Tailor-made heavy plates for pressure vessels T - Dillinger Hütte GTS

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Tailor-made heavy plates for
pressure vessels
F. Gottwalles, Dr. I. Detemple, Dr. P. Wolf
Dillinger Hütte GTS
Introduction
T
echnical requirements for petrochemical
reactor steels have proliferated in the last
decade. The need to increase economic benefi
ts together with higher operating temperatures
and pressures are leading to the construction of
higher capacity reactors with thicker walls.
Also, more severe requirements in these steel specifi -
cations leave the steelmaker little scope to fi nd a good
balance between all the infl uencing parameters, in
order to achieve the best possible steel design.
These two points can lead to a situation where the
material’s physical limits are reached. It is not only
the heat treatment, but also the chemical composition
and thickness of the plate that strongly infl uence the
steel’s mechanical properties.
By taking the example of carbon manganese steel
SA516-70 edition 2004 addendum 2006 [2], we will look
at how the microstructure is modifi ed and thus the
main mechanical properties.
It may be noted that the 2007 edition of this standard
provides for new possibilities − it now permits maximum
manganese content of 1.50% as against 1.20% so
long as the maximum carbon content is also reduced.
However, this cannot be implemented if the reactor
has to be constructed according to the 2004 edition
addendum 2006 or previous editions.
The hollomon parameter (HP) concept
The Hollomon parameter (HP) [1] is used to describe
the various heat treatments applied to a steel plate, especially
tempering and stress relief treatments as well
as their combination (see fi gure 1). This parameter is
useful as it combines the various infl uencing factors of
heat treatment into a single value.
Thus different heat treatments having an identical HP
will have an identical effect on the material’s metallurgy
− the higher the HP, the more signifi cant the
metallurgical effect.
Infl uence of the HP on the plate’s
mechanical properties
Figure 2 shows the evolution of yield strength and tensile
strength for a typical chemical composition and
for plates with thicknesses between 30 and 50 mm,
against the HP (N representing the normalised state).
The blue-shaded band represents yield strength values
calculated with a confi dence interval of ±2.3σ. The
red-shaded band represents tensile strength values
calculated with a confi dence interval of ±2.3σ. The
continuous red and blue lines represent the minimum
and maximum requirements set by standard SA516-70
ed. 2004 ad. 2006 [2].
Figure 1 Figure 2 D
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Figure 3
Figure 4 and 5
We can see that yield strength and tensile strength are
affected by the heat treatment: the harsher the heat
treatment, the more the material’s yield strength and
tensile strength decrease.
Figure 3 shows the evolution of toughness against HP.
The coloured lines represent, for various test temperatures,
the lower toughness limits calculated for a typical
chemical composition and for plates with thicknesses
between 30 and 50 mm. 99% of the results are
thus found above this limit. The two horizontal black
lines represent typical minimum values 25 J average
(17 J lowest single value).
We can see that the toughness is strongly affected by
the heat treatment − the harsher the heat treatment,
the more the material’s toughness decreases.
Therefore, by keeping the chemical composition
constant whatever the HP, the requirements set by
a standard or customer may no longer be complied
with. To counter this effect, the steel’s carbon content
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PRODUCTION/PROCESSING/HANDLING 2
can be adjusted. Figures 4 and 5 are curves of the
expected values of tensile strength and toughness at
–29°C for plate thickness of 50 mm and typical chemical
composition by only varying the carbon content
and the HP.
As fi gures 4 and 5 show, the carbon content has to
be increased to ensure the minimum tensile strength
requirement, and it has to be decreased to improve
toughness. For certain carbon contents, and according
to the heat treatment applied, it is no longer possible
to ensure the product’s mechanical requirements
without employing other solutions. These other solutions
will be examined below.
Infl uence of plate thickness on
mechanical properties
Figure 6 shows the evolution of yield strength and
tensile strength, for a typical chemical composition
and for normalised plates, against thickness.
The blue-shaded band represents yield strength
values calculated with a confi dence interval of ±2.3σ.
Figure 6
Published in The International Oil&Gas Engineer February 2008
The red-shaded band represents
tensile strength values calculated
with a confi dence interval
of ±2.3σ. The continuous red and
blue lines represent the minimum
and maximum requirements set
by standard SA516-70 ed. 2004 ad.
2006 [2].
When plate thickness increases,
the material’s yield strength and
tensile strength decrease for a
constant chemical composition.
Therefore, from a certain thickness,
it is no longer possible to
D

Figure 7
ensure the minimum tensile strength requirements set
by the standard by keeping the steel’s same chemical
composition. Normally the carbon content has to be
increased.
Figure 7 shows the evolution of toughness against
plate thickness. The coloured lines represent, for
various test temperatures, the lower toughness limits
calculated for a typical chemical composition. 99% of
the results are thus found above this limit. The two
horizontal black lines represent typical minimum values
25 J average (17 J lowest single value).
For low plate thicknesses, the cooling rate (in air)
is relatively high and can produce a ferritic-perlitic
microstructure containing bainite. This negatively
affects toughness in the untempered state. Therefore
the chemical composition and/or heat treatment
requires adjustment.
Above a certain thickness where toughness has
reached a maximum, the material’s toughness de-
Figure 8 and 9
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creases with increased plate thickness, all other
parameters remaining constant. This phenomenon is
mainly due to increased grain size of the microstructure
since the plate’s cooling rate decreases.
Therefore, to ensure the minimum requirements of the
standard (or customer), for a given test temperature
and thickness, it is necessary to modify the steel’s
chemical composition and/or heat treatment.
Let us look again at the solution of varying the carbon
content to alleviate the negative effect of increasing
the product’s thickness on the material’s mechanical
properties. Figures 8 and 9 show the expected values
of tensile strength and toughness at –29°C for a typical
chemical composition of plate in normalised condition
by varying the carbon content and plate thickness. As
in the previous case, it appears necessary both for the
carbon content to be increased to ensure the minimum
tensile strength requirement, and to be reduced
to improve toughness.
Finally for certain combinations “product thickness –
heat treatment – toughness test temperature – standard
or customer specifi cation”, it can easily be seen
that the single carbon content variation is no longer
suffi cient. Furthermore, carbon negatively affects certain
steel forming properties, especially its suitability
for oxycutting or its weldability.
Finally, carbon comes directly into the calculation of
the carbon equivalent which is often limited by the
standards or customer specifi cations.
Therefore other methods have to be used to adjust the
mechanical properties of the plates. However, these
have to comply with the standard or be accepted by
the customer.
D

