Inspecting the quality of weld joints produced by diffusion welding

INSPECTING THE QUALITY OF WELD JOINTS

PRODUCED BY DIFFUSION WELDING

A. A. Kotel'nikov, A. V. Bashurin,

and T. I. Boreb'eva

UDC 620.138.6

In c o n t r a s t to fus ion we ld ing , w h e r e t h e r e a r e m a c r o s c o p i c d e f e c t s ( c r a c k s , v o i d s , f o r e i g n i nc lu - s i ons ) , in d i f fu s ion we ld ing t h e r e a r e d e f e c t s of s i g n i f i c a n t l y s m a l l e r s i z e [1]. As a r e s u l t the u s e of such m e t hods of i n s p e c t i o n as x - r a y , m a g n e t o g r a p h i c , and u l t r a s o n i c a r e v e r y d i f f i cu l t .

T h i s a r t i c l e c o n s i d e r s the i n s p e c t i o n of weld jo in t s by m e a s u r i n g the e l e c t r i c a l r e s i s t a n c e and with t he u s e of m e c h a n i c a l h y s t e r e s i s .

In p a s s i n g an e l e c t r i c c u r r e n t t h r o u g h a we lded s a m p l e with d e f e c t s in the f o r m of oxide i n c l u s i o n s , m i c r o v o i d s , and m i c r o a r e a s of p o o r p e n e t r a t i o n the d r o p in vo l t a ge in the a r e a ad jo in ing the ioint is s i g - n i f i c an t l y g r e a t e r than in the b a s e m e t a l b e c a u s e of the much h i g h e r e l e c t r i c a l r e s i s t a n c e of the weld zone . The e l e c t r i c a l r e s i s t a n c e is d e t e r m i n e d by the i nd iv idua l p r o p e r t i e s of the d e f e c t ( form, d i m e n s i o n s ) and b y t h e i r c o n c e n t r a t i o n and i n c r e a s e s a s a r e s u l t of the d i s s i p a t i o n of e l e c t r o n s in the i n t e r i o r void and the e l a s t i c f i e lds of the d e f e c t s [2].

The e l e c t r i c a l r e s i s t a n c e of we lded s a m p l e s and b a s e m e t a l was m e a s u r e d with a type R348 p r e c i - s i o n p o t e n t i o m e t e r by the f o u r - c o n t a c t method , which m a k e s i t p o s s i b l e to d i s r e g a r d the r e f i n e m e n t of the c o n - t a c t b e t w e e n the s a m p l e and the m e a s u r i n g p r o b e fo r the va lue of the e l e c t r i c a l r e s i s t a n c e be ing m e a s u r e d . Holes with a d i a m e t e r of 1 m m and a depth of 4 m m w e r e d r i l l e d in the s a m p l e s at a d i s t a n c e of 15 m m f r o m the jo in t . S tee l p r o b e - e l e c t r o d e s w e r e p l a c e d in the ho le s and ca lked in. A r e c t i f i e d and s t a b i l i z e d c u r r e n t with a d e n s i t y of 5-6 A / c m 2 was p a s s e d t h r o u g h the s a m p l e s . The e l e c t r i c a l r e s i s t a n c e of the w e l d e d s a m p l e was c o m p a r e d with the e l e c t r i c a l r e s i s t a n c e of a sound one and the jo int was c o n s i d e r e d good i f the e l e c t r i c a l r e s i s t a n c e in the a r e a of the weld zone was equa l to tha t of the b a s e m e t a l .

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Fig. 2

Fig . 1. D e v i c e for m e a s u r i n g the r e s i d u a l ang le of t o r s i o n : 1) s a m p l e ; 2) s u p p o r t i n g d e v i c e ; 3) i n d i c a - t o r .

F ig . 2. R e l a t i o n s h i p of r e s i d u a l d e f o r m a t i o n of the b a s e m e t a l and the weld jo in t s to the va lue of t o r s i o n - a l m o m e n t (sol id l i ne s a r e 45 s t e e l and b r o k e n St.3 s t e e l ) .

