EVALUATION OF THE EFFECT OF CENTRAL

RUPTURE (CAVITY) OF A BILLET

ON PIERCEABILITY

E. S. Umanskii and Yu. L. Kanter UDC 621.774.35

In the manufacture of seamless tubes the main and most essential operation is to produce a hollow shell by piercing a uniformly heated billet in a helical-rolling mill. Underlying the process is the forma- tion in the center and its vicinity of a cavity which the mandrel transforms into a hole.

Despite the numerous experimental data [1-3] and certain theoretical developments in this area [3- 4], so far there is no common opinion on the nature of the axial rupture. A more thorough study of the mechanics of this process will permit piercing billets without the formation of cracks in front of the man- drel nose, which, as is known, leads to rolling-out of scabs on the inner surface of the tubes, i.e., to de- fective products.

We note that if reduction in advance of the mandrel nose is less than the critical 5e, the presence of defects can be minimized. Therefore, it is quite important to establish the actual value of 5 c by physical modeling of the process. Among tbe main methods of determining the critical reduction, the main ones are tensile tests, hot twisting, and pierceability [5].

The first two methods model the process of rolling and do not permit obtaining directly the relation between the deformation conditions and the instant of rupture during piercing. Only a determination of the optimal temperature regime and deformability for a state of stress differing fromthat occuring during helical rolling is possible.

Several types of piereeability tests exist: helical rolling of cylindrical specimens with an evaluation of the plastic properties on the basis of the volume of the cavity formed; rolling of gradually tapering

~p

a P p

E . . . . . . . . . q .......... ,

~P IS b

Fig. I. Diagram of modeling the pro- cess of piercing a solid billet: a) first cycle; b) n-th cycle (encounter with mandrel).

Kiev Polytechnic Institute. Translated from Problemy Prochnosti, No. 8, pp. 114-117, August, 1975. Original article submitted May 8, 1974.

�9 Plenum Publishing Corporation, 22 7 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 Jbr $15.00.

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/

2 6 7 8 #

Fig . 2. D i a g r a m of the d e v i c e fo r m o d e l i n g the p r o c e s s of p i e r c i n g a b i l l e t in a h e l i - c a l - r o l l i n g m i l l .

i 0 2 J 5 h, mm

Fig . 3. Ax ia l p r e s s u r e of the m a n - d r e l P~ as a funct ion of h in the a b - s ence of i ts d e f o r m a t i o n by p a r a l l e l d i e s .

s p e c i m e n s with d e t e r m i n a t i o n of the c r i t i c a l r educ t ion ; r e t a r d a - t ion of c y l i n d r i c a l s p e c i m e n s with d e t e r m i n a t i o n of the a v e r a g e p a r t i a l and c r i t i c a l r educ t ion ; i nves t i ga t i on of the p l a s t i c i t y of s t e e l by the c r o s s - r o l l i n g method [6], e tc . H o w e v e r , these t e s t s do not p e r m i t m o d e l i n g the p r o c e s s of p i e r c i n g a so l id b i l l e t by a m a n d r e l with an a c c u r a c y su f f i c i en t fo r p r a c t i c e .

In the p r e s e n t work we a t t e m p t e d to eva lua t e the e f fec t of the c a v i t y on the p i e r c e a b i l i t y of the b i l l e t . The p r o b l e m w a s so lved e x p e r i m e n t a l l y by a p p r o x i m a t e m o d e l i n g of the p i e r c i n g p r o c e s s .

In the h e l i c a l - r o l l i n g mi l l the b i l l e t p a s s e s s u c c e s s i v e l y through t~'o d i f f e r e n t s e c t i o n s of the d e f o r m a t i o n zone which d i f f e r in the i n t ens i t y and c h a r a c t e r of d e f o r m a t i o n .

The f i r s t s e c t i o n is l o c a t e d f r o m the p lane of g r i p of the work p i ece by the r o l l s to the e n c o u n t e r with the m a n d r e l nose .

The a v e r a g e p a r t i a l r e d u c t i o n s of the p i ece in the go rge a t 5-20 c y c l e s / m i n a r e 0 .5-1.2%. The upper l i m i t of the a v e r a g e p a r t i a l r e d u c t i o n , a c c o r d i n g to the da t a of the Moscow Ins t i tu te of S tee l and A l l o y s , can be i n c r e a s e d to 5% [3].

A s m a l l s e c t i o n of s t a t i o n a r y gu ides d i r e c t i n g the b i l l e t to the m a n d r e l nose o p e r a t e s in the go rge . T h e r e f o r e , we can neg l ec t t h e i r e f f ec t in the given s ec t i on .

