Use of an austenitic stainless alloy

Abstract

The invention relates to the use of an object of an austenitic stainless alloy in high temperature and high-pressure environment wherein said austenitic stainless alloy comprises in weight%:C less than 0.03; Si less than 1.0; Mn less than or equal to 1.2; Cr26.0 to 30.0; Ni 29.0 to 37.0; Mo 6.1 to 7.1; N 0.25 to 0.36;P less than or equal to 0.04; Sless than or equal to 0.03; Cu less than or equal to 0.4; balance Fe and unavoidable impurities. The invention also relates to superheater comprising at least one object made of said steel.

Description

[0001] USE OF AN AUSTENITIC STAINLESS ALLOY

[0002] Technical field

[0003] The present disclosure relates to use of an object of an austenitic stainless alloy comprising a low content of manganese in combination with a high content of nitrogen and a high content of molybdenum and nickel and chromium in a high temperature and high-pressure environment.

[0004] Background

[0005] A recovery boiler is an important part of the kraft process in the pulp and paper industry. It comprises of several components such as water walls, superheaters and economizers. Black liquor is formed during the kraft process, and it contains a lot of compounds which will affect the surfaces of the components of the recovery boiler by introducing corrosion. In order to withstand the most corrosive environment of the recovery boilers, such as those found in superheaters, composite tubes are used. However, these tubes may have a problem with general corrosion as well as a reduced heat transfer due to thick tube walls and may also have problems with mechanical strength.

[0006] There is a need to reduce the costs of operation and one way to achieve this is to increase the size of recovery boilers and this in turn will increase the problems mentioned above. Another way to reduce costs is to reduce the intervals between maintenance stops. The maintenance stops are usually necessary due to replacement of superheater tubes which have corroded and therefore have to be replaced.

[0007] Hence, there is a need for a material which will be able to solve or at least mitigate the problems mentioned above and the present invention therefore aims at solving or at least reducing the afore mentioned problems. Summery

[0008] The present disclosure therefore relates a use of an object, which comprises an austenitic stainless alloy having the following composition weight% (wt%):

[0009] C less than 0.03;

[0010] Si less than 1.0;

[0011] Mn less than or equal to 1.2;

[0012] Cr 26.0 to 30.0;

[0013] Ni 29.0 to 37.0;

[0014] Mo 6.1 to 7.1;

[0015] N 0.25 to 0.36;

[0016] P less than or equal to 0.04

[0017] S less than or equal to 0.03;

[0018] Cu less than or equal to 0.4; balance Fe and unavoidable impurities in high temperature and high-pressure environments. According to the present invention, the term “high temperature” is intended to mean temperatures in the range from 450 to 550 °C and by the term “high pressure” is intended to mean a pressure from 60 to 150 bar, such as about 140 to 150 bar, such as 140 bar. This type of environment is for example found in a superheater in a recover boiler, such as a black liquor recovery boiler having a flue gas environment. A superheater of a recovery boiler is placed either in the furnace of the recovery boiler, typically at the top of the furnace, or in the flue gas duct downstream of the furnace, where the flue gases from the furnace are led. The superheaters may be placed either in the same flue gas duct or in parallel flue gas ducts.

[0019] The present invention thus also relates to a superheater comprising at least one object consisting or comprising of the alloy as defined hereinabove or hereinafter. According to embodiments, the object may be a formed object, such as a tube, such as a superheater tube, which may be a straight or bent. According to embodiments, the superheater tube may only consist of the alloy as defined hereinabove or hereinafter. According to embodiments, the superheater may comprise superheater tubes of other alloys as well as superheater tubes of the alloy as defined hereinabove or hereinafter. According to embodiments, the superheater may comprise at least one tube of the alloy as defined hereinabove or hereinafter. According to embodiments, the superheater may only comprise of superheater tubes of the alloy as defined hereinabove or hereinafter

[0020] The present invention also relates to a recovery boiler comprising the superheater as defined hereinabove or hereinafter.

