WROUGHT NI-MO ALLOYS WITH EXCELLENT CORROSION RESISTANCE IN REDUCING ACIDS AND IMPROVED FABRICABILITY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This patent application claims priority to U.S. Provisional Patent Application No. 63/742,971, which was filed on Jan. 8, 2025.

FIELD

[0002]This invention relates to nickel-base alloy compositions centered around 28 wt. % Mo and critical amounts of the additional alloying elements Fe and Cr that resulted in enhanced thermal stability of the metallurgical structure as required for the fabrication of large structures and their repair in the chemical processing and related industries while still providing adequate corrosion resistance in key industrial chemicals.

BACKGROUND

[0003]It has long been recognized and well established that the B-type alloys based on Ni-28Mo (compositions will be stated in weight percent unless otherwise noted) provide outstanding corrosion resistance to the key industrial chemicals hydrochloric acid and sulfuric acid. In fact, the first patent on alloy “B” dates to 1921 (Clement, U.S. Pat. No. 1,375,083), the patent rights of which were acquired by the predecessor to the assignee prior to the introduction of Hastelloy B alloy. However, the need for improved fabricability of large structures in the chemical processing and related industries, especially regarding formability and weldability, led to several successor alloys.

[0004]One notable invention by Flint (U.S. Pat. No. 2,959,480 issued Nov. 8, 1960) describes an alloy with preferentially 26-30% molybdenum and 1.2-2.3% vanadium, the latter element being critical and presumably acting as a scavenger of carbon to prevent formation of secondary carbides in the heat-affected zones (HAZ) of weldments.

[0005]Another alloy, alloy B-2, commercialized by the assignee under the tradename HASTELLOY® B-2 alloy, is characterized by very low levels of carbon and silicon, designed to minimize sensitization of the HAZ of weldments by virtue of suppression of precipitation of grain boundary carbides in these zones. This advance was enabled by the advent of the argon-oxygen-decarburization (AOD) melting technology. The development of alloy B-2 and its commercial versions was reviewed by Hodge and Kirchner (NACE 1975, Paper No. 60). While the issue of HAZ sensitization was addressed, it was later discovered that alloy B-2 was prone to the formation of embrittling intermetallic phases (particularly Ni₄Mo) when exposed during fabrication to temperatures in the 1200-1500° F. (649-816° C.) range, even for relatively short periods of time.

[0006]This deficiency led to the development of alloy B-3, known commercially as HASTELLOY® B-3® alloy (Klarstrom, U.S. Pat. No. 6,610,119 B2 issued Aug. 26, 2003) produced by Haynes International, Inc. This alloy is characterized by significantly slower precipitation kinetics in the aforementioned temperature range by virtue of crucial minor additions of several substitutional alloying elements from the groups VI, VII, or VIII of the periodic table. All the while, the excellent corrosion resistance of this type of alloy in key industrial chemicals was not compromised.

[0007]Another effort by VDM Germany resulted in the development of alloy B-10 with a nominal composition of Ni-24Mo-8Cr-6Fe (NACE 1998, Paper No. 481). This alloy is reported to exhibit excellent corrosion resistance at intermediate concentrations of sulfuric acid and hydrochloric acid, even in the presence of small amounts of oxidizing agents. While good thermal stability was achieved also on weldments, the alloy's significantly reduced Mo content would not match the corrosion performance of the Ni-28Mo base alloys.

[0008]More recently, a Ni-30Mo-4Fe-3Cr alloy with crucial amounts of 0.015 B and 0.01 Y was disclosed by Tawancy (U.S. Pat. No. 7,922,969 B2, filed on Jun. 28, 2007, now lapsed). Tawancy teaches the criticality of both B and Y for achieving acceptable levels of thermal stability. However, when attempting to reproduce the claimed properties of Tawancy's typical alloy at the lab scale, massive processing issues were encountered that prevented making any useful material. To a practitioner of the art, this is not too surprising because both B and Y are known to be grain boundary active and their presence at the elevated levels Tawancy requires is likely to result in embrittlement. Moreover, fabricability (in particular, weldability) of such wrought alloy would also be compromised by the elevated levels.

