High molybdenum duplex stainless steel

Abstract

Disclosed herein is a duplex stainless steel alloy containing 40%-60% ferrite and 60%-40% austenite by weight, and a method for making the same, the alloy comprising, or consisting essentially of, 10% to 20% chromium (Cr); 6% to 13% molybdenum (Mo); 0.5% to 6.5% nickel (Ni); 2.25% to 12% manganese (Mn); 0.05% to 5% copper (Cu); 0.05% to 0.4% nitrogen (N); 0.05% to 0.35% carbon (C); 0.01% to 3.5% cobalt (Co); less than 2% silicon (Si); less than 2% tungsten (W); and the balance iron (Fe). The duplex stainless steel alloy may comprise cast steel or wrought steel, or may be in powder form.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 391,512, filed July 22, 2022, which is incorporated by reference in its entirety for all purposes. [Background technology]

[0002] Duplex stainless steels (DSS) are a subfamily of stainless steel alloys. These materials are commonly used in industrial and energy markets due to their combination of corrosion resistance, strength, and impact toughness. Applications include oil and gas equipment and pipelines; subsea, marine equipment, and ship components; and industrial heat exchangers, piping, and chemical processes.

[0003] The microstructure of DSS is unique, exhibiting both austenite and ferrite phases at approximately 60-40% and 40-60%, respectively.

[0004] The four primary alloying elements in this steel family are chromium (Cr), molybdenum (Mo), nickel (Ni), and nitrogen (N). Cr and Mo act as ferrite stabilizers, while Ni and N act as austenite stabilizers. Referring to Figure 1, the microstructure of 2507DSS contains both ferrite (black / gray) and austenite (white) stable zones.

[0005] For dual phase stainless steels, the general trend to achieve higher mechanical properties and corrosion resistance behavior has been to increase the Cr and Ni content, thereby balancing the phases. Some exemplary DSS compositions are shown below: Lean duplex stainless steels - 2101, 2102, 2202, 2304: 21-23 wt% Cr, 1-4 wt% Ni, <1 wt% Mo Duplex stainless steels - 2205, 2003, 2404: 20-24 wt% Cr, 3-5 wt% Ni, 1-3.5 wt% Mo Super duplex stainless steels - 2507, 255, Z100: 25% by weight Cr, 5.5-7% by weight Ni, 3.5-4.5% by weight Mo Hyper Duplex Stainless Steel - 2707: >27 wt% Cr, 6.5 wt% Ni, 4.8 wt% Mo Hyper Duplex Stainless Steel - 3207: >32 wt% Cr, 7 wt% Ni, 3.5 wt% Mo Summary of the Invention

[0006] A potential drawback of having a high Cr and / or Ni content is the formation of detrimental phases (sigma and chi phases) during processing, because increasing the Cr or Ni content increases the solvus temperature of the detrimental phases. For example, see Figure 2, which illustrates how the sigma solvus temperature of DSS2507 increases with Ni content. If detrimental sigma and / or chi phases are abundant in the alloy during processing, e.g., hot working, the risk of forming processing defects (cracks, fissures, pores, etc.) increases. Furthermore, higher Ni contents can result in higher costs. If the phase balance is not properly achieved, the temperature difference between the detrimental phase formation temperature and the temperature of the duplex stainless steel (50 / 50 austenite and ferrite) remains small, presenting processing challenges and adversely affecting mechanical properties and corrosion behavior. Ideally, the temperature difference between the solvus temperature and the temperature of the duplex stainless steel (the "delta temperature") is as large as possible to improve alloy production and processing.

[0007] In one embodiment, an embodiment of the present invention relates to a duplex stainless steel alloy comprising, or consisting essentially of, 10% to 20% by weight chromium (Cr); 6% to 13% by weight molybdenum (Mo); 0.5% to 6.5% by weight nickel (Ni); 2.25% to 12% by weight manganese (Mn); 0.05% to 5% by weight copper (Cu); 0.05% to 0.4% by weight nitrogen (N); 0.05% to 0.35% by weight carbon (C); 0.01% to 3.5% by weight cobalt (Co); less than 2% by weight silicon (Si); less than 2% by weight tungsten (W); and the balance iron (Fe).

[0008] The stainless steel alloy contains 40%-60% ferrite and 60%-40% austenite by weight, and also has a nickel equivalent and a chromium equivalent. The nickel equivalent and chromium equivalent are defined as: (i) the nickel equivalent (Ni eq ) = wt% of Ni + (30 × wt% of C) + (0.5 × wt% of Mn), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 7 respectively <Ni eq Values of <20 and 21 <Cr eq or (ii) has a nickel equivalent (Ni eq ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 10 each <Ni eq Values <18 and 21 <Cr eq <27.

[0009] One or more features may be included. The stainless steel alloy may include 12% to 17% by weight chromium (Cr); 7.25% to 11% by weight molybdenum (Mo); 0.75% to 5% by weight nickel (Ni); 2.5% to 8% by weight manganese (Mn); 1.75% to 3.5% by weight copper (Cu); 0.1% to 0.3% by weight nitrogen (N); 0.075% to 0.2% by weight carbon (C); 0.01% to 3% by weight cobalt (Co); less than 1.5% by weight silicon (Si); less than 1.5% by weight tungsten (W); and the balance iron (Fe).

