Patent Grant
7785532
The invention relates to corrosion-resistant, nickel-based alloys.
In the nineteen twenties, it was discovered by Becket (U.S. Pat. No. 1,710,445) that the addition of 15 to 40 wt. % molybdenum to nickel resulted in alloys highly resistant to non-oxidizing acids, notably hydrochloric and sulfuric, two of the most important industrial chemicals. Since the least expensive source of molybdenum was ferro-molybdenum, a significant quantity of iron was included in these alloys. At about the same time, it was also discovered by Franks (U.S. Pat. No. 1,836,317) that nickel alloys containing significant quantities of molybdenum, chromium, and iron, could cope with an even wider range of corrosive chemicals. We now know that this is because chromium encourages the formation of protective (passive) films in so-called oxidizing acids (such as nitric), which induce cathodic reactions of high potential. These inventions led to the introduction of the cast HASTELLOY A, B, and C alloys, and subsequently to the wrought B, C, and C-276 alloys. The need to minimize the carbon and silicon contents of such alloys, to improve their thermal stability (Scheil, U.S. Pat. No. 3,203,792) was factored into the composition of HASTELLOY C-276 alloy.
With regard to the quantities of molybdenum and chromium that can be added to nickel, these are dependent upon thermal stability. Nickel itself possesses a face-centered cubic structure, at all temperatures below its melting point. Such a structure provides excellent ductility and resistance to stress corrosion cracking. Thus, it is desirable that alloys of nickel designed to resist corrosion also possess this structure, or phase. However, if the combined additions exceed their limit of solubility in nickel, second phases of a less-desirable nature are possible. Metastable or supersaturated nickel alloys are possible if high temperature annealing (to dissolve unwanted second phases), followed by rapid quenching (to lock in the high temperature structure) are employed. The Ni—Mo alloys and most of the Ni—Cr—Mo alloys fall into this category. The main concern with such alloys is their propensity to form second phase precipitates, particularly at microstructural imperfections such a grain boundaries, when reheated to temperatures in excess of about 500° C., where diffusion becomes appreciable. Such elevated temperature excursions are common during welding. The term thermal stability relates to the propensity for second phase precipitation at elevated temperatures.
In the nineteen fifties, Ni—Mo and Ni—Cr—Mo alloys with low iron contents, covered by G.B. Patent 869,753 (Junker and Scherzer) were introduced, with narrower compositional ranges and stricter controls on carbon and silicon, to ensure corrosion resistance yet minimize thermal instability. The molybdenum range of the nickel-molybdenum (Ni—Mo) alloys was 19 to 32 wt. %, and the molybdenum and chromium ranges of the nickel-chromium-molybdenum (Ni—Cr—Mo) alloys were 10 to 19 wt. % and 10 to 18 wt. %, respectively. These led to the introduction of wrought HASTELLOY B-2 and C-4 alloys in the nineteen seventies.
Since then, it has been discovered that HASTELLOY B-2 alloy is prone to rapid, deleterious phase transformations during welding. To remedy this, HASTELLOY B-3 alloy, the phase transformations of which are much slower, was introduced in the nineteen nineties after discoveries by Klarstrom (U.S. Pat. No. 6,503,345). With regard to recent developments in the field of Ni—Cr—Mo alloys, these include HASTELLOY C-22 alloy (Asphahani, U.S. Pat. No. 4,533,414), HASTELLOY C-2000 alloy (Crook, U.S. Pat. No. 6,280,540), NICROFER 5923 hMo (Heubner, Köhler, Rockel, and Wallis, U.S. Pat. No. 4,906,437), and INCONEL 686 alloy (Crum, Poole, and Hibner, U.S. Pat. No. 5,019,184). These newer alloys require molybdenum within the approximate range 13 to 18 wt. %, and chromium within the approximate range 19 to 24.5 wt. %.
With a view to enhancing the corrosion performance of the Ni—Cr—Mo alloys, additions of tantalum (of the so-called reactive element series) have been used. Notably, U.S. Pat. No. 5,529,642 describes an alloy containing from 1.1 to 8 wt. % tantalum. This has been commercialized as MAT-21 alloy.
