Duplex Stainless Steel and Its Uses

Abstract

This invention relates to a corrosion-resistant duplex stainless steel (ferritic austenitic alloy) for use in an apparatus for urea production; and its applications. This disclosure also relates to objects made of said duplex stainless steel. Furthermore, this disclosure relates to a method for urea production and an apparatus for urea production comprising one or more components made of said duplex stainless steel, and to a method for modifying an existing apparatus for urea production.

Description

[0001] This application is a divisional application of the patent application filed on July 20, 2016, with application number 201680042538.6 and invention title "Duplex Stainless Steel and Its Uses". Technical Field

[0002] This disclosure relates to a corrosion-resistant duplex stainless steel (ferritic austenitic alloy) for use in an apparatus for urea production. This disclosure also relates to objects made of said duplex stainless steel and its uses. Furthermore, this disclosure relates to a method for urea production and an apparatus for urea production, the apparatus including one or more components made of said duplex stainless steel, and to a method for modifying an existing apparatus for urea production. Background Technology

[0003] Duplex stainless steel refers to ferritic austenitic alloys. These alloys have a microstructure comprising both ferrite and austenitic phases. Background references in this area include WO 95 / 00674 and US 7,347,903. The duplex stainless steels described therein exhibit high corrosion resistance and are therefore suitable, for example, for use in the highly corrosive environments of urea manufacturing plants.

[0004] Urea and its production

[0005] Urea (NH2CONH2) can be produced from ammonia and carbon dioxide in the urea synthesis section of a urea plant at high temperatures (typically between 150°C and 250°C) and pressures (typically between 12 MPa and 40 MPa). In this synthesis process, two consecutive reaction steps can be considered. In the first step, ammonium carbamate is formed, and in the next step, this ammonium carbamate is dehydrated to provide urea. This first step (i) is exothermic, and the second step can be represented as an endothermic equilibrium reaction (ii):

[0006] (i) 2NH3 + CO2 → H2N-CO-ONH4

[0007]

[0008] In a typical urea production unit, the aforementioned reaction is carried out in the urea synthesis section to obtain an aqueous solution containing urea. In one or more subsequent concentration sections, the solution is concentrated to ultimately obtain urea in melt form rather than solution form. The melt is further subjected to one or more finishing steps, such as granulation, pelletizing, forming, or compaction.

[0009] A commonly used process for producing urea based on stripping is the carbon dioxide stripping process, as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp. 333-350. In this process, a synthesis section is followed by one or more recovery sections. The synthesis section includes a reactor, a stripper, a condenser, and a preferably but not essential scrubber, wherein the operating pressure is in the range of 12 MPa to 18 MPa, such as from 13 MPa to 16 MPa.

[0010] In the synthesis section, the urea solution leaving the urea reactor is fed to the stripper, where a large amount of unconverted ammonia and carbon dioxide are separated from the urea aqueous solution.

[0011] Such a stripper can be a shell-and-tube heat exchanger in which a urea solution is fed to the top at the tube side and carbon dioxide feed for urea synthesis is added to the bottom of the stripper. On the shell side, steam is added to heat the solution. The urea solution exits the heat exchanger at the bottom, while the vapor phase exits the stripper at the top. The vapor exiting the stripper contains ammonia, carbon dioxide, inert gases, and a small amount of water.

[0012] The vapor is typically condensed in a falling film heat exchanger or in a submerged condenser, which can be horizontal or vertical. A horizontal submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp. 333-350. The resulting solution, containing condensed ammonia, carbon dioxide, water, and urea, as well as non-condensed ammonia, carbon dioxide, and inert vapor, is recycled.

[0013] The processing conditions are highly corrosive, especially due to the hot and concentrated carbamate solution. In an attempt to prevent corrosion, oxygen, typically in the form of passive air, has been added to the urea process as a passive agent, meaning that a portion of the oxygen will combine with the chromium in the steel to form a protective chromium oxide layer on the stainless steel surface of the equipment.

[0014] In the past, corrosion posed a problem that even urea manufacturing equipment made of stainless steel and with the addition of passive air corroded very quickly and was prone to premature replacement, also creating inherent safety hazards due to the presence of oxygen. This problem was solved by making the equipment, specifically the relevant components subjected to the aforementioned corrosive conditions, of duplex stainless steel, and more specifically of so-called super duplex stainless steel as described in WO 95 / 00674 (which is trademarked as such). The problem has been solved (as described in the previous sentence). This super duplex stainless steel has an increased chromium content, and the combination of oxygen and duplex steel has allowed for a significant reduction in the amount of oxygen required for coarse passive corrosion and a lower level of passive corrosion. Therefore, super duplex stainless steel is very effective for use in urethane environments (e.g., in urea production plants), but at high temperatures, i.e., temperatures above 200°C, such as reaching 205°C, the level of passive corrosion may be higher than desired. Therefore, there is still a need for more corrosion-resistant duplex stainless steels that will increase the lifespan of specific equipment in urea production plants operating at higher temperatures, such as, for example, HP (high-pressure) strippers.

