Note: Descriptions are shown in the official language in which they were submitted.
CARBURIZED WORKPIECE AND PREPARATION METHOD THEREFOR
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority of Chinese Patent Application No.
202011138231.0,
filed with the China National Intellectual Property Administration on October
22, 2020, and titled
with "CARBURIZED WORKPIECE AND PREPARATION METHOD THEREFOR", which is
hereby incorporated by reference in its entirety.
FIELD
[0002] The present belongs to the technical field of material and workpiece
preparation, and in
particular to a carburized workpiece and a preparation method therefor.
BACKGROUND
[0003] With the improvement of air compressor and rock drilling equipment
technology, the
output air pressure of the drilling rig is getting higher and higher,
resulting in higher drilling
frequency, higher impact force on the drill bit, and greater probability of
breakage of drill bit spline.
[0004] Failure research shows that the crack source of the breakage on the
carburized drill bit
spline is caused by the secondary hardening caused by the frictional heat
between the spline of the
drill bit and the spline of the drive chuck. The higher the concentration of
the carburized layer of
the spline of the drill bit, the higher the austenite content retained in the
structure, and the greater
the risk of thermal crack caused by secondary hardening. However, in the field
of down-the-hole
drill bits, the carburizing concentration in carburized drill bits is
generally controlled at a high level,
and there is often much retained austenite in the carburized layer. Therefore,
under the continuously
increasing working air pressure, the probability of thermal crack caused by
friction at the spline of
the drill bit continues to increase, and the probability of breakage of the
spline also continues to
increase. In addition, at the beginning, since the contact between the spline
of the drill bit and the
spline of the drive chuck is linear contact, the surface hardness on the
spline of the drill bit will be
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higher, thus it is more difficult for the running-in between the spline of the
drill and the spline of
the drive chuck, and it is easier to cause localized overload, Furthermore,
the generation and
propagation of thermal cracks are accelerated.
SUMMARY
[0005] In view of this, the technical problem to be solved by the present
invention is to provide
a carburized workpiece and a preparation method that can effectively reduce
the probability of
spline breakage.
[0006] The present invention provides a carburized workpiece. The carburized
layer of the
carburized workpiece, from surface to inside, has a carbon concentration first
increasing and then
decreasing. The surface carbon concentration of the carburized layer is not
lower than 0.46%, the
highest carbon concentration of the carburized layer is 0.58-0.68%, and a
carburization depth
reaching the highest carbon concentration is 15-50% of the total depth of the
carburized layer.
[0007] Preferably, the carburized depth reaching the highest hardness in the
carburized layer is
no more than 0.5 mm, and the hardness in the carburized layer decreases
gradually after reaching
the highest hardness.
[0008] Preferably, a total depth of the carburized layer is 1.6-2.2 mm; the
carburized depth
reaching the highest carbon concentration is 0.4-1.1 mm.
[0009] Preferably, a material of the carburized workpiece is selected from the
group consisting
of 18CrNi3Mo, 23CrN3iMo and 30CrNi4Mo.
[0010] The present invention further provides a method for preparing a
carburized workpiece,
comprising:
[0011] Si) carburizing a workpiece at a temperature of 920 C-930 C, a boost
carbon potential
of 1%-1.15% and a diffusion carbon potential of 0.78%-0.85%;
[0012] S2) quenching the carburized workpiece;
[0013] S3) performing high-temperature tempering on the quenched workpiece;
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[0014] S4) performing isothermal salt-bath quenching treatment on the high-
temperature
tempered workpiece;
[0015] S5) performing low-temperature tempering on the isothermal salt-bath
quenched
workpiece to obtain a carburized workpiece.
[0016] Preferably, in the step Si), the boost stage is performed for 4.5-7 h;
and the diffusion stage
is performed for 4-6 h.
[0017] Preferably, in the step Si), after the carburizing treatment, the
workpiece is cooled to
855 C or lower with the furnace, and then cooled to room temperature by gas
quenching.
[0018] Preferably, in the step S2), the quenching is performed at a
temperature of 830 C-850 C
and a protective carbon potential of 0.45%-0.5% for 3-5 h.
[0019] Preferably, in the step S3), the high-temperature tempering is
performed at a temperature
of 660 C-700 C in an inert atmosphere for 5-7 h.