[0021] The present invention also relates to a method for converting energy into heat and / or electricity in a pulp and / or paper production plant, wherein at least one part of the equipment of the plant is made from an austenitic stainless alloy as defined hereinabove or hereinafter. According to embodiment, said at least one part is a tube, such as a superheater tube as defined hereinabove or hereinafter.

[0022] The present invention also relates to a method for modifying a superheater in a pulp and / or paper production plant by replacing at least one of the superheater tubes with a superheater tube comprising or consisting of the alloy as defined hereinabove or hereinafter.

[0023] Figures

[0024] Figures la and lb show a comparison of the thermal transfer between a composite tube and a tube of ; and

[0025] Figure 2 shows impact toughness result after aging a sample of Alloy 1 of the

[0026] Examples.

[0027] Detailed description

[0028] Hence, the present invention relates to a use of an object which comprises an austenitic stainless alloy having the following composition weight% (wt%): C less than 0.03;

[0029] Si less than 1.0;

[0030] Mn less than or equal to 1.2;

[0031] Cr 26.0 to 30.0;

[0032] Ni 29.0 to 37.0;

[0033] Mo 6.1 to 7.1;

[0034] N 0.25 to 0.36;

[0035] P less than or equal to 0.04

[0036] S less than or equal to 0.03;

[0037] Cu less than or equal to 0.4; balance Fe and unavoidable impurities in high temperature and high-pressure environments.

[0038] It has surprisingly been shown that when an object of the present austenitic stainless alloy as defined hereinabove or hereinafter is used in a superheater, , the strength and impact toughness even after prolonged exposure to temperature will be excellent even though the high temperature environment. This is very surprising, especially in regard to the excellent impact toughness even after prolonged exposure to temperature, as alloys having this high Mo content in combination with the Cr content would normally not have this high impact toughness after ageing at these high temperatures. Thus, alloys having the present composition are normally used in lower temperatures due to the content of Mo and Cr as these should not work in these temperatures. As this alloy will provide an object with higher strength and excellent impact toughness even though the harsh environments, an object made thereof will be able to have thinner walls, from 3 to 10 mm wall thickness, but still withstand high pressure, which in turn will provide for lower weight of the object as well as better heat transfer.

[0039] Furthermore, it has been found that the present alloy as defined hereinabove or hereinafter will also have corrosion resistance in the present environment which is surprising as alloys containing high amounts of nickel are usually sensitive to corrosion due to the high sulphur content in the flue gasses of the recovery boilers.

[0040] According to embodiments of the present disclosure, the tube, such as the superheater tube, is consisting of the alloy as defined hereinabove or hereinafter. The advantage of having a such a tube is that no post-welding processes are required in order to achieve the desired properties of a superheater tube.

[0041] As stated above, an object composed or consisting of the austenitic stainless alloy as defined hereinabove or hereinafter will have high corrosion resistance and good structure stability. By good structure stability is meant that there will almost be no precipitates of intermetallic phases formed in the austenitic stainless alloy during the manufacturing process or during use of the object. Furthermore, an object composed or consisting of the austenitic stainless alloy as defined hereinabove or hereinafter will have a combination of high impact toughness, high strength, such as yield strength and tensile strength, and good ductility as well as very good corrosion properties and good weldability even in harsh environment.

[0042] Hereinafter, the alloying elements of the austenitic stainless alloy as defined hereinabove or hereinafter are discussed, wherein wt% is weight%:

[0043] Carbon (C): less than or equal to 0.03 wt%

[0044] C is an impurity contained in the austenitic stainless alloy. When the content of C exceeds 0.03 wt%, the corrosion resistance is reduced due to the precipitation of chromium carbide in the grain boundaries. Thus, the content of C is less than or equal to 0.03 wt%, such as less than or equal to 0.02 wt%.

