US3929473A

US3929473A - Chromium, molybdenum ferritic stainless steels

Info

Publication number: US3929473A
Application number: US474543A
Authority: US
Inventors: Michael A Streicher
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1971-03-09
Filing date: 1974-05-30
Publication date: 1975-12-30
Anticipated expiration: 1992-12-30

Abstract

A ferritic alloy containing, in general ranges, 27-32.50% chromium, 1.8-5.8% molybdenum, 0.25-3.0 nickel, 100 ppm carbon maximum, 200 ppm nitrogen maximum, the sum of carbon plus nitrogen being 250 ppm maximum, having inherent post-welding ductility and high corrosion resistance.

Description

United States Patent Streicher Dec. 30, 1975 CHROMIUM, MOLYBDENUM FERRITIC [56] References Cited STAINLESS STEELS UNITED STATES PATENTS [75] Inventor: Michael A. Streicher, Wilmington, 2,183,715 12/1939 Franks 75/126 F Del. 2,274,999 3/1942 Allen 75/126 C v 2,624,671 1/1953 Binder.... 75/126 C Asslgneelde Nemours & 3,672,876 6/1972 Sipos 75 124 I Company, Wilmington, Del. [22] Ffl d; M 30, 1974 Primary ExaminerL. Dewayne Rutledge pp NOI: 474,543 Assistant ExaminerArthur J. Steiner Related US. Application Data [57] ABSTRACT- Division March 1971, A ferritic alloy containing, in general ranges, abandoned, which is a continuation-impart of Ser. 27 32 5 Chromium, 1 g 5 g% l bd 46,428 l 1970 abandoned- O.253.0 nickel, 100 ppm carbon maximum, 200 ppm nitrogen maximum, the sum of carbon plus nitrogen [52] US. Cl; /128 W; 75/128 N being 2 0 pp maximum having inherent p [51] Int. Cl. C22C 38/44 welding ductility and high corrosion resistance [58] Field of Search 75/128 W, 128 N 2 Claims, 2 Drawing Figures m l a "50' b PP" E 6 660 25521 1 Mae lflflpprw l T mill/z 200 1rb r 04 PP 5 \w 2 M Fame mag/mama 5-0 2 an 51.9 I m we g 404: r 41 I0 .9 .418 521 512 53%1 Q 650 29 a 50 571 A .217 29 m w 508* 419 0 e9 & 9I" z 67 E I I .957 um l l i l 1 l I 1- 22' 24 26 2a 50 32 34 Per cent qy vt eyfltabro Passed W -6'L Test butFazYedBeadTesb Failed R961 50110 41m 22m FazMRCIg mum dikzl rem +aemz may US. Patent Dec. 30, 1975 Sheet 1 of2 3,929,473

w mw

Perlezzi 1 29 WeyZM ybdenw/L US. Patent Dec. 30, 1975 Sheet 2 of2 3,929,473

CHROMIUM, MOLYBDENUM FERRITIC STAINLESS STEELS CROSS REFERENCE TO RELATED APPLICATION This is a division of application Ser. No. 122,529, filed Mar. 9, 1971, now abandoned which in turn is a continuation-in-part of U.S. Pat. application Ser.-No. 46,428 filed June 15, 1970, now abandoned.

BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises a corrosion-, resistant especially pitting-resistant ferritic alloy having good post-welding ductility containing, as principal alloying element, chromium and molybdenum in the combinations lying within areas A A B, C C and D of FIG. 1 of this application, carbon 100 ppm maximum, nitrogen 200 ppm maximum, and carbon plus nitrogen 250 ppm maximum, the remainder being iron and incidental impurities.

The essential components of the alloys of this invention are Fe, Cr, Mo and certain metal additives hereinafter identified. As in all alloys of the class involved, there may also be present incidental impurities. In commercial practice these might consist of the following, in the approximate weight percentages reported:v S, 0.010%, P, 0.010% (together with, typically, 0.80% Mn and 0.50% Si as deliberate additions).

DRAWINGS The following drawings present the essential requirements in terms of percent chromium as abscissa and percent molybdenum as ordinate together with the permissible carbon and nitrogen contents required according to this invention, in which:

FIG. 1 is a plot of four different regions of different corrosion resistance and post-weld ductility for alloys containing C equal to or below 100 ppm, N equal to or below 200 ppm, and C+N equal to or below 250 ppm, and

FIG. 2 is an overlay of the same regions of corrosion resistance and post-weld ductility as FIG. 1 within which are plotted typical ferritic Cr, Mo alloy compositions matching those of FIG. 1, except that the C content is above 100 ppm, or the N content is above 200 ppm, or C+N is above 250 ppm.

In the early development of the stainless steels, chromium steels containing 12-14% Cr and 14% C were the first, large-volume products. Attempts were soon made (Br. Pat. No. 18,212 accepted on July 9, 1914) to improve the corrosion resistance properties by the addition of molybdenum; however, it was noted that molybdenum, when applied in sufficient quantity to make the alloy passive, also made it too hard and brittle. Brittleness contributed by Mo addition was confirmed by Reitz et al. in U.S. Pat. Nos. 2,110,891 and 2,207,554. Franks U.S. Pat. No. 2,183,715 taught additions of 1-5% of Mo to iron, chromium alloys but found this addition insufficient to overcome even his mild service exposures and recommended the addition of niobium to the extent of four times the carbon content, at least, to overcome his problems of pitting corrosion.

Finally, Moneypenny, in Stainlesslr on and Steel, Vol.

1, Chapman & Hall, London, 1947, p. 48, reported certain contemporaneous work done in Germany to improve the usefulness of iron chrcmium alloys by adding about 2% Mo to them. While resistance to corrosion by a number of organic acids and other'com- 2 pounds was reported-to be markedly increased, especially at Cr contents above about 18%, the mechanical properties were'not improved. Thus, the alloys were classed as notch-brittle and subject to marked grain growth when heated to high temperatures, as, for example, during welding.

It has been generally recognized, up to this date, that Fe, Cralloysas a class develop a high degree of brittleness in or adjacent to welds, and this inadequacy has severely limited uses of the alloys containing more than about 20% Cr wherever welding is essential as, for example, in the manufacture of chemical processing and other vessels, pipes and similar equipment.

