Metallurgy, engineering codes & welding practices - Stainless Steel ...

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DUPLEX

Duplex Stainless Steel:

Metallurgy, engineering codes & welding practices (Part 1 of 2) Duplex Stainless Steels (DSS) are extremely high grade engineering alloys combining good corrosion resistance with high strength and ease of fabrication. These steels are frequently used in the oil and gas industries both in upstream and downstream applications for assemblies such as pipe work systems, manifolds and risers. Furthermore, these are used in the petrochemical industry in the form of pipelines and pressure vessels. These steels are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, in addition to having a higher strength than standard austenitic stainless steels such as Type 304 or 316, enabling thinner sections to be used and providing significant cost benefits.

By Ramesh Bapat, Senior Principal Engineer, Foster Wheeler Upstream & Pradip Goswami, P. Eng., IWE, Welding and Metallurgical Specialist - Ontario, Canada

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lthough duplex stainless steels are highly corrosion-resistant and oxidation-resistant they cannot be used at elevated temperatures. This is due to the formation of brittle phases in the

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ferrite at relatively low temperatures as subsequently mentioned; these phases have a catastrophic effect on the toughness of the steels. Therefore, the American Society of Mechanical Engineers

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(ASME) pressure vessel codes restrict the service temperature of all grades to below 315°C; other codes specify even lower service temperatures, even as low as 250°C for super duplex steels. www.stainless-steel-world.net

DUPLEX Duplex alloys can be divided into three main groups: lean duplex, 22% Cr duplex and 25% Cr super duplex; recently, a hyper duplex grade has been developed. The division is based primarily on the alloying level in terms of Pitting Resistance Equivalent Number (PREN) which measures the alloy’s resistance for pitting corrosion. This PREN is calculated from a simple formula: PREN = % Cr + 3.3% Mo +16% N; an allowance for W is sometimes made, having a factor of 1.65. Typically, duplex steel has a PREN less than 40; a super duplex PREN is typically between 40 and 45, a hyper duplex is above 45, whilst the lean grades typically have lower nickel content and hence lower price. The metallurgy of the duplex stainless steel family is complex and requires very close control of weld metal composition and heat treatment regimes to ensure that mechanical properties and/or corrosion resistance would not be adversely affected. To produce the optimum mechanical properties and corrosion resistance, the microstructure or phase balance of both the parent and weld metal should be approximately 50% ferrite and 50% austenite. This precise value is impossible to achieve with accuracy, but a range of phase balances between 35 - 60% ferrite and the rest austenite is acceptable in the various industry standards. While composition and, perhaps more importantly, heat treatment parameters are relatively easy to control, this is not the case during welding. The amount of ferrite is dependant not only on composition but also on the cooling rate; fast cooling rates retain more of the ferrite that forms at elevated temperature. Therefore, to minimize the risk of producing very high ferrite levels in the weld metal, it is necessary to ensure that there is a minimum heat input and, therefore, a maximum cooling rate. A rule of thumb is that heat input for duplex and super duplex steels should not be less than 0.5kJ/mm, although thick sections will need this lower limit to be increased.

Introduction Duplex stainless steels are two-phase alloys based on the iron-chromium-nickel (Fe-Cr-Ni) system, comprising of approximately equal amounts of BodyCentered Cubic (BCC) ferrite, -phase, and Face-Centered Cubic (FCC) austenite, -

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phase, in their microstructure. Over last 30 years, duplex stainless steels have seen extremely widespread application in the offshore oil, gas, pulp & paper, power, petrochemical and a host of other industries. An attractive combination of high strength and excellent resistance to general corrosion and various forms of stress corrosion cracking and good weldability makes duplex and super duplex alloys as candidate materials for applications as stated above. Corrosion resistance (both against pitting, chloride) and other forms of Stress Corrosion Cracking (SCC) is the result of the presence of ferrite and austenite phases in the microstructure. In the ‘sweet’ and ‘sour’ corrosive environments for upstream oil and gas environments, duplex stainless steels are the standard design basis materials. In regards to the downstream side for many engineering applications in the petroleum and refining industry, DSS are currently the preferred material over conventional austenitic stainless steels. A newer grade of duplex alloys termed as ‘hyper duplex’ is the latest addition to the duplex family and is accepted by the offshore oil and gas industry. Sandvik SAF 3207 HD (UNS S32707, C-max-0.03, Cr32, Ni-7, Mo- 3.5, N-0.5, PRE- 50) is the alloy which is termed as hyper duplex stainless steel. This new material has a yield strength 20% higher than that of the super duplex stainless steel and a service temperature up to 90°C as an umbilical tubing. Although this alloy is referred to in this article, further details are not provided as this alloy is fairly recent alloy and is the process of establishing credentials in oil and gas industry. In addition to possessing good resistance to general corrosion, pitting and chloride stress corrosion cracking, these steels have high allowable/proof stresses which are beneficial when weight control is important; for example, this application is crucial for topsides plant on offshore platforms, piping and other critical equipments. Also, even at low temperatures, good toughness is exhibited by the parent materials and properly welded weldments, and is of consequence especially when installations are subject to “blowdown.” However, maximum operating temperatures are limited by

