THERMALLY STABLE Ni-Cr-Mo ALLOY FOR ELEVATED TEMPERATURE SERVICE

Abstract

A nickel chromium alloy containing 16.48 to 20.57 wt.% chromium, 10.46 to 13.10 wt.% molybdenum, 0.03 to 1.67 wt.% iron, 0.24 to 0.98 wt.% manganese, 0.02 to 0.43 wt.% silicon, 0.08 to 0.38 wt.% aluminum, 0.002 to 0.047 wt.% carbon, up to 0.006 wt.% boron and the balance nickel has sufficiently high thermal stability for service in the 750 to 1500⁰ F temperature range, and a sufficiently high corrosion resistance to tolerate hydrochloric acid-based and sulfuric-acid-based condensates for short periods of time after the alloy has been subjected to a homogenization treatment at a temperature of 2050⁰ F and hot worked at a start temperature of 2050⁰ F.

Description

[0001] THERMALLY STABLE Ni-Cr-Mo ALLOY FOR ELEVATED TEMPERATURE SERVICE

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This patent application claims priority to U.S. Patent Application Serial No. 19 / 012,016, which was filed on January 7, 2025.

[0004] FIELD OF INVENTION

[0005] The invention is related to corrosion resistant nickel-chromium-molybdenum alloys suitable for use at temperatures between 750° F to 1500° F.

[0006] BACKGROUND

[0007] The nickel-chromium-molybdenum (Ni-Cr-Mo) alloys have been used by the chemical industry since the 1930’s to resist the corrosive effects of reducing, inorganic acids, notably hydrochloric acid, sulfuric acid, hydrobromic acid, and hydrofluoric acid (HC1, H2SO4, HBr, and HF). These alloys also possess resistance to oxidizing, inorganic acids, such as nitric (HNO3), for which passivation, i.e. the formation of chromium-rich films, is the protective mechanism.

[0008] Other important attributes of the Ni-Cr-Mo alloys are their resistance to chloride-induced, localized attack (pitting and crevice corrosion) in the event of passive film breakdown, their resistance to chloride-induced, stress corrosion cracking, and their tolerance of so-called oxidizing impurities in HC1, H2SO4, HBr, and HF, such as dissolved oxygen, ferric ions, and cupric ions.

[0009] The first Ni-Cr-Mo alloys were discovered by Franks (U.S. Patent 1,836,317). These also contained smaller, deliberate additions of iron (to enable the use of ferro-compounds during furnace charging), tungsten, and vanadium. They also contained small amounts of carbon and silicon, as impurities. The first commercial material was designated HASTELLOY C alloy andcontained approximately 16 wt.% chromium and 16 wt.% molybdenum. Initially, this alloy was used in the form of castings; wrought products followed in the 1940’s. The following information refers to wrought Ni-Cr-Mo alloys.

[0010] With the advent of argon -oxygen decarburization (AOD) in the 1960’s, a new, low-carbon / low-silicon version became available as HASTELLOY C-276 alloy. This wrought alloy also contained approximately 16 wt.% of both chromium and molybdenum.

[0011] Pure nickel has a face-centered cubic (FCC or gamma) atomic structure, and this can be highly alloyed with elements such as chromium and molybdenum without causing other phases to occur in the microstructure. This has been regarded as highly beneficial from the standpoint of corrosion resistance and mechanical properties (especially tensile ductility). However, the alloying additions in HASTELLOY C and HASTELLOY C-276 alloys exceed their solubility limits at temperatures below about 2100° F. Furthermore, carbon impurities in Ni-Cr-Mo alloys can result in the formation of carbides in microstructural grain boundaries.

[0012] To overcome these issues, Ni-Cr-Mo alloys are typically solution annealed (at temperatures above those required to dissolve secondary intermetallic phases, such as mu, and to dissolve any secondary carbides), and water quenched, to “freeze-in” the high temperature, single phase (FCC or gamma) atomic structure. In other words, materials such as HASTELLOY C-276 alloy are meta-stable FCC at room temperature.