Early investigators were able to reduce the impact brittleness of ferritic chromium alloys by limiting combined carbon and nitrogen contents to about 0.023% maximum, as reported in U.S. Pat. No. 2,624,671; however, marked post-welding brittleness persisted and, in U.S. Pat. No. 2,624,670, it was reported necessary to convert the alloys to at least a pa'rtially austenitic state in order to cure the difficulty. But austenitic alloys are subject to chloride stress-corrosion cracking, and so one valuable attribute was lost in the acquisition .of another. Moreover, these investigators deemed it necessary to heat treat by annealing at 900C., followed by rapid quenching, in order to minimize brittleness in weldments, and this is an exceedingly troublesome and expensive expedient.

Corrosion is anextremely complex combination of phenomena constituting numerous well-recognized types. To detect and overcome susceptibility to the individual types of corrosionrequires individually designed techniques for each. It is also not generally true that a material resistant to one form of corrosion is resistant also to others. For example, a nickel-bearing stainless steel may be highly resistant to nitric acid, and yet prone to disastrous cracking when exposed under stress to chloride environments.

The alloys of this invention have been developed to resist exposures to a wide variety of corrosive environments, while still having high post-weld ductility and good economy in the fabrication.

Important types of corrosion include the following:

1. Pitting corrosion in halide environments a. Extreme exposure, as in oxidizing chloride environments, e.g., 10% FeCl .6 H O at 50C., accentuated by crevices,

b. Severe exposure, as in chloride waters containing permanganate ions at C.,

2. Intergranular corrosion in acid and chloride environments 3.'Stress-corrosion cracking in chloride-containing environments 4. General surface corrosion a. Organic acids, such as sulfamic, formic, acetic, and

oxalic acids,

b. Oxidizing acids, such as 65% nitric,

c. Inorganic reducing acids, such as boiling 10% sulfuric,

(This latter category can best be appraised in three different aspects:

(-1) Active alloys, which are active at once, or. within a few hours, these dissolving at rates in excess of 50,000 mils per year, (II) Passive alloys, which are passive upon immersion in the corrosive media, dissolving relatively uniformly therein at rates less than mils/yr. These alloys become activated when contacted with an activating electrode and remain active A. SPECIMEN PREPARATION 1. Ingredients All specimens were prepared by the technique hereinafter described, using high purity ingredients as detailed in Table I:

TABLE I Ingredient Supplier Analysis Iron Glidden Co. 99.9l% Fe, C 20 ppm,

N 40 ppm Chromium Union Carbide 99.957: Cr, (LUV/l Fe,

Corp. C 50 ppm, N 60 ppm Chromium Shicldalloy Corp. 982% Cr, C 85 ppm,

N 284 ppm Molybdc- Fansteel Co. 99.9% Mo, C 20 ppm,

num

N l ppm Molybdc- Climax Molybdenum 99.771 Mo, C 32 ppm,

num

N 12 ppm Where nickel was utilized, the ribbon form was employed. Silicon was reagent grade, aluminum was in lump form analyzing 99.992% Al, carbon was of High Purity lump grade, free of filler or in the form of high carbon ferro-chrome alloy, and nitrogen was supplied as Cr N powder.

2. Melting The alloying ingredients were melted in high purity alumina crucibles in a vacuum induction furnace, which was sealed and evacuated to 10 to 10 Torr before the power was switched on. The power was increased gradually to minimize thermal shock and, when melting was incipient, the furnace was filled with gettered argon (a purified commercial grade of argon especially low in oxygen and nitrogen content) to an absolute pressure of 5 inches Hg in order to inhibit vaporization of the alloying ingredients. At the completion of the melting operation, the heat was east through a fire brick funnel into a vertically disposed cylindrical copper mold placed in the argon atmosphere. After cooling, the ingot was removed and the hot top containing the shrinkage cavity was cut off.

3. Heat Treatment and Working Each ingot was soaked for 3 hours at 2200F. in an electric furnace (air atmosphere) and then forged to a rectangular cross section.

The forged ingot was then reheated to 2150F. and rolled to a thickness of 100 mils in light passes, interspersed with four reheats to 2l50F., each requiring about mins.

4 After the final rolling, the sheet was heated at 2000F. for 1 hour and water-quenched. Alloys containing titanium as a stabilizing additive were given a lower final heat treatment of 2 hours at l750F.

Specimens subjected to corrosion, mechanical and analytical tests were cut with a power saw and were thereafter ground to an grit finish using a watercooled silicon carbide belt.

4. Welding To investigate the effects of welding on corrosion resistance and on mechanical properties, autogenous welds were made as follows:

Welded samples for bend and stress corrosion tests measured approximately 3 inches long X 1 inch wide by 0.1 inch thick, and these were given a lengthwise fusion weld using the argon gas-tungsten arc welding process and an energy input per pass of approximately 16,000 joules/inch [the'energy input per pass in joules/inch arc voltage (volts) X are current (amperes)/torch travel speed, in./sec. During the welding, the back of the sample was concurrently shielded with argon, to reduce oxidization and safeguard against pickup of nitrogen. In further explanation, there was no fusion of two pieces of alloy here, the electrode simply being given a single pass longitudinally of the sample piece. During this pass, the energy input was suffieient to melt the metal in the immediate region of the electrode traverse for almost the entire thickness of the sample and for a width of approximately Aiinch. The specimens were then allowed to cool in the air to room temperature, thereby duplicating usual welding practice.

Three specific sample regions are of particular interest in tests hereinafter reported, these being the visually apparent weld zone, where the torch had melted the surface metal, the remote base-plate zone (abbreviated BP), which is all metal one-half inch or more away from the weld, and the intervening heat-affected zone (HA2).

5. Analyses The data hereinafter reported, and plotted in FIGS. 1 and 2, are based on weighed out proportions of iron, chromium and molybdenum. Confidence in this approach has been provided by a weight balance established by weighing cast ingots and rolled sheets made from these ingots and comparing the results with the total weight of the metals charged in making the alloys. The average detectable change in weight between the weighed-in ingredients, the ingots and the rolled sheets amounted to only 0.1 gm out of a total weight of 400 gms. Additional confidence in the practice arises from the consistency and sharp definition of the pitting test results plotted in the FIGURES.

Carbon was determined by combustion with a Leco Carbon Analyzer. Nitrogen analyses were made by the micro Kjeldahl method using Nesslers Reagent.

