US2671050A - Stainless steel alloy and apparatus for converting hydrocarbons - Google Patents

Description

F. J. SHORTSLEEVE ET AL 2, 050 STAINLESS STEEL ALLOY AND APPARATUS FOR OCARBONS March 2, 1954 CONVERTING HYDR Filed March 31, 1950 '3 Sheets-Sheet 1 L y?! I v w vwv vvw/vvv 35 3O Z5 20 W777; CHROM/UM O AYAVAY AYV R 4 INVENTORS:

Franc/ls J Shorfs/eeve Morris Nich lson Ym 7% .dTTO/PNEY March 1954 F. J. SHORTSLEEVE El AL 2,671,050

STAINLESS STEEL ALLOY AND APPARATUS FOR CONVERTING HYDROCARBONS 3 Sheets-Sheet 2 Filed March 31, 1950 Quick? R E:

ON b 8 m3 2a m? mwmfiw an M PM W

nrfomvar M r 1954 F ORTSLEEVE ET AL 2,671 50 SH STAINLESS STEEL ALLOY AND APPARATUS FOR CONVERTING HYDROCARBONS Filed March 31, 1950 3 Sheets-Sheet 3 W7." 7. NICKEL 0v 6 6 a IO :2 l4 l6 1s 26 W77 2, CHROM/UM- I INVENTORY: 27 Franc/L9 J Shorfs/eeve J? Morris E. Nicgolson Em A ATTORNEY Patented Mar. 2, 1954 STAINLESS STEEL ALLOY AND APPARATUS FOR CONVERTING HYDROCAR-BON S Francis J. Shortsleeve and Morris E. Nicholson, Chicago, Ill., assignors to Standard OilCompany, Chicago,

111., a corporation of Indiana Application March 31, 1950, Serial No. 153,186

Claims.

This invention relates to improved oxidation and corrosion resistant chromium-nickel-silicon stainless steel alloys which are not susceptible to embrittlement caused by the formation of sigma phase at high temperatures. Further the invention relates to apparatus for converting hydrocarbons which is adapted to resist the action of corrosive substances contained in the oil, and to withstand continued exposure to the temperatures employed in heating the oil that is undergOlIlg treatment without forming the embrittling sigma phase.

In order to increase the resistance of iron to corrosion and to oxidation at elevated temperatures, it is customary to add chromium. While excellent oxidation resistance results, the high temperature strength is not improved materially with increase in chromium content. This deficiency can be overcome by adding nickel but, for some compositions, only at the expense of decreasing the resistance of the alloy to sulfur corrosion at the elevated temperatures. Both the iron-chromium alloys and the iron-chromiumnickel alloys in which the chromium exceeds about 20 Weight percent have the very serious disability that long exposure to temperatures of the order of 900 F. to 1700 F. eventually results in a very great loss in creep strength and duetility-so great a loss that the metal often can be shattered with a blow from a hammer. This phenomenon has been described as embrittlement due to the formation of the so-called sigma phasea complex compound which is basically iron-chromium but may contain also some of the other alloying elements found in stainless steels, such as, silicon, manganese, nickel, etc. It is known that addition of nickel, in moderate amounts, to chromium-iron alloys promotes the formation of the sigma phase, which diiliculty has limited the use of the chromium-nickel-iron alloys in corrosive atmospheres when the metal is exposed to the range of temperature at which sigma forms, 1. e., 900-1700" F., depending upon the particular composition. Above about 150 1700 F., depending upon the particular composition, the sigma phase changes into either the ferritic (alpha) phase or the more desirable austenitic (gamma) phase and no embrittlement due to sigma exists.

As has been mentioned, the presence of nickel in some percentage range reduces the resistance of the iron-chromium alloy to sulfur corrosion. It has been found that the addition of silicon overcomes this harmful effect of nickel, and also greases the oxidation and. ca burizaii n resist- 2 ance of the alloy. Besides the fact that silicon in amounts of 3% (all compositions given herein are in weight percent) or more reduces the workability of the alloy to the point that it is no longer practical to make wrought articles, it has been believed that the addition of silicon in any amount was harmful in that it not only promoted the formation of sigma but also broadened the range of chromium contents over which sigma could form. For this reason, in the chromiumnickel stainless steels, the amount of silicon usually has been kept as low as efiicient steel making methods permit; normally in typical steels such as the 18Cr-8Ni and 25Cr-20Ni' compositions the silicon'conte'nt is less than 1%, for example, about 0.5%.