K u r s k P o l y t e c h n i c I n s t i t u t e . T r a n s l a t e d f r o m P r o b l e m y P r o c h n o s t i , No. 2, pp . 103-104, F e b r u a r y , 1976. O r i g i n a l a r t i c l e s u b m i t t e d S e p t e m b e r 26, 1974.

�9 Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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F i g . 4 F ig . 3. R e l a t i o n s h i p of the s t r e n g t h of weld io in t s to t he quan t i t y of d e f e c t s in the jo in t zone d e t e r m i n e d by the va lue of e l e c t r i c a l r e s i s t a n c e and the r e s i d u a l d e - f o r m a t i o n (sol id l i ne s a r e 45 s t e e l and b r o k e n St.3 s t e e l ) .

F ig . 4. R e l a t i o n s h i p of e l e c t r i c a l r e s i s t a n c e (1) and r e s i d u a l d e f o r m a t i o n (2) to the quan t i ty of d e f e c t s in the jo in t zone .

In d e t e r m i n i n g jo in t qua l i ty u s i n g the m e c h a n i c a l h y s t e r e s i s me thod the s a m p l e was loaded with a t o r s i o n a l m o m e n t , the h y s t e r e s i s loop was d r a w n , and the a m o u n t of d i s s i p a t i o n of e n e r g y as a r e s u l t of i n t e r n a l f r i c t i o n and the p r e s e n c e of d e f e c t s in the weld zone was judged on the b a s i s of the d i m e n s i o n s of t he loop [3].

T h e e x p e r i m e n t a l i n v e s t i g a t i o n s w e r e m a d e on a KN-50 t o r s i o n m a c h i n e . A p l a n of i t i s shown in F ig . 1. F o r m e a s u r i n g the r e s i d u a l ang le of t o r s i o n a d i a l i n d i c a t o r with d i v i s i o n s of 1 p was u s e d . The d i s t a n c e b e t w e e n the p ins ( m e a s u r i n g b a s e ) was 8 +~176 ram.

W e l d e d s a m p l e s with a l ength of 60 m m and a d i a m e t e r of 16 m m and s a m p l e s of b a s e m e t a l of the s a m e d i m e n s i o n s , t r e a t e d us ing a weld c y c l e , w e r e t e s t e d . 4"

Weld jo in t s of 45 and St.3 s t e e l s w e r e t e s t e d . F o r the p u r p o s e of check ing the e f f e c t i v e n e s s of the i n s p e c t i o n , s a m p l e s wi th a notch in the weld zone w e r e t e n s i l e t e s t e d [4]. The s a m p l e s w e r e we lded u n d e r t h e fo l lowing c o n d i t i o n s :

We ld ing t e m p e r a t u r e - - 1000~ (St.3), 1050~ (45 s t ee l ) ,

Unit p r e s s u r e -- 1.5 k g f / m m 2,

W e l d i n g t i m e - - 10 min ,

V a c u u m - 2 . 1 0 -4 m m Hg.

To ob t a in weld jo in t s with v a r i o u s q u a n t i t i e s of d e f e c t s the s u r f a c e s be ing w e l d e d w e r e g round to d i f f e r e n t s u r f a c e f i n i s h e s ( v 3 - ~ 6 ) .

F i g u r e 2 shows the r e l a t i o n s h i p of the r e s i d u a l d e f o r m a t i o n of the b a s e m e t a l and the weld jo in t s of the two m a t e r i a l s to the va lue of t o r s i o n a l m o m e n t in i n s p e c t i n g by m e c h a n i c a l h y s t e r e s i s . Wi th an in - c r e a s e in t o r s i o n a l m o m e n t the s a m p l e s with d e f e c t s in the weld zone s t a r t to d e f o r m p l a s t i c a l l y s i g n i f i - c an t l y e a r l i e r t han the s a m p l e s of b a s e m e t a l .