Titanium, niobium and aluminum were determined by X-ray fluorescence.

B. ALLOY TESTING l. Pitting Corrosion: Potassium Permanganate-Sodium Chloride Test This is a new test applied by applicant to simulate chloride pitting in severely corrosive natural waters, such as Ohio River water used in heat exchangers. Such waters contain some manganese and must be chlorinated to prevent the accumulation of organic slime in the heat exchangers. A propensity towards severe pitting attack results, probably due to the conversion of tetravalent, insoluble manganese to soluble permanganate (Mn by chlorine and the simultaneous reduction of chlorine to chloride (Cl) ions.

Service tests at plant locations require relatively large amounts of material and 6-18 month test exposures for alloy evaluation, so that this accelerated test was developed as a substitute. 1

A 2% KMnO -2% NaCl water solution with pH ad justed to 7.5 was employed. Large test tubes 11 /2 inches long X 1 /2 inches dia. containing 150 ml of the test solution were immersed in a 90C. thermostatically controlled water bath. (The 90C. temperature was selected to simulate conditions in heat exchangers.) The test tubes were covered with a rubber stopper fitted with a glass tube for venting, and the specimens placed therein were 1 X 2 X 0.08 inch thick pieces ground to an 80 grit finish.

Pitting attack in the solution is evidenced by extensive formation of a surface coating of insoluble manganese oxides. It appears that, as the alloy dissolves at anodic sites (pits), insoluble manganese oxide is precipitated at the unpitted cathodic areas where permanganate ions are reduced to the tetravalent state in an electrochemically equivalent reaction.

The coating is removed at room temperature without attack on the metal by immersion of the specimen in a solution disclosed in applicants US. Pat. No. 3,48l,882, consisting of: 900 ml H O, 27.4 ml 96.5% H 30 l4.4g oxalic acid, 0.2g Alkanol WXN and 0.2g diorthotolylthiourea. The cleaned specimen clearly reveals evidence of pitting attack to the unaided eye.

Only specimens which were free of all pitting attack, and of manganese oxide coating, were classified resistant. Those which displayed any pitting at all were rated failed. Commercially available ferritic and austenitic stainless steels (e.g., A.l.S.l. 446, 316 and 310) were readily pitted by this solution at room temperature. Generally, specimens resistant to attack for the first-24 hours were found to be resistant for as long as 16 months.

1n the tests hereinafter reported, samples resistant to this hot permanganate-chloride test were classified as highly resistant and of high resistance to pitting corrosion.

2. Pitting Corrosion: Ferric Chloride Test This test is commonly used when conducted at room temperature; however, applicant chose to accelerate it by elevating the test temperature to 50C. and by providing tight crevices. As accelerated, this test is more severe than the permanganate-chloride pitting test at 90C.

The testwas conducted in a thermostatically controlled water bath at a temperature of 50C. using 150 ml of FeCl 6H O in water in individual I 1 /2 X 1 /2 inch dia. test tubes vented through tube-fitted rubber stoppers. The unwelded test specimens, ground to 80 grit finish, measured 1 X 2. X 0.08 inch thick. Crevices were created on the edges and surfaces of the specimens by employing polytetrafluoroethylene blocks on the front and back held. in positionby pairs of rubber bands stretched at 90 to one another in both longitudinal and transverse directions. This created two sharp crevices at top and bottom of the specimen where the longitudinal elastic touched the metal, two somewhat 6 less sharp crevices at the side edges and two crevices under the polymer blocks. Contraction of the elastics provided constant crevice conditions during progressive metal corrosion at the points of contact.

; At room temperatures, it was found that, if an alloy pits with a crevice it will eventually also pit without a crevice, but the exposure required to reveal this may be as long as 4 months duration. ln applicants accelerated test, pitting occurred within 24 hours in the case of alloys susceptible to this type of pitting. Resistant alloys were exposed for weeks, and, in some cases, for as long as 12 months, without any pitting attack.

As hereinafter reported, samples that resisted attack.

in the hot ferric chloride test were classified as extremely resistant. Almost all of the same analyses that passed this test had already passed the permanganatechloride test.

3. Stress Corrosion: Boiling Magnesium Chloride Test This test, while not yet actually adopted as a standard by the American Society of Testing Materials, is nevertheless already widely utilized. It is conducted in accordance with the procedures described by applicant in association with A. J. Sweet, published in Corrosion,

Vol. 25, No. 1, pp. 1-6 (1969) January.

The test solution is boiling (155C.) 45% MgCl The test specimens were 3 X A wide, mil thick, in most cases having a lengthwise autogenous weld, because welded specimens reveal susceptibility to stress corrosion more readily than unwelded specimens. The

welded specimens were bent 180 over a 0.366 inch dia. cylindrical mandrel. Stress was applied by tightening a Hastelloy C bolt through holes at each end of the specimen, the bolt being electrically insulated from the specimen by polytetrafluoroethylene bushings.

Austenitic stainless steels fail by cracking in 1-4 hours during exposure to this test. In contrast, it was found that alloys according to this invention did not crack within days of exposure. Alloys which did not fail sooner were routinely left on test for 100 days to demonstrate their immunity to stress corrosion.

The boiling MgCl test is a very severe one, not usually encountered in industry. Neverthless, I have found a correlation between it and the stress corrosion propensity of such Crcontaining alloys as AISl-430 and -446 to cracking in NaCl solutions containing only 50 ppm C1. The latter is much more like a simulated service corrosion test; however, test exposures of 250 hours or more are often required to, detect corrosion susceptibility. Thus, for ferritic alloys, the MgCl test can be considered to be a valid, rapid test for evaluating stress corrosion cracking.

Since preparation of welded stress-corrosion crack- 1 ing specimens requires cold bending welded specimens transversely of the weld, there was incidentally afforded a severe test of ductility. Some test alloys outside this invention cracked during bending and were therefore not tested in the MgCl solution. Consolidated test data are set out in the Table I1 hereinafter set forth.

