Kuppan T. Heat Exchanger Design Handbook
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814
Chapter
I3
2.
To overcome the cost disadvantage of Grade 7, a new alloy designated Grade 12 with
0.8%
Ni and
0.3%
MO has been developed for increased corrosion resistance
in
specific
media [217].
3.
Lower alloy Grade
9
(3%
A1 and 2.5% V) is used for pressure-vessel applications.
4. For cryogenic applications the basic titanium alloy Ti-6A1-4V or Ti-5A1-2.5Sn-EL1 is used.
ASTM and ASME Specifications for Mill Product Forms
ASTM and ASME specifications that govern unalloyed titanium and alloyed titanium in vari-
ous mill product forms such as strip, sheet and plate, seamless and welded pipe, seamless and
welded tube for condensers and heat exchangers, bars and billets, and forgings in various
grades are given in Table 48. For these alloys the design temperatures should be less than
approximately 806°F (430°C) to avoid excessive oxidation and oxygen embrittlement
in
con-
tinuous service.
Industry uses titanium in sheet and plate form in addition to tubes and plate used
in
heat
exchangers. Construction can be solid titanium, titanium-clad steel, or with titanium linings.
Solid titanium construction is generally more cost-effective than clad construction when vessel
wall thickness is below approximately 1-1.5 in (25.4-38.1 mm).
28 TITANIUM CORROSION RESISTANCE
28.1 Surface Oxide
Film
Titanium exhibits excellent corrosion resistance to general corrosion, pitting, erosion-corrosion,
and galvanic corrosion. The superior corrosion resistance of titanium alloys is derived from
the formation of a highly stable, tenacious, protective oxide film on its surface. This film,
typically
50
to 200
A
thick, is primarily titanium dioxide (TiO?) in rutile (highly crystalline)
andor anatase (somewhat amorphous) form [218]. Even if the oxide film is damaged, being a
reactive metal and with a high affinity for oxygen, the protective oxide films form spontane-
ously and instantaneously whenever fresh metal surfaces are exposed to traces
of
oxygen or
moisture [2 131. Film growth is also accelerated under strongly oxidizing conditions such as in
HN03
and Cr03 media [219]. Another desirable feature
is
that at low temperatures, this surface
oxide film provides a barrier for penetration of hydrogen into titanium surfaces. Conversely,
titanium is severely corroded
in
a reducing environment wherein it is not readily repassivated.
Once the brakedown of the passive film occurs, the corrosion process is unhindered.
Loss
of
oxide film could occur due to abrasion, chemical reduction of the oxide, or surface contamina-
tion, notably by iron oxide [220].
28.2 General Corrosion
The oxide film on titanium is attacked only by a few media, including hot concentrated reduc-
ing acids, most notably hydrofluoric acid. Anhydrous conditions
in
the absence of oxygen
Table
48
Titanium Product Shapes, Grades, and ASTWASME Specifications
Product description Grades ASTWASME
Strip, sheet, and plate
Grades
1,
2, 3,
4,
5,
7,
9,
11,
12
B
265/SB 265
Seamless and welded pipe
Grades
1,
2,
3,
4,
7,
9,
11,
12
B
337ISB33 7
Seamless and welded tube for Grades
1,
2,
3,
4,
6,
7,
9,
11,
12
B338/SB338
condensers and heat exchangers
Bars and billets
Grades
1,
2,
3,
4,
5,
6, 7,
11,
12
B348/SB348
Forgings
Grades
1,
2,
3,
4,
5,
6, 7,
11,
12
B38 1/SB38
1
Material Selection and Fabrication
81
5
should be avoided since the protective film may not be regenerated if damaged. General meth-
ods to improve corrosion resistance of titanium in a reducing environment are suggested by
Schutz et al. [213]. Such measures include the following:
Alloying with metals such as palladium, nickel, andor molybdenum.
Addition of inhibitor to the environment, such as oxidizing metal cations, oxidizing anions,
precious metal ions, oxidizing organic compounds; adding oxidizing species (inhibitors)
to the reducing environment is to permit oxide film stabilization; additions of minute
amounts
of
water to certain anhydrous environments is to maintain passivity.