[0013] To avoid the precipitation of secondary (i.e. diffusion-induced) intermetallic phases and carbides, the Ni-Cr-Mo alloys are normally restricted to use up to approximately 1000° F, beyond which significant diffusion of elements, and subsequent secondary phase formation, are possible. However, thermal cycles associated with welding (in so-called heat-affected zones orHAZ) can cause secondary phase precipitation at appropriate nucleation and growth sites, such as grain boundaries.

[0014] Given these facts, the third wrought Ni-Cr-Mo material, HASTELLOY C-4 alloy, discovered by Hodge et al. and covered by U.S. Patent 4,080,201, exhibited less tendency for secondary intermetallic precipitation and secondary carbide precipitation. It too contained 16 wt.% chromium and 16 wt.% molybdenum, but did not include deliberate additions of iron and tungsten. HASTELLOY C-4 alloy also contained a small addition of titanium to react with carbon (forming primary carbides) at the melting stage of production, thus leaving less carbon in solution to form secondary carbides in the grain boundaries of weld heat-affected zones.

[0015] The term used to describe resistance to secondary intermetallic and secondary carbide precipitation is “thermal stability.” These precipitates occur over a range of temperatures, but 1400° F is a temperature at which many of their negative effects upon mechanical properties peak. It is therefore common to measure the thermal stability of the Ni-Cr-Mo alloys by exposing them to temperatures such as 1400° F or thereabouts for a significant period (such as 1,000 hours) and measuring their percentage elongation values in subsequent room temperature tensile tests. Elongation values in the approximate range 40% to 60% are typical for such alloys in the annealed condition, so any significant reduction can be attributed to secondary precipitation, thus lack of thermal stability.

[0016] It should be mentioned that some of the Ni-Cr-Mo alloys are also prone to atomic ordering reactions at lower temperatures, which greatly enhance their strengths, but reduce their ductilities.More recent Ni-Cr-Mo materials include HASTELLOY C-22 alloy (from the 1980’s), HASTELLOY C-2000 alloy (from the 1990’s), and HASTELLOY HYBRID-BCl alloy (from the 2000’ s). The first two of these were discovered in response to the need for higher resistance to process streams and flue gas condensates containing high levels of oxidizing contaminants. Thus HASTELLOY C-22 alloy contains 22 wt.% chromium, and HASTELLOY C-2000 alloy contains 23 wt.% chromium (chromium being beneficial to the formation of protective, passive films in oxidizing environments).

[0017] HASTELLOY HYBRID-BCl was discovered in response to a need for greater resistance to non-oxidizing acid solutions, such as hydrochloric acid, but with some resistance to oxidizing contaminants, in case of upset conditions (hence the inclusion of 15 wt.% chromium, in addition to 22 wt.% molybdenum).

[0018] HASTELLOY C-22 alloy was discovered by Asphahani and is covered by U.S. Patent 4,533,414. HASTELLOY C-2000 alloy was discovered by Crook and is covered in U.S. Patent 6,280,540. HASTELLOY HYBRID-BCl was discovered by Crook and is covered in U.S. Patent 7,785,532. Two other Ni-Cr-Mo materials were discovered circa 1990 by Heubner et al. (U.S. Patent 4.906,437) and by Crum et al. (U.S. Patent 5,019,184).

[0019] The patent of most relevance to the current discovery is U.S. Patent 9,970,091 (Crook et al.) that describes a method by which the thermal stability of highly alloyed Ni-Cr-Mo materials can be enhanced. Essentially, it involves maintaining the other intermetallic phases (such as mu phase or P phase) that occur during solidification of such materials in a primary form throughout subsequent hot working and annealing steps, by sub-solvus processing, or more precisely “sub-intermetallic-solvus” and “super-carbide-solvus” processing.While not regarded commercially as members of the corrosion-resistant, Ni-Cr-Mo (C-Type) family of materials, intended for use up to approximately 1000° F, it is important to mention two other nickel-based alloys. One is INCONEL 625 alloy, which contains 21.5 wt.% chromium, 9 wt.% molybdenum, and a significant (3.6 wt.%) addition of niobium (possibly with a partial tantalum content). INCONEL 625 alloy is used within the temperature range of interest (i.e. 1000° F to 1500° F) but undergoes a strengthening reaction within this temperature range due to the formation of fine precipitates of a secondary phase known as gamma double prime, which has the composition NisNb. As a result, it exhibits a significant reduction in ductility and cannot be considered thermally stable.

