steel esr cern
TRANSCRIPT
-
8/10/2019 Steel Esr Cern
1/11
-
8/10/2019 Steel Esr Cern
2/11
C Geyari: Design considerations in the use of stainle ss steel for vacuum and cryogenic equipment
(American Iron and Steel Institute) system and many manu-
facturers give equivalent AISI specifications in their literature.
In the AISI system the basic 18 /8 type austenit ic steel with
approx. 0.1% carbon bears the designation 302, and starting
from this, many modified types have been developed. The
purpose of the changes in alloy composition are improvement
of:
Corrosion resistance (CR)
Weldability (W)
Machinability (M)
Mechanical properties-Yield strength (Y).
The relationship of the various alloys of interest for vacuum
service is shown in Figure I. Arrows point in the direction of
improved properties.
304L+N
316L N
I
Y
I
Y
304 L
316 L
317L
I
W
I
w
CR CR
304
- 316
t
W
M
303- 302
Figure
1. Stainles s steel used in vacuum equipment.
(AJSJ designa-
tion.)
CR = Corrosion Resistance.
W = Weldability.
Y = Yield strength.
M = Machinability.
Improvement of welding properties of stainless steel
A question which arises frequently when stainless steel has to
be selected for welded vacuum equipment is whether Low
Carbon or Stabi lized types should be used; or whether
standard types are adequate.
During the first years of widespread use of stainless steels
the manufacturers often met with the problem of corrosion in
the vicinity of, and parallel to the welding seam as illustrated
in Figure 2. This used to be called Weld Decay but the proper
designation of this phenomenon is Intercrystalline Disinte-
gration, Intergranular Corrosion, or Carbide Precipitation.
The factors causing this type of corrosion are now well
understood. Normally al l carbon is dissolved in the austenite
Figure 2. Intergranular corrosion next to the
grade 832
(AJSJ 302).4
-
8/10/2019 Steel Esr Cern
3/11
-
8/10/2019 Steel Esr Cern
4/11
C
Geyari:
Design considerations in the use of stainless steel for vacuum and cryogenic equipment
C
800
600
400
zoo
0
-
8/10/2019 Steel Esr Cern
5/11
C Geyari: Design considerations in the use of stainles s steel for vacuum and cryogenic equipment
stainles s steel, as well as the definition of 0.2?: proof yield
strength is shown in Figure 8.
When comparing the yield strength of the base metal and the
filler metal in properly executed stainles s steel welds we find
A Deformotion, mm
B Deformation, mm
Figure 8.
A. Strain curve for typical structural steel.
B. Strain curve for typical austen itic stainles s ste el (ductile).
og = Ultimate tensile stress.
o,,.~ = Proof yield stress at 0.2% elongation.
os = Yield stress.
that the yield strength of the filler metal is never lower than the
yield strength of the base metal. In many case s it is considerably
higher-see Table I9
Consequently the weld seam should not be considered to be
the weak link in a properly designed and correctly welded stain-
less steel structure.
The design of low temperature (cryogenic) equipment bene-
fits from the favourable mechan ical properties of the austenitic
stainle ss steels at these temperatures. There is no decrease in
the yield strength and in the ultimate tensile strength, although
there is a slight loss of ductility. The most critical property at
ultra-low temperatures is the impact stren
increase of notch sensitivity at low temper
negligible . The retention of impact streng
ure 9.
Temperature,
Figure 9. Representative Charpy V-notch imp
and 304L stainless steel.
The designer of vacuum equipment
using stronger stainles s steel material, with
the other favourable properties of the st
cryogenic vacuum insulated vessels in whic
reduction of wall thickness in order to
Usually these vessels have to withstand a
1 bar, at very low temperatures. The m
increase the yield strength are:
I. Addition of nitrogen to the alloy.
2. Cold stretching.
3. A combination of 1 and 2.
This is illustrated in Figure IO. *
The following Figure I I I shows the
strength by nitrogen alloying or cold-stretchi
As an indication of the savings obtainab
that an increase of 40% in the 0.2 % proof
a reduction of wall thickness in the order
The influence of cold working in mechanica
Due to the fact that most vacuum vessels
working processes , s uch as
Deep drawing (Dished
Necking
Us)
Bending and flanging
Table 3. Yield strength in austenitic stainles s s teels and weld metal
-
8/10/2019 Steel Esr Cern
6/11
C Geyari: Design considerations in the use of stainle ss steel for vacuum and cryogenic equipment
I. Nitrogen alloying
2. Restretching of plates ond
sheejs
3. Coidstrefching of welded
vessels
Pf i
4. Combinofions of i-3.
o- 0.2
kg/mm2
40
30
20
:
#
OO- 0.20 % N
IO-
OOU
Cold stretching %
Figure10.Methods or raisingheyield strength f austenitic tainless
steels.
%C %O %NI %N
-832MV DO4 16.0 95 OD4
---8rn 0.04 16.6
a5
0.1 I
Figure 12 demonstratesclearly the
yield strength due to cold working. The
larger rate or increase,consequently th
shown n the diagram by the distancebe
lines,decreases.he considerablencreas
100
IO-
o---1
0 IO 20 30 40
% Coldslrelchmg
Figure12. Yield strength, ensile trength,e
versushe degree f cold stretchingor gra
MV).
should also be noted. However, the be
considerable hanges hat take place n t
ties s to try and drill a hole into stainle
gone a 30 deformation. The shift in
due to cold working is shown n Figure 1
Pressureest
In this connection it might be opportun
usual method of pressure esting of ves
of 1.5 x working pressure hould be us
where stainless teelvessels re concern
the vesselwhich are stressed y normal
below the 0.2 proof strength it is qui
parts will be stressedo more than 0.2
application of 1.5 x working pressure,
will be cold-deformed. Thus a change
-
8/10/2019 Steel Esr Cern
7/11
C Geyari: Design considerations in the use of stainles s steel for vacuum and cryogenic equipment
Kg/mm
Magnetic properties at low temperatures
60
50
40
30
20
10
0
0 10
20 30 40 50
60
70"
I.
Figure 13. Stress/strain curvesrz.
832 MV = Type 304 stainles s steel.
the austenitic structure into martensite. Since we are concerned
with vacuum apparatus which is often used in high magnetic
fields, or for exact magnetic measurements the probability of
the non-magnetic stainle ss steel being transformed into magnetic
metal by cold working operations without subsequent annealing
must be borne in mind. T he degree of martensite formation by
cold working is influenced by the alloy composition and the
temperature conditions Austenite forming elem ents (Ni, C, N)
are instrumental in preventing a significan t increase in magnetic
permeability, without eliminating the increase in tensile strength.
The combined effects of cold working and low temperature
on martensite formation in a type 304 (18/8) austen itic steel is
shown in Figure 14. The broken room temperature line is of a
similar shape as the bottom line in Figure 12. The formation
of martensite (magnetic phase) at cryogenic temperatures is
evident.
The austenite-martensite transition takes
tures in annealed stainles s ste el without
or cold work being applied. The tempera
austenitic structure loses its stability and
to transform into a martensitic structure
mainly de pendent upon the alloy compositi
forrners Ni, C, N determine Ms as shown
u
c
- +2GGc .
Ni content, %
Figure 15. Lowering of the Ms point by nickel
0.04% C and 4-12% Ni.
The add ition of 0.01% of N or C low
approximately I7 K.r3 Obviously the nom
the alloy does not give a definite indication
to the fact that the commercial tolerances
sition will cause the MS point to vary b
Since the Ms point for most of the commercia
steel is placed at about 270 K it is advisabl
with a nominal compos ition indicating a
in those cases where the non-magnetic
maintained at very low temperatures.
alloys (approximately 0.2% N) are one p
tion. Table 4 shows the ranking of auste
(AISI Type Numbers) for stability of meta
the 4.2 . . . 300 K temperature range:
Table 4. Structural stability of austenitic stain-
less steels at low temperatures (4.2 . . . 300 K)
Doubtful Stable
Stable 316 316 LN
316
L
316
LN
304 304
LN
304 L 304N
321
-
8/10/2019 Steel Esr Cern
8/11
C Geyari: Design considerations in the use of stainle ss steel for vacuum and cryogenic equipment
Porosity problems
Where equipment is used under high or ultra-high vacuum
attention has to be paid to the appearance of minute flaws in
the steel. The microsco pic size of these imperfections is very
troublesome, it being highly probable that during production
the flaw will be blocked by lubricating media, polishing paste,
or even by the solutions used for cleaning the metal surface.
