The C-type nickel base alloys of the type containing significant amounts of chromium (about 16 to 25%) and molybdenum (about 12 to 18%) may be improved by adding small but critical amounts of copper (about 1 to 3.5%) which their general corrosion resistance to a wide range of both oxidizing and non-oxidizing industrial media.

FIELD OF THE INVENTION
 This invention relates generally to non-ferrous metal alloy compositions
 and more specifically to a particular family, called C-types, of nickel
 base alloys containing significant amounts of chromium and molybdenum
 along with minor, but important, amounts of other alloying elements which
 impart general corrosion resistance to the alloys.
 BACKGROUND OF THE INVENTION
 The forerunner of today's general purpose corrosion resistant Ni--Cr--Mo
 alloys was developed and patented in the 1930's (U.S. Pat. No. 1,836,317)
 by Russell Franks, working at the time for a predecessor to the developer
 of the present invention. The commercial embodiment of this invention was
 marketed under the name Alloy C and included, besides chromium and
 molybdenum, smaller amounts of iron, the option of a tungsten addition,
 and minor additions of manganese, silicon, and vanadium to aid in
 manufacturing. Alloys within this compositional range were found to
 exhibit passive behavior in many oxidizing acids by virtue of the chromium
 addition. Also, they exhibited good resistance to many non-oxidizing acids
 by virtue of the enhancement of nickel's natural nobility by molybdenum
 and tungsten additions.
 Over the years, several discoveries related to this alloy family or system
 have been made. First, it was identified that carbon and silicon are quite
 deleterious to the corrosion resistance of these alloys, because they
 promote the formation of carbides and intermetallic precipitates (such as
 mu-phase) at grain boundaries within the microstructure. At high carbon
 and/or silicon levels, these compounds can form upon cooling after
 annealing, or during elevated temperature excursions, such as those
 experienced by weld-heat-affected-zones. Since the formation of these
 compounds depletes the surrounding regions of chromium, molybdenum (and,
 if present, tungsten), those regions become much more prone to chemical
 attack, or become "sensitized". The compounds themselves can also be
 attacked preferentially. A key patent relating to low carbon and low
 silicon Ni--Cr--Mo alloys (U.S. Pat. No. 3,203,792) having improved
 thermal stability was issued in 1965. The commercial embodiment of that
 patent was developed and marketed as Alloy C-276 by the successor to the
 Haynes Stellite Company and is still the most widely used alloy of this
 family.
 Even with low carbon and low silicon levels, the Ni--Cr--Mo alloys are
 metastable, i.e. in combination, the alloying elements exceed their
 equilibrium solubility limits and eventually cause microstructural changes
 in the products. Exposure of the alloys to the approximate temperature
 range of 1200.degree. F. to 1800.degree. F. (or about 650-1000.degree.
 C.)quickly induces metallurgical changes, in particular the precipitation
 of intemetallic compounds in the grain boundaries, which weaken the
 structure. To reduce further the tendency for deleterious compounds to
 form, a tungsten-free, low iron composition called Alloy C-4 was developed
 and patented (U.S. Pat. No. 4,080,201) by co-workers of the present
 inventor. This patent required a carefully controlled composition and also
 included small but important amounts of titanium to combine with any
 residual carbon and nitrogen. Similarly, U.S. Pat. No. 5,019,184 again
 teaches that low iron and low carbon plus some titanium reduces Mu phase
 formation by enhancing thermal stability in these alloys.
 Another important discovery with regard to C-type alloys containing both
 molybdenum and tungsten was that optimum corrosion and pitting resistance
 is dependent upon certain important elemental ratios. It was discovered
 during the development of C-22 Alloy that the Mo:W ratio should lie
 between about 5:1 and 3:1 and that the ratio of 2.times.Cr:
 Mo+(0.5.times.W) should fall in the range of about 2.1 to 3.7. See U.S.
 Pat. No. 4,533,414, also assigned to the assignee of the present
 invention.
 More recently, U.S. Pat. No. 4,906,437 disclosed the subtle effects of the
 deoxidizing elements aluminum, magnesium, and calcium if kept within
 certain narrow, specified ranges, with regard to hot workability and
 influence on corrosion performance. The base composition described in U.S.
