US3573901A

US3573901A - Alloys resistant to stress-corrosion cracking in leaded high purity water

Info

Publication number: US3573901A
Application number: US743674A
Authority: US
Inventors: George Economy
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1968-07-10
Filing date: 1968-07-10
Publication date: 1971-04-06
Anticipated expiration: 1988-04-06

Abstract

A HIGH NICKEL ALLOY CONTAINING SUBSTANTIAL BUT CONTROLLED AMOUNTS OF CHROMIUM, E.G., 28%, AND ADVANTAGEOUSLY IRON, HABING IMPROVED RESISTANCE TO STRESS-CORROSION CRACKING WHEN USED IN HIGH PURITY WATER ENVIRONMENTS CONTAMINATED BY LEAD. IRON IS BENEFICIAL, PARTICULARLY IN MINIMIZING SCALE FORMATION. OTHER CONSTITUENTS CAN BE UTILIZED TO ADVANTAGE, INCLUDING COLUMBIUM, MOLYBDENUM, VANADIUM, AND TUNGSTEN.

Description

April 6 1971 Maven/r eace-fir Gwen/0M ALLOYS RESISTANT IO STRESS-CORROSION CRACKING s ECONOMY 3,573,901

IN LEADED HIGH PURITY WATER Filed July 1.0, 1968 Z 4 6 5 10 I2 l4 l6 l8 Z0 Z2 24 6 M96;- Papas/yr av INVENTOR. 660/966 [CO/YOM) BY h -Fwd 14 T P YD United States Patent 3,573,901 ALLOYS RESISTANT TO STRESS-CORROSION CRACKING IN LEADED HIGH PURITY WATER George Economy, Monsey, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y.

Continuation-impart of abandoned application Ser. No.

653,665, July 17, 1967. This application July 10, 1968,

Ser. No. 743,674

Int. Cl. C22c 19/00 US. Cl. 75-171 39 Claims ABSTRACT OF THE DISCLOSURE A high nickel alloy containing substantial but controlled amounts of chromium, e.g., 28%, and advantageously iron, having improved resistance to stress-corrosion cracking when used in high purity water environments contaminated by lead. Iron is beneficial, particularly in minimizing scale formation. Other constituents can be utilized to advantage, including columbium, molybdenum, vanadium, and tungsten.

This application is a continuation-in-part of application Ser. No. 653,665, filed July 17, 1967, now abandoned.

Some years ago, circa 1960, it was reported in the literature that, based upon laboratory tests, nickel-chromium-iron alloys, 75% to 80% nickel, 14% to 16% chromium and up to 7% or 8% iron, manifested an ostensible susceptibility to undergo intergranular stress-corrosion cracking in pressurized water of high purity at elevated temperatures. Theretofore, it was considered that such alloys were immune to this form of attack in view of their outstanding record in service, a record still intact insofar as I am aware. However, the fact that these alloys performed vital roles in pressurized water systems, e.g., reactor components and the like, coupled with the serious ramifications of possible failure in service, dictated that research efforts be conducted to explore whether indeed a problem existed, and, if so, to devise appropriate solutions thereto.

As a result of intensive investigation, it was determined that provided certain environmental conditions existed, including oxygen (air or otherwise) contamination of the pressurized water (temperature above about 300 F., e.g., 600 F.) plus alloy surface defects (crevices), cracking could be brought about under conditions of high stress. And as a result of that research effort it was discovered that close compositional control of various con stituents, notably titanium, aluminum and silicon, or alternatively, the use of higher chromium contents, markedly enhanced resistance to stress-corrosion attack under such conditions of operation.

During the course of the investigation referred to above, it was also found that nickel-chromium and nickel-chromium-iron alloys of the type above mentioned were prone to stress-corrosion cracking in pressurized high purity water containing lead as a contaminant as well as air. In a paper presented before the annual conference of National Association of Corrosion Engineers in 1964 and which appeared in Corrosion, vol. 21, page 3 (1965), H. R. Copson and S. W. Dean (co-inventors of one solution to the aerated water problem) demonstrated that materials such as lead powder, lead oxide, etc., functioned subversively by actually promoting stress-corrosion cracking, notwithstanding that the alloys were characterized by the absence of detrimental surface defects.

Subsequent experimentation has since shown that cracking can be induced in lead-contaminated de-aerated water (lead being the only known contaminant). This finding was of noteworthy interest in many respects, including the determination that various alloys which con- Patented Apr. 6, 1971 ferred substantially improved resistance to cracking in aerated high purity water offered no appreciable improvement over conventional alloys when utilized in lead-contaminated circuits. Indeed, even the mode of failure appeared to be somewhat different, for lead contamination caused transgangular cracking in contrast with the intergranular type experienced with aerated water. At any rate, while the possibility of air incidents occurring in pressurized water reactors was and is indeed small (in boiling water reactors air would normally be present), there is a recognized danger of lead contamination in hot water systems. As a consequence, rigid precautions are undertaken to exclude lead incidents even to the point of using lead-free pipe joint compounds. But the fact that contamination can occur is no less real.

