GB2133037A - Stainless duplex ferritic- austenitic steel, articles made therefrom and method of enhancing intergranular corrosion resistance of a weld of the stainless duplex ferritic austenitic steel - Google Patents

Abstract

The steel consists of about: .03 w/o Max. carbon, 3.0 w/o Max. manganese, 1.0 w/o Max. silicon, 26.0 to 29.0 w/o chromium, 3.5 to 5.2 w/o nickel, 3.5 w/o Max. molybdenum, 0.15 w/o Min. nitrogen, 2 w/o Max. copper, 0.005 w/o Max. boron and the balance essentially iron. The steel preferably contains about 0.17 to 0.35 w/o nitrogen for improved pitting resistance and for increased austenite content. Welds of the steel preferably contain at least about 17% austenite in the as-welded condition for improved pitting and intergranular corrosion resistance. There is also disclosed a method of improving the intergranular corrosion resistance of an as-welded steel of similar composition but extending to 11 to 30 w/o chromium and 3.5 to 20 w/o nickel.

Description

SPECIFICATION Stainless duplex ferritic-austenitic steel, articles made therefrom and method of enhancing inter granular corrosion resistance of a weld of the stainless duplex ferritic austenitic steel This invention generally relates to a stainless duplex ferritic-austenitic steel. It relates more particu larlyto such steel having a unique combination of good mechanical properties and good corrosion resistance properties.

Heretofore, a stainless duplex ferritic-austenitic steel, designated AISI Type 329, has been commer ciallyavailablewith: a) good mechanical properties such as high annealed yield strength; and b) good corrosion resistance such as resistance to general corrosion in the presence of strong oxidizing agents (e.g., boiling nitric acid). Typical uses for Type 329 steel have included tube or pipe for heat exchange applications involving severely corrosive, oxidizing environments such as are found in the petroleum refining, petrochemical, chemical, and pulp and paper industries (e.g., in nitric acid cooler-conde nsers).Type 329 steel has typically had a composition of about 0.08 weight percent (w/o) Max. carbon, 1.0 w/o Max. manganese, 0.75 w/o Max. silicon, 23.0 to 28.0 w/o chromium, 2.5 to 5.0w/o nickel, 1.0 to 2.0 w/o molybdenum, with the balance essentially iron.

Compositions similarto Type 329 steel have been sold, containing down to about.02w/o carbon, upto about2 w/o manganese, up to about6 wiz nickel, up to about 30 w/o chromium, up to about 3.5w/o molybdenum andlor u p to about 0.25 w/o nitrogen.

- However, the resistance to interg ranular corrosion in the presence of strong oxidizing agents and the resistance to pitting in the presence of halides, particularly chlorides, of Type 329 steel has left something to be desired in areas ofthe steel which have been welded, particularly in areas which have been welded but not subsequently annealed (e.g., in areas of a tube, formed from the steel, which have been welded into a tube sheet of a heat exchanger).

Hence, a steel has been sought with mechanical properties and corrosion resistance properties at least as good as Type 329 steel and with intergranular corrosion resistance and pitting resistance, as welded or as welded plus annealed,that are superior to Type 329 steel.

In accordance with this invention, a stainless duplex ferritic-austenitic steel is provided, the broad, preferred and particularly preferred forms of which are conveniently summarized as consisting essential ly of about: Particularly Broad Preferred Preferred Ranges Ranges Ranges Elements (w/o) -. (w/o) (w/o) C .03 Max. .01-.028 .015-.025 Mn 3.0 Max. 1.0 Max. 0.5 Max.

Si 1.0 Max. 0.75 Max. 0.5 Max.

Cr 26.0-29.0 26.0-28.0 26.5-27.5 Ni 3.5-5.2 4.0-5.0 4.5-5.0 Mo 3.5 Max. 1.0-2.5 1.25-2.25 Cu 2.0 Max. 1.0 Max. 1.0 Max.

B .005 Max.

N 0.15 Min. 0.17-0.35 0.18-0.28 the balance ofthe steel being essentially iron, and wherein when Mo is greaterthan 2.5 the total of chromium w/o plus nickel w/o plus molybdenum w/o is no more than about 34.0 and the total of nickel w/o plus molybdenum w/o is no more than about 7.0.

Incidental impurities in the steel can comprise: up to about .04w/o, preferably up to about .025 w/o, phosphorous; upto about .03 w/o, preferably up to about .005w/o, sulphur; up to about 0.2 w/o tungsten; up to about 0.25 w/o vanadium; up to about 0.2 w/o cobalt; and up to about 0.1 w/o of elements such as aluminum, calcium, magnesium and titanium and up to about 0.1 w/o of misch metal which can be used in refiningthesteel.

In the foregoing tabulation, it is not intended to restrict the preferred ranges of the elements ofthe steel ofthis invention for use solely in combination with each other or to restrict the particularly preferred ranges ofthe elements ofthe steel for use solely in combination with each other. Thus, one or more of the preferred ranges can be used with one or more of the broad rangesforthe remaining elements and/or with one or more of the particularly preferred ranges for the remaining elements. In addition, a preferred range limitfor an element can be used with a broad range limit orwith a particularly preferred range limit for that element.