Other alloying elements that can be used to increase high temperature strength and increase oxidation and corrosion resistance are believed to promote sigma formation. Some of these elements are aluminum, columbium, titanium, molybdenum, manganese and cobalt.

In general the metallurgists in their search for alloys with high oxidation and corrosion resistance have avoided the elements that were-believed to be sigma phase promoters and have tried to stay out of the sigma-forming range of chromium contents. An example of such a stainless steel, supposedly non-sigma-forming is the nominal 18Cr-8Ni. However, failures have occurred in such steels at temperatures in the sigma-forming range .that have not been explained because no consideration was given to sigma formation. One of the objects of this invention is to develop a chromium-nickel-iron-silicon alloy that is highly resistant to oxidation and corrosion at high temperatures and embrittlement. Another object is to develop a silicon containing chromium nickel stainless steel that does not form the sigma phase when subjected to temperatures between 900-l700 F; for prolonged periods. Still another object. isto. provide a highly corrosion and oxidation resistant and non-sigma-forming alloy, containing at least about 1% and less than about 3% of silicon, that can be Wrought into such articles as furnace tubes, jet turbine blades, etc. Yet another object is to provide a stainless steel that is non-sigma forming and is an austenitic or mixed austeniticferritic type. f Since many hydrocarbon oils contain sulfur compounds that decompose, while being heated for separation by distillation or conversion into lower boiling materials, into highly corrosive hy-; drogen. sulphide, it is an object of this inventionis not subject to sigma J to develop an apparatus that is resistant to sulfur compound attack at elevated temperatures and is not subject to high temperature embrittlement from sigma-phase formation.

The invention by means of which these objects are achieved is based on the discovery that a maximum effective-content otchromium existsat which, ifthe" nicheliccntent is; suitably adjusted, up to about 3% of silicon can be employed without introducing any susceptibility to form the sigma phase. The invention comprises a nonsigma forming stainless steel containing-notmore than about 17.5 effective percent of' chromium, at least about 1% but less; than about 3.%: of. silicon, and an amount of nickel. which is: suitably adjusted at or above a hereinafter described particular minimum requirementior theparticular chromium and silicon content.

At an efiective chromium content oi 17.5%, the minimum nickel content required to maintain a:- substantially austenitictype non-sigma-forming steel will vary from at least about 1.0% at 1%: silicon content to at least about 15%:at 3% silicon. content. If an alloy containing both austenite and ferrite is desired,- the nickel content must be reduced: to? less than at 1%. silicon and less than at 3% silicon and imorder to prevent sigma-iormation at these lower nickelcontents the: chromium content also must bec'reduced as will be shown later; The remainder ot the alloy is principally iron and the-usual elements contained: in commercial stainless: steels,1 e:- g., manganese-which normally may be present in amounts lessthan" 2 carbon is preferably less than 011%: but may be present in amounts up to 0.5 and steel making amounts oi phosphorus; sulfur, and miscellaneous elements such as nitrogen; cobalt, vanadium, columbium, titanium, etc.

The stainless steel compositions of our invention areillustrated in: the following figures which form a part of: this specification.

Figures" 1. and 2 are plots on. triangular coordimates of. the iron-chromium-nickel system. with variousamounts of silicon present, and

Figure}; is a cartesian plot. showing the relationshi-p between nickel and chromium at various sihcon. contents.

The invention can. be better" understood by reference to Figure 1 which shows on triangular coordinates the phase diagram ABCDHJE of the highv purity irmi-chromiunr-nickel; system at- 1200 F. The high purity boundaries are based onithe best: results published and our own work. 'Ihecompositions falling within the ABCDI-IJ portion of the ternary diagram are not susceptible to sigma formation. This portion is divided into the'austenitic (7) phase area ABH, the ferritio (al phase area JCI) and the mixed austenitic plusferritic (7+0) phase area HBCJ; B is the gamma; triple point and C is the alpha triple point.

