Microstructural, compositional and mechanical investigation of Shielded Metal Arc Welding (SMAW) welded superaustenitic UNS N08028 (Alloy 28) stainless steel
Graphical abstract
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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:
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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.
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The fracture surfaces observed after tensile tests or impact tests
References (24)
- et al.
Comparative studies of the seawater corrosion behaviour of a range of materials
Desalination
(2003) - et al.
A comparative evaluation of gas tungsten and shielded metal arc welds of a ‘‘ferritic’’ stainless steel
J Mater Process Technol
(1999) - et al.
Microstructural, compositional and residual stress evaluation of CO2 laser welded superaustenitic AISI 904L stainless steel
Mater Sci Eng A
(2006) - et al.
Recent developments in stainless steels
Mater Sci Eng R
(2009) - et al.
A shielded metal arc welding expert system
Comput Ind
(1993) - et al.
Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds
J Mater Process Technol
(2005) - et al.
Fatigue, monotonic and fracture toughness properties of a Cr–Mn–N steel
Int J Fatigue
(2001) - et al.
An investigation of microstructure/property relationships in dissimilar welds between martensitic and austenitic stainless steels
Mater Des
(2004) - et al.
Low cycle fatigue of welded joints: new experimental approach
Nucl Eng Des
(2004) - et al.
Stainless Steel
(1986)