The steel ofthis invention has: a) the good mechanical properties of Type 329 steel; and b) corrosion resistance properties, particularly resist anceto intergranularcorrosion and pitting in weld areas, that are superiorto Type 329 steel.

In the stainless duplexferritic-austenitic steel of this invention, carbon, which is a strong austenite former, is keptto a minimum to minimize the formation of chromium-rich carbonitrides or carbides (e.g., M23C6) at grain boundaries when the steel is heated. In this regard, no more than about .03 w/o carbon, preferably no more than about 0.25 w/o carbon (e.g., down to about .001 to .005 w/o carbon), is utilized. Thereby, the susceptibility of the steel to intergranular corrosion is reduced. About .01 w/o carbon is considered a practical and hence preferred, but not an essential, minimum because ofthe cost of reducing the carbon below about 0.1 w/o. A particularly preferred range for carbon is about .015 to .025 w/o.

Manganese is an austenite former and also increases the solubility of nitrogen in the steel ofthis invention. In addition, manganese is a scavenger for unwanted elements (e.g., sulfur). Hence, at least about 0.2 w/o manganese is preferably present in the steel. However, manganese can promotetheformation of sigma phase which, if present: a) makes the steel hard and brittle and thereby makes it difficult to handle and workthe steel; and b) makes the steel prone to corrosion. Also, most ofthe benefit from having manganese present can be attained with upto about 3.0 w/o manganese, and more than about 1.0 w/o manganese may adversely affect the pitting resistance ofthe steel. Hence, only up to about 3.0 w/o manganese is utilized in the steel.Preferably, no morethan aboutl.0w/o,betteryetno morethan about 0.5 w/o, manganese is present in the steel.

Silicon acts as a deoxidizing agent and a strong ferrite former. Silicon also improves the weldability ofthe steel by increasing the fluidity of the steel when it is molten. Hence, at least about 0.2 w/o silicon is preferably present in the steel. However, silicon promotes the formation of stigma phase. Hence, only upto about 1.0 w/o silicon is utilized in the steel.

Preferably, no more than about 0.75 w/o, better yet no more than about 0.5 w/o, silicon is present in the steel.

Chromium is a ferrite former and provides signifi cant corrosion resistance to the steel ofthis invention.

In this regard, chromium provides significant resist anceto: a) general and intergranularcorrosion in the presence of strong oxidizing agents such as nitric acid heated above its atmospheric boiling point; and b) pitting in the presence of chlorides. Chromium also increases the solubility of nitrogen in the steel. Hence, at least about 26.0 w/o chromium is present in the steel. However, chromium promotes the formation of sigma phase. Hence, no more than about 29.0 w/o chromium is utilized in the steel, and preferably no morethan about28.0w/ochromium is utilized. The use of about 26.5 to 27.5 w/o chromium is particularly preferred in the steel, but the use of about 28.0 to 29.0 w/o chromium may be preferred for providing corrosion resistance if little or no (e.g., about 0.2 w/o Max.) molybdenum is used in the steel.

Nickel is a strong austenite former, and for this reason, at least about 3.5 wlo nickel is present in the steel ofthis invention. Nickel also provides general corrosion resistance in acid environments, particular ly in sulfuric acid. However, nickel is relatively expensive. Nickel also decreases the solubility of nitrogen in the steel and promotes the formation of sigma phase. Moreover, most ofthe corrosion resistance benefits, obtained by adding nickel, can be attained with upto about 5.2 w/o nickel. Hence, not more than about 5.2 w/o nickel is present in the steel.

Preferably, about 4.0 to 5.0 w/o, better yet about 4.5 to 5.0 w/o, nickel is used in the steel.

Molybdenum is a strong ferrite former and, if added to the steel ofthis invention, provides significant corrosion resistance, particularly pitting resistance. Molybdenum also increases the solubility of nitrogen in the steel. However, molybdenum prom otestheformationofsigma phase, and hence, not more than about3.5w/o molybdenum, preferably not more than about 2.5 w/o molybdenum, is used.

Preferably, at least about 1.0 w/o molybdenum is present in the steel for pitting resistance. It is particularly preferred thatthe steel contain about 1.25 to 2.25 w/o molybdenum for use in a wide variety of corrosive environments, particularly those contain ing chlorides. However, for corrosive environments containing little or no chlorides, it is contemplated thatthe steel can contain little or no (e.g., about 0.2 w/o Max.) molybdenum.

