Method for bonding dual-phase stainless steel

The method provides a diffusion-bonding of dual-phase stainless steel material having excellent strength and corrosion. The method includes: cold-working a material to be bonded so as to enhance the proof stress; inserting into the bonding portion an insert material; applying pressure thereto while performing shielding by mixed nitrogen/argon gas; heating the restricted bonding portion; cooling at a restricted rate; so as to form a bonded portion having the ferritic phase percentage of 30-70% by volume.

This application claims priority under 35 U.S.C. .sctn. .sctn. 119 and/or 
365 to 8-307577 filed in Japan on Nov. 19, 1996, the entire content of 
which is herein incorporated by reference. 
TECHNICAL FIELD 
The present invention relates to a method for bonding dual-phase stainless 
steel material, particularly dual-phase stainless steel pipes, for use in 
drilling oil wells, oil purification, oil transportation, and like 
applications to obtain joints having excellent strength and corrosion 
resistance. 
BACKGROUND ART 
Both high strength (proof stress) and high corrosion resistance are 
required of steel materials, particularly steel pipes, for use in drilling 
oil wells, oil production, oil purification, oil transportation, and like 
applications; thus, various materials have been developed in accordance 
with working environment and applications. Particularly, for application 
to a wet carbon dioxide environment containing a trace quantity of 
hydrogen sulfide, dual-phase stainless steel having excellent corrosion 
resistance is often used. Dual-phase stainless steel has a microstructure 
of dual phase, i.e. a ferritic phase (hereinafter referred to as ferrite) 
and an austenitic phase (hereinafter referred as austenite). Since 
dual-phase stainless steel in the solid-solution heat-treated state is low 
in proof strength, proof strength is enhanced through cold working. 
When dual-phase stainless steel is used as, for example, oil country 
tubular goods, dual-phase stainless steel pipes having a length of 10 m to 
15 m are joined one to another through use of threaded joints as in the 
case of steel pipes of other materials. Since oil country tubular goods 
for use in drilling an oil well reach a final overall length of thousands 
of meters, hundreds of portions joined by means of threaded joints are 
involved. This method which uses threaded joints involves the following 
problems. 
a) Since accurate threads must be cut, considerable cost is involved. 
b) Since a joining force varies among threaded joints, skilled workers must 
perform the joining work in order to ensure reliability of joined 
portions. 
c) Threads of pipes are susceptible to damage during transport. 
To solve these problems, a welding method such as TIG welding is proposed 
as an alternative to the joining method which uses threaded joints. The 
welding method is a highly efficient joining method, but involves the 
following problems. 
A) The ferritic phase percentage becomes relatively high over a weld zone 
and over a wide heat affected zone, causing impairment in toughness and 
corrosion resistance. 
B) Carbon-nitrides, intermetallic compounds, and the like precipitate over 
a wide heat affected zone, causing impairment in toughness. 
As a simple, convenient method to solve these problems involved in the 
welding method, a diffusion-bonding method is proposed (Japanese Patent 
Laid-Open (kokai) No. 3-86367). However, even the diffusion-bonding method 
has a drawback that softening occurs in a zone heated during 
diffusion-bonding, causing impairment in proof stress of a joint. 
Hereinafter, a joint refers to a wide zone which includes a bond portion, 
a heat affected zone, and a portion of base metal adjacent to the heat 
affected zone. 
To solve the above-mentioned drawback, there is disclosed a method in which 
a zone including an end surface of base metal, which zone will become a 
joint, is previously cold-worked so as to improve the proof stress of the 
zone and in which a zone to be heated is narrowed so as to prevent 
impairment in strength (Japanese Patent Laid-Open (kokai) No. 6-7967). 
However, since the method described in Japanese Patent Laid-Open (kokai) 
No. 6-7967 is primarily concerned with a high-alloy steel pipe having an 
austenitic microstructure, an approach in relation to control of 
dual-phase microstructure composed of ferrite and austenite is not 
addressed. Accordingly, when the method of Japanese Patent Laid-Open 
(kokai) No. 6-7967 is applied to dual-phase stainless steel, the following 
problems arise. 
1) Since the ferritic phase percentage of a bond portion becomes high as 
compared to that of base metal, the bond portion is impaired in corrosion 
resistance and toughness. 
2) Since carbon-nitrides and intermetallic compounds are formed from 
ferrite in a bond portion, the bond portion is impaired in toughness and 
corrosion resistance. 
