Development ofSuperaustenitic Stainless Steels

byMats Liljas, Avesta Sheffield, R&D, S-774 80 Avesta, Sweden

AbstractThe evolution of austenitic stainlesssteels started more than eighty yearsago. Early, it involved also high alloygrades today often called super-austenitic or high performancegrades. This paper tries to describethis important part of the develop-ment. Alloy development aimed forboth wet corrosion and high tempe-ratures are covered. The driving forcefor the evolution has been the needfor better properties of the alloys,such as higher corrosion resistance ina specific environment. Importantprerequisites for the developmenthave been improved steel productionprocesses, better tools for assessingproperties and simulation or model-ling techniques for production anduse of the alloys. Characteristic prop-erties of superaustenitic stainlesssteels are described, particularlymechanical properties, corrosionresistance and weldability.

IntroductionSince the first commercial productionof stainless steels in the beginning ofthis century the austenitic family ofstainless steels has been the totallydominating type. The main reasonsfor this are superior properties com-bined with the comparable ease ofproduction and of fabrication, and,not the least, their excellent weld-ability. Over the years there has beena continuous development andimprovement of the austenitic gradesfrequently resulting in higher alloyedvariants. The evolution has beendriven by the increased requirementsfrom users and fulfilled by producersthrough R&D efforts and improvedsteelmaking capabilities. This has

resulted in several high performancestainless steels providing manyspecial property profiles.

The concept SuperausteniticStainless Steel has not been clearlydefined and depending on thedefinition, the time of the first super-austenitic steel can be between the1930s and the 1970s. The term hasprobably been formed analogous tonickel-base superalloys, used forhighly alloyed nickel-base alloys.A current interpretation of super-austenitic stainless steel is anaustenitic steel composition with highamounts of chromium, nickel, molyb-denum and nitrogen (Cr, Ni, Mo andN) resulting in an iron content closeto or less than 50%. Typical examplesare the so called 6% Mo steels thatwill be discussed to some extent inthis paper. However, other highlyalloyed austenitic steels for bothaqueous corrosion resistance andhigh temperature service should beincluded in this group. The object ofthis paper is to outline the historicaldevelopment of superausteniticstainless steels and to review theirimportant properties.

HistoryThe first austenitic stainless steel,V2A, was developed at Krupp in1912, and had a composition ofabout 0.3% C, 20% Cr and 7% Ni11). It was the forerunner to the Cr-Nistainless steels most known as type304. Important commercial produc-tion of this type of austenitic steels didnot start until after World War I,around 1920-24. The evolution ofaustenitic stainless steels followeddifferent paths depending on thevarious applications. Additions of Mo

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and Si were used at an early stage toimprove the corrosion resistance toacids, one important industry beingthe pulp and paper industry (sulfiteindustry), Increased Cr levels wereused to get a better high temperatureoxidation resistance. Type 310, withabout 25% Cr and 20% Ni, wasproduced already in the 1930s.

The driving force for developmentof the austenitic stainless steels hasbeen the need from the end user sidefor materials that are resistant toincreasingly harsh environments. Aparticular medium that gave rise todevelopment of special stainless steelalloys already in the 1930s issulphuric acid. In Europe (France)Uranus B6, with approximately 20%Cr, 25% Ni, 4.5% Mo and 1.5% Cuwas developed and in USA, Alloy 20,containing 20% Cr, 30% Ni, 2.5% Moand 3.5% Cu, was developed. Alloy20 had, due to the high alloy content,poor hot workability and thereforelimited availability in wrought form.This was later remedied by the

addition of various trace elementsand the alloy is still used extensivelyfor sulphuric acid service (Carpenter20Cb-3), see Table 1 (2)1. Alloy B6,more known today as 904L, came toa very widespread use starting fromthe 1970s in applications such aspulp and paper and also chemicalindustry. One reason for the in-creased use was improved produc-tion capability through the introduc-tion of new refining technologies,such as AOD (Argon OxygenDecarburisation) in the early 1970s.These technologies allowed a bettercontrol of alloy additions and im-proved the removal of detrimentaltramp elements considerably. Thus,the possibility to produce extra lowcarbon (ELC) steels was improveddrastically.

