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Welding of Zirconium lloys

R. Terrence Webster Consultant

ZIRCONIUM is a corrosion-res is tant s t ruc-

tural metal that has many physical and mechani-

cal proper t ies s imilar to t i tanium and austeni t ic

s tainless s teels . Zirconium al loys are weldable;

procedures and equipment are s imilar to those

used for welding t i tanium and austeni t ic s tain-

less steels . Zirconium h as a low coeff icien t of

thermal expansion, which contr ibutes to low dis-

tor t ion dur ing welding.

Because of t he r eact iv i ty of z i r conium wi th

oxygen, n i t rogen, and hydrogen, t he meta l mus t

be shielded dur ing welding with high-pur i ty in-

er t gas or a good vacuum. Fur thermore , z i r co-

nium should be f ree of oi l , grease, and dir t to

avoid the d i s solving of carbon- and oxygen-con-

taining mater ials , which can embri t t le the metal

or create porosi ty and may reduce the corrosion-

res is tant proper t ies of the metal .

Zirconium and i ts al loys are avai lable in two

general ca t egori es : commercia l gr ade and r eac-

tor grade. Commercia l -grade z i r conium des ig-

nates zi rconium that contains hafnium as an im-

pur i ty . Reactor -grade z i r conium des ignates

zi r conium f rom which mos t o f t he hafnium has

been r emoved to make i t su i t able for nuclear

reactor appl icat ions . Because pure zi rconium

has relat ively low mechanical proper t ies , var i -

ous al loying elements are added to enha nce i ts

mechanical proper t ies .

Zirco nium and i ts al loys are avai lable in plate,

sheet , bar , rod, and tubing form in a var iety of

mater i a l speci f ica t ions . The four mos t comm on

grades of commercia l z i r conium al loys , as spec-

i f i ed in ASTM B 550, ar e g iven in Table 1 . For

reactor grades , the hafnium content is reduced to

a minim um and other impur it i es ar e c losely con-

t ro ll ed . Che mical compos i t ions of t he mos t

common grades of nuclear -grade Zi r caloys , as

speci f ied in ASTM B 351, ar e g iven in Table 2 .

Zi r conium can only be welded to i t s e l f or t o

other react ive metal al loys , such as t i tanium,

niobium, or t anta lum. Wh en w elded to nonreac-

t ive metals , intermetal l ics that embri t t le the

weld meta l ar e formed.

Process Selection

Zi rconium al loys ar e h ighly r eact ive to oxy-

gen and ni t rogen in ai r at high temperatures .

Consequent ly , t he s e l ec t ed welding proces ses

and procedures mus t be capable of sh i e ld ing the

weldment and heat - af f ect ed zones (HAZ) f rom

contaminat ion. The use of f luxes is general ly

avoided, because r eact iv i ty wi th the chemical s

in the f luxes causes br i t t leness and may reduce

the cor ros ion r es i s t ance of z i r conium weld-

ments .

The fo l lowing welding proces ses can be used

for welding any of t he z i r conium al loys :

• Gas - tungs ten ar c welding (GTA W)

• Gas -meta l ar c welding (GMA W)

• Plasma arc welding (PAW)

• Elect ron-beam welding (EBW)

• Laser -beam welding (LBW)

• Fr i c t ion welding (FRW)

• Res i s t ance welding (RW)

• Res i s t ance spot welding (RSW)

• Res i s t ance seam welding (RSEW )

The se l ec t ion of a welding proces s depends on

several factors: weld joint , tensi le and corrosion-

res is tant proper ty requirements , cost , and design

configurat ion.

Gas tungsten arc welding i s the mo s t widely

used proces s for jo in ing z i r conium al loys . I t em-

ploys techniques s imilar to those used for weld-

ing s tainless s teel , that is , the direct current

power supply is connected for s t raight polar i ty

( e l ec trode negat ive , DCEN ) . Two des i r able f ea-

tures are a conta ctor for making and breaking the

arc and high- f r equency arc s t ar t i ng . Recom-

mended e l ec t rodes ar e thori a t ed tungs t en , Type

E W T h2 .

Plasma arc welding

i s a l so commonly used,

especia l ly for autogenous welding o f but t j o in t

th i cknesses f rom 3.18 to 1 .52 mm (0 .125 to

0.600 in . ).

