New hyper duplex stainless steels Guocai Chai1 and Pasi Kangas 2,

1R&D centre, Sandvik Materials Technology, 811 81 Sandviken, Sweden 2PA Wire and Heating Technology, 811 81 Sandviken, Sweden Abstract. Hyper duplex stainless steels, HDSS, are a new group of duplex stainless steels with PRE-values

higher than 48. These duplex stainless steels show both highest corrosion pitting resistance and highest

strength among the existing modern DSS. Two newly developed grades are Sandvik SAF 2707 HD and

Sandvik SAF 3207 HD. This paper will provide a general description of the main characters of these two

materials. Their corrosion and mechanical properties are compared with those of the super duplex stainless

steels. Their applications in the chemical and petrochemical industries, oil refinery, subsea or deep subsea

fields and other potential areas are analyzed and discussed. 1 Introduction Since the launch of the first duplex steel in the 1960s, duplex stainless steels are today an important group of stainless steels, and regarded as workhorses in many applications. Several generations of commercial duplex stainless steels have been developed. In the middle of 1980’s, several highly alloyed duplex stainless steels, such as UNS S32750, S32760 and S32520, were introduced. Since the PRE-values of these alloys are higher than 40, they were designated as super duplex stainless steels, SDSS [1, 2]. These alloys show an attractive combination of excellent corrosion resistance, high mechanical properties and good weldability. They have been widely used in the industries such as oil and gas, petrochemical and chemical processing since then, and have become an attractive alternative to other high performance materials such as super austenitic stainless steels and Ni-based alloys. Despite the very successful applications or experiences of super duplex stainless steels during last 15 years, the development in highly alloyed duplex stainless steels has been very active because there are still areas where the corrosion resistance of these alloys is insufficient for a long service life or at higher temperatures. This indicates that there are desires for new duplex stainless steel alloys with higher corrosion resistances. In other areas, new duplex stainless steels with a combination of excellent corrosion resistance and even higher strength are needed. For these challenges, two new highly alloyed duplex stainless steels, UNS S32707 (Sandvik SAF 2707HD) and UNS S33207 (Sandvik SAF 3207HD), have recently been developed [3-5]. The nitrogen content in these alloys is now up to 0,5%. They have PRE-values close to 50. These new alloys show both highest pitting corrosion resistance and highest strength among the existing modern DSS. They have also good fabricability and weldability, and have now been designated as hyper duplex stainless steel, HDSS. This paper will therefore provide a general description of these two hyper duplex stainless steels and their applications.

2 Development of hyper duplex stainless steels

In order to improve corrosion resistance and strength, modern duplex stainless steels have been alloyed with Cr, Mo/W, Ni and N. The challenge to develop new highly alloyed duplex stainless steels by increasing these alloying elements is to balance the alloying level to control the risk for formation of intermetallic phase such as sigma phase (Fig 1a) or nitride (Fig 1b) which increases with increasing content of elements such as Cr and Mo or N in highly alloyed duplex stainless steels.

Fig 1. Influence of Cr and N contents on the formation of sigma and Cr2N at 900°C, (a). In 0.03C-0.6Si-1.5Mn-

4Mo-7Ni-0.4N-xCr system, (b). In 0.03C-0.6Si-1.5Mn-4Mo-7Ni-30Cr-xN system.

During the development of hyper duplex stainless steels, however, it was found that addition of the elements such as Cr, Mo and N to high levels simultaneously can have an unexpected positive synergistic effects. From this study, it was found that the solid solution content of N in a duplex stainless steel can be much higher than expected. Fig 2a shows the influence of addition of Cr, Mo and N simultaneously on the temperature for nitride solution or formation. The influence of variation or increase of Mo and Cr contents on this temperature is relatively small. Low Cr and Mo contents in the alloy do not correspond to a low temperature for nitride formation. This shows that increase in amounts of Cr and Mo can increase the solubility of N.

As shown in Fig 1b, increase in nitrogen can suppress the precipitation of sigma phase. This phenomenon can be observed in a wide range of chromium contents in duplex stainless steels (Fig 2b).

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in the matrix, (b). Influence of nitrogen content on the formation of sigma phase in highly alloyed duplex stainless steels.

The synergistic effects of the alloying elements Cr, Mo and N in highly alloyed duplex stainless steels can facilitate the possibility to manufacture the alloys into various product forms. However, there are some limitations. Fig 3 shows the influence of solubility of N and Cr in the austenitic phases of highly alloyed duplex stainless steels on the ductility of the alloys. With a combination of an N content of less than about 0.7% and a Cr content of less than about 31% in the austenitic phase, the duplex stainless steel alloys can obtain good ductility and also be easily manufactured.

Fig 3. Influence of synergistic effect of the alloying elements Cr and N on the ductility of the highly alloyed

duplex stainless steels.