With the foregoing in mind, it would, of course, be obviously desirable to have available an alloy which exhibited substantial resistance to stress-corrosion cracking in pressurized water whether it be contaminated by lead, air, or both, and whether surface defects be present in the material or not. This would greatly obviate concern that might stem from uncertainties as to what type of cracking might arise. In any event, a possible approach to the problem seemed to suggest itself from prior research. Of the two solutions to the air contamination difficulty discussed above herein, the higher chromium content concept seemed to be the more attractive from a commercial viewpoint. Thus, an investigation was initiated to determine whether such alloys would resist stress-corrosion cracking induced by lead contamination. This avenue of pursuit did not prove to be a panacea unto itself. Other problems were soon apparent. Moreover, as will be shown herein, the highest chromium contents brought on an undesirable scaling problem.

It has now been found that certain nickel-chromium and nickel-chromium-iron alloys of specially balanced composition, particularly in respect of nickel content, afiord outstanding resistance to stress-corrosion cracking in pressurized water notwithstanding the fact that lead be present as a contaminant. Furthermore, the aforementioned ditficulty attendant excessive scaling can be substantially eliminated, if not completely obviated. It is to be understood, however, that while the invention is primarily directed to overcoming the lead problem, it is not restricted thereto. Certain special alloys within the invention also afford outstanding resistance to stresscorrosion cracking in aerated high purity water and in chloride media generally. In this connection, it might be mentioned that chlorides are an additional possible source of high purity water contamination.

It is an object of the invention to provide new and improved nickel-chromium and nickel-chromium-iron alloys.

It is another object of the invention to provide nickelbase alloys which exhibit a high degree of resistance to stress-corrosion cracking when in contact with lead-contaminated high purity water environments.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which various relationships, as explained herein, are graphically depicted.

Generally speaking and in accordance herewith, stresscorrosion cracking of nickel-base alloys in lead-contaminated water, particularly at temperatures of about 300 F. to 660 F., e.g., about 450 F. or 500 -F. to about 625 F., can be substantially lessened by using alloys of the following composition (based on weight percent): about 24% to not more than 32% and, for best results, not in excess of 30%, chromium; from 50% to less than 67% nickel; up to 10%, e.g., 1% to 8%, molybdenum;

up to 4% or 6%, e.g., 1% to 4%, columbium; up to 0.1% carbon, up to 1% or 2% each of titanium, aluminum and manganese; up to 2.5%, e.g., up to 1% silicon; and the balance essentially iron, the iron being not more than 25%, e.g., not exceeding about 20% Or but beneficially being present in an amount of at least 4%. Vanadium or tungsten in an amount up to 10% individually, e.g., 1% to 8% of each, can be present in the alloys to considerable advantage; however, the total sum of these constituents together with molybdenum and/or columbium should not exceed about In consistently achieving highly satisfactory results, it is of quite significent benefit that the chromium, iron, columbium, molybdenum, vanadium and tungsten be correlated such that the following stress-cracking relationship, SC, is satisfied percent Cr+0.25 (percent Fe) (0.9) (percent Mo) +percent Cb+ 1.25 (percent VH-percent W) is at least 28% and most advantageously greater than 30.5%. (In the relationship expressed above, and wherever it appears herein, the coefficient 0.25 can be varied by $0.02, the coefficient 0.9 by $0.1, the coefficient unity before Cb by $0.1, and the coefficient 1.25 by $0.25.)

In accordance with the present invention, it has been found that nickel contents in excess of about 67% apparently promote stress-corrosion cracking (or at least do not assist in overcoming or suppressing such cracking) of nickel-chromium and nickel-chromium-iron alloys in lead-contaminated high purity water at elevated temperatures, an effect not experienced in aerated pressurized water. The theoretical explanation for this phenomenon is not at present completely understood. In any event, it is most advantageous that the nickel content not exceed 65% or 66% but at least 50%, e.g., at least 52% or 55%, should be present to provide satisfactory mechanical and other characteristics, and to assure, in applications so requiring, good resistance to chloride stress-corrosion cracking.

