Sens It Ization

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The

ize of the ensitization Zone in

304

tainless teel Welds

N .

S TSAI an d T.

W

EAGAR

Factors influencing the size and shape of the sensitization zone in 3 4 stainless steel w elds

have be en studied through a statistically designed ex perim ent. The results indicate that the

width of the sensitization zone is in proportion to the magnitude of the heat input except

when very little fusion occ urs. Th e minimum sensitization size can be obtained with

a

broadly distributed heat source traveling at high speed and the maximum size can be

obtained with low speed and a narrow heat distribution.

Under certain welding con dition s one can obtain welds which are free of sensitization

on the surface and hence this process m ay lead to a new method o preventing

sensitization-induced intergranular corrosion.

By

depositing surface sensitization-free

welds on either side of a previously sensitized w eld the chrom ium carbide precipitates

disso lve and the sensitization on the plate surface is eliminated.

INTRODUCTION

Sensitization-induced intergranular corrosion in 304 stain-

less steel weldments has bec ome a major problem in recent

years. Factors controlling the sensitization are alloy con tent,

carbon content, and weld thermal cycles. Among these, the

metallurgical factors are well known, * and a number of

methods of preventing intergranula r corrosion a re available:

Employing solution heat treatment

Lowering carbon content

Adding stabilizers

Employing internal cooling of pipes to produce fa-

vorable compressive residual stress on the surface

However, these methods are either costly or difficult to

W ly , particularly to weldments that have already been sen-

sitized. A practical and inexpensive method of reducing

sensitization of these existing welds is desirable. This paper

studies the ef fec ts of the welding process parameters on the

location and size of the sensitization zone and presents a

N

TSAI

and

T

W. EAGAR are

with

the Department of

Materials Science and Engineering, Massachuset ts Institute of

Technology, Cambridge,

M A

02139

new method of preventing sensitization-induced inter-

granular corrosion.

EXPERIMENT L PROCEDURE

A Box-Behnken experimental design was used with current,

arc length, and travel speed employed as the primary vari-

ables. Single-pass, bead-on-plate welds were made by gas

tungsten arc welding on 30-cm/30-cm /1.3-cm 304 stainless

steel plates using argon shielding gas. The chemical com-

position of the stainless steel is given in Table 1

The weld cross section was etched in 10 pet oxalic acid

(per ASTM-262 practice

A).

The location and the siz e of the

sensitization zone were measured under an optical micro-

scope at 200-times ma gnification, and the sensitization zone

is defined as the region in which more than 30 pet of the

grain boundary is attacked.