[0015] As those skilled in the art will understand, generally speaking, the use of duplex steel in a carbamate environment involves exposing the duplex steel to the carbamate. Such use means contacting the duplex steel with a fluid containing carbamate, such as a carbamate solution. This specifically refers to concentrated carbamate solutions, such as ammonium carbamate solutions with a concentration of 15% to 95% by weight, or, for example, ammonium carbamate solutions with a concentration of 45% to 95% by weight. More specifically, the carbamate-containing fluid has a high temperature, such as above 180°C, or above 200°C.

[0016] Furthermore, another issue with the use of duplex stainless steel is that its initial microstructure (i.e., the microstructure it possesses when manufactured by the steel manufacturer) can be altered during further processing, such as welding. The microstructural stability of duplex stainless steel depends on its composition, and when manufacturing complex components, it is important to have materials with a stable microstructure during operation to ensure adequate corrosion resistance and sufficient mechanical properties. Therefore, there is also a demand for duplex stainless steel with a stable microstructure.

[0017] Therefore, there remains a need for further improvements to the duplex stainless steel materials used in urea production equipment, especially for components exposed to high temperatures and corrosive fluids, such as stripper tubes.

[0018] Therefore, there is a need to provide corrosion-resistant materials with improved passive corrosion rates (especially when exposed to fluids containing carbamates at high temperatures, such as in stripper tubes), thereby extending the life of the stripper while maintaining sufficiently good structural stability of the stripper material, and more specifically, structural stability in the heat-affected areas of the weld joints where the heat exchanger is connected to the tube sheet. Summary of the Invention

[0019] To address one or more of the aforementioned expectations, this disclosure provides, in one aspect, a duplex stainless steel comprising, by weight percent (wt%):

[0020] C is at most 0.030;

[0021] Si is at most 0.8;

[0022] Mn is at most 2.0;

[0023] Cr 29.0 to 31.0;

[0024] Ni 5.0 to 9.0;

[0025] Mo is less than 4.0;

[0026] W is less than 4.0;

[0027] N 0.25-0.45;

[0028] Cu maximum 2.0;

[0029] S is at most 0.02;

[0030] P is at most 0.03;

[0031] The remaining Fe and the unavoidable impurities; and

[0032] The content of Mo+W is greater than 3.0 but less than 4.0.

[0033] In this disclosure, the terms "carbamate" and "ammonium carbamate" are used interchangeably. Ammonium carbamate is preferably a carbamate.

[0034] Additionally, this disclosure relates to the use of duplex stainless steel in urethane environments, the duplex stainless steel comprising, by weight percent (wt%):

[0035] C is at most 0.030;

[0036] Si is at most 0.8;

[0037] Mn is at most 2.0;

[0038] Cr 29.0 to 31.0;

[0039] Ni 5.0 to 9.0;

[0040] Mo is less than 5.0;

[0041] W is less than 5.0;

[0042] N 0.25 to 0.45;

[0043] Cu maximum 2.0;

[0044] S is at most 0.02;

[0045] P is at most 0.03;

[0046] The remaining Fe and the unavoidable impurities; and

[0047] The content of Mo+W is greater than 3.0 but less than 4.0.

[0048] Furthermore, this disclosure relates to the forming objects of duplex stainless steel as defined above or below, and to the use of stainless steel as defined above or below in an apparatus for urea production.

[0049] This disclosure also relates to a method for producing urea, wherein at least one component of the equipment is made of duplex stainless steel as defined above or below, and the apparatus for producing urea includes one or more components comprising duplex stainless steel as defined above or below.

[0050] In addition, this disclosure also provides a method for modifying an existing plant for urea production, and a method for reducing the passive corrosion rate of a urea plant by using duplex stainless steel as defined above or below. Detailed Implementation

[0051] This disclosure relates to duplex stainless steel, which comprises, by weight percent (wt%):

[0052] C is at most 0.030;

[0053] Si is at most 0.8;

[0054] Mn is at most 2.0;

[0055] Cr 29.0 to 31.0;

[0056] Ni 5.0 to 9.0;

[0057] Mo is less than 4.0;

[0058] W is less than 4.0;

[0059] N 0.25-0.45;

[0060] Cu maximum 2.0;

[0061] S is at most 0.02;

[0062] P is at most 0.03;

[0063] The remaining Fe and the unavoidable impurities; and

[0064] The content of Mo+W is greater than 3.0 but less than 4.0.

[0065] Therefore, for example, this disclosure relates to duplex stainless steel, which comprises, by weight percent (wt%):

[0066] C is at most 0.020;

[0067] Si is at most 0.8;

[0068] Mn is at most 2.0;

[0069] Cr 29.0 to 31.0;

[0070] Ni 5.0 to 9.0;

[0071] Mo is less than 4.0;

[0072] W is less than 4.0;

[0073] N 0.25 to 0.45;

[0074] Cu maximum 2.0;

[0075] S is at most 0.01;

[0076] P is at most 0.02;

[0077] The remaining Fe and the unavoidable impurities; and

[0078] The content of Mo+W is greater than 3.0 but less than 4.0.

