TPi October 2009

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Mater ialsTube and pipe
48 Tube Products International October 2009
FOR energy-conducting tubes (oilfield,
conduits and boiler tubes), the trend is
moving towards higher-tensile alloys as
preferred materials.
Pipeline producers must operate their
pipelines under increasingly harsh
environmental conditions and at higher
pressure, in high-tensile steel grades
such as X80, X100 and higher.
In addition, the trend in the large
tube sector is moving towards thicker
walls and increased resistance to low
temperatures.
New hot strip grades for
oil and gas pipelines
Germany’s largest steel producer,
ThyssenKrupp Steel AG, is a
manufacturer of feedstock for tube
production. Since the middle of the
1980s the company has been supplying
hot strips for tubes welded with
longitudinal and spiral seams.
In doing so, the supplier specialised in
input stock for tubes with diameters of
more than 20" (508mm) and in sour-gas
resistant qualities. ThyssenKrupp Steel
delivers around 500,000 tons per year to
tube manufacturers around the globe.
The company is also aware of the
increasing demands of the input
stock. Customers mainly demand
higher strengths in order to be able to
transport larger oil or gas volumes at
higher pressure, among other things.
While a few years ago, an operating
pressure of 80 bar was standard for
gas pipelines, today pipes are run at a
pressure of up to 100 bar.
To keep pace with this trend, the
development of higher-tensile tube
steel grades with a classification of
X80 for the standard grades and X70
for sour-gas resistant materials is
underway.
The ‘X’ classification is based on
the American measuring units and
measures the strength in kilopond per
square inch.
The approximate value for the minimum
limit of elasticity in Megapascal (MPa)
results from the multiplication of the
X-value with a factor of seven.
Micro-alloyed tube steel grades
contain exact amounts of niobium,
vanadium and titanium, which are used
to precisely determine the strength
of the material. The viscosity of the
materials is also controlled via the alloy
concept.
For example, high viscosity can prevent
the creation of kilometre-long tears
caused by small defects exposed to
the pressure in a pipeline.
High-tensile, sour-gas-resistant
tube steel grades
Just under a quarter of the pipe casings
supplied by ThyssenKrupp Steel
are comprised of the so-called HIC
(hydrogen induced cracking) grades.
It is planned to expand the strength
range of these sour-gas resistant tube
steel grades and offer HIC steel grades
with strengths of up to X70.
If the transported oil or gas contains
hydrogen sulphide, hydrogen-induced
cracks can appear in the pipeline
tubes. Such sour-gas wells exist in the
Gulf region, as well as in Canada and
Mexico.
During hydrogen induced cracking, the
hydrogen sulphide connects to the
water or water vapour in the oil or gas
to form sulphuric acid, which attacks
the tube walls.
During the corrosion process, hydrogen
is produced, which diffuses into the steel
and can lead to cracking. This can only
be avoided by producing steel grades
with a high purity level and an even
texture.
HIC grade steel must be desulphurated,
so it contains less than 0.0015%sulphur.
In addition, calcium is injected into the
liquefied material during production.
Composite materials with
customised characteristics
Tribond®, a product from ThyssenKrupp
Steel, is described as a customised
composite material off the coil. The hot-
rolled material combines various, in part
oppositional, characteristics in a three-
layer steel strip.
The basic idea revolved around
the development of a material that
combines high resistance to wear and
tear with good plasticity.
In the meantime, there are further
developments that combine a
comparatively low-cost, quality, hot
rolled steel strip with a thin corrosion-
resistant stainless steel surface.
Further research concerns MultiBond®
products that expand the original
three-layer TriBond design to five or
more layers.
New tube materials in development
During the hot roll bonding process, a homogenous metalline bond at the atomic level is ▼ ▼
formed