HIGH-STRENGTH LIGHT

FILAMENTS

FOR

REINFORCING

ALLOYS O. A. Bannykh, Yu. E. Busalov, A. A. Klekovin, I. M. Kop'ev, and D. I. Prokof'ev

UDC

620.17:539.562:669.7

Among the most promising composite materials are those consisting of light alloys reinforced with high-strength steel filaments. As compared with aluminum alloys, composites of aluminum high-strength steel filament have the following advantages: I) higher strength and strength-to-weight ratio; 2) higher modulus of elasticity; 3) higher heat resistance. Figure 1 shows the variation of the strength-to-weight ratio of the composite with the volume per cent of filaments with different strengths. With relatively low strength of the filament (100-150 kg/mm 2) an increase in the volume per cent of filament above 50-60% has little effect on the strength-to-weight ratio of the composite. With a filament strength of 200-350 kg/mm 2 the strength-to-weight ratio increases at a decreasing rate at large volumes of filament. In view of the technological difficulties in obtaining composites with a high volume per cent of filament it is expedient to use only 20-40% filament (hatched zone in Fig. i). It should be noted that even with a filament strength as high as 350 kg/mm 2 the advantage of the composite over aluminum alloys in terms of strength-to-weight ratio is not so high at room temperature as at elevated temperatures. For example, at 400~ one can obtain a substantial increase of the strength-to-weight ratio with use of the composite material even with a filament strength of i00 kg/mm 2. The modulus of elasticity of reinforced aluminum alloys is high - with 30 vol. % filament Young's modulus is around 12,000 kg/mm 2 and the density is 4.2 g/cm 3 (there are no aluminum alloys with such high values of Young's modulus). It can be assumed that the use of heat resistant steel filament would permit operation of reinforced aluminum alloys at temperatures up to 500~ Also, composite

materials

reinforced with filament should have a high resistance to crack propaga-

tion. Considering the applications of composite materials based on light alloys and methods of manufacturing them (hot extrusion, explosion, diffusion welding, etc. ) associated with thermal interactions, one can formulate the basic requirements for the high-strength reinforcing filament: high strength, low density, retention of high-strength in heat treatment (i. e., slow decline of the strength during brief exposure to high temperatures in the process of manufacturing the composite), and ductility to ensure the possibility of preparing thin filaments and netting from the material. For heat resistant composites of the aluminum alloy-high strength steel filament type there are additional requirements: high long-term strength at operating temperatures, stability of the transition layer

/

.sO

~o

~o

z:o

so

80 v~,~,

Fig. i. Strength-to-weight ratio of aluminumsteel filament composite in relation to volume per cent of filament of different strengths. The hatched region indicates the strength-to-weight ratio of the strongest aluminum alloys at different temperatures.

A. A. B a i k o v I n s t i t u t e of M e t a l l u r g y . M e t a l l o v , No. 7, pp. 4 0 - 4 5 , J u l y , 1973.

Translated from Metallovedenie i Termicheskaya

Obrabotka

9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 ff'est 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval syste,n, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. ,4 copy of this article is available from the publisher for $15.00.

592

ob, kg/mm2 '

- ,w.-,

200

Ob, kg/mm z

/,

o

coo

zoo

soo

6oo

zo

~

o

coo

zoo

i

3oo

a

5oo

~

m

b

F i g . 2. E f f e c t of t e m p e r i n g t e m p e r a t u r e o n t h e s t r e n g t h and d u e t i l i t y of f i l a m e n t s , a) T e m p e r e d 15 min; b) 2 h. 1) 40KNKhlKVTYu; 2) 2Kh15N5AM3; 3) N18K9M5T; 4) 0Khl4N14M2; 5) USA. b e t w e e n the a l l o y ( m a t r i x ) and the r e i n f o r c i n g f i l a m e n t u n d e r the p r l o n g e d i n f l u e n c e of t e m p e r a t u r e stress.

