Unraveling the effects of solution treatment on the microstructure and corrosion behavior of HT-HAZ in duplex stainless steel welded joints

In this work, the influence of solution treatment on the corrosion behavior and microstructure of HT-HAZ was studied. The results show that the balanced two-phase ratio is discovered after solution treatment. After solution treatment at 960°C, a small amount of chromium nitride remains at the two-phase interface and in the ferrite. With the solution treatment temperature increasing to 1120°C, chromium nitride can be fully dissolved. The chemical elements of the two-phase are redistributed during the solution treatment process, which results in changes in PREN. After solution treatment, the proportion of recrystallized grains increases whereas that of sub-structured grains decreases, indicating the reduction of residual stresses and deformations. The percentage of CSL boundaries increases as the solution treatment temperature rises. Compared with HT-HAZ without solution treatment, the improved corrosion resistance after solution treatment is ascribed to the balanced two-phase ratio, less chromium nitride precipitates, smaller residual deformations, and more CSL boundaries. However, the pitting resistance after solution treatment at 960°C is still relatively poor on account of the residual chromium nitride. The pitting nucleation sites are closely associated with the chromium nitride precipitates and PREN of two-phase.


Introduction
Duplex stainless steel (DSS) has a wide range of applications in petrochemical industries, marine architecture, and nuclear power plants on account of its high strength and good corrosion performance.In the application process of DSS, welding is the most common manufacturing method, which degrades corrosion resistance.Solution treatment is a viable method to enhance the corrosion property of DSS [1] .In the solution treatment process, the dissolution of chromium nitride, equilibrium of two-phase structure, and redistribution of chemical elements can result in a change in corrosion performance.However, the study on the influence of solution treatment on corrosion performance and microstructure of DSS is mainly concentrated on weld metal (WM) and base metal (BM).The corrosion property and microstructure evolution of high-temperature heat-affected zone (HT-HAZ) have received less attention, and HT-HAZ is the weakest part in the welded joints [2] .Thus, it is vital to probe the influence of solution treatment on HT-HAZ to assure the application security of DSS welded joints.
In this paper, the microstructural characteristics of HT-HAZ before and after solution treatment were analyzed, including metallographic structure, chemical composition, and grain characteristics.In addition, the electrochemical corrosion behavior of HT-HAZ was investigated.This study is hoped to offer practical guidance for enhancing the reliability of HT-HAZ.

Sample preparation
The base metal and filler of DSS welded joints were 2205 DSS and E2209, respectively.The machining process and definition of HT-HAZ have been reported in our previous studies [2] .To obtain a balanced two-phase structure and fewer precipitates, the temperatures of solution treatment were selected as 960°C and 1120°C, respectively.Subsequently, the specimen was quenched in water.

Microstructural characterization
Beraha II reagent was used to etch the polished specimens of HT-HAZ, and the austenite and ferrite were characterized by optical microscopy (OM).The chromium nitride precipitates could be investigated with a 10% oxalic acid solution.The chemical elements were inspected by energydispersive X-ray spectroscopy (EDS), and the grain characteristics were measured by electron backscatter diffraction (EBSD).

Corrosion measurement
All corrosion tests were implemented using a Gamry interface 1000E potentiostat.The sample (exposed area: 1 cm 2 ) was used as the working electrode, a saturated calomel electrode (SCE) played the role of the reference electrode, and a carbon rod acted as the counter electrode.The corrosion behavior was measured in 3.5 wt% NaCl solution.When the open-circuit potential (OCP) was stable, the polarization curves were recorded over a potential range from -0.5 V SCE to 1.2 V SCE with a scan speed of 0.166 mV/s.The corrosion morphology after polarization was examined by scanning electron microscope (SEM).

Effect of solution treatment on microstructure
3.1.1Evolution of microstructure.Figure 1 shows the optical micrographs and two-phase fraction of HT-HAZ.Compared with HT-HAZ without solution treatment, solution treatment promotes the formation and growth of austenite, which mainly includes Widmanstätten austenite (WA), partially transformed austenite (PTA), grain boundary austenite (GBA), secondary austenite (γ 2 ), and intragranular austenite (IGA).At the solution treatment temperature of 960°C, the secondary austenite and intragranular austenite increases significantly.With the solution treatment temperature rising to 1120°C, secondary austenite disappears, and the remaining austenite increases in size.In addition, part of intragranular austenite undergoes spheroidization at higher solution treatment temperature and longer holding time, forming spherical austenite (SA).After solution treatment, the equilibrium two-phase ratio is found for the HT-HAZ.After oxalic acid etching, the morphologies of HT-HAZ are exhibited in Figure 2. Before solution treatment, there is numerous pitting nucleation inside the ferrite and at the two-phase interface, which is caused by the dissolution of chromium nitride [3] .After solution treatment at 960°C, a small amount of chromium nitride remains at the two-phase interface and inside the ferrite.The chromium nitride is fully dissolved with the solution treatment temperature rising to 1120°C.