Table 1 Chemica l compositions of

commercial zirconium and zirconium

alloys per AST M specification B 550 92

Gas metal arc welding

i s occas ional ly use

for j o in t t h i cknesses f rom 3.18 mm (0 .125 in .

or more , because of i t s more- r apid weld t im

and the consequent s avings in shi e ld ing gas an

product ion t ime. Weld qual i ty is more d i f f i cu

to maintain, because of weld spat ter and ar

ins tabi l i ty, which resul t in weld contaminat io

and weld defect s .

Electron beam welding i s rarely used, be

cause of h igh equipment opera t ing cos t as we

as weld chamber s ize l imitat ions .

Laser beam weld ing

has had very l imi t e

use in jo in ing z i r conium and has been appl i e

pr imari ly in nuclear reactors .

Friction welding i s used to jo in z i r conium

tubes to zi rconium rods , as wel l as to diss imila

metal al loys ( for example, zi rconium to s tainles

s teel) for heat-exchanger appl icat ions .

Resistance welding

i s especial ly useful fo

the s eam or spot welding of t h in sheets , becaus

no shielding is required.

Shielding ases

High-pur i ty argon (99.996 Ar ; -6 5 °C, o

-8 5 °F, dew point ), he l ium, or a mixture of t h

two gases ar e used to weld z i r conium al loys

Argon has the advantage o ver hel ium in terms o

lower cost , and i t provides good s tabi l i ty an

weld meta l cont rol . Hel ium i s used to obta i

deeper penet r a t ion in to the jo in t , and m ixtures o

argon and hel ium are f r equent ly used in auto

Table 2 Chemical compositions of the

most common grades of nuclear grade

Zircaloys per ASTM specification B 351 92

Compmifion,

wt%

Element R60702 R60704 R60705

Tin 1.20-1.70 1.20-1.70 - .-

Zirconium + hafnium, Iron 0.07-0 .20 0.18-0. 24 - .-

min 99.2 97.5 95.5 Chromium 0.05-0.1 5 0.07-0. 13 .--

Hafnium, max 4 .5 4. 5 4. 5 Ni cke l 0 .0 3~ ). 08 . . . . . .

Iron + ch romi um 0 .2 ma x 0 . 2- 0. 4 0 .2 ma x N iob ium . . . . . . 2 .4 0- 2.

Ti n ... 1 .0 -2 .0 -- Oxygen . . . . . . 0.09-- 0.

Hydrogen, max 0.005 0.005 0.005 Iron + chromium

Ni tr og en, ma x 0 .0 25 0 .0 25 0 .0 25 + ni ck el 0 .1 8- 0. 38 . . . . . .

Carbon, max 0.05 0.05 0.05 Iron + chromium -.. 0.2 8-0. 37 -..

Ni ob iu m . . . . . . 2 . 0- 3 .0 Ha fn iu m 0 .0 10 0 .0 10 0 .0 10

Oxygen, max 0.16 0.18 0.18 Zirconium bal bal bal

Composition, wt R60802 R60804 R60901

Element (Zircaloy-2) (Zircaloy-4) (Zr-2.5N

ASM Handbook, Volume 6: Welding, Brazing, and Soldering

D.L. Olson, T.A. Siewert, S. Liu, and G.R. Edwards, editors, p 787-788

DOI: 10.1361/asmhba0001441

Copyright © 1993 ASM International®All rights reserved.

www.asminternational.org

8/10/2019 Welding of Zirconium Alloys

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788 / Weld ing Consumables and Procedures

matic operations. Helium is the gas most often

used to fabricate nuclear reactor equipment.

Filler Metal Selection

Three zirconium and zirconium-alloy elec-

trodes are identifi ed in American Weld ing Soci-

ety (AWS) specification A5.24-90 (Table 3).

The fil ler-metal composition is selected to match

the base-meta l composition. Type ERZr2 is used

to weld commercially pure zirconium (grade

R60702); type ERZr3 to weld Zr-l.5Sn alloy

(grade R60704); and type ERZr4 to weld Zr-

2.5Nb alloy (grades R60705 and R60706). For

critical nuclear applications, the impurities are

more closely controlled and the hafnium level is

minimized. The nuclear-grade zircon ium alloys

(Table 1) are welded with f iller wire of the same

grade chemical composition.