4. lntergranular Attack (lGA): Ferric Sulfate-Sulfuric Acid Test To detect susceptibility to intergranular attack (hereinafter abbreviated lGA), welded specimens were exposed for hours to boiling 50% H 80 containing 41.6 gm'll Fe2( 4)3 X H O. This rapid test was originally developed by applicant for austenitic stainless steels (M. A. Streicher, ASTM Bulletin No. 229, pg. 77 (1958) April, and ASTM-A262-68 Recommended Practice for Detecting Susceptibility to lntergranular Attack in Stainless Steels). Applieants extensive investigation has now established that this test is also valid for the determination of susceptibility to IGA in commercial ferritic stainless steels of the class represented by AISI-430, -446 and of this invention, as a function of heat treatment and Cr, C and N contents.

The test was conducted on specimens ground to 80 grit finish, measuring about 1 X 2 X 0.08 inch thick with an autogenous weld across the width of the specimens. The specimens were immersed in 600 ml of test solution held in a 1 liter Erlenmeyer flask fitted with an Allihn condenser.

Specimens tested were evaluated by both weight-loss measurements and, especially, by 80 X microscopic examination for evidence of grain dropping. Three zones were particularly examined for dislodged grains, the base plate (BP), the weld metal (Weld) and the heat-affected zone (HAZ). Any evidence of dislodged grains was cause for rejection of the particular alloy sample. The results are tabulated in Table II.

5. General Corrosion in Acids As hereinafter set out in Table III, a comparison was made of commercial alloys with alloys within the limits of this invention as regards general corrosion occurring in representative acid environments, including oxidizing, reducing, organic and inorganic. The acids, techniques and data for commercial alloys have been previously published by applicant in Corrosion, Vol. 14, No. 2, p. 59t-70t, February (1958).

Briefly, all tests were conducted on unwelded specimens measuring l X 2 inches about 80 mils thick, with surfaces ground to an 80-grit finish. Boiling test solutions of 600 ml volume were employed using Erlenmeyer flasks fitted with reflux condensers. Tests showing astronomica1" corrosion rates lasted only 5 minutes, but for samples corroding at less than 100 mils/- year, the tests were prolonged for 100 hours.

Especially significant, as detailed later, is a group of tests utilized to show the development and/or loss of passivity, and the corrosion rate in boiling sulfuric acid.

6. Mechanical Tests In addition to the bend tests made preliminary to the MgCl stress corrosion test of Section B( 3) supra, a number of additional mechanical tests were made to obtain a comparison with commercial steels of the same general class and, in any case, to establish critical strength data.

Thus, a tensile test was conducted on alloy Q-202-H made according to this invention, the analysis of which was 28.5% Cr, 4.0% Mo, C, 23 ppm, N, 130 ppm. The results, as compared with commercial steels having properties tabulated in the Stainless Steel Handbook published by the Allegheny Ludlum Steel Corp., pp. 2-5 (1951) were as follows:

Alloy 75F. -25F. 50F.

O-433 bent hent bent cracked [Cr 28.5%, M0. 4.0%

C18 ppm, N 37 ppm] Q-436 bent bent ICr 28.0%, M0. 4.0%

C 28 ppm, N 83 ppm] Q-437 bent cracked {Cr 27.5%, M0 4.0% C 29 ppm, N 65 ppm] Yet another mechanical test was a cold rolling test in which the following alloys of this invention, which had previously been hot-rolled to a thickness of about mils, were cold-rolled to about 25 mils, the limit of the rolls:

Per Cent Alloy Cr(%) Mo(%) C(ppm) N(ppm) Reduction Q-l 20 30.0 3.0 90 Q-ZOZA 28.5 4.0 20 25 81 Q-562 35.0 3.5 14 20 69 Q-557 33.0 4.5 28 35 70 Q-514 30.5 4.0 5 I70 67 In every case, there was excellent ductility, i.e., there was no cracking, either at the edges or in the surfaces.

In still another investigation, comparative Charpy impact tests were run on a 29.0% Cr, 4.3% Mo, 25 ppm C, ppm N specimen according to this invention, labeled Invention in the tabulation infra, along with AISI-446 and -316 commercial steels.

All Charpy specimens were half-size, i.e., 2.16 X 0.197 X 0.394 inch, with a 45 notch having a 0.010 inch radius. These specimens were machined from inch thick plates with the root of the notch lying in the rolling direction.

Type

Alloy of Fracture AlSl-446 Charpy Impact (ft-lb.)

AISI-3l6 42.75, 47.5 45.0

lnvention 44, 5]

From the foregoing, the Charpy impact values for alloys of this invention were about the same as for AISI-3 16 and much superior to those of AISI-446.

C. EVALUATION OF Fe-Cr-Mo ALLOYS LIMITED IN C AND N CONTENTS BUT CONTAINING NO OTHER Additives Beyond Incidental Impurities Referring to FIG. 1, a great number of alloy composi- 9 tions are plotted which collectively precisely define a number of different regions A, and A, (which can, for some purposes, be considered together to be an entity A), B, C, and C (which can, for some purposes, be considered together to be an entity C) and D according to this invention which are characterized by improved corrosion resistance, especially pitting resistance, over the prior art. In addition, these several regions are characterized by different corrosion resistances among themselves, generally showing increasing corrosion immunity with increase in both Cr and Mo contents within the overall perimeter enclosing all of the regions.

The vertical division line at 27.5% Cr defining the areas made up of regions A, and C, to the left and A and C to the right can be disregarded in the general consideration of corrosion resistance as to which Table ll pertains; however, this dividing line has significance TABLE ll a. Regions A, and A collectively, characterized by resistance to pitting under exposure to (l) the permanganate-chloride test and (2) the ferric chloride test, (3)

15 resistant'to intergranular corrosion attack [lGA] under exposure to the ferric sulfate-sulfuric acid test, (4) ductile in the 180 transverse weld bend test of asreceived (unannealed) welded specimens and (5) resistant to-stress corrosion [S.C.].

Composition in Per Cent by Wt.

