Duplex Stainless Steel: A Critical Review of Metallurgy, Engineering Codes and Welding Practices

Ramesh Bapat, CMfgE, P.E.

 Chief Engineer I, Western Region Materials Engineering

FMC Technologies

ramesh.bapat@fmcti.com 

Pradip Goswami, P.Eng., IWE

Welding and Metallurgical Specialist

Ontario, Canada

pgoswami@sympatico.ca

October 17,2012 

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Agenda

1. Safety / Quality Moments

2. Introduction

3. Metallurgy

4. Corrosion Resistance / Testing Methods –PREN, PREW

5. CCT / CPT Temperatures

6. Design Codes and Specifications

7. Role of Alloying Elements

8. Welding of Duplex and Super Duplex

9. Micrograph and Schaffer Diagram

10. Concluding Remarks

11. References

12. Acknowledgements

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Introduction

• There are four (4) types of duplex – lean duplex (20%Cr), duplex (22%Cr), super duplex (25%Cr) and hyper duplex (30%+Cr)

• Lean duplex is better than 316 grade stainless steel but not as good as duplex• Duplex has widespread application in offshore oil and gas, petrochemical and 

other industries due to high strength, excellent resistance to SCC and good weldability

• Presence of dual microstructure – ferritic / austenitic; excellent corrosion resistance and strength, pitting resistance and resistance to chloride stress  cracking

• Applications – down hole tubulars, flow lines, manifolds• Maximum operating temperatures; limited to 230°C (447°F)

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Metallurgy of Duplex Stainless Steels (DSS)

• DSS solidify as fully ferritic, some of which transforms to austenite as temperature falls to 1000°C (1832°F)

• Nitrogen promotes austenite formation from the ferrite at a higher temperature; however, while cooling, carbides, nitrides, sigma and other intermetallic phases are possible (microstructural constituents)

• Solidification diagram of DSS Fe-Cr-Ni system at 68% iron

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Corrosion Resistance

• DSS exhibit a high level of corrosion resistance in most environments• If microstructure contains at least 35% ferrite, duplex grades are more resistant to 

chloride stress cracking than austenitic grades like 304 and 316. Ferrite is susceptible to hydrogen embrittlement; hence, duplex grades are not suitable where hydrogen may be charged into the metal

• In the 1980s, a higher alloyed DSS grade was developed called super duplex• Super duplex grades are made to withstand more aggressive environments, but 

bear risk of precipitating unfavorable phases due to higher alloying elements• Super duplex grades are usually characterized by a Pitting Resistance Equivalent 

Number (PREN) higher than 40• The higher the PREN number, the better the corrosion resistance• PREN= (%Cr) + (3.3x%Mo) + (16x%N)• PREW= (%Cr) + (3.3x%Mo) +(0.5%W) + (16x%N)

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Role of Alloying Elements in Duplex

• Chromium (Cr) – Ferrite Stabilizer – A minimum of 10.5% chromium is necessary to form a stable passive film to protect steel from mild atmospheric corrosion. The corrosion resistance is higher with increasing chromium content

• Molybdenum (Mo) – Ferrite Stabilizer – Molybdenum acts to support chromium to provide chloride corrosion resistance and resistance to pitting corrosion; molybdenum also resists tendency to form detrimental intermetallic phases (e.g.: sigma, chi)

• Nickel (Ni) – Changes lattice structure from BCC to FCC; FCC gives excellent toughness; Ni additions help delay the formation of detrimental intermetallic phases

• Nitrogen (N2) – Austenite Stabilizer – It delays formation of sigma phase; increases resistance to pitting and crevice corrosion; increases toughness; nitrogen is adjusted based on nickel content to achieve desired phase balance

• Manganese (Mn) – Somewhat controversial; stabilizes austenite, but may reduce pitting corrosion resistance