So far, there has not been developed a method for bonding dual-phase 
stainless steel capable of simply and conveniently obtaining a joint 
having excellent strength and corrosion resistance. 
An object of the present invention is to provide a simple, convenient 
method for bonding dual-phase stainless steel, particularly dual-phase 
stainless steel pipes, capable of providing a joint having excellent 
strength and corrosion resistance. 
DISCLOSURE OF THE INVENTION 
Inventors of the present invention solved the above problems based on 
following their acknowledgment. 
(a) Through the prevention of release of nitrogen (N) from a bonded portion 
and the vicinity thereof, there can be prevented an increase in the 
ferritic phase percentage thereof. The prevention of release of N can be 
implemented by adjustment of the atmosphere around the bond portion and 
the vicinity thereof. 
(b) In order to prevent an increase in the ferritic phase percentage in a 
bond portion and the vicinity thereof, in addition to the above-mentioned 
prevention of release of N, the upper limit of a rate of cooling from a 
diffusion-bonding temperature must be controlled to a predetermined value 
or lower. Also, by limitation of the lower limit of the cooling rate to a 
predetermined value or higher, the precipitation of intermetallic 
compounds, which would otherwise significantly impair toughness, is very 
effectively prevented. 
(c) Through the quantitative determination of the length of a heating zone 
which is not impaired in it's toughness or corrosion resistance during 
diffusion-bonding, and on the basis of the obtained quantitative data, the 
zone where carbon-nitrides and intermetallic compounds are generated is 
strictly limited. This limitation of a heating zone is also effective in 
obtaining a required proof stress to a joint. 
(d) In order to obtain a required proof stress to a joint, a material to be 
bonded is cold-worked at the vicinity of an end surface to be bonded or 
over the entire length so as to endow the end portion or the entire 
material with a proof stress which is at least a predetermined amount 
higher than the required proof stress of the joint. 
In the present invention, a diffusion bonding method in which the 
above-mentioned conditions were used in combination was actually applied 
to dual-phase stainless steel materials to form joints, and the formed 
joints were subjected to a corrosion test and the like to confirm the 
effect of the method. The present invention was thus accomplished. The 
gist of the present invention resides in the following method for bonding 
dual-phase stainless steel material through diffusion-bonding. 
(1) A method for bonding dual-phase stainless steel material, comprising 
the steps of: cold-working in at least a zone including a butting surface 
of the material to be bonded so as to enhance the proof stress of the 
cold-worked zone to (1.3.times.(minimal necessary proof stress of 
joint)-10) kgf/mm.sup.2 or higher; arranging the materials to be bonded 
such that the butting surfaces of the materials butt against each other 
via an insert material having a melting point not higher than 1150.degree. 
C. and a thickness of 10-80 .mu.m; applying a pressure of 0.5-2 
kgf/mm.sup.2 in a butting direction while performing shielding, by means 
of a mixed nitrogen/argon gas which contains argon in an amount of 0-80% 
by volume; diffusion-bonding the butting surfaces through heating such 
that the following conditions 1) and 2) are satisfied; and cooling a 
bonded portion and the vicinity thereof such that the following condition 
3) is satisfied, so as to adjust the ferritic phase percentage of the 
bonded portion and the vicinity thereof to 30-70% by volume; 
1) Butting portions and the vicinities thereof are heated to a temperature 
not lower than the melting point of the insert material and lower than the 
melting point of the dual-phase stainless steel and are held at the 
temperature for 120 seconds or longer, 
2) During heating as described above in 1), heating the zone not longer 
than 20 mm from the butting surface to a temperature not lower than 
800.degree. C., and heating the zone not longer than 40 mm from the 
butting surface to a temperature not lower than 600.degree. C., 
3) After the heating and diffusion-bonding are performed, under the above 
conditions 1) and 2), cooling the bonded portion to a temperature of 
400.degree. C., at a cooling rate of 50-150.degree. C./s. 
(2) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the dual-phase stainless steel contains N in an 
amount of 0.1-0.3% by weight. 
(3) A method for bonding dual-phase stainless steel material, as described 
above in (2), wherein the dual-phase stainless steel material is a pipe. 
(4) A method for bonding dual-phase stainless steel material, as described 
above in (2), wherein the dual-phase stainless steel material is a coiled 
tubing. 
(5) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein a mixed nitrogen/argon gas, which contains argon in 
an amount of 20-80% by volume, is used as shield gas. 