[0010] The stainless steel alloy may include cast steel and / or wrought steel. The stainless steel alloy may have a yield strength of at least 70 ksi, an ultimate tensile strength of at least 115 ksi, an elongation of >30%, a reduction in area of >50%, and / or a pitting corrosion resistance equivalent index value of at least 30.

[0011] In another aspect, an embodiment of the present invention relates to a duplex stainless steel alloy powder for additive manufacturing, comprising, or consisting essentially of, 10% to 20% by weight chromium (Cr); 6% to 13% by weight molybdenum (Mo); 0.5% to 6.5% by weight nickel (Ni); 2.25% to 12% by weight manganese (Mn); 0.05% to 5% by weight copper (Cu); 0.05% to 0.4% by weight nitrogen (N); 0.05% to 0.35% by weight carbon (C); 0.01% to 3.5% by weight cobalt (Co); less than 2% by weight silicon (Si); less than 2% by weight tungsten (W); and the balance iron (Fe).

[0012] Stainless steel alloys contain 40%-60% ferrite and 60%-40% austenite by weight, and also have nickel equivalents and chromium equivalents. The nickel equivalents and chromium equivalents are defined as: (i) nickel equivalent (Ni eq ) = wt% of Ni + (30 × wt% of C) + (0.5 × wt% of Mn), and chromium equivalent (Cr eq) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 7 respectively <Ni eq Values of <20 and 21 <Cr eq or (ii) has a nickel equivalent (Ni eq ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq Each of the <Ni eq Values <18 and 21 <Cr eq <27. The powder comprises a plurality of spherical particles having an average particle size selected from the ranges of 15-53 micrometers or 45-103 micrometers.

[0013] The powder comprises, or consists essentially of, 12% to 17% by weight chromium (Cr); 7.25% to 11% by weight molybdenum (Mo); 0.75% to 5% by weight nickel (Ni); 2.5% to 8% by weight manganese (Mn); 1.75% to 3.5% by weight copper (Cu); 0.1% to 0.3% by weight nitrogen (N); 0.075% to 0.2% by weight carbon (C); 0.01% to 3% by weight cobalt (Co); less than 1.5% by weight silicon (Si); less than 1.5% by weight tungsten (W); and the balance iron (Fe).

[0014] In yet another aspect, an embodiment of the present invention relates to a method for forming a duplex stainless steel alloy, the method comprising melting a mixture of elements to form a molten metal alloy consisting of, or consisting essentially of, 10 wt.% to 20 wt.% chromium (Cr); 6 wt.% to 13 wt.% molybdenum (Mo); 0.5 wt.% to 6.5 wt.% nickel (Ni); 2.25 wt.% to 12 wt.% manganese (Mn); 0.05 wt.% to 5 wt.% copper (Cu); 0.05 wt.% to 0.4 wt.% nitrogen (N); 0.05 wt.% to 0.35 wt.% carbon (C); 0.01 wt.% to 3.5 wt.% cobalt (Co); less than 2 wt.% silicon (Si); less than 2 wt.% tungsten (W); and the balance iron (Fe). The stainless steel alloy contains 40%-60% by weight ferrite and 60%-40% by weight austenite, and also has a nickel equivalent and a chromium equivalent, which are (i) the nickel equivalent (Ni eq ) = wt% of Ni + (30 × wt% of C) + (0.5 × wt% of Mn), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 7 respectively <Ni eq Values of <20 and 21 <Cr eq or (ii) has a nickel equivalent (Ni eq ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 10 each <Ni eq Values <18 and 21 <Cr eq <27. The molten metal alloy is rapidly cooled to solidify the metal alloy.

[0015] The method may include forging the solidified metal alloy. The forged metal alloy may be heat treated.

[0016] In another aspect, an embodiment of the present invention relates to a method for forming a duplex stainless steel alloy powder having a composition consisting essentially of, or consisting of, 10 wt.% to 20 wt.% chromium (Cr); 6 wt.% to 13 wt.% molybdenum (Mo); 0.5 wt.% to 6.5 wt.% nickel (Ni); 2.25 wt.% to 12 wt.% manganese (Mn); 0.05 wt.% to 5 wt.% copper (Cu); 0.05 wt.% to 0.4 wt.% nitrogen (N); 0.05 wt.% to 0.35 wt.% carbon (C); 0.01 wt.% to 3.5 wt.% cobalt (Co); less than 2 wt.% silicon (Si); less than 2 wt.% tungsten (W); and the balance iron (Fe). The stainless steel alloy comprises 40 wt.%-60 wt.% ferrite and 60 wt.%-40 wt.% austenite and has a nickel equivalent and a chromium equivalent, which are determined by: (i) the nickel equivalent (Ni eq ) = wt% of Ni + (30 × wt% of C) + (0.5 × wt% of Mn), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 7 respectively <Ni eq Values of <20 and 21 <Cr eq or (ii) has a nickel equivalent (Ni eq ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), where Ni eq and Cr eq are 10 each <Ni eq Values <18 and 21 <Cr eqor having a value of <27. The powder comprises a plurality of spherical particles having an average particle size selected from the ranges of 15-53 micrometers or 45-103 micrometers. The method includes melting a charge comprising the composition to form a molten metal bath. A molten metal stream is produced from the molten metal bath. The molten metal stream is atomized to form a plurality of metal droplets. The metal droplets are cooled, causing the metal droplets to solidify and form a powder.

