Influence of cooling rate on microstructure evolution and pitting corrosion resistance in the simulated heat-affected zone of 2304 duplex stainless steels
Highlights
► Effect of cooling rate on microstructure evolution was studied. ► Effect of cooling rate on pitting corrosion resistance was studied. ► Using CPT to evaluate the pitting corrosion resistance after thermal cycle of welding is studied. ► The weaker region of DSS HAZ was determined, which can be explained by PREN.
Introduction
Duplex stainless steels (DSS) have become important alternatives to conventional austenitic stainless steels due to their higher mechanical strength, better corrosion resistance and lower alloying costs compared with their counterparts. Their perfect performances rely on the microstructure comprised of approximately equal amounts of austenitic and ferrite phases, as well as the lack of precipitation of detrimental secondary phases such as σ and (Cr2N). These microstructure features are generally obtained through alloy chemical composition design and solution heat treatment [1], [2], [3], [4], [5], [6], [7], [8], [9].
However, the fusion welding process required for assembling the construction modifies the ferrite/austenite phase balance (1:1) in duplex stainless steels and promotes the intermediate-phases’ precipitation in the welded joints. The main consequence of this phenomenon is that corrosion resistance and mechanical properties of these materials are dramatically affected, particularly in the heat affected zone (HAZ) [10], [11], [12], [13], [14], [15], [16]. During the thermal cycle of HAZ, the alloy is heated to a very high temperature above 1350 °C in a short time, held for a few seconds, and subsequently cooled to room temperature. In the heating and holding period, most of the γ islands in the duplex structure dissolve into the δ-ferrite matrix, and δ-ferrite grains grow coarse. During the cooling stage in the temperature range between 1350 and 800 °C, austenite re-precipitates around ferrite grain boundaries. The final microstructure depends on the parameters of thermal cycles such as holding time, peak temperature, and the cooling rate from 1350 to 800 °C. Among these parameters, the cooling rate from 1350 to 800 °C is the most important one. Generally, high cooling rate result in excessively coarse ferrite grains, which is harmful to both mechanical properties and corrosion resistance. By comparison, low cooling rate promote a favorable high austenite fraction, but bring on brittle intermetallic phases, especially for the high-alloyed DSS. In the actual welding process, the cooling rate was determined by the heat input and thickness of the plate. Higher heat input and thinner plates result in a slower cooling rate. Therefore, welding parameters should be adjusted to ensure that the cooling rate is low enough to allow adequate austenite formation, and yet high enough to avoid precipitating of intermetallic phases [17], [18], [19], [20], [21], [22], [23], [24].
High pitting corrosion resistance of stainless steels is due to alloying elements such as Cr, Mo and N. The influence of the alloying elements on pitting corrosion resistance can be simplified by pitting resistance equivalent number (PREN), given by PREN = %Cr + 3.3%Mo + 20%N [25]. In a duplex stainless steel microstructure, the main alloying elements, i.e. chromium, molybdenum, nickel and nitrogen, are not evenly distributed in ferrite and austenite. Chromium and molybdenum enrich in ferrite, whereas nickel and nitrogen are concentrated in austenite. The PREN of the two single phases can vary, resulting in different pitting corrosion resistance of single phase. Therefore, the pitting corrosion resistance of duplex stainless steels is determined by the PREN value of the weak phase [26], [27]. As for the influence of cooling rate on the pitting corrosion resistance of the DSS heat-affected zone, several reports focus on precipitation of harmful precipitators [22], [28], [29], [30]. In this paper, we investigate another aspect of this issue. Here, special attention was paid to the alloying elements’ distribution between two phases with respect to different cooling rates, which could be the intrinsic cause of the deterioration of the DSS heat-affected zone.
Section snippets
Material
The material investigated in this work was a commercial standard duplex stainless steel plate of DSS 2304 (manufactured by Baosteel), with the chemical composition listed in Table 1. The samples were melted in a 50 kg vacuum furnace and then cast as a single square ingot. After removing the oxide skin, the ingot was forged into square bloom at the temperature ranging from 900 to 1200 °C and divided into several blooms with a dimension of 150 mm × 100 mm × 42 mm. The blooms were reheated at 1250 °C for 2 h
Results and discussion
Under the solution-treated condition, the microstructure of the base metal (BM) is consisted of γ (white) and α (relatively dark), as shown in Fig. 2.
Conclusions
From the above studies, three simple conclusions can be drawn:
- (1)
Ferrite content increased gradually as the cooling rate increased from 10 s to 100 °C/s.
- (2)
As the cooling rate increased, the CPT of the specimens tended to decrease, indicating a deterioration of pitting corrosion resistance. All the pits are found in ferrite phase.
- (3)
The pitting corrosion resistance of the simulated HAZ was determined by the PREN of the ferrite phase.
Acknowledgements
The authors would like to thank Gang Hu for his great help with SEM characterization, whilst gratefully acknowledge the helpful collaboration of Baosteel. This work is supported by the National Science Foundation of China (Grant Nos. 50871031, 51131008, 51134010 and 51071049) and Industrialization Project of Shanghai New and High technology.
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