RESULTS

ND

DISCUSSIONS

Sensitization occurs in the region which experiences peak

temperatures of 650Â to 900 OC for a number of seconds

during the welding process. Two primary variables deter-

mining the location and width of the sensitization zone are

~~~~~~~~ FOR

ENER GY

SYSTEMS 1984 MERIC N

SOCIETY

FOR METALS

VOL..

6 NO I

J L N E

1984

33



Table I. Chemical Composition of the

3 4

Stainless Steel

peak

temperature distribution and the time spent between

these temperatures. Although there will always

be

a region

in the heat-affected zone of any weldment that reaches these

temperatures, it is possible to adjust the time at temperature

by appropriate choice of the welding parameters.

Th e experimental data of the location and the width of the

sensitization zone produced in this study are given in

Table 11. The sensitization width is in proportion to the heat

input per unit length, as shown in Figure

1,

except when

little or no melting occurs. It is seen that the width of the

sensitization zone can vary from nil to over o ne millimeter

at heat inputs of less than

100

J /mm, and hence, the area

of the sensitization can be very large at low heat inputs when

melting is minimized. This is because much of the thermal

energy

is

expended in heating the steel to the sensitization

range when melting is minimized.

A I n t h r o ug h plate t h i c k n e s s 4 i r i ~ L o n

I n t r a ns v e r se d i r e c t l c n

0 3 0 0 0 6 0 0 0 9 0 0 . 0 1 2 0 0 .0

Heat

input

CJ rn rn

5

Fig. 1 -Th e sensitization width is in proportion

to

the heat input p r unit

length. except when little or no melting occurs.

Table 11 Values of the Welding Process Variables and the Location and Size of the Sensitization Zone

Heat

Weld

AL

Speed Voltage Input

N iY 1\71

SY

5YW

S P

No.

AA

rnrn) rnrn/s) V ) W/mrn) mm ) mm ) rnrn) rnrn) rnrn)

34 VOL

6 N O

1.

J U N

1984

J

MATERIALS FOR

ENERGY SYSTEMS



No 3

290A

10mm

2mmI

s

No

4

142A

1

u

No

12

131A

Â§ u

9 m l

No 8

21 A

5mm

2mmI s

Fig. 2 Configurations of the sensitized heat affected zon s

MATERIALS FOR ENERGY SYSTEMS

P

X .

c :

I

No

1

2

90A

lOmm

15mm/



c )

d

Fig.

3

B y depositing high-speed, low-current welds surface-sensitization-freeweld s), the previously sensitized region can

be

eliminated.

a)

The relative

location of the surface-sensitization-freeweld,

with

respect to the previously sensitized weld;

b )

the region between the large weld and the small weld; c) the

sensitization to the left of the large weld, extending all the way up to the surface; d ) he sensitization. magnified.

Above

300

J/mm the width of the sensitized zone is

proportional to the heat input. It will be noted that the

sensitization width is about twice as large in the through-

plate thickness direction as in the transverse direction. This

is due to the finite thickness of the plate which effectively

reflects the heat from the bottom surface and raises the

temperature

in

the through-plate thickness direction.

In welding it is often assumed that the location of the

sensitization zone is determined solely by the peak tem-

perature distribution. This assumption suggests that the

cooling rates are approximately the same at either the inner

or the outer regions of the heat-affected zone. To a first

approximation this simplification describes the region of

peak temperatures between

650

to

950

OC which produce

sensitization. The study of Solomon3supports the peak tem-

perature as the most important variable influencing the de-

gree of sensitization.

Figure

2

shows the configurations of the sensitized heat-

affected zones. Those in the right column have been en-

larged four times. It is interesting to note that some welds

viz.

1

5 6 13 14 and 15 in Figure

2

have no sensi-

tization in the transverse direction. It seems that under the

conditions of high travel speed and low heat input one can

produce a surface-sensitization-freeweld. Under these con-

ditions the simplification that peak temperature alone

controls sensitization breaks down. Under these condi-

tions the time in the sensitization range is insufficient to

cause sensitization.

The discovery of such surface-sensitization-free welds

leads to an important possible application in preventing

sensitization-induced intergranular corrosion.

It

is possible

to deposit two surface-sensitization-freewelds on either side

of a previous weld at the location of the sensitized region.

This assumes a knowledge of the location of these sensitized

7 V I 6 . NO.

I

J l lNF 984 J . MATERIALS FOR ENERGY SYSTEMS



regions. Fortunately, several heat-flow model^^ ^ are avail-

able from which to predict the peak temperature distribu-

tions for a given set of welding parameters. The simplified

prediction of the sensitized region, based solely on peak

temperature distribution, can give a position of the sensi-

tized heat-affected zone which is sufficient for purposes of

locating these surface-sensitization-free weld beads.

I f a

high-spee d, low-heat-input pass is deposited near these sen-

sitized regions, it eliminates the sensitization, as demon-

strated in Figure 3.

The heat of the high-speed weld causes the dissolution of

the chromium-carbide precipitates. without producing suf-

ficient time for new surface sensitization to occur. The sen-

sitization in the bulk of the plate will not influence the

corrosion resistance, since it is not in contact with the corro-

sive environment. In some cases, this may be an inexpen-

sive and practical method for repair of field welds that have

been sensitized,

Figure 3 (a) indicates the relative location of the surface -

sensitization-free weld with respect to the previously sensi-

tized weld. Note that the small weld is not located exactly

at the sensitization zone of the large weld . Th e sensitization

to the left of the large weld extends all the way up to the

surface, w hile the sensitization on the right, w here the repair

weld has been m ade, does not. Figure 3(b ) shows the region

between the large and small weld at higher magnification. I t

is clear that the sensitization is eliminated near the surface.

Figure 3(c ) shows that the sensitization to the left of the

large weld extends all the way up to the surface. Figure 3( d)

shows the magnified sensitization region.

ACKNOWLEDGMENTS

The authors are grateful for support of this work by both the

U.S. Department of Energy (Basic Energy Sciences) under

contract DE-AC02-78ER04799 and the Office of Naval

Research under contract N00014-80-C-0384.

REFERENCES

I .

M

G . Fontana and N . D. Greene:

C or ro si on h g i n e e r i n ~ .

. 61

McGraw-Hill. 1978.

2 14

H. Uhlig: Corrosion ami Corro.\Iun C t~titrol, . 305 Wile). New

York. 1971.

3. H .

D. Solomon: Variables Influe nci n~Weld Sensitization

of

Austenitic

Stainless Steel.

Weldrnmls: P /~~,vIcaletallurgy-

and

F ai lu re P / I C I I O I ~ I

emi, Pro cee din p of the Fifth Bollon Landi118Conference.

1979.

p.

149.

4 .

N S. Tsai and T. W. Eagar: Temperature Field Produced by a Distrib-

uted Heat Source.

Weld.

J 1983. vol.

62. no

12. p.

346.

5 N . S. Tsai and T. W. Eagar: Change u Weld ool Shape

by

Variations

in the Distribution

of

Heat Source in Arc Welding,

Mo~lell i t i f ;

Cust

I I I R

iind W~ld111gr oces s e .~ .

enniker. New Hampshire, 1983.

1

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