Kuppan T. Heat Exchanger Design Handbook

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834

Chapter

Types of Ceramic Heat-Exchanger Construction

Tubular Construction.

Applications for tubular ceramic units include

[255]:

Waste heat recovery in metals industries.

Chemical plant waste heat recovery.

Incinerators.

Closed-cycle gas turbine heat-source exchangers.

Open-cycle gas turbines (exhaust-heated cycle).

Fluidized-bed heat exchangers.

Gas-to-gas heat recovery units.

Nuclear process heat intermediate heat exchangers.

Plate Fin Construction.

Applications for ceramic plate fin units include [255]:

Vehicular gas turbine recuperators.

Industrial gas turbine recuperators.

Stirling engine heat exchangers.

4. High-temperature gas-to-gas heat exchangers,

Rotary generators.

Applications requiring very compact, small-volume heat exchangers.

33 ALLOYS

FOR

SUBZERO TEMPERATURES

Steels and nonferrous materials are used for containment, handling, and transporting

lique-

fied gas and liquification of gases. Other applications include stationary structures and mobile

equipment exposed

adverse climates or operating conditions or both. Temperatures below

-150°C

(-238°F)

often are identified as cryogenic temperatures. The motive for utilizing

low-

temperature technology is that at cryogenic temperatures liquid gases occupy much less volume

than their pressurized gaseous state. Therefore, the containment vessels for liquid gases may

be smaller, thinner (because of lower pressure), and less costly

[256].

33.1 Ductile-Brittle Transition Temperature

(DBTT)

Some metals display a marked

loss

of ductility in a narrow temperature range below room

temperature. This is called the ductile-brittle transition temperature (DBTT). The application

steels below their nil ductility temperature (NDT)

avoided because of the danger created

by brittle crack propagation which could lead

the catastrophic failure of a component or

entire system

[

161.

Below the nil ductility temperature, very little energy is required for crack

propagation

[

141. The following factors can improve the toughness of steels

[

171:

(1)

ASTM

grain size has a strong effect on a given steel’s NDT. As the grain size number increases (i.e.,

the grains become smaller), the NDT drops. Therefore, fine-grained steels usually are used for

low-temperature applications. (2) Proper heat treatment can be very effective in increasing a

steel’s fracture toughness. Quenching and tempering

an effective heat treatment for improv-

ing toughness.

(3)

Among alloying elements, nickel, titanium, and manganese tend

increase

steel’s toughness.

(4)

Another variable that affects DBTT is thickness.

33.2 Crystal Structure Determines Low-Temperature Behavior

Below ambient temperatures, a metal’s behavior is characterized somewhat by crystal structure

[256]:

Material Selection and Fabrication

835

The yield and tensile strength

body-centered cubic (bcc) structure metals depend greatly

on temperature and generally display a marked loss of ductility in a narrow temperature

range below room temperature (e.g., iron and molybdenum).

Face-centered cubic (fcc) structure metals such as aluminum, copper, nickel, and austenitic

stainless steel often increase in ductility as temperatures decrease, and their tensile strength

is more temperature dependent than their yield strength.

The tensile and yield strengths of hexagonal close-packed (hcp) structure metals are more

temperature dependent, and they usually suffer a severe loss of ductility at subzero temper-

atures (e.g., zinc, titanium, some alpha-titanium alloys); however, they have good ductility

down to cryogenic temperatures.

Plastic deformation and cryogenic temperature can cause a normally ductile and tough

stainless steel such as

301,

302, 304, and 321 to partially transform to a bcc structure, thus

affecting ductility and toughness.

Minor variations in composition can affect ductility in certain materials. For example, an

increase in oxygen from

0.10

to 0.20% in a ductile titanium alloy can lower ductility at

-423°F

(-253°F)

from

to almost

0%.

33.3

Requirements of Materials for Low-Temperature Applications

A major requirement of materials for liquefaction equipment and for containment and transport

of liquefied gases is toughness at the handling temperatures of the liquid. Additional require-

ments include minimum weight, weldability characteristics to resist hot cracking, cold crack-

ing, and embrittlement of heat-affected zones, high strength and toughness of the welded joints,

and corrosion resistance, low thermal expansion to minimize dimensional changes due to tem-

perature difference between the ambient and service temperature, relatively low specific heats,

low thermal conductivity to minimize thermal conduction, etc. Carbon steels have only limited

corrosion resistance and must be replaced with corrosion-resistant alloys when metal loss be-

comes severe.

