History of Duplex Stainless Steel

Transcription

1 Karakteristik Dan Pemilihan Material Duplex Stainless Steel Dr.-Ing. Bambang Suharno Dr. Ir. Sri Harjanto Metallurgy and Materials Engineering Department 2007 History of Duplex Stainless Steel 1929 : Avesta Jernverke (25% Cr, 5% Ni) 1933 : J-Holtzer Company, France, salah komposisi dari 18%Cr-9%Ni-2.5%Mo (Austenitik SS) menjadi 20%Cr-8%ni-2.5%Mo (Duplex SS) dimana terdapat fasa ferrite dalam matriks austenite, ketika di heat treatment ternyata tidak sensitif terhadap intergranular corrosion 1970 : Duplex di kembangkan di Germany dan Swedia (sejalan dengan ditemukannya AOD process) 1980 : Dikembangkan Super Duplex SS, ditandai dengan nilai PREN > 40 (= %Cr + 3.3% Mo + 16% N) Phase Diagram of Duplex SS Metallurgy of Duplex SS Selama solidification, duplex pertama kali membentuk ferrite Pada penurununan temperatur austenite terbentuk Pada cast duplex a structure of austenite islands in a ferrite matrix (dark) Pada wrought alloys the microstructure has a morphology of laths of austenite in ferrite matrix (dark)

2 Typical Microstructure of Duplex SS (wrought and cast) Microstructure of Duplex SS wrought Duplex stainless steels have microstructures a mixture of austenite and ferrite ferrite cast The microstructure shows an image of a duplex 3RE60 grade Ferrite Phase Karakteristik Duplex Stainless Steels Keberadaan ferrite dalam austenite menyebabkan: better intergranular corrosion resistance and stress corrosion cracking resistance Dibandingkan dengan austenitic stainless steel Tetapi keberadaan ferrite dalam austenite juga: Menyebabkan terjadinya reaksi pembentukan berbagai variety of secondary phases, Berakibat terhadap corrosion resistance mechanical properties (khususnya impact toughness) Austenitic stainless steel: good weldability and low-temperature toughness, their chloride SCC resistance and strength are comparatively poor. Ferritic stainless steel: good resistance to chloride SCC but have poor toughness, especially in the welded condition. A duplex microstructure with high ferrite content can therefore have poor low-temperature notch toughness, whereas a structure with high austenite content can possess low strength and reduced resistance to chloride SCC.

3 Karakteristik Duplex Stainless Steel Memiliki mikrostruktur a mixture of austenite and ferrite. resistant to stress corrosion cracking, meski tidak sebaik ferritic stainless steel Ketangguhannya (toughness) Lebih tinggi dibanding ferritic stainless steel Tetapi lebih rendah dibanding austenitic stainless steel Their strength is 2 X greater than austenitic SS (annealed) Have general corrosion resistances = or >> than 304 and 316 in general their pitting corrosion resistances are >> than 316. They suffer reduced toughness below 50 C and after exposure above 300 C so are only used between these temperatures are also magnetic easily differentiate them from common austenitic SS (non magnetic) Stess-Strain Curves for Duplex SS Sifat Mekanik Duplex SS Application of Duplex SS Chemical processing, transport and storage Oil and gas exploration, offshore rigs and refining Marine environments Pollution control equipment Pulp & paper industry Desalination plants and seawater systems Pressure vessels, reactor tanks and heat exchangers Rotors, impellers and shafts in industrial equipment Digesters, storage and clarifier tanks, stock washers for the pulp and paper industry Absorber towers, FGD systems in pollution control

4 Duplex Stainless Steels ELEMENTS AND University PROCESS of VARIABLES METALLURGY OF DUPLEX STAINLESS STEELS AIM OTHER EFFECTS TO CONSIDER COMMENTS Duplex stainless steel: usually have a low carbon additions of molybdenum, nitrogen, tungsten, and copper. Typical Cr contents are 20 to 30% and Ni contents are 5 to 10 %. The specific advantages offered by duplex stainless steels over conventional austenitic stainless steels are strength, chloride stress-corrosion cracking resistance, and pitting corrosion resistance. Nitrogen Nickel Chromium Molybdenum (Tungsten) Heat treatment temperature Strong gamma forming effect. Efficient even for rapid thermal cycles (HAZ). Positive effect on localized corrosion at ambient improves mechanical properties Strong gamma forming effect. Improves low temperature impact strength. Strong alpha forming effect. Improve general and localized corrosion resistance. PREN=Cr+3,3(Mo+0,5W)+16N Fixes the phase balance and the partition of elements between phases. Dissolves intermetallic phases. Risk of porosities (welding). Chromium nitrides precipitates. Cost is high! High Cr+Mo+W Increase drastically the risk of brittle intermetallic phases precipitations. Temperature and cooling rate (water cooling) essential to produce a sound structure. UR 35N (2304) = 0,12% UR 45N (2205) = 0,17% UR 47/52N+ (2507) = 0,25% (Be careful with low N 2 contents!) UR 35N (2304) = 4% UR 45N (2205) = 5,5% UR 47/52N+ (2507) = 7% PREN values UR 35N (2304) = 26% UR 45N (2205) = 33/36% UR 47/52N+ (2507) = 42% Heat treatment UR 35N (2304) = 980 C UR 45N (2205) = 1050 C UR 47/52N+ (2507) = 1120 C Group of Duplex Stainless Steel Pitting Resistance Index 1. Lean duplex, 2304 (S32304), which contain little or free of Mo Fe-23Cr-4Ni-0.1N 2. Standard 2205 (S32205) for more than 80% of duplex use Fe-23Cr-5.5Ni-3Mo-0.15N PREN = (Pitting Resistant Index) 3. High alloy 25 %Cr duplex such as 255 (S32550) and S31260 Fe-25Cr-5Ni-2.5Mo-0.17N-Cu PREN = Superduplex, with %Cr and increased Mo and N such as 2507 (S32750) Fe-25Cr-7Ni-3.5Mo-0.25N-W-Cu PREN > 40 Composition also plays a major role in the corrosion resistance of duplex stainless steels.

