슈퍼듀플렉스강 용접금속의 내식성에 미치는 σ상의 영향

Abstract

Duplex stainless steels are alloys consisting of a two phase microstructure with a balance between ferrite and austenite and combine very good corrosion properties, high yield strength and good toughness. Modern developments of these steels concern the so-called super duplex stainless steel grades with increased Cr, Ni, Mo, and N contents, for enhanced corrosion resistance and better weldability. Due to their good corrosion and mechanical properties, super duplex stainless steels are used in highly demanding applications such as in petrochemical industries and in off shore constructions. One of the most critical and commonly used fabrication processes used with these alloys is welding. During welding, the precipitation of harmful intermetallic phase called as sigma(σ) phase resulting in brittleness and especially in reduction of corrosion resistance. A specimen of weld metal was prepared by GTA welding with weld wire of super duplex stainless steel ER 2594(Φ2.4) to know the effect of σ phase on corrosion resistance of super duplex stainless steel weld metal. Aging treatment was conducted for the sample at the temperature range of 700 to 900°C for 5 to 300 minutes. Electrochemical polarization tests of uniform corrosion in 0.5M H2SO4 solution, pitting corrosion in 3.5% NaCl solution and intergranular corrosion in 2M H2SO4＋0.01M KSCN＋0.5M NaCl solution were conducted. The effect of σ phase to uniform corrosion, pitting corrosion and intergranular corrosion of weld metal has been investigated and the results were derived as follows.

1. Corrosion current density was decreased a little with an increase of aging time over 60 minutes at 700 to 900°C and uniform corrosion of weld metal was more influenced to the volume fraction of ferrite and austenite than the volume fraction of σ phase.

2. Pitting potential Ep representing pitting corrosion was found to tend to decrease with an increase of aging time at 700 to 900°C. The volume fraction of σ phase tends to increase with an increase of aging temperature and time and pitting corrosion of weld metal was increased by an increase of σ phase. And most of the pits formed near the σ phase in the ferrite and propagated to austenite.

3. The volume fraction of σ phase tends to increase with an increase of aging temperature and time and intergranular corrosion of weld metal was increased by an increase of σ phase. Degree of sensitization(DOS) representing intergranular corrosion was found to tend to increase with an increase of aging time at 700 to 800°C, while it decreased by an increase of aging time at 900°C.