(6) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein after heating and diffusion-bonding are performed, 
cooling of the bonded portion to a temperature of 400.degree. C., at a 
cooling rate of 80-120.degree. C./s. 
(7) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the microstructure of the bonded portion and the 
vicinity thereof has a ferritic phase percentage of 40-70%, by volume. 
(8) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein a heated zone, corresponding to either of the heated 
portions is substantially symmetric with respect to the bonded interface. 
(9) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the butting portions and the vicinities thereof are 
heated to a temperature not lower than the melting point of the insert 
material and lower than the melting point of the dual-phase stainless 
steel and are held at the temperature for 200-400 seconds. 
(10) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the proof stress of at least the cold-worked zone, 
including the butting surface ranges between (1.3.times.(minimal necessary 
proof stress of joint)-8) and (1.5.times.(minimal necessary proof stress 
of joint)-5) kgf/mm.sup.2. 
(11) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the dual-phase stainless steel material contains N 
in an amount of 0.1-0.3% by weight, shielding is performed by means of a 
mixed nitrogen/argon gas which contains argon in an amount of 20-80% by 
volume, cooling to a temperature of 400.degree. C. is performed at a 
cooling rate of 80-120.degree. C./s after the heating and 
diffusion-bonding are performed, and the bonded portion and the vicinity 
thereof has a ferritic phase percentage of 40-70% by volume. 
(12) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the vicinity of the butting surface is hot-worked so 
as to increase the thickness thereof, and then the entire material to be 
bonded is cold-worked so as to obtain a uniform thickness to thereby 
enhance the proof stress of the vicinity of the butting surface over that 
of the remainder of the material to be bonded. 
(13) A method for bonding dual-phase stainless steel material, as descrided 
above in (1), wherin the material to be bonded is cold-worked entirely so 
as to enhance the proof stress of the material. 
(14) A method for bonding dual-phase stainless steel material, as described 
above in (13), wherein the dual-phase stainless steel material contains N 
in an amount of 0.1-0.3% by weight. 
(15) A method for bonding dual-phase stainless steel material, as described 
above in (14), wherein the dualphase stainless steel material is a pipe. 
(16) A method for bonding dual-phase stainless steel material, as described 
above in (14), wherein the dualphase stainless steel material is a coiled 
tubing. 
(17) A method for bonding dual-phase stainless steel material, as described 
above in (13), wherein the proof stress of the material is enhanced 
through cold drawing. 
(18) A method for bonding dual-phase stainless steel material, as described 
above in (13), wherein the proof stress of the material is enhanced 
through use of a pilger mill. 
(19) A method for bonding dual-phase stainless steel material, as described 
above in (1), wherein the ferritic phase percentage of the material ranges 
between 30% by volume and 70% by volume. 
(20) A method for bonding dual-phase stainless steel material, as described 
above in (13), wherein the ferritic phase percentage of the material 
ranges between 30% by volume and 70% by volume. 
In the above description, "dual-phase stainless steel" refers to stainless 
steel whose major microstructure is comprised of a austenitic phase and a 
ferritic phase at room temperature. 
A "zone including a butting surface" refers to the portion of a material to 
be bonded, which ranges between the butting surface, whose location is 
taken as 0, and a location about 50 mm from the butting surface. 
A "minimal necessary proof stress of a joint" refers to a stress of the 
material as measured in a tensile test of a bonded joint. 
A "bonding portion" refers to, before and during bonding, an insert 
material and the narrow portions, including butting surfaces of both 
materials to be bonded, and "bonded portion" refers to, after bonding, the 
portion of bonded material where the elements of the insert material are 
densely present and the vicinities of the butting surfaces where the 
elements of the insert material are recognizably diffused. A "bonded 
portion and the vicinity thereof" refers to a bonded portion and a heat 
affected zone adjacent to the bonded portion. Accordingly, the "bonded 
portion and the vicinity thereof" and a heat unaffected portion of a 
material constitutes the entirety of a bonded article. In a bonding 
portion, the interface between the butting surfaces of bonded materials, 
which is formed after an excessive amount of a liquid-phase insert 
material is pushed out as a result of application of pressure, is 
hereinafter referred to as a "bonded interface." Herein, "material" and 
"material to be bonded" are not discriminated from each other, but refer 
to dual-phase stainless steel material to be bonded.