Precious metal surface treatments; this includes precious metals such as platinum and palla-
dium that have been ion implanted, ion plated, or diffused thermally.
Anodizing and thermal oxidation:
As stated earlier, titanium relies on the presence
of
an inert
surface oxide film for its corrosion resistance, especially where hydrogen uptake is of
concern. Therefore, surface treatment processes such as anodizing and thermal oxide treat-
ment are practised to thicken and toughen the oxide film, which
in
turn will improve the
metal’s corrosion resistance.
Anodic protection by impressed current or galvanic coupling with
a
more noble metal in order
to maintain the surface oxide film.
Surface pickling
in
HN03-HF solution to remove smeared surface iron.
Metallic coatings.
Noble alloy contact or by the applications of their oxides such as Ni, Cu, Fe, and
MO
on
titanium alloy surfaces, which is effective for crevice corrosion.
28.3
Resistance to Chemicals and Solutions
Titanium resists corrosion in seawater, brines, aqueous chlorides, organic and oxidizing acids,
and neutral and inhibited reducing conditions better than other metals. Titanium is unique
in
its resistance to chlorides. Titanium, being immune to corrosion
in
wet chlorine, would face
little maintenance problems. Since the heat transfer rates of titanium are
so
much better than
glass, the titanium coolers require only about 12-15% the space to do the same job as glass
exchangers for chlorine applications. High corrosion resistance to oxidizing and chloride solu-
tion led to widespread use in marine, chemical process industries, power plants, and petrochem-
ical industries.
Titanium alloys are attacked severally by red fuming nitric acid or nitrogen tetroxide.
Titanium and its alloys are sensitive
to
reducing acids, and the breakdown of the surface oxide
films can take place when the temperature andor concentration of pure acid exceeds certain
values. Typical reducing acids includes hydrochloric, hydrobromic, hydroiodic, hydrofluoric,
phosphoric, sulfuric, and sulfamic acids. In some acids that do attack titanium, the addition of
small amounts
of
an oxidizing acid such as nitric or salts (such as copper sulfate) inhibits
attack.
Resistance to Waters
Titanium exhibits good corrosion resistance to fresh water, industrial cooling waters including
raw seawater, brackish estuary water, and polluted water. Titanium (like many other metals)
is subject to the formation of mineral scales when water temperatures are excessive [220]. It
resists all forms of corrosive attack by fresh water and steam to temperatures as high as 600°F
(316°C) and corrosion by seawater to temperatures as high as 500°F (260°C) [219]. The pres-
ence of sulfides in seawater does not affect the corrosion resistance.
81
6
Chapter
13
Forms of Corrosion
Galvanic Corrosion.
From its position in the galvanic series, Titanium is highly immune to
galvanic corrosion but tends to accelerate the corrosion of the other metal of the galvanic
couple. The exception is in highly reducing acid environments where titanium may not passi-
vate. Under these conditions,
it
has a potential similar to aluminum and will undergo acceler-
ated corrosion when coupled to other more noble metals [219]. While using titanium tubes,
extreme care should be taken to avoid galvanic corrosion of tubesheets and water boxes of a
condenser, which have different corrosion potential. Solid titanium or titanium-clad tubesheets
are preferred in order
to
eliminate galvanic corrosion of a tubesheet of
an
anodic material,
such as a copper alloy or carbon steel. Water boxes should be coated for protection. If
a
titanium-clad tubesheet is used (a minimum of 3/16 in or
4.5
mm thick cladding is required),
the tube-to-tubesheet joints should be welded, but if a solid tubesheet is used, roller expansion
can be performed.
Hydrogen Embrittlement.
In a galvanic couple, if titanium is the cathodic member, hydrogen
may be evolved on its surface. Normally the surface oxide film on titanium acts as an effective
barrier to penetration by hydrogen. By specification, hydrogen is limited to 150 ppm maximum
in all the common tube metal grades [220]. Within the range of pH 3 to 12, the oxide film is
stable and presents a good barrier to penetration by hydrogen [219]. Under certain conditions,
titanium can absorb hydrogen and become embrittled:
1.
Dry,
nonoxidizing hydrocarbon or hydrogen stream-the oxide films breaks down since
there is no mechanism for replacing oxygen lost to the process stream.
