This article discusses the effect of heat treatment on microstructure development and corrosion resistance of 2205 duplex stainless steel (DSS) welded joints. It also describes the properties and applications of DSS.
Due to its excellent corrosion resistance and high strength, DSS is the best metal for use in harsh environments such as reactors, tankers, deep sea pipelines for petroleum product transportation, seawater desalination, refining chemical and petrochemical industries. Its chemical composition includes Cr, Mo, Ni and N.
Annealing temperature, cooling rate, solidification and heat input after the welding process are the main factors affecting the volume fraction of ferrite and the precipitation of intermetallic phases in harmful phases, such as σ-phase, χ-phase, secondary austenite, nitrides (CrN and Cr2N) ) and carbides (M23C6).
Soldering is the main and important activity. During heat treatment, three zones are formed: the base metal (VM), the heat-affected zone (HAZ) and the weld zone (WZ). After welding, DSS identified three different regions of ferritic and austenitic phase chemistry, each contributing to a different degree of corrosion resistance.
In addition to affecting the chemical composition of the ferrite, the heat generated during welding also affects the volume fraction of the ferrite. Multiple passes allow excess secondary austenite to form. Therefore, the corrosion resistance of the weld zone is limited. In addition, the selection of a suitable welding electrode is critical for controlling the microstructure of the weld zone as well as its post-weld characteristics.
The effect of filler metals on the microstructure, mechanical properties and solidification of dissimilar welds between API X70 high strength low alloy pipe steel and 2507 super duplex stainless steel was investigated. The 309L filler weld contains skeletal ferrite in an austenitic matrix. Another study used gas tungsten arc welding (GTAW) and filler metals ER2553 and ERNiCrMo-4 to study the weldability, metallurgical and mechanical properties of joints made of super duplex stainless steel UNS 32750. Due to the improved mechanical quality of welded joints using ER 2553 compared to welded joints using ER NiCrMo-4, the authors recommend its use for welding super duplex stainless steels. These studies show that controlling the microstructure of the weld zone is critical.
The most well-known and important method for improving post-weld microstructure is post-weld heat treatment (PWHT), which includes several elements such as heating temperature, holding time, cooling rate, and process atmosphere. PWHT, improper annealing temperature, shielding gas and low cooling rate can contribute to the deposition of intermetallic compounds, which depends on the presence of Cr, Mo and C. PWHT welds in general and DSS welds in particular have been the subject of numerous studies.
According to several studies, annealing should be carried out between 1000 and 1200 °C followed by water quenching. Some studies have shown that the ideal annealing temperature for DSS without precipitation of intermetallic compounds is in the range of 1050 to 1100 °C. During PWHT, the duration of the annealing and the cooling rate should be considered. Many researchers have studied the effect of annealing time on mortar and have found that increasing the annealing time leads to a decrease in corrosion resistance.
The formation of hazardous phases is caused by the slower cooling rate after grouting, although the cooling rate during PWHT is a major concern. The presence of intermetallic phases and σ-phases rich in chromium and molybdenum makes DSS steels more prone to embrittlement, resulting in reduced mechanical properties and low corrosion resistance.
In order to prevent dangerous phases, the PWHT methods must be correctly followed. On the other hand, proper PWHT can increase the volume fraction of austenite, thereby improving the corrosion resistance of DSS welds. This presentation highlights the importance of choosing the optimal PWHT settings. There are few research reports on the effect of heat treatment atmospheres on the microstructure of DSS welded joints.
In a recent report published in Scientific Reports, the authors discuss postweld heat treatment of 2205 DSS at 1050°C followed by water quenching to prevent the formation of harmful phases such as sigma, secondary austenite, chi, nitride and carbides. , will reduce the microstructure, mechanical properties and corrosion resistance.
Energy dispersive X-ray spectroscopy (EDS) showed that the particular EMF spot studied had significant nitrogen levels and, in turn, low chromium nitride levels. Argon and nitrogen respectively were used as controlled atmospheres compared to unprotected samples. A study was made on how the environment in which PWHT is used affects the microstructure and corrosion resistance of duplex stainless steel welds.
