CA1263041A

CA1263041A - Nickel-chromium-molybdenum alloy

Info

Publication number: CA1263041A
Application number: CA000493553A
Authority: CA
Inventors: William Lawrence Mankins; David Gary Tipton
Original assignee: Inco Alloys International Inc
Current assignee: Huntington Alloys Corp
Priority date: 1984-11-13
Filing date: 1985-10-22
Publication date: 1989-11-21
Also published as: JPS61153262A; EP0184038A1

Abstract

ABSTRACT

A highly carburization resistant alloy characterized by good structural stability at elevated temperatures, the alloy containing correlated percentages of iron, nickel, chromium, molybdenum, carbon, titanium, etc.

Description

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The sub~lect lnvention is directed to a novel iron-nickel-chrom:LIlm (Pe Nl-Cr~ alloy characteri~ecl by a high degree of resistance to carburlzation and which affords a combination of other desirable metallurgical properties, including structural stability at elevated temperatures, circa lgO0-2000F, the ability to be both hot and cold worked, good resistance to corrosion includlng resistanGe to chloride attacks, etc.
, : INVENTION BAC~GROUND
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As is known, iron-base, nickel chromium alloys are e~tensively used in a host of diverse applications by reason of one or more (and within limits~ strength, ductility~ corrosion resistance, etc. Such attributes notwithstanding, this type of alloy generally suff rs from an inability to reslst~satisfactorily the destructi~e toll occasioned by ~carburization, a phenomenon by ~hich the alloy structure is environmentally degraded from the surface inward. As a consequence~ the load bearing capacity of the alloy is adversely affected as manifested by impaired~strength (stress rupture, creep), lowered ductility, etc.
Usually~the initial attack is along the grain bou~daries and this tends to accelerate failure, or at least premature removal of a given alloy component from its operational environment.
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2 PC-1225 In any case, if the carburization problem could be substantially minimized wlthout subverting other properties, such an alloy would find expanded use for such applications as the petrochemical and coal gasification fields, ethylene pyrolysis, etc., areas in which alloys are exposed to a combination of carbonaceous environments and high temperature.

But in addressing the p~oblem of carburiæation resistance, it would bs self-defeating to achieve success at the expense of other desired properties as contemplated herein, e.g., high temperature structural stability over prolonged periods of time, elevated temperature stress-rupture strength, workability, etc.

SUMMARY OP TFIF INVENTION

It has now been discovered that an Iron-nickel-chromium alloy of special chemistry and containing carefully correlated percentages of iron, nickel, chromium, molybdenum and carbon and certain other constituents discussed her~in results in a (i) markedly enhanced carburization resistant material at temperature levels at least as high as 180~-2000F. Moreover, the sub1ect alloy is (ii) workable , (iii) not prone to form deleterious amounts oE topological closepacked phases prematurely such as sigma, and otherwise offers ~iv) structural stability over substantial periods of time upon exposure to elevated temperature.
Further, the alloy is (v) weldable and (vi) affords a high degree of resistance to pitting attack in aggressive corrosive media.

In addition to the foregoing, it has been also found that the contemplated alloy offers enhanced o~idation resistance~ a phenomenon by which the alloy surface undergoes attack in oxygen-containing environments at high temperature. As a consequence, the material continuously undergoes weight loss, the surface "spalls off." As would be expected the oxidation problem is particularly acute in "thin section"
mill product forms, strip, sheet, thin wall tubing, etc.

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3 6l790-1589 DESCRIPTION OF T~E INV~NTION
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~eneral~y speakln~, the sub~ect invention eontemplates an iron-nlckel~chromium alloy containing about 24~ to 35% nickel, about 19 o 24% chromium, about 1.5 to 4.5~ molybdenum, carbon in an ~mount not o-IZ%
exceeding about ~ %, O to 1.5 or 2% man~anese, O to 1% aluminum, up to 1~ tltanium, o to 1% sllicon, O to nbout 0.3Z nitro~en, the balance being e~sentially lron. As contemplated herein, the expresslons "balance"
or "bal.1nce essentially" ln referrin~ to lron content do not p~eclu(lc the presence of other element~s commonly present as lncldental constituents, tnc1udin~ daoxidl~in~ and cleanin~ elements, and usual impurittes associnted therew~t~ ln amounts which do not adverselv affect the bAsic characteriscics of the ~11oy.