and

Here we consider the properties of filaments of high-strength steels from commercial heats - carbon steel USA, high-strength austenitic-martensitic steel V N S 9 (2KhlSNSAM3), maraging steel M S 2 0 0 (NISKgMST), austenitic steel E P 3 2 2 (0KhI4NI4M2), and an aging alloy based on F e - C o - N i - C r (40 K N K H 9 M V T Y u ) (Table I). The filament prepared from these steels with a diameter of 0.2 m m has a strength over 250 kg/mm2. W e investigated the effect of temperature (20-700 ~ and holding time (15 min to 2 h) on the strength, ductility, and phase composition of the filament. Filament samples (base length i00 m m ) were tested in the Instron machine at a pull rate of 2 ram/rain at 20 ~ (after tempering) and at the tempering temperature. The amount of austenite and martensite in the samples tested was determined by x-ray analysis, using the integral intensity of lines (lll)y and (ll0)ff, which was possible in most cases due to the w e a k deformation texture (except for filament of steel N I 8 K g M 5 T ) . The results of tensile tests at 20 ~ are shown in Fig. 2. In the original condition the strength was highest for the filament of steel U S A (385 kg/mm2). Raising the tempering temperature to 300 ~ causes a sharp reduction in the strength of U 8 A filament due to the low thermal stability of the structure of carbon steel. After tempering at 400 ~ for 15 min the strength was around 250 k g / m m 2, and 215 k g / m m 2 after 2 h, which was lower than the strength of other steels tested. The strength of steels 2 K h I 5 N 5 A M 3 , N I S K g M 5 T , and 0 K h I 4 N I 4 M 2 was constant after tempering at temperatures as high as 450-500 ~. Above 500 ~ the steels weaken. The most heat resistant of these three steels at 500 ~ was steel 2 K h I 5 N 5 A M 3 ((rb = 310 k g / m m 2 after tempering for 15 min and 302 k g / m m 2 after 2 h). The strength of steel N I 8 K 9 M 5 T decreased from 280 to 260 k g / m m 2 after 15 min at 500 ~ and that of steel 0KhI4NI4NI2 from 270 to 250 k g / m m 2. T e m p e r i n g at 400-500 ~ led to an increase in the strength of alloy 40KNKhMVTYu. At 600 ~ the strength of this alloy was about 315 k g / m m 2 after 15 rain, but dropped considerably after 2 h (270 kg/mm2). The ductility of all these steels remained practically constant after tempering at temperatures up to 600 ~ (6 = 2-4%), but increased sharply above 600 ~ (Fig. 2). The decrease in the strength of the alloy steels at 500 and 600 ~ is shown in Fig. 3. TABLE

I

Steel C

2Kh15N5AM3 N18K9M5T 0Kh14N14M2 40KNKhMVTYu

0,24 0,02 0,04 0,03

Sl It

0,39 0,01 0,21 0,40

Note: Not over 0. 02~ S and P.

Mn

cr

Comp6*ifion, ~o N, I Co [ ~o

4,971 12,9o

0,42 14,71 18,2 ] 8,,5 4,7 0,05 2,15 0,34 12~-95 12,74 1,90 12,5 19,6 40,,9 3,4

1

W

T!

-

052

C8

17

AI

-

593

TABLE Toc

2 Aatemper [ A~ kg/mm 2

300 4OO 50O 6OO 7OO O

t

~_

200 460

I 30

60/40

135/50 2201130~ 2701175'

The strength of alloy steel filaments can be increased by heat treat,ment and cold drawing and aging. The high-strength condition depends on the effect of such factors as: i) strain hardening of austenite; 2) the martensitic transformation; 3) strain hardening of martensite; 4) precipitation h a r d e n i n g of a u s t e n i t e ; 5) p r e c i p i t a t i o n h a r d e n i n g of m a r t e n s t t e (or, m o r e p r e c i s e l y , a p h a s e ) . T h e r o l e of e a c h of t h e s e f a c t o r s d i f f e r s in the s t e e l s investigated.

J20 280

20/20

AOtemper = reductionofstrength (Ob) at20 ~ after temperingat T ~ ascompared with original; AOTernp is the difference between the strength at T ~and 20~ Data for2KhlSN5AM3 in numerators, 0Khl4N14M2 in denominators,

The strength of heat resistant filaments of steels 0KhI4NI4M2 and 2KhI5N5AM3 at elevated testing temperatures decreases more rapidly than at 20 ~ after tempering at the same temperature, although the temperature of the sharp decline in strength is the same (Table 2).

