Nitrogen removal from 26 pet Cr ferritic stainless steel
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its effect on n i t r o g e n . At 26 wt pct C r , the oxygen s o l u b i l i t y i s about t w e l v e t i m e s g r e a t e r than in p u r e i r o n . 2 3) F r o m s t u d i e s on the k i n e t i c s of n i t r o g e n r e m o v a l f r o m liquid i r o n , t h e r e is good e v i d e n c e that the p r e s e n c e of oxygen in the m e t a l s e r i o u s l y r e t a r d s the d e s o r p t i o n r e a c t i o n . F i s c h e r and H o f f m an 3 h a v e shown
Table I. Equilibrium Concentrations at 1600~ for 26 Pet Cr Alloy
Pressure
N, ppm
O at 0.005 Pct C, ppm
O at 0.01 Pct C, ppm
10p 50p
21 48 68 97
2 11 22 44
1 5
100p
2001
11
22
The aboveconcentrations were obtained from numericalsolution of the following equations: Kco = Peo/nc X A o Kn2 =An/(Pnz)
ac =re • %C
ao =f0 X %O
an=FnX%N
logfc = *0.22 X %C+ 0.11 l X %N- 0.097 X ~ - 0.24 X %Cr logJo = 0.2 • %O+ 0.057 • %N- 0.097 X %O 0.24 X %Cr logfn = 0.13 X %C+ 0.05 • %O- 0.45 • %Cr *Values for interactioncoefficientsfrom Refs. 7 and 8.
10
o
i 0 nI-
Z- 0.1
0.01
,,,t,,,,I
0
20
I
I
40
t
I
I
60
Fe - Cr, w/o
I
80
l
100
Cr
Fig. i--Solubility of nitrogen in liquid Fe- Cr alloys at 1 atm pressure of nitrogen and 1600~ VOLUME 3, MAY 1972-1163
0.050
6oo]~
0.040'
Oxy~;e n Contents
:E z"
"
400
o
. ~
Oxygen
"~.]%
OOlO~X 9
|
~0.01%
o
,o
~-
~~~~ ,o
300
i00
,oo
0. uJ
TO TWO COLOUR OPTICAL ~" PYROMETER
400
>
x
"
(G)
o- (L) o
•GAS
(0)
f1
(I)~ (P)~
I'
L__
1__1 (KI
Fig. 3--Levitation apparatus with rapid quenching device. (a) 15 mm Vycor tube; (b) O-ring and clamp; (c) mild steel cylinder; (d) pendulum; (g) sliding b r a s s contact; (b) b r a s s slide-rod; (i) copper blocks; (j, k) solenoid v a n e s ; (l) [evitatiou coil; (m) steel piston; (n) i2 lb. lead hammer; (p) gas inlet.
that the r a t e constant is about one o r d e r of m a g n i t u d e s m a l l e r in liquid i r o n containing 0.15 to 0.24 wt pct O c o m p a r e d with that in m e l t s containing 0.001 to 0.02 wt pct. T h i s b e h a v i o r is a t t r i b u t e d to the fact that oxygen, which is highly s u r f a c e active, occupies m o s t of the p o s s i b l e d e s o r p t i o n s i t e s on the m e t a l s u r f a c e which would o t h e r w i s e be occupied by n i t r o g e n . N e l son, 4 Fig. 2, h a s r e p o r t e d that the s u r f a c e c o v e r a g e of a v a i l a b l e s i t e s i s r e l a t i v e l y high at oxygen l e v e l s above about 0.02 wt pct, and this r e s u l t s in a d r a s t i c r e d u c tion in the n i t r o g e n r e m o v a l r a t e by gas r i n s i n g . LEVITATION EXPERIMENTS Levitation melting experiments were conducted at McMaster University, Hamilton, Ontario to determine nitrogen loss as a function of time. A schematic diagram of the levitation apparatus is shown in Fig. 3. The power required for melting was supplied by a I0 1164-VOLUME 3, MAY 1972
I
-
0~ 0
n
I
200
I00
76r RAIDIU5
(H).
l
20
o
~(A)
A
I z
(J)
\
200
Oxygen
ARGON BUBBLING TIME, MIN. Fig. 2--Influence of dissolve
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