Title of Invention

MONEL ALLOY RESISTANT TO STRESS CORROSION CRACK IN HYDROFLUORIC ACID.

Abstract

Monel alloy, in particular Monel 400, which is stress corrosion crack resistant in hydrofluoric acid or where the acid formation is a possibility through chemical reactions with fluoride salts and its process for manufacture. The stress corrosion crack protected Monel-400 is selected from atleast one of (i) Monel-400 cold worked atleast 60% (ii) grain refined Monel-400 and (iii) Monel-400 galvanic coupled with a protective metal which is anodic to Monel-400.The Monel alloy is directed to wide variety of industrial applications even in environments where the alloy comes in direct contact with hydrofluoric acid. It would thus be possible to use of articles / components made of Monel 400 in conditions wherein the said articles would come in direct contact with hydrofluoric acid.

Full Text

FIELD OF INVENTION The present invention relates to a variety of an alloy metal and in particular, to Monel alloy which is stress corrosion crack resistant in hydrofluoric acid or where the acid formation is a possibility through chemical reactions with fluoride salts. The Monel alloy and more particularly, the Monel 400 of the invention which is stress corrosion crack resistant is directed to wide variety of industrial applications even in environments where the alloy comes in direct contact with hydrofluoric acid. The invention is also directed to a simple and effective manner of manufacture of Monel-400 to make it resistant to stress corrosion cracking in hydrofluoric acid and facilitate production of Monel-400 for more reliable and wider applications in environments where the alloy is subject to exposure to stress corrosion conditions. BACKGROUND ART Monel-400 is known to be naturally resistant to hydrofluoric acid (HF) but stress corrosion cracking (SCC) occurs in it when stress is present. Such stress corrosion of Monel 400 in Hydrofluoric acid was reported by Frazer, O.B.3., "Symposium on Stress Corrosion of Metals" ASTM-AIME, Philadelphia (1944). Copson and Cheng, "Corrosion 12, 647t (1956)" also disclosed and reported the failure of the alloy 400 in hydrofluoric acid vapour where the U-bend specimens were exposed over 48 % hydrofluoric acid in closed polyethylene containers partially immersed in a water bath 60°C. It was further reported that the mode of cracking was intergranular (IG) in the annealed material but transgranular (TG) in cold drawn materials. The SCC of Monel-400 in other environment media such as in caustic solutions, in primary water and in mercury salts have also been reported. Everhart and Price "Corrosion Testing and Evaluation: Silver Anniversary Volume, ASTM STP 1000, R. Baboian and S.W. Dean, Eds, ASTM, Philadelphia (1990)" have also reported use of slow strain testing (SSRT) to investigate SCC of Monel-400. They have reported no cracking in 26% of HF solution but the development of extensive transverse cracks of up to 3 grains deep in 0.063M CuF2 + 0.38M HF solution. In this case the cracking was inter granular (IG) at low strain levels in the specimens but trans granular (TG) at higher strains. It is also known as suggested by Boyd and Berry "Stress Corrosion Cracking-A state of art" ASTM, Philadelphia (1972)" based on the reduction of susceptibility to TGSCC in Fe-Ni-Cr alloys with higher nickel addition due to an increase in the stacking fault energy that this increase in stacking fault energy of nickel would account for the predominantly intercrystalline cracking of nickel base alloys. Also a transition in the mode of cracking in cold worked alpha brass in ammoniacal solution is also reported (Althof, F.C., Z. Metallkunde, 36,177 (1944); Chatterjee, U.K., PHD thesis, IIT Kharagpur 1968). Presumably, beyond a critical value of cold deformation, the increasing structural heterogeneity inside the grains reach a condition when the grain interior becomes baser with respect to grain boundaries. The dissolution is favoured more inside the grains creating sites for transgranular cracking. It is well known Monel-400 has many industrial applications in direct contact hydrofluoric acid or where the acid formation is a possibility through chemical reaction with fluoride salt. In particular it is known to be useful in the fluorination of uranium oxide in the production steps of Uranium but is found to be vulnerable