionally other inorganic materials.
[0026] The preferable amorphous substance includes boron oxide-silicon oxide, boron oxide-silicon
oxide-clay, boron oxide-silicon oxide-alumina, etc.
[0027] In the amorphous substance of boron oxide-silicon oxide, B
2O
3 : SiO
2 is 65 to 95 : 35 to 5, preferably 70 to 90 : 30 to 10 (ratio by weight).
[0028] The boron-based inorganic compound or boron oxide-based amorphous substance is preferably
that with a water content of 10% or less, more preferably 5% or less. If the water
content in the boron-based inorganic compound or boron oxide-based amorphous substance
is too high whether water is that of adhesion or of crystallization, evaporation of
the water may cause pinhole defects etc. to be formed in the resulting masking film
for preventing carburizing and nitriding, so that a reliable carburizing and nitriding
preventing film is hardly formed or the carburizing and nitriding preventing film
may not be formed at all.
[0029] To form the masking film having a thickness necessary and adequate for carburizing
and nitriding prevention, the content of the boron-based inorganic compound or boron
oxide-based amorphous substance in said powder is set preferably in the range of 20
to 80% by weight in terms of anhydrides thereof, and if its content is deficient,
the carburizing and nitriding preventing film may become thin and uneven, and pinhole
defects may occur after decomposition and combustion of the resin, making it difficult
to achieve reliable carburizing and nitriding preventing effects. In contrast, if
its content is too high, the absolute amount of the resin is made relatively deficient,
and said powder may not adequately adhere to the portion to be prevented from being
carburized and nitrided, so adhesion of the masking film formed at the time of carburizing
and nitriding treatment is worsened, making it difficult to achieve reliable carburizing
and nitriding preventing effects.
[0030] On the other hand, the characteristics of the thermally fusible resin are that it
acts as an adhesive component for permitting the powder for prevention of carburizing
and nitriding to adhere to the portion to be prevented from being carburized and nitrided
and that it is thermally decomposed to disappear under the carburizing and nitriding
preventing treatment. That is, it is necessary to permit said powder to adhere to
the portion to be prevented from carburized and nitrided; in the present invention,
the adhesion of said powder to the portion to be prevented from being carburized and
nitrided can be easily effected by incorporation of a predetermined amount of the
thermal fusible resin as an essential component into said boron-based inorganic compound
or boron oxide-based amorphous substance. As the thermally fusible resin, thermoplastic
resin and thermosetting resin can be used, and these can easily adhere to the portion
to be prevented from being carburized and nitrided because the thermoplastic resin
is rendered softened and molten by heating and the thermosetting resin is rendered
plasticized and fusible prior to hardening reaction.
[0031] Accordingly, the first characteristic of this thermally fusible resin is that it
is rendered softened and molten by heating or plasticized and fused and bonded prior
to thermal hardening reaction. It is desired to select the resin which is thermally
fused and bonded to the material preferably at a temperature of 350°C or less, more
preferably about 300°C or less. In the method described below, however, it is hard
to permit the resin thermally fused and bonded at too high temperature, e.g. about
350°C or more, to reliably adhere to the portion to be prevented from carburized and
nitrided, making it often difficult to achieve satisfactory carburizing and nitriding
preventing effects.
[0032] The second characteristic of the fusible resin is that after the powder for prevention
of carburizing and nitriding is allowed to adhere to the portion to be prevented from
carburised and nitrided and before the carburizing and nitriding treatment is conducted,
the resin should be thermally decomposed to disappear so as not to impair formation
of a masking film consisting exclusively of the boron-based inorganic compound or
boron oxide-based amorphous substance. Thus, it is desired to select the resin thermally
decomposed to disappear preferably in the range of 400 to 600°C. This is because the
resin which is thermally decomposed at too high temperature e.g. 700°C or more will
undergo thermal decomposition during carburizing and nitriding to generate pinhole
defects in the resulting carburizing and nitriding preventing film, making it difficult
to achieve reliable carburizing and nitriding preventing effects.
[0033] As thermally decomposable resin satisfying these required characteristics, mention
can be made of various kinds of resin, among which particularly preferably used are
thermoplastic resins such as polyolefin type resin such as polyethylene, polypropylene
etc., polyester type resin, and acryl type resin, and thermosetting resins such as
thermosetting polyester type resin and acryl type resin. If the thermosetting resin
is used, it is preferable to select a hardening agent to show hardening reaction by
releasing a blocking agent after the main agent is thermally plasticized by blocking
a cross-reactive functional group.