[0017] The charge material may be melted in an atmosphere including air, an inert gas, and / or a vacuum. The molten metal stream may be atomized in a high pressure gas including argon, nitrogen, and / or helium.

[0018] These and other objects, along with advantages and features of the embodiments of the present invention disclosed herein, will become more apparent through reference to the following description, drawings, and claims. Furthermore, it is understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. [Brief explanation of the drawings]

[0019] In the drawings, like reference numbers generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings:

[0020] FIG. 1 is a photomicrograph illustrating a prior art duplex stainless steel;

[0021] Figure 2 is a graph illustrating the increase in solvus temperature of the sigma phase of DSS with Ni content;

[0022] FIG. 3A is a Schaeffler diagram illustrating the phases formed in stainless steels depending on their composition;

[0023] Figure 3B is a Long / DeLong diagram illustrating the phases formed in stainless steels depending on their composition;

[0024] FIG. 4 is a photomicrograph illustrating the microstructure of a duplex stainless steel according to an embodiment of the present invention; and

[0025] 5A and 5B are images illustrating nitrogen outgassing and blowholes when the carbon level in a stainless steel composition is too low (eg, less than 0.05%). DETAILED DESCRIPTION OF THE INVENTION

[0026] In many conventional duplex stainless steels, high chromium content was necessary to aid corrosion resistance. Increasing the alloying elements may be considered for DSS to achieve high mechanical properties and corrosion resistance (e.g., Cr>30%). However, higher Cr levels increase cost and also increase the formation of deleterious phases (sigma and chi phases) during processing. Furthermore, with continued increases in Cr and Ni in DSS compositions, gains in corrosion resistance are typically offset by an increase in the solvus temperature of the sigma and / or chi phases, correspondingly reducing the hot working window.

[0027] The alloy composition according to an embodiment of the present invention includes rebalancing the levels of Cr, Mo, Ni, and N to lower the solvus temperature of deleterious sigma and chi phases, thereby aiding processability and supporting mechanical properties and corrosion resistance. In particular, the weight percent of Cr is reduced to a more suitable level while maintaining corrosion resistance. In parallel, the weight percent of Mo is increased to a higher level than that of conventional DSS because Mo is better than Cr for solid solution strengthening and corrosion resistance. The amounts of Cr and Mo are selected to still allow for the formation of a dual-phase stainless steel structure (40-60% ferrite and 60-40% austenite). Phase balancing is achieved with Ni, N, Mn, and Cu. The alloy composition includes less than 2 wt.% Si and W, and less than 3.5 wt.% Co.

[0028] Exemplary composition ranges according to embodiments of the present invention are provided in Tables 1 and 2.

[0029] [Table 1]

[0030] [Table 2]

[0031] The relationship between certain elements of the compositions described herein may be determined by calculating the nickel equivalent and chromium equivalent and by reference to a phase diagram. In particular, the nickel equivalent and chromium equivalent for a given composition may be calculated as follows: Nickel equivalent (Ni eq ) = wt% Ni + (0.5 × wt% Mn) + (30 × wt% C); and Chromium equivalent (Cr eq ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%).

[0032] Referring to the Schaefflerian diagram in Figure 3A (adapted from "Alloying Elements and Constituent Diagrams" by Lippold, JC & Kotecki, DJ (2005a) in "Welding Metallurgy and Weldability of Stainless Steels" (pp. 31-34) published by John Wiley), the stable phases of both austenite and ferrite are 18 <Cr eq <38 and 5 <Ni eq <33 (phase region 310 in FIG. 3A). In other embodiments, the stable phases of both austenite and ferrite may form when 18 <Cr eq <35 and 5 <Ni eq < 33 and 18 <Cr eq <30 and 5 <Ni eq <33.

[0033] In duplex stainless steels, the austenite and ferrite are roughly equal, i.e., 40-60% austenite and 60-40% ferrite. Thus, duplex stainless steels are formed when the nickel equivalent and chromium equivalent are within the range of amounts defined by phase region 320. Here, 21 <Cr eq <38 and 7 <Ni eq <20 (phase region 320 in FIG. 3A).

