STP48440S.2307

Weldablity o f Austeniti c Stainles s Steel sas Affected b y Residua l Elements

REFERENCE: G . E . Linnert , "Weldabit y o Austeniti c Steel s a s Af -fected by Residua ] Elements," Effects of Residual Elements on Propertiesof Austenitic Stainless Steels, ASTM STP 418, Am . Soc . Testin g Mats. ,1967, p . 105 .

ABSTRACT: Th e effect s o f residua l element s i n austeniti c stainles s steel sare o f considerabl e importanc e i n welding . Man y unusua l occurrence ssuspected t o b e relate d t o residua l element s hav e bee n encountered , in -cluding cracking , porosity , sla g formation , corrosio n susceptibility , an dembrittlement. Residual element s ar e difficul t t o contro l an d trace becaus ethe wel d meta l ma y accumulat e th e residua l elemen t fro m suc h source sas the bas e metal , fille r metal , fluxes , an d shieldin g gases . Becaus e grea teffort regularl y i s mad e t o circumven t difficultie s throug h th e weldin gprocedure, man y phenomen a believe d t o b e relate d t o residual s escap einvestigation an d ar e no t documented . Thre e proble m area s o f curren tinterest ar e reviewed : (a) slag formatio n o n wel d meta l whic h interfere swith th e joinin g process, (b) porosity i n weld metal a s produce d b y nitro-gen containin g compound s i n th e bas e metal , an d (c ) crackin g i n wel dmetal an d th e heat-affecte d zones . A nee d i s show n fo r mor e exactin gknowledge of the atomistic distributio n of residua l elements in the variou sforms o f austenitic stainless steels, such as weld metal an d wrough t or cas tbase metal , becaus e thei r behavio r i n weldin g varie s greatly wit h micro -structural condition .

KEY WORDS : metals , stainles s steels , precipitatio n hardening , welding ,austenitic stainles s steels , slags , blowholes , porosity , cracking , fluxes ,heat-affected zone , residua l elements , ho t cracking , arc welding , electro nbeam welding

The effect s o f residua l element s in weldin g austeniti c stainles s steel sare considerabl y more complex tha n i n thei r manufacture o r an y otherphase o f fabricatio n an d use . Complication s aris e i n weldin g becauseresidual elements which have the propensity to affec t weldability , in addi-tion to being present in the base metal, also may be introduced by othermaterials employe d in th e joinin g operation. Many times, w e ar e con -cerned not with the level of a residual in the base metal, but whether the

1 Supervisor, weldin g research , researc h an d technology , Armc o Stee l Corp. ,Middletown, Ohio .

105

STP418-EB/Jul. 1967

Copyright 1967 by ASTM International www.astm.org

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1 06 RESIDUA L ELEMENT S IN STAINLES S STEEL S

accumulated amoun t of an elemen t from al l contributing source s ha s in -creased t o a n influentia l level . Th e element s carbon , boron , sulfur , an dhydrogen are but a few residuals which can be introduced into a particularregion o f a weld joint by a surprising number of sources .

Many unusual occurrences, suspecte d t o b e the effect s o f residual ele -ments, hav e bee n encountere d i n weldin g th e austeniti c stainles s steels .These incident s hav e include d a wide variet y o f difficultie s an d defects ;such a s cracking , porosity , sla g formation, corrosio n susceptibility , an dembrittlement. These effect s ca n vary with the welding process bein g em-ployed. Consequently , th e weldin g enginee r an d th e welde r ar e ofte ntempted t o try an y new procedure tha t offer s promis e o f overcoming theunwanted effect . Whil e a successful procedure spell s relief fo r th e imme -diate problem, little is learned abou t the basic cause to prevent recurrence .In general , to o littl e wor k ha s bee n conducte d t o firml y establis h th emechanisms by which the suspec t residuals exer t thei r effects .

As illogica l a s it ma y seem , a grea t man y o f th e so-calle d "residual "elements which can b e pointe d ou t a s harmfu l t o th e weldabilit y o f on egroup o f th e austeniti c stainles s steel s als o wil l b e foun d usefull y em -ployed a s alloyin g elements in othe r group s o f thes e steels . Briefly , th eeffect imparte d b y a given element will vary greatl y wit h the overall allo ycomposition, microstructure, and condition of the steel. The complexitiesof this situation preven t th e formulation o f simple, generalized statement sregarding th e influence o f particular residual elements upon weldability .Furthermore, thos e wit h preconceive d idea s abou t widesprea d adverseeffects o f residual elements will be surprised to learn tha t important bene-fits hav e been found attributabl e to certain residua l elements i n particula rwelding operations. There have been a number of cases wher e fervor fo rproducing stainles s stee l with the lowes t possible leve l of al l residual ele -ments resulted in a final product tha t displayed inferio r weldability .

