Elsevier

Materials Characterization

Volume 82, August 2013, Pages 113-119
Materials Characterization

Corrosion behavior in high heat input welded heat-affected zone of Ni-free high-nitrogen Fe–18Cr–10Mn–N austenitic stainless steel

https://doi.org/10.1016/j.matchar.2013.05.011Get rights and content

Highlights

  • Corrosion behavior in the weld HAZ of high-nitrogen austenitic alloy was studied.

  • Cr2N particle was not precipitated in high heat input welded HAZ of tested alloy.

  • Pitting corrosion and interphase corrosion show a different behavior.

  • Pitting corrosion resistance was affected by whether or not δ-ferrite forms.

  • Interphase corrosion resistance was affected by the total amount of δ-ferrite.

Abstract

The pitting corrosion and interphase corrosion behaviors in high heat input welded heat-affected zone (HAZ) of a metastable high-nitrogen Fe–18Cr–10Mn–N austenitic stainless steel were explored through electrochemical tests. The HAZs were simulated using Gleeble simulator with high heat input welding condition of 300 kJ/cm and the peak temperature of the HAZs was changed from 1200 °C to 1350 °C, aiming to examine the effect of δ-ferrite formation on corrosion behavior. The electrochemical test results show that both pitting corrosion resistance and interphase corrosion resistance were seriously deteriorated by δ-ferrite formation in the HAZ and their aspects were different with increasing δ-ferrite fraction. The pitting corrosion resistance was decreased by the formation of Cr-depleted zone along δ-ferrite/austenite (γ) interphase resulting from δ-ferrite formation; however it didn't depend on δ-ferrite fraction. The interphase corrosion resistance depends on the total amount of Cr-depleted zone as well as ferrite area and thus continuously decreased with increasing δ-ferrite fraction. The different effects of δ-ferrite fraction on pitting corrosion and interphase corrosion were carefully discussed in terms of alloying elements partitioning in the HAZ based on thermodynamic consideration.

Introduction

In the economic and environment concerns, the high-nitrogen austenitic stainless steel (HNS) is very attractive candidate for replacing conventional Fe–Cr–Ni austenitic stainless steels [1]. That is, HNS can achieve the excellent combination of mechanical property and corrosion resistance for relatively low production cost, by replacing nickel (Ni) in conventional Fe–Cr–Ni austenitic stainless steel with nitrogen (N) [2], [3], [4], and the reduction of Ni is also beneficial to suppress the environmental contamination. N, as a strong austenite (γ) stabilizer, makes it possible to retain austenite phase without Ni addition.

The extensive studies have reported the excellent mechanical properties of high-nitrogen alloying steel [4], [5], [6], [7], i.e. the addition of N can increase both tensile and yield strength without significant loss of ductility. Simmons [4] reviewed the effect of N on a variety of material properties and showed that tensile strength as well as yield strength and strain hardening rate linearly increase with increasing N content. Müller et al. [5] also examined the effect of N on the strengthening and work hardening of HNS, and reported that N increases strength by making tight stacking of twins. Mathew et al. [6] evaluated the creep strength of austenitic 316L stainless steel and found that N enhances creep strength by solid-solution hardening and precipitation hardening. Hwang et al. [7] studied the ductile-to-brittle transition temperature (DBTT) of HNS and reported that low DBTT can be achieved by N addition, improving γ stability.

The corrosion resistance is another important characteristic to determine the performance of HNS [1], [3], [8], [9]. Ha et al. [1], [3] investigated the pitting corrosion resistance of HNS and showed that combination of N and C addition enhances pitting corrosion resistance by achieving the more protective passive film; however, the pitting corrosion resistance of HNS deteriorated by lamellar Cr2N precipitate formed during aging at 900 °C [8]. According to Hänninen et al. [9], the corrosion resistance increases with increasing N until the solubility limit of N is reached, and then deteriorated by Cr2N precipitation. Metikoš-Huković et al. [10] explained the corrosion resistance changes according to N addition in terms of the electronic structure of passive film. N-induced short range ordering decreases the defect density, resulting in the improvement of the corrosion resistance.

