Kuppan T. Heat Exchanger Design Handbook

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784

Chctptrr

Silicon Bronzes (Cu-Si-Ag), C64700-C66 100

C65 100 silicon bronze B (98.5Cu- 1 SSi) and C65500 silicon bronze A (97Cu-3Si) are notable

in this group. These alloys contain about

silicon and manganese at about 1%. They are

known for excellent mechanical properties, workability (both cold working and hot forming),

good joinability, and corrosion resistance, particularly corrosion fatigue.

Aluminum Brass

Aluminum brass owes its resistance to impingement attack in high-velocity seawater and brack-

ish water to a strong tenacious oxide film on its surfaces. It is prone to dezincification; how-

ever, this is overcome by inhibiting with arsenic (0.02-1

%).

The inhibited arsenical aluminum

brass (C68700) finds extensive use in fossil power plant condensers, boiler feedwater heaters,

and shipboard condensers. It is an alternative to inhibited admiralty brass, where cooling-water

velocity is high.

Copper-Nickels (Cu-Ni-Fe), C70ooo-C79900

Copper-nickels or cupronickels, C70400, C70600, C7

1000,

1500,

and C72200, are single-

phase solid solution alloys, with nickel

the principal alloying element. Alloys containing 10

and 30% nickel are important from the heat-exchanger construction point of view. They

are

resistant to fresh, brackish, and seawater. Copper-nickel alloys with the addition of iron and

sometimes manganese are resistant to erosion-corrosion and SCC. Iron enhances the resistance

to impingement attack of these alloys, if it is in solid solution. The presence of iron in small

microprecipitates can be detrimental to corrosion resistance. Copper-nickels are an alternative

to inhibited admiralty metal tubes where cooling-water velocity is high. General corrosion rates

for C70600 and C71500 in seawater are about

mpy. Maximum design velocities for con-

denser tubes are 3.6

m/s

ftjs) for C70600 and 4.6

m/s

fth) for C7 1500.

At elevated temperatures, the creep strength of cupronickels is greater than that of copper

and brasses. Therefore, they find use in high-temperature and high-pressure feedwater heaters

and heat exchangers [187].

The

cupronickels are highly resistant to SCC. Of all the copper

alloys, they are the most resistant to SCC in ammonia environments. Therefore, they are some-

times installed in the air removal sections (Fig. 26)

large-surface condensers.

They are prone to nonuniform and pitting corrosion,

sulfide is present, for instance,

during intermittent service

[

1091. Dealloying has rarely been seen in the cupronickels.

EXHAUST

STEAtl

AIR

EXTRACT ION DUCTS

Figure

Air removal section in a surface condenser

[196].

Material Selection and Fabrication

785

The

10%

copper-nickel (90Cu- 1 ONi), C70600, finds applications

heat exchangers and

condensers in seawater applications, feedwater heaters, and refinery heat exchangers.

The 30% copper-nickel (70Cu-30Ni), C7 1500, is also known as chromium copper-nickel

(67.2Cu-30Ni-2.8Cr). Among all the copper alloys, this alloy exhibits excellent resistance to

impingement attack, SCC, and corrosion of most acids and waters and steam condensates. It

is used

applications involving severe corrosion problems such as power-plant condensers,

feedwater heaters, shipboard heat exchangers and condensers, refinery heat exchangers like

overhead condensers and coolers, and aftercoolers. Because of its lower thermal conductivity

than brass, some designers provide

5-10%

more heat-transfer area

[

1871.

21.5 Designation

Copper and Copper

Alloys

Used as Heat-

Exchanger Materials

A detailed list

copper and its alloys and the nominal compositions used

various types of

heat exchangers is given

Table 38.

21.6 Product

Forms

Copper alloys are available in shapes like plates, sheet and strip, heat exchanger tubes (bare

and integral fin), and ferrule stock. ASTM designations for heat exchanger tubes are shown

Table 39, and for plates, sheets, and strips in Table

40.

