AvestaPolarit Welding Stainless steels

Stainless steels – their propertiesand their suitability for welding

Interim reprint

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The revisions made to this brochure concern the cover, company name and logotype only,which now adhere to AvestaPolarit’s graphic profile. In all other respects, the contents are

identical with the information supplied in brochure 9473:2.

INDEXINTRODUCTION .................................................................................................................................. 1COMPOSITION AND MECHANICAL PROPERTIES .......................................................................... 1PHYSICAL PROPERTIES ................................................................................................................... 2CORROSION RESISTANCE PROPERTIES ....................................................................................... 2WELDABILITY ..................................................................................................................................... 3FILLER METALS FOR STAINLESS STEELS ..................................................................................... 4FILLER METAL FORMS ...................................................................................................................... 5WELD DEFECTS/PRACTICAL ADVICE ............................................................................................. 6POST-WELD TREATMENT ................................................................................................................. 7

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INTRODUCTIONWhen we speak of stainless steels in everyday speech,we mean steels alloyed with at least 12% chromium.As a result of reactions with the oxygen in the air, a pro-tective oxide film forms on this alloy and prevents fur-ther rapid oxidation. Modern-day stainless steels arealso usually alloyed with nickel and molybdenum, whichfurther enhances their corrosion resistance properties.

The purpose of this lecture is to:– shed light on the importance of microstructure for

the corrosion resistance properties, physical proper-ties and mechanical properties of the steels andprovide information on their weldability

– give advice on the selection of filler metals for differ-ent steel grades

– inform briefly on different filler metal forms– provide practical advice for the welding of stainless

steels.

COMPOSITION AND MECHANICALPROPERTIESThe mechanical properties, corrosion resistance andweldability of a steel are largely determined by itsmicrostructure. This is in turn determined chiefly by thechemical composition of the steel. Steels are dividedinto different groups in the following tables (page 2)based on the predominant microstructure.

Austenitic steelsThis type of stainless steel is dominant in the market.The group includes the very common AISI 304 and AISI316 steels, but also the higher-alloy AISI 310S andASTM N08904. Austenitic steels are characterizedby their high content of austenite-formers, especiallynickel. They are also alloyed with chromium, molyb-denum and sometimes with copper, titanium, niobiumand nitrogen. Alloying with nitrogen raises the yieldstrength of the steels.

Austenitic stainless steels have a very wide range ofapplications, e.g. in the chemical industry and the food

processing industry. The molybdenum-free steelsalso have very good high-temperature properties andare therefore used in furnaces and heat exchangers.Their good impact strength at low temperatures isoften exploited in apparatus such as vessels for cryo-genic liquids.

Austenitic steels cannot be hardened by heat treat-ment. They are normally supplied in the quench-annealed state, which means that they are soft and highlyformable. Their hardness and strength are increased bycold working. Certain steel grades are therefore sup-plied in the cold-stretched or hard-rolled condition.

Ferritic steelsThese steels are, in principle, ferritic at all tempera-tures. This is achieved by a low content of austenite-formers, mainly nickel, and a high content of ferrite-formers, mainly chromium.

The older type, such as AISI 430, was mainly used forhousehold utensils and other purposes where corro-sion conditions were not particularly demanding.Steels with a high chromium content, such as AISI 446with 27% chromium, are used at high temperatureswhere their resistance to sulphurous flue gases is anadvantage. However, the risk of 475°C embrittlementand precipitation of brittle sigma phase in high-chromi-um steels must always be taken into consideration.Today’s ferritic steels, such as S44400 with extremelylow carbon and nitrogen contents, find greatest usewhere there is a risk of stress corrosion cracking.

Ferritic steels have a slightly higher yield strength (Rp0.2) than austenitic steels, but they have less elon-gation at fracture. Another characteristic that distin-guishes ferritic steel from austenitic material is that fer-ritic steels have much lower strain hardening.

Ferritic-austenitic steelsThis group of steels is intermediate in terms of struc-ture and alloy content between ferritic and austeniticsteels. The main characteristic that differentiates fer-ritic-austenitic steels from austenitic and ferritic steelsis that they have a higher yield strength and tensilestrength. They are therefore often used in dynamicallystressed machine parts, e.g. suction rolls for papermachines. New areas of application are within the oil,gas and petrochemical sector, seawater-bearingsystems and the offshore industry.

