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CHAPTER 17

Welding

NEARLY ALL WELDING of stainless steel

is done by end users or processors. Like thermalprocessing, it is complex in theory and practice.This chapter gives a basis for understanding theinfluence of alloy composition and metallurgyon the welding process, which must be re-spected as a process that combines melting, re-fining, and thermal processing. Knowledge of each aspect is required for the process to be de-signed and executed properly.

The welding and joining of stainless steels re-quires knowledge of both the technology of thewelding or joining process and the response of

the steel to the thermal and mechanical effectsof the process. The welding process must, of course, produce a sound joint, but it must alsoresult in the weld and its surrounding affectedmetal having correct strength, toughness, corro-sion resistance, etc. for the intended serviceconditions. This chapter does not attempt toteach welding. The main objective is to showhow standard welding technology is correctlyapplied to stainless steels.

The foremost special consideration of weld-

ing stainless steel as opposed to carbon steel isthat the chromium in stainless steel, which iswhat makes it stainless, must be protected fromoxidation, so that:

1. It stays in solution as a corrosion-resistingelement.

2. It does not form refractory oxides that woulddiminish weld soundness.

Welding Characteristics of Stainless Steels

Austenitic stainless steels are readilywelded by nearly all welding techniques. The

characteristics of austenitic stainless steels that

distinguish them from ordinary carbon steels inwelding are:

• Austenitic stainless steels have lower thermalconductivity and higher thermal expansionthan carbon steels or ferritic stainless steels,which can localize the heating, thus increas-ing the potential for residual stress andtherefore hot cracking.

• Stainless steels contain readily oxidizedchromium, which must be protected.

• Surface oxidation during welding depletes

chromium in all types of stainless steel fromthe underlying surface, resulting in reducedcorrosion resistance unless this layer is re-moved.

• The possible formation of chromium car-bides in the heat-affected zone (HAZ) cancause susceptibility to grain boundary corro-sion (sensitization).

• The possible precipitation of intermetallicphases in the HAZ can lower toughness andcorrosion resistance.

• There is increased microsegregation in thefusion zone with increasing alloy content.

• There are thermodynamically metastableconditions due to the low diffusion rates inthe face-centered cubic (fcc) matrix.

The influence of carbon has been well ad-dressed using low-carbon versions of all gradeswhenever welding involves significant time be-tween 600 and 900 °C (1110 and 1650 °F). Thisprevents rapid precipitation by reducing the su-persaturation of carbon. The older method of preventing sensitization is to stabilize the alloyswith titanium, as in type 321, or with niobium,as in 347. This is foolproof only if carbon levelsare low, less than 0.04%, since TiC can dissoci-ate at elevated temperatures and not be able to

Stainless Steels for Design Engineers

Michael F. McGuire, p 201-212

DOI: 10.1361/ssde2008p201

Copyright © 2008 ASM International®

All rights reserved.

www.asminternational.org



202 / Stainless Steels for Design Engineers

recombine successfully with titanium duringcooling, permitting a thin zone of sensitizationcalled knife-line attack . Fortunately, most 321and 347 are produced with carbon levels below0.03%. The higher carbon-stabilized alloys andthe high-carbon (>0.03%) unstabilized alloysmust be annealed after welding to redissolvechromium carbides if the cooling was suffi-ciently slow for the carbides to have formed.This is avoided only in thin-gauge (>1.5 mm,0.06 in.) material or when the HAZ is drasti-cally reduced, as in laser welding.

The high thermal expansion of austeniticstainless steel can cause high residual stressaround welds, which may require annealing toeliminate. Another serious threat posed by ther-

mal stresses is hot cracking. This can occur tomaterial that has just solidified when geometricconstraints to contraction imposed by the sur-rounding material imposed act on weak grainboundaries. This weakness occurs when thesteel solidifies in an austenitic mode. Whenaustenite freezes, it strongly rejects sulfur to theintergranular areas, where it forms weak films.This is solved by balancing the composition sothat alloys solidify first as ferrite, which doesnot reject the sulfur, forcing it to precipitate as

sulfide inclusions within the grains. This ap-proach is highly effective but cannot be used for

some highly alloyed grades with compositionsthat do not permit a ferritic solidification mode.In such alloys, sulfur and other contaminants,such as phosphorus, oxygen, zinc, and copper,must be excluded from the weld zone. Welds of less highly alloyed austenitics, generally thosewith less than 20% chromium, which are bal-anced to freeze in a ferritic mode, retain someferrite at room temperature, normally between3 and 10%. This is not harmful since the ferriteis richer in chromium and in molybdenum, if present.

