WO2009044135A2

WO2009044135A2 - Duplex stainless steel casting alloy composition

Info

Publication number: WO2009044135A2
Application number: PCT/GB2008/003329
Authority: WO
Inventors: Stanley Alan Hebdon; Roger Francis
Original assignee: Weir Materials Ltd
Priority date: 2007-10-03
Filing date: 2008-10-02
Publication date: 2009-04-09
Also published as: GB0719288D0; GB0809316D0; WO2009044135A3

Abstract

The present invention provides a duplex stainless steel casting alloy composition comprising, in weight percent: up to around 0.05 % carbon; around 0.5 % to 1.2 % silicon; around 1 % to 2 % manganese; around 3.7 % to 4.5 % nickel; around 21 % to 23 % chromium; around 1.5 % to 2.5 % molybdenum; around 0.15 % to 0.25 % nitrogen; and balance iron and incidental impurities.

Description

DUPLEX STAINLESS STEEL CASTING ALLOY COMPOSITION

The present invention relates to a duplex stainless steel casting alloy composition. Stainless steels, iron-carbon alloys containing at least 10.5 % chromium, are employed in a great many applications where particular properties, most commonly corrosion and oxidation resistance are required. A large number of different stainless steel alloys are available for use in different environments. A large proportion of all stainless steel produced is Austenitic, or 300 series, stainless steel. A feature common to all Austenitic stainless steels is that they must contain sufficient nickel and/or manganese to retain an Austenitic structure at all temperatures from cryogenic regions to the melting point of the alloy. There are however a number of disadvantages associated with Austenitic stainless steels, such as 316 and 316L. For example, nickel and, to a lesser extent, manganese are both relatively expensive materials and so the relatively high nickel and/or manganese content required in Austenitic steel alloys provides a significant case disadvantage for steels of this kind. Moreover, the corrosion resistance and strength of, for example 316 and 316L, stainless steels may not be sufficiently high for many potential applications. It is therefore desirable to produce a cheaper stainless steel alloy exhibiting greater corrosion resistance and higher strength than conventional Austenitic stainless steel alloys.

Duplex stainless steel alloys have been developed in order to address some of the above disadvantages associated with Austenitic stainless steels. Duplex stainless steel alloys possess a mixed microstructure of austenite and ferrite. Duplex stainless steels typically exhibit greater strength and improved corrosion resistance compared to Austenitic stainless steels. Duplex stainless steels are typically characterised by relatively high chromium and molybdenum contents and lower nickel contents as compared to typical Austenitic stainless steels. Unfortunately, while duplex stainless steel alloys have been developed for use in wrought product forms, significant further development is still required in order to optimise the properties of such alloys for use in certain applications and/or for use to produce cast products. An object of the present invention is to provide a duplex stainless steel alloy which overcomes one or more of the problems associated with known stainless steel alloys.

The alloy composition according to the present invention has been optimised to provide enhanced corrosion resistance and strength compared to known duplex stainless steel alloys and Austenitic stainless steel alloys. Moreover, it will be appreciated by the skilled person that the above alloy composition is eminently suitable for use as a casting alloy. In particular, the alloy according to the present invention possesses a stable and balanced composition which provides a high degree of castability in casting applications together with a balanced microstructure which ensures superior corrosion resistance and strength compared to, for example, 316L type Austenitic stainless steels.

The present invention further provides a method for producing a cast product, said method comprising introducing a duplex stainless steel alloy composition into a mould, causing said composition to at least partly solidify within said mould, and removing said at least partly solidified composition from said mould, said alloy composition comprising, in weight percent: up to around 0.05 % carbon; around 0.5 % to 1.2 % silicon; around 1 % to 2 % manganese; around 3.7 % to 4.5 % nickel; around 21 % to 23 % chromium; around 1.5 % to 2.5 % molybdenum; around 0.15 % to 0.25 % nitrogen; and balance iron and incidental impurities.