The microstructural analysis of the heat-treated samples showed that when argon was used as a controlled atmosphere, no separation of the second phase occurred. However, this occurs in the microstructure of the sample when nitrogen is used in the atmosphere. In addition, the unprotected samples formed intermetallic deposits, the microstructure of which was very similar to that of the nitrogen protected samples.
Samples heated in nitrogen and a nitrogen controlled atmosphere showed nitride deposition in the ferrite region, but samples heated in an argon controlled atmosphere did not. An in-depth analysis of the microstructure using optical microscopy and scanning electron microscopy showed that nitride precipitates often appeared in the soldering zone of nitrogen-protected samples. Corrosion resistance of heat-treated welded joints is increased by reducing the volume fraction of ferrite in post-weld heat-treated samples compared to welded samples without heat treatment.
The hardness of the weld zone after welding of heat-treated samples is higher than that of the base alloy. The initial hardness of the duplex stainless steel was 286 Hv, and the average hardness of the weld zone during PWHT in air, argon, and nitrogen was 340, 411, 343, and 391 Hv, respectively.
The hardness of the welding zone increased to 33, 44, 20 and 37%, respectively. After PWHT, a significant reduction in elongation and tensile strength was observed. The initial maximum tensile strength of the base metal 2025 DSS was 734.9 MPa, and the tensile strength of welded joints in PHE welding in an atmosphere of air, argon and nitrogen was 769.3, 628.4, 737.8 and 681.4 MPa.
In conclusion, this study of the effect of heat treatment conditions on the microstructure of welded joints in duplex stainless steels shows that PWHT improves grain refinement and increases the proportion of austenite in the HAZ and weld zone. The formation of nitride precipitates is caused by the use of nitrogen as a protective gas during heat treatment. The same effect was also observed when the sample was heated without a protective gas. However, no precipitation of nitrides occurs when argon is used as the shielding gas.
Samples heated with nitrogen during PHT had the highest volume fraction of ferrite compared to samples heated with argon and air during heat treatment. After PWHT, it was found that the tensile strength (UTS) and ductility decreased significantly, especially when using nitrogen and air as the heat treatment atmosphere. Higher Vickers hardness values were observed for HAir and HNitrogen welds. This is due to the release of nitrides. Corrosion resistance of HNitrogen samples subjected to heat treatment with nitrogen after welding and W samples after welding will be reduced due to the precipitation of nitrides and secondary austenite.
Muhammad A.Yu. et al. (2023). Effect of Heat Treatment Atmosphere on Microstructure Development and Corrosion Resistance of 2205 Duplex Stainless Steel Weldments. Scientific Reports, 13, 4592. https://doi.org/10.1038/s41598-023-31803-5
Pitkala, J., et al. (2022). Investigation of the influence of alloying elements and temperature on the solubility of nitrogen in the industrial production of stainless steel. Transactions in Metallurgy and Materials B, 53, 2364–2376. https://doi.org/10.1007/s11663-022-02534-1
Wang, H. et al. (2022). Influence of secondary phase precipitation on mechanical properties and corrosion resistance of 00Cr27Ni7Mo5N super duplex stainless steel after solid solution treatment. Materials, 15(21), 7533. https://doi.org/10.3390/ma1521753
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Surbhi Jain is a freelance technology writer based in Delhi, India. She has a Ph.D. He holds a PhD in Physics from the University of Delhi and has participated in several scientific, cultural and sports activities. Her academic background is in materials science research with a specialization in the development of optical devices and sensors. She has extensive experience in content writing, editing, experimental data analysis and project management, and has published 7 research articles in Scopus indexed journals and filed 2 Indian patents based on her research work. She is passionate about reading, writing, research and technology and enjoys cooking, playing, gardening and sports.
Post time: Apr-28-2023