WO2003044238A1

WO2003044238A1 - Super-austenitic stainless steel

Info

Publication number: WO2003044238A1
Application number: PCT/SE2002/002156
Authority: WO
Inventors: Babak Bahar; Johan Frodigh; Pasi Kangas; Ulf Kivisakk
Original assignee: Sandvik AB
Current assignee: Sandvik AB
Priority date: 2001-11-22
Filing date: 2002-11-22
Publication date: 2003-05-30
Anticipated expiration: 2004-05-22
Also published as: SE0103938L; US7081173B2; CN1589335A; AU2002353725A1; SE525252C2; WO2003044239A1; BR0214346A; EP1446513A1; NO20042103L; CN1293223C; KR20050044557A; AU2002366113A1; JP2005509751A; US20030143105A1

Abstract

Super-austenitic stainless steel alloy with a composition, balanced in such a way that the alloy and products made from the alloy fulfil high requirements on a combination of high corrosion resistance, especially in inorgnic and organic acids and mixtures thereof, good general corrosion resistance, good structure stability as well as improved mechanical properties in combination with good workability, in particular in the embodiment tubes, especially seamless tubes and seam welded tubes for application in said environments and with following composition, in wight-percent: Cr 23.0-30.0, Ni 25.0-35.0, Mo 2.0-6.0, Mn 1.0-6.0, N 0-0.4, C up to 0.05, Si up to 1.0, S up to 0.02, Cu up to 3.0, W 0-6.0 a or more element of the Group Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0 and the balance being Fe and normally occurring impurities and steel making additions.

Description

Super-austenitic stainless steel

Field of the invention

The present invention relates to a super-austenitic stainless steel alloy with a composition, balanced in such a way that alloy and products produced of the alloy fulfills high requirements on a combination of high corrosion resistance, especially in inorganic and organic acids and mixtures thereof, good general corrosion resistance, good structure stability as well as improved mechanical properties in combination with good workability, in particular in the embodiment tubes, specially seamless tubes and seam-welded tubes for use in said environments.

Background of the invention

Austenitic steel with optimized properties is used in many different applications and is a common alternative to e.g. nickel-base alloys.

The disadvantage with the latter is the permanently pressed price for the raw material. Primarily, the choice of steel grade is determined by the requirements on corrosion resistance, workability as well as structure stability. High alloyed austenitic stainless steels are found in a range of different embodiments for corrosive environments within e.g. the chemical industry, especially in the production of acids, as well organic as inorganic, for the production of oil products, and for sea water cooling.

The use is to a large extend limited by the corrosion resistance and that high alloying levels imply problem with both the workability by hot-extrusion and cold- rolling as well as that the structure stability diminishes simultaneously to an increasing alloying level of especially the elements Cr, Mo, N, W, Cu and Mn.

The developed alloys are generally characterized in that one tries to find a composition, which obtains high corrosion resistance within a broad range of chemical environments. The high alloying level implies rise in the price compared to lower alloyed material. In particular, nickel-base alloys are considered being very expensive and high alloyed austenitic alloys with lower content of nickel but with high alloying level are frequently limited by their workability, which means that it is difficult to hot-extrude seamless tubes of the alloy as well as cold-rolling the material to suitable final dimension.

The high price makes that market for this type of alloys is relatively limited which is a occasion to that one is going to develop all-round material in order to be able to offer a type of alloy for different applications and whereby win advantages in the form of cost savings for preparation and stock-keeping. It is a disadvantage with the known high alloyed austenitic steel grades as e.g. alloy as is described in SE465373, which hereby is included as reference, or nickel- base alloys as e.g. Alloy 59 that structure stability can only be managed within a very narrow temperature range, which implies problems with the production of heavier structures as well as that the subsequent treatment such as welding becomes more complicated. Deteriorated structure stability entails deteriorated corrosion resistance and shorter life time for products produced of these alloys in applications in the above-mentioned environments.

Summary of the Invention

It is therefore an object of the present invention to provide a stainless steel alloy, in particular a super-austenitic stainless steel alloy with high corrosion resistance in inorganic and organic acids and mixtures thereof, good general corrosion resistance.