Alloy Cr and Mo, ppm

No. I C and N Remarks Region A, Cr Mo C N 665 1 25.0 5.5 75 150 Not tested for stress corrosion 438 27.0 4.0 24 68 Passed all 5 tests 577 25:5 5.5 r 25 63 Test 3 [IGA] omitted 549 g 27.5 5.5 195 Passed all 5 tests 548 v 27.5 5.0 10 5 Tests Nos. 1 & 3 [lGA] omitted I 496 27.5 4.5 31 155 .489 26.0 5.5 19 I 108 Test No. l (KMn0,NaCl) omitted 488 26.0 5.0 22 1 l0 Passed all 5 tests Composition in Per Cent by Wt. Alloy Cr and Mo, ppm

No. C and N Remarks Within Region A, Cr Mo C N 656 28.5 4.0 23 100 Tests No. 2 and No. 5 for FeCl;

and stress corrosion, respectively, omitted 611 29.5 4.7 25 118 Tests No. 3 [lGALand No. 5

[S.C.] omitted 610 28.5 3.5 25 Tests No. 1, No. 3 and No. 5 omitted 585 28.5 4.5 20 93 Passed all 5 tests 559 30.0 4.0 24 I50 Tests No. 3 [10A] and No. 5

' [S.C.] omitted 554 28.5 4.2 23 17 Tests No. 3 [16A] and No. 5

i [S.C.] omitted 548 27.5 5.0 10 5 Tests No. 1 and No. 3 [lGA] omitted 547 27.5 3.8 15 Tests No. 3-5 omitted 544 29.5 3.2 24 1'18 Tests No. 3 [IGA] and No. 5

[S.C.] omitted 543 29.0 4.7 27- 13 Test No. l KMnO,NaCl omitted 54] 29.5 4.5 38 Tests No. l-3, incl., omitted 539A 30.0 3.5 15 I 128 Test No. 3 [lGA] omitted 538 28.5 4.5 29 15 Passed all 5 tests 537 28.5 4.5 23 133 518 31.0 v 4.0 21 88 Tests No. l and No. 3 [16A] omitted 517 31.0 3.0 14 188 Test No. 3 [10A] omitted 513 30.0 4.5 19 150 Tests No. l and No. 3 [lGA] omitted 436 28.0 4.0 28 v 83 Passed all 5 tests and, in addition, was ductile at 75 F.

Composition in Per Cent by Wt. Alloy Cr and Mo, ppm

No. C and N Remarks Peripheral Cr Mo C N Analyses Outside Regions A, and A, (Underscorcd Alloy Nos. plotted on FIG. 2)

. .-cont1nued Composition in I L 2.: 1 Per Cent b'yawt. 1'

h Alloy A Cr and Mo,-ppm j p No. i. I y C and N i Remarks. 'Peripheral' Cr" Mo C' i v N i Analyses;

fiutside Regions i g i Tcst N o. 4 (Bend).

- Tests No. l. 3 and 5 omitted 494 27.0 .0 10 305 Failed Tcst No.4 tBendl.

' Tests No. l and 5 omitted 502 28.0 6.0 9 165 504w 2x5 5.5 10 m Faild TcstNo. s (so g I i Test No. L- omitted x 51 l v 29.5 5.0 ll l' ailcd Test No. 4 (bend).

'Tests' No." I. No. 3 & No. 5 rr- 1 omitt'ed- 48l 29.5 4.8 9,3 $8 Failed TestNo. 5 (8.0. L g

; Test Noni omitted I I I Y i 558 33.0 5.0 22 '5" Failed T est Nb. 4 (Bend). I i

' Tests No. 3 84 No.5 omitted M6 35.0 5.0 20 ;203 Failed TestNo. 4 (Bend). 1'

' i Test'No'. 5 omittcd 603 35.0 4.5 l I 1 l5 Failed Test No. 4 (Bend).

Tests No. 3 and No. 5 omitted c. Region B, characterized by resistance to pitting under exposure to (l) permanganate-chloride test and (2) ferric chloride test, (3) resistant to intergranular corrosion attack (lGA) under exposure to the ferric 0 sulfate-sulfuric acid test, (4) ductile in the 180 transverse weld bend test of as-received (unannealed) welded specimens and (5) resistant to stress corrosion (S.C.). In addition, all region B and D specimens are passive in boiling H 50 as hereinafter set out in Table IV; however, region D specimens otherwise have the properties of regions C and C i.e., they fail the ferric chloride Test No. 2.

b. Regions C and C collectively, characterized'by resistance to pitting under exposure to (l) permanganate-chloride test, (3) resistance "to intergranular co rrosion attack (lGA) under exposure to ferric sulfatesulfuric acid test,\(4) ductileuin the 180 transverse weld bend test of as-receivcd (unannealed) welded specimens and (5) possessedof stress-corrosion resis-T tance to extent tested. The following specimens all failed Test No. 2, the ferric chloride pitting test.

Composition in Per Cent by Wt. Alloy Cr and Mo. ppm I No. C and N Remarks Regions C and C (except Alloy No. 568. which is just below) Cr 0 C N I 625 27.0 4.0 l5 190 Passed Tests No. l. 3 and 4 -Not tested for SC. (No. 51- 624 26.0 3.5 1.7 .150 576 23.0 6.0 6 43 .Test No. 3 lGA omitted.

; Passed S.C. test 57l 26.5 3.0 10 l IS in addition to Test No. 2'. Test No. l (KMnO NaCl) alone run g (and passed) 568 Y 27.0 2.5 5 120 Failed Test No. l. Tests No. 3

- and No. 5 omitted 567 25.5 4.0 5 75 In addition to Test No. 2. Test No. l (KMnO NaCl) alone run (and passed) 666 22.0 I 6.0 l 52 ll0 Passed Tests No. l. 3 8L '4.

' Not tested for SC. 597 30.0 570 28.0 13 -98 In addition to Test No. 2. Test No. l (KMnO.,-NaCl) alone run 1 (and passed r 520 32.0 2.0 I7 50 Passed Tests No. I. 3 8L 4.

- Not tested for S.C.

Mid

Slfi 31.0 2.5 7 175 508 29.5 3.0 15 163 Tests No.2. 'No. 3 84 No. 4 alone run.

Failed No. 2 and No; 3 (.IGA) 457 29.0 3.0 I28 Tests Nofl. No. 2 8L No. 3 alone run.

Failed No. 2. passed No. 1 {it No. 3 503 28.5 3.4 5 160 Tests No. 2. No. '3 and No. 4 alone 1' run. Passed No. 3 and No.4 I 435 29.0 3.0 46 PassedTests No. 1.3.4 & 5.

1 failed No. 2 v 1 Composition in,- Per Cent by Wt.

- Passed all tests. t v Passed all 5 tests Passed Tests No. l-4. incl. Test No. 5 (S.C.) omitted I V, i I P Passed'l'ests No. lb 2,4 and "5; Test No. 3 (lGA) omitted v 555 33.0 3.0 48 23 52| 32.0 4.0 I5 45 Passed Tests No. 2, 4 & 5.

