Formation of Secondary Phases and Their Effect on the Mechanical Properties of Joints Formed by TLP Bonding Using Fe-B-Si Insert Metal in Duplex Stainless Steel

Abstract

We studied the effects of secondary phases on the mechanical properties of joints formed by transient liquid-phase bonding in nitrogen-containing duplex stainless steel UNS S32750. Transient liquid-phase bonding was carried out using Fe-B-Si amorphous insert metal over the temperature range of 1423-1473K for 0-1000s under a vacuum of 5x10(-5)Torr. Area fraction of austenite () in the joint region decreased with increasing bonding temperature and holding time. In particular, when the holding time was sufficiently prolonged, phase-depleted region were observed in the base metal adjacent to the joint. Secondary phases formed at the joint after short holding times were predominantly chromium borides. For specimens bonded with longer holding times (up to 1000s), boron nitrides were formed at the interface with the joint. Formation of secondary phases in a joint area during the bonding process was expressed by diffusion equation. Tensile strength was improved in accordance with increase of bonding temperature and holding time. Especially, the bonding efficiency was over 98% for the specimen held for 1000s at 1473K. Tensile strength of the joint after short holding times was affected by brittle residual eutectic constituents and borides, whereas tensile strength after longer holding time was influenced by boron nitride formed at the joint interface. The results showed that the mechanical properties were strongly related to the secondary phases (CrB, BN) formed during the bonding process.