Superduplex stainless steel having high corrosion resistance and high yield strength in the solution annealed condition

Abstract: The steel, object of our invention is highly resistant to stress corrosion in acid and alkaline environment, and has furthermore high pitting and crevice corrosion resistance. This steel has a very high yield strength, even in the solution annealed condition. The steel consists of: (in weight percent) C 0.025 max Mn 0.80 max Si 0.80 max Cr 24 - 26 Ni 6.5 - 8.0 Mo 3.0 - 4.0 Cu 1.2 - 2.0 W 0.8 - 1.2 N 0.23 - 0.33 S 0.002 max P 0.025 max Fe balance with the proviso: Pitting Index = Cr + 3.3 Mo + 16N > 42

Description: The Austenitic/Ferritic stainless steels presently on the market are those reported in the ASTM A790 specification with the addition of the Zeron 100, equivalent to UNS S32760, which is not yet included in the ASTM specification. The steel grades which are mainly requested for the oil field applications are the UNS S32750 and UNS S32760. There are however many other kind of steels which can be included in the same class and which are listed and described in the following publications : Superduplex Stainless Steel : Structure and Properties. by J. Charles Superduplex Stainless Steel : Structure and Properties. by J. Charles The Corrosion Resistance of Duplex Stainless Steel. by S. Bernhardsson The invention concerns the production of austenitic-ferritic stainless steel of the superduplex class, manufactured by electrical arc melting and A.O.D. refining. The A.O.D. refining allows a deep deoxydation and desulphurization of the steel as well as to reach the required chemical composition in a repeatable way. The analytical balancing of the alloying elements Cr, Mo, W, Cu and N is very important for obtaining the mechanical properties and the corrosion resistance on hot forged and/or rolled products in the solu tion annealed condition. Our invention has taken advantage of the combined action of Mo, W, Cu, N which allows to reach a yield strength value higher than 620 MPa (90,000 PSI) as well as a generalized resistance to the stress corrosion, to the pitting and crevice corrosions. The optimized chemical composition is the following : (weight in percent) C 0.025 max Mn 0.80 max Si 0.80 max Cr 24 - 26 Ni 6.5 - 8.0 Mo 3.0 - 4.0 S 0.002 max P 0.025 max Cu 1.2 - 2.0 W 0.80 - 1.2 N 0.23 - 0.33 Fe balance The ferrite on the product solution annealed at the temperature of 1070 - 1100°C is 40 - 55% Pitting index = Cr + 3.3 Mo + 16 N > 42 1. Mechanical characteristics of the product solution annealed at 1080 °C : Room Temp. 80°C 150°C Rs 0.2 MPa (KSI) 630 (91) 550 (80) 480 (70) R MPa (KSI) 865 (125) 800 (116) 750 (110) A % 40 HB 270 °C -45 -65 -100 KV Joule 300 144 38 2. Comparisons with other steels of the same class on the basis of the corrosion resistance : 2.1 Chemical analysis of steel (A = Steel subject of the invention) Cr Ni Mo Cu W N A) 25.5 7.4 3.9 1.7 0.9 0.27 B) 25.5 7.1 3.6 0.7 0.7 0.26 C) 25.4 7.0 3.6 0.2 --- 0.28 2.2 Pitting corrosion resistance (ASTM G48) at 50°C - 24 h. A B C weight loss (g/m 2 ) 0.049 0.070 0.76 N° of pitting/m 2 0 0 270 Max pit depth (micron) 0 0 100 Critical Pitting Temp.(°C) 80 60 50 2.3 Corrosion resistance in 5% Na Cl at PH = 4 - 6 at 600 mV : A B C CPT (°C) 80 72 63 2.4 Crevice corrosion resistance (ASTM G78 modified) 100 days . Environment : Aerated sea water containing 20,000 ppm cloride plus 2 ppm residual chlorine with occasional shock dosing up to 10 ppm chlorine ar 35°C A B C N° of crevice (min/max) Absent 1 / 3 Traces 2.5 Sulfide Stress Cracking Resistance of the steel object of the invention (NACE TM-01-77) at 80°C - 760 h : Applied load = 550 MPa no cracking

Claims: Alloy having the chemical composition (in percent by weight) : C 0.025 max Mn 0.80 max Si 0.80 max Cr 24 - 26 Ni 6.5 - 8.0 Mo 3.0 - 4.0 S 0.002 max P 0.025 max Cu 1.2 - 2.0 W 0.8 - 1.2 N 0.23 - 0.33 Fe balance Pitting Index = Cr + 3.3 Mo + 16 N > 42 Balanced chemical composition to obtain after solution heat treatment at 1070-1100°C an alfa Ferrite of 40 - 60%. Alloy according to claim 1, characterized in that the content of C is 0.01 - 0.02%. Alloy according to claim 1, characterized in that the content of Si is 0.35 - 0.60%. Alloy according to claim 1, characterized in that the content of Ni is 7.0 - 7.5%. Alloy according to claim 1, characterized in that the content of Mo is 3.7 - 4.0% Alloy according to claim 1, characterized in that the content of Cu is 1.6- 1.9%. Alloy according to claim 1, characterized in that the content of W is 0.9- 1.1%. Alloy according to claim 1, characterized in that the content of N is 0.25- 0.30%.

Inventors: FORONI LUCA, FORONI,LUCA
Applicants: FORONI SPA, FORONI S.P.A.
Application year: 1992
Main classification: C22C38
IPC codes: C22C38