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Table 1 shows the nominal chemical compositions of the hyper duplex stainless steel grades: UNS S32707 (Sandvik SAF 2707HD) and UNS S33207 (Sandvik SAF 3207HD), and super duplex stainless steel grade: UNS S32750 (Sandvik SAF 2507). Sandvik SAF 3207 HD contains highest amounts of the alloying elements. The nitrogen content is up to 0.5%.

Table 1. Nominal chemical compositions and PRE values of three duplex stainless steels (wt_ %)

Grade UNS C_max Cr Ni Mo N PRE*

Sandvik SAF 2507 S32750 0.03 25 7 4 0.3 42.5 Sandvik SAF 2707 HD S32707 0.03 27 7 5 0.4 48 Sandvik SAF 3207 HD S33207 0.03 32 7 3.5 0.5 50

*minimum PRE value for tube materials For duplex stainless steels, the pitting corrosion resistance is proportional to pitting corrosion resistance equivalent value, PRE, of an alloy, which can be calculated as follows:

PRE= %Cr + 3.3%Mo + 16%N (% by weight) (1)

The pitting corrosion resistance equivalent values, PRE, of these three alloys are also shown in Table 1. SAF 3207 HD has a minimum PRE value of 50 and SAF 2707 HD has a minimum PRE value of 48, comparing with 42.5 on SAF 2507. A duplex stainless steel with a PRE value above 48 is nowadays designated as hyper duplex stainless steel, HDSS [3, 5]. Duplex stainless steels UNS S33207 and UNS S32707 are therefore hyper duplex stainless steels.

3 Corrosion properties Fig 4a shows a comparison of the critical pitting temperatures, CPT, of these three DSS, determined in 6% FeCl3, the ASTM G48A test solution. As known, the pitting corrosion resistance of duplex stainless steels is related to the PRE values of the alloys. Generally, it is true. However, SAF 3207 HD shows slightly lower CPT than SAF 2707 HD although SAF 3207 HD has a slightly larger PRE value. Fig 4a also shows a comparison of the critical crevice corrosion temperatures, CCT, of these materials, determined with the MTI-2 crevice former. SAF 3207 HD and SAF 2707 HD show similar CCT. As expected, hyper duplex stainless steels show much higher both CPT and CCT than super duplex stainless steel SAF 2507. This phenomenon is more apparent in the concentrated sodium chloride solution measured using potentiostatic tests at +600mV (Fig 4b).

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Fig 4. Comparison of CPT and CCT between super and hyper duplex stainless steels; (a). Critical temperature

assessed using modified G-48A and MTI-2 testing, (b). Critical pitting temperatures (CPT) at varying concentrations of sodium chloride, from 3 to 25% (potentiostatic determination at +600mV SCE with surface

ground with 220 grit paper). Stress corrosion cracking, SCC, is perhaps the most serious form of corrosion encountered in industrial processes as it can lead to rapid material failure. Fig 5 shows a comparison of the stress corrosion cracking resistances of the hyper duplex stainless steel and the austenitic stainless steels. The SCC resistance of hyper duplex stainless steel in chloride solutions at high temperatures is much higher than those of the compared austenitic stainless steels. There were no signs of SCC in the hyper duplex stainless steel in the 1000 ppm Cl- solution at up to 300°C and in the 10000 ppm Cl- solution at up to 250°C. The standard austenitic grades AISI 304 and 316 can crack at very low chloride levels. The austenitic steels in many cases become more resistant at Ni levels above 25% (Sanicro 28 in the Fig 5). The hyper duplex alloys, however, have even higher resistance due to its dual phase structure.

Fig 5. Comparison of stress corrosion cracking resistance of hyper duplex stainless steel and austenitic stainless steels. The tests were performed in an autoclave, where the samples were loaded to the yield strength level. The pressure was approximately 100 bar, Oxygen content was 8 ppm. Fresh NaCl solution was pumped constantly

into the chamber and duration of the test was 1000h. Sanicro 28 = UNS N08028. For hyper DSS SAF 2707HD, no failure after 1000 hours was observed.

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4 Mechanical properties 4.1 Tensile properties Due to high amounts of alloying elements, hyper duplex stainless steels show very high tensile properties. Actually, hyper duplex stainless steels SAF 3207 HD in the quenched - annealed condition shows the highest strength among the existing duplex stainless steels. It is more than 20% higher than that of super duplex Sandvik SAF 2507. Fig 6a shows the influence of temperature on the 0.2% proof strength of two hyper duplex stainless steels, SAF 2707 HD and SAF 3207 HD and super duplex stainless steel SAF 2507 tube materials with a wall thickness of up to 4 mm. The strength slightly decreases with increasing temperature.