In a preferred steel ofthis invention, the total of chromium w/o plus nickel w/o plus molybdenum w/o in the steel does not exceed about 34.0 and the total of nickel w/o plus molybdenum w/o does not exceed about7.0. This inhibits sigma phase formation during the processing ofthis preferred steel which could adversely affect the workability and the corrosion resistance ofthe steel. By so limiting the total of chromium, nickel and molybdenum, the workability ofthis preferred steel is made comparable to Type 329 steel, and this steel can be processed in the same general manner as Type 329 steel, as will be described below,to remove anyminoramounts of sigma phase that mightform.In this regard, by controlling the total of chromium, nickel and molybdenum in this preferred steel, the hardness ofthe steel is kept from exceeding about 30 on the Rockwell C (Rc) scale when the steel is sensitized by heating at 1400"F (760"C) fortwo hours and then air cooling. By so limiting the total of chromium, nickel and molyb denum,theriskofforming sigma phase in weld areas ofthis preferred steel, as a result ofthe welding process, is also substantially reduced. Of course,the total of chromium, nickel and molybdenum in the steel ofthis invention need not be so limited, provided sigma phase formation is nota problem in the processing or welding ofthe steel.For example, the total of chromium, nickel and molybdenum need not be so limited: a) if the dimensions ofthe articles (including intermediate and final shaped articles), formed from the steel, allow the articles to be rapidly cooled through the sigma phase sensitization range of about 1250 to 1650"F (about 675 to 900"C); orb) if any sigma phase can subsequently be removed in a conventional mannerfrom the articles.

Copper, if if added to the ofthis invention, can provide significant corrosion resistance, particularly resistance to general corrosion in acids such as sulfuric acid. Copper is also an austenite former.

However, mostofthe benefitfrom adding copper can be attained with up to about 2.0 w/o copper, and more than about 1.0 w/o copper can adversely affect pitting resistance. Forthese reasons and to minimize the cost ofthe steel, copper is limited to 2.0 w/o maximum, preferably 1.0 w/o maximum.

Nitrogen is a strong austeniteformerand contributestothetensilestrength and pitting resistance of the steel ofthis invention. Nitrogen also seems to inhibit the formation of sigma phase. Hence, nitrogen can be present in the steel up to its limit of solubility, which may be up to about 0.4 w/o, provided the steel is notto be welded or heated for a prolonged period at a temperature at which nitrides or carbonitrides could form, i.e., at about 1050to 17500F (about 565 to 955"C). In accordance with this invention, the steel contains a minimum of about 0.15 w/o nitrogen.

When the steel is to be welded, it is preferred that the steel contain at least about 0.17 w/o, better yet at least about 0.18 w/o, nitrogen to provide enhanced pitting resistance and high levels, i.e., at least about 17%, of austenite in weld areas of the steel, even without subsequent annealing.When the steel isto be welded, it is also preferred that the nitrogen content not exceed about 0.35 w/o, betteryet about 0.28 w/o, to avoid porosity in the weld. Inthis regard, when nitrogen exceeds the stated preferred limits, some of the nitrogen in solid solution can come outofsolution during welding and can be trapped during subsequent solidification ofthe steel. This can produce pores in the weld area, thereby making the weld area proneto corrosion and mechanical failure. Thus, to assure good weldability of the steel by conventional welding techniques, nitrogen in the steel is preferably limited, for example, to: about 0.28 w/o Max. when using autogenous gas tungsten arc (GTA) welding techniques; and about 0.35 w/o Max when using electron beam welding or laserwelding techniques.

In the steel of this invention, it is preferred thatthe austeniteformers, nickel, manganese, copper and carbon, not be present in minimum amounts in the steel when the ferrite formers, chromium, silicon and molybdenum, are present in maximum amounts. In this regard, one should not rely on using nitrogen to form austenite in the steel when the remainder ofthe alloy balance would produce atotallyferriticstruc- ture such as would be obtained with a significant excess of ferrite formers, beyond the levels required to produce 100% ferrite. This is because, when the steel is heated (e.g.,welded), nitrogen mayform chromium nitrides, thereby reducing the amountof nitrogen that is present interstitially in the austenite and that stabilizes the austenite.

Upto about 0.005 w/o boron can be present in the steel ofthis invention. In this regard, a small but effective amount (e.g., 0.0005 w/o or more) of boron can be used, because it is believed to have a beneficial effect on corrosion resistance, as well as hotworkability.

Small amounts of one or more other elements may also be present in the steel because of their beneficial effect in refining (e.g., deoxidizing and/or desulfurizing)the melt. For example, elements such as calcium, magnesium, aluminum and/ortitanium, in addition to silicon, can be added to the melt to aid in deoxidizing and also to benefit hotworkability as measured by high temperature ductility. When added, the amounts of such elements should be adjusted so that the amounts retained in the steel do not undesirably affect corrosion resistance or other desired properties. Misch metal (a mixture of rare earths primarily comprising cerium and lanthanum) can also be added to the meltfor. interalia, removing sulfur, and its use is believed to have a beneficial effect upon hotworkability.However, forthat effect, no definite amount of misch metal need be retained in the steel because its beneficial effect is provided during the melting process when, if used, upto about 0.4w/o, preferably no more than about 0.3 w/o, is added.

In a preferred steel of this invention containing about 0.17 to 0.35 w/o nitrogen, the elements are preferably balanced so that the value ofthe-chromium equivalent ("Cr Eq.") minus the nickel equivalent ("Ni Eq."), calculated bythefollowing equations, is no more than about 16.4, preferably no more than about 15.3: CrEq. = Crw/o -i- + Mow/o + 1.5 x Siwlo Ni Eq. = 40 (Cw1o +N w/o) + Niwlo + 0.5x (Cuwlo+Mnwlo).

