CORROSION
RESISTANCE
IN AGGRESSIVE
MEDIA
(POLYACRYLONITRIL E.
M.
Frolikova
OF WELDED FROM
E)
Kh17N13M2T
PRODUCTION
OF
STEEL*
NITRON
FIBRE
and
T. K.
Prokop'eva
UDC 677.4-620.193.56
Stainless Kh17N13M2T steel was used in the fabrication of a pilot spinning and finishing treatment plant for Nitron p o l y a c r y l o n i t r i l e fibre production. T e s t s on this steel in a 12% sodium thiocyanate solution a t r o o m t e m p e r a t u r e (the operational conditions of the spinning baths) showed that welded joints, made m a n ually by e l e c t r i c - a r c and a r g o n - a r c welding, showed inadequate c o r r o s i o n r e s i s t a n c e in the heat-affected zone. The s u r f a c e of the welded joint b e c a m e c o v e r e d with c o r r o s i o n deposit and the solution was d i s c o l oured with a brown tinge; the parent metal was not corroded. L a b o r a t o r y tests have shown that welded joints having s a t i s f a c t o r y c o r r o s i o n r e s i s t a n c e in the abovementioned medium can be obtained by chemical cleaning after welding. EXPERIMENTAL
RESULTS
The chemical composition of the Khl7N13M2T steel used for test is given below (%): C ........... Si . . . . . . . . . . . Mn . . . . . . . . . . Cr . . . . . . . . . .
0.07 0.36 1.37 16.18
Ni . . . . . . . . . . . . . Ti . . . . . . . . . . . . . Mo . . . . . . . . . . . .
12.27 0.5 1.74
The dimensions of the test specimens w e r e 5 0 x 8 0 × 1 . 5 mm. The steel plates were welded to two different specifications: manual a r c - w e l d i n g using EA400/10-U e l e c t r o d e s and SV-04Kh19NllM3 filter and by manual a r g o n - a r c welding using SV-04Khl9NllMe or Khl7N13M2T filler wire. Surplus metal was not r e m o v e d from the weld s e a m s . Specimens of parent metal and of welded joints w e r e checked for tendency towards i n t e r c r y s t a l l i n e c o r r o s i o n by the AM method (GOST 6032-58) before c o r r o s i o n t e s t . No i n t e r c r y s t a l l i n e c o r r o s i o n w a s observed. Specimens w e r e tested after welding and after chemical cleaning. Following t r i a l s on m o r e than 20 known compositions of chemical cleaning solution, solution I as used by the "Uralkhimmash" plant [1] and solution II as used by the tube industry [2, 3] were selected. * Titanium steel with 17% Cr, 13% Ni, and 2% Mo.
Fig. 1. C o r r o s i o n pitting at the heat-affected zone of welded joints after 1200 h test in the spinning bath: a) overall picture of welded joint; b) c o r r o s i o n - p i t t i n g section (x 10). All-Union Scientific R e s e a r c h and Design Institute of Chemical Machinery Construction (NIIKhIMMASh). T r a n s l a t e d f r o m Khimicheskie Volokna, No. 5, pp. 60-62, S e p t e m b e r - O c t o b e r , 1969. Original a r ticle submitted April 4, 1968.
564
T A B L E 1. Specifications for C h e m i c a l ly Cleaning Welded Joints Cleaning agent Temperature, °C
Solution I SolutionII Paste
2 21 50 20
Duration, h 5
0.55 1 12
A smooth, matte, metal s u r f a c e was obtained after c h e m ical cleaning with no t r a c e of c o r r o s i o n . The given solutions have p r a c t i c a l l y no effect on s c a l e - f r e e metal. The welded joints were cleaned completely of scale and there remained only a ring of slag inclusion after e l e c t r i c a r c welding and of scale, introduced into the weld s e a m f r o m the w e l d - r o o t side, after a r g o n - a r c welding. As a rule, i n c r e a s i n g the duration of c h e m ical-cleaning time did not produce a higher degree of cleaning of the surface.
