Super duplex stainless steels are alloys consisting of 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 grad...
Super duplex stainless steels are alloys consisting of 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 offshore constructions. One of their the most critical and commonly used fabrication processes is welding. But the weld material for super duplex stainless steel was almost not developed and not evaluated in Korea. In Chapter 3, GTAW, SMAW, and FCAW were applied in actual process in order to evaluate and develop the performance caused by the development of super duplex stainless steel weld material. As a comparison material, weld material from KOBELCO was used at the same weld condition with the developing material. It is also applied in the actual welding process. Especially during the welding process, arc monitoring system was used to analyze arc wave form ; then, the workability evaluation for actual field was also preceded. A sound all-deposited metal(weld metal) with no lack of fusion was obtained by manual GTAW(Gas Tungsten Arc welding) with a limit of 25.56~27.77kJ/g welding heat input for each gram of deposited metal. Tensile strength at room temperature, elongation, and charpy impact values at -40℃ of weld metal obtained by using developed GTAW material showed similar level with the ones of KOBELCO. SMAW-3, a new weld material, was produced by changing flux component and wire. Workability in welding was examined by measuring the changes of resistance in welding, ΔR ; and as a result, SMAW-3 showed almost equally improved weldability with the one of KOBELCO. Charpy impact values at -40℃ for the new weld material, which is developed by changing flux component and wire for SMA welding, also showed similar level with the one of KOBELCO. FCAW weld metal showed similar charpy impact values at -40℃ and tensile strength at room temperature with developed weld material when it was welded with same welding conditions. However, there was a large difference in the elongation. The reason is that because some slags, which were not properly removed after they were formed on the weld beads, were mixed with weld metal during the multi-pass welding. A wire of the first test specimen, FCAW-1, had significantly high level of weld resistance, 6.23mΩ/㎜, compared to the one of KOBELCO, 3.83mΩ/㎜. FCAW-1 has relatively thinner cross section, 0.17㎜, than the one of KOBELCO, 0.24㎜, it has higher resistance to the electric current. FCAW weld metal structure showed a typical structure with a phase fraction of 55 : 45 of austenite and ferrite after welding, which is fairly good. FCAW-4, a material with controlled flux components and tube thickness, showed better performance than KOBELCO product. And also a field workability test was done by a welder under a same condition for both developing and KOBELCO products. In chapter 4, TIG welding of super duplex stainless steels is performed with argon shielding gas only, nitrogen gets lost from the weld pool, which can result in a ferrite-rich weld metal, with an inferior corrosion resistance than parent metal. Nitrogen permeation model from the shield gas which gets into the weld metal in DCEN-TIG welding has suggested. This plasma stream model shows characteristics of permeation of nitrogen ions into the molten metal due to the strong physical effect of plasma stream which formed by the arc pressure rather than the permeation of nitrogen ions caused by electric effect. In Chapter 5, a specimen of weld metal was prepared by TIG welding with super duplex stainless steel to know the effect of σ(Sigma) 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. The effect of sigma(σ) phase to corrosion characteristics of weld metal by the Electrochemical polarization tests has been investigated. Corrosion current density was decreased a little with an increase of aging time over 60 minutes at 700 to 900°C and uniform corrosion was more influenced to the volume fraction of ferrite and austenite than the volume fraction of σ phase. Pitting potential was found to tend to decrease with an increase of aging time at 700 to 900°C. Most of the pits formed near the σ phase in the ferrite and propagated to austenite. Intergranular corrosion of weld metal was increased by an increase of σ phase. Degree of sensitization 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.
Super duplex stainless steels are alloys consisting of 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 offshore constructions. One of their the most critical and commonly used fabrication processes is welding. But the weld material for super duplex stainless steel was almost not developed and not evaluated in Korea. In Chapter 3, GTAW, SMAW, and FCAW were applied in actual process in order to evaluate and develop the performance caused by the development of super duplex stainless steel weld material. As a comparison material, weld material from KOBELCO was used at the same weld condition with the developing material. It is also applied in the actual welding process. Especially during the welding process, arc monitoring system was used to analyze arc wave form ; then, the workability evaluation for actual field was also preceded. A sound all-deposited metal(weld metal) with no lack of fusion was obtained by manual GTAW(Gas Tungsten Arc welding) with a limit of 25.56~27.77kJ/g welding heat input for each gram of deposited metal. Tensile strength at room temperature, elongation, and charpy impact values at -40℃ of weld metal obtained by using developed GTAW material showed similar level with the ones of KOBELCO. SMAW-3, a new weld material, was produced by changing flux component and wire. Workability in welding was examined by measuring the changes of resistance in welding, ΔR ; and as a result, SMAW-3 showed almost equally improved weldability with the one of KOBELCO. Charpy impact values at -40℃ for the new weld material, which is developed by changing flux component and wire for SMA welding, also showed similar level with the one of KOBELCO. FCAW weld metal showed similar charpy impact values at -40℃ and tensile strength at room temperature with developed weld material when it was welded with same welding conditions. However, there was a large difference in the elongation. The reason is that because some slags, which were not properly removed after they were formed on the weld beads, were mixed with weld metal during the multi-pass welding. A wire of the first test specimen, FCAW-1, had significantly high level of weld resistance, 6.23mΩ/㎜, compared to the one of KOBELCO, 3.83mΩ/㎜. FCAW-1 has relatively thinner cross section, 0.17㎜, than the one of KOBELCO, 0.24㎜, it has higher resistance to the electric current. FCAW weld metal structure showed a typical structure with a phase fraction of 55 : 45 of austenite and ferrite after welding, which is fairly good. FCAW-4, a material with controlled flux components and tube thickness, showed better performance than KOBELCO product. And also a field workability test was done by a welder under a same condition for both developing and KOBELCO products. In chapter 4, TIG welding of super duplex stainless steels is performed with argon shielding gas only, nitrogen gets lost from the weld pool, which can result in a ferrite-rich weld metal, with an inferior corrosion resistance than parent metal. Nitrogen permeation model from the shield gas which gets into the weld metal in DCEN-TIG welding has suggested. This plasma stream model shows characteristics of permeation of nitrogen ions into the molten metal due to the strong physical effect of plasma stream which formed by the arc pressure rather than the permeation of nitrogen ions caused by electric effect. In Chapter 5, a specimen of weld metal was prepared by TIG welding with super duplex stainless steel to know the effect of σ(Sigma) 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. The effect of sigma(σ) phase to corrosion characteristics of weld metal by the Electrochemical polarization tests has been investigated. Corrosion current density was decreased a little with an increase of aging time over 60 minutes at 700 to 900°C and uniform corrosion was more influenced to the volume fraction of ferrite and austenite than the volume fraction of σ phase. Pitting potential was found to tend to decrease with an increase of aging time at 700 to 900°C. Most of the pits formed near the σ phase in the ferrite and propagated to austenite. Intergranular corrosion of weld metal was increased by an increase of σ phase. Degree of sensitization 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.
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