Wrought nickel-molybdenum alloy with enhanced resistance to thermally-induced embrittlement

Abstract

A wrought nickel-molybdenum alloy containing 27.22 wt. % molybdenum, 4.73 wt. % iron, 0.47 wt. % chromium, 0.67 wt. % manganese, and 0.25 wt. % aluminum, and the balance nickel, as well as this alloy plus or minus the manufacturing variances typically encountered by each element during air melting, vacuum melting, and electro-slag remelting, have improved resistance to thermally-induced embrittlement over 100 hours at 1200° F.

Description

FIELD OF INVENTION

[0001] This invention relates generally to nickel-base alloy compositions and more specifically to a family of nickel-base alloys containing about 26 wt. % to 30 wt. % molybdenum which are resistant to hydrochloric acid and sulfuric acid.BACKGROUND

[0002] By virtue of their high molybdenum contents, the nickel-molybdenum alloys are very resistant to pure hydrochloric acid and to pure sulfuric acid. As a result, they have been used since the early 1930's in the chemical process industries, in both cast and wrought forms. The original, commercial, cast material, designated HASTELLOY alloy B, had a molybdenum content of about 28 wt. %.

[0003] There is confusion as to the origin of HASTELLOY alloy B. However, it is likely that U.S. Pat. No. 1,375,083 (Clement), describing an alloy containing molybdenum and a metal with the properties of nickel, cobalt, and iron, wherein the iron content should not be greater than 10%, was relevant. Similarly, some aspects of HASTELLOY alloy B appear to have been associated with U.S. Pat. No. 1,710,445 (Becket), in particular a molybdenum content in the range 15% to 40%, and the use of a small addition of vanadium to act as a scavenger, presumably of unwanted impurities (during melting), such as oxygen and sulfur.

[0004] One significant inconsistency between U.S. Pat. No. 1,710,445 and the likely commercial embodiment (i.e. HASTELLOY alloy B) is that the patent calls for an iron content of at least 10%, whereas HASTELLOY alloy B contains 4% to 6% iron. All percentages are presumed to be percentages by weight, as is usual in the commercial realm of nickel-based alloys.

[0005] It should be mentioned that HASTELLOY alloy B was still commercially available in both cast and wrought forms in 1970. A Cabot (STELLITE Division) brochure from 1970 provides the chemical compositions of both the cast and wrought forms. Both forms contain 26% to 30% molybdenum, and 4% to 6% iron. The differences lie in their vanadium contents (0.2% to 0.6% for the cast product, and 0.2% to 0.4% for the wrought product) and their maximum allowed carbon contents (0.12% for the cast product, and 0.05% for the wrought product). The cast form is still commercially available today, under the American Casting Association designation N-12 MV. Wrought HASTELLOY alloy B is also still commercially available today.

[0006] A second important U.S. Patent in the history of the nickel-molybdenum alloys is that of George N. Flint (U.S. Pat. No. 2,959,480, issued in November 1960). It describes nickel-molybdenum alloys that resist intergranular corrosion in heat-affected zones (HAZ) of welds, and that can be hot-worked without the occurrence of cracking. It states that these alloys should contain special amounts of vanadium, in the range 1.2% to 2.3%. It also states that particularly good corrosion resistance and hot-workability are exhibited within this vanadium range when combined with molybdenum in the range 26% to 30%, and iron up to 7%.

[0007] The advent of argon-oxygen de-carburization (AOD) technology in the mid-1960's, which enabled the manufacture of nickel-based alloys with very low carbon contents, was an important step in the evolution of the nickel-molybdenum alloy system. In particular, it led to the development and introduction of HASTELLOY alloy B-2 (a wrought material) in the 1970's.