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loss of toughness over long periods at temperatures exceeding approximately 300°C. Understanding and development of the steels has proceeded to an extent that substantial increase in use is envisaged, and investigative work continues apace. There has been considerable application experience with DSS and Super Duplex Stainless Steels (SDSS) for items such as downhole and well head tubular, well heads, flow lines, pipelines, manifolds, coolers, heat exchangers, valves and water pumps.

Metallurgy of Duplex Stainless Steels Figure 1 shows a schematic isoplethal section of the Fe-Cr-Ni diagram for an iron content of 68%. As shown in Figure 1, DSS solidify as a fully ferritic structure, which partly transforms to austenite as the temperature falls to approximately 10,000C (18,320F) depending on the composition of the alloys. Throughout the transformation, there is little change in the

˚C

L

˚F

L +γ +α

L +γ

L+α

1400

2552

2192

1200 α

γ

1000

1832 α +

γ

[Ν]

800 %Ni %Cr

1472 5

0 30

15

10 25

20

15

Figure 1: Solidification diagram of duplex Stainless steels Fe-Cr-Ni system at 68% iron(3).

equilibrium ferrite–austenite balance even at lower temperatures. This allows the desired phase balance and the diffusion of all elements closer to their preferred equilibrium positions during initial solidification. Nitrogen promotes austenite formation from the ferrite at a higher temperature. However, as cooling proceeds to lower temperatures, carbides, nitrides, sigma and other intermetallic phases are all possible as micro structural constituents. The austenite/ferrite ratio depends on the chemical composition of the alloy and the

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DUPLEX Table 1: The Role of the Alloying Elements in Duplex Stainless Steels; the roles of various alloying elements in duplex stainless steels are as presented below: Alloying Elements

Role

Effects on Duplex S.S Metallurgy

Chromium

Ferrite stabilizer

• Increase corrosion resistance. • The duplex grades are designed for superior corrosion resistance than ferritic or 304/316 austenitic grades

Molybdenum Ferrite stabilizer

• Together with Cr improves chloride corrosion resistance to stainless steels. • With at least 18%, chromium content, additions of molybdenum become approximately three times as effective as chromium additions against pitting and crevice corrosion in chloridecontaining environments. • Molybdenum increases the tendency DSS to form detrimental intermetallic phases. Typically restricted to 4% in duplex stainless steels

Nickel

Austenite stabilizer

• DSS/SDSS contain an intermediate amount of nickel such as 4 to 7%. • Prevents formation of detrimental intermetallic phases in austenitic stainless steels • Increases notch-toughness of the austenitic stainless steels. • Balances the austenite Ferrite Ratio

Nitrogen

Austenite stabilizer

• Nitrogen is added to offset the effects of chromium and molybdenum contents to form sigma phase. • Increases pitting and crevice corrosion resistance • Substantially increases strength(mechanical properties) and toughness. • The most effective solid solution strengthening element. • Delays formation of intermetallic phases during weldign and fabrication.

Manganese

Austenite stabilizer

The addition of manganese has a controversial effect; they are known to increase the nitrogen solubility and in some respect to stabilize the austenitic phase directly or indirectly. On the contrary, most of the localized, mainly pitting corrosion resistance investigations have underlined a negative effect of manganese additions.

Sulphur

Among the detrimental alloying elements, Sulphur likely has the worst effect. Sulphur is generally combined with manganese or several oxides to form precipitates. The worst case is a large oxide inclusion surrounded by sulphur species. Locally, the passive film is unable to resist and severe local corrosion may start.

heat treatment. Too fast cooling rates during solidification may reduce this retransformation process. Small changes in the nickel and chromium content have a large influence on the amount of austenite and ferrite in duplex stainless steels.

Current density

Si P

Higher alloyed Super Duplex Stainless Steels (SDSS) developed typically in the late 1980s are designed to withstand more aggressive environments, but also bear a higher risk of precipitation unfavorable phases due to the higher alloying element content. SDSSs are

Reducing Conditions

Ni Mo

Mo Ni

Cr Ni Mo W Cu

Cr

Si

Oxidizing Conditions

Cr, Mo, N

Mn, Ni

usually characterized by having a PREN greater than 40. The higher the PREN, the better the predicted corrosion properties of a DSS are. This is an increasingly common specification for certain offshore applications. However, PREN numbers only provide an approximate grading of alloys and do not account for the microstructure of the material. An acceptance corrosion test on material in the supply condition is much more meaningful and a necessity in many offshore applications. The most common way of ranking stainless steels for their PREN according to the relation between the amount of the essential elements and the corrosion properties can be formulated by using this relation: PREN = (%Cr) + (3.3 x %Mo) + (16 x %N)

Activity

Passivity

Transpassivity

Potental

PREN = (%Cr) + (3.3 %Mo) + (0.5 %W) + (16 %N) (Including Tungsten, if any)

Figure 2: Influence of alloying elements on the corrosion resistance (2).

