Kutz M. Handbook of materials selection

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78 STAINLESS STEELS

maximum. Usually, stainless intended for plate is reﬁned to a low-sulfur level

to improve hot workability. Plate is generally formed and welded, with rather

little machining by the customer. Low sulfur is quite detrimental to machinabil-

ity. As bar products are commonly meant to be machined, most stainless bar

actually must be resulfurized to some level, about 0.02%, for improved mach-

inability. When this plate is intended to be drilled for a tubesheet, machineability

becomes important and a resulfurized grade, still within the old 0.030 sulfur

maximum, may be chosen.

It is precise control of chemistry, in particular nitrogen, that has permitted

development of the superaustenitic 6% molybdenum grades. The ability to

closely control nitrogen as an alloying addition has also tremendously improved

the weldability of duplex stainless. Whereas formerly the only duplex stainless

used in North America was 3RE60, and that only in tubing, today several grades

of duplex stainless plate, pipe, and bar products are used in signiﬁcant and

growing amounts.

Chemistry control also means that the producer will minimize the use of

expensive alloying elements. To remain competitive, mills now melt to the bot-

tom end of the allowable range. One consequence for the user is that 316L

stainless, speciﬁed as 2.00–3.00% molybdenum, now has a melt range of about

2.00–2.10% Mo. Through the 1970s the typical Mo level of 316L was about

2.3%. In addition, the average nickel content has dropped from just below 12%

in the 1970s, down to around 10.2% Ni as currently produced. The result has

occasionally been that 25-yr-old 316L equipment, replaced in kind, gave unex-

pectedly short life. In part, of course, this is due to increased corrosive conditions

from recycling, rather than dumping, waste.

4 AVAILABILITY

The metallurgical aspects of alloy selection are well covered in this and other

works. Along with corrosion resistance, strength, and fabricability, one of the

most important material properties is availability. On a large project most alloys

can be made in all product forms, in full mill heat lots. A contemporary example

is 317LMX

威, used as ﬂat-rolled product, in mill heat lot quantities, for ﬂue gas

desulfurization scrubbers. This grade is rarely, if ever, carried in stock by mills

or service centers.

Few mills produce all the forms, such as sheet, plate, bar, pipe, pipe ﬁttings,

and appropriate weld ﬁllers, needed to fabricate a chemical process vessel. The

minimum quantity a mill requires per product form is large, and lead times can

be signiﬁcant. This may limit ﬂexibility in the event of error, last minute design

changes, or later maintenance needs.

Consider what alternate, if more expensive, grade may be used to ﬁll in for

unavailable product forms or sizes. For example, AL-6XN has occasionally been

used to ﬁll out a bill of materials speciﬁed 317LMX

威. The stainless grades 304L

and 316L are used in such quantity that availability is unlikely to be an issue.

The same may not be true for more highly alloyed or specialized materials.

5 FERRITIC STAINLESS STEEL

The ferritic grades listed below exhibit corrosion resistance over a very wide

range. All are subject to severe embrittlement after prolonged exposure in the

6 MARTENSITIC STAINLESS STEELS 79

700–1100⬚F (370–600⬚C) temperature range. Alloy 409 is weldable and is the

alloy used for automotive exhaust systems and catalytic converter shells. It has

some useful high-temperature oxidation resistance to about 1200

⬚F (650⬚C). In

plate gauges it may be welded with ERNiCr-3 for better toughness. Alloy 409

is not suited for decorative applications as it may rust from exposure to weather.

Alloy 416 is free machining, available both as a ferritic and a martensitic version;

430 is commonly used for decorative purposes as well as food-handling equip-

ment. E-BRITE has chloride pitting resistance equal or superior to 316L

stainless, and practical immunity to chloride SCC. E-BRITE is currently avail-

able only as tubing for heat exchangers. AL29-4C is extremely resistant to chlo-

ride pitting and used for heat exchanger applications, available only as tubing

or thin strip.

