HK1070396B - Duplex stainless steel - Google Patents

Description

Duplex stainless steel

Background

Technical Field

The present invention relates generally to duplex stainless steels. In particular, the present invention relates to a duplex stainless steel that can economically replace certain known duplex stainless steels while providing improved corrosion resistance as compared to certain austenitic stainless steels, such as AISI type 304, 316 and 317 austenitic stainless steels. The invention also relates to a method of manufacturing the duplex stainless steel of the invention. The duplex stainless steels of the present invention can be applied, for example, in corrosive environments and can be manufactured into articles of manufacture, such as strips, rods, plates, sheets, castings, pipes, or tubes.

Background

Duplex stainless steels are alloys containing a microstructure consisting of a mixture of austenite and ferrite phases. Generally, they exhibit certain characteristics of two phases while having relatively higher strength and ductility. Various duplex stainless steels have been proposed, some of which are described in US patents US3650709, US4340432, US4798635, US4828630, US5238508, US5298093, US 56245804 and US 6069441.

Early dual phase alloys were moderately resistant to common corrosion and chloride stress corrosion cracking, but suffered from substantial loss of properties when used under as-welded conditions. Currently, one of the most widely used second generation duplex stainless steels is available from AlleghenyLudlum, Pittsburgh, Pa. under the trademark AL2205(UNS 31803 and/or S32205). The duplex stainless steel is an alloy containing nominal chromium of 22%, nickel of 5.5%, molybdenum of 3%, nitrogen of 0.16%, which provides corrosion resistance in many environments beyond that of AISI type 304, 316 and 317 austenitic stainless steels (all percentages herein refer to weight percentages of total alloy weight unless otherwise specified). AL2205 is a nitrogen augmented duplex stainless steel that imparts the metallurgical benefits of nitrogen to improve corrosion performance and as-welded properties while also exhibiting twice the yield strength over conventional austenitic stainless steels. In environments where resistance to common corrosion and chloride stress corrosion cracking ("SCC") is important, the duplex stainless steel is often used in the form of welded pipes or tubular components, as well as formed and welded sheet products. Its increased strength creates opportunities for reduction in the wall thickness of the tube and inhibits losses in operation.

As just described, the AL2205 has been terminated by a tube or pipeWidespread acceptance by users, especially when SCC is a concern, has been widely recognized as a low cost alternative to type 316 stainless steel. This is due in large part to the fact that AL2205 is significantly more resistant to crevice corrosion than the type 316 and type 317 austenitic stainless steels. This excellent resistance to chloride ion crevice corrosion can be seen in the following table which lists the results of ASTM procedure G48B using a 10% ferric chloride solution. The 10% ferric chloride solution referred to is based on the weight of the hexahydrate salt and corresponds to 6% by weight of anhydrous ferric chloride salt.

Crevice corrosion data in 10% ferric chloride
		Alloy (I)	Temperature at which crevice corrosion starts
Type 316	27℉(-3℃)
		Type 317	35℉(2℃)
AL2205	68℉(20℃)

However, the extraordinary corrosion resistance (and other properties) of AL2205 is beyond what is needed in some applications. In certain SCC applications, while AL2205 provides an acceptable technical solution, it is not an economically practical alternative to type 304, 316 or 317 stainless steel. The higher cost of AL2205 is mainly due to the amount of the alloying elements nickel (nominal 5.5%) and molybdenum (nominal 3%).

It is therefore desirable to provide a weldable, formable duplex stainless steel having a higher corrosion resistance than the type 304, 316 or 317 austenitic stainless steels and a lower production cost than the commonly used AL2205 duplex stainless steels.

Summary of the invention

The invention relates to a duplex stainless steel comprising, in weight percent, up to 0.06% carbon, 15% to 25% chromium or less, greater than 3% to 6% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% and 2.5% molybdenum, up to 0.5% copper, up to 0.2% cobalt or less, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron; as well as iron and unavoidable impurities. The duplex stainless steel is a steel that is weldable, formable, and exhibits higher corrosion resistance than austenitic stainless steels of types 304, 316, and 317.

