U.S. patent number 3,877,281 [Application Number 05/394,134] was granted by the patent office on 1975-04-15 for method for producing a high strength bolt.
This patent grant is currently assigned to Kobe Steel, Ltd.. Invention is credited to Toshihiro Minami, Eiji Niina, Kiyoshi Shimizu.
United States Patent |
3,877,281 |
Shimizu , et al. |
April 15, 1975 |
Method for producing a high strength bolt
Abstract
A method for producing a high strength bolt from a carbon steel
or a low alloy steel material, which comprises the steps of
subjecting said material, in turn, to cold working at the
reduction-of-area percentage of 10 percent and over rapid heating
to a temperature range from 450.degree. C to A.sub.1 transformation
point, warm-forming to a bolt shape and air-cooling or cooling at a
cooling rate higher than that of the air-cooling. The steel
material adapted for use herein includes a steel having a pearlite
structure or a tempered martensite structure, and particularly the
steel having the latter structure presents excellent resistance to
the delayed rupture phenomenon with an accompanied high tensile
strength of over 100 kg/mm.sup.2, particularly, in the range from
130 to 140 kg/mm.sup.2 and over.
Inventors: |
Shimizu; Kiyoshi (Kobe,
JA), Minami; Toshihiro (Kobe, JA), Niina;
Eiji (Nishinomiya, JA) |
Assignee: |
Kobe Steel, Ltd. (Kobe,
JA)
|
Family
ID: |
14467659 |
Appl.
No.: |
05/394,134 |
Filed: |
September 4, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Oct 27, 1972 [JA] |
|
|
47-107774 |
|
Current U.S.
Class: |
72/364; 148/653;
470/17; 148/654 |
Current CPC
Class: |
C21D
8/06 (20130101) |
Current International
Class: |
C21D
8/06 (20060101); B21k 001/46 () |
Field of
Search: |
;72/364,377 ;10/27
;148/12,12.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Larson; Lowell A.
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
What is claimed as new and desired to be secured by letters patent
of the United States:
1. A method for producing a high strength bolt from a carbon steel
or a low alloy steel material, comprising the steps of:
subjecting said material to cold working at a reduction-of-area
percentage of 10 percent and over;
rapidly heating said material thus worked to a temperature range
from 450.degree.C to the A.sub.1 transformation point and at a rate
of at least 50.degree.C/min.;
warm-forming said material to a bolt shape; and
air cooling said material or cooling said material at a cooling
rate higher than that of said air cooling, said cooling rate being
at least 50.degree.C/min.
2. A method as defined in claim 1, wherein a carbon steel or a low
alloy steel material is subjected at least once to the
strengthening treatment consisting of cold working at a
reduction-of-area percentage of 10 percent and over, rapid heating
to a temperature range from 250.degree.C to the A.sub.1
transformation point, and rapid cooling, after which said material
thus treated is subjected in turn to cold working at a
reduction-of-area percentage of 10 percent and over, rapid heating
to a temperature range from 450.degree.C to the A.sub.1
transformation point; warm-forming to a bolt shape, and air cooling
or cooling at a cooling rate higher than that of said air
cooling.
3. A method as defined in claim 1, wherein the high strength bolt
produced according to claim 1 is subjected in turn to reheating to
a temperature range from 450.degree.C to the A.sub.1 transformation
point, air-cooling or cooling at a cooling rate higher than that of
said air cooling.
4. A method as defined in claim 3, wherein the heating time for
bolts at the reheating temperature during the reheating treatment
is within 5 minutes.
5. A method as defined in claim 1, wherein said steel material is
that which is hot-rolled or has been subjected to a normalizing
treatment or annealing treatment after hot-rolling, thereby
presenting a pearlite structure.
6. A method as defined in claim 1 wherein said steel material is
that which has been subjected to hardening and tempering treatments
after hot-rolling, thereby presenting a tempered martensite
structure and wherein the reduction-of-area percentage for cold
working used ranges from 10 to 40 percent.
7. A method as defined in claim 1, wherein said steel material is
subjected at least once to a cycle of hot-rolling, rapid heating
and hardening, thereby rendering finer the austenite crystal grain
size to over No. 10 and inclusive according to ASTM grain size,
after which said material is subjected to a tempering treatment,
thereby presenting a tempered martensite structure, and wherein the
reduction-of-area percentage of cold working ranges from 10 to 40
percent.
8. A method as defined in claim 1, wherein the content of carbon is
0.5 percent, or lower.
9. A method as defined in claim 1 wherein the cold working is
wire-drawing.
10. A method as defined in claim 1, wherein the heating time for
bolts at the heating temperature during the strengthening treatment
is within 3 minutes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for producing a high strength
bolt from a carbon steel or a low alloy steel material which has
been subjected to hot-rolling, normalizing or annealing, thereby
presenting a ferrite-pearlite structure or which has been subjected
to hardening and tempering, thereby presenting a tempered
martensite structure, and more particularly to a method which
comprises the steps of subjecting said steel material, in turn, to
cold working at a reduction-of-area percentage of 10 percent and
over, rapid heating to a temperature range from 450.degree. C to
A.sub.1 transformation point, hot forming to a bolt shape and air
cooling or cooling at a cooling rate higher than that of the
air-cooling, thereby presenting a high strength bolt having a
tensile strength of 70 kg/mm.sup.2 and over. (Meant by the term,
"strengthening treatment" as used herein is a combination of
working and heat-treatment which includes a cycle of cold working,
rapid heating and rapid cooling.)