In the 1970s a special high alloyaustenitic stainless steel was devel-oped for phosphoric acid service. Thegrade, Sanicro 28, contained high Crand Ni amounts (Table 1) andshowed also a high resistance to

stress corrosion cracking as did 904Land Alloy 20 (3). This alloy has beenused extensively in the form of pipesand tubes in phosphoric andsulphuric acid environments but alsoas casing and liners in deep sour gaswells.

Alloy 20 and 904L formed a basefor further development of super-austenitic steels. In the 1960s, analloy that showed high resistance toe.g. seawater with a Mo contentabove 5%, NSCD, was introduced byUgine (4). In 1967, INCO applied fora patent of an alloy containing 14-21% Cr, 20-40% Ni and 6-12% Mowith enhanced resistance to corrosionin chloride media such as seawater(5). A steel according to this patent,introduced in the early 1970s, wasAL-6X with 20% Cr, 25% Ni and 6%Mo. In 1975 Allegheny Ludlum madea patent application on the samealloy ranges in the INCO patent butclaimed improved hot workabilitythrough controlled additions of Ce(6). AL-6X was mainly used in thin

Table 1.Compositions of austenitic stainless steels for wet corrosion applications.

Alloy Cmax Mn Cr Ni Mo Cu N Other PRE EN UNS Trademark304 0.08 18 9 18 1.4301 S30400316L 0.03 17 12 2 24 1.4404 S31603Alloy 20 0.05 20 30 2.5 3.5 32 - -

20 Cb-3 0.06 20 34 2.5 3.5 Nb, Ta 32 - N08020 Carpenter TechnologyB6, 904L 0.02 20 25 4.5 1.5 35 1.4539 N08904Sanicro 28 0.02 27 31 3.5 1 39 1.4563 N08028 Sandvik

NSCD 0.03 17 16 5.5 2.5 35 - - UgineAL-6X 0.03 20 25 6 40 - N08366 Allegheny LudlumAL-6XN 0.03 20.5 24 6.3 0.22 45 - N08367 Allegheny Ludlum

1.4439 0.03 17 14 4 0.15 33 1.4439 S317262RE69 0.02 25 22 2 0.12 37 1.4466 S31050 SandvikASN 7W 0.04 18 16 7 2 0.15 43 - - Böhler

VEW 963 0.03 17 16 6.3 1.6 0.15 40 - - VEWAntinit 3974 0.03 6 23 17 3 0.40 0.2Nb 39 - - Thyssen254 SMO 0.02 20 18 6.1 0.7 0.2 43 1.4547 S31254 Avesta Sheffield

1925hMo 0.02 20 25 6.2 0.7 0.2 44 1.4529 N08925 VDMSX 0.02 17.5 20 1 2 5 Si 21 - S32615 SandvikAlloy 31 0.02 27 31 6.5 1.2 0.2 52 1.4562 N08031 VDM

934LN 0.02 10 20 15 4.5 0.4 41 - -1.4565 0.03 6 24 18 4.5 0.4 45 1.4565 S34565654 SMO 0.02 3 24 22 7.3 0.5 0.5 56 1.4652 S32654 Avesta Sheffield

1Commercial Trademarks used in this paper are attributed once in Tables 1 and 2.

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walled condenser tubing for seawatercooled power plants. The high alloycontent made the steel prone toprecipitation of intermetallic phasespreventing fabrication in heaviersections.

In the late 1960s it was shown thatN addition retards both carbide andintermetallic phase precipitation inaustenitic steels (71. The Germangrade 1.4439 (∼317 LMN) withminimum 4% Mo and 0.15% N wasan example of a steel where thisknowledge was used. This grade hassince been used in many applicationswith severe corrosion environmentssuch as heat exchangers, flue gasdesulphurization (FGD) and pulp andpaper bleach plants. A furtherdevelopment of 1.4439 was BöhlerAntinit ASN 7W with as high as 7%Mo, see Table 1 (8). This alloy did forsome reason not see any importantcommercial use and a successor withlower molybdenum content, VEWA963 (Table 1), was presented later(9). Also, other higher alloyedaustenitic steels were developedwhere the strong austenitizing effectof N was utilized to an even greaterextent. To achieve higher N solubilityand thereby even higher N levels, Mnalloying was practised. This resultedin highly alloyed austenitic gradeswith high corrosion resistance as wellas a much improved strength. Oneexample was Amaganit 3974 with thecomposition as shown in Table 1. Thegrade was used in non-magneticsubmarines as one example.