[0020] Preferably, in the step S4), a salt-bath quenching in the isothermal
salt-bath quenching
treatment is performed at a heating temperature of 790 C-850 C for 25-90 min,
and a nitrate bath
isothermal quenching in the isothermal salt-bath quenching treatment is
performed at a temperature
of 230 C-280 for 20-70 min; in the step S5), the low-temperature tempering is
performed at
180 C-230 C for 4-6 h.
[0021] The present invention provides a carburized workpiece. The carburized
layer of the
carburized workpiece, from surface to inside, has a carbon concentration first
increasing and then
decreasing, wherein the carbon concentration on the surface is no less than
0.46%, the highest
carbon concentration is 0.58%-0.68 %, and a carburized depth reaching the
highest carbon
concentration is 15%-50% of the total depth of the carburized layer. Compared
with the existing
technology, by means of controlling a change curve of the carbon concentration
of the carburized
layer and reducing the carbon concentration of an outer surface layer, the
present invention can
effectively control the amount of retained austenite on the outermost layer,
reduce the hardness of
the outermost layer, increase the running-in degree of the spline of the
carburized workpiece and
the spline of the drive chuck, and reduce the thermal crack phenomenon of the
carburized layer
under high-frequency friction, such that the probability of breakage of the
spline of the carburized
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workpiece can be effectively reduced, the service life of the carburized
workpiece can be greatly
prolonged, and the carburized workpiece is widely used in the field of rock
drilling using high-
frequency high-load down-the-hole drill bits.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1 shows the gradient curves of the carburizing concentration and
hardness of the
carburized layer of HD55A-152 drill bit obtained in the Example 1 of the
present invention;
[0023] FIG. 2 shows the metallographic structure micrograph of the carburized
layer of HD55A-
152 drill bit obtained in Example 1 of the present invention.
DETAILED DESCRIPTION
[0024] The technical solutions in the embodiments of the present invention
will be clearly and
completely described below in conjunction with the examples of the present
invention. Apparently,
the described examples are only some of the embodiments of the present
invention, but not all of
them. Based on the examples of the present invention, all other embodiments
obtained by those of
ordinary skill in the art without making creative efforts belong to the
protection scope of the present
invention.
[0025] The present invention provides a carburized workpiece, which has a
carburized layer,
from surface to inside, having a carbon concentration first increasing and
then decreasing, wherein
the carbon concentration on the surface is not less than 0.46%, the highest
carbon concentration is
0.58%-0.68 %, and a carburized depth reaching the highest carbon concentration
is 15%-50% of
the total depth of the carburized layer.
[0026] In the present invention, the carburized workpiece is preferably a
carburized drill bit; the
carburized workpiece is preferably formed by a high-strength structural steel;
a material of the
carburized workpiece is more preferably selected from the group consisting of
18CrNi3Mo,
23CrN3iMo and 30CrNi4Mo; the carburized drill bit has a spline shank with a
diameter of
preferably 50-180 mm, and an insert bit with a diameter of preferably 100-380
mm.
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[0027] The surface of the carburized workpiece is a carburized layer; the
carburized layer, from
surface to inside, has a carbon concentration first increasing and then
decreasing, i.e., the carbon
concentration distribution curve is in a shape of analogous parabola. The
carbon concentration on
the surface of the carburized layer is preferably the concentration at 0.2 mm.
The carbon
concentration on the surface is preferably 0.48%-0.5%. The carbon
concentration on the surface is
preferably the concentration at 0.2 mm from the surface layer. The highest
carbon concentration of
the carburized layer is preferably 0.60%-0.68%, more preferably 0.62%-0.66%,
further more
preferably 0.62%-0.65%. In the carburized layer, the carburized depth reaching
the highest
carburizing concentration is preferably 20%-45%, more preferably 25%-40%,
further more
preferably 28%-38%, most preferably 29.4%-35.2% of the total depth of the
carburized layer. In
the present invention, the carburized depth reaching the highest carbon
concentration is preferably
0.4-1.1 mm, more preferably 0.4-0.8 mm, further more preferably 0.4-0.7 mm,
most preferably 0.5-
0.6 mm. A total depth of the carburized layer is preferably 1%-1.5%, more
preferably 1%-1.4%,
further more preferably 1%-1.3%, yet further more preferably 1.1%-1.2%, most
preferably 1.1%-
1.15% of the diameter of the carburized workpiece. In the present invention, a
total depth of the
carburized layer is preferably 1.6-2.2 mm, more preferably 1.6-2 mm, further
more preferably 1.6-
1.8 mm, most preferably 1.7 mm.
