DUPLEX STAINLESS STEELS AND USES THEREOF

Abstract

A duplex stainless steel for use in a urea production plant and/or in a urea production process, containing in weight percentage (% w): C 0.03 or less; Si 0.5 or less; Mn 2.5 or less; Cr from more than 30.0 to 35.0; Ni from 5.5 to 8.0; Co from 0.01 to 0.8; Mo from 2.0 to 2.5; W 2.5 or less; N from 0.3 to 0.6; Cu 1.0 or less; and having one or more of: Ca 0.0040 or less; Mg 0.0040 or less; one or more rare-earth elements in a total amount of 0.1 or less; the balance being Fe and impurities; and satisfying the relationship: Z=1.062 (Ni+Co)+4.185 Mo is between 14.95 and 19.80.

Claims

1-31. (canceled)

32. A urea production process comprising: at least one step performed in a device comprising duplex stainless steel containing: carbon in weight percentage of 0.03% or less, silicon in weight percentage of 0.5% or less, manganese in weight percentage of 2.5% or less, chromium in weight percentage from more than 30.0% to 35.0%, nickel in weight percentage from 5.5% to 8.0%, cobalt in weight percentage from 0.01% to 0.8%, molybdenum in weight percentage from 2.0% to 2.5%, tungsten in weight percentage of 2.5% or less, nitrogen in weight percentage from 0.3% to 0.6%, copper in weight percentage of 1.0% or less, at least one of: calcium in weight percentage of 0.0040% or less, magnesium in weight percentage of 0.0040% or less, and at least one rare-earth element in a total amount in weight percentage of 0.1% or less, and a balance being iron and any impurities; wherein the duplex stainless steel has a composition parameter representative of the combined contents of nickel, cobalt, molybdenum defined by a formula of: composition parameter=1.062 (nickel+cobalt)+4.185 molybdenum wherein in the formula, nickel indicates the weight percentage of nickel, cobalt indicates the weight percentage of cobalt, molybdenum indicates the weight percentage of molybdenum, and the composition parameter ranges from between 14.95 and 19.80.

33. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.001% to 0.02% in weight percentage of carbon.

34. The urea production process of claim 32, wherein the duplex stainless steel contains from 30.5% to 35% in weight percentage of chromium.

35. The urea production process of claim 32, wherein the duplex stainless steel contains from 30.5% to 32% in weight percentage of chromium.

36. The urea production process of claim 32, wherein the duplex stainless steel contains from 30.5% to 31.6% in weight percentage of chromium.

37. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.001% to 0.9% in weight percentage of copper.

38. The urea production process of claim 32, wherein the duplex stainless steel contains one of: from 0.10% to 0.90% in weight percentage of copper, and from 0.10% to 0.40% in weight percentage of copper.

39. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.02% to 0.6% in weight percentage of cobalt.

40. The urea production process of claim 32, wherein the duplex stainless steel contains from 6.0% to 7.5% in weight percentage of nickel.

41. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.5% to 2.5% in weight percentage of manganese.

42. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.5% to 2.2% in weight percentage of manganese.

43. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.001% to 2.5% in weight percentage of tungsten.

44. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.02% to 1.0% in weight percentageof tungsten.

45. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.001% to 0.004% in weight percentage of calcium.

46. The urea production process of claim 32, wherein the duplex stainless steel contains from 0.001% to 0.004% in weight percentage of magnesium.

47. The urea production process of claim 32, wherein the duplex stainless steel contains at least one rare-earth element in a total amount of 0.05% or less.

48. The urea production process of claim 32, wherein the duplex stainless steel contains at least one rare-earth element selected from the group consisting of: lanthanum, cerium, praseodymium and at least one mixture thereof.

49. The urea production process of claim 32, wherein the duplex stainless steel contains, as impurities, no more than at least one of: 0.025% in weight percentage of of phosphorus (P) and 0.005% in weight percentage of sulphur.

50. The urea production process of claim 32, wherein the duplex stainless steel is in contact with an ammonium carbamate solution having a concentration of ammonium carbamate ranging from 15% in weight percentage to 95% in weight percentage.

51. The urea production process of claim 32, wherein the duplex stainless steel is in contact with an ammonium carbamate solution having a concentration of ammonium carbamate ranging from 50% in weight percentage to 95% in weight percentage.

52. The urea production process of claim 32, wherein the duplex stainless steel is in contact with an ammonium carbamate solution at a temperature of at least 185 C.