2. Description of the Prior Art
Hithereto, a method for producing a high strength bolt has recourse
to the steps wherein, to form a bolt shape, a cold-or-hot-working
or machining operation is used, followed by a refining treatment
such as hardening and tempering, thus obtaining a desired strength
and toughness.
However, such a method for producing a high strength bolt tends to
incur problems in quality assurance of products. The conventional
method necessarily dictates the use of a refining treatment at an
elevated temperature, such as hardening and tempering after forming
into a bolt shape, such that such a refining treatment results in
the necessity for close adjustment of the atmosphere used in a
furnace. Unless such an atmosphere furnace is used, there will
result an oxidation and decarbonization phenomena on the surface
layer of the bolt, thereby bringing about a wider range of
variation in strength with the resultant failure to present a
stable level of quality for the bolts produced.
In general, the production of a high strength bolt having a tensile
strength exceeding the range of 120 kg/mm.sup.2 to 130 kg/mm.sup.2
has been deemed difficult, because when such a bolt is subjected to
tension under a substantial static load for a certain period of
time, there tends to occur a delayed rupture phenomenon due to a
sudden occurrence of embrittlement, with the appearance thereof
exhibiting no plastic deformation.
Accordingly, it remains desirable to have a method for producing a
high strength bolt which has a high tensile strength but does not
create a delayed rupture phenomenon due to the sudden occurrence of
embrittlement.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a
method for producing a high strength bolt which dispenses with the
use of a refining treatment using an atmosphere furnace as well as
heat-treatment at an elevated temperature, yet providing the
quality equivalent or superior to that of a high strength bolt
which is produced using a conventional refining treatment.
Briefly, according to one embodiment of the present invention,
there is provided a method for producing a high strength bolt from
a carbon steel or low alloy steel material, wherein, for the
production of a high strength bolt having a tensile strength
ranging from 100 to 120 kg/mm.sup.2, a steel wire (rod) as rolled
or as normalized and annealed is in turn subjected to cold working
at a reduction-of-area percentage of 10 percent and over, heating
at a heating rate of over 50.degree.C/min. to a temperature range
from 450.degree. C to A.sub.1 transformation point by using heating
means such as resistance heating under air, high frequency
induction heating, flame heating, or the like, and then hot-forming
to a bolt shape, followed by air cooling or cooling at a cooling
rate higher than that of the air cooling. For the production of a
high strength bolt having a tensile strength of over 100
kg/mm.sup.2, a carbon steel or low alloy steel material in a wire
or rod form is subjected in turn to a refining treatment of
hardening and tempering, cold working at a reduction-of-area
percentage of 10 percent and over, heating at a heating rate of
over 50.degree. C/min. to a temperature range from 450.degree. C to
A.sub.1 transformation point by using the aforesaid heating means,
warm-forming to a bolt shape and finally air-cooling or cooling at
a cooling rate higher than that of the air cooling. The level of
tensile strength referred to herein is not necessarily limited to
that exceeding 110 kg/mm.sup.2.
A high strength bolt, which is made of carbon steel or low alloy
steel material and which is produced according to the method of the
present invention and has a tensile strength of over 110
kg/mm.sup.2, presents excellent resistance to the delayed rupture
phenomenon due to a sudden occurrence of embrittlement, and
particularly presents a bolt which has a tensile strength of a
range from 130 to 140 kg/mm.sup.2 and over, affording excellent
resistance to the delayed rupture pheonomenon, as compared with
conventional bolts subjected to the refining treatment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an electron-microscopic picture at 22,500X, of the
structure of a steel `A` as rolled and the structure of the steel A
which has been subjected to 43 percent wire-drawing, followed by a
strengthening treatment at 550.degree.C;
FIG. 2 is a plot showing the relationship of the temperature used
in hot-forming a rolled steel A to a bolt shape versus the hardness
of a bolt at bolt head and shank;
FIG. 3 is a diagram showing distribution of hardness at bolt head
and shank, in connection to the bolt produced according to the
present invention and the bolt produced according to the prior art
(cold-forming);
and
FIG. 4 is a diagram showing distribution of hardness at bolt head
and shank of a bolt produced from the refined steel C according to
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The types of steels applicable to the method of the present
invention should preferably be carbon steels having a carbon
content of no more than 0.5 percent and low alloy steels.
The reason why the content of carbon should be no more than 0.5
percent is as follows: Although a high strength bolt having a
tensile strength of 100 to 120 kg/mm.sup.2 may be produced of
rolled or tempered steels, the use of low and medium carbon steels
rather than high carbon steels is best suited and advantageous for
the method of the present invention using the strengthening
treatment from the viewpoints of tensile strength and toughness.
Carbon is an essential component for a material for use in
producing a high strength bolt having a tensile strength of over
110 kg/mm.sup.2 for the purpose of applying a refining treatment of
hardening and tempering as well as for achieving a required
hardenability and strength. However, from the viewpoint of the
delayed rupture phenomenon, the content of the carbon should
preferably be no more than 0.5 percent.
