WO2008054300A9

WO2008054300A9 - Duplex stainless steel alloy and use of this alloy

Info

Publication number: WO2008054300A9
Application number: PCT/SE2007/050694
Authority: WO
Inventors: Kenneth Goeransson
Original assignee: Sandvik Intellectual Property; Kenneth Goeransson
Priority date: 2006-10-30
Filing date: 2007-10-02
Publication date: 2009-06-11
Also published as: CA2667436A1; MX2009004535A; SE530711C2; EP2082073A1; BRPI0717944A2; KR20090078813A; SE0602287L; JP2010508439A; EP2082073A4; CN101558180A; UA93587C2; EA200970433A1; EA014812B1; US20100316522A1; WO2008054300A1

Abstract

The present invention relates to a duplex stainless steel alloy containing in weight-%: C max 0.03%, Si < 0.30%, Mn 0-3.0%, P max 0,030%, S max 0,050%, Cr 25-29%, Ni 5-9%, Mo 4.5-8%, W 0-3%, Cu 0-2%, Co 0-3%, Ti 0-2%, Al 0-0.05%, B 0-0.01 %, Ca 0-0.01 %, and N 0.35-0.60%, balance Fe and normal occurring impurities, wherein the ferrite content is 30-70 volume- %, and wherein each weight-% of Mo above may optionally be replaced by two (2) weight-% W.

Description

Duplex stainless steel alloy

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a duplex stainless steel alloy, which is a steel alloy having a ferritic-austenitic matrix and especially high resistance to corrosion in combination with good structural stability and hotworkability. The ferrite content is 30- 70 volume-% and such steel alloys have a well balanced composition, which imparts the material corrosion properties, which make it suitable for use in for instance chloride-containing environments, such as in the sea.

BACKGROUND OF THE INVENTION AND PRIOR ART

Over the recent years, the environments in which corrosion resistant metallic materials were used became more aggressive and the requirements on the corrosion properties as well as on their mechanical properties increased. Duplex steel alloys, which were established as an alternative to other steel grades used until then, for example high alloyed austenitic steels, nickel-base alloys or other high alloyed steels, were also a part of that development. An established measure for the corrosion resistance in chloride-containing environments is the so-called Pitting Resistance Equivalent (abbreviated PRE), which is defined as

PRE=%Cr+3.3%Mo+16%N

where the percentage for each element allude to weight-percent. A higher numerical value indicates a better corrosion resistance in particular against pitting corrosion. The essential alloying elements, which affect this property, are according to the for- mula Cr, Mo₁ N. An example for such a steel grade is evident from EP0220141 , which hereby through this reference is included in this description. This steel grade with the denotation SAF2507 (UNS S32750) was mainly alloyed with high contents of Cr, Mo and N. It is consequently developed against this prop- erty with above all good resistance to corrosion in chloride environments.

In recent times also the elements Cu and W have shown to be efficient alloying additions for further optimization of the steel's corrosion properties in chloride environments. The element W has by then been used as substitute for a portion of Mo₁ as for example in the commercial alloy DP3W (UNS S39274) or Ze- ronl OO, which contain 2.0% respectively 0.7% W. The latter contains even 0.7% Cu with the purpose to increase the corrosion resistance of the alloy in acid environments.

The alloying addition of tungsten led to a further development of the measure for the corrosion resistance and thereby the PRE- formula to the PREW-formula, which also makes the relationship between the influence of Mo and W on the alloys corrosion resistance clearer:

PREW=%Cr+3.3(%Mo+0.5%W)+16%N,

such as described for example in EP 0 545 753. This publication refers to a duplex stainless alloy with generally improved corrosion properties.

The above-described steel grades have a PRE/PREW-number, irrespective method of calculation, which lies above 40. From the alloys with good corrosion resistance in chloride environments also SAF 2906 shall be mentioned, which composition appears from EP 0 708 845. This alloy, which is characterized by higher contents of Cr and N compared to for example SAF2507, has shown being especially suitable for use in environments, where resistance to intergranular corrosion and corrosion in ammonium carbamate is of importance, but it has also a high corrosion resistance in chloride-containing environments.