 Pat. No. 4,906,437 is quite similar to that discovered in 1964 by R. B.
 Leonard who, at that time, was researching C-type alloys for the assignee
 of the present invention. See G. B. Pat. No. 1,160,836. By performing
 potentiostatic studies on several compositional variants, Leonard
 identified Ni--23Cr--15Mo as a suitable design base for developing cast
 Ni--Cr--Mo alloys.
 Of course, different families of alloys, containing some of the same
 elements but in differing proportions, have been developed to have
 different properties so as to satisfy different needs in the metallurgical
 arts. One example of such a different type of alloy is Alloy G, developed
 by the predecessor of the present assignee during the 1950's to resist
 phosphoric acid. It superficially resembles the C-type alloys except for
 containing much more iron and less molybdenum along with some cooper. It
 is more fully disclosed in U.S. Pat. No. 2,777,766.
 Published information relating to the nominal compositions and corrosion
 properties of these prior art C-type alloys is summarized in Tables A and
 B.
 The aforementioned patents are only representative of the many alloying
 situations reported to date in which many of the same elements are
 combined to achieve distinctly different functional relationships such
 that various phases form providing the alloy system with different
 physical and mechanical characteristics. Nevertheless, despite the large
 amount of data available concerning these types of nickel-base alloys, it
 is still not possible for workers in this art to predict with any degree
 of accuracy or confidence the physical and mechanical properties that will
 be displayed by certain concentrations of known elements even though such
 combinations may fall within broad, generalized teachings in the art,
 particularly when the new combinations may be thermo-mechanically
 processed somewhat differently from those alloys previously employed in
 the art.
 SUMMARY OF THE INVENTION
 The most desirable attribute of the Ni--Cr--Mo alloys from a chemical
 process industry standpoint is their successful application in a wide
 range of corrosive environments. However, it is inappropriate to consider
 the existing alloys as equal entities, since they vary considerably in
 their resistance to specific media, depending upon the precise chromium,
 molybdenum, and tungsten levels. High chromium alloys provide enhanced
 resistance to oxidizing media, such as nitric acid, for example while low
 chromium alloys perform better in non-oxidizing solutions such as
 hydrochloric acid.
 Accordingly, a principal object of this invention is to provide a new
 corrosion resistant alloy with as wide an application range as possible,
 so as to overcome the limitations of the existing Ni--Cr--Mo alloys, by
 incorporating many of the best uniform corrosion characteristics of each
 of the previous alloys in a single new product. This enhanced versatility
 in both oxidizing and non-oxidizing media should also reduce the risks of
 premature failure in ill-defined process environments, and under the
 occasional upset or changing conditions, found in the chemical industry.
 It has been found that the above object, as well as other advantages which
 will become apparent, may be achieved by adding small but critical amounts
 of copper to C-type base alloys so as to provide new and improved products
 having compositions generally falling within the following preferred
 ranges, in weight percent:

Preferred Most Preferred
 Chromium: 22.0 to 24.5 22.35 to 23.65
 Molybdenum: 14.0 to 18.0 15.35 to 16.65
 Copper: 1.0 to 3.5 1.40 to 1.80
 Iron: Up to 5.0 0.30 to 1.50
 Silicon: Up to 0.1 Up to 0.05
 Manganese: Up to 2.0 0.10 to 0.30
 Magnesium Up to 0.1 Up to 0.05
 Cobalt: Up to 2.0 Up to 1.95
 Aluminum: Up to 0.5 0.15 to 0.30
 Calcium: Up to 0.05 Up to 0.02
 Carbon: Up to 0.015 Up to 0.007
 Nitrogen: Up to 0.15 Up to 0.06
 Tungsten: Up to 0.5 Up to 0.50
 Carbide forming elements: Up to 0.75 Up to 0.35 (in total)
 Nickel: Remainder
 Subsequent data herein will show that copper, within a narrow critical
 range, can be added to many existing high chromium Ni--Cr--Mo alloys to
 enhance their resistance to non-oxidizing media. The benefits in
 hydrochloric acid were opposed to previous experimental evidence, and the
 improved effects, as a function of copper content, are quite unexpected
 and non-linear, that is more copper does not give better properties.