Another development in the early1970s using N addition was an alloyintended for urea production. Thealloy, 2RE69 (Table 1), with high Crand Ni levels, showed excellentcorrosion resistance also in nitric acidand chloride environments (10, 11).

In 1976 Avesta Jernverks ABpatented and introduced 254 SMO,a 6Mo superaustenitic stainless steelwith a balanced compositioncontaining 20% Cr, 18% Ni, 6% Mo,0.7% Cu and 0.2% Ni (12). Theaddition of N made the precipitationof intermetallic phases more sluggish,facilitating production of heaviergauges. It also improved the mechan-ical and corrosion properties. Later,other 6% Mo steels followed this Nalloying approach. Examples include

AL-6XN and Cronifer 1925hMo, seeTable 1. Common for this family of socalled 6Mo super austenitic steels is avery high resistance to pitting andcrevice corrosion. Therefore, theyhave been used extensively in theoffshore and desalination industriesfor seawater handling, in chlorineand chlorine dioxide stages in bleachplants, and in flue gas desulphuriza-tion plants.

A special development in the 1970swas a high Si austenitic stainless steelfor use in sulphuric acid at hightemperature and concentrations andhighly concentrated nitric acid.Avesta Jernverk and Sandvik Steeljointly developed and patented thisalloy (13). It was later denominatedSX and contains as high as 5% Siand 2% Cu (Table 1) providing thesteel excellent properties for thosespecial environments.

Further developments in the 1980shave been to use the Sanicro 28 baseand add more alloying elements.Alloy 31 is one example where theMo content has been increased andN addition has been practised, seeTable 1 (14). This grade has, due toits very high Cr level, a higher pittingresistance than the current 6Mogrades.

The concept of N alloying auste-nitic grades has been used fordecades and very high N levels, upto about 1%, have been achieved(15). Well known commercial gradesare for example the Nitronic seriesfrom Armco. The positive influence ofMn on N solubility has been used inmany developments but Cr and Mohave similar and maybe synergeticeffects. In the 1980s alloys, high inthese three elements, and with veryhigh N levels were developed inSweden and Germany (16, 17). Bothgrades, 934LN and 1.4565, con-tained about 0.4% N, as seen inTable 1. These grades show similarcorrosion resistance but superiorstrength compared to the 6Mo steels.The use of thermodynamic databasesto predict the N solubility in highalloy austenitic steels has resulted infurther development. If the alloyinglevels of Cr and Mo are furtherincreased, even higher N contentscan be reached yet with quite lowMn addition. This was utilized in the

development of 654 SMO, with just3% Mn and still 0.5% N as listed inTable 1 (18). 654 SMO is one of themost highly alloyed superausteniticstainless steels produced to date andas a consequence, it has a corrosionresistance on level with the bestnickel-base alloys, as will be shownbelow.

Parallel to the evolution of austeniticstainless steels there was also anearly development of austeniticnickel-base alloys for corrosive en-vironments. Thus for example variousHastelloys were developed in the1920s and 1930s. The productionwas made in quite a small scaleduring the first decades. As with theaustenitic stainless steels, the intro-duction of new refining technologiessuch as electroslag remelting (ESR),vacuum arc remelting (VAR) and AODresulted in increased yields andimproved production economy.