SUMMARY

[0009]While significant improvements in thermal stability have been made since the invention of alloy B, further improvements thereof are highly desirable while not compromising the B-type alloys' excellent corrosion resistance in key industrial chemicals. Those improvements will translate into greater ease in the manufacture (forming, welding) of large chemical reactor vessels and could conceivably even allow for explosion cladding of the alloy to carbon steel, a feat that has hitherto not been achieved with commercial B-type alloys. The further improvement in fabricability is the aim of the present invention.

[0010]Surprisingly it was discovered that further alloying with certain substitutional alloying elements (most notably Cr and Fe) well beyond the ranges claimed for aforementioned commercial alloys (B, B-2, B-3) did not diminish the corrosion resistance in key industrial chemicals to any significant degree, as often presumed. For example, Klarstrom in U.S. Pat. No. 6,610,119 B2 limited the addition of Cr and Fe to no more than 5 at. % each for that reason (note that these atom percentages for Cr and Fe convert to 4.0 wt. % and 4.2 wt. %, respectively). Yet, thermal stability of the alloys of this invention (as measured by retained room temperature tensile elongation after thermal exposure) after certain very aggressive processing steps was markedly improved.

[0011]Metallurgically, this improvement is believed to have been brought about by moving the alloy's composition to a different phase field at the relevant intermediate temperatures. For example, the compositions of alloys B-2 and B-3 can at 1300° F. (704° C.) be expected to lie in the austenitic gamma matrix+Ni₄Mo and gamma+Ni₄Mo+Ni₃Mo phase fields (ASM International Phase Diagram Center, 2019), respectively, both encompassing the undesirable brittle intermetallic phase Ni₄Mo. In contrast, the composition of the inventive alloys is projected to lie in the gamma+NiMo phase field. While NiMo is also an intermetallic phase with limited ductility, its equilibrium volume fraction would be much reduced and its formation would necessitate the diffusion of greater amounts of Mo, rendering its formation kinetics more sluggish than that of Ni₄Mo and Ni₃Mo. Equally importantly, the solubility of Mo in the gamma matrix, key to the alloy's corrosion resistance, would not be diminished significantly when shifting to this new phase field. This is a unique and previously untested approach.

[0012]Other details, objects, and advantages of the wrought, nickel-molybdenum alloy as well as methods of making and using such an alloy will become apparent as the following description of certain exemplary embodiments thereof proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a table containing the chemical compositions of the studied alloys in weight percent.

[0014]FIG. 2 is a graph showing a pseudo-TTT diagram designed by Klarstrom for alloys B-2 and B-3 (U.S. Pat. No. 6,610,119 B2) based on hardness readings. The chosen thermal stability test condition—1450° F. (788° C.)/48 h—depicted by the bull's eye symbol, is located well within the precipitation range of the intermetallic Ni₄Mo/Ni₃Mo phases in these alloys. To further enhance precipitation kinetics, annealed samples were 40% cold-rolled prior to thermal exposure. Note that Klarstrom's TTT curves do not account for the impact of prior cold work.

[0015]FIG. 3 is a graph depicting the compositional space in the quaternary Ni-28Mo-Cr—Fe system covered by alloys B, B-2, B-3, other studied experimental alloys, and the inventive alloys delineated by the dashed parallelogram. Note the shading of the bullets and how this relates to thermal stability. Open bullets indicate that this property could not be measured. Instances of poor corrosion resistance are annotated.

DETAILED DESCRIPTION

[0016]This disclosure concerns alloys in the Ni-28Mo-Cr—Fe system. Another alloying element (Cu) was also explored initially. However, massive hot workability problems were encountered as already noted by Klarstrom (U.S. Pat. No. 6,610,119 B2) in this type of alloys bearing Cu. While the Ni—Mo—Cr—Fe system is a primary aspect, additional minor additions in Al, Mn, and W were made to replicate the typical composition of HASTELLOY® B-3® alloy that served as a benchmark. Note that these minor additions are considered processing aids for deoxidation (Al) and desulfurization (Mn), and a residual (W). Commercially produced mill-annealed 0.125″ (3.2 mm) thick sheet of HASTELLOY® B-3® alloy was used for all comparative testing.