/miTl2

b ...._t

0/0 0/0 8/0 70/90 130/170

Notes

60

.90

min

In the p r o c e s s of m a n u f a c t u r i n g the f i l a m e n t , s t e e l 0Khl4N14M2 u n d e r g o e s the m a r t e n s i t i e t r a n s f o r m a t i o n d u r i n g c o l d p l a s t i c d e f o r m a t i o m F i g . 3. S t r e n g t h of f i l a T h e f i r s t t h r e e f a c t o r s p r o v e d to have a s u b s t a n t i a l e f f e c t on the s t r e n g t h . m e n t s in r e l a t i o n to t e m In the o r i g i n a l c o n d i t i o n (before t e n s i l e t e s t s ) the w i r e c o n s i s t e d of 40.7% p e r i n g t i m e a t 500 (a) a n d m a r t e n s i t e . A g i n g f o r a s long as 100 h at 450 and 500 ~ did n o t c h a n g e the 600~ (b). N o t a t i o n s the a m o u n t of m a r t e n s i t e o r a u s t e n i t e . T h e l o w e r i n g of the s t r e n g t h a f t e r s a m e a s in F i g . 2. h o l d i n g at 500 ~ is due to r e c o v e r y p r o c e s s e s , o c c u r r i n g l a r g e l y in t h e d e f o r m e d m a r t e n s i t e and to a l e s s e x t e n t in a u s t e n i t e . M e a s u r e m e n t s of the w i d t h of l i n e s ( l l l ) T and (ll0)ce on the x - r a y d i a g r a m s of t h i s s t e e l s h o w e d that e v e n 15 rain at 500 ~ r e d u c e s the w i d t h of l i n e (ll0)oe b y 50-60% and (111)~ b y 20-30% a s c o m p a r e d with the o r i g i n a l c o n d i t i o n . At 600 ~ the r e c o v e r y p r o c e s s e s in m a r t e n s i t e and a u s t e n i t e o c c u r at a h i g h e r r a t e , a l o n g w i t h a ( M ) ~ T transformat i o n , c a u s i n g a g r a d u a l r e d u c t i o n in the a m o u n t of m a r t e n s i t e . N e i t h e r in t h e o r i g i n M c o n d i t i o n o r a f t e r t e m p e r i n g at 400 and 500 ~ d o e s f i l a m e n t of m a r a g i n g s t e e l N18K9M5T c o n t a i n 7 p h a s e . X - r a y a n a l y s i s s h o w e d the p r e s e n c e of m a r t e n s i t e and d i s t i n c t t e x t u r e . T h e i n c r e a s e in the s t r e n g t h is due m a i n l y to aging of d e f o r m e d m a r t e n s i t e . A m a r t e n s i t e t e x t u r e w a s a l s o f i x e d a f t e r t e m p e r i n g at 600% w h i c h c a u s e d t r a n s f o r m a t i o n of p a r t of the m a r t e n s i t e to a u s t e n i t e . T e m p e r i n g at 700 ~ r e s u l t s in an a +/3 s t r u c t u r e , and a f t e r 30 m i n at t h i s t e m p e r a t u r e the t e x t u r e d i s a p p e a r s . T h e a m o u n t of a u s t e n i t e in s t e e l N1SK9M5T w a s 78% a f t e r 2 h at 700 ~ T h u s , the w e a k e n i n g e f f e c t of t e m p e r i n g on 0Khl4N14M2 f i l a m e n t is due to r e c o v e r y and the a(M}--~ 7 t r a n s f o r m a t i o n , w h i l e f o r f i l a m e n t of s t e e l N18K9M5T t h e " o v e r a g i n g " of m a r t e n s i t e a n d s o l u t i o n of e x c e s s p h a s e s in the a u s t e n i t e f o r m e d d u r i n g t e m p e r i n g m u s t a l s o be c o n s i d e r e d a l o n g w i t h t h e s e f a c t o r s . T h e high h e a t r e s i s t a n c e of s t e e l 2Khl5N5AM3 as c o m p a r e d with s t e e l s N18K9M5T and 0 K h l 4 M 2 i s due to the h i g h e r i n i t i a l t e m p e r a t u r e of the a - - fl t r a n s f o r m a t i o n and the p r e s e n c e of t h e r m a l l y s t a b l e c a r b i d e p a r t i c l e s in the s t r u c t u r e . T h e high h e a t r e s i s t a n c e of a l l o y 40KNKhMVTYu is due to h a r d e n i n g with i n t e r m e t a l l i c p a r t i c l e s . T e m p e r i n g i n d u c e s c o n s i d e r a b l e p r e c i p i t a t i o n h a r d e n i n g of the a l l o y , and the i n c r e a s e in s t r e n g t h is g r e a t e s t a r o u n d 600 ~. In c o n s i d e r i n g the u s e of t h e s e s t e e l s f o r r e i n f o r c i n g l i g h t a l l o y s one m u s t k e e p in m i n d t h a t c a r b o n s t e e l of the U8A t y p e c a n b e u s e d only in a l i m i t e d n u m b e r of c a s e s . T h e high s t r e n g t h of t h i s s t e e l c a n b e r e t a i n e d in a c o m p o s i t e m a t e r i a l only in the c a s e of b r i e f h e a t i n g in the p r o c e s s of m a n u f a c t u r i n g the c o m p o s i t e (for e x a m p l e , in c o m b i n i n g the f i l a m e n t w i t h the m a t r i x b y m e a n s of e x p l o s i v e i m p u l s e ) . M e t h o d s of p r e p a r i n g c o m p o s i t e s t h a t r e q u i r e h e a t i n g to t e m p e r a t u r e s above 300 ~ and h o l d i n g at s u c h t e m p e r a t u r e s a r e h a r d l y s u i t a b l e f o r t h i s s t e e l , s i n c e a s u b s t a n t i a l r e d u c t i o n of the s t r e n g t h i s u n a v o i d a b l e . F i l a m e n t of s t e e l U8A (or o t h e r c a r b o n s t e e l s ) c a n be r e c o m m e n d e d f o r c o m p o s i t e s o n l y f o r o p e r a t i o n at t e m p e r a t u r e s near room temperature. 594