An evolution has also occurredduring the years of heat resistingaustenitic stainless steels. A deter-mining factor for this field of applica-tion is the fact that the austeniticstructure implies a high creep strength.Types 309 and 310 were thus usedearly for high temperature service dueto both high oxidation resistance andcreep strength. For more criticalapplications nickel-base alloys with∼20% Cr and up to ∼70% Ni wereused. They contained various otheradditions for improved high tempera-ture performance. Austenitic stainlesssteel grades with ∼20% Cr and 30-40% Ni have also been availablesince the early 1920s as oxidationresistant alloys for use in furnaceparts and heating elements. Due tonickel shortage in the early 1950s,INCO introduced Incoloy 800, con-taining about 20% Cr and 30-35%Ni (19). The alloy also contained Aland Ti additions that were found toimprove the high temperature prop-erties considerably. Later, severalmodified versions have resulted inAlloy 800 representing a family ofmaterials with a range of properties.

Minor alloy additions to the hightemperature materials have beenused to improve the scaling resistanceand creep strength. Elements mostcommonly used to reduce the scaleformation are Al and Si but trace

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additions of Th, Y, Zr and REM canhave drastic effects. Elements thatenhance creep properties are e.g. C,N, Mo, AI+Ti. Different alloyingphilosophies have been used duringthe years resulting in a great varietyof heat resisting superausteniticgrades. Avesta Sheffield has success-fully used a combination of Si, N andREM to achieve superior high tem-perature properties of a 21% Cr, 11%Ni steel, 253 MA (20). Recently,Avesta Sheffield introduced a hightemperature grade containing 25%Cr, 35% Ni utilizing this combinationof additions (21). This superausteniticsteel, 353 MA, shows superior oxida-tion resistance and excellent creepstrength (Table 2). Silicon and REMadditions in combination with a highchromium content, give the goodoxidation resistance. The high solidustemperature compared to nickel-basealloys, 1360°C for 353 MA, is also anadvantage at very high temperatures.The high creep strength is particularlyuseful at temperatures above 1000°C.This is a result of the carbon andnitrogen additions, but also to acertain extent, the REM additions.

MechanicalPropertiesThe prime target for superausteniticstainless steel has been to achievehigh corrosion performance bycertain alloying additions and not todevelop or establish specific mechan-ical properties. Resulting mechanicalproperties have therfore been ofsecondary importance also bearingin mind the fact that austenitic steelsseldom display any dramatic effects.Characteristic for the austenitic stain-less steels is a moderate strengthcombined with a high ductility. Inci-dental to the improvements achievedin corrosion resistance, the highnitrogen superaustenitic stainless steelmay show increases of 50-100% inyield strength while retaining thesuperb ductility and toughness thatcharacterize austenitic stainless steels.This is illustrated in Table 3, where

typical mechanical data for severalsuperaustenitic steels are listed. It canbe seen that the alloys with high con-tents of N show the highest strengths,in some cases on level with those ofduplex stainless steels. Despite thehigh strength, the elongation is con-siderable and even superior to that ofmany lower alloyed grades. This isexplained by another feature perti-nent to high N; a high work harden-ing rate. Thus, very high strengths canbe obtained in cold worked com-ponents. Applications where thisproperty can be used are for tubularsin deep sour wells and for bolts etc.The impact toughness of practicallyall austenitic stainless steels is veryhigh, in many cases exceeding themaximum value obtainable in thecurrent Charpy V test at roomtemperature. The tough-ness onlydeclines moderately with loweredtemperatures and is still high at-196°C.

Corrosion resistanceAs described earlier, the develop-ment of austenitic stainless steel hasto a great extent been driven by theneed to satisfy requirements oncorrosion performance in variousenvironments. Many new steels havethus been developed for particularenvironments or applications such assulphuric acid and nitric acid. Mostsuperaustenitic stainless steels are forthis reason superior to the standardgrades. In this paper the discussionwill be concentrated to four maincorrosion types of great importancefor the high performance steels,namely uniform corrosion, pitting,crevice corrosion and stress corrosioncracking.

Table 2.Compositions of austenitic heat resistant stainless steels.

Alloy Cmax Si Cr Ni N Other EN UNS

309S 0.08 23 13 1.4833 S30908

310S 0.08 25 20 1.4845 S31008Alloy 800* 0.05 0.5 20 31 AI, Ti 1.4958 N08800

253 MA** 0.10 1.5 21 11 0.1 REM 1.4835 S30815353 MA** 0.05 1.2 25 35 0.1 REM 1.48XX S35315

* Trademark INCO ** Trademark Avesta Sheffield

Table 3.Typical mechanical properties of some standard and super austenitic grades,hot rolled plate.