to SCC. There is thus a requirement in the art to provide for a Monel variety which could guard against such SCC susceptibility and favour a much wider and reliable effective use of the alloy. OBJECTS OF THE INVENTION It is thus the basic of the present invention to provide for Monel alloy with stress corrosion crack resistant characteristic so that it can be effectively and advantageously used also in environments were it is in direct contact with hydrofluoric acid or where the acid formation is a possibility through chemical reactions with fluoride salts. Another object of the present invention is directed to provide stress corrosion crack resistant Monel-400 by way of simple and cost effective process such that Monel-400 which would be adapted for wide variety of applications including industrial applications in direct contact in hydrofluoric acid or where such acid formation is possibility due to chemical reaction with fluoride salt. Another object of the present invention is directed to effectively and advantageously make use of Monel-400 in adverse conditions subject to exposure to hydrofluoric acid especially such as in the fluorination of Uranium Oxide in the production of Uranium and resist the problem of component vulnerability due to stress corrosion cracking. 2 Yet another object of present invention is directed to advantageous use of articles / components made of Monel 400 in conditions wherein the said articles would come in direct contact with hydrofluoric acid. SUMMARY OF THE INVENTION Thus according to the basic aspect of the present invention there is provided Monel alloy rendered resistant to stress corrosion cracking in hydrofluoric acid comprising Monel alloy, basically a Nickel-Copper alloy having elemental constitution in the range of Ni 61-67 % by wt., Cu 28-32 % by wt., Fe 0.7-2 % by wt., Mn. 0.7-1% by wt., Si 0.1-0.5% by wt., Ti In accordance with yet another aspect the stress corrosion crack resistant Monel-400 comprise Monel-400 galvanic coupled with stainless steel preferably 316 stainless steel. In the above disclosed stress corrosion crack resistant Monel-400 the Monel alloy comprises elemental constitution of Ni in the range of 63 to 66% by weight. Cu in the range of 29 to 31 % of weight. Fe in the range of 0.9 to 2 % of weight, Si in the range of 3 to 4 % of weight. Al in the range of 2 to 3% by weight, C in the range of 0.10 to 0.15 % by weight, Mn less than 1% by weight, Ti and Co in the range of 0.05 toO.1% by weight. In accordance with the preferred aspect the Monel-400 stress corrosion crack resistant in hydrofluoric acid comprises Monel-400 alloy having and elemental constitution of Ni 64.89% by weight, Cu 31.80% by weight, Fe 1.95% by weight, Mn 0.73% by weight, Si 0.12% by weight, Ti 0.05% by weight, S 0.004% by weight, Al 0.07% by weight, Co 0.09% by weight and C 0.122% by weight. Importantly the above disclosed Monel-400 of the invention provides for improvement in ductility ratio (DR) of the alloy thereby confirming the desired decrees in the SCC 3 sususceptibility of the alloy. In accordance with a preferred aspect the Ductility Ratio (DR) of the Monel-400 is above 0.85. In accordance with another aspect of present invention there is disclosed a process for rendering Monel alloy, basically a nickel-copper alloy, with elemental constitution in the range of Ni 61-67 % by wt., Cu 28-32% by wt, Fe 0.7-2% by wt, Mn 0.7-1% by wt, Si 0.1-0.5% by wt, Ti which comprises subjecting the alloy to treatment selected from any one or more of the following: (a) subjecting the said alloy to cold working to at least 60% by cold rolling for sheets and by cold drawing though dies for bars or rods, (b) subjecting the said alloy to grain refinement to 4 micron diameter or less ; and (c) subjecting the said alloy to galvanic coupling with a metal which is anodic to the said alloy. In accordance with a preferred aspect the process for manufacture of stress corrosion crack resistant Monel comprising subjecting Monel 400 to treatment selected from any one or more of tne following; (a) subjecting the Monel 400 to step of cold working to at least 60% (b) subjecting the Monel 400 to grain refinement to less than 6micron and more preferably less than 4micron; and (c) subjecting the Monel 400 to galvanic coupling with a protecting metal such as 316 SS which is anodic to Monel alloy. It is found that 60% cold