[0028] According to the present invention, the carburized depth reaching the
highest hardness in
the carburized layer is preferably no more than 0.5 mm, more preferably 0.25-
0.45 mm, further
more preferably 0.3-0.4 mm, most preferably 0.3 mm. After reaching the highest
hardness in the
carburized layer, the hardness will gradually decrease. In the present
invention, the hardness
preferably decreases in a gradient after reaching the highest hardness in the
carburized layer.
[0029] The metallographic structure of the carburized layer of the carburized
workpiece provided
by the present invention has almost no retained austenite in the 50-500 times
microscope view.
[0030] In the present invention, by means of controlling a change curve of the
carbon
concentration of the carburized layer and reducing the carbon concentration of
an outer surface
layer, the amount of retained austenite on the outermost layer is effectively
controlled, the hardness
of the outermost of the drive chuck is increased, and the thermal crack
phenomenon of the
carburized layer under high-frequency friction is reduced, such that the
probability of breakage of
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the spline of the carburized workpiece can be effectively reduced, the service
life of the carburized
workpiece can be greatly prolonged, and the carburized workpiece is widely
used in high-frequency
and high-load down-the-hole drill bits in the field of rock drilling.
[0031] The present invention further provides a method for preparing the above
carburized
workpiece, comprising: Si) carburizing a workpiece at a temperature of 920 C-
930 C, a boost
carbon potential of 1%-1.15% and a diffusion carbon potential of 0.78%485%;
S2) quenching the
carburized workpiece; S3) performing high-temperature tempering on the
quenched workpiece; S4)
performing isothermal salt-bath quenching treatment on the high-temperature
tempered workpiece;
S5) performing low-temperature tempering on the isothermal salt-bath quenched
workpiece to
obtain a carburized workpiece.
[0032] The present invention has no special limitation on the sources of all
raw materials, which
can be commercially available.
[0033] In the present invention, the workpiece can be prepared according to
methods well-known
to those skilled in the art, preferably obtained by blanking, forging and
machining; where the
forging is performed at an initial forging temperature of preferably 1180 C-
1250 C and a final
forging temperature of preferably 870 C-890 C.
[0034] The workpiece is carburized at a temperature of preferably 922 C-928 C,
more preferably
924 C-926 C, further more preferably 925 C; the boost carbon potential is
preferably 1.05%-
1.15%, and the boost stage is performed for preferably 4.5-7 h, more
preferably 5-6 h; the diffusion
carbon potential is preferably 0.78%-0.83%, and the diffusion stage is
performed for preferably 4-
6 h, more preferably 4.5-5 h. The carburizing agent in the carburizing
treatment is preferably
methanol and acetone, and the combination of the two can control the carbon
potential in the
carburizing process.
[0035] After the carburizing treatment, the workpiece is preferably cooled to
855 C or lower
with the furnace, and then cooled to room temperature by gas quenching; where
the gas quenching
is performed using high-pressure nitrogen as a medium to realize rapid
cooling.
[0036] The carburized workpiece is subjected to quenching treatment at a
temperature of
preferably 830 C-850 C and a protective carbon potential of preferably 0.45%-
0.5% for preferably
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3-5 h. After the quenching treatment, the workpiece is preferably cooled using
a rapid-quenching
cooling oil.
[0037] The quenched workpiece is subjected to high-temperature tempering at a
temperature of
preferably 660 C-700 C and preferably in an inert atmosphere for preferably 5-
7 h; wherein the
inert atmosphere is preferably nitrogen.
