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ORNL- TM- 3561
C o n t r a c t No. W-7405-eng- 26 METALS AND CERAMICS D I V I S I O N
REMOTE I N S P E C T I O N OF WELDED J O I N T S R. W. M c C l u n g This report was prepared as a n account of work sponsored by the United States Government. Neither the United States nor the IJnited States Atomic Energy Commission, nor any of ?heir empioyees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes an)legal liability or responsibility â‚Źor the accuracy, completeness or usefulness of any information, apparatus, product o r process disctosed, or represents that its use would not infringe privately owned rights.
SEPTEMBER 1971
OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37330 ope rated by
UNION CARBIDE CORPORATION for the
U.S. ATOMIC ENEBGY COMMISSION
iii
CONTENTS Page
............................ Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Environment and Other Parameters . . . . . . . . . . . . . . . . Customary Methods f o r Nondestructive Examination .of Welds . . . V i s u a l Examination . . . . . . . . . . . . . . . . . . . . Liquid P e n e t r a n t Examination . . . . . . . . . . . . . . . Radiographic Examination . . . . . . . . . . . . . . . . . Ultras o n i c Examinat i o n . . . . . . . . . . . . . . . . . . Advanced Methods f o r Nondestructive Examination of Welds . . . . Abstract
Eddy Current Examination
.................
Acoustic Emission Techniques
1
2
4 4 5 5
7 9
9
. . . . . . . . . . . . . . . 11
. . . . . . . . . . . . . . . 12 . . . . . . . . . . . . . . . . . . 12
Other Methods of Examination Summary and Recommendations
1
REMOTE INSPECTION OF WELDED JOINTS
R. W. McClung
ABSTRACT
Maintenance and r e p a i r of r e a c t o r components by remote welding w i l l r e q u i r e development of adequate n o n d e s t r u c t i v e examination techniques t o a s s u r e t h e i n t e g r i t y of t h e weld. High temperature and r a d i a t i o n l e v e l s w i l l r e q u i r e remote i n s p e c t i o n s and add t o t h e complexity o f t h e performance. Recommended developments i n c l u d e improved c l o s e d - c i r c u i t t e l e v i s i o n f o r v i s u a l examination, e v a l u a t i o n of new hight e m p e r a t u r e p e n e t r a n t s , s t u d y of n e u t r o n radiography, improved t r a n s d u c e r s f o r u l t r a s o n i c s , and a p p l i c a t i o n of a c o u s t i c emission and eddy c u r r e n t s . S o p h i s t i c a t e d mechani c a l p o s i t i o n i n g , manipulation, and scanning equipment w i l l be r e q u i r e d f o r most of t h e t e c h n i q u e s . INTRODUCTION
The long-range economic u t i l i z a t i o n of n u c l e a r r e a c t o r s f o r p r o d u c t i o n of e l e c t r i c a l power r e q u i r e s t h a t p r a c t i c a l economic maint e n a n c e and r e p a i r be p o s s i b l e .
The l a t t e r f u n c t i o n s become more
complicated t h a n t h o s e encountered i n o t h e r i n d u s t r y because of t h e h i g h t e m p e r a t u r e and r a d i a t i o n background a s s o c i a t e d w i t h some of t h e components.
However, d e s p i t e t h e d i f f i c u l t i e s , n e c e s s a r y maintenance
o p e r a t i o n s i n n u c l e a r r e a c t o r s have been conducted s u c c e s s f u l l y .
In
r e g i o n s of v e r y high r a d i a t i o n background, ingenious mechanical d e v i c e s have been developed t o allow simple r e p a i r s .
For example, t h e
Homogeneous Reactor T e s t (HRT) w a s a proving ground f o r a number of remote maintenance methods.
The Molten S a l t Reactor Experiment (MSRE)
'J. S. Culver, Viewing Equipment f o r Use i n t h e HRT Core and Blanket V e s s e l s , OWL-2886 (Mar. 1, 1960).
2B. D. Draper and E. C. Hise, Remote Maintenance Procedure Report, CF-59-11-128 (Nov. 26, 1959). 3P. P. Holz, Some Miscellaneous Maintenance Tools Used t o Manipulate Loose Objects i n t h e HRT Core, CF-59-11-123 (Nov. 17, 1959). 'P. P. Holz, A d d i t i o n a l Miscellaneous Maintenance Tools Used i n t h e HRT Core, CF-60-9-103 (Sept. 26, 1960).
L
a l s o demonstrated t h e f e a s i b i l i t y o f remote maintenance and r e p a i r of r a d i o a c t i v e components. mechanical (e.g.,
In both of t h e s e examples much of t h e work w a s
flanges, b o l t s , e t c . ).
Relatively l i t t l e of t h e p a s t
work has included s t u d y of remote welding techniques f o r r e p a i r or replacement of components.
Even less has been done on remote i n s p e c t i o n
techniques for welds made remotely.