US-A-4 985 091 describes an alloy intended for use in hydrochloric and sulfuric acid environments, where mainly intergranular corrosion arises. It is primarily intended as alternative to recently used austenitic steels.

US-A-6 048 413 describes a duplex stainless alloy as alternative to austenitic stainless steels, intended for use in chloride-containing environments.

EP 0 683 241 discloses a duplex stainless steel alloy having a composition resulting in improved properties with respect to resistance to both stress corrosion cracking and pitting in chlo- rideion-containing environments than most other duplex stainless steel alloys known. However, this alloy as well as the alloys discussed above is highly susceptible to intermetallic pre- cipitation, especially sigma phase precipitation, which makes the material hard and brittle. Accordingly, the production of a material with good ductility by use of the duplex stainless steel alloy according to EP 0 683 241 is made very difficult.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a duplex stainless steel alloy of the type defined above and especially in the European patent 0 683 241 , which has improved properties, especially ductility and toughness, with respect to such an alloy already known while maintaining at least similar levels of corrosion resistance as such an alloy. The alloy should have a good hotworkability.

This object is according to the invention obtained by providing a duplex stainless steel alloy, which contains in weight-%: C max

0.03%, Si < 0.30%, Mn 0-3.0%, P max 0,030%, S max 0,050%,

Cr 25-29%, Ni 5-9%, Mo 4.5-8%, W 0-3%, Cu 0-2%, Co 0-3%, Ti

0-2%, Al 0-0.05%, B 0-0.01 %, Ca 0-0.01 %, and N 0.35-0.60%, balance Fe and normal occurring impurities, wherein the ferrite content is 30-70 volume-%, and wherein each weight-% of Mo above may optionally be replaced by two (2) weight-% W.

It has been found that a duplex stainless steel alloy with this composition has especially an increased ductility and toughness with respect to the alloy according to EP 0 683 241 , and it has also an increased corrosion resistance. By reducing the Si content to be below 0.30 weight-% a significant reduction in sigma phase precipitation is achieved, which is the key to the increased ductility and toughness of the steel alloy according to the invention. Thus, it has been found that when using a comparatively high content of Mo it is highly efficient to reduce the content of Si for reducing the risk for intermetallic precipitations.

According to an embodiment of the invention the content of Si is max 0.25 weight-%, which makes the steel alloy even less prone to sigma-formation for increasing the ductility and toughness of the material. It is expected that the same would be valid if Molybdenum would be partly or entirely replaced by Tungsten.

According to another embodiment of the invention the content of Si is max 0.23 weight-%.

According to another embodiment of the invention the content of Mo is a weight-% and the content of W is b weight-%, wherein a+b/2>5.0. Such a high content of Mo and/or W results in excellent resistance to corrosion, especially pitting- and crevice corrosion, but the increased risk for intermetallic precipitations with such high contents of these elements is efficiently counteracted by the combination thereof with the low content of Si. According to another embodiment of the invention a>5.0. It is pointed out that claim 1 is to be interpreted as when starting from the content intervals of Mo (4.5-8%) and W (0-3%) it is possible to replace each % of Mo by 2% of W or conversely, so that the content of Mo may for example be 3% when the content of W is at least 3%. According to a preferred embodiment a+b/2<8, i.e. the total content of Mo and W does not exceed 8%, for keeping the costs thereof at a reasonable level. According to another preferred embodiment b=0, i.e. the alloy contains only Mo.

According to yet another embodiment of the invention the content of Co is 0-0.010 weight.-%. Co is an expensive material, and it has been found that the structure's ability as well as the corrosion resistance improvement influence thereof is not an essential factor in a steel alloy with a composition according to the present invention.

According to another embodiment of the invention the content of ferrite is 40-60 volume-%.

According to another embodiment of the invention the average PRE- or PREW-value of the two phases of the alloy exceeds 44, whereby PRE = %Cr + 3.3%Mo + 16%N and PREW = %Cr + 3.3(%Mo + 0.5%W) + 16%N, wherein % is weight-%. The PRE- or PREW-value for both the ferrite and austenite phase may be higher than 47, preferably higher than 48,5, and said average PRE- or PREW-value may be higher than 48, preferably higher than 49. It has turned out that the pitting and crevice corrosion resistance in the steel alloy according to the invention is especially increased by increasing the PRE- or PREW-value of the phase with the lowest such value. It has been found that the steel alloy according to the invention will still have a good hot- workability with a PRE- or PREW-value higher than 49.