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versus 0.2% proof strength of SAF 3207 HD tube materials. In spite of high strength, hyper duplex stainless steels still show high ductility. The elongation of the material in the quenched - annealed condition is higher than 20% (Fig 6b). It is interesting to mention that the elongation of SAF 2707 HD tube materials changes little with increasing the strength as shown in Fig 6b. This can be attributed to the very fine grained austenitic and ferritic phases in the materials as shown in Fig 7.

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Fig 7 (a). Microstructure of hyper DSS SAF 3207 HD tube material with a dimension of 14.7x1 mm. The white phase is austenite, and the grey phase is ferrite, (b). Grain structure, color grains are austenitic phase (3.6µm),

and grey grains are ferritic phase (5.1µm)

4.2 Impact toughness Impact toughness is the amount of energy required to fracture a material at high velocity by impact testing. This method can be used to predict the likelihood of brittle fracture. Fig 8 shows the change in impact toughness of two hyper duplex stainless steels SAF 2707 HD and SAF 3207 HD materials at various testing temperatures. Both grades show very high impact toughness in the temperature range used. The ductile to brittle transition temperature is below -50°C. It seems that although the tensile strength of hyper SAF 2707 HD is higher than that of super DSS S32750, but their impact toughness are comparable (Fig 8a)

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(Charpy-V 10x10mm) in the longitudinal directions. The arrows mean that the values should be higher because this is the maximum value or limit of the impact tester, and (b). SAF 3207 HD in the longitudinal direction,

(Charpy-V 10x5mm), 4.2 Fracture toughness Fracture toughness is the resistance of a material containing pre-existing sharp cracks to fracture under conditions of crack tip plane strain or stress. For duplex stainless steels, fracture toughness is usually evaluated through the fracture toughness testing such as CTOD (crack-tip opening

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displacement) [6, 7]. Fig 9a shows a comparison of the CTOD values of the hyper and super duplex stainless steels. Hyper DSS SAF 2707 HD has a CTOD value that is even higher than that of super DSS SAF 2507. Hyper DSS SAF 3207 HD shows also a high CTOD value. It should be mentioned that CTOD value depends largely on the thickness and geometry of the testing specimen. The CTOD value decreases with increasing specimen thickness. Fig 9b shows the influence of specimen thickness on the CTOD requirements [8] for the super or hyper duplex stainless steel plate materials with surface flaws. It seems that the CTOD values of the hyper duplex stainless steels are much higher than the CTOD requirements.

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Fig 9. (a). A comparison of CTOD values of hyper and super duplex grades, (b). CTOD requirements for the

plate materials with surface flaws. 4.3 Fatigue properties Fatigue is the progressive, localised, permanent microstructure change in a material suffering fluctuating stresses that are usually much lower than the static tensile strengths. The fatigue limit or endurance and fatigue crack growth are the most important fatigue properties to be considered. Fig 10 shows the fatigue and corrosion fatigue properties of the hyper and super duplex grades. Hyper duplex stainless steels show higher fatigue strength or limit than super duplex stainless steel (Fig 10a). This is mainly due to the fact that hyper duplex grades have higher tensile strength and similar elongation. Since hyper duplex stainless steels show excellent corrosion resistance, the influence of artificial seawater on the fatigue life of hyper duplex stainless steel materials is relatively small. Therefore, they also show higher corrosion fatigue strength than super duplex stainless steel (Fig 10b).

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Fig 10. Fatigue properties of hyper and super duplex stainless steels evaluated using WohlerFullA. The line after 2x106 cycles indicates a fatigue endurance or strength at 2x106 cycles; (a). HCF in

air, tubes, (b). Corrosion fatigue in artificial seawater, tubes.

5 Weldability A welding consumable designated Sandvik 27.9.5.L has been specially developed for welding hyper duplex stainless steels [9]. High nitrogen content in the material gives a rapid austenite formation during welding. A high amount of nickel content will promote austenite reformation. It is important to control the ferrite content in order to obtain a microstructure free from intermetallic precipitates, and it should generally be that the weld metal and HAZ have a ferrite content in the range of 30-70%. Fig 11 shows the microstructures in weld metal and heat affected zone (HAZ) of hyper DSS S32707 (SAF 2707HD). The morphology and phase balance are typical one pass welds in duplex stainless steels. There are no visible intermetallic precipitates in the weld or HAZ.

Fig 11. Microstructures of the welded SAF 2707HD tube material, (a). In centre of weld metal, (b). In HAZ and

fusion line [9] The results from extensive welding trials show that the weldability of hyper duplex stainless steels are good. Both welded joints and overlay welds show good strength and ductility, and high pitting resistance.