Martensitic steelsMartensitic steels have the highest strength but alsothe lowest corrosion resistance of the stainless steels.Martensitic steels with high carbon contents may beregarded as tool steels.

Owing to their high strength in combination with somecorrosion resistance, martensitic steels are suitable forapplications which subject the material to both corro-sion and wear. An example is in hydro-electric turbines.

Martensitic-austenitic steelsA martensitic-austenitic structure is obtained by in-creasing the nickel content slightly compared with themartensitic steels. These steels also often have aslightly lower carbon content. The range of applica-tions is largely the same as for martensitic steels.

STAINLESS STEELS Their properties and theirsuitability for weldingby Björn Holmberg, M.Sc.

1 Stainless Steels

Stainless Steel 02-06-26 12.40 Sida 1

* Due to the high mechanical strength of ferritic-austenitic steels,machining and joint preparation may demand certain considera-tion. The use of planar machines or lathes has proven to be theeasiest method of joint preparation. If the milling method is to beused, feed and cutting speeds should be reduced by a minimum20% compared to conventional cutting data for austenitic stain-less steels.** quenched and tempered condition.

PHYSICAL PROPERTIESStainless steels differ from unalloyed materials withrespect to thermal expansion, thermal conductivity andelectrical conductivity, as illustrated below for severaldifferent steels.

Table 3

Steel type Type � � � Ex10-6°C W/m C n�m kN/mm2

Carbonsteels 1016 13 47 150 205Ferritic S44400 12.5 24 600 225Ferritic-austenitic 329 13.5 20 850 205Austenitic 304 19.5 15 700 200

� = coefficient of thermal expansion at 20-800°C� = thermal conductivity at 20°C � = electrical resistance at 20°C E = modulus of elasticity at 20°C

The differences have to be taken into consideration byboth designer and welder. The high thermal expansionand low thermal conductivity of the austenitic steelslead to higher shrinkage stresses in the weld thanwhen carbon and ferritic steels are used. Thin sectionsof austenitic steels may therefore be deformed whenan abnormally high heat input is used.

CORROSION RESISTANCE PROPERTIES

Austenitic steelsThese steels are mainly used in wet environments. Withincreasing chromium and molybdenum contents, thesteels become increasingly resistant to aggressive solu-tions. The higher nickel content reduces the risk ofstress corrosion cracking. Austenitic steels are more orless resistant to general corrosion, crevice corrosionand pitting, depending on the quantity of alloying ele-ments. Resistance to pitting and crevice corrosion isvery important if the steel is to be used in chloride-con-taining environments. Resistance to pitting and crevicecorrosion increases with increasing contents of chromi-um, molybdenum and nitrogen.

2Stainless Steels

Table 1

Microstructure Type C % Cr % Ni % Mo % Other(max.) elements %

Ferritic 430 0.10 16.0-18.0 max. 0.5 – –S44400 0.025 17.0-19.0 max. 0.5 2.0-2.5 Ti-stab.

Ferritic- 329 0.10 24.0-27.0 4.5- 6.0 1.3-1.8austenitic S31803 0.03 21.0-23.0 4.5- 6.5 2.5-3.5 N=0.10-0.20(Duplex steels)Austenitic 304 0.05 17.0-19.0 8.0-11.0 –

321 0.08 17.0-19.0 9.0-12.0 – Ti-stab.316 0.05 16.0-18.5 10.5-14.0 2.5-3.0304L 0.030 17.0-19.0 9.0-12.0 –316L 0.030 16.0-18.5 11.5-14.5 2.5-3.0310S 0.08 24.0-26.0 19.0-22.0317L 0.030 17.5-19.5 14.0-17.0 3.0-4.0

N08904 0.025 19.0-21.0 24.0-26.0 4.0-5.0 Cu 1.2-2.0Martensitic 420 0.4 12.0-14.0 max. 1.0 – –Martensitic-austenitic – 0.1 12.0-14.0 5.0- 6.0 – –

Table 2

Microstructure Type Rp 0.2 N/mm2 Rm N/mm2 A5 % Hardness HB(min.) (min.) (max.)