22 COPPER CORROSION

22.1 Corrosion Resistance

Copper is classed as a noble metal, and therefore copper is corrosion resistant. The formation

of a thin protective layer on the surface is the reason for the corrosion resistance of the copper

alloys, such as brass, copper-nickel, and bronze. The presence of oxygen or other oxidizing

agent is essential for corrosion to take place. Copper alloys are resistant to neutral and slightly

alkaline solutions with the exception of those containing ammonia, which causes SCC

[

11.

They are resistant to urban, marine, and industrial atmospheres. Copper and most

the copper

alloys are sensitive to velocity effects. They are resistant to water and find use

domestic

and industrial systems and in seawater applications. They are attacked by oxidizing acids,

NH,OH plus

can cause SCC, and hydrogen sulfide, sulfur, and its other compounds attack

[189].

Important sources on corrosion of copper are Ref. 183 and Cieslewicz et al. [190].

22.2 Galvanic Corrosion

Copper and copper alloys occupy a mid position in the galvanic series. Copper is cathodic to

aluminum and stainless steel, yet anodic to passive stainless steel, high-nickel alloys, and

titanium. In seawater applications, tubes made of cupronickels, stainless steel, and titanium are

more noble than tubesheets made of Muntz and aluminum bronze. Therefore, there will be a

galvanic corrosion of tubesheets made of the latter alloys when the tubes are made of the

former materials. Galvanic corrosion will also take place between tube inserts and tubes when

the stainless steel inserts are fitted into copper tubes to overcome erosion-corrosion.

22.3 Pitting Corrosion

The main drawback of copper alloys is their susceptibility to pitting corrosion in water contain-

ing the combination of oxygen and sulfide. Hence, copper is not suitable for coastal water

786

Table

Copper and Its Alloys Used

Heat Exchangers

UNS

no. Common name

Cl0100

(OFE)

Oxygen-free electrolytic (electronic) copper

C 10200

(OF)

Oxygen-free copper without residual

deoxidants

C10300

Oxygen-free extra-low-phosphorus copper

Cl0400

Oxygen-free copper

with

silver

C10500

C10700

C 10800

loo0

Oxygen-free low-phosphorus copper

Electrolytic tough-pitch copper

Cl 1300

Silver-bearing tough-pitch copper

C11400

Cl1500

C116OO

C12000 (DLP)

Phosphorus-deoxidized copper (low-resid-

ual copper)

C 12200 (DHP)

Phosphorus-deoxidized copper (high-resid-

ual copper)

C12300 (DHP)

Phosphorus-deoxidized copper (high-resid-

ual copper)