While for some purposes a non-sigma forming ferritic' steel may be desirable, the austenitic and austenitic-ferritic steels have the most desirable high temperature properties. When an extremely tough alloy that is particularly resistant to oxidation at elevated temperatures is needed, an alloy that is wholly austenitic or substantially austenitic is required. Our' invention is primarily concerned with iron-chromiumniokel-silicon alloys that are austenitic or mixed austenitic-ferritic in nature;

In addition to the high purity alloy boundaries there-appear Figure 1 theboundary lines sep- 4 arating the sigma and non-sigma forming regions for the iron-chromiurn-nickel alloys to which 1, 2 and 3% of silicon have been added; these boundaries appear as the solid lines that diverge from the line AB.

The dotted lines in Figure 1 represent the previouslyaccepted: positions of the silicon addition phase boundariesof iron-chromium-nickel alloys. These boundaries were constructed as the best possible synthesis of the conclusions expressed in these articles: Schafmeister and Ergang, Archiv" fiir: das Eisenhiittenwesen, vol. 12, 459 (1939') Gowand Harder, Trans Amer Soc Metals, vol. 30,. 855 (1942); Payson and Savage, Trans Amer: Son. Metals, vol. 39, 404. (1947).

In ohd'erto show the eiiect of silicon addition moreclearlmthe area enclosed by the heavy lines WXYZ' has been replotted on a larger scale in Figure. 2. Our work indicates that when about 1% of silicon is added. to a high purity ironchromium-nickel alloy the gamma triple point shimzto thepoint B at Ni=10-.O-% and the alpha shiitsto the point C at Ni=3.5%= and Cr=l,4a5;%-. When about 2% of silicon are added the gamma triple: point shifts to the point B at Ni=12.587 and. the alpha. triple point shifts to the point. C? at Ni=4.0.%-. and Cr=12.5 At: a, silicorraddition of about 3% the gamma triplepoi-nt shiftsto B at Ni=-1-5.0 and the alpha triple point shifts to C at Ni=4.5-% and. Cr=10.5;%. The lines B 8 B 0 andB3C which form the phase bound.- ary between the sigma and non-sigma forming (1+ area and the a+- +r area is a. straight line.

Figures 1 and; 2 indicate that in so far as. experimental accuracy permitsin the location of the line, the phase boundary between the austenitic. area and the austenitic plus sigma area: for the. silicon addition alloy-up? to about 3% of silicon addition-coincides. with the high purity alloy boundary at about 11.5%. chromium content. (The experimental error in the determination of this position. is. believed to be less; than about 0.5% absolute.)

Ihe short lines pointing toward the. iron scale of the chart indicate the. approximate position of the boundary between the gamma. and gamma plus alpha areas and the gamma plus alpha. and the alpha areas respectively.

While our experiments have not established with certainty the point, where the line AB be gins to bend significantly, it appears: that the departure. from a straight line, or nearly straight, begins to beapprcciable at about 20% nickel content.

The dotted. lines in Figure 2 illustrate the shape and location of the silicon addition boundaries as previously accepted. Lines M N P M N P and M3N P correspond to silicon contents of 1%, 2% and 3% respectively.

Our discovery explains the heretofore unexplainable failures of the nominal 18Cr-8Ni steels inprolonged service. at temperatures in the range of 9410-4500 F. These alloys are sigma-forming because the nickel content is too low to overcome the sigma-forming tendency of the about 0.5% of silicon usually present in the commercial puritystainless steels. Figure 2 indicates that about 1% more nickel atabout 17.5% chromium, instead of the 18-20% specification, would render the nominal 18-8 steels non-sigma forming.

Thusour invention comprises iron-chromiumnickel .alloys containing from at least about 1%- to not more than3% V of silicon which are not sus-- ceptible tosigma phase formation and which confiist of the austenitic or mixed austenitic-ferriticphases. The area bounded-by the lines MNPTBA represents the composition of our stainless steel alloy at silicon contents ranging from at least about 1% to not more than about 3%; the line MNP represents the minimum amount of chromium content and the line ABT represents the maximum amount of chromium content in our composition; the line ABT represents the minimum amount of nickel needed to overcome the sigma forming tendency of the chromium and silicon but the nickel content can be greater than this minimum but preferably should be less than about 20 (Of course when We speak of the lines MNP and ABT, we mean to imply the line with the superscript corresponding to the desired silicon content.)