It is further object of the present invention to provide a super-austenitic stainless steel alloy with good structure stability and improved mechanical properties in combination with good workability, in particular in the embodiment tubes, especially seamless tubes for application in said environments.

These objects are fulfilled with an alloy according to the present invention, which contains (in weight-%): Cr 23.0-30.0

Ni 25.0-35.0

Mo 2.0-6.0 Mn 1.0-6.0

N 0-0.4

C up to 0.05

Si up to 1.0

S up to 0.02 Cu up to 3.0

W 0-6.0 one or more elements of the Group Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0 and the balance being Fe and normally occurring impurities and steel making additions.

Short description of the drawings

Figure 1 shows yield point in tension for the heats 1 to 10 according to the invention at room temperature.

Figure 2 shows yield point in tension for the heats 1 to 10 according to the invention at temperature of 100°C.

Figure 3 shows yield point in tension for the heats 1 to 10 according to the invention at temperature of 200°C. Figure 4 shows result of impact test for half size specimen of the heats 1 to 10 according to the invention at room temperature, average of three tests.

Figure 5 shows result of impact test for half size specimen of the heats 1 to 10 according to the invention at -196°C, average of three tests.

Figure 6 shows, elongation for heats 1 to 10 according to the invention at temperature of 200°C.

Figure 7 shows elongation for heats 1 to 10 according to invention at room temperature. Figure 8 shows elongation for heats 1 to 10 according to invention at temperature of 100°C.

Detailed description of the Invention

A systematic development work has surprisingly shown that an alloy with an alloying content according to the present invention shows these properties.

The alloy according to the invention contains therefore, in weight-percent:

Cr 23.0-30.0 Ni 25.0-35.0

Mo 2.0-6.0

Mn 1.0-6.0

N 0-0.4

C up to 0.05 Si up to 1.0

S up to 0.02

Cu up to 3.0

W 0-6.0 one or more element of the Group of Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0 and the balance being Fe and normally occurring impurities and steel making additions.

The effect of the alloying elements for the alloys in the framework of the present invention is the following:

Chromium (Cr) is a very active element with the purpose to improve the resistance to the plurality of corrosion types, such as general corrosion and corrosion in acid environments, especially where contaminated acids occur. Besides, a high content of Chromium is desirable in order to enable the addition of nitrogen into the alloy in sufficient contents. Thus, it is desirable to keep the content of Chromium as high as possible in order to improve the corrosion resistance. Therefor, the content of Chromium should lie in the range of 23.0-30.0 weight-% and be preferably at least 24.0 weight-%, more preferably at least 27.0 weight- %. However, too high contents of Chromium increase the risk for intermetallic precipitations, for what reason this content has to be limited up to max 30.0 weight-%, preferably to 29.0 weight-%.

Nickel (Ni)

A high content of nickel homogenizes high alloyed steel by increasing the solubility of Cr and Mo. Thereby the austenite stabilizing nickel suppresses the forming of the unwanted phases sigma-, laves- and chi-phase, which to a large extend consist of the alloying elements chromium and molybdenum. However, a disadvantage is that nickel decreases the solubility of nitrogen in the alloy and detonates the hot-workability, which entails an upper limitation for the content of nickel in the alloy. However, the present invention has shown that high contents of nitrogen can be allowed at contents of nickel according to the above-mentioned by balancing the high content of nickel to high Chromium- and Manganese-contents.

Therefore the content of nickel of the alloy should be limited to 25.0-35.0 weight-%, preferably being at least 26.0 weight-%, more preferably at least 30.0 weight-% most preferably 31.0 weight-% and preferably highest 34.0 weight-%.

Molybdenum (Mo)

In modern corrosion resistant austenitic steels frequently a high alloying addition of molybdenum in order to increase the resistance to corrosion attacks in e.g. reducing acids as well as oxidizing chloride environments. Molybdenum in high contents can, dependent on the total composition of the alloy increase the corrosion rate respective decrease the corrosion resistance. The explanation is the precipitation inclination of molybdenum, which can give rise to unwanted phases. Thereby a high content of Chromium is chosen in favor of a high content of Molybdenum, and also in order to obtain an optimum structure stability of the alloy. Certainly both alloying elements increase the precipitation inclination, but tests show that molybdenum performs this more than double as much as chromium. In the present alloy it is possible to wholly or partly replace the amount of molybdenum with tungsten. However, the alloy should preferably contain at least 2.0 weight-% molybdenum. The content of molybdenum should therefore be limited to between 2.0 and up to 6.0 weight-%, preferably to at least 3.7 weight-%, more preferably to at least 4.0 weight-%. The upper limit for the content of molybdenum is 6.0 weight-%, preferably 5.5 weight-%.

