TINSI-STEEL ​Duplex stainless steel

TINSI-STEEL Duplex stainless steel

Duplex Steel ,Duplex stainless Steel, Super Duplex steel,Super Duplex stainless steel,

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Introduction of Duplex stainless steels

Duplex stainless steels (DSS) are finding increasing use in the refining industry, primarily because they often offer an economical combination of strength and corrosion resistance. These stainless steels (SS) typically have an annealed structure that is generally half ferrite and half austenite, although the ratios can vary from approximately 35/65 to 55/45. The benefits expected from the use of DSS are maintained even to a ratio of 75/25 ferrite/austenite volume fraction (except for possible problems with weldability). Most refinery applications where DSS are used are corrosive, and DSS or other higher alloys are required for adequate corrosion resistance. However, some plants are also starting to consider DSS as a “baseline” material. They are using it in applications where carbon steel may be acceptable, but DSS have been shown to be more economical considering their higher strength and better long-term reliability.

Duplex stainless steels (DSS) have existed since the 1930s. However, the first generation steels such as Type 329 (UNS S32900) had unacceptable corrosion resistance and toughness at weldments. the initial applications were almost exclusively heat exchanger tubing, particularly in corrosive cooling water services, and shafting or forgings. In the 1980s, second generation DSS became commercially available which helped overcome the problems at the welds. These new grades had nitrogen additions, which along with improved welding practices designed for the DSS, led to the welds’ mechanical (strength and toughness) and corrosion properties being comparable to the annealed base metal. The DSS most commonly used today in refineries include those with 22 % and 25 % Cr. The 25 % Cr (super duplex grades) usually also contain more molybdenum and nitrogen, and so have higher PREN values than the 22 % Cr duplex steels.

2205 Duplex steel

All duplex stainless steels resist the Achilles heel of the 300 series - chloride stress corrosion cracking. The original duplex alloy Type 329 offered useful corrosion resistance and high strength while using less nickel. Modern day duplex alloys, like 2205, further optimized performance and ease of fabrication. Duplex alloys overall offer good general corrosion resistance and higher strength with the benefit of a cost saving. We know today that 2205 has approximately double the strength of 300 series stainless and is similar to 317L in regards to general corrosion. Due to the cost competitive supply of 2205, it has replaced 304L, 316L, 317L and even carbon steel for many applications. The higher strength allows for weight savings to counteract any cost increase. The improved corrosion resistance lengthens the overall life expectancy. The strength and corrosion benefits have made 2205 a competitor to many different austenitic stainless steels.

Super Duplex steel

Super duplex alloys extend the range of the same benefits offered by the lower duplex grades. Similar to lean duplex, the super duplex market is divided among several primary super duplex grades including ZERON® 100 (S32760), SAF 2507® (S32750)Super duplex has replaced many stainless steels and even high nickel grades due to lower cost and comparable corrosion resistance. Historically, alloys such as alloy 400,alloy 600, 90-10 Cu-Ni, and 6% Mo grades have dominated applications in seawater. Recently super duplex has replaced these alloys for marine applications, seawater reverse osmosis, offshore oil rigs and even subsea applications. In fact, super duplex has allowed the oil and gas industry to produce pipe that handles pressures of up to 15,000 psi (1,034 MPa) using heavy wall pipe that is stronger than 625 nickel alloy. With a fraction of the cost, combined with high strength and resistance to seawater, the upstream oil and gas market widely utilizes super duplex stainless. By employing critical production methods and careful fabrication, duplex grades are expected to be even more widely used as oil wells become deeper with higher pressures.

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TINSI-STEEL offered Table 1 lists the chemistries and UNS numbers of various common DSS, including some first generation DSS for comparison. Note that UNS S32205 is a “newer version” of UNS S31803 and is produced with higher nitrogen, chromium, and molybdenum contents. ASME and ASTM standards for these grades are given in Table 2, while Table 3 provides the mechanical properties. Type 316L and other austenitic SS are included for comparison.