C 12500

Fire-refined copper

C 1270

C 1280

C 12900

C13000

C14200

Phosphorized, arsenical copper

C 19200

Phosphorized copper

with

1% iron

Cl9400

High-strength modified copper

C23000

Red brass

c26000

Cartridge brass

C26800

Yellow brass

C27000

C28000

Muntz metal

C36500

Leaded Muntz metal, uninhibited

C36600

Arsenical inhibited leaded Muntz metal

C36700

Antimonial inhibited

C36800

Phosphorus inhibited

c44300

Admiralty brass

c44300

Arsenical inhibited admiralty brass

c44400

Antimonial inhibited admiralty brass

C44500

Phosphor inhibited admiralty brass

C46400

Uninhibited naval brass

C46500

Arsenical inhibited naval brass

C46600

Antimonial inhibited

C46700

Phosphorus inhibited

C60800

Aluminum bronze

Chapter

Composition

99.99% Cu min

99.95% Cu min

99.9% Cu min

99.9% Cu

min

99.88% Cu min, remainder Ag, As,

Sb, and others

99.94% Cu min, 0.1-0.5% As,

0.015-

0.04%

98.97% Cu,

1.0%

Fe,

0.03%

97.4% Cu, 2.4% Fe, 0.13% Zn,

0.04%

8446% Cu, remainder Zn

70% Cu,

30%

65% Cu, 35% Zn

60% Cu, 40% Zn

5841%

Cu, 0.4-0.94 Pb, 0.25% Sn

max, remainder Zn

5841% Cu, 0.4-0.94 Pb, 0.259 Sn

max, remainder Zn

7&73% CU, 0.9-1.2%

Sn, remainder

59-62% CU, 0.5-1

.O%

Sn, remainder

95% Cu,

Material Selection and Fabrication

787

Table

Continued

UNS no. Common name Composition

C6 1300

Aluminum bronze, 7%

90% Cu, 67.5% Al,

0.15%

Ni, 2-3%

C6 1400

Aluminum bronze D

91% Cu, 7% AI, 270 Fe

C63000

10% Aluminum-nickel bronze

82% Cu, 10% AI,

Ni,

370

C63200

9% Aluminum-nickel bronze

82% Cu, 9% AI,

Ni,

4% Fe

C65

100

Low-Si bronze B

98.5% Cu, 1.5-2.0%

Si,

0.7% max

C65500

High-Si bronze A

97% Cu,

C68700

Aluminum brass D, arsenical inhibited

77.5% Cu, 20.5% Zn, 2% AI

C70400

Copper-nickel,

95% Cu,

C70600

Copper-nickel, 10%

90% Cu,

10%

C71000

Copper-nickel, 20% 80% Cu, 20% Ni

1500

Copper-nickel, 30% 70% Cu, 30% Ni

C7 1640

Copper-nickel-iron-manganese

29-32% Ni, 1.7-2.3% Fe, 1.5-2.5%

C7 1900

Chromium copper-nickel

67.2% Cu, 30% Ni, 2.8% Cr

C72200

Chromium copper-nickel

83% Cu, 16.5% Ni,

0.5%

Note.

This table has been compiled

from

the ASM Metals Handbook

[

1881.

Table

ASTM Specifications for Copper Alloy Heat Exchanger Tubes

ASTM

spec.

Description Alloys UNS no.

Blll Copper and copper alloy seam-

C10100, C10200, C10300, C10800, C12000, C12200,

less condenser tubes and fer-

rule stock

C 14200, C 19200, C23000, C28000, C44300,

C44400,

C44500,

C60800, C61300, C61400,

(268700, C70400, C70600, C7 1000, C7 1500,

C7 1640, C72200

B359 Copper and copper alloy seam-

C10100, C10200, C10300, C10800, C12000, C12200,

less condenser and heat ex-

C14200, C19200, C23000, C44300, C44400,

changer tubes with integral

C44500, C60800, C68700, C70400, C70600,

fins

C71000, C71500, C72200

B395 U-bend seamless copper and

C 10200, C 10300, C 10800, C

2000, C

2200, C

4200,

copper alloy heat exchanger

C19200, C23000, C44300, C44400, C44500,

and condenser tubes

C60800, C68700, C70400, C70600, C7 1000,

C7 1500, C72200

B543 Welded copper and copper alloy

C 10800, C 12200, C 19400, C23000, C44300, C44400,

heat exchanger tube

C44500, C68700, C70400, C70600, C7

OOO,

C71500, C71640, C72200

B552 Seamless and welded copper-

nickel tubes for water desalt-

C70600, C7 1500, C7 1640, C72200

ing plants

788

Chapter

Table

Copper Alloy Plates, Sheets, and Strips

ASTM

spec. Description Alloys

UNS

no.

B171

Copper alloy plate and sheet for

C36500, C36600, C36700, C36800, C44300, C44400,

pressure vessels, condensers,

C44500, C46600, C46700, C6

300,

C6 1400,

and heat exchangers

C63000, C63200, C70600, C7

1500,

C72200

B569

Brass strip in narrow widths and

C26000

light gage for heat-exchanger

tubing

B96

Copper-silicon alloy plate,

C65100, C65400, C65500, C65800

sheet, strip, and rolled

bar

for

general purposes and pressure

vessels

applications. However, alloys

C70600

and

1500 display excellent resistance to pitting

seawater.

