U.S. patent number 3,922,038 [Application Number 05/387,566] was granted by the patent office on 1975-11-25 for wear resistant boronized surfaces and boronizing methods.
This patent grant is currently assigned to Hughes Tool Company. Invention is credited to Stanley R. Scales.
United States Patent |
3,922,038 |
Scales |
November 25, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Wear resistant boronized surfaces and boronizing methods
Abstract
Disclosed herein are treatments for the manufacture of wear
resistant steel surfaces, which are carburized, boronized, quenched
and tempered for extreme surface hardness, with a strong, tough
supporting base that minimizes fracturing of the brittle boronized
case.
Inventors: |
Scales; Stanley R. (Houston,
TX) |
Assignee: |
Hughes Tool Company (Houston,
TX)
|
Family
ID: |
23530435 |
Appl.
No.: |
05/387,566 |
Filed: |
August 10, 1973 |
Current U.S.
Class: |
384/95; 148/279;
148/217; 175/374; 428/682 |
Current CPC
Class: |
C23C
8/00 (20130101); E21B 10/22 (20130101); Y10T
428/12958 (20150115) |
Current International
Class: |
C23C
8/00 (20060101); E21B 10/22 (20060101); E21B
10/08 (20060101); F16C 019/00 () |
Field of
Search: |
;308/8.2,241
;148/31.5,16.5 ;175/374 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Myhre; Charles J.
Assistant Examiner: Liles; James D.
Attorney, Agent or Firm: Felsman; Robert A.
Claims
I claim:
1. In an earth boring drill bit having at least one rotatable
cutter supported on a steel bearing pin, the improvement which
comprises:
a carburized friction bearing surface formed on said steel bearing
pin;
a boronized surface formed on said carburized bearing surface;
said carburized surface being hardened and tempered by heat
treatment;
whereby the wear resistant boronized surface has a tough and strong
supporting base to sustain large loads imposed by a mating friction
bearing in said rotatable cutter and avoid brittle fracture.
2. In an earth boring drill bit having at least one rotatable
cutter supported on a steel bearing pin, the improvement which
comprises:
a friction bearing surface formed on said steel bearing pin and
having a carburized case of a depth of at least 0.030 inch;
a boronized surface having a depth of at least 0.001 inch formed on
said carburized case;
said carburized surface being hardened and tempered by heat
treatment;
whereby the wear resistant boronized surface has a tough and strong
supporting base to sustain large loads imposed by a mating friction
bearing in said rotatable cutter and avoid brittle fracture.
3. In an earth boring drill bit having at least one rotatable
cutter supported on a steel bearing pin, the improvement which
comprises:
a carburized friction bearing surface formed on said steel bearing
pin;
a boronized surface formed on said carburized bearing surface;
said carburized case being quenched from a temperature to produce a
martensitic grain structure;
said carburized case being tempered to produce a tempered
martensitic grain structure;
whereby the wear resistant boronized surface has a tough and strong
supporting base to sustain large loads imposed by a mating friction
bearing in said rotatable cutter and avoid brittle fracture.
4. In an earth boring drill bit having at least one rotatable
cutter supported on a steel bearing pin, the improvement which
comprises:
a friction bearing surface formed on said steel bearing pin and
having a carburized case of a depth of at least 0.030 inch;
a boronized surface having a depth of at least 0.001 inch formed on
said carburized case;
said carburized case being quenched from a temperature to produce a
martensitic grain structure;
said carburized case being tempered to produce a tempered
martensitic grain structure;
whereby the wear resistant boronized surface has a tough and strong
supporting base to sustain large loads imposed by a mating friction
bearing in said rotatable cutter and avoid brittle fracture.
5. In an earth boring drill bit having at least one rotatable
cutter supported on a steel bearing pin, the improvement which
comprises:
a friction bearing surface formed on said steel bearing pin and
having a carburized case of a depth of at least 0.030 inch;
a boronized surface having a depth of at least 0.001 inch formed on
said carburized case;
said carburized case being quenched from a temperature of at least
1390.degree.F. to produce a martensitic grain structure;
said carburized case being tempered from a temperature within a
range of 290.degree. through 510.degree.F. for about 1 hour to
produce a tempered martinsitic grain structure;
whereby the wear resistant boronized surface has a tough and strong
supporting base to sustain large loads imposed by a mating friction
bearing in said rotatable cutter and avoid brittle fracture.
6. The method of manufacturing earth boring drill bits having at
least one rotatable cutter supported on a steel bearing pin, said
method comprising the steps of:
forming a friction bearing surface on said bearing pin to produce a
carburized case of selected depth;
boronizing said carburized case;
hardening said carburized case;
tempering said carburized case;
whereby the wear resistant boronized surface has a tough and strong
supporting base to sustain large loads and avoid brittle
fracture.
7. The method of claim 11 wherein said carburizing is to a depth of
at least 0.030 inch and said boronizing is to a depth of at least
0.001 inch.