In carrying the invention into pr~ctice, molybdenum plavs a ma10r positlve sole ~n maxlmi7.in~ resistance to carburization. AdvantaKeously, it should be maintained at A level of about 2Z or more ln seekin~ optimum carburization reslstance. Percenta~es much beyond 4~ do not offer an appreciable advanta~e, gIven c09t considerations. ~lowever, ~here resistance to corrosion, particularly chloride attack is important, ehe molybdenum can be as high a~s about fiZ.

Chromium imparts reslstance to corrosion but should not exceed about 24 or 25% slnce lt lends to sig~a formation at elevated temperature~
and attendanc embrittlement problems. A ran~e of 20-23~ is quite s~ttsFflctory. The total chromlum plu~ molybden~m content preferably does not exceed 26~ nr 27% since mol~bdenum also lends to si~ma formation.
Where hi~h eemperature applications are not involved, the chromi~m plus molybrlenum can he extended to 29%.

; Nickel contrlhutr to good workahillty and mechanical properties.
Should the nlckel level fall much below 24~the stabilitv of the alloy could be impairedj particularl~ if the chromium andlor molvbdenum is at the hi~her end of their respective ranges. On the other hand, nickel percentaRes above 35% have be~n explored (up to 42%) without si~nificane ~a636~

4 PC-1225 property degradation, hut nickel does increase cost. A nickel range of 28~ to 35% is considered most beneficial.

Carbon to the excess, say 0.3%, detracts from pitting resistance.
In addition, workability is adversely affected~ h~wever, it does add to strength and other properties and, accordingly, a range of about 0.04 or 0.05 to 0.1% is deemed distinctly ad~antageous.

For workabllity and other benefits titanium should be present but amounts above 1% are not required. A range from O.l or 0.2 to 0.75% is quite beneficial. Aluminum can be used as a deoxidizer and as an aid to workability. A range of 0.05 to 0.5% is quite satisfactory.
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By so controlling the carbon, titanium, and aluminum as well as the high percentage cons~ltuents (Mo, Cr, Ni) the alloys are not only workable but can be produced using air melting practice. This is not to say vacuum processing ls precluded but there is an economic advantaga to the former.

In terms of such constituents as manganese and siliconJ both can be present in amounts up to 2% and 1%, respectively. Higher amounts are unnecessary. Where oxidation resistance is of importance manganese should not exceed about 0.6%. Manganese promotes weldability, particularIy at the higher end of its range with aluminum at the lower end of its range. It ls de0med that nitrogen, a potent austenite performer, can be present, a range of 0. 05 to 0. 25% being considered satisfactory. Nitrogen is considered to be beneficial at the lower nickel levels.

25~ The following information and data are given as illustrative of the invention.

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Carburization Resistance 14kg. samples of various compositions were alr melted and forged, the compositions being given in Table 1, Alloys A, B and C being beyond and Alloys 1 and 2 being within the invention.
TABLE I
Allov Mo C Cr ~i Ti Al Mn Si A 0.01. .0621.01 31~84 .38 .30 .14 .23 B 0.92 .0620.96 32.16 .37 .32 .11 .18 1 1.98 .1220.27 32.27 .35 .26 .26 .27 2 3.94 .1419.93 32.49 .31 .25 .37 .32 C 7.87 .1120.32 32.45 .34 .31 .30 .46 Balance iron plus impurities, e.g., sulfur and phosphorus In respect of the above alloy compositions, they were sub~ected to a gaseous carburization test in which specimens were machined into cylinders approximately ~" diameter and 1" :Ln length. These were placed in a tray and put in~o a muFfle type Eurnace, the temperature being 1800F. The test was conducted for 100 hours using a gaseous atmosphere of 2% methane plus hydrogen. After exposure, the samples were water quenched and then weighed to determine weight gain data. The results are reported in Table II.
TABLE II
Carburization Data: ~ormalized Weight Gain MoWeight Gain Alloy (%)(mg/cm2) A .01 11.7 0.92 9.3 1 1.98 6.3 2 3.94 6.2 C 7.87 4.9 As can be observed from the data in Table II, a rather dramatic improvement obtained in respect of carburization resistance with regard to Alloys 1 and 2. Alloy C (7.87% Mo) showed some further improvement but the cost associated with such molybdenum levels would not likely warrant such percentages on a commercial~scale.

Weight gain is essentially a measure o how many atoms of carbon have been absorbed but without re8ard as to to the depth o ef~ect.
Thus, concentration versus depth profiles were dete~nined and F~gure 1 reflects this information. Figure 1 confirms, in essence, the data of S Table II. As i9 manifest, with increasing molybdenum percentages the penetration profile shrinks indicating that less diffusion has occurred.

