LOW-CYCLE STEELS

FATIGUE IN

OF

AGGRESSIVE

STIIUCTURAL MEDIA

V. V. Larionov, K h . M. a n d Oo N. M u r o m t s e v a

UDC g69.15-194.2:~20.193:g20. 178.35

Khanukhov,

C e r t a i n f o r m s of s t r u c t u r a l m e t a l c o n s t r u c t i o n (gas cleaning s y s t e m s c r u b b e r s , g a s h o l d e r s , etc.) a r e u s e d u n d e r conditions of the action of a g g r e s s i v e m e d i a and c y c l i c a l l y changing loads, which leads to t h e i r e a r l y w e a r and f a i l u r e [ 1 ]. Until the p r e s e n t the influence of m e d i a on low cycle fatigue has b e e n i n v e s t i g a t e d p r i m a r i l y on m a c h i n c r y s t e e l s u n d e r pulsating s t r e s s e s (bending, tension) in m e d i a not c h a r a c t e r i s t i c of the o p e r a t i o n of s t r u c t u r a l m e t a l c o n s t r u c t i o n [ 2 - 5 ] . In connection with this it is of i n t e r e s t to study the influence of a c o r r o s i v e m e d i u m on the cyclic s t r e n g t h of s t r u c t u r a l s t e e l s with a s m a l l n u m b e r of load c y c l e s . The s e r v i c e m e d i a for c y c l i c a l l y loaded welded m e t a l s t r u c t u r e s a r e c o r r o s i v e m e d i a eontaining s u l f a t e s t o g e t h e r with c h l o r i d e s and also m e d i a with fluorides. F o r the investigations the following solutions w e r e s e l e c t e d : 0.1 N H2SO 4 + 3% NaC1 (I) and ~0 g / l i t e r NaC1 + 5 g / l i t e r NaF, pH = ~.8 (II). The s y m m e t r i c soft and rigid loading t e s t s w e r e made both on the b a s e m e t a l and on weld joints of 10KhSND and I~G2AF s t e e l s . The c h e m i c a l c o m p o s i t i o n s and m e c h a n i c a l p r o p e r t i e s of the m a t e r i a l s a r e shown in Table 1. The influence of c o r r o s i v e m e d i u m I on low c y c l e s t r e n g t h was i n v e s t i g a t e d on unnotched s a m p l e s with a weld joint a f t e r a p r e l i m i n a r y hold of t h e m in this m e d i u m . The solution c o n c e n t r a t i o n s w e r e chosen so that the c o r r o s i o n d a m a g e s of the m a t e r i a l c o r r e s p o n d e d to d a m a g e s o c c u r r i n g in p r a c t i c e . The t i m e of p r e l i m i n a r y hold of the s a m p l e s c o r r e s p o n d e d to the length of subsequent cyclic loading using the p r o p o s e d t e s t b a s e . Taking into c o n s i d e r a t i o n the change in loading f r e q u e n c y (which depends upon loading l e v e l ) , which was 0.751.25 c y c l e s p e r minute, the length of the hold v a r i e d f r o m 5 h (with 200 c y c l e s ) to 134 h (with 10,000 c y c l e s ) . A f t e r holding in the m e d i u m the s a m p l e s w e r e d r i e d and until the s t a r t of t e s t s w e r e s t o r e d in a d e s i c c a t o r . At l e a s t two s a m p l e s w e r e t e s t e d at each level. B e f o r e i m m e r s i o n in the v e s s e l with the c o r r o s i v e liquid the c l a m p p o r t i o n s of the s a m p l e s w e r e c o v e r e d with a l a y e r of molten paraffin, which p r o t e c t e d t h e m f r o m c o r rosion. The influence of c o r r o s i v e m e d i u m II on low cycle fatigue s t r e n g t h w a s i n v e s t i g a t e d on tubular s a m p l e s only on the b a s e m e t a l of 10KhSND steel. The solution was p o u r e d inside the s a m p l e with the l o w e r opening closed with a r u b b e r s t o p p e r . The s a m p l e s w e r e held for 30 and 90 days in this condition. B e c a u s e of the low v o l u m e of solution it was changed e v e r y five days. During testing the solution was inside the s a m p l e . The d e f o r m a t i o n of the gage length was m e a s u r e d on the outside. The t e s t s of both types of s a m p l e s w e r e m a d e at r o o m t e m p e r a t u r e on a U~.M-10T m a c h i n e under soft and r i g i d s y m m e t r i c loading conditions with a f r e q u e n c y of 1-2 c y c l e s p e r minute. The t e s t s a m p l e s w e r e cut f r o m 30-40 m m thick p l a t e t r a n s v e r s e to the roiling direction. During the e x p e r i m e n t s load and d e f o r m a t i o n were recorded automatically. TABLE