In order to show still more clearly the variation of the maximum chromium and minimum nickel content of the iron-chromium-nickel alloy with varying silicon, the phase boundary lines ABC have been placed on a cartesian plot of nickel vs. chromium which is illustrated as Figure 3. Also shown in Figure 3 are the previously accepted silicon addition boundary lines MNP. The letters have the same significance in all three figures. 'The lines PT, PT and P T represent a portion of the previously accepted boundary between the ferritic area and the ferritic plus sigma phase area; T represents the junction of this previously accepted boundary and the line BC of our phase diagram. The line T T T indicates the manner in which this intersection point shifts its location with changes in silicon content from 1% to 3%.

Although the line T T T has been drawn as a straight line, it is more probably somewhat curved. However, the deviation from the straight line relationship is believed to be within the experimental error. When T T T is considered to be a straight line, the location of any point on the line can be calculated using the following relationship (for convenience the following symbols have been adopted for use in equations: Si=wt. per cent silicon; Ni=wt. per cent nickel; Cr=wt. per cent efiective chromium) Cr=l7.0-1.7Si and Ni=4.5+0.7Si. The relationship between nickel and chromium for a T T T line alloy is approximately, Ni=10.00.3Cr.

The silicon addition lines, in Figure 3, AB C AB C and A38 0 can be divided into two segments for easier analysis. Line AB is relatively straight at an effective chromium content of 17.5% from a nickel content of about 8.5% to about 20%. The gamma triple points represent the minimum content of nickel, at the particular silicon percentage, necessary to maintain a nonsigma forming composition; this amount is the gamma triple point nickel content. The minimum nickel percentage can be calculated for any variation of silicon between about 1% and 3% by the formula: minimum Ni=7.5+2.5Si, when the effective chromium content is about 17.5%.

The boundaries B C B 0 B 0 form a family of straight lines whose slope may be determined from the percentage of silicon present, as follows:

Figure 3 indicates that the amount of nickel 7.

present can be greater thanthe calculated mini- 'mum amount without adversely affecting the sigma susceptibility; however, it is preferred to maintain'the nickel content at less than about 20%.

In much the same manner the previously accepted silicon addition lines, in Figure 3, MW P M N P and M N P can be divided into two segments for analysis; each of these segments can be considered as approximately a straight line. It is estimated that line LPN! is approximately a straight line at a constant chromium content from a nickel content of about 13% to about 20%. A generalized relationship has been determined between the previously accepted maximum amount of chromium that can be tolerated in the composition and the silicon content, and between the minimum amount of nickel previously thought necessary to maintain a non-sigma forming composition at the particular silicon content, as follows: maximum Cr=17.52.0Si and minimum Ni=9.0+2.0Si.

The previously accepted boundaries N P N P and N P forms a family of approximately straight lines whose general slope is=0.2Si. The relationship between the minimum amount of nickel previously thought to be needed to maintain a non-sigma forming alloy at a particular chromium and silicon content is,

The PT lines shown in Figure 3 represent the previously accepted boundary between the ferritic area and the ferritic plus sigma area. These lines form a family whose relationship can be generalized as:

This line is the low chromium-low nickel section of the polygonal area which represents the stainless steel composition of our invention.

Thus our invention comprises a stainless steel (of commercial purity in respect to elements other than chromium, nickel and silicon) which contains, at least about 1% but less than 3% of silicon, an effective amount of chromium of not more than about 17.5% and a lower limit dependent on the silicon content, at least'a mini mum amount of nickel which amount varies with silicon and chromium content (which minimum amount of nickel, and, lower limit of chromium, can be determined by reference to the area enclosed by the letters ABTPNM at the desired silicon and chromium content) and the remainder principally iron. Figure 3 indicates that at about 2% silicon content a non-sigma forming composition can be produced which may contain from at least 13.5% to about 17.5 eiiective percent of chromium, from at least about 12.5% to 20% of nickel and the remainder principally iron, which composition is substantially all austenitic phase; also a composition can be produced which may contain from at least about 11.5% to about 15% of effective chromium at a nickel content of about 8%, from at least 13.5% to 17.5% of efiecti've chromium at a nickel content of 12.5% and the remainder principally iron, which composition is a mixed austenitic-ferritic phase alloy;