Although the content of other elements to be added are not
specifically limited, it is preferable that a small amount of
elements such as Al, N, Ti, Nb, etc. may be added for achieving a
finer grain size of austenite crystals.
The steel material, of which the high strength bolt having a
tensile strength of over 110 kg/mm.sup.2 is made, has a tempered
martensite structure which has been subjected to hardening and
annealing treatments. In this respect, it should be noted that the
tempering to be used depends on desired strength and toughness of
the steel, and the workability of the subsequent working steps of
the steel material, and it should also be recognized that the
hardening treatment is effective for achieving the desired
resistance to the delayed rupture pheonomenon, if the grain size of
the austenite crystals over ASTM No. 10 is obtained by utilizing a
rapid heating for achieving finer grain size, in addition to the
ordinary heat-treatment.
The reason why the reduction-of-area percentage of the cold working
should be over 10 percent is that an increase in strength of a bolt
shank portion, which has been warm-worked, may be expected from a
rapid heating by using air-atmosphere resistance-heating and high
frequency induction heating during the warm-working, whereas the
strength in the bolt head portion will be decreased due to the
aforesaid warm-working. As a result, in order to accommodate such
incompatibility, the reduction-of-area percentage of 10 percent and
over is required for achieving uniformity in the strength of a
bolt. Furthermore, although the efficacy of the strengthening
treatment, according to the method of the present invention,
depends on the types of the materials to be worked, the increase in
the strength of a bolt is lower in the case of the
reduction-of-area percentage of below 10 percent, while the working
at the reduction-of-area percentage of over 40 percent is difficult
to apply to a martensite steel. For this reason, the
reduction-of-area percentage of cold working of over 10 percent is
suited for rolled steel material or normalized steel material,
while the reduction-of-area percentage of cold working of 10
percent to 40 percent is suited for a hardened and tempered steel.
Cold working as used herein should preferably be cold drawing (wire
drawing), but is not limited thereto. For instance, the cold
working used may include roller-dies working.
The temperature as used for heating in the present invention should
fall in a range from 450.degree. C to A.sub.1 transformation point.
This is because the heating temperature of a range from 449.degree.
C to 250.degree. C tends to cause cracking in the head portion of a
bolt, since the heating temperature is within the range of blue
shortness temperatures, while heating temperatures of over the
A.sub.1 transformation point will lead to a poor strengthening
effect. Meant by the heating temperature is the temperature
obtained by using electric-resistance heating means, high frequency
induction heating means, etc.
Alternatively, in case the strength of a bolt thus produced can not
satisfy the value required, then the bolt may be rapidly heated in
a salt bath, lead bath, air atmosphere furnace, or the like, which
has been maintained at a temperature ranging from 450.degree.C to
700.degree.C, and soaked for no more than 5 minutes therein,
followed by air cooling or cooling at a cooling rate higher than
that of the air cooling, thereby achieving the desired
strengthening effect.
As is apparent from the foregoing description, the method of the
present invention can obviate the use of heat treatment at an
elevated temperature, such as hardening and tempering, after
forming to a bolt shape as in the conventional method. In addition,
since the temperature for warm-forming or re-heating treatment is
limited to 700.degree.C, which is much lower than the hardening
temperatures, there can be achieved improvements in the skin
condition as well as dimensional accuracy for the bolts, without
incurring the possibility of oxidation or decarbonization, thereby
enabling the production of consistent high quality bolts.
The present invention will now be described in more detail with
reference to the ensuing examples.
Table 1 shows the chemical composition of steel wires tested.
Table 1
__________________________________________________________________________
Chemical Composition of Samples C Si Mn P S Cr Mo B
__________________________________________________________________________
AISI 1024 Steel 0.22 0.28 1.55 0.021 0.020 B AISI 1040 Steel 0.38
0.23 0.75 0.022 0.016 C AISI 1027 Steel 0.25 0.23 1.51 0.017 0.015
D AISI 1035 Steel 0.35 0.24 0.84 0.016 0.024 0.13 0.23 0.0016 E
AISI 4140 Steel 0.37 0.26 0.76 0.014 0.020 1.07 0.20
__________________________________________________________________________
In the above Table 1, steels A and B represent the samples which
were subjected in turn to cold wire-drawing at a reduction-of-area
percentage of 5 to 43 percent, heating at a heating rate of
40.degree.C/sec. by using an electric heating means, warm-forming
to M10 bolt (shank: 48 L) and water-quenching immediately
thereafter. In addition to this, the bolts thus produced were again
placed in a salt bath which had been heated to a temperature of
550.degree.C and held therein for 20 seconds, followed by water
quenching.
The shank portions of the M10 bolts were machined to a diameter of
8.0 and then subjected to No. 4 tensile test applicable to bolts.
The results of the test are shown in Table 2.