Tests No. l and ,N o."3, (lGAl omitted Region D 560 33.0 2.0 1 I6 85 Passed Tests No. l, 3 and 4.

|.. No. 5 (S.C.) omitted;

As hereinbefore mentioned in- Section 8(5), compar- The following tests, reported in Table IV, illustrate atlve general corrosion resistance to typical common the critical compositional relationship necessary to acid environments, including oxidizing, reducing, orachieve the high resistance to boiling 'l0% sulfuric acid game and inorganic acids, is set out in the following corrosion possessed by alloys lying within regions B and Table III: -D, FIG. 1. 1

TABLE III COMPARISON OF GENERAL CORROSION OF ALLOYS lN ,AClDS* General Corrosion (Boiling) (mils per year) 507: Sulfuric 1 with Ferric Sodium Sulfuric Alloy Nitric Sulfate Sulfamic Formic Acetic Oxalic Bisulfate Acid 657: 10% 45% l0%. l0% 10% Y AlSl 430 20 i 312 l44,000 84,700 3,000 6,400 9l,200 252,000 AlSl 446 8 36 150,000 9,700 0 7,000 64,800 270,000 AlSl 304 8 23 l,300 1,715 300 570 2,760 l6,420 AlSl 316 ll 25 75 520 2 96 I70 855 Carpenter 20 8 9 7 l6 7 2 7 ll 43 Hustelloy C 450 240 8 5 0 I 8 i 8 17 Titanium 1 140 285 1873 0 950 I 250 6,290 Fe-Zli'k Cr-4"/ Mo (1) 2 .l 6 0 h l 0 l3 9 '52,l80 l e-33% Cr-3% Mo (2) f v 60 (1) Alloy O 202. having C 23 ppm, N l ppm (2) Alloy O 555. having C 48 ppm. N 23 ppm Acid concentrations-in per cent by weight TABLE IV CORROSION OF Fe-Cr-M o Aeeovs mnoluno l0% SULFURIC A'clb' v Composition I I Corrosion Per Cent by Wt. ppm v State (I) Rate (2) Alloy No. Cr Mn C N (mils/yr.)

513 30.0 4.5 9 150 active 44,200 539-A 30.0 3.5 l5 l28 active l95,200

6l 2 (FIG. 2) 3l.0 5.0 25 290 .active 48,000

51'9 I 31.0 4.5 l8 100 7 active 53.200 518. 3L0 4.0 2| 88 I 5 active 62,500

627 510,2). 31.0 5.5 l0 265 active 72,100

628 (FIG. -2) 3l.5 3.0 7 235 active 83,400

52l 1 32.0 4.0 I l5 passive 4 v .75 .629 32.0 3.0 l6 H passive I 1 4 5 659 32.0 2.75 '45 140 passive A '58) (FIG Z) 32.0" 2.5 22 215 passive 55 520 ii; 32.0 2-,0 l7 50 active 11.6.000

- 32.0 M 0.0. 25 l 70 active 54,000 33.0 '4.5 i 28 '35 passive 70 33.0 4.0 25 53 passive I, 65

v v 33.0 300 48 23 passive 60 I 33.0 i 2.5 46 98 passive 50 33:0 2.0" 16 passive 45 33.0 1.5 22 passive 40 35.0 v 39 320 passive 50 -Paxsivi.- n6 sihle evolution of hydrogen, low'c'orrosion rate. (2) Rate inactive alloys de'termiped in 5-r n in .-te st. Rates on passive alloys determined in l00-hr. test.

The following Table V lists the analyses and test results for a large number of Fe-Cr-Mo alloys which do not meet the compositional limits of this invention, particularly as regards C and N contents. These Alloy respects. For example, below region C the alloys suffer both serious pitting corrosion in the less severe Test No. 1 (permanganate-chloride exposure) and may also be subject to intergranular attack, with resultant grain Nos. are plotted w1th1n the overlay of HO. 2, and the dropping, although they may be ductile after weldlng. several causes of test failure are denoted by character- Below region D, the alloys suffer not only pitting 1st1c point symbols defined 1n the drawtng legend. From corrosion and intergranular attack but are also brittle Table V, taken in conjunction with FIG. 2, it can be after welding. To the right of regions B and D, the seen that the contents of both C and N are sharply alloys are brittle after welding, whereas, above area A cr1t1cal, and that this criticality is also affected, to some and region B, the alloys are either brittle, so that they degree, by the associated Cr and Mo. break during bending after welding, or otherwise they TABLE V FIG. 2 DATA TEST RESULTS STRESS COMPOSITIONS lN KMnO,- CORRO- ALLOY wT. PER CENT Cr 82 M0, NaCl FeCl Fe.,1SO.. BEND 510M NO. PPM c AND N Test No. 1 Test NO. 2 H. .so Test NO. 3 TEST NO. 4 TEST NO. 5

Cr M0 C N HAZ WELD BP 529 27.5 4.2 16 208 P P P P P P P "532 28.5 4.5 24 353 P P F P P P P 627 31.0 3.5 10 265 P P P P P P P 668 35.0 4.0 39 320 P P P P P P 493 27.0 5.5 223 P P P P P P 453 29.0 4.0 18 239 P P P P P P 492 27.0 5.0 10 283 P P P P P. F 628 31.5 3.0 7 235 P P F P P P(F)* 612 31.0 5.0 25 290 P P P P P F 615 35.0 2.5 23 100 P F P P P F 630 35.0 3.5 7 185 P P P P P F 657 28.5 4.0 56 198 P P P P P P 458 28.5 4.0 114 208 P F F P P F 459 28.5 4.0 118 F P F F P F 599 33.0 3.0 109 68 P F P F P P P 494 27.0 6.0 10 305 P P P P F 613 34.0 2.0 26 300 P F P P P F 497 28.0 3.5 29 209 F F P P P 594 25.0 5.0 18 268 P F F P P F 463 28.5 4.0 14 239 F P P F F 4098 29.0 4.7 856 219 P P F F P F 450 27.5 3.0 14 204 P F P F F 452 28.5 3.0 33 267 P F F F P 460 28.5 4.0 171 P F F F F P F 464 28.5 4.0 22 239 P F F P P F 487 26.0 1.0 26 204 F F F F P P 589 32.0 2.5 22 215 P F F F P F "*531 28.5 4.5 .334 25 P P F F F F 461 28.5 4.0 189 89 P F F F F F 582 27.0 3.0 48 255 F F F P P P P 587 33.0 1.5 22 195 P F P P P F 530 26.0 1.0 15 90 F F F F P P P 408 29.0 4.7 48 372 F F F F Second Sample "Deficiency cured by heat I hr. at 2(10UF. and water quenching. Deficiency not cured by heating l hr. at 20U0F. and water quenching,

not tested {P Passed F Failed D. SUMMARY From the foregoing, it will be seen that the alloys of my invention have post-welding ductility and good stress corrosion resistance besides being,