In the s econd s e c t i o n ( r o l l - c l e a r s ec t ion ) , l o c a t e d be tween the m a n d r e l nose and w i thd ra w a l of the hollow she l l f r o m the m a n d r e l , the p a r t i a l r educ t i on amoun t s to 35--40% [3].

On the b a s i s of p r e l i m i n a r y c a l c u l a t i o n s of the s t r e s s e d s t a t e and e x p e r i m e n t a l da t a we can note four ma in f a c t o r s of d e f o r m a t i o n of tile b i l l e t in the d e f o r m a t i o n zone of the h e l i c a l - r o l l i n g m i l l : c o m p r e s s i o n , c o n s t r a i n e d t o r s i o n , b ind ing , and s h e a r . The a c c u m u l a t i o n and r e p e a t e d a l t e r n a t i n g change of t ens i l e and s h e a r i n g d e f o r m a t i o n s p r o b a b l y have a d e c i s i v e e f fec t on rup tu r e of the a x i a l p a r t of the work p iece b e f o r e the m a n d r e l nose .

D e f o r m a t i o n of the b i l l e t in the go rge can be r e a l i z e d by a s i m p l e r method in the c a s e of c r o s s f o r g - ing of a c y l i n d r i c a l body with an e c c e n t r i c i t y of the e x t e r n a l f o r c e s if we a s s u m e that the c y l i n d e r is d e - f o r m e d g r a d u a l l y , i . e . , f o r a c e r t a i n r educ t i on the length of the d e f o r m a t i o n zone is s m a l l and equal to a s.

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o r 1 6 2 ,, , 0 ~ 8 I2 �9 h; mm [ ,l]

Fig. 4 Fig. 5 Fig. 4. Depth of penetration of the mandrel h as a function of the number of reductions of the specimen

n 1 .

Fig. 5. Qualitative picture of twisting of a bar in-

serted into the center of a cylindrical billet after rol-

ling it between parallel plates.

A f t e r a h a l f - t u r n of the w o r k p i e c e the length of the d e f o r m a t i o n zone i n c r e a s e s to 2 a l , a c c o r d i n g l y in - c r e a s i n g the r e d u c t i o n , e t c . , to A (Fig . 1). If in th is c a s e a m a n d r e l is i n s t a l l e d in the m i d d l e of the d e - f o r m a t i o n zone on the t r a v e l path of the b i l l e t , then the c a v i t y o b t a i n e d wil l enab le us, a p p a r e n t l y , to rnodel the p i e r c i n g p r o c e s s .

To m o d e l the p r o c e s s of p i e r c i n g a so l id b i l l e t in the c a s e of he l i c a l r o l l i n g , a dev i ce was d e v e l o p e d and m a n u f a c t u r e d which p r o v i d e s : a c o n s t a n t p r e s s u r e of the m a n d r e l on the mode l b i l l e t ; c o m p a r a t i v e l y wide r a n g e of d i m e n s i o n s of the w o r k p i e c e and a m o u n t of r educ t ion ; d e f o r m a t i o n of the s p e c i m e n by p a r a l - l e l d i e s d u r i n g p i e r c i n g ; c r o s s f o rg ing of c y l i n d r i c a l b i l l e t s with e c c e n t r i c i t y of the a pp l i e d e x t e r n a l f o r c e s and wi thout it.

The d e v i c e (Fig . 2) c o n s i s t s of l o w e r and uppe r p l a t e s , c l a m p i n g a s s e m b l i e s , and a DS-02 d y n a m o - m e t e r , The g r e a t e r p a r t of the d e v i c e is a s s e m b l e d on the l o w e r p la te 1. In p la t e 1 a r e gu ides 2 m o v i n g f r e e l y in the open ings of i n s e r t s which a r e r i g i d l y connec t ed with t ipper p la te 3. A mMe die 4 is a l so s e c u r e d to the u p p e r p l a t e by s c r e w s . The c y l i n d r i c a l s t e m 5 of the p la te s e r v e s f o r f a s t e n i n g the uppe r p a r t of the d e v i c e to the s t a t i o n a r y bed of the 10- ton t e s t i n g m a c h i n e . The r e c i p r o c a l mot ion of the l o w e r bed of the t e s t i n g mach ine p r o v i d e s the n e c e s s a r y d e f o r m a t i o n of the s p e c i m e n by t r a n s m i t t i n g the p r e s - s u r e to the p r o j e c t i n g c y l i n d r i c a l s t e m of the f e m a l e d ie 6. On the l o w e r p la te is i n s t a l l e d l e f t p r o p 13 w i th a s t op s c r e w , by mov ing which a c e r t a i n p r e s s u r e is c r e a t e d on m a n d r e l 7. The magn i tude of the p r e s s u r e i s m e a s u r e d by d y n a m o m e t e r 11. The chock 9 and r e a r s u p p o r t with a s t op s c r e w 10 a r e in tended fo r s e t - t ing the s p e c i m e n in the n e c e s s a r y pos i t i on .