[0079] Broadly speaking, this disclosure is based on the prudent insight that duplex stainless steel, as defined above or below, provides better corrosion resistance to areas exposed to carbamates under high pressure and high temperature. Therefore, the duplex stainless steel is specifically designed for manufacturing components exposed to concentrated ammonium carbamate at high temperatures (above about 180°C), such as components in heat exchanger tubes or tubes in strippers, for example. As those skilled in the art will understand, specifically for carbamate exposure in equipment within urea plants, typical concentrated carbamate solutions have a concentration of ammonium carbamate ranging from 15 wt% to 95 wt%, preferably from 45 wt% to 95 wt%, and more preferably from 47 wt% to 92 wt%. In one embodiment, such an ammonium carbamate stream comprises 85 wt% to 92 wt% ammonium carbamate, such as that present in a high-pressure carbamate condenser in a urea production plant. In another embodiment, such ammonium carbamate feed streams contain from 45% to 65% by weight of ammonium carbamate, such as in the pool condenser and / or the inlet of a stripper in a urea production unit, such as a high-pressure stripper in a urea production unit, specifically a CO2 stripping type unit.

[0080] Even though the super duplex stainless steel described in WO 95 / 00674 exhibits excellent corrosion resistance in urethane solutions at temperatures above 180°C (even in the absence of oxygen), there is still room for improvement in the passive corrosion rate of duplex stainless steel, especially at temperatures above approximately 180°C (typically in stripper tubes). Duplex stainless steels, as defined above or below, exhibit significantly lower passive corrosion rates at these extreme temperatures. One advantage of duplex stainless steel is that it provides improved lifespan expectations for strippers, specifically for heat exchanger tubes.

[0081] This disclosure also relates to the use of duplex stainless steels as defined above or below in urethane environments (such as ammonium urethane environments), wherein the duplex stainless steel preferably comprises, by weight percent (wt%):

[0082] C is at most 0.030;

[0083] Si is at most 0.8;

[0084] Mn is at most 2.0;

[0085] Cr 29.0 to 31.0;

[0086] Ni 5.0 to 9.0;

[0087] Mo is less than 5.0;

[0088] W is less than 5.0;

[0089] N 0.25 to 0.45;

[0090] Cu maximum 2.0;

[0091] S is at most 0.02;

[0092] P is at most 0.03;

[0093] The remaining Fe and the unavoidable impurities; and

[0094] The content of Mo+W is greater than 3.0 but less than 4.0.

[0095] Therefore, for example, this disclosure relates to the use of duplex stainless steel in urethane environments (such as ammonium urethane environments) as defined above or below, wherein the duplex stainless steel comprises, by weight percent (wt%):

[0096] C is at most 0.020;

[0097] Si is at most 0.8;

[0098] Mn is at most 2.0;

[0099] Cr 29.0 to 31.0;

[0100] Ni 5.0 to 9.0;

[0101] Mo is less than 5.0;

[0102] W is less than 5.0;

[0103] N 0.25 to 0.45;

[0104] Cu maximum 2.0;

[0105] S is at most 0.01;

[0106] P is at most 0.02;

[0107] The remaining Fe and the unavoidable impurities; and

[0108] The content of Mo+W is greater than 3.0 but less than 5.0.

[0109] The present invention has surprisingly discovered that by manufacturing stripper tubes from duplex stainless steel as defined above or below, the addition of oxygen in the process can be reduced to almost zero while still achieving a low passive corrosion rate in the stripper tubes, a rate not found in any other component of the urea plant. Furthermore, the inventors have found that conventionally used tests for evaluating corrosion of stainless steel (such as the Streicher test performed at 127°C using a ferric sulfate-sulfuric acid test solution) used to form duplex stainless steel (as described in WO 95 / 00674) are not relevant to the corrosion actually observed in specific equipment (stripper tubes) within the urea plant. Therefore, further improvements in the passive corrosion rate of duplex stainless steel can only be achieved through corrosion testing in a high-pressure autoclave, which simulates the actual process conditions common in specific equipment such as stripper tubes.

[0110] The elemental composition of duplex stainless steel is generally defined above or below, and the function of each alloy element is further described below.

[0111] In this disclosure, carbon (C) is considered an impurity element and has limited solubility in both ferrite and austenitic phases. This limited solubility implies a risk of excessively high percentages of carbide precipitation, which reduces corrosion resistance. Therefore, the amount of C should be limited to a maximum of 0.030% by weight, such as a maximum of 0.020% by weight, such as a maximum of 0.017% by weight, such as a maximum of 0.015% by weight, such as a maximum of 0.010% by weight.

[0112] Silicon (Si) is used as a reducing additive in steel manufacturing. However, excessive Si content increases the tendency for intermetallic phases to precipitate and reduces the solubility of nitrogen. Therefore, the Si content should be limited to a maximum of 0.8 wt%, such as a maximum of 0.5 wt%, such as in the range of 0.05 wt% to 0.50 wt%, such as 0.1 wt% to 0.5 wt%.