Table 2
__________________________________________________________________________
Reduction-of-area percentage of cold wire-drawing versus mechanical
properties
__________________________________________________________________________
Reduction Yield Tensile Elonga- Final Type Production of-area point
strength tion reduction of % of cold 4A of-area Steel condition
wire dra- (kg/mm.sup.2) (kg/mm.sup.2) (%) percen- wing (%) tage (%)
__________________________________________________________________________
5 64.7 72.6 28.0 64.2 64.3 72.8 29.0 64.9 Hot-forming 10 69.6 77.2
27.0 62.3 (550.degree.C) 69.6 79.0 26.0 63.8 20 74.6 84.6 25.5 60.9
A 75.6 86.5 25.0 62.0 30 76.0 86.5 24.5 60.7 AISI 76.5 87.0 24.0
61.5 1024 43 77.6 90.3 22.6 59.2 77.2 92.5 21.9 58.8 5 65.7 74.0
29.5 64.3 Hot-forming 66.7 75.6 28.0 64.2 (550.degree.C) 10 74.6
81.6 26.5 60.3 and reheat- 74.8 81.0 25.5 62.0 ing treat- 20 77.8
87.5 25.0 59.5 ment 77.8 86.5 23.8 58.7 (550.degree.C) 30 77.5 86.0
24.0 59.3 78.0 87.6 23.5 59.0 5 67.3 75.8 23.5 45.9 65.8 76.8 24.5
46.3 Hot-forming 10 69.6 81.8 18.0 42.7 B (550.degree.C) 67.6 80.0
21.0 44.7 AISI 20 77.8 90.9 16.0 40.9 1040 76.1 88.8 18.0 40.4 30
77.5 89.1 17.5 40.2 79.9 91.2 17.1 39.2 Hot-forming 5 67.1 77.8
20.0 45.0 (550.degree.C) 68.0 77.8 19.0 45.5 and reheat- 10 70.6
82.0 18.0 43.7 ing treat- 72.4 83.6 18.5 41.5 ment 20 77.2 90.9
15.0 38.6 (550.degree.C) 76.8 91.5 17.0 33.8 30 78.8 93.6 16.3 38.3
79.9 92.1 17.0 38.8
__________________________________________________________________________
As can be seen from Table 2, cold wire-drawing prior to
warm-forming exerts a great effect on the mechanical properties of
bolts which have been warm-formed. The tensile strength and yield
point show an increase with an increase in the reduction-of-area
percentage of cold working, despite the heating used for
warm-forming. On the other hand, elongation, and toughness required
for the final reduction-of-area percentage show a tendency to
slightly decrease with the increase in strength. Another fact is
that bolts, which have been re-heated, present improved strength,
as compared with those which have not been subjected to re-heating
treatment.
FIG. 1 shows the relationship of the mechanical properties and the
electron-microscopic pictures of materials which are made of rolled
steel of sample A and have been subjected to 43 percent cold
wire-drawing. The cold working breaks in pieces the cementite
included in the pearlite, and such broken cementite may be
spheoroidized due to the use of the thermal cycle of rapid heating
and rapid cooling. This well explains that the steel wire which has
been subjected to the strengthening treatment gives greater
elongation together with increased strength, as compared with a
cold worked steel. Thus, the aforesaid fact is considered to have
bearing on the increase in strength of bolts, because the shank
portion of a bolt does not undergo the influence by hot-forming and
hence retains improved properties given by the strengthening
treatment of the present invention.
On the other hand, the steel wires B in Table 1 having diameters of
10.8 and 9.68 were at a reduction-of-area percentage of 20 percent
then rapidly heated to a temperature of 550.degree.C at a heating
rate of 5.degree.C/min. to 120.degree.C/sec. warm-formed to M10
bolt and then water-quenched to thereby test the influence of the
heating rate. Table 3 shows the results of the B01t No. 4` tensile
test given to M10 bolts whose shank portions were machined to a
diameter of 8 after warm-forming at varying heating rates.
Table 3
__________________________________________________________________________
Heating Rate Versus Mechanical Properties Tpye of Heating Yield
Tensile Elonga- Final Reduc- Steel Rate Point Strength tion (%)
tion-of-area (kg/mm.sup.2) (kg/mm.sup.2) 4 A percentage(%)
__________________________________________________________________________
120.degree.C/sec. 83.7 97.3 21.0 42.0 83.3 96.5 21.3 42.3
40.degree.C/sec. 83.0 97.0 21.0 41.8 82.8 96.3 21.5 42.5 Steel `B`
240.degree.C/min. 82.0 95.5 21.3 42.0 83.5 96.0 21.8 41.6 (AISI
1040 180.degree.C/min. 78.0 92.3 22.0 42.3 Steel) 80.8 94.0 21.0
41.8 100.degree.C/min. 77.5 90.1 21.8 43.1 76.8 87.8 22.2 43.8
50.degree.C/min. 69.6 79.3 22.3 44.7 68.5 80.0 22.5 43.5
5.degree.C/min. 65.3 72.6 23.8 45.7 65.8 73.8 22.0 46.0
__________________________________________________________________________
FIG. 2 shows the relationship of a warm-forming temperature versus
hardness of head portions and shank portions of bolts which were
produced by subjecting steel A, in turn, to cold wire-drawing at a
reduction-of-area of 20 percent, rapid heating at a heating rate of
40.degree.C/sec., and forming to M10 bolt, followed by water
quenching.