Distribution of typical elements.
The pitting resistance equivalent number (PREN) of austenite and ferrite is computed by the formula [4] : PREN = 20 wt% N + 3.3 wt% Mo + wt% Cr (1)  The typical elements of austenite and ferrite are measured, as summarized in Table 1.It is discovered that Mo and Cr enrich inside the ferrite whereas Ni enriches inside the austenite.After solution treatment, the ferrite has a larger PREN, and the PREN of two-phase increases.3 presents the relative frequency of sub-structured, deformed, and recrystallized grains before and after solution treatment.At the as-weld state, lots of substructured grains are found, indicating that larger deformation occurs inside the HT-HAZ.After solution treatment, the proportion of recrystallized grains increases while that of sub-structured grains decreases.
In comparison with HT-HAZ at the solution temperature of 960°C, the grain types of ferrites after solution treatment at 1120°C do not change significantly.In the austenite, the proportion of substructured grains decreases, implying the reduction of residual deformations and stresses.The corrosion resistance can be reinforced by the coincidence site lattice (CSL) boundary on account of its lower interfacial energy [5] .The proportion of CSL boundaries before and after solution treatment is shown in Figure 4.After solution treatment, the fraction of CSL boundaries increases.Higher solution treatment temperature promotes the formation of more CSL boundaries.

Effect of solution treatment on corrosion behavior 3.2.1 Polarization results.
The polarization curves of HT-HAZ before and after solution treatment are illustrated in Figure 5 (a).All polarization curves have a similar shape, suggesting that solution treatment does not lead to obvious changes in corrosion kinetics.As seen from Figure 5 (a), the anodic polarization curves consist of four regions according to the evolution of these polarization curves.The parameters of passive current density (i p ) and breakdown potential (E pit ) are presented in Figure 5 (b).Compared with HT-HAZ before solution treatment, the E pit increases slightly, which is attributed to the presence of residual chromium nitride (Figure 2 (b) and (d)).With the solution treatment temperature rising, the chromium nitride is fully dissolved.Besides, the residual deformation of HT-HAZ decreases, and the fraction of CSL boundaries increases.In this case, the pitting resistance of HT-HAZ is reinforced significantly.The evolution of i p coincides with that of E pit .

Corrosion morphology.
The corrosion morphologies of HT-HAZ before and after solution treatment are shown in Figure 6.At the as-weld state, lots of pits nucleate at the two-phase interface and in the ferrite, resulting from the existence of chromium nitride.Pits occur in both ferrite and austenite on account of the similar PREN of the two-phase.In addition, the location of secondary austenite is also sensitive to pitting nucleation due to the lower Mo and Cr contents.After solution treatment, pits decrease markedly.At the solution temperature of 960°C, there are still a small number of pits at the austenite/ferrite interface and inside the ferrite, resulting from the existence of residual chromium nitride.With the solution temperature increasing to 1120°C, the higher phase boundary energy and lower PREN of austenite are the main reasons for pitting nucleation.

Conclusions
The corrosion performance and microstructure evolution of HT-HAZ before and after solution treatment are investigated, and the effects of microstructure on corrosion behavior are discussed.The conclusions are as follows: (1) After solution treatment, the balanced two-phase fraction is found.At the solution temperature of 960°C, a small amount of undissolved chromium nitride remains at the austenite/ferrite interface and inside the ferrite.With the solution treatment temperature rising, the chromium nitride precipitate is completely dissolved, and the fraction of recrystallized grains and CSL boundaries increases.
(2) Solution treatment does not lead to significant changes in the corrosion kinetics of HT-HAZ.After solution treatment at 960°C, the E pit of HT-HAZ increases slightly.With the solution treatment temperature increasing to 1120°C, the corrosion performance is improved due to the disappearance of chromium nitride, small residual deformation, and more CSL boundaries.
(3) Pits are prone to nucleate around secondary austenite and chromium nitride precipitate.Simultaneously, pits are also easy to nucleate at the two-phase interface and inside the austenite resulting from the higher phase boundary energy and lower PREN of austenite.
(4) The association of corrosion behavior and microstructure evolution during the solution treatment process is clarified.It is found that the lower solution temperature cannot significantly change the corrosion property of HT-HAZ.Furthermore, the solution treatment temperature of 1120°C is suitable for the engineering application of DSS welded joints.
(5) This work focuses on the influence of solution treatment on the corrosion property and microstructure of HT-HAZ.However, the welding parameters are also important factors affecting the corrosion property of HT-HAZ.In future work, the influence of welding parameters on the corrosion performance and microstructure of HT-HAZ should be further studied to ensure the application safety of DSS welded joints.

Figure 3 .
Figure 3.The relative frequency of grain types before and after solution treatment (a) as-weld; (b) 960°C; (c) 1120°C.

Figure 4 .
Figure 4.The relative frequency of CSL boundaries before and after solution treatment (a) ferrite; (b) austenite.

Figure 5 .
Figure 5. Polarization results of HT-HAZ (a) polarization curves; (b) E pit and i p .

Table 1 .
Chemical element concentrations of HT-HAZ before and after solution treatment.