Filament of the aging alloy 40KNKhMVTYu is the most widely applicable for manufacturing composites by various technological p r o c e s s e s . Aside f r o m methods of joining materials in the solid state, the filament can be embedded in the liquid metal or the liquid metal can be poured over the filament so long as the matrix metal has a melting point below 700 ~ The composite r e i n f o r c e d with 40KNKhMVTYu filament (or a s i m i l a r alloy) is suitable for prolonged operation at t e m p e r a t u r e s up to 500-550~176 When the three steels investigated (2Khl5N5AM3, 0Khl4N14M2, and NlSK9M5T) are treated to the same strength it is difficult to choose one over the other. The difference in the thermal stability of the structure and the mechanical properties is relatively small. The thermal stability of these steels makes it possible to obtain composites by hot extrusion, rolling, diffusion welding, etc. Composites reinforced with filament of these steels can be used not only at room and low temperatures but also at temperatures as high as 400-450 ~ The prospects of these steels will depend above all on the strength. steel NI8KgM5T (as well as other maraging steels).

The most promising

is maraging

It should be noted that the suitability of high-strength filament for composite materials was determined only in relation to the changes in'the mechanical properties of the filament at elevated temperatures. The workability of the steels from which the filaments were prepared and the interaction between the matrix and the filament during manufacture of the composite and in operation were not taken into account. However, this should not be the decisive factor in selecting a reinforcing material, since the technique of manufacturing the filament can be improved by careful control of the technological parameters and the interaction between the matrix and the filament can be controlled by means of special barrier layers and additional alloying of the matrix. CONCLUSIONS 1. Comparative tests were made of the mechanical p r o p e r t i e s of high-strength filament of various steels (USA, 0Kh14N14M2, 2Kh15NSAM3, N18K9M5T)and alloy 40KNKhMVTYu in relation to the t e m p e r i n g t e m p e r a t u r e and time. 2. The effect of tempering on the mechanical p r o p e r t i e s of steels is due to the changes o c c u r r i n g in the s t r u c t u r e of the filament. X - r a y analysis indicated that the d e c r e a s e in the strength of the filament at elevated tempering t e m p e r a t u r e s is due mainly to r e c o v e r y p r o c e s s e s in cold worked m a r t e n s i t e , the a(M) ~ 7 t r a n s f o r m a t i o n , and also the coalescence of particles of excess phases. 3. The strength of filaments of h i g h - s t r e n g t h steel U8 d e c r e a s e s above 300% that of maraging steel N18K91K5T, steel of the transition c l a s s 2Khl5N5AM3, and austenitic steel 0Khl4N14M2 above 500 ~ and that of alloy 40KNKhMVTYu above 650 ~ 4. The technology used for manufacturing composites of the light alloy-high strength filament type depends on the temperature at which the filament weakens. Liquid-phase methods can be used to manufacture composites with filament of alloy 40KNKhMVTYu (embedding the filament in the liquid metal, or casting), while only solid-phase methods can be used for the other alloy steels investigated (rolling, extrusion, etc.), with heating to temperatures not over 500% Composites with filament of carbon steel U8 can be obtained only with brief heating at temperatures not over 300 ~ which sharply limits the methods of preparing composite materials with filaments of this steel.

595