CHALLENGES OF
HARSH SUBZERO
ENVIRONMENTS
Some of the most inhospitable locations on the planet present unimaginable
challenges not only for human beings and animals, but just as significantly
for materials, explains Johanna Haapasalmi, Ovako, Finland.
C
onsider installations in a subzero environment and the
implications on the materials used when temperatures
dip below -40˚C. Such installations within the Arctic
Circle – as well as other subzero environments on the
planet – see pipeline installations, power plants, oil and gas
exploration and production, LNG plants and mining, even shipping,
all exposed to these extreme conditions.
Of course, steel has been used in such locations for some
considerable time and, as with all environments, there has been
development of materials to specifically suit prevailing conditions.
For all of these installations, exhaustive and thorough risk
assessments have to be conducted to ensure sound operational
capability of the steel components and the equipment they are
being used in.
Understanding the effects on materials, particularly steel,
of subzero temperatures is imperative for the safe operation
not only of equipment, but also for personnel; a topic that is
increasingly important for companies.
Steels undergo a sudden transition from ductile behaviour
at high temperatures to brittle cleavage failure at lower
temperatures (Figure 1). In certain steel grades, as a load is applied
this can manifest as a sudden and unexplained fracture, which
can have disastrous effects on an operation and the personnel
involved.
Manufacturing process
A better understanding as to why conventional steels exhibit
declining levels of performance in subzero conditions can
be gained by examining the production methods. By closely
controlling the complete process from raw material selection,
chemical composition through to precision of the melt cycle
ensures the correct properties. The secondary metallurgy
uses a proprietary Ovako technique critical in controlling the
final chemical composition and cleanliness of the steel. The
importance of managing the chemical composition, concentration
of non-metallic inclusions, grain size and impurity levels, cannot be
emphasised enough.
Manufacturing and production processes employed by Ovako
are based on decades of research including the close analysis of
thousands of fatigue failures with scanning electron microscopy,
which allows the determination of the position, type and size of
failures. This data provides an advantage in facilitating treatment
and modification in order to produce engineering steels, such as
SZ-Steel, which stands for subzero steel (Figure 2).
Figure 1. Ductile to brittle transition temperature of steel is
dependent on the composition of the material grade.
Figure 5. Instrumented impact testing of grade Ovako 277.
One of the company’s attribute brands it comprises a carefully
selected range of steel grades, all of which are operationally
proven in some of the most demanding environments on the
planet. SZ-Steel retains its properties in temperatures down to
-40˚C and beyond, and – in extreme cases – is well tested to
withstand temperatures down to -101˚C.
Load case
Figure 2. Developed for Arctic conditions, SZ-Steel from Ovako.
Figure 3. Comparisons between inclusion engineered steel
Ovako 277Q and standard grade.
As companies push the limits of performance in cold climates and
harsh subzero environments demand for strong, safe and wellproven materials is on the rise. Components such as pipeline bolts,
hydraulic parts and offshore lifting devices must offer greater
reliability at -40˚C or lower subzero temperatures.
The ability of steel to handle loads in all directions is often
overlooked, as standards mainly specify impact strength in the
longitudinal direction. Traditional steels are manufactured in
a process where there is little or no control over inclusions.
During the rolling process, the inclusions within the steel
become elongated. As a result, they weaken the fatigue and
impact properties of the steel by disrupting the structural
homogeneity. Subsequently, components manufactured from
such material can exhibit very different properties when
handling transverse or longitudinal loads, and this can make
conventional steel unsuitable for safety critical applications.
Ovako has developed a process to overcome this problem by
reducing the number and size of inclusions in both longitudinal
and transverse direction, IQ-Steel. By combining the isotropic
properties in IQ-Steel with the SZ-Steel properties this ensures
a high level of strength in both the transverse and longitudinal
directions. This is illustrated in Figure 3, where Ovako material
grade 277L – with normal processing – is compared with the
isotropic material grade Ovako 277Q, which combines inclusion
engineering. In the latter case, transverse comes close to
longitudinal sampling giving the material greater strength.