The amount of ferrite expected can be meas-ured by magnetic devices and estimated fromthe Schaeffler diagram, a useful empirical map-ping of weld metal phase composition shown in

Fig. 1. This diagram has an arbitrary coolingrate resembling that of tungsten inert gas (TIG;described in a separate section of this chapter)welds. Faster or slower cooling will change therelative amounts of ferrite and austenite becauseof the need for diffusion to achieve the most sta-ble phase balance. Very rapid cooling, as withlaser welding, tends to make austenitic weldsless ferritic and has the opposite effect in duplexalloys.

The Schaeffler diagram has been improved

by the Welding Research Council’s adoption of the modification shown in Fig. 2, which super-

Fig. 1 The Schaeffler diagram. Source: Ref 1




on corrosion resistance of a highly alloyedaustenitic grade. Excess nitrogen in the shield-ing gas (e.g., more than 10%) can cause poros-ity in the weld, and greater than 5% is detrimen-tal to the life of the tungsten electrode.

The heat from welding can produce a surfaceoxide composed mainly of iron and chromium.The underlying surface can be significantly de-pleted of chromium because of the loss of

chromium to this scale and therefore signifi-cantly lower in corrosion resistance. Pits canstart in this thin layer and propagate into soundmetal beneath. For heat-tinted surfaces, thedarker the tint, the stronger will be the effect. Tofully restore corrosion resistance, the area mustbe ground to remove the oxide and any depletedbase metal. This should be followed by acidpickling, which completes the removal of theoxide and depleted zone.

Duplex stainless steels differ from austeniticstainless steels in their metallurgical response towelding mainly because their approximately50% ferrite causes greater thermal conductivityat lower temperatures, and ferrite has greaterdiffusion rates. These alloys solidify in a com-pletely ferritic mode, and since ferrite rejectslittle sulfur on solidification, hot shortness is nota problem. So, compared to austenitic stainlesssteels, duplex stainless steels have the followingdistinguishing factors:

• The ferritic solidification mode of duplex

stainless steels provides very good hotcracking resistance. The rapid cooling of welds produces welds and HAZ with moreferrite than the parent metal by quenching inthe high-temperature ferrite.

• Duplex alloys are more sensitive to prob-lems in the HAZ because their generallyhigh chromium and molybdenum contentplus their ferritic content make the precipita-tion of embrittling intermetallic phases morerapid than in austenitics, so minimizing thetotal time at high temperature is the overrid-ing concern.

• While carbide sensitization is not an issuewith the duplex alloys, the formation of in-termetallic phases can cause loss of corro-sion resistance.

• Duplex, like all stainless types, must be pro-tected from oxidation by shielding gas, andsince nitrogen is a crucial alloying element,especially in duplex alloys, it must be a

component of the gas mixture.• Cleaning before and after welding is equally

important in duplex as in austenitics.

Modern duplex alloys derive their impressivestrength, toughness, and corrosion resistancefrom their nearly equal percentage of ferrite andaustenite. The nitrogen content of the austenitebrings its corrosion resistance up to that of theferrite phase, which is richer in chromium andmolybdenum. Nitrogen additions partition tothe austenite and thus both strengthens it and in-

creases its corrosion resistance to close to thatof the ferrite. The early duplex alloys had a ten-dency to form excessive ferrite when weldedand formed embrittling intermetallic phasesrather rapidly. The additions of larger amountsof nitrogen stabilized the austenite to highertemperatures, so welds did not become so fer-ritic. The nitrogen also decreased the speed atwhich intermetallic phases form, enlarging thetime window for welding without their precipi-tation. And, by promoting greater austenite for-

mation at high temperature, the addition of high(>0.12%) nitrogen actually reduces the ten-dency for chromium nitride precipitation. De-spite these advances, the key precaution inwelding duplex alloys is to prevent the forma-tion of embrittling phases while preserving asclose to a 50/50 austenite/ferrite structure aspossible. Minimizing time at red heat tempera-tures (500 to 900 °C, 930 to 1650 °F) is the ob-

jective. But, sufficient time must be spent aboveabout 1000 °C (1830 °F) to promote the forma-tion of sufficient austenite. If the weld cannot be

annealed, increased nickel filler metal (e.g.,2209 with 2205 base metal) should be used.Thus, joint preparation must be done correctlyand not left to the welder to correct using time-consuming remedial procedures.