Ferritic Alloys (%)

Alloy UNS Cr Ni Mo Si Mn Cu C Fe Other

409 S40900 11 — — 0.4 0.3 — 0.015 87 —

405 S40500 12 — — 0.3 0.5 — 0.06 86 0.3Al

416 S41600 13 — — 0.5 1 — 0.05 85 0.2S 0.04P

430 S43000 16.5 — — 0.5 0.5 — 0.08 83 —

439 S43035 17.2 — — 0.5 0.4 — 0.015 81 0.5Ti

E-BRITE S44627 26 — 1 0.2 0.1 — 0.002 72 0.1Cb

AL29-4C S44735 29 — 4 0.35 0.3 — 0.02 66 0.5Ti

6 MARTENSITIC STAINLESS STEELS

The only one of these that is readily welded is 410S. The higher carbon 410

may require signiﬁcant preheat and postweld anneal. Consider the nickel alloy

ﬁller ERNiCr-3 for maximum resistance to weldment cracking, at a sacriﬁce in

strength and hardness. The corrosion resistance of 410 depends upon the heat-

treated condition. For maximum corrosion resistance temper either below 750

⬚F

(400

⬚C) or above 1100⬚F (600⬚C). Both 410 and the martensitic 416 are used in

sporting arms construction. The 440 series are used for cutting tools, 440A and

B for pocket knives; 440C has the best edge retention and is used for ﬁne custom

knives and surgical tools.

Martensitic Alloys (%)

Alloy UNS Cr Ni Mo Si Mn C Fe Other

410 S41000 12 — — 0.3 0.4 0.14 87 —

410S S41008 12 — — 0.3 0.4 0.05 97 —

416 S41610 13 — — 0.4 0.2 0.1 84 0.2S 0.05P

420 S42000 13 — — 0.4 0.4 0.2 86 —

431 S43100 15.6 1.9 — 0.2 0.4 0.1 81 —

440A S44002 17 — 0.5 0.2 0.4 0.67 81 —

440B S44003 17 — 0.5 0.2 0.4 0.8 81 —

440C S44004 17 — 0.5 0.2 0.4 1.07 80 —

80 STAINLESS STEELS

7 AGE-HARDENING MARTENSITIC STAINLESS STEELS

These grades combine useful corrosion resistance with simple heat treatment.

The most widely used is 17-4PH. They are susceptible to chloride stress–

corrosion cracking. Susceptibility depends somewhat upon grade and aging tem-

perature.

Age-Hardening Alloys (%)

Alloy UNS Cr Ni Mo Cu Mn Al Ti C Fe Other

455 S45500 11.5 8.5 — 2 0.2 — 1.1 0.2 76 0.3Cb

PH15-7Mo

威 S15700 15 7 2.5 — 0.4 1 — 0.04 74 —

17-4PH

威 S17400 15.5 4.7 — 3.3 0.5 — — 0.05 75 0.3Cb

17-7PH

威 S17780 17 7.3 — — 0.7 1.2 — 0.06 74 —

PH13-8Mo

威 S13800 13 8 2.5 — — 1.2 — 0.03 76 —

8 DUPLEX STAINLESS STEELS

Duplex stainless steels are characterized by high strength and good resistance

to chloride stress–corrosion cracking. They represent a more economical solu-

tion to the problem of stainless SCC than the use of a high-nickel alloy. The

lower grades, such as 2304, may be used to replace carbon steel in large part

because the higher strength permits such weight reduction. The two grades most

available from North American service centers, in a range of product forms, are

2205 and 2507. Alloys Zeron

威 100, 2507, and 255 have sufﬁcient chloride pit-

ting resistance for seawater service.