The composition of our stainless steel alloy can be more accurately defined by mathematically describing the perimeter of the composition as "1 cente iie 'waQhmm -u1n-v E m;- en ice. lcr-si co ten ircm z stabout 1%. o than about 35711. e mb sition. oi our nyention may be considered as an area obtained by plotting thechromium and nickel contents of the. phase boundary between the sigma containing: and; the sigma-free 'y and 'Y.+G- phases for the phase diag am as. t is. ow lo at d. by our work and the previously accepted location, The upperlimit on nickel content our composition about 20% which limit is imposed by the. fact that: nickel contents above 20% make the stainlessstccl unduly: susceptible to high temperature corrosion. The silicon of our alloy should be atleastabout 1% in order toobtain the benefit oiincreased oxidation resistance but should be held below about 3% in order to permit the manufacture of; wrought articles. The area med by the invention is bounded: the maxiamount. of nickel is less than about 20 the maximum effective amount of chromium; is about.17.5% when the amount of nickel is at least, Ni =,'7.5+2.5si; between the limits (Cr: 17.5, Ni=7 .5+2.5S'1) m=(1.0+2.0Si) /(4.5+2.0Si)

between h im ts.

(C1=17.0-=.1.7Sl, Ni=4.5+0.7Si)

and (Cr=17.5-3.0Si, Ni=4.0+2.0Si), the effective chromium content and the minimum nickel content vary in accordance with equation:

Cr=21.5Si-Ni between the limits,

(Cr=l'7.5-3.0Si, Ni=4.0.+2.0Si)

and (Cr=17.5-2.0Si, Ni=9.0+2.0Si), the effective chromium content and the minimum nickel content vary in accordance with the equation: =19.0.-5.5S i+0.2Si(Ni) between the limits (Qr=17.52.0Si, Ni=9.0+2 .0Si) and Ni=about goathe effective chromium content is at least about Cr=1'7.5-2.0S i; and the remainder of the alloy principally iron. As has been pointfld out elsewhere in this specification; the equations of the sides of the polygon that bounds our compo? cities. and he cal la e p rip ry breakp s are general z r m a t p ific l c on tents and allowance must be made for some deviation from the actu l o by t llowable. exp rimen al r o in i type Qt o Although we speak of at leastabout 1% of silicQ we realize that silicon may be present in comm ci l purity stainle ssteels. n amount to. about 0.7% but silicon is considered to be an adde l o m nt n se n o n greater than 0.7%. When. We, Speak of silicon content, we mean the total silicon present as def termined by analysis.

Reference to Figure 1 will show that the above compositions vary in phase content from all gamma, phase to a mixture of alpha and gamma phases. For some purposes an extremely tough alloy that is particularly resistant to oxidation at. elevated temperatures is desired; for these requirements an alloy that is entirely austenitic (gamma phase) or issubstantially allaustenitic s ce se r- Fer u ses the. s icoacou cnt s ld a ea t bou .7 in order t set the ben fit: 7. n r a ed oxidation. esis ce bu sno d ekep below abou 3% in or er t erm t the alloys to bemanufactured into wroughtafiticles; the effective chromium content should be kept aboveabout 10% order to afford sufficient oxidation resistance at temperatures in the range 1000,-.15 00 F. and must be kept below about 17.5% inorder to avoid sigma formation; the nickel content must be high enough to avoid sigma formation and improve. workability but should be held below about 20% in ordertoprevent making the alloyunduly susceptible. to. high temperature corrosion. The minimum amount of nickel that is necessary in an alloy that is nonsigma forming and is all or substantially all austenitic is that defined by the formula;

The susceptibility of a stainless steel alloy to form sigmav is primarily determined by the amount of chromium in the solid solution phase. Itis well known in the art that the carbon pres,- ent in stainless steels of commercial purityfmay combine with chromium to form a carbide which may precipitate out of the solid solution and thereby reduce the amount of chromium availe able in th solid solution. This carbide formation makes the effective amount of chromium lower than is shown as present by the, chemical analysis of the alloy. The net effect is to cause the alloy to behave. as though it has a lower chromium content and is less susceptible to sigma formation. Since it is estimated that 0.1% of carbon will remove on the order of 1%v of 1 chromiumfrom solid solution, many borderline stainless steel ay be rendered non-sigma forming by the presence of carbon. Our work has taken into account the effect of carbon and we prefer to speak of the efiective chromium content and thereby eliminate the uncertainty resulting from varying carbon content.

While we recognize the beneficial efi'ects of carbon on sigma-forming tendencies, we prefer to keep the carbon content of our compositions low because of the deleterious effect of carbon on other properties. The presence of the carbides generally is undesirable except where wear resistance is wanted. Their presence may seriously impair the ductility and impact resistance cf the stainless steel.