It is believed that such a value of chromium equivalent minus nickel equivalent can be used to provide a weld ofthis preferred steel (containing about 0.17 to 0.35 w/o nitrogen) with an austenite content of at least about 17%, aswelded. Of course, reasonable care should betaken in welding andthen cooling this preferred steel in order to be sure of obtaining at least about 17% austenite in the weld.Nevertheless, a weld can be provided with at least about 17% austenite simply by: a) welding this preferred steel using techniques conventionally employed in com mercial welding of stainless duplex austenitic-ferritic steel tubing orvessels (e.g., byGTA); and b)then allowing theweld area to cool in anymannerthatis i) conventionally used in commercial welding of such steel tubing our vessels and ii) slow enough so that at least about 17% austenite forms in the weld as the weld cools. However, the cooling oftheweld should not be so slow as to cause excessive carbonitride precipitation in the weld which could reduce its pitting and/or intergranular corrosion resistance.

It is believed that an austenite content of at least about 17% in a weld of a preferred steel of this invention, containing about 0.17 to 0.35 w/o nitrogen, provides the weld and the high-temperature heat affected zone ofthe steel, in the as-welded condition, with improved pitting and intergranular corrosion resistance, even without subsequent annealing ofthe weld area. In this regard, a weld in a preferred steel of the invention can contain upto about 50%, but typically no morethan about 25%, austenite in the as-welded condition. Austenite reduces the continuity and the amount offerriteto-ferrite grain boundaries in welds ofthe steel.As a result, austenite reduces the amount and continuity chromium-rich carbides and carbonitrideswhich can form at ferriteto-ferrite grain boundaries in thewelds.This prevents the chromium from being depleted from the adjacent ferrite matrix.

However, the advantages of providing at least about 17% austenite in a weld are not confined to the preferred steel ofthis invention containing about 0.1 7 to 0.35 w/o nitrogen. The intergranular corrosion resistance of a weld, in the as-welded condition, can be be improved by providing at leastabout 17% austenite in the weld for any stainless duplex ferritic-austenitic steel consisting essentially of about: Elements w/o C .01-.03 Mn 3 Max.

Si 1 Max.

Cr 11-30 Ni 3.5-20 Mo 3.5 Max.

Cu 2 Max.

B .005 Max.

N 0.10-0.35 wherethe balance ofthe steel is essentially iron. In addition, the pitting resistance of a weld, in the as-welded condition, can be improved by providing at least about 17% austenite in the weld for any ofthe aforementioned duplex steels wherein nickel is limited to about3.5to5.2w1o.

Where largeferrite grains with extensive and continuous ferrite-to-ferrite grain boundaries, containing carbodes and carbonitrides, are more likely to beformed in the steel ofthis invention during processing, as in large section-size pieces, it is also preferred that the parent or base metal ofthe steel have an austenite content of at least about 30%, betteryetatleastabout40%, upto about60%.The austenite present in the base metal reduces the tendency to form largerferriticgrains and thereby improves the impact strength and tensile ductility of the steel.As in the case ofthe weld area, the austenite present also reducesthe continuity and amount of the carbides and carbonitrides which can form at ferrite-to-ferrite grain boundaries and thereby improves the pitting and intergranularcorrosion resistance ofthesteel. However, the base metal ofthesteel ofthis invention can, if desired, contain somewhat less than the preferred amountofaustenite, i.e., down to about 25% austenite.

No special techniques are required in melting, casting and working the steel ofthis invention. In general, arc melting with argon-oxygen decarburization is preferred, but other practices can be used. In some instances, an initial ingot, cast as an electrode, can be remelted, or powder metallurgy techniques can be used to provide better control of unwanted constituents or phases. Good hotworkability is attained by hot working from a furnace temperature of about 20500F (about 1120 C), preferably from about 1950"F (about 1065"C), and reheating as necessary.

Process annealing is carried out above about 17500F (about 955"C), preferably at about 1850 to 1950"F (about 1010 to 1065"C), for a time depending upon the dimensions of the article which is then preferably quenched in water.

The steel of this invention is suitableforforming to a greatvariety of shapes and productsforawide variety of uses, for which Type 329 steel has heretofore been used. The steel of this invention lends itself to the formation of billets, bars, rod, wire, strip, plate or sheet using conventional practices. The steel ofthis invention is particularly suited to be used in cold rolled, annealed sheet or strip and hot rolled, annealed plate that are to be welded.As compared to Type 329 steel, the steel ofthis invention has, inter alia: superior resistance to embrittlement when heated at about 700 to 1 0000F (about 370 to 540"C)for prolonged periods; highertensile strength in the base metal; and higher tensile strength in weld areas.

