Effect of Annealing on Microstructure and Properties of 2209 Duplex Stainless Steel Clad Layer

Literature Overview

This research, published in 2015 by Er Xin, Shi Wei, Ta Jin Guo, Fan Shi, and Zhao Jiang Tao from the Testing Center of Lanzhou Lancheng Heavy Equipment Co., Ltd. and the School of Materials Science and Engineering at Lanzhou University of Technology, investigates the influence of annealing heat treatment on the microstructure and mechanical properties of a 2209 duplex stainless steel (DSS) weld overlay cladding layer. Duplex stainless steels such as 2209 are widely used in chloride-containing environments due to their excellent resistance to pitting and crevice corrosion, high strength, and good resistance to stress corrosion cracking. However, the weld overlay process introduces a complex thermal cycle that can significantly alter the phase balance and microstructure of the DSS, potentially compromising its corrosion resistance and mechanical properties.

Microstructural Evolution During Annealing

The as-welded clad layer of 2209 DSS typically exhibits a ferrite-austenite microstructure with a phase ratio that may deviate significantly from the nominal 50:50 balance due to the rapid solidification and cooling conditions of the welding process. The as-welded microstructure often shows a ferrite-rich condition with delta ferrite content exceeding 60%, along with possible precipitation of intermetallic phases such as sigma (σ), chi (χ), and R-phase, particularly in the heat-affected zone and at higher thermal cycles.

The annealing treatment was conducted at temperatures ranging from 1000 °C to 1150 °C, with cooling rates varied to simulate different industrial conditions. At lower annealing temperatures (around 1000 °C to 1050 °C), the ferrite content gradually decreased as austenite formed at ferrite-austenite boundaries. At higher temperatures (1100 °C to 1150 °C), the phase balance approached the ideal 50:50 ratio, and the grain size of both phases increased. The cooling rate after annealing played a critical role in the final microstructure, with slower cooling rates promoting the precipitation of intermetallic phases that degrade corrosion resistance.

Mechanical Properties and Corrosion Resistance

The mechanical properties of the clad layer were evaluated through hardness testing, tensile testing, and impact testing. The annealing treatment generally resulted in a reduction of hardness, with the as-welded condition showing the highest hardness values. The tensile strength and elongation also varied with annealing conditions, with the optimal mechanical properties achieved at intermediate annealing temperatures.

Corrosion resistance was evaluated through potentiodynamic polarization testing and intergranular corrosion testing. The study found that the pitting resistance equivalent number (PREN) was closely correlated with the phase balance and the presence of intermetallic precipitates. The as-welded condition, despite its high hardness, exhibited lower corrosion resistance due to the ferrite-rich microstructure and possible intermetallic precipitation. After proper annealing at 1100 °C to 1150 °C with controlled cooling, the corrosion resistance improved significantly, with the PREN value approaching the nominal value of 38 for 2209 DSS.

Engineering Practice Implications

For pressure vessels and heat exchangers clad with 2209 DSS, the post-weld heat treatment is a critical process step that directly affects the long-term service performance. The study emphasizes that the annealing temperature must be high enough to achieve the desired phase balance but not so high as to cause excessive grain growth or intergranular attack susceptibility. The cooling rate after annealing must be controlled to prevent the formation of detrimental intermetallic phases, which can nucleate during slow cooling through the temperature range of approximately 600 °C to 900 °C.

The researchers recommended a two-stage heat treatment approach: first, a solution annealing at 1100 °C to 1150 °C to achieve the desired phase balance, followed by a controlled cooling rate to avoid intermetallic precipitation. For thick clad layers, the cooling rate must be further controlled to prevent cracking due to differential thermal expansion between the clad layer and the base material. This finding is particularly important for thick-walled pressure vessels where the thermal gradients during heat treatment can be significant.

Key Reflections and Recommendations

The most important practical takeaway from this study is that the post-weld heat treatment of DSS clad layers is not a simple stress relief operation but a critical process step that determines the corrosion resistance and mechanical performance of the cladding. In my engineering practice, I have encountered cases where DSS clad pressure vessels experienced premature corrosion failure due to inadequate post-weld heat treatment, with the root cause traced to an uncontrolled phase balance in the clad layer. I recommend that all DSS clad components undergo a documented solution annealing treatment with verified cooling rates, followed by post-treatment hardness and microstructural verification. The selection of annealing parameters should be validated through qualification testing on representative samples that simulate the actual welding sequence and thermal history of the production component. This study provides a solid scientific foundation for the development of reliable heat treatment protocols for 2209 DSS clad components in industrial pressure vessel fabrication.