[0017]Eighteen experimental alloys, numbered 1-18, were melted and processed at the laboratory scale. This encompassed primary melting in vacuum, electro-slag re-melting to a 4″ (10 cm) diameter ingot, forging to a slab, and final hot rolling to 0.125″ (3.2 mm) thick sheet. The actual chemical compositions of these alloys are listed in FIG. 1. Carbon and Si aims were low as possible, typically achieving less than 0.002% and less than 0.02%, respectively.

[0018]Annealing temperatures of the hot-rolled sheets were adjusted to the chemical compositions of the alloys to result in clean single-phase microstructures (considered crucial for adequate corrosion resistance) of preferably ASTM no. 3 grain size. The respective annealing temperatures and grain sizes are compiled in Table I.

Note that HASTELLOY® B-3® alloy is usually annealed at relatively low temperatures in the mill, resulting in a comparatively fine grain. Note also that the same (mill) annealing temperature resulted in all experimental alloys in two-phase microstructures. In fact, in several cases (marked by an asterisk), even annealing at 2200° F. (1204° C.) did leave some precipitates.

[0019]A major goal of this invention was to further improve the thermal stability compared to that of HASTELLOY® B-3® alloy, the alloy known to be the most stable one in the B-type family. This required identifying conditions under which HASTELLOY® B-3® alloy would not be stable and form detrimental embrittling phases. The pseudo TTT diagram of alloys B-2 and B-3 shown in FIG. 2 provided some guidance: aging at 1450° F. (788° C.) (the nose of the upper C-curve) for 48 hours would place the alloys well into the transformation range of HASTELLOY® B-3® alloy. In addition, to mimic constrained heavy section welds, the annealed samples were 40% cold rolled prior to the aging treatment. This amount of cold work would further enhance the kinetics of any phase transformations and, thus, represent a highly aggressive worst-case scenario. Note that a temperature of 1450° F. (788° C.) is well below the stress-relieving temperature of these types of alloys. The outcome of this thermal stability test in terms of retained room temperature ductility (tensile elongation) is summarized in Table II.

[0020]The benchmark alloy, HASTELLOY® B-3® alloy, is virtually brittle (elongation less than 1 percent) in this particular cold-worked+aged condition, as projected. A significant differentiation between the various experimental alloys is seen, ranging from virtually brittle to quite ductile with double-digit elongations. Note that in the as-annealed condition, ductility of all tested alloys was in the 50-70% range.

[0021]Any improvements in thermal stability were not to come at the expense of corrosion resistance in key industrial chemicals. The two key chemicals B-type alloys are commonly exposed to are hydrochloric acid and sulfuric acid. Accordingly, several aggressive immersion corrosion tests were conducted, the results of which are compiled in Tables III through XV. Note that not all experimental alloys could be tested in all immersion tests. Yet, to ensure consistency, control coupons of the same commercial heat of B-3 alloy were included in all tests. To put the variances in corrosion rate between B-3 alloy and the various experimental alloys in perspective, corrosion rates are provided for several commercial Ni—Cr—Mo C-type alloys. The latter alloys are well established in the chemical processing industry, albeit not being considered optimal for use in the chemicals of interest for embodiments of our alloy.

[0022]Within the typical scatter of the results of such corrosion immersion tests (compare the variances for B-3 alloy between repeat runs), all experimental alloys except alloys 8 and 17 (highlighted in the tables) exhibited a corrosion resistance comparable to that of HASTELLOY® B-3® alloy. Alloy 8 features a combination of high Cr and high Fe. In alloy 17, a significant portion of Mo was (on an equi-atomic basis) replaced by W. These compositional changes significantly impaired corrosion resistance in these media.

