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Failure analysis of a commercially pure titanium tube in an air conditioner condenser

Failure analysis of a commercially pure titanium tube in an air conditioner condenser

Joining of titanium and stainless steel is challenging due to the formation of hard, brittle
intermetallics. This study focuses on engineering ductile materials for joining transition metals. Friction
welding of tube to tube-plate by an external tool, a novel solid state welding process was employed to join
titanium
tube
and stainless steel tube plate. The interlayers engineered were copper, silver and Cu–Zn alloy.
The micrographs revealed phase transformations in titanium tube and unaffected stainless steel base.
Interface peak microhardness of 458 HV was observed for Ti/Cu–Zn/SS welded sample. The intermetallics
formed were characterized by X-ray diffraction and scanning electron microscopy with energy dispersive
spectroscopy. A novel shear test procedure was developed to evaluate the maximum shear load. It was found
that joints with silver as interlayer withstood the maximum shear load of 56 kN. The shear surfaces were
further analyzed and investigated for fracture study.

Titanium has today replaced copper alloys as the most favoured tube material for salt water cooled
condensers. Main reason is the excellent corrosion resistance of titanium in chloride containing
environments. The experience of <a href="http://www.bjchangli.com/titanium-raw-materials/titanium-bar/"
target="_self">titanium bar condensers is usually more than satisfactory, even if a few tube leaks have
occurred. Possible damage mechanisms by high cycle fatigue, galvanic corrosion, water-droplet erosion and
by flow-assisted corrosion are discussed. These perils can be handled by a number of adequate
countermeasures analysed in laboratory work and meanwhile proven by plant service.

The corrosion resistance of titanium in sea water is extremely excellent, but <a
href="http://www.bjchangli.com/titanium-nickel-zirconium-tube/" target="_self">titanium 、nickel 、
zirconium tube are expensive, and the copper alloy tubes resistant in polluted sea water were
developed, therefore they were not used practically. In 1970, ammonia attack was found on the copper alloy
tubes in the air-cooled portion of condensers, and titanium tubes have been used as the countermeasure. As
the result of the use, the galvanic attack on copper alloy tube plates with titanium tubes as cathode and
the hydrogen absorption at titanium tube ends owing to excess electrolytic protection was observed, but the
corrosion resistance of titanium tubes was perfect. These problems can be controlled by the application of
proper electrolytic protection. The condensers with all titanium tubes adopted recently in USA are intended
to realize perfectly no-leak condensers as the countermeasure to the corrosion in steam generators of PWR
plants. Regarding large condensers of nowadays, three problems are pointed out, namely the vibration of
condenser tubes, the method of joining tubes and tube plates, and the tubes of no coolant leak. These three
problems in case of titanium tubes were studied, and the problem of the fouling of tubes was also examined.
The intervals of supporting plates for titanium tubes should be narrowed. The joining of titanium tubes and
titanium tube plates by welding is feasible and promising. The cleaning with sponge balls is effective to
control fouling.

Titanium is the ninth most abundant element in the earth's crust and the fourth most commonly used
structural metal. In nature, it occurs only as a mineral (ore) in combination with oxygen or iron (rutile,
TiO2, or ilmenite, FeTiO3).

Titanium is a lightweight material whose density is approximately 60 percent of steel's and 50
percent of nickel and copper alloys'. It was recognized in the 1950s as a desirable material for
aerospace applications—especially airframe and engine components. In the 1960s and 1970s, titanium was
considered for use in vessels and heat exchangers in corrosive chemical process environments. Typical
applications included marine, refinery, pulp and paper, chlorine and chlorate production, hydrometallurgy,
and various other oxidizing and mildly reducing chemical services.

In the 1980s and 1990s, titanium began to be used for many nontraditional applications, including
tubulars for geothermal energy extraction and oil and gas production, consumer goods (such as sporting
equipment), food processing, biomedical implants, and automotive components.

According to the U.S. Geological Survey (USGS), 52 million pounds of titanium were produced in the U.S.
in 2000; worldwide, more than 100 million pounds were produced.

Titanium sponge is obtained by reacting rutile ore with chlorine and coke, followed by magnesium
(Kroll) reduction and then vacuum distillation to remove excess magnesium and magnesium chloride. Titanium
sponge is pressed into blocks to make a consumable electrode and then melted in an inert environment under
vacuum to produce a titanium ingot.

Titanium is well-known for its unique combination of properties, which include low modulus of
elasticity, stable and steadfast oxide film (which provides excellent corrosion and erosion resistance),
and a high strength-to-density ratio.

Titanium's fabricability, weldability, and formability make possible its use in many shop and field
operations. Although gas tungsten arc welding (GTAW) is the primary joining process, many other procedures
are suitable. Titanium's weld characteristics are similar to those of stainless steels' or nickel
alloys', with surface cleanliness and inert gas shielding being important. Fabricators often perform
seal welding and butt welding operations in the shop and the field.