Process Procedures

Joints and welding wire should be cleaned

before welding. Most welding wire is ade-

quately acid cleaned prior to spooling or packag-

ing. When cut lengths of wire are found to be

dirty, it is good practice to wipe each piece with

a clean cloth and solvent prior to use. I f it is

necessary to clean spooled wire, then a wipe

Table 3 Chem icalcompositionsof

zirconium and zirconium alloy electrodes

Composition, wt%

ERZr2

ERZr3 ERZr4

E le me nt R60702) R6 07 04 ) (R60705)

Carbon 0.05 0.05 0.05

Chromium + iron 0.20 0.204).40 0.20

Niobium . . . . . . 2.0-3.0

Hafnium 4.5 4.5 4.5

Hydrogen 0.005 0.005 0.005

Nitrogen 0.025 0.025 0.025

Tin , 1.0(02.00 ...

Zirconium

+ hafni um 99.01 min 97.5 min 95.5 min

Source: AWS A5.24-90

with solvents will usual ly be satisfactory. If wire

is acid cleaned, then the acid bath should be

fresh and should not contain iron or any other

contaminant that will remain on the wire sur-

face.

Heavy grease and machining oil on the work-

piece can be removed using degreasing solvents

or reagent methanol, acetone, methyl ethyl ke-

tone (MEK), or the equivalent, If acetone is

used, then an alcohol rinse should follow.

Heavy mill scale (usually gray in color)

should be removed by grinding. The blue sur-

face oxide that develops when the metal is

heated above 315 to 425 °C (600 to 800 °F) can

be removed either by wire brush ing (with a clean

austenitic stainless steel wire brush not used for

other metals) or by grinding or acid pickling,

which are more effective than wire brushing. A

common treatment is to soak from 2 to 5 min in

room-temperature solution of 35% HNO 3 and

5% HF in water, followed by a thorough clean-

water rinse and air drying.

A joint cleaned by degreasing can still be cov-

ered by a very l ight oil film that is approximately

the equivalent thickness of the residue left by

fingerprints. This can be effectively removed by

abrasive or detergent cleaning, using common

household cleansers or detergents, followed by

air drying. Ai r drying can be accelerated by rins -

ing with methanol, acetone, or other solvents.

Joints cleaned by abrasive, detergent, or acid

pickling should be kept entirely clean of dirt,

including fingerprints. Generally, it is good

practice to clean and to weld a joint within the

same work shift. When this is not possible, the

joint should be covered with paper or bagged

with plastic sheet or another material to mini-

mize the accumulat ion of dust and dirt.

ASTM specification B 614 ( Descaling and

Cleaning Zirconium and Zirconium Alloy Sur-

faces ) should be consulted for cleaning details.

reheating

s not normally required for zirco-

nium welding. The commercial alloys have

enough ductility to prevent weld cracking (of

uncontaminated welds). Preheating is an aid to

moisture removal and is good practice where the

shop temperature is low or the humidity is high

(it is better to control temperature and humidit

in the welding area). Heat can be applied by any

conventiona l means. However, if torch heating

is used, then the torch should be moved continu

ously to avoid local heating of the metal.

Interpass cleaning is not required if the weld

deposit is bright and shiny. Light temper colors

(straw to light blue) should be removed by

brushing with a clean austenitic stainless stee

brush.

To prevent weld contamination from the oxy

gen and nitrogen in air, the weld must be

shielded during welding and cooling to below

480 °C (896 °F).

The commercial grades of zirconium wil

most often be welded in air, and the weldment

will require gas shielding on the torch side and

the back side of the weld. For sheet, plate, an

pipe welds, this requires a gas trailing shield on

the torch side and a back-up gas shield for the

back side. The shielding gases must envelope

the weldment during welding and cooling.

Complex assemblies that are difficult to shield

in air can be welded in a chamber. The chambe

can be capable of either being evacuated an

back filled with argon or helium or it can b

supply purged with shielding gas before and dur

ing the welding operation. Taping a plastic ba

over the part to be welded and sealing the bag a

the point where the welding cables enter the ba

will simulate an enclosed chamber when helium

is introduced into the bag system as a shieldin

gas.

Heat Treatment.

Preweld heat treatment i

not required. Postweld stress rel ief for commer

cial grades R60702 and R60704 can be use

when high residual stresses occur. Grad

R60705 must be stress relieved within 14 day

after welding to prevent delayed hydride crack

ing (DHC). Commercial-grade zirconium alloy

can be heat treated in air or in an inert atmo

sphere. Nuclear-grade z ircon ium alloys are usu

ally heat treated in an inert atmosphere.