As compared to Type 329 steel, the steel of this invention also has superior corrosion resistance, particularly intergranular corrosion and pitting resistance. The steel of this invention has especially superior intergranular corrosion and pitting resistance in weld areas, particularly in the as-welded condition. Moreover, like Type 329 steel, the corrosion resistance in weld areas ofthe steel ofthis invention can be improved by annealing to increase the austenite in the weld areas and to dissolve carbides, particularly intergranular carbides.In this regard, the steel ofthis invention can be annealed at about 1750 to 2050"F (about 950 to 1 1 20"C), preferably about 1825 to 1950"F (about 995 to 1065"C), for as short as a few seconds or up to about 30 minutes, followed by air cooling.

The steel ofthis invention is advantageously used in the manufacture of tubing for use in heat exchangers or condensers. Because of its goodweldability by conventional welding techniques, this steel is suitableforthe manufacture of welded tubing, preferably by GTAwelding. For some purposes, it is useful to provide this steel in theform of a weld filler wire.

Anyminoramountsofsigma phase which may form in a steel ofthis invention, such as a preferred steel in which the total of chromium w/o plus nickel w/o plus molybdenum w/o is no more than about 34.0 and the total of nickel w/o plus molybdenum w/o is no more than about 7.0, can be removed in a conventional manner such as would besatisfactoryforType329 steel. In this regard, sigma phase can be removed by heating or heating plus working ofthe steel followed by rapid cooling (e.g., air cooling of small sectionsizes or water quenching or large section-sizes).

The heatsAto V used in the examples, which follow, were prepared as small experimental heats, induction melted under argon. Heats A, B, J, N, R, T, U and V were each a steel ofthis invention ("invent."), and none ofthe other heats was a steel ofthis invention. The heatswere analyzed as set forth in Table I below. The tolerances forthe analyses did not exceed: +.003w/oforcarbon; +.02w/oforman- ganese and for silicon; +.08 w/o for nickel; +.05w/o for molybdenum; f0.18 w/o for chromium; +.01 w/o for 0.1 0to 0.19w/o nitrogen; and +.02w/ofor0.20to 0.49w/o nitrogen.

Each heat was hot worked to form a strip, annealed as required, cold rolled to .125 inch (.3cm) thickness, annealed in neutral salt at 1 8500F (101 00C) forthree minutes and then air cooled. The austenite content of the base metal of each strip was determined by x-ray diffraction to +2% ofthe reported value. The austenite content of the base metal of each strip is set forth in Table I, below.

Welding of a strip from each heat, when carried out in the examples, was carried out with a GTA apparatus, and afterwelding, the strip was cooled at a rate which approximated conventional commercial weld-cooling rates. The austenite content ofthe weld area of each strip was determined by point counting of one typical field using 300 intersections at 500X magnification. The austenite content ofthe weld of each strip is set forth in Table I, below.

TABLE I Austenite Elements* Base (w/o) Metal Weld Heats C Mn Si P S Cr Ni Mo Cu** N (E (%) A (invent.) .024 .37 .31 .020 .008 26.19 4.79 174 N.A. .21 45 N.A.

B (invent.) .024 .38 .32 .021 .008 26.47 4.83 1.44 N.A. .20 40 24 C .056 .40 .32 .020 .008 26.36 4.81 1.44 N.A. .22 54 30 D .052 .39 .32 .021 .008 27.00 4.86 1.44 N.A. .20 42 21 E .023 .39 .32 .020 .007 26.55 5.55 1.44 N.A. .19 48 21 F .026 .38 .32 .021 .007 26.76 5.56 1.44 N.A. .18 44 20 G .025 .38 .33 .023 .007 26.95 6.13 1.43 N.A. .16 46 18 H .027 .40 .33 .022 .007 27.11 6.21 1.45 N.A. .15 44 11 I .025 .42 .32 .021 .008 25.73 4.84 1.43 N.A. .17 48 N.A.

J (invent.) .026 .42 .32 .021 .008 26.98 4.82 1.43 N.A. .15 40 15 K .026 .42 .34 .023 .007 26.64 4.76 1.44 N.A. .13 36 5 L .025 .42 .32 .021 .007 26.88 4.78 1.42 N.A. .13 40 6 M .021 .41 .34 .021 .008 27.19 5.47 1.43 N.A. .15 39 11 N (invent.) .030 .38 .34 .022 .007 26.54 4.94 1.46 N.A. .19 40 19 0 .023 .40 .35 .020 .008 26.68 6.32 1.47 N.A. .20 47 22 P .027 .38 .32 .019 .008 27.21 6.20 1.41 N.A. .20 43 23 Q .026 .39 .32 .017 .007 25.48 4.92 1.43 N.A. .20 43 N.A.

R (invent.) .028 .38 .34 .022 .007 27.15 4.76 1.48 N.A. .20 41 22 S .028 .40 .34 .021 .007 27.06 5.45 1.46 N.A. .19 43 N.A.

T (invent.) .021 .42 .36 .019 .008 26.69 4.80 2.36 .02 .21 N.A. N.A.

U (invent.) .021 .44 .40 .023 .008 26.29 4.70 2.35 .02 .19 N.A. N.A.

V (invent.) .021 .44 .39 .025 .008 26.25 4.82 2.36 .84 .18 N.A. N.A.

*Oxygen was no more than about .02 w/o.