[0034] Alternatively, the relationships between certain elements of the compositions described herein may be determined by the following formula and the Long / DeLong structural diagram of FIG. 3B: Nickel equivalent (Ni eq ) = wt% Ni + (0.5 × wt% Mn) + (30 × wt% C) + (30 × wt% N) Chromium equivalent (Cr eq ) = Cr weight % + Mo weight % + (1.5 × Si weight %) + (0.5 × Nb weight %)

[0035] Referring to the Long / Dillon diagram in Figure 3B (adapted from "Alloying Elements and Constituent Diagrams" by Lippold, JC & Kotecki, DJ (2005a), in "Welding Metallurgy and Weldability of Stainless Steels" (pp. 31-34), John Wiley), the stable phases of both austenite and ferrite are 21 <Cr eq <27 and 10 <Ni eq <18 (phase region 350 in FIG. 3B). The large difference between the Ni equivalent range and the Cr equivalent range based on the Schaeffler and Long / DeLong diagrams is due to the fact that the latter includes nitrogen in experiments and tables, while the former does not.

[0036] Duplex stainless steel alloys according to embodiments of the present invention do not contain niobium; therefore, Cr eq The weight percent of Nb in the formula is equal to zero for the compositions described herein.

[0037] Typically, alloys with too much ferrite have poor corrosion resistance and low toughness but high strength, while alloys with too much austenite have good corrosion resistance and high toughness but low strength.

[0038] The function of various elements in embodiments of the present invention is described below, along with criteria for selecting the levels of these elements.

[0039] Cr: Chromium (Cr) is an important element for both ferrite stabilization and corrosion resistance. Compared to conventional duplex alloys, the required level is reduced to 10-20 wt%. Cr in this range still provides the necessary corrosion resistance for stainless steels, but also lowers the solvus temperature of deleterious phases such as sigma and chi phases. Preferably, the Cr level is in the range of 12-17 wt%.

[0040] Mo: Molybdenum (Mo) is another ferrite stabilizer, but it also greatly increases the corrosion resistance of the alloy. This element is essentially in the range of 6-13 wt%. Also like Cr, too high a Mo level can result in the formation of deleterious phases such as sigma and chi phases, which affect processability. At Mo levels below 6 wt%, the material, combined with the low Cr level, does not form the amount of ferrite phase necessary for the required processability and corrosion resistance. For these reasons, the preferred Mo range is 7.25-11 wt%.

[0041] A lower limit of molybdenum is required primarily for two reasons: corrosion resistance and solid solution strengthening. Due to the inherently low Cr levels in the alloy, a minimum of 6 wt. % Mo, e.g., a minimum of 7.25 wt. % Mo, is required to reach the corrosion resistance levels found in most duplex stainless steels. With regard to mechanical properties and strength, Mo is a stronger solid solution strengthener than Cr in stainless steels, and thus a level of at least 6 wt. %, e.g., at least 7.25 wt. %, is required to maintain high levels of strength. If the Mo level is high enough, it is possible to combine corrosion resistance with high strength.

[0042] The primary reason for the upper molybdenum limit is to avoid the formation of deleterious phases. In particular, at levels above 11 wt. % Mo, e.g., above 13 wt. %, deleterious phases (sigma and chi phases) form at high temperatures. The high solvus temperature of the deleterious phases can pose significant processing challenges for the alloy. Higher solvus temperatures result in large volumes of these brittle intermetallic phases, leading to cracks and fissures during hot working. In addition to processability, the formation of these deleterious phases during heat treatment and cooling adversely affects mechanical properties and corrosion resistance. Mo levels below 11 wt. % compromise the processability (hot working and heat treatment) of the alloy and its performance. Pushing the Mo level above 13 wt. % may result in the resulting alloy being unprocessable by conventional methods. Furthermore, to maintain the duplex structure, Mo levels above 13 wt. % require the inclusion of less than 10 wt. % C; the resulting alloy is no longer a stainless steel and is highly susceptible to corrosion.

[0043] Ni: Nickel (Ni) is primarily used to stabilize austenite and balance the austenite / ferrite phase fraction. Therefore, a range of 0.5-6.5 wt% is appropriate for this major alloying element. However, excess Ni can increase the solvus temperature of deleterious phases, which can cause processing challenges. Thus, the preferred Ni range is 0.75-5 wt%.

[0044] Mn: Manganese (Mn), like Ni, is an austenite stabilizer and is used to promote the formation of austenite. Mn can also increase the solubility of nitrogen, another major alloying element. If the Ni level is at the lower end of the Ni ranges listed above, more Mn may be needed to balance the austenite phase fraction, and vice versa; therefore, the preferred Mn range is 2.25-12 wt.%. However, while excess Mn may reduce the overall corrosion resistance of the alloy, it can be used as a cost-effective austenite stabilizer when Ni prices fluctuate. For these reasons, the preferred Mn range is 2.5-8 wt.%.