Figure 2 depicts surface potential versus molybdenum content.
This may be viewed as the chemical effect of molybdenum on carbon diffusion, or specifically the effect of molybdenum on gas-metal reaction at the surface, carbon solubility, or carbon activity coefficient. The surface potential appears to be a quite linear decreasing function of molybdenum, at least up to 4%. The behavior at 8% molybdenum is not clearly understood.

We have also determLned that molybdenum decreases the carbon ; 15 diffusion coefficient.

~; ~ Oxidation Resistance ; Tables III(chemistry) and IV (data) afford a comparison of the oxidation resistance behavior of aLloys within the invention versus commercial (control): alloys of somewhat similar composition.

The oxidation test was one of cyclic oxidation using 14,kg.
samples ~air melted) forged to flats, hot rolled to 0.3I2 inch and cold rolled to 0.125 inch. The test comprised sub~ecting specimens for 15 mlnotes at 2noooF~ cooling for 5 minutes in air, heating again to 2000F, holding for 15 minutes, again cooling 5 minutes in air, until testing was completed. Specimens were checked at 100 hr. intervals. Prior to test the~specimens were annealed at 2I50F and water quenched. Oxide was removed~by grinding to 120 grit.

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TABLE III
Alloy Mo C Cr Nl Ti Al Mn Fe % % % ~ % % %

6347 3 l.o9 .05 20~82 32.73 .30 .32 .09 Bal.
6348 4 3.92 .04 20.85 32.37 .40 .29 .OB
h349 D 9.62 .04 20.70 32.40 .35 .28 .08 Control ~1 .05 21 32 .5 .5 l.O "
Control #2* .Ol .05 20.93 32.93 .5 .45 .10 *Contained .54 Si and .07 Cu TABLE IV
2000F Cyclic Oxidation Data Heat No. Weight Change/Unit Area, mg/cm2 Depth of AlloylOOhr. 200hr. 300hr. 400hr. 500hr. 700hr. 1000hr. Attack,in.
3 ~1.0 +1.~ ~2.0 -~2.3 +1.7-2~.9 -81.7 .004 4 +1.1 +1.6 -~2.2 +2.6 +3.1-15.5 -66.9 .006 D -0.3 -0.4 -0.2 ~0.2 -0.8-8.9 -40.2 .005 Control #1 +2.6 -~l0.1 -86.6 -12~.4 -156.8 -223.1 -316.~ .020 Control #2 +1.5 - 1.7 -25.0 -65.3 - 98.8 -180.9 -294.5 .019 As will be observed, the alloys within the inventlon compar~d more than favorably wlth the Control alloys. Maintaining manganese at low levels, i.e., below 0.6 or 0.5% contributes to enhanced oxidat:Lon resistance.

Cyclic oxidation Test on Alloy 4 in the form 0.030 thin gauge sheet also compared favorably with Control Alloy No. 1 as reflected in Table V.
TABLE V
1000F Cyclic Oxidation Data, .030 inch sheet Alloy lOOhr. 200hr 300hr. 400hr. 500hr. 700hr. 1000hr~

4 -~1.6 -0.1 -21.1 -26.~ 42.5 ~75.5 -95.3 *Control ~ 2.6 -40.1 -86.6 -12~.4 -156.8 -223.1 -316.
*=.125 gage ~2~3~

Testing of thil~ gauge specimens is markedly ~ore severe because warpage ~9 much more likely to occur on cooling thus increasing the tendency Eor oxida scaling.
Struct _al (Phase) Stabl]ity In Table VII infra are given the results oE various impact (ability to absorb impact) tests. Charpy V-Notch Lmpact testing is often used as a means of predlcting whether an alloy wilL under~o embr-lttlement on being exposed to elevated temperatures for prolonged periods.
While a 1000 hour test period might normally be deemed lQ sufficiently severe, tests were also conducted for 3000 hours at temperatures of 1400F and 1500Fo The composition of the alloys tested are given in Table III.
TA~LE VI
Alloy Mo C Cr Ni Ti Al Mn Fe 3 1~89 ~05 20~82 32~73 ~31 ~32 ~09 Aal.
4 3~92 ~04 20~85 32~37 ~40 ~29 ~08 Bal.
D 9.62 .04 20~ 70 32 ~ 40 ~ 35 ~ 28 ~ 08 Bal.
TABLE VII
Temperature Time Charpy V-Notch, ft. lbs.
(F) (hr.) Alloy 3 Alloy 4 Alloy D

1400 3000 90 14 *
1500 1000 87 3~ *
1500 3000 81 17 *
*discontinued All samples annealed at 2150F and water quenche~ prior to exposure.

