Transient Liquid Phase Bonding behavior of Duplex Stainless Steel UNS S32750 Using Fe-B-Si Insert Metal

Abstract

In this dissertation, microstructural evolution, the bonding behavior and mechanical properties of the joints during transient liquid phase (TLP) bonding for nitrogen-containing duplex stainless steel UNS S32750 were investigated to establish TLP diffusion bonding technology of duplex stainless steel. The ultimate objective of the bonding is to obtain a uniform microstructure and mechanical property with base metal. However, it is difficult to achieve a uniform microstructure and mechanical property with base metal because commercial Ni-base insert metals used for the bonding of stainless steels have large constituent difference from base metal. To maintain a uniform microstructure and excellent mechanical property for the joint of duplex stainless steel, the development of transient liquid phase diffusion bonding with Fe-base insert metal is necessary. Transient liquid-phase bonding was carried out using Fe-B-Si amorphous insert metal at the temperature range of 1423–1473 K for 0~1000 s under 5×10−5 Torr vacuum. Volume fraction of austenite (γ) in the joint region decreased with an increase of bonding temperature and holding time. In particular, when the holding time was sufficiently prolonged, γ phase-depleted regions were observed at the base metal adjacent to the joint. γ–δ transformation by inter-diffusion occurred at the base metal near the joint, as mentioned previously. The primary reason for this might be the diffusion of γ-stabilizing element (N) from the base metal into the insert metal during the bonding, while a δ-stabilizing element (Si) diffused into the base metal. Thus, the γ–δ transformation occurred in the base metal adjacent to the joint by interdiffusion of alloying elements. Base metal dissolution occurred in the early stages due to melting-point depressant elements contributed by the insert metal. Boride and nitride initially formed at the joint but disappeared gradually with increased holding time, and the disappearance time decreased with an increase of bonding temperature. In the case of a prolonged holding time, a depleted area of the γ phase was observed at the base metal adjacent to joints. In order to prevent this phenomenon, an austenite regeneration heat treatment process was performed. The austenite volume fraction of the base metal and joint area increased with an increase in austenite regeneration heat treatment time; deviation of the austenite volume fraction between the base metal and joint area decreased. After bonding, the austenite volume fraction increased with an increase in holding time during the austenite regeneration heat treatment at 1273 K. For the following austenite regeneration heat treatment conditions, 1273 K for 1000 s, after bonding at 1423 K for 1000 s, the austenite volume fraction in the base metal and joints increased to 51% and 43% respectively. With regard to homogenization behavior, tendencies between the calculated and measured results of the diffusion mechanism were nearly identical. Although the actual constituent homogenization periods were shorter than those calculated by the diffusion model, predictions for joint homogenization were theoretically possible. From electron probe microanalyzer (EPMA) analyses in the joint bonded at 1423 K for 400s and 1000s, the concentration of the alloy elements (Fe, Cr, Ni) coexisted in the γ + δ area. Therefore, dual phase joint microstructure similar to base metal could be obtained as performing austenite regeneration heat treatment. Microstructural changes of the joints during bonding are progressed with 4 stage, base metal dissolution → boride and nitride disappearance + joint widening → austenite growth to the joint → homogenization. Austenite in the base metal grew into the joint area during the austenite formation and growth steps. Secondary phases of Cr boride and BN were observed using EPMA analysis. Secondary phases formed at the joint after short holding times (0 s, 10 s, 50 s) were predominantly chromium borides. For specimens bonded with longer holding times (up to 1000 s), boron nitrides were created at the interface with the joint, and the amount of boride in the specimen was less than the case of shorter bonding times. Secondary phases can be formed around a joint due to interdiffusion of elements between dissolved base metal and alloying elements of insert metal during bonding. When secondary phases are present in the joint area, they usually influence on the mechanical properties of the joint. The formation of secondary phases in the joint area during bonding proceeds as follows. During the early stages of bonding, the base metal at the joint interface dissolves due to the reaction between the base metal and the melting-point depressants from the insert metal. Elements contained in the dissolved base metal diffuse into the liquid insert metal, and elements contained in the liquid insert metal diffuse into the base metal. During this process, the most thermodynamically stable phase, i.e., that with the lowest Gibbs free energy (BN in this case), forms preferentially in the joint interface. Bonding strength increased with increase of bonding temperature and holding time. Although all tensile strength specimens were fractured at the joint area, the specimen bonded at the highest temperature and for the longest time (1473 K and 1000 s, respectively) had a bonding strength similar to that of the base metal with high elongation prior to fracture. In the TLP bonding of duplex stainless steel using Fe-B-Si insert metal, the mechanical properties were mainly influenced by BN formed at the bonding interface. The amount and distribution of each secondary phase also influenced the mechanical properties in terms of bonding strength.