[0045] A lower manganese limit is required for three reasons: suppression of deleterious phases, phase balance, and nitrogen solubility. Because high levels of Ni can lead to an increase in the solvus temperature of deleterious phases, Mn can be used to partially replace Ni and maintain phase balance. At Mn levels below 2%, either the Ni concentration becomes too high, resulting in the formation of deleterious phases, or the Ni concentration remains low, resulting in a phase imbalance and the material no longer being a duplex stainless steel. In addition, some Mn is required for nitrogen solubility in the material; if the nitrogen solubility is too low, nitrogen will be released from the solid / liquid as the material solidifies, forming bubbles. The upper manganese limit is primarily required for corrosion resistance. Levels above 8 or 12 wt.% significantly reduce pitting corrosion, as given by the PREN (Pitting Resistance Index) formula: %Cr + 3.3(%Mo) + 16(%N) - 1.6(%Mn). Similarly, excess Mn reduces the stability of the passive layer by increasing the defect density of the passive film, making the material more susceptible to general corrosion.

[0046] Cu: Copper (Cu) has a strong effect on corrosion resistance in reducing acids. Cu can also help by lowering the solvus temperature of deleterious phases and is preferably added at levels between 0.05-5 wt.%. Excess Cu has been shown to cause processing problems such as hot embrittlement. Therefore, the preferred Cu range is 1.75-3.5 wt.%.

[0047] The lower copper limit is needed for two reasons: slight suppression of deleterious phases and corrosion resistance in reducing acids. Similar to Mn, some Cu can be used to replace Ni and stabilize austenite, thus reducing the formation of deleterious phases. With regard to corrosion resistance, levels below 1.75 wt.% do not have a significant effect on corrosion resistance. The upper copper limit is set at 3.5 wt.% due to the negative effect Cu can have on hot workability / hot ductility, leading to severe cracking during mechanical deformation. Severe cracking can render the material unprocessable.

[0048] N: Nitrogen (N) is a very strong austenite stabilizer and can also increase the strength and corrosion resistance of the alloy. The range of N required to form a strong, corrosion-resistant duplex alloy is 0.05-0.4 wt%. Elements such as Cr, Mn, and C affect the solubility of nitrogen in the material. Thus, too high an N level can lead to "as-cast" porosity and other processing defects. Consequently, the preferred range of N is 0.1-0.3 wt%.

[0049] C: Carbon (C) is an austenite stabilizer and is required in excess of 0.05 wt.% to maintain a proper phase balance between 40-60% ferrite and 60-40% austenite. The presence of C can also aid in the solubility of nitrogen. Thus, in a preferred embodiment, the minimum C level is set at 0.075 wt.%. Excessive carbon levels above 0.35 wt.% C can result in the formation of carbides, which reduces corrosion resistance. In a preferred embodiment, the minimum C level is 0.2 wt.%.

[0050] More specifically, a lower carbon limit is required for two reasons: nitrogen solubility and austenite stabilization (phase balance). If the carbon level is less than 0.05%, the nitrogen solubility will be too low, resulting in nitrogen outgassing and blowholes, as shown in Figures 5A and 5B. In addition, if the carbon level is less than 0.05 wt%, the austenite phase fraction will be too low. This may be compensated for by increasing Ni, Mn, or Cu, but excessive amounts of these elements can introduce additional problems, as discussed above. At levels above 0.2 or 0.35 wt%, carbon forms carbides with Cr and Mo, pulling these two elements out of solution and thereby reducing the corrosion resistance of the alloy. Furthermore, as the carbon level increases, the material becomes more susceptible to sensitization at high temperatures (during component use), further reducing Cr, an important element for corrosion resistance. The carbides Cr (M 23 When C6) and Mo (MC) are formed, the corrosion resistance of the alloy decreases and the phase stability of ferrite also decreases.

[0051] Cobalt (Co) may be used as a Ni substitute in some of its performance capabilities. However, Co is not a strong austenite stabilizer and typically has a higher raw material cost than Ni. Therefore, the Co content is less than 3.5 wt.%, preferably less than 3 wt.%.

[0052] The lower limit of 0.01 wt% cobalt is needed for one reason - it is an austenite stabilizer (Ni substitute). Since Co is only needed to replace Ni as an austenite stabilizer, it is needed at significant levels unless Ni is at high levels, thus not increasing the solvus temperature of deleterious phases. The upper limit of 3.5 wt% Co is set, at least in part, for economic reasons - the cost of Co is higher than other austenite stabilizers, so replacing Ni or Mn with more expensive alloying elements like Co is not needed unless there is a significant change in mechanical properties or corrosion resistance.

[0053] Si: Silicon (Si) is commonly used as an effective deoxidizing element. Si can also be used as a ferrite stabilizer, but has also been shown to adversely affect the processability of the material. Therefore, the Si content is less than 2 wt. %, and preferably less than 1.5 wt. %.

[0054] W: Tungsten (W) can be used as a substitute for Mo in some of its properties. When W is present in the presence of Mo and Cr, it can improve the corrosion resistance of stainless steel. However, high levels of W can lead to the formation of sigma and chi phases, which are detrimental to the properties and characteristics of the material. Therefore, the W content is less than 2 wt%, preferably less than 1.5 wt%.