[0020] The other is HASTELLOY S alloy, which is compositionally similar to HASTELLOY C-4 alloy (in having a chromium content of 16 wt.% and a molybdenum content of 15 wt.%), but contains small deliberate additions of silicon, carbon, boron, and lanthanum, to enable usage in severe cyclic heating conditions up to 2000° F.

[0021] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The objective of the work leading to the present discovery was a Ni-Cr-Mo material of sufficiently high thermal stability for service in the 750° F to 1500° F temperature range, and of sufficiently high corrosion resistance to resist exposure to hydrochloric acid-based and sulfuric acid-based condensates for short periods of time during equipment shutdown. Such condensates occur if chlorine and / or sulfur are present in the gases encountered within the systems operating at these high temperatures.

[0023] Thermal stability is a term which refers to the ability of an alloy to maintain its as-produced, room temperature microstructure during thermal cycles or periods at temperatures(typically above 1000° F) where the diffusion of alloying elements is significant, and can lead to diffusion-induced changes in microstructure. Such changes occur if the alloy is unstable, due to one or more elements being in excess of their solubility limits. In the Ni-Cr-Mo alloys, it is common for secondary (i.e. diffusion-induced) precipitates to form in the grain boundaries (carbides, for example) and within the grains (mu phase, P phase, and sigma phase intermetallics, for example), as discussed in the Background section. The outcome of secondary precipitation is reduced ductility and toughness.

[0024] The approach taken during this work was first to identify those experimental alloy compositions which exhibit good ductility after exposure to 1400° F for 1,000 hours, as defined by elongation values and reduction of area (ROA) values in excess of 40% in subsequent room temperature tensile tests.

[0025] In the belief that some carbon is necessary for adequate creep resistance within the proposed operating temperature range, the most promising carbon-bearing alloy (Alloy K) to meet this criterion was then tested for its resistance to simulated HCl-based and H2SO4-based condensates, relative to INCONEL 625 and HASTELLOY C-22 alloys, both of which materials are regarded as possessing adequate resistance to such condensates for short periods of time (during equipment cool-down, for maintenance).

[0026] All of the experimental alloys were vacuum induction melted (VIM), then electro-slag remelted (ESR). The resulting ingots were homogenized at 2050° F and hot worked to 0.5 inch thick plate and 0.125 inch sheet using a start temperature of 2050° F.

[0027] The purpose of this method is to retain other intermetallic phases (believed to be mu phase and / or P phase) that form within the predominant gamma phase solution duringsolidification of the ingot, throughout the wrought manufacturing process. This should then reduce the driving force for secondary (i.e. diffusion-induced) intermetallic formation, particularly in the grain boundaries, during service at temperatures above about 1000°F (where diffusion becomes significant), or during the thermal cycles involved with welding.

[0028] TABLE 1 : Aim and Actual Compositions

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[0037] *Alloy of this invention

[0038] LAP=Low as possible

[0039] Table 1 gives the aim and actual compositions of the experimental materials used in the studies leading up to the present discovery. The chromium (Cr) and molybdenum (Mo) contents of Alloy A were based on those of HASTELLOY C-4 alloy. The Cr and Mo contents of Alloy Dwere significantly higher (20 wt.% for both elements). Alloys B and C represent intermediates using the same chromium to molybdenum weight percentage ratio (1:1).

[0040] Alloys A-D also included minor additions of manganese (to control the sulfur content of the molten material), aluminum (to control the oxygen content of the molten material), silicon (to enhance oxidation resistance at high operating temperatures), and carbon (to provide creep strength at high operating temperatures). Alloy C also included a minor addition of titanium to enable the formation of a stable dispersion of primary MC carbides in the microstructure, which it was thought might help with creep strength and grain size control.

[0041] The composition of Alloy E was similar to that of HASTELLOY S alloy. Consequently, there was no deliberate addition of carbon, but instead Alloy E included a minor addition of boron (to provide creep strength). A minor addition of the rare earth, lanthanum, was also included (to enhance oxidation resistance).