[0038] After the high-temperature tempering treatment, the workpiece is
preferably machined to
obtain a carburized workpiece of a specific size, which is then subjected to a
isothermal salt-bath
quenching treatment; where a salt-bath quenching in the isothermal salt-bath
quenching treatment
is performed at a heating temperature of preferably 790 C-850 C, more
preferably 800 C-830 C
for preferably 25-90 min, more preferably 35-60 min, further more preferably
40-50 min, most
preferably 40-45 min; and a nitrate bath isothermal quenching in the
isothermal salt-bath quenching
treatment is performed at a temperature of preferably 230 C-280 for
preferably 20-70 min, more
preferably 30-50 min, further more preferably 30-45 min, most preferably 35-40
min.
[0039] The workpiece after isothermal salt-bath quenching treatment is
subjected to low-
temperature tempering at a temperature of preferably 180 C-230 C for
preferably 4-6 h. After the
low-temperature tempering, the workpiece is preferably taken out of the
furnace and air-cooled to
obtain a carburized workpiece.
[0040] The present invention precisely controls the key process parameters of
the heat treatment
process to obtain a carburized layer with a carbon concentration showing an
analogous parabolic
gradient change and having no retained austenite near the surface, thereby
reducing the key
influencing factors of the thermal crack caused by the friction of the spline
of the drill bit and
promoting the matching degree of the spline and the drive chuck, and
effectively reducing the
probability of the breakage of the spline of the drill bit.
[0041] The selection of the temperature and carbon potential in the
carburizing treatment, the
selection of quenching temperature, carbon potential and cooling method, and
the selection of
temperature of the high-temperature tempering and protective atmosphere are
the keys to obtain a
carbon concentration gradient changing in an analogous parabolic curve, and
are the prerequisite
to effectively control the amount of retained austenite near the surface of
the carburized layer. The
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selection of the temperature of the salt bath in the salt-bath quenching and
the temperature of the
nitrate bath isothermal quenching, and the selection of the temperature of the
high-temperature
tempering and cooling method are key measures to further effectively control
the retained austenite.
[0042] In order to further illustrate the present invention, the carburized
workpiece provided by
the present invention and the preparation method thereof are described in
detail below in
conjunction with examples.
[0043] The reagents used in the following examples are all commercially
available.
Example 1
[0044] 23CrNi3Mo was used as base material of the drill bit, which was
subjected to blanking,
forging, machining, carburizing treatment, quenching treatment, high-
temperature tempering,
machining, isothermal salt-bath quenching, low-temperature tempering, drilling
alloy insert holes
and inserting alloy inserts to prepare a HD55A-152 drill bit. The forging was
performed at an initial
forging temperature of 1180 C and a final forging temperature of 870 C. The
carburizing was
performed at a temperature of 925 C, at a boost carbon potential of 1.05% for
6 h, and at a diffusion
carbon potential of 0.78% for 5 h. After cooling to 855 C with the furnace,
the resulting material
was cooled to room temperature by gas-quenching using the high-pressure
nitrogen as a medium.
The quenching was performed using a rapid-quenching cooling oil as a medium at
a temperature
of 850 C and a protective carbon potential of 0.45% for 3 h. The high-
temperature tempering was
performed at a temperature of 700 C for 5 h under nitrogen protection. The
salt bath was performed
at a temperature of 830 C for 40 min, and the nitrate bath isothermal
quenching was performed at
a temperature of 230 C for 35 min. The low-temperature tempering was performed
at 180 C for 4
h. Then the material was taken out of the furnace and air-cooled.
[0045] The carburizing concentration and hardness of the carburized layer of
the HD55A-152
drill bit obtained in Example 1 were analyzed to obtain carbon concentration
and hardness curves.
The results are shown in FIG. 1.
[0046] The HD55A-152 drill bit obtained in Example 1 was analyzed using a
metallographic
microscope to obtain a metallographic structure micrograph of the carburized
layer, as shown in
FIG. 2. It can be seen from FIG. 2 that there was almost no retained austenite
in the metallographic
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structure in 500 times metallographic microscopic view.
[0047] The test results of the carburized layer show that the carbon
concentration gradient of the
carburized layer was in a shape of analogous parabola, the concentration at
0.2 mm from the surface
was 0.48%, the highest carbon concentration was 0.62%, the carburized depth
reaching the highest
concentration was 0.6 mm, the total depth of the carburized layer was 1.7 mm,
and the carburized
depth reaching the highest hardness in the carburized layer was 0.3 mm, and
the hardness of the
carburized layer gradually decreased after reaching the highest hardness;
there was almost no
retained austenite in the metallographic structure in 500 times metallographic
microscopic view.