1

Material

! i c

[ ~,r

Chemical composition, % s I P I s' I Or! N ' I c "

10KhSND weldbasemetaljoint 0,11 0.680,0120,02

0,920,870,560.55

~y, V -

-

ey, [at, MPaseals/MPaseals *,

0,2020'176 458411

16GA2F base metal 0,18 1,29 0,022 0,024 0,48 0,09 0,08 0.09 0,125 0:I~91 ~37 weld joint t

I

566566

58,65' 8'6

655 555

65,2 f5,2

Note: ey is the d e f o r m a t i o n c o r r e s p o n d i n g to the yield strength. C e n t r a l S c i e n t i f i c - R e s e a r c h and Design Institute for S t r u c t u r a l Metal Construction. T r a n s l a t e d f r o m F i z i k o - K h i m i e h e s k a y a Mekhanika M a t e r i a l o v , Vol. 1~, No. 3, pp. 39-43, M a y - J u n e , 1980. Original a r t i c l e s u b mitted J a n u a r y 7, 1979.

0038-5565/80/1603-0227507.50

9 1981 Plenum Publishing C o r p o r a t i o n

227

e~

m~

D

450o 8001 400 500J $00 400 z $00

10214 "~ rO~110~ Joint

c,l

~

~

_

o

10-~ I0~

I0~

I0e

10s h

Fig. 1. Low c y c l e s t r e n g t h in soft (a a = const) and rigid (~a = e a / e y = const) l o a d ing of s a m p l e s with a weld joint of 10KhSND and 16G2AF s t e e l s a f t e r holding t h e m in a c o r r o s i v e m e d i u m containing c M o r i d e s and sulfates: 9 I ) t e s t s in the c o r r o s i v e m e dium; A, [] ) t e s t s i n a i r . Judging f r o m the low cycle t e s t r e s u l t s of c y l i n d r i c a l s a m p l e s with a weld joint of 10KhSND and I~G2AF s t e e l s (Fig. 1) the hold in c o r r o s i v e m e d i u m I does not r e d u c e t h e i r static and cyclic strength. The low cycle life in soft and rigid loading of s a m p l e s in the c o r r o s i v e m e d i u m and in a i r is d e s c r i b e d by a single fatigue c u r v e . Inspection of the o u t e r s u r f a c e of the gage length of the s a m p l e s r e v e a l e d t r a c e s of pitting c o r r o s i o n and in s o m e c a s e s the zone n e a r the joint showed d a m a g e by it to a g r e a t e r d e g r e e . However, f a i l u r e in static and low c y c l e loading o c c u r r e d ; as a rule, in the b a s e metal. The r e c o r d e d reduction in cyclic s t r e n g t h p r o p e r t i e s of the b a s e m e t a l in the weld joint is s o m e w h a t conditional since it is r e l a t e d to f e a t u r e s of the method of m e a s u r i n g longitudinal d e f o r m a t i o n s on unnotched s a m p l e s , which p r o v i d e s the i n t e g r a l value of d e f o r m a tion of the s a m p l e gage length. Actually the d e f o r m a t i o n in thr b a s e m e t a l is higher than m e a s u r e d b e c a u s e of r e d i s t r i b u t i o n of e l a s t o p l a s t i c d e f o r m a t i o n s in the weld joint [6 ]. The a l m o s t c o m p l e t e a b s e n c e of the influence of p r e l i m i n a r y c o r r o s i o n d a m a g e s on r e s i s t a n c e to lowcycle d e f o r m a t i o n is c o n f i r m e d by an a n a l y s i s of cyclic e l a s t o p l a s t i c d e f o r m a t i o n c u r v e s (Fig. 2). C u r v e s of the change in the width of the h y s t e r e s i s loop -'(K) e and of the a c c u m u l a t i o n of p l a s t i c d e f o r m a t i o n s ~ ( K ) in r e l a t i o n to the n u m b e r of load