Thus no leak is indicated during room temperature
leak
testing.
However, at a later stage, such as bake-out or cooling. a minute
leak appears. At that time it is extremely difficult to determine
whether the leak is an acute or a virtual one, or whether the
pressure rise noted is due to outgassing.
The reason for the occurrence of these flaws, or for the
apparent porosity of the steel is found in the process of making
sheets or bars at the steel mill. There the steel is cast at very high
temperature into ingots. During the cooling of the ingots
various impurities, mainly sulph ides and oxides, float on top
of the ingot. Although the top part of the ingot is cut off before
the steel is rolled into sheets or bars, there is a possibility of
voids and inclusio ns remaining in the centre of the ingot. These
are reduced in diameter during rolling, but do not disappear
entirely. This is shown schem atically in Figure lb.
Rolling 1
BARS BARS 8 PLATES TUBES
Figure 16.
Schem atic-inclusions in steel during casting and rolling.
These remaining voids and inclusio ns are potential leak
paths, which become apparent only after machining of the
metal. It is therefore mandatory to consider the direction of
rolling of the material when designing vacuum components
made of stainless steel. l s
These considerations are illustrated in Figure 17. A-shows
a typical flange for high vacuum. If this is cut out of a plate B
LEAK
Figure 17. High vacuum flanges porosity.
(b) Using Electra Slag Refined (ESR)
material is more expensive it is reco
critical applications . Table 5 co
stainless steel with ESR stainless ste
Table 5. Typica l values for austenitic stainles s
Electric arc furnace
with oxygen blowing
Area of oxides
Area of sulphide s
Inclusion index
Oxygen content
0.010-0.030%
0.0304050 %
60-140 mm/dm2
0.006-0.100%
Selection of materials for the intersecting
CERN
-
8/10/2019 Steel Esr Cern
9/11
C Geyari: Design considerations in the use of stainle ss steel for vacuum and cryogenic equipment
Figure 18. Schem atic diagram-relationships between the CERN
Proton Synchrotron (PS) and the Intersecting Storage Ring s (ISR).
two circular vacuum chambers in such a way that the required
vacuum (IO- torr) can be continuously maintained. A vacuum
of IO- i to IO- 1 must be maintained in the intersection regions.
A few years ago 10m9 torr could only be obtained in certain
laboratory equipment.
Maintaining such a vacuum in a chamb
approximately 2 km and with thousands
makes the ISR one of the most advanced
the world.
The configuration of the proton beam
of approximately 160 :: 52 mm along the
of the rings. Where possib le, tubes with a
of 169 mm were used. A number of magnets
beam have been placed at regular interspac
In the sections lying within the poles of the
gap is only 60 mm. Tubes with elliptica l cr
for these regions. The outside dimension s
long axis and 60 mm for the short axis.
The following principal requirements w
the material for the tube components.
I. Very low magnetic permeability (ma
in the sheet as well as in the welded joints.
2. High yield strength to prevent the
collapsin g due to the outside pressure an
to use sufficiently thin-walled tubes. A y
min. 42.5 kg/mm was required for a wall
3. Low outgassing rate of the steel
means freedom from dissolved and absorbe
Due to these requirements a nitrogen-b
SKRN (equivalent to AISI 316 + N), with
composition was chosen :
C Si Mn Cr Ni
0.025 0.5 1.8
17.5 13.5
-
8/10/2019 Steel Esr Cern
10/11
-
8/10/2019 Steel Esr Cern
11/11
L
S
p
,
a
J
U
S
S
O
S
,
J
O
J
p
m
o
d
u
m
d
m
d
a
q
a
s
a
m
d
c
p
q
~
I
Z
L
E