33.4

Notch Toughness

Notch toughness is defined as the ability to resist brittle fracture at high stresses, such

can

be caused by impact loading. Notch toughness

measured by various means. The favored

method recommended by the American Petroleum Institute as well as the ASTM and ASME

is the Charpy V-notch test. It is considered the most appropriate because a part or structure

will generally fail due to a notch or other stress concentration

[257]

or a defect such as a

gouge, weld crack, arc strike, or a sharp discontinuity

[258].

Using the Charpy V-notch test,

one can determine the transition temperature at which

material becomes brittle. This informa-

tion helps the designer to choose a steel that will remain ductile through the range of tempera-

tures or stresses it will be subjected to in service.

Notch Toughness-ASME Code Requirements

The ASME Code should be consulted for allowable stress at low temperatures and for the

testing required of the material to ascertain that it is suitable for low-temperature service. The

ASME Code stress tables designate

-20°F

as the beginning of low temperature.

33.5

Selection of Material for Low-Temperature Applicatlons

Selecting materials for low-temperature and cryogenic applications calls for thorough under-

standing of the application and knowledge of the mechanical properties that each grade of

metal provides. Since various low-temperature materials are available, the designer must con-

836

Chapter I3

sider the merits of each material according to the application. Some factors suggested

Marshal1 [257] include cost by strength ratio, welding, and fabricating costs, and where extra

high strength and good impact properties to below -75°F (-59.4"C) are called for, an alloy

steel should be chosen.

33.6

Materials for Low-Temperature and Cryogenic Applications

Aluminum, copper, Titanium, nickel-base alloys, ferritic steels including 9% Ni steels, and

stainless steels offer designers a wide choice and have been used successfully for liquefaction,

containment, and transport of liquefied gases. The selection of aluminum, copper, nickel- and

cobalt-base alloys, titanium, low-alloy ferritic steels including 9% Ni steels, and stainless steels

is discussed next. Table

shows a list

materials for low-temperature application along

with the minimum temperature applicability.

Aluminum for Cryogenic Applications

Aluminum and aluminum alloys have fcc crystal structures and retain good ductility at subzero

temperatures. Aluminum can be strengthened by alloying and heat treatment while still retain-

ing

good

ductility along with adequate toughness at subzero temperatures. Among the alumi-

num and aluminum alloys, 1100, 2014 to 2024, 2219-T87,

3003,

5052,

5083-0,

5086 and

5456, 6061-Tb, and 7005, 7075, 7079, and 7178 are recommended for cryogenic applications.

Table

Materials for Low-Temperature Application

Alloy designation Specification Approximate

lowest temperature

Low-carbon steel

442, A 516

-50°F (-46°C)

A 537

-75°F (-59°C)

A 662,

724

Alloy steel A5 17

-75°F (-59°C)

A203, Gr. A and

2.25% Ni

-90°F (-68°C)

A203, Gr.

E, and F, 3.50%

-150°F (-101°C)

A645,

5.0%

-320°F (-196°C)

A353

-320°F (-196°C)

A553, Type I1 8%

-275°F (-170°C)

A553, Type

-320°F (-196°C)

A543

-180°F

(-1

18°C)

A736

-50°F (-46°C)

A844 (9% Ni)

-320°F (-196°C)

Stainless steel

304, 304L, 316, 316L, 347

-452°F (-269°C)

Aluminurn

1100, 2014

2024, 2219-T87, 3003, 5083-0, 5456,

-452°F (-269°C)

6061-Tb, 7005

Copper

C10200, C12200 (DHP), C17200, C22000, C26000,

-325°F (-198°C)

C5 10o0, C70600, C7 1500

Nickel

Monel-K, Hastelloy

Hastelloy C, Inconel alloy 600,

-452°F (-269°C)

706, Inconel alloy 718, Invar-36, Inconel alloy

-441°F (-263°C)

x-700

Titanium Ti-5A1-2.5Sn, Ti-6A1-4V (ELI)

-320°F

(-1

96°C)

Ti-5A1-2.5Sn

(ELI)

-423°F (-253°C)

Note: Refer

code

for

applicable lowest temperature.