There is further provided according to the present invention a cast product obtained by casting a duplex stainless steel casting alloy composition comprising, in weight percent: up to around 0.05 % carbon; around 0.5 % to 1.2 % silicon; around 1 % to 2 % manganese; around 3.7 % to 4.5 % nickel; around 21 % to 23 % chromium; around 1.5 % to 2.5 % molybdenum; around 0.15 % to 0.25 % nitrogen; and balance iron and incidental impurities. It is preferred that the alloy composition of the present invention comprises around 0.02 to 0.05 weight percent carbon, more preferably 0.03 to 0.05 weight percent carbon. Most preferably the alloy composition of the present invention comprises around 0.03 weight percent carbon. The skilled person will appreciate that the upper limit for the carbon content of the alloy composition according to the present invention is higher than many conventional stainless steel alloys. Incorporating higher than usual amounts of carbon in the alloy reduces the overall cost of the alloy since it allows the use of a wider range of cheaper melting stock to be employed in the production of the alloy composition, without causing detriment to the strength or corrosion resistance of the alloy composition. As mentioned above, the silicon content of the duplex stainless steel alloy composition may be around 0.5 to 1.2 weight percent. More preferably the alloy composition comprises around 0.5 to 1 weight percent silicon. It is particularly preferred that the alloy composition comprises no less than 0.5 weight percent silicon to avoid deoxidation problems, which may occur on employing the alloy in casting applications. Moreover, ensuring the silicon content is no lower than around 0.5 weight percent contributes to ensuring that the appropriate balance between austenite and ferrite phases is obtained. Most preferably the silicon content contained in the alloy composition of the first aspect of the present invention is around 0.8 weight percent.

With regard to the manganese content of the alloy composition of the present invention, it is preferred that the alloy comprises at least 1.0 weight percent manganese, preferably around 1.25 to 2 weight percent manganese, more preferably around 1.25 to 1.75 weight percent manganese. Most preferably the manganese content of the alloy of the first aspect of the present invention is around 1.5 weight percent. The aforementioned ranges for the manganese content of the alloy are important in ensuring the correct phase balance of the alloy composition (increasing the amount of manganese increases the austenite phase content of the alloy, but to a lesser extent than increasing the amount of nitrogen or nickel) and for ensuring that the nitrogen present in the composition is held in solution, which is important for providing good casting properties.

The nickel content of the alloy composition of the present invention is important since it is related to the phase balance (increasing the amount of nickel increases the austenite phase content of the alloy) and strength of the austenite phase of a final product incorporating the alloy composition. It will be appreciated from the definition of the present invention set out above, that the range of nickel present in the alloy composition is lower than many common duplex stainless steel alloys. In view of the relatively high cost of nickel, it will be appreciated that this fact has a benefit in terms of reducing the cost of the alloy composition. Preferably the amount of nickel present in the alloy composition of the present invention is around 4.0 to 4.5 weight percent, more preferably around 4.1 to 4.3 weight percent. Most preferably the alloy composition of the present invention comprises around 4.2 weight percent nickel. The alloy composition of the present invention may contain around 21.0 to

23.0 weight percent chromium as set out above in respect of the present invention. This level of chromium contributes to pitting corrosion resistance and ensures the appropriate phase balance between austenite and ferrite phases (increasing the amount of chromium increases the ferrite phase content of the alloy). Thus, while the chromium content may be in the range of around 21 to 23 weight percent, the chromium content is more preferably in the range around 21 to 22 weight percent and most preferably around 21.5 weight percent. Molybdenum is a relatively expensive element and so from a cost point of view it is desirable to minimise the amount of molybdenum present in the alloy composition of the present invention. Molybdenum does however positively influence the pitting corrosion resistance and phase balance stability of the alloy composition (increasing the amount of molybdenum increases the ferrite phase content of the alloy). It is therefore preferred that the alloy composition of the present invention comprises a minimum of around 1.7 weight percent, more preferably at least around 1.8 weight percent molybdenum. Still more preferably the alloy composition of the present invention comprises around 1.75 to 2.25 weight percent, more preferably around 1.9 to 2 weight percent, and most preferably around 1.95 weight percent molybdenum.