It is known that, in general, manganese, 00-. belt and copper act much like nickel their effects on the sigma transformation; however, it is believed that their effect is somewhat less than that of a similar amount of nickel. While stainless steels normally contain less than l% of. man ane mum d contain a uc as without being classed as manganese steels. Al} though we prefer to use nickel as our alloying agent to obtain workability, we could decrease the amount of nickel present and make up the necessary percentage by adding suitable amounts of manganese, cobalt, copper or combinations thereof to the composition' Thus when we re,- fer to nickel, we means to include those compositions wherein manganese, cobalt or copper are added to the chromiumenickel stainless steel to replace some of the nickel in our composition.

Our experiments indicate that the presence of; minor amounts of the elements usually found in commercial purity stainless steels will not affect the i m o ming op r e o r Pre erred. compositions. Thes el nt a be etc. Nitrogen is known phosphcr s, an um to p mo eusteni c ph se. iqrmatiqa so that addition of minor amounts may be desirable. The terms substantially all iron or principally iron are intended to include these minor elements as well as the usual content of manganese in chromium-nickel stainless steels.

The following alloy, after 500 hours exposure to a temperature of 1200" F., was found to be a mixture of gamma and alpha phases: chromium, 14.77%; nickel, 7.87%; silicon, 0.90%; carbon, 0.06%; manganese, 0.78%; molybdenum, 0.03%; sulfur, 0.010% phosphorus, 0.022%; and the remainder iron. Other compositions which do not form the sigma phase when exposed for long periods to temperatures in the range of 900-1700 F. are:

Percent P the remainder of the alloy being iron and steelmaking amounts of the other elements, such as given for the complete composition above.

Since our composition has excellent oxidation and corrosion resistance as well as high strength at elevated temperatures and is not susceptible to sigma embrittlement at these temperatures, it can be used anywhere that such properties are desirable. A few of these fields are: tubes in high pressure steam boilers, turbosuperchargers, et engine parts, gas turbines, chemical roasting equipment, high temperature chemical process equipment, etc. A principal field of use for our composition is in the heating of hy drocarbon materials to temperatures above 800 F. and in the thermal or catalytic cracking of hydrocarbons to produce gasoline. The heating for such conversions is ordinarily carried out in tubes or coils which may be subjected to oxidizing conditions on the outside and simultaneously to the action of sulfur compounds associated with the material being cracked within the tubes. Our alloys are admirably suited to the fabrication of tubes and containers for such high temperature operations. The heating of hydrocarbons is a particularly suitable field for the use of our alloy because of the variety of stocks and operating conditions which must be met. Therefore We illustrate the use of our alloy in this field by several operating examples.

In a coil only type thermal cracking process, the feed to the process often is a high sulfur gas oil, for example, a 25 API West Texas gas oil containing 2.0% total sulfur. The oil is heated at a pressure of about 400 p. s. i. g. to a transfer line temperature of about 925 F. before it leaves the furnace and passes to a coldoil quench which stops the cracking reaction and prevents excessive formation of unwanted tarry material. During its passage through the tubes of the furnace the sulfur compounds in the oil decompose to form hydrogen sulfide, which is extremely corrosive at high temperatures; simul taneously the outer surface of the tube is exposed to the radiant heat of the burners and the oxidizing action of the hot combustion gasesoften the tubes operate at red heat. Experience shows that a relatively high chromium-nickel alloy is needed to overcome the simultaneous corrosive and oxidizing conditions; however, these alloys have been subject to embrittlement failures and so are not used by many refiners who employ instead low chromium alloy tubes and accept a shorter tube life. It is ordinary practice to operate this type of cracking process for run lengths of about 2000 hours, which may be sufficient time to form harmful amounts of sigma if the alloy is susceptible to sigma formation.

Another type of thermal cracking operation is the so-called tube and tank process wherein the feed is heated to 8'75-900 F. and about 500 p. s. i. g. before the oil is transferred to an insulated drum, where it is held and allowed to continue the cracking reaction until the desired amount of conversion has taken place. The heavy tarry materialwhich is withdrawn from the reactor may have a sulfur content of between 3 and 4% so that a highly corrosion-resistant material is required. As the liquids in the vessel may be at a temperature of above 800 F. and the run lengths for this type of operation may reach 3000 hours, it is probable that materials that are susceptible to the formation of sigma will form such in the furnace tubes and may do so in the reaction drum.