A number of residual elements exer t multipl e effects . Instance s ca n b ecited where increasing amounts of a residual element will cause deteriora -tion i n on e property , bu t a t th e sam e tim e wil l exer t improvemen t i nanother property . Suc h case s usuall y requir e determinatio n o f a com -promise range for the particular element , an d thi s may cal l fo r consider -able skil l on the part o f the steelmake r to kee p a trace elemen t withi n arelatively narrow range. Ther e ar e effect s i n welding which are produce dby two or more residual elements. At this time, little quantitativ e data areavailable to show whether the tota l influenc e o f these element s is additiveor multiplicative .

Although a n element-by-element review would be useful , a better per -spective o f the residua l element problem i n welding can b e presented b ydiscussing selected area s which are of current interest an d wherein weld-ing researc h investigator s hav e bee n mos t active . Othe r propertie s an d


LINNERT O N WELDABILIT Y 10 7

FIG. l Slag or dross on top surface of solidified welds made in five differentaustenitic stainless steels by the gas tungsten-arc process. The specimens representprogressively increasing slag-forming propensity from virtually no slag (No. 1) toconsiderable slag (No. 5) .

FIG. 2 Slag or dross on root of full-penetration weld bead of same specimensas illustrated in Fig. 1. Note that even though welding was performed in flat posi-tion, slag is accumulated on bottom of weld by surface tension forces. Steel inSpecimen 3 produces small amount of slag which coalesces in small spots dis-tributed at intervals. Specimens numbered 4 and 5 have more-or-less continuousslag covering along weld bead.

characteristics o f stainless steels which are affecte d b y residual elements,such as corrosion resistance, als o can be o f importance i n a weldment.

Slag Formatio n i n Weldin g

Certain residual elements may cause viscous, refractory slag or dross toform on the molten weld pool o f stainless steel during fusion welding . Theappearance o f the sla g remaining on the surface s of the solidifie d weld is


l 08 RESIDUA L ELEMENT S IN STAINLESS STEEL S

illustrated in Figs. 1 and 2. This is a troublesome conditio n because (a) theslag ma y interfer e wit h manipulatio n o f th e molte n metal , (b) th e sla gmay become entrapped in the weld, and (c) the slag may reduce the exten tof penetration by the weld bead. The sla g is more apt to be noticed andregarded as objectionable in welding processes which do not use a protec -tive flux or slag cover, suc h as oxyacetylene, gas tungsten-arc, ga s metal -arc, o r electron-bea m welding .

The slag problem ha s been more troublesome i n gas tungsten-arc weld-ing (GTAW), wher e the weld pool normall y is expected to be free o f slagor othe r oxide . Occasionally , a particular hea t o f base metal , o r o f fillerwire, will be employed which persists in forming small patches o r globulesof sla g on the surfac e o f the molten weld . In manua l welding operations ,the welder ofte n ca n manipulat e the torc h t o contro l thes e sla g globulesand disperse them on the solidifying wel d surface. However, i n automaticGTAW operations , suc h a s makin g an autogenou s wel d i n tubing , th eslag globules can cause several problems.

As th e sla g globules quickly move abou t o n th e wel d pool , the y fre-quently move into the area directly under the electrode and cause momen-tary arc instability. This is reflected in a variation i n weld bead size, shape,

, and penetration. Lack of complete penetration may b e cause for rejection.i Usually, slag particles cling very tenaciously to th e surfac e and hav e beenthe cause of defects in subsequent rolling and cold drawin g operations .

The origin of the slag, or the mechanism b y which it forms, is complex .Laboratory stud y to date has not trace d th e sla g back t o the observabl enonmetallic inclusions in the stee l prio r t o welding . Yet , ther e i s stron gevidence that the slag-making elements are already in the oxidized form .In work with Types 304 and 316 steels , specimen s from steels containin gwidely varyin g amount s o f nonmetalli c inclusions , a s observe d metallo -graphically, produced equa l amounts of weld slag using the same GTA Wprocedure. Weldin g experiments performe d i n a "dr y box " ( a speciall ypurged welding chamber) on very clean steel s which previously producedweld sla g in regula r GTA W operation s continue d t o produc e a n equa lamount o f slag . Thi s performanc e suggested , o f course , tha t th e sla gglobules represente d a n accumulatio n o f submicroscopi c oxid e particle salready in the steel rather than the formation of new particles by oxidation.When th e sla g globule s wer e mechanicall y dislodge d fro m th e surfac eof the solid weld made in a dry box an d only the weld metal again meltedunder th e gas tungsten-arc torch , no sla g appeared during th e remelting .This resul t suggested that the offending , slag-formin g compounds o r par -ticles were eliminated from the weld metal during the first fusion operation .