The presence of nitrogen in the alloy composition of the present invention provides a number of important benefits. For example, providing the alloy composition with the appropriate amount of nitrogen contributes to improved corrosion resistance, phase balance (increased nitrogen results in increased austenite phase content) and the strength of the austenite phase in a product incorporating the alloy composition. Thus, while the amount of nitrogen present in the alloy composition may be around 0.15 to 0.25 weight percent as set out above, it is preferred that the alloy composition comprises around 0.15 to 0.18 weight percent nitrogen and yet more preferably around 0.15 to 0.2 weight percent. Most preferably, the alloy composition comprises around 0.155 weight percent nitrogen.

The alloy composition according to the present invention preferably comprises austenite and ferrite phases. The phase balance influences the mechanical and corrosion properties of the alloy. Increasing the proportion of the ferrite phase increases the strength of the alloy but reduces the impact toughness, whereas increasing the proportion of the austenite phase in the alloy increases the toughness of the alloy but reduces its tensile and proof strength. The ferrite phase may represent around 40 to 60 weight percent of the alloy composition, with the balance made up of austenite. Alternatively, the alloy composition may comprise around 40 to 60 weight percent of the austenite phase with the balance ferrite. If the proportion of one of the two phases falls below 40 weight percent or exceeds 60 weight percent then this can adversely affect the corrosion resistance of the alloy. Preferably the alloy composition comprises approximately equal amounts of the austenite and ferrite phases and most preferably around 50 weight percent ferrite and around 50 weight percent austenite phases. It is particularly preferred that the alloy composition is a two-phase alloy containing only austenite and ferrite phases, and does not contain any other phases, such as sigma phases, which could compromise the mechanical and/or corrosion properties of the alloy.

In order to ensure that the microstructure of the cast alloy of the present invention contains only two phases (austenite and ferrite), and does not contain any undesirable nitrides or third phases which might compromise corrosion resistance and toughness properties, it is preferred that the alloy composition is subjected to an appropriate heat treatment process. In a preferred embodiment of the casting method forming one aspect of the present invention the alloy composition of the present invention is solution annealed and, preferably, water quenched. Preferably said solution annealing is carried out by heating the alloy to a temperature of around 1000 °C to around 1150 °C for a sufficient period of time to dissolve all undesirable carbides into the austenitic phase. It is preferred that the alloy is then cooled sufficiently quickly, preferably by rapid water quenching, to ensure that the dissolved carbon atoms do not have sufficient time to precipitate out and thereby remain in solution. As well as ensuring that the cast alloy produced in this way possesses only austenite and ferrite phases, and does not contain any undesirable nitrides, providing an alloy composition that can be processed in this way, as well as a method incorporating these steps, makes the alloy and the method eminently suitable for use in relation to applications subject to the Norwegian NORSOK rules which specify that an alloy must be solution annealed and water quenched, and that the microstructure of the alloy must be free from precipitates and third phases.

According to a still further aspect of the present invention there is provided a duplex stainless steel casting alloy composition comprising, in weight percent: up to around 0.03 % carbon; around 0.8 % silicon; around 1.5 % manganese; around 4.2 % nickel; around 21.5 % chromium; around 1.95 % molybdenum; around 0.155 % nitrogen; and balance iron and incidental impurities.

The alloy composition of the present invention may further comprise copper and/or tungsten. The inclusion of copper in the alloy composition of the present invention may contribute to corrosion resistance, and in particular corrosion resistance to sulphuric acid which can be of importance in certain applications. The alloy composition may comprise around 0.2 to 1 weight percent copper, more preferably around 0.2 to 0.6 weight percent copper. Most preferably the alloy composition comprises around 0.4 weight percent copper. With regard to tungsten, this element may contribute to improved pitting corrosion resistance and phase balance. It has also been observed that the presence of tungsten may aid re-passivation of a surface of a final product incorporating the alloy composition of the present invention. The alloy composition may comprise around 0.2 to 1 weight percent tungsten, more preferably around 0.2 to 0.5 weight percent tungsten. Yet more preferably the alloy composition comprises around 0.2 to 0.25 weight percent tungsten and it is most preferred that the alloy composition comprises around 0.2 weight percent tungsten.