Duplex Alloys (%)

Alloy UNS Cr Ni Mo Cu Mn Si N C Fe Other

2304 S32304 23 4 0.3 0.3 2 — 0.1 0.03 70 —

3RE60 S31500 18.4 4.8 2.7 — 1.6 1.7 0.07 0.03 70 —

19D S32001 21.2 1.3 — — 5 0.4 0.15 0.02 71 —

2205 S31803 22 5.6 3.0 — 1.5 0.5 0.14 0.02 67 —

Zeron

威 100 S32760 25 6.5 3.5 0.5 — — 0.25 0.02 63 0.7W

2507 S32750 25 7 4 — 0.1 0.2 0.3 0.02 63 —

7-MoPLUS

威 S32950 26.5 4.8 1.5 — 0.4 0.3 0.2 0.02 66 —

255 S32550 25.5 5.7 3.1 1.8 0.8 0.5 0.17 0.02 62 —

9 AUSTENITIC STAINLESS AND NICKEL ALLOYS

These nickel bearing alloys are used for the majority of corrosive environments.

Alloy 316L is by far the most common choice for chemical process equipment.

Formerly, when 316L was inadequate, the designer switched to the high end of

the spectrum, alloy C-276, which is still considered the most broadly useful of

the high alloys. Today there are numerous other choices, ranging from those of

intermediate cost to alloys superior to C-276 in speciﬁc environments.

Types S30600, S32615, and alloy D-205

are specialized high-silicon ma-

terials intended for concentrated sulfuric acid heat exchanger service. Weldabil-

ity is limited. A very high chromium grade, R20033, is also intended for hot

9 AUSTENITIC STAINLESS AND NICKEL ALLOYS 81

concentrated sulfuric acid. It is weldable and has a broader range of potential

uses than do the high-silicon alloys.

Materials in the 5–7% molybdenum range are used for seawater service and

chemical process vessels in general. Alloys N08367, S31254, and N08926 con-

stitute the ‘‘6 moly’’ alloys. These have both corrosion resistance and cost in-

termediate between 316L and alloy C-276. All are readily fabricated and broadly

available. Nitrogen in these alloys retards sigma formation, in spite of rather

high Mo and moderate Ni levels. Prolonged exposure in the 1000–1800

⬚F (540–

980

⬚C) temperature range may cause sigma-phase precipitation.

Alloys in the 13–16% Mo range include C-276, C-22, 59, C-2000, and 686.

The latter four are regarded as improvements over C-276, with higher Cr for

oxidizing environments. The newest, and possibly best, of the very high end

nickel alloys, MAT 21, makes use of a tantalum addition, as well as 19% Mo.

Alloy selection for corrosive process environments is a complex process. It

should include experience with similar equipment, extensive testing in the exact

corrosive environment of interest, and detailed knowledge of the various alloys

to be considered. Oftentimes, apparently minor contaminants can cause major

changes in corrosion rates. One example is contamination of organic chlorides

with small amounts of water. This can permit the organic compound to hydro-

lyze, forming hydrochloric acid. The HCl in turn may aggressively pit or stress

corrode the standard 18–8 stainless steels. Other examples include the alloys

B-2, 200, and 400, which contain no chromium. While they have excellent

corrosion resistance in reducing environments, they have little or no resis-

tance to oxidizing environments. Unexpected failures may therefore arise from

contamination by small amounts of oxidizing salts (e.g., FeCl

, CuCl

,or

NaClO

), or even dissolved oxygen. Titanium behaves in the opposite manner

and requires the presence of oxidizing species to develop its protective oxide

ﬁlm.

Austenitic Alloys (%)

