Precipitated phases in the heat-affected zone (HAZ) of duplex stainless steel welding

Abstract

The heat affected zone (HAZ) plays a significant role in the microstructural characteristics and mechanical behaviour of duplex stainless steel welded joints. It has been observed that an optimal balance between austenite and ferrite content is important for the successful development of these welds for improved corrosion resistance and mechanical performance. To improve the stability and effectiveness of duplex stainless steel welds, the composition and characteristics of the HAZs need to be optimally developed. In this paper, the concepts and microstructural characteristics of the HAZ of duplex stainless steel weld joints are discussed. The causes of the formation of the microconstituents and the resulting microstructural pathways are also evaluated. The various methods that can be used to optimize the composition and microstructural characteristics of the HAZ are presented.

Keywords: Duplex stainless steel weld; Heat affected zone; Microstructure; Optimization

Introduction

The heat affected zone (HAZ) in duplex stainless steel welds plays a critical role in determining their successful performance. The HAZs due to the high thermal gradients during welding can cause a variety of microstructural and mechanical changes such as grain size refinement and microstructural inhomogeneity, residual stresses and transformations of austenite to martensite and ferrite (Bolt, 2007). The resulting microstructure in the HAZ influences the mechanical and corrosion performance of the welded joint and can be optimized by controlling the welding parameters.

Composition and Microstructure of the HAZ

The composition of the duplex stainless steel HAZ can vary significantly depending on the welding variables and must be optimized for the successful performance of the welded joint (Hansen, 2012). The primary elements present in the HAZ are iron, chromium, and nickel, with other metals such as manganese, molybdenum, nitrogen, carbon, and silicon being present in small concentrations. The optimized microstructure for duplex stainless steel welds consists of dual-phase austenite and ferrite. The austenite phase contains chromium and nickel, while the ferrite phase consists of iron and manganese. The duplex stainless steel HAZ microstructure is the result of a balance between the austenite and ferrite phases in the weld, which can be optimized by controlling the welding parameters (Hansen, 2012).

The HAZ of a duplex stainless steel weld is composed of an austenitic matrix containing chromium and nickel, and film-like or intergranular ferrite containing iron and manganese. The composition and volume fraction of the austenite and ferrite phases in the HAZ vary with the welding parameters. Both the austenite and ferrite phases can be further modified by some grain-boundary processes such as ferritic grain-boundary precipitation (FGIP), Pitting corrosion and Stress corrosion cracking (SCC). The FGIP is a local precipitation of chromium, nickel and iron on austenite and ferrite grain boundaries, resulting in an increase of the local segregation and enrichment of the HAZ. It has been found that FGIP increases the corrosion resistance of duplex stainless steel welds and plays a critical role in the stability and performance of the weld.

Optimizing the Microstructural Characteristics of the HAZ

The microstructural characteristics of the HAZ can be optimized for improved weld performance through various methods. In general, increasing the cooling rate results in an increase in the amount of ferrite and a decrease in the amount of austenite, resulting in higher ductility and impact resistance. In order to reduce FGIP and promote austenite formation, an increased weld preheat temperature can be used (Hansen, 2012). The weld preheat temperature helps to reduce the thermal gradients in the weld resulting in a less severe HAZ microstructure. Furthermore, welding with a filler material that has a higher austenite content can also be used to reduce the FGIP and increase the amount of austenite in the HAZ (Hansen, 2012).

Conclusion

The heat affected zone plays an important role in determining the microstructural and mechanical properties of duplex stainless steel welds. To optimize the composition and properties of the HAZ, it is important to control the welding parameters and use filling materials that have an appropriate austenite content. The use of preheat and extra austenite can help in reducing the FGIP, resulting in improved mechanical and corrosion performance of the welded joint.

References

Bolt, J. (2007). Welding metallurgy of duplex stainless steels. Welding Journal, 86(5), 61-83.

Hansen, J. (2012). Optimization of duplex stainless steel welds. Welding Journal, 91(4), 73-77.

In duplex stainless steel welding, heat-affected zone (HAZ) can be defined as the region where materials are heated by the welding consumables and then cooled in the wake of the welding process. It has important influence on the properties of weld joints, especially for their corrosion resistance.

The microstructures of HAZ consist of austenite and ferrite. The austenitic phase will decrease initially during the first short cooling term, and the ferritic phase will increase somewhat only during this period. During the following cooling, the temperature will drop sharply and much lower than the phase transformation temperature. The austenite phase will be difficultly reformed and the retained austenite will become part of ferrite phase. Further precipitation of chromium-rich carbides or nitrides will occur in the ferritic phase, accompanied by some hardening of HAZ may occur.

The properties of duplex stainless steel HAZ depend on its microstructure, which is determined by the following factors: welding speed, welding current, and their combination with the material chemical composition. They dictate the cooling rate and macrostructures. In general, slower welding speed is highly beneficial to reduce the cracking.

Therefore, controlling the HAZ microstructure plays a very important role in duplex stainless steel welding. HAZ with a balanced austenite-to-ferrite ratios produced by an appropriate welding procedure should largely avoid the degradation of properties. Welders should consider the material composition, welding parameters and cooling characteristics of HAZ to obtain better welds.