Attempts hav e bee n mad e t o identif y th e slag-formin g elements b yanalyzing the globules whic h can be scraped of f the weld surface. Result sexchanged and discussed among investigators all point t o elements whichform high-melting-poin t oxides a s being a t the roo t o f the problem . Th e


LINNERT O N WELDABILIT Y 1 09

elements aluminum , calcium, magnesium , titanium, an d zirconiu m havebeen mentione d mos t frequently . O f course , th e analys t als o find s th eoxides o f chromium, manganese, silicon , an d iro n presen t i n substantia lquantities. The analytical work on weld slag has not been extensive enoug hto clearly define the circumstances under which certain elements will enterinto the formation of an objectionable slag . It i s not possible to single outparticular element s suc h a s aluminum , or titanium , an d maintai n tha tthese elements cannot be tolerated abov e certain levels without weld slagformation. Indeed, certain types of stainless steels are made which containsubstantial alloying amounts of these elements and yet they can be GTAWwelded without objectionable slag formation.

The answe r to the weld slag puzzle appear s t o li e in the overal l com-position o f the slag that forms a s nonmetallic inclusions in the base metaland filler metal accumulat e on the surfac e o f the weld . It i s well knownthat weld metal formed by the GTAW process contains noticeabl y fewe rnonmetallic inclusion s tha n the base metal fused t o form th e weld. Ther ehas not bee n a methodical investigation , however , of the compositio n ofthe final slag as affected by (a) nonmetallics present in base or filler metals,and (> ) chemical reactions between metal and slag during slag formation .Even thoug h element s whic h for m refractor y oxides ma y b e present , i tappears possibl e t o hav e the fina l sla g o f a compositio n whic h i s quit efluid a t weldin g temperatures . However , i f circumstance s lea d t o th eformation o f a slag composition whic h is very viscous at welding tempera-tures, th e accumulatio n o f slag globules o r patche s suddenl y makes th ewelder aware that his manipulative efforts ar e being hampered by trouble-some foreign material on the surfac e o f the weld pool.

Another subtle form of slag formation occasionally is experienced in gastungsten-arc welding the stainless steels which appears a s a very light filmor dros s over th e entir e surface of the molte n weld pool. Thi s film is n omore than a cloudy effec t o n the molten wel d metal, an d it does not seemto affec t th e fluidity of the molten meta l o r t o preclude th e welde r fro mmaking a sound joint . However, th e film sometimes noticeably decrease sthe extent of penetration o f the weld into the base metal. In a productionoperation usin g the GTAW process, th e unexpected presence of the drossfilm on th e wel d poo l ma y resul t i n defectiv e (incompletely penetrated )welds. Althoug h n o researc h wor k has bee n reporte d o n th e mechanicsby which the film forms o n the poo l an d reduce s wel d penetration , i t i shypothesized tha t th e film represents a readily oxidized residua l element(or elements), the oxide of which enters into the thermionic syste m of thearc. Th e dissimila r liberatio n o f hea t a t th e anod e an d cathod e o f atungsten-arc i n inert gas is well known, but little information i s availabl eon the changes which occur in heat balance when unusual ion or electronemitters appea r o n th e electrod e o r th e wel d pool . Som e evidenc e ha s


l l O RESIDUA L ELEMENT S I N STAINLESS STEEL S

appeared tha t the presence of rare earths (that is , cerium, lanthanum) instainless steels can promote thi s effec t i n GTAW welding.

Remedial change s whic h ca n be made in welding procedur e to over -come sla g or dros s film vary greatly with the weldmen t being produced.While the introductio n o f a fluxin g agen t might seem to b e a n effectiv ecountermeasure, this seldom is a convenient or practical remedy. The fluxapproach ha s bee n explore d t o th e poin t o f addin g smal l amount s ofspecial "fluxing gases" to the inert gas shield used in the GTAW process .These attempt s at gas fluxing have not achieve d complet e succes s i n anyinstance because of problems wit h toxicity , wel d contamination, an d s oforth. Th e us e o f helium shielding is more effectiv e tha n argo n i n con -trolling wel d sla g o r dros s film . Smal l addition s o f hydroge n (approxi -mately 2 t o 5 per cent ) t o th e iner t ga s shiel d als o ma y b e helpfu l i nraising the thermal output of the GTAW arc and dispersing a troublesomeslag.