The alloys of the invention (Alloys 3 and 4) were quite resistant to premature embrittlement as evident from Table VII. Even upon 3000 hour ~9~3~

testing the alloy~ wlthin the in~rention performed satisfactorily. Alloy n (g . 62~ Mo) did not stand up at 1400~/100 hr. It was sigma prone.
To further study stability a commercial size, (450 lb.) centrifugally cast hollow billet was e~ctruded to a tube shell and cold worked to 2.25 inch dia. x 0.270 inch wall tube. (Compositlon: 0.06 C, 0.03 Mn, 0.33 Si, 31.9B Ni, 21.55 Cr, 0.18 Al, 0.32 Ti, 3.12 Mo, Fe balance). The specimen was annealed at 2150F Eor an hour and air cooled prior to test. The tube was rupture tested at 1200F/12KS1 for the tremendously long period of 26,394 hours (3 years) and then discontinued no failure having occurred. A metallographic study showed M23C6 carb~des and very fine particles of sigma within the grains w~ich were deemed innocuous, particularly since a portion of the specimen was placed in a vise and bent to ascertain if embrittlemen~ had occurred. The ductile nature of the specimen was obvious.
t~eldability -Compositions for weldability are given in Table VIII. In this connection, two alloy serie.s were evaluated one involving variations in aluminum and manganese (Alloys 5-8), the other (Alloys A, B, 1, 2, and C) exploring the effect of molybdenum.
Material was provided as ~" thick x 2" wide hot forged flats which were overhauled and rolled to 0.310" thick x 2" wide for Vareetraint test samples. Included for purposes of comparison is a well lcnown commercial alloy (Control).

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TABL~ VIII
Alloy Mo C Cr ~i Ti Al Mn Fe A 0.01 .06 21.01 31.84 .38 .30 .14Bal.
B 0.92 .06 20.96 32.16 .37 .32 .11Bal.
1 1.98 .12 2~.27 32.27 .35 .26 .26 "
2 3.g~ .14 19.93 32.49 .31 .25 .37 "
C 7.87 .11 20.32 32.45 .3~ .31 .30 "
3.93 .05 20.32 32.14 .40 .27 .07 "
6 3.82 .05 21.08 32.25 .31 .0~ .15 "
7 3.90 .05 20.50 32.14 .~2 .30 .56 "
8 3.87 .08 20~88 32.25 .28 .04 .56 "
Control.26 .08 19.89 32.80 .44 .32 .83 "
Alloy *contained 0.04~ copper. All heats contained small amounts Si.
Bal. = balance and impurities.

A travel speed of 5"/min, an amperage of 190 amps and a voltage over the range of t3.8-15.0 volts were employed. The Varestraint teæt, one of relatively constderahle severity, was conducted on hoth a 50" and 25" radius block with the results given in Table IX.

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11 PC-~225 TARLE_IX
Varestralnt Test Resul~,s 50 Inch Radius Block 25 Inch Radius Block Test MCL Ave. TCL Ave. Test MCL Ave. TCL Ave.
Alloy Thick (mils) MCL (mi].s) TCL Thick (mils~ MCL (mils) TCL
.3030 0 .302 18 79 A .3040 0 0 .303 13 1550 75 .3030 n 0 .304 15 96 .30~0 0 .314 12 26 B .3090 0 0 0 .315 15 1445 53 .3~80 0 .313 15 ~7 .31~0 0 .31~ 30 105 1 .3110 0 0 0 .312 20 2432 B4 .3100 0 .311 22 12~
.3090 0 .320 35 68 2 .3140 0 0 0 .320 25 31 g3 86 .3140 0 .315 33 118 ,3140 0 .314' 36 ~ 161 C .3150 7 0 13 .313 28 34 97 123 .31~2l ~10 .313 38 112 .3130 0 .313 12 20 .3l30 0 0 0 .315 26 l9 114 87 .3130 0 .316 28 128 3Q .3000 0 .299 28 96 6 .3000 0 0 0 .302 26 15 123 117 .3030 .3130 0 .31~ 38 126 7 .3150 0 0 0 .313 22 3065 96 ~ .304 ~ - b 0 - .306 i6 ~ 63 8 O3070 0 0 .30~ 0 8 0 .3050 0 _ _ ~ .306~7 101 .303 38 197 ControlO306 26 35 41 71 .303 38 197 Alloy .30932 72 .307 30 131 ~ MCL -Maxim~m Crack Length Amperage 190 l'CL -Total Crack Length Voltage 13.8 - 15.0 Travel Speed 5"/min.
All the specimens perEormed at least as ~more) satisfactorily as the commercial control alloy. Of the molybdenum series, the high ~2~;3~