Fig. 3 Effect of weld shielding gas composition on crevicecorrosion resistance of autogenous welds in AL-6XN

alloy tested per American Society for Testing and Materials

(ASTM) G-48B at 35 °C (95 °F)



Chapter 17: Welding / 205

Duplex stainless steels, because of their mod-erate thermal expansion and higher thermalconductivity, can tolerate relatively high heatinputs since these factors determine the stressintensity that will be generated by thermal gra-dients. However, excessively low heat inputscan result in fusion zones that are predomi-nantly ferritic, with a resultant loss of toughnessand corrosion resistance. At the other extreme,heat inputs that are too high lead to the forma-tion of embrittling intermetallic phases. Thisissue concerns the HAZ, which must dwell inσ-forming temperatures for some period of time. The key is to limit the time at those tem-peratures by not permitting interpass tempera-tures to exceed 150 °C (300 °F) because work-

piece temperature has the greatest influence ontime at σ-forming temperatures. It is prudent toimpose this limitation when qualifying the weldprocedure and then monitoring the productionwelding interpass temperature electronically toensure qualifying procedures are not more le-nient than are those of production.

Postweld stress relief is not needed for duplexweldments and indeed could be harmful be-cause of the danger of embrittlement. Full an-nealing can be done and can restore the original

phase balance and composition that gives theoptimal toughness and corrosion resistancefound in wrought material.

Ferritic stainless steels can be split into twogroups for purposes of welding: the older semi-ferritic group and the more prevalent stabilizedferritic group. The first group, in whichchromium is between 16 and 18% with carbonup to 0.08%, is exemplified by the alloy 430.These alloys form appreciable amounts of austenite when heated above 800 °C (1470 °F).Unless they are cooled extremely slowly, more

slowly than can be done in welds, the austenitetransforms to martensite, which is very brittle.

The stabilized grades commonly use titaniumor niobium to combine with the carbon and ni-trogen, which otherwise would cause the forma-tion of the high-temperature austenite, render-ing the alloys ferritic at all temperatures.

The salient metallurgical characteristics forwelding of the two groups are:

• Both groups offer good thermal conductivityand low thermal expansion.

• Both groups require protection from oxida-tion by shielding gases. The stabilized groupshould not be exposed to nitrogen.

• The semiferritic group will form martensite,which requires annealing to eliminate.

• The stabilized group can lose toughness viaexcessive grain growth.

• The grades more highly alloyed withchromium and molybdenum can form α' andσ, leading to embrittlement.

The semiferritic alloys such as 430, 434, and436 are seldom welded and often called un-weldable. The reason is that the welds are in-variably partially martensitic and thus normallybrittle. Only very specially controlled composi-tions of 430 can be welded successfully, andthese are not generally available commercially.While the technical remedy for this is simplyannealing, it is seldom economically viable. It israre to see any welding more extensive than

spot welding of unexposed surfaces with thesealloys. If for some reason they must be used andwelded, then the techniques for weldingmartensitic stainless steels should be employed.

The stabilized ferritic stainless steels arecommonly welded. The levels of stabilizing ele-ments required to prevent austenite formationand sensitization are well known and are re-flected in the alloys’ chemistry specifications.For 409, the required titanium level is Ti > 0.08+ 8(C + N), while the requirement for the higherchromium 439 is 0.20 + 4(C + N). These are

empirical relationships that take into accountthat some titanium oxidizes before it can stabi-lize carbon and nitrogen. Niobium can replacesome titanium. This is discussed in detail inChapter 8 on ferritic stainless steels. Because of the low toughness these alloys have in largecross sections, these alloys are only rarely seenwith minimum section size of more than 3 mm(0.11 in.) and normally have sections less than 2mm (0.08 in.). Thus, successful welding is sim-plified to making a sound, well-shielded weld

without producing excessive grain growth in theHAZ. In practice, this can be achieved by limit-ing heat input to less than 6 kJ/cm. An empiricalrelationship between grain diameter D and heatinput E (kJ/cm) has been reported (Ref 2). Inthe fusion zone, the relationship is:

D = 206 × E – 585.6 (Eq 1)

In the HAZ, it is:

D = 29.6 × E – 50.6 for up to 6.6 kJ/cm (Eq 2)

and

D = 75 × E – 350 above 6.6 kJ/cm (Eq 3)

The light gauges ensure sufficiently shorttimes at high temperature that precipitation of




intermetallic phases should not be a concern,even though they can form, especially in super-ferritic alloys.

The impact properties of ferritic stainlesssteels are always a concern because their transi-tion temperature can become elevated to ambi-ent levels. It has been determined that there ex-ists an optimum level of titanium around 0.10%,which ensures this minimum transition temper-ature (Ref 3). Because it is difficult to have lowenough carbon plus nitrogen to stabilize at thistitanium level, dual stabilization with titaniumand niobium as well as not having excessiveheat input are the best way to ensure weldtoughness.