50 test drill bits were tested under such working condition that the
proportion of bit breakage was
20%. No breakage occurred.
Example 2
[0048] 30CrNi3Mo was used as the base material of the drill bit, which was
subjected to blanking,
forging, machining, carburizing treatment, quenching treatment, high-
temperature tempering,
machining, isothermal salt-bath quenching, low-temperature tempering, drilling
alloy insert holes
and inserting alloy inserts to prepare a HD55A-152 drill bit. The forging was
performed at an initial
forging temperature of 1180 C and a final forging temperature of 870 C. The
carburizing was
performed at a temperature of 925 C, at a boost carbon potential of 1.15% for
5 h and at a diffusion
carbon potential of 0.80% for 4.5 h. After cooling to 855 C with the furnace,
the resulting material
was cooled to room temperature by gas-quenching using the high-pressure
nitrogen as a medium.
The quenching was performed using a rapid-quenching cooling oil as a medium at
a temperature
of 830 C and a protective carbon potential of 0.50% for 3 h. The high-
temperature tempering was
performed at a temperature of 680 C for 5 h under nitrogen protection. The
salt bath was performed
at a temperature of 810 C for 45 min, and the nitrate bath isothermal
quenching was performed at
a temperature of 260 C for 40 min. The low-temperature tempering was performed
at 230 C for 4
h. Then the material was taken out of the furnace and air-cooled.
[0049] The test results of the carburized layer show that the carbon
concentration gradient of the
carburized layer was in a shape of analogous parabola, the concentration at
0.2 mm from the surface
was 0.50%, the highest carbon concentration was 0.65%, the carburized depth
reaching the highest
concentration was 0.5 mm, the total depth of the carburized layer was 1.7 mm,
and the carburized
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depth reaching the highest hardness in the carburized layer was 0.4 mm, and
the hardness of the
carburized layer gradually decreased after reaching the highest hardness;
there was almost no
retained austenite in the metallographic structure in 500 times metallographic
microscopic view.
50 test drill bits were tested under such working condition that the
proportion of bit breakage was
20%. No breakage occurred.
Example 3
[0050] 30CrNi4Mo was used as the base material of the drill bit, which was
subjected to blanking,
forging, machining, carburizing treatment, quenching treatment, high-
temperature tempering,
machining, isothermal salt-bath quenching, low-temperature tempering, drilling
alloy insert holes
and inserting alloy inserts to prepare a HD55A-152 drill bit. The forging was
performed at an initial
forging temperature of 1180 C and a final forging temperature of 870 C. The
carburizing was
performed at a temperature of 925 C, at a boost carbon potential of 1.05% for
5.5 h and at a
diffusion carbon potential of 0.83% for 4.5 h. After cooling to 855 C with the
furnace, the resulting
material was cooled to room temperature by gas-quenching using the high-
pressure nitrogen as a
medium. The quenching was performed using a rapid-quenching cooling oil as a
medium at a
temperature of 830 C and a protective carbon potential of 0.48% for 3 h. The
high-temperature
tempering was performed at a temperature of 670 C for 5 h under nitrogen
protection. The salt bath
was performed at a temperature of 800 C for 45 min, and the nitrate bath
isothermal quenching
was performed at a temperature of 280 C for 40 min. The low-temperature
tempering was
performed at 230 C for 4 h. Then the material was taken out of the furnace and
air-cooled.
[0051] The test results of the carburized layer show that the carbon
concentration gradient of the
carburized layer was in a shape of analogous parabola, the concentration at
0.2 mm from the surface
was 0.50%, the highest carbon concentration was 0.65%, the carburized depth
reaching the highest
concentration was 0.5 mm, the total depth of the carburized layer was 1.7 mm,
and the carburized
depth reaching the highest hardness in the carburized layer was 0.35 mm, and
the hardness of the
carburized layer gradually decreased after reaching the highest hardness;
there was almost no
retained austenite in the metallographic structure in 500 times metallographic
microscopic view.
50 test drill bits were tested under such working condition that the
proportion of bit breakage was
20%. No breakage occurred.
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