[0045] Silicon (Si): less than or equal to 1.0 wt%

[0046] Si is an element which may be added for deoxidization. However, Si will promote the precipitation of the intermetallic phases, such as the sigma phase, therefore Si is contained in a content of 1.0 wt% or less, such as 0.5 wt% or less. According to one embodiment, Si is more than 0.01 wt%. According to one embodiment, Si is less than 0.3 wt%. According to yet an embodiment, Si is from 0.1 to 0.3 wt%.

[0047] Manganese (Mn): less than or equal to 1.2 wt%

[0048] Mn is used in most stainless alloys because Mn will form MnS, which will improve the hot ductility. Mn is also considered to be beneficial for increasing strength in most austenitic stainless alloys when added in high amounts (such as around 4 wt%). However, it has, for the austenitic stainless alloy as defined hereinabove or hereinafter, surprisingly been found that a content of Mn above 1.5 wt%, will reduce the strength of the austenitic stainless alloy, therefore, the content of Mn is less than or equal to 1.2 wt%, such as less than or equal to 1.1 wt%, such as less than or equal to 1.0 wt%. According to one embodiment, the content of Mn is from 0.01 to 1.1 wt%. According to another embodiment, Mn is from 0.6 to 1.1 wt%.

[0049] Nickel (Ni): 29 wt% to 37 wt%

[0050] Nickel is together with Cr and Mo beneficial for improving the resistance to stress corrosion cracking in the austenitic stainless alloys. Additionally, nickel is also an austenite stabilizing element and will also reduce the precipitation of intermetallic phases in the grain boundaries of the austenitic stainless steel, especially when it is exposed to a temperature interval of 600- 1100°C. The grain boundary precipitates may affect the corrosion resistance negatively. The nickel content is therefore at least or equal to 29 wt%, such as at least 31 wt%, such as at least 34 wt%. However, increased nickel content will decrease the solubility ofN. Therefore, the maximum content of Ni is less than or equal to 37 wt%, such as less than or equal to 36 wt%. According to one embodiment, the Ni content is from 34 to 36 wt%.

[0051] Chromium (Cr): 26 to 30 wt%

[0052] Cr is the most important element in stainless alloys as Cr is essential for creating the passive film, protecting the stainless alloy from corroding. Also, the addition of Cr will increase the solubility of N. When the content of Cr is less than 26 wt%, the pitting corrosion resistance for the present austenitic stainless alloy will not be sufficient. Additionally, when the content of Cr is more than 30 wt%, secondary phases, such as nitrides and sigma phase will be formed, which will adversely affect the corrosion resistance. Accordingly, the content of Cr is therefore from 26 to 30 wt%, such as more than 26 wt%, such as from 26 to 29 wt%, such as from 26 to 28 wt%, such as of more than 26 to 29 wt%, such as of more than 26 to 28 wt%.

[0053] Molybdenum: (Mo): 6.1 to 7.1 wt%

[0054] Mo is effective in stabilizing the passive film formed on the surface of the austenitic stainless alloy and is also effective in improving the pitting resistance. When the content of Mo is less than 6.1 wt%, the corrosion resistance against pitting will not be high enough for the austenitic stainless alloy as defined hereinabove or hereinafter. However, a too high content of Mo will promote the precipitation of intermetallic phases, such as sigma phase and also deteriorate the hot workability. Accordingly, the content of Mo is from 6.1 to 7.1 wt%, such as from 6.3 to 6.8 wt%.