half c y c l e s a r e q u a l i t a t i v e l y s i m i l a r f o r the two s t e e l s . T h e s e d a m a g e s a r e insufficient to a c c e l e r a t e the origin of a s u r f a c e m i c r o c r a c k . In addition the h a r m f u l influence of c o r r o s i o n pits is a p p a r e n t l y l e s s e n e d in the initial p e r i o d of loading when at the g i v e n r a t e of loading p l a s t i c d e f o r m a t i o n is u n i f o r m l y d i s t r i b u t e d o v e r the whole working s u r f a c e of the s a m p l e . However, the l o n g e r p r e l i m i n a r y hold in c o r r o s i v e m e d i u m II and s u b s e q u e n t cyclic loading under c o n ditions of action of this m e d i u m c a u s e a s u b s t a n t i a l d e c r e a s e in the life of the tubular s a m p l e s . F o r e x a m p l e , in soft loading of 10KhSND s t e e l s a m p l e s at the l e v e l of the yield s t r e n g t h of the m a t e r i a l the n u m b e r of c y cles until f a i l u r e is halved (Fig. 3). In this c a s e s a m p l e s with a p r e l i m i n a r y hold of 30 days w e r e tested. In r i g i d loading of s a m p l e s held in the c o r r o s i v e m e d i u m f o r 90 d a y s the life d r o p p e d b y t h r e e to four t i m e s d e pending upon the n u m b e r of load c y c l e s . H e r e the r o l e of the m e d i u m also p r e d o m i n a t e s but the l a y e r of d a m a g e d m e t a l is t h i c k e r (Fig. 4b, c ) . However, the cyclic e l a s t o p l a s t i c p r o p e r t i e s do not change (Fig. 2) but in c o r r o s i o n - m e c h a n i c a l t e s t s u n d e r rigid loading conditions t h e r e is a significant d e c r e a s e in the t i m e until a p p e a r a n c e of a crack, as indicated by the change in the m a x i m u m s p r e a d Sma x in s t r e s s e s in r e l a t i o n to the n u m b e r of half c y c l e s K in rigid loading. A s h a r p d r o p in cyclic s t r e s s e s c o r r e s p o n d s to the s t a r t of f o r m a t i o n of a m a c r o c r a c k developing f r o m c o r r o s i o n d a m a g e s (Fig. 4c). As a r u l e c r a c k s in tubular s a m p l e s o r i g i n a t e in the i n n e r s u r f a c e (the side of action of the c o r r o s i v e m e d i u m ) and p r o p a g a t e p r e f e r e n t i a l l y on the i n n e r p e r i m e t e r , s u b s e q u e n t l y growing to the o u t e r s u r f a c e of the s a m p l e gage length (Fig. 4b, c ). At the s a m e t i m e on the control s a m p l e s c r a c k s develop both on the inside and on the outside (Fig. 4a). The v a l u e s of the index m in the Coffin - M a n s o n equation d e s c r i b i n g the rigid loading fatigue c u r v e w e r e unchanged in this work. The r a t i o of the n u m b e r of c y c l e s until the a p p e a r a n c e of a c r a c k ( d e t e r m i n e d f r o m the c u r v e in Fig. 2) in the control s a m p l e s and in the s a m p l e s with c o r r o s i o n d a m a g e was about equal to the r a t i o of the n u m b e r of c y c l e s f o r f a i l u r e f o r t h e s e s a m p l e s . This is an indication of the fact that the c o r r o s i v e 228

2

5"0

101 6 4

100

150 {K-f)

104

6~ 2 j~

O00 -

/yo

gO0.