Although the Ni—Mo alloys possess outstanding resistance to non-oxidizing acids (i.e. those which induce the evolution of hydrogen at cathodic sites), they are intolerant of additions, residuals, or impurities which result in cathodic reactions of higher potential. One of these so-called “oxidizing species” is oxygen, which is hard to avoid. While the Ni—Cr—Mo alloys can tolerate such species, they do not possess sufficient resistance to the non-oxidizing acids for many applications. Thus there is a need for materials which possess the attributes of both the Ni—Mo and Ni—Cr—Mo alloys.
Materials with compositions between those of the Ni—Mo and Ni—Cr—Mo alloys do exist. For example, a Ni—Mo—Cr alloy containing approximately 25 wt. % molybdenum and 8 wt. % chromium (242 alloy, U.S. Pat. No. 4,818,486) was developed for use at high temperatures in gas turbines, but has been used to resist aqueous environments involving hydrofluoric acid. Also, B-10 alloy, a nickel-based material containing about 24 wt. % molybdenum, 8 wt. % chromium, and 6 wt. % iron was promoted as being tolerant of oxidizing species in strong non-oxidizing acids. As will be shown, however, the properties of these two Ni—Mo—Cr alloys are generally similar to those of the Ni—Mo alloys, and do not provide the desired versatility.
The principal object of this invention is to provide wrought alloys which exhibit characteristics of both the Ni—Mo and Ni—Cr—Mo alloys, possess good thermal stability, and are thus extremely versatile. These highly desirable properties have been unexpectedly attained using a nickel base, molybdenum between 20.0 and 23.5 wt. %, and chromium between 13.0 and 16.5 wt. %. To enable the removal of oxygen and sulfur during the melting process, such alloys typically contain small quantities of aluminum and manganese (up to about 0.5 and 1 wt. %, respectively, in the Ni—Cr—Mo alloys), and possibly traces of magnesium and rare earth elements (up to about 0.05 wt. %).
Iron is the most likely impurity in such alloys, due to contamination from other nickel alloys melted in the same furnaces, and maxima of 2.0 wt. % or 3.0 wt. % are typical of those Ni—Cr—Mo alloys that do not require an iron addition. Thus a maximum of 2.0 wt. % iron is proposed for the alloys of this invention. Other metallic impurities are possible, including, tungsten (up to 0.75 wt. %), cobalt (up to 1.0 wt. %), copper (up to 0.5 wt. %), titanium (up to 0.2 wt. %), niobium (up to 0.5 wt. %), tantalum (up to 0.2 wt. %), and vanadium (up to 0.2 wt. %).
By use of special melting techniques, in particular argon-oxygen decarburization, it is possible to achieve very low carbon and silicon contents in such alloys, to enhance their thermal stability. However, it is not possible to exclude these elements completely.
With regard to carbon content, the preferred experimental alloy of the study which led to this discovery contained 0.013 wt. % carbon (because it was not possible to apply the argon-oxygen decarburization process during melting of the experimental alloys). Thus it is evident that at least 0.013 wt. % carbon can be tolerated in the alloys of this invention. This is therefore the proposed maximum for carbon in the alloys of this invention.
With regard to silicon, a maximum of 0.08 wt. % is typical of the wrought Ni—Cr—Mo alloys; thus a maximum of 0.08 wt. % is proposed for the alloys of this invention.
It is believed that the extreme versatility of the alloys of this invention is best illustrated by
The discovery of these extremely versatile alloys involved the testing of small, experimental heats of material (each about 22.7 kg in weight). These were produced by vacuum induction melting, electroslag remelting, ingot homogenizing (50 h at 1232° C.), hot forging, and hot rolling into 3.2 mm thick sheets at 1149 to 1177° C. For each experimental alloy, an appropriate solution annealing treatment (in most cases at 1149° C.) was determined by furnace trials. As may be deduced from Tables 2 and 3 (nominal compositions and chemical analyses of experimental alloys), deliberate additions of manganese and aluminum were used to help minimize the sulfur and oxygen contents of all the alloys. Except in the case of the HYBRID alloy, the experimental materials also contained traces of rare earth elements, for enhanced sulfur and oxygen control.
The upper compositional boundaries were determined without corrosion testing, since it was not possible to generate a single phase microstructure in alloy EN1406. Thus, 23.67 wt. % molybdenum and 16.85 wt. % chromium are regarded as outside the compositional range of this invention.
