Mechanical and Microstructural Characterization of TIG Welded Dissimilar Joints between 304L Austenitic Stainless Steel and Incoloy 800HT Nickel Alloy

Grzegorz Rogalski,Aleksandra Świerczyńska,Dariusz Fydrych,Michał Landowski

doi:10.3390/met10050559

Abstract

In this article, the mechanical properties and microstructure of 304L austenitic stainless steel/Incoloy 800HT nickel alloy dissimilar welded joints are investigated. The joints were made of 21.3 mm × 7.47 mm tubes using the TIG process with the use of S Ni 6082 nickel filler metal. No welding imperfections were found and high strength properties of joints were obtained, meeting the assumed acceptance criteria of the product’s standards. The tensile strength of the welded joints was higher than for the joined materials (Incoloy 800HT). Macro- and microscopic metallographic tests revealed the correct morphology of the joints and the appropriate structures in their critical zones. However, differences were found in the morphologies of the zones between the weld and the base materials. In fusion boundary from the side of the Incoloy 800HT alloy, no clear outline of the fusion line was observed (type A fusion boundary), while increased grain size and an epitaxial structure were observed. In turn, in the zone: weld–304L steel, a distinct fusion line was observed with areas with an increased amount of high-temperature δ ferrite (type B fusion boundary). No precipitates were found that could reduce the resistance of the joints to intergranular corrosion. A hardness decrease (approximately 30 HV0.1) in the transition zone: austenitic steel–weld and an increase of hardness (approximately 10 HV0.1) on the opposite side of the welded joint were observed.

Highlights

Many design solutions require the use of components made of materials with specific properties in terms of strength and work, e.g., at high temperatures or in environments promoting various forms of material degradation [1,2,3,4]
The design of the heat exchanger enabled the use of an Incoloy 800HT coil only in a space exposed to a medium characterized by the risk of stress corrosion cracking (SCC)
The difficult operating conditions of heat exchanger components necessitate the usage of materials with properties which are suitably matched to the environment, and the welding of dissimilar joints [9,47,48]

Summary

Introduction

Many design solutions require the use of components made of materials with specific properties in terms of strength and work, e.g., at high temperatures or in environments promoting various forms of material degradation [1,2,3,4]. Their use is often the result of the use of a calculation code and of the operating conditions of the device, e.g., the type of working medium, temperature, mechanical and thermal load, etc. Austenitic stainless steels, as well as nickel and Metals 2020, 10, 559; doi:10.3390/met10050559 www.mdpi.com/journal/metals

Methods

Discussion

Conclusion