However, d e s p i t e t h e p a u c i t y of
e f f o r t on i n s p e c t i o n of remotely made welds, work has been accomplished toward t h e remote a p p l i c a t i o n of i n s p e c t i o n t e c h n i q u e s , i n b o t h n u c l e a r reactorsh-'
and h o t c e l l s . l''
This r e p o r t d i s c u s s e s t h e p r e s e n t t e c h -
nology t h a t may be a p p l i c a b l e t o t h e remote examination o f welds made during maintenance o p e r a t i o n s for Molten S a l t Reactors (MSR).
Most of
t h e d i s c u s s i o n would b e e q u a l l y a p p l i c a b l e t o o t h e r r e a c t o r systems. ENVIRONMENT AND OTHER PARAMETERS''
The materials and environmental c o n d i t i o n s assumed a r e t h o s e assoc i a t e d w i t h an I G R and encountered i n e v a l u a t i o n o f a remotely made weld
'P. N. Haubenreich and J. R. Engel, "Experience w i t h t h e Molten8(2), 118-136 (1970). S a l t Reactor Experiment ,"Nucl. Appl. Technol. 'R. W. McClung, " S t a t e o f t h e A r t of Nondestructive Testing for S e r v i c e and Postoperation Examination of Reactor P r e s s u r e Vessels , I 1 pp. 1C5-167 i n I n c i p i e n t F a i l u r e Diagnosis for Assuring S a f e t y and A v a i l a b i l i t y of Nuclear Power P l a n t s (Conference Proceedings, Gatlinburg, Tennessee, October 3cC-November 1, 1967), COW-671011 (January 1968).
"R. D. Wylie, "Design f o r In-Service Inspection," pp. 220-227 i n I n c i p i e n t F a i l u r e Diagnosis f o r Assuring S a f e t y and A v a i l a b i l i t y of Nuclear Power P l a n t s (Conference Proceedings, Gatlinburg, Tennessee, October 3C-November 1, 1 a 6 7 ) , COW-671011 (January 1968). 8C. E. Lautzenheiser, "Requirements for Pre-Service and In-Service I n s p e c t i o n o f Nuclear Coolant Pressure Boundaries ,"pp. 2-10 i n Proceedings of S i x t h I n t e r n a t i o n a l Conference on Nondestructive T e s t i n g , June 1-5, 1379, Kannover, Germany, Vol. N, Dortmund, Germany, March 1970. 'R. W. McClung and K. V. Cook, Developmi f o r t h e R e m t e Measurement o f t h e HRT Core VI 'OR. W. McClung and D. A. Douglas, "Nondestructive T e s t i n g of I r r a d i a t e d Materials i n t h e United S t a t e s , " pp. 17S202 i n High A c t i v i t y Hot Laboratories Working Fiethods, Vol. 1, Proceedings of an I n t e r n a t i o n a l Symposium Organized by ENEA and EURATOM, M c G r a w - H i l l , New York, 1965.
"Based on R. B. Briggs, Oak Ridge N a t i o n a l Laboratory, p e r s o n a l communication, October 1970.
3
(e.g.
, after
removal and replacement of a major component, such as a
h e a t exchanger).
The primary metal for c o n s i d e r a t i o n i s H a s t e l l o y N,
and a t y p e 300 s e r i e s s t a i n l e s s s t e e l i s considered a p o t e n t i a l a l t e r nate.
The welds a r e predominantly c y l i n d r i c a l i n pipes having diameters
from 1 t o 20 in. and w a l l t h i c k n e s s e s up t o 1 i n .
Larger c y l i n d e r s
i n c l u d e t h e h e a t exchanger and t h e core v e s s e l , w i t h diameters of approximately 5 and 30 f t , r e s p e c t i v e l y . A n t i c i p a t e d temperatures i n t h e r e a c t o r c e l l range from 1000 t o 1200째F.
However, l o c a l i z e d cooling f o r both welding and i n s p e c t i o n can
probably b r i n g temperatures down t o t h e range 200 t o 600째F and p o s s i b l y even t o 200 t o 400째F. The a n t i c i p a t e d l e v e l o f r a d i a t i o n ' * i n t h e r e a c t o r c e l l t e n days a f t e r t h e system i s shut dawn and d r a i n e d i s expected t o be approximately
lo5
R/hr.
The dominant r a d i a t i o n w i l l be gamma r a y s from r e l a t i v e l y
noble f i s s i o n products d e p o s i t e d on t h e metal s u r f a c e s of t h e h e a t exchanger t u b e s and on t h e g r a p h i t e i n t h e core v e s s e l . h i g h e s t dose r a t e ( c a l c u l a t e d t o be 1 . 4 x
lo5
The a r e a of
R/hr) i s at t h e midplane
immediately a d j a c e n t t o a h e a t exchanger.
Values i n o t h e r p o r t i o n s of
t h e c e l l may be 25 t o 30% of t h e maximum.
Most of t h e r a d i a t i o n w i l l
have photon e n e r g i e s of 0 . 8 MeV and below. Care must be e x e r c i s e d during t h e examination t o avoid t h e i n t r o d u c t i o n of m a t e r i a l s t h a t could be d e t r i m e n t a l t o t h e continued s e r v i c e a b i l i t y of t h e components.