According to a particular embodiment of the invention, the duplex stainless steel comprises, in weight percent, up to 0.03% carbon, 19.5% to 22.5% chromium, greater than 3% to 4% nickel, up to 2% manganese, 0.14% to 0.20% nitrogen, up to 1% silicon, 1.5% to 2.0% molybdenum, up to 0.4% copper, up to 0.3% phosphorus, 0.001% sulfur, and 0.0015% to 0.0030% boron; iron and inevitable impurities.

The duplex stainless steel of the present invention may also be composed mainly of: at most 0.03% carbon, 19.5% to 22.5% chromium, greater than 3% to 4% nickel, at most 2% manganese, 0.14% to 0.20% nitrogen, at most 1% silicon, 1.5% to 2.0% molybdenum, at most 0.4% copper, at most 0.3% phosphorus, 0.001% sulfur, and 0.0015% to 0.0030% boron, by weight percent; iron and inevitable impurities.

The invention also relates to producing an article, for example, a strip, rod, plate, sheet, casting, tube or pipe made from or comprising the duplex stainless steel of the invention. Articles formed from the duplex stainless steels of the present invention are particularly advantageous when intended for use in chlorine-containing environments.

In addition, the present invention relates to a method of manufacturing a duplex stainless steel, according to which a duplex stainless steel including at most 0.06% of carbon, 15% to 25% or less of chromium, more than 3% to 6% of nickel, at most 3.75% of manganese, 0.14% to 0.35% of nitrogen, at most 2% of silicon, more than 1.4% and 2.5% or less of molybdenum, at most 0.5% of copper, at most 0.2% or less of cobalt, at most 0.05% of phosphorus, at most 0.005% of sulfur, 0.001% to 0.0035% of boron; and duplex stainless steel of iron and inevitable impurities. The steel is solution annealed and cooled. The steel may be further processed to produce articles of manufacture or any other desired form.

The present invention includes the following aspects:

1. a duplex stainless steel comprising, in weight percent, up to 0.06% carbon, 15% to 22.5% chromium, greater than 3% to equal to or less than 4% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to equal to or less than 2.5% molybdenum, equal to or less than 0.5% copper, equal to or less than 0.2% cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron; iron and inevitable impurities.

2. The duplex stainless steel of item 1, comprising up to 0.03% carbon.

3. The duplex stainless steel of item 1, wherein 15% to 21% or less of chromium is contained.

4. The duplex stainless steel of clause 1, wherein the nickel is contained by more than 3% to 3.5%.

5. The duplex stainless steel of clause 1, wherein the manganese is included at most 2%.

6. The duplex stainless steel of item 1, comprising 0.14 to 0.20% nitrogen.

7. The duplex stainless steel of item 1, comprising up to 1% silicon.

8. The duplex stainless steel of item 1, comprising 1.5 to 2.0% molybdenum.

9. The duplex stainless steel of clause 1, comprising up to 0.4% copper.

10. The duplex stainless steel of item 1, comprising up to 0.03% phosphorus.

11. The duplex stainless steel of item 1, comprising up to 0.001% sulfur.

12. The duplex stainless steel of item 1, comprising 0.0015% to 0.003% boron.

13. The duplex stainless steel of item 1, wherein the steel is weldable and formable.

14. A duplex stainless steel consisting essentially of, in weight percent, up to 0.06% carbon, 15% to 22.5% chromium, greater than 3% to equal to or less than 4% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to equal to or less than 2.5% molybdenum, equal to or less than 0.5% copper, equal to or less than 0.2% cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron; iron and inevitable impurities.

15. A duplex stainless steel comprising, in weight percent, up to 0.03% carbon, 15% to 21% chromium, greater than 3% to 4% nickel, up to 2% manganese, 0.14% to 0.20% nitrogen, up to 1% silicon, 1.5% to 2% molybdenum, up to 0.4% copper, less than 0.2% cobalt, up to 0.03% phosphorus, 0.001% to 0.0035% boron; iron and inevitable impurities.