The alloy composition of the present invention may further comprise sulphur and/or phosphorous. Preferably the sulphur content of the alloy composition is no greater than around 0.02 weight percent, and more preferably falls in the range around

0.005 to 0.02 weight percent. It is most preferred that the sulphur content of the alloy composition is no greater than around 0.01 weight percent. It is preferred that the maximum amount of phosphorous in the alloy composition of the present invention is around 0.025 weight percent, more preferably in the range around 0.01 to 0.025 weight percent. Most preferably the alloy composition comprises no more than around 0.02 weight percent phosphorous.

The presence of aluminium in the alloy composition of the present invention may have a negative effect on the impact properties of a product incorporating the alloy composition of the present invention. Moreover, limiting the amount of aluminium present in the alloy composition should improve the toughness of products incorporating the alloy composition and prevent the formation of undesirable nitrides in the alloy composition, which might compromise corrosion resistance and toughness properties. For the aforementioned reasons, it is therefore preferable to retain as little aluminium as possible in the alloy composition. To this end, it is preferred that the alloy composition comprises no more than around 0.01 weight percent aluminium, more preferably no more than around 0.005 weight percent aluminium. In addition to the importance of including certain elements within the alloy composition of the present invention, as specified above, it is preferred that the presence of certain other elements is minimised, and as far as practicable, expressly excluded. It is particularly preferred that the alloy composition of the present invention has not been subjected to the deliberate addition of one or more elements selected from the group consisting of titanium, magnesium, vanadiuim, niobium, cobalt and any rare earth element, namely scandium, yttrium, and the fifteen lanthanides. It is thus preferred that the alloy composition of the present invention contains essentially none of one or more of the elements selected from the group consisting of titanium, magnesium, vanadiuim, niobium, cobalt and any rare earth element.

It is preferred that the alloy composition of the present invention exhibits a 0.2 % proof strength of at least around 420 MPa, more preferably around 440 MPa. It is preferred that the ultimate tensile strength (UTS) of the alloy composition is at least around 650 MPa. With regard to impact toughness (Charpy V notch) of the alloy composition, it is preferred that this is at least 35 to 45 J at -46 °C. More preferably, the impact toughness is in the range 50 to 85 J at -46 °C and most preferably around 80 to 85 J at -46 ⁰C.

With regard to pitting corrosion resistance, it is preferred that the alloy composition of the present invention exhibits sufficient corrosion resistance to pass the ASTM G 48 "A" standard test at 20 °C over a 24 hour period.

It is preferred that the alloy composition of the present invention possesses a Pitting Resistance Equivalent Number (PREN) of at least around 28 or 29, more preferably a PREN of at least around 30. The PREN of an alloy composition is calculated as (wt.% chromium) + (3.3 x wt.% molybdenum) + (16 x wt. % nitrogen). The NACE MR-0175 / ISO 15156 Part 3 standard specifies that solution annealed duplex stainless steels with a PREN between 30 and 40 are satisfactory up to 0.1 bar H₂S. Therefore, preferred alloy compositions possessing a minimum PREN of around 30 are suitable for applications to which the NACE MR-Ol 75 / ISO 15156 Part 3 standard applies. The alloy composition of the present invention may possess a PREN of around 30 to 35, more preferably around 30 to 33, and most preferably a PREN of around 30 to 31.

The invention will now be further described with reference to the following non-limiting Example, in which, Figure 1 is a graph of critical crevice corrosion temperature as a function of chloride content of a solution in which a test sample of the alloy composition of the present invention is immersed.

EXAMPLE

Three test samples of a duplex stainless steel casting alloy composition of the present invention were prepared according to the elemental specification shown below in Table 1.