Especially in the superferritics, maintaining

the benefits of having the fairly precise balanceof carbon plus nitrogen to the stabilizing ele-ments titanium and niobium requires that nei-ther carbon nor nitrogen come into contact withthe weld pool. Likewise, oxygen must be rigor-ously avoided because it will quickly depletethe essential titanium, which is even more read-ily oxidized than chromium. Extraordinary sur-face cleaning at and near the weld will pay divi-dends in final quality.

Martensitic stainless steels vary little in alloy

content, ranging from 11 to 18% chromium withsmall amounts of nickel and molybdenum.Their carbon content ranges from 0.10 to over0.30%. Thus, the major challenge they presentis avoiding the potential cracking, which ismost likely to occur in the HAZ from stressescaused by the austenite-to-martensite transfor-mation on cooling. Since this transformationcannot be avoided, the desired approach is tostart with a well-tempered or annealed materialand then preheat and maintain high interpasstemperatures. For low carbon levels, below

0.10%, preheat can be omitted, but between0.10 and 0.20% carbon, preheating to 250 °C(480 °F) is advised and for higher carbon levels,300 °C (570 °F). The problem becomes moresevere with increasing carbon level because thetransformation takes place at lower tempera-tures in more brittle material. Even with pre-heating, distortion may be encountered. For allnormal uses of martensitic stainless steels, afinal heat treatment is required to achieve thequenched and tempered properties for which

these alloys are designed.Aside from the cracking consideration,martensitic welding considerations are similarto, but less stringent than, those of low-alloystabilized ferritic stainless steels with regard to

cleanliness and shielding. If mechanical re-quirements permit, the use of austenitic (309L)weld filler metal should be considered. The soft

joint may deform to accommodate thermalstrains and thus minimize weld cracking.

Precipitation-Hardening Stainless Steels.Last, precipitation-hardening (PH) stainlesssteels, while very complex metallurgically, arestraightforward from a welding perspective.Obviously, any heat treatment to achieve theproperties of which these alloys are capablemust be a final step. The considerations in weld-ing them are:

• Shielding must be sufficient to prevent lossof oxidizable alloying elements such as tita-

nium, aluminum, and, of course, chromium.• Filler metal must match the base metal if like properties are required.

• Postweld heat treatment solution annealingmust be adequate to homogenize weld solid-ification segregation.

• Austenitic PH grades are fully austenitic andsubject to hot short cracking.

• The high aluminum or titanium contents of many PH alloys cause their welds to be“slaggy,” and these slaggy welds have are ir-regular with objectionable recesses,

crevices, or prominences.

These alloys are easily welded and not proneto cracking or developing embrittling phases.But, because these alloys are designed for ex-treme mechanical performance, it is essential topreserve their correct chemistry by shieldingwith a fully inert gas mixture. If mechanicalproperties equal to that of the base metal are notrequired in the weld, then austenitic filler, suchas 309L, can be used.

Table 1 summarizes the major metallurgically

important parameters for the various types of stainless alloys. It is prudent to consult with themanufacturer’s data sheets for specific recom-mendations on alloys that they produce as theyare often privy to test data and user experiencethat cannot be found elsewhere in the literature.

Material Selection and Performance

Stainless alloys that are prone to precipitation

of intermetallic phases require special preweld-ing consideration. Such alloys include duplex,superferritic, and superaustenitic alloys. Anyamount of time for which these alloys havebeen exposed to temperatures at which inter-




metallic phases form without full subsequenthomogenization anneal is time that the weldercannot use to complete a satisfactory weld be-fore precipitation occurs. Thus, accurate knowl-edge of material history is vital. Likewise, vari-ations within specification of nitrogen contentinfluence the time it takes intermetallic phasesto form. Once a welding procedure is qualifiedfor an alloy with given nitrogen content, use of lower nitrogen alloys would not be prudent en-gineering practice.

Austenitic stainless steels that are intendedfor autogenous welding are often specified withelevated sulfur levels, on the order of 0.005 to0.015%. This is done to improve weld penetra-tion through the so-called Marangoni effect.

This effect exploits the temperature-dependentsurface concentration of sulfur in the weld pool,which causes a decreased surface tension to-ward the hotter center of the pool, causing themolten pool to flow toward the center on thesurface and then flow downward, shooting thehottest metal to the bottom of the weld pool, asshown in Fig. 4. This speeds welding and mini-mizes weld and HAZ width, which is a goodthing. The effect on corrosion resistance is lessdesirable since the abundant MnS inclusions

that result from the higher sulfur levels decreasepitting resistance. This decrease in corrosion re-sistance can only be eliminated by a long an-neal. Unfortunately, the pipe purchaser cannotknow if the pipe has had a sufficient anneal.