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Figure 3: Iso-corrosion curves, 0.1 mm/year in sulphuric acid.

Figure 4: Iso-corrosion curves, 0.1 mm/year in sulphuric acid with 2000 ppm chloride ions.

Table 2: Examples of environments which SCC of duplex stainless steels may be expected (3). Alloy Type

42% MgCl2 35% MgCl2 40% CaCl2 Boiling 154°C Boiling 125°C 100°C 0.9xY.S. U-Bend U-Bend

25-28% NaCl 26% NaCl 600 ppm Cl Boiling 106°C Autoclave (NaCl) U-Bend 200°C U-Bend Autoclave 300°C U-Bend

100 ppm Cl (Sea Salt+O2) Autoclave 230°C U-Bend

S.S 304L/316L Sandvik 3RE60 Duplex 2205 25 Cr Duplex Super Duplex Cracking anticipated Cracking not anticipated Insufficient data least 12% chromium, which is true with all types of DSS and ASME or ASTM Specifications Product Form SDSS. Duplex stainless steels exhibiting high level of corrosion SA-240 Plate, Sheet resistance in most corrosive SA-479, A276 Bar Products environments are often used in lieu of austenitic SS. Such SA-790, A928 Pipe applications are common where SA-789 Tubing austenitic stainless steels would SA-815 Fittings have problems with chloride pitting or chloride SCC. SA-182 Forgings High chromium content, which is SA-351, A890, A995 Castings beneficial in oxidizing acids, along with sufficient ASTM A923 Testing molybdenum and nickel to provide resistance in mildly reducing acid Corrosion resistance environments, provides a balanced Stainless steels are corrosion resistant due corrosion resistance to DSS and SDSS. to formation of an invisible, 2-4 nm thick, The relatively high chromium, molybdenum passive film established in oxidizing and nitrogen also give them very good environments when the steel contains at resistance to chloride pitting and crevice Table 3: ASME and ASTM specifications for DSS (1).

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corrosion. Due to presence of at least 2530% ferrite, DSS/SDSS have an advantage in potential CSCC environments over Types 304 or 316.

References 1.

2.

3.

API Technical Report 938-C. 2005. Use of Duplex Stainless Steels in the Oil Refining Industry The History of Duplex Developments. J. Charles and P.Chemelle. 8th Duplex Stainless Steels Conference. 13-15 October 2010. Beaune, France The Physical Metallurgy of Duplex Stainless Steels. J.O. Nilsson and G. Chai. Sandvik Materials Technology

Be sure to catch Part Two of this technical article in the upcoming January/February issue of Stainless Steel World.

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DUPLEX

Ramesh Bapat Ramesh Bapat has 44 years of diversified experience as a Materials and Metallurgical Engineer in automotive, aerospace and oil and gas industries following under graduate (BS) in Metallurgical Engineering in 1969 from University Of Pune in India. Ramesh completed MS in Material Science from Arizona State University in 1994. He served for one year as a RAND Fellow as a Science & Technology Policy Fellow to support The White House Office of Science & Technology Policy (OSTP) through RAND Think Tank in 2000 to 2001 and his employer gave him a leave of absence to complete this Prestigious Fellowship. Ramesh is a Licensed Professional Engineer (PE) and also a Certified Manufacturing Engineer (CMfgE). He served as a Chief Engineer at FMC Technologies Inc. and at this time he is working as a Senior Principal Engineer in Foster Wheeler Corporation. Ramesh is an active member of various Professional Organizations and in 2010 was inducted into the "Hall of Fame" by the Training Dept. of FMC Technologies for teaching various engineering classes to their employees.

Pradip Goswami Pradip Goswami has over 29 years of experience as a Welding & Metallurgical Engineer/Specialist in Power Generation, Engineering activities in Oil and Gas Industries, Pressure Vessel Fabrication Industry. Following completion of Masters in Welding & Metallurgical Engineering from Indian Institute of Technology in 1983, he gained extensive knowledge & experience in the in the above mentioned industries. Mr. Goswami has the following professional credentials:- Registered Professional Engineer in Ontario Canada, International Welding Engineer (IWE), Canadian Welding Bureau Certified Welding Engineer – “Welding Procedure & Practices and keeps himself always up-to-date with the latest developments in areas related to welding –metallurgy-corrosion- NDE through active participation in various professional forums such as NACE Professional Networking, LinkedIn, Eng-Tips.