2.
Disruption of the oxide film allows easy penetration by hydrogen.
3.
If the temperature is above 176°F (80"C), hydrogen may diffuse into the metal and cause
hydrogen embrittlement.
Even the risk of failure due to hydrogen embrittlement can be greatly reduced if care is taken
to maintain the potential of the tubedtubesheets in the range
-0.5
to -0.07
V
on the saturated
calomel electrode (SCE) scale
[
1931. TIMET's recommendation for the limiting temperature
for titanium usage in hydrogen environments is
71°C
(160°F) [219]. This temperature is based
on the barrier effect or near-zero diffusion rate of hydrogen at that temperature [220].
Crevice Corrosion.
Titanium alloy as well as commercially pure titanium will be subjected
to localized attack in tight crevices exposed to hot (>158"F/7OoC) aqueous solutions, or chlo-
ride, bromide, iodide, or fluoride solutions. Crevice corrosion of unalloyed titanium may occur
in seawater at temperatures above the boiling point. One major exception is the alloys contain-
ing 0.2% palladium [221]. Titanium alloy Grade 12 and Ti-Pd (Grades
7
and 11) offer resis-
tance to crevice corrosion in seawater at temperatures up to
500°F
(260°C) [219]. Both alloys
also provide improved resistance to reducing acid conditions, and Grade 12 offers increased
strength particularly as the temperature is increased. The projected immunity of titanium and
its palladium alloy from pitting corrosion, as would occur in crevices, is shown in Fig. 30 as
a function of chloride concentration and temperature. Where corrosion of titanium is a problem
under deposits, molybdenum-bearing titanium grade, Grade 12, R53400 (Ti-O.3Mo-O.gNi),
should be used.
Erosion-Corrosion.
An important property of titanium is its excellent resistance to erosion-
corrosion and its various forms such as cavitation and impingement attack. Titanium is consid-
ered one of the best cavitation-resistant materials available for seawater service. Titanium has
the ability to resist erosion due to high-velocity seawater up to 30
m/s
[218].
Stress
Corrosion Cracking.
SCC of commercial titanium and titanium alloys may occur in
red fuming nitric acid at room temperatures. The possibilities of
SCC
in natural seawater can
Material Selection and Fabrication
81
7
OJ
I
I
I
I
0
M
$00
t
so
9.00
Rmp
C"
Figure
30
Crevice corrosion
of
titanium;
crevice
corrosion
in titanium
is
a
function
of
temperature
and
chloride concentration. [From
Ref.
196.1
be reduced by alloy selection and heat treatment. The presence of defects combined with an
unfavorable stress condition can account for almost instantaneous failure of welded structures
in contact with trichloroethylene
[79].
Corrosion
Fatigue.
Titanium, unlike many other materials, does not suffer a significant loss
of fatigue properties in seawater. In fatigue-limited applications,
ASME
Code Criteria
or
actual
in situ fatigue should be considered.
MIC. Titanium is highly resistant to MIC. More than
30
years of extensive Titanium alloy
use in biologically active process and raw cooling waters, especially seawater, appears to
substantiate titanium's resistance to MIC
[218].
28.4
Thermal Performance
With its low fouling resistance values in combination with thinner tube walls, the overall heat-
transfer coefficient has been more than adequate in all installations. Titanium's resistance to
erosion-corrosion permits high flow velocity (up to
120
ft/s if pressure drop considerations
permit). High velocity means higher heat-transfer rates and lower fouling factors. Experience
has shown that titanium exchangers handling seawater can be designed with a fouling factor
as low as
0.0005
h ft2 "F/BTU
[219].
28.5
Fouling
As
titanium
is
perfectly passivated, its surface has no biostatic action and would be fouled
biologically. Fouling, mussels, and barnacles are a problem in a power station installed on the
seashore. Therefore, tubes must be kept clean by an online tube cleaning system such as a
sponge rubber ball system, or by a correct system cooling water velocity, or by chlorination
treatment.
28.6
Applicatfons
Titanium Tubing for Surface Condensers
Titanium tubing in steam condensers has proven to be the most reliable of all tubing materials.