[0008] Promotional literature from Cabot (STELLITE Division) in 1977 states that HASTELLOY alloy B-2 resists the formation of grain boundary carbide precipitates in weld heat-affected zones (HAZ's), thus making it suitable for use in the as-welded condition (avoiding the need for solution-annealing and rapid quenching of industrial components after welding). Notably, the composition of HASTELLOY alloy B-2 did not include a deliberate addition of iron, limiting it to a maximum allowed content of 2%. Likewise, HASTELLOY alloy B-2 did not require a deliberate addition of vanadium, an element regarded as important in previous nickel-molybdenum alloys. Also, the use of AOD technology enabled the maximum allowed carbon content to be reduced to 0.02% in HASTELLOY alloy B-2.

[0009] While the advent of AOD technology enabled the production of a wrought, nickel-molybdenum alloy with a very low carbon content, thus reducing the tendency for carbides to precipitate in its grain boundaries during welding, it was not able to overcome another limitation of such alloys, and that is their tendency to embrittle due to ordering reactions within the metastable, face-centered cubic, atomic structure, if exposed to temperatures above about 1000° F.

[0010] This limitation was addressed in U.S. Pat. No. 6,610,119, issued in August 2003, the inventor being Dwaine L. Klarstrom. The commercial embodiment of the Klarstrom patent was HASTELLOY B-3 alloy.

[0011] U.S. Pat. No. 6,610,119 is unusual in that it uses atomic percentages, rather than weight percentages, making comparison with other nickel-molybdenum alloy patents more complicated. In addition to atomic percentage ranges for nickel (73 to 77) and for molybdenum (18 to 23), it defines other selected, substitutional, alloying elements that can be present up to 5 atomic % (for any single element), one or more optional, substitutional alloying elements that can be present up to 1 atomic % (for any single element), and one or more interstitial elements which should be as low as possible, and not greater than 0.2 atomic % in total.

[0012] Examples of other selected, substitutional, alloying elements, defined by Klarstrom as “preferred substitutional alloying elements” are chromium, cobalt, iron, manganese, and tungsten. Copper is defined as an “undesirable substitutional alloying element”, which can be tolerated up to about 0.5 atomic %. Silicon, vanadium, niobium, and tantalum were defined by Klarstrom as very (or most) undesirable, and limited to less than or equal to 1 atomic % maximum. Klarstrom's examples of interstitial elements, which should be as low as possible, are nitrogen, oxygen, phosphorous, sulfur, and carbon.

[0013] The wrought, nickel-based alloys typically possess face-centered cubic (FCC or gamma-phase) atomic structures in the solution-annealed and rapidly quenched condition. However, most nickel-based alloys intended for use in acids are super-saturated with alloying elements at temperatures lower than the solution-annealing temperature; this can cause the precipitation of second phases within the microstructure, especially at temperatures above 1000° F., where elemental diffusion is enabled.

[0014] Service temperatures rarely reach more than 500° F., but temperatures can reach in excess of 1000° F. during wrought processing (i.e. hot-forging and hot-rolling), and during welding of the nickel-molybdenum alloys.

[0015] The wrought, nickel-molybdenum alloys are prone to four distinct reactions at temperatures in excess of about 1000° F., as follows:

[0016] 1. The nucleation and growth of M6C carbides within the alloy grain boundaries.

[0017] 2. Long-range ordering to form the intermetallic phase Ni2Mo.

[0018] 3. Long-range ordering to form the intermetallic phase Ni3Mo.

[0019] 4. Long-range ordering to form the intermetallic phase Ni4Mo.

[0020] The nucleation and growth of M6C carbides within the grain boundaries of the nickel-molybdenum alloys can lead to preferential corrosive attack of the grain boundaries during industrial service. However, the formation of intermetallic phases in the nickel-molybdenum alloys can cause embrittlement, which can in turn lead to halide-induced stress corrosion cracking, an insidious and unpredictable form of attack.

[0021] The potential for embrittlement is most acute during the welding of thick plates, in the fabrication of large, industrial components, such as reaction vessels. Embrittlement can be overcome by subsequent solution-annealing treatments (at temperatures close to 2000° F.), followed by rapid quenching, but such processes become less practical as plate thicknesses and component sizes increase.