16. An article of manufacture comprising duplex stainless steel comprising, in weight percent, up to 0.06% carbon, 15% to 22.5% chromium, greater than 3% to equal to or less than 4% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to equal to or less than 2.5% molybdenum, equal to or less than 0.5% copper, equal to or less than 0.2% cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron; iron and inevitable impurities.

17. The article of claim 16, wherein the steel is in the form of an article selected from the group consisting of a bar, a rod, a plate, a sheet, a casting, a pipe, and a pipe.

18. A method of manufacturing a duplex stainless steel, the method comprising:

providing, in weight percent, a duplex stainless steel alloy including up to 0.06% carbon, 15% to 22.5% chromium, greater than 3% to 4% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to 2.5% molybdenum, up to 0.5% copper, up to 0.2% cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron, and iron and inevitable impurities;

solution annealing the steel; and are

The steel is cooled.

19. The duplex stainless steel of clause 3, wherein the chromium is contained in an amount of 15% to 20.5%.

20. The duplex stainless steel of clause 3, wherein the nickel is contained by more than 3% to 3.5%.

21. The duplex stainless steel of clause 1, wherein the impurity is tungsten.

22. The duplex stainless steel of item 14, which consists essentially of 15% to less than or equal to 21% chromium.

23. The duplex stainless steel of item 22, consisting essentially of 15% to 20.5% chromium.

24. The duplex stainless steel of item 14, consisting essentially of greater than 3% to 3.5% nickel.

25. The duplex stainless steel of clause 22, consisting essentially of greater than 3 to 3.5% nickel.

26. The duplex stainless steel of item 14, consisting essentially of up to 0.4% copper.

27. The duplex stainless steel of clause 15, wherein the chromium is contained between 15% and 20.5%.

28. The duplex stainless steel of clause 15, wherein the nickel is contained by more than 3% to 3.5%.

29. The duplex stainless steel of clause 27, wherein the nickel is contained by more than 3% to 3.5%.

30. The duplex stainless steel of clause 15, wherein less than 0.4% copper is included.

31. The duplex stainless steel of item 15, wherein the impurity is tungsten.

32. The article of manufacture of clause 16, wherein the duplex stainless steel comprises 15% to less than or equal to 21% chromium.

33. The article of manufacture of item 32, wherein the duplex stainless steel comprises 15% to 20.5% chromium.

34. The article of manufacture of item 16, wherein the duplex stainless steel comprises greater than 3% to 3.5% nickel.

35. The article of manufacture of item 32, wherein the duplex stainless steel comprises greater than 3% to 3.5% nickel.

36. The article of manufacture of clause 16, wherein the duplex stainless steel comprises up to 0.4% copper.

37. The article of manufacture of item 16, wherein the impurity is tungsten.

38. The method of clause 18, wherein the duplex stainless steel comprises 15% to less than or equal to 21% chromium.

39. The method of clause 38, wherein the duplex stainless steel comprises 15% to 20.5% chromium.

40. The method of clause 18, wherein the duplex stainless steel comprises 3% to 3.5% nickel.

41. The method of clause 38, wherein the duplex stainless steel comprises 3% to 3.5% nickel.

42. The method of clause 18, wherein the duplex stainless steel comprises up to 0.4% copper.

43. The method of clause 18, wherein the impurity is tungsten.

Detailed description of the invention

The invention relates to a duplex stainless steel comprising, in weight percent, up to 0.06% carbon, 15% to 25% chromium or less, greater than 3% to 6% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% and 2.5% molybdenum, up to 0.5% copper, up to 0.2% cobalt or less, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron; as well as iron and unavoidable impurities. The aforementioned duplex stainless steels of the present invention preferably comprise 20-80 vol% of each of the austenitic and ferritic phases in the annealed condition. The duplex stainless steels of the present invention are weldable, formable, and more corrosion resistant materials than the type 304, type 316, and type 317 austenitic stainless steels.