8. The method of claim 12 wherein said hardening is by quenching
from a temperature of at least 1390.degree.F., and said tempering
is from a temperature within a range of 290.degree. through
510.degree.F. for about 1 hour.
Description
BACKGROUND OF THE INVENTION:
1. Field of the Invention:
This invention relates in general to surface treatments of metals,
particularly to those for steels requiring exceptional wear
resistance under heavy loads such as those imposed upon earth
boring drill bit bearings.
2. Description of the Prior Art:
It has long been known that the surfaces of metals such as steel
may be treated with a variety of elements or compositions that
combine with the constituents of steel to produce hard wear
resistant surfaces. One of the known prior art treatments is
carburization. Another is boronizing.
The bearing surfaces of rotatable cutters in earth boring drill
bits are commonly carburized, hardened and tempered to increase
their wear resistance. Such surfaces sometimes have regions of soft
anti-galling material such as silver or silver alloy, as disclosed
in U.S. Pat. No. 3,235,316. The mating surfaces of the opposing
bearing shafts often include deposits of hard metal of cobalt based
alloy of the "Stellite" series.
Boronizing of the bearing surfaces of earth boring drill bits has
not heretofore been commercially successful, principally due to the
brittle nature of the hard surfaces and their shallow depth.
SUMMARY OF THE INVENTION
This invention relates to the discovery that a wear resistant
surface for steel such as a bearing surface in an earth boring
drill bit may be constructed advantageously by a process that
includes carburization, boronizing the resulting carburized
surface, hardening in a manner to protect the boronized case from
decarburizing, and then tempering. By carburizing and boronizing to
the requisite depths, hardening in a manner to produce a
martensitic grain structure in the carburized case, and tempering
to thereafter produce tempered martensite, a surface of extreme
hardness results with a strong and tough supporting base. This base
minimizes fracturing of the brittle boronized case since it is
resistant to deformation and provides a satisfactory support for
the boronized case.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a fragmentary perspective view, partially in section,
showing an earth boring drill bit and typical bearing which
receives the metallurgical treatment described herein.
FIG. 2 is a cross-sectional view as seen looking along the lines
II--II of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The numeral 11 in the drawing designates an earth boring drill bit
having a plurality of head sections or legs 13 joined by suitable
means such as welding (not shown) and threaded at 15 for attachment
to a supportive drill string member. One of the prior art
lubrication and compensator systems 17, such as illustrated in FIG.
1, feeds lubricant through a passageway 19 and into a bearing
region formed between a cantilevered shaft or pin 21 of the head
section and a rotatable cutter 23 having teeth 25 for
disintegrating earth. Suitable seal means 27 prevent the escape of
lubricant from the bearing region, as explained in U.S. Pat. No.
3,397,928.
The above mentioned bearings in an earth boring drill often include
a primary friction region 29, an anti-friction ball bearing and
cutter retainer region 31, and radial and thrust friction bearing
regions designated respectively 33 and 35.
FIG. 2 shows in cross-section the cantilevered bearing pin 21 and
rotatable cutter 23. The cylindrical surface 35 of the bearing pin
21 receives the special treatment of the present invention, while
the opposing surface of the rotatable cutter 23 receives a prior
art treatment such as that described in U.S. Pat. No. 3,235,316,
preferably that using silver alloy. This patent teaches that
improved friction bearings for earth boring drill bits result from
the use of indentations filled with a soft anti-galling material
such as silver or silver alloy 37 in a surface having a hardness
and wear resistance comparable to carburized and hardened
steel.
As previously stated this invention utilizes four primary method or
process steps: carburizing, boronizing, hardening and tempering,
each of which will be described separately by way of example. The
initial step in the method of this invention requires carburization
of a steel bearing surface such as the primary friction bearing
region 29. One of the prior art carburization methods may be used.
Gas carburizing is a well known art and is the preferred method. It
is described on pp. 93-114 of Volume 2 of the 8th Edition of the
Metals Handbook, "Heat Treating, Cleaning and Finishing" (1964,
American Society for Metals). An example of the gas carburizing of
the bearing surfaces of a selectively copper plated rock bit head
section made of A.I.S.I. 4815 steel is as follows:
Carburizing temperature: 1700.degree.F.
Carburizing time: 9 hours at 1700.degree.F.
Carburizing atmosphere: Generated endothermic gas enriched with
methane to have a carbon potential of 1.35% carbon.
A typical analysis of the carburizing gas (atmosphere) is as
follows:
40% N.sub.2
20% co (co.sub.2 about 0.05%)
38% H.sub.2
2% ch.sub.4
this produces a carburized case depth of about 0.065 inch with
carbon content at the surface about 1.00%.
Pack carburizing is another well known art that may be used. It is
described on pp. 114-118 of Volume 2 of the same Metals Handbook.