For example, no s u l f u r - b e a r i n g o i l s and no
aluminum or low-melting a l l o y s should be i n t r o d u c e d f o r c o n t a c t w i t h H a s t e l l o y N because t h e y may r e a c t t o cause l o s s of d e s i r e d p r o p e r t i e s . S i m i l a r l y , f o r s t a i n l e s s s t e e l s , c h l o r i n e - f r e e m a t e r i a l s must be used. Because of t h e h i g h r a d i a t i o n and temperature l e v e l s i n t h e r e a c t o r c e l l , p e r s o n n e l a c c e s s i s obviously forbidden.
Therefore, mechanical
systems would have t o be designed and b u i l t t o allow remote performance o f t h e examination.
Some a t t e n d a n t r e s t r i c t i o n s on t h e mechanical
equipment i n c l u d e t h e f a c t t h a t examination equipment must be i n s e r t e d 12J. R. Tallackson, p r i v a t e communication t o R. W. McClung, Jan. 29, 1971.
4 i n t o t h e r e a c t o r c e l l from above through a c c e s s p o r t s (perhaps 1 f t i n diameter) and be manipulated over d i s t a n c e s up t o ?O o r 40 f t (not n e c e s s a r i l y immediately below an access p o r t ) .
In a d d i t i o n , because of
l i m i t e d space w i t h i n t h e c e l l , t h e examination equipment should not extend outward more t h a n 1 f t from t h e component being examined.
CUSTOMARY METHODS FOR NONDESTRUCTIVE EXAMINATION OF WELDS Under normal circumstances s e v e r a l methods of n o n d e s t r u c t i v e examination would be a p p l i c a b l e t o t h e examination of welds i n H a s t e l l o y N or stainless steel.
penetrants
These i n c l u d e v i s u a l examination, use of l i q u i d
, radiography,
arid u l t r a s o n i c methods.
of t h e b a s i c methods i s intended i n t h i s r e p o r t .
No d e t a i l e d d i s c u s s i o n Descriptions w i l l be
l i m i t e d t o t h o s e a s p e c t s p e r t i n e n t t o remote o p e r a t i o n w i t h i n t h e ‘ r e s t r i c t i o n s t h a t were p r e v i o u s l y mentioned.
Those w i s h i n g more infor-
mation about t h e methods a r e r e f e r r e d t o t h e Nondestructive Testing Handbook’?
and Welding I n s p e c t i o n .
l4
V i s u a l Examination Many f e a t u r e s about t h e q u a l i t y of a w e l d can be noted by v i s u a l examination.
.Among t h e c o n d i t i o n s t h a t may be observed a r e s u r f a c e
cracks, undercut, o v e r l a p , w e l d i r r e g u l a r i t y , and dimensional inaccuracies.
However, t h e s u r f a c e appearance a l o n e i s inadequate t o prove
c a r e f u l workmanship o r good i n t e r n a l q u a l i t y .
The v i s u a l examination
should be made b e f o r e , during, and a f t e r t h e welding and be f u r t h e r supplemented w i t h o t h e r methods of n o n d e s t r u c t i v e examination. I n t h e s t r i n g e n t environment being discussed, such v i s u a l examinat i o n must be done w i t h o p t i c a l a i d s , such as c l o s e d - c i r c u i t t e l e v i s i o n ’?American S o c i e t y f o r Nondestructive T e s t i n g , Nondestructive 2 , ed. by Robert C. McMaster, Ronald Press , New York, T e s t i n g Handbook 1959. I4AWS Committee on Methods of I n s p e c t i o n , Welding I n s p e c t i o n , American Welding S o c i e t y , New York, 1968.
1
5 (CCTV).
The CCTV has been s u c c e s s f u l l y a p p l i e d i n r a d i o a c t i v e environs
f o r surveying r e a c t o r i n t e r n a l s and monitoring o p e r a t i o n s i n h o t c e l l s .
157
I"
Care must be taken t o minimize t h e e f f e c t s of h i g h temperature and r a d i a t i o n on t h e components of t h e camera t o avoid excessive image degradation and f a i l u r e . Liquid Penetrant Examination Liquid p e n e t r a n t s c o n t a i n i n g v i s i b l e or f l u o r e s c e n t dyes a r e commonly used t o d e t e c t t h e presence of surface-connected f l a w s i n welds. However, t h e commonly used m a t e r i a l s are not a p p l i c a b l e a t t h e h i g h temperature expected i n t h e r e a c t o r c e l l during t h e examination.
A
r e c e n t experimental product17 shows promise of b e i n g a p p l i c a b l e f o r p e n e t r a n t examination a t temperatures as h i g h as 600 t o 000째F. A t room temperature t h e p e n e t r a n t , remover, and developer are i n crayon form. The high-temperature p e n e t r a n t w a s developed f o r use on multipass welds. No information i s a v a i l a b l e on t h e s e n s i t i v i t y t o cracks o r o t h e r d i s -
continuities.
Use of p e n e t r a n t techniques i n t h e remote environment
would r e q u i r e t h e development of procedures f o r performance and examfnat i o n of r e s u l t s .
The CCTV p r e v i o u s l y d i s c u s s e d could probably be used
t o advantage. Radiographic Examinat i o n Radiography i s a primary requirement f o r t h e examination and acceptance of welds.