Chemical cleaning of the welded joints was p e r f o r m e d to the following p r o c e s s sequence: d e g r e a s i n g {by m a g n e s i u m oxide paste or dipping in an alkaline solution); washing; chemical cleaning (dipping in the pickling solution); wash; drying. A chemical cleaning paste was also tried, this being applied to a previously--degreased and dried w e l d joint surface. A high d e g r e e of cleaning was obtained by repeating application of paste for some 6 h. Howe v e r , the s u r f a c e quality after t r e a t m e n t with paste was substantially below that obtained with solution c l e a n ing; t h e r e f o r e , paste was used only for cleaning welded joints before r e - f i t t i n g plant equipment. The compositions of the two solutions and of the paste a r e given below: Solution I (g/litre) H2SO4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NaNO 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NaF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solution II (%) HNO 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P a s t e (g/litre) H2SO4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NaC1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Asbestos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
200 25 20 8 1.5 200 20 400
The weld-joint specimens w e r e c h e m i c a l l y cleaned to the specifications given in Table 1. Specimens of welded joints f r o m KhlTN13M2T steel w e r e tested in various sections of the spinning and fibre finishing p r o c e s s e s at the Nitron fibre production plant of the Saratov Chemical Combine. The s p e c i mens w e r e exposed to the p r o c e s s solutions in completely s u b m e r g e d conditions and also at interface conditions for periods of 1200 to 3000 h. C o r r o s i o n r e s i s t a n c e was evaluated to a ten-point scale as r e c o m m e n d e d in GOST 5272-51. C o r r o sion test results on these specimens a r e given in Table 2.
Fig. 2
Fig. 3
Fig. 2. Welded joint specimen chemically cleaned with solution I after 300 h test in spinning bath. Fig. 3. I n t e r c r y s t a l l i n e c o r r o s i o n at heat-affected zone of specimen manually e l e c t r i c - a r c welded with no solution chemical cleaning, a f t e r 1200 h in spinning bath (× 500).
565
T A B L E 2. C o r r o s i o n T e s t R e s u l t s on S p e c i m e n s Held u n d e r D i f f e r e n t C o n d i t i o n s
Testc°n iti°ns°fw Item, lded °ints liquidC°rr°si°nrat 'Year
Welding method
anti-corrosion duration of treatment
h in
phase
at interface
S p i n n i n g bath (8-11% NaCNS, t = 9-11*C) Argon-arc welding (filler wire sv-04Khl9NllM3)
After welding Solution I Solution II Paste
1200 3000 3000 1200
0.002* 0.0001 0.0002 0.002
Electric-arc welding (electrodes EA 400/10U)
After welding Solution I Solution II Paste
1200/3000 3000 3000 3000
0.008 0.0001 0.0002 0.0008
0.0002 0.0001 0.0006
0.009* 0.0003 0.0001
o.oo7~
P r e l i m i n a r y h e a t i n g bath (4% NaCNS, t = 50"C) Argon-arc welding (filler wire sv-04Khl9NllM3)
After welding Solution I Solution II Paste
1200 3000 3000 1200
0.003* 0.0003 0.0006 0.002
0.002* 0.0003 0 0.002*
Electric-arc welding (electrodes EA 400/lOU)
After welding Solution I Solution II
1200/3000 3000 3000
0.008 0.001i"
0.003~" 0.0003* 0.0004*
0.004~
Spun yarn w a s h i n g bath ( 0 . 5 - 0 . 8 % NaCNS, t = 5 0 ° C Argon-arc welding (filler wire sv- 04Khl9NllM3)
After welding Solution I Solution II
1200/3000 1200/3000 1200/3000
0.001 0.0005 0.0003
0.001. 0.0002 0.0001
Electric-arc welding (electrodes EA 400/10U)
After welding Solution I Solution II
1200/3000 3000 3000
0.007 0.002 0.00124
0.0034 0.0012~f 0.0001
* Corrosion pits were observed in heat-affected zone. Intercrystalline corrosion observed in heat-affected zone. Note: First figure given for duration of test is for submergence in liquid, second figure is for test at air- liquid interface.