As indicated above herein, should chromium beto the excess, an undue amount of scale forms. In the absence of lead, there is apparently little problem in this respect indicating some form of chemical activity ensues once lead is present. Whatever be the explanation, it has been ascertained, for example, that high chromium contents, say about 35%, result in a detrimentally high degree of scaling. With time this scale exfoliates and serves to introduce a crud problem, a severe drawback in a reactor system. However, it has been found that the amount of scale formation is considerably reduced with chromium contents not exceeding about 30%, although it can be as high as 32% particularly in the presencev of at least 10% iron. Iron, as will be illustrated herein, restricts or inhibits the tendency for scale to form at the higher chromium levels, i.e., upwards of 26% or 28% to 30% chromium, and also contributes to the overall resistance of the alloys to stress-corrosion attack in lead-contaminated high purity water, although to a significantly lesser extent than does chromium. Accordingly, it is highly desirable that the alloys contain 4% or more of iron and up to as high as 15% or or possibly as high as particularly when the chromium content is at least 26% (an amount above which gives rise to the beginning of a scale problem). Together With nickel at the higher end of the nickel range, substantial amounts of iron tend to exert somewhat of a negative influence with regard to stress-corrosion cracking; thus, it is beneficial that the sum of the nickel plus 0.7 times the iron content not exceed 70%.

At least 24%, and for best results at least 26%, chromium should be present to assure highly satisfactory resistance to stress-corrosion cracking in lead-contaminated pressurized water, good strength characteristics and overall resistance to corrosive media generally. However, it can be as low as 22% when the alloys contain an equivalent amount by weight of moly denum nd 0r colum- 4 bium as illustrated hereinafter, and particularly when the stress-cracking relationship, SC,

percent Cr+0.25 (percent Fe) (0.9) (percent Mo) +percent Cb+1.25 (percent V i-percent W) is greater than 30.5%, e.g., at least about 31%. It might be mentioned, however, that provided this relationship is observed, chromium contents below 22% and even down to near 16% might be used if the only danger of contamination was from lead. But entertaining such a risk is neither warranted nor recommended. Columbium, molybdenum, vanadium and tungsten, in addition to their capability of resisting stress-corrosion cracking in leadcontaminated water, confer strengthening characteristics, resistance to other corrosive media, etc. Accordingly, alloys containing at least 2% or 4% of one or more of these constituents are of exceptional utility. Moreover, should the nickel exceed about 65% or 65.5%, it is of benefit that the alloys contain at least 3% and advantageously at least 6% of one or more of the constituents columbium, molybdenum, vanadium and tungsten. This has the advantage of being able to use lower chromium contents and thus lessen the tendency for scale to form.

The capability of alloys contemplated herein to resist stress-corrosion cracking and scale formation in leadcontaminated pressurized water is graphically illustrated in the accompanying drawing wherein there is depicted a correlation between chromium, and iron, nickel being essentially the base. The drawing also graphically delineates alloys which consistently afiord an outstandingly high degree of resistance to cracking in chloride environments. Turning 'first to the line AB, the region to the left and upwards thereof indicates those alloys in which severe and objectionable scaling occurs. While between lines AB and CD a moderate degree of scaling results, it is much preferred to control the percentages of chromium and iron as to represent a point at least below CD. Between CD and EF scaling is very slight and below EF, particularly with chromium at a maximum of 30%, no scaling of any consequence is encountered. Thus, an ironfree alloy containing about 35% chromium would be expected to exhibit a severe scaling problem. Similarly, an alloy containing 29% chromium and 10% iron should afford a greater degree of scale resistance than an ironfree alloy containing the same chromium content. The data set forth in Table II confirm these points.

In respect of resistance to lead cracking, the area to the right and above line GH represents those alloys which offer exceptional resistance to lead attack. This line is represented by the equation percent Cr+0.25 (percent Fe)+percent Me is greater than 30.5%

The value for the symbol Me [Me is the same as the component(0.9) (percent Mo) +percent Cb+1.25 (percent V+percent W)in the SC relationship discussed above herein] in the above equation is zero (0), but only for the line GH. Thus, it is to be understood that when the alloys contain one or more of the constituents columbium, molybdenum, 'vanadium or tungsten, the line GH is, in eifect, correspondingly lowered downward and to the left in a manner parallel to GH. For example, the line I] is represented by this sameequation but the value of Me, representing the total amount of one or more of the elements molybdenum, columbium, vanadium and tungsten, is 2%. Similarly, the value for Me in respect of line KL is 4%. As with the case of the area below and to the left of line GH, the areas below lines I] and KL, respectively (depending upon the value of Me present), are indicative of less crack-resistant alloys. As to stress-corrosion attack induced by chlorides, alloys to the right of line MN are more prone or susceptible to this form of attack.