[0113] Manganese (Mn) is added to increase the solubility of nitrogen and to replace nitrogen as an alloying element, as Mn is considered austenite-stabilizing. However, Mn can have a negative impact on structural stability, and therefore its content is limited to a maximum of 2.0% by weight, such as a maximum of 1.5% by weight, such as in the range of 0.5% to 1.5% by weight.

[0114] Chromium (Cr) is the most reactive element for achieving resistance to most types of corrosion. In urea synthesis, the Cr content is particularly important for corrosion resistance and therefore should be as high as possible. However, there is a balance between high Cr content and good structural stability. Therefore, in this disclosure, to achieve sufficient corrosion resistance while ensuring structural stability, the Cr content should be in the range of 29.0 wt% to 31.0 wt%. Thus, the Cr content ranges from 29.0 wt% to 31.0 wt%, such as from 29.00 wt% to 30.00 wt%.

[0115] Nickel (Ni) is primarily used as an austenite stabilizing element. An advantage of Ni is that it has no negative effect on structural stability. A Ni content of at least 5.0 wt% is required to ensure stability, as chromium nitride may form during heat treatment if the Ni content is below 5 wt%. However, Ni can form strong complexes with ammonium, so the Ni content should be kept as low as possible. Therefore, Ni content is typically in the range of 5.0 wt% to 9.0 wt%, such as from 5.5 wt% to 8.5 wt%, or from 5.5 wt% to 7.5 wt%.

[0116] Molybdenum (Mo) is used to improve the passivity of duplex stainless steel. However, excessive Mo content increases the risk of intermetallic phase precipitation. Therefore, Mo is less than 5.0 wt%, for example, less than 4.0 wt%. Tungsten (W) increases resistance to pitting and crevice corrosion. However, excessive W content increases the risk of intermetallic phase precipitation, especially when combined with high Cr and Mo contents. Therefore, W is less than 5.0 wt%, for example, less than 4.0 wt%. To obtain the best possible corrosion properties, the Mo+W content should be as high as possible without unreasonably high sensitivity to the σ phase. If the Mo+W content is higher than 5.0 wt%, the driving force for the σ phase will be very high, making it difficult to prepare a composition without the σ phase. However, according to this disclosure, it has been shown that if W+Mo is higher than 3.0 wt%, duplex stainless steel will exhibit less corrosion in stripper tubes. Therefore, the Mo+W content is higher than 3.0 wt%, but less than 5.0 wt%, for example, less than 4.0 wt%. Furthermore, if the W+Mo content is higher than 3.0 wt% but less than 4.0 wt%, then the duplex stainless steel as defined above or below contains a low content of the σ phase, for example, essentially no σ phase, such as a maximum of 0.5 wt%, such as a maximum of 0.05 wt%. The σ phase should be avoided as it can cause embrittlement in duplex stainless steel and thereby reduce corrosion resistance.

[0117] Nitrogen (N) is a strong austenite-forming element and enhances austenite reconstruction. Additionally, N influences the distribution of Cr and Mo, as well as Ni, in both the austenitic and ferrite phases. Therefore, higher N content increases the relative proportions of Cr and Mo in the austenitic phase. This means that austenite becomes more corrosion-resistant and that higher Cr and Mo contents can be added to duplex stainless steels while maintaining structural stability. Therefore, the N content should be at least 0.25 wt%. However, nitrogen has limited solubility, and excessively high nitrogen content increases the risk of chromium nitride formation, which in turn affects corrosion resistance. Therefore, N should not exceed 0.45 wt%. Thus, N content ranges from 0.25 wt% to 0.45 wt%, such as from 0.28 wt% to 0.40 wt%.

[0118] Copper (Cu) is an optional element in this disclosure, and its addition improves general corrosion resistance in acidic environments such as sulfuric acid. However, high Cu content will reduce resistance to pitting and crevice corrosion. Therefore, the Cu content should be limited to a maximum of 2.0% by weight, such as a maximum of 1.0% by weight, such as a maximum of 0.8% by weight.

[0119] Sulfur (S) adversely affects corrosion resistance by forming readily soluble sulfides. Therefore, the S content should be limited to a maximum of 0.02% by weight, such as a maximum of 0.01% by weight.

[0120] Phosphorus (P) is a common impurity element. If present in amounts greater than about 0.03% by weight, it can have adverse effects on properties such as thermal ductility, weldability, and corrosion resistance. The P content in alloys should be limited to a maximum of 0.03% by weight, such as a maximum of 0.02% by weight.

[0121] When the term "maximum" is used, unless otherwise specified, a person skilled in the art knows that the lower limit of the range is 0% by weight. Therefore, for C, Si, Mn, Cu, S, and P, the lower limit is 0% by weight, as they are optional components.