Alloy UNS Cr Ni Mo Si Mn Cu C Fe Other

302 S30200 18.5 8.2 — 0.5 0.75 — 0.1 72 —

304L S30403 18.3 9 — 0.5 1.7 — 0.02 70 —

321 S32100 17.3 9.3 — 0.7 1.8 — 0.01 70 0.2Ti

347 S34700 17 9.5 — 0.7 1.5 — 0.04 70 0.5Cb

316L S31603 16.4 10.2 2.1 0.5 1.6 — 0.02 69 —

317L S31703 18 11.6 3.1 0.4 1.5 — 0.02 65 —

317LMN S31726 17 13 4.2 0.5 1.5 — 0.03 62 0.15N

A610 S30600 18 15 — 4 0.7 — 0.01 62 —

254 SMO S31254 20 18 6.1 0.4 0.7 0.7 0.015 54 0.2N

SX S32615 18 20.5 0.9 5.5 1.5 2 0.04 51 —

654 SMO

威 S32654 24 22 7.3 — 3 0.5 0.01 42 0.5N

B66 S31266 24.5 22 5.6 — 3 1.5 0.02 44 2W

904L N08904 21 25 4.5 0.5 1.7 1.6 0.015 45 —

1925hMO

威,

25-6MO

N08926 20 25 6.2 0.4 0.7 0.9 0.01 46 0.2N

AL-6XN N08367 20.5 24 6.3 0.4 0.3 — 0.02 48 0.22N

82 STAINLESS STEELS

Alloy UNS Cr Ni Mo Si Mn Cu C Fe Other

28 N08028 27 31 3.5 0.2 1.8 1 0.01 35 —

31 N08031 27 31 6.5 0.2 1 1.2 0.01 33 0.2N

33 R20033 33 31 1.4 0.3 0.7 0.7 0.01 32 —

20Cb-3 N08020 20 33 2.2 0.4 0.4 3.3 0.02 40 0.5Cb

3620 Nb N08020 20 37 2.1 0.3 1.6 3.4 0.02 35 0.6Cb

825 N08825 21.5 40 2.8 0.3 0.6 2 0.01 29 0.8Ti

RA333 N06333 25 45 3 1 1.5 — 0.05 18 3Wm3Co

G-30 N06030 29.5 45 5.5 — — 1.9 0.01 15 0.7Cb 2.5W

G N06007 22 45 6.5 0.3 1.5 2 0.01 20 2.1Cb 0.8W

G-3 N06985 22 48 7 0.4 0.8 2 0.01 18 0.3Cb 0.8W

625 N06625 21.5 61 9 0.1 0.1 — 0.05 4 3.6Cb

C-276 N10276 15.5 57 15.5 0.05 0.5 — 0.005 5.5 0.2V 4W

686 N06686 20.5 57 16.3 0.1 0.2 — 0.005 1 3.9W 0.2Al

C-2000

威 N06200 23 58.5 16 0.02 0.2 1.6 0.003 0.3 0.25Al

C-22 N06022 21 57 13 0.05 0.3 — 0.003 4 0.2V 3W

59 N06059 23 59 16 0.05 0.4 — 0.005 0.3 0.2Al

MAT 21 — 19 60 19 — — — — — 1.8Ta

D-205 — 20 64 2.5 5 — 2 0.02 6 —

B-2 N10665 — 71 28 0.01 0.15 — 0.002 0.8 —

B N10001 — 66 30 — 0.5 — 0.06 3 0.3V

B-3 N10675 1.3 67.7 28 0.03 0.5 — 0.002 1.5 —

B-4 N10629 1.3 67 28 — — — 0.005 3 —

400 N04400 — 66 — 0.02 1 31 0.1 1.5 —

201 N02201 — 99.5 — 0.2 — — 0.01 0.2 —

10 WELDING

There are some important distinctions between welding carbon steel and welding

stainless steel. Alloys containing more than about 20% nickel have somewhat

different requirements than the lower nickel austenitic stainless steels. There are

speciﬁc issues regarding duplex stainless steels and weld ﬁllers for the high-

molybdenum grades.

10.1 Carbon Steel versus Stainless

Six important differences between welding the carbon or low-alloy structural

steels and the austenitic stainless and nickel alloys are surface preparation,

shielding gases, cold cracking versus hot cracking, distortion, penetration, and

fabrication time.

1. Surface Preparation. When fabricating carbon steel, it is common prac-

tice to weld right over the scale (a so-called mill ﬁnish is a layer of blue-black

oxide, or scale, on the metal surface), red rust, and even paint. The steel weld

ﬁllers normally contain sufﬁcient deoxidizing agents, such as manganese and

silicon, to reduce these surface iron oxides back to metallic iron. The resultant

Mn–Si slag ﬂoats to the weld surface. Iron oxide, or scale, melts at a lower

temperature, 2500

⬚F (1371⬚C), than does the steel itself. One can see this in a

10 WELDING 83

steel mill when a large ingot is removed from the soaking pit for forging—the

molten scale literally drips off the white-hot steel.