BEST MODE FOR CARRYING OUT THE INVENTION 
1. Dual-Phase Stainless Steel 
Dual-phase stainless steel may be stainless steel having two phases of a 
austenitic phase and a ferritic phase and corresponds to the following 
stainless steel in terms of chemical composition. In the following 
description, "%" accompanying an alloying element refers to "wt. %." 
Dual-phase stainless steel may be stainless steel which contains C (not 
greater than 0.08%), Si (not greater than 2%), Mn (not greater than 2%), 
Ni (3-8%), Cr (18-28%), and N (0.002-0.35%). In order to stably attain a 
required austenitic phase percentage so as to obtain good corrosion 
resistance and toughness, the nitrogen content more preferably ranges 
between 0.1% and 0.3%. The ferritic phase percentage of dual-phase 
stainless steel is preferably 30-70% by volume; that is, the volume ratio 
between ferritic phase and austenitic phase preferably ranges 
substantially between 3:7 and 7:3 and is more preferably near 1:1. 
Dual-phase stainless steel material may be of any shape, but the present 
invention is concerned particularly with the form of pipe. Pipe may be an 
ordinary seamless steel pipe or an ordinary straight steel pipe having a 
longitudinal seam, a spiral weld seam, or a like seam; or a coiled 
seamless steel pipe, i.e. coiled tubing. The method of the present 
invention is particularly suited for the bonding of coiled tubing. 
2. Heating and Cooling Conditions 
2-1) Heating temperature for bond portion and holding time for holding the 
bonding portion at the temperature 
A bonding portion is heated to a temperature not lower than the melting 
point of an insert material and not higher than the melting point of a 
material to be bonded and is held at the temperature for 120 seconds or 
longer. 
In order to complete bonding in a short period of time, the bonding portion 
must be heated to a temperature not lower than the melting point of the 
insert material so as to bring the insert material into the liquid phase, 
to thereby bring the liquid-phase insert material in contact with the 
entire butting surfaces of materials to be bonded, i.e. dual-phase 
stainless steel, and to diffuse the elements of the liquid-phase insert 
material into the dual-phase stainless steel. When the heating temperature 
is lower than the melting point of the insert material, the insert 
material is in the solid phase; thus, the above-mentioned good contact or 
diffusion cannot be expected. 
By contrast, when the insert material is heated to a temperature in excess 
of the melting point of the dual-phase stainless steel, the material 
melts, causing the coarsening of a microstructure of the dual-phase 
stainless steel. As a result, the features of the present invention fail 
to be yielded. 
In order to establish a chemical composition similar to that of the 
dual-phase stainless steel at a bonded interface, which serves as the 
center of the bond portion, through sufficient diffusion of elements of 
the liquid-phase insert material at the bonded interface so as to reliably 
impart sufficient strength and corrosion resistance to a bonded joint, the 
bonding portion must be held at a temperature of the above-mentioned 
temperature zone for 120 seconds or longer. The upper limit of the holding 
time is not particularly specified. However, heating for a long time is 
disadvantageous in terms of coarsening of microstructure and economical 
efficiency. Thus, the holding time is preferably about 1800 seconds or 
shorter. In order to establish better balance between economical 
efficiency and the properties of a bonded joint, such as strength and 
corrosion resistance, the bonding portion is preferably held at a 
temperature of the above-mentioned temperature zone for 200-400 seconds. 
2-2) Restriction on heating zone 
A zone including both butting surfaces of a bonding portion which is heated 
to a temperature not lower than 800.degree. C. during bonding is 
restricted in length to 20 mm or shorter, and a zone including both 
butting surfaces of a bonding portion which is heated to a temperature not 
lower than 600.degree. C. is restricted in length to 40 mm or shorter. 
Proof stress which has been enhanced through cold working, which will be 
described later, is impaired as a result of heating to a temperature of 
800.degree. C. or higher. Thus, by restricting to 20 mm or shorter the 
length of a zone, including both butting surfaces, to be heated to a 
temperature not lower than 800.degree. C., impairment in the proof stress 
of a bonded joint can be suppressed within a relatively small range. As 
mentioned above, even when a proof stress decreased zone is partially 
present in a bonded joint, if the zone is narrow, a surrounding 
high-strength zone restrains the proof stress decreased zone from 
plastically deforming upon subjection to a tensile load. 