006

400D

~ 700

~7~_....~

Rubberstopper ]

10' ~4

800 #Oa

10~ lO s

10 e

Fig. 2

10 e

K

lO*

10-'

N o

101

10"~

103

// f

Fig. 3

Fig. 2. Change in width of the h y s t e r e s i s loop ~ ( K ) = 5(K)/ey and the a c c u m u l a t i o n of p l a s t i c d e f o r m a t i o n s e ( K ) = e~K)/e, in r e l a t i o n to the n u m b e r i l J of loading half c y c l e s f o r the i n v e s t i g a t e d m a t e r i a l s in a i r ( ~ , [], o ) , with a p r e l i m i n a r y hold in m e d i u m I ( i , []), and with a p r e l i m i n a r y hold and s u b s e q u e n t testing in m e d i u m II ($) and a l s o the change in m a x i m u m r a n g e of cyclic s t r e s s e s Sma x in rigid leading of tubular s a m p l e s with a c o r r o s i v e liquid (li) and control s a m p l e s ( o ) . Fig. 3. Low c y c l e s t r e n g t h in soft and rigid loading of 10KhSND s t e e l tubul a r s a m p l e s with a p r e l i m i n a r y hold and subsequent testing in a c o r r o s i v e m e d i u m containing c h l o r i d e s and fluorides: ll) hold of 30 days; i ) 90 days; o) tested in a i r .

Fig. 4. Fractures of corrosion and control samples: a) soft loading, control sample, Nf = 2524 cycles, e(0) = 1.3; b) soft loading, preliminary hold of 30 days, Nf = 509 cycles, ~(0) = 1.8; c) rigid loading, preliminary hold of 90 days, Nf = 757 cycles, ~a = 1.7.

m e d i u m during a t i m e of cyclic t e s t s up to 13 h does not influence the r a t e of p r o p a g a t i o n of c r a c k s in 10KhSND steel. The results of the investigations of the influence of aggressive media on low cycle fatigue of structural steels make it possible to draw the following conclusions. Corrosive media containing chlorides with sulfates or fluorides do not influence the resistance of steels to deformation and propagation of cracks in low cycle loading. The decrease in their life in media in low cycle loading is related to the start of formation of a crack

229

initiated b y c o r r o s i o n d a m a g e , the amount of which depends upon the length of contact of the m e t a l with the aggressive medium. LITERAT.URE 1.

2Q 3. 4. 5. 6.

CITED

F. L. Levin, A. D. Goronkova, T. S. K i r e e v a , V. V. Romanov, and O. N. M u r o m t s e v a , " T h e c o r r o s i o n r e s i s t a n c e of s t e e l s u n d e r conditions of b l a s t f u r n a c e g a s c l e a n e r , " in: Special Quality Steels and Alloys [in R u s s i a n ] , No. 1, M e t a l l u r g i y a , Moscow (1976), pp. 81-87. G. V. Karpenko, K. B. Katsov, I. V. I~okotailo, and V. P. Rudenko, The Low Cycle Fatigue of Steel in Working Media [in R u s s i a n ] , Naukova Dumka, Kiev (1977). A. B. Kuslitskii, V. I. Tkachev, I. V. Kokatailo, et al., " T h e influence of s u r f a c e active m e d i a on the low cycle fatigue of s t e e l , " F i z . - K h i m . Mekh. M a t e r . , No. 1, 107-108 (19~7). I. E. Z a m o s t y a n i k , K. B. Katsov, A. B. Kuslitskii, and G. V. Karpenko, " T h e low c y c l e hydrogen fatigue of steel," F i z . - K h i m . Mekh. M a t e r . , No. 2, 117-118 (1972). G. V. K a r p e n k o and V. T. Stepurenko, The Influence of Working Media on the Strength of Steel [in R u s s i a n ] , Izd. Akad. Nauk UkrSSR, Kiev (1961). S. V. S e r e n s e n and N. A. Makhutov, " T h e r e s i s t a n c e of a weld joint of low c a r b o n s t e e l to low c y c l e f a i l u r e in r e l a t i o n to the p r o p e r t i e s of individual zones," P r o b l . P r o c h n . , No. 12, 25-33 (1970).