For alloys capable of delivering the greatest resistance to both lead and chloride attack and also to sc li g, he

, 600 F. plus or minus 5. Overall, the

Ti, Other, Annealed 1; percent percent percent plus H.T. Annealed 0.15 OK(4) K(2) Subsequent to the foregoing processing, the specimens were formed into single U-bends and bolted to achieve and maintain a highly stressed condition. Distilled and deionized water was placed into a one gallon autoclave 5 together with about grams of metallic lead powder used as the contaminant. The U-bends were suspended on a fixture and then immersed in the autoclave solution. The autoclave was thereafter sealed, pressurized to 500 pounds per square inch (p.s.i.) with argon or hydrogen,

10 vented to a pressure of one atmosphere and aspirated. This procedure was twice repeated, ending with one atmosphere of argon or hydrogen whereby virtually all air was removed. The autoclave was brought to test temperature, to wit two-week periods. At the end of each of the first three two-week intervals, the autoclave was cooled to room temperature, opened, and the samples removed and visually examined at 45 magnifications (45 x) for evibetter appreciation of the invention the following illusdcnce of cracking. If a Crack Wa Clearly evident, the specimen was removed from test and the depth of crack determined. The remaining specimens were put back into test using fresh solution and lead metal powder. At the end of the final test period, the specimens were again ing were (a) soaked at about 2200 F. to 2300 F. for visually examined at 45X. If cracks were observed, depth thereof was measured. If cracks were not visually detected, the specimens were sectioned in several places and given a thorough metallographic examination at a magnification of at least 125x. Crack depths were rolled about 40% to a thickness of 0.15 inch, (f) again measured.

With regard to the data given below, the symbol OK denotes that no cracking was observed either visually or metallographically (a numeral following, i.e., OK(2), indicates the number of specimens tested); 2/30, for

an additional heat treatment consisting of heating at example, indicates cracking visual after the first twoweek period and depth of crack was 30 mils (4/ would mean visual crack detected at end of second two-week period and crack depth was 60 mils). 8 m./ 110 indicates crack not visually observed but detected metallographi- TABLE I.COMPOSITION1 Fe, Ni, C, percent percent percen Alloy No.

nickel, chromium, and iron should be correlated such that the alloy falls within the area OPQRO. It is to be understood, however, particularly with regard to the scope of the claims, that when one or more of the elements columbium, molybdenum, vanadium and tungsten are present, the boundary line segment OR would be represented by a line segment downward of and in a manner substantially parallel to OR. Thus, for example, when the alloys contain 2% of rnetal from the group molybdenum, columbium, vanadium and tungsten, this boundary segment line would be O'R rather than OR; the boundary line would be 0"R" and encompass an area O"PQR"O when 4% of such constituents is present in the alloys contemplated, etc. With consideration being given also to pressurized water contaminated by air, the 15 test was conducted for eight weeks consisting of four chromium content should be at least 26% irrespective of a the amount, if any, of metal from the group molybdenum, columbium, vanadium and tungsten.

For the purpose of giving those skilled in the art a trative information and data are given.

A substantial number of alloys was prepared, processed, and tested, the compositions of the alloys being set forth in Table I. Ingots obtained upon melting and coolabout two hours, (b) forged to a thickness of one inch, (c) reheated to about 2200 F. and hot rolled to a thick ness of about A inch, (d) annealed for one hour at about 2100 F. followed by a water quench, (e) cold annealed at 2100" F. and water quenched, and then (g) machined into specimen blanks approximately 3.25 inches in length, 0.5 inch in width and 0.12 inch in thickness. A substantial number of the specimens were subjected to 1250 F. for two hours and air cooling prior to being machined. Thus, samples were either tested in the annealed and/or annealed plus heat treated (H.T.) conditions.

54\ I\ I )\l\l) .0 .0 .00 .0 0 0 0 me. 0&0. Q @QQQ "B "H HM% "H "m n "w "w msK 0 KKK K KKKK "m m m "m n n H; n; & n "O00 O 0000 ".1" H; A n "w "w 5 W .m v .w .m Me n n n n n U n u "8 m u 0O .m 8 a a S U u U n n u n n n l\I\ I 0 005902059 -)0)000 0 0 Q0 M w 6 n B HHMMHM%BMW%WM%% "m K KK K K K H n m m 1,... O0 0 0O 0 O 0 n 1.. .y .1 .1.: .y m mofinv 5 0 05 5 8 a a a an a 8 n n 8 co 8 88 8 u n n u n n H u n .bOo o oun OJ 0a. .b .oonua II 1 1 1 i 1 I Catcher CWLCVWVSAT "LT .C HMCLCT 3 .5443 flennscesns u u .2 u Je ne 185279 7 .0012311134 4 71 H H "0 .v "15678 ti l all 4 5 3 00 4 6 9 1 1 1 1 2% 2 1 3mm 2 zmmwmwt m nmmmnwmuwzwnammmmmwlumm 0 00 0 0 00 0 0 00900 0 n00 00 00 00 0 0a000o 0 0 0 0 00 7 63 4 5 6 6666 1206253 12696 5965 5 0 00 00a 0 eaodeaa 0 0 00 0 0000a00000a0eao00 7 35 6 7 9003 6 5054414 0 94456000360459850928 L as 6 6 ate 5 5 5 6 0 .s5 .tztssaactta 5 s2 2 5 7 at 3 55 9 "482 0 2524831 2 4647870613319645 1 1 0 aa a 0 3 2 1 sas 6 L0 0 7 0 2 a6 7 0 0 0 1 0 mm 3 44133 8 s 7 121 w n 2 52 5 6 047 6 7930254 1042558906829706209718 7 2 2 m n W 1 Alloys also contained about 0.2% or less of each of silicon and manganese and generally about 0.1% or less of each of aluminum and copper plus other impuritles. Nickel is by difference.