Porosity i n Wel d Meta l

Blowholes sometime s appear in the surfac e o f austenitic stainles s stee lweld metal. Internal porosity, also caused by a gas, may be encountered.The mos t frequen t caus e o f thi s unsoundnes s i s th e elemen t hydrogen.The amoun t o f residua l hydroge n containe d i n th e austeniti c stainles ssteel base metal and filler metal usually is in the orde r o f 1 to 1 2 ppm,which ordinarily proves to be within the solubility limit for the conditionsof temperatur e an d solidificatio n rate during welding. Therefore , hydro -gen-induced porosity occurs when the residual hydrogen is supplementedby anothe r sourc e which raises the tota l amoun t o f hydrogen abov e th esolubility limit . O f course, al l materials intende d fo r weldin g operation son stainless steels are designed to be "low-hydrogen" i n character. How -ever, a few examples of deceptive sources o f hydrogen ar e dam p fluxes ,improperly compounded electrode coverings, and imperfectly sealed joint sin welding torch cooling systems which bleed water vapor .

Another caus e o f wel d meta l porosity i s residua l nitrogen . However ,the mechanism by which this elemen t produces unsoundness i s quite un -like that involvin g hydrogen. The residua l nitrogen content o f austeniticstainless steel s generally is in the rang e o f abou t 0.0 2 t o 0.0 5 pe r cent .Some specia l steel s whic h mak e us e o f nitroge n a s a n alloyin g elemen tmay contai n a s much as 0. 5 pe r cent . Th e for m an d distributio n o f th enitrogen i s determined by the natur e o f alloyin g elements present i n th esteel's composition . Nitroge n hel d a s a compoun d sometime s threaten sthe soundnes s o f th e wel d metal .

Stainless steels containing only residual nitrogen, but bearing additionsof stron g nitride-forming elements , suc h a s aluminum , titanium , zirco -nium, an d columbium , often wil l contain nitrides , o r carbonitrides , dis -tributed a s microscopic inclusions . When these steel s are fusio n welded ,


LINNERT O N WELDABILIT Y 111

FIG. 3 Longitudinal section through GTAW weld showing small gas pocketsalong fusion line at upper boundary of weld in Cr-Ni-Al stainless steel. Porosity wascaused by thermal decomposition of aluminum nitride inclusions in base metal im-mediately adjacent to weld. Etchant: mixed acids (XIO).

FIG. 4 Typical gas pocket along fusion line of GTAW weld in Cr-Ni-Al stain-less steel containing approximately I per cent aluminum. Traces of undissociatedaluminum nitride are shown remaining along lower edge of cavity. Unetched(X500).

inclusions i n th e bas e meta l a t th e ver y edg e o f th e wel d ar e pron e t odissociate under the thermal conditions imposed by welding. If the gaseousproduct of decomposition i s not abl e t o escape throug h the molte n weldmetal, the nitrogen remains entrapped as a tiny gas pocket along the fusio n


1 1 2 RESIDUA L ELEMENT S I N STAINLES S STEEL S

interface becaus e o f its origin i n the bas e meta l immediatel y adjacen t t othe weld .

The propensity t o fusion line porosity wil l be dependent upo n the num-ber, size , an d kind of nitride inclusions in the base metal, and th e weldingconditions employed . Aluminu m nitride s ar e readil y dissociate d b y th eheat of welding. The nitrides of titanium and zirconium have a somewha thigher decompositio n temperatur e an d displa y a significantl y lesse rtendency to produce fusion lin e porosity. Weldin g conditions pla y a largepart i n determinin g whether the ga s which is liberate d a t th e fusio n lin e

FIG. 5 Typical cracking in austenitic stainless steel weld metal. Open crackat surface is readily visible to the naked eye and would be called a "hot crack."Several internal "microfissures" are present. Weld metal was formed by GTAWprocess and has a wholly austenitic microstructure. Etchant: mixed acids (X50).

will escap e t o th e wel d surface . Trave l spee d i s especially importan t be -cause rapi d solidificatio n increases the numbe r of gas bubbles entrapped .

Fusion lin e porosit y occasionall y ha s bee n troublesom e i n weldin gTypes 32 1 and 347 . Thes e steel s frequentl y contain varyin g amounts oftitanium o r columbiu m carbonitride s a s microscopi c inclusions . Newe rstainless steels ar e being introduced whic h contain stron g nitride-formingelements and weld metal porosity ma y prove a problem unles s correctiv emeasures ar e taken. Some years ago , 17-7PH , a semiaustenitic Cr-Ni-Alprecipitation hardenin g stainless steel, was reported t o give difficulty wit hfusion line porosity, as shown in Fig. 3 , in joints welded b y the automati cGTAW proces s [I].2 Th e presenc e o f aluminu m nitrid e inclusion s wa sestablished as the cause, as illustrated in Fig. 4 [2] . A remedy was effecte d

The itali c numbers i n bracket s refe r t o th e lis t o f reference s appende d t o thi spaper.


UNNERT O N WELDABILiT Y 1 1 3

by adding a small amount of titanium or zirconium to preferentially formnitride inclusions havin g bette r thermal stabilit y [3].

