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.

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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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