Stainless steel, by contrast, must be clean and free of any black scale from

hot rolling, forging, or annealing operations. Such scale is predominately chro-

mium oxide. Normally, stainless is produced with a descaled white or bright

ﬁnish. Stainless steel melts at a lower temperature than does its chromium oxide

scale, and the stainless weld ﬁller chemistry is not capable of reducing this scale

back to metallic chromium. As a result, with gas-shielded processes it is difﬁcult

to get the weld bead to even ‘‘wet,’’ or stick to, a scaled piece of stainless.

The need to clean or grind down to bright metal is more likely to cause trouble

when stainless is being joined to carbon steel. In this dissimilar metal joint it is

necessary to grind that carbon steel to bright metal, on both sides of the joint,

free of all rust, mill scale, grease, and paint. The preferred weld ﬁllers for this

particular joint, to minimize the hard martensitic layer on the steel side, are

ENiCrFe-3-covered electrodes or ERNiCr-3 bare ﬁller wire; E309 electrodes are

commonly used but are not the optimum choice.

Both stainless and high-nickel alloys designed for corrosion resistance are

produced to very low carbon contents, less than 0.03% and sometimes less than

0.01% carbon. Any higher carbon will reduce the metal’s corrosion resistance.

For this reason it is necessary to clean the alloy thoroughly of all grease and oil

before welding. Also the very high nickel alloys, such as Monel 400 or com-

mercially pure nickel 200/201, are very sensitive to weld cracking from the

sulfur in oil or marking crayons.

Metallic zinc paint is a common way to protect structural steel from corrosion.

Even a small amount of that zinc paint overspray on stainless will cause the

stainless to crack badly when welded. Consider ﬁnishing all stainless welding

before painting the structural steel in the area.

2. Shielding Gases. For gas–metal arc welding (GMAW, a.k.a. MIG) carbon

steel the shielding gases are usually 95% argon 5% oxygen, 75% argon 25%

carbon dioxide, or 100% CO

. While suitable for carbon or low-alloy steel weld-

ing wire, such gases are far too oxidizing for use with stainless or nickel alloys.

It is not unknown to hear the complaint ‘‘clouds of red smoke are coming off

when I weld this stainless . . . heavy spatter’’ and then learn that the shielding

gas used was 75% argon 25% CO

. A ﬁne gas for carbon steel but not for any

stainless.

One exception to this high CO

prohibition is when using ﬂux-cored wire,

either stainless or nickel alloy. Some of these wires are speciﬁcally formulated

to run best with 75% Ar 25% CO

Stainless and nickel alloys may be GMAW spray-arc welded using 100%

argon shielding gas, though it is not necessarily the ideal choice. Stainless steels

possess a passive chromium oxide ﬁlm. The basis of stainless corrosion resis-

tance, this ﬁlm is not desirable when welding. Some 80% argon is necessary to

get into a spray-transfer mode. Beyond this, a helium addition, up to about 20%,

gives a hotter arc and helps break up the oxide ﬁlm. A very slight amount of

carbon dioxide, perhaps 1–2%, will prevent arc wander. Too much CO

may

begin to add carbon to the weld, undesirable for corrosion resistance. Nitrogen

may be added in gases speciﬁcally designed for welding the duplex stainless

steels.

84 STAINLESS STEELS

Pulse-arc welding is usually done with a 75% argon 25% helium mix. Short-

circuiting arc transfer may be done with this 75% Ar–25% He mix but a 90%

helium 7 argon 2 CO

‘‘tri-mix’’ is more commonly used.