The SZ-Steel classification helps to reduce risks of
embrittlement and fracturing, and also contributes in safeguarding
the natural environment while improving safety for workers and
service crews.
Measure for better control
Figure 4. Prior to Charpy testing, steels are place in the ‘Freeze
Box’ and taken to temperatures as low as -101˚C.
World Pipelines / REPRINTED FROM JANUARY 2016
Established specifically to measure the effects of subzero
temperatures on steel, the company has test facilities suitable for
the need at the various production plants producing SZ-Steel. In
Imatra, in south east Finland, the so called ‘Freeze Box’ is capable
of achieving test temperatures as low as -101˚C (-214˚F).
Once at the required test temperature, the material is then put
through a Charpy V-notch test in order to determine the amount
of energy absorbed by the material during fracture. The results are
recorded in accordance with the Standard Methods for Notched
Bar Impact Testing of Metallic Materials, as specified by EN/ISO
148-1, EN 10045-1 and ASTM E23. As determined by the standard, if
the material breaks on the flat plane then the fracture is regarded
as brittle. However, if it breaks with jagged edges or shear lips,
then the fracture is considered to be ductile.
To further ensure and develop materials that can withstand
sudden force in service, instrumented impact tests are performed
to study the behaviour of steels. The instrumented testing is
measuring not only the total energy absorbed by the sample
during the impact test, as a normal test is doing, but also the force
absorbed through the cut.
By using the instrumented testing, Ovako is able to better
describe and understand when and why the fracture moves from
tough/plastic to a brittle behaviour, as shown in Figure 5.
Operational use
Nowhere is a material better trialled than in true operational
conditions. Many applications in the oil and gas, refining,
petrochemical, marine, energy and mining industries successfully
use SZ-Steel in subzero environments. For many years Ovako
has been supplying bar, ring and seamless tubes to the offshore
industry for manufacture into a wide range of products that have
to endure high shock loads and fatigue on land, on the high seas
and in the depths in mooring chains, fixing bolts, lifting equipment
and hydraulic components.
Fixing studs, bolts and associated equipment, such as flanges
and pump bodies as well as service and installation equipment
in SZ-Steel grades are widely used in pipeline installations
throughout the Arctic region. Ovako SZ-Steel grade Imatra L7, used
for pipe fixing stud bolt, is tested at temperatures down to -101˚C
(Figure 7). Typically, the material returns minimum figures for yield
strength 970 MPa, tensile strength 1085 MPa and impact strength
27 J. It can also be used on exposed installations where a
pipeline is raised above ground and fixed on piles to
avoid subsidence – brought about by permafrost
and melt conditions – in order to avoid any
risk of the pipeline bending and serious
buckling.
A leading supplier of power
equipment to operators
working in the area relies on
the performance of SZ-Steel
for its rotor shafts, a critical
component in the electric
motors to ensure operational
efficiency and reliability. Used
in safety critical applications
they deliver power to winches,
compressors, conveyors and mining machines. Typically, the
motors operate in average temperatures between -20˚C and
20˚C, but are designed to handle operating conditions as low
as -55˚C.
Figure 6. Shackles and master links in SZ-Steel. Image courtesy
of Gunnebo Industries.
For another customer, world leading manufacturer of lifting
and lashing equipment SZ-Steel is incorporated in its chain, wire
rope slings, shackles and master link products (Figure 6). All are
produced in compliance
with the DNV (Det Norske
Veritas) 2.7-1 certification –
an accreditation designed
to safeguard life, property
and the environment, both at
sea and onshore, in cold climates.
The company also prides itself on
certifying its products to perform at
temperatures of -40˚C and below.
Complete traceability of the material is
assured with accompanying mill
certificates that provide key data
Figure 7. For stud
bolt material, SZ-Steel
such as toughness, tensile strength,
grade Imatra L7 is
elongation and hardness, along
used and tested at
with details of each specific heat
-101˚C.
treatment.
