Metal treatment – Stock – Ferrous
Patent
1996-11-06
1998-12-15
Yee, Deborah
Metal treatment
Stock
Ferrous
420 67, 420 68, 420 69, C22C 3844
Patent
active
058491112
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a duplex stainless steel consisting of an austenitic phase and a ferritic phase. More specifically, it relates to a super duplex stainless steel suitable for welding providing high resistance to stress corrosion cracking and high toughness of the weld zones, which can be applied to heat exchangers exposed to sea water, brine-resistant chemical equipment and structures, pipings in chemical plants, line pipes, and oil well pipes.
BACKGROUND
Duplex stainless steel with high corrosion resistance and weldability has recently been in great demand for heat exchangers exposed to sea water, brine-resistant chemical equipment and structures, pipings in chemical plants, line pipes, and oil well pipes. This requirement of corrosion resistance is particularly stringent.
Many types of duplex stainless steel are commercially available. For example, "Weldable duplex stainless steels and super duplex stainless steels" by L. van Nassau, H. Meelker and J. Hilker (Dutch Welding Association, 1991) discloses four alloys listed as (a)-(d) below in the order of increasing corrosion resistance: steel (PREN=32-40), and stainless steel (PREN>40), %Cr+3.3.times.%Mo+16.times.%N. The greater the PREN is, the higher pitting resistance.
Super duplex stainless steel is designed to have desirable mechanical properties and high corrosion resistance as represented by PREN, defined above, greater than 40, by incorporating a high concentration of N in a 25% Cr steel as a basic component.
JP62-56556 proposes a highly corrosion-resistant super duplex stainless steel with a highly stable microstructure containing a relatively high amount of N, specified in relation to concentrations of other components, and a specified amount of the ferrite phase. This steel has a PREN defined by
JP05-132741 proposes a super duplex stainless steel which has a PREW defined by
In addition, the present inventors have proposed in JP04-293844 a super duplex stainless steel showing high corrosion resistance of weld zones which have a PREW of at least 43, a machinability index of up to 65, a difference in pitting resistance of the ferritic phase and that of the austenitic phase of -3.0 to 3.0, a composition less susceptible to formation of .sigma., .chi., and other intermetallic phases than conventional super duplex stainless steels.
The composition of conventional super duplex stainless steels based on 25% Cr steel, comprising larger amounts of Mo and N, enhances greatly the precipitation of .sigma., .chi., and other intermetallic phases during the steelmaking process or in the welding of such steels. The precipitation around weld zones decreases the corrosion resistance considerably, presenting a serious problem in practical applications.
The inner pressure of oil well pipes has been increased in recent years to lower operation costs by increasing the flow rate of the working fluid, hence requirements for duplex stainless steel for well pipes of high resistance to stress corrosion cracking, specifically critical stress for cracking .sigma..sub.th of at least 45.5 kgf/mm.sup.2 (65 ksi) in a pressurized corrosion environment, and of sufficient toughness of weld joints, specifically Charpy impact value of at least 200 J/cm.sup.2 at -30.degree. C.
PREN and PREW described above, determined uniquely by the initial composition of the alloy, have been used as indices of pitting resistance, and regarded as good representations of corrosion rate or pitting resistance of pressurized corrosion environments containing a chloride ion. Super duplex stainless steel has been defined as an alloy with PREN or PREW greater than 40, and is regarded as the most corrosion-resistant alloy in the present state of art.
PREN and PREW are useful, however, only when the steel has an austenite-ferrite duplex structure as a result of appropriate solid solution treatment after hot working. The resistance to stress corrosion cracking of the solidification structure of weld zones or heat affected zones (HAZs hereinafter) that have experien
REFERENCES:
L. Van Nassau et al, "Welding Duplex and Super-Duplex Stainless Steel" (A Guide for Industry pp. 1-15 & 4pp of figures, 1988.
Igarashi Masaaki
Kondo Kunio
Mori Tomoki
Ogawa Kazuhiro
Ueda Masakatsu
Sumitomo Metal Industries Ltd.
Yee Deborah
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