Yield Tensile Elonga- Charpy V Brinell

Alloy strength strength tion Impact hardnessMPa MPa % J, RT

304 280 590 50 300 170316L 290 600 50 300 170904L 270 600 50 300 160

1.4439 310 640 50 300 180Sanicro 28 300 620 50 300 1701.4465 450 820 60

254 SMO 340 700 50 250 180654 SMO 470 840 60 250 215

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Uniform corrosionUniform corrosion on stainless steelsoccurs mainly in acids and hotalkaline solutions that are able todestroy the passive layer. The mostimportant alloying additions to in-crease the passivity of a stainlesssteel are Cr and Mo. Thus, the alloysvery high in these elements showexcellent corrosion resistance in manysolutions. Ni increases the resistancein some non-oxidizing acids and incertain environments additions suchas Si, Cu and W give increasedresistance. Corrosion data for varioussteels could be presented as cor-rosion rates in mm/year or in iso-corrosion diagrams covering differentconcentrations and temperatures of acertain chemical. Such information isavailable for example in AvestaSheffield Corrosion Handbook (22). InFigure 1, an iso-corrosion diagram fora number of austenitic stainless steelsin sulphuric acid is shown. The higheralloyed steels show a superior resist-ance in a wide range of concentra-tions than the standard grades 304and 316. The diagram also showsthat the high silicon steel, SX, has avery high resistance to concentratedsulphuric acid. The iso-corrosion dia-gram in hydrochloric acid is shown inFigure 2. Also here, the great advan-tage of using a high alloy superaus-tenitic grade is apparent. In sulphuricand hydrochloric acid the very highalloyed grade 654 SMO showssuperior corrosion resistance at mostconcentrations.

Pitting and crevice corrosionPitting and crevice corrosion are twoclosely related types of corrosion thatregularly cause failures on stainlesssteels exposed to chloride containingenvironments. It is also a well knownfact that increased contents of Cr andMo enhance the pitting corrosionresistance. By adopting a pittingindex or pitting resistance equivalent(PRE) the influence of various elementson the chloride pitting corrosionresistance can be estimated. The PREis the result of application of linearregression mathematics to the valuesof a particular corrosion result as afunction of the composition of thegrades tested. The most current PREexpression involves Cr, Mo and N, asshown below, but also other elementscould have some effect on pittingresistance.

PRE = % Cr + 3.3 x % Mo + k x % N,

where k is reported as between13 and 30.

A positive factor has been pro-posed for W and a strong negativefactor has been reported for S (23).The pitting indices, using a factor forN of 16, are listed in Table 1. Acommon laboratory test method forranking different alloys is a modifiedASTM G48 test where a criticalpitting temperature (CPT) can beassessed in a ferric chloride solution.The CPT is normally defined as thelowest temperature where a pittingattack occurs. In Figure 3 (page 6) the

CPT values for a number of austeniticstainless steels are plotted against thePRE value using a factor for N of 16.The 6Mo steels, represented by254 SMO, show quite high PRE num-bers and CPT values. This family ofsuperaustenitic stainless steels hasalso been used extensively in appli-cations where a high pitting resist-ance is required. The highest PREvalues are reached in Alloy 31 and,particularly, in 654 SMO. With thesehigh values the pitting resistance is onthe level with many high performancenickel-base alloys. The ranking of thecrevice corrosion resistance is largelysimilar as that of pitting resistance.

Stress corrosion cracking(SCC)Stress corrosion cracking is caused bya combination of tensile stresses andcorrosive environment. For stainlesssteels, SCC occurs mainly in chlorideenvironments above about 50°C. Thestandard austenitic stainless steels aremore prone to SCC than ferritic andduplex stainless steels. However,when it comes to superausteniticstainless steels, very high resistancecan be obtained, in many instancessuperior to the duplex grades. This isillustrated in Figure 4 (page 6), wherethe threshold stresses for SCC, undersevere evaporative conditions, deter-mined by the drop evaporation testare shown. Increased risk of attack isencountered if hydrogen sulphide(H2S) is present, as common in oil and

Figure 1.Isocorrosion diagram, 0.1 mm/year, in sulphuric acid.