––

3. Cold Cracking versus Hot Cracking. Carbon steel weldments may harden,

and crack, as they cool from welding. High hardness, and the resulting cracking,

may occur when the steel contains more than 0.25% carbon. Alloying elements

that increase hardenability, such as manganese, chromium, molybdenum, etc.,

can make steels of lower carbon content also harden. Hydrogen pickup from

moisture in the air causes underbead cracking in steels that harden as they cool

from welding. To prevent such cracking, the steel is usually preheated before

welding. This retards the cooling rate of the weld and avoids martensite for-

mation. Postweld heat treatment, or stress relief, is also applied to some steels,

or for certain applications. The martensitic stainless steels behave in this fashion

and because of their high hardenability are quite difﬁcult to weld.

Austenitic stainless and nickel alloys do NOT harden, no matter how fast they

cool from welding. So, it is not necessary to preheat austenitic stainless, nor to

postweld heat treatment. Indeed preheating austenitic alloys, beyond what may

be necessary to dry the metal, can be positively harmful. Stress relief 1100–

1200

⬚F (600–650⬚C) as applied to carbon steel is ineffective with stainless or

nickel alloys and may damage the corrosion resistance of some grades.

Stainless steel weldments are usually quite resistant to cracking, unless con-

taminated, possibly by zinc or copper, more rarely by aluminum. A small amount

of ferrite in the austenitic weld bead provides this hot cracking resistance.

High-nickel alloys are less forgiving and may be susceptible to cracking in

restrained joints, including heavy sections. This is a hot tearing, not a cold crack.

That is, the weld bead tears rather than stretching, as the bead contracts upon

solidifying.

This hot tearing/hot cracking has nothing to do with hardness. The faster a

nickel alloy weld freezes solid, the less time it spends in the temperature range

where it can tear. For this reason preheating, which slows down the cooling rate,

is actually harmful, as it permits more opportunity for hot tearing to occur.

4. Distortion. Stainless steel has poor thermal conductivity, only about one

fourth that of ASTM A36 structural steel. This means the welding heat tends to

remain concentrated, rather than spread out. Stainless also expands with heat

about half again as much as does carbon steel. The combination of these two

factors means that stainless or nickel alloy fabrications distort signiﬁcantly more

than similar designs in carbon steel.

Tack welds should be closer than with carbon steel and sequenced in a pattern,

left side, right side, middle, etc. If the tacks are simply done in order from one

end, the plate edges close up. To balance stresses, weld runs should be done

symmetrically about the joint’s center of gravity. Back step welding is helpful.

This subject is well covered in Refs. 9 and 10.

Reducing heat input reduces the stresses and distortions from the welding

operation. Heat into the workpiece is controlled by welding current, arc voltage,

travel speed, and the speciﬁc welding process used. For the same amperes, volts

and speed, submerged arc welding (SAW) transfers the most heat; shielded metal

arc (SMAW), and gas–metal arc (GMAW) with argon shielding next and roughly

10 WELDING 85

equivalent. Gas–tungsten arc welding (GTAW) can put the least heat into the

work.

5. Penetration. There is a tendency to increase heat input with the stainless

and high-nickel alloys. First, the weld ﬁllers tend to be sluggish and not ﬂow

well, as compared with carbon steel. Second, the arc simply will not penetrate

stainless steel as it does carbon steel. The higher the nickel, the less the pene-

tration. Increasing welding current will not solve the problem, and may cause

cracking in the higher nickel grades. Stainless, and especially nickel alloy, joint

designs must be more open. The base metal should be single or double beveled,

with a root gap, so that the weld metal may be placed in the joint.

6. Fabrication Time. Cleanliness, distortion control measures, maintaining

low interpass temperatures, and even machining add up to more time spent

fabricating stainless than carbon steel. A shop experienced with stainless may

require 1.6 times as long to complete the same fabrication in stainless, as in

carbon steel. A good carbon steel shop encountering stainless or nickel alloys

for the ﬁrst time may easily spend twice as long, maybe even three times as

long, to do the stainless fabrication, as it would the same job in carbon steel.

10.2 Austenitic Alloys

The fundamental problem to be overcome in welding austenitic nickel bearing

alloys is the tendency of the weld to hot tear upon solidiﬁcation from the melt.

This matter is readily handled in stainless steels of up to about 15% or so nickel.