According to another embodiment of the invention the ratio be- tween PRE(W)-value for the austenite phase and PRE(W)-value for the ferrite phase lies between 0.90 and 1.15, preferably between 0.95 and 1.05.

An alloy according to the present invention is suitable to be used in chloride-containing environments in product forms such as bars, tubes, such as welded and seamless tubes, plate, strip, wire, welding wire, constructive parts, such as for example pump, valves, flanges and couplings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows a calculated phase content of a duplex stainless steel alloy according to an embodiment of the invention as a function of temperature,

Fig 2 is a graph similar to Fig 1 for a reference steel alloy according to EP 0 683 241 , and

Fig 3 is a micrograph of continuously cooled samples of the al- loys according to Fig 1 and Fig 2 according to three different cooling speeds.

DETAILED DESCRIPTION OF THE INVENTION

Good corrosion resistance properties as well a high ductility and toughness is obtained by the combination of elements in a duplex stainless steel alloy according to the invention. This steel alloy has also good workability, which enables for example extrusion to seamless tubes. The alloy according to the invention contains (in weight-%): C max 0.03%

Si < 0.30%

Mn 0-3.0%

P max 0.030%

S max 0.050%

Cr 25-29%

Ni 5-9%

Mo 4.5-8%

W 0-3%

Cu 0-2%

Co 0-3%

Ti 0-2%

Al 0-0.05%

B 0-0.01 %

Ca 0-0.01 %

N 0.35-0.60%

balance Fe and normal occurring impurities, wherein the ferrite content is 30-70 volume-%, and wherein each weight-% of Mo above may optionally be replaced by two (2) weight-% W.

Carbon (C) has limited solubility in both ferrite and austenite. The limited solubility implies a risk of precipitation of chromium carbides and the content should therefore not exceed 0.03 weight-%, preferably not exceed 0.02 weight-%.

Silicon (Si) is utilized as desoxidation agent in the steel production and it increases the flowability during production and welding. However, too high contents of Si lead to precipitation of unwanted intermetallic phase, wherefore the content is limited to below 0.30 weight-%, preferably max 0.25 weight-%, more preferably max 0.23 weight-%.

Manganese (Mn) is added in order to increase the N-solubility in the material. However, it has shown that Mn only has a limited influence on the N-solubility in the type of alloy in question. In- stead there are found other elements with higher influence on the solubility. Besides, MrV in combination with high contents of sulfur can give rise to formation of manganese sulfides, which act as initiation-points for pitting corrosion. The content of Mn should therefore be limited to between 0-3.0 weight-%, preferably 0.5-1.2 weight-%.

Phosphorus (P) is a common impurity element. If present in amounts greater than approximately 0,05%, it can result in adverse effects on e.g. hot ductility, weldability and corrosion resistance. The amount of P in the alloy should therefore not exceed 0.05%.

Sulfur (S) influences the corrosion resistance negatively by forming soluble sulfides. Furthermore, the hotworkability deteriorates, for what reason the content of sulfur is limited to max 0.030 weight-%, preferably less than 0.010 weight-%.

Chromium (Cr) is a much active element in order to improve the resistance to a majority of corrosion types. Furthermore, a high content of chromium implies that one gets a very good N-solu- bility in the material. Thus, it is desirable to keep the Cr-content as high as possible in order to improve the corrosion resistance. For very good amounts of corrosion resistance the content of chromium should be at least 25 weight-%. However, high contents of Cr increase the risk for intermetallic precipitations, for what reason the content of chromium must be limited up to max 29 weight-%, preferably 25.5-28 weight-%.

Nickel (Ni) is used as austenite stabilizing element and is added in suitable contents in order to obtain the desired content of fer- rite. In order to obtain the desired relationship between the austenitic and the ferritic phase with between 30-70 volume-% ferrite, an addition of 5-9 weight-% nickel is required, and it is preferably 6-8 weight-%. Molybdenum (Mo) is an active element which improves the resistance to corrosion in chloride environments as well as preferably in reducing acids. A too high Mo-content in combination with high Cr-contents, implies that the risk for intermetallic precipitations increases. The Mo-content in the present invention should lie in the range of 4.5-8 weight-%, preferably above 5.0 weight-%, in which each weight-% of Mo may optionally be replaced by 2 weight-% W.

