Metal treatment – Stock – Ferrous
Patent
1996-07-24
1997-09-30
Yee, Deborah
Metal treatment
Stock
Ferrous
148326, 420 60, 420 61, C22C 3842
Patent
active
056722150
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an inexpensive, highly corrosion-resistant duplex stainless steel suitable for use in constructing piping systems, heat exchangers and the like, to be installed in plants of oil refining industries, chemical industries and such.
BACKGROUND ART
Generally, carbon steels are used most prevalently as materials for constructing piping systems for handling fluids, such as industrial water and halfway products, and heat exchangers to be installed in plants of oil refining industries and chemical industries. Since the corrosion resistance of carbon steels with industrial water is not necessarily satisfactory, those facilities need to be renewed everythree to ten years depending on the degree of progress of corrosion. Recently, extension of the period of routine inspection of plants and extension of the life of equipment have been desired to reduce progressively increasing maintenance costs and facility renewal costs. Consequently, it is the trend of the times to use corrosion-resistant materials including stainless steels for constructing equipment that has been constructed from carbon steels.
Austenitic stainless steels of JIS (Japanese Industrial Standards) designations SUS304, SUS316, SUS304L and SUS316L are most commonly used because of their excellent corrosion resistance and weldability. Although highly resistant to pitting and crevice corrosion, austenitic stainless steels are susceptible to stress corrosion cracking (SCC). Although plants of oil refining industries and chemical industries use industrial water on a massive scale for cooling and the like, there is the possibility that chlorides contained in a very small concentration of 300 ppm or less in industrial water cause SCC.
Although very excellent in SCC resistance, ferritic stainless steels are inferior to austenitic stainless steels in weldability and toughness, hence it is not quite satisfactory to substitute austenitic stainless steels with ferritic stainless steels. Duplex stainless steels which have been developed to solve such a problem have a duplex structure consisting of ferrite and austenite, are excellent in SCC resistance, weldability and toughness as compared with austenitic stainless steel, and have a higher strength than those of ferritic stainless steels and austenitic stainless steels.
Standard duplex stainless steels specified in JIS and ASTM Standards are 18% Cr-3% Mo system stainless steel of ASTM-UNS-S31500, 23% Cr-0.4% Mo system stainless steel of ASTM-UNS-S32304, 22% Cr-3% Mo system stainless steel of SUS329J3L (UNS-S31803), and 25% Cr-3% Mo system stainless steel of SUS329J4L (UNS-S31260, S32550). These steels, having a greater Cr and Mo content, have higher corrosion resistance. The stainless steel UNS-S32304 having a relatively small Cr and Mo content is a relatively inexpensive duplex stainless steel and its corrosion resistance is substantially the same as those of austenitic stainless steels SUS316 and SUS316L.
The field of application of duplex stainless steels has been expanding owing to the foregoing characteristics. Disclosed in JP-A No. 52-716 is a duplex stainless steel of UNS-S31260 system containing over 20% but not more than 35% Cr, 3 to 12% Ni, 0.5 to 5.0% Mo, 0.2 to 1.5% Cu, W and V, and excellent in resistance to intergranular corrosion by sea water. Disclosed in JP-A No. 56-142855 is a duplex stainless steel containing 20 to 35% Cr, 3 to 10% Ni, 0.5 to 6.0% Mo, W or V, and B, and excellent in hot workability and resistance to localized corrosion. Disclosed in JP-A No. 61-56267 is a duplex stainless steel containing 21 to 24.5% Cr, 2 to 5.5% Ni, 0.01 to 1.0% Mo and 0.01 to 1.0% Cu, and excellent in weldability and corrosion resistance. Disclosed in EP 0 337 846 B1 (Oct. 18, 1989) is an invention relating to a steel having enhanced wear corrosion resistance to the corrosive action of sulfuric acid and obtained by adding 1 to 3.5% Cu to a 23%Cr-0.4%Mo steel analogous to that of UNS-S32304.
Duplex stainless steels are inferior in hot workability, and surface
REFERENCES:
patent: 4798635 (1989-01-01), Bernhardsson et al.
European Search Report dated Mar. 19,1996.
Azuma Shigeki
Ogawa Kazuhiro
Sumitomo Metal Industries Ltd.
Yee Deborah
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