Manganese (Mn)

Manganese is of vital importance for the alloy because of three reasons. For the final product a high strength will be aimed at, for what reason the alloy should be strain hardened during cold working. Both nitrogen and manganese are known for decreasing the stacking fault energy, which in turn leads to that dislocations in the material dissociate and form Shockley-particles. The higher the stacking fault energy the greater the distance between the Shockley- particles and the more aggravated the transversal sideslipping of the dislocations will be which makes that the material get tendencies to strain harden. Of these reasons are high contents of manganese and nitrogen are very important for the alloy. Furthermore, Manganese increases the solubility of nitrogen in the smelt, which further speaks in favor of a high content of Manganese. Solely the high content of Chromium does not make the solubility sufficient since the content of nickel, which decreases the solubility, was chosen higher than the content of Chromium. A third motive for a content of Manganese in the range for the present invention is that a yield stress analysis made at increased temperature has surprisingly shown the improving effect of Manganese on the hot workability of the alloy. The higher alloyed the steels become, the more difficult they will be worked and the more important additions for the workability improvement become, which both simplify and make the production cheaper. The good hot workability makes the alloy excellent for the production of tubes, wire and strip etc.

Therefore should alloying content of Manganese lie in the range 1.0-6.0 weight- %, but preferably be higher than 2.0 weight-%, preferably higher than 3.0 and preferably lie within the range between 4.0 and 6.0 weight-%.

Carbon (C) has limited solubility in both ferrite and austenite. The limited solubility implies a threat to precipitation of chromium carbides and therefore the content should not exceed 0.05 weight-%, preferably not exceed 0.03 weight-%.

Silicon (Si) is utilized as desoxidation agent at the steel production as well as it increases the flowability during preparation and welding. However, too high contents of silicon lead to precipitation of unwanted intermetallic phase, for what reason the content should be limited to max 1.0 weight-%, preferably max 0.8 weight-%, more preferably to 0.4 weight-%.

Sulfur (S) influences the corrosion resistance negatively by forming easy soluble sulfides. Besides, it deteriorates the hot workability, for what reason the content of Sulfur is limited to max 0.02 weight-%.

Nitrogen (N) is like molybdenum a popular alloying element in modern corrosion resistant austenites in order to strongly elevate the corrosion resistance in oxidizing chloride environment, but also the mechanical strength of an alloy. Besides, nitrogen has the positive effect that it subdues the forming of intermetallic phase strongly. The upper content is limited by the solubility of nitrogen in digest and at casting, while the lower is limited of structure stability and austenite stability. For the present alloy it is foremost the impact of nitrogen on the increase of the mechanical strength as is utilized. By nitrogen like manganese decreases the stacking fault energy of the alloy attains a strong increase in tensile strength at cold-deformation, such as mentioned above. The invention utilizes even that nitrogen elevates the mechanical strength of the alloy as a result of interstitial soluted atoms, which cause tensions in the crystal structure. By using a high- strength material the possibility is given to obtain the same strength, but with less material consumption and thereby lower weight. Simultaneously this increases the requirements on the ductility of the material. Therefore the content of nitrogen should be 0.20-0.40 weight-%.

Copper (Cu)

The influence of copper on the corrosion properties of austenitic steel grades is disputed. However, it is considers to be clarified that copper strongly improves the corrosion resistance in sulfuric acid, which is of large importance for the alloys field of application. In tests copper showed being an element, which is favorable from a production point of view, especially for the production of tubes, for what reason an addition of copper is particularly important for material made for tube applications. However, it is acquired by experience that a high content of copper in combination with a high content of Manganese strongly detonates the hot-ductility, for what reason the upper limit for the content of copper is determined to 3.0 weight-%. The content of copper is preferably highest 1.5 weight-%.