FIG. 2 reveals that the relationship of the difference in
reduction-of-area percentage is maintained in fact for the
difference in hardness for the bolts which have been cold formed,
because the head portion of a cold formed bolt has been subjected
to cold working at about 75 percent reduction-of-area percentage,
whereas the shank portion of the bolt has been subjected to cold
wire drawing of only 20 percent reduction-of-area percentage. In
such cold working, the head portion of the bolt gives extremely
great hardness and high strength as compared with those of the
shank portion, while presenting reduction in toughness therewith.
Accordingly, it is not preferable that the rupture occurs at the
neck portion of the bolt, in case the bolt is subjected to a great
load and broken, in contrast to the normal rupture at the thread
portion. As the forming temperature is increased, there results a
lesser difference in hardness between the head and shank portions
of the bolt. Although the increase in hardness of the shank portion
of the bolt depends on the reduction-of-area percentage of cold
working, soaking time at a heating temperature and cooling rate,
such an increase shows a peak at 400.degree.C and thereafter
gradually goes down as the temperature becomes higher than the peak
temperature. On the other hand, the hardness of the head portion of
the bolt varies, to some extent, with the varying reduction-of-area
percentage of cold-wire-drawing and compression percentage of the
head portion of the bolt, while the hardness increases in
temperature range from 200.degree.C to 300.degree.C with the
increase in warm-forming temperatures, but decreases thereafter,
showing a sharp decrease at 400.degree.C thus presenting the least
difference in hardness between the shank and head portions of the
bolt. With such a bolt, there occurs normal rupture at the thread
portion, when the bolt ruptures due to a great load. Accordingly,
such a bolt is preferable. The cooling after warm-forming should
terminate as rapidly as possible. In other words, if a bolt is
slowly cooled, then the bolt will be annealed, thereby failing to
present the tensile strength of over 80 kg/mm.sup.2 which is
essential for the high strength bolt.
Table 4 shows the tensile test results of bolts which have been
produced by heating the steel B to 550.degree.C at a heating rate
of 40.degree.C/sec. and hot-forming the same to M10 bolt, and
subjecting the bolt to three types of cooling, i.e.,
water-quenching, air-cooling and slow cooling. This test reveals
that the slow cooling results in the failure to obtain a high
strength bolt.
Table 4
__________________________________________________________________________
Cooling Conditions After Hot-Forming Versus Mechanical Properties
Final Reduc- Type of Cooling Yield Tensile Elonga- tion-of- Steel
Condition Point Strength tion (%) area (kg/mm.sup.2) (kg/mm.sup.2)
4 A percen- tage (%)
__________________________________________________________________________
Air Cooling 74.4 84.8 17.0 41.5 Steel `B` 74.7 85.0 18.0 41.7 Water
77.8 90.9 16.0 40.9 (AISI 1040) Quenching 76.1 88.8 18.0 40.4 Slow
Cooling 58.9 73.1 24.5 48.6 57.0 72.6 23.0 49.5
__________________________________________________________________________
In such cooling, air cooling may be suitably used for a bolt of a
small diameter, whereas a bolt of a great diameter should be
subjected to oil cooling or water cooling.
After being subjected to the method for producing a high strength
bolt according to the present invention, if a bolt is still short
of the strength required, then the bolt may be re-heated to achieve
the required strength.
For reheating, a bolt is placed in a salt bath or lead bath which
has been heated to a temperature of 450.degree.C to 700.degree.C,
then allowed to stand therein for 5 minutes, and quenched.
In this respect, the re-heating should be carried out at a high
heating rate and thus the bolt should be directly charged into a
heating bath. The soaking temperature of the bolt should be limited
to a range between 450.degree.C and 700.degree.C, because a
temperature over 450.degree.C can avoid the blue shortness, while a
temperature below 700.degree.C can prevent a decrease in strength.
While the soaking time depends on the type of heating furnace, size
of bolt, heating temperatures and the like, the shorter soaking
time is preferable because of its greater effectiveness, and thus
limited up to 5 minutes.
Table 5 shows the relationship of the mechanical properties and
soaking time when M10 bolt is re-heated to 450.degree.C in an air
atmosphere furnace, the M10 bolt being produced from a steel wire B
according to the method of the invention.
Table 5 ______________________________________ Re-Heating Time and
Mechanical Properties Type of Tensile Rupture Steel Soaking Time
Strength Position (kg/mm.sup.2)
______________________________________ 20 seconds 94.8 thread
portion 94.3 thread portion 1 minute 93.5 thread portion 94.0 Steel
`B` 3 minutes 93.7 thread portion 92.8 thread portion (AISI 1040) 5
minutes 93.0 thread portion 92.3 thread portion 10 minutes 91.5
thread portion 91.0 thread portion 15 minutes 74.6 thread portion
75.0 thread portion ______________________________________
In this connection, cooling after re-heating treatment should be
accelerated to a cooling rate higher than that of air cooling,
because slow cooling results in an annealed bolt having a low
strength, as in the case of hot-forming.
The steel wires, as used in the present invention, should include
those as rolled or normalized, and the type of steel should cover
carbon steel or low alloy steels which are generally used as
materials for producing bolts.