For concrete pumping and other
hydraulic applications, hydraulic
cylinders are critical components that are also used in subzero
conditions. Hydraulic cylinders are put under pressure meaning they
are subject to a three dimensional load. This means they must be
able to handle high levels of impact strength in both longitudinal
REPRINTED FROM JANUARY 2016 / World Pipelines
and transverse direction. As described above it is not uncommon
with material showing excellent impact strength in longitudinal
having dramatically lower transverse impact strength. By combining
properties from the company’s IQ-steel with SZ-steel, Ovako 277Q
(Figure 3) is proven to fulfil the critical requirements of cylinders
used by a major concrete pump manufacturer.
Table 1 shows a range of areas and grades from Ovako,
where SZ-Steel brings superior performance for use in extreme
environments where subzero temperatures and exacting
conditions prevail.
account of the prevailing climatic and operational conditions
has to be available when specifying material for use in subzero
applications. Ensuring that the material complies with the
relevant standards and ideally has a proven operational record
in such environments.
Essential is the ability to deliver material with the
right heat treatment and the right control over steel
microstructure. A delicate balance between ductility and
strength. Wrong heat treatment and the result is a hard
material that could risk brittle fracture, too ductile a material
and the high strength required might not be present. Low
impurity levels and controlled grain size ensures the right
material performance to meet the most demanding Arctic
conditions.
Standards
There are a number of relevant international standards that refer
to the use of materials in subzero applications, such as ISO, EN, API,
ASTM, DIN, DNV and JIS. Use of the correct steel in these areas is
imperative given the low temperatures and prevailing conditions.
Note
This materials research and innovation was recognised with the
presentation of an ATC Spotlight on Arctic Technology award
for the company’s range of SZ-Steel grades at the OTC’s Arctic
Technology Conference in March 2015.
Conclusions
The importance of understanding a material’s limitations in
subzero temperatures is paramount. Similarly, a comprehensive
Table 1. Typical applications where SZ-Steel material grades are being used
Application
area
Oil and gas
Construction
Mobile cranes
and lifting
devices
Examples of
standards
Examples of suitable Ovako grades
Typical data of Ovako grades**
Ovako grade
Yield
strength
Tensile
strength
Impact strength
MPa
(min.)
MPa (min.)
Temp.
(˚C)
Joule
(min.)
Corresponding
standard*
Sample
direction
Products
API 6A
OVAKO 326C
AISI 4140
600
740
-50
150
L
Bars,
tubes,
rolled
rings
API 6D
OVAKO 281T
19MNV5
620
740
-40
115
L
Rolled
rings
API 16A
OVAKO 322D
AISI 4130
620
730
-50
100
L
Bars,
tubes,
rolled
rings
API 16C
OVAKO 322D
or 322Q
AISI 4130
620
730
-50
100
L
Tubes
EN ISO
898-1
Cromobolt
42CrMo6F
980
1100
-40
40
L
Bars
Max dia./
wall (mm)
60
ImacroM
5Cr16F
980
1100
-55
40
L
Bars
60
SB27M12CB
27MnCrB5-2
1000
1070
-40
85
L
Bars
35
SB30M12CB
30MnCrB5-2F
1030
1090
-40
65
L
Bars
42
SB29M10CB
1050
1100
-40
55
L
Bars
50
EN 10269
25CrMo4
460
650
-40
120
L
Bars
100
ASTM A320
Imatra L7
970
1085
-101
27
L
Bars
30
EN 130013-1
520M
400
520
-40
27
L
Bars
100
EN 17115
1100
1200
-40
50
L
Bars
70
Ovako 277L
23MnNiMoCr54
23MnNiMoCr5-4
660
800
-40
110
L
Bars,
tubes,
rolled
rings
160
Ovako 277Q
660
800
-40
90
T
* Designation followed by ‘F’ is not a formal standard grade, but named according to the rates of EN10027.
** Typical data is not guaranteed.
World Pipelines / REPRINTED FROM JANUARY 2016
Bars,
tubes,
rolled
rings
120