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6 Applications Extremely high critical pitting temperature, CPT, and critical crevice corrosion temperature, CCT, of hyper duplex stainless steels allow the materials to be used in the areas where high corrosion resistance and high service temperature are required (Fig 12). The good combination of extra high strength and high ductility makes it possible for hyper duplex stainless steels to allow substantial reduction in wall thickness, which leads to a reduction of weight in applications such as ultra-deep seawater, energy and refinery sectors. In all these situations the alloy’s superior properties can be fully utilized to ensure reliability and safe service [10, 11]. The followings are some examples.

Fig 12. Possible application areas of hyper duplex stainless steels

6.1 Chemical and petrochemical industry In condensers cooled with seawater there is a risk of chloride corrosion, pitting and crevice corrosion and stress corrosion cracking, from the seawater side. In a condenser cooled with seawater at an average temperature of 30°C on the tube side and a shell side with a temperature of ~70°C, super DSS S32750 due to chloride corrosion can fail after one year. Hyper DSS Sandvik SAF2707 HD has been installed in several units since the beginning of 2008 and has been performing well so far. In a seawater cooled ethylene oxide condenser the seawater outlet temperature was 40°C. The shell side of the condenser had a temperature of 132°C. Al-Brass tubing has failed after 2 years due to crevice corrosion. In 2005, Sandvik SAF2707 HD was installed and has performed well.

Another application is reboilers where chemicals are heated by steam at 104°C and the chemicals may contain solids and chlorides and other corrosive species. Super DSS S32750 has suffered from erosion corrosion. S32707 was chosen as a replacement and was installed in 2009. 6.2 Oil refinery In oil refineries overhead condensers in crude distillation units contain hydrochloric acid condensate that is formed when the overhead gases are cooled down. As cooling media seawater may be used and this may cause corrosion problems on both tube side and shell side. Sandvik SAF2707 HD has been proven to withstand corrosion from both the condensate and the seawater side.

In a refinery where the seawater temperature on the tube side was up to 39°C and overhead gases were cooled down and condensed from 140°C at inlet to 45°C at outlet there was a problem with hydrochloric acid corrosion from the overhead gases on Al-brass tubing. Sandvik SAF2707 HD tubes were installed in March 2009 and were in a perfect shape when inspected after one year in operation.

Other areas where Sandvik SAF2707 HD has been installed is in crude oil preheaters where crude oil with chloride contaminations is preheated at 120°C. 6.3 Subsea or deep subsea applications In steel tube umbilical applications deeper waters and in some cases higher temperatures call for stronger and more corrosion resistant materials than existing duplex and superduplex stainless steels can handle reliably. Hyper DSS Sandvik SAF 3207 HD has been developed as a problem solver for deep wells and corrosive conditions. The proof strength of typical umbilical sizes is roughly 20% higher compared to the commonly used UNS S32750, which means lower weight for long umbilicals and the ability to withstand higher external pressure in deep sea applications.

Another application of Sandvik SAF 3207 HD is raw seawater injection where untreated seawater is injected into the well in order to replace the retrieved oil and hence increase the output from the well. The string shall support its own weight and threaded connections in combination with warm seawater which results in that very good crevice corrosion resistance in combination with high mechanical strength is of vital importance. Sandvik SAF 3207 HD is an excellent solution for such application.

Other potential applications for Sandvik SAF3207 HD are for hydraulic lines and in production tubing.

Summary With positive synergistic effects of the elements Cr, Mo and N, it is possible to develop highly alloyed duplex stainless steels. The hyper duplex stainless steels with high contents of Cr, Mo and N together have a clean microstructure, good properties and simultaneously a good fabricability. This is a major technology leap for the development of high alloyed duplex stainless steels.

Hyper duplex stainless steels have the highest critical pitting temperature, CPT, and critical crevice corrosion temperature, CCT, among the modern duplex stainless steels. Hyper duplex stainless steels have the highest tensile and fatigue strengths among the modern duplex stainless steels. Hyper duplex stainless steels have a good weldability and high toughness. With a good combination of extreme high corrosion resistance and strength, hyper duplex stainless steels have been successfully used in the chemical and petrochemical industry and oil refinery, and can be used in the areas where even higher corrosion resistance and service temperature or high strength/weight ratios are required.

Acknowledgements This paper is published by permission of Sandvik Materials Technology. The supports from Mr M. Klockars and Ms S. Åkesson are acknowledged. The contributions from Mr P. Stenvall and Dr U. Kivisäkk are appreciated. References 1. J. Charles, Proc. Conference on DSS ’91, Les Ulis, France, Les Editions de Physique, 3 (1991) 2. J-O Nilsson, Mater. Sci. and Tech., 8, 685 (1992). 3. K. Göransson, M-L Nyman, M. Holmquist and E. Gomes, Sandvik SAF 2707 HD, Sandvik SAF

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