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Resistance to Acids

• DSS have good corrosion resistance against strong acids• Both 2205 and 2507 DSS outperform many high nickel austenitic stainless steels in

solutions containing up to approximately 15% acid• DSS do not have sufficient nickel to resist the strong reducing conditions of mid-

concentration of hydrochloric acid• Their resistance to oxidizing conditions makes DSS viable candidates for nitric acid

service and strong organic acids• The DSS are also used in processes involving halogenated hydrocarbons because of

their resistance to pitting and stress corrosion

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Resistance to Caustics

• The high chromium content and presence of ferrite provides for good performance of duplex stainless steels in caustic environments

• At moderate temperatures, corrosion rates are lower than those of the standard austenitic grades

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Pitting and Crevice Corrosion Resistance

• DSS can be characterized by a temperature above which pitting corrosion will initiate and propagate within approximately 24 hours to a visibly detectable extent

– This temperature is known as Critical Pitting Temperature (CPT)– Pitting initiation below this temperature will not occur for long periods of time

• Pit initiation is random, and the CPT is sensitive to minor variations• With a new research tool described in ASTM G 150, CPT can now be accurately and

reliably measured by electro potential measurements

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Pitting and Crevice Corrosion Resistance (Cont’d.)

• The critical temperature for initiation of crevice corrosion is called the Critical Crevice Temperature (CCT)

– Dependent on the individual sample of stainless steel, chloride environment and nature of the crevice

– Geometrical factor of the crevices causes more scatter for the measurement of CCT than for the CPT

– CCT could be 15 to 20°C (27 to 36°F) lower than the CPT• Generally, higher critical pitting or crevice corrosion temperatures indicate greater

resistance to the initiation of these forms of corrosion• CPT and CCT in welded condition would be expected to be somewhat lower than the

equivalent base metal. It is advisable for the welding consumable to be slightly overmatching compared to the base metal.( e.g. 2205 grade base metal is welded with 2209 welding consumable)

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Pitting and Crevice Corrosion Resistance (Cont’d.)

• CPT and CCT of 2205 and all other grades of DSS are well above those of Type 316– This makes 2205 and other DSS / SDSS a versatile preferred material for

applications involving chlorides and H2S environment (Refer MR0175/ISO 15156-3, Table A24)

– For critical seawater applications involving superior corrosion resistance, the SDSS are extremely popular compared to higher alloyed austenitic grade stainless steels

– CPT = Constant + %Cr + 3.3x%Mo + 16x%N (Per IMOA guidelines)

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Resistance to Stress Corrosion Cracking

• In many refining, petrochemical process industries, DSS are replacements for austenitic grades in applications with a significantly lower risk of SCC

• DSS/SDSS are generally not susceptible to stress corrosion cracking in oil and gas production environment

• Examples of environments in which SCC of DSS may be expected, including the boiling 42% magnesium chloride test, are shown below

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Requirements of Design Codes and Specifications

• DSS can have good notch toughness for low (arctic) temperature and ambient temperatures, but not for cryogenic applications

• Minimum allowable temperatures are –51°C (–60°F) in the B31.3 Code• Minimum allowable temperatures are –29°C (–20°F) for some cases in the ASME

Section VIII Code– Actual limits are determined by reviewing the applicable code

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Requirements of Design Codes and Specifications (Cont’d.)

• ASME Section VIII requirements for impact testing for DSS base and weld metals are given in UHA-51(d) (3)

– Requires impact testing of all DSS thicker than 10 mm (3/8 in.) or those with a Minimum Design Metal Temperature (MDMT) less than –29°C (–20°F)

– Maximum operating temperatures are limited by the susceptibility of the sigma phase embrittlement

– Most design codes applicable to refinery and oil and gas processing plants limits upper application temperature of various DSS grades to between 260°C to 300°C (500°F to 572°F) as long as it is intermittent and not continuous

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Requirements of Design Codes and Specifications (Cont’d.)