However, t h e excessive r a d i a t i o n l e v e l s and h i g h
temperatures t o b e encountered i n t h e r e a c t o r c e l l raise genuine doubts
"J. P. P e l e , G. Hoyaux, and J. M. Baugnet, "Underwater Nondestructive T e s t i n g a t t h e BR2 Reactor," pp. 203-219 i n High A c t i v i t y Hot Laboratories Working Methods, Vol. 1, Proceedings of an I n t e r n a t i o n a l Symposium Organized b y ENEA and EURATOM, M c G r a w - H i l l , New York, 1965.
%. K. A. Walrave , "Equipment f o r Nondestructive Examinat i o n , 'I pp. 221-228 i n High Activity- Hot Laboratories Working Methods, Vol. 1, Proceedings o f an I n t e r n a t i o n a l Symposium Organized b y ENEA and EURATOM, M c G r a w - H i l l , New York, 1965. '7H. G. Bogart, Magnaflux Corporation, p e r s o n a l communication, January 1971.
G
about t h e p o s s i b i l i t y of v a l i d radiography f o r t h e examination o f t h e remotely made welds.
Radiography has been demonstrated and used on
h i g h l y r a d i o a c t i v e o b j e c t s i n s p e c i a l f a c i l i t i e s ( e . g . , h o t c e l l s ) and w i t h s p e c i a l t e c h n i q u e s , b u t i n every case s i g n i f i c a n t e f f o r t w a s taken t o minimize t h e exposure of t h e f i l m t o t h e background r a d i a t i o n .
'
8-20
The f i l m i s u s u a l l y placed f o r a v e r y s h o r t t i m e i n a f i x e d p o s i t i o n and
massive s h i e l d i n g prevents most of t h e background r a d i a t i o n from g i v i n g an overexposure o f fog.
S i m i l a r s t e p s f o r radiography on any of s e v e r a l
c l o s e l y a s s o c i a t e d r a d i o a c t i v e components i n a r e a c t o r becomes s o d i f f i c u l t as t o be i m p r a c t i c a l . The above d i s c u s s i o n and extreme pessimism a p p l i e s t o x- or gammaradiography s i n c e t h e customary x-ray f i l m i s s e n s i t i v e t o t h e background radiation.
A p o t e n t i a l a l t e r n a t i v e method o f radiography u s e s neutrons
as t h e p e n e t r a t i n g r a d i a t i o n . 2 1
Some of t h e primary d e t e c t i o n methods
f o r neutron radiography ( e . g . , f o i l s of indium or gadolinium, t r a c k - e t c h p l a t e s such as mica or c e l l u l o s e n i t r a t e , and n e u t r o n - s e n s i t i v e image i n t e n s i f i e r s ) a r e not s e n s i t i v e t o x- and gamma-rays and t h u s allow radiography o f h i g h l y r a d i o a c t i v e specimens ( e . g . , 31"
i r r a d i a t i o n capsules
used f u e l elements from n u c l e a r r e a c t o r s ) or radiography of nonradio-
a c t i v e specimens i n t h e presence of h i g h background r a d i a t i o n .
For
example, u s e f u l neutron radiography i s commonly performed u s i n g a neutron beam accompanied b y high i n t e n s i t y gamma r a d i a t i o n from a n u c l e a r r e a c t o r . A t t h e p r e s e n t s t a g e of development of b o t h techniques and equipment,
t h e s e n s i t i v i t y and r e s o l u t i o n of neutron radiography f o r metals ( i . e . , welds) i s l e s s t h a n i s expected of x-radiography.
Another problem i s
t h e l i m i t e d choice and c a p a b i l i t y of p o r t a b l e sources f o r neutrons. "R. W. McClung and K. V. Cook, F e a s i b i l i t y S t u d i e s f o r t h e Nondes t r u c t i v e T e s t i n g of t h e EGCR Through-Tube Weldment, ORNLTM-46 (Nov. 14, 1961). I9R. W. McClung, "Radiography i n t h e Presence of Background 23 (1), 4 1 4 5 (January 1965). Radiation," Mater. Evaluation -
"High A c t i v i t y Hot Laboratories Working Methods, Proceedings o f an I n t e r n a t i o n a l Symposium Organized by ENEA and EURATOM, Grenoble, France, June 1965, n o t e p a r t i c u l a r l y papers on pp. 25-89 and 1 3 5 1 9 7 of Vol. 1.
21H. Berger , Neutron Radiography, E l s e v i e r Publishing Company, Amsterdam, 1965.
7 Despite t h e problems, l i m i t e d work has been done on welds, and w i t h advances i n b o t h t h e technology of neutron radiography and a v a i l a b i l i t y of improved sources ( e . g . ,
2 5 2 C f ) , b e t t e r performance c a p a b i l i t i e s f o r
t h e neutron radiography of welds should be expected.
Extensive develop-
ment would be r e q u i r e d f o r equipment t o p l a c e both source and d e t e c t o r i n proper p o s i t i o n n e a r t h e weld t o be examined i n t h e r e a c t o r c e l l . U l t r a s o n i c Examination U l t r a s o n i c methods of n o n d e s t r u c t i v e examination of welds have, i n r e c e n t y e a r s , been used i n c r e a s i n g l y .