22.4

Intergranular

Corrosion

The most susceptible alloys for this form of corrosion are Muntz metal, admiralty metal, alumi-

num brasses, and silicon bronzes.

22.5

Dealloying (Dezincification)

Various forms of dealloying, including dezincification, have been covered in the chapter on

corrosion. Dealloying of copper-base materials is briefly discussed here. Brasses containing

more than

15%

Zn, such as leaded Muntz metal, admiralty brass, and naval brass, are suscepti-

ble to dealloying when subjected to prolonged contact with slow-moving water or mildly acidic

water. Trends of dezincification, stress corrosion cracking, and impingement attack with

in-

creasing zinc content in copper-zinc alloys are shown

Fig.

27.

few methods to control

dezinc

fic at ion are

Copper alloys with copper contents more than

85%

resist dezincification.

Small amounts (0.02-0.1 1%) of

As,

Sb, or P added to admiralty brass (C44300) impart

high resistance to dezincification, and the inhibited alloys are called arsenical admiralty

brass, antimonial admiralty brass, and phosphorous admiralty brass. Similarly, the addition

As,

Sb, or P tends to inhibit dezincification in leaded Muntz metal C36500, and in

naval brass (C46400).

The

uninhibited alloys and the corresponding inhibited alloys are

given in Table 4

Dealuminification

the

aluminum bronzes

occasionally reported, but it is not a

significant problem until aluminum reaches the

to 11% range [191]. The microstructure is

particularly important in the dealloying resistance of the alloys. Dealloying is rarely seen in

all alpha, single-phase alloys such as aluminum bronzes

C60800,

C61300, and C61400, and

when seen, conditions of low pH

and

high temperatures are usually present. But duplex struc-

tures are more prone to dealloying. Proper heat treatment can control dealloying in the duplex

a1 loy

789

Material Selection and Fabrication

High

Sensitivity

dezincificotion

to stress

High

Rate

Low

Rate

impingement

attack

lmmun

Low

Immune

Figure

Trends

dezincification, stress corrosion cracking, and impingement attack with increas-

ing zinc content in copper-zinc alloys (brasses).

[From

Ref.

81.1

Table

Inhibited Alloys

~ ~ ~~~~~~~~~~~~~

Uninhibited alloys Inhibited alloys

C36500

Leaded Muntz metal

C36600, C36700, C3680&As,

Sb,

inhibited leaded Muntz

metal, respectively

C44300

Uninhibited admiralty brass

C44300, C44400, C44500-As,

Sb,

inhibited admiralty brass,

respectively

C46400

Uninhibited naval brass

C46500, C46600, C46700-As,

Sb,

inhibited naval brass,

respectively

Denickelification

Denickelification is occasionally reported in the higher nickel copper-nickel alloys, 70Cu-30Ni

(C7

1500), in feedwater pressure tubes at temperatures over

12°F (lOO"C), in low-flow condi-

tions, and in high local heat flux [135], and in refinery overhead condensers, where hydrocar-

bon streams condense at temperatures above 300°F (149°C)

[

1911. According to Tuthill

[

1911,

denickelification problem appears to be associated with hot sports that develop in the tubing

as a result of fouling and thermogalvanic differences that arise,

22.6

rosi

on-Co

rosi

Erosion-corrosion is the common tube-side phenomenon in copper and some copper alloys

with water on the tube side

the result of excessively high velocity and turbulence.

improve resistance to erosion-corrosion, copper is alloyed with iron, chromium, and titanium

[

1091. Aluminum brass and copper-nickels are highly resistant to erosion-corrosion. Remedial

measures to overcome erosion-corrosion include (1) proper design and sizing of the system

that maximum velocity is kept about 4-5 ft/s,

(2)

redesigning the waterbox or install vanes

and diffusers to reduce turbulence in the inlet region,

(3)

installing cathodic protection system

790

Chapter

in the waterbox, (4) installing tube inserts made of stainless steel or plastics, and

(5)

using

chromium modified copper-nickel alloys or copper-nickel-iron alloys.

