Abstract
The main objective of this research is to better understand the correlation between the constituent phases presented in the super-duplex steel SAF 2507 when it is under welding process by arc shielding gas MIG-MAG (Metal Inert Gas-Metal Active Gas). Conventional short circuit transfer and derivative STT (Surface Tension Transfer) using the 2594 welding wire as a filler metal and the effects on welding power in hardness, toughness and pitting corrosion are considered here. The results showed that the welding energy (Ew) changed the α/γ-phase’s balance and occasionally formed σ-phase in ferrite grain boundaries which led to changes in hardness, toughness and pitting corrosion resistance in molten zone (MZ), heat activated zone (HAZ) and metal base regions (MB). Furthermore, the increased amount of γ-phase improved the pitting corrosion resistance index (PRENγ) mainly in the MZ. This is due to decrease of α-phase fraction and formation of coarser grains, for higher welding energy. The toughness in the MZ decreased with less formation of γ-phase, coalescence of ferritic grains and localized formation of σ-phase, raising the hardness in the HAZ when the welding energy was lower.
Highlights
The results showed that the welding energy (Ew) changed the α/γ-phase’s balance and occasionally formed σ-phase in ferrite grain boundaries which led to changes in hardness, toughness and pitting corrosion resistance in molten zone (MZ), heat activated zone (HAZ) and metal base regions (MB)
This research aims to clarify the correlation between the phases presented in the super-duplex steel SAF 2507, in the MZ-HAZ-MB regions of a welded joint under the arc shielding gas MIG-MAG welding process, while considering the effects of welding energy on hardness, toughness and pitting corrosion
In the HAZ there is the formation of a columnar mixed microstructure near the MZ with the development of ferrite grains
Summary
Stainless super-duplex steels (SSDS) are alloys of the type Fe-Cr-Ni-Mo-N, and are used in a wide variety of applications, such as chemical, petrochemical, oil and gas industrial plants, manufacturing of tubular products and offshore structural industries [3] [4]. More than 50% of all stainless steel delivered to the world has undergone some welding process [6] In this way, the application of this material which presents excellent mechanical and corrosion properties is essential. The super-duplex steels are a family of steels which contain approximately equal fractions of ferrite (α) and austenite (γ) phases, 50/50 by volume [10] [11] This microstructure can be achieved by balancing the alloying elements, heat treatment and/or thermomechanical processes. Some of these elements, which are added as an alloying element in austenite, can precipitate and lead to pitting corrosion [13] [14]
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