working provides the much required immunity against stress corrosion cracking in hydrofluoric add. Cold working higher than 60% would also enable maintaining the intended immunity from stress corrosion cracking. As regards the other aspect of prevention of stress corrosion cracking by controlling the grain size it is found that while a grain size of less than 6 microns and more preferably less than 4 microns is more advantageous to resist stress corrosion cracking. The galvanic coupling of Monel to arrest stress corrosion cracking can be achieved by carrying out galvanic coupling involving a protective metal which should be anodic to the Monel alloy being protected. For such galvanic coupling of Monel stainless steal can be advantageously used. In accordance with a preferred aspect of the invention the method of manufacture of stress corrosion crack resistant Monel in hydrofluoric acid can be obtained by subjecting 4 the Monel alloy to a combination of 60% of cold working and galvanic coupling or small grain size and galvanic coupling for better protection. In accordance with yet another aspect of the present invention there is provided use of Monel 400 stress corrosion crack resistant in hydrofluoric acid for use as components immune to 5CC in hydrofluoric acid such as in fluorination of uranium oxide in production of Uranium. The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to non limiting exemplary illustrations of the Monel alloy of the invention and its manner of manufacture as per the following example: EXAMPLES: A selective composition of Monel-400 was used to carry out the following exemplary illustration. The elemental constitution of the Monel-400 used is given in Table 1 hereunder. Table 1 Element Weight % Ni 64.89 Cu 31.80 Fe 1.95 Mn 0.73 Si 0.12 Ti 0.05 S 0.004 A1 0.07 Co 0.09 C 0.122 Under the following Examples I to III the stress corrosion crack characteristic of Monel-400 after hot rolling, after 50% cold working and after 60% work condition respectively were studied. For the purpose of stress corrosion crack study in hydrofluoric acid tests were conducted in a CORTEST slow strain rate testing machine. Cylindrical specimens of 60 mm length, 4 mm diameter and 25mm gauge length were use. The corrodent 40% HF was contained in a temperature vessel made of Hastelloy. The level of acid in the vessel was 5 maintained at just below the gauge length of the specimen. The solution was heated to a temperature of 70 degree centigrade by external electrical heating of the container. The result of stress corrosion crack test were noted as discussed hereunder: Example I: SCC result in Monel-400 obtained after hot rolling. Under this example the above Monel-400 composition as obtained after hot rolling was studied to determined its stress corrosion crack characteristics. The results obtained and in particular the SSRT data noted were as shown in Table 2 hereunder Table 2: SSRT data for Monel-400 (hot rolled condition) S.No. Medium Galvanic Coupling UTS fMpa) S.R Ductility % elong D.R Strain Rate Remarks 1 Air - 622 - 20 - 3.327X10-6 - 2 HF at 70°C - 698.26 1.12 13.92 0.696 " IG 3 HF at 70°C - 654.04 1.05 13.05 0.652 " IG 4 HF at 70°C SS316 618.4 0.994 17.3 0.865 " IG 5 HF at 70°C SS316 628.69 1.01 19.7 0.985 " IG As would be apparent from the result in Table 2 above, a considerable low value of ductility ratio (OR) shows that the material is susceptible to SCC under the test condition.The cracking mode is intergranular. Metaiiographic and Factrographic evidences of such intergranular cracking are further shown in Figures 1 and 2 respective. Example II: SCC result of 50% cold work material. The SSRT data noted under the above condition are reproduce in Table 3 hereunder : Table 3: SSRT data for Monel-400 (50% cold worked). S.No. Medium Galvanic Coupling UTS (MPa) S.R. Ductility D.R Strain Rate Remark s 1 Air - 757.36 - 20.15 - 3.327x10-6 2 Air - 736.59 - 18.04 - " 3 HF at 70°C - 667.97 0.89 14.03 0.73 " TG 4 HF at 70°C - 622.51 0.83 15.15 0.79 " TG As would be evident from the results in Table 3 above, even 50% cold working can not arrest of problem of stress corrosion cracking of Monel-400 in hydrofluoric acid . The factrographic shown in Figure 3 gives evidence of transgranular cracking. 