DETAILED DESCRIPTION OF THE INVENTION
 The discovery of the compositional range defined above involved three
 stages. First, stag with a base composition (Example C-1) somewhat similar
 to that proposed by R. B. Leonard (Sample A-5), the corrosion resistance
 effects of copper were determined at several increments by adding up to
 about 6.0 wt. % Cu to the base. Examples C-2 to C-7 show the compositions
 and test results. Then, having established that the optimum copper level
 is about 1.6% +/-0.3% from a versatility standpoint (see FIGS. 1 & 2), the
 effects of iron, nitrogen, and tungsten (as a partial replacement for
 molybdenum) were determined. Finally, the useful ranges of chromium,
 molybdenum, and a variety of minor elements (typically found in wrought,
 Ni--Cr--Mo alloys) were established.
 The investigation of copper as a possible useful addition to this alloy
 system was initially prompted by its known benefits in other types of
 alloy systems, such as the Fe--Ni--Cr--Mo and Ni--Fe--Cr--Mo alloy
 systems, particularly with regard to its frequent improvement to sulfuric
 acid resistance. The only previous data concerning the effects of copper
 in high chromium Ni--Cr--Mo alloys (R. B. Leonard, 1965) inferred a
 slightly negative effect upon resistance to hydrochloric acid, but a
 positive effect on resistance to moderate concentrations of sulfuric acid.
 Only one copper level (2.36 wt. %) was studied by R. B. Leonard, however,
 and at a relatively low chromium content (21.16 wt. %). Also, the work of
 R. B. Leonard involved only castings, whereas the primary focus of this
 invention is wrought products, i.e. sheets, plates, bars, wires (for
 welding), and tubular products, forged and/or rolled from cast ingots.
 For each stage of the project, small heats (usually about 20-25 Kg.) of
 experimental materials were produced by vacuum-induction melting,
 electroslag remelting, hot forging, homogenizing (e.g. 50 hrs. at
 2250.degree. F. or 1240.degree. C.) and hot rolling at about 2240.degree.
 F. into plates or sheets about 0.125 in. (3 mm) thick for testing. For
 each alloy, an appropriate solution annealing treatment (e.g 10-20 min. at
 2050-2150.degree. F. or 1130-1190.degree. C. followed by water quenching)
 was determined by furnace trials. As may be deduced from the list of
 experimental compositions given in Table C, most of these alloys contained
 small amounts of aluminum (for deoxidation), manganese (to tie up sulfur),
 carbon, cobalt, and silicon (typical mill impurities). Small amounts of
 magnesium were also added to the experimental melts for deoxidation
 purposes but only traces appear in the final products.
 The effects of copper on the uniform corrosion behavior of high chromium,
 Ni--Cr--Mo alloys are evident from the test results for the first batch of
 alloys (Alloys C-1 to C-7 in Table C) and FIG. 1. In both concentrations
 of sulfuric acid (70% and 90%), copper was found to be extremely
 beneficial, even at a level of only 0.6 wt. %. In dilute hydrochloric
 acid, the relationship between copper content and corrosion rate was found
 to be complex and unexpected. It was discovered that significant benefits
 accrue from additions of copper in the range 0.6 wt. % to 3.1 wt. %. The
 corrosion rate at 6.1 wt. % copper was also low, probably because most of
 the copper partitioned to primary precipitates in the microstructure
 leaving the matrix with a lower effective concentration. None of the other
 experimental alloys contained such primary (solidification) precipitates.
 With regard to the resistance of the experimental alloys to boiling 65%
 nitric acid, an unexpected relationship with the copper content was
 measured. In particular, a peak in the corrosion rate was measured at
 about 0.6 wt. % copper then lower values until above about 5% as shown in
 FIG. 2.
 Testing of the second batch of alloys (Examples C-8 to C-1 in Table C)
 revealed that iron, when added in the range 1.0 wt % to 4.2 wt. % has
 little effect on the general corrosion resistance of the system, at least
 in alloys with near the optimum copper content (approximately 1.6 wt. %).