Material Selection and Fabrication

The salient features of these alloys are discussed next. Before that, the factors that favor alumi-

num use in cryogenics are listed [259]:

1. Unlike other metals, aluminum has no ductile-to-brittle (DBT) transition regardless of the

direction of stress.

2. Inertness to cryogenics like methane, ethylene, argon, helium, neon,

O?,

N?,

H,.

3. Absence of corrosion at cryogenic temperatures; protective coatings are rarely necessary,

but the designer should be cautious of galvanic couple.

4. The tensile strength increases proportionately more than yield strength in the cryogenics,

heavily stressed tanks can warp more at cold than at room temperature without failing.

Tear resistance, another measure

toughness, is the energy needed to propagate a crack.

Tear resistance of 5083-0 and its welds is as high at -196°C as it is at room temperature.

6. Emissivity and reflectivity: aluminum's low emissivity improves the effectiveness of insu-

lation systems [260].

7. Weldable and provide excellent mechanical properties in the as-welded condition.

3000

series:

Aluminum alloy 3003 is used in fabrication of brazed plate fin heat exchangers

and other equipment in gas liquefaction plants. It is available as tubing (including finned

tubing), pipe, sheet, and plate. It is readily joined by brazing or welding.

3003

meets the

requirements of the

ASME

Boiler and Pressure Vessel Code for working temperatures

upto -321°F (-196°C).

5000

series:

Alloys such as 5052, 5083, 5086, and 5456 exhibit a combination of properties

that make them popular for most applications. Their moderate strength, good toughness,

and good weldability have resulted in their selection for oceangoing tankers for carrying

oxygen, LNG, and other cryogenic gases, and for tank trailers, stationary storage contain-

ers, and processing equipment. For plate fin heat exchangers, 5083-0 is used.

6000

series:

Alloy 6061 offers the advantage listed for

5000

series alloys except that in the

as-welded condition its strength is low.

7000

series:

Alloys such as 7075, 7079, and 7178 display the highest strength of all aluminum

alloys. But they lose toughness below -320°F (-196°C). They are generally nonweldable

and found only in limited application. Two newer alloys, 7039 and X7007, show promise

for cryogenic service because they are readily welded and retain adequate toughness at all

temperatures [256].

Copper and Copper Alloys

Copper and copper alloys have fcc crystal structures similar to those of aluminum and retain

a high degree

ductility and toughness at subzero temperatures, down to -423°F (-253°C).

Copper alloys that might be considered for use at subzero temperatures are C10200 oxygen-

free copper, C12200

(DHP),

C17200, C22000, C26000, C51000, C70600, and C71500 [205].

The development of light-weight brazed aluminum heat exchangers for cryogenic applications

caused copper to be replaced by aluminum for many of these components.

Titanium and Titanium Alloys

Commercially pure titanium may be used for tubing and small-scale cryogenic applications

that involve only low stresses in service. For temperatures down to -320°F (-196"C), the

normal interstitial (NI) grade alloys Ti-5A1-2.5Sn and Ti-6A

-4V are suitable. Interstitial impu-

rities such as iron, oxygen, carbon, nitrogen, and hydrogen reduce the toughness of these alloys

at both room and subzero temperatures. For temperatures below -196"C, extra low interstitial

(ELI) grades of Ti-5A1-2.5Sn and Ti-6A1-4V are used [205]. The lower strength, all-alpha Ti-

5A1-2.5Sn

EL1

is used down to -423°F (-253"C), the temperature of liquid hydrogen. Titanium

838

Chapter

and titanium alloys are not recommended for containment or other use with either liquid or

gaseous oxygen in cryogenic service, because any fresh surface caused due to abrasion or

impact exposed to oxygen will cause ignition and hence possible explosion.

Nickel and High-Nickel Alloys

Nickel is an fcc metal that retains good ductility and toughness at subzero temperatures. Unal-

loyed nickel is low in strength and has only limited applications at subzero temperatures.

However, several nickel-base alloys, including some superalloys, exhibit excellent combina-

tions of strength, ductility, and toughness up to

-44

(-263°C). Typical nickel-base alloys

for cryogenic applications include Monel K-450, Hastelloy

Hastelloy C, Inconel alloy

600,

Inconel alloy 706, Inconel alloy 718, Inconel X-700, and Invar-36 (36% Ni-iron) [205].