It is immune to corrosion from chlorides. It returns no harmful metal ions to the environment
81
8
Chapter
13
or to the condensate feedwater stream. Titanium overcomes the problems of random tube
failure caused by erosion-corrosion at tube inlets due to turbulence or partial blockage by
shells, mussels, debris, and ammonia condensate, and is highly resistant to erosion attack by
steam impingement. In addition to fresh tubing, titanium tubings are used for retubing the
leaky tubes. When retubing an existing condenser that has been designed for use with tubes
other than titanium, there are six areas to consider
[219].
They are:
(1)
Titanium tube heat
exchangers require shorter baffle spacings to accommodate thinner tubes and low Young’s
modulus to overcome
FIV
problems;
(2)
tubesheet material for a possible galvanic couple with
titanium as cathodic material (titanium can essentially be welded only
to
titanium,
so
that the
tubesheets must also be made of titanium or be overlaid with titanium if a welded joint is
desired);
(3)
effect of reduced weight due to use of thin-walled tubes and low density;
(4)
effect of reduced water velocity since the inner diameter has increased due to thin-walled
tubes;
(5)
fouling; and
(6)
if there is already a cathodic protection system
in
operation, make
it compatible with titanium; such measureskautions include that hydriding can take place at
potentials greater than
-0.75
V
(SCE)--do not exceed
-0.90
V
(SCE)-that the system must
have an automatic potential control, and that sacrificial anodes should not be placed closer
than
30
in from the tubesheet.
Refinery and Chemical Processing: Service Experience
The primary reason for titanium’s expanded application in the refinery industries, particularly
in
the condenser service, is due to its superior resistance to handle refinery process streams
containing hydrogen sulfide, chlorides, dilute hydrochloric acid, ammonia, and hydrogen
[220,222].
Titanium Grade
2
tubes are used in overhead coolers and condensers. Titanium for
refinery services is limited to operating temperature of
260°C.
If hydrogen is present, tempera-
ture should not exceed
175°C
(350°F)
to prevent hydrogen embrittlement.
Chemical Processing
The primary basis for titanium’s expanded application in the chemical processing industries
has been its superior resistance to the aggressive, mildly reducing, oxidizing, andlor chloride-
containing process streams.
PHE
Due to its inherent corrosion resistance, titanium is used as a plate heat exchanger material
in
corrosive applications.
29 TITANIUM FABRICATION
29.1
Welding
Titanium
Unalloyed titanium and all alpha titanium alloys are weldable. Alpha-beta alloys, alloy Ti-6Al-
4V,
and weakly beta-stabilized alloys are also weldable. Beta-phase alloys and strongly stabi-
lized alpha-beta-phase alloys are difficult to weld and they will embrittle.
Welding Methods
Commercially pure titanium and weldable alloys can be arc welded successfully by the manual
gas tungsten arc, plasma arc, and gas metal arc processes. If economically justified by thick-
ness, run length, or repetitive work, automatic hot wire TIG with special trailing shields is
used. Procedures and equipment are generally similar to those used for welding austenitic
stainless steel or aluminum, with two important exceptions
[215]:
(1)
Titanium requires greater
cleanliness and
(2)
it requires auxiliary gas shielding. In general, fluxes are not used due to
the high-temperature reactivity of titanium.
Material Selection and Fabrication
81
9
Weldability Considerations
When heated in air, titanium oxidizes rapidly and, at temperatures near its melting point
(3272"F), (1
)
molten titanium reacts with most materials, which embrittles the weldment and
reduce the corrosion resistance, and (2) it dissolves its own oxide, leaving inclusions
in
welds.
In its solid state, titanium above 1200°F absorbs oxygen, nitrogen, and hydrogen from the
atmosphere. Even small amounts of these elements can cause embrittlement
in
the weld and
loss
of corrosion resistance [223]. Hydrogen in concentrations exceeding 150 ppm can embrit-
tle the pure metal and also the various commercial alloys available [224]. Consequently, to
assure a successful titanium weld, air must be excluded from both the face and back of the weld
metal and a secondary inert-gas shield must protect the weld area
until
it
cools to 600-800°F
(3 15425°C).
Shielding Gases.
In arc welding, this is accomplished by replacing the air surrounding the
weld area by argon or helium with argon.