[0122] Additionally, other components may optionally be added to the duplex stainless steel as defined above or below during the manufacturing process to improve machinability, such as hot workability, machinability, etc. Examples of such elements include, but are not limited to, Ti, Nb, Hf, Ca, Al, Ba, V, Ce, and B. If added, these elements will be added in a total amount of up to 0.5% by weight. Optionally, for example, for alloys as defined above or below (containing specified amounts of the defined elements C, Si, Mn, Cr, Ni, Mo, W, N, Cu, S, and P, and the remainder Fe and unavoidable impurities), the alloy may consist of the specified amounts of the defined elements plus up to 0.5% by weight of optional added elements, such as those added for machinability, such as Ti, Nb, Hf, Ca, Al, Ba, V, Ce, and B, and the remainder Fe and unavoidable impurities.

[0123] The remaining material in duplex stainless steel as defined above or below is Fe, and unavoidable impurities. Examples of unavoidable impurities are elements and compounds that are not intentionally added but cannot be completely avoided, as they typically appear as impurities in materials used to manufacture duplex stainless steel.

[0124] According to this disclosure, the ferrite content of duplex stainless steel is important for corrosion resistance. Therefore, the ferrite content is preferably in the range of 30% to 70% by volume, such as in the range of 30% to 60% by volume, such as in the range of 30% to 55% by volume, such as in the range of 40% to 60% by volume.

[0125] Duplex stainless steels as defined above or below can be prepared using conventional methods, i.e., hot working and / or cold working after casting, and optional additional heat treatment. Duplex stainless steels as defined above or below can also be prepared, for example, in powder form by hot isostatic pressing (HIP).

[0126] Duplex stainless steel, as defined above or below, can be used in other applications where equipment requires good corrosion resistance. Some examples of possible uses for duplex stainless steel include its use as a construction material for process chemical components intended for use in nitric acid environments, melamine production, the paper and pulp industry (such as white liquor environments), and as a welding wire material. The steel can be used, for example, to manufacture seamless pipes, welded pipes, flanges, couplings, and sheet metal.

[0127] This disclosure also relates to a molded object comprising duplex stainless steel, wherein, according to one embodiment, the object is a tube, such as, for example, a stripper tube for a urea production unit, or a liquid distributor for a stripper in a urea production unit. This disclosure also relates to the use of duplex stainless steel, as defined above or below, in any of the embodiments described above and below, in a urea synthesis process. This use of duplex stainless steel, as defined above or below, is to reduce corrosion of one or more components of equipment used in said process, such as one or more components of a high-pressure urea synthesis section, such as components in contact with carbamate solutions.

[0128] Another aspect of this disclosure is to provide a method for producing urea, wherein at least one of the equipment components, such as a component in contact with a carbamate solution, is made of duplex stainless steel as defined above or below. The carbamate solution may have an oxygen content of less than 0.1 ppm, such as less than 0.04 ppm (by weight).

[0129] Another aspect of this disclosure provides an apparatus for urea production, wherein the apparatus includes one or more components comprising duplex stainless steel as defined above or below. According to one embodiment, one or more stripper tubes in a stripper tube comprise or are made of duplex stainless steel as defined above or below. According to another embodiment, the apparatus includes a high-pressure urea synthesis section comprising a stripper, wherein the stripper includes at least one liquid distributor comprising duplex stainless steel as defined above or below. The duplex stainless steel can be used in methods for retrofitting existing urea production apparatuses comprising one or more components selected from the group consisting of liquid distributors, radomes, (control) valves, and injectors, wherein the method is characterized in that one or more stripper tubes are replaced by stripper tubes comprising duplex stainless steel as defined above or below. This method can also be used in methods for reducing the corrosion rate of a urea plant by replacing at least one stripper tube with a stripper tube comprising duplex stainless steel as defined above or below.

[0130] This disclosure also relates to the following numbered non-limiting embodiments:

[0131] Implementation Scheme 1.0. Use of duplex stainless steel in urethane environments, wherein the duplex stainless steel comprises, by weight % (wt%):

[0132] C is at most 0.030;

[0133] Si is at most 0.8;

[0134] Mn is at most 2.0;

[0135] Cr 29.0 to 31.0;

[0136] Ni 5.0 to 9.0;

[0137] Mo is less than 4.0;

[0138] W is less than 4.0;

[0139] N 0.25-0.45;

[0140] Cu maximum 2.0;

[0141] S is at most 0.02;

[0142] P is at most 0.03;

[0143] The remaining Fe and the unavoidable impurities; and

[0144] The content of Mo+W is greater than 3.0 but less than 4.0.

[0145] Implementation Scheme 1.1. Use of duplex stainless steel in urethane environments, wherein the duplex stainless steel comprises, by weight % (wt%):

[0146] C is at most 0.020;

[0147] Si is at most 0.8;

[0148] Mn is at most 2.0;

[0149] Cr 29.0 to 31.0;

[0150] Ni 5.0 to 9.0;

[0151] Mo is less than 5.0;

[0152] W is less than 5.0;

[0153] N 0.25 to 0.45;

[0154] Cu maximum 2.0;

[0155] S is at most 0.01;

[0156] P is at most 0.02;

[0157] The remaining Fe and the unavoidable impurities; and

[0158] The content of Mo+W is greater than 3.0 but less than 5.0.

