Challenges of Diffusion Bonding of Different Classes of Stainless Steels

Abstract

Solid state diffusion bonding is used to produce monolithic parts exhibiting mechanical properties comparable to those of the bulk material. This requires diffusion of atoms across mating surfaces at high temperatures, accompanied by grain growth. In case of steel, polymorphy helps to limit the grain size, since the microstructure is transformed twice. The diffusion coefficient differs extremely for ferritic and austenitic phases. Alloying elements may shift or suppress phase transformation until the melting range. In this paper, diffusion bonding experiments are reported for austenitic, ferritic, and martensitic stainless steels possessing varying alloying elements and contents. Passivation layers of different compositions are formed, thus affecting the local diffusion coefficient and impeding diffusion across faying surfaces. As a consequence, different bonding temperatures are needed to obtain good bonding results, making it difficult to control the deformation of parts, since strong nonlinearities exist between temperature, bonding time, and bearing pressure. For martensitic stainless steel, it is shown that it is very easy to obtain good bonding results at low deformation, whereas ferritic and austenitic stainless steels require much more extreme bonding parameters.

Evaluation of Passivation Process for Stainless Steel Hypotubes Used in Coronary Angioplasty Technique

“…One of the most generally accepted is the formation of a stable OH-(oxy-hydroxide hydrate) film in the area of a few millivolts around the redox potential, E corr . More recent articles have brought new information about the passivation process of austenitic steels [31][32][33][34][35], including the role of molybdenum in their passivation [36].…”

Nonlinear Ultrasonic C-Scan Imaging for Contact-Type Defects in Diffusion-Bonded Joints—A Case Study

“…Of particular note is the contact-type defect that may occur in diffusion-bonded joints, the nature of which has not been clearly studied, and it often represents the lack of welding at the two interfaces. Since the surfaces of the defect are in close contact, conventional ultrasound is almost completely transmitted at the interface, making it difficult to detect [ 5 ]. The lack of accurate detection methods for contact-type defects in diffusion-bonded interface seriously affects the large-scale application of diffusion-bonded structures in important fields such as aerospace, and therefore, a reliable non-destructive testing method is urgently needed to evaluate the quality of diffusion bonding.…”

Diffusion Bonding and Transient Liquid Phase (TLP) Bonding of Type 304 and 316 Austenitic Stainless Steel—A Review of Similar and Dissimilar Material Joints

“…Compared with other joining techniques, diffusion bonding is expensive and only widespread in aerospace engineering. Besides being able to join dissimilar materials, diffusion bonding can be used for full cross-section joining that may be particularly interesting for holohedral joints, such as internal structures of plate-type heat exchangers [5,6]. Transient liquid phase (TLP) bonding differs from solid-state diffusion bonding in a way that a thin interlayer, that has a lower melting point than the base materials and can melt below the bonding temperature, is placed between the base materials that are to be joined.…”