53. The urea production process of claim 32, wherein the duplex stainless steel is in contact with an ammonium carbamate solution at a temperature of at least 205 C.

54. The urea production process of claim 32, wherein the duplex stainless steel is used in at least one of: at a maximum temperature of at least 185 C., at a maximum pressure of at least 150 bar, and in an environment having an ammonia/carbon dioxide molar ratio in the range of 3.2 to 3.6.

55. The urea production process of claim 32, wherein the duplex stainless steel is used in at least one of: at a maximum temperature in a range of 205 C. to 215 C., at a maximum pressure of at least 160 bar, and in an environment having a ammonia/carbon dioxide molar ratio in the range of 3.2 to 3.6.

56. The urea production process of claim 32, wherein the at least one step performed in the device comprising duplex stainless steel occurs in a urea environment under oxygen-free conditions.

57. The urea production process of claim 32, wherein the at least one step performed in the device comprising duplex stainless steel comprises using the duplex stainless steel in a high pressure section of a urea plant.

58. The urea production process of claim 32, wherein the at least one step performed in the device comprising duplex stainless steel comprises using the duplex stainless steel in association with performing one of: an ammonia-stripping process and a self-stripping process.

59. The urea production process of claim 32, wherein the at least one step performed in the device comprising duplex stainless steel comprises using the duplex stainless steel in a high pressure stripper configured for one of: ammonia-stripping and self-stripping.

60. The urea production process of claim 32, wherein the duplex stainless steel is coupled with a covering layer which covers at least a surface portion of the duplex stainless steel, the covering layer being made of one of: zirconium and a zirconium alloy.

61. The urea production process of claim 32, wherein the composition parameter ranges between 14.95 and 17.50.

62. A urea production plant device comprising: a duplex stainless steel containing: carbon in weight percentage of 0.03% or less, silicon in weight percentage of 0.5% or less, manganese in weight percentage of 2.5% or less, chromium in weight percentage from more than 30.0% to 35.0%, nickel in weight percentage from 5.5% to 8.0%, cobalt in weight percentage from 0.01% to 0.8%, molybdenum in weight percentage from 2.0% to 2.5%, tungsten in weight percentage of 2.5% or less, nitrogen in weight percentage from 0.3% to 0.6%, copper in weight percentage of 1.0% or less, at least one of: calcium in weight percentage of 0.0040% or less, magnesium in weight percentage of 0.0040% or less, and at least one rare-earth element in a total amount in weight percentage of 0.1% or less, and a balance being iron and any impurities; wherein the duplex stainless steel has a composition parameter representative of the combined contents of nickel, cobalt, molybdenum defined by a formula of:
composition parameter=1.062 (nickel+cobalt)+4.185 molybdenum wherein in the formula, nickel indicates the weight percentage of nickel, cobalt indicates the weight percentage of cobalt, molybdenum indicates the weight percentage of molybdenum, and the composition parameter ranges between 14.95 and 19.80.

63. A method of modifying a urea production plant, the method comprising: replacing at least part of the urea production plant with a device comprising a duplex stainless steel containing: carbon in weight percentage of 0.03% or less, silicon in weight percentage of 0.5% or less, manganese in weight percentage of 2.5% or less, chromium in weight percentage from more than 30.0% to 35.0%, nickel in weight percentage from 5.5% to 8.0%, cobalt in weight percentage from 0.01% to 0.8%, molybdenum in weight percentage from 2.0% to 2.5%, tungsten in weight percentage of 2.5% or less, nitrogen in weight percentage from 0.3% to 0.6%, copper in weight percentage of 1.0% or less, at least one of: calcium in weight percentage of 0.0040% or less, magnesium in weight percentage of 0.0040% or less, and at least one rare-earth element in a total amount in weight percentage of 0.1% or less, and a balance being iron and any impurities; wherein the duplex stainless steel has a composition parameter representative of the combined contents of nickel, cobalt, molybdenum defined by a formula of:
composition parameter=1.062 (nickel+cobalt)+4.185 molybdenum wherein in the formula, nickel indicates the weight percentage of nickel, cobalt indicates the weight percentage of cobalt, molybdenum indicates the weight percentage of molybdenum, and the composition parameter ranges between 14.95 and 19.80.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Further features and advantages of the present disclosure will be apparent from the following description of non-limiting embodiments thereof, with reference to the figures of the accompanying drawings, wherein:

[0096] FIG. 1 contains a table (Table 1) reporting the composition of exemplary samples of duplex stainless steels according to the disclosure, as well as of some reference samples; and

[0097] FIG. 2 contains a table (Table 2) reporting the results of corrosion resistance tests performed on the samples of Table 1.