Table 1 Welding parameters for various stainless steels

Alloy group Filler

Heat input

kJ/cm(max)

Shielding

gas Preheat

Interpass

max

Postweld

heat treat

Austenitic . . . 20–40 Ar+2% O2,

Ar/3% CO2 /2% H

2

He+7.5%Ar+2.5 CO2

150 oC 150 oC None or full anneal

301, 302, 304 308, 308L Same Same . . . . . . . . .

304L 308L Same Same . . . . . . . . .

309 309, 310 Same Same . . . . . . . . .

310 310 Same Same . . . . . . . . .

316L, 316Ti 316L, 317L Same Same . . . . . . . . .

321, 347 347, 308L Same Same . . . . . . . . .

Superaustenitic 22, 675, 276 16 Argon/helium or

argon + 3–5%

N2:no O

2

50 oC 100 oC None or full anneal

PH grades Same as base alloy 20–40 Argon/helium no . . . Full solution anneal

Martensitic

410 410, 308, 309L 20–40 Ar+2% O2,

He+7.5%Ar+2.5 CO2

250oC 250 oC min Slow cool

420 420, 308, 309L, 310 20–40 Ar+2% O2,

He+7.5%Ar+2.5 CO2

250 oC 250 oC min Anneal

440 440, 308, 309L, 310 20–40 Ar+2% O2,

He+7.5%Ar+2.5 CO2

250 oC . . . . . .

Supermartensiic Same as base metal 20–40 Argon/helium no . . . Full solution anneal

Ferritic

430 430, 309L 20–40 Ar+2% O2,

He+7.5%Ar+2.5 CO2

no . . . Subcritical anneal

434 309 Mo L 20–40 Ar+2% O2,

He+7.5%Ar+2.5 CO2

no . . . Subcritical anneal

409 410L, 308, 309L 6.0 Ar+2% O2,

He+7.5%Ar+2.5 CO2

no n.a. none

439 439L, 309L, 316L 6.0 Ar+2% O2,

He+7.5%Ar+2.5 CO2

no n.a. none

Superferritic 29-4C 6.0 Argon/helium no n.a. None or full anneal

2003, 2101, 2304,

19-D

2209 5–25 Argon + 3% N2

no 150 oC None or full anneal

2205 2209 5–25 Argon + 3% N2


25 Cr duplex 25Cr-10Ni-4Mo-N 5–25 Argon + 3% N2


2507

superduplex

25Cr-10Ni-4Mo-N 5-25 Argon + 3% N2


PH, precipitation hardenable




In-line induction annealing is insufficient forthis purpose. Furnace anneals of about an hourare required. For alloys like 304L and 316L, theuser should always require material chemistrycertifications and assume that any sulfur levels

above 0.003% are going to result in decreasedpitting resistance of 1 to 5 PREN (pitting resist-ance equivalent number), which means up to 10°C (18 °F) decrease in critical pitting tempera-ture, roughly the difference between 304 and316 in performance. This also applies to girthwelds done by the pipe user.

Welds are essentially a casting in the midst of wrought material. In addition to inclusions de-creasing weld corrosion resistance as men-tioned, solidification segregation can also causemicroscopic regions to be poorer in corrosion-

resisting alloying elements chromium, molyb-denum, and nitrogen. This effect is minimal forlow-alloy material, but for highly alloyedaustenitic grades, it is a major effect, as shownin Fig. 5. Eliminating this effect requires a thor-ough homogenization anneal.

The use of filler metal with higher corrosionresistance does not totally offset the influence of welding on corrosion resistance because someof the base metal melts and is not altered incomposition by the filler metal. This is called

the unmixed zone. It is essentially a zone withproperties equal to that which would occur in anautogenous weld, that is, the corrosion resist-ance is lower depending on total alloy level andsulfur content.

Welding Processes

All stainless steels should be very clean priorto welding. The chemistries of both base metals

and filler metals are carefully formulated to pro-duce the mechanical and corrosion propertiesthat these alloys have been designed to produce.Virtually any contaminant can either interferewith the welding procedure or detrimentally

alter the composition of the welded joint, whichin turn can alter corrosion and mechanical prop-erties and compromise the entire structure.Moisture, paint, dirt or grease, oil, and oxidesall can negate good material, good weldingtechnique, and good procedural qualification.

Cutting fluids, especially sulfurized oils, are es-pecially detrimental and should be removedcompletely prior to welding. Preheating is neverstrictly forbidden since it is required to elimi-nate moisture.

Joint design does not differ in principle fromthat of other steel weldments. There is, howeveran increased need for dimensional uniformityfor the alloys susceptible to intermetallic precip-itation since minimizing time at temperature is apriority, and variations in joint geometry impedethe swift completion of the weld. This is also

true for alloys that are susceptible to excessivegrain growth, such as the stabilized ferritics, orto sensitization.

