슈퍼듀플렉스 스테인리스강 용접 재료 및 용접금속의 특성 연구
- Abstract
- 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.
- Issued Date
- 2016
- Awarded Date
- 2016. 2
- Type
- Dissertation
- Publisher
- 부경대학교 대학원
- Affiliation
- 부경대학교 대학원
- Department
- 대학원 금속공학과
- Advisor
- 정병호
- Table Of Contents
- 목 차
Abstract...........................................................................................................................ⅳ
제1장 서론....................................................................................................................1
1.1 연구 목적 및 필요성.............................................................................................1
1.2 연구내용...................................................................................................................4
참고 문헌......................................................................................................................6
제2장 이론적 배경.....................................................................................................8
2.1 듀플렉스 스테인리스강의 개발과 전망............................................................8
2.2 듀플렉스 스테인리스강의 용접 및 기계적 특성..........................................13
2.2.1 용접공정 개요................................................................................................13
2.2.2 용접 공정상의 유의사항.............................................................................14
2.2.3 듀플렉스강 용접부의 기계적 성질...........................................................18
2.3 듀플렉스 스테인리스강에서 σ상의 석출기구................................................25
2.3.1 듀플렉스 스테인리스강에서 σ상의 석출.................................................25
2.3.2 듀플렉스 스테인리스강에서 σ상의 석출기구.........................................29
2.4 듀플렉스 스테인리스강의 용접금속과 질소..................................................31
2.4.1 개요...................................................................................................................31
2.4.2 용접금속에서 N의 손실과 기공 발생......................................................31
2.5 GTA 용접 시 아크전압과 전류 특성...................................................35
2.5.1 GTA 용접 시 아크전압과 전류 특성............................................35
2.5.2 GTA 용접 시 아크압력의 발생......................................................35
2.6 듀플렉스 스테인리스강 용접부의 부식 특성............................................40
2.6.1 입계부식................................................................................................41
2.6.2 균일부식..................................................................................................42
2.6.3 공식..........................................................................................................43
참고 문헌............................................................................................................48
제3장 슈퍼듀플렉스강 용접재료의 성능평가와 용접공정
개발.....................................................................................................................54
3.1 서언............................................................................................................................54
3.2 실험 방법.................................................................................................................56
3.2.1 TIG 용접 시험.................................................................................................56
3.2.2 SMAW 용접 시험..........................................................................................59
3.2.3 FCAW 용접 시험...........................................................................................61
3.2.4 현장 작업성 시험............................................................................................65
3.3 실험결과 및 고찰...................................................................................................67
3.3.1 TIG 용접...........................................................................................................67
3.3.2 SMAW 용접.....................................................................................................73
3.3.3 FCAW 용접......................................................................................................80
3.3.4 현장 작업성.......................................................................................................89
3.4 결언.............................................................................................................................95
참고 문헌........................................................................................................................98
제4장 슈퍼듀플렉스강 TIG 용접에서의 질소 침투 특성..................100
4.1 서언...........................................................................................................................100
4.2 기존의 질소 침투 모델................................................................................102
4.2.1 가스-금속계에서 용접금속 내로의 질소 침투........................................102
4.2.2 가스-플라즈마계에서 용접금속 내로의 질소 침투................................104
4.2.3 용접금속 내의 질소 침투 개선 모델.........................................................107
4.3 용접금속 내의 질소 침투에 관한 기초 실험................................................111
4.3.1 서언...................................................................................................................111
4.3.2 실험재료 및 방법..........................................................................................111
4.3.3 실험결과 및 고찰..........................................................................................113
4.4 플라즈마 스트림 모델의 타당성 검증 실험..................................................116
4.4.1 서언..................................................................................................................116
4.4.2 실험재료 및 방법..........................................................................................117
4.4.3 실험결과 및 고찰..........................................................................................118
4.5 결언..........................................................................................................................124
참고 문헌.....................................................................................................................125
제5장 슈퍼듀플렉스강 용접재료의 부식특성...........................................127
5.1 서언.................................................................................................................127
5.2 사용재료 및 실험방법.........................................................................................132
5.2.1 사용 용접재료................................................................................................131
5.2.2 열처리 및 조직관찰......................................................................................133
5.2.3 전기화학적 분극시험....................................................................................134
5.3 실험결과 및 고찰.................................................................................................138
5.3.1 미세조직 및 경도..........................................................................................138
5.3.2 경도...................................................................................................................147
5.3.3 시효온도 및 시간에 따른 σ상 및 α상의 체적분율...............................149
5.3.4 전기화학적 부식 특성...................................................................................153
5.4 결언...........................................................................................................................190
참고 문헌.....................................................................................................................191
제6장 결론....................................................................................................................196
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