DETAILED DESCRIPTION

[0098] Duplex stainless steels according to the disclosure contain in weight % (% w): [0099] C max 0.03 [0100] Si max 0.5 [0101] Mn max 2.5 [0102] Cr from more than 30.0 to 35.0 [0103] Ni 5.5 to 8.0 [0104] Co 0.01 to 0.8 [0105] Mo 2.0 to 2.5 [0106] W max 2.5 [0107] N 0.3 to 0.6 [0108] Cu max 1.0 [0109] and has one or more of: [0110] Ca max 0.0040 [0111] Mg max 0.0040 [0112] one or more rare-earth elements max 0.1 [0113] the balance being Fe and impurities (as commonly understood, impurities are all those elements and compounds which are not purposively added to the steel formulation, but are however present in small amounts being contained in the raw materials used for manufacturing the duplex stainless steel).

[0114] The duplex stainless steels of the disclosure are further characterized in that the content of Ni, Co, Mo is such that:

Z.sub.min[1.062 (Ni+Co)+4.185 Mo]Z.sub.max(II) [0115] where: [0116] Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively; [0117] Z.sub.min=14.95; [0118] Z.sub.max=19.80.

[0119] In other words, the duplex stainless steels of the disclosure have a composition parameter Z, representative of the combined contents of Ni, Co, Mo and defined by formula (I):

Z=1.062 (Ni+Co)+4.185 Mo(I) [0120] where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively; [0121] and wherein 14.95Z19.80.

[0122] In certain embodiments, the duplex stainless steels according to the disclosure contain in weight % (% w): [0123] C 0.001 to 0.03 [0124] Si 0.001 to 0.5 [0125] Mn 0.001 to 2.5 [0126] Cr from more than 30.0 to 35.0 [0127] Ni 5.5 to 8.0 [0128] Co 0.01 to 0.8 [0129] Mo 2.0 to 2.5 [0130] W 0.001 to 2.5 [0131] N 0.3 to 0.6 [0132] Cu 0.001 to 1.0 [0133] and has one or more of: [0134] Ca max 0.0040 [0135] Mg max 0.0040 [0136] one or more rare-earth elements, in particular selected in the group consisting of La, Ce, Pr and mixture thereof, in a total amount of max 0.1 [0137] the balance being Fe and impurities; [0138] and wherein the content of Ni, Co, Mo is such that:

Z.sub.min [1.062 (Ni+Co)+4.185 Mo]Z.sub.max(II) [0139] where: [0140] Z.sub.min=14.95; [0141] Z.sub.max=19.80.

[0142] According to the disclosure, the composition parameter Z as above defined ranges between 14.95 and 19.80, preferably between 14.95 and 19.00, more preferably between 14.95 and 18.00, more preferably between 14.95 and 17.50.