[0055] Nitrogen (N): 0.25 to 0.36 wt%

[0056] N is an effective element for increasing the strength in austenitic stainless alloy by using solution hardening. N is also beneficial for the structure stability. Furthermore, N will improve the deformation hardening during cold working. When the content of N is less than 0.25 wt%, the neither the strength or nor the ductility will be high enough. If the content of N is more than 0.36 wt%, the flow stress will be too high for obtaining efficient hot workability. Thus, in the present disclosure, the inventors have surprisingly found that an austenitic stainless alloy having a combination of both improved ductility and yield strength will be obtained if the content of N is from 0.25 to 0.36 wt%, such as from 0.26 wt% to 0.33 wt%, such as 0.26 to 0.30 wt%. Phosphorus (P): less than or equal to 0.04 wt%

[0057] P is considered to be an impurity and it is well known that P will affect the hot workability negatively. Accordingly, the content of P is set at less than or equal to 0.04 wt% or less such as less than or equal to 0.03 wt%.

[0058] Sulphur (S): less than or equal to 0.03 wt%

[0059] S is considered to be an impurity as it will deteriorate the hot workability. Accordingly, the allowable content of S is less than or equal to 0.03 wt%, such as less than or equal to 0.02 wt%.

[0060] Copper (Cu): less than or equal to 0.4 wt%

[0061] Cu is an optional element and is considered as an impurity. The present stainless alloy comprises Cu due to the raw material used as the manufacturing material. The content of Cu should be as low as possible, and therefore the level of Cu for the present alloy is less than or equal to 0.4 wt% as above this level the mechanical properties will be negatively affected. According to one embodiment, Cu may be present in an amount from 0.001 to 0.4 wt%. .

[0062] The austenitic stainless alloy as defined hereinabove or herein after may optionally comprise one or more of the following elements selected from the group of Al, V, Nb, Ti, O, Zr, Hf, Ta, Mg, Pb, Co, Bi, Ca, La, Ce, Y and B. These elements may be added during the manufacturing process in order to enhance e.g. deoxidation, corrosion resistance, hot ductility and / or machinability. However, as known in the art, the addition of these elements has to be limited depending on which element is present. Thus, if added the total content of these elements is less than or equal to 1.0 wt%.

[0063] The term "impurities" as referred to herein is intended to mean substances that will contaminate the austenitic stainless alloy when it is industrially produced, due to the raw materials such as ores and scraps, and due to various other factors in the production process and are allowed to contaminate within the ranges not adversely affecting the austenitic stainless alloy as defined hereinabove or hereinafter.

[0064] According to one embodiment, the alloy as defined hereinabove or hereinafter consist of the following:

[0065] C less than 0.03;

[0066] Si less than 1.0;

[0067] Mn less than or equal to 1.2;

[0068] Cr 26.0 to 30.0;

[0069] Ni 29.0 to 37.0;

[0070] Mo 6.1 to 7.1;

[0071] N 0.25 to 0.36;

[0072] P less than or equal to 0.04

[0073] S less than or equal to 0.03;

[0074] Cu less than or equal to 0.4; and optionally one or more elements of the group of Al, V, Nb, Ti, O, Zr, Hf, Ta, Mg, Pb, Co, Bi, Ca, La, Ce, Y and B less than or equal to 1.0 wt; balance Fe and unavoidable impurities.

[0075] Further, when the expression “less than” is used, it is to be understood that unless stated otherwise, the lower limit is 0 wt%.

[0076] The present disclosure is further illustrated by the following non-limiting examples:

[0077] Examples

[0078] In order to compare thermal transfer between a composite tube and a tube consisting only of the alloy according to Table 1, the temperature profiles were calculated for a temperature of 550 °C. The result is shown in Figure la and Figure lb. SI is furnace side and S2 is steam side. Figure la shows the thermal transfer of a composite tube consisting of an alloy sold under the trademark Sanicro® 28 (UNS N08028) and an alloy of HT 7 (UNS N08605). The outer layer of the composite tube was of Sanicro® 28.

[0079] The dimensions were accordingly: 63.5 x 6.35 mm.

[0080] (4.88 mm min wall HT7)

[0081] (1.65 mm wall Sanicro® 28)

[0082] Calculations:

[0083] 1 1.65 4.88

[0084] Which gave an overall heat transfer coefficient K:

[0085] K=4240 W / m2K

[0086] Figure lb shows the thermal transfer of a tube of Alloy 1.