Ferritic 430 250 440-640 18 200S44400 340 440-640 25 210

Ferritic- 329 440 590-780 20 260austenitic* S31803 480 680-880 25 290Austenitic 304 210 490-690 45 200

321 210 490-690 40 210316 220 490-640 45 200304L 190 460-640 45 190316L 210 490-690 45 200310S – max. 780 – 220317L 220 490-640 45 200

N08904 220 500-750 35 220Martensitic 420 450 650-850 15 220Martensitic-austenitic – 620 830-1030 15 320

**

**

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The rich chloride content of seawater makes it a par-ticularly harsh environment which can attack stainlesssteel by causing pitting and crevice corrosion. However,two stainless steel grades designed to cope with thisenvironment have been developed by AvestaPolarit,254 SMO (ASTM S31254) and 654 SMO (ASTMS32654). 254 SMO has a long record of successfulinstallations for seawater handling within offshore, de-salination, and coastal located process industries.Some crevice corrosion has still been reported and formore severe situations, i.e. severe crevice geometriesand elevated temperatures, the natural selection shouldbe 654 SMO.Most molybdenum-free steels can be used at high tem-peratures in contact with hot gases. An adhesive oxidelayer then forms on the surface of the steel. It is impor-tant that the oxide is impervious so that further oxidationis prevented and the oxide film adheres tightly to thesteel. At very high temperatures, the oxide begins tocome loose (scaling temperature). This temperature in-creases with increasing chromium content. A commonhigh-temperature steel is 310S. Another steel that hasproved to be very good at high temperatures is AvestaPolarit 253 MA. Due to a balanced composition and theaddition of cerium, among other elements, the steel canbe used at temperatures of up to 1150-1200°C in air.

Ferritic steelsThe modern molybdenum-alloyed ferritic steels havelargely the same corrosion resistance as AISI 316 but aresuperior to most austenitic steels in terms of their resist-ance to stress corrosion cracking. A typical applicationexample for these steels is hot-water heaters.For chlorine-containing environments, where there is aparticular risk of pitting, e.g. in seawater, the high-alloysteel S44635 (25Cr 4Ni 4Mo) can be used.

Ferritic steels with high chromium contents have goodhigh-temperature properties. As mentioned previously, thesteels readily form brittle sigma phase within the tempera-ture range 550-950°C, but this is of minor importance aslong as the product, e.g. a furnace, operates at its servicetemperature. AISI 446 with 27 % chromium has a scalingtemperature in air of about 1070°C.

Ferritic-austenitic steels (duplex/super duplex)The most widely exploited property of this category ofsteels is their good resistance to stress corrosion crack-ing. They are quite superior to common austenitic steelsin this respect. Today’s modern steels with correctlybalanced compositions, for example AvestaPolarit 2205(UNS S31803), also possess good pitting properties andare not sensitive to intergranular corrosion after welding,as were the “old” ferritic-austenitic steels.The latest developed duplex stainless steels with veryhigh Cr, Mo and N-contents (super duplex = AvestaPolarit SAF 2507) have better corrosion resistance thanthe 2205-type and are in many cases comparable to the6-Mo steels (254 SMO).

Martensitic and austenitic steelsCompared with the steels discussed above, these steelshave much poorer corrosion resistance properties owingto lower contents of chromium and molybdenum.

WELDABILITY The Schaeffler-de-Long diagramAn aid in determining which structural constituents canoccur in a weld metal is the Schaeffler-de-Long dia-

gram. With knowledge of the properties of differentphases, it is possible to judge the extent to which theyaffect the service life of the weldment. The diagram canbe used for rough estimates of the weldability of differentsteel grades as well as when welding dissimilar steels toeach other. See page 4.

A new method of determining the ferrite content from thechemical composition of the weld metals has been devel-oped by Sievert et al. See page 4.

Austenitic steelsThe steels of type 304, 316, 304L and 316L have verygood weldability. The old problem of intergranular corro-sion after welding is very seldom encountered today. Thesteels suitable for wet corrosion either have carbon con-tents below 0.05% or are niobium or titanium stabilized.They are also very unsusceptible to hot cracking, mainlybecause they solidify with a high ferrite content. Thehigher-alloy steels such as 310S and N08904 solidify witha fully austenitic structure when welded. They shouldtherefore be welded using a controlled heat input. Steeland weld metal with high chromium and molybdenumcontents may undergo precipitation of brittle sigma phasein their microstructure if they are exposed to high tempera-tures for a certain length of time. The transformation fromferrite to sigma or directly from austenite to sigma pro-ceeds most rapidly within the temperature range 750-850°C. Welding with a high heat input leads to slowcooling, especially in light-gauge weldments. The weld’sholding time between 750-850°C then increases, andalong with it the risk of sigma phase formation.