Cracking i n Wcldmcnt s

Cracking i s the principal defec t to be avoided in welding the austeniti cstainless steels , an d residua l element s appea r t o pla y importan t role s i nmany of the cracking phenomena observed. The literature contains a largenumber o f excellen t dissertation s o n th e variou s form s o f crackin g i nwelded joints and their suspected cause s [4]. Crackin g has been foun d t o

FIG. 6 Photomicrograph of typical microfissure in wholly austenitic stainlesssteel weld metal deposited by E3W-I5 shielded metal-arc electrode. Grain boundarymarked "A" is in earliest detectable stage of fissuring. Grain boundary marked "B"contains concentration of particles that apparently induces fissuring susceptibility.Etchant: mixed acids (X500) .

occur i n welded joint s of austenitic stainless stee l both i n the wel d metaland in the base metal heat-affected zone.

Weld metal cracking i n austeniti c stainles s steel s ha s bee n separate dinto four types , whic h have acquire d suc h common names as : (1 ) crate rcracks, (2 ) star cracks, (3 ) hot cracks or microfissures, an d (4) root crack s[5]. There are reasons t o believe that al l four types of cracking ar e simplymanifestations of the same basic kind of cracking; namely, "hot cracking "or, whe n present i n it s earlies t stage , "microfissuring " [6]. Ho t crackin gand microfissuring, as illustrated in Fig. 5 , gave much difficulty som e yearsago, but toda y enough is known about thi s form o f cracking to avoi d itsoccurrence i n the majority of weldments. Nevertheless, th e mechanics bywhich microfissures develop ar e not completely understood , an d thi s de -


114 RESIDUA L ELEMENT S I N STAINLES S STEEL S

feet stil l pose s a seriou s weldin g problem fo r certai n type s o f stainles ssteels. Althoug h investigator s hav e continue d t o stud y th e proble m o fmicrofissures, there is much to be learned abou t the influenc e o f residua lelements in controlling their occurrence.

Hot cracks or microfissures occur intergranularly or interdendritically asshown i n Fig . 6 . Th e grai n boundar y identifie d b y th e lette r "A " ha sfissured, and wil l propagat e a s a crac k unde r certai n condition s o r willopen a s a tear or crack durin g plastic deformation . Grain boundary "B"has not fissured but contains an abnormal concentratio n o f particles tha tappears to induc e fissuring susceptibility. The formatio n of microfissures

seems to be controlled by five principal factors : (1) the microstructure ofthe weld metal upon solidification, (2) the composition o f the weld metal,particularly the level of certain residual elements, (3) the amoun t of stressimposed on the weld as it cools through the high temperature range, (4) thehot ductility of the weld metal a t high temperatures, an d (5) the presenc eof notche s that form incipien t crack s a t the edg e of the weld .

Microfissures ca n develop in the as-deposite d wel d metal shortl y afte rsolidification. Recen t wor k has show n tha t microfissure s als o can occu rin th e heat-affecte d zone s of previously deposited (sound ) beads o f weldmetal [7,8]. Surprisingly , heat treatmen t o f wel d beads a t hig h tempera -tures intende d to disperse segregate s does no t necessaril y eliminate sus -ceptibility to fissuring in multipass heat-affected zones .

The generalized theory on microfissuring proposes tha t the segregationof certain elements as complex compounds i n grain boundaries an d inter -

FIG. 7 Microstructure o f weld metal containing approximately 5 pe r centdelta or free jerrite in the austenitic matrix. Etchant: mixed acids (X500).


LINNERT O N WELDABILIT Y 1 1 5

dendritic region s durin g solidification i s responsible fo r crac k formatio nat these locations. Severa l hypothese s have been advance d to explai n themechanics through which the defec t initiates . The experimenta l evidenc eas a whol e indicate s tha t mor e tha n on e ho t crackin g mechanis m ca noperate to form microfissures in weld metal. Regardless of the uncertaintyover the exact nature of the rupture mechanics, there is good agreement onthe residual elements which promote microfissuring. Th e mos t frequentlydealt wit h offendin g elements , i n approximat e orde r o f decreasin g po -tency, ar e boron , phosphorus , sulfur , selenium , tin , silicon , columbium ,and tantalum . Eve n oxyge n an d hydroge n hav e bee n pointe d ou t a spromoters o f microfissure s in austeniti c stainles s stee l wel d metal , bu twith limited evidence to substantiate th e assertion .