EXAMPLES

[0143] Exemplary steel compositions according to the disclosure comprise, in percentages by weight: [0144] C: 0.03% or less; [0145] Si: 0.5% or less; [0146] Mn: 2.5% or less; [0147] Cr: 30.5% to 35%; [0148] Ni: 5.5% to 8%; [0149] Mo: 2% to 2.5%; [0150] W: 0.02% to 1.0%; [0151] Co: 0.01% to 0.8%; [0152] N: 0.3% to 0.6%; [0153] Cu: 1% or less; [0154] one or more of the following: [0155] Ca: 0.004% or less; [0156] Mg: 0.004% or less; [0157] one or more rare earth elements in a total amount of 0.05% or less; [0158] the remainder being Fe and unavoidable impurities; [0159] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0160] Other embodiments of the steel of the disclosure comprise, in percentages by weight: [0161] C: 0.001% to 0.03%; [0162] Si: 0.001% to 0.5%; [0163] Mn: 0.001% to 2.5%; [0164] Cr: more than 30% to 35%; [0165] Ni: 5.5% to 8%; [0166] Mo: 2% to 2.5%; [0167] W: 0.4% to 0.8%; [0168] Co: 0.01% to 0.8%; [0169] N: 0.3% to 0.6%; [0170] Cu: 0.001% to 1%; [0171] one or more of the following: [0172] Ca: 0.001% to 0.004%; [0173] Mg: 0.001% to 0.004%; [0174] one or more rare earth elements in a total amount of 0.001% to 0.1%; [0175] the remainder being Fe and unavoidable impurities; [0176] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0177] Other compositions according to the disclosure comprise, in percentages by weight: [0178] C: 0.001% to 0.03%; [0179] Si: 0.5% or less; [0180] Mn: 0.5% to 2.2%; [0181] Cr: 30.5% to 34%; [0182] Ni: 5.5% to 8%; [0183] Mo: 2% to 2.5%; [0184] W: 2.5% or less; [0185] Co: 0.01% to 0.8%; [0186] N: 0.3% to 0.6%; [0187] Cu: 1% or less; [0188] one or more of the following: [0189] Ca: 0.004% or less; [0190] Mg: 0.004% or less; [0191] one or more rare earth elements in a total amount of 0.05% or less; [0192] the remainder being Fe and unavoidable impurities; [0193] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0194] Yet other compositions according to the disclosure comprises, in percentages by weight: [0195] C: 0.02% or less; [0196] Si: 0.001% to 0.5%; [0197] Mn: 2.5% or less; [0198] Cr: 30.5% to 32%; [0199] Ni: 5.5% to 8%; [0200] Mo: 2% to 2.5%; [0201] W: 0.1% to 1%; [0202] Co: 0.01% to 0.8%; [0203] N: 0.3% to 0.6%; [0204] Cu: 0.15% to 0.25%; [0205] having one or more of the following: [0206] Ca: 0.004% or less; [0207] Mg: 0.004% or less; [0208] La, Ce, Pr or other rare earth elements: 0.05% or less [0209] the remainder being Fe and unavoidable impurities; [0210] satisfying the relationship CRC=1.062*(Ni+Co)+4.185*Mo is between 14.95 to 19.80.

[0211] Other compositions according to the disclosure comprises, in percentages by weight: [0212] C: 0.03% or less; [0213] Si: 0.5% or less; [0214] Mn: 0.001% to 2.2%; [0215] Cr: 31% to 35%; [0216] Ni: 6% to 7.5%; [0217] Mo: 2% to 2.5%; [0218] W: 2.5% or less; [0219] Co: 0.01% to 0.8%; [0220] N: 0.4% to 0.6%; [0221] Cu: 0.9% or less; [0222] one or more of the following: [0223] Ca: 0.004% or less; [0224] Mg: 0.004% or less; [0225] one or more rare earth elements in a total amount of 0.05% or less; [0226] the remainder being Fe and unavoidable impurities; [0227] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0228] Other examples according to this disclosure comprise, in percentages by weight: [0229] C: 0.03% or less; [0230] Si: 0.5% or less; [0231] Mn: 0.5% to 2.2%; [0232] Cr: 30.5% to 35%; [0233] Ni: 5.5% to 6.5%; [0234] Mo: 2% to 2.5%; [0235] W: 0.001% to 2.5%; [0236] Co: 0.01% to 0.6%; [0237] N: 0.35% to 0.6%; [0238] Cu: 1% or less; [0239] one or more of the following: [0240] Ca: 0.004% or less; [0241] Mg: 0.004% or less; [0242] one rare earth element selected from La, Ce, Pr or a combination thereof: 0.05% or less [0243] the remainder being Fe and unavoidable impurities; [0244] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0245] For example, the present disclosure relates to elementary steel compositions that comprise, in percentages by weight: [0246] C: 0.03% or less; [0247] Si: 0.5% or less; [0248] Mn: 2.2% or less; [0249] Cr: 31% to 32%; [0250] Ni: 5.5% to 8%; [0251] Mo: 2% to 2.5%; [0252] W: 2.5% or less; [0253] Co: 0.02% to 0.4%; [0254] N: 0.3% to 0.6%; [0255] Cu: 0.001% to 1%; [0256] one or more of the following: [0257] Ca: 0.004% or less; [0258] Mg: 0.004% or less; [0259] one rare earth element selected from La, Ce, Pr or a combination thereof: 0.05% or less [0260] the remainder being Fe and unavoidable impurities; [0261] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0262] Other exemplifying composition according to the disclosure comprise, in percentages by weight: [0263] C: 0.03% or less; [0264] Si: 0.5% or less; [0265] Mn: 2% or less; [0266] Cr: 30.5% to 33%; [0267] Ni: 5.5% to 8%; [0268] Mo: 2% to 2.5%; [0269] W: 0.2% to 1%; [0270] Co: 0.02% to 0.4%; [0271] N: 0.3% to 0.6%; [0272] Cu: 1% or less; [0273] one or more of the following: [0274] Ca: 0.001% to 0.004%; [0275] Mg: 0.001% to 0.004%; [0276] La, Ce, Pr or other rare earth elements: 0.001% to 0.05% [0277] the remainder being Fe and unavoidable impurities; [0278] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0279] Further example compositions according to the disclosure comprise, in percentages by weight: [0280] C: 0.02% or less; [0281] Si: 0.5% or less; [0282] Mn: 0.5% to 2.2%; [0283] Cr: 30.5% to 34%; [0284] Ni: 5.5 to 8%; [0285] Mo: 2 to 2.5%; [0286] W: 0.02 to 1%; [0287] Co: 0.02 to 0.6%; [0288] N: 0.3 to 0.6%; [0289] Cu: 0.20% to 0.9%; [0290] one or more of the following: [0291] Ca: 0.004% or less; [0292] Mg: 0.004% or less; [0293] one or more rare earth elements in a total amount of 0.05% or less; [0294] the remainder being Fe and unavoidable impurities; [0295] satisfying the relationship: Z=1.062*(Ni+Co)+4.185*Mo is between 14.95 and 19.80.