In a catalytic cracking process, the principal duty of the furnace is to heat the feed very rapidly to a temperature of about 900 F. so that little cracking takes place, as it is desired that the cracking take place in the presence of the catalyst, away from the furnace. The most economical operation of catalytic cracking processes requires extremely long run lengths of the order of 6000-8000 hours, and a trouble-free furnace is essential to such run lengths.

While we have listed many examples where our compositions could be used advantageously, we do not intend that this list be considered exhaustive and we intend to include any and all uses that can be made of this alloy.

We claim:

1. An apparatus for heating hydrocarbons comprising a furnace, tubes positioned within said furnace and means for heating said tubes to temperatures on the order of about 900 to 1700 F., said tubes being formed of an austenitic-type stainless steel consisting of, on a weight basis, silicon: between at least about 1% and not more than 3%, nickel: between at least equal to the sum of 7.5+2.5 per cent silicon and about 20%,

chromium: about 17.5%, and the remainder substantially all iron, which stainless steel is characterized by freedom from sigma phase after prolonged exposure to temperatures on the order of about 900 to 1700 F.

2. The apparatus of claim 1 wherein the tubes are formed of a composition consisting of chromium, 17%; silicon, 2.0%; nickel, 15.5%, and the remainder substantially all iron.

3. The apparatus of claim 1 wherein the tubes are formed of a composition consisting of chromium, 17%; silicon, 2.8%; nickel, 14.5% and the remainder substantially all iron.

4. An apparatus for heating hydrocarbons to a cracking temperature comprising a furnace, a heating coil positioned within said furnace and means for heating said coil to temperatures within the range of about 900 and 1700 R, which coil is formed of an austenitic-type stainless steel consisting of, on a weight basis, silicon: between at least about 1 and not more than 8%, nickel: between at least the sum of 7.5+2.5 per cent silicon and about 20%, chromium in substeiitielllfir an man, which steel iiecharaeteri'zd by the ability ti) W'ith's'fi'alnd prblofiged exposure tetemp'erahires of between about 900 'a'nd'1700 F. Without forming sigma phases 5. An apparatus fqrheating hydrocarbons 00-1 prising '2, 'furnece tubes positioned Within said furnace and means for heating said tubes to atiiiperature on the order of about 900 to about 1700" R, which tubes are ffqrmed'of an austenitictype Stainless steel consisting of, on a weight basis, silicon about 2%, nickel between at least 12,576 and about 20%, chromium about 17 .5% and the remainder substantially all iron, "which tubes *aifle cha'recfiermzd by freedom from sigma phase after prolonged. exposure'to 'temperatures en-the ofder'0f900'to 1700 F.

FRANCIS J. SHORTSLEEVE. MORRIS E. NICHOLSON.

bate Q 'Niinib'er 7 June 2'7, 1922 OTHER RE."FIEHREIENCES Stainless Iron and Steel, pages 396 anh 580'; edited by Monypenny. Published in 1931 by Chapman andyHall, London, England.

Archer, Refiner and Natural Gasoline Manufacturer, v01. -20,- pages262-268 and 281, July Morton, The Oil and Gas Jbiirne of June 26,1941, pags 60-63.

Claims (1)

1. AN APPARATUS FOR HEATING HYDROCARBON COMPRISING A FURNACE, TUBES POSITIONED WITHIN SAID FURNACE AND MEANS FOR HEATING SAID TUBES TO TEMPERATURE ON THE ORDER OF ABOUT 900* TO 1700* F., SAID TUBES BEING FORMED OF AN AUSTENITIC-TYPE STAINLESS STEEL CONSISTING OF, ON A WEIGHT BASIS, SILICON: BETWEEN AT LEAST ABOUT 1% AND NOT MORE THAN 3%, NICKEL: BETWEEN AT LEAST EQUAL TO THE SUM OF 7.5+2.5X PER CENT SILICON AND ABOUT 20%, CHROMIUM: ABOUT 17.5%, AND THE REMAINDER SUBSTANTIALLY ALL IRON, WHICH STAINLESS STEEL IS CHARACTERIZED BY FREEDOM FROM SIGMA PHASE AFTER PROLONGED EXPOSURE TO TEMPERATURES ON THE ORDER OF ABOUT 900* TO 1700* F.

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