NOVEMBER 2016
| WORLD FERTILIZER |
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microstructure is influenced by the chemical composition and
also, very importantly, by the manufacturing process.
So, as it has been just mentioned, the corrosion resistance of a
urea grade is not only determined by its chemical composition,
and a strict compliance to specifications regarding corrosion limits
is not sufficient to assure the best quality.
Top product quality is closely related to the productive
process used, knowledge and ‘care’ in each manufacturing stage. In
this regard, key factors in the manufacture of high-quality and
corrosion-resistant tubes are:
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Rolling process.
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Heat treatment.
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Inspection process.
Cold-rolling
During the rolling process, the tube is physically formed adopting
its final dimensional properties. Despite internal stresses
generated, which will be released in the subsequent thermal
treatment, any micro-crack generated in a deformation process –
which is not correctly calculated (tooling design,
deformation/material resistance calculation, installation operation
parameters, etc.) – may entail a shorter tube life. Any possible oil
trapped in a micro-crack may cause premature corrosion as it is
difficult to completely clean oil.
Therefore, major technical knowledge and control of cold
deformation processes is required, including the use of FEM
calculation to improve tube corrosion resistance.
Heat treatment
Treatment applied to tubes after cold rolling provides the material
with metallurgical properties. The process intends to achieve the
following main aims: removal of undesired chemical compounds,
strain softening and setting of desired grain structure forming and
homogeneous oxide layer.
However, a major impact factor on heat treatment is derived
from the previous process: comprehensive cleaning of traces of oil
and grease (with high carbon content) from rolling process, as well
as physical particles or chemical components. Using the most
advanced techniques for heat exchange tubes cleaning is essential
to guarantee a correct furnace entry surface. Any carbon trace will
reduce chromium content from the surface made up of chromium
carbides, thus reducing the corrosion resistance of the material.
Such cleaning can only be guaranteed by using an automated
system suitable for more demanding tube applications (fertilizers
and umbilical tubing, etc.).
Inspection process
Even with the most-in-depth knowledge of the manufacturing
production process and ideal facilities, a complex defect
detection system is required to ensure the quality of tubes used in
the vessels of urea synthesis loop.
Apart from ultrasound and Eddy Current inspection required
to ensure dimension control and absence of defects in the
material, it is important to highlight sigma phase continuous
control – in particular, duplex grades, given their tendency to form
this inter-metallic phase and high risk derived from its presence.
A urea grade for each need
As mentioned at the beginning of the article, each urea
manufacturing process, as well as each area within this process, has
its own characteristics. Such characteristics make materials used
nowadays diverse yet limited to three in the urea HP synthesis
loop:
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TP 316 LUG: material employed in piping systems and
scarcely used today in heat exchangers of the HP section.
Still used in low-pressure vessels, it is a cost-effective
and widely available material, although greater wall
thicknesses are required and therefore higher weight for
the whole section. It still has a risk of condensation
corrosion, not to mention the least passive urea material
being prone to more corrosion failure modes than the
following two materials.
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TP 310 MoLN: commonly known as 25.22.2, its higher
alloying elements content has made this the most
popular material in recent decades. It is a long-term
experienced material with lower corrosion rates and no
condensation corrosion. As a characteristic of fully
austenitic microstructures, this steel grade is susceptible
to chloride stress corrosion cracking. It is also widely
available on the market and less O
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for passivation is
needed, compared to 316L UG.
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UNS S32906: due to its duplex structure, it has much
higher mechanical properties, leading to lower wall
thicknesses and less total system weight. Moreover, due
to this ferrite-austenite structure, it behaves better
against pitting and crevice corrosion. It is not even prone
to chloride SCC, thereby resulting in fewer failure modes.
It is also the most passive behaving material among those
used in the urea field (active corrosion has never been
detected). On the other hand, it is a more costly material
– although an important cost saving can be obtained via
total weight reduction. It is only offered by two top
seamless steel tube producers, Tubacex is among them.
Conclusion
As explained through this article, there are many factors to be
taken into account when selecting the best material for each case.
General corrosion resistance is a determining factor; however,
there are other aspects, such as failure modes in each grade,
passive/active behaviour of the material, its mechanical properties
or even availability and costs, which may influence material
selection.
To ensure the best possible corrosion ratios, as well as
chemical composition and metallurgical properties, it is
paramount to know if the production process used is appropriate
and guarantees the desired properties.
To achieve this, working with a company that supplies value
through a stable modern industrial process, technical knowledge
as well as R&D orientation is necessary.
Figure 3.
Tubacex Group’s manufacturing plant in Austria
(SBER).
