Method for bonding dual-phase stainless steel

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.