[0022] The Alloy Digest for HASTELLOY B-3 alloy (published by Alloy Digest, Inc. in August 1994 indicates that HASTELLOY alloy B-2 is prone to long-range ordering to both Ni3Mo and Ni4Mo, with a response peak at approximately 1400° F., whereas HASTELLOY B-3 alloy is associated with a much slower long-range ordering response involving all three intermetallic phases, peaking at 1200° F. (where Ni2Mo and Ni4Mo occur).

[0023] FIG. 5 in U.S. Pat. No. 6,610,119 is a similar graph (known as a Time-Temperature-Transformation plot) for one of the experimental alloys (i.e. No. 17). It again illustrates that the Klarstrom alloys exhibit a much slower, long-range ordering response, in this case to Ni2Mo and Ni3Mo, but not to Ni4Mo. Interestingly, alloy No. 17 exhibited a double peak, one at 1200° F., and the other at 1480° F.

[0024] There is a need for a nickel-molybdenum material with significantly greater resistance to embrittlement than HASTELLOY B-3 alloy.SUMMARY OF THE INVENTION

[0025] We provide a wrought nickel-molybdenum alloy containing 27.22 wt. % molybdenum, 4.73 wt. % iron, 0.47 wt. % chromium, 0.67 wt. % manganese, and 0.25 wt. % aluminum, and the balance nickel plus impurities, plus or minus the manufacturing variances typically encountered by each element during air melting, vacuum melting, and electro-slag remelting. The alloy may also contain residual quantities of copper, cobalt, and tungsten, and impurity levels of silicon and carbon. This alloy composition has improved resistance over HASTELLOY B-3 alloy to thermally-induced embrittlement over 100 hours at 1200° F.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a graph of Brinell Hardness vs. Aging Time at 1200° F. for the tested alloys.

[0027] FIG. 2 is a graph of Brinell Hardness vs. Aging Time at 1400° F. for the tested alloys.DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] A method by which the ductility (and conversely brittleness) of a metallic material can be assessed is hardness testing. While not a direct measurement, it avoids the cost and complexity of tensile testing. Thus, hardness testing was used in the study leading to this discovery.

[0029] Rockwell “B” and Rockwell “C” hardness readings were taken on samples of multiple alloys, aged for various times (0.5, 1, 2, 4, 8, 24, and 100 hours) at both 1200° F. and 1400° F., using a NEWAGE INDENTRON hardness tester. The Rockwell “B” and “C” readings were converted to a unilinear scale, i.e. Brinell (3000 kg, 10 mm ball steel), so that sensible plots could be created.

[0030] The materials studied included commercial HASTELLOY B-3 alloy (Batch No. 2675-8-6606) and several experimental, nickel-molybdenum alloys with molybdenum contents in the range 27.22 wt. % to 31.14 wt. %. The actual (analyzed) chemical compositions of the experimental alloys are given in Table 1. Several of the experimental alloys contained moderate levels of iron and / or chromium; others also contained moderate levels of copper. The elements cobalt, tungsten, manganese, and aluminum were deliberately added to the experimental materials, but at low levels of less than 1 wt. %. Silicon and carbon were kept as low as possible in these experimental alloys, as is usual for corrosion-resistant, nickel-based alloys.TABLE 1Actual Compositions of Experimental Alloys and CommercialHASTELLOY B-3 Alloy Control Heat (Weight %).AlloyNiMoFeCrCuCoWMnAlSiCB-366.6228.601.481.95<0.100.040.180.590.41<0.10<0.001163.0127.361.246.790.040.100.220.640.100.02<0.0022*65.3327.224.730.470.020.110.220.670.250.01<0.002361.6631.144.720.480.010.100.190.660.250.01<0.002463.8028.380.560.525.350.110.240.720.320.02<0.002563.8828.332.780.543.060.100.240.760.280.02<0.002660.4927.643.785.580.010.110.220.660.260.02<0.002762.1828.622.162.153.490.100.230.720.310.03<0.002860.9028.820.564.034.320.110.240.700.280.03<0.002*Alloy of the invention