[0034] The amount of the thermally fusible resin in the powder for prevention of carburizing
and nitriding according to the present invention is preferably in the range of 20
to 80% by weight, more preferably 40 to 60% by weight, and if the amount of the incorporated
resin is deficient, the powder tends to be difficult in the following method to be
fused and bonded to the portion to be prevented from being carburized and nitrided,
whereas given a large amount of the incorporated resin, the absolute amount of the
boron-based inorganic compound or boron oxide-based amorphous substance becomes deficient,
thus making it difficult to form a close masking film, and in either case, none of
satisfactory carburizing and nitriding preventing effects may be obtained.
[0035] However, if a powder containing a suitable amount of the boron-based inorganic compound
or boron oxide-based amorphous substance and the thermally fusible resin is used,
the powder which adhered to the surface of the metal to be treated is uniformly fused
and bonded onto the heated portion (i.e. the portion to be prevented from being carburized
and nitrided) of the material treated by virtue of the adhesiveness of the thermally
fusible resin accompanying its thermal plasticization, and when it is exposed to a
carburizing and nitriding atmosphere at 400°C or more, the thermally fusible resin
disappears by thermal decomposition, and simultaneously the sufficient amount of the
boron oxide-based amorphous substance in the powder is molten or fused and bonded
onto the material thereby forming a close masking film thereon with good adhesion
in the absence of pinhole defects etc., thus bringing about reliable prevention of
carburizing and nitriding.
[0036] Although the essential constitutional components in the powder for prevention of
carburizing and nitriding according to the present invention are the 2 components
described above. As other components, water glass, frint, low-melting glass or metal
powder such as tin, aluminum, zinc etc. or disrupted metal foil etc. can be added
in suitable amounts to improve the carburizing and nitriding preventing effects. Additional
materials such as titanium oxide, iron oxide, zinc oxide, talk, calcium carbonate,
mica, silica (molten silica, aerosol etc.), alumina, magnesium, silicon carbide, fly
ash, graphite, silicic acid, kaolinite, clay etc. are added in small amounts to improve
the adhesive properties or denseness of the carburizing and nitriding preventing film
which is fusion-bonded on the material treated, or to prevent the fluidity (sagging
properties) of the carburizing and nitriding preventing coating at the time of carburizing
or nitriding treatment.
[0037] In addition, the powder for prevention of carburizing and nitriding is preferably
colored with various pigments to improve degrees of absorption of the laser. Selection
of the color depends on the thermal absorption of laser rays, and colors such as green,
red etc. are relatively desirable. To improve degrees of absorption, colloidal graphite
etc. may be applied onto the surface of the material treated.
[0038] The powder for prevention of carburizing and nitriding according to the present invention
is a mixture of the above constitutional components, and the process for preparation
thereof is not limited at all, and for example, the boron oxide-based amorphous substance
in a powder form and the thermally fusible resin in a powder form and if necessary
other auxiliary additive may be blended and uniformly mixed with one another. Alternatively,
in a preferable process, the thermally fusible resin is heated and softened, and then
the powdery boron oxide-based amorphous substance and other powdery auxiliary additives
are uniformly dispersed in it, and the mixture is cooled, solidified and disrupted
so that the powder with uniform components can be easily obtained without separation
of individual constitutional components from each other. In a similar preferable process,
the thermally fusible resin is dissolved in suitable solvent, and then the powdery
boron oxide-based amorphous substance and other powdery auxiliary additives are uniformly
mixed with it, followed by spray-drying.
[0039] To permit the powder for prevention of carburizing and nitriding to adhere evenly
and uniformly to the portion to be prevented from being carburized and nitrided, the
diameter of the powder is preferably regulated in the range of 10 to 250 µm in average,
more preferably 50 to 200 µm. If the powder is too course, there is a tendency for
an uniform carburizing and nitriding preventing film to be hardly formed, whereas
it is too fine, there is a tendency for a thick carburizing and nitriding preventing
film to be hardly formed, making it difficult to achieve satisfactory carburizing
and nitriding preventing effects.
[0040] The content of the boron-based inorganic compound or boron oxide-based amorphous
substance having a carburizing or nitriding preventing action in this powder for prevention
of carburizing or nitriding is in the range of 20 to 80% by weight, preferably 40
to 60% by weight in terms of anhydrides thereof, and the preferable content of the
thermally fusible resin to be thermally decomposed under carburizing or nitriding
conditions is in the range of 20 to 80% by weight, preferably 40 to 60% by weight.
[0041] A particularly preferable example of such a boron-based inorganic compound or boron
oxide-based amorphous substance is that with a water content of 10% by weight or less,
more preferably 5% by weight, and a preferable example of thermally fusible resin
is one member selected from polyolefin type resin, polyester type resin and acryl
type resin or a mixture thereof.