**Copper, when not analyzed ("N.A."), did not exceed about .05 w/o.

Example 1 The hardness of strips from certain heats was determoned after: a) annealing each strip in salt at 1850"F (1010 C) forthree minutes and then air cooling; and b) annealing each strip as in a),followed by heat treating each strip at 14000F (760"C) fortwo hours and then air cooling. The results are setforth in Table II, below.

TABLE II Heat Elements Annealed Treated Cr Ni Mo Hardness Hardness Heats (w/o) (w/o) (w/o) (Rc) (Rc) R (invent.) 27.15 4.76 1.48 21.8 24.5 J (invent.) 26.98 4.82 1.43 20.5 24.7 B (invent.) 26.47 4.83 1.44 21.9 23.1 N (invent.) 26.54 4.94 1.46 21.1 23.1 S 27.06 5.45 1.46 21.7 31.8 M 27.19 5.47 1.43 21.2 31.5 E 26.55 5.55 1.44 22.7 32.2 F 26.76 5.56 1.44 22.4 32.5 G 26.95 6.13 1.43 22.0 37.9 P 27.21 6.20 1.41 22.6 35 8 27.11 6.21 1.45 21.0 40.5 0 26.68 6.32 1.47 22.3 37.5 T (invent.) 26.69 4.80 2.36 23.3 36.0 U (invent.) 26.29 4.70 2.35 22.2 36.9 V (invent.) 26.25 4.82 2.36 22.8 36.7 Table II shows that, in a preferred steel ofthis invention in heats B, J, N and R, the use of no more than about 5.2 w/o nickel, a total of chromium w/o plus nickel w/o plus molybdenum w/o of no more than about 34.0, and a total of nickel w/o plus molybdenum w/o of no more than about 7.0 prevents the hardness of the preferred steel from exceeding about Rye 30 when the steel is heated at about 1400"F (760"C) fortwo hours and then air cooled. This indicatesthat any sigma phase, which mayform in the preferred steel, will not significantly impair the hot workability orthe corrosion resistance of the steel and can be removed by conventional heating or heating plus working techniques in making a finished product.

Example 2 The intergranular corrosion resistance, as welded, of stripsfrom certain heats was determined in ferric sulfate plus sulfuric acid (ASTM A262-B). The test was significantly more severe than ASTM A262-B, because three periods of 120 hours each were used.

Each strip had been welded and machine ground to a 1.25 x 1 x .125 inch (3.2 x 2.5 x 0.3 cm) sample with a 120 gritfinish before being tested.

The results are setforth in Table IIIA and IIIB, below.

Corrosion rates were determined in mils per year (MPY) and converted to millimeters per year (MMPY).

The depth of attack in the weld and the hightemperature heat affected zone (HAZ), immediately adjacent the weld, was measured in inches, using cross-sections ofthe weld areas, and converted to centimeters.

SABLE IIIA Corrosion Rates In 120 Hour Periods Elements Austenite C N in Weld 1st Period 2nd Period 3rd Period Heats w/o w/o (%) (MPY) (MMPy) (MPY) (MMPY) (MPY) (MMPT) C - .056 .22 30 19.4 .49 32.2 .82 56.6 1.44 D .052 .20 21 21.0 .53 44.1 1.12 56.2 1.43 B (invent.) .024 .20 24 16.4 .42 19.1 .49 22.0 .56 J (invent.) .026 .15 15 199 .51 35.8 .91 46.8 1.19 K .026 .13 5 22.7 .58 38.8 .99 48.9 1.24 L .025 .13 6 22.9 .58 57.0 1.45 81.1 2.06 N (invent.) .030 .19 19 20.1 .51 29.7 .75 31.2 .79 R (invent.) .028 .20 22 21.5 .55 26.1 .66 25.0 .64 TABLE IIIB Depth of Attack After 1st Depth of Attack After 3rd 120 Hr. Period 120 Hr. Period Weld Weld HAZ HAZ Weld Weld HAS HAZ Heats (inches) (cm) (inches) (cm) (inches) (cm) (inches) (cm) C .0042 .011 .0065 .017 .0076 .019 .0118 .030 D .0057 .014 .0053 .014 .0118 .030 .0193 .049 B (invent.) .0025 .006 .0021 .005 .0047 .012 .0069 .018 J (invent.) .0082 .021 .0086 .022 .0114 .029 .0224 .057 K .0037 .009 .0065 .017 .0155 .039 .0264 .067 L .0074 .019 .0043 .011 .0215 .055 .0396 .101 N (invent.) .0036 .009 .0024 .006 .0076 .019 .0068 .017 R (invent.) .0031 .008 .0029 .007 .0060 .015 .0066 .017 Tables IllAand IIIB showthat, in a preferred steel of this invention in heats B, N and R as welded, the use of morethan about 0.15 w/o (i.e., at least about 0.17 woe) nitrogen and no more than about 0.3 w/o carbon and the presence of more than about 15% (i.e., at least about 17%) austenite in the weld provides improved intergranular corrosion resistance in the weld and the heat affected zone ofthe steel, particularly after the third period of exposure to ferric sulfate plus sulfuric acid.