No
other gases can be used.
When titanium parts are heated to more than
500°F
(260"C), contact with adhesive tapes,
papers, marking crayons, etc. containing more than
50
ppm chlorine or other halogen com-
pounds should be avoided [225].
Welding
of
Titanium to Dissimilar Metals.
Satisfactory fusion welding of titanium to dissimi-
lar metals, except with vanadium and silver, is not possible because of the formation of brittle
intermetallic compounds. Therefore, interconnections between dissimilar metals are made by
mechanical means such as by bolting. Lining vessels with titanium cannot be accomplished
directly by fusion welding. In some instances silver filler metal is used for seal welds [217].
Manufacturing Facilities.
While it is not always possible to fabricate large titanium assembl-
ies
in
a closed clean room as demanded for a clean environment, designating areas of the heavy
workshop area to be screened off, paying great attention to preventing weld contamination with
shop dirtddust by screening, and stringent solvent cleaning of weld preparations and filler wire
before welding are necessary to achieve the proper weld metal characteristics and corrosion
resistance.
Edge Preparation
Prepare joints by sawing, machining, or grinding. Joint surfaces should be smooth and free of
crevices, roughness, and overlaps that can trap dirt and cleaning solution.
If
burrs should occur,
remove them with a clean, sharp hand file or rotary file. Dirty shearing equipment is likely to
contaminate sheared or slit edges and ultimately cause weld metal porosity.
Joint Design
Joint designs for titanium welding are essentially the same as those found
in
the inert-gas arc
welding of other metals. The joint design should allow for full shielding of both sides or permit
the exclusion of air from parts of the joint reaching temperatures of 1000°F (538°C) or greater
[226]. For example, argon purge holes are provided in vessel shell nozzle reinforcing plates
and tubular stiffeners. Special configurations are necessary on titanium-clad tubesheet joints to
give adequate back shielding.
Precleaning and Surface Preparation
A
good titanium weld begins with cleanliness, both in the immediate weld area and
in
the shop
floor. Foreign matters that contaminate the weld lead to porosity and low mechanical proper-
ties. Consequently, surface residues such as scale, oxide, grease, dust, grinding wheel grit, etc.
820
Chapter
I3
must be removed before welding. The surface oxide and scale resulting from normal hot-
forming operations can be removed mechanically or chemically.
Cleaning Titanium.
Titanium can be cleaned by steam cleaning, alkaline cleaning, vapor
degreasing, and solvent cleaning methods. Cleaning solutions for titanium must maintain pas-
sivity and avoid possible hydrogen uptake by the titanium. Titanium cleaning is discussed by
Chevalier [225], and ASTM Specification B600 details procedure for descaling and cleaning
titanium and its alloys.
Degreasing.
Titanium parts that are free of scale or oxide require only degreasing. Light or
medium contamination is degreased by immersion in hot alkaline solutions or by spraying.
Heavy contamination is degreased by cleaning in alkaline solution in two stages, preferably
after degreasing. Use nonchlorinated degreasing solvents, reagent methanol, acetone, or methyl
ethyl ketone to degrease
or
remove machining oil. Avoid using chlorinated solvents such
as
trichloroethylene vapor, alcohols, and methanol in particular [226]. Deposits remaining from
solvents of this type may cause cracking in high-strength alloys if the metal is subsequently
heated in stress relief or actual service.
Descaling
or
Oxide Removal.
Light oxides may be removed by brushing with
a
stainless
steel wire brush, sand blasting, or draw filing. Steel wool or abrasives should never be used
because of the danger of contamination.
If
grinding is required, the use of silicon carbide is
preferred. Heavy oxide scales may be removed by grinding, machining, liquid abrasive blast-
ing, salt-bath descaling, or alkaline cleaning. To remove oxide scale formed at temperatures
below
1
150°F, a pickling solution is used that consists of
25-30%
nitric acid and
2-3%
hydro-
fluoric acid by weight mixed with water. To remove oxide scale formed at temperatures above
1
150°F, sandblasting or solutions of sodium hydride, Virgo, Kolene, or similar salt baths fol-
lowed by acid pickling are required [226]. Proper inhibition is most critical when reducing
acids, such as HCl, H2S04, sulfamic, or oxalic acids, are used to remove scaling. Since rapid
titanium attack may occur, hydrofluoric acid cleaning solutions are not to be used for scale
removal.