[0042] The material to be treated to which the present invention is applied includes various
metal materials whose surface hardening is carried out by partial carburizing and
nitriding treatment, and the commonest materials are steel materials and alloy steels,
and the present invention is applied to the portion to be prevented from, for example,
carburizing and nitriding machine parts such as shafts and bearings or abrasive and
grinding members so that the portion thereof undergoing strong friction and abrasion
is hardened by carburizing and nitriding and the remaining portion is prevented from
being carburized and nitrided, thus maintaining high toughness. In this manner, the
portion to be prevented from being carburized and nitrided maintain its inherent toughness,
whereas the non-adhered portion can be restrictively hardened by carburizing and nitriding
treatment.
Examples
[0043] Hereinafter, the constitution and working effect of the present invention are described
specifically by reference to Example and Comparative Example.
Example 1
[0044] As a test specimen for carburizing treatment, a cylindrical steel material of 22
mm diameter and 70 mm length was divided equally in the longitudinal direction to
give semicylindrical specimens. This specimen was transferred to a coating booth (2)
by the 6-axis robot shown in FIG. 1, and a powder for prevention of carburizing and
nitriding (average particle diameter of 100 µm) consisting of 55% by weight boron
oxide-silicon oxide type amorphous substance (i.e. 90% by weight boron oxide plus
10% by weight silicon oxide) and 45% by weight polyethylene powder was applied to
the whole of the test specimen by electrostatic coating. Then, this test specimen
was transferred to the laser irradiation position in YAG laser marker (4) by the 6-axis
robot and the flat portion (bottom) of the semicylindrical specimen was irradiated
with the laser under the control of a computer so as to draw a square pattern with
a side of 5 mm. The test specimen after irradiation was transferred again to the coating
booth (2) by the 6-axis robot, and the powder for prevention of carburizing and nitriding
on the test specimen was blow away by air blow.
[0045] The powder for prevention of carburizing and nitriding was fusion-bonded on the test
specimen to form a masking film at an accuracy of ± 0.5 mm.
[0046] Then, this test specimen was introduced into a carburizing heating furnace and subjected
to carburizing treatment using gas type carburizing. After treatment, the test specimen
was evaluated by observation under a microscope and its hardness was determined, and
as a result, the hardness of only the portion whose wear characteristics were intended
to be improved by carburizing treatment was accurately improved, and the portion protected
with the powder for prevention of carburizing and nitriding was completely protected
from the carburizing action.
1. A method for masking a portion of a metal material to be carburized or nitrided, which
comprises coating the powder for prevention of carburizing or nitriding over a region
including at least said portion to be prevented from being carburized and nitrided,
heating only said portion by laser beam, thereby fusing and bonding the powder to
only the portion to be prevented from being carburized and nitrided.
2. A method for masking according to claim 1, wherein the laser beam heating is effected
by a laser beam emitted from an apparatus including a laser beam emitting unit and
a computer for automatically controlling and driving the movement of a scanning head
emitting a laser beam such that only the portion of the metal material to be prevented
from being carburized and nitrided is irradiated with the laser beam.
3. A method for masking according to claim 2, wherein the scanning is controlled by a
computer, in which the movement in the X- and Y-axial directions (horizontal biaxial
directions with a right angle to each other) is conducted by the movement of the laser
beam, and scanning in the vertical direction are conducted by a robot.
4. A method for masking according to any one of claims 1, 2 and 3, wherein the powder
for prevention of carburizing or nitriding comprises as essential components a substance
having a carburizing or nitriding preventing action and thermally fusible resins to
be thermally decomposed under carburizing or nitriding conditions.
5. A method for masking according to claim 4, wherein the substance having a carburizing
or nitriding preventing action is a boron-based inorganic compound or boron oxide-based
amorphous substance.
6. A method for masking according to claim 4 or 5, wherein the content of the boron-based
inorganic compound or boron oxide-based amorphous substance having a carburizing or
nitriding preventing action in the powder for prevention of carburizing or nitriding
is in the range of 20 to 80% by weight in terms of the content of anhydrides thereof,
and the content of the thermally fusible resin to be thermally decomposed under carburizing
or nitriding conditions is in the range of 20 to 80% by weight.
7. A method for masking according to claim 5 or 6, wherein the boron-based inorganic
compound is a boron oxide with a water content of 10% by weight or less.
8. A method for masking according to claim 5 or 6, wherein the boron oxide-based amorphous
substance consists of 60 to 90% by weight B2O3 and 40 to 10% by weight SiO2, and the water content thereof is 10% by weight or less.
9. A method for masking according to any one of claims 4 to 8, wherein the thermally
fusible resin is at least one kind of resin selected from
polyolefins, polyesters and acrylic resins.