With the worst offending residua l elements designated, it might seem asimple task to ascertain the tolerable level for each. This has not provedpracticable becaus e o f the amoun t o f an y elemen t tha t ca n b e tolerate dis dependen t upo n th e fiv e principa l factor s mentione d earlier . Micro -structure wa s th e firs t wel d meta l featur e mentione d whic h strongl yaffects microfissuring suspectibility. Weld metal having a wholly austeniticmicrostructure is considerably more sensitive to conditions whic h promotemicrofissuring than weld metal containing some delta or free ferrit e in anaustenitic matri x a s show n i n Fig . 7 .

To illustrat e th e influenc e o f microstructure , wel d meta l o f Typ e 31 0(25Cr-20Ni) composition, which ordinarily has a wholly austenitic micro-structure, can be shown to undergo an increase in microfissuring suscepti-bility unde r rigorou s tes t condition s a s it s phosphoru s conten t i s raisedabove 0.01 0 pe r cent . O n th e othe r hand , i f th e chromiu m an d nicke lcontents of a weld metal of similar total alloy content are adjusted to secureabout 5 per cent delta ferrite in the weld deposit , the n a phosphorus con -tent exceeding abou t 0.1 per cent would be required to produce a similarincrease in microfissuring susceptibility. Eve n when consideration i s lim-ited t o wel d metal s wit h a wholl y austeniti c microstructure, th e secon dfeature mentioned, overall chemica l composition, will affec t toleranc e forresidual elements . A s examples , microfissurin g ha s bee n show n t o b ereduced by : (a ) a smal l increase i n carbo n conten t [9] , (> ) a substantialincrease in manganese [10], an d (c ) increasing the nitroge n conten t [11].Consequently, attempt s a t quantitativ e evaluatio n o f th e influenc e o fresidual elements o n cracking have been limited t o specifi c type s of steeland weldin g condition s [72] .

To summarize the present statu s of the weld metal microfissuring prob -lem in terms of residual elements, it can be said that two different practice sare followed dependin g upo n th e microstructur e o f th e weld . Whereve rpossible, a ferrite-containing austenitic weld structure i s employed . Withproper contro l of wel d meta l ferrit e content , the toleranc e for residua lelements exceeds b y a very comfortable margin the quantitie s of residual


1 1 6 RESIDUAL ELEMENT S I N STAINLES S STEEL S

elements present in the base metal and all other material s which contributeto th e wel d composition .

When a wholly austenitic weld structure must be employed, the practicewith respec t t o residua l element s i s simpl y to : (a ) hol d al l recognize dcrack-promoters a s low as can be achieved with available technology an dpermissible costs, an d (b ) make all possible adjustment s in regula r alloy-ing elements (tha t is , carbon, manganese , silicon , columbium ) known toimprove cracking resistance. Even wit h the bes t obtainable materials an dmost favorabl e weldin g procedure , wholl y austeniti c wel d deposit s ar e

recognized t o b e mor e crack-sensitiv e tha n th e ferrite-containin g types .Therefore, greate r car e mus t b e exercise d i n al l phase s o f weldin g andinspection whe n weld metal of this microstructural character i s employed.

Base metal heat-affected zone cracking i n welde d austeniti c stainles ssteels wa s no t know n t o b e a proble m unti l greate r us e o f (a ) heavie rsections, o r (b) mor e elaboratel y alloye d types o f steel s began . Perhap sthe first serious encounte r wit h bas e meta l crackin g occurre d wit h Typ e347 (18Cr-8Ni-Cb) i n about 194 9 when this steel was employed i n heavy-wall pressure vessel s for nuclea r power units . Although Type 34 7 ha d agood reputatio n a s a weldable steel i n th e for m o f shee t an d ligh t plate ,some items displaye d a n alarmin g propensit y t o crac k i n heavy section sof bas e meta l i n the zon e immediatel y adjacen t t o weld s as illustrated in

FIG. 8 Underbead microcracking in Type 34 7 base metal o f shielded metal-arcwelded joint in as-welded condition. Etchant: mixed acids (X500).


LINNERT ON WELDABILITY 1 l 7

Fig. 8. Cracking was observed to occur in this region both in the as-welde dcondition an d afte r postwel d stress-relie f hea t treatment . Shortl y after -ward, an d muc h to th e disma y of stea m powe r engineers , Typ e 34 7 i nheavy-wall pipin g of higher-temperature central stea m station s began t oexhibit susceptibility to failure by cracking in the base metal heat-affecte dzone after seemingl y sound weld joints had been in elevated temperatureservice fo r som e tim e [13].

Much research has been directed to the weld joint cracking in Type 347and at least two mechanisms appear to operate: (1 ) an intergranular for mof ho t shortnes s whic h occurs durin g welding , whereupo n th e crackin gdevelops because of grain boundary liquation, o r embrittlement , and (2 )a complex phenomenon involving strain-induced precipitation in the heat-affected zon e during postweld heat treatment , or servic e at elevated tem -perature, wherei n th e chang e i n mechanica l propertie s result s i n stress -rupture failur e unde r certai n condition s [14]. Th e Britis h painstakinglydetermined that the precipitation o f columbium carbide in the dislocationspresent i n th e stresse d heat-affecte d zone s wa s the ke y t o th e problem .The British als o found tha t a s little as 0.10 per cen t columbium , presentas a residual element , coul d produce th e strain-induced precipitatio n an dset the stage for failure by cracking [15].