According to certain embodiments of the present invention, the duplex stainless steel may include, in weight percent, up to 0.03% carbon, 19.5% to 22.5% chromium, 3% to 4% nickel, up to 2% manganese, 0.14% to 0.20% nitrogen, up to 1% silicon, 1.5% to 2.0% molybdenum, up to 0.4% copper, up to 0.3% phosphorus, 0.001% sulfur, and/or 0.0015% to 0.0030% boron; iron and inevitable impurities. These ranges are particularly suitable in the use of pipes where formability and strength are required, while maintaining the necessary level of corrosion resistance. The duplex stainless steels of the present invention may include various other alloying additions and additives known in the art. Embodiments of the duplex stainless steel of the present invention are less expensive to produce than the commonly used AL2205 duplex stainless steel due to the low content of alloying additions, particularly nickel and molybdenum. However, duplex stainless steels of the present invention still provide a stable austenite phase (relative to the martensite causing deformation) and a desired level of corrosion resistance. The following are nickel and certain embodiments of the invention compared to AL2205The molybdenum content.

Amount of elements Ni and Mo (weight percent)
			Alloy (I)	Duplex stainless steel embodiments of the invention	AL2205
Ni	Greater than 3.0 to 4.0	The rated weight is 5.5%
			Mo	1.5 to 2.0	The rated weight is 3%

Despite the reduced nickel and molybdenum content compared to AL2205, the evaluated embodiments of the duplex stainless steel according to the invention exhibit a significantly stronger point/gap corrosion resistance than the austenitic stainless steels of types 304, 316 and 317. As is well known in the art, type 316 and 317 stainless steels are more resistant to pitting/crevice corrosion than type 304 stainless steels.

As an example of the present invention, the present inventors produced a single pass of duplex stainless steel comprising the following composition in weight percent: 0.018% carbon, 0.46% manganese, 0.022% phosphorus, 0.0034% sulfur, 0.45% silicon, 20.18% chromium, 3.24% nickel, 1.84% molybdenum, 0.21% copper, 0.166% nitrogen, and 0.0016% boron (hereinafter "example 1"). As explained below, this embodiment of duplex stainless steel according to the invention shows a significantly higher point-like corrosion resistance than austenitic stainless steels of type 316 and 317, as well asIt maintains a lower production cost compared to AL2205 due to the reduced content of nickel and molybdenum.

Point-like corrosion resistance
		Alloy (I)	Critical pitting corrosion temperature ("CPT")
Type 316 stainless steel	59℉(15.0℃)
		317 type stainless steel	66℉(18.9℃)
Example 1	88.3℉(31.3℃)

The CTP of the type 316 and 317 austenitic stainless steels is based on ASTM procedure G-48A. According to this procedure, samples of the material were immersed in beakers containing 6% ferric chloride solution at the required temperature for 72 hours, and then evaluated for signs of pitting. The test was repeated with increasing temperature to determine the temperature at which pitting occurred. The CPT of example 1 was measured by ASTM procedure G150. According to this procedure, the same value, CPT, as determined by ASTM procedure G-48A, was determined by placing a sample of the material in an electrochemical cell containing a 1 molar (approximately 5.85% by weight) solution of sodium chloride and polarizing it to a potential of +700mV against a Saturated Calomel Electrode (SCE). The temperature of the solution was increased at a rate of 1 c per minute and the corrosion current was monitored. At a certain temperature, the current rapidly increases and exceeds the critical value of 100 milliamps per square centimeter, and this temperature is recorded as CPT. The pitting on the sample can be visually determined.

In addition, the present inventors also developed another duplex stainless steel within the scope of the present invention, which comprises, in weight percent, 0.021% of carbon, 0.50% of manganese, 0.022% of phosphorus, 0.0014% of sulfur, 0.44% of silicon, 20.25% of chromium, 3.27% of nickel, 1.80% of molybdenum, 0.21% of copper, 0.167% of nitrogen, and 0.0016% of boron (hereinafter referred to as "example 2"), and evaluated various mechanical properties of the steel. The results are shown below. As expected, the mechanical properties of example 2 exceeded the minimum requirements of ASTM specification a240 for AL 2205. Moreover, although the yield strength and tensile strength of example 2 were lower than AL2205, they were comparable. Importantly, however, these values significantly exceed the minimum strength requirements of ASTM specification a240 for austenitic stainless steels of types 304, 316 and 317.