Table 2—ASME ad ASTM Specifications for Duplex stainless steels

Table 3—Mechanical Properties of Various Duplex and 316L SS

Normative References Duplex stainless steels

ASME Boiler and Pressure Vessel Code (BPVC) 1, Section VIII : Pressure Vessels; Division 1, Division 2

ASME B31.3, Process Piping

ASME SA-182,Specification for Forged or Rolled Alloy-Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service

ASME SA-240, Specification for Heat-Resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels

ASME SA-351, Specification for Castings, Austenitic, Austenitic-Ferritic (Duplex), for Pressure-Containing Parts

ASME SA-479, Specification for Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure Vessels

ASME SA-789, Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing for General Service

ASME SA-790, Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe

ASME SA-815, Specification for Wrought Ferritic, Ferritic/Austenitic, and Martensitic Stainless Steel Piping Fittings

ASTM A276 2, Standard Specification for Stainless Steel Bars and Shapes

ASTM A890, Standard Specification for Castings, Iron-Chromium-Nickel-Molybdenum Corrosion-Resistant, Duplex (Austenitic/Ferritic) for General Application

ASTM A923, Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels

ASTM A928, Standard Specification for Ferritic/Austenitic (Duplex) Stainless Steel Pipe Electric Fusion Welded with Addition of Filler Metal

ASTM A995, Standard Specification for Castings, Austenitic-Ferritic (Duplex) Stainless Steel, for Pressure-Containing Parts

ASTM E140, Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness

Tubes&pipes Requirements for production of Duplex stainless steels

1.Tubing shall be seamless or welded tubing, and if welded tubing is approved, it shall receive both hydrostatic testing and nondestructive electric (eddy current) testing in accordance with ASME SA-789. Tubing shall be manufactured from steel produced by the electric furnace process, and subsequently VOD or AOD. Secondary melting processes such as VAR and ESR are permitted.

2.Tubing shall be solution annealed at the minimum temperature or in the temperature range listed for the particular grade in ASME SA-789 for sufficient time to eliminate intermetallic precipitates and then rapid cooled by water quenching, or air or inert gas cooling to below 315 °C (600 °F). Other heat treatments and quenching media other than water must be approved by the purchaser.

3.The hardness testing criteria given in ASME SA-789 shall be modified to Rockwell C 28 maximum for lean and standard 22 % Cr DSS. Failure to meet this hardness requirement shall constitute failure of the specimen, and shall result in hardness testing being required on each length in the heat lot represented by the specimen.

4.For standard 22 % Cr and 25 % Cr DSS, one specimen representing each heat lot shall receive a microstructural examination per the requirements of ASTM A923 Test Method A. The presence of affected or centerline structures shall be grounds for rejection of the solution anneal batch represented by the specimen, and the batch shall require reheat treatment and retesting. The sample size shall be a 2.54 cm (1 in.) long tube specimen for each heat lot. In addition, ASTM A923 Test Method C shall also be done and meet the given criteria, or if not given, then a criteria agreed to by purchaser and supplier.

5.No weld repairs to tubes are permitted.

6.Tubes and tube bends shall not be heat treated after bending or straightening. For standard grades, cold work shall be limited to 15 % maximum which is equivalent to limiting U-bending to 3.3D bend radii minimum. For 25 % Cr DSS grades, bends can be made down to 1.5D bend radii with no heat treatment required. If the hardness requirement given in A.2.3 is exceeded doing to working or bending, re-solution annealing per A.2.2 will be required.

7.Lubricants shall be removed from tube surfaces prior to heat treatment. The fabricator shall submit heat treatment plans for approval, which will include precautions to minimize exposure of any sections to 700 °C to 950 °C (1300 °F to 1750 °F) as this could cause unacceptable intermetallic precipitation. The preferred heat treatment method is electric resistance, which is done for only seconds.

8.After heat treating, other than bright anneal procedures, all tubing shall receive a descaling treatment of pickling followed by a neutralizing and appropriate rinsing treatment. All mill scale shall be removed.