[0055] Advantages of compositions according to embodiments of the present invention may include a greater temperature difference (delta) between the solvus temperature of deleterious phases and the hot-working temperature than that offered by other DSS compositions. This greater temperature difference improves hot workability / processability, thereby allowing for easier processing of the material and reducing the risk of sigma and chi phase formation during heat treatment and quenching. In addition, these compositions offer improved mechanical properties compared to other DSSs, thereby resulting in weight savings and design flexibility. These improved properties may include, for example, a yield strength (YS) of 70 ksi, an ultimate tensile strength (UTS) of 115 ksi, an elongation of >30%, and a reduction in area (RA) of >50%, as described below for alloy AH in Example 1. Finally, the compositions described herein offer corrosion resistance comparable to other DSSs, such as a Pitting Resistance Index (PREN) value of 30-45, calculated as %Cr + 3.3(%Mo) + 16(%N) - 1.6(%Mn).

[0056] The alloys described herein may be formed by melting using conventional arc melting or vacuum induction melting (VIM). Suitable systems may be vacuum furnaces manufactured by Consarc or ALD Vacuum Technologies. All elements of the desired composition are added to a crucible and melted together. Final and subsequent additions (for volatile elements) may be made to achieve the final chemistry just before the material is tapped. The melting process may be followed by a homogenization process using a standard atmospheric furnace. Once the molten metal is homogenized, it is poured into a mold and solidified as it reaches room temperature.

[0057] The solidified metal may then be strengthened and shaped by forging. Examples of suitable forging methods include forging in a hydraulic or mechanical press at temperatures ranging from 1800-2300°F (e.g., using a GFM SX-65 rotary forging machine, a 4500 ton forging press, or other similar hot working equipment). The ingot / billet may be heated to an appropriate temperature (1800-2300°F), followed by plastic deformation of the ingot / billet into the shape and size(s) required for the project.

[0058] Other methods for processing this material include hot rolling, using, for example, a 20 Hi reverse cold / hot roll mill, cold rolling, drawing, and extrusion. Before and after these metal working processes, the material is solution annealed to relieve stress and / or dissolve undesirable third phases. This annealing step is preferably performed at the duplexing temperature, i.e., the temperature at which the alloy is approximately 50% austenite and 50% ferrite. Depending on the size of the product, the heat treatment may take 1-4 hours and is followed by a water quench to room temperature.

[0059] The alloys described herein may be provided in powder form for additive manufacturing. Thus, spherical particles are a suitable shape. For powder-based laser fusion (selective laser sintering (SLS) and selective laser melting (SLM)), the preferred particle size range is 15-53 micrometers in diameter. For electron beam additive manufacturing, the preferred particle size range is 45-103 micrometers. The powder may be formed by atomization. Related techniques include melting a charge material into a metal bath in an air atmosphere, an inert gas, or a vacuum, or other melting environment, where the charge material includes the alloy composition described herein. A molten metal stream is generated from the molten metal bath. The molten metal stream is broken into multiple metal droplets by high-pressure gas: argon, nitrogen, helium, or other mixtures. The metal droplets cool and solidify as they fall downward into a collection chamber, forming a powder. Suitable atomization systems include atomizers manufactured by Arcast, Retech, or ALD. [Example]

[0060] Example 1 - High Molybdenum Dual Phase Alloy Heats were melted using compositions according to embodiments of the present invention as shown in Table 3.

[0061] [Table 3]

[0062] Tensile test results for alloys according to embodiments of the present invention and comparisons with alloys 2205 and 2507 are shown in Table 4 below, demonstrating the superior mechanical properties of the high molybdenum DSS exemplified by alloy AH. In particular, these alloys exhibit comparable or higher yield strength, ultimate tensile strength, elongation percentage, area reduction, and pitting resistance index when compared to alloys 2205 and 2507.

[0063] Alloy AH was heat treated at temperatures corresponding to each of the different duplexing temperatures. For example, alloy G was heat treated as follows: the charge was held in a furnace at 2200°F for 1 hour, followed by a rapid water quench. Furthermore, if the alternative heat treatment produces an alloy with a higher ferrite content than the austenite content, the mechanical properties are increased, i.e., improved. Conversely, if the alternative heat treatment produces an alloy with a higher austenite content than the ferrite content, the mechanical properties are decreased, i.e., deteriorated.

[0064] [Table 4]

[0065] Corrosion testing was performed using salt spray and was performed at 25°C per ASTM A923-C and at 22°C per ASTM G48A. Each of these ASTM standards is incorporated herein by reference in its entirety. Additional details regarding the effect of oxygen content on inclusion formation and the pitting corrosion resistance of hyperduplex stainless steels are found in Jeon, S.-H. et al., MATERIALS TRANSACTIONS, 55(12), 1872-1877, 2014, which is incorporated herein by reference.

[0066] Typical sigma / chi solvus temperatures, biphasing temperatures, and delta temperatures for various DDS alloys are as follows: 2205 - Sigma / Chi solvus temperature 1775°F, diphasing temperature 1950°F, delta temperature 175°F 2507 - Sigma / Chi solvus temperature 1925°F, diphasing temperature 2075°F, delta temperature 150°F 2707 - Sigma / Chi solvus temperature 1950°F, diphasing temperature 2000°F, delta temperature 50°F 3207 - Sigma / Chi solvus temperature 1950°F, diphasing temperature 1900°F, delta temperature 50°F High molybdenum duplex stainless steels - 1600-2100°F depending on the Ni, Cr and Mo content range.