On the other hand, the excellent mechanical properties and corrosion resistance of stainless steels can be deteriorated due to the formation of brittle zone in the heat-affected zone (HAZ) [11], [12], [13], [14], [15], [16], [17], [18] during welding, and thus the application of stainless steels has been strongly affected by its weldability. Many studies have been carried out related to the corrosion behavior in the HAZ of commercial austenitic and duplex stainless steels (DSS) [11], [12], [13], [14], [15], while only a few investigations for weldability of HNS can be found [16], [17], [18]. Besides, it is very difficult to find the published data on the corrosion behavior in the HAZ of Ni-free HNS, which is examined in this study.

Therefore, here we carefully evaluate the resistance of two types of corrosion phenomena, which are pitting corrosion and interphase corrosion, in the HAZ of a HNS with a composition of Fe–18Cr–10Mn–N alloy. The weld HAZs were simulated using Gleeble simulator and the corrosion behavior was evaluated using electrochemical tests. The better understanding of the different aspects of pitting and interphase corrosion has been achieved through electron probe micro analysis (EPMA), electron microscopy analysis and thermodynamic consideration.

Section snippets

Experimental Procedures

The chemical composition of the HNS examined in this study is Fe–17.96Cr–9.74Mn–0.03Si–0.33N–0.03C (wt%), and balance Fe. To obtain fully austenite microstructure without Ni, our alloy contains high Mn as well as N addition, and high Mn also leads to increase N solubility in HNS. Ingot was fabricated using a commercial pressurized vacuum-induction melting furnace (VIM 4 III-P, ALD, Germany) under nitrogen partial pressure of 1 bar. The ingot was first annealed for homogenization at 1250 °C for 2 

Microstructure before Electrochemical Test

Fig. 3 shows the optical micrographs of the base steel and the HAZs. The base steel is fully austenite, while the HAZs are composed of mixed phases of austenite and newly formed phase which was identified as δ-ferrite from TEM analysis in Fig. 4.

The equilibrium fraction of δ-ferrite in Fig. 1 linearly increases with increasing peak temperature, which is confirmed by the microstructure observation shown in Fig. 3. Here, it is possible that the actual fraction of each phase in HAZs is smaller

Conclusion Remarks

The pitting corrosion resistance and interphase corrosion resistance in high heat input welded HAZ of metastable austenitic Fe–18Cr–10Mn–N alloy are systematically examined. From the electrochemical test results, it was found that the pitting corrosion and interphase corrosion behaviors in the HAZs showed a different aspect with increasing δ-ferrite fraction, and the following conclusions are drawn.

  • (1)

    The pitting corrosion resistance (Epit) of HAZs deteriorated as compared with base steel and this

Acknowledgments

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

References (25)

Cited by (54)

  • Effects of Ar-N<inf>2</inf>-He shielding gas on microstructure, mechanical properties and corrosion resistance of the Laser-MIG additive manufacturing 316L stainless steel

    2023, Journal of Materials Processing Technology
    Citation Excerpt :

    Then, the Cr-Mo-Ni atomic segregation at the δ-γ interface can be suppressed, and the precipitation of the brittle phase (δ→σ) when isothermally held at 600–900℃ can also be avoided. As Moon et al. (2013) affirmed, the increased fraction of δ-ferrite in the austenitic stainless steel weld can significantly deteriorate the pitting corrosion resistance due to the Cr-Mo-Ni atomic segregation at the δ-γ interface and formation of Cr-depleted zones at the γ boundaries. In addition, N micro-alloying is believed to decrease the diffusion ability of C atoms and Cr atoms, which helps prevent the precipitation of the detrimental intermetallic phase Cr23C6 and eliminate the Cr-depleted zone (Hänninen et al., 2001).

  • Effect of environmental condition on corrosion susceptibility of GTA welded AISI 202 austenitic stainless steel

    2022, Materials Today: Proceedings
    Citation Excerpt :

    It can be noted from the Nyquist plots that the capacitive loop is identified in the form of a half-circle for BZ, HAZ, and WZ and that this is associated with a charge transfer rate at the interface region of the film and solution. The capacitive loop with slight depression seen in the high-frequency area suggests a rapid charge transfer process occurring at the interface of the alloy [16]. The amplitude of these impedance loops shows the alloy's polarisation resistance and, as a result, its corrosion resistance.

View all citing articles on Scopus
View full text