Mechanical Properties
				Alloy standard	0.2% offset yield strength	Ultimate tensile strength	Percent elongation
ASTM A240 minimum 304 stainless Steel	30,000	75,000	40.0
				ASTM A240 minimum 316 stainless Steel	30,000	75,000	40.0

ASTM A240 minimum 317 stainless Steel	30,000	75,000	35.0
				ASTM A240 minimum AL2205 duplex stainless steel	65,000	90,000	25
AL2205 stainless steel	85,000	125,000	30
				Example 2	83,000	114,000	37

Therefore, the duplex stainless steel of the present invention can become a lower cost alternative to AL 2205. As illustrated in inventive examples 1 and 2, embodiments of the duplex stainless steel of the present invention are mechanically comparable to AL2205 and have significantly better corrosion resistance to the pores/crevices than type 316 and 317 stainless steels.

The invention also relates to the production of articles, for example, strips, rods, plates, sheets, castings, pipes and pipes, made of or comprising the duplex stainless steels of the invention. According to these embodiments of the invention, the duplex stainless steel article comprises, in weight percent, up to 0.06% carbon, 15% to 25% chromium or less, greater than 3% to 6% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to 2.5% molybdenum, 0.5% or less copper, 0.2% or less cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, and 0.001 to 0.0035 boron; iron and inevitable impurities. Articles formed from the duplex stainless steels of the present invention are particularly advantageous for use in chlorine-containing environments.

In addition, the present invention relates to a method of manufacturing duplex stainless steel. According to the method of the invention, a duplex stainless steel is provided, comprising, in weight percent, up to 0.06% carbon, 15% to less than or equal to 25% chromium, greater than 3% to 6% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to less than or equal to 2.5% molybdenum, less than or equal to 0.5% copper, less than or equal to 0.2% cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, and 0.001% to 0.0035% boron; iron and inevitable impurities. According to the method, this steel is subsequently solution annealed and cooled. Such steel may be further processed into articles, such as those mentioned above, or into any other desired form using techniques well known to those of ordinary skill in the art.

It is to be understood that the description herein illustrates aspects of the invention relevant to a clear understanding of the invention. Certain aspects that will be apparent to those skilled in the art and that are not helpful in a better understanding of the invention have not been presented in order to simplify the description of the invention. While the invention has been described in terms of only certain embodiments, those skilled in the art will recognize, in view of the foregoing description, that many embodiments, adaptations, and variations of the invention may be implemented. All such variations and modifications of the invention are intended to be covered by the foregoing description and the following claims.

Claims (37)

1. A duplex stainless steel, consisting of: at most 0.06% carbon, 15% to 22.5% chromium, greater than 3% to equal to or less than 4% nickel, at most 3.75% manganese, 0.14% to 0.35% nitrogen, at most 2% silicon, greater than 1.4% to equal to or less than 2.5% molybdenum, equal to or less than 0.5% copper, equal to or less than 0.2% cobalt, at most 0.05% phosphorus, at most 0.005% sulfur, 0.001% to 0.0035% boron, in weight percent; the balance being iron and unavoidable impurities.