molyb<lenum mater:Lal (Alloy C, 7,87% Mo) was more susceptible to cracklng.
Regarding the Al/Mn series, the low alumlnum, high manganese material (Alloy 8~ was the most crack resistant. Accordingly, by using molyhdenum levels within the :Lnvention, particularly with low aluminum, 0.04 to 0.35, and high manganese, ~say 0.3 to 0.6%, weldabi]ity is improved.
Pitt:Lng Corro ion Resistance Data reported in Table X give an indlcation of pittlng resistance. Samples were cold-rolled to 0.125" antl annealed at either 2l50F or 2350F for one hour, followed by water quench:Lng. Specimens (approximately 7"x3"~ were prepared by grLnding to 320 grit and then exposed 4 hours at 95F in acldified 10.8 2/o FeC1. 6H2 (Smith Test).
Af~er exposure, welght loss per unit surface area was determined and the specimens visually evaluated for the appearance of pits.
TABI.E X
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15 ~ C Mo Cr Ni Ti Al Mn Sl Pitting Mg/cm2 % % % ~ ~ % %
E .29 1.98 20.86 32.70 .42 .33 .1l 1.84 Yes n.d.
9 .05 1.89 20.82 32.73 .30 .32 .09 .20 Yes n.d.
F .28 3.79 20.95 32.28 .29 .29 .07 .17 Yes 7.773 .04 3.29 20.85 32.37 .40 .29 .08 .21 No 0.334 G .28 2.26 20.86 32.47 .32 .31 .07 .18 Yes 10.181 n.d. = not determined As can be seen from Table X, carbon at the higher levels is detrimental to pitting resistance. It detracts from the resistance to pitting imparted by molybdenum. Accordingly, where corrosion resistance is important carbon should not exceed about 0.12% Also, for such purposes the molybdenum can be extended to 6%.
Irrespective of carburization resistance and other attributes) if the alloys arP unworkable, then they would find little utility. However, alloys within the invention are both hot and cold workable. Using Alloys 3, 4 and D of Table VI, these alloys forged readily and the forgings upon inspection were of high quality.

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Hardne~ss data are given in Table XI for ~iven annealing temperAture.s. Also included is hardness in the cold worked condition. In this connectlon~ specimens were cold rolled to about 0.125" thick from thickness given in Table XII.
TABLE XI
_ Annealing Meat Annealing Hardness As Cold Worked Alloy Treatment, F ~b Hardness, Rc 3 2150/ 1 hr~ 66.5 33 4 2150/ 1 hr. 71.5 33 0 n 2]50/ 1 h~. 84.5 33 TABLE XII
Starting Final y Thickness Thickness %Reduction 3 .524 .126 76 4 .473 .127 73 D .500 .125 75 ?
Considering both the data from Tables XI and XII, the hardness measurements reflect that the alloys are relatlvely readily workable.
From Table XII, it will be noted that cold reductions of more than 60%
could be achieved without intermedia-te annealing. This together with the hardness data reflects that the alloy0 have excellent cold workability and a low work hardening rate. It might be added that high carbon is not beneficial to workability.
Although the present invention has been described in connection with preferred embodiments, 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.
Such modifications and variations are considered to be within the purview and scope of the inventlon and appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An iron-nickel-chromium alloy characterized by good carburization resistance together with structural stability at elevated temperatures, the alloy consisting of (weight percent) about 28 to 35% nickel, 20 to 24% chromium, at least 1.5% and up to 4.5%
molybdenum, carbon present up to 0.12%, titanium present up to 1%, 0 to 1% aluminum, 0 to 2% manganese, 0 to 1% silicon, 0 to about 0.25%
nitrogen and the balance essentially iron.

2. The alloy of claim 1 containing 29 to 33% nickel, 20.5 to 23% chromium about 2 to 4% molybdenum, 0-04 to 0.1% carbon and 0.2 to 0.75% titanium.

3. The alloy of claim 1 in which the sum of chromium plus molybdenum does not exceed 26%.

4. The alloy of claim 1 in which weldability is enhanced by combination of aluminum 0 to 0.5% and from 0.5 to 1% manganese.