[0042] From Alloy F, a higher chromium to molybdenum ratio was adopted, assuming that chromium is generally beneficial in gaseous environments at high temperatures, and that molybdenum might be detrimental to thermal stability in the presence of even a small quantity of silicon. Thus Alloys F-M included aim chromium contents in the range 17 to 21 wt.% and aim molybdenum contents in the range 10.75 to 13.5 wt.%.

[0043] Equally important were experiments with no deliberate carbon addition (in Alloys F, G, and I), and an intermediate carbon level of 0.03 wt.% (in Alloy K). The addition of boron (to enhance creep strength) was retained in Alloys F-M.

[0044] Alloys L and M were based upon the premise that Alloy K might fulfdl the objectives of the study, in that it exhibited good thermal stability (as indicated by the elongation and ROAvalues in Table 2) and good resistance to simulated condensates containing hydrochloric acid and others containing sulfuric acid, relative to HASTELLOY C-22 alloy and INCONEL 625 alloy (as indicated by the corrosion rate values in Table 3). It also exhibited good oxidation resistance at 16000F and good creep resistance at 14000F, equivalent to those of INCONEL 625 alloy. Thus, commercially acceptable ranges (for key elements) centered upon Alloy K were projected and “all-el ements-low” and “all-elements-high” compositions were melted as Alloys L and M, respectively, and subjected to room temperature tensile tests after aging for 1,000 hours at 1400° F.

[0045] An annealing temperature of 2125° F was selected for Alloys A-D; this temperature provided optimum microstructures, i.e. grain boundaries free from secondary precipitation, no remnants of prior grain boundaries from upstream processing, and grain sizes in the range ASTM 3-5. However, this annealing temperature is slightly above the primary intermetallic solvus (2100° F) projected for these alloys by the work leading up to U.S. Patent 9,970,091.

[0046] Annealing trials of subsequent experimental materials (Alloys E-K) indicated that optimum microstructures could be created at lower temperatures, ensuring less or no dissolution of the primary intermetallic precipitates. Alloys E-J were annealed at 2025° F, Alloy K was annealed at 2050° F, and Alloys L and M were annealed at 2100° F. All annealing trials and processes involved a water quench.

[0047] The discoveries made during these studies were:

[0048] 1. That the use of a chromium to molybdenum weight percentage ratio of 1 : 1 in Ni-Cr-Mo alloys containing small quantities of carbon and silicon, and chromiumcontents within the approximate range 16 to 20 wt.%, results in poor thermal stability, even using the processing method described in U.S. Patent 9,970,091.

[0049] 2. Good thermal stability, as defined by room temperature elongation and reduction of area (ROA) values in excess of 40% after 1,000 hours at 1400° F, is possible at molybdenum contents in the range 10.46 wt.% (Alloy L) to 13.10 wt.% (Alloy M), together with chromium contents in the range 16.48 wt.% (Alloy F) to 20.57 wt.% (Alloy M), using the processing method described in U.S. Patent 9,970,091.

[0050] 3. Good thermal stability is possible even with significant additions of silicon and carbon, which are well-known destabilizers of Ni-Cr-Mo alloys, using the aforementioned chromium and molybdenum ranges and the processing method described in U.S. Patent 9,970,091; This was discovered by testing Alloy H, containing 0.43 wt.% silicon and 0.041 wt.% carbon.

[0051] 4. Surprisingly, even a 0.39 wt.% addition of titanium is detrimental to these alloys, reducing the elongation and reduction of area values to below 40% (Alloy J).

[0052] 5. That the most promising alloy from a commercial standpoint is Alloy K. Not only does this alloy meet the thermal stability criterion, but also it contains deliberate additions of both carbon and boron (for creep strength) and has been shown to equal INCONEL 625 alloy in creep-rupture test at 1400° F, and in an oxidation test at 1600° F.TABLE 2: Room Temperature Tensile Data after 1000 h Age at 1400° F

[0053]

[0054] *Alloy of this invention

[0055] The laboratory-simulated condensates used to test Alloy K, HASTELLOY C-22 alloy, and INCONEL 625 alloy were as follows:

[0056] 1. The ASTM G28B testing solution at 250° F, purged with nitrogen.

[0057] 2. The ASTM G28B testing solution at 250° F, purged with oxygen.