2 Three split heats with carbon contents of about 0.04% to about 0.1%.

7 cally with crack depth being 110 trnils. Alloys 1 to 1 8 are within the invention (alloys 17 and 18 being within the invention only in the broadest sense as explained herein) whereas A through W are outside the scope thereof;

however, the latter alloys afford a reasonably good basis for comparison.

prerequisites of the invention are otherwise met. Aluminum and titanium can be present up to 5% of each where only lead contamination would be a problem and do lessen the susceptibility to cracking; however, they tend to promote cracking in air-contaminated pressurized water at high levels, say, 3% or 4%. Accordingly, for overall best results the presence of these elements should be held to a maximum not exceeding about 1% or 2% each.

In Table II there is reported data concerning scaling effect with respect to alloys prepared and tested in the same manner as Table I (although a lead-free environment was used and is included for purposes of comparison).

TABLE II MDD in 14 days Cr, Fe, Ti, Other, Lead-conleadpercent percent percent percent percent percent taminated free MDD=Milllgrams per square decimeter per day.

curred. This is in marked contrast to alloys within the scope of the invention. The very slight cracking detected after the completion of the full test period concerning alloy 2 was only discernible by metallographic means and, more importantly, the cracks were extremely shallow. This alloy did not exhibit even the shallow cracking in the annealed plus heat treated condition and represents a very substantial improvement over alloys A through W.

Viewed in another perspective, no specimen of any alloy in which the hereindefined stress-cracking relationship, SC, exceeded 30.5 exhibited cracking. That this is not attributable solely to using alloys of higher chromium content than found in alloys commercially employed is reflected by a comparison of alloy 6 (22.6% Cr) and, say, alloy A, B, D, K and L (all of which contain at least 24% Cr). The value for SC in alloy 6 (which contained both columbium and molybdenum) is well over 30.5 but never exceeds about 26.5 for alloys A, B, D, K and L. It will be further observed that alloy 4, the other alloy within the invention containing less than 24% chromium but which contained a substantial amount of molybdenum, also manifested a high degree of resistance in the lead-contaminated environment. Thus, these constituents molybdenum and columbium (also vanadium and tungsten) are not only suitable additions in maintaining the nickel content below about 67% but also are considered beneficial in resisting stresscorrosion cracking which would be otherwise attributable to lead contamination at chromium levels below 24%.

Elements such as cobalt, manganese, copper and tantalum do not contribute to resisting cracking as evident from alloys T, U, V and W. If necessary for other purposes, such constituents can be present provided the It will be observed that a quite substantial amount of scaling formed in respect of alloy AA which contained 35% chromium. While alloys B B through DD (as is alloy AA) are outside the invention by virtue of nickel content, they nonetheless afford a ready comparison with alloy 1 regarding the scale inhibiting effect of iron. A similar comparison can be made between

alloys

15, 8 and 12 and virtually iron-free alloy EE.

The data given in Table III reflect the manner in which the strength characteristics of alloys contemplated within the invention are sharply enhanced by utilization of one or more of the constituents columbium, molybdenum, vanadium and tungsten. Alloy FF, representative of an alloy used extensively commercially, is included for purposes of comparison. The yield and ultimate tensile strengths are given in pounds per square inch (p.s.i.), the elongation and reduction of area being given in percent Results in both the annealed and aged conditions are given. Thus, with regard to the group of alloys FF and 18-21, each of the alloys was annealed at a temperature between 2050 F. to about 2100 F. for about one hour and water quenched. Data was obtained in this (the annealed) condition. Other specimens were thereafter aged at about 1300 F. for about five hours and air cooled and the results were again obtained. A different annealing treatment was used in respect of the group of alloys FF and 22, 11, 23 and 24. In this instance, a temperature of about 1750 F. was used rather than 2050" F. to 2100 F. For convenience, the nickel content is given as the balance (Bal.) of the compositions though small amounts of impurities and incidental elements were also present.