[0023]Having verified a comparable corrosion resistance of many of the experimental alloys, their thermal stability performance revealed the pattern shown in FIG. 3: the alloys with excellent retained ductility (the solid bullets) reside inside the parallelogram delineated by the dashed lines. Mathematically, for a given Fe content up to 5%, the Cr content would thus be bound by the equations 5-0.5*Fe (lower limit) and 7-0.5*Fe (upper limit). Outside this range, retained ductility decreases significantly to single digits (the gray bullets) to eventual virtual brittleness (the cross-hatched bullets). This combination of excellent thermal stability AND comparable corrosion resistance was unexpected, especially since data in this higher-dimensional compositional space is scarce.

[0024]It is important to stress that these favorable results were obtained with a Ni-28Mo base alloy. Exploring compositions with lower (alloys 6 and 10) and higher Mo contents (alloys 7 and 12) suggests a sensible Mo range of 28±1%: lesser Mo contents tend to impair the corrosion resistance (see Tables IV and VII) whereas greater Mo contents rapidly degrade the alloy's thermal stability. In fact, at 31% Mo (alloy 7), the material was virtually brittle in the chosen condition (Table II).

[0025]Commercially melting to a point or line is obviously not feasible. Hence, sensible ranges for the other two key alloying elements, Cr and Fe, need to be defined. Production experience suggests that both elements can be well controlled within the 2% range shown in FIG. 3. Added to FIG. 3 are also the typical compositions of the commercial alloys B, B-2, B-3, and the ranges of Cr and Fe claimed in the respective patents.

[0026]Using the chemical composition of alloy 11 as a base, experimental alloys 13-18 were melted adding Cu, Co, W, and Mn in varying amounts. None of these additions improved the combination of thermal stability and corrosion resistance to any significant degree.

[0027]Summarizing the role of additional alloying elements, prior experience with the B-type alloys and the presented data teach the following:

[0028]Aluminum (Al) chiefly serves as a melting aid (deoxidizer). Its preferred range is from 0.1% to 0.75%. It is not anticipated that higher Al contents would improve the properties of the alloys.

[0029]Boron (B) should be kept as low as possible. Boron could lead to the precipitation of Mo-rich borides in the grain boundaries during certain thermal treatments and, thus, potentially cause sensitization of the material, degrading its corrosion resistance. Boron should not exceed 0.03%.

[0030]Carbon (C), likewise, should be kept as low as possible to suppress formation of Mo-rich carbides during certain thermal treatments. Carbon should not exceed 0.02%.

[0031]Cobalt (Co) is a common substitutional alloying element in Ni-base alloys. However, the solubility of the crucial element Mo in Co is much less than that in Ni. Hence, while a Co content up to 5% may be permissible, no benefits of alloying with Co can be projected.

[0032]Copper (Cu) is an undesirable substitutional alloying element, degrading hot workability. Indeed, Cu-bearing variants of the alloys may exhibit massive cracking during forging, at least at levels of 2.5% and higher. However, Cu levels up to 2% should be tolerable.

[0033]Manganese (Mn) is, like Al, considered a processing aid for these types of alloys. It may be present in amounts up to 3%. However, Mn, like Co, tends to decrease the solubility of Mo in the Ni-base. Hence, no beneficial effects of Mn beyond 3% are anticipated.

[0034]Silicon (Si) is also a highly undesirable alloying element believed to promote carbide formation and, hence, potential sensitization of the material. Up to 0.2% may be tolerable but preferably less than 0.05%.

[0035]Tungsten (W) is not anticipated to add benefit to the inventive alloys (will likely diminish the solubility for Mo) even though W might be tolerable up to 2%. In fact, W additions of 15% greatly impaired corrosion resistance in many of the tested media.

[0036]Vanadium (V) is not anticipated to add benefit to the inventive alloys since its scavenging capacity of the undesirable elements C and Si is not required on account of AOD melting but may be tolerated up to 1%.

[0037]The interstitials nitrogen (N), oxygen (O), phosphorus (P), and sulfur(S) are all considered undesirable and should be kept as low as possible, preferably below 0.02% each.

[0038]Although we have described certain present preferred embodiments of our alloy, it should be understood that our invention is not limited thereto but may be embodied within the scope of the following claims.