 The partial replacement of molybdenum with about 4.0 wt. % tungsten was
 found to degrade significantly the resistance to 2.5% hydrochloric acid
 and 70% sulfuric acid. Nitrogen, at a level of 0.1 wt. % was found to
 reduce the resistance of the alloy system to 2.5% hydrochloric acid but
 this disadvantage may be offset by its usually beneficial strengthening
 effects.
 The third batch of alloys (designated Examples C-12 to C-15 in Table C)
 enabled the preferred boundaries of the alloy system to be better
 identified. With regard to the minor elements, the effects of these at low
 levels were studied in Alloy C-12. Their effects at higher levels were
 studied in Alloy C-13. It was determine that, within the ranges studied,
 the favorable properties of the system are maintained. The effects of
 chromium and molybdenum were determined by testing Alloys C-14 and C-15.
 At low chromium and molybdenum levels (21.6 wt. % and 14.6 wt. %
 respectively), the resistance of the alloy system to 65% nitric acid was
 considerably reduced. At high chromium and molybdenum levels (24.2 wt. %
 and 16.6 wt. %), enhanced uniform corrosion properties were discovered,
 but the annealed and quenched microstructure exhibited an abundance of
 grain boundary precipitates, which would be deleterious to the mechanical
 properties, and promote grain boundary attack in certain media. However, a
 high chromium content with a low molybdenum content, or a low chromium
 content with a high molybdenum content would generally be acceptable.
 In addition to testing the experimental alloys, certain of the commercial
 wrought, Ni--Cr--Mo compositions (corresponding to specific patents) were
 tested also, to allow direct comparisons with the most preferred alloy of
 this invention (Alloy C-4). Comparative corrosion data are presented in
 Tables B and C, to further illustrate the advantages or improvements
 created by this invention.
 Several observations may be made concerning the general effects of the
 various other alloying elements from the foregoing test results (or
 previous work with similar alloys) as follows:
 Aluminum (Al) is an optional alloying element. It is usually used as a
 deoxidizer during the melting process and is generally present in the
 resultant alloy in amounts over about 0.1 percent. Aluminum may also be
 added to the alloy to increase strength but too much will form detrimental
 Ni.sub.3 Al phases. Preferably, up to about 0.50 percent, and more
 preferably 0.15 to 0.30 percent, of aluminum is present in the alloys of
 this invention.
 Boron (B) is an optional alloying element which may be unintentionally
 introduced into the alloy during the melting process (e.g., from scrap or
 flux) or added as a strengthening element. In the preferred alloys, boron
 may be present up to about 0.05 percent but, more preferably, less than
 0.01 percent for better ductility.
 Carbon (C) is an undesirable alloying element which is difficult to
 eliminate completely from these alloys. It is preferably as low as
 possible since corrosion resistance falls off rapidly with increasing
 carbon content. It should not exceed about 0.015 percent, but may be
 tolerated at somewhat higher levels up to 0.05 percent in castngs if less
 corrosion resistance is acceptable.
 Chromium (Cr) is a necessary alloying element in these alloys as explained
 above. While it may be present from about 16 to 25 percent, the most
 preferred alloys contain about 22 to 24.5 percent chromium. It seems to
 form a stable passive film during corrosion of these alloys in oxidizing
 media. At much higher concentrations, the chromium cannot remain in
 solution but partitions into second phases which embrittle the alloy.
 Cobalt (Co) is almost always present in nickel-base alloys since it is
 mutually soluble in the nickel matrix. The alloys of the present invention
 may contain up to about 2 or 3 percent, above which the hot working
 properties of the alloys may deteriorate.
 Copper (Cu) is often an undesirable alloying element in these types of
 alloys because it generally reduces hot workability. However, as explained
 above, it is a key component of this invention.
 Iron (Fe) is a permissive alloying element. It is commonly present in these
 types of alloys since the use of ferro-alloys is convenient for adding
 other necessary allowing elements. However, as the amount of iron
 increases above about 5%, the corrosion rate increases.
 Manganese (Mn) is a preferred alloying element. It is used herein to tie up
 sulphur and improve hot workability, and is preferably present in alloys
 of this invention in amounts up to about 2 percent. The most preferred
 alloys contain at least about 0.1 to 0.3 percent manganese.