5. An iron-nickel-chromium alloy characterized by good carburization resistance together with structural stability at elevated temperatures, the alloy consisting of (weight percent) about 24 to 35% nickel, 19 to 25% chromium, at least 1.5% and up to about 6% molybdenum, carbon present up to 0.12%, titanium present up to 1%, 0 to 1% aluminum, 0 to 2% manganese, 0 to about 1% silicon, 0 to about 0.25% nitrogen and the balance essentially iron.

6. A (i) wrought, (ii) weldable iron-nickel-chromium alloy characterized by excellent (iii) hot and (iv) cold workability (v) a low work hardening rate, (vi) high strength at elevated temperature and being further characterized at temperatures at least as high as 1800° to 2000°F by (vii) good carburization and (viii) oxidation resistance together with (ix) good structural stability when exposed at a temperature at least as high as 1200°F for at least 1000 hours, the alloy consisting essentially of (weight percent) about 28 to 35%

nickel, 20 to 24% chromium, at least 1.5% and up to 4.5% molybdenum, carbon present up to 0.12%, titanium from 0.2 to 1%, 0 to 1%
aluminum, 0 to 2% manganese, 0 to 1% silicon, 0 to 0.25% nitrogen and the balance iron, said alloy being in contact with a carbonaceous environment at high temperature which is conducive to causing carburization.

7. The alloy of claim 6 containing 29 to 33% nickel, 20.5 to 23% chromium, about 2 to 4% molybdenum, 0.04 to 0.01% carbon, 0.2 to 0.5% titanium and about 0.05 to 0.75% aluminum.

8. The alloy of claim 6 in which the sum of chromium plus molybdenum does not exceed 26%, manganese does not exceed 0.6% and aluminum is from 0.05 to 0.5%.

9. The alloy of claim 6 in which weldability is enhanced by controlling the percentages of aluminum and manganese such that the aluminum is from 0.04 to 0.35% and the manganese is about 0.3 to 0.6%.

10. A weldable, iron-nickel-chromium alloy characterized by excellent hot and cold workability, a low work hardening rate, which strength at elevated temperature and being further characterized at temperatures at least as high as 1800° to 2000°F by good carburization and oxidation resistance together with good structural stability when exposed to temperatures at least as high as 1200°F
for at least 1000 hours, the alloy consisting essentially of (weight percent) about 28 to 35% nickel, 19 to 23% chromium, at least 1.5%
and up to about 4% molybdenum, carbon present up to 0.15%, titanium present from 0.25 to 1%, 0 to 1% aluminum, 0 to 2% manganese, 0 to 1% silicon, 0 to 0.25% nitrogen and the balance essentially iron.

11. The alloy of claim 10 in which the manganese content does not exceed 0.6% to thereby enhance the property of oxidation resistance.

12. The alloy of claim 10 in which the carbon content is at least about 0.05% to provide greater strength properties.

13. A weldable, hot and cold workable, iron-nickel-chromium alloy in the wrought condition characterized by a low work hardening rate, high strength at elevated temperature and further being characterized by good carburization and oxidation resistance at temperatures at least as high as 1800°F, together with good structural stability when exposed at a temperature at least as high as 1200°F for at least 1000 hours, the alloy consisting essentially of (weight percent) about 28 to 35% nickel, is to 24% chromium, at least 1.5% and up to about 4% molybdenum the sum of the chromium plus molybdenum not exceeding 27%, carbon present up to 0.12%, from 0.2 to 1% titanium, 0 to 1% aluminum 0 to 2% manganese, 0 to 1%
sllicon, 0 to 0.25% nitrogen and the balance iron.

14. The alloy of claim 13 which contains at least 0.05%
aluminum and in which the chromium plus molybdenum does not exceed about 26%.

15. A weldable, hot and cold workable iron-nickel-chromium alloy characterized by a low work hardening rate, high strength at elevated temperature and further being characterized by good carburization and oxidation resistance at temperatures at least as high as 1800°F together with good structural stability at a temperature at least as high as 1200°F for at least 1000 hours, the alloy consisting essentially of (weight percent) about 28 to 35%
nickel, 19 to 25% chromium, at least 1.5% and up to about 4%
molybdenum, carbon present up to 0.12%, 0 to 1% titanium, from 0.05 to 1% aluminum, 0 to about 0.6% manganese, 0 to 1% silicon, 0 to 0.25% nitrogen and the balance iron.