High-Temperature Brazing versus Transient Liquid Phase Diffusion Welding of 20 Steel Tubes: Microstructure and Mechanical Properties

Literature Overview

Wang Xuegang, Yan Qian, and Li Xingeng from Shandong Electric Power College conducted a comparative study of two joining methods for 20 steel tubes: high-temperature brazing using nickel-based filler metal BNi2, and transient liquid phase (TLP) diffusion welding using a self-developed iron-nickel based interlayer alloy. This study is directly relevant to steel tube manufacturing and repair operations, particularly in power generation and high-temperature applications where 20 steel (20# carbon steel) tubes are extensively used for boiler tubes, heat exchanger tubes, and structural piping.

Comparative Microstructural Analysis

The study employs scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA) to characterize the weld microstructures and composition distributions of both joining methods. The results reveal fundamental differences in the metallurgical behavior of the two processes.

High-Temperature Brazing with BNi2:

• Joint microstructure exhibits typical Ni solid solution brazing seam characteristics
• A distinct brazing seam boundary exists between the filler metal and the base metal
• Composition distribution in the joint zone is non-uniform
• Silicon-based brittle phases (Si compounds) are present in the joint
• The joint exhibits no toughness—failure occurs in a brittle manner

TLP Diffusion Welding with Iron-Nickel Interlayer:

• Joint microstructure is similar to and continuous with the base metal
• No distinct seam boundary exists—the joint is metallurgically homogeneous
• Composition distribution is uniform throughout the joint
• No silicon-based brittle phases are formed
• Joint strength and toughness reach the base metal level
• Performance significantly exceeds that of high-temperature brazing

Metallurgical Mechanism Interpretation

The fundamental difference between the two processes lies in the extent of elemental interdiffusion during and after solidification. In high-temperature brazing, the BNi2 filler metal solidifies as a Ni-rich solid solution with limited diffusion into the 20 steel base metal during the brazing cycle. The cooling rate after brazing is typically too fast to allow complete homogenization, resulting in a composition gradient and the precipitation of brittle intermetallic phases containing silicon. The silicon originates from the flux or the filler metal composition and forms Si-rich brittle phases that act as crack initiation sites.

In contrast, TLP diffusion welding involves a longer isothermal holding period at elevated temperature after the interlayer has fully melted and reacted with the base metal. During this holding period, the concentration gradients drive extensive atomic diffusion, progressively homogenizing the joint composition. The iron-nickel interlayer was specifically designed to have a melting point and solidus/liquidus temperature range compatible with the 20 steel base metal, ensuring that the interlayer dissolves completely and the resulting solid solution is compositionally compatible with the base metal. The absence of silicon in the interlayer composition eliminates the formation of brittle Si phases entirely.

Engineering Practice Implications

For steel tube manufacturing and repair operations, this study provides a clear technology selection guideline. When joining 20 steel tubes in applications requiring full-strength, ductile joints—such as boiler tube splicing, heat exchanger tube repairs, or structural pipe connections—TLP diffusion welding is the preferred method. The joint's metallurgical homogeneity and mechanical properties matching the base metal mean that the joint can be treated as part of the parent material for design purposes, eliminating the need for joint efficiency factors or derating.

However, TLP diffusion welding requires specialized equipment (vacuum or inert gas furnaces capable of precise temperature control), longer cycle times, and more stringent process control compared to conventional brazing. The process window is narrower, and the holding time must be sufficient for complete homogenization but not so long as to cause excessive grain growth or softening of the base metal. For high-volume production or field repair applications where speed and simplicity are prioritized, high-temperature brazing may still be acceptable if the joint is subjected only to low-stress, non-cyclic loading conditions.

Critical Reflections

The study's comparison is compelling but focuses exclusively on 20 steel, a plain carbon steel with relatively simple metallurgy. The TLP approach's advantages may not translate directly to alloy steels, stainless steels, or nickel-base alloys, where the interlayer composition must be carefully tailored to avoid unwanted phase formation and where the diffusion kinetics are more complex. Additionally, the study does not report on the thermal residual stress distribution in the welded joints, which is a critical parameter for fatigue performance and dimensional stability. For power plant applications where 20 steel tubes are subjected to cyclic thermal loading, the residual stress state and its interaction with the applied stress field would determine the fatigue life of the joint, and this aspect should be investigated in follow-up work. The study nonetheless establishes a clear metallurgical basis for preferring TLP diffusion welding over conventional brazing for 20 steel tube joints, and the self-developed iron-nickel interlayer alloy represents a valuable process innovation that could be extended to other steel grades with appropriate compositional modification.