6 Example III : SCC result of 60% cold work material. SSRT data obtain of 60% cold worked material in accordance with the present invention are presented in Table 4 hereunder. Table 4: SSRT data for Monel-400 (60% cold worked) S.N 0. Mediu m Galvanic Coupling UTS (MPa) S.R. Ductilit y D.R Strain Rate Remarks 1 Air - 831.88 - 15.6 - 4.05X10-6 2 Air - 819.55 - 15.97 - " 3 HF at 70°C 833.95 1.017 19.14 1.19 " NC (No Cracking) 4 HF at 70°C - 702.35 0.857 16.89 1.057 " NC 5 HF at 70°C 787.34 0.960 7 16.04 1.004 " NC 6 Air 798.5 - 14.08 - 3.327x10-6 - 7 HF at 70°C 731.5 0.916 14.01 0.946 " NC 8 Air 897.42 - 17.28 - 2.314x10-6 - 9 HF at 70°C 860.7 0.96 15.45 0.89 2.314x10-6 NC 10 HF at 70°C 790.53 0.88 16.52 0.95 2.314x10-6 NC As would be apparent from the results in Table 4, the 60% of cold working of Monel-400 in accordance with the invention achieved a ductility ratio (DR) around 1 indicating the material has been rendered immune to the SCC. The above therefore sufficiently confirm the advantages following 60% cold working in Monel-400 which avoids the problems of stress corrosion cracking in hydrofluoric acid. Example IV : Effect of grain size Under the example the possible arresting of stress corrosion cracking by way of refined grain size of the Monel-400 in accordance with a further aspect of invention was studied. For the purpose 50% cold worked material which showed under example II above to be susceptible to SCC was heat treated at different temperature for varied lengths of time in a tube furnace flushed with hydrogen gas. Grain size obtained was varied from 4 to 13 microns. The SCC susceptibility of such varied grain sized Monel-400 was noted. As represented in Figure 4, it was found that decrease in the grain size led to increase in DR and therefore decreased SCC susceptibility. Importantly at the lower grain size 4 micron there was a surprising and synergetic drastic reduction in susceptibility to SCC. 7 Example V : Effect of galvanic coupling Under this example the effect of galvanic coupling of Monel 400 with 360 stainless steel was studied. For the purpose 60% cold worked Monel was used and the SSRT data was noted as shown in Table 5 hereunder. Table 5 : SSRT data for Monel-400 coupled with SS316 S.No. Medium Galvanic Coupling UTS (MPa) S.R. Ductility D.R Strain Rate Remarks 1 HFat 70°C SS316 704.02 0.859 15.087 0.945 4.05X10-6 NC 2 HF at 70°C SS316 769.5 0.938 19.35 1.21 " NC 3 HF at 70°C SS316 640.86 0.782 16.88 1.057 " NC As shown in Table 5 above galvanic coupling was found to the effective in arresting SCC susceptibility (DR=0.86-0.98) of Monel-400 in HF vapour. It is found that use of stainless steel- Monel-400 coupling safeguards the Monel-400 from corrosive attack in HF environment. The above therefore clearly and sufficiently confirms the advantages in arresting and controlling the stress corrosion cracking in Monel-400 involving any one and or more of (i) 60% of more cold working to render immune to SCC of Monel-400 (ii) decreasing the grain size to less than 6 micron preferably less than 4 micron to surprisingly and drastically arrest susceptibility to SCC of Monel (iii) galvanic coupling of Monel-400 to 316 stainless steel to appreciably reduce the susceptibility to SCC. The invention therefore favours advantageous and wide scale application of Monel-400 specially in industrial application in direct contact with hydrofluoric acid or where acid formation is a possibility through chemical reaction with fluoride salt. Importantly and more particularly the present invention would enable use of Monel-400 based components in environments subject to generation /presence of hydrofluoric acid such as in the fluorination of Uranium oxide in the production steps of Uranium without the problems of SCC susceptibility. 8 WE CLAIM: 1. Monel alloy rendered resistant to stress corrosion cracking in hydrofluoric acid comprising Monel alloy, basically a Nickel-Copper alloy having elemental constitution in the range of Ni 61-67 % by wt., Cu 28-32 % by wt., Fe 0.7-2 % by wt., Mn. 0.7-1% by wt., Si 0.1-0.5% by wt., Ti 2. Monel alloy rendered resistant to stress corrosion cracking in hydrofluoric acid as claimed in claim 1 comprising said Monel alloy galvanic coupled with stainless steel, preferably 316 stainless steel. 3. Monel alloy rendered resistant to stress corrosion cracking in hydrofluoric acid as claimed in anyone of claims 1 to 2 wherein said Monel alloy comprises elemental constitution of Ni 64.89 % by weight. Cu 31.8 % of weight. Fe 1.95 % of weight, Si 0.12 % of weight. Al 0.07 % by weight, C 0.122 % by weight, Mn 0.73 % by weight, Ti 0.05 % by wt., S 0.004% by wt. and Co 0.09 % by weight. 