22.7

Stress

Corrosion Cracking

Pure copper is immune to SCC, but the higher zinc content brasses with Zn content

the

range of 2040% are subject to stress corrosion cracking. Susceptibility increases as zinc con-

tent increases from 20 to 40%. Prominent copper alloys susceptible to SCC include C23000,

C26000, C26800, C27000, C28000, C36500, C44300 (uninhibited), and C46400. Copper-nick-

els and pure copper are more resistant.

The combined action of at least three substances usually is necessary to produce stress

corrosion cracking in stressed copper-base alloys

[

1911:

(

1) ammonia or ammonia-producing

material (organic and inorganic substances containing nitrogen), (2) moisture (or water), and

(3) oxygen. It is rarely encountered in feedwater heaters and condensers because of their low

oxygen content.

keep the oxygen level to the lowest amount, air in-leakage during down-

time periods should be minimized. Environments like sulfates, nitrites, and nitrates under cer-

tain circumstances also cause SCC of copper alloys. SCC in copper alloys is usually intergranu-

lar,

whereas admiralty and aluminum brass almost invariably crack transgranularly and

does

brass [192]. Cracks can be detected by eddy current inspection. Relative resistance of the

various

copper alloys to SCC is as follows [190,192].

Low resistance

Brass containing over

20%

zinc.

Brass containing over 20% zinc and small amounts

lead, tin, or aluminum such as leaded

high brass, naval brass, admiralty and aluminum brass.

Medium resistance

Brasses containing less than 20% zinc, such as red brass.

Aluminum bronze.

Phosphorus bronze.

High resistance

Silicon bronze.

Phosphorized copper.

Very high resistance

Cupronickel.

Tough-pitch copper.

Commercially pure copper.

SCC can be controlled by

SCC can be controlled, and sometimes prevented, by selecting copper alloys with less than

15% Zn. Brasses containing less than

15%

Zn are resistant SCC. DHP copper and tough-

pitch copper are generally immune to SCC.

Reduce the stress to a safe level by stress relieving or annealing.

Controlling the aggressiveness of the environment by addition

inhibitors.

Since the water-side SCC is often associated with surface deposits, it is important to keep

the tubes free from deposits by proper tube cleaning.

Steam-Side Stress Corrosion Cracking

Among the copper alloys typically used for condenser tubes, the brasses (copper-zinc) are the

most susceptible to SCC in environments containing ammonia. Other susceptible materials

include arsenical aluminum brass and admiralty brass. The environmental conditions leading

Material Selection and Fabrication

791

to SCC on the steam side of copper alloy condenser tubes are ammoniacal solutions containing

dissolved oxygen. The ammonia is derived from the water treatment compounds used for boiler

feedwater, which decompose due to thermal effects in the boiler

[

193-1 951. The ammonia

concentration is particularly high in the air removal section, and this location suffers SCC most

often. The high ammonia concentrations develop because the baffles locally shield the tubes

and prevent the condensate in this area from being continually washed away by condensate

falling from above. The other necessary ingredient for SCC to occur, oxygen, comes from the

air drawn into the condenser through leaking joints, valve packing glands, etc. In addition to

usual methods, some methods of controlling SCC in condenser tubes include the following

[

1931:

1. Use alloys resistant to SCC. Brass tubes may be replaced by cupronickel, stainless steel,

or titanium tubes, particularly in the air removal section where steam-side environments

can be quite aggressive.

Reduce or minimize the addition of ammonia and ammonia-producing additives to the

boiler feedwater.