The rolled steels of two types, i.e., steel A and steel B, as shown
in Table 1, were cold-drawn at a reduction-of-area percentage of 5
to 20 percent, then heated at a heating rate of 40.degree.C/sec. in
an electric furnace, warm-formed to M10 bolt and water-quenched
immediately thereafter. On the other hand, part of the bolts thus
treated were directly charged into a salt bath which has been
heated to 550.degree.C for re-heating treatment, allowed to stand
therein for 20 seconds, and then water quenched. Those M10 bolts
thus prepared were subjected to a tensile test using a wedge. The
results thereof are shown in Tables 6 and 7.
Table 6
__________________________________________________________________________
Results of Tensile Test Using A Wedge for Bolts of Steel `A`
Production Condition Wedge Angle 0 Wedge Angle 10 Remarks
Reduction- of-area percentage Tensile Tensile in wire draw-
Strength Rupture Strength Rupture ing (%) Forming (kg/mm.sup.2)
Position (kg/mm.sup.2) Position
__________________________________________________________________________
5 74.1 Thread 72.6 Thread Comparative 74.5 Portion 72.0 Portion
Example Warm 10 Forming 80.0 " 79.2 " This Inven- (550.degree.C)
80.8 " 78.6 " tion 20 86.7 " 83.0 " This Inven- 86.1 " 83.9 " tion
5 77.2 " 76.0 " Comparative Warm 76.8 " 75.5 " Example Forming 10
(550.degree.C) 83.9 " 81.0 "Inven- This inven- and Re- 83.0 " 80.6
" tion Heating 20 (550.degree.C) 89.9 " 87.5 " This Inven- 89.4 "
88.0 " tion 10 Cold 75.0 " 71.5 " Prior Forming 74.5 " 70.5 " Art
__________________________________________________________________________
Table 7
__________________________________________________________________________
Results of Tensile Test With Wedge for Bolts of Steel Production
Condition Wedge Angle 0 Wedge Angle 10 Remarks Reduction- of-area
percentage Tensile Tensile in wire draw- Strength Rupture Strength
Rupture ing (%) Forming (kg/mm.sup.2) Position (kg/mm.sup.2)
Position
__________________________________________________________________________
5 75.3 Thread 74.2 Thread Comparative 85.8 Portion 73.3 Portion
Example Warm 10 Forming 83.3 " 81.6 " This Inven- (550.degree.C)
83.0 " 82.2 " tion 20 90.5 " 88.9 " This Inven- 91.3 " 89.9 " tion
5 Warm 79.4 " 77.8 " Comparative Forming 78.8 " 76.8 " Example
(550.degree.C) 10 and Re- 84.6 " 83.0 " This Inven- Heating 85.3 "
82.7 " tion (550.degree.C) 20 93.0 " 90.9 " This Inven- 92.3 " 91.5
" tion 10 Cold 76.5 " 72.5 " Prior Forming 76.0 " 73.0 " Art
__________________________________________________________________________
FIG. 3 shows the hardness distribution at the heads and shanks of
M10 bolts of two groups, one of which has been subjected in turn to
cold water-drawing at a reduction-of-area percentage of 20 percent
and warm-forming at 550.degree.C according to the present
invention, and the other of which has been subjected to cold
working according to the conventional method. The test results
reveal that the high strength bolts produced, according to the
present invention, as shown in Tables 6 and 7 and FIG. 3, present a
lesser hardness difference at the head and shank portions of the
bolts, as compared with bolts produced according to the
conventional method as well as the bolts produced for comparison
purpose, while the former exhibits normal rupture.
The sample steels, C, D and E, as shown in Table 1, were subjected
to a refining treatment of hardening and tempering, thereby
attaining the tensile strength of over 100 kg/mm.sup.2 to obtain
high strength bolts.
The sample steel C was oil-hardened at 870.degree.C and subjected
to a refining treatment of tempering at 570.degree.C, thus
attaining mechanical proporties as a refined steel, as shown in
Table 4. Thereafter, the steel C was subjected to preliminary cold
wire drawing at a reduction-of-area percentage of 20 percent,
heated at a heating rate of 5.degree.C/min. to 40.degree.C/sec. to
a temperature of 550.degree.C, allowed to stand thereat for 5
seconds, and water quenched. The heating rates used and the
mechanical properties obtained are shown in Table 8.
Table 2 shows the mechanical properties including the tensile
strength of 10 steel wire which has been subjected in turn to
preliminary cold wire-drawing, rapid heating and water quenching,
while the strength of the bolt shown therein represents that of the
shank portion of the bolt. Accordingly, the 10 wire should present
properties the same as shown in Table 8, because the sample steel C
was subjected, like a 10 steel wire to the preliminary cold wire
drawing, rapid heating for hot-forming, and rapid cooling.
Table 8 ______________________________________ Heating Rate Versus
Mechanical Properties (Sample steel `C`) Heating Yield Tensile
Elonga- Final Reduction- Rate Point Strength tion (%) of-area
percent- (kg/mm.sup.2) (kg/mm.sup.2) 4 A age (%)
______________________________________ 40.degree.C/sec. 90.0 105.7
19.7 68.0 240.degree.C/min. 89.8 105.3 19.8 68.0 180.degree.C/min.