Figure 6 shows some joint designs appropri-ate to stainless steels, including the more sensi-tive alloys. These, like all joint designs, aim toensure full penetration without burn through.

Gas tungsten arc welding (GTAW)/tung-sten inert gas (TIG) is commonly used for theautomated production of stainless steel pipe andtube, as well as manual short runs. It is versatile

and generally used when thicknesses are lessthan 6 mm (0.2 in.). It can produce very high-quality welds. A constant-current power supply ispreferred. It is best performed with the DCSP(di-rect current straight polarity) electrode negative

Fig. 4 Metal flow directions in a weld pool with (left) andwithout (right) sulfur. Source: Adapted from Ref 4

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

−5

194

185

176

167

158

149

140131

122

113

104

95

86

77

68

59

50

41

32

23

C r i t i c a l p i t t i n g t e m p e r a t u r e i n 6 %

F e C I 3 ,

° F

C r i t i c a l p i t t i n g t e m p e r a t u r e i n 6 %

F e C I 3 ,

° C

Molybdenum, wt%

Unwelded

Welded

1 2 3 4 5 6 7

Fig. 5 The influence of molybdenum on critical pitting tem-perature. Source: Adapted from Ref 5




technique. It is helpful to incorporate a high-frequency circuit to aid in establishing the arc.Thoriated electrodes containing 1.7 to 2.2% tho-ria are recommended because they have betteremissive properties and provide better arc stabil-ity at higher currents. If consumable electrodesare used, the shielding gas precludes the need forcoatings. The weld metal alloys are not necessar-ily the same as the parent alloys but are chosenbased on their ability as weld metals to providethe most acceptable corrosion and mechanical

properties. This sometimes means usingaustenitic filler with a ferritic base or highernickel content in an austenitic or duplex base tocompensate for the solidification rate or inher-ently lower corrosion resistance of the weld.

The shielding gas must replicate the con-trolled gas mixtures used to refine stainless steeland establish the original composition. Theweld pool exposes a great deal of surface area tothe atmosphere in a very turbulent manner. Gasflows, usually 12 to 18 L/min, must be adequateto prevent air infiltration by aspiration or turbu-lence before arc contact, ideally until tempera-tures cool to below oxidation temperatures.

For manual GTAW using a filler wire, thewire should be fed continuously into the weld

pool. Intermittent wire addition can lead to cre-ation of zones of essentially autogenous weld,negating many of the benefits of filler metal ad-dition. Moving the tip of the wire in and out of the protection of the gas shield is especially

t

d

t

d

d

a

d

a

d

a

k

d

a

k

d

a

r = 6-8mm

r = 6-8mm

Fig. 6 Joint designs. Courtesy Ugine S.A.

Thickness Gap d, Root K,

Groove Process th, mm (in.) mm (in.) mm (in.) Bevel α(°)

GTAW 3–5 1–3 . . . . . .

GMAW 3–5 1–3 . . . . . .

SMAW 3–4 1–3 . . . . . .

SMAW 4–15 1–3 1–2 55–65

GTAW 3–8 1–3 1–2 60–70

GMAW 5–12 1–3 1–2 60–70

SAW 9–12 0 5 60

SMAW >10 1.5–3 1–3 55–65

GMAW >10 1.5–3 1–3 60–70

SAW >10 0 3–5 80

SMAW >25 1–3 1–3 10–15

GMAW >25 1–3 1–3 10–15

SAW >25 0 3–5 10–15

GTAW >3 0–2 . . . . . .

GMAW >3 0–2 . . . . . .

SMAW >3 0–2 . . . . . .

SMAW 3–15 2–3 1–2 60–70

GTAW 25–8 2–3 1–2 60–70

GMAW 3–12 2–3 1–2 60–70

SAW 4–12 2–3 1–2 70–80

SMAW 12–50 1–2 2–3 10–15

GTAW >8 1–2 1–2 10–15

GMAW >12 1–2 2–3 10–15

SAW >10 1–2 1–2 10–15

GMAW, gas metal arc welding; GTAW, gas tungsten arc welding; SAW, submerged arc welding; SMAW, shielded metal arc welding




bad. The hot tip can carry oxides and nitridesinto the weld, defeating the action of the shieldgas and impairing weld quality.