[0159] Implementation Scheme 1.2. The use of duplex stainless steel according to Implementation Scheme 1.0 or 1.1, wherein Mn is 0.5% to 1.5% by weight.

[0160] Implementation Scheme 1.3. The use of duplex stainless steel according to Implementation Scheme 1.0, 1.1, or 1.2, wherein Si is 0.010% by weight to 0.50% by weight.

[0161] Implementation Scheme 1.4. Use of duplex stainless steel according to any one of Implementation Schemes 1.0 to 1.3, wherein Ni is 5.5 wt% to 8.5 wt%, such as from 5.5 wt% to 7.5 wt%.

[0162] Implementation Scheme 1.5. Use of duplex stainless steel according to any one of Implementation Schemes 1.0 to 1.4, wherein N is 0.28% to 0.40% by weight.

[0163] Implementation Scheme 1.6. The duplex stainless steel according to any one of Implementation Schemes 1.0 to 1.5, wherein the duplex stainless steel comprises, by weight % (wt%):

[0164] C is at most 0.030;

[0165] Si is at most 0.8;

[0166] Mn is at most 2.0;

[0167] Cr 29.0 to 31.0;

[0168] Ni 5.0 to 9.0;

[0169] Mo is less than 4.0;

[0170] W is less than 4.0;

[0171] N 0.25-0.45;

[0172] Cu maximum 2.0;

[0173] S is at most 0.02;

[0174] P is at most 0.03;

[0175] Implementation Scheme 1.7. The use of duplex stainless steel according to Implementation Scheme 1.6, wherein the duplex stainless steel comprises, by weight % (wt%):

[0176] C is at most 0.020;

[0177] Si is at most 0.8;

[0178] Mn is at most 2.0;

[0179] Cr 29.0 to 31.0;

[0180] Ni 5.0 to 9.0;

[0181] Mo is less than 4.0;

[0182] W is less than 4.0;

[0183] N 0.25 to 0.45;

[0184] Cu maximum 2.0;

[0185] S is at most 0.01;

[0186] P is at most 0.02;

[0187] The remaining Fe and the unavoidable impurities; and

[0188] The content of Mo+W is greater than 3.0 but less than 4.0.

[0189] Implementation Scheme 1.8. Use of duplex stainless steel in a urea synthesis process according to any one of Implementation Schemes 1.0 to 1.7, wherein the duplex stainless steel is used to reduce corrosion of one or more components of a high-pressure urea synthesis section in contact with ammonium carbamate solution.

[0190] Implementation Scheme 1.9. A molded object comprising duplex stainless steel as defined in any one of Implementation Schemes 1.0 to 1.7, wherein the molded object is a tube, a stripper tube for a urea production apparatus, or a liquid distributor for a stripper of a urea production apparatus.

[0191] Implementation Scheme 1.10. A method for producing urea, wherein at least one component of the equipment is made of duplex stainless steel as defined in any one of Implementation Schemes 1.0 to 1.7, the method preferably comprising forming ammonium carbamate and dehydrating the ammonium carbamate to provide urea.

[0192] Implementation Scheme 1.11. An apparatus for urea production, wherein the apparatus includes one or more components comprising duplex stainless steel as defined in any one of Implementation Schemes 1.0 to 1.7.

[0193] Implementation Scheme 1.12. The apparatus according to Implementation Scheme 1.11, wherein the one or more components are one or more stripper tubes.

[0194] Implementation Scheme 1.13. The apparatus according to Implementation Scheme 1.11 or 1.12 includes a high-pressure urea synthesis section, the high-pressure urea synthesis section including a stripper, wherein the stripper includes at least one liquid distributor, the at least one liquid distributor comprising duplex stainless steel as defined in any one of Implementation Schemes 1.0 to 1.7.

[0195] Implementation Scheme 1.14. A method for modifying an existing apparatus for urea production, the apparatus comprising one or more components selected from a group consisting of a liquid distributor, a radome, (control) valves, and injectors, wherein the method is characterized in that one or more stripper tubes are replaced by stripper tubes comprising duplex stainless steel as defined in any one of Implementation Schemes 1.0 to 1.7.

[0196] Implementation Scheme 1.15. A method for reducing the passive corrosion rate of a urea plant by replacing at least one stripper tube with a stripper tube comprising duplex stainless steel as defined in any one of Implementation Schemes 1.0 to 1.7.

[0197] Implementation Scheme 2.0. A duplex stainless steel comprising, by weight percent (wt%):

[0198] C is at most 0.030;

[0199] Si is at most 0.8;

[0200] Mn is at most 2.0;

[0201] Cr 29.0 to 31.0;

[0202] Ni 5.0 to 9.0;

[0203] Mo is less than 4.0;

[0204] W is less than 4.0;

[0205] N 0.25 to 0.45;

[0206] Cu maximum 2.0;

[0207] S is at most 0.02;

[0208] P is at most 0.03;

[0209] The remaining Fe and the unavoidable impurities; and

[0210] The content of Mo+W is greater than 3.0 but less than 4.0.