Ammonia concentrations in the air removal section can be reduced by modifying the de-

sign to eliminate horizontal baffles or by installing a water showering system to dilute the

ammoniacal condensates.

4. If SCC is due to ammonia liberated from putrefying organisms during shutdown periods,

flush the system with fresh water for a short period every day.

Eliminate oxygen from the system. All potential sources of air leakages should be attended

and rectified.

Test

for

SCC-ASTM

BI54.

ASTM

154, mercurous nitrate test,

standard test method

for detecting SCC of copper and copper alloys. This test method is an accelerated test for

detecting the presence of residual (internal) stress that might result in SCC of individual parts

in storage or in service.

22.8

Condensate Corrosion

Copper alloy tubes, particularly brass, can withstand the action of steam condensate in a very

satisfactory manner except where the steam condensate contains high concentrations of ammo-

nia and oxygen [194]. Condensate corrosion, also known as “ammonia attack,” on the steam

side of condensers, generally takes place in the air removal section where ammonia and oxygen

concentrations are particularly high. Many of the methods of preventing ammonia SCC are

also helpful to prevent condensate attack.

22.9

Deposit Attack

Deposit attack on condenser tubes occurs under conditions of stagnant or low water velocities,

generally less than 3.1 ft/s (1

ds).

Deposition

waterborne particles on tube surfaces leads

to differential aeration and anodic dissolution

the tube material [196]. Deposit attack is

overcome by these measures

[194]:

Maintain tube-side water velocity above 1

m/s

and avoid stagnant conditions in aggressive

media by flushing with treated water during shutdown.

Use more resistant materials like

70/30

and 90/10 copper-nickels and titanium.

22.1

Hot-Spot Corrosion

Hot-spot corrosion is a localized form of pitting corrosion that takes place at “hot spots” on

the condenser tube wall due to low water velocity and/or high heat

flux

[

1961. These conditions

792

Chapter

can be caused by poor steam distribution or the absence of water on the cooling-water side of

the tube. This form of corrosion rarely takes place in main steam condensers.

22.11 Snake Skin

Snake skin is as the result of deposition of copper corrosion product carried from low-pressulse

feedwaters employing admiralty brass tubes or high-pressure feedwater heater tubes made of

Monel400 or 70Cu-30Ni. The deposition results in thin flaky skin formation similar in appear-

ance to skin shed by snake. This snake skin falls from the tubes when dried

[

1941.

22.1

Corrosion Fatigue

Copper and copper alloys such as beryllium copper, cupronickels, phosphorus bronzes, and

aluminum bronzes exhibit resistance to corrosion fatigue in many environments involving cor-

rosiveness and applied stress.

22.1

Biofouling

Copper and copper alloys including copper-nickel exhibit excellent resistance to marine biofou-

ling such as barnacles, mussels, and marine invertebrates.

22.1

Cooling-Water Applications

The principal constituents of water that affect the performance of copper alloys are dissolved

oxygen, nutrients, bacteria, organisms, biofouling, sediment, and residual chlorine from the

cholorination practice

[

Inhibited admiralty brass, aluminum brass, and cupronickels ex-

hibit excellent resistance in seawater, and show erosion-corrosion and biofouling resistance.

Hence, these alloys are now extensively used in fossil power-plant heat exchangers.

22.1

Resistance

Seawater Corrosion

Corrosion resistance of copper and copper alloys in seawater is determined by the nature of

the naturally occurring and protective corrosion product film [191]. The main drawback of

copper alloys is their susceptibility to pitting corrosion in sulfide-bearing seawater in the pres-

ence

oxygen. This is especially true with polluted coastal water. Sulfide attack is further

discussed next.

22.16 Sulfide Attack

Sulfide attack is accelerated corrosion of copper alloys that occurs when the cooling water,

most often brackish water or seawater, is polluted with sulfides, polysulfides, or elemental

sulfur, and no copper alloy is resistant to sulfide attack

[

1941. Various sources of sulfides

include

(1)

the natural sulfates present in sea water and

(2)

the putrification of organic suifur

compounds from decaying plant and animal matter during periods of extended shutdown

[

1931.