CHROME-TITANIUM

COATING

OF

CARBON

STEELS L. G. Voroshnin, B. and F. I. Panteleenko

S.

UDC 621.785.5

Kukharev,

Alloying of c h r o m i u m b a s e diffusion c a r b i d e l a y e r s with titanium significantly i n c r e a s e s t h e i r h a r d n e s s , w e a r r e s i s t a n c e , and a n t i c o r r o s i o n p r o p e r t i e s [1]. H o w e v e r , as a r e s u l t of t h e i r thinness (up to 25 It) c h r o m e - t i t a n i u m l a y e r s a r e f r e q u e n t l y defective, which l i m i t s t h e i r wide use. T h e r e f o r e a study of the p o s sibility of intensification of the p r o c e s s of c h r o m e - t i t a n i u m coating with a s i m u l t a n e o u s i n c r e a s e in the thickn e s s of the p r o t e c t i v e coating is of significant i n t e r e s t . We have solved this p r o b l e m b y using the effect of the liquid m e t a l p h a s e in the diffusion l a y e r being f o r m e d o c c u r r i n g as a r e s u l t of p r e l i m i n a r y t r e a t m e n t of the m a t e r i a l being s t r e n g t h e n e d [2 ]. The p r e l i m i n a r y t r e a t m e n t of the m e t a l s (technically p u r e iron, 45, US, and U12 s t e e l s ) was diffusion zinc coating, which was done at 700 deg C f o r 1 h in a 78A1203 + 20Zn + 2NH4C1 b y weight m i x t u r e . The c h r o m e - t i t a n i u m coating was done at t e m p e r a t u r e s of 850-1050 deg C for 2-8 h in a r e d u c e d a l u m i n o t h e r m i c 28A1203 + 14A1 + 40Cr203 + 16TiO 2 + 2NH4C1 m i x t u r e . It was e s t a b l i s h e d that t h e s e conditions of t r e a t m e n t a r e the o p t i m u m in r e l a t i o n to the quality of the t r e a t e d s u r f a c e and the t h i c k n e s s of the diffusion l a y e r s obtained. The influence of final t r e a t m e n t conditions on the thickness of the c h r o m e - t i t a n i u m l a y e r is i l l u s t r a t e d by the data given in T a b l e 1. P r e l i m i n a r y zinc coating m a k e s it p o s s i b l e to i n c r e a s e the diffusion l a y e r thickn e s s by 2 to 15 t i m e s (depending upon the c h r o m e - t i t a n i u m coating conditions and the type of s t e e l ) . F o r e x a m p l e , on U12 s t e e l c h r o m e - t i t a n i u m coated using this method at 1050 deg C for 8 h a 95 t~ thick c a r b i d e l a y e r was obtained while in t r e a t m e n t using the traditional method it is 25 ~. The s t r u c t u r e of the diffusion l a y e r f o r m e d is d e t e r m i n e d b y the c h e m i c a l c o m p o s i t i o n of the s t e e l and the conditions of p r e l i m i n a r y and final c h e m i c o t h e r m a l t r e a t m e n t . The diffusion l a y e r of p r e l i m i n a r i l y z i n c coated technical i r o n obtained with the t r e a t m e n t conditions c h o s e n by us is a solid solution of c h r o m i u m , titanium, and zinc in a - i r o n with a s u r f a c e content of the e l e m e n t s of 52, 4.7, and 0.25 wt.%, r e s p e c t i v e l y . In m e d i u m c a r b o n s t e e l s the diffusion l a y e r c o n s i s t s of a zone of (Cr, Ti, Fe)23C 6 and (Cr, Ti, Fe)TC 3 c a r b i d e s with inclusions of a - p h a s e and a zone of solid solution of c h r o m i u m , titanium, and zinc in a - i r o n . The t i t a B e l o r u s s i a n Polytechnic Institute, Minsk. T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika M a t e r i a l o v , Vol. 16, No. 3, pp. 43-46, M a y - J u n e , 1980. Original 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 28, 1978.

230

0038-5565/80/1603-0230507.50

9 1981 Plenum Publishing C o r p o r a t i o n