Gas metal arc welding (GMAW) is arcwelding in which a consumable electrode pro-vides larger amounts of filler weld metal thanpractical in GTAW. There are three GMAWtechniques:

• Pulsed arc transfer• Spray transfer• Short-circuiting transfer

Pulsed arc transfer employs a power sourcethat is switched rapidly to provide transfer of weld metal droplets at regular intervals. Spraytransfer uses a high current to form a stream of

fine drops from the end of the electrode. This isdone with high power, resulting in a large fluidweld pool, and therefore limits the technique tohorizontal orientations and thick material.Short-circuiting transfer uses arc contact withthe workpiece at low power to melt the elec-trode, after which the short circuit is broken,and material transfer ceases. The technique cre-ates a minimal weld pool and is viable in manyorientations. It is a low-heat process suitable forthin material but may cause lack of penetration

defects if used for thick-section welding.For all GMAW processes, excessive protru-sion of the wire should be avoided; otherwise,the full benefit of the inert gas shielding may belost.

Submerged arc welding (SAW) employs aconsumable electrode immersed in a conductiveflux that acts as a protective shield from the at-mosphere. The arc is struck through the flux,and gravity deposits the molten metal to theworkpiece. The large weld pool has high heatinput and can deposit large amounts of metal

relatively quickly. Thus, SAW may be prefer-able to multipass techniques for alloys such asduplex for which time at temperature is limited.It is restricted to horizontal orientations and re-quires postweld slag (flux) removal.

Shielded metal arc welding (SMAW) isdone manually with short lengths (“sticks”) of coated electrodes. This method has great versa-tility with some trade-off in cost and quality.This last aspect is arguable, but the lack of shielding gas may introduce oxygen to the weld

metal, which can be detrimental to toughness.Flux cored wire (FCW) welding is a methodthat is able to accommodate a large range of thickness and orientations while providing highdeposition rates. The equipment is the same as

for GMAW, but the consumable electrode, theFCW filler metal, has a flux core that supple-ments the shielding gas. Because of the flux, theshielding requirements are reduced; gases canbe argon/25% carbon dioxide for horizontalwelding with current and voltages from 150 to200 amp and 22 to 38 V, respectively. Verticalwelds can use 100% carbon dioxide with am-perage of 60 to 110 amp and voltage of 20 to 24V. Flow rates of gas are 20 to 25 L/min. It ispossible to get high-carbon welds, which maynot resist corrosion as well as desired, so as al-ways, weld qualification, including corrosionevaluation, is critical.

Oxyfuel gas welding (OFW), “torch” weld-ing, uses oxygen to accelerate fuel (typically

acetylene) combustion to produce temperaturesthat can melt steels. By controlling the fuel-airmixture, the flame can be made nonoxidizingfor low-alloy steels. However, these “neutral”flames can simultaneously oxidize and carbur-ize stainless steels. Thus, the OFW process isnot suitable for use with stainless steels.

Laser welding has become a major produc-tion method when it can be automated, as forpipe and tube or high-production manufactureditems, such as air-bag canisters. Metallurgically,

it resembles resistance welding in that bothhave minimal HAZ and very high quenchingrates, both of which can have a pronounced ef-fect on some types of stainless steel. The effectis to undercool the molten metal and suppressthe transformation that would normally occur.So, an austenitic alloy that normally solidifies ina ferritic mode before transforming to austenitefreezes directly as austenite. The freezing is sorapid that the normal hot shortness of austeniticsolidification is avoided, so quality is not com-promised. In fact, laser welds quench the mate-

rial so rapidly that corrosion resistance is en-hanced since inclusions cannot nucleate andgrow. Duplex alloys, on the other hand, freezein their high-temperature ferrite structure be-cause the fast quench prevents the nucleationand growth of austenite. Unless this ferrite isheated to permit austenite to form, lower-tough-ness welds will result. Ferritic, martensitic, andPH alloys are not harmed by the rapid quench.

Resistance welding is readily done on mosttypes of stainless steel. Allowance must be made

for the lower thermal and electrical conductivityof stainless steels compared to other commonmaterials. Most resistance welds, including bothseam and spot welds, have deep, tight crevicesadjacent to the welds. The possibility of crevice




corrosion in these regions should be consideredwhen contemplating the use of spot welds instainless materials. The possibility of entrap-ment of foreign material and the difficulty of re-moving it from such crevices should also beconsidered, especially in equipment for foodhandling, pharmaceutical production, etc.

High-frequency induction welding of stainless steel is more difficult than for low-alloy steel because of the refractory nature of chromium oxide, which has a higher meltingtemperature than does the stainless base metal.This is opposite from the situation in low-alloysteels, for which the iron oxide melts at a lowertemperature than does the iron base metal. Thepresence of this refractory oxide on the surfaces

to be joined makes it more difficult to obtain adefect-free weld.