[0031] The wrought, experimental alloys involved with this study were made by vacuum induction melting (VIM), followed by electro-slag re-melting (ESR), to produce ingots of material (of diameter 4 in, length 7 in, and approximate weight 25 lb.) amenable to hot working.

[0032] Ingots of these experimental alloys were homogenized for 24 hours at 2200° F., prior to hot forging. Hot forging, and subsequently hot rolling, of these alloys were carried out using start temperatures of 2200° F. and 2150° F., respectively. The alloys were hot rolled to plates, of thickness 0.5 in, and sheets, of thickness 0.125 in.

[0033] Trials were performed to determine an appropriate solution annealing temperature for each alloy, i.e. temperatures that resulted in structures free from second phase remnants from the hot working processes, precipitation-free grain boundaries, and moderate grain sizes (approximately ASTM Sizes 3 to 4). These solution annealing temperatures were as follows:

[0034] Alloy 1:2100° F.

[0035] Alloy 2:2000° F.

[0036] Alloy 3:2200° F.

[0037] Alloy 4:2100° F.

[0038] Alloy 5:2100° F.

[0039] Alloy 6:2200° F.

[0040] Alloy 7:2100° F.

[0041] Alloy 8:2200° F.

[0042] The results of the post-aging hardness tests are presented in Table 2, and in FIGS. 1 and 2. A very surprising discovery was the performance of Alloy 2, which did not exhibit significant hardening, even after 100 hours, during aging at 1200° F. This was in stark contrast to the other experimental alloys, and HASTELLOY B-3 alloy, all of which had hardened significantly after 8 hours at 1200° F. The differences were not quite as stark at 1400° F., since Alloy 2 did harden after an exposure time of 100 hours, and three of the other alloys (including HASTELLOY B-3 alloy), reached an exposure time of 8 hours before hardening.TABLE 2Brinell Hardnesses after Aging for Various Times at 1200° F. and 1400° F.AgingAgingBrinell Hardness (3000 kg, 10 mm Ball Steel)Temp., ° F.Time, hB-312*34567812000.521026419021618018024718525312001216271185247185185271240258120022163011902532001902712402581200425831918528622819030128630112008279327195301294216327286311120024336336195319344294344336344120010034435319535335335336234435314000.5222319195286185176264185301140012223271952861851762791953271400222233619529419018530119532714004222344195311200185301195336140082223441953112001853012713361400243363531953272402863112793361400100400362279409362286327327371*Alloy of the invention

[0043] The results for HASTELLOY B-3 alloy are consistent with a chart in the Alloy Digest of August 1994, in that the embrittling / hardening reaction is slower at 1400° F. than it is at 1200° F.

[0044] The results presented in Table 2, FIG. 1, and FIG. 2 reveal that Alloy 2 possesses unusually high resistance to embrittlement, which also infers more protection against halide-induced, stress corrosion cracking, for a given set of manufacturing, welding, and fabrication conditions. Moreover, this level of resistance to embrittlement was not predicted by the prior art.

[0045] An addition of about 5 wt. % iron was used in wrought products up to the advent of HASTELLOY alloy B-2, and is still used in cast products, both with significant vanadium additions. The iron-bearing, wrought products also had significant carbon impurity contents, as did the cast products, carbon being advantageous to fluidity during the filling of molds.

[0046] There appears to be no compositional overlap with prior art, given that the maximum iron content in the Klarstrom patent is 5 atomic %, and the atomic percentage of iron in Alloy 2 is 5.59. Also, the alloy of this invention (Alloy 2) contains deliberate additions of chromium (0.47 wt. %) and manganese (0.67 wt. %), which may be having an influence upon the long-range ordering kinetics.