4. A process for rendering Monel alloy, basically a nickel-copper alloy, with elemental constitution in the range of Ni 61-67 % by wt., Cu 28-32% by wt, Fe 0.7-2% by wt, Mn 0.7-1% by wt, Si 0.1-0.5% by wt, Ti by wt, Co 0.1-0.2% by wt and C in hydrofluoric acid, which comprises subjecting the alloy to treatment selected from any one or more of the following: (a) subjecting the said alloy to cold working to at least 60% by cold rolling for sheets and by cold drawing though dies for bars or rods, (b) subjecting the said alloy to grain refinement to 4 micron diameter or less; and (c) subjecting the said alloy to galvanic coupling with a metal which is anodic to the said alloy. 5. A process as claimed in claim 4, wherein the said metal which is anodic to the said alloy used is Type 316 stainless steel, 6. A process as claimed in anyone of claims 4 to 5, wherein the said alloy used comprises elemental constitution of Ni 64.89%, Cu 31.8%, Fe 1.95%, Mn 0.73%, Si 0.12%, Ti 0.05%, S 0.004%, Al 0.07%, Co 0.09% and C 0.122% (all wt%), 9 10 7. A process as claimed in anyone of claims 4 to 6, carried out such that the ductility ratio (DR) of the alloy is above 0.85 in slow strain rate test (SSRT), 8. A process for rendering Monel resistant to stress corrosion cracking, as claimed in anyone of claims 4 to 7, comprising the lowering down of grain size to 4 micron or below, as a singular measure, 9. A process for rendering Monel resistant to stress corrosion cracking, as claimed in anyone of claims 4 to 7, comprising cold working Monel to 60% or higher, as a singular measure. 10. A process for rendering Monel resistant to stress corrosion cracking, as claimed in anyone of claims 4 to 7, galvanic coupling with Type 316 SS, as a singular measure. 11. A process for rendering Monel resistant to stress corrosion cracking, as claimed in anyone of claims 4 to 7, comprising subjecting the said alloy to a combination of cold working 60% or more and galvanic coupling with Type 316 SS or small grain size of 4 micron and lower and galvanic coupling with Type 316 SS. 12. A process for rendering Monel resistant to stress corrosion cracking, as claimed in anyone of claims 6 to 9, comprising subjecting the said alloy to a combination of cold working of 60% or more and galvanic coupling with Type 316 SS, with or without a grain size of 4 micron or below, 13. A process for rendering Monel resistant to stress corrosion cracking, as claimed in anyone of claims 4 to 7, comprising subjecting the said alloy to a combination of galvanic coupling with Type 316 SS and controlling of grain size to 4 micron or below, with or without a of cold working of 60% or more, 14. A process for rendering Monel resistant to stress corrosion cracking in hydrofluoric add substantially as herein described and illustrated with reference to the accompanying examples. Monel alloy, in particular Monel 400, which is stress corrosion crack resistant in hydrofluoric acid or where the acid formation is a possibility through chemical reactions with fluoride salts and its process for manufacture. The stress corrosion crack protected Monel-400 is selected from atleast one of (i) Monel-400 cold worked atleast 60% (ii) grain refined Monel-400 and (iii) Monel-400 galvanic coupled with a protective metal which is anodic to Monel-400.The Monel alloy is directed to wide variety of industrial applications even in environments where the alloy comes in direct contact with hydrofluoric acid. It would thus be possible to use of articles / components made of Monel 400 in conditions wherein the said articles would come in direct contact with hydrofluoric acid.

Documents:

00384-kol-2004-abstract.pdf

00384-kol-2004-claims.pdf

00384-kol-2004-correspondence.pdf

00384-kol-2004-description(complete).pdf

00384-kol-2004-form-1.pdf

00384-kol-2004-form-13.pdf

00384-kol-2004-form-18.pdf

00384-kol-2004-form-2.pdf

00384-kol-2004-form-3.pdf

00384-kol-2004-letters patent.pdf

00384-kol-2004-p.a.pdf

384-KOL-2004-CORRESPONDENCE 1.1.pdf

384-KOL-2004-FORM 27.pdf

384-kol-2004-granted-abstract.pdf

384-kol-2004-granted-claims.pdf

384-kol-2004-granted-description (complete).pdf

384-kol-2004-granted-form 2.pdf

384-kol-2004-granted-specification.pdf