There are probably s e v e r a l r e a s o n s ,
i n c l u d i n g (1) s i g n i f i c a n t advances i n c a p a b i l i t y of b o t h equipment and personnel; ( 2 ) i n c r e a s e d awareness of t h e a b i l i t y of u l t r a s o u n d t o d e t e c t d i s c o n t i n u i t i e s such as cracks and l a c k of f u s i o n , which are d i f f i c u l t t o d e t e c t r a d i o g r a p h i c a l l y ; and (3) d e s i r e f o r t h e u l t i m a t e i n confidence i n c r i t i c a l components (as a r e s u l t many complementary examination methods a r e used).
Many of t h e mechanical scanning systems used for u l t r a s o n i c
examination of materials l e n d confidence t o t h e a b i l i t y t o manipulate probes remotely.
The mechanical c a p a b i l i t y coupled w i t h t h e minimal
e f f e c t of r a d i a t i o n on u l t r a s o n i c t r a n s d u c e r s has l e d t o i t s u t i l i z a t i o n on many occasions f o r t h e examination of r a d i o a c t i v e components i n both
react o r s
2-24
and h o t c e l l s .
22R. D. Wylie, "Design for In-Service I n s p e c t i o n , " pp. 22C-227 i n I n c i p i e n t F a i l u r e Diagnosis f o r Assuring S a f e t y and A v a i l a b i l i t y of Nuclear Power P l a n t s (Conference Proceedings, Gatlinburg, Tennessee, October 30-Noveniber 1, 1967), COJ!P-671011 (January 1968). 23C. E. Lautzenheiser, "Requirements f o r Pre-Service and In-Service I n s p e c t i o n of Nuclear Coolant P r e s s u r e Boundaries," pp. 2-10 i n Troceedings of S i x t h I n t e r n a t i o n a l Conference on Nondestructive T e s t i n g , June 1-5, 1970, Hannover, Germany, Vol. N, Dortmund, Germany, %rch 1970. 24R. W. McClung and K. V. Cook, Development of U l t r a s o n i c Techniques f o r t h e Remote Measurement of t h e HRT Core Vessel W a l l Thickness, ORNL-TM-103 (Mar. 15, 1962). 25R. W. McClung and D. A. Douglas, "Nondestructive T e s t i n g of Irradiated Materials i n t h e United S t a t e s , " pp. 179-202 i n High A c t i v i t y Hot Laboratories Working Methods, Vol. 1, Proceedings of an I n t e r n a t i o n a l Symposium Organized by ENEA and EURATOM, McGraw-Hill, New York, 1965.
8 However, s e v e r a l problems must be r e s o l v e d t o allow use of u l t r a s o n i c s f o r examination o f MSR welds.
Welds have been s u c c e s s f u l l y
i n s p e c t e d on many materials, i n c l u d i n g carbon s t e e l s and aluminum. However, d i f f i c u l t y has been encountered i n t h e a p p l i c a t i o n of u l t r a sound t o welds i n s t a i n l e s s s t e e l and nickel-base a l l o y s .
A large
v a r i a b l e a t t e n u a t i o n o f t h e sound and t h e presence of r e f l e c t i o n s from g r a i n boundaries i n t e r f e r e s w i t h c a l i b r a t i o n and u l t r a s o n i c examination on such materials.
Laboratory s t u d i e s should be undertaken t o determine
t h e cause of i n s p e c t i o n d i f f i c u l t y and t o devise techniques and/or equipment modifications necessary t o overcome t h e problems, allowing highconfidence i n s p e c t i o n . Materials t o couple u l t r a s o u n d from t h e t r a n s d u c e r i n t o t h e weldment w i l l have t o be s e l e c t e d on t h e b a s i s o f s u s c e p t i b i l i t y t o r e a c t o r contamination o r a c t i v a t i o n , remote a p p l i c a b i l i t y , and adequate u l t r a s o u n d t r a n s m i s s i o n a t t h e expected temperatures.
A p o t e n t i a l a l t e r n a t e solu-
t i o n t o t h e problem of coupling t h e sound i s t h e u s e of e l e c t r o m a g n e t i c i n d u c t i o n t r a n s d u c e r s not r e q u i r i n g a l i q u i d coupling.
2 i 7 2'1
Recent work
has shown t h a t u s e f u l u l t r a s o n i c v i b r a t i o n s can be g e n e r a t e d i n specimens by electromagnetic t e c h n i q u e s , b u t t h e s e n s i t i v i t y i s s i g n i f i c a n t l y below
t h a t p o s s i b l e w i t h conventional p i e z o e l e c t r i c devices.
F u r t h e r work i s
needed t o improve t h e o p e r a t i o n of t h e s e new t r a n s d u c e r s b e f o r e t h i s method could be considered a p p l i c a b l e f o r t h e c i t e d i n s p e c t i o n of welds. Kechanical scanning devices w i l l be r e q u i r e d t o move t h e t r a n s d u c e r I n a preprogrammed p l a n over t h e weldment. t r a n s d u c e r may be a s s u r e d by CCTV.