Tungsten (W) increases the resistance to pitting- and crevice corrosion. But the addition of too high contents of tungsten in combination with that the Cr-contents as well as Mo-contents are high, means that the risk for intermetallic precipitations increases. The W-content in the present invention should lie in the range of 0-3.0 weight-%.

Copper (Cu) may be added in order to improve the general corrosion resistance in acid environments such as sulfuric acid. At the same time Cu influences the structural stability. However, thigh contents of Cu imply that the solid solubility will be exceeded. Therefore the Cu-content should be limited to max 2.0 weight-%, preferably between 0 and 1.5 weight-%, more preferred 0.1-0.5 weight-%.

Cobalt (Co) has properties that are intermediate between those of iron and nickel. Therefore, a minor replacement of these elements with Co, or the use of Co-containing raw materials (Ni scrap metal usually contains some Co, in some cases in quantities greater than 10%) will not result in any major change in properties. Co can be used to replace some Ni as an austenite-stabilizing element. Co is a relatively expensive element, so the addition of Co is limited to be within the range of 0-3 weight-%.

Titanium (Ti) has a high affinity for N. It can therefore be used e.g. to increase the solubility of N in the melt and to avoid the formation of nitrogen bubbles during casting. However, excessive amounts of Ti in the material causes precipitation of nitrides during casting, which can disrupt the casting process and the formed nitrides can act as defects causing reduction in corrosion resistance, toughness and ductility. Therefore, the addition of Ti is limited to 2 weight-%.

Aluminium (Al) and Calcium (Ca) are used as desoxidation agents at the steel production. The content of Al should be lim- ited to max 0.05 weight-%, preferably max 0.03%, in order to limit the forming of nitrides. Ca has a favourable effect on the hotductility. However, the Ca-content should be limited to max 0.010 weight-% in order to avoid an unwanted amount of slag.

Boron (B) may be added in order to increase the hotworkability of the material. At a too high content of Boron the weldability as well as the corrosion resistance could deteriorate. Therefore, the content of boron should be limited to max 0.01 weight-%.

Nitrogen (N) is a very active element, which increases the corrosion resistance, the structural stability as well as the strength of the material. Furthermore, a high N-content improves the recovering of the austenite after welding, which gives good properties within the welded joint. In order to obtain a good effect of N, at least 0.35 weight-% N should be added. At high contents of N the risk for precipitation of chromium nitrides increases, especially when simultaneously the chromium content is high. Furthermore, a high N-content implies that the risk for porosity increases because of the exceeded solubility of N in the smelt. For these reasons the N-content should be limited to max 0.60 weight-%, preferably >0.35-0.45 weight-% N is added.

The content of ferrite is important in order to obtain good mechanical properties and corrosion properties as well as good weldability. From a corrosion point of view and a point of view of weldability a content of ferrite between 30-70% is desirable in order to obtain good properties. Furthermore, high contents of ferrite imply that the impact strength at low temperatures as well as the resistance to hydrogen-induced brittleness risks deteriorating. The content of ferrite is therefore 30-70 volume-%, preferably 40-60 volume-%.

DESCRIPTION OF A PREFERRED EMBODIMENT

Two experimental alloys were produced in order to test primarily the effect of different concentrations of Si. Table 1 below shows the content of the two alloys No. 1 and No. 2, in which No. 1 is a duplex stainless steel alloy according to an embodiment of the present invention and alloy No. 2 is such an alloy according to EP 0683241.

Table 1

Furthermore, the alloys were modelled using the Thermo-Calc software with database CCTSS (a slightly modified version of the commercial database TCFE3 with improved models for e.g. duplex alloy compositions). Fig 1 and 2 show the calculated phase contents of alloy No. 1 and alloy No. 2, respectively, as a function of the temperature. In these Figures:

1 : The ferrite content. It is seen that for the alloy according to the present invention (Fig 1 ) a heat treatment in the region of 1 100 - 1 300⁰C is needed for obtaining a ferrite content desired. 2: The austenite content. The heat treatment is carried out so that only a ferrite phase and an austenite phase are obtained.