Alloying
A reduction of carbon in the steel can be offset by
adjusting the steel‘s chemistry using different alloying
elements such as chromium, molybdenum, copper,
nickel and/or niobium. Thus the alloying elements,
by their effect, contribute directly to improving the
steel‘s toughness, or allow to reduce the carbon content
without reducing tensile strength and so indirectly
improving the product‘s toughness.
Figure 10
Figure 10 shows the effect of alloying on the mechanical
properties in the form of zones. These zones
are functions of the thickness and heat treatment
depending on whether a „CMn“ concept or a
„CMn + alloying“ concept is used. These zones represent
(t; HP) combinations for which a plate can be
produced according to standard SA516-70 ed. 2004
ad. 2006 with minimum toughnesss in the transverse
direction at quarter thickness of 25 J average
(17 J lowest single value) at –29°C.
However, this concept also has limits and certain
„t; HP“ combinations cannot be reached. Standards
and specifi cations restrict the alloying element content
and these elements infl uence the carbon equivalent
(the most commonly used formula is:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15). Furthermore,
it should not be forgotten that the production
cost of an alloyed plate is signifi cantly higher than
that of a carbon-manganese plate.
Therefore, it is necessary to explore other avenues, for
example by directly adjusting the parameters of the
heavy plate production process.
PRODUCTION/PROCESSING/HANDLING 4
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Accelerated cooling followed by
tempering
This solution consists in the accelerated cooling of
the plates in order to produce a microstructure having
a fi ner grain size and comprised of bainite and
martensite. Nevertheless, to achieve good toughness
on these plates, they have to receive a heat treatment
(tempering) after quenching.
However, certain standards or specifi cations do not
permit this special treatment. Yet, according to standard
SA516, for thick plates it is possible to reproduce
cooling similar to that undergone by a thin plate after
normalisation. This is not a matter of quenching but
just the accelerated and controlled cooling of the thick
plates. This accelerated cooling must be followed by
tempering. The plate‘s delivery condition is no longer
„N“ but „N + AC + T“.
The goal is to produce the same microstructure as
thin plate for thick plate. Grain size directly infl uences
toughness. By acting on this parameter it is possible
to satisfy the requirements in terms of toughness,
while keeping enough carbon and alloying element
content to reach the required levels of yield strength
and tensile strength.
Figure 11 shows typical plate microstructures obtained
by means of these two types of process.
Figure 11
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Figure 12, like fi gure 10, shows the effect of the
„N + AC + T“ treatment on the mechanical properties.
Here the zones also represent (t; HP) combinations for
which a plate can be produced according to standard
SA516-70 ed. 2004 ad. 2006, with minimum toughness
D

Figure 12
in the transverse direction at quarter thickness of
25 (17) J at –29°C, by the production method used.
By implementing this type of process (when the
standard or the customer specifi cation permits) the
steelmaker can accept more severe requirements in
terms of the (t; HP) combination or in terms of a pure
requirement with constant (t; HP) combination.
Conclusion
By means of various examples we have seen that the
mechanical properties of a heavy plate depend on
many parameters. Not only the chemistry employed
in preparing the steel in the steelplant, but also the
thickness of the fi nished product and the type of process
implemented strongly affect the characteristics of
the fi nished product. Thus requirements that are too
severe compared with those defi ned in the standard,
especially when they only take account of part of
the infl uencing parameters or when they are applied
uniformly to an entire range of thicknesses, limit the
steelmaker‘s options to adjust the mechanical properties
of heavy plate. For highly demanding specifi cations,
as increasingly required by customers, it is
therefore important to hold detail discussions with
the steelmaker before ordering plate.
PRODUCTION/PROCESSING/HANDLING 5
BIBLIOGRAPHIC REFERENCES
[1] HOLLOMON, John Herbert & JAFFE, L.D.
Time-Temperature relations in Tempering
Steel. Transaction of the AIME, 162, 1645,
223-249
[2] ASME Code Section II Part A, Edition 2004
+ Addenda 2006, SA-516/SA-516M, Specifi -
cation for pressure vessel plates, carbon
steel, for moderate- and lower-temperature
service
[3] DETEMPLE Ingo, DEMMERATH Anja,
The Effect of Heat Treatment, Hydrocarbon
Engineering, November 1998, p. 67.
[4] DETEMPLE Ingo, Alloying Concepts,
Hydrocarbon Engineering, December 2003,
p. 64.
ABBREVIATIONS
HP Hollomon Parameter
t Thickness
CE Carbon equivalent
N Normalised
AC Accelerated Cooling
T Tempering
Table 1: Table of abbreviations
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