F i g u r e 3 shows the r e l a t i o n s h i p of the s t r e n g t h of weld jo in t s to the quan t i ty of d e f e c t s in the jo in t zone d e t e r m i n e d by the va lue of e l e c t r i c a l r e s i s t a n c e and r e s i d u a l d e f o r m a t i o n . As may be s e e n , t he m o r e d e f e c t s in the jo in t and the g r e a t e r the a r e a occup ied by t h e m the h i g h e r the va lue s of h y s t e r e s i s l o s s e s and e l e c t r i c a l r e s i s t a n c e of the s a m p l e s , which l e a d s to a r e d u c t i o n in the s t r e s s for f a i l u r e .

The e l e c t r i c a l r e s i s t a n c e of p o r t i o n s of b a s e m e t a l 30 m m long and 16 m m in d i a m . t r e a t e d us ing t h e weld c y c l e was (0.288-0.294) �9 10 -4 ~ f o r 4 5 s t e e l a n d (0.248-00254) �9 1 0 - 4 ~ fo r St.3 s t e e l . The r e s i d u a l d e f o r m a t i o n was 3 . 7 . 1 0 -5 and 3 �9 10 -5 r a d , r e s p e c t i v e l y . F o r weld jo in t s t h e s e p r o p e r t i e s w e r e in a l l c a s e s h i g h e r t han t h o s e of the b a s e m e t a l . Th i s i s an i n d i c a t i o n of the fact tha t of the c h o s e n we ld ing m e t h o d s not one l e a d s to the f o r m a t i o n of a d e f e c t - f r e e jo in t . In m e c h a n i c a l t e s t i n g a l l of the s a m p l e s f a i l e d in the jo in t .

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With an increase in the weld t empera tu re of the St.3 samples to 1050~ and of those of 45 steel to l l00~ with the other pa r ame te r s unchanged the e lec t r ica l r es i s t ance and the res idual angle of to rs ion of the weld joint became the same at those proper t ies of the base metal. Tensi le tes ts of notched samples showed the s t rength of the joint to be equal to that of the base metal.

To check the sensit ivity of these methods to discontinuity and lack of penetrat ion-type defects, a r t i - fical defects" with a known area were applied to weld samples . On one of the faces of the samples being joined a hole 0.5-1 mm in diam. and 0.1-0.2 mm in depth was made.

In welding under the conditions which gave s imi lar values of e lec t r ica l res i s tance and residual de- format ion for the welded samples and the samples of base metal (ll00~ for 45 steel) defects with the ex- ternal form of unfused a reas remained in the a reas of the holes.

Figure 4 shows the relat ionship of the inspection resu l t s to the percent quantity of defects in the joint zone. With application of ar t i f ic ia l defects to a total a rea of 1.5% of the a rea of the sample c ross sect ion the e lec t r ica l r es i s t ance of the joint differs f rom that of the base metal by an average of 1.37%. This is an indication of the quite high sensit ivity of the method to lack of penetrat ion and discontinuity defects . The accuracy in measur ing the e lec t r ica l r es i s t ance was ~0.3%. Consequently the method of measur ing the e lec t r ica l r es i s t ance may be used to determine defects with a total a rea of not less than 0.6% of the a rea of the sample c ross section. For mechanical hys te res i s the measur ing accuracy was ~=2-3%.

L I T E R A T U R E C I T E D

I. E.S. Karakozov, "Determining the quality of joints produced by welding in the solid phase," in: Reviews of Papers of the Interunivers i ty Conference on the Strength of Welded Par t s and the Ac- curacy of their Product ion [in Russian], MVTU, Moscow (1973).

2. V . I . Vladimirov and R. G. Lupashku, "An investigation of c racks by the e lec t r ica l r es i s t ance method," Probl . Prochnost i , No. 4 (1973).

3. V . I . Maksak and K. P. Zakharov, "Determining the s t rength and degree of lack of penetrat ion of a weld joint produced by fr ict ion welding on the basis of mechanical hys te res i s , " Svarchn. pro izv . , No. 10 (1972).

4. A . A . Kotel 'nikov and A. V. Bashurin, in: Diffusion Welding of 38KhMYuA Chromium- -Aluminum Steel in Vacuum [in Russian], N. F. Kazakov (editor), MTIMMP, Moscow (1973).

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