Accurate p o s i t i o n i n g of t h e
It may be p o s s i b l e t o design and
b u i l d a c c e s s o r i e s t h a t can be a t t a c h e d t o t h e mechanical system t o be used f o r t h e performance of t h e remote welding. "H. Wuestenberg , "BerEhrungslos e elektrodynamis che Ultras challwandler und i h r e Verwendung i n der Ultraschallpriifung," pp. 3 7 4 8 i n Proceedings o f t h e S i x t h I n t e r n a t i o n a l Conference on Nondestructive Testing, June 1-5, 1970, Hannover, Germany, Vol. B, Dortmund, Germany, March 1970.
27E. R. Dobbs and 2. D. L l e w e l l w , "Generation of U l t r a s o n i c Waves Without Using a Transducer , I ' NondestEuctive T e s t i n g ( B r i t i s h ) 4(l),4+56 (l(T1).
9
W
ADVANCED METHODS FOR NONDESTRUCTIVE EXAMINATION OF WELDS
Because of t h e d i f f i c u l t i e s a n t i c i p a t e d i n applying t h e common methods f o r n o n d e s t r u c t i v e i n s p e c t i o n of welds, it may be d e s i r a b l e , or even necessary, t o develop or improve c e r t a i n advanced methods of nondes t r u c t i v e examination t o provide assurance of weld i n t e g r i t y .
Among t h e
methods t h a t o f f e r promise a r e eddy c u r r e n t s and a c o u s t i c emission. Eddy Current Examination Methods using electromagnetic i n d u c t i o n of eddy c u r r e n t s have been used f o r d e t e c t i n g flaws and measuring e l e c t r i c a l c o n d u c t i v i t y , permeabili t y , t h i c k n e s s , and t h e space between a d j a c e n t components.
Application
t o welds has been s p a r s e , i n p a r t because t h e e f f e c t i v e depth of p e n e t r a t i o n of t h e eddy c u r r e n t s i s u s u a l l y small ( t y p i c a l l y l e s s t h a n 0.1 i n . ) , and a l s o because unwanted s i g n a l s due t o v a r i a t i o n s i n coil-to-specimen spacing ( l i f t - o f f ) a r e d i f f i c u l t t o e l i m i n a t e .
The l a t t e r would be s i g -
n i f i c a n t l y a f f e c t e d by roughness o f t h e weld bead.
Recent advances i n
eddy-current t h e o r y and modeling have l e d t o optimum design of c o i l s and systems t h a t now allow performance s i g n i f i c a n t l y s u p e r i o r t o any p r e viously attainable.
28-3"
Advanced i n s t r u m e n t a t i o n has reduced t h e
v a r i a t i o n s i n s i g n a l due t o l i f t - o f f t o a n e g l i g i b l e q u a n t i t y .
31-32
Therefore, t h e eddy-current method deserves new c o n s i d e r a t i o n f o r t h e examination of s e v e r a l a s p e c t s of remotely made welds. 28C. V. Dodd and W. E. Deeds, " A n a l y t i c a l S o l u t i o n s t o Eddy-Current Probe C o i l Problems," J. Appl. Phys. 3 9 ( 6 ) , 2829-2838 (1968). -
-
29C. V. Dodd, W. E. Deeds, J. W. Luquire, and W. G. Spoeri, Some Eddy-Current Problems and Their I n t e g r a l S o l u t i o n s , ORNL-4384 ( A p r i l 1969). 3 0 C . V. Dodd, W. E. Deeds, J. W. Luquire, and W. G. Spoeri, "Analysis of Eddy-Current Problems w i t h a Time-sharing Computer, Mater. Evaluation 27(7), 1 6 5 1 6 8 (1969). 31C. V. Dodd, Design and Construction of Eddy-Current Coolant-Channel Spacing Probes, ORNL-3580 ( A p r i l 1964).
2C. V. Dodd, "A P o r t a b l e Phase-Sensitive Eddy Current Instrument , ' I Mater. Evaluation 26(3), 33-36 (1968). -
10
1.
Eddy-current techniques t h a t w e r e developed f o r t h e measurement
of space between components 3 3 7 " (e. g.
, coolant
channel s p a c i n g between
n u c l e a r f u e l p l a t e s ) could be u s e f u l f o r measuring t h e s e p a r a t i o n of s u r f a c e s t o be j o i n e d by welding.
2. Although improvements have been made t o t h e eddy-current method,
it i s s t i l l l i m i t e d i n i t s depth of p e n e t r a t i o n (even though g r e a t e r depths can now be i n s p e c t e d t h a n b e f o r e ) .
However, because of t h e
q u e s t i o n s concerning t h e a p p l i c a b i l i t y and b e n e f i t o f l i q u i d p e n e t r a n t and v i s u a l techniques, eddy c u r r e n t s could be considered as a replacement f o r t h e d e t e c t i o n of s u r f a c e o r near-surface d e f e c t s .
3.