Figure 2.Isocorrosion diagram, 0.1 mm/year, in hydrochloric acid.

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Figure 3.Plot of CPT in Ferric chloride versus PRE for several austenitic grades.

Figure 4.Threshold stresses determined by the drop evaporation test.

gas wells. Such wells are currentlycalled sour wells. Superausteniticstainless steels show high resistancein sour environments while the duplexstainless steels are more prone tocracking due to hydrogen embrittle-ment of the ferrite phase. In the mostsevere oil and gas environments withhigh hydrogen sulphide contents,austenitic alloys are the optimalchoice of material.

The general observation is that thehigh alloy superaustenitic steels showsuperior resistance to all four corro-sion types described above. There-fore a single alloy can be used inmany harsh environments which is ofgreat importance in some industries.The evolution has been that super-austenitic grades can serve as multi-purpose grades similarly to the role of316L for less severe conditions. Thus317L and 904L have been multi-purpose grades for the pulp andpaper industry for many years. Thisrole has today been overtaken by the6Mo grades.

WeldingThe welding of common austeniticstainless steels is very well establishedand is generally considered to givevery few problems provided appro-priate procedures and recommendedconsumables are used. One classicalproblem is that of weld decay due to

carbide precipitation followed bysusceptibility to intergranular cor-rosion of the heat affected zone. Thisproblem is today eliminated bycontrol of the carbon levels in thesteels. Due to a stable structure thereis no need to preheat or post weldheat the weldment. Normally, there isno minimum heat input limitation andquite high arc energies can be usedwithout any adverse effects.

For the more highly alloyed super-austenitic grades, particular concernhas to be paid to two phenomenarelated to the solidification, namelythose of hot cracking and elementalsegregation. Hot cracking occurseither directly during the solidification,and is referred to as solidificationcracking, or upon reheating of suc-cessive weld runs and is then con-sequently called reheat cracking.Both types of cracking are related tothe solidification mode of the welddeposit and presence of certainimpurities such as sulphur and phos-phorous. A primary ferritic solidifica-tion as is the case in standard aus-tenitic grades such as 304 and most316 gives high crack resistance.Superaustenitic stainless steels thatnormally have fully austenitic solid-ification are therefore more prone toboth types of cracking. A classicalcase is grade 310 showing a well-known susceptibility to hot cracking.Some of the highly alloyed, heatresistant grades have a similar

susceptibility. Although having anaustenitic solidification, 6Mo gradesexhibit less sensivitiy. However, thenickel-base fillers that are normallyused for this group of steels maybring the deposit to a more suscep-tible composition. To avoid or reducethe hot cracking, welding should bedone with low arc energy and withjoints giving low restraints.

One alloying element in steels witha high tendency to segregate duringsolidification is molybdenum. Ac-cordingly, superaustenitic grades withhigh molybdenum levels such as the6Mo steels show large molybdenummicrosegregations in autogenousweld metals. The areas depleted inmolybdenum have a lower localpitting index and are thus less resis-tant to chloride pitting corrosion. Toovercome this reduction in corrosionresistance, fillers overalloyed withmolybdenum, preferably nickel-basealloys, are used. Alternatively, a postweld solution heat treatment canrestore the pitting resistance of auto-genous welds. Another possibility iswelding with nitrogen addition to theshielding gas that enhances thenitrogen level in the weld to such adegree that improved pitting resist-ance is achieved. This, however, maynot be fully sufficient to restore thepitting resistance.

