二相ステンレス鋼管の液相拡散接合法の開発

概要

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The transition liquid-phase (TLP) diffusion bonding process was recently tailored to join the tubes of duplex stainless steel, Fe-25Cr-6.5Ni-3Mo-2W-0.5Cu-0.3N, with a Ni brazing filler, Ni-7Cr-3Fe-4.5Si-3B. The butts cramped by a hydraulic system are heated up locally with an induction coil in Ar shield gas. While the bonding at the holding temperatures lower than 1250°C introduces a large amount of boron nitrides at the interface which degrade extremely the tensile strength of the joint, the higher temperature bonding eliminates them from the joint, whose strength is about 850MPa close to 1000MPa for the base metal. The coarser recrystalized grains in the stronger joint scatter the ultrasonic wave so significantly that the joint efficiency is judged non-destructively with the ultrasonic transmission test. The precipitates Fe-Cr-Mo in HAZ reduce substantially the impact value of the joint, which recovers to the original level after the post-heat treatment at 1100°C to dissolve them. Another precipitates Cr7C3 in HAZ dont impair the localized corrosion and the stress corrosion cracking resistance. The developed TLP process is a promising technology for joining the duplex stainless steel tubes at the site of oil fields.