Example 3 The general corrosion resistance of strips from certain heats was determined in boiling 65w/o nitric acid forfive 48 hour periods (ASTM A262-C). The test was significantly more severe than ASTMA-262C, because the nitric acid contained 0.5 g/l potassium dichromate so that it provided a severe oxidizing environmentsuch as isfound in nitric acid heated above its atmospheric boiling point (e.g., in a nitric acid cooler-condenser). Each strip had been hand ground to an approximately 1.5 x 0.5 x 0.125 inch (3.8 x 1.3 x 0.3 cm) samplewith a 120 gritfinish before being tested.

The results are setforth in Tables IVA and IVB, below,for duplicateteststrips. Corrosion rates were determined in mils peryear (MPY) and converted to millimeters per year (MMPY).

TABLE IVA Corrosion Rates In Elements 48 Hour Periods Cr N 1st Period 2nd Period 3rd Period Heats w/o w/o (MPY) (MMPy) MPY (MMPy) (MPY) (MMPT) Q 25.48 .20. 581/554 14.8/14.1 284/353 7.2/9.0 474/523 12.0/13.3 I 25.73 .17 297/281 7.5/7.1 407/365 10.3/9.3 246/242 6.2/6.1 A (invent.) 26.19 .21 257/243 6.5/6.2 392/356 10.0/9.0 456/437 11.6/13.1 J (invent.) 26.98 .15 75/77 1.9/2.0 283/272 7.2/6.9 208/199 5.3/5.1 TABLE IVB Corrosion Rates In 48 Hour Periods Average of 4th Period 5th Period Periods Tested Heats (MPT) (MMPY) (MPY) (MMPT) (MPY) (MMPY) Q 337/469 8.6/11.9 198/192 5.0/4.9 375/418 9.5/10.6 I 419/355 10.6/9.0 435/532 11.0/13.5 361/355 9.2/9.0 n A (invent.) 300/308 7.6/7.8 146/154 3.7/3.9 311/300 7.9/7.6 J (invent.) 164/134 4.2/3.4 318/319 8.1/8.1 210/200 5.3/5.1 Tables IVA and IVB show that, in the steel ofthis invention in heats A and J, the use of at least about 26.0w/o chromium provides improved general corrosion resistance.

Example 4 The pitting resistance of strips from certain heats was determined in 6 w/o ferric chloride (solution from ASTM-G48). The tests were carried out at 40"C, and each strip was immersed in 150 ml of ferric chloride solution for 72 hours. Each strip had been welded, annealed at 18500F (1010 C) for 10 minutes, air cooled and then machine ground to a 1.25 x 1 x .125 inch (3 x 2.5 x 0.3 cm) samplewith a 120 gritfinish before being tested.

The results are setforth inTableV, below,for duplicateteststrips. Corrosion rates were determined in milligrams per square centimeter (Mg/cm2).

No strip was observed to have suffered preferential attack in its weld area.

TABLE V Elements Corrosion C Cr Ni N Rates Heats w/o w/o w/o w/o (Mg/cm2) B (invent.) .024 26.47 4.83 .20 0/0 i: " .026 26.64 4.76 .13 1.4/2.0 L .025 26-88 4.78 .13 2.3/3.0 N (invent.) .030 26.54 4.94 .19 .1/1.3 R (invent.) t .028 27.15 4.76 .21 1.2/0 TableVshowsthat, in a preferred steel ofthis invention in heats B, N and R as welded plus annealed,the use of more than about 0.13 w/o (i.e., at leastaboutO.17w/o) nitrogen provides improved pitting resistance.

Example 5 The pitting resistance of strips from certain heats was determined in 6 w/o ferric chloride at 22 C for three days (ASTM-G48). Unlike ASTM-G48, each strip was immersed in 150 ml offerric chloride solution in the tests. Each strip had been welded and then machine ground as in Example 4 before being tested.

The results are setforth in Table VI, below, for duplicate test strips. Corrosion rates were deter mined in milligrams per square centimeter (Mg/cm2).

The test strips also were visually compared at the end ofthe tests to determine the relative extent of pitting which had been suffered. The pitting resistance ofthe stripswas rated eithergood (G), moderate (M) orbad (B) from this visual comparison.

TABLE VI Elements Austenite Corrosion C Cr Ni N in Weld Rates Visual Heats w/o w/o w/o w/o (%) (Mg/cm2) Ratings B (invent.) .024 26.47 4.83 .20 24 1.7/2.6 G J (invent.) .026 26.98 4.82 .15 15 13.4/14.0 B E .023 26.55 5.55 .19 21 .8/.9 G F .026 26.76 5.56 .18 20 1.1/2.9 G M .021 27.19 5.47 .15 11 7.7/8.7 B G .025 26.95 6.13 .16 18 1.3/2.3 G/M H .027 27.11 6.21 .15 11 6.9/8.1 B Table VI shows that,, in a stainless duplex ferriticaustenitic steel such as the steel ofthis invention, as welded, the presence of at least about 17% austenite in the weld provides improved pitting resistance in the weld areas ofthe steel. Table VI also shows that, in a steel ofthis invention in heats B and J as welded, the use of more than about 0.15 w/o (i.e., at least about 0.17 w/o) nitrogen and the presence of more than about 15% (i.e., at least about 17%) austenite in the weld is preferred to provide improved pitting resist anceintheweld areas ofthe steel.