The corrosion rates for the other experimental alloys (i.e. those which responded well to solution annealing and water quenching, yielding a single phase microstructure) and commercial materials in the strong, oxidizing and strong, reducing acid media previously mentioned are given in Table 4. The steep decline in resistance to the strong, oxidizing solution (oxygenated 2.5% HCl at 121° C.) associated with reducing the chromium content from 14.86 to 12.67 wt. % in alloys containing about 23 wt. % molybdenum (EN1106 versus EN5900) indicates that the chromium content should be at least 13.0 wt. %. Also, the steep decline in resistance to the strong, reducing solution (nitrogenated 2.5% HCl at 121° C.) associated with reducing the molybdenum content from 21.64 to 19.82 wt. % in alloys containing about 15 wt. % chromium (the HYBRID alloy versus EN1006) indicates that the molybdenum content should be at least 20.0 wt. %.
To provide additional evidence of the unique behavior and versatility of the HYBRID alloy, it was compared with B-3 alloy (as the representative of the Ni—Mo system) and C-276 alloy (as the representative of the Ni—Cr—Mo system) in several other oxidizing and reducing environments. The results of these comparative tests are given in Table 5. In hydrochloric acid (HCl), hydrofluoric acid (HF), and sulfuric acid (H2SO4), which are reducing, the HYBRID alloy provides resistance approaching that of the Ni—Mo alloys. In nitric acid (HNO3) and a mixture of ferric chloride (FeCl3) plus hydrochloric acid, which is oxidizing, the HYBRID alloy approaches the performance of the Ni—Cr—Mo alloys, whereas the Ni—Mo alloys exhibit extremely high corrosion rates in such environments.
Even though the samples tested were all wrought sheets, the alloys should exhibit comparable properties in other wrought forms (such as plates, bars, tubes, pipes, forgings, and wires) and in cast and powder metallurgy forms. Consequently, the present invention encompasses all forms of the alloy composition.
Although I have disclosed certain present preferred embodiments of the alloys, it should be distinctly understood that the present invention is not limited thereto but may be variously embodied within the scope of the following claims.
This application claims the benefit of provisional application Ser. No. 60/836,609, filed Aug. 9, 2006.
| Number | Name | Date | Kind |
|---|---|---|---|
| 1710445 | Becket | Apr 1929 | A |
| 1836317 | Franks | Dec 1931 | A |
| 3203792 | Scheil et al. | Aug 1965 | A |
| 3838981 | Foley et al. | Oct 1974 | A |
| 4043810 | Acuncius et al. | Aug 1977 | A |
| 4129464 | Matthews et al. | Dec 1978 | A |
| 4245698 | Berkowitz et al. | Jan 1981 | A |
| 4533414 | Asphahani | Aug 1985 | A |
| 4818486 | Rothman et al. | Apr 1989 | A |
| 4906437 | Heubner et al. | Mar 1990 | A |
| 5019184 | Crum et al. | May 1991 | A |
| 5374323 | Kuhlman et al. | Dec 1994 | A |
| 5417918 | Kohler et al. | May 1995 | A |
| 5429690 | Heubner et al. | Jul 1995 | A |
| 5529642 | Sugahara et al. | Jun 1996 | A |
| 6280540 | Crook | Aug 2001 | B1 |
| 6503345 | Klarstrom | Jan 2003 | B1 |
| 6544362 | Pike et al. | Apr 2003 | B2 |
| 20030051783 | Pike, Jr. et al. | Mar 2003 | A1 |
| 20030070733 | Pike, Jr. et al. | Apr 2003 | A1 |
| 20030084975 | Pike et al. | May 2003 | A1 |
| 20030091460 | Klarstrom | May 2003 | A1 |
| 20050053513 | Pike, Jr. | Mar 2005 | A1 |
| Number | Date | Country |
|---|---|---|
| 0 365 884 | Dec 1993 | EP |
| 0 628 088 | Apr 1997 | EP |
| 869753 | Jun 1961 | GB |
| 2 003 179 | Mar 1979 | GB |
| Number | Date | Country | |
|---|---|---|---|
| 20080038148 A1 | Feb 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60836609 | Aug 2006 | US |