When a bonding portion is heated, carbon-nitrides and intermetallic 
compounds are noticeably generated at a heating temperature of 
600-1000.degree. C. Thus, the length of a zone, including both butting 
surfaces, to be heated to a temperature not lower than 600.degree. C. must 
be limited to 40 mm or shorter. When the length of the zone to be heated 
to a temperature not lower than 600.degree. C. is in excess of 40 mm, 
impairment in toughness and corrosion resistance is unavoidable due to 
generation of precipitates such as carbon-nitrides. 
It is prefer that a heating zone is restricted not only by the length of a 
heating zone to 20 mm or shorter or to 40 mm or shorter but also by equal 
distribution of the length of 20 mm or 40 mm between both materials to be 
bonded so that the length is restricted to 10 mm or shorter or to 20 mm or 
shorter on each of the materials to be bonded. In other words, restriction 
on a heating zone is effected symmetrically with respect to a bonded 
interface, i.e. the restriction is preferably effected on both materials 
to be bonded over the same width on both sides with respect to the bonded 
interface. 
The above-mentioned restriction on a heating zone can be performed by the 
following method. 
1) The width of a device for heating a bonding portion is adjusted. 
2) Jackets (see FIG. 1) manufactured of a material having a high thermal 
conductivity, for example, copper, and used for cooling materials such as 
steel pipes are attached onto the materials on both sides of the device 
for heating the bonding portion, thereby suppressing temperature increase, 
which would otherwise occur due to thermal conduction, at the portions of 
the materials located outside the heating device. The cooling capability 
of the cooling jackets is adjusted by changing the flow rate of cooling 
water and the location of the cooling jackets attached onto the materials 
to be bonded. 
For a zone heated to a temperature not lower than 800.degree. C. and a zone 
heated to a temperature not lower than 600.degree. C., during bonding 
temperature can be measured through use of a sensor such as a 
thermocouple. After bonding, heating temperature can be highly accurately 
estimated by observation of the microstructure of a bonded portion through 
an optical microscope and observation of the amount of precipitated 
carbon-nitrides and intermetallic compounds through an electron 
microscope, while comparison is made with standard samples, which are 
prepared through the application of various heat cycles to samples of the 
same kind. 
2-3) Cooling rate of bonded portion 
A bonded portion is cooled from a bonding temperature down to 400.degree. 
C. at a rate of 50-150.degree. C./s. Carbon-nitrides and intermetallic 
compounds are remarkably generated at a heating temperature of 
600-1000.degree. C., causing impairment in properties of a bonded portion. 
Thus, cooling from a bonding temperature to 400.degree. C. must be 
performed at a rate not lower than 50.degree. C./s. By contrast, when 
cooling is performed at an excessively high rate, the generation of 
austenitic phase in a bond portion is retarded and, in turn, the ferritic 
phase percentage increases, resulting in a failure to bring the ferritic 
phase volume percentage into a required range, which will be described 
later. Accordingly, the upper limit of a cooling rate in the 
above-mentioned temperature zone is determined to be 150.degree. C./s. In 
order to further reliably suppress the generation of carbon-nitrides and 
intermetallic compounds and obtain an appropriate ferritic phase 
percentage in a bonded portion, the cooling rate is preferably 
80-120.degree. C./s. 
As in the above-mentioned case of restriction on a heating zone, the 
cooling rate is controlled by adjustment of the flow rate of cooling water 
within cooling jackets and adjustment of the location of the cooling 
jackets attached onto the materials to be bonded. 
3. Atmosphere 
Bonding is performed in a mixed nitrogen/argon gas atmosphere which 
contains argon in an amount of 0-80% by volume. In order to provide 
sufficient strength and corrosion resistance, dual-phase stainless steel 
contains N in a very high content near a steel-making limit. During 
bonding, in the vicinities of butting surfaces of materials to be bonded, 
nitrogen in excess of solution by solid solution diffuses into a 
liquid-phase insert material to thereby be released into the atmosphere. 
Accordingly, the ferritic phase percentage increases, causing impairment 
in the proof stress and corrosion resistance of a bonded joint. Also, 
nitrogen in process of release becomes a cause of a defect of a bonded 
zone. These phenomena begin to explicitly occur when the argon gas content 
and the nitrogen gas content become greater than 80% by volume and less 
than 20% by volume, respectively. 