Tungsten (W) increases the resistance to pitting and stress corrosion cracking. But alloying with too high contents of tungsten in combination with that the content of chromium as well as the content of molybdenum are high, involves that the risk for intermetallic precipitations increases. Therefor the content of tungsten should lie within the range of 0 to 6.0 weight-%, preferably 0 to 4.0 weight-%.

Ductility addition

At least one of the elements of the group of Magnesium (Mg), Calcium (Ca), Cerium (Ce), Boron (B), Lanthanum (La), Praseodynium (Pr), Zirconium (Zr), Titanium (Ti) and Neodynium (Nd) should be added in a content of up to 2.0 weight-% in order to improve the hot-workability.

Description of Embodiments In illustrating but non-limiting purpose some embodiments of the present invention are presented.

In Table 1 shows the compositions for tested alloys according to the invention and for known alloy, which are presented in comparing purpose. Totally 11 pieces 170- kg test ingots were produced in a HF-vacuum furnace. Further, a 2.2 tons full-scale-ingot was produced whose composition is shown as heat no. 12. The heat number and composition for the test ingots appear from Table 1 :

Table 1. Composition of tested material, (weight-%)

Heat no. C Si Mn Cr Ni Mo Cu N Ce

1 0,015 0,22 5,16 27,00 34,12 6,60 1 ,42 0,38 0,06

2 0,015 0,24 4,92 23,19 34,13 3,77 0,54 0,24 0,06

3 0,015 0,22 1 ,03 27.71 34,86 3,97 0,50 0,41 0,03

4 0,014 0,24 1 ,02 23,60 34,88 6,88 1 ,44 0,26 0,05

5 0,015 0,23 4,99 23,68 24,67 3,89 1 ,45 0,37 0,03

6 0,016 0,26 1 ,10 24,16 25,10 7,00 0,50 0,38 0,02

7 0,017 0,27 5,06 26,23 29,48 6,20 0,45 0,22 0,04

8 0,017 0,24 1 ,14 27.72 29,87 3,91 1 ,48 0,25 0,04

9 0,015 0,23 1 ,07 24,16 25,07 6,91 0,52 0,37 0,04

10 0,019 0,24 4,71 27,44 34,17 6,54 1 ,38 0,39 <0,01

11 0,011 0,27 5,1 26,5 33,7 5,9 0,011 0,38 0,03

12 0,012 0,34 5,04 26,44 33,96 5,26 0,080 0,080 0,01

A 0,004 0,05 0,03 22,3 60,0 16,0 0,011 0,002

B 0,020 3 24 22 7,3 0,5 0,50

C < 0,02 < 1 < 1 20 25 6,5 1 0,2 Heat A means Alloy 59, heat B means 654 SMO and heat C means UNS N08926. From all ingots test material was produced by forging, extrusion, heat- treating, turning/milling and finally heat-treating, which was executed at 1120°C under 30 min followed by water quenching.

For the known alloys being used as references are - in case that they have been used for testing - the intervals indicated define the composition which was tested and which lies within the standard composition for the alloy.

EXAMPLE 1

The resistance to general corrosion was measured by exposing the steel according to the present invention for the following environments: - 1.5% HCI at boiling temperature,

- 30% H₂S0₄ at 80°C - 50% H₂S0₄ at 90°C

- mixture of 25% formic acid + 50% acetic acid and 2000 ppm Cl~^~

- 43% H₃P0₄ contaminated with 41.9 % P₂0₅ + 1.8 %F~ at 90°C

On each material double tests were made in respective solution. The testing was performed according to the following procedure: exposure in three periods,

1 +3+3 days, activating in the beginning of each period with strip of Zn. Results on the individual specimen was taken as average of corrosion rate during periods 2 and 3. The results from the tests can be summarized according to the following:

Corrosion rate (mm/year)

1.5% HCI at boiling temperature 1 -2.5

30% H₂S0₄ at 80°C 0

50% H2SO₄ AT 90°C, 0.35-0.55 mixture of 25% formic acid + 50% acetic acid and 2000 ppm Cl~