[0067] For the eight alloys listed above, the sigma / chi solvus temperatures, duplexing temperatures, and delta temperatures are: Alloy A Sigma / Chi solvus temperature 1860°F, duplex temperature 2180°F, delta temperature 300°F Alloy B Sigma / Chi solvus temperature 1810°F, dual phase temperature 2240°F, delta temperature 430°F Alloy C Sigma / Chi solvus temperature 1945°F, diphasing temperature 2000°F, delta temperature 55°F Alloy D Sigma / Chi Solvus Temperature 2050°F, Duplex Temperature 2115°F, Delta Temperature 65°F Alloy E Sigma / Chi Solvus Temperature 2055°F, Duplex Temperature 2185°F, Delta Temperature 130°F Alloy F Sigma / Chi solvus temperature 2100°F, dual phase temperature 2300°F, delta temperature 200°F Alloy G Sigma / Chi solvus temperature 2080°F, diphasing temperature 2200°F, delta temperature 120°F Alloy H Sigma / Chi phase solvus temperature 2050°F, dual phase temperature 2250°F, delta temperature 200°F

[0068] The solvus temperatures of some materials according to embodiments of the present invention are comparable to those of other DSS alloys. In some embodiments, Cr and Ni may each affect the sigma phase solvus temperature. All eight alloys described above, including those with relatively high sigma / chi solvus temperatures, can be processed and achieve excellent properties because the delta temperature between the sigma / chi solvus temperature and the solution heat treatment temperature is large enough to produce high-quality material. The larger delta temperature allows for the production of products with larger diameter morphologies without precipitation of sigma / chi phase upon cooling / quenching. Without the presence of sigma / chi phase, degradation of mechanical and corrosion properties is reduced / eliminated as the product morphology increases.

[0069] While the present invention has been described in detail herein in connection with one or more preferred embodiments, it is understood that this disclosure is exemplary and explanatory of the invention only, and is made solely for the purpose of providing a complete and enabling disclosure of the invention. The foregoing disclosure is not intended to be construed as limiting the invention, or in any way excluding other embodiments, alterations, variations, modifications, or equivalents; the present invention is limited only by the claims and their equivalents.

Claims

1. 1. A duplex stainless steel alloy comprising: 10% to 20% by weight of chromium (Cr); 6 wt.% to 13 wt.% molybdenum (Mo); 0.5 wt.% to 6.5 wt.% nickel (Ni); 2.25 wt% to 12 wt% manganese (Mn); 0.05% to 5% by weight of copper (Cu); 0.05% to 0.4% by weight of nitrogen (N); 0.05% to 0.35% by weight of carbon (C); 0.01 wt.% to 3.5 wt.% cobalt (Co); less than 2 wt.% silicon (Si); less than 2 wt.% tungsten (W); and The balance is iron (Fe), wherein the stainless steel alloy comprises 40%-60% by weight ferrite and 60%-40% by weight austenite, and has a nickel equivalent and a chromium equivalent; Here, the nickel equivalent and the chromium equivalent are: (i) nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 7<Ni ｅｑ <20 and 21<Cr ｅｑ or (ii) has a nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 10<Ni ｅｑ <18 and 21<Cr ｅｑ <27.

2. 12% to 17% by weight of chromium (Cr); 7.25 wt% to 11 wt% molybdenum (Mo); 0.75 wt.% to 5 wt.% nickel (Ni); 2.5 wt% to 8 wt% manganese (Mn); 1.75 wt% to 3.5 wt% copper (Cu); 0.1% to 0.3% by weight of nitrogen (N); 0.075% to 0.2% by weight of carbon (C); 0.01% to 3% by weight of cobalt (Co); less than 1.5 wt.% silicon (Si); less than 1.5 wt.% tungsten (W); and 10. The composition of claim 1, comprising the balance iron (Fe).

3. 10. The duplex stainless steel alloy of claim 1, wherein the stainless steel alloy comprises cast steel.

4. The duplex stainless steel alloy of claim 1 , wherein the stainless steel alloy comprises wrought steel.

5. 10. The duplex stainless steel alloy of claim 1, wherein the stainless steel alloy has a yield strength of at least 70 ksi.

6. 10. The duplex stainless steel alloy of claim 1, wherein the stainless steel alloy has an ultimate tensile strength cast of at least 115 ksi.

7. 10. The duplex stainless steel alloy of claim 1, wherein the stainless steel alloy has an elongation >30%.

8. 10. The duplex stainless steel alloy of claim 1, wherein the stainless steel alloy has an area reduction of >50%.

9. 10. The duplex stainless steel alloy of claim 1, wherein the stainless steel alloy has a pitting corrosion resistance index of at least 30.