[0058] 3. 20% HC1 at 250° F, purged with nitrogen.

[0059] 4. 20% HC1 at 250° F, purged with oxygen.

[0060] 5. 70% H2SO4 at 250° F, purged with nitrogen.

[0061] 6. 70% H2SO4 at 250° F, purged with oxygen.

[0062] All of the corrosion tests were carried out in autoclaves. The test durations were 4 hours without interruption in the case of 20% HC1 (since it is known to be an aggressive solution, and could damage the autoclave), and 24 hours without interruption in the case of the other test solutions.

[0063] The ASTM G28B was designed to simulate condensates encountered in flue gas desulfurization (FGD) systems in power stations using fossil fuels and comprises 11.5% H2SO4 + 1.2% HC1 + 1% FeCh + 1% CuCl2.The reason for testing in solutions purged in nitrogen is that it avoids the presence of dissolved oxygen, a well-known oxidizing agent, and one that can change the cathodic reaction of a corrosion process from one of hydrogen evolution to one which encourages the formation of passive films. The reason for testing in solutions purged with oxygen is that it provides a measure of control over the influence of oxygen, i.e. it will match the solubility limit at the chosen test temperature.

[0064] TABLE 3: Corrosion Rates in Simulated Condensates, mpy

[0065]

[0066] The unit mpy stands for mils per year, i.e. thousandths of an inch per year. Study of these corrosion test results indicates that Alloy K is more resistant to all of the simulated condensate solutions, except one (Solution 2), than INCONEL 625 alloy, and even exceeds the performance of HASTELLOY C-22 alloy in Solutions 3 and 4. It is therefore concluded that the alloys of this invention possess sufficient condensate corrosion resistance to be considered for elevated temperature service in the presence of gases containing chlorine and / or sulfur, and where hydrochloric acid and / or sulfuric acid might condense during the cooling-down of systems to room temperature.

[0067] It should be noted that, in service, the alloys would be exposed to these extremely aggressive solutions for a very short periods of time (during equipment cool-down), so the high rates recorded during these longer-term corrosion tests are not of concern, but did enable comparisons.Seven of the eleven experimental alloys whose compositions are set forth in Table 1 have the desired properties. These experimental alloys were all nickel-based and had a chromium content which ranged from 16.48 to 20.57 wt.%. The molybdenum content ranged from 10.46 to 13.10 wt.% and iron was present from 0.03 up to 1.67 wt.%. The manganese content was from 0.24 to 0.98 wt.%. The aluminum content ranged from 0.08 to 0.38 wt.%. The silicon content ranged from 0.02 up to 0.43 wt.%. Carbon was present in an amount between 0.002 and 0.047 wt.% and boron was present up to 0.006 wt.%.

[0068] Although we have described certain present preferred embodiments of our alloy it should be understood that the invention is not limited thereto, but may be variously embodied within the following claims.

Claims

We claim:

1. A wrought, nickel-chromium-molybdenum alloy having sufficiently high thermal stability for service in the 750 to 1500° F temperature range, and a sufficiently high corrosion resistance to tolerate hydrochloric acid-based and sulfuric-acid-based condensates for short periods of time, comprising:16.48 to 20.57 wt.% chromium,10.46 to 13.10 wt.% molybdenum,0.03 to 1.67 wt.% iron,0.24 to 0.98 wt.% manganese,0.02 to 0.43 wt.% silicon,0.08 to 0.38 wt.% aluminum,0.002 to 0.047 wt.% carbon,up to 0.006 wt.% boron, andthe balance nickel,wherein the alloy has been subjected to a homogenization treatment at a temperature between 2025° F and 2100° F and hot worked at a start temperature between 2025° F and 2100° F.

2. The alloy of claim 1 wherein the alloy has elongation values and a reduction of surface area greater than 40% after being aged for 1,000 hours at 1400° F.

3. The alloy of claim 1 wherein the alloy is comprised of 19.5 wt.% chromium, 11.71 wt.% molybdenum, 0.83 wt.% iron, 0.59 wt.% manganese, 0.03 wt.% silicon, 0.28 wt.% aluminum, 0.032 wt.% carbon, 0.004 wt.% boron and the balance nickel.

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