Phase transformation characteristics of duplex stainless steel welding heat-affected zone (HAZ)

Characteristics of Phase Transformation in Heat Affected Zone (HAZ) of Gas Metal Arc Welding (GMAW) of Duplex Stainless Steel

Abstract

Gas metal arc welding (GMAW) is one of the most important welding processes used in industry, especially for welding duplex stainless steels. Due to the high temperature during welding process, the microstructure of the heat affected zone (HAZ) significantly changes and thus alters its properties. A comprehensive review of HAZ phase transformation in duplex stainless steels was conducted for the impact on toughness, corrosion resistance and other properties. This paper presents a brief overview of the various phase transformations occurring during GMAW and their effects on the mechanical and corrosion properties of duplex stainless steel welds.

Key Words: duplex stainless steel, gas metal arc welding (GMAW), heat affected zone (HAZ), phase transformation

Introduction

Duplex stainless steels are a specific type of stainless steel alloy developed for superior properties such as tensile and stress-rupture strength, superior corrosion resistance, good weldability and formability, ease of fabrication and long-term durability. These stainless steels are composed of 50-70% ferrite and 25-30% austenite, along with small amounts of other elements such as carbon, nitrogen, and manganese. Due to the high strength and corrosion resistance, duplex stainless steels are often used for applications in hostile environments, such as in chemical, petrochemical and marine industries. Gas metal arc welding (GMAW) is one of the most commonly used welding processes for duplex stainless steel. It is a highly efficient, reliable and cost effective method of welding. However, GMAW leads to significant HAZ temperature, which can cause microstructural changes that can alter the mechanical and corrosion resistant properties of the welded joint. The aim of this paper is to provide a comprehensive review of the various phase transformations that occur during GMAW and their effects on the mechanical and corrosion resistances of duplex stainless steel welds.

Discussion

During GMAW in duplex stainless steels, the high temperature HAZ (<1000 °C) causes a variety of phase transformations to occur. The most significant of these transformations are austenite-ferrite transformation, CIgrain growth, γ→αmartensite transformation, and γ′→α′martensite transformation.

Austenite-Ferrite Transformation

When the temperature of duplex stainless steel HAZ drops below the A3 point, ferrite will start to form from austenite. This transformation increases the amount of ferrite in the HAZ and decreases the amount of austenite. This transformation is thermodynamically favoured by the decrease in free energy as it occurs. The amount of ferrite that forms depends on the cooling rate and the heating rate. Fast cooling rates promote austenite-ferrite transformation and slow cooling rates promote α-ferrite formation.

Grain Growth

In duplex stainless steels, the intermetallic phases (mostly CI phases) are prone to grain growth during GMAW due to the high temperature HAZ. This eventually leads to embrittlement of the HAZ, which can affect the weld strength and resistance to corrosion damage. In some cases, this grain growth can be prevented by using a Nitrogen-rich shielding gas during the welding process, as nitrogen is known to stabilize the CI phases and thus reduce grain growth.

γ→αMartensite Transformation

During GMAW, the temperature of the HAZ can reach high enough to cause γ→αmartensite transformation. This transformation causes significant changes to the microstructure of the HAZ, including martensite formed in the place of ferrite and a decrease in austenite. The presence of martensite decreases the toughness of the HAZ, meaning it is less resistant to sudden changes in temperature and pressure, as well as fracture.

γ→αMartensite Transformation

Similar to the γ→αmartensite transformation, GMAW can also cause the γ→αmartensite transformation. This transformation is usually caused by an increase in nitrogen content in the shielding gas or flux medium, which can lead to the martensite to form instead of austenite. This transformation reduces the corrosion resistance of duplex stainless steel welds, as austenite is generally more resistant to corrosion than martensite.

Effects on Mechanical and Corrosion Properties

The various phase transformations that occur during GMAW of duplex stainless steel can have significant effects on its mechanical and corrosion properties. The most significant effects include decreased ductility and increased hardness leading to embrittlement; decreased toughness leading to increased risk of crack formation; and decreased corrosion resistance due to the presence of martensite.

Conclusion

GMAW of duplex stainless steel leads to high temperature HAZ which can cause significant microstructural changes. The most significant phase transformations that occur during GMAW are austenite-ferrite transformation, CIgrain growth, γ→αmartensite transformation, and γ′→α′martensite transformation, all of which can have a significant effect on the mechanical and corrosion properties of the welds. Hence, in order to ensure that welds have the desired properties, proper welding parameters must be chosen and appropriate shielding gases and fluxes must be used.

The Heat-Affected Zone (HAZ) is the area in a material, particularly metals, which is changed by exposure to elevated temperatures and subsequent cooling. In the welding process, the temperature in the HAZ will vary according to the weld joint, welding parameters and the materials used. This is particularly true of duplex stainless steel, which has different austenetic and ferritic phases, and shows different effects in the HAZ.

Duplex stainless steel has a two-phase microstructure and the HAZ has both a ferritic and an austenitic (or some intermediate) phase. After welding, the austenite tends to transform to ferrite with further cooling from elevated temperatures (> 500°C) . In an ideal situation, in the centre of the weld the ferrite would start to transform to austenite at about 350°C before further cooling to room temperature. However, due to the high cooling rates associated with welding and the high heat input rates, austenitic grains may still remain after welding. The ratio of ferrite to austenite in the HAZ will depend on the base materials, the heat input and the cooling rate.

The characteristic of this alloy mean that the HAZ can be subject to a wider range of stresses and temperatures than standard austenitic stainless steel. This can cause metallurgical constraints, such as high hardness and a low toughness through stage. Low toughness can also arise as a consequence of carbon precipitation and sensitisation, which can be caused by prolonged exposure to elevated temperatures.

In addition, the grain structures located in the HAZ can be weakened leading to an increased risk of grain boundary cracking. The formation of intermetallic compounds on the grain boundaries can also reduce the corrosion resistance and can reduce the HAZ strength and fracture toughness. Careful selection of welding parameters, as well as pre- and post-weld heat treatments are needed to minimise the formation of intermetallic compounds and grain boundary cracking.

In summary, duplex stainless steels have different austenite and ferrite phases present in their microstructure. During the welding process, the austenite may transform to ferrite in the HAZ due to cooling from elevated temperatures, and this can cause metallurgical constraints. To minimise these risks, careful selection of welding parameters and pre- and post-weld heat treatments are needed.