– Code limit applies to the risks associated with continuous long-term exposures above the limiting temperature

– Brief infrequent excursions of the actual metal temperature into the embrittlement range may be tolerated without significant loss of properties

• Damage from overheating is cumulative; the code does not address the issue• Typical design limits for DSS and SDSS are specified below

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Welding of Duplex and Super Duplex SS

• Duplex and super duplex stainless steels are weldable by all conventional arc welding processes such as:

– Shielded Metal Arc Welding (SMAW) is particularly excellent for position welding, single-sided welding and weld joints where access is limited

– Gas Metal Arc Welding (GMAW) is a particularly good method for welding sheet metal up to around 6 mm thick

– Gas Tungsten Arc Welding (GTAW) is normally used for thin (up to around 4 mm) work pieces; it is specifically common in the welding of pipe joints

– Flux Core Arc Welding (FCAW) is suitable for material thicknesses above approximately 2.5 mm

– Submerged Arc Welding (SAW) is widely used with duplex steels due to its high productivity, and the beautiful weld finishes are advantageous

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Welding of Duplex and Super Duplex SS (Cont’d.)

• Welding duplex and super duplex stainless steels to design code / specification requirements is challenging to any welding engineer

• Common arc welding processes are suitable for joining duplex and super duplex stainless steels depending on process and economy related conditions as narrated below

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Welding of Duplex and Super Duplex SS (Cont’d.)

• Recommended shielding gases for MIG, TIG, FCAW for duplex / super duplex stainless steels

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Commonalities for DSS / SDSS

• Duplex stainless steel has an approximate balanced 50/50 microstructure obtained by controlled chemical composition and heat treatment

• Austenite / ferrite spacing is important to prevent Hydrogen Induced Stress Cracking (HISC)

• 30 microns austenite/ferrite spacing is usually considered good for forgings; 10 microns for small diameter tubing

• Micrographs of both duplex and austenitic alloys are shown to the right (common industry practice ferrite is normally 40 – 60%)

• The Schaeffer Diagram shows the predicted ferrite content of various duplex weld metals

• Ni Equivalent=%Ni+30x%C+0.5% Mn

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Concluding Remarks

• Duplex stainless steels are extremely versatile and are engineering alloys of very high integrity

• Careful selection of these alloys for the right design and service environment leads to much better performance, design life and higher integrity

• The modern duplex stainless steels have as good weldability as the austenitic stainless steels

• Good corrosion resistance and mechanical properties of DSS are the result of well crafted WPS / PQR that define heat inputs and cooling rates to achieve weldments with optimum ferrite to austenite balance

• The presence of ferrite in DSS imparts the superior chloride SCC resistance and high strength

• Austenite in DSS provides higher resistance to aqueous corrosion and low temperature impact toughness

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Concluding Remarks (Cont’d.)

• The recommended phase balance of DSS and SDSS should contain 40% – 60% ferrite in the base metal and the weld metal

• For DSS, too low and too high heat inputs should both be avoided, as both extremes can lower the corrosion resistance

• Nitrogen additions to the shielding gas and the purging gas can be used with advantage, when a higher corrosion resistance is desired in the weld, than normally can be obtained by pure argon

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References

1. API Technical Report 938-C. 2005. Use of Duplex Stainless Steels in the Oil Refining Industry.

2. The History of Duplex Developments. J. Charles and P.Chemelle. 8th Duplex Stainless Steels Conference. 13-15 October 2010. Beaune, France.

3. The Physical Metallurgy of Duplex Stainless Steels. J.O. Nilsson and G. Chai. Sandvik Materials Technology.

4. Practical Guidelines for the Fabrication of Duplex Stainless Steels. International Molybdenum Association.

5. Welding Duplex and Super Duplex Stainless Steels. L. van Nassau, H. Meelker and J. Hilkes.

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Acknowledgements

• Thanks to FMC Management and particular thanks to the following experts for their support of this paper: Brian SKEELS, Greg Glidden, Michael Coles, Elliott Turbeville, Joel Russo, Tina Kruse, Mike Robinson, Mike Williams, Randy Shipley, Randy Wester and Jill Bell.

• Thanks to the organizing committee of the Stainless Steel World Americas 2012 conference for the opportunity to present this paper.

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