10. 1. A duplex stainless steel alloy powder for additive manufacturing comprising: 10% to 20% by weight of chromium (Cr); 6 wt.% to 13 wt.% molybdenum (Mo); 0.5 wt.% to 6.5 wt.% nickel (Ni); 2.25 wt% to 12 wt% manganese (Mn); 0.05% to 5% by weight of copper (Cu); 0.05% to 0.4% by weight of nitrogen (N); 0.05% to 0.35% by weight of carbon (C); 0.01 wt.% to 3.5 wt.% cobalt (Co); less than 2 wt.% silicon (Si); less than 2 wt.% tungsten (W); and The balance is iron (Fe), wherein the stainless steel alloy comprises 40 wt%-60 wt% ferrite and 60 wt%-40 wt% austenite, and has a nickel equivalent and a chromium equivalent; Here, the nickel equivalent and the chromium equivalent are: (i) nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 7<Ni ｅｑ <20 and 21<Cr ｅｑ or (ii) has a nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 10<Ni ｅｑ <18 and 21<Cr ｅｑ having a value < 27; and wherein the powder comprises a plurality of spherical particles having an average particle size selected from the range of 15-53 micrometers or 45-103 micrometers.

11. The composition comprises: 12% to 17% by weight of chromium (Cr); 7.25 wt% to 11 wt% molybdenum (Mo); 0.75 wt.% to 5 wt.% nickel (Ni); 2.5 wt% to 8 wt% manganese (Mn); 1.75 wt% to 3.5 wt% copper (Cu); 0.1% to 0.3% by weight of nitrogen (N); 0.075% to 0.2% by weight of carbon (C); 0.01% to 3% by weight of cobalt (Co); less than 1.5 wt.% silicon (Si); less than 1.5 wt.% tungsten (W); and 11. The powder of claim 10, comprising the balance iron (Fe).

12. 1. A method of forming a duplex stainless steel alloy, comprising: Melting a mixture of elements 10% to 20% by weight of chromium (Cr); 6 wt.% to 13 wt.% molybdenum (Mo); 0.5 wt.% to 6.5 wt.% nickel (Ni); 2.25 wt% to 12 wt% manganese (Mn); 0.05% to 5% by weight of copper (Cu); 0.05% to 0.4% by weight of nitrogen (N); 0.05% to 0.35% by weight of carbon (C); 0.01 wt.% to 3.5 wt.% cobalt (Co); less than 2 wt.% silicon (Si); less than 2 wt.% tungsten (W); and forming a molten metal alloy comprising a balance of iron (Fe); wherein the stainless steel alloy comprises 40 wt%-60 wt% ferrite and 60 wt%-40 wt% austenite, and has a nickel equivalent and a chromium equivalent; Here, the nickel equivalent and the chromium equivalent are: (i) nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 7<Ni ｅｑ <20 and 21<Cr ｅｑ or (ii) has a nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 10<Ni ｅｑ <18 and 21<Cr ｅｑ having a value < 27; and quenching the molten metal alloy to solidify the metal alloy.

13. The method of claim 12 further comprising forging the solidified metal alloy.

14. 14. The method of claim 13, further comprising heat treating the forged metal alloy.

15. 1. A method of forming a duplex stainless steel alloy powder, the powder comprising: 10% to 20% by weight of chromium (Cr); 6 wt.% to 13 wt.% molybdenum (Mo); 0.5 wt.% to 6.5 wt.% nickel (Ni); 2.25 wt% to 12 wt% manganese (Mn); 0.05% to 5% by weight of copper (Cu); 0.05% to 0.4% by weight of nitrogen (N); 0.05% to 0.35% by weight of carbon (C); 0.01 wt.% to 3.5 wt.% cobalt (Co); less than 2 wt.% silicon (Si); less than 2 wt.% tungsten (W); and the balance being iron (Fe), wherein the stainless steel alloy comprises 40%-60% by weight ferrite and 60%-40% by weight austenite, and has a nickel equivalent and a chromium equivalent; Here, the nickel equivalent and the chromium equivalent are: (i) nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 7<Ni ｅｑ <20 and 21<Cr ｅｑ or (ii) has a nickel equivalent (Ni ｅｑ ) = wt% Ni + (30 × wt% C) + (0.5 × wt% Mn) + (30 × wt% N), and chromium equivalent (Cr ｅｑ ) = Cr wt% + Mo wt% + (1.5 × Si wt%) + (0.5 × Nb wt%), Ni ｅｑ and Cr ｅｑ are 10<Ni ｅｑ <18 and 21<Cr ｅｑ having a value < 27; and wherein the powder comprises a plurality of spherical particles having an average particle size selected from the range of 15-53 micrometers or 45-103 micrometers; melting a charge containing the composition to form a molten metal bath; generating a molten metal stream from a molten metal bath; atomizing the molten metal stream to form a plurality of metal droplets; and The method includes cooling the metal droplets, whereby the metal droplets solidify to form a powder.

16. 16. The method of claim 15, wherein the charge material is melted in an atmosphere comprising air, an inert gas, or a vacuum.

17. 16. The method of claim 15, wherein the molten metal stream is atomized in a high pressure gas comprising at least one of argon, nitrogen, or helium.

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