Carbon Steels and Alloy Plate Steels

ASTM specifications A203, A353, A442, A516, A517, A537, A553, A612, A645, A662, and

A724 describe steel plates with minimum Charpy V-notch energy or lateral expansion require-

ments at testing temperatures from -15 to -320°F (-26 to -196°C).

Carbon Steels.

Carbon steels provide service to -75°F. Less costly than alloy steels, they

combine better weldability with low coefficients of thermal expansion and thermal conductiv-

ity. In carbon steels, the principal means of improving notch toughness is through changes in

composition of C, Mn, Si, and A1 contents. Carbon lowers toughness, whereas Mn increases

it. Si and A1 are added as deoxidizers. Silicon-killed steel has slightly better notch toughness

than semikilled steel, and silicon-aluminum-killed steel has still higher toughness

[

2581.

ASTM A516.

The major advantage of A516 steels is their low initial cost. But they

feature the lowest ASME stresses, 13,750 to 17,500 psi. Thus, a given design strength requires

heavier gages than are needed with high-strength steels. A516 steel is used widely in air

liquefaction plants, refrigerating plants, transport equipment, and containment vessels operating

down to -50°F (-46°C) [257). For these applications, the steel is normally made to meet

impact test requirements of ASTM A300 Class 1 specification, which calls for plates

normalized and to meet a Charpy keyhole minimum of 15 ft-lb at -50°F.

A51

Of the low-temperature alloy steels, A517 Grade F has the highest allowable stresses.

At -50°F its impact strength (Charpy V-notch) is

ft-lb, and its notch and crack resistance

are sufficient to encourage wide usage.

ASTM A537

Grades.

Higher strength with good notch toughness is available in carbon steels

such as the two classes listed in ASTM A537 grades, normalized (Class A) or quenched and

tempered (Class

B),

which provides 60,000 min psi yield strength plus 15 ft-lb of impact

strength (Charpy V-notch) at -75°F.

2.25%

Nickel Steels.

ASTM specification A203 Grades A and

are used in service down to

-90°F (-68°C). The low-temperature requirements are given in ASTM specification A300.

3.5%

Nickel Steels.

ASTM specification A203 Grades

and

are used in service down to

-148°F (-100°C). Forgings and bolting materials are also covered in ASTM specifications.

The low-temperature requirements are given in ASTM specification A300.

Nickel Steel.

This is used as

wrought material for service down to approximately

-185°F (-12O"C), for the fabrication of welded vessels for handling and storage of liquid

ethylene in land based plant and marine tankers. Quenched, temperized, and reversion

an-

nealed, ASTM

A645

specifies 5% nickel steel designed for LNG service. Armco Cryonic

covered by ASTM A645.

ASTM A353

Alloy Steel.

This is one grade

Ni steel, normalized and tempered for subzero

use. Yield strength is 75 ksi; specification for longitudinal and transverse impact (Charpy V-

notch) energy is 20 ft-lb minimum at -320°F (-196°C).

Material Selection and Fabrication

839

ASTM A553 steels contain

or 9% nickel and are essentially quenched and tempered

ksi yield strength (minimum). Impact energy minimum is 25 ft-lb (longitudinal) and 20

ft-lb (transverse) at -196°C (-320°F) for Type I,

Ni, or -170°C (-275°F) for

Ni steel

(Type 11). The welding considerations are the same for both types of steel regardless of heat

treatment. The 9% nickel steels have long been used for ethylene, methane, LNG, oxygen, and

nitrogen applications. One of the benefits of nickel steels in design of LNG vessels is volume

and weight savings due to their relatively high strengths. Selection criteria and fabrication

aspect of 9% nickel steel are discussed in detail later.

36%

Ni-Iron Alloy.

Low expansion 36% nickel-iron alloy is marketed under various trade

names, including Invar 36, Nilo 36, and Dilavar. It is used principally for sea transportation

and land-based storage of liquefied gases. It is also used at service temperatures down to that

of liquid helium -269°C (-452°F). The nickel-iron alloy plate for pressure vessels is covered

in ASTM A658.

Products Other Than Plate

A partial list of ASTM specifications for other than plate steel products for subzero service is:

A333

Seamless and welded steel pipe.

A334

Seamless and welded carbon and alloy steel tubes.

A350

Forged or rolled carbon and alloy steel flanges, fittings, and valves.

A352

Ferritic steel castings.

A420

Piping and fittings of wrought carbon steel and alloy steel.