Rinsing.
Water rinsing after degreasing or acid pickling must be carried out with deionized
water having a resistivity of more than
50,000
ohdcc when intending to do an operation such
as heat treatment, welding, etc. above 500°F (260°C) [225].
Handling
of
Cleaned Components.
Handling of cleaned components before being welded
must be with clean gloves to avoid fingerprints; during waiting periods the parts must be stored
in
a packed condition and protected from external contamination.
Fitup
Uniform fitup avoids burn-through, controls underbead contour, and simplifies backup shield-
ing
or
purging.
Preheating
Titanium welding does not normally require preheating [215]. For very low ambient tempera-
ture and low humidity conditions, preheating to
100-1
50°F (3745°C) helps moisture removal.
Filler Metal
Thirteen titanium and titanium alloy filler metal (or electrode) classifications are given in
AWS
A5.16-90. A partial list is shown in Table
49.
Maximums are set on carbon, oxygen, hydrogen,
and nitrogen contents. Filler metal composition is usually matched to the grade
of
titanium
being welded. ERTi-2 and ERTi-0.2Pd are good overall choices. ERTi-2 can be used on grades
1, 2, and 3 base metal [215]. Recommended filler wire grades and the base-metal grades are
given in Table
50.
Material Selection and Fabrication
82
1
Table
49
AWS A5.16 Classifications
of
Titanium Filler Metal
AWS
classifications Composition
ERTi-
1
Ti-0.500
ERTi-2
Ti-0.050
ERTi-3
Ti-0.130
ERTi-4
Ti -0.200
ERTi-5
Ti-6A1-4V
ERTi-5ELI
Ti-6A1-4V
ERTi-6
Ti-5A1-2.5Sn
ERTi-6ELI
Ti-5A1-2.5Sn
ERTi-7
Ti-0.050-0.2Pd
ERTi-9
Ti-3A1-2.5V
ERTi-9ELI
Ti-3A1-2.5V
ERTi- 12
Ti-0.3Mo-0.8Ni
Table
50
Recommended Titanium
Filler Metals [215]
Filler grade Base metal grade
ERTi-
1
1
ERTi-2
1
ERTi-3
2
ERTi-4
3
ERTI-7
7
ERTi-
12
12
Welding Procedures
Gas Tungsten Arc Welding.
Due to the inert-gas shielding characteristics and the high degree
of arc control, GTAW is ideally suited for titanium. Use a drooping-characteristic power supply
and dcsp. Titanium can be successfully welded with GTAW in thickness ranging from several
thousandths of an inch to more than several inches. Use the
same
techniques as for welding of
stainless steel. To avoid contamination, weld joint and wire must
be
clean, and the shielding
gas, usually argon, must be free from moisture and impurities. A facility to ignite the arc is
required that prevents contamination of the titanium with tungsten inclusions from the elec-
trode. AWS
D10.6,
Gas Tungsten Arc Welding of Titanium Piping and Tubing, covers various
aspects of GTAW of titanium.
Shielding.
Inert-gas shielding (either by argon or helium) of the weld area is accomplished
by
‘S
everal methods:
1.
A vacuum chamber
or
special inert-gas-filled chamber, which eliminates the need for
elaborate jigs and other fixtures that normally would be required to adequately shield
complex assemblies in air; it is generally considered that the highest weld quality is ob-
tained by employing vacuum or argon purged chamber welding.
2.
A backup gas shielding with trailing shield by welding in open air.
822
Chapter
13
A section of AWS D10.6 on gas shielding offers several alternatives for shielding, includ-
ing work-mounted trailing shields and torch-mounted trailing shields,
Welding Titanium in an Open-Air Environment With Three Shielding Gases Techniques.
Al-
though historically titanium has been welded in chambers that are either vacuum or argon
purged, increased usage of open-atmosphere welding is taking place in the chemical industries
and marine sectors with proper shielding techniques. The shielding for welding titanium in
an
open atmosphere can be accomplished by using a combination of three shielding gases tech-
niques. They are as follows [227]:
1.