Base metal heat-affected zone cracking has been reported i n relativel ylight sections of the more complex-alloyed austenitic stainless steels [16].Although the microstructures of these steels contain many kinds of com-pounds whic h alon e giv e caus e fo r concer n i n welding , there ha s bee nenough variation in the susceptibilit y of differen t lot s o f the sam e type ofalloy to suspec t tha t residual element s ma y b e exertin g a n influenc e o noverall weldin g performance.

Summary

Residual elements in austenitic stainless steels exert both favorable andunfavorable influence s o n th e weldabilit y o f thes e steels . Th e circum -stances throug h whic h various residua l element s reac h influentia l level soften ar e deceptive , an d th e mechanism s b y whic h th e element s exer tspecific effect s ar e no t wel l understood . T o illustrat e th e complexit y ofthe rol e o f residual elements , severa l troublesom e condition s hav e beenreviewed; t o wit :

1. Sla g formation on the surface of the molten weld metal during fusionjoining is promoted b y such elements as aluminum, calcium, magnesium ,titanium, an d zirconium , bu t th e mer e presenc e o f the elemen t doe s no tinvariably result i n slag . Studies sugges t that submicroscopi c particle s o fcompounds o f thes e element s agglomerat e t o for m sla g durin g fusion .When the agglomeration represents a high-melting point, viscous slag , orinfluences th e arc , it s presence o n the weld is troublesome .

2. Porosit y an d blowhole s sometime s occu r i n wel d metal , an d ar e


l l 8 RESIDUA L ELEMENT S IN STAINLESS STEEL S

commonly caused by hydrogen. However, nitroge n also can produce thes eforms o f unsoundness . Th e mechanic s involv e thermal decompositio n o fmetal nitrid e paritcles alon g the fusion lin e during welding.

3. Crackin g i n wel d meta l i s promoted b y a numbe r o f elements , in -cluding boron , phosphorus , sulfur , an d columbium . Thi s defect , com -monly called "ho t cracking " or "microfissuring" is highly dependent upo nweld metal composition, microstructure , an d high-temperature properties ;as well a s the weldin g procedure an d condition s associate d wit h the jointbeing welded.

4. Crackin g i n the base meta l heat-affected zones ha s proved t o be anunexpectedly sever e problem , bu t onl y i n certai n steel s unde r specifi cconditions. A t leas t tw o cracking mechanism s have been detecte d b y in -vestigators: namely ; (1 ) intergranula r crackin g resultin g fro m grai nboundary liquidation , o r embrittlement , an d (2 ) stress-ruptur e failur earising out of strain-induced precipitatio n hardening .

Concluding Remark s

There i s a strong need fo r mor e exacting knowledge o f the atomisti cdistribution o f residua l elements i n the austeniti c stainles s steel s an d th eformation an d behavio r o f compound s an d phase s involvin g these ele -ments during the usual thermal cycles of welding. With the increasing us eof ne w research tool s an d techniques, there i s good reason t o believe tha tmore welding problems wil l be analyzed to greater depth in the near future .While some work of this kind has been carried ou t [77,75] , the importanc eof welding as our most widely used joining process and the need for greate rweld reliability calls for muc h more effor t i n this direction .

References

[] Funk , C . W. , an d Granger , M . J. , "Metallurgica l Aspect s o f Weldin g Pre -cipitation-Hardening Stainless Steels," The Welding Journal, Researc h Supple-ment, Vol. 23, No. 10, Oct. 1954 , pp. 496s-508s.

[2] Linnert , G. E. , "Weldin g Precipitation-Hardening Stainless Steels," The Weld-ing Journal, Vol. 36 , No. 1 , Jan. 1957 , pp. 9-27 .

[3] U. S. Paten t No. 3,117,861 , Jan . 14, 1964 , Linnert , G. E. , and Espy , R. H. ,"Stainless Stee l and Article. "

'f[4] Borland , J . C. , an d Younger , R . N. , "Som e Aspect s o f Crackin g in Welde dCr-Ni Austeniti c Steels," British Welding Journal, Vol . 7 , No . 1 , Jan. I960 ,pp. 22-59.

[5] Poole , L . K. , "Th e Incidenc e o f Crackin g in Weldin g Type 34 7 Steels, " TheWelding Journal, Vol . 32 , No . 8 , Researc h Supplement , Aug . 1953 , pp .403s-412s.