A522

Forged or rolled

and 9% nickel steel flanges, fittings and valves.

A67 1

Electric fusion welded steel pipe.

A751

Carbon and alloy steel castings.

In addition

the steels listed, a number

proprietary ferritic steels have been developed by

several steel producers to meet certain requirements for service at subzero temperatures.

Austenitic Stainless Steel

In the last decade, there has been a considerable increase in the use of austenitic stainless steels

for cryogenic services at temperatures between -240 and -452°F (-151 and -268.9"C). They

play an important role in LNG ships for containment tanks, storage tanks, cargo piping systems,

and a variety

ancillary equipment.

The AISI 300 type steels such as 304, 304L,

310S,

316, 316L, 321, and 347 offer a fine

combination

toughness and weldability for service down to

-452°F

(-269°C). Among these

alloys, Types 304 and 304L are the most commonly used alloys. Consequently, they have the

largest service experience and coverage in design codes. These grades have moderate strength

and excellent toughness, and they are selected for their formability

fabricability, and ready

availability in a variety of product forms [14,261].

Among the modified varieties, nitrogen-containing, high-proof-strength stainless (e.g.,

Types 304N, 316L

N) is used for cryogenic processing plants and in liquid oxygen and

nitrogen storage and transportation applications. The addition of 0.2% N raises the proof

strength by about 40% and moderately increases the tensile strength without sacrificing ductil-

ity or fracture toughness

[

141.

33.7

Fabrication

Cryogenic Vessels and Heat Exchangers

Most of the cryogenic materials are formable and weldable. Most of the units are welded.

Brazing is limited to manufacture of

PFHE

[256]. Important considerations for fabrication of

cryogenic plants are discussed

detail in Ref. 262. Some

the considerations are:

Chapter

All raw materials and consumables should undergo rigorous quality and specification tests.

2. Edge preparation is considered to be of the utmost importance for all types of welding,

and clear specifications should be laid down and strictly adhered

for all joints.

3. Welding plants should be checked for proper functioning.

4. It is essential that the weld metal and heat-affected zone achieve a fracture toughness

greater than or equal to that of the base metal. The weldment should resist hot and cold

cracking and embrittlement of the heat-affected zone.

33.8

Nickel

Steel

Low-carbon 9% nickel steel was developed in the United States by the International Nickel

Company, Inc. The fundamental mechanical properties of 9% Ni steel must meet the minimum

requirements of ASTM 353 or ASTM 553 Type I specifications. The steel is used for vessels

and plant for processing, transportation, and storage of liquefied gases down to -196°C.

The

excellent low-temperature toughness of this steel results from the alloying addition of approxi-

mately 9% nickel and the presence of stable retained austenite at cryogenic temperature. Nickel

also suppresses the formation of ferritelpearlite high-temperature transformation products; thus,

a microstructure is produced that is higher in strength and notch toughness [263].

Merits of 9% Nickel Steel

Materials previously used, such as copper and austenitic corrosion-resistant steels, presented

no particular difficulties in construction but the cost of these materials was high [264]. The

use

9% nickel steel has increased because it is a cheaper material than those materials

previously used for operation down

-321°F (-196°C). The higher strength of this steel

compared with aluminum, copper, or even austenitic steels makes possible the use

higher

design stresses and hence offers high strength-to-weight ratio. The major factors that favor 9%

nickel steel include

[

141:

High design stress coupled with good fracture toughness characteristics.

Relatively low thermal expansion compared with austenitic stainless steel and aluminum

alloys.

Good weldability by a variety of processes, including shielded metal arc, gas metal arc,

and submerged arc welding.

High melting point and retention of strength at elevated temperatures; this property is

important for the structural integrity under shipboard fire conditions.

Chemically resistant to liquid oxygen and nitrogen, producing no corrosion products that

would hamper the operation of valves and meters or cause unsafe conditions [265].

6. Low thermal conductivity.

Good weldability [266].

Forming of 9% Nickel Steel

The material is readily machinable and may be hot or cold formed. After hot forming, however,

the steel must be heat-treated by double normalizing and tempering unless the hot-forming

temperature approximates the first normalizing temperature, when the first normalizing treat-

ment can be omitted.