As the m a t e r i a l f o r the s p e c i m e n we s e l e c t e d l e a d , whose p h y s i c a l and m e c h a n i c a l p r o p e r t i e s a r e c l o s e to those of c e r t a i n g r a d e s of s t e e l a t the r o l l i n g t e m p e r a t u r e . The shape and r e l a t i o n s h i p of the d i - m e n s i o n s of the m a n d r e l w e r e s i m i l a r to t hose used on p i e r c i n g m i l l s [2].

A va lue of P ; = 120 kg of the c o n s t a n t p r e s s u r e on the m a n d r e l was used on the b a s i s of p r e l i m i n a r y e x p e r i m e n t s (Fig . 3). In th is c a s e the depth of p e n e t r a t i o n of the c y l i n d r i c a l p a r t of the 5 - r a m - d i a m e t e r tool w a s 2 m m . It was n e c e s s a r y to take into accoun t that the m a n d r e l , p e n e t r a t i n g , c o u n t e r a c t e d the r o t a - t ion of the b i l l e t d u r i n g i t s d e f o r m a t i o n , and this in the c a s e of the p r e s e n c e of e c c e n t r i c i t y of the c o m p r e s - s ive f o r c e s app l i ed to the b i l l e t s u r f a c e l ed to t w i s t i n g of a c e r t a i n p a r t of it .

The t angen t i a l s t r e s s e s in the r e g i o n of p e n e t r a t i o n of the m a n d r e l can be e s t i m a t e d a p p r o x i m a t e l y if we a s s u m e that the tool p e n e t r a t e s c o n s t a n t l y into a d i sk of t h i c k n e s s b~. A c c o r d i n g to [7], we have

M, (i)

w h e r e r is the r a d i u s of a m a n d r e l tha t p e n e t r a t e d . t o depth bi .

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Although th i s f o r m u l a p e r t a i n s to a d i sk and, c o n s e q u e n t l y , does not t ake into accoun t the s u p p o r t i n g e f f ec t of the r e m a i n i n g p a r t of the b i l l e t , i t i n d i c a t e s that r a t h e r l a r g e t angen t i a l f o r c e s T r 0 should ac t in the zone of p e n e t r a t i o n , s i n c e the r a d i u s of the m a n d r e l r is s m a l l in c o m p a r i s o n with the r a d i u s of the work p i e c e R and a m o u n t s to 0 .2 -0 .5 R.

E x p e r i m e n t s c a r r i e d out a c c o r d i n g to the i nd i ca t ed s c h e m e showed tha t in t ense flow of me ta l in a d - vance of the m a n d r e l nose is a n e c e s s a r y cond i t ion fo r p e n e t r a t i o n of the m a n d r e l into the b i l l e t . T h e r e - f o r e we mo lded a p r o c e s s such tha t b e g i n n i n g with the Very f i r s t r e d u c t i o n (5 = 15%) the tool p e n e t r a t e d in - to the s p e c i m e n .

The e c c e n t r i c i t y of the e x t e r n a l f o r c e s , d e p e n d i n g on the r a t i o H / b and angle of r o t a t i o n of the s p e c i - men ~ 2, was d e t e r m i n e d by the fo l lowing f o r m u l a :

We note that the m a x i m u m p e n e t r a t i o n of the m a n d r e l into the s p e c i m e n was o b s e r v e d in the p r e s e n c e of a m a x i m u m t w i s t i n g m o m e n t .

I n v e s t i g a t i o n s in the a b s e n c e of e c c e n t r i c i t y w e r e a l so c a r r i e d out. In th is c a s e the depth of p e n e - t r a t i on of the m a n d r e l was 35-45% l e s s than that when e ~ 0. Some r e s u l t s of the i nves t i ga t i on a r e p r e - sen ted in F ig . 4.