5 Pitting Resistance Index Komposisi Duplex SS C Cr Ni Mo N Seri 2205 PREN = [Cr%] = 3.3 [Mo%] = 16 [N%] 34 C Cr Ni Mo N Seri 2304 PREN (Cr%) (Mo%) = 16 (N%) 24 C Cr Ni Mo N Seri 2507 PREN = [Cr%] [Mo%] + 16 [N%] 40 Others S=0.001 Others S = Others S=0.001 UNS Type C Mn P S Si Cr Ni Mo N Cu Other Number S S W S S S S S S S c S S d S Composition Grade (%) THE DUPLEX STAINLESS STEELS FAMILY Corrosion Composition Grade (%) C Cr Ni Mo Cu N PREN resistance similar to UR 35N (2304) 0, > 0,1 0,2 0,12 > L UR 45N (2205) 0, ,7 3,1-0,18 33/36 317L, 317LMN, 904 UR 47N+/52N+ (2507) 0,02 25,5 7 3,7 -/1,5 0,25 >40 904L, 6MO YS Rp 0,2 (Mpa) UTS/Rm (Mpa) A% USA EU UR 35N (2304) > 400 > 600 > UR 45N (2205) > 480 > 680 > 25 UR 47N+/52N+ (2507) > 560 > 760 > , (+A923) / HIGH STRENGTH EU Standard DUPLEX Austenitic (47 N+) 4410 (52 N+) 4507 (45 N) 4462 (35 N) 4362 (B26) 4529 (B25) (B6) Rp 0.2% (MPA) ASTM (47N+) (52N+) (45N) (35N) (B25) LMN 904L 317L 316L 304L Duplex stainless steels yield strength range from 400Mpa (UR 35N) to 560Mpa (UR 52N+) Yield strength (Ksi) DUPLEX Austenitic

6 SCHAEFFLER-DELONG Diagram Ni equivalent AUSTENITE A+M 317 LN 316L 304L MARTENSITE A+M+F F+M % UR B66 /S31266 UR B26 / 6 Mo N % SR50A 20% UR B25 S31254 DUPLEX 50α / 50γ S32520/550/750/760 UR 52N + /S32205 UR45N/S31803 UR 35N/S % FERRITE Cr equivalent Cr% + Mo% + 1,5 Si% + 0,5 Nb% + 5 V% + 3 Al% STRESS CORROSION CRACKING C UR 45N 390 UR 35N UR 52N No cracking AISI 316 Constant-strain tests in autoclave Above the curve, SCC occurs. AISI 304 Open circles mean no SCC for UNS 32520/UR 52N Cl - (%) Duplex stainless steels behave much better than austenitic stainless steels Duplex vs Austenitic SS Sifat korosi Duplex SS Sifat ketahanan korosi Memiliki ketahanan SCC lebih baik dari Austenitic SS Memiliki ketahanan korosi pitting dan korosi batas butir

7 Pembentukan Fasa Kedua (Intermetallic phase) The high alloy content of duplex stainless steels also makes them susceptible to the formation of intermetallic phases from extended exposure to high temperatures. Significant intermetallic precipitation may lead to a loss of corrosion resistance and sometimes to a loss of toughness. PRECIPITATIONS OBSERVED IN FUNCTION OF TIME AND TEMPERATURE University Temperature of ( C) Maximum service limit (UR 45 N ) FERRITE 100 % Increase of ferrite content Heat treatment (α= γ = 50 %) α > σ α > χ SAFE SERVICE RANGE (- 50, 300 C) LONG TERM SERVICE EMBRITTLEMENT α > α ' Cu --> ε EMBRITTLEMENT LOW TEMPERATURE BRITTLENESS (θ <- 50 C) Heat treatment range DO NOT USE s (Seconds) Hours 10 4 h 10 5 h Time Possible Precipitates in Duplex SS TTT diagrams S32304 S31803 S32520 C F SIGMA Alpha Seconds 10 hours

8 Sigma Phase Morphology of Sigma Phase Sigma (σ) = Fe-30Cr-4Ni and 4-7% Mo Has harmful effects on the mechanical properties, ductility and toughness it is detrimental to corrosion resistance Sigma phase precipitates in Duplex SS over a wider temp range and in a shorter time The presence of ferrite enhances the precipitation of sigma phase The diffusion rate of sigma phase forming elements (Cr, Mo, W) in ferrite is 100 times faster than in austenite Sigma phase can form from O C Chi Phase Chi Phase Chi (λ) phase is commonly found in duplex SS but it is usually in much smaller quantities than sigma phase It is also as harmful as sigma phase to the properties It is more detrimental to pitting resistance than sigma phase Chi and Sigma phase are not distinguishable using optical microscope (can be distinguished by TEM) Chi phase precipitates in the range O C Upon long term aging, chi phase will convert to sigma phase

9 Welding Welding Duplex Stainless Steels Duplex stainless steels have good weldability They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding. The recommended filler material for 2205 stainless steel is 2209 Weldability of duplex stainless steels are better than ferritic stainless steels, but generally not as good as austenitic materials. Control of heat input is important. Solidification cracking and hydrogen cracking are concerns when welding duplex stainless steels Modifications to the alloy compositions have been made to improve corrosion resistance, workability, and weldability. In particular, nitrogen additions have been effective in improving pitting corrosion resistance and weldability