2. The duplex stainless steel of claim 1 wherein carbon is at most 0.03%.

3. The duplex stainless steel of claim 1 wherein chromium is 15% to 21% or less.

4. The duplex stainless steel of claim 1 wherein nickel is greater than 3% to 3.5%.

5. The duplex stainless steel of claim 1 wherein manganese is at most 2%.

6. The duplex stainless steel of claim 1 wherein the nitrogen is between 0.14% and 0.20%.

7. The duplex stainless steel of claim 1 wherein silicon is at most 1%.

8. The duplex stainless steel of claim 1 wherein molybdenum is 1.5% to 2.0%.

9. The duplex stainless steel of claim 1 wherein copper is at most 0.4%.

10. The duplex stainless steel of claim 1 wherein phosphorous is at most 0.03%.

11. The duplex stainless steel of claim 1 wherein sulfur is at most 0.001%.

12. The duplex stainless steel of claim 1 wherein boron is 0.0015% to 0.003%.

13. The duplex stainless steel of claim 1 wherein the steel is weldable and formable.

14. A duplex stainless steel, consisting of: at most 0.03% carbon, 15% to less than or equal to 21% chromium, greater than 3% to less than or equal to 4% nickel, at most 2% manganese, 0.14% to 0.20% nitrogen, at most 1% silicon, 1.5% to 2% molybdenum, up to 0.4% copper, less than 0.2% cobalt, at most 0.03% phosphorus, 0.001% to 0.0035% boron, in weight percent; the balance being iron and unavoidable impurities.

15. An article of manufacture comprising duplex stainless steel, the composition of which is as follows: at most 0.06% carbon, 15% to 22.5% chromium, greater than 3% to equal to or less than 4% nickel, at most 3.75% manganese, 0.14% to 0.35% nitrogen, at most 2% silicon, greater than 1.4% to equal to or less than 2.5% molybdenum, equal to or less than 0.5% copper, equal to or less than 0.2% cobalt, at most 0.05% phosphorus, at most 0.005% sulfur, 0.001% to 0.0035% boron, in weight percent; the balance being iron and unavoidable impurities.

16. The article of claim 15, wherein the steel is in the form of an article selected from the group consisting of a bar, a rod, a plate, a sheet, a casting, a pipe, and a pipe.

17. A method of manufacturing a duplex stainless steel, the method comprising:

providing a duplex stainless steel alloy consisting of, in weight percent: up to 0.06% carbon, 15% to 22.5% chromium, greater than 3% to equal to or less than 4% nickel, up to 3.75% manganese, 0.14% to 0.35% nitrogen, up to 2% silicon, greater than 1.4% to equal to or less than 2.5% molybdenum, equal to or less than 0.5% copper, equal to or less than 0.2% cobalt, up to 0.05% phosphorus, up to 0.005% sulfur, 0.001% to 0.0035% boron, and the balance iron and unavoidable impurities;

solution annealing the steel; and are

The steel is cooled.

18. The duplex stainless steel of claim 3 wherein the chromium is between 15% and 20.5%.

19. The duplex stainless steel of claim 3 wherein the nickel is greater than 3% to 3.5%.

20. The duplex stainless steel of claim 1 wherein the impurity is tungsten.

21. The duplex stainless steel of claim 14 wherein the chromium is comprised between 15% and 20.5%.

22. The duplex stainless steel of claim 14 wherein the nickel is greater than 3% to 3.5%.

23. The duplex stainless steel of claim 21 wherein the nickel is greater than 3% to 3.5%.

24. The duplex stainless steel of claim 14 wherein the copper is less than 0.4%.

25. The duplex stainless steel of claim 14 wherein the impurity is tungsten.

26. The article of manufacture of claim 15, wherein the chromium in the duplex stainless steel is from 15% to 21% or less.

27. The article of manufacture of claim 26, wherein the chromium in the duplex stainless steel is from 15% to 20.5%.

28. The article of manufacture of claim 15, wherein the nickel in the duplex stainless steel is from greater than 3% to 3.5%.

29. The article of manufacture of claim 26, wherein the nickel in the duplex stainless steel is from greater than 3% to 3.5%.

30. The article of manufacture of claim 15, wherein the copper in the duplex stainless steel is at most 0.4%.

31. The article of manufacture of claim 15, wherein the impurity is tungsten.

32. The method of claim 17, wherein the chromium in the duplex stainless steel is 15% to 21% or less.

33. The method of claim 32, wherein the chromium in the duplex stainless steel is from 15% to 20.5%.

34. The method of claim 17, wherein the nickel in the duplex stainless steel is 3% to 3.5%.

35. The method of claim 32, wherein the nickel in the duplex stainless steel is 3% to 3.5%.

36. The method of claim 17, wherein the copper in the duplex stainless steel is at most 0.4%.

37. The method of claim 17, wherein the impurity is tungsten.