[0047] During the manufacture of nickel-based alloys, it is not possible to attain precise control of elemental contents to two decimal places, given that variances can occur in chemical analyses, and during the melting processes. Ranges are therefore established, based on prior knowledge of such variances. Any commercial embodiment of the alloys of this invention (including silicon and carbon impurity levels, and copper, cobalt, and tungsten residual levels) would therefore be subject to such typical manufacturing variances.

[0048] The variances expected for the elements molybdenum, iron, chromium, manganese, and aluminum in HASTELLOY B-3 alloy are as follows:

[0049] Molybdenum: plus or minus 1.00 wt. %

[0050] Iron: plus or minus 0.35 wt. %

[0051] Chromium: plus or minus 0.35 wt. %

[0052] Manganese: plus or minus 0.25 wt. %

[0053] Aluminum: plus or minus 0.14 wt. %

[0054] In addition, silicon and carbon should be kept as low as possible in the commercial embodiment of the alloy of this invention, but might be as high as 0.050 wt. % (in the case of silicon) and 0.015 (in the case of carbon), according to the expectations for HASTELLOY B-3 alloy (which also calls for silicon and carbon to be as low as possible).

[0055] With regard to the residual elements, cobalt, copper, and tungsten, which can be carried over from prior materials melted in the same furnace, HASTELLOY B-3 alloy allows for amounts not exceeding 1.00 wt. % in the case of cobalt, 0.10 wt. % in the case of copper, and 1.00 wt. % in the case of tungsten.

[0056] If these expected variances are added to the composition of Alloy 2, they result in the following compositional ranges for a commercial embodiment of Alloy 2:

[0057] Nickel: Balance

[0058] Molybdenum: 26.22 wt. % to 28.22 wt. %

[0059] Iron: 4.38 wt. % to 5.08 wt. %

[0060] Chromium: 0.12 wt. % to 0.82 wt. %

[0061] Manganese: 0.42 wt. % to 0.92 wt. %

[0062] Aluminum: 0.11 wt. % to 0.39 wt. %

[0063] Silicon: up to 0.050 wt. %

[0064] Carbon: up to 0.015 wt. %

[0065] Cobalt: up to 1.00 wt. %

[0066] Copper: up to 0.10 wt. %

[0067] Tungsten: up to 1.00 wt. %

[0068] Although we have described and illustrated certain present preferred embodiments of our wrought nickel-molybdenum alloy with enhanced resistance to thermally-induced embrittlement it should be distinctly understood that our invention is not limited thereto but may be variously embodied within the scope of the following claims.

Claims

1. A wrought, nickel-molybdenum alloy resistant to thermally-induced embrittlement over 100 hours at 1200° F. comprising:26.22 wt. % to 28.22 wt. % molybdenum,4.38 wt. % to 5.08 wt. % iron,0.12 wt. % to 0.82 wt. % chromium,0.42 wt. % to 0.92 wt. % manganese,0.11 wt. % to 0.39 wt. % aluminum,up to 0.050 wt. % silicon,up to 0.015 wt. % carbon,up to 1.00 wt. % cobalt,up to 0.10 wt. % copper,up to 1.00 wt. % tungsten, andbalance nickel.

2. The alloy of claim 1 comprising 27.22 wt. % molybdenum, 4.73 wt. % iron, 0.47 wt. % chromium, 0.67 wt. % manganese, and 0.25 wt. % aluminum.

3. The alloy of claim 1 comprising, 0.02 wt. % copper, 0.11 wt. % cobalt, 0.22 wt. % tungsten, 0.01 wt. % silicon, and carbon less than 0.002 wt. %.

4. The alloy of claim 1 wherein the alloy is in a form selected from the group consisting of plates, sheets, bars, tubes, wires, and billets.

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