TABLE III Reduction of Yield strength Ult. tens. strength Elongation, percent area, percent Cr, F 0, Others Ni, percent percent percent percent percent Annealed Aged Annealed Aged Annealed Aged Annealed Aged 16 6. 7 O. 05 Bal 35, 800 37, 700 97, 900 96, 800 51, 000 47, 200 67, 000 64, 000 26. 9 11. 3 0. 06 5. 8 Mo 49, 200 51, 500 111, 800 114, 600 52, 700 51, 700 69, 700 65, 500 26. 2 0. 1 0.05 7. 2 V 52, 600 120, 500 00 66, 000 26.7 0. 1 0.06 6. 1 W 52, 600 43,500 26.7 2.0 0.06 4.9 Cb 46,100 57,200

6. 7 0. 05 42, 200 500 99, 700 100, 900 39, 000 27. 6 3. 0 0.05 0. 9 Ch Bal. 70, 600 71, 700 126, 900 129, 700

1. 0 Mo 0. 9 W 11 27. 7 3. 2 0. 05 2. 8 Cb Bal 66, 800 67, 000 128, 128, 400 42, 700 42, 700 58, 000 56, 000 23 27. 8 1. 9 0. 05 gSgVAl Bal--- 68, 200 89, 300 127, 600 145, 700 43, 500 36, 500 61, 700 58, 5 00 24 27. 8 0. 1 0. 05 7. 3 W BaL. 70, 500 74, 200 128, 900 131,000 39, 000 40, 000 61, 500 62, 000

It will be noted that strength levels as much as 50% to 100% above the conventional alloy are obtainable. Since these same alloys offer a high degree of resistance to stress-corrosion cracking, it follows that such alloys can, therefore, take a much greater stress than conventionally used alloys without cracking. Put another way, it is reasonable to expect that higher strength alloys than those currently used may very Well be required for the types of applications referred to herein. Since stress is a contributing cause to cracking, it would appear that additional stress can be applied to alloys of the invention without the likelihood of an increased propensity toward cracking.

Various alloys were also tested for susceptibility to stress-corrosion cracking in chloride environments. In this connection, the well known boiling magnesium chloride test was used. Specimens of Alloys 8 and 9 (three of each), which alloys contain over 50% nickel, were formed into U-bends and immersed in boiling (about 154 C.) magnesium chloride in an autoclave for a period of 30 days. Upon removal therefrom and inspection thereof, no evidence of cracks was ascertained in any of the six specimens. The specimens had been solution treated at 2100 F. for about one hour, water quenched, and thereafter held at 1250 F. for about two hours and air cooled. In contrast thereto, a similarly heat treated and processed alloy but which contained 42.2% nickel, 28.8% chromium, 28.0% iron, 0.05% carbon and minor amounts of aluminum, titanium and impurities exhibited cracking. Each of the three tested specimens cracked and the measured depths ranged from 20 mils to 60 mils.

Particularly good alloys which would afford markedly improved resistance (versus conventionally used compositions) to attack in both airand lead-contamined high purity water environments, chloride media and scale formation in lead-contaminated water consist essentially of a nickel-base and contain from 24% to about 30% chromium, from 4%, e.g., 8%, and up to 20% iron, the nickel, chromium and iron being correlated as to represent a point falling within the area OPQRO' (the boundary line segment OR being as defined herein), up to 4% columbium, up to 8% molybdenum, up to 8% vanadium, and up to 8% tungsten, with the provisos that (a) the sum of the columbium, molybdenum, vanadium and tungsten not exceed and (b) the stress-cracking relationship, SC,

percent Cr+ 0.25 (percent Fe) +(0.9) (percent Mo) +percent Cb+1.25 (percent V-l-percent W) is at least 30.5%, up to 0.1% carbon, up to 1% titanium, up to 1% aluminum, up to 2% manganese and up to 1% silicon, and the balance essentially nickel, the nickel being at least 50% and not exceeding 65%. From 1% to 4% of columbium and/or 1% to 6% each of molybdenum, vanadium, tungsten, the total of the four elements not exceeding 15%, is markedly beneficial.

Another range of composition which would offer an improvement in lead-contaminated water over conventional alloys that might otherwise be employed is as follows: about 58% or 60% to 65% or 66% nickel, 24% to 30% chromium, up to 8% molybdenum, up to 3% columbium, about 0.01% to 0.09% carbon, up to 1% titanium, the balance being essentially iron, the iron being from 4% to 10%. An alloy containing about 28% chromium, about 8% to 13% iron, balance essentially nickel gives excellent results in all aspects regarding cracking with respect to lead, air, or chloride contamination and also scaling.

In terms of application, heat exchangers, pressure vessels, tubing, primary water piping are illustrative of the articles of manufacture which can be produced using the alloys contemplated herein.