Even when the argon gas content is zero, i.e. the nitrogen gas content is 
100% by volume, there merely occurs a slight increase in the amount of 
nitride precipitates in a bond portion. Accordingly, bonding is performed 
in an atmospheric gas whose argon gas content ranges between 0 and 80% by 
volume. When the argon gas content is 0, i.e. the nitrogen gas content is 
100% by volume, the amount of nitride precipitates increases in a bonded 
portion as mentioned above, causing a potential impairment in ductility as 
measured in a tensile test of a boded joint. Thus, the argon gas content 
is more preferably 20-80% by volume. 
4. Microstructure of a Bonded Portion 
A bonded portion and the vicinity thereof is adjusted to have a ferritic 
phase volume percentage of 30-70% through the appropriate combination of 
the above-mentioned conditions. When the ferritic phase volume percentage 
is less than 30%, sufficient toughens and corrosion resistance cannot be 
obtained. By contrast, when the volume ratio is in excess of 70%, 
toughness and corrosion resistance are impaired. More preferably, the 
ferritic phase percentage ranges between 40-70% by volume. It is 
preferable that the entirety of a bonded member, which is composed of a 
bond portion and the vicinity thereof and a heat unaffected portion of a 
member, have a ferritic phase percentage ranging between 30 and 70% by 
volume. The ferritic phase volume percentage can easily be brought within 
the range through use of the above-mentioned manufacturing method. 
5. Insert Material 
An insert material must have a melting point not higher than 1150.degree. 
C. and a thickness of 10-80 .mu.m. An insert material for use in bonding 
dual-phase stainless steel is preferably a metallic material. 
When the melting point of an insert material is in excess of 1150.degree. 
C., the insert material fails to have sufficient fluidity when it assumes 
the liquid phase at a bonding temperature, for example, 1200.degree. C., 
which is comprehensively determined in consideration of diffusion rate and 
the like at a temperature higher than the melting point of the insert 
material. As a result, the contact between the insert material and the 
entire butting surfaces of materials to be bonded becomes insufficient, 
resulting in a failure to obtain a good bonded portion. 
In order for a metallic insert material to have a melting point not higher 
than 1150.degree. C., the metallic insert material is preferably a thin 
strip which is formed through the rapid solidification of molten metal, 
for example, having the following chemical composition: Cr: not less than 
5%; Mo: not more than 9%; Si: 6.5-10%; B: not more than 4%; balance: Ni. 
When the melting point is not higher than 1150.degree. C., the boron and 
silicon contents may be varied so long as the sum of them does not exceed 
12%. 
A lower melting point of an insert material is more preferred. However, an 
insert material having a melting point lower than 1050.degree. C. is 
rarely present. Even when such an insert material is available and brought 
in the liquid phase at a relatively low heating temperature so as to 
utilize the feature of a low melting point, the rate of diffusion into 
materials to be bonded decreases, resulting in insufficient bonding. 
Accordingly, the lower limit of the melting point of an insert material is 
preferably about 1050.degree. C. 
When the thickness of an insert material is less than 10 .mu.m, a roughness 
of the butting surface cannot be completely filled with insert material. 
This causes the occurrence of defect with a resultant impairment in 
bonding strength. By contrast, when the thickness is in excess of 80 
.mu.m, the diffusion of Si and B requires heating for a relatively long 
time, causing impairment in bonding efficiency. If bonding is completed in 
a relatively short time, Si and B segregate in a bonded interface portion 
and corrosion resistance is deteriorated. Accordingly, the thickness of an 
insert material is determined to be 10-80 .mu.m. 
Such a thickness can be obtained easily by, for example, the rapid 
solidification method of molten metal. Specifically, a molten metal having 
the above-mentioned chemical composition is dropped onto twin rolls or 
single roll rotating at a high speed. Instead of self-manufacture of an 
insert material, many kinds of amorphous thin strips usable as an insert 
material are procurable on the market. 
6. Improvement of Proof Stress Through Cold Working 
In the present invention, cold working must be performed on at least the 
vicinities of the butting surfaces of dual-phase stainless steel, which 
serves as materials to be bonded, so as to improve the proof stress of the 
cold-worked zone to (1.3.times.(minimal necessary proof stress of 
joint)-10) kgf/mm.sup.2 or higher. Alternatively, the entire materials to 
be bonded may be previously cold-worked. In some case, cold-working the 
entire material may be easier to perform in view of a manufacturing 
process. When the entire material is to be cold-worked, a target proof 
stress of the base metal must be similar to that of the case of 
cold-working the vicinity of a butting surface. 