'*
Welds i n a u s t e n i t i c s t a i n l e s s s t e e l s may c r a c k u n l e s s t h e
d e p o s i t c o n t a i n s 4 t o 9$ f e r r i t e .
An eddy-current method can measure
t h e amount and d i s t r i b u t i o n of f e r r i t e i n t h e weld (as w e l l as o t h e r c o n d i t i o n s l e a d i n g t o v a r i a t i o n s i n e l e c t r i c a l or magnetic p r o p e r t i e s ) and determine t h e tendency f o r c r a c k formation. The eddy-current technique has been used i n areas of h i g h background radiat i o n 3
9
-'
and a t t h e expected temperatures.
With proper design of
t h e t e s t system, t h e s e environmental c o n d i t i o n s do not a d v e r s e l y a f f e c t t h e performance.
A s d i s c u s s e d under U l t r a s o n i c Examination, t h e same
mechanism used f o r welding could probably be used as t h e b a s i c scanner f o r eddy c u r r e n t s . 33C. V. Dodd and R. W. McClung, "Fuel Element Coolant and Other Spacing Measurements," Trans. Am. Nucl. SOC. 4(l),136 (1961). 34C. V. Dodd, "Design and Construction of Eddy-Current Coolant18(6), Channel Spacing Probes," Microtecnic (Lausanne) 1 8 ( 5 ) , 286-289 and -
369-371 (1964).
35C. V. Dodd, W. E. Deeds, and W. G. S p o e r i , "Optimizing Defect Detection i n Eddy-Current Testing," pp. 35-46 i n Proceedings o f t h e S i x t h I n t e r n a t i o n a l Conference on Nondestructive Testing, June 1-5, 1970, Hmnover, Germany, Vol. C , Dortmund, Germany, March 19'70.
"-R. W. McClung and D. A. Douglas, "Nondestructive T e s t i n g of I r r a d i a t e d Materials i n t h e United S t a t e s , " pp. 1?+202 i n High A c t i v i t y Hot Laboratories Working Methods, Vol. 1, Proceedings o f an I n t e r n a t i o n a l Symposium Organized by ENEA and EURATOM, McGraw-Hill, New York, 1965. 3r7C. V.
Dodd, unpublished work, 1971.
11 Acoustic Emission Techniques A l l of t h e p r e v i o u s l y discussed techniques a r e a p p l i e d t o determine
t h e a s - f a b r i c a t e d c o n d i t i o n of t h e weld only a f t e r t h e welding o p e r a t i o n
i s completed.
Each i s an a c t i v e technique i n t h a t energy o r m a t t e r i s
a p p l i e d t o t h e area t o be examined ( e . g . ,
s o n i c v i b r a t i o n s , x rays,
p e n e t r a n t s ) , and changes i n t h e energy or matter are i n t e r p r e t e d i n terms o f t h e specimen.
The a c o u s t i c emission method i s a p a s s i v e t e c h -
nique i n which d e t e c t o r s a r e used t o l i s t e n f o r energy produced w i t h i n t h e specimen as it undergoes a change (e.g., c r a c k pop-in or p r o p a g a t i o n ) .
s t r e s s waves r e l e a s e d a t
Extensive development on a c o u s t i c emission
has been performed r e c e n t l y a t s e v e r a l s i t e s . 38-40
Included has been t h e
r e a l - t i m e monitoring of simple weldments b o t h d u r i n g and a f t e r t h e a c t u a l welding process. 4 1
Preliminary r e s u l t s on purposely cracked welds have
shown c o r r e l a t i o n between a c o u s t i c emission and cracking i n t h e welds. F u r t h e r s t u d y i s needed t o determine i t s degree of a p p l i c a b i l i t y t o t h e m a t e r i a l s and c o n f i g u r a t i o n s being considered.
No mechanical scanning
would be r e q u i r e d f o r a c o u s t i c emission s i n c e f i x e d t r a n s d u c e r s are s u f f i c i e n t t o d e t e c t t h e e m i t t e d energy.
'D. L. Parry, "Nondestructive Flaw Evaluation i n Nuclear Power I n s t a l l a t i o n s , I ' pp. 107-126 i n I n c i p i e n t F a i l u r e Diagnosis f o r Assuring S a f e t y and A v a i l a b i l i t y of Nuclear Power P l a n t s (Conference Proceedings, Gatlinburg , Tennessee, October 30-November 1, 1967), CONF-671011 (January 196$). 3qA. T. Green and C. E. Hartbower, "Stress-Wave 4nalysis Techniques for Detection of I n c i p i e n t F a i l u r e , " pp. 127-162 i n I n c i p i e n t F a i l u r e Diagnosis f o r Assuring S a f e t y and A v a i l a b i l i t y o f Nuclear Power P l a n t s (Conference Proceedings, Gatlinburg, Tennessee, October 30-November 1, 1967), COW-671011 (January 1968).
"E. N. Pedersen and J. C. Spanner, "Detection, Location and C h a r a c t e r i z a t i o n of Flaw Growth i n Metals Using Acoustic Emission Methods , I f pp. 163-164 i n I n c i p i e n t F a i l u r e Diagnosis for-Assuring S a f e t y and Availa b i l i t y of Nuclear Power P l a n t s (Conference Proceedings, Gatlinburg , Tennessee, October 30-November 1, 1967), COW-671011 (January 1968).