[0296] In particular, duplex stainless steels having the compositions in Table 1 were prepared and tested (in Table 1, some components are not indicated, being however in the amounts as previously disclosed).

[0297] The samples were prepared as common in the field and tested according to standard testing procedure. Samples A1 to A5 were prepared by using laboratory produced materials, while sample B1 was prepared by using material from an industrial production.

[0298] In particular, corrosion tests were performed in a high pressure autoclave in ammonium carbamate solution at high pressure and high temperature (conditions representative of typical operation conditions in urea plants, in particular in the tubes of a urea stripper).

[0299] In particular, the corrosion resistance of the duplex stainless steels of the disclosure was tested in an oxygen-free carbamate solution, having a composition simulating the worst conditions normally occurring in the tubes of a high pressure section urea stripper of a urea plant, and at a temperature of 208 C.

[0300] In more detail: the corrosion behavior of the laboratory heats was checked via immersion tests that were conducted in a 5-liter Zirconium autoclave. The autoclave was equipped with adequate feed and discharge lines and a stirrer. The test solution contained a mixture of urea, ammonia and water, at concentrations similar to those of the urea synthesis process. Temperature and pressure for the experiments were set in the upper level of the typical ranges measured in a urea stripper, 180-210 C. and 140-200 bar, respectively. The test solution was degassed before starting the tests to eliminate oxygen from the system. These experiments were designed to simulate the most severe conditions in a stripper of a urea plant without oxygen injection; note that under current working conditions in a urea plant, the stainless steel would perform even better, due to the presence of low amounts of oxygen and less aggressive conditions.

[0301] Test duration was 13 and 30 days. ASTM G31 (Standard Practice for Laboratory Immersion Corrosion Testing of Metals) standard indications were followed for test specimen preparation and the corrosion rate was measured by the gravimetric method.

[0302] After exposures of 13 days and 30 days respectively in the oxygen-free carbamate solution, the corrosion resistance was evaluated by calculating the corrosion rate (expressed in mm/year).

[0303] The results are shown in Table 2.

[0304] The results confirm that the samples (A1-A5; B1) made of a duplex stainless steel according to the disclosure (i.e., satisfying the composition requirements of the disclosure (in particular with respect to the combined content of Ni, Co, Mo), have a corrosion rate significantly lower than comparative samples Ref1, Ref2, Ref3 and thus a better corrosion resistance).

[0305] It should be appreciated that the experimental tests confirm that when Z satisfies the requirement: 14.95Z19.80, corrosion values are significantly lower than those exhibited by reference materials.

[0306] Corrosion values would be even significantly lower in working conditions in a urea plant, since the experimental set-up conditions are much more aggressive.

[0307] Finally, although the disclosure has been disclosed in relation to the above-mentioned embodiments, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the appended claims. Accordingly, various changes and modifications to the presently disclosed embodiments will be apparent to those skilled in the art.