The fully austenitic steel AvestaPolarit 254 SMO shouldbe welded like all other fully austenitic steels, in otherwords with some caution to reduce the risk of hot crack-ing. For further information on the welding of AvestaPolarit 254 SMO, see separate brochure.

Ferritic steelsThese steels are generally more difficult to weld thanaustenitic steels. This is the main reason they are notused to the same extent as austenitic steels. The oldertypes, such as AISI 430, had greatly reduced ductility inthe weld. This was mainly due to strong grain growth inthe heat-affected zone (HAZ), but also to precipitation ofmartensite in the HAZ. They were also susceptible tointergranular corrosion after welding. These steels aretherefore often welded with preheating and postweldannealing. Today’s ferritic steels of type S44400 andS44635 have considerably better weldability due to lowcarbon and nitrogen contents and stabilization with titani-um/niobium. However, there is always a risk of unfavour-able grain enlargement if they are not welded under con-trolled conditions using a low heat input. They do not nor-mally have to be annealed after welding.

These steels are welded with matching or austeniticsuperalloyed filler metal (such as Avesta P5).

Ferritic-austenitic steelsToday’s ferritic-austenitic steels have considerably bet-ter weldability than earlier grades. They can be weldedmore or less as common austenitic steels. Besidesbeing susceptible to intergranular corrosion, the oldsteels were also susceptible to ferrite grain growth in theHAZ and poor ferrite to austenite transformation,resulting in reduced ductility. Today’s steels, which havea higher nickel content and are alloyed with nitrogen,exhibit austenite transformation in the HAZ that is suffi-cient in most cases. However, extremely rapid cooling

3 Stainless Steels

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after welding, for example in a tack or in a strike mark,can lead to an unfavourably high ferrite content.Extremely high heat input can also lead to heavy ferritegrain growth in the HAZ.

FN = Ferrite number

U . IHeat input = � x1000 . v

� = constant dependent on welding method (0.7-1.0)

U = voltage (V)

I = current (A)

v = welding speed (mm/s)

When welding UNS S31803 (AvestaPolarit 2205) in aconventional way (0.6-2.0 kJ/mm) and using filler metalsat the same time, a satisfactory ferrite-austenite balancecan be obtained. For the new super duplex stainless steel(AvestaPolarit SAF 2507) a somewhat different heat inputis recommended (0.2-1.5 kJ/mm). The reason for loweringthe minimum value is that this steel has a much highernitrogen content than 2205. The nitrogen favours a fastreformation of austenite which is important when weldingwith a low heat input. The maximum level is lowered inorder to minimize the risk of secondary phases.

The steels are welded with ferritic-austenitic or austeni-tic filler metals. Welding without filler metal is not recom-mended without subsequent quench annealing.Nitrogen affects not only the microstructure, but also theweld pool penetration. Increased nitrogen content re-duces the penetration into the parent metal. To avoidporosity in TlG-welding it is recommended to producethin beads. To achieve the highest possible pitting cor-rosion resistance at the root side in ordinary 2205 weldmetals, the root gas should be Ar + N2 or Ar + N2 + H2.The use of H2 in the shielding gas is not recommendedwhen welding super duplex steels. When welding 2205with plasma, a shielding gas containing Ar + 5% H2 issometimes used in combination with filler metal and fol-lowed by quench annealing. For further information onthe welding of AvestaPolarit 2205 and AvestaPolaritSAF 2507, see separate brochures.

Martensitic and martensitic-austenitic steelsThe quantity of martensite and its hardness are the maincauses of the weldability problems encountered withthese steels. The fully martensitic steels are air-harden-ing. The steels are therefore very susceptible to hydro-gen embrittlement. By welding at an elevated tempera-ture (= the steel’s Ms temperature), the HAZ can be keptaustenitic and tough throughout the welding process.After cooling, the formed martensite must always betempered at about 650-850°C, preferably as a conclud-ing heat treatment. However, the weld must first havebeen allowed to cool to below about 150°C.

Martensitic-austenitic steels, such as 13Cr/6Ni and16Cr/5Ni/2Mo, can often be welded without preheatingand without postweld annealing. Steels of the 13Cr/4Nitype with a low austenite content must, however, bepreheated to a working temperature of about 100°C. Ifoptimal strength properties are desired, they can beheat treated at 600°C after welding. The steels are weld-ed with matching or austenitic filler metals.