Rad ia l c o l u m n s w e r e app l i ed to the end of the s p e c i m e n s in the r eg ion of p e n e t r a t i o n fo r a qua l i t a t ive eva lua t i on of the c h a r a c t e r of the s t r e s s e d s t a t e . It was found that in the c a s e of e c c e n t r i c i t y of the app l i ed e x t e r n a l f o r c e s the end tw i s t ed t o w a r d the s ide o p p o s i t e the d i r e c t i o n of ro t a t i on .

B e s i d e s e x p e r i m e n t s on l e a d s p e c i m e n s the c h a r a c t e r of d e f o r m a t i o n was eva lua t ed a l so on m o d e l - i n g - c l a y c y l i n d e r s with p r e l i m i n a r i l y i n s e r t e d c l a y b a r s of a d i f f e r e n t c o l o r . The s p e c i m e n s w e r e ro l l ed be tween two p a r a l l e l p l a t e s .

In th is c a s e t h r e e ma in f a c t o r s of d e f o r m a t i o n o c c u r r e d : c o m p r e s s i o n , s h e a r , and b ind ing .

D u r i n g r o l l i n g , in t ense b ind ing was noted on the ends of the s p e c i m e n s . The i n s e r t e d b a r s w e r e twi s t ed in a d i r e c t i o n o p p o s i t e to the r o t a t i o n of the work p i ece . T h e i r m a x i m u m twi s t i ng was noted in the c e n t e r of the d e f o r m a t i o n zone . It is obv ious that such a c h a r a c t e r of d e f o r m a t i o n of the s p e c i m e n is due to the e c c e n t r i c i t y of the app l i ed e x t e r n a l f o r c e s and is r e l a t e d with the e f f ec t of t angen t i a l s t r e s s e s Tr0 (Fig. 5).

C O N C L U S I O N S

1. F o u r ma in f a c t o r s of d e f o r m a t i o n of the b i l l e t a c t in the d e f o r m a t i o n zone of the he l i c a l r o l l i n g mi l l : c o m p r e s s i o n , c o n s t r a i n e d tw i s t i ng , b ind ing , and s h e a r .

2. The a c c u m u l a t i o n and r e p e a t e d a l t e r n a t i n g change of t e n s i l e and s h e a r d e f o r m a t i o n s have the main e f fec t on the s i ze of the c a v i t y of the ax ia l p a r t of the work p i ece in f ron t of the m a n d r e l nose . In - t ense flow of m e t a l in advance of the m a n d r e l nose is a n e c e s s a r y cond i t ion in t h i s ' c a s e .

3. We can r e c o m m e n d a r a t h e r s i m p l e t e s t fo r p i e r c e a b i l i t y which c o n s i s t s in r o l l i n g a d i sk of the i nves t i ga t ed m a t e r i a l be tween two p a r a l l e l p l a t e s a t the a p p r o p r i a t e t e m p e r a t u r e and with a c e r t a i n p a r t i a l r educ t ion until r u p t u r e in the c e n t e r of the d i s k a f t e r a c e r t a i n n u m b e r of c y c l e s . T h e s e da ta p e r m i t m a k - ing the o p t i m a l a d j u s t m e n t of the m i l l .

].

2. 3. 4.

5. 6.

7.

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

P. K. T e t e r i n , T h e o r y of H e l i c a l Ro l l ing [in Russ i an ] , M e t a l l u r g i y a , Moscow (1971). A. P. C h e k m a r e v et a l . , P i e r c i n g in H e l i c a l - R o l l i n g M i l l s [in Russ i an ] , M e t a l l u r g i y a , Moscow (1967}. V. S. S m i r n o v , T h e o r y of P r e s s u r e Shaping of Me ta l s [in Russ i an ] , M e t a l l u r g i y a , Moscow (1973). K. N. Shevchenko, P r i n c i p l e s of M a t h e m a t i c a l Methods in the T h e o r y of P r e s s u r e Shaping of Me ta l s [in Russ i an ] , V y s s h a y a Shkola , Moscow (1970}. A. P . C h e k m a r e v e t a l . , I n t e n s i f i c a t i o n of H e l i c a l Rol l ing [in Russ i an ] , M e t a l l u r g i y a , Moscow (1967}. I. A. Sap ryk in , " Inves t i ga t i on of the p l a s t i c i t y of s t e e l by the c r o s s - r o l l i n g me thod , " in: P r e s s u r e Shaping of Me ta l s and Weld ing [in Russ i an ] , T r u d y L P I im. Ka l in ina , No. 308 (1969). S. P . T i m o s h e n k o , and J . N. Good ie r , T h e o r y of E l a s t i c i t y , McGraw (1970}.

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