[0087] The tube had the following dimensions:

[0088] 63.5 x 3.1 mm

[0089] The calculations were accordingly:

[0090] 1 _ 3.1

[0091] K “ 193

[0092] Heat transfer coefficient

[0093] K=6225 W / m2K

[0094] As can be seen from these calculations, the heat transfer coefficient of the alloy of Table 1 is higher than the heat transfer coefficient of the composite tube. This means that a tube consisting only an alloy within the ranges as defined hereinabove or hereinafter will have a much better ability to transfer heat from the furnace to the steam compared to a composite tube. An additional benefit of this is that for a given steam temperature, the metal temperature of a tube will be lower, which in turn will reduce the corrosion rate. Experiment 2

[0095] To investigate the microstructural stability of the material, an ageing experiment was performed.

[0096] Samples were taken from a solution annealed tube of size 170x12 with a composition as per the table below:

[0097] Semi-finished impact toughness specimens of size 15x12x57 mm were cut and left to age in lab furnaces at 550, 600, 650 and 700°C. The specimens were taken out of the furnaces after 3,000 h and machined to final Charpy-V specimen size (10x10x55). These were then tested at room temperature in a Zwick / Roell PSW750 impact testing machine. The diagram of Figure 2 shows the measured impact toughness as an average of three specimens per ageing temperature after 3,000 h ageing. At 550°C the toughness remains on the same order as in unaged material, while at higher temperature it drops due precipitation of embrittling phases. This shows that the material has excellent structure stability up to 550°C and therefore an excellent impact toughness.

Claims

Claims1. Use of an object of an austenitic stainless alloy in high temperature and high-pressure environment wherein said austenitic stainless alloy comprises in weight%:C less than 0.03;Si less than 1.0;Mn less than or equal to 1.2;Cr 26.0 to 30.0;Ni 29.0 to 37.0;Mo 6.1 to 7.1;N 0.25 to 0.36;P less than or equal to 0.04S less than or equal to 0.03;Cu less than or equal to 0.4; balance Fe and unavoidable impurities.

2. The use according to claim 1, wherein the austenitic stainless alloy has a content of Si which is less than 0.5 wt%.

3. The use according to claim 1 or claim 2, wherein the austenitic stainless alloy has a content of Si which is more than 0.01 wt%4. The use according to any one of claims 1 to 3, wherein the austenitic stainless alloy has a content of Si which is from 0.1 to 0.3 wt%.

5. The use according to any one of claims 1 to 4, wherein the austenitic stainless alloy has a content of Mn which is less than or equal to 1.1 wt%.

6. The use according to any preceding claims, wherein the austenitic stainless alloy has a content Mn which is from 0.01 to 1.1 wt%, such as from 0.6 to 1.1 wt%7. The use according to any preceding claims, wherein the austenitic stainless alloy has a content of Cu which is from 0.001 to 0.4 wt%8. The use according to any preceding claims, wherein the austenitic stainless alloy has a content of Ni is from 31 to 36 wt%, such as from 34 to 36 wt%.

9. The use according to any preceding claims, wherein the austenitic stainless alloy has a content of Cr which is from 26 to 29 wt%, such as from 26 to 28 wt%.

10. The use according to any preceding claims, wherein the austenitic stainless alloy has a content of Cr which is more than 26 wt%.

11. The use according to any preceding claims, wherein the austenitic stainless alloy has a content of Mo which is from 6.1 to 7.1 wt%, such as from 6.3 to 6.8 wt%.

12. The use according to any one of claims 1 to 11, wherein the object is a formed object, such as a tube, such as a superheater tube.

13. The use according to claim 12, wherein the tube, such as the superheater tube, consist of the alloy as defined in claims 1 to 11.

14. A superheater comprising at least one object according to any one of claims 1 to 13.

15. A recovery boiler comprising the superheater according to claim 14.

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