FILLER METALS FOR STAINLESS STEELS Austenitic filler metalsA. Weld metals with up to 40% ferrite.Most common stainless steels are welded with fillermetals that produce weld metal with 2-12 FN* at roomtemperature. The reason for this is that the risk of hotcracking can be greatly reduced with a few per cent fer-rite in the metal, since ferrite has much better solubilityof impurities than austenite. These filler metals havevery good weldability. Heat treatment is generally notrequired.

High-alloy filler metals with chromium equivalents ofmore than about 20 can, if the weld metal is heat treatedat 550-950°C, give rise to embrittling sigma phase. Highmolybdenum contents in the filler metal, in combinationwith ferrite, can cause sigma phase during welding if ahigh heat input is used. Multipass welding has the sameeffect. Sigma phase reduces ductility and can promotehot cracking. Heat input should be limited for these fillermetals. Nitrogen-alloyed filler metals produce weldmetals that do not precipitate sigma phase as readily.

* FN = Ferrite Number, which is an international measure of theferrite content of the weld metal at room temperature.For ferrite contents of 0-6%, FN = % ferrite.For contents between 6 and 25%, FN is a unit or so higher.For contents over about 25 %, only the % concept is used.An extension of the FN scale to levels above 25 FN is beingdiscussed within IIW. The designation EFN (E = Extended) isthen used.

4Stainless Steels

10

15

20

25

30

Nickel equivalent =% Ni + 0.5 x % Mn + 30 x % C +30 x % N

Chromium equivalent =% Cr + % Mo + 1.5 x % Si +0.5 x % Nb

5 10 15 20 25 30

5

M + A

F+M

40% F

A=AUSTENITE

M=MARTENSITE

M + F

M + A + F

F=FERRITE

A + F

100% F

P10 P16

353 MAP12

254 SFER

904L

310P6

OFN

2 FN

6 FN

12 FN

SKR-NF

253 MA

SLR P5

SKNb2205

308L/MVR2304

347/MVNb

248 SV

2507/P100

P7

453 S

739 S

316L/SKR

10

12

14

16

18

18 20 22 24 26 28 30

Nickel equivalent =Ni + 35C + 20N + 0.25Cu

Chromium equivalent =Cr + Mo + 0.7NB

WRC-1992

10

12

14

16

1818 20 22 24 26 28 30

2507/P100

2205

2304

A

AF

F

FA

0

46

8

2 FN

10

12

14

16

18

20

22

24

26

28

3035

4045

50

60

70

80

90

100 FN

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Non stabilized filler metals, with carbon contents higherthan 0.05%, can give rise to chromium carbides in theweld metal, resulting in poorer wet corrosion proper-ties. Today’s non stabilized filler metals, however,generally have no more than 0.04% carbon unless theyare intended for high-temperature applications.

Superalloyed filler metals with high ferrite numbers (15-40%) are often used in mixed weld connectionsbetween low-alloy and stainless steel. Weldability isvery good. By using such filler metals, mixed weldmetals of the 18/8 type can be obtained. The use of fil-ler metals of the ordinary 18/8 type for welding low-alloy to stainless steel can, owing to dilution, result in abrittle martensitic-austenitic weld metal.

Other applications for superalloyed filler metals are inthe welding of ferritic and ferritic-austenitic steels. Themost highly alloyed, with 29Cr9Ni, are often usedwhere the weld is exposed to heavy wear or for weld-ing of difficult-to-weld steels, such as 14% Mn steel,tool steel and spring steel.

B. Fully austenitic weld metalsSometimes ferrite-free metals are required. The reasonis that there is usually a risk of selective corrosion ofthe ferrite. Fully austenitic weld metals are naturallymore susceptible to hot cracking than weld metals witha few per cent ferrite. In order to reduce the risk, theyare often alloyed with manganese and the level of traceelements is minimized. Large weld pools also increase the risk of hot cracks.

A large fully austenitic weld pool solidifies slowly with acoarse structure and a small effective grain boundaryarea. A small weld pool solidifies quickly, resulting in amore fine-grained structure. Since trace elements areoften precipitated at the grain boundaries, the precipi-tations are larger in a coarse structure, which increasesthe risk that the precipitations will weaken the grainboundaries to such an extent that microfissures form.Many microfissures can combine to form visible hotcracks.

Fully austenitic filler metals should therefore be weldedwith low heat input. Since the filler metal generally haslower trace element contents than the parent metal,the risk of hot cracking will be reduced if a large quan-tity of filler metal is fed down into the weld pool.Because the weld metal is ferrite-free, its impactstrength at low temperature is very good. This is impor-tant to manufacturers of, for example, welded tanksused to transport cryogenic liquids.