An example of pack carburizing of the bearing surfaces of a rock
bit head section made of A.I.S.I. 4815 steel is as follows:
Carburizing compound (packed around the surface to be carburized):
Charcoal, 90% (6 to 14 mesh size), energized with about 4%
BaCO.sub.3 and about 1.5% CaCO.sub.3.
Carburizing temperature: 1700.degree.F.
Carburizing time: Nine hours at 1700.degree.F.
This produces a carburized case depth of about 0.065 inch with
carbon content at the surface about 1.00%.
Liquid carburizing is another well known art to the metals
industry. It is described on pp. 133-145 of Volume 2 of the same
Metals Handbook. While not actually used for this invention, it is
anticipated that 9 hours at 1700.degree.F. in a salt bath
containing about 6 - 16% sodium cyanide and 30 - 55% barium
chloride would produce a satisfactory carburized case to a depth of
about 0.065 inch.
The second step of the method of this invention is boronizing of
the previously carburized surfaces. Pack boronizing is the
preferred technique and is a relatively new art.
An example of boronizing a carburized bearing surface of an
A.I.S.I. 4815 steel head section is as follows:
Compound: Boronizing powder was packed around the carburized
bearing surface. This powder was 90% finer than 150 mesh, had
40-80% B.sub.4 C, 2-40% (graphite), 1-4% KHCO.sub.3, with remainder
up to 20% in impurities.
Boronizing temperature: 1650.degree.F. (in a carburizing
atmosphere).
Boronizing time: 5 hours at temperature in a furnace with a carbon
potential of 1.00%.
This produced a boronized case depth of about 0.004 inch. Longer
boronizing times and/or higher boronizing temperatures can be used
for deeper boronized case depth, but a 0.001 to 0.010 inch deep
boronized case has less tendency to crack or spall than a deeper
case.
Gas boronizing is an alternate technique in the prior art. It is
described in U.S. Pat. No. 2,494,267, "Surface Hardening of Ferrous
Metals," Schlesinger and Schaffer, Jan. 10, 1950. The method
described utilizes gaseous diborane (B.sub.2 H.sub.6) at about
700.degree.C (1292.degree.F).
Liquid boronizing is another prior art boronizing method. It is
described in two papers: "Boronizing of Steel" by D. C. Durrill and
Dr. Donald D. Allen, Magnetic Propulsion Systems, Inc. and
"Boriding Steels for Wear Resistance" by Howard C. Fielder and
Richard J. Sieraski, General Electric Co. (Metal Progress, Feb.
1971, pp. 101-107).
Neither gives the liquid salt bath composition, but the latter
paper states that it contains flourides of lithium, sodium,
potassium, and boron. Temperatures and times reported vary from
1450.degree. - 1650.degree.F. and from 15 minutes to 36 hours.
The third and fourth steps in the method of this invention are
hardening and tempering of the carburized, boronized and cleaned
steel surfaces.
The hardening and tempering of carburized steel is a well known
art. The hardening, usually quenching in agitated oil, from a
temperature of at least 1390.degree.F. can be performed using one
of several procedures, such as the following two for carburized
A.I.S.I. 4815 steel and produces a martensitic case:
a. Single quench from a carburizing or reducing atmosphere and a
temperature of 1500.degree.F.
b. Double quench from a carburizing or reducing atmosphere and
temperatures of respectively 1550.degree.F. and 1435.degree.F.
Quenching from a carburizing or reducing atmosphere prevents
decarburization or oxidation of the boronized case. A suitable
coating such as copper plating may be used. A suitable atmosphere
is one similar to the previously explained methane enriched
endothermic gas except slightly higher in CO.sub.2 (about 0.4 to
0.8%) because of the lower temperatures.
The tempering temperature is usually low, 290.degree. -
510.degree.F., preferably about 330.degree.F. for 1 hour, to
toughen the carburized case without appreciably lowering its
strength (hardness) to produce tempered martensite.
The above described procedures are utilized to produce a carburized
foundation on the steel bearing pin 21 of at least 0.060 inch. The
boronizing procedure produces a boronized case of at least 0.001
inch (average about 0.004 inch) with a surface hardness in a range
of 900 to 2100 KHN. The hardening and tempering procedure develops
a hardness in the carburized foundation in a range of 50 to 64
Rockwell C (550 to 800 KHN). When such a bearing pin is assembled
with a rotatable cutter of the type shown in the drawing and
described in U.S. Pat. No. 3,235,316, overall bearing performance
is much improved over that obtainable with a carburized bearing
surface. Results indicate that such a surface performs comparably
with much more expensive procedures such as when utilizing a
deposit of hard metal of the cobalt based "Stellite" series in a
groove on the bearing pin.
While the invention has been described in only a few of its forms
it should be apparent to those skilled in the art that it is not so
limited but is susceptible to various changes and modifications
without departing from the spirit thereof.
* * * * *