DISCUSSION
OF
RESULTS
T a b l e 2 shows that the g e n e r a l c o r r o s i o n r a t e of s p e c i m e n s w e l d e d b y a r g o n a r c o r by e l e c t r i c a r c w e l d i n g does not e x c e e d 0.01 m m / y e a r i n any of the t e s t m e d i a (within the s c a l e 3 g r o u p of Wexceptionally r e s i s t a n t materialW). V i s u a l e x a m i n a t i o n of the s u r f a c e of s p e c i m e n s , a r g o n a r c w e l d e d without s u b s e q u e n t c h e m i c a l c l e a n i n g and t h e n s u b m e r g e d i n the t h r e e s o l u t i o n b a t h s i n d i c a t e d ( s e e T a b l e 1), shows m o r e d i s t i n c t e v i d e n c e of c o r r o s i o n p i t s at the h e a t - a f f e c t e d zone t h a n that found with s p e c i m e n s t e s t e d at the a i r - l i q u i d i n t e r f a c e (Fig. 1). T h e depth of i n d i v i d u a l c o r r o s i o n p i t s w a s a s m u c h as 0.7 m m . F o r c o m p a r i s o n , F i g . 2 shows s p e c i m e n s w h i c h w e r e c h e m i c a l l y c l e a n e d u s i n g s o l u t i o n I a f t e r w e l d i n g . No c o r r o s i o n p i t t i n g w a s found a f t e r 3000 h of t e s t . I n t e r c r y s t a l l i n e c o r r o s i o n (Fig. 3) w a s o b s e r v e d at the h e a t - a f f e c t e d zone i n e l e c t r i c - a r c w e l d e d s p e c i m e n s (not c h e m i c a l l y c l e a n e d ) a f t e r 1200 h at i n t e r f a c e c o n d i t i o n s i n the s p i n n i n g bath and i n the p r e heating bath. M e t a l l u r g i c a l a n a l y s i s showed that the depth of the i n t e r c r y s t a l l i n e c o r r o s i o n did not e x c e e d 15-20 p. I n t e r c r y s t a l l i n e c o r r o s i o n is p r o b a b l y l i n k e d with the p r e s e n c e of a c a r b u r i s e d l a y e r at the s u r f a c e of the s t e e l p l a t e s a f t e r they w e r e r o l l e d followed b y a c t i o n of the a g g r e s s i v e m e d i u m . I n t e r c r y s t a l l i n e c o r r o s i o n w a s a l s o o b s e r v e d i n s p e c i m e n s , w e l d e d b y the s a m e m e t h o d , but g i v e n s u b s e q u e n t c h e m i c a l c l e a n i n g i n s o l u t i o n s I and II. In t h i s c a s e i t s e x t e n t i n depth w a s q u i t e i n c o n s i d e r a b l e ,
566
amounting to only 5-10 ~. It a p p e a r s that p a r t of the c a r b u r i s e d l a y e r was r e m o v e d f r o m the s u r f a c e of the p l a t e b y c h e m i c a l cleaning. When t i m e was i n c r e a s e d f r o m 1200 to 3000 h, the depth of p e n e t r a t i o n of i n t e r c r y s t a l l i n e c o r r o s i o n b e c a m e no greater~ t h e r e f o r e this f o r m of c o r r o s i o n does not a p p e a r to be dangerous. S p e c i m e n s , c h e m i c a l l y cleaned with p a s t e , showed h i g h - c o r r o s i o n r e s i s t a n c e ( c o r r o s i o n r a t e 0.0010.002 m m / y e a r ) . The r e s u l t s of t h e s e investigations w e r e taken into account when f a b r i c a t i n g the spinning and fibre finishing plant designed by VNIILTEKMASh {Research Institute for Textile Machinery). This plant was installed in the Nitron f i b r e production shop at the Saratov Chemical Combine and has a l r e a d y been in o p e r a t i o n f o r m o r e than one y e a r . No s u r f a c e c o r r o s i o n has been o b s e r v e d on this equipment. CONCLUSIONS 1. Scale, f o r m e d when welding Khl7N13M2T steel, c a u s e s discolouration of sodium thiocyanate s o l u tions and this cannot be t o l e r a t e d in Nitron f i b r e production. 2. Joints, welded both by a r g o n - a r c and by e l e c t r i c - a r c welding, can be f r e e d c o m p l e t e l y of s c a l e by c h e m i c a l cleaning. 3. No c o r r o s i o n pitting w a s o b s e r v e d at the h e a t - a f f e c t e d zone with a r g o n - a r c welded joints which had been c h e m i c a l l y cleaned. 4. It is i m p o r t a n t to check for p o s s i b l e i n t e r c r y s t a l l i n e c o r r o s i o n of the steel. equipment operating in solutions of sodium thiocyanate (GOST 6032-58). LITERATURE 1o
2.
3.
This m a y o c c u r with
CITED
M. B. V i z e l ' b e r g , I. P. Gonibesova, and T s . G. Maizel, Khimicheskoe i Neftyanoe M a s h i n o s t r o e n i e , No. 1, 43 (1967). N. V. Bogoyavlenskaya and Ya. N. Lipkin, Stal', No. 4, 346 (1959). N. V Bogoyavlenskaya, Ya. N. Lipkin, and L. A P e t r u s e n k o , in:Tube Production [in Russian], No. 14, Metallurgiya (1964), p. 110.
567