To avoid cracks in fully austenitic weld metals the fol-lowing rules should be observed:– when welding thick plates in possibly high restraint

situations, consideration should be given at the design stage to avoiding the creation of crevices

– do not weave the electrode (less than 2 x core wirediameter)

– weld width ~ 1.5-2.5weld depth

– never leave crater cracks before the next bead is welded.

Ferritic filler metalsFully ferritic filler metals have previously been regardedas very difficult to weld. They also required heat treat-ment of the weld metal after welding. Those that areused today have very low carbon and nitrogen con-tents and are often stabilized with titanium. Today’s fil-ler metals therefore produce weld metals that are less

sensitive to intergranular corrosion. Nor is anypostweld heat treatment necessary.

Another very important phenomenon that applies to allfully ferritic filler metals is that they tend to give riseto a coarse crystalline structure in the weld metal.Ductility decreases greatly with increasing grain size.These filler metals must therefore be welded using lowheat input.

Ferritic filler metals are mostly used for welding match-ing work metal.

Ferritic-austenitic filler metalsIn order to achieve good ferrite-austenite balance inthe weld metal, the filler metals are often superalloyedwith regard to nickel and/or nitrogen. Welding withoutfiller metal can therefore produce 80-100% ferrite insome steels, with a consequential reduction in the duc-tility and corrosion resistance of the weld metal.

The ferritic-austenitic filler metals are not susceptibleto hot cracking, since they have a high ferrite content.

Weldability as a whole is considerably better than forthe fully ferritic steels. There is some susceptibility tograin coarsening, but not very much. In order to keepgrain size down, heat input should be limited.

The first ferritic-austenitic filler metals (type 329)were sensitive to so-called 475°C embrittlement. Sub-sequent stress relieving was therefore unsuitable forthese filler metals. Today’s ferritic-austenitic fillermetals (type 22Cr9Ni3MoN and 25Cr10Ni4MoN) arerelatively unsusceptible to 475°C embrittlement. Thereason for this is that they have higher nickel contentsand are alloyed with nitrogen.

Ferritic-austenitic filler metals are mainly used for weld-ing matching base metals for use in environmentswhere there is a risk of stress corrosion cracking. Sometypes are also used for welding ferritic chromiumsteels or ferritic-martensitic steels. Ferritic-austeniticfiller metals have higher strength than the commonaustenitic filler metals. The higher ferrite contentresults in lower impact strength, however, especially atlow temperatures.

Martensitic-Martensitic/austenitic filler metalsWelding with matching filler metal is recommended ifoptimal mechanical properties are desired.

FILLER METAL FORMSCovered electrodes are available with many differenttypes of coverings. They can be roughly classified intobasic and rutile. There are a number of variants ofthese types, for example rutile-basic and rutile-acidic.The latter type is the most common. Rutile-acidicelectrodes are often easy to weld with alternating cur-rent. These coverings are therefore sometimes desig-nated AC/DC (= Alternating Current / Direct Current).There are also special position electrodes speciallysuited for position welding and for pipe welding. Theposition welding electrodes sometimes have the suffix-PW (= Position Welding) or -VDX (Vertical Down).There are special high-recovery electrodes for weldingthick plate in the horizontal position.

Different coverings give the electrodes special proper-ties. Basic electrodes are particularly suitable forrestrained weldments, where the risk of hot cracking ishigh. Basic electrodes give good penetration in theparent metal. This is advantageous if the root gap istoo narrow in some cases, due to shrinkage. This can

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then minimize the grinding work from the root side.One disadvantage of basic electrodes is that they havepoorer weldability and deslagging characteristics thanrutile and rutile-acidic electrodes. Basic electrodesproduce a convex profile in fillet joints. Rutile-acidicelectrodes produce a concave profile in fillet joints. Interms of corrosion, it is less important which type ofcovering is used, provided that there are no defects inthe weld metal.

Wire for MIG and plasma-arc welding is layer woundon a spool. TIG wire is normally supplied in one-metrelengths. Layer wound wire should lie flat if a few turns arecut off the spool and laid freely on the floor. The resultantloop should have a diameter of 400-1200 mm (cast). Ifthe loop rises more than about 25 mm from the floor(helix), the wire may flop about during welding, disruptingthe welding procedure. Too little cast will result in slug-gish wire feed.