Thermal cutting of stainless steels is rou-tinely practiced, but the processes and parame-ters used are determined by the refractory na-ture of the chromium oxides that form onstainless steels. The high temperatures attain-able with lasers or plasma arc torches providegood cutting action, and these processes are fre-quently used. To expand the range of thick-nesses that can be cut or to increase cutting

speed, supplemental oxygen or nitrogen blast jets may be used. Stainless steels may also becut using oxyfuel equipment if supplementaliron powder is used. Combustion of the iron in-creases the temperature, while the iron oxidehelps flux the refractory chromium oxide. Ther-mally cut edges of stainless steel usually requiresubsequent cleaning, typically by grinding ormilling. Chemical cleaning of all surfaces of cutpieces to remove heat tint, fume deposits, andother contaminants is advisable.

Soldering and brazing are possible with all

stainless steels. Soldering is done below 450 °C(840 °F), while brazing is done above 450 °C(840 °F). Solders are generally alloys of tin andbismuth, lead, silver, or antimony or combina-tions of several of these. Brazes are normally ei-ther silver based or nickel based. Thechromium-rich oxide coating must be removedby a suitable flux for bonding to occur. Fluxesare typically acid type with chlorides. Thus,after the soldering or brazing, the flux must bethoroughly removed to prevent subsequent pit-

ting corrosion. Brazing temperatures must bechosen to avoid ranges at which unfavorablephases form. The best range can be determinedfrom examining temperature ranges to beavoided in the thermal processing chapter

(Chapter 13) of this book. Brazes and soldersrarely match the corrosion resistance of stain-less steels, and careful attention should be givento the potential for galvanic and other forms of corrosion when considering the use of solderedor brazed joints with stainless steels.

Welding Practices

Safety must always be considered whenwelding. In addition to the normal hazards(which are not discussed here) associated withwelding, welding of stainless steels presents aspecial hazard: hexavalent chromium. The fume

created by welding stainless steel contains sig-nificant concentrations of chromium trioxideand other forms of hexavalent (Cr+6) chromium.Hexavalent chromium is a carcinogen and regu-lated by the Occupational Safety and HealthAdministration (OSHA). Exposure to and in-halation of stainless steel welding fumes mustbe avoided. The product exposure limit forhexavalent chromium is 5 μg/m3 as of Decem-ber 31, 2008. Refer to OSHA for further updateson this limit. Use of fume extraction equipment

is generally the preferred method of minimizinghexavalent chromium exposures. Positioningand operation of the fume extraction devicemust be done precisely to ensure effective fumeremoval while avoiding excess turbulence,which can cause loss of effective inert gasshielding of the weld pool. Thermal cutting of stainless steels also generates hexavalentchromium, and similar procedures are requiredto minimize exposure during such operations.

Nondestructive Evaluation (NDE) is usedalmost universally to ensure weld quality. All of

the standard NDE techniques used with othermaterials are applicable to stainless steel weld-ments. Allowance must be made for the differ-ing physical properties of stainless steels, andappropriate reference defect standards must beprovided. However, one technique—magneticparticle inspection—is problematic. The pres-ence of bands of persistent austenite in marten-sitic or PH stainless steels can lead to spuriousdefect indications. For this reason, magneticparticle examination of stainless steel welds is

best avoided.Recent developments in stainless steel havebeen made with weldability as a major consid-eration. Highly alloyed, low-carbon martensiticalloys for line pipe have been developed with




the express purpose of use in the as-welded con-dition. The low carbon makes welds of this ma-terial that are tough and do not require temper-ing, so girth welds in the field are possible.

Likewise, the lean duplex alloys have verydelayed precipitation of intermetallic phases be-cause of their higher nitrogen and lowerchromium and molybdenum contents. Thismakes welding of these alloys much more fool-proof than with the early duplex alloys, such asS31803. The dual-stabilized ferritic alloys havetougher welds than those stabilized with only ti-tanium or niobium.

New developments in welding also have animpact on stainless steels. The friction stir weld-ing (FSW) process offers the promise of reliable

solid-state joining. By avoiding melting andresolidification, issues associated with soluteredistribution are eliminated. The relatively lowtemperatures involved essentially eliminate

generation of weld fume (see the discussion of safety). Other new welding processes, such asmultiple (GTA or GMA) torch welding, laser-assisted GMA or GTA welding, etc. promisegreater productivity.

REFERENCES

1. D.J. Kotecki, Welding of Stainless Steels,Welding, Brazing, and Soldering, Vol 6, ASM Handbook , ASM International, 1993,p 677–707

2. B. Aziez and R. Feen, Sheet Metal Ind ., 1,1983, p 28–34

3. S.D. Washko and J.F. Grubb, Proc. Int’l

Conf on Stainless Steel, 1991, Chiba, ISIJ4. Stainless Steels, Les Editions de Physiques,

1992, p 7865. A. Garner, Corrosion, 37, 1981, p 178

soldadura aceros inoxidables

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