Steel Esr Cern

8/10/2019 Steel Esr Cern

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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


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C

Geyari:

Design considerations in the use of stainless steel for vacuum and cryogenic equipment

C

800

600

400

zoo

0


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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


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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


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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


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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


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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


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