Surface Preparation and Scale Removal for Welding

Prior to the welding operation, all extraneous material and surface oxides should be removed

from the weld joint area by wire brushing. Grinding is a more effective procedure. Residual

scale can cause porosity in succeeding layers of weld metal. As an alternative to grinding, it

Material Selection and Fabrication

has been found possible to remove the scale from

nickel steel by flame cleaning, using

conventional oxyacetylene equipment

[267].

Edge Preparation

The edges should preferably be machined or cut by employing flame cutting. The shape should

be such as to afford easily full penetration. Normally the

bevel with open root or an asym-

metrical double

bevel is used

[268].

If the edge is prepared by flame cutting, it will be

necessary to grind it, to remove the surface oxide formed and to eliminate the underlying layer

where the carbon content has become higher.

Welding Procedures

The welding procedures most widely used are manual metal arc welding with covered elec-

trodes, automatic or semiautomatic submerged arc welding with continuous wire, MIG weld-

ing, cored wire welding, and for the root pass TIG welding and plasma arc welding. Employ

MIG welding with a spray mode of metal transfer to overcome lack of fusion problems, to

obtain clean welds while keeping the heat input to relatively low levels

[269].

Electrodes

The electrodes that have been used for

nickel steel can be considered in three classes

[270]:

1. Nickel-based electrodes of the Inconel or Hastelloy type; the trade names are Inco weld

A, Inconel

192,

and lnconel

182.

Chromium-nickel stainless steel electrodes to a very limited extent.

Low-alloy ferritic electrodes.

Guidelines for Welding of

Ni Steel

As is well known,

Ni steel derives its properties through double normalizing and tempering

or quenching and tempering. The welding processes and the weld metal must be such as not

to alter, beyond acceptable limits, the structural characteristics of the parent metal in the heat-

affected zone.

ensure tensile strength and toughness in the fusion zone, and to avoid weld-

ing flaws, a few practical rules are suggested by Pozzolini et al.

[268]:

Use low-hydrogen electrodes with a diameter less than 4 mm and wires with a diameter

less than

3.2

mm.

Lay narrow beads in a number of passes rather than large beads partially overlapping.

Preferably weld in the horizontal position; avoid the vertical position.

not preheat unless absolutely necessary.

Use relatively low heat inputs

(9-16

kJ/cm, according to the welding process) and current

settings to keep the dilution to a minimum.

Use welding processes with high cooling rates, and avoid keeping the material at tempera-

tures exceeding the lowest critical point of transformation.

Avoid any stress-relief heat treatment. If really necessary, it should be carried out at

temperatures below the lowest critical point

transformation.

All the welding materials should be dried prior to welding and the shielding gases should

of controlled purity.

Suggestions for welding

nickel steel are also given by Thorneycroft

[267]

and include the

following:

For thicknesses less than

(25.4

mm),

a single-vee included joint angle of

75-80'

recommended. This practice should

also

be followed for double-vee joints in heavier plate.

10.

The use of chill bars as backup plates is not recommended. The underside of the root

run

should be cooled in air.

842

Chapter 13

11.

It is recommended that the root run size be larger than normal.

12.

The welding gun should be inclined

as to lead the welding arc in the welding direction

order properly to shield the molten deposited weld metal from the atmosphere.

13.

Use electrodes with lime-fluorspar coatings, properly dried to ensure low hydrogen con-

tents in the weld zone.

Welding Problems With 9% Ni Steel

Conventionally, welding of 9% Ni steel is made using high-nickel alloy of the austenitic type.

This process presents some critical problems [27

11:

The yield strength of the weld metal is low compared with that of the base metal.

2. During welding there is a high susceptibility to various forms of hot cracking such

longitudinal bead cracking, and crater cracking of the weld metal.

The high-alloy filler material is relatively expensive and results in higher construction

cost.

Postweld Heat Treatment (PWHT)

Since the 9% nickel steel retains its toughness after welding, there is no need for postweld

stress-relieving treatment. In October 1960 (see

Operation

Cryogenics),

a number of welded

pressure vessels in 9% nickel steel were tested to destruction in the United States

the

as-

welded condition to illustrate that the 9% steel would behave in a tough and ductile manner at

-196°C without stress relief [272]. ASME Code Case 1308-S allows use of the steel in the as

welded condition in thicknesses up to 2 in

(50.8

mm). Fabricators of vessels for some chemical

applications may for sections above a certain thickness, typically above

mm, wish to apply

a stress-relieving operation at a temperature of 1050°F. Further, in some instances it may be

necessary to produce large components by welding the plate required size followed by hot

forming; in such work a double-normalizing and tempering treatment will normally be required

on the welded and formed material [273]. Temperatures for PWHT should

kept below the

plate tempering temperature

prevent the formation

excessive amounts of austenite, which

would affect the toughness, and very slow cooling rates should not

used;

ASME

Code

specifies cooling rates in excess of 166”C/min down to 320°C to minimize the possibility of

“temper embrittlement” [266].

