Deformation and failure response of 304L stainless steel SMAW joint under dynamic shear loading

Abstract

The dynamic shear deformation behavior and fracture characteristics of 304L stainless steel shielded metal arc welding (SMAW) joint are studied experimentally with regard to the relations between mechanical properties and strain rate. Thin-wall tubular specimens are deformed at room temperature under strain rates in the range of 8 × 10² to 2.8 × 10³ s⁻¹ using a torsional split-Hopkinson bar. The results indicate that the strain rate has a significant influence on the mechanical properties and fracture response of the tested SMAW joints. It is found that the flow stress, total shear strain to failure, work hardening rate and strain rate sensitivity all increase with increasing strain rate, but that the activation volume decreases. The observed dynamic shear deformation behavior is modeled using the Kobayashi–Dodd constitutive law, and it is shown that the predicted results are in good agreement with the experimental data. Fractographic analysis using scanning electron microscopy reveals that the tested specimens all fracture within their fusion zones, and that the primary failure mechanism is one of the extensive localized shearing. The fracture surfaces are characterized by the presence of many dimples. A higher strain rate tends to reduce the size of the dimples and to increase their density. The observed fracture features are closely related to the preceding flow behavior.

Introduction

Austenitic stainless steels have been the focus of considerable research recently because of their high strength, good ductility, excellent corrosion resistance and a reasonable weldability [1], [2], [3]. These properties make austenitic stainless steels attractive candidate materials for use in the fabrication of piping systems, automotive exhaust gas systems and in a variety of equipment associated with the chemical and nuclear power industries. Generally, the fabrication process involves the joining of stainless steel components by means of a suitable fusion welding process such as shielded metal arc welding (SMAW). However, the thermal effects associated with the welding process generally cause a structure to fail at its welded joints, and consequently, a number of researchers have investigated the relative influences of the welding structure, the welding parameters, the nitrogen content, the number of welding passes and the solidification morphology on the mechanical properties of welded austenitic stainless steel joints [4], [5], [6], [7], [8]. Previous studies have also investigated the tensile strength, impact properties and neutron irradiation resistance of austenitic steel weldments [9], [10], [11], [12]. The scope of these studies has generally been restricted to low strain rate conditions. However, in practice, austenitic stainless steel weldments tend to be applied within complex dynamic loading environments, and hence, their responses are significantly different to those observed when they are subjected to low strain rates. Consequently, if the performance of structures containing welded stainless steel components is to be improved under realistic loading conditions, it is essential to develop a thorough understanding of the dynamic deformation and fracture behavior of 304L SS weldments under high strain rate conditions.

Although it is well known that the flow phenomenon, work hardening characteristics and ductility of 304L stainless steel are all affected by the degree of pre-strain and by the strain rate [13], [14], [15], the precise influence of high strain rates on the flow and fracture characteristics of 304L SS weldments is still unclear. Since 304L SS weldments are frequently subjected to dynamic loading conditions in their service environments, it is essential to evaluate their mechanical properties and fracture behaviors under realistic high strain rate loading conditions. A review of the available literature demonstrates a lack of data relating to the effects of strain rate upon the mechanical properties and fracture characteristics of these weldments, especially under dynamic shear loading conditions. Consequently, the present study aims to investigate the influence of strain rate on the dynamic shear properties and fracture behavior of 304L SS weldments. Specifically, the study examines the correlation between the high strain rate shear stress–strain response and the fracture characteristics, and discusses this relationship in terms of the loading conditions.