The Effect of Holding Time on Dissimilar Transient Liquid-Phase-Bonded Properties of Super-Ferritic Stainless Steel 446 to Martensitic Stainless Steel 410 Using a Nickel-Based Interlayer

Abstract

The dissimilar joining of martensitic and ferritic stainless steels have been developed that needs corrosion resistance and enhanced mechanical properties. In this study, the transient liquid-phase bonding of martensitic stainless steel 410 and super-ferritic stainless steel 446 was conducted with a nickel-based amorphous interlayer (BNi-2) at constant temperature (1050 °C) and increasing times of 1, 15, 30, 45, and 60 min. For characterization of the TLP-bonded samples, optical microscopy and scanning emission microscopy equipped with energy-dispersive X-ray spectroscopy were used. To investigate the mechanical properties of TLP-bonded samples, the shear strength test method was used. Finally, the X-ray diffraction method was used for microstructural investigation and phase identification. The microstructural study showed that the microstructure of base metals changed: the martensitic structure transited to tempered martensite, including ferrite + cementite colonies, and the delta phase in super-ferritic stainless steel dissolved in the matrix. During the transient liquid-phase bonding, the aggregation of boron due to its diffusion to base metals resulted in the precipitation of a secondary phase, including iron-chromium-rich borides with blocky and needle-like morphologies at the interface of the molten interlayer and base metals. On the other hand, the segregation of boron in the bonding zone resulted from a low solubility limit, and the distribution coefficient has induced some destructive and brittle phases, such as nickel-rich (Ni₃B) and chromium-rich boride (CrB/Cr₂B). By increasing the time, significant amounts of boron have been diffused to a base metal, and diffusion-induced isothermal solidification has happened, such that the isothermal solidification of the assembly has been completed under the 1050 °C/60 min condition. The distribution of the hardness profile is relatively uniform at the bonding zone after completing isothermal solidification, except the diffusion-affected zone, which has a higher hardness. The shear strength test showed that increasing the holding time was effective in achieving the strength near the base metals such that the maximum shear strength of about 472 MPa was achieved.

Keywords: bonding temperature and time; martensitic stainless steel; mechanical property; microstructure; super-ferritic stainless steel; transient liquid phase bonding.

Figure 1

The fixture used to transient liquid phase bond the SFSS 446 and MSS 410.

Figure 2

Schematic of experimental setup of the diffusion bonding processes of super-ferritic and martensitic stainless steel by BNi-2 amorphous foil.

Figure 3

The optical microscopy of the microstructure of martensitic stainless steel410: (a) as-received and (b) under 1050 °C/60 min conditions.

Figure 4

The optical microscopy of the microstructure of super-ferritic stainless steel 446: (a) as-received and (b) 1050 °C/60 min conditions.

Figure 5

The optical microscopy of TLP-bonded sample at 1050 °C/1 min condition.

Figure 6

The microstructure of joint interface of the TLP-bonded sample at 1050 °C/60 min condition.

Figure 7

(a) The SEM micrograph and (b) line scans across the TLP bonded sample at 1050 °C/60 min condition.

Figure 8

The SEM microscopy of a bonded sample at 1050 °C/15 min condition.

Figure 9

(a) The SEM micrograph and (b) EDS spectrum of nickel-rich boride in centerline joint of the TLP-bonded sample at 1050 °C/60 min condition; (c) the SEM micrograph and (d) EDS spectrum of chromium-rich boride in the centerline joint of the TLP-bonded sample at 1050 °C/15 min condition.

Figure 10

The microstructure of diffusion-affected zone on the side of: (a) super-ferritic stainless steel 446 an (b) martensitic stainless steel 410.

Figure 11

The EDS analysis of secondary-phase precipitations: (a) Point A, (b) Point B, and (c) Point C in Figure 10.

Figure 12

Hardness profile across the joint TLP-bonded region under constant temperature 1050 °C by increasing time: (a) 1 min, (b) 30 min, and (c) 60 min. (d) Shear strength of base metals and TLP-bonded metals under constant temperature of 1050 °C by increasing time. (e) The XRD pattern of TLP-bonded fracture surface under 1050 °C/15 min conditions.