0-0.02

43% H₃P0₄ contaminated with 41.9 % P₂0₅ + 1.8 %F^~ at 90°C 654 SMO 0.0581

Heat 10 0.0469

Heat 11 0.0438

EXAMPLE 2

Within among others the process-, refinery- and oil- and gas-industry it is usual with cooling of different agents by treated or untreated seawater. A common construction is that one uses a tubular heat exchanger with tubes that either are welded or introduced into in a tube sheet. A not totally unusual style for a tube heat exchanger is that the tubes are bent in U-shape and both the inlet and the outlet is done in the same tube gable. When these u-shaped tubes are produced a cold working are located in the bend for which a stress-relieving annealing may be performed. The tubular part is cooled with seawater whereby good corrosion resistance in chloride containing environments, especially seawater, is required. Corrosion in seawater is characterized by chloride induced local corrosion. The standard-method ASTM G48A will be used as test method for local corrosion in seawater, which is thought to simulate chlorinated seawater, the most corrosive state of seawater. It is established that cold working diminishes resistance to local corrosion. Subsequently test specimen were taken out, which were cold-worked with a reduction rate of 60 % and which then were tested according to the standard ASTM G48C, whereby a value for the Critical Pitting Temperature (CPT) of 92.5°C was obtained. For cold-worked specimen with a reduction rate of 60% for the reference steel UNS N08926 a CPT-value of 64°C was obtained. 254 SMO, which has a CPT-value of 87°C in annealed condition, obtains only 62.5°C to 72.5°C in CPT-value in cold-worked condition. However, the CPT- value of 92.5°C for the alloy according to the invention in cold-worked condition is very close the CPT-value of 100°C, which was obtained in tests of the same material in annealed condition. Accordingly, the alloy according to the invention shows a very good resistance to local corrosion in seawater irrespective the degree of cold working or whether the stress-retaining annealing was done or not. This makes the alloy and products manufactured of this alloy, such as e.g. tubes, especially seamless and seam-welded tubes very suitable for use in the application sea water cooling.

EXAMPLE 3

In order to find a suitable temperature for heat-treating annealing tests were performed on 8 heats at different temperatures during 1 hour. After studies on the micro structure the results can be summarized according to table 2:

Table 2 shows micro structure stability at different temperatures (°C).

O - no precipitations

"-" - not tested x - trace

X - phase

The annealing series made show that all variants show a clean austenitic structure at 1250°C.

EXAMPLE 4

In order to examine the hot-workability variants 1-10 were tested in Gleeble in order to determine the suitable forging temperature. The obtained data was evaluated with regard to the maximum ductility as well as the burning temperature defined as 0% ductility. The results can be summarized with the assistance of following equations: Maximum ductility:

129.8 - 1.86 % Mn - 87.86 % N - 7.48 % Mo

I burning-

1269 - 1.09 % Ni - 3.1 % Mn + 4.1 % Cr - 128.6 % N - 8.6 % Mo

The results for these equations and heats according to the invention and the reference heats are shown in table 3:

Table 3.

Heat Max. ductility [%] I burning L CJ

1 37.4476 1221.113

2 71.3628 1248.483

3 62.1660 1254.799

4 53.5968 1232.131

5 58.9132 1242.915

6 42.0072 1228.447

7 54.6832 1247.244

9 43.6148 1230.627

10 37.8548 1223.494

11 42.7952 1225.727

12 74.0520 1269.288

A 9.88848 1157.081

B 25.6860 1207.340

C 61.7480 1239.150

That manganese at Gleeble-testing detoriates the maximum ductility correlates with the forming of manganese sulfides in the grain boundaries. Besides manganese nitrogen and molybdenum are negative for the hot-ductility. Molybdenum and nitrogen have a solution hardening effect as well as they make the recrystallization more difficult, which gives a distinct result on the hot- ductility. Nickel, manganese, nitrogen and molybdenum decrease the burning temperature, while chromium increases it. In order to achieve a steel that is good from hot-working point of view the content of Chromium should instead be held as high as possible. In order to stabilize the alloy, nickel should to certain content replace nitrogen. Then nitrogen and molybdenum are added up to the desired corrosion resistance. Manganese will be totally avoided and the desired nitrogen solubility will instead be obtained by increasing the content of chromium.

