Microstructure, mechanical and corrosion properties of friction stir welded high nitrogen nickel-free austenitic stainless steel
Graphical abstract
Introduction
High nitrogen austenitic steels (HNAS) are attractive candidates for the replacement of conventional Fe–Cr–Ni austenitic steels [1]. Nitrogen in steels has a significant effect on stabilizing austenite, enabling partial or complete replacement of expensive Ni used to stabilize the austenitic phase [2]. Additionally, the synergetic effect of nitrogen and other alloying elements (Cr, Mo, V, etc.) simultaneously improves the mechanical properties and corrosion resistance of steels [3], [4]. With the development of pressurized smelting equipment, HNAS with excellent comprehensive properties containing more than 1.0 wt.% of nitrogen are being widely considered for transportation, medical applications, power generation, and cryogenic industries, etc. [5], [6], [7].
However, the wide application of HNAS for structural applications is largely dependent on its welding characteristics. Conventional fusion welding processes, such as GTAW, TIG, GMAW and laser welding; of HNAS exhibit a number of limitations: (a) loss of nitrogen by desorption of nitrogen or formation of nitrogen associated pore in the weld zone, (b) precipitation of Cr-nitrides and carbides in weld zone and heat-affected zone (HAZ), and (c) solidification cracks in weld zone and liquation cracks in the HAZ [8], [9], [10]. When welding of HNAS with nitrogen content exceeding the nitrogen solubility under atmospheric pressure is carried out using fusion welding methods, the weld defects are prominent, and lead to significant deterioration in mechanical properties and corrosion resistance of the weld.
To alleviate and even avoid problems associated with the conventional fusion welding processes of HNAS, solid-state joining technique is considered appropriate. Friction stir welding (FSW) was invented as an innovative solid-state welding technology by TWI in 1991 [11]. In the FSW process, a rotating tool containing a pin and a shoulder is plunged into the joint between two workpieces, generating heat by friction. Once the material is heated to a visco-plastic state, the tool is translated along the weld line. Plasticized base material passes around the tool, where it is consolidated because of the force applied by the shoulder of the tool. Finally, it leaves a solid phase bond between the two pieces. Initially, FSW was successfully applied to metals with moderate melting point, such as Al, Cu and Mg alloys [12], [13], [14]. The ability to use FSW for joining of high softening-temperature materials has recently been brought to fruition because of the development of specialized tool material [15], [16]. The feasibility of FSW for steels, Ti alloys, and Ni-base alloys has been reported to exhibit sound weld joint [17], [18], [19], [20].
In recent years, several studies on the FSW of HNAS have been carried out [21], [22], [23]. Park et al. [21] conducted a feasibility work and demonstrated that the defects associated with melting/solidification phenomena in fusion welding process of the HNAS joint can be eliminated. Miyano et al. [22] also performed FSW of 2-mm thick Fe–23Cr–0Ni–1Mo–1N austenitic steel using different parameters. Although optimum welding parameters were determined in relation to the mechanical properties of the joint, the microstructural characteristics were not explored. D. Wang et al. [23] carried out a detailed examination of the microstructure and the mechanical properties of the friction stir welded Fe–18.4Cr–15.8Mn–2.1Mo–0.69N–0.04C austenitic stainless steel. The authors reported that the FSW significantly refined the austenitic grains and increased hardness and strength in the stir zone (SZ).
In the present study, a high nitrogen nickel-free austenitic stainless steel with 0.96 wt.% nitrogen was processed with FSW. The microstructural evolution, mechanical, and corrosion properties were studied to reveal the microstructure and properties of friction stir welded HNAS.
Section snippets
Material preparation and friction stir welding process
The HNAS material used in this study was made using vacuum induction furnace and electroslag remelting furnace in nitrogen atmosphere [24]. The chemical composition of the steel is listed in Table 1. The experimental cold rolled plate was of dimensions 150 mm × 60 mm × 2.4 mm. The plates were solution treated at 1100 °C for 90 min, followed by water quenching and then ground to remove the oxide and contamination of both the top surface and joint surface. In order to assist tool penetration as well as
Joint appearance and inspection result
Fig. 3 shows a typical joint appearance processed at welding speed of 100 mm/min and tool speed rotating of 400 rpm from the top surface. No groove-like defects [25] were observed along the weld line, indicating that the heat input was sufficient to ensure the material flow during FSW process [26], [27]. Dye penetrant inspection was conducted to reveal surface cracks and the inspection results confirmed the absence of surface cracks in the weld. It should be pointed out that the dyed parts
Conclusions
In this study, the FSW of a high-nitrogen nickel-free austenitic stainless steel plate was performed at a tool rotational speed of 400 rpm and welding speed of 100 mm/min. Microstructural characteristics, mechanical and corrosion properties were explored. The results of the study can be summarized as follows:
- (1)
A sound FSW joint was acquired without any defects. The SZ did not show nitrogen loss, and was characterized by fine-grained microstructure.
- (2)
Based on TEM and EBSD studies, a small amount of Cr
Acknowledgments
The present research was financially supported by National Natural Science Foundation of China (No.51304041, 51434004 and U1435205) and Program for New Century Excellent Talents in University (No. N130502001). RDKM gratefully acknowledges support from the University of Texas at El Paso, USA.
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