Master of Philosophy Course
Modelling of Materials
Power Plant Steels: Remanent Life
A. N. Other
The steels used in the power generation industry are almost always
given a severe tempering heat-treatment before they enter service.
This would, in most contexts, give them a highly stable microstructure
which is close to equilibrium. In fact, they undergo many changes
over long periods of time. This article is a review of some of the
methods which exploit the changes in order to estimate
the life that remains in alloys which are only partly exhausted.
Many of the safety-critical components in power plant are made of
steels developed to resist deformation when used in the range
480-565 oC and 15-90 MPa. They are expected to serve reliably
for a period of about 30 years, giving a maximum tolerable creep strain rate
(approximately two percent elongation over the 30 years). The design stress must be set to
be small enough to prevent creep rupture over the intended life of the plant.
The steels are able to survive for such long periods because the
operating temperature is only about half of the absolute melting
temperature, making the migration of atoms very slow indeed. Creep
therefore depends on the ability of dislocations to overcome obstacles
with the help of thermal energy. The obstacles are mainly carbide
particles which are dispersed throughout the microstructure.
Suppose that the microstructure and the operating conditions do not
change during service. The accuracy with which component life might then
be predicted would depend only on the quality of the experimental data.
The so-called safety factors common in design could then be
greatly reduced with obvious benefits. Of course, this never happens in
practice; the steels are always heterogeneous and the service
conditions vary over a range of scales and locations. The design life is
therefore set conservatively to account for the fact that measured
creep data follow a Gaussian distribution with a significant width.
In spite of this, experience has shown that decommissioned plant could
have been kept in service without sacrificing safety. To take
advantage of this observation requires methods for the reliable
estimation of the remaining life. The techniques used for this purpose are
summarised in Table 1. Of the properties listed, no single measurement is
comprehensive enough to describe the steel with all requisite
completeness. However, the present survey is confined to just two
topics, damage parameters and hardness changes. It is recognised that
the implementation of a life-extension procedure must be
based on much wider considerations backed by more frequent
Table 1: Methods used in the
estimation of remaining life.
| Property ||
|Damage summation || Evans, 1984  |
|Hardness|| Goto, 1984  |
|Tensile test || Cane and Williams, 1987  |
|Interparticle spacing || Askins and Menzies, 1985  |
|Cavitation parameter || Gooch, 1989  |
|Number density of cavities || Dyson and McLean, 1972 |
|Fraction of cavities || Walker and Evans, 1970  |
|Impact toughness || Wignarajah et al. 1990  |
A satisfactory way of representing creep damage (C) is to use a
parameter () which is normalised by its value at failure
(). The magnitude of will depend on the precise
values of stress
, temperature (T) and any other variable which influences the
creep process. Since these variables are not necessarily constant, the
extent of damage is often written [1,9]
is typically the time or the creep strain. Failure occurs when
the sum achieves a value of unity.
Evans  argues that it is more appropriate to use the strain rather
than the time, since the latter is not considered as a state
variable. In the context of thermodynamics, the state of a system can
in principal be specified completely by a number of state variables (such
as temperature, pressure) such that its properties do not depend at all on
the path by which those variables were achieved. This clearly cannot be
the case even for the creep strain. This is because the extent of
damage is expected to depend on the path by which a given value of
strain is achieved, for example, whether the strain is localised at grain
boundaries or uniformly distributed. This necessarily means that the above
equation is an approximation; as Evans states, it should be a
reasonable approximation if the mechanism of creep does not change
between the components of the summation. Thus, Cane and Townsend 
conclude that the use of the life fraction rule in taking
account of temperature variations is more justified than for
variations in stress. This is because for the latter case, the
dislocation networks become finer (relative to the carbide
spacings) at large stresses. The network nodes then do not
coincide with carbide particles, thus changing the mechanism of
deformation. This is not the case with variations in temperature
because the dislocation network then scales with the particle
spacing. The failure of the life fraction rule is sometimes
accommodated by empirically setting the limiting value of
C to some positive value which is not unity.
HARDNESS, INTERPARTICLE SPACING
The hardness can be used as an indicator for the state of the
steel in its life cycle. Changes in hardness occur due to recovery,
coarsening of carbide particles, and recrystallisation. All
creep-resistant power plant steels are severely tempered before they
enter service. They are therefore beyond the state where secondary
hardening is expected and the hardness can, during service, be expected
to decrease monotonically. In these circumstances, an Avrami equation
adequately represents the changes in hardness,
where t is the time, and n rate constants and is given
where is the initial hardness,
is its hardness at the end of useful life and the
hardness at time t.
The hardness at the point where the
microstructure is exhaustively annealed is likely to be around
most power plant steels. Its main components include the intrinsic strength of iron and
solid solution strengthening. The starting hardness is likely to be in the range HV.
Therefore, all that can be expected is a change in hardness of about 30-70 HV over a period
of some 30 years. Thus, Roberts and Strang  have shown that the hardness can decrease
by about 20% in the stressed regions of long-term creep test specimens; this is consistent
with an approximately 25% reduction found by Maguire and Gooch . Figure 1 shows the
nature of the changes in hardness to be expected typically, as reported by Maguire and
Gooch  for a 1CrMoV steel which was tempered at 700 oC for 18 h prior to
the ageing at temperatures in the range 600-640 oC.
Figure 1: Changes in the
hardness of a 1CrMoV steel during ageing in the temperature range
600-640. The open circles are data from the grips of a creep test
specimen, and the filled circles represent measurements from the gauge
length. Data from Maguire and Gooch.