[0211] Implementation Scheme 2.1. A duplex stainless steel, comprising, by weight percent (wt%):

[0212] C is at most 0.020;

[0213] Si is at most 0.8;

[0214] Mn is at most 2.0;

[0215] Cr 29.0 to 31.0;

[0216] Ni 5.0 to 9.0;

[0217] Mo is less than 4.0;

[0218] W is less than 4.0;

[0219] N 0.25 to 0.45;

[0220] Cu maximum 2.0;

[0221] S is at most 0.01;

[0222] P is at most 0.02;

[0223] The remaining Fe and the unavoidable impurities; and

[0224] The content of Mo+W is greater than 3.0 but less than 4.0.

[0225] Implementation Scheme 2.2. The duplex stainless steel according to Implementation Scheme 2.0 or 2.1, wherein Mn is 0.5 wt% - 1.5 wt%.

[0226] Implementation Scheme 2.3. The duplex stainless steel according to Implementation Scheme 2.0, 2.1, or 2.2, wherein Si is 0.010% to 0.50% by weight.

[0227] Implementation Scheme 2.4. The duplex stainless steel according to any one of Implementation Schemes 2.0 to 2.3, wherein Ni is 5.5 wt% to 8.5 wt%, such as from 5.5 wt% to 7.5 wt%.

[0228] Implementation Scheme 2.5. The duplex stainless steel according to any one of Implementation Schemes 2.0 to 2.4, wherein N is from 0.28% to 0.40% by weight.

[0229] Implementation Scheme 2.6. The duplex stainless steel according to Implementation Scheme 2.0 or 2.1, wherein Mn is 0.5 wt% to 1.5 wt%, Si is 0.010 wt% to 0.50 wt%, Ni is 5.5 wt% to 8.5 wt%, and N is 0.28 wt% to 0.40 wt%.

[0230] Implementation Scheme 2.7. A molded object comprising duplex stainless steel according to any one of Implementation Schemes 2.0 to 2.6.

[0231] This disclosure is further illustrated by the following non-limiting embodiments.

[0232] Example

[0233] Table 1 shows the composition of the duplex stainless steel used in the examples. The test specimens were made from 270 kg steel billets that were hot-forged, hot-rolled, cold-rolled, and then heat-treated.

[0234] Corrosion testing using an autoclave

[0235] Samples were cut from 5mm strips prepared by heating to approximately 1200°C, followed by cold rolling at an intermediate stage (approximately 1100°C) at room temperature, and finally annealing at 1070°C. Samples used for testing were in the form of specimen blocks, approximately 20 × 10 × 3mm in size. All surfaces were machined and finished using wet grinding.

[0236] The corrosion resistance of duplex stainless steel in an oxygen-free carbamate solution was evaluated. The composition of the carbamate solution was selected to simulate conditions more severe than those commonly found in stripper heat exchanger tubes in urea plants. The temperature during the test was 210 °C. The corrosion rate was calculated after 14 days of exposure to the oxygen-free carbamate solution. The results are shown in Table 3. As can be seen in the table, packs 1 and 2 exhibit better corrosion resistance compared to comparative packs 3-5, which indicate lower corrosion rates.

[0237] The following procedure is used for exposure. Thoroughly clean the autoclave using ultrapure water and ethanol. Clean the sample blocks (strips) in acetone and ethanol, weigh them, and measure their dimensions. Then mount these on Teflon sample holders.

[0238] Water and urea are added to the autoclave. The autoclave is then purged with nitrogen to remove oxygen and other gases. Ammonia is then added to the autoclave.

[0239] Heating began the following day according to the temperature profile shown in Table 2. This sequence was designed to avoid overheating. The specimens were exposed at 210°C for 14 days.

[0240] surface 1 Components of the filling in the embodiment

[0241] Filling C Si Mn P S Cr Ni Mo W Mo+W N Cu 1 0.012 0.08 1 0.008 0.008 29.07 5.76 0.48 2.55 3.03 0.35 0.01 2 0.012 0.23 1.05 0.005 0.005 29.92 7.17 3.01 - 3.01 0.3 - 3 0.011 0.48 1.06 0.004 0.006 28.74 6.84 2.24 - 2.24 0.34 <0.010 4 0.010 0.11 1.09 0.005 0.006 33.31 6.5 0.48 - 0.48 0.41 <0.010 5 0.010 0.48 4.07 0.004 0.007 30.77 5.08 0.33 - 0.33 0.33 <0.010

[0242] surface 2 Heating sequence of the autoclave .

[0243]

[0244] Table 3

[0245]

[0246] Mechanical testing

[0247] Mechanical properties were evaluated using tensile testing, impact testing, and hardness measurement. A 5mm cold-rolled and annealed strip was used for tensile testing and hardness measurement. An 11mm hot-rolled strip was used for impact testing. The strips were prepared as described above.