The major effect of the sulfide is to destroy the existing protective surface oxide film,

any.

It can greatly increase general corrosion, and it can induce or accelerate dealloying, pitting,

and erosion-corrosion

[

1941. The corrosion attack is enhanced in the presence of oxygen. The

following remedial measures will overcome sulfide attack

[

1931.

If sulfide forms during periods of extended shutdown due to the putrification of organic

sulfur compounds, it can be removed daily by flushing the system using fresh water.

Precondition the copper alloy surfaces by ferrous sulfate dosing to make them more corro-

sion resistant.

Material Selection and Fabrication

793

Use alternative materials such as stainless steel and titanium tubes in place of copper.

22.1

Exfoliation

Copper-nickel alloy feedwater heater tubes are subjected to exfoliation

[

189,1971.

Exfoliation

is due to pronounced schistous oxidation of the materials, and the resulting loss in strength of

the feedheater tubes, which is no longer sufficient to withstand the water pressure and hence

burst. The factors that cause exfoliation are

(1)

cyclic and peak loads, i.e., where the plant was

frequently started up and shut down, and

(2)

the presence of oxygen, water vapor, and water.

Among the copper-nickels,

copper-nickel was the most susceptible, but

and

are better. Monel alloy

400,

Inconel alloy

600

and titanium retain their “as-installed”

appearance under test conditions

[

1971.

22.18 Copper and Aquatic Life

Copper is declared poisonous to aquatic life. Hence its discharge into the source of cooling

water is prohibited by laws and environmental regulations.

23 COPPER WELDING

Copper and its alloys are welded by shielded metal arc welding (SMAW), gas tungsten arc

welding (GTAW), plasma arc welding (PAW), submerged arc welding (SAW), and oxyacety-

lene welding (OAW). Of these, GTAW and GMAW are the most popular, with argon, helium,

or nitrogen gas shielding. The primary criteria for choosing between GMAW and GTAW are

thickness of the metal to be welded and amount of welding to be performed. GMAW is gener-

ally preferred for thicknesses greater than about

1/4

(6.4

mm). The GMAW process provides

intense heat generated by the arc, high deposition, dense deposits, low preheat and interpass

temperatures, good properties, and minimum distortion

[184].

GTAW is used for thickness less

than about

0.08

mm). Generally, welding is carried out using dcsp with a

mix of

helium and argon. For thickness between

and

mm, both processes can be used. Other

joining methods include resistance welding and induction welding, brazing, and soldering.

Welding of copper and its alloys are discussed in detail by Gaffoglio

[184],

Ref.

198,

Dawson

1199-2011,

by Callcut et al.

[202],

and in Ref.

203.

23.1 Weldability

In all welding processes, the dominant factor in establishing weldability is thermal conductivity

of the base metal. When welding copper, allowances are necessary for the chilling of the

welds because of the very high thermal conductivity of the metal. Preheating is often required.

Considerable distortion is possible in coppers because of the higher thermal expansion than

other commercial metals. These characteristics pose difficulties that must be overcome for

satisfactory welding.

Factors Affecting Weldabili ty

Other than the elements that comprise a specific alloy, the principal factors that influence

weldability are

(

thermal conductivity,

(2)

preheating,

(3)

thermal expansion,

(4)

alloying

elements,

(5)

surface cleanliness and surface oxide,

(6)

shielding gas,

(7)

joint design,

(8)

welding position,

(9)

consideration for precipitation hardenable copper alloys,

(

10)

hot crack-

ing, and

porosity.

Thermal Conductivity.

The high thermal conductivities of copper and certain high-copper

alloys such

aluminum bronze, commercial bronze

(90%),

and red brass

(85%)

pose problems