[6] Linnert , G. E. , "Weldin g Type 34 7 Stainles s Stee l Pipin g and Tubing, " Weld-ing Research Council Bulletin Series , No. 43 , Oct. 1958.

[7] Cordea , J . N. , Rammer , P . A. , an d Martin , D . C. , "Cause s o f Fissurin g inNickel-Base an d Stainles s Stee l Allo y Wel d Metals, " The Welding Journal,Vol. 43, No. 11 , Research Supplement, Nov. 1964 , pp. 481s^91s.

[8] Haddrill , D. M. , an d Baker , R. G., "Microcracking i n Austeniti c Weld Metal, "British Welding Journal, Vol . 12 , No. 8 , Aug. 1965 , pp. 411-419 .

[9] Campbell , H. C. , an d Thomas , R . D. , Jr. , "Th e Effec t o f Alloyin g Element s


DISCUSSION O N WELDABILIT Y 1 1 9

on th e Tensil e Propertie s o f 25-2 0 Weld Metal, " The Welding Journal, Vol .25, No . 11 , Research Supplement , Nov. 1946 , pp. 760s-768s . Als o discussio nVol. 26, No. 2 , 1947, pp. 119s-120s .

[10] U . S . Paten t No . 2,894,833 , Jul y 14 , 1959 , Linnert , G . E. , an d Larrimore ,R. M., "Stainless Steel for Weld. "

[11] U . S . Patent No . 2,871,118 , Jan . 27 , 1959 , Perkins, R. A. , and Binder , W. O. ,"Resistance to Hot-Cracking of Chromium-Nickel Steel Welds. "

[72] Hoerl , A. , an d Moore , T . J. , "Th e Weldin g o f Typ e 34 7 Stainles s Steels, "The Welding Journal, Vol . 36 , No . 9 , Researc h Supplement , Sept . 1957 ,pp. 442s-448s.

[13] Moore , N . E. , an d Griffiths , J . A. , "Microstructura l Cause s o f Heat-Affecte dZone Crackin g i n Heav y Sectio n 18-12-N b Austeniti c Stainles s Stee l WeldedJoints," Journal of the Iron and Steel Institute, Vol . 197 , Jan. 1961 , pp . 29 -39.

[14] Younger , R . N. , Haddrill , D . M. , an d Baker , R . G. , "Post-Wel d Hea t Treat -ment o f Hig h Temperatur e Austeniti c Steels, " Journal of the Iron and SteelInstitute, Vol . 201, Aug. 1963 , pp. 693-698.

[75] Haddrill , D . M. , and Baker , R . G. , "Effec t o f Cobal t o n th e Susceptibilit y ofWelded Austeniti c Steel s t o Heat-Affected Zone Crackin g During Heat Treat -ment," British Welding Journal, Vol. 11 , No. 9 , Sept. 1964 , pp . 453-461.

[76] Linnert , G . E. , "Th e Weldabilit y o f Alloy s fo r High-Temperatur e Service, "The Welding Journal, Vol . 27 , No . 8 , Research Supplement , Aug . 1948 , pp .385s-405s.

[77] Heuschkel , J., "Initial Characteristic s of Chromium-Nickel Steel Weld Metals, "The Welding Journal, Vol. 34, No. 10 , Oct. 1955 , pp. 484s-504s.

[75] Makin , S . M., et al , "Distribution o f Phosphorus and Sulfu r i n Fully AusteniticStainless Stee l Welds," British Welding Journal, Vol. 7 , No. 10 , Oct. 1960 , pp .595-599.

DISCUSSION

H. C. Campbell1 (written discussion)Do residua l metalli c element saffect weldabilit y when present a s atoms , o r mus t they becom e sulfides ,oxides, etc., in order to be noticed?

G. E. Linnert (author)In almos t all cases, residual elements combinewith other availabl e elements in the stee l to for m compounds , or alloyedphases. Naturally , th e propertie s o f the compoun d o r phas e the n deter -mine it s influenc e upo n weldability . Surprisin g effect s ma y b e observe ddepending upon such properties a s density, melting point, solid and liquidsolubility, an d dissociatio n temperature . A s indicate d a t th e conclusio nof the paper, ver y little quantitative data have been obtained on the com-position an d propertie s o f offendin g phase s an d compound s observed t otake part i n the mechanic s of welding problems. Consequently , many ofour evaluation s of residual elements as they affec t weldabilit y ar e purelyempirical. Hydroge n i s a n exceptiona l residua l elemen t inasmuc h a s i tapparently act s t o for m blowhole s an d porosit y withou t enterin g int ocombination with other elements in the steel .

1 Director o f research, Arcos Corp., Philadelphia, Pa.


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STP48440S.2307