1. in area A, made up of regions A, and A collectively, extremely resistant to pitting corrosion as regards both Tests No. l, permanganate-chloride, and No. 2, ferric-chloride,

2. ln area C, made up of regions C and C collectively, highly resistant to pitting corrosion as regards Test No. l,

3. ln region B, equally resistant as area A, plus passive and resistant to corrosion in boiling 10% H 4. In region D, equally resistant as area C, plus passive and resistant to corrosion in boiling 10% H 80 Outside of areas A and C and regions B and C, taken together, Fe-Cr-Mo alloys are deficient in one or more E. ADDITION OF OTHER METALS TO Fe-Cr-Mo ALLOYS In order to determine possible benefits of other additives, a number of specimens were made up containing 28-29% Cr, 4-4.5% Mo, plus single metals in the ranges set forth in Table VI. The specific purposes for which the several additions were made are indicated, together with a brief report of side effects noted.

TABLE VI Component Achievement of Purpose and Amounts Purpose Other Effects Altlrnii'ium Grain refitiel' Yes '-0.l0 0.60% g Titanium or a) To prevent [GA a) No. l.G.A. above inven- Nio'bium 3 5 tion's specified C, N 0.20 0.6054 b) Grain refi'rier limits. Bend cracking tendency increased. I h) Yes. Grain was refined. Plu'tihtim Field A- Cz 0.006 0.30% passivity in Yes. Continued region A- boiling 1072 C properties.

I H 50 Ptllludium Passivity in Yes. Lost pitting rc- (l.02 0.2071 hoiling I071. sistance in both H- SO Tests No. l and No. 2. iridium Yes. Continued region 0.015 0.10% A C properties. Rhodium Yes. Resistant in Test No. I 0.005 0.l0'/r but not in Test No. 2.

One sample. near the N limit of 200 ppm? showed l.G.A. Osmium Yes. Osmium oxide has 0.02 0.l0 /i high vapor pressure and is toxic. Continued A C properties. Ruthenium Yes. No deleterious effects 0.020 L507: observed up to 0.30%

Ru. Suffered stress corrosion above 0.30% level. 0.02% Ruthenium Yes. No deleterious 0.307! Aluminum effects observed.

' Grain refinement I noted.

0.01% Ruthenium Yes. Region A require- 0.20% Nickel merits met, and no stress corrosion on welded specimen despite Ni. I 0.20% Gold Yes. Resistant in Test No. l.

but not Test No. 2. Nickel 0.25 to Yes. Stress corrosion 2.071 resistance progressively dei creases as nickel content increases; Nickel 2.0 3.0'7: Yes. Sc|f-repassivating.

' and resistant in Test No. l. but not Test No. 2. Cobalt 2.0 4.0% Yes. Stress corrosion re sistance seriously decreased. Not rcsistant in Test No. 2. Addition of sili- Mo re- Yes. Resistant in Tests con in range l.5 placement No. l and No.2. 2.071 to alloys Containing 27 3071 Cr and 1.5 2.0%

Mo. 0.80% Mn 0.5071 Commonly Yes. No harm done to Si 7 present in a any Region A:

commercial properties. heats. 0.20% Cu or Commonly present Yes. No harm done to 0.1571 Ni. singly, in commercial Region A or 010% Cu heats properties.

tivein producing the d For the additions of ruthenium and nickel; respec esired results.

'tively, the entries of Table V! are expanded as Tables Vll and VIII, where the individual results'for several samples are shown. In addition, these Tables show the self-repassivating effect obtained ,when sufficient of TABLE v11 either additive, Ru or Ni, respectively, is present.

EFFECT OF RUTHENIUM ADDITIONS TO Fe 28% Cr 4 4% Mo Al .LOY

Behavior in Ruthenium Boiling l lll H 80, Stress Alloy Addition Corrosion Rate Pitting Corrosion v Corrosion (3) No. ('7: by Weight) State (mils/year) KMnO.,NaCl (l) FeCl;; (2) (Boiling 45% MgCL) 338' 0.015 active 62,200 477-/\ 0.017 active P 334 0.020 passive 60 P P Resistant (not welded) 542 0.20 passive 9 P TABLE VII-continued EFFECT OF RUTHENIUM ADDITIONS To Fe '28%-Cr 47; Mo ALLOY Behavior in Ruthenium Boiling I07: H 80 Stress Alloy Addition Corrosion Rate Pitting Corrosion Corrosion (3) No. (/1 by Weight) State (mils/year) KMnO..NaCl (I) FeCl; (2) (Boiling 45% MgCl 475 0.30 passive 2 Resistant (welded) 683 (1.50 passive* 7 Failed (welded) 671 0.75 passive" 2 Failed (welded) 684 l.50 passive" 2 Failed (welded) plus 0.20 Ni passive 40 1 P I P Resistant (welded) self-repassivating (112% KMnO. 2% NaCl at 10"C. (2) 1071 FcCl o H O at 50C. with crevices.

(3) Magnesium chloride test.