88.3 102.0 20.0 68.5 120.degree.C/min. 85.2 100.5 21.1 68.8
50.degree.C/min. 83.5 98.3 21.5 69.7 5.degree.C/min. 78.2 92.5 22.0
70.3 ______________________________________
As is apparent from Table 8, the greater the heating rate at
warm-forming, the higher will be the strength. Accordingly, a
heating rate of at least over 50.degree.C/min. should be adopted
for attaining the desired increase in strength.
Sample steel C was subjected in turn to a refining treatment of oil
hardening at 870.degree.C, then tempering at 570.degree.C, then to
the prelimiary cold working at a reduction-of-area percentage of 20
percent, heating to 550.degree.C at a heating rate of
75.degree.C/min. in a direct electric-current-flowing heating means
placed immediately ahead of a press forming machine, and
hot-forming to M10 bolt 5 minutes thereafter. In this connection,
the cooling after warm-forming should be carried out as rapidly as
possible, since slow cooling results in an annealed bolt due to
self-retained heat of the bolt given during-hot-forming, thus
failing to achieve the intended strengthening effect.
Table 9 shows the tensile test results of bolts which have been
subjected to three types of cooling after hot-forming, i.e., water
quenching air cooling and slow cooling, revealing that slow cooling
does not give a high strength.
Table 9
__________________________________________________________________________
Cooling Condition After Hot-Forming And Mechanical Properties Type
of Cooling Yield Tensile Elonga- Final Reduc- Steel Condition Point
Strength tion (%) tion-of-area (kg/mm.sup.2) (kg/mm.sup.2) 4 A
percentage(%)
__________________________________________________________________________
Water 94.3 110.0 19.8 67.5 Steel Quenching `C` (AISI Air 1027
Cooling 84.0 100.0 21.5 68.9 Steel) Slow Cooling 71.4 85.4 23.5
77.5
__________________________________________________________________________
The intended properties may be obtained for a bolt of small
diameter by air cooling. However, a bolt of great diameter requires
oil-cooling or water quenching.
The sample steel C was subjected to a refining treatment of
oil-hardening at 870.degree.C and then tempering at 570.degree.C,
then preliminary cold working at a reduction-of-area percentage of
5 to 40 percent, then heating at 550.degree.C at a heating rate of
75.degree.C/sec., warm-forming to M10 bolt 5 seconds thereafter,
and finally water-quenching. Table 10 shows the influence of the
preliminary cold working on the mechanical properties obtained.
Table 10
__________________________________________________________________________
Preliminary Cold Working and Mechanical Properties Type of Prelim-
Yield Tensile Elonga- Final Reduc- Steel inary cold Point Strength
tion (%) tion-of-area working (%) (kg/mm.sup.2) (kg/mm.sup.2) 4 A
percentage (%)
__________________________________________________________________________
78.0 92.0 22.1 70.3 Steel `C` 5 80.5 94.3 22.0 69.5 (AISI 1027 10
89.3 105.5 21.0 68.5 steel) 20 94.3 110.0 19.8 67.5 30 97.0 116.5
18.8 67.8 40 100.5 119.3 16.5 67.0
__________________________________________________________________________
As can be seen from Table 10, the application of preliminary cold
working results in the increase in strength, whereas the toughness
thereof decreases. The preliminary cold working at a
reduction-of-area percentage of over 40 percent causes transverse
cracking in the refined steel material, thus interrupting further
working.
The sample steel C was in turn subjected to a refining treatment of
oil-hardening at 870.degree.C and then tempering at 570.degree.C,
then preliminary cold working at a reduction-of-area percentage of
20 percent, heating to 550.degree.C at a heating rate of
75.degree.C/sec. in a direct current-flowing heating means
positioned immediately ahead of a press forming machine,
warm-forming to a M10 bolt 5 seconds thereafter, and then
water-quenching. FIG. 4 shows hardness distributions at the head
and shank portions of bolts.
In general, the head of a bolt has been subjected to 75 percent
working, whereas the shank portion has been subjected to only 20
percent preliminary working, such that the difference in such
working percentage is reflected in tact to the hardness difference.
As a result, the bolt head exhibits abnormally high hardness
(strength) as compared with those of the shank portion, while the
toughness thereof decreases accordingly. Rupture will take place at
the neck portion of a bolt, when the bolt is subjected to a great
load and ruptured.
In contrast thereto, a warm-formed bolt according to the present
invention presents lowered hardness for the head portion, because,
when warm-formed as shown in FIG. 2, the deformation resistance of
the sample steel will decrease with the accompanied lesser
work-hardening, while the shank portion of the bolt presents a
hardness increase due to the strengthening treatment of the
invention, i.e., a cycle of the preliminary cold working at a
reduction-of-area percentage of 20 percent, rapid heating used for
warm-forming and rapid cooling thereafter, thereby lessening the
difference in hardness between the head and shank portions of the
bolt.
The sample steels D (boron steel) and E (Cr-Mo steel) were
subjected twice to a cycle of hardening treatment by a
high-frequency heating means, then a tempering treatment in a head
bath, preliminary cold working at a reduction-of-area percentage of
20 percent, heating to 550.degree.C at a heating rate of
75.degree.C/sec. and warm forming to M10 bolt.