In these stainless grades the weld metal composition is adjusted, usually by

slightly higher chromium and reduced nickel, to form a small amount of ferrite

upon solidiﬁcation. The amount of ferrite in the weld may be measured mag-

netically, and is reported as a ferrite number (FN). This ferrite acts to nullify

the effects of the elements responsible for hot cracking in the Ni–Cr–Fe aus-

tenitics. These elements are chieﬂy phosphorus, sulfur, silicon, and boron. In

higher nickel grades, about 20% nickel and over, it is metallurgically not possible

to form any measurable amount of ferrite. Therefore other means of minimizing

hot cracking must be used. Foremost among these is to use high purity raw

materials in the manufacture of weld ﬁllers. Simply reducing the amounts of

harmful P, S, Si, and B in the weld metal improves its ability to make a sound

weld. Phosphorus, in particular, must be kept below 0.015% in the weld wire

itself. Certain alloy additions such as manganese, columbium (niobium), and

molybdenum serve in one way or another to reduce the austenitic propensity for

weld hot cracking. Manganese ranges from about 2% in AWS E310-15-covered

electrodes to 8% in ENiCrFe-3-nickel-alloy-covered electrodes. Columbium at

the 0.5% level, as in 347 or 20Cb-3 stainless, is harmful whereas 2–4% Cb is

quite beneﬁcial in many nickel base weld ﬁllers. Molybdenum is not added with

the intention of increasing weldability, nevertheless it does so. The 2% Mo

contributes to 316L being the most weldable of the stainless steels, and 15% in

C-276 accounts for the popularity of the various ‘‘C-type’’ electrodes in repair

welding.

The distinction between the lower nickel stainless grades, which depend upon

ferrite to ensure weldability, and the high-nickel alloys, which require high purity

weld ﬁllers, is an important one to remember. Most ferrite-containing (stainless)

86 STAINLESS STEELS

weld ﬁllers are useless with nickel alloy base metal, as dilution of the weld bead

with nickel from the base metal eliminates this ferrite. Likewise a high purity

nickel alloy weld ﬁller, such as ER320LR, may not be quite so crack resistant

when contaminated by phosphorus from use on 316L or carbon steel base metal.

Alloys under 20% Nickel

Most austenitic grades containing less than 20% nickel are joined with weld

ﬁllers that utilize some ferrite, perhaps 4-12 FN, to ensure freedom from hot

cracks. In practice, this means stainless steels up through 317LMN or 309S,

both about 13% nickel. The 310S type stands in an odd position between the

stainless and the nickel alloys, having neither ferrite nor any particular alloy

addition to the weld metal to ensure sound welds. Not surprisingly, 310S welds

have a reputation for ﬁssuring.

Alloys over 20% Nickel

Corrosion-resistant alloys in this category begin with the 18% nickel 254 SMO,

for this and other ‘‘6 moly’’ grades are welded with an overmatching nickel

alloy ﬁller such as ERNiCrMo-3.

Other nickel alloys are joined with matching composition weld ﬁllers modi-

ﬁed only by restrictions on P, S, Si, and B. A minor amount of titanium may

be added for deoxidation. Other ﬁllers contain signiﬁcant manganese or colum-

bium to improve resistance to ﬁssuring and hot cracking.

Such chemistry modiﬁcations are rarely as effective as is the use of ferrite in

the lower nickel stainless weld ﬁllers. Welding technique and attention to clean-

liness, then, become increasingly important to ensure the soundness of fully

austenitic welds. These techniques include bead contour and low interpass tem-

perature. Reinforced, convex stringer beads are much more resistant to center-

bead cracking than are shallow, concave beads. Interpass temperature for most

nickel alloy weldments is kept below 300

⬚F (150⬚C). Cleanliness includes NOT

using oxygen additions to the GMAW shielding gases for nickel alloys.

It is worth repeating here that high-nickel alloys cannot be welded using

stainless steel weld ﬁllers. Stainless steel ﬁllers (308, 309, etc.) depend upon a

chemistry that will solidify from the melt as a duplex structure, containing a

small amount of ferrite in with the austenite.