The primary shielding is provided by the welding torch.
2. The primary shielding is supplemented by a trailing shield, which is known as the secon-
dary shielding; the trailing shield permits cooling
of
the weld deposit and the adjacent
HAZ under a blanket
of
argon gas.
3.
The back shielding protects the backside of the weld and its adjacent heat-affected zone.
Typical gas shielding for tube-to-tubesheet welding is shown in Fig.
31.
Narrow-Groove Welding
of
Titanium Using
the
Hot Wire Gas Tungsten Arc Process.
Though
narrow-groove welding has been dominated by the submerged arc and the gas metal arc weld-
ing processes, the use of narrow-groove joint designs with the automatic GTAW process is
promising for welding of titanium. The automatic hot wire GTAW process with narrow-groove
joint designs combines the advantages of automatic hot wire GTAW (high deposition rates,
excellent shielding characteristics) with narrow-groove welding (high groove
fill
rate, reduced
weld distortion) [227]. As is true with all of the narrow-groove welding processes, a problem
that must be addressed is inadequate control of face fusion.
MIG Welding.
The
MIG
process employs a direct current supply and reverse polarity. The
power supply generally consists of a constant-potential rectifier, although a power supply with
a drooping load characteristic is also applicable.
29.2
In-Process Quality Control and
Weld
Tests
Method to Evaluate the Shielding
Since titanium readily reacts with air at elevated temperatures to form oxides that exhibit
specific colors, the oxide color of the weld surface can be used as an effective method to
c
-
Argon
required
if
8affk
prerdltd
to
snug
fit
Figure
31
Gas
shielding for titanium tube-to-tube sheet welding. (From Ref.
215.)
Material Selection and Fabrication
823
evaluate the inert-gas shielding. Welds made with proper shielding will exhibit a bright, metal-
lic, shiny silvery weld color; the change in color represents increasing amounts of weld contam-
ination, which usually takes place due to faulty or inadequate trailing shielding [227]. Table
5
1 differentiates the acceptance criteria and required disposition for each oxide colour category.
Hardness
A clean weld should not be more than
30
points BHN higher than the base metal. A greater
increase indicates contamination [2 151.
Bend Test
Make a simple test by butt welding two narrow strips of titanium together and bending the
piece over a mandrel with a radius of about three to four times the thickness of the sheet held
in a vice. Welds with satisfactory ductility will bend over the following radii without cracking
[215]:
Grades 1, 2, 7, and 12
4T
Grade
3
5T
Weld Defects
1. Titanium and its alloys are not prone to solidification cracking. However, under conditions
of severe restraint, solidification cracking has been observed sometimes [229].
2. Weld metal porosity is probably the most frequently encountered weld defect in titanium
weld metals [228]. Porosity is generally caused by hydrogen, which may originate from
gas entrapment, filler metal, or base metal. Careful shielding of the weld metal and clean-
ing of the surfaces will help to reduce the weld metal porosity. Welding at low speeds
gives extra time for gas escape, and quality of cut edges also affects the incidence of
porosity.
3.
Contamination cracking will occur when substantial pickup
of
interstitial elements occurs.
This problem can be solved by careful cleaning
of
the surfaces and the weld joint area,
and proper shielding.
29.3
Heat Treatment
Annealing
Titanium and its alloys are annealed to improve ductility, dimensional or thermal stability,
fracture toughness, and creep resistance. Since improvement in one or more properties is usu-
ally obtained at the expense of some other property, the material producer should be consulted
for heat-treatment considerations.
Table
51
Weld Colors and Their Meanings (Visual Colour Acceptance Criteria) [219,227,228]
Color
of
weld zone Interpretation
~~~ ~ ~ ~ ~ ~~~~~~~~~
Silver
Correct shielding, noncontaminated weld, satisfactory
Light straw
Slight contamination, but acceptable
Dark
straw
Slight contamination, but acceptable
Dark
blue
Heavier contamination, may be acceptable depending on service
Light blue
Heavier contamination, unlikely to be acceptable
Grey blue, grey, white
Very heavy contamination, unacceptable; remove by grinding, repair,
powdery (loose deposit)
then weld again