When the proof stress of the vicinity of a butting surface or the proof 
stress of the entire base metal is less than (1.3.times.(minimal necessary 
proof stress of joint)-10) kgf/mm.sup.2 before bonding is performed, the 
proof stress of a bonded joint becomes lower than the minimal necessary 
proof stress of material. 
By contrast, excessively intensive cold working causes an impairment in 
corrosion resistance. Thus, preferably, the proof stress of butting 
portions and the vicinities thereof is enhanced through cold working to 
(1.5.times.(minimal necessary proof stress of joint)-5) kgf/mm.sup.2 or 
less. In order to reliably impart sufficient strength and corrosion 
resistance to a bonded joint, the enhanced proof stress preferably ranges 
between (1.3.times.(minimal necessary proof stress of joint)-8) and 
(1.5.times.(minimal necessary proof stress of joint)-5) kgf/mm.sup.2. 
In the case of pipes and coiled tubing, cold working can be carried out 
through cold drawing, rolling by a pilger mill, or local cold forging 
while a mandrel is inserted. In the case of shapes other than pipes, cold 
working can be carried out through cold rolling, forging, hammering, or 
the like. When only the vicinity of an end surface of a pipe or coiled 
tubing is to be cold-worked, the vicinity of an end surface may be 
hot-upset so as to increase its thickness and then be subjected to cold 
working so as to make the once increased thickness decreased to the same 
thickness as that of the remainder of the pipe or coiled tubing, to 
thereby enhance the proof stress of the cold-worked zone. By any of these 
methods, cold-working must be performed at least in the zone which ranges 
between a butting surface, whose location is taken as 0 mm, and a location 
about 0-50 mm away from the butting surface. Of course, as mentioned 
above, the entire material to be bonded may be cold-worked. The reason why 
a portion which extends at least 50 mm from an end must be cold-worked is 
to restrain a softened portion of a heat affected zone including a bond 
portion by a surrounding high-proof-stress portion adjacent to the 
softened portion to thereby reliably impart a required minimum proof 
stress to base metal. For example, 5% cold working easily imparts a proof 
stress of about 760 MPa (110 ksi) to dual-phase stainless steel, 
irrespective of whether the vicinity of an end surface or the entire base 
metal is cold-worked. The proof stress of this portion may be directly 
obtained by a tensile test or, for example, through conversion of hardness 
obtained by a hardness test to a proof stress. 
7. Applied Pressure 
A pressure applied during bonding is 0.5-2 kgf/mm.sup.2 as measured on a 
surface perpendicular to a butting direction. When the applied pressure is 
less than 0.5 kgf/mm.sup.2, reliable adhesion during bonding is not 
established between a liquid-phase insert material and the butting 
surfaces of materials to be bonded and also between the materials to be 
bonded each other. 
When the applied pressure is in excess of 2 kgf/mm.sup.2, the deformation 
of a bonding portion becomes excessively large. Thus, the upper limit of 
the applied pressure is determined to be 2 kgf/mm.sup.2. 
The above-mentioned pressure may be applied in the form of a reaction force 
which is generated in a spring attached to the clamps of materials to be 
bonded as a result of the thermal expansion of the materials, or may be 
externally applied in the form of a hydraulic pressure. 
EXAMPLES 
The effects of the present invention will next be described by way of 
example. In examples of the present invention, a dual-phase stainless 
steel pipe was used as materials to be bonded. 
FIG. 2 shows the chemical composition and other properties of insert 
materials used in an experiment. Insert material I-1 had a melting point 
of 1140.degree. C., and insert material I-2 had a melting point of 
1130.degree. C. The insert materials I-1 had thickness of 30 .mu.m and 120 
.mu.m, and insert material I-2 had a thickness of 30 .mu.m. 
FIG. 3 shows the chemical composition of three kinds of dual-phase 
stainless steel pipes serving as materials to be bonded. These test 
materials all have a melting point not lower than 1550.degree. C. The test 
materials are seamless steel pipes having an outer diameter of 130 mm and 
a wall thickness of 15 mm and manufactured by the steps of: piercing in an 
ordinary manner; rolling by a mandrel mill; finish rolling by a stretch 
reducer; performing solid solution heat treatment at a temperature of 
1150.degree. C.; and cold drawing, during which the cold working ratio was 
varied so as to vary the proof stress. 
