Elsevier

Materials & Design

Volume 63, November 2014, Pages 278-285
Materials & Design

Microstructural, compositional and mechanical investigation of Shielded Metal Arc Welding (SMAW) welded superaustenitic UNS N08028 (Alloy 28) stainless steel

https://doi.org/10.1016/j.matdes.2014.06.014Get rights and content

Highlights

  • The microstructures of the base metal and weld metal are different.

  • Tensile properties of weld metal are higher than base metal excluding ductility.

  • Base metal presents a higher plastic stain than weld metal.

  • Base metal presents a higher cyclic hardening compared to weld metal.

  • Cyclic behavior of welded joint is the same than weld metal specimens.

Abstract

Shielded Metal Arc Welding (SMAW) was performed on UNS N08028 (Alloy 28) superaustenitic stainless steel sheets. In the present work, the microstructure and the mechanical properties of base metal (BM), weld metal (WM), and welded joint (WJ) are investigated. Optical micrographs show that the base metal presents austenitic grains, and the weld metal exhibits a fully austenitic dendritic structure, confirming the Schaeffler diagram estimations. Microhardness measurements indicate that the hardness increases in the weld bead due to the rapid cooling and thermal cycle during welding procedure. The measured mechanical properties and the analysis of the fracture profiles show that the two materials are ductile but the ductility is less pronounced in the weld metal. Consistently the yield stress, the plastic strength and the impact toughness are lower than in the base metal. In addition, the BM presents a higher cyclic hardening and plastic strain compared to those of WM. Cyclic stress–strain hysteresis loops show that WM and WJ have almost the same cyclic behavior and especially at high imposed strain levels.

Introduction

Austenitic stainless steels have found wide applications because of their excellent mechanical and corrosion properties, and a reasonable weldability [1], [2]. The superaustenitic grades of stainless steel, containing more than 20% of nickel and a high amount of alloying elements as Mo provide an improved corrosion resistance associated with a high strength level [3]. Some investigations have already been carried out on mechanical properties of superaustenitic stainless steels (SASS) such as tensile strength and impact properties [4], [5]. In particular, it has been found that the augmentation of the strain rate does not affect the yield stress or tensile strength, but it is beneficial to the ductility [6].

The SASS are especially suitable for the manufacturing of piping and a variety of other components in chemical, petrochemical and nuclear industries. These piping systems require generally a welding of many components in order to assembly different equipments of the structure and to make them resistant. Previous studies have focused on investigating chemical and mechanical properties of welded stainless steel as regards the different welding processes.

In this framework, it has been found that CO2 laser welded superaustenitic stainless steel AISI 904L exhibits a fully austenitic, dendritic structure [7]. The studies of the solidification behavior of laser welded 304 austenitic stainless steel have shown a cellular-dendritic structure and a complete absence of (δ) ferrite and microsegregation [8]. The microstructure of the AISI 304L ASS weld metals reveals that the ferrite content in the deposited metals raises with an increasing of Cr/Ni ratio [9]. In addition, different welded austenitic stainless steel microstructure of 302, 304, 316L, 310S and 347 ASS are almost fully austenitic due to the rapid solidification rate [10]. These results show that the microstructure and the mechanical properties of these steels are very dependent on the preparation conditions. In addition the specific study of the influence of welded joint on the mechanical behavior of a steel pipe under monotonic and cyclic loading [11] has shown that the maximum load carrying capacity is affected by the presence of residual stresses in the welded joint.

In the present work, the investigations have been focused on the microstructural and mechanical properties of SMAW welded Alloy 28 (UNS N08028) which is a superaustenitic stainless steel containing 26–28% of Cr and 29.5–32.5% of Ni that gives to this material excellent anti-corrosion and mechanical properties [12]. In order to optimize further applications as piping, the microstructural properties and their incidence on mechanical properties such as tensile strength, ductility, impact resistance and cyclic stress–strain behavior have to be characterized. Thus, in the following, both base metal (BM), weld metal (WM) and welded joint (WJ) have been studied.

Section snippets

Material and test procedures

Hot rolled superaustenitic stainless steels were welded using Shielded Metal Arc Welding (SMAW). The welding procedure was made on base metal sheets of Sanicro28 commercial filler metal. This process can be used to weld a lot of various metals such as carbon steels, copper, brass and aluminum and it is the most used in the stainless steel welding industry [13]. It is based on the arc force which provides digging action for electrode penetration in the base metal [14]. The steel sheet was first

Chemical and microstructural investigation

The chemical composition of the BM and the WM were identified using EDS X-ray analysis as shown in Table 2.

As expected, the composition of the two materials BM and WM (Sanicro) provided from the industrial supplier is nearly similar, especially Cr and Ni contents. In fact, Cr, Si and Mo are alphagenic elements; they favor the presence of the ferritic phase. It is therefore necessary to introduce a gammagenic element such as Ni to allow the desired austenite structure to form at annealing

Conclusions

The investigation of the microstructural and mechanical properties of a SWAW welded superaustenitic Alloy 28 stainless steel enables the following conclusions:

  • Concerning the microstructure analysis, austenitic grains with the presence of twins are evidenced in the base metal. In the weld metal, a fine dendritic structure that transforms from columnar to equiaxed from the fusion line to the center of the welded bead is observed.

  • The fracture surfaces observed after tensile tests or impact tests

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