As for formability, titanium can be bent, cold-formed, and drawn readily. Furthermore, most industrial
titanium alloys do not require stress relief annealing after cold forming.

Titanium Tube and Pipe—Types and Uses

Welded titanium tube is available in outside diameters (ODs) from 0.5 to 2.5 inches and wall
thicknesses from 0.020 to 0.109 in. Welded pipe is available in standard industry sizes from 0.75 to 8 in.
nominal OD with nominal wall thicknesses in Schedules 5, 10, and 40. Seamless pipe with ODs from 2 to 20
in., wall thicknesses from 0.25 to 2.0 in., and lengths to 60 feet also can be made.

Welded titanium raw materials
and pipe can be tested with many of the same techniques used for steel tube and pipe. Eddy current,
pneumatic, and ultrasonic testing all are applicable to titanium. Procedures for eddy current and
ultrasonic testing can be used to meet or exceed American Society for Testing and Materials (ASTM) B-338
and to help ensure tube reliability.

In terms of cost, titanium is competitive with higher-end specialty steels and alloys. In fact, if
analyzed on a life cycle basis, titanium often is more attractive economically. This stems from
titanium's useful life—20 to 40 years or more—and ease of maintenance. Furthermore, titanium's
exceptional corrosion resistance often allows a zero corrosion allowance. This means that thinner-walled <a
href="http://www.bjchangli.com/titanium-raw-materials/titanium-plate/" target="_self">titanium plate or
pipe may be substituted for other materials with heavier walls.

When titanium and other materials are analyzed, they must be compared by their cost per linear foot,
not by their cost per pound. Because titanium is a relatively low-density material, its cost per pound is
greater than for most other metals.

With their increasing availability, titanium and titanium-alloy tubulars will continue to meet many
challenges in chemical processing, oil and gas production, automotive, and consumer applications. The
titanium industry's large excess capacity means it should be able to accommodate new applications and
emerging markets for titanium with little or no trouble.

R.L. Porter is a corrosion engineer and C.P. Clancy is general manager of commercially pure products
for RMI Titanium Co., 1000 Warren Ave., Niles, OH 44446-0269, phone 330-544-7633, fax 330-544-7796, e-mail
PorterRLP@aol.com, Web site www.rti-intl.com. RMI Titanium Co. provides titanium in a variety of forms—
bloom, billet, sheet, welded tube, seamless pipe, and plate—for applications such as aerospace,
automotive, deep-sea oil and gas exploration and mining, and sports equipment; parent company RTI
International Metals Inc. manufactures and distributes extruded shapes and provides engineered systems for
energy-related markets and environmental engineering services.

The commercial production of titanium plate, sheet, strips, and bars is carried out using hot and cold
rolling mills to achieve the necessary reductions and desired shapes. Rolling may be defined as the
reduction of the cross-sectional area of a piece by compressive forces applied through rolls. Cold rolling
is carried out at temperatures below which the rate of strain hardening is greater than the rate of
recrystallization. When reduction is carried out above such a temperature, the process is termed hot
rolling. The major quantity of titanium plate, sheet, strips and bars is processed using hot rolling
techniques.

The commercial production of titanium plate, sheet, strips, and bars is carried out using hot and cold
rolling mills to achieve the necessary reductions and desired shapes. Rolling may be defined as the
reduction of the cross-sectional area of a piece by compressive forces applied through rolls.

Cold rolling is carried out at temperatures below which the rate of strain hardening is greater than
the rate of recrystallization. When reduction is carried out above such a temperature, the process is
termed hot rolling. The major quantity of titanium plate, sheet, strips and bars is processed using hot
rolling techniques.

The forged billets, whose surfaces have been descaled, are rolled between 1350 and 1500°F (730 and 815
°C). This temperature is approximately 200°F (110°C) lower than the forging temperature. Titanium can be
continuously rolled at temperatures as low as 1100°F (595°C).

As the thickness of the material to be rolled is decreased, the temperature of the piece must be
considerably lowered to minimize surface contamination. A careful choice of pass sequences to obtain a
certain reduction must be made when rolling titanium. Pass sequence refers to the number of reductions
taken and percentage reduction of the piece per pass.

Continuous sheet and strip are best cold- or hot rolled with the application of back and forward
tensions to reduce the friction in the roll gap. In cold rolling thin sheet, extremely tight roll settings
are required to produce uniform cross section.

Extrusion is the shaping of metal into a chosen continuous form by forcing it through a die of the
desired shape. Titanium can be extruded to produce rounds, squares, tubes, and other simple shapes. Typical
extrusion temperatures range between 1800 and 1900°F (980 and 1040°C).

Titanium metal has been observed to have better flow characteristics than steel. It more readily fills
the die, causes less die wear, and maintains closer tolerances than do steels.

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