3: The content of N

4: Liquid metal 5: Sigma-phase. The formation thereof may be avoided by rapid cooling.

6: Content of Cr₂N, which cause brittleness and reduction in corrosion resistance.

7: Carbide content, which should be kept low for not influencing welds. A high tendency for carbide precipitation leads to the risk of reduced corrosion resistance near welds. The equilibrium amount of carbides should therefore be kept low.

8: lntermetallic phase. The sum of this and the sigma phase is to be kept as low as possible.

Table 2

Table 2 above shows the total PRE of the two alloys and the predicted PRE for each phase when quenched from 1 100⁰C, as well as the ratio between PRE in the austenite and in the ferrite. It also shows the predicted ferrite content after a quench from 1100°C and finally the predicted dissolution temperatures for Cr₂N and sigma (σ) phase, and the predicted presence of any precipitates at 1100⁰C. Since the precipitation of Cr₂N is more rapid than that of σ phase, two T_max,c_r2N are presented, one for the case for slow cooling when equilibrium amounts of σ are allowed to precipitate ("with σ") and another for rapid cooling when σ does not precipitate ("without s"). It is clear that both alloys fulfil the requirements on ferrite content, total PRE as well as PRE balance and minimum PRE in each phase as stated in our WO 03020994.

Sample manufacturing

The alloys were produced by melting, casting of ingots and finally press forging. Table 3 shows the results of the forging. The forging was interrupted when severe surface defects began to form, and the total reduction of cross-sectional area during the forging process can thus be used as an estimate of the forge- ability of the two alloys.

Table 3

The forged bars were annealed at 1 100⁰C, followed by quenching in water before any further processing was begun. The pre- material used for samples was annealed once more, after sectioning into smaller pieces, at 1 100⁰C for 1 h, followed by water quench. After this treatment, the different samples were machined. Testing

Impact testing

Impact testing was performed on 10x10mm Charpy v-notch samples (55mm long) in four different materials conditions: as- annealed (i.e. 1 100°C/water quench) and with an additional anneal of the impact samples at a lower temperature. Table 4 shows the different materials conditions as well as the resulting impact toughness values. Two samples were tested for each composition and annealing condition.

Table 4

Alloy 1 , with a high Mo content and low Si and Co contents has a good impact toughness provided a sufficiently high annealing temperature is used. This Table illustrates a weakness of the alloy 2 according to EP 0 683 241 , namely that a Si content higher than 0.5% together with a high Mo content gives a potentially brittle material. Just reducing the Si content (as in the alloy 1 according to the present invention) gives a large improvement in toughness.

Continuous cooling

9 samples from each heat was annealed at 1100⁰C and then reheated to three different temperatures: 1050, 1 100 and 1150⁰C from each heat, respectively. The samples were cooled at three different constant cooling rates from the different holding temperatures: 20, 60 and 140°C/min. This means that 9 different annealing cycles were used for each heat. No nitrides were found in any of the samples. Table 5 summarises observations made by optical microscopy. A relative ranking index is used for the σ phase content of different samples, where:

0: no σ phase detected

1 : 1-2 σ phase particles on average detected in a field of view at

50Ox magnification

2: small amounts of σ phase detected at 50Ox magnification (but more than 2 particles/field of view)

3: relatively large amounts of σ, but with less than 5% of ferrite transformed

4: more than 5% of the ferrite transformed to σ

5. more than 25% of the ferrite transformed to σ

6: more than 50% of the ferrite transformed to σ

Table 5

It is shown that alloy 1 is slightly less prone to σ precipitation than alloy 2. It is pointed out that a "note" of 2, preferably 1 , is necessary for making it possible to properly manufacture the material in question.

Fig 3 shows micrographs of the continuously cooled samples heated to 1100⁰C. Light colour is austenite, brown is ferrite and blackish is σ-phase. It is shown that the formation of σ-phase (blackish) is remarkably weaker for alloy No. 1 according to the present invention than for alloy No. 2 according to EP 0 683 241 , which is obviously due to the lower content of Si.