EXAMPLE 5 Tests according to the standard ASTM G48 A were executed on material from all variants, except heat 8. The starting temperature was 25°C for all variants, except heats 11 and 12, which were tested at a starting temperature of 50°C. Double tests were made. The rise of the temperature was 5°C for all samples. The test solution, which was used, was the usual, 6% FeCI₃ without any addition of HCI. The results was taken as average of CPT for the two specimen. As the result from the best variants it appeared that pitting corrosion does not occur at the highest test temperature, which was 100°C. The electro-chemical testing was performed on all heats, except heat no. 8. In this case the environment was 3% NaCI-solution and the applied potential 600 mV, SCE. The starting temperature was 20°C, which then was stepped up by 5°C. Six specimen from each material heat were tested. The results from electrochemical testing appeared to be a CPT-value of between 85-95°C. EXAMPLE 6

The tensile strength was measured by tensile test at room temperature (RT) Figure 1 , 100°C Figure 2, and 200°C Figure 3. At each temperature two specimen of each material variant were tested. Variant 8 was not tested at 100°C. The result (yield strength and elongation) is presented as average of the two values from each material variant. The impact strength by impact testing at room temperature, see e 4 and -196°C, see figure 5. Generally three specimen were used at each temperature and the results are presented as average of these three. For heats 1-8 half specimen (5x10 mm cross section area) were used and for heats 11-12 entire test specimen (10x10 mm cross section area) were used. The yield strength for the best heats lies at 450 MPa at room temperature and at 320 MPa at 200°C. Elongation values (A) were generally high, 60-70 %, see Figures 6-8. The impact strength for the best heats is 300J/cm² at RT and ca 220 J/cm² at -196°C.

EXAMPLE 7

In order to measure the degree of intergranular corrosion Huey-testing was executed according to standard ASTM A262-C in 65% HN0₃, during 5 X 48 hours with double tests.

All heats were tested, except heat no. 8. The results are shown as average of two specimens average corrosion rate during the five periods. The corrosion rate for the tested heats is shown in Figure 9. It appears that the corrosion rate varies between 0.06 and 0.16 mm/year.

Claims

1. Super-austenitic stainless alloy with a combination of high corrosion resistance, good general corrosion resistance and good structure stability c h a racterized in, that it contains, (in weight-percent):

Cr 23.0-30.0

Ni 25.0-35.0

Mo 2.0-6.0

Mn 1.0-6.0 N 0-0.4

C up to 0.05

Si up to 1.0

S up to 0.02

Cu up to 3.0 W 0-6.0 a or more element of the Group Mg, Ce, Ca, B, La, Pr, Zr, Ti, Nd up to 2.0 and the balance being Fe and normally occurring impurities and steel making additions and with improved workability for the production of tubes, such as seamless tubes, especially suitable for use in acid environments and for chloride environments, such as sea water environments, especially sea water cooling.

2. Super-austenitic alloy according to claim ^characterized in, that content of Chromium is 24.0-30.0 weight-%, the content of nickel is 26.0-

35.0 weight-%, the content of Molybdenum is 3.7-6.0 weight-%, the content of Manganese is 2.0-6.0 weight-%, the content of carbon is max 0.03 weight-% and the content of Silicon is max 0.8 weight-%.

3. Super-austenitic alloy according to claim 1-2, ch a ra cte r i zed in, that the content of Sulfur is max 0.002 weight-%.

4. Super-austenitic alloy according to claim 1-3, c h a r a c t e r i z e d in, that the content of chromium is 27.0-29.0 weight-%, the content of nickel is 30.0-35.0 weight-%, the content of Molybdenum is 4.0-5.5 weight-%, the content of manganese is 3.0-6.0 weight-%, the content of carbon is max 0.03 weight-%, the content of Silicon is max 0,5 weight-% and the content of Sulfur is max 0,002 weight-%.

5. Super-austenitic alloy according to claim 1-4, c h a r a c t e r i z e d in, that the content of Copper is max 1 ,5 weight-% and the content of tungsten is 0- 4,0 weight-%.

6. Use of an alloy according to any of the preceding claims, which is cold bent to U-bends and which is used as heat-exchanger tubes in applications where high requirements on the resistance to chloride environments, such as sea water have to be fulfilled.