The selection of material for any specific environment is directly
dependent on the material’s properties, especially those properties that
are affected by that special environment.
Metal properties are classified in terms of Mechanical, Physical and
Chemical properties. These are further subdivided into Structure
Sensitive or Structure Insensitive properties. The following table
describes these properties.
Table 1: Metal properties.
In this article, we are concerned only with the structure-sensitive
mechanical properties of metal. Metals are favored as a construction
material because they offer a combination of mechanical properties that
are unique and not found among non-metals. Metals are generally strong
and many can be loaded or stressed to very high levels before breaking.
One property of metals of interest is their capacity to exhibit a high
degree of elastic behavior in their early load-carrying capacity. This
is a very important property for effective use of the metal as a
construction material. When these metals are loaded beyond their elastic
range they exhibit another set of important properties called ductility
and toughness. These properties and how they are affected by change in
temperature are the point of this article.
Pipeline Steels
We will focus on carbon and low-alloy steels. It may be noted that the
bulk of the material that is used in conventional pipeline engineering
comes from this generic group. Aptly, it is the ductility and toughness
of these metals and how they are affected by the variation of
temperature that is our subject. The emphasis is made on the variation
under low temperature. For this purpose it is essential to know what is
meant by these metal properties and by low temperature. The following
definitions are understood by fracture mechanics.
Ductility is defined as the amount of plastic deformation that metal
undergoes in resisting the fracture under stress. This is a
structure-sensitive property and is affected by the chemical
composition.
Toughness is the ability of the metal to deform plastically and
absorb energy in the process before fracturing. This mechanical and
structure sensitive property is the indicator of how the given metal
would fail at the application of stress beyond the capacity of the
metal, and whether that failure will be ductile or brittle. Only one
assessment of toughness can be made with some reasonable accuracy from
ordinary tensile testing, and that is the metal displays either ductile
or brittle behavior. From that it can be assumed that the metal
displaying little ductility is unlikely to display a ductile failure if
stressed beyond its limits. The failure in this case would be brittle.
The temperature of metal is found to have profound influence on the
brittle/ductile behavior. The influence of higher temperature on metal
behavior is considerable. The rise in temperature is often associated
with increased ductility and corresponding lowering of the yield
strength. The rupture at elevated temperatures is often intergranular,
and little or no deformation of the fractured surface may have occurred.
When lowered below room temperature, the propensity for brittle
fracture increases.
- The term fracture is strictly defined as irregular surface that
forms when metal is broken into separate parts. If the fracture has
propagated only part way in the metal and metal is still in one piece,
it is called a crack.
- A crack is defined as two coincident-free surfaces in a metal that
join along a common front called the crack tip, which is usually very
sharp.
- The term fracture is used when the separation in metal occurs at
relatively low temperature and metal ductility and toughness performance
is the chief topic.
- The term rupture is more associated with the discussion of metal separation at elevated temperatures.
As noted previously, two basic types of fracture occur in metals:
ductile and brittle. These two modes are easily recognized when they
occur in exclusion, but fractures in metal often have mixed morphology
and that is aptly called
mixed mode. The mechanisms that initiate the fracture are
shear fracture,
cleavage fracture, and
intergranular fracture. Only the shear mechanism produces
ductile fracture. It may be noted that like the modes discussed here, the failure mechanisms also have no exclusivity.
A crack is defined above as two coincident-free surfaces in a metal
that join along a common front called the crack tip, which is usually
very sharp. Irrespective of the fracture being ductile or brittle, the
fracture process is viewed as having two principal steps:
1. Crack initiation, and
2. Crack propagation.
Knowledge of these two steps is essential as there is a noticeable
difference in the amount of energy required to execute them. The
relative level of energy required for initiation and for propagation
determines the course of events which will occur when the metal is
subjected to stress.
There are several aspects to the fracture mechanics that tie in with
the subject of metal ductility and toughness but this article is not
planned for detailed information on fracture mechanics. Hence, these are
not discussed in detail but some specific-related topics are listed in
Table 2.
Table 2: Topics related to fracture mechanics.