The terms and expressions which have been employed are used as termsofdescription and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents ofthe features shown and described or portions thereof, but it is recognizedthatvarious modifications are possible within the scope ofthe invention

Claims (28)

claimed. CLAIMS

1. A stainlessduplexferritic-austenitic steel consisting essentiallyofabout: Elements w/o C .03 Max.

Mn 3.0 Max.

Si 1.0 Max.

Cr 26.0-29.0 Ni 3.5-5.2 Mo 3.5 Max.

Cu 2.0 Max.

B .005 Max.

N 0.15 Min.

the balance of the steel being essentially iron, and wherein when Mo is greaterthan 2.5 the total of chromium w/o plus nickel w/o plus molybdenum w/o is no more than about 34.0 andthetotal of nickel w/o plus molybdenumw/o is no morethan about 7.0.

2. The steel of claim 1 whichcontainsabout: Elements wlo C .01-.028 Mn 1.0 Max.

Si 0.75 Max.

Cr 26.0-28.0 Ni 4.0-5.0 Mo 1.0-2.5 Cu 1.0 Max.

N 0.17-0.35

3. Thesteelofclaim 1 which contains about: Elements w/o C .015-.025 Mn 0.5 Max.

Si 0.5 Max.

Cr 26.5-27.5 Ni 4.5-5.0 Mo 1.25-2.25 Cu 1.0 Max.

N 0.18-0.28

4. The steel of claim 1 wherein nitrogen is about 0.17 to 0.35 w/o.

5. The steel of claim 4 wherein, when used for welding, the elements are balanced to provide a weld ofthe steel with at least about 17% austenite and enhanced intergranular corrosion resistance and pitting resistance in the as-welded condition ofthe steel.

6. The steel of any one of claims 1 to 3 wherein the chromium equivalent minus the nickel equivalent is no more than about 16.4.

7. The steel of any of claims 1 to 3 wherein the chromium equivalent minus the nickel equivalent is no more than about 15.3.

8. The steel of any claims 1 to 6 wherein nickel is about 4.0 to 5.0 w/o.

9. The steel of claim 8 wherein molybdenum is about 1 to 2.5 w/o.

10. The steel of claim 9wherein chromium is about 26.0 to 28.0 w/o.

11. The steel of claim 10 wherein manganese is about 1.0 w/o Max.

12. The steel of any of claims 1 to 1 1,wherein the total of chromium w/o plus nickel w/o plus molybdenum w/o is no more than about 34.0 and the total of nickel w/o plus molybdenum w/o is no more than about7.0 regardless of molybdenum content.

13. A hot rolled, annealed plus welded plate comprising the stainless duplex ferritic-austenitic steel of claim 6.

14. The plate of claim 13 wherein the chromium equivalent minus the nickel equivalent ofthe steel is no more than about 15.3.

15. A cold rolled, annealed plus welded sheet or strip comprising the stainless duplexferritic-austenitic steel of claim 6.

16. The sheet of strip of claim 15 wherein the ehromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.

17. Awelded article comprising the stainless duplexferritic-austenitic steel of claim 6.

18. The article of claim 17, wherein the chromium equivalentminusthe nickel equivalent of the steel is no more than about 15.3.

19. A method of enhancing intergranular corrosion resistance of a weld of a stainless duplex ferritic-austenitic steel in the as-welded condition; the steel consisting essentially of about: Elements w/o C .01-.03 Mn 3 Max.

Si 1 Max.

Cr 11-30 Ni 3.5-20 Mo 3.5 Max.

Cu 9 Max.

B .005 Max.

N 0.10-0.35 he balance ofthe steel being essentially iron; said nethod comprising: providing at least about 17% austenite in the weld n the as-welded condition.

20. The method of claim 19wherein nickel is about 3.5 to 5.2 w/o in the steel to provide the weld with enhanced pitting resistance in the as-welded condition.

21. The method of claim 20 wherein the steel contains about: Elements w/o Mn 1.0 Max.

Si 0.75 Max.

Cr 26.0-28.0 Ni 4.0-5.0 Mo 1.25-2.5 Cu 1.0 Max.

N 0.17-0.35

22. The method of claim 19 wherein the weld has upto about 25% austenite, as welded.

23. A stainless du plexferritic-austenitic steel substantially as hereinbefore described.

24. A hot rolled, annealed plus welded plate substantially as hereinbefore described.

25. A cold rolled, annealed plus welded sheet or strip substantially as hereinbefore described.

26. Awelded article substantially as hereinbefore described.

27. A method of enhancing the intergranular corrosion resistance of a weld of a stainless duplex ferritic-austenitic steel in the as-welded condition substantially as hereinbefore described.

28. A stainless duplex ferritic-austenitic steel, substantially as hereinbefore described with reference to any one of the Examples.