[0248] Perform tensile tests at room temperature according to ISO 6892-1:2009.

[0249] The impact test specimens were standard V-notch test pieces (SSV1). Testing was performed according to ISO 14556. The tests were conducted at two temperatures: room temperature and -35°C.

[0250] Hardness measurements were performed on the cross-section of a longitudinal sample taken from a 5 mm strip. The measurement was taken at the center of the strip. The Vickers hardness measurement was performed with a load of 10 kg (HV10).

[0251] Perform austenite spacing measurements on the same specimen used for hardness measurements. Perform the measurements according to recommended practice DNV-RP-F112, Section 7 (October 2008).

[0252] The results of the mechanical tests are shown in the table below:

[0253] surface 4A Results from tensile tests

[0254] Filling Rp 0.2 (MPa) Rp0.1 (MPa) Rm(MPa) A(％) 1 626 717 865 30 2 648 744 878 27 3 566 669 831 31 4 669 755 883 28 5 617 703 817 28

[0255] surface 4B Results from impact test

[0256] Filling RT-1 RT-2 RT-3 1 146 169 153 2 194 188 178 3 202 208 213 4 172 178 178 5 143 135 150

[0257] surface 4C From - 35 Results of ℃ impact test

[0258] Filling Test 1(J) Test 2(J) Test 3(J) Average value (J) 1 107 146 130 128 2 145 141 137 141 3 165 179 186 177 4 37 40 39 39 5 24 23 20 22

[0259] surface 4D Results of hardness test

[0260]

[0261] surface 4E Results of austenite spacing

[0262] Austenite spacing (μm) 9,7 12,1 4,5 9 12,2

Claims

1. Use of duplex stainless steel in a carbamate environment, said use including exposing said steel to a fluid containing 45% to 65% by weight of ammonium carbamate at a temperature above 180°C, said duplex stainless steel comprising, by weight (wt%): C is at most 0.030; Si is at most 0.8; Mn is at most 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo is less than 4.0; W is less than 4.0; N0.25-0.45； Cu maximum 2.0; S is at most 0.02; P is at most 0.03; The remaining Fe and the unavoidable impurities; and The content of Mo+W is greater than 3.0 but less than 4.

0.

2. The use of the duplex stainless steel according to claim 1, wherein the duplex stainless steel comprises, by weight % (wt%): C is at most 0.020; Si is at most 0.8; Mn is at most 2.0; Cr 29.0 to 31.0; Ni 5.0 to 9.0; Mo is less than 4.0; W is less than 4.0; N0.25-0.45； Cu maximum 2.0; S is at most 0.01; P is at most 0.02; The remaining Fe and the unavoidable impurities; and The content of Mo+W is greater than 3.0 but less than 4.

0.

3. The use of duplex stainless steel according to claim 1, wherein Mn is 0.5%-1.5% by weight.

4. Use of the duplex stainless steel according to claim 1, wherein Si is 0.010% to 0.50% by weight.

5. Use of the duplex stainless steel according to claim 1, wherein Ni is 5.5% to 8.5% by weight.

6. Use of the duplex stainless steel according to claim 1, wherein N is 0.28% to 0.40% by weight.

7. Use of the duplex stainless steel according to claim 1, wherein Mn is 0.5 wt% to 1.5 wt%, Si is 0.010 wt% to 0.50 wt%, Ni is 5.5 wt% to 8.5 wt%, and N is 0.28 wt% to 0.40 wt%.

8. Use of the duplex stainless steel according to any one of claims 1 to 7, in a urea synthesis process, the duplex stainless steel is used to reduce corrosion of one or more components in a high-pressure urea synthesis section that is in contact with a fluid containing 45% to 65% by weight of ammonium carbamate and at a temperature above 180°C.

9. A molded object comprising the duplex stainless steel as defined in any one of claims 1 to 7, wherein the molded object is a stripper tube for an apparatus for urea production.

10. A method for producing urea, the method comprising reacting ammonia with carbon dioxide in a urea production apparatus under urea-forming conditions, wherein at least one component of the equipment of the urea production apparatus is made of duplex stainless steel as defined in any one of claims 1 to 7, the method comprising forming ammonium carbamate and dehydrating the ammonium carbamate to provide urea, and exposing the component of the equipment to a fluid containing 45% to 65% by weight of ammonium carbamate and at a temperature above 180°C.

11. An apparatus for urea production, wherein the apparatus comprises one or more components comprising duplex stainless steel as defined in any one of claims 1 to 7, wherein the one or more components are one or more stripper tubes.

12. A method for modifying an existing apparatus for urea production, the apparatus comprising one or more components selected from liquid distributors, radomes, valves, and injectors, wherein the method includes the step of replacing one or more stripper tubes with stripper tubes comprising duplex stainless steel as defined in any one of claims 1 to 7.

13. A method for reducing the passive corrosion rate of a urea plant by replacing at least one stripper tube with a stripper tube comprising duplex stainless steel as defined in any one of claims 1 to 7.

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