Mechanical properties

Table 6 shows results from tensile testing. Alloy No. 2 is apparently less ductile than alloy No. 1 according to the invention.

Table 6. Results from tensile testing. Two samples from each heat

Corrosion testing

Critical crevice temperature (CCT) according to MTI-2 and criti- cal pitting temperature (CPT) in "Green Death" solution (1 %FeCI₃+1 %CuCl₂+11 %H₂Sθ₄+1.2%HCI) is shown in Table 7. There is very little difference in crevice corrosion resistance between the different alloys. The assumption that pitting and crevice corrosion resistance in duplex alloy is mainly determined by the PRE of the phase with lowest PRE agrees with the fact that alloy 1 has the highest CCT. Furthermore, improved behaviour of Alloy 1 with respect to Alloy 2 appears in the form of a lower weight loss due to corrosion and higher maximum temperatures.

Table 7. Results from crevice corrosion testing according to MTI-2, pitting corrosion in Green Death solution and pitting corrosion in ferric chloride. Two samples/alloys were used for each test.

Summary

The alloy (No. 2) corresponding to EP 0 683 241 is highly susceptible to sigma phase precipitation, which makes the production of a material with good ductility very difficult. This problem is solved by lowering the Si content and a good balance between the PRE-values of the two phases. Furthermore, the alloy No. 2 has a low forgeability. By reducing the Si content of an alloy of the type defined in EP 0 683 241 , i.e. by using a composition of alloy No. 1 , not only will the ductility and toughness increase, the corrosion resistance is increased as well, which in fact is an effect that was quite unexpected.

Claims

1. A duplex stainless steel alloy, characterized in that it con- tains in weight-%:

C max 0.03%

Si < 0.30%

Mn 0-3.0%

P max 0.030%

S max 0.050%

Cr 25-29%

Ni 5-9%

Mo 4.5-8%

W 0-3%

Cu 0-2%

Co 0-3%

Ti 0-2%

Al 0-0.05%

B 0-0.01 %

Ca 0-0.01 %

N 0.35-0.60% balance Fe and normal occurring impurities, wherein the ferrite content is 30-70 volume-%, and wherein each weight-

% of Mo above may optionally be replaced by two (2) weight-% W.

2. Alloy according to claim 1 , characterized in that the content of Si is max 0.25 weight-%.

3. Alloy according to claim 1 , characterized in that the con- \ tent of Si is max 0.23 weight-%.

4. Alloy according to any of the preceding claims, characterized in that the content of Mo is a weight-% and the content of W is b weight-%, wherein a + b/2 > 5.0.

5. Alloy according to claim 4, characterized in that a > 5.0.

6. Alloy according to claim 4, characterized in that a + b/2 < 8.

7. Alloy according to any of the preceding claims, characterized in that the content of Co is 0-0.010 weight-%.

8. Alloy according to any of the preceding claims, characterized in that the content of Cr is 25.5-28 weight-%.

9. Alloy according to any of the preceding claims, characterized in that the content of Ni is 6-8 weight-%.

10. Alloy according to any of the preceding claims, characterized in that the content of N is 0.35-0.45 weight- %.

1 1. Alloy according to any of the preceding claims, character- ized in that the content of ferrite is 40-60 volume-%.

12. Alloy according to any of the preceding claims, characterized in that the average PRE- or PREW-value of the two phases of the alloy exceeds 44, whereby PRE = %Cr + 3.3%Mo + 16%N and PREW = %Cr + 3.3(%Mo + 0.5%W) +

16%N, wherein % is weight-%.

13. Alloy according to claim 12, characterized in that the PRE- or PREW-value for both the ferrite and the austenite phase is higher than 47, preferably higher than 48.5, and that said average PRE- or PREW-value is higher than 48, preferably higher than 49.

14. Alloy according to claim 12 or 13, characterized in that the ratio between PRE(W)-value for the austenite phase and PRE(W)-value for the ferrite phase lies between 0.90 and 1.15, preferably between 0.95 and 1.05.

15. Use of an alloy according to any of the preceding claims in chloride containing environments in product forms such as bars, tubes, such as welded and seamless tubes, plate, strip, wire, welding wire, constructive parts, such as for example pumps, valves, flanges and couplings.