Element Range/Limit (vveiuht %)

TABLE 1

The elemental compositions of the three test samples were as shown below in Table 2:

Element Melt Analysis : Corrosion test samples (weight percent)

TABLE 2 CHLORIDE SCC

C-rings made of the alloy composition of the present invention were stressed to 100 % of the actual 0.2 % proof stress according to EFC 17 (European Federation of Corrosion - "A WORKING PARTY REPORT ON: Corrosion Resistant Alloys for

Oil and Gas Production: Guidance on general requirements and test methods for

Hydrogen Sulphide service").

When tested in boiling 25 weight percent sodium chloride at pH~6, the alloy composition of the present invention showed no cracks after 1,000 hours. 316L austenitic steel alloy samples cracked after 504 to 840 hours.

SULPHIDE SCC

4-point bent beams made of the alloy composition of the present invention were stressed to 100 % of the actual 0.2 % proof stress and exposed in 20 weight percent sodium chloride at 90 °C with 0.1 bar H₂S and 1.7 bar CO₂ (pH=3.5) in accordance with EFC 17. This simulates an aggressive produced water.

No cracking was seen after 30 days on triplicate samples according to the present invention. 4-point bent beams made of the alloy composition of the present invention were stressed at 100 % of the actual 0.2 % proof stress and were exposed to water containing 1000 mg/1 of chloride at 90 °C with 0.1 bar H₂S and a pH of 3.0. This environment simulates aggressive condensed water. No cracking was seen on triplicate samples after 30 days exposure. The NACE MR-0175 / ISO 15156 Part 3 standard specifies that solution annealed duplex stainless steels with a PREN between 30 and 40 are satisfactory up to 0.1 bar H₂S. PREN = (wt.% Cr) + (3.3 x wt.% Mo) + (16 x wt.% N). The PREN of the alloy according to the present invention used in these tests possessed a PREN of 30.249. The above results demonstrate that the alloy of the present invention is satisfactory for use up to 0.1 bar H₂S in both aggressive produced and condensed waters. CREVICE CORROSION

Samples of the alloy composition of the present invention were tested as discs, of 75 mm diameter with PEEK crevice washers (single ring) fastened to a torque of 7 nm. The discs were immersed in solutions containing 70 mg/1 sulphate and various concentrations of chloride, both as sodium salts. Tests were conducted in static solution at 23 °C, 40 °C, 60 °C and 80 °C for 30 days.

The highest chloride at which no crevice corrosion occurred was plotted against temperature, as shown in Figure 1. The data for 316L is taken from Efird et al, Mat. Perf., j_8, 7 (1979) 34. As can be seen in Figure 1, the samples made from an alloy composition according to the present invention exhibited greater resistance to chloride ion induced crevice corrosion than 316L steel.

Claims

1. A duplex stainless steel casting alloy composition comprising, in weight percent: up to around 0.05 % carbon; around 0.5 % to 1.2 % silicon; around 1 % to 2 % manganese; around 3.7 % to 4.5 % nickel; around 21 % to 23 % chromium; around 1.5 % to 2.5 % molybdenum; around 0.15 % to 0.25 % nitrogen; and balance iron and incidental impurities.

2. An alloy composition according to claim 1, wherein said composition comprises around 0.02 to 0.05 weight percent carbon.

3. An alloy composition according to claim 1, wherein the composition comprises around 0.03 to 0.05 weight percent carbon.

4. An alloy composition according to any one of claims 1, 2 or 3, wherein said alloy composition comprises around 0.5 to 1 weight percent silicon.

5. An alloy composition according to any one of claims 1, 2 or 3, wherein said alloy composition comprises around 0.8 weight percent silicon.

6. An alloy composition according to any preceding claim, wherein the alloy composition comprises around 1.25 to 1.75 weight percent manganese.

7. An alloy composition according to any one of claims 1 to 5, wherein said alloy composition comprises around 1.25 to 1.5 weight percent manganese.

8. An alloy composition according to any one of claims 1 to 5, wherein said alloy composition comprises around 1.5 weight percent manganese.

9. An alloy composition according to any preceding claim, wherein said alloy composition comprises around 4 to 4.5 weight percent nickel.