33.9

Welding

Austenitic Stainless Steels

for

Cryogenic Application

The austenitic stainless steels with or without nitrogen strengthening are readily welded by all

of the common welding processes, provided appropriate procedures and consumables are used.

The welding processes include SMAW, SAW, TIG, and MIG welding processes. Guidelines

on welding of stainless steels for cryogenic applications are discussed in Ref. 261. “Guide to

the Welding and Weldability of Cryogenic Steels” (IISAIW-844-87), issued by the IIW, is

principally devoted to the consideration of welding of fine-grain aluminum-killed steels and

nickel-alloyed ferritic steels up to

nickel.

Charpy V-Notch Impact Properties

Due to service conditions, the selection of welding consumables is guided by the mechanical

property requirements for the weld metal. The most important mechanical property is Charpy

V-notch impact at

-450°F

(-268°C); the minimum energy absorption

20 ft-lb and lateral

expansion is

0.015

minimum.

Material Selection and Fabrication

843

Problems in Welding

Four main factors affect the strength and toughness of the as deposited weld metal and take

on added significance at cryogenic temperatures [274]:

1. Sensitization.

Ferrite content.

3. Nitrogen pickup.

4. Oxide inclusions.

These four phenomena are discussed in detail in Ref. 261. The following discussion is

based on that reference.

Sensitization.

The grain boundary precipitation

chromium carbides reduces weld metal

toughness at low temperatures. Weld metal toughness can be improved by using very low

carbon fillers such as 308L

(0.04%

C max.) and 316L (0.03% C max.). For stainless steels

with carbon content higher than 0.03%, annealing of the welds at temperatures greater than

950°C, followed by rapid cooling, dissolves the carbides and improves the toughness.

Ferrite Content.

To avoid microfissuring, for a wide range of stainless steel, the weld metal

is usually balanced to provide

4-8

FN.

However, for cryogenic service, higher ferrite reduces

toughness at -196°C and lower temperature.

obtain a good combination of strength and

toughness of the weld, ferrite content should be maintained in the range of 0-2

[275].

Nitrogen Pickup.

Nitrogen increases the yield strength and decreases the toughness of stain-

less steel weld metals [275,276]. When GTAW and GMAW processes are followed, careful

gas coverage should be provided

avoid nitrogen pickup in the weld metal. While employing

SMAW, lime coatings of the electrode, generally give better coverage and less nitrogen pickup

than titanium coatings.

Oxide Inclusion Content.

Oxide inclusions form sites for the initiation of microvoids. The

toughness at cryogenic temperatures increases with decreasing inclusion contents [276].

reduce inclusions, use basic-coated electrodes, since they generally provide better toughness

than rutile ones, due to lower oxygen content and oxide inclusions than the rutile coatings

[

2771.

CLADDING

Many applications require resistance to corrosive media. It is well known that various grades

of austenitic stainless steels

Types 304, 304L,

308,

316, and 347, nickel, Monel, Inconel,

cupronickel, aluminum, zirconium, and titanium exhibit excellent corrosion resistance

many

corrosive environments. However, the construction

large assemblies such as pressure vessels

and heat exchangers in corrosion-resistant metal involves costs. Consequently, increasing use

is being made of clad materials to achieve the optimum balance of strength and surface proper-

ties to overcome corrosion by an economical means. The potential for severe corrosion

various coal combustion and incineration environments has renewed interest

cladding tech-

nology for both Code-approved and enhanced strength developmental alloys [21]. Apart from

cladding, the other methods of protecting the base metals are lining, which refers to sheet or

strip attached internally to the component by mechanical or intermittent fusion techniques, and

sheathing by external attachment.

34.1

Cladding

A clad plate is a composite plate consisting of a base metal and a cladding of corrosion-

resistant or heat-resistant metal or a plastic (polyvinyl chloride plastic-clad steel plate is a