- Effects of axiality of stress,
- Crack arrest theory,
- Stress intensity representation,
- Stress gradient,
- Rate of Strain,
- Effect of Cyclic Stress,
- Fatigue Crack,
- Crack Propagation, (KIc= σ √πa)
- Griffith’s theory of fracture mechanics,
- Irwin’s K = √E x G,
- Crack Surface Displacement Mode,
- Crack Tip Opening Displacement (CTOD), (BS 5762-1979 and BS 7448 part-I)
- R-Curve Test methods
- J- Integral Test method,
- Linear-Elastic Fracture Mechanics (LEFM) (ASTM E 399),
- Elastic-Plastic Fracture Mechanics (EPFM),
- Nil Ductility Temperature (NDT).
Though the topics in Table 2 are not commonly taken into
consideration when selecting suitable material for an onshore pipeline,
these are essential parts of subsea pipeline and riser technology. In
fact, some of the specification (e.g. API 1104, DNV-OS F101 etc.)
suggest the use of fracture mechanics to determine the failure behavior
of metal in these services.
Returning to our earlier discussion, lowering the temperature of
metal profoundly affects fracture behavior. Strength, ductility,
toughness and other properties are changed in all metals when they are
exposed to temperature near absolute zero. The properties of metals at
very low temperatures are of more than casual interest because
pipelines, welded pressure equipment and vessels are expected to operate
satisfactorily at levels below room temperatures. For example, moderate
sub-zero temperatures are imposed on equipment for dewaxing petroleum
and for storage of nitrogen, liquefied fuel gases and pipelines.
Much lower temperatures are involved in cryogenic services where
metal temperature falls to –100 C (-150 F) and below. The cryogenic
service may involve storage of liquefied industrial gases like oxygen
and nitrogen. Toward the very bottom of the temperature scale, there is a
real challenge for metals that are used in the construction of
equipment for producing and containing liquid hydrogen and liquid
helium,because these elements in liquefied form are increasingly
important in new technologies. Helium in liquefied form is only slightly
above absolute zero, which is 1 Kelvin (-273.16 C or – 459.69 F).
Absolute zero (1 K) is the theoretical temperature at which matter
has no kinetic energy and atoms no longer exhibit motion. Man has yet to
cool any material to absolute zero, so it is unknown how metals would
behave when cooled to this boundary condition.
However, metal components have been brought to the temperatures very
close to absolute zero, hence it presents a special challenge to metals
and welded components as they would be required to serve in this
extremely low temperature.
When cooled below room temperature every metal will reach a
temperature where the kinetic energy will be reduced to nil. The atoms
of the element will move closer and the lattice parameters will become
smaller. All these changes would affect the mechanical properties of the
metal.
Metal Strength At Low Temperature
As we have seen, as temperature is lowered from room temperature, 75oF
(24oC or 297oK), to absolute zero, 1oK, the atoms of an element move
closer together by dimensions easily compounded from the coefficient of
thermal expansion. Several changes occur as a result of this smaller
lattice parameter. For example, the elastic module increases. In
general, the tensile strength and yield strength of all materials
increase as the temperature is lowered to the nil ductility temperature
(NDT) , where the yield and tensile strength are equal (σo = σu). The
change in these properties is variable in degree for different metals
but change does occur.
When the temperature of low-carbon or low-alloy steel is lowered, the
corresponding increase in strength of metals occurs. This is attributed
to an increase in resistance to plastic flow. Because plastic flow is
strongly dependent upon the nature of the crystalline structure, it
would be logical to assume that metals with the same kind of structure
would react similarly.
A cautionary note: The material in ASTM A 333 Grades 1,3,4,6,9 and 10
is required to have minimum of 10 ft-lbs absorbed energy (impact
values).This is the same as ASTM A 350 LF1, but material ASTM A 350 LF2
and LF3 are required to have minimum of 12 ft-lbs absorbed energy
(impact values). This is at any given temperature, respective of that
material.
Selecting Material From Specification And Codebooks
There are several ASME/ASTM specifications specifically tailored for
low-temperature services, but it is important to check if the specified
test temperatures for the metal in use is in tally with the design
temperature of the system. ASTM-A/ASME -SA105 is not a low-temperature
material; however, it may be used for low temperature if all the other
factors are conforming to the requirements and an additional impact test
on the material is carried out at a temperature that is in tally with
the design temperature.
Similarly, ASTM A 106 pipes (grade A, B or C) must be checked for the
test temperatures because ASTM A 106 is specified as “high-temperature”
material and rightfully the impact test is not even included in the
non-mandatory requirement. The same is the case with ASTM A 105 forged
material discussed above. Concerning ASTM A 333 grades 1, 3, 4, 6, 9 and
10 pipes for the acceptable impact values and their test temperatures,
the specification must be referenced before arbitrarily using them for
any service temperature range. ASTM A 350 LF1 (-20 F), LF2 (-50 F), LF 3
(-150 F) are suitable for low-temperature service to the limits set by
the specification, but one should check the specified energy absorption
value Cv to ensure it is in tally with the system design parameters.