The surface finish of the MIG wire has great importancefor the wire’s feeding properties. The finish should beneither too rough nor too smooth. Electrolytically pol-ished wire, which is very smooth, often runs heavily in thewire guide. Scratched wire also runs poorly. If the wire istoo soft, it may bend and get stuck at the feed rolls.

It is often advantageous to use filler metal in TIG andplasma-arc welding. The quantity of trace elements in theparent material is normally higher than in the filler metalwire. Using filler metal wire dilutes the trace elements,reducing the risk of hot cracking. The melting of the wirealso reduces the temperature of the molten metal, whichalso reduces the risk of hot cracking.

For MIG welding of common steels of type AISI 304, 316,304L and 316L, wire with an elevated Si content is alsoavailable. Such wire produces a more stable arc and themolten metal flows out better than when a wire with a lowSi content is used. Wire with a high Si content cannot beused in fully austenitic steels of type N08904 since therisk of hot cracking increases with increasing Si contentin fully austenitic steels.

Wire intended for submerged-arc welding (SAW)should not be too large in diameter, since there is some-times a risk of hot cracking. Wire with a maximum dia-meter of 3.2 mm is therefore normally used.

A flux is used in submerged-arc welding to protect themolten metal against oxidation, but many fluxes also addchromium to the molten metal. An elevated chromiumcontent and thereby elevated ferrite content counteractshot cracking.

Flux-cored wire electrodes for stainless steel weldingare becoming increasingly popular. Some of the wiresavailable today have very good welding characteristicsand produce adequately corrosion-resistant weld metals.Unfortunately, their impact strength is not as good as thatof MIG weld metal. Another advantage of cored-wireelectrodes is that they can be welded with a wide rangeof currents and perform well in different positions.Welding with high current in thick sections gives veryhigh deposition rates.

Another important point to consider is that if welding canbe carried out in the horizontal position, the learning timefor the welder is much shorter, compared to TIG orcovered electrode welding.

There are flux-cored wire electrodes that can be weldedwithout shielding gas. However, this type of wire doesnot possess as good welding properties as the wireswelded with shielding gas.

WELD DEFECTS/PRACTICAL ADVICESome of the most common types of defects are de-fined below.– Hot crackingThis is the most common type of weld defect, and iscaused by, among other things, excessively large weldpools, high impurity levels, high weldment restraint,and too thin welds. Weld-crater cracks are a type ofhot cracking and occur if the arc is extinguished tooquickly. Ferrite in the weld metal counteracts hotcracking. Hot cracks must be ground away.– Strike scarsStrike scars occur if the arc strays outside the jointbriefly while the electrode is being struck. This type ofdefect has high inherent stress, often in combinationwith a sharp crack. It can cause stress corrosion crack-ing and crevice corrosion. Strike scars in duplex steelscan give rise to 90-100% ferrite, resulting in embrittle-ment and reduced corrosion resistance. Strike scarsmust be ground away.– PorosityPorosity is caused by moisture on the work metal,moisture in the electrodes, moisture in the gas (TIG,MIG), contamination of the joint (oil, paint etc).– Slag inclusionsThese may result from the use of an electrode with toolarge a diameter in a narrow joint, or by careless weld-ing.– Incomplete penetrationThis results from using the wrong type of joint, orincorrect welding parameters.– Root defectIncomplete penetration can cause crevice corrosionand stress corrosion cracking.– Incomplete fusionThis is caused by an incorrect travel speed in MIGwelding, an excessively narrow joint, excessively lowwelding current, or the wrong electrode angle. – Hydrogen cracking in 13 Cr weld metalPreheat temperature too low, moisture content incovering too high.– Excessive local penetration (pipe welding) Gap too large, heat input too high. – Sink or concavity (pipe welding) Incorrect joint design. – Oxidized root side Poor shielding can cause corrosion attacks. Removethe oxide. – Spatter Grinding spatter can cause pitting and must thereforebe removed. Weld spatter can also cause pitting.– Grinding scratchesCoarse grinding of the welded joint must be followedby fine grinding and possibly polishing.

Practical advice– Use standardized joint types. A single-U butt joint is

recommended for pipe welding with TIG. The single-Ubutt joint is particularly advantageous in the over-head position. A tip is to machine single-V butt jointsbut grind up the single-V butt joint to a single-U buttjoint in the overhead position. Tack with a gap ofabout 1.0-2.5 mm.