Precipitates impede the motion of dislocations and any strength in
excess of is often related to the spacing
() between the particles (Cane, 1986, 1987):
where it is assumed that in
order to be consistent with coarsening theory.
It is well known that differences in hardness develop in the grip and
gauge length of a creep test specimen (Figure 1). The strain in the gauge
length leads to accelerated softening. In fact, the hardness reduction
that occurs in the gauge length is roughly proportional to that in
the grip . Tack
 have therefore taken H to be a function of strain:
empirical constants. This illustrates the fact that hardness is a crude indicator of
remaining life. Furthermore, it's ability to account for creep damage in the form of
voids is not represented in any theory. Hardness tests can therefore only serve a useful
purpose in the regime of steady-state creep, before the onset of gross damage. This is
evident from where it is seen that specimens with the same hardness are at different
life-fractions. There is a further complication, that the hardness of welded regions is
likely to be inhomogeneous even when the welds are made with matching
compositions. The potential location of failure is then difficult to
identify since creep ductility, creep strength and creep strain may vary
with position. The weld metal always has a larger oxide content than
the parent steel, and hence has a lower creep ductility. It cannot
therefore be assumed that failure will always occur in the softest part
of the joint. A harder weld may be needed to ensure the same rupture
life as the parent steel (Figure 2).
Figure 2: Data for
Cr1Mo steel and matching weld metal. The curve
represents the locus of all points along which the weld metal and parent
metal have equal rupture lives. Note that for a given life, the weld
must be harder than the parent steel (after Tack, ).
The hardness can be related inversely to the spacing
between precipitate particles; this is illustrated in Figure 3 for a
1CrMo steel . The spacings are typically measured using
transmission electron microscopy at a magnification of about with approximately 100 fields of view covering 30 taken at random. The actual measurement involves counting the
number of particles per unit area () and it is assumed that
. The amount of material examined in any
transmission microscope experiment is incredibly small, so care has to
exercised in choosing representative samples of steel. In some cases,
the microstructure may be inherently inhomogeneous. One example is the
12Cr and 9Cr type steels where there is a possibility of regions of delta-ferrite
where the precipitation is quite different from the majority tempered martensite
Obviously, hardness tests are much simpler to conduct when
compared with the effort required to properly measure particle spacings.
A further complication is that there is frequently a mixture of many
kinds of particles present, some of which continue precipitation during
service whereas others dissolve. Thus, Battaini  found
that in a 12CrMoV steel, precipitation continues to such an
extent during service that there is a monotonic decrease
. In fact, the distribution of particles was bimodal with
peaks at 30 nm and 300 nm diameters. It is strange that they were only
able to correlate the hardness against the changes in the coarser
particles. For another steel (12CrMoVW), Battaini found an even more
complex variation in the interparticle spacing with a maximum value in
for the coarse particles.
It should be emphasised that the standard error in
measurements is quite large, frequently larger than the variations
observed. Given the difficulties of interpretation, and the
experimental error, it is unlikely that measurements can be
used as a satisfactory general measure of remaining life. The data can
nevertheless be of use in the design of physically based creep models.
Figure 3: The hardness
as a function of the near neighbour spacing of carbides following creep
tests at 630 oC for a variety of time periods. After Askins and
Menzies, 1985 .
The present work is
by no means a comprehensive review of the available literature.
There is a vast array of methods
available for the assessment of remaining life in power plant
steels. Of these, the use of damage parameters and hardness
measurements have been the subject of this review. Although there are
difficulties of interpretation and considerable uncertainties in the
data, both of these parameters can, with care, be used as approximate
indicators of remanent life.
 H. E. Evans: Mechanisms of Creep Fracture, Elsevier
Applied Science Publishers, Essex, England (1984)
 T. Goto: Creep and fracture of engineering materials and
structures, Swansea, Pineridge Press, (1984) 1135.
 B. J. Cane and J. A. Williams: International Materials
Reviews 32 (1987) 241-262.
 M. C. Askins and K. Menzies: Unpublished work,
referred to in Gooch and Townsend, (1987) 1985
 D. J. Gooch, M. S. Shammas, M. C. Coleman, S. J. Brett
and R. A. Stevens: Proc. Conf. Fossil Power Plant
Rehabillitation, Cincinnati, Ohio, ASM International,
Ohio (1989) Paper 8901-004.
 B. F. Dyson and D. McLean: Metal Science 6 (1972)
 G. K. Walker and H. E. Evans: Metal Science 4 (1970)
 S. Wignarajah, I. Masumoto and T. Hara: ISIJ
international 30 (1990) 58-63.
 E. L. Robinson: Trans. Am. Soc. Mech. Engrs. 74
 B. J. Cane and R. D. Townsend: CEGB report
TPRD/L/2674/N84, Leatherhead, Surrey (1984)
 B. W. Roberts and A. Strang: Refurbishment and Life
Extension of Steam Plant, Institution of Mechanical
Engineers, London (1987) 205-213.
 J. Maguire and D. J. Gooch: Proc.
Int. Conf. Life Assessment and Extension, The Hague,
 A. J. Tack, J. M. Brear and F. J. Seco: Creep:
Characterisation, Damage and Life Assessment, eds D. A. Woodford,
C. H. A. Townley and M. Ohnami, ASM International, Ohio, U.S.A.
 P. Battaini, D. D'Angelo, G. Marino and J. Hald: Creep
and Fracture of Engineering Materials and Structures,
Institute of Metals, London (1990) 1039-1054.
H. K. D. H. Bhadeshia
Mon Jun 14 09:24:27 BST 1999