Effects of cooling regimes on the microstructural and mechanical properties of the transient liquid phase joints of UNS S32750 super duplex stainless steel/BNi-2/AISI 304 stainless steel

UNS S32750 super duplex stainless steel and AISI 304 stainless steel were transient liquid phase (TLP) bonded using BNi-2 at a bonding temperature of 1050 degrees C and for two bonding times of 10 and 45 min. After TLP bonding, the samples were subjected to either furnace-cooling or water-cooling regimes. The optical microscope, scanning electron microscope, and energy-dispersive X-ray analysis were employed to investigate the microstructural characteristics of joints, as well as base metals. Moreover, the mechanical properties of the TLP bonds were assessed using microhardness and shear strength examinations. The results showed that the joints fabricated at a bonding time of 10 min exhibited isothermal solidification zone, non-isothermal solidification zone, and diffusion-affected zone, whereas the samples held for 45 min showed a complete isothermal solidification. The only phase in the isothermal solidification zone was gamma-Ni solid solution. However, the non-isothermal solidification zone of the water-cooled bond consisted of Ni-rich boride compounds and a gamma-Ni solid solution, while Ni-rich and Cr-rich eutectic constituents, as well as Ni3Si precipitates, existed in the bonding zone of the furnace-cooled sample. Microstructural analysis showed the formation of the sigma-sigma phase in the microstructure of the furnace-cooled super duplex stainless steel, which was the primary reason for the increase in the hardness values of the alloy to around 415 Hv. The water-cooled samples, in contrast, did not contain the sigma-sigma phase and showed notably lower hardness values (approximately 320 Hv). The non-isothermal solidification zones and diffusion-affected zones of super duplex stainless steels exhibited the highest hardness levels of nearly 525 and 450 Hv, respectively. This work has demonstrated that the joints containing the complete isothermal solidification possessed the highest shear strength, around 380 MPa. [GRAPHICS] .

Automated summary

This study investigates the TLP bonding of UNS S32750 super duplex stainless steel and AISI 304 stainless steel using BNi-2 and explores the microstructural characteristics and mechanical properties of the bonded joints. The results show that the bonding time, cooling regime, and base metal composition have significant effects on the microstructure and hardness of the joints.