Although the present invention has been described in conjunction with lead-contaminated pressurized water systems, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. For example, it is contemplated that alloys in accordance herewith can be used in other fields of application, e.g., the chemical processing and power industries, the high temperature field where stress-rupture characteristics and resistance to oxidation are of importance and generally where severe corrosive environments would be encountered. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1.'A process for improving the resistance to stresscorrosion cracking of metal articles in contact with high purity water, particularly water contaminated with lead, which comprises flowing high purity water past and in contact with such a metal article, the article being formed from an alloy composed of about 24% to 32% chromium, from 50% to less than 67% nickel, up to 10% molybdenum, up to 6% columbium, up to 10% vanadium, up to 10% tungsten with the proviso that the total sum of the molybdenum, columbium, vanadium and tungsten does not exceed 15% and with the further proviso that the chromium, iron, molybdenum, columbium, vanadium and tungsten are correlated such that the stresscracking relationship, SC,

Percent Cr+-0.25 (percent Fe)+( 0.9) (percent Mo) +percent Cb+1.25 (percent V+percent W) is at least 28%, up to 0.1% carbon, up to 5% titanium, up to 5% aluminum, up to 2% manganese, up to 2.5% silicon, and the balance essentially iron, the iron not exceeding 25%.

2. A process in accordance with claim 1 in which the alloy described therein contains from 26% to 30% chromium, 52% to 65% nickel, and in which titanium and aluminum, if any, does not exceed 3% respectively, and the iron content, if any, does not exceed 20%.

3. A process in accordance with claim 2 in which the stress-cracking relationship, SC, is greater than 30.5%.

4. A process in accordance with claim 3 in which the alloy described therein contains at least 4% iron.

5. A process in accordance with claim 3 in which the alloy described therein contains from 8% to 20% iron.

6. A process in accordance 'with claim 3 in which the alloy described therein contains 1% to 8% of at least one metal selected from the group consisting of molybdenum, columbium, vanadium and tungsten.

7. A process in accordance with claim 3 in which the alloy described therein contains at least 4% iron, metal from the group consisting of 1% to 4% columbium and 1% to 6% each of molybdenum, vanadium and tungsten.

8. A process in accordance with claim 1 in which the alloy described therein is such that the chromium and iron contents are correlated so as to represent a point within the area OPQRO in the accompanying drawing.

9. A process in accordance with claim 8 in which the alloy described therein contains at least 1% of metal from the group consisting of molybdenum, columbium, vanadium and tungsten.

10. A process in accordance with claim 3 in which the alloy described therein is such that the chromium and iron contents are correlated so as to represent a point within the area OPQRO in the accompanying drawing.

11. A process in accordance with claim 8 in which the alloy described therein contains at least 1% to 8% of metal selected from the group consisting of molybdenum, columbium, vanadium, and tungsten.

12. A nickel-chromium alloy characterized by improved resistance to stress-corrosion cracking when in contact with lead-contaminated high purity water, said alloy consisting of from about 26% to 32% chromium, from 52% to less than 67% nickel, up to 4% molybdenum, up to 6% columbium, up to 10% vanadium, up to 10% tungsten with the proviso that the total sum of the molybdenum, columbium, vanadium, and tungsten does not exceed 15% and with the further proviso that the chromium, iron, molybdenum, columbium, vanadium, and tungsten are correlated such that the stresscracking relationship, SC,

Percent Cr+0.25 (percent Fe)+(0.9) (percent Mo) +percent Cb+l.25 (percent V+percent W) is at least 28%, up to 0.1% carbon, up to titanium, up to 5% alumium, up to 2% manganese, up to 2.5% silicon, and the balance essentially iron, the iron not exceeding 25%.

13. An alloy in accordance with claim 12 in which titanium and aluminum, if any, do not exceed 3% each, the iron content does not exceed 20% and in which the value for SC is greater than 30.5%.

14. An alloy in accordance with claim 13 which contains at least 4% iron.

15. An alloy in accordance with claim 13 which contains a total of 1% to 8% of metal from the group molybdenum, columbium, vanadium and tungsten.

16. An alloy in accordance with claim 12 in which the chromium and iron amounts are correlated so as to represent a point within the area OPQRO in the accompanying drawing.

17. An alloy in accordance with claim 16 in which the alloy described therein contains at least 1% of metal from the group consisting of molybdenum, columbium, vanadium and tungsten.

18. An alloy in accordance with claim 13 in which the chromium and iron amounts are correlated so as to represent a point within the area OPQRO in the accompanying drawing.

19. An alloy in accordance with claim 18 in which the alloy contains 1% to 8% of metal from the group consisting of molybdenum, columbium, vanadium and tungstem.

20. A process for improving the resistance to stresscorrosion cracking of metal articles in contact with leadcontaminated high purity water which comprises flowing high purity water past and in contact with such a metal article, the article being formed from an alloy composed of from 24% to 30% chromium, from 50% to less than 67% nickel, up to molybdenum, up to 4% columbium, with the proviso that when the nickel content exceeds about 65.5% the alloys contain at least about 3% of metal from the group consisting of molybdenum and columbium, up to about 0.1% carbon, up to about 2% manganese, up to about 1% silicon, up to 1% titanium, up to 1% aluminum, and the balance essentially iron, the iron not exceeding about 10% when the alloys contain less than about 26% chromium.