FIG. 4 shows actual proof stresses of these steel pipes, the minimal 
necessary proof stress of a joint (kgf/mm.sup.2), and the lower limit of a 
proof stress which must be previously added to a material in the present 
invention (1.3.times.(minimal necessary proof stress of 
joint)-10)(kgf/mm.sup.2). 
As shown in FIG. 1, a bonding apparatus used in the experiments includes a 
one-turn coil for heating and gas shielding. The width of zone heated to a 
temperature not lower than 800.degree. C. and of zone heated to a 
temperature not lower than 600.degree. C. was varied by varying the width 
of the heating coil 1 within the range between 10 mm and 50 mm and varying 
the cooling capability of cooling jackets 3 for cooling a steel pipe 2 
(the cooling jackets 3 are directly attached onto the steel pipe 2, and 
cooling water is circulated within the jackets) located on the outer sides 
of the steel pipe 2. 
In order to apply pressure to a steel pipe to be tested, the steel pipe was 
clamped and pressure was applied by the following two methods: &lt;1&gt; a 
spring was incorporated into a damper so as to release a portion of stress 
accompanying thermal expansion to thereby adjust an applied pressure; and 
&lt;2&gt; pressure was externally applied in the form of a hydraulic pressure. 
Steel pipes were arranged in such a manner as to butt against each other, 
while an insert material was inserted between the butting surfaces of 
steel pipes. The butting portions were heated and held at the heating 
temperature, while pressure was applied, to thereby perform bonding. The 
ferritic phase content of the bonded portion of the thus-obtained joint 
was measured through use of a ferrite scope. 
FIG. 5 is a table showing conditions under which diffusion-bonding was 
performed. 
Also, the bonded portion were evaluated for strength and corrosion 
resistance by a tensile test and a corrosion test. 
FIG. 6 shows the shape of a test piece used in the tensile test on the 
bonded portion. 
FIG. 7 shows a test piece used in the corrosion test, and FIG. 8 shows the 
condition when the test piece was attached onto a test jig. 
The corrosion test was conducted in a 5% NaCl solution having a temperature 
of 80.degree. C. and saturated with H.sub.2 S having a partial pressure of 
0.001 MPa and CO.sub.2 having a partial pressure of 3.0 MPa. A test piece 
was placed in the solution in a bent state as shown in FIG. 8 so as to 
induce a stress corresponding to the proof stress of material at the 
center of the test piece. After the elapse of 336 hours, the test piece 
was checked for corrosion cracking. 
FIG. 9 shows the test results. 
In comparative examples B1, B2, and B6 in which enhancement through the 
cold working of material was insufficient and in comparative example B4 in 
which the zone heated to a temperature not lower than 800.degree. C. was 
in excess of 20 mm, the proof stress of a bonded portion failed to reach a 
required value. 
In comparative example B3 in which an insert material was excessively 
thick, corrosion resistance was impaired due to segregation of Si and B in 
a bonded layer. 
In comparative examples B4 and B5, since a zone heated to a temperature not 
lower than 600.degree. C. was in excess of 40 mm, carbon-nitrides and 
intermetallic compounds precipitated in relatively large amounts, causing 
impairment in corrosion resistance. As a result, cracking occurred in the 
corrosion test. 
In comparative example B7, since the argon gas content of a bonding 
atmosphere was 90% by volume, which fell outside the corresponding range 
as specified in the present invention, the ferritic phase volume 
percentage of a bonded portion was in excess of 70%, and corrosion 
resistance was impaired. As a result, cracking occurred in the corrosion 
test. 
In comparative example B8, since the cooling rate was excessively high, the 
ferritic phase percentage became relatively high. As a result, cracking 
occurred in the corrosion test. 
In comparative example B9, since the cooling rate was excessively low, 
carbon-nitrides and intermetallic compounds precipitated in relatively 
large amounts, causing impairment in corrosion resistance. As a result, 
cracking occurred in the corrosion test. 
By contrast, in examples A1 to A6 which satisfy all conditions specified in 
the present invention, obtained joints were sufficiently satisfactory in 
terms of proof stress and corrosion resistance. 
INDUSTRIAL APPLICABILITY 
The present invention provides a method for bonding dual-phase stainless 
steel material capable of providing a bonded portion having excellent 
proof stress and corrosion resistance. Thus, the bonding method is useful 
as a basic technique related to dual-phase stainless steel material, whose 
use is increasing in oil-related industries and other relevant industries 
in order to improve the degree of safety.