 Molybdenum (Mo) is a major alloying element of the present invention as
 explained above. Amounts greater than about 12 percent are necessary to
 provide the desired corrosion resistance to the nickel base and amounts
 greater than 14 percent are preferred. However, amounts greater than about
 18 percent embrittle the alloys due to the promotion of secondary phases
 and are difficult to hot work into wrought products.
 Nickel (Ni) is the base metal of the present invention and should be
 present in amounts greater than about 45 percent, in order to provide
 adequate physical properties and good resistance to stress corrosion
 cracking to the alloy. However, the exact amount of nickel present in the
 alloys of the invention is determined by the required minimum or maximum
 amounts of chromium, molybdenum, copper and other alloying elements
 present in the alloy.
 Nitrogen (N) is an optional strengthening alloying element which may be
 present up to about 0.015 percent without significant detriment to the
 general corrosion resistance properties of the alloy even though there is
 some reduction to resistance to HCl.
 Oxygen (O), Phosphorus (P) and Sulphur (S) are all undesirable elements
 which, however, are usually present in small amounts in all alloys. While
 such elements may be present in amounts up to about 0.1 percent without
 substantial harm to alloys of the present invention, they are preferably
 present only up to about 0.02 percent each.
 Silicon (Si) is a undesirable alloying element because it has been shown to
 to promote the formation of harmful precipitates. While it may be present
 up to about one percent to promote fluidity during casting into less
 corrosion-resistant near net shape articles, the preferred alloys contain
 no more than about 0.1 percent, and, most preferably, less Man about 0.05
 percent silicon in wrought products.
 Tungsten (W) is an often an optional alloying element which may take the
 place of some of the molybdenum in these types of alloys. However, because
 it degrades the corrosion resistance and is a relatively expensive and
 heavy element, the preferred alloys of this invention contain no more that
 about one half percent of tungsten.
 It is generally known to those skilled in the art that the carbide-forming
 elements such as titanium, vanadium, niobium, tantalum, and hafnium may be
 added to the Ni--Cr--Mo alloys (to tie up any carbon) without detriment to
 the physical properties. Accordingly, it is believed that these elements
 could be added at levels up to about 0.75 wt. % in total but preferably
 are only up to 0.35% in this new alloy system.
 While in order to comply with the statutes, this present invention has been
 described in terms more or less specific to one preferred embodiment, it
 is expected that various alterations, modifications, or permutations
 thereof will be readily apparent to those skilled in the art. Therefore,
 it should be understood that the invention is not to be limited to the
 specific features shown or described, but it is intended that all
 equivalents be embraced within the spirit and scope of the invention as
 defined by the appended claims.
 TABLE A
 Prior Art Alloys
 Nominal Compositions
 SAMPLE # A-1 A-2 A-3 A-4 A-5 A-6
 A-7
 U.S. Pat. No. 1,836,317 3,203,792 4,080,201 4,533,414 4,906,437 5,019,184
 2,777,766
 Alloy Name C C-276 C-4 C-22 59 686 G
 Alloy Digest Ni-23 Ni-164 Ni-211 Ni-317 -- --
 Ni-113
 Nickel Balance Balance Balance Balance Balance Balance
 Balance
 Cobalt &lt;2.5 &lt;2.0 &lt;2.5
 Chromium 16 16 16 22 23 20.5
 22.25
 Molybdenum 16 16 16 13 16 16.3
 6.5
 Tungsten 4 4 3 3.9
 0.5
 Iron 5 5 &lt;3 3 1 1
 19.5
 Manganese &lt;1 &lt;1 &lt;1 &lt;0.5
 1.3
 Silicon &lt;1 &lt;0.08 &lt;0.08 &lt;0.08 0.04
 0.35
 Carbon &lt;0.08 &lt;0.01 &lt;0.01 &lt;0.01 0.005 0.006
 0.03
 Aluminum
 Vanadium &lt;0.35 &lt;0.35 &lt;0.35
 Titanium &lt;0.7
 Copper
 2.0
 Others
 2.12