P No pitting Not tested TABLE VIII. 1 1

EFFECT OF NICKEL ADDITIONS TO Fe 28% Cr 4% 'Mo ALLOY Behavior in Nickel Boiling 10% H SO. Alloy Addition. Corrosion Rate Fitting Corrosion Stress No. by Weight) State (mils/year) KMnO NaClfi FeCl;,(2,) Corrosion(3) 436 0.00 active 52,000 -P" P Resistant (welded) 677 0.10 active 63000 P P Resistant (welded) 239 0.20 active Pf P Resistant 217 0.25 passive 56 P P Failed (welded) 183 0.30 passive 52 P P Failed after i 119 hours 191 0.40 passive 29 P I P Failed after I 26l hours 241 0.50 passive 24 P i P Failed after i 16 hours 245 1.50 passive 6 P l P Failed in less than 16 hours 245 1.50 passive 6 P Pv Failed in less 1 4 than 16 hours 681 1.80 passive l l P. 1 P 664 2.00 passive* X P P 658 2.50 passive* 10 P I F 649 3.00 passive* 9 P F Footnotes for Table Vlll (1)271 KMnO -2'7( NaCl at 90C. 1 (2)1054 FcCl;..o,H O at 50C. with crevices. I (3)Magnesium chloride test on unwelded specimens except as noted. P Passed. no pitting F Failed. pitted Not tested These alloys are also self-repassivating.

The effectiveness of nickel in conferring passivity in of demarcation setting off area A from A andC from H 50 is a function of both chromium and molybde- C in FIG. 1. num, as shown in TABLE IX. Thus, positive benefits 50 H In addition, as indicated by Alloy No. 634 in TABLE accrue above a molybdenum content of about 2.0%- lX,i,alloy's'containing the specified minimum of rutheand with the approximate lower essential limit for chroniumappear to require the same 27.5% minimum chromium 27.5%, thereby locating the broken vertical line Y mium.

TABLE 1x I i NICKEL RUTHENIUM ADDITIQNS IO. F1:-C r-Mo ALLOXS Boiling 1071 Compt'isit ionil 'i Sulfuric Acid i Pitting'Corrosion l Stress 1 Alloy Cr 1 Mo Nickel 2 a State KMnO 2NaCl(2) .FeC.l,('3) CorrosionM) 1 -15. r (not welded) 1 0-231 25.0 410- 0.40 active F F 'F'ailed'after447 hrs.

0-232 26.0 4.0 0.40 h v active P F Resistant 0-233 27.0 4.0 0.40" 1 active P F Failed after 447 hrs.

0-191 28.0 i 4.0 "0.40 passive i P" P Failed after 261 hrs. Q-l96 28.5 0.0 0.40 u iv e F F 0195 211.5 Y 1.0 h 0.40 active F I F .0194 zx s 3.0? 0.40 I passive F 0.193. 221.5 .tlo 0.40 passive P F 0-192 28.5 3.5 0.40 passive P P TABLE IX-continued EFFECT OF NICKEL AND RUTHENIUM ADDITIONS TO Fe-Cr-Mo ALLOYS Boiling Composition( 1 Sulfuric Acid Pitting Corrosion Stress Alloy Cr Mo Nickel State KMnO,-NaCl(2) FeCl;,(3) Corrosion(4) (not welded) Ruthenium O-634 26.0 1.0 0.02 active F F (HPer cent by weight,

(2)271 KMnO, 2% NaCl at 90C.

(3}[0'71 FeCl h H O at 50C. with crevices. (4)Magncsium chloride test on unweldcd specimen. P resistant F pitted The research on additives of Table VI indicates that:

1. Aluminum can be added up to 0.60% to the compositions of this invention in order to obtain grain refmement.

2. Titanium and niobium, in contrast with the opposite expectation based on prior art, were not effective in my Fe-Cr-Mo-containing alloys to fix excessive C or N, although they did produce a grain refinement similar to that obtained with Al.

3. The noble metals aided region A compositions to achieve passivity in boiling 10% H 80 but palladium especially, and rhodium to a lesser degree, reduced the pitting corrosion resistance. Of the noble metals, ruthenium is especially attractive because of moderate cost, effectiveness in small amounts, and freedom from loss in pitting corrosion resistance.

4. Nickel is effective in producing passivation, but the quantities required make the alloys prone to stress corrosion cracking in MgCl solution. However, 0.01% Ru 0.20% Ni provided passivation without loss of stress corrosion resistance.

5. Nickel in the range of 2.0-3.0% causes the alloy to acquire the property of self-repassivation (refer Table VIII). There is, however, accompanying loss in pitting resistance in the ferric chloride test, and in the magnesium chloride stress corrosion test.

6. In alloys containing 2730% Cr and 15-20% Mo minima, it is feasible to obtain enhanced corrosion resistance (i.e., the properties of Region A by additions of 1.5-2.0% Si.

What is claimed is:

1. molybdenum alloy having good post-welding ductility consisting essentially of chromium and molybdenum in the weight percentages within areas A and C of FIG. 1, 0.25 -3.0 weight per cent Ni, carbon ppm maximum, nitrogen 200 ppm maximum, and carbon plus nitrogen 250 ppm maximum, the balance being iron and incidental impurities.

2. A corrosion-resistant ferritic iron-chromiummolybdenum alloy according to claim 1 incorporating 28.530.5% Cr and 3.54.5% Mo.

A corrosion-resistant ferritic iron-chromium IJNHEE STATES PATENT GFFEQE QER'EEHQCAEE @F QQRREQ'HGN Q1 PATENT NO. 3,929,475

DATED December 30 1975 INVENTOR(S) Michael A Streicher It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown beiow:

Front page, under "RELATED U58. APPLICATION DATA", after March 9 1971, delete "abandoned".,

Column 6, line 2'? before widefl, insert --inch--. 6

7 Column ll in Table at top of page, under "Alloy E10,", the

numerals 49a and 616 should be underscored.

Column l t, Table IV, first line, the "0" value for Alloy No, 513 should be -=-19-- instead of "9". 0

gigned and Sealed this twenty-seventh Day 0f April 1976 [SEAL] Arrest:

RUTH c. MASON c. MARSHALL DANN a Arresting Officer Commissioneroflalents and Trademarks

Claims

1. A CORROSION-RESISTANT FERRITIC IRON-CHROMIUM-MOLYBDENUM ALLOY HAVING GOOD POST-WELDING DUCTILITY CONSISTING ESSENTIALLY OF CHROMIUM AND MOLYBDENUM IN THE WEIGHT PERCENTAGES WITHIN AREAS A2 AND C2 OF FIG. 1, 0.25-3.0 WEIGHT PER CENT NI, CARBON 100 PPM MAXIMUM, NITROGEN 200 PPM

2. A corrosion-resistant ferritic iron-chromium-molybdenum alloy according to claim 1 incorporating 28.5-30.5% Cr and 3.5-4.5% Mo.