10. An alloy composition according to any one of claims 1 to 8, wherein said alloy composition comprises around 4.1 to 4.3 weight percent nickel.

11. An alloy composition according to any one of claims 1 to 8, wherein said alloy composition comprises around 4.2 weight percent nickel.

12. An alloy composition according to any preceding claim, wherein said alloy composition comprises around 21 to 22 weight percent chromium.

13. An alloy composition according to any one of claims 1 to 11, wherein said alloy composition comprises around 21.5 weight percent chromium.

14. An alloy composition according to any preceding claim, wherein said alloy composition comprises around 1.75 to 2.25 weight percent molybdenum.

15. An alloy composition according to any one of claims 1 to 13, wherein said alloy composition comprises around 1.9 to 2 weight percent molybdenum.

16. An alloy composition according to any one of claims 1 to 13, wherein said alloy composition comprises around 1.95 weight percent molybdenum.

17. An alloy composition according to any preceding claim, wherein said alloy composition comprises around 0.15 to 0.2 weight percent nitrogen.

18. An alloy composition according to any one of claims 1 to 16, wherein said alloy composition comprises around 0.15 to 0.18 weight percent nitrogen.

19. An alloy composition according to any one of claims 1 to 16, wherein said alloy composition comprises around 0.155 weight percent nitrogen.

20. An alloy composition according to any preceding claim, wherein the composition comprises ferrite and austenite phases.

21. An alloy composition according to claim 20, wherein said alloy composition comprises around 40 to 60 weight percent of said ferrite phase.

22. An alloy composition according to claim 20, wherein said alloy composition comprises around 50 weight percent of said ferrite phase.

23. An alloy composition according to any preceding claim, wherein said alloy composition further comprises around 0.2 to 1 weight percent copper.

24. An alloy composition according to any preceding claim, wherein said alloy composition further comprises around 0.2 to 1 weight percent tungsten.

25. An alloy composition according to any preceding claim, wherein said alloy composition further comprises up to 0.02 weight percent sulphur.

26. An alloy composition according to any preceding claim, wherein said alloy composition further comprises up to around 0.025 weight percent phosphorous.

27. An alloy composition according to any preceding claim, wherein said alloy composition further comprises up to around 0.01 weight percent aluminium.

28. An alloy composition according to any preceding claim, wherein said alloy composition possesses a minimum Pitting Resistance Equivalent Number of around

30.

29. An alloy composition according to any preceding claim, wherein said alloy composition possesses a minimum Pitting Resistance Equivalent Number of around 30 to 35.

30. A method for producing a cast product, said method comprising introducing a duplex stainless steel alloy composition into a mould, causing said composition to at least partly solidify within said mould, and removing said at least partly solidified composition from said mould, said alloy composition comprising, in weight percent: up to around 0.05 % carbon; around 0.5 % to 1.2 % silicon; around 1 % to 2 % manganese; around 3.7 % to 4.5 % nickel; around 21 % to 23 % chromium; around 1.5 % to 2.5 % molybdenum; around 0.15 % to 0.25 % nitrogen; and balance iron and incidental impurities.

31. A method according to claim 30, wherein the alloy composition is subjected to a heat treatment process comprising solution annealing and water quenching.

32. A method according to claim 31, wherein said solution annealing is carried out by heating the alloy composition to a temperature of around 1000 °C to around 1150 °C.

33. A cast product obtained by casting a duplex stainless steel casting alloy composition comprising, in weight percent: up to around 0.05 % carbon; around 0.5 % to 1.2 % silicon; around 1 % to 2 % manganese; around 3.7 % to 4.5 % nickel; around 21 % to 23 % chromium; around 1.5 % to 2.5 % molybdenum; around 0.15 % to 0.25 % nitrogen; and balance iron and incidental impurities.

34. A duplex stainless steel casting alloy composition comprising, in weight percent: up to around 0.03 % carbon; around 0.8 % silicon; around 1.5 % manganese; around 4.2 % nickel; around 21.5 % chromium; around 1.95 % molybdenum; around 0.155 % nitrogen; and balance iron and incidental impurities.