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ApplicationsSome examples of applications forthe superaustenitic stainless steelswere described already above. Dueto their good combination of cor-rosion resistance, many superauste-nitic grades are used under a greatvariety of conditions. The 6Mo gradeswere originally and are still usedmuch in the pulp and paper industryin C- and D-stage bleach washersand in flue gas scrubbers. In theseapplications the 6Mo grades re-placed other stainless steels such as317L and 904L that suffered pittingcorrosion. The largest single field ofapplication for the 6Mo steels is theNorth Sea offshore industry wherelarge quantities are being used in theseawater systems, mainly in the formof pipes and fittings. Other Importantapplications where seawater orsealine water is the environment arecondensers, heat exchangers andpiping in desalination plants. Due tothe good combination of pitting anduniform corrosion resistance of the6Mo steels, many installations inchemical and pharmaceuticalindustry have been made.

The most highly alloyed super-austenitic steel, 654 SMO, has foundapplications where the 6Mo steelsare not sufficiently resistant and takesup the competition with nickel-basealloys and titanium. Examples of thisare plate heat exchangers andflanges for seawater at higher tempe-ratures, D-stage bleach washers andcondenser tubing.

Superaustenitic heat resistantgrades, like 353 MA, show excellenthigh temperature properties thatbring them on level with certainnickel-base alloys, particularly con-cerning oxidation, carbon andnitrogen pick-up and creep resist-ance. Important areas of applicationare equipment for heat treatment,power industry, thermal destruction-incineration units and cementindustry.

Concluding remarksThe above short review cannot coverthe complete picture of the super-austenitic stainless steels. It is, how-ever, the hope that the long existenceand strong present position of thissteel family are illustrated. The devel-opment of superaustenitic stainlesssteels has been guided by newdemands and made possible throughnew or improved production proces-ses. However, a deep understandingof the correlations between propertiesand structure has been crucial for thedevelopment. Today, the highestalloyed superaustenitic stainless steelsexhibit characteristics that make themon the level with some of the nickel-base alloys. Certainly, the evolutionhas not ceased. Due to even moreadvanced or new process techniquesand better simulation and modellingcapabilities there are great potentialsalso for further evolution.

References1. German Patent No. 304, 126 (1912)2. Black H L, Lherbier L W, ASTM Special

Technical Publication No. 369, (1963),p. 312

3. Bernhardsson S, Österholm R, SandvikLecture 52/57E, FSI, June, (1979)

4. Baroux B, Maitrepierre Ph, Revue deMétallurgie -CIT, February, (1981), p. 145

5. US Patent No. 3,547,625 (1967)

6. US Patent No. 4,007,038 (1975)

7. Thier, H et al, Arch. Eisenh. 40 No 4,(1969), p. 333

8. Katz W, Werkstoffe u. Korrosion, (1973),p. 790

9. Kohl H et al, Proceedings "AdvancedStainless Steels for Seawater Applications",Piacenza, Climax Molybdenum, (1980),p. 59

10.Blom U, Kvarnbäck B, Materials Per-formance, July, (1975), p. 43

11. Wallén B et al, Stainless Steel Industry, 6,No 34, (1978), p.

12. Swedish Patent No. 7601070-1 (1985)

13. British Patent No. 1.534.926, (1976)14. Heubner U et al, Werkstoffe u. Korrosion

40, (1989), p. 418

15. Uggowitzer et al, Applications of StainlessSteels, Stockholm. (1992), p. 62

16. Swedish Patent No. 8305795-0 (1986)

17. German Patent No. DE 3729577 C1 (1988)

18. Swedish Patent No. 9000129-8 (1992)

19. Van de Vorde M, Alloy 800. Petten IntConf, North Holland Publ. Company,(1978), p. xvi

20. Swedish Patent No. 7410791-3 (1985)

21. Swedish Patent No. 8804178-5 (1990)

22. Avesta Sheffield Corrosion Handbook forStainless Steels (1994)

23. Neubert V et al, Final Report COST 504 II,Clausthal, Aug. (1993)

Paper presented at IIW-95 Annual Assembly, International Conference on Welding ofStainless Steels, June 12-13, 1995, Stockholm, Sweden.

Although Avesta Sheffield has made every effort to ensure the accuracy of this publication, neither it nor any contributor can accept any legalresponsibility whatsoever for errors or omissions or information found to be misleading or any opinions or advice given.

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