M10 bolt was formed with a notch of 0.05R (stress concentration
factor 6) and was subjected to a so-called loop type delayed
rupture test in a high humidity atmosphere, in which the bolt was
immersed in a water bath, with the tension being applied thereto.
Table 11 shows the results of such a test.
As is clear from Table 11, in case a bolt is produced by
hot-forming according to the strengthening treatment of the
invention, after the austenite crystal grain size has been rendered
finer by means of the refining treatment in combination with the
high frequency induction heating treatment, the bolt thus produced
presents excellent resistance to delayed rupture, as compared with
a conventional bolt having the same strength.
As is apparent from the foregoing description, a high strength bolt
according to the present invention presents a high tensile strength
of over 100 kg/mm.sup.2 without impairing the toughness thereof, in
contrast to the conventional heat-treated bolt.
On the other hand, since rapid heating in the high frequency
induction heating means used for the refining treatment for
preliminary cold working can render finer the austenite crystal
grain size, the bolts produced according to the present invention
can present excellent resistance to delayed rupture, as compared
with conventional high strength bolts.
Table 11
__________________________________________________________________________
Results of the Delayed-Rupture Test Mechanical Properties Sample
Steel Heat-treatment Yield Tensile Elonga- Final Reduc- Austenite
Notch Rupture Point Strength tion (%) tion-of-area Grain Size
Strength Time (kg/mm.sup.2) (kg/mm.sup.2) *1 percentage (%) *2
(kg/mm.sup.2) *3
__________________________________________________________________________
(HV) D-O (compara- Formed to a bolt tive material) and then heat-
130.8 141.3 17.6 55.2 7.0 208.5 50 treatment, 850.degree.C O.Q. --
420.degree.C A.C. -D-1 (this High frequency in- invention) duction
heating heat 135.6 137.6 18.3 59.1 11.8 220.3 3256 treatment -- 20%
preliminary working W.H. -- 850.degree. C O.Q. 380.degree.C A.C. --
500.degree.C W.H. E-O (compara- Formed to bolt and tive material)
then heat-treatment, 128.2 138.3 15.6 53.4 8.0 207.0 43
850.degree.C O.Q. -- 490.degree.C A.C. E-1 (this High frequency in-
invention) duction heating heat 137.8 138.1 18.3 58.0 12.0 225.0
8634 treatment -- 20% preliminary working W.H. -- 850.degree.C O.Q.
490.degree.C A.C. -- 500.degree.C W.H.
__________________________________________________________________________
*1 Elongation was measured by gauge length 4 A; *2 Grain size after
heat-treatment; *3 The rupture time in the delayed rupture test is
carried out under the nominal load of 175 kg/mm.sup.2 and using the
water immersion at room temperature.
The sample steel C was subjected in turn to a refining treatment of
oil-hardening at 870.degree.C and tempering at 650.degree.C, cold
wire-drawing at a reduction-of-area percentage of 20 percent,
heating to 550.degree.C at a heating rate of 240.degree.C/sec.,
holding at said temperature for 5 seconds and water quenching.
The material thus treated was then subjected twice to a
strengthening treatment consisting of cold working at a
reduction-of-area percentage of 20 percent, heating to 500.degree.C
at a heating rate of 240.degree.C/sec., holding at said temperature
for 5 seconds, and water quenching.
The materials, which have been subjected to one and two cycles of
strengthening treatment of the invention as described, were further
subjected in turn to a wire drawing at a reduction-of-area
percentage of 20 percent, heating to 500.degree.C at a heating rate
of 75.degree.C/sec., warm-forming to M10 bolt 5 seconds thereafter
and water quenching.
The mechanical properties obtained are shown in Table 12.
Table 12 ______________________________________ Mechanical
Properties Material Worked Yield Tensile Elonga- Final Re- Point
Strength tion (%) duction-of- (kg/mm.sup.2) (kg/mm.sup.2) 4 A area
per- centage (%) ______________________________________ Refined
Material 76.0 90.0 23.0 70.5 One cycle of strengthening treatment
87.6 97.5 22.6 68.0 Two cycles of strengthening treatments 96.3
105.6 22.0 64.5 M10 bolt of refined material 90.0 98.6 22.3 67.3
M10 bolt subjected to one cycle of strengthening treatment 97.5
105.8 22.0 66.5 M10 bolt subjected to two cycles of strengthening
treatments 104.2 112.3 21.5 63.2
______________________________________
As can be seen from Table 12, in case the refined steel is
subjected in turn to one or two cycles of a strengthening
treatment, wire drawing at a reduction-of-area percentage of 20
percent, and then hot-forming to M10 bolt, then there results a
decrease in both elongation and toughness required for the final
reduction-of-area percentage, while there are obtained improved
tensile strength and yield point. In addition, the test results
show that the repeated strengthening treatments can present further
increased tensile strength for bolts which have been
warm-formed.
Obviously, numerous modifications and variations of the present
invention are possible in light of the above teachings. It is to be
understood therefore that within the scope of the appended claims,
the present invention may be practiced otherwise than as
specifically described herein .
* * * * *