– Never leave grinding burr.– Clean the joint before welding.– When tacking with TIG, use shielding gas and grind

off or thin out the tacks.– When welding pipe with TIG, use pure argon and

gas hoses of good quality.

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Stainless Steel 02-06-26 12.40 Sida 6

– Spread out the gas on the root side. Gas flow (2)-20 I/min.

– Purge the pipe with 7-10 x the enclosed volume.– Keep the shielding gas on until the weld has cooled

to below about 200°C. – Using a gas lens is recommended–it provides a

better gas shield. Good in deep joint types, forexample weldolets.

– MIG welding can be carried out with pure argon or agas mix of argon + 30% helium + 1% oxygen.

– Heat input 0.5-1.5 kJ/mm (normal).– If welding with covered electrodes, do not exceed

the maximum recommended current.– Extinguish the arc carefully at the end of the weld. – Do not exceed the recommended welding current. – Interpass temperature <100°C (150°C).– The joint must be completely free of low-melting

phases such as metallic copper, zinc or lead. Suchphases can otherwise cause metal penetrationduring welding.

– Submerged-arc welding and resistance welding canbe used, but require special welding parameters.Information can be obtained from our technicalcustomer service.

POST-WELD TREATMENTTo ensure satisfactory corrosion resistance for the weld-ed joint, slag, spatter and oxides must be removed.Welding oxide is rich in chromium, which means thatthe material underneath the oxide has been depletedof chromium, thereby reducing its resistance to pittingcorrosion. Post-weld treatment is therefore very impor-tant if the weld is to be exposed to acidic or neutral,chloride containing solutions such as seawater andpulp bleach plant liquids.

In these cases, pickling should be carried out to re-move this oxide and enable the formation of a new pro-tective and passivating oxide layer.

Note that failure to use sufficient shielding gas duringpipe welding may result in oxidation of the root side. Insuch cases the root side has to be cleaned by mechani-cal or chemical means.

AnnealingStress-relief annealing of a non-stabilized stainlesssteel at temperatures within the range 550-650°Cinvolves a risk of chromium carbide precipitation andmight reduce the resistance to wet corrosion.Stabilized material however can undergo stress-reliefannealing within the temperature range 550-650°Cwithout any problems.

The safest method is to carry out stress-relief anneal-ing at temperatures in excess of 1,000°C. The tem-perature levels can be provided by the manufacturer.

Brushing/grindingSpatter and strike scars should be ground off, whileoxide and other discoloration should be removed bybrushing.

Grinding should be carried out in several stages andfinished using an emery cloth with a 120 mesh or finer.

If steel brushing is preferred, stainless steel brushesmust be used.

Surfaces which have undergone a process of grindingshould preferably be pickled or washed with dilutenitric acid to ensure full protection against corrosion.

BlastingIf blasting is used, the blasting medium must be cleanand free of iron particles, iron oxides, zink, or othersimilar materials.

Pickling or washing with dilute nitric acid is recom-mended after blasting.

PicklingFrom a corrosion point of view, pickling is consideredto be the best method for cleaning a welded joint. Inaddition to the actual cleaning process which occursduring pickling, the welded area also undergoes a newprocess of passivation.

This method restores the welded joint’s resistance tocorrosion, partly by removing the chromium depletedlayer and partly by forming a new layer of the protec-tive oxide film.

Pickling can be performed at the location of the jointusing either pickling paste or pickling fluid. All residuecaused by the pickling process should be thoroughlyrinsed away using clean water and dealt with in ac-cordance with the recommendations provided by therelevant authorities.

AvestaPolarit Welding offers a comprehensive range ofpickling products for effective pickling and can alsoprovide advice on how the pickling process can bestbe carried out in different environments.

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Stainless Steel 02-06-26 12.40 Sida 7

Information given in this brochure may be subject to alteration without notice.Care has been taken to ensure that the contents of this publication are accurate but AvestaPolarit AB and its subsidiary companies do

not accept responsibility for errors or for information which is found to be misleading. Suggestions for or descriptions of the end use orapplication of products or methods of working are for information only and the company and its subsidiaries accept no liability in respect

thereof. Before using products supplied or manufactured by the company the customer should satisfy himself of their suitability.

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AvestaPolarit Welding ABP.O. Box 501SE-774 27 Avesta, SwedenTel.: +46 (0)226 815 00Fax: +46 (0)226 815 75www.avestapolarit.com/welding

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