"W. D. J o l l y , "The Application of Acoustic Emission t o In-Process I n s p e c t i o n of Welds," Maker. Evaluation 28(6), 135139, 144 (June 1970). -
12 The t r a n s d u c e r m a t e r i a l s (not n e c e s s a r i l y t h e d e s i g n ) would probably be s i m i l a r t o t h o s e used f o r a c t i v e u l t r a s o n i c techniques and could t o l e r a t e t h e s h o r t - t e r m exposure t o t h e environmental temperature and radiation.
The coupling o f t h e t r a n s d u c e r s t o t h e weldment would b e
simpler t h a n f o r t h e a c t i v e u l t r a s o n i c technique s i n c e movement of t h e t r a n s d u c e r during t h e examination would not be r e q u i r e d .
If s i g n a l s of
a c o u s t i c emission a r e d e t e c t e d , supplementary examinations would be neces-
sary t o p i n p o i n t t h e l o c a t i o n of t h e f l a w . Other Methods of Examination I n a d d i t i o n t o t h e methods mentioned above on which some experience e x i s t s f o r examination of welds, o t h e r new methods of n o n d e s t r u c t i v e examination should be considered, although near-term a p p l i c a b i l i t y may not be obvious.
Among t h e s e a r e thermal techniques and holography.
Development programs should be continuously surveyed, and, i f promising p o s s i b i l i t i e s occur, f e a s i b i l i t y f o r a p p l i c a t i o n t o welds should be studied. S L W Y AND RECOMMENDATIONS
Yondestructively i n s p e c t i n g a remotely made weld a t h i g h temperatures and i n a high r a d i a t i o n background i s a d i f f i c u l t problem b u t one f o r which worthwhile s o l u t i o n s can be Dbtained.
The a n t i c i p a t e d environment
includes temperatures ( a f t e r c o o l i n g ) i n t h e range 200 t o 600째F and r a . d i a t i o n l e v e l s ( t e n days a f t e r d r a i n i n g ) i n excess of lo5 R/hr. S o p h i s t i c a t e d mechanical devices w i l l be r e q u i r e d t o move t h e i n s p e c t i o n probes and devices remotely i n t o p o s i t i o n a t t h e weld t o be examined, and mechanical scanning c a p a b i l i t y of t h e weld a r e a must b e provided f o r s e v e r a l of t h e methods.
Comments on t h e following methods b r i e f l y sum-
marize recommendations t o achieve a working system of n o n d e s t r u c t i v e i n s p e c t ion. 1.
C l o s e d - c i r c u i t t e l e v i s i o n i s recommended f o r v i s u a l examination
of t h e weld, b e f o r e , during, and a f t e r f a b r i c a t i o n and t o provide v i s u a l monitoring of t h e performance of o t h e r examination methods.
Specially
designed equipment w i l l be r e q u i r e d t o a s s u r e proper performance i n t h e environment. 2.
Conventional p e n e t r a n t techniques a r e i n a p p l i c a b l e .
Advanced
high-temperature p e n e t r a n t techniques should be s t u d i e d and procedures developed i f f e a s i b i l i t y i s demonstrated. 3.
X- or gamma-ray radiography i s probably i m p r a c t i c a l because of
t h e extremely h i g h i n t e n s i t i e s of ambient r a d i a t i o n a n t i c i p a t e d .
Advanced
n e u t r o n r a d i o g r a p h i c techniques should be i n v e s t i g a t e d . 4.
U l t r a s o n i c techniques can be performed remotely i n t h e environ-
Work i s needed t o improve t h e c a p a b i l i t y of u l t r a s o n i c s t o perform
ment.
c a l i b r a t e d , meaningful i n s p e c t i o n s on welds i n s t a i n l e s s s t e e l and n i c k e l base a l l o y s .
Techniques must be developed f o r remotely coupling t h e
sound f r o m t h e t r a n s d u c e r i n t o t h e specimen.
Advanced s t u d i e s f o r
e l e c t r o m a g n e t i c i n d u c t i o n of u l t r a s o u n d (which would n o t r e q u i r e c o u p l i n g ) should be made t o improve s e n s i t i v i t y . 5.
Eddy-current techniques should b e developed f o r measurement of
j o i n t f i t - u p , d e t e c t i o n of n e a r - s u r f a c e f l a w s , and measurement of v a r i a t i o n s i n t h e m i c r o s t r u c t u r e of t h e welds as i n d i c a t e d b y v a r i a t i o n s i n t h e e l e c t r i c a l and magnetic p r o p e r t i e s . 6.
Acoustic emission t e c h n i q u e s should b e pursued f o r r e a l - t i m e
monitoring of t h e weld i n t e g r i t y through d e t e c t i o n of t h e s t r e s s waves e m i t t e d by t h e formation or propagation of cracks.
7.
Other more s o p h i s t i c a t e d examination methods should be considered
f o r a p p l i c a t i o n t o t h i s d i f f i c u l t problem.
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