An informed selection has to be made. There are several
boiler-quality plate materials specified by the ASTM specifications and
ASME codes but not all are suitable for low-temperature services. Some
are so designed metallurgically that they are not suitable for
low-temperature service. Plate material conforming to the ASTM A 515
specification is an example. Most of the metals that are fit for low
temperature are generally tested to 32oF (0 C) unless specified
otherwise. So, the general assumption that all ASME material is good up
to -20oF will not be correct, unless it is tested and material test
report so declares.
API mandates that PSL2 pipes be tested at 32oF (0 C) or any lower
temperature as agreed between the buyer and manufacturer and is expected
to have 20 ft-lbf (27 J) absorbed energy. The same is not true for PSL1
pipes. In either case, it is important to determine what was the actual
test temperature and what responsibility engineers have to ensure that
the test temperature is in tally with the design temperature of the
system.
Among pipeliners, a question is often raised if, in designing a
buried pipeline, one needs to consider the low temperature. The answer
is not metallurgical since it is unrelated to the material property as
much as it is geographical and environmental, that is, the design
conditions. The data provided by the user (clients) and the
specification must be consulted.
Generally, a buried pipeline will not be subject to very low
temperatures unless buried in permafrost, so no specific caution beyond
the general design considerations would be required. However, the
general guidance in such case should be to look at the product
properties, risk analysis, product leakage, and will a reduction in
pressure at a certain point reduce the temperature to what is considered
a low-temperature range.
If there is a cause to expect lower temperature, then determine to
what extent lower temperature will occur during the life of service. If
the temperature is ever in the critical low range, it will be prudent to
identify those conditions and take them into account while selecting
the material.
Similar consideration applies to the aboveground pipe and components.
Aboveground valves flanges and pipes are more exposed to the weather
and are also carrying the similar product. Therefore, they have greater
propensity to face low temperature in their service lives. The following
questions must be asked and answered: Are they insulated? Are they
heated? Is there any possibility of depressurization that would lead to
extensive temperature reduction, etc? There is a multiplicity of factors
that affect the understanding of the material behavior in extreme
stress conditions. All possible factors must be identified and
addressed.
Conclusion
The questions we have tried to explore are more complex than this
discussion which is an attempt to simplify the basic understanding of
the subject. This discussion is intended to bring out the importance of
the subject and direct readers to available resources for material
selection issues.
Important Additional Information
The sub-ambient temperature dependence of yield strength σo (Rp0.2) and
ultimate tensile strength σu in a bcc metal is shown in Figure 1.
Consider the graph, the material is ductile until a very low
temperature, point A, where Y.S. equals the UTS of the material (σo =
σu). Point A represents the NDT temperature for a flaw-free material.
The curve BCD represents the fracture strength of a specimen containing a
small flaw (a < 0.1mm). The temperature corresponding to point C is
the highest temperature at which the fracture strength σf ≈ σo. Thus
point C represents the NDT for a specimen with a small flaw.
The presence of a small flaw raises the NDT of steel by about 200°F
(110°C). Increasing the flaw size decreases the fracture stress curve,
as in curve EF, until with increasing flaw size a limiting curve of
fracture stress HJKL is reached. Below the NDT the limiting safe stress
is 5,000-8,000 psi (~35 to 55 MPa).
Above the NDT the stress required for the unstable propagation of a long
flaw (JKL) rises sharply with increasing temperature. This is the
crack-arrest temperature curve (CAT). The CAT curve defines the highest
temperature at which unstable crack propagation can occur at any stress
level. Fracture will not occur for any point to the right of the CAT
curve.
The temperature above which elastic stresses cannot propagate a crack
is the fracture transition elastic (FTE). The temperature defines the
FTE, at the point K, when the CAT curve crosses the Yield Strength, σo
curve. The fracture transition plastic (FTP) is the temperature where
the CAT curve crosses the Ultimate Tensile Strength σu curve (point L).
Above this temperature, the material behaves as if it is flaw-free, for
any crack, no matter how large, cannot propagate as an unstable
fracture.
Reference :
http://www.pipelineandgasjournal.com/selection-pipe-material-low-temperature-service-0