21. A process in accordance with claim 20 in which the iron content does not exceed about 10% and is correlated With the nickel such that the percentage of nickel plus 0.7 times the percentage of iron does not exceed 70%.

22. A process in accordance with claim 20 in which the article is formed from an alloy containing from 1% to 8% molybdenum and from 1% to 4% columbium.

23. A process in accordance with claim 20 in which the article is formed from an alloy containing 60% to 66% nickel, 26% to 30% chromium, up to 8% molybdenum, up to 3% columbium, about 0.01% to 0.9% carbon, up to 1% titanium, and the balance essentially iron, the iron not constituting more than 10% of the alloy.

24. A process in accordance with claim 20 in which the article is formed from an alloy containing about 28% chromium, about 8% to 13% iron, the balance being essentially nickel.

25. A nickel-chromium alloy characterized by improved resistance to stress-corrosion cracking when in contact with lead-contaminated high purity water, said alloy consisting of 26% to 30% chromium, from 52% to less than 67% nickel, up to 4% molybdenum, up to 4% columbium, with the proviso that when the nickel content exceeds about 65.5 the alloys contain at least about 3% of metal from the group consisting of molybdenum and columbium, up to about 0.1% carbon, up to about 2% manganese, up to about 1% silicon, up to 1% titanium, up to 1% aluminum, and the balance essentially iron, the iron not exceeding about 10% when the alloys contain less than about 26% chromium.

26. An alloy in accordance with claim 25 containing at least 55% and not more than 66% nickel.

27. An alloy in accordance with claim 25 containing from 1% to 4% molybdenum and from 1% to 4% columbium.

2.8. An alloy in accordance with claim 25 containing 60% to 66% nickel, up to 4% molybdenum, up to 3% columbium, about 0.01% to 0.09% carbon, up to 1% titanium, and up to not more than 10% iron.

29. An alloy in accordance with claim 25 containing about 28% chromium, about 8% to 13% iron, the balance being essentially nickel.

30. An alloy in accordance with claim 25 containing at least 3% of metal from the group consisting of molybdenum and columbium.

31. An alloy in accordance with claim 25 containing at least 6% of metal from the group consisting of molybdenum and columbium.

32. A process for improving the resistance .to stresscorrosion cracking of metal articles in contact with high purity Water, particularly water contaminated with lead, which comprises flowing high purity water past and in contact with such a metal article, the article being formed from an alloy composed of about 26% to 32% chromium, from 4% to 15% iron, the chromium and iron being correlated such that the stress-cracking relationship, SC, comprised of the percentage of chromium plus 0.25 (percent Fe) is at least 28%, up to 0.1% carbon, up to 5% titanium, up to 5% aluminum, up to 2% manganese, up to 2.5% silicon and the balance essentially nickel, the nickel being at least 50% but not more than 67%.

33. A nickel-chromium alloy characterized by improved resistance to stress-corrosion cracking when in contact with lead-contaminated high purity water, said alloy consisting of from about 26% to 32% chromium, from 4% to 15% iron, the chromium and iron being correlated such that the stress-cracking relationship, SC, comprised of the percentage of chromium plus 0.25 (percent Fe) is at least 28%, up to 0.1% carbon, up to 5% titanium, up to 5% aluminum, up to 2% manganese, up to 2.5% silicon and the balance essentially nickel, the nickel being at least 52% but not more than 67%.

34. A process in accordance with claim 3 in which the alloy described therein contains about 1% to 4% columbium.

35. A process in accordance with claim 3 in which the alloy described therein contains about 1% to 3% aluminum.

36. A process in accordance with claim 3 in which the alloy described therein contains about 1% to 3% titanium 37. An alloy in accordance with claim 13 in which the alloy contains about 1% to 3% aluminum.

38. An alloy in accordance with claim 13 in which the alloy contains about 1% to 3% titanium.

13 39. An alloy in accordance with claim 15 which c0ntains about 1% to 4% columbium.

References Cited UNITED STATES PATENTS 6/1925 Girin 75--128 7/1946 Scott et a1. 75-171 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,573,901 Dated April 5, lQ'Tl Inventor) GEORGE ECONOMY It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column Column Column Column Column (SEAL) Attest:

EDWARD M.FIETCHER,JR. Attesting Officer 6, Table I, under columheading "Ni percent" Alloy No.

7, line 1, for "18" read --l7--.

7, line 2, for "17" read --l6--; same line, for "l8" 12, line 8 (Claim 25, line 4), before "26%" insert ---from---. 5 12, line 16 (Claim 25, line 12 after "iron" first occurrence delete the comma and insert a period and delete remainder of the claim.

si ned and sealed this 26th day of March 1 972.

ROBERT GOTTSCHALK Commi ssioner of Patents