10.3 Duplex Stainless Steels

When welding duplex stainless the issues are maintaining the austenite–ferrite

balance in the weldment and avoiding precipitation of nitrides and sigma. Rather

high heat input is used to weld duplex, similar to welding 316L stainless. This

is in contrast to the lower heat preferred with the high-nickel alloys. Low heat

input is positively NOT desirable with a duplex, as it may not permit sufﬁcient

transformation of ferrite to austenite upon cooling. Current duplex weld ﬁllers

have nickel content somewhat increased over that of the base metal. This is to

ensure that enough weld metal ferrite transforms to austenite, to maintain a

balance of the two phases.

Toughness and critical pitting temperature of duplex weldments vary with the

choice of welding process, in opposite directions. In order of increasing notch

impact toughness: SMAW AC/DC, FCAW (ﬂux cored arc welding), SMAW

REFERENCES 87

DC-basic, SAW, GMAW argon shielding, GMAW 95AR 3He 2N

shielding, and

GTAW. In order of increasing pitting resistance, as measured by critical pitting

temperature: GTAW, GMAW, SAW, FCAW, SMAW DC-bsic, and SMAW AC/

DC.

10.4 High-Molybdenum Alloys

High-molybdenum-containing stainless and nickel alloys are welded with an

overmatching ﬁller metal. This is necessary to maintain corrosion resistance in

the weld metal at least equal to the base metal. The reason is that molybdenum

and chromium segregate as the weld metal solidiﬁes from the melt. This leaves

local areas with high, and with low, molybdenum. Pitting corrosion can start in

the low-Mo area, with the pits eventually growing even into metal with high

molybdenum. This occurs in alloys ranging from 316L to C-276, being more

severe at higher alloy contents.

This matter began to receive attention when the 6% molybdenum stainless

steels came on the market. If any of these 6% Mo grades are welded without

ﬁller metal, the result is a weld bead that may be as low as 3% Mo in areas.

The end result can be that this weld has only the pitting corrosion resistance of

317L stainless. In the case of tubular products autogenously welded in produc-

tion, a high-temperature anneal is used to homogenize the metal. In addition, a

small amount of nitrogen, 3–5%, is added to the torch gas. Fabrications of thin

sheet, which cannot be annealed after welding, should have this nitrogen addition

to minimize the loss of corrosion. In addition, thin sheet welds solidify more

quickly, hence the segregation is less severe.

In normal fabrication of a 6% molybdenum grade, alloy 625 (ERNiCrMo-3)

ﬁller metal is used. The weld metal contains 9% Mo. After welding, segregation

causes some areas to have as little as 6% molybdenum. The result is that the

alloy 625 weld bead has approximately the same corrosion resistance as the 6%

molybdenum base metal. Higher alloy weld ﬁllers, such as ERNiCrMo-10 or

ERNiCrMo-14, may also be used, though the beneﬁt may be more theoretical

than real. ERNiCrMo-4 is not suggested, as it has 5% less chromium than does

AL-6XN, for example. Since the mid-1980s nearly all of the 6% molybdenum

alloy fabrications have been made, and put into service, using a 9% molybdenum

weld ﬁller.

ERNiCrMo-3 weld ﬁller is widely available and is appropriate for welding

lower alloys such as 317L, 317LMN, and 904L for chloride service. This

molybdenum, and chromium, segregation problem exists with the 13–16% Mo

nickel alloys as well. In this case there is not much choice in higher alloy weld

ﬁllers.

WEB SITES

Detailed technical information on heat and corrosion resistant alloys is avail-

able at www.rolledalloys.com. Complete stainless steel data is at www.

alleghenyludlum.com. Nickel alloys in general are covered at www.nidi.org.

REFERENCES

1. H. H. Uhlig, The Corrosion Handbook. Wiley, New York, 1948.

2. James Kelly, Corrosion Resistant Alloy Speciﬁcations & Operating Data, Bulletin 151, Rolled

Alloys, Temperance, MI, 2001.