Advanced high chromium ferritic steels for boiler components operating at high temperature
G. Cumino (*), S. Di Cuonzo (*), A. Di Gianfrancesco(+), O. Tassa(+)
(*) Dalmine S.p.A, 24044 Dalmine (BG), Italy
cum@Dalmine.it, sdicuonzo@dalmine.it
(+) Centro Sviluppo Materiali S.p.A., 00129 Roma, Italy
a.digianfrancesco@c-s-m.it, o.tassa@c-s-m.it
Abstract — In the last 15 years Dalmine and Centro
Sviluppo Materiali (CSM) have worked together to
develop the high Chromium ferritic steels for tubes and
pipes to manufacture high temperature boiler
components used in new high efficiency power plants.
The activities started in 1985 with the development of
P/T91 process and metallurgy, in the frame of ECSC
project, followed, in 1994, by the evolution of P/T91
with the addition of 1% W. This new European steel
called E911 was introduced during the COST 501
European Programme to improve the high temperature
mechanical properties.
This paper describes the process routes and compares
the mechanical and creep behaviour of the two grades
as well as the evolution of microstructure and
precipitates after aging.
Keywords — high chromium ferritic steel, creep, microstructure evolution, boiler material.
1. INTRODUCTION
The development of the family of 9-12%Cr martensitic
steels for power generation components has been a
significant feature in the 1980s. However, the basic
compositions of 9Cr1Mo have been widely used for
many years. The driving force for new developments
has been the need to increase the temperature and
pressure of the steam circuit.
The Steel Grade P91, which has the base composition
of 9%Cr 1%Mo with additions of niobium, vanadium
and nitrogen, developed in the ORNL Research
Laboratories, has provided a platform for further
developments in Japan (Naoi et al.,1995; Sawarahi et
al., 1995). In Europe, a new steel called E911 derived
from the Grade 91 (9Cr1MoNbVN) with the addition
of 1% W was developed in the frame of the programme
COST 501 Round III WP11.
The aim of this paper is to present the work carried out
by Dalmine and CSM to develop and qualify P91 and
E911 in terms of mechanical properties and
microstructure evolutions (Di Gianfrancesco et al.,
1996).
2. MATERIALS
External qualified suppliers manufactured ingots of the
two grades: Cogne for P91, UES (United Engineering
Steel, UK) for E911.
The chemical compositions are shown in Table I.
Dalmine has manufactured the pipes with the following
process route:
• ingot heating at 1300°C
• partially boring on a horizontal press
• piercing
• rolling at the pilger mill
• heat treatment (Grade E911 = normalising
1060°C/air + tempering 760°C; Grade 91 =
normalising 1040°C/air + tempering 740°C)
• finishing operations
• machining
2.1 Characterisation of the as treated material
The E911 transformation temperatures have been evaluated by dilatometer. As shown in Table II they are higher than for steel P91.
The tensile properties from room temperature up to 700°C are shown in Fig. 1 for both the steels.
2.2 Creep tests
Creep tests have been carried out on pipes in the range 550 - 650°C. Creep rupture tests have now reached 40000 hours for E911 and P91 at 650°C. The oldest creep specimen still running for P91 at 550°C has reach 89000 hours. Figs 2 and 3 show a comparison between the behaviour of E911 and P91 pipes at 600°C and 650°C respectively.
The ductility of both materials decreases just a little after 15000 hours for the specimens tested at higher temperature, but in any case, up to now the elongation to rupture is still more than 17 % and the reduction of area is more than 50%. The lowest values of the reduction of area were found at the highest test temperature (650°C).
2.3 Mechanical Testing of Aged Materials
Grade E911 and P91 materials have been subject to a long-term exposure in order to evaluate the variation in tensile and impact properties at room temperature. This has been made after an isothermal exposure of the materials at 625 and 650 °C up to 1000, 3000 and 10000 hours made by an electrical laboratory furnace showing a maximum temperature deviation in the chamber of + 4 °C. The results of the tests are summarised in Figs 4-7.
The results of the long-term exposure show:
- a 10% maximum reduction of the tensile properties
after 1000 hours (Fig. 4),
- a constant values of ductility (Fig. 5),
- a strong reduction of the impact properties during the
first 3000 hours of exposure (Fig.6) with an increase of
% of brittle fracture (Fig.7).
After the same long-time exposure, steel P91 shows a similar trend in term of tensile properties, but impact properties of P91 keep always better than E911.
2.4 Microstructural Evolution
STEM investigations on thin foils under the as received and aged conditions, have been performed on both steels E911 and P91. The aim was to characterise the evolution of the tempered martensite and to determine the different types of second phase, the dimensional distribution of different types of precipitate and their composition evolution with ageing treatment.Quantitative analysis on automatic image analysis system, X-ray diffraction and EDS microanalyses have been carried out by extraction replicas. The microstructures of the as treated materials are presented in Fig. 8.
Fig. 9 show an evident recovery process due to formation of subgrains, with a reduction of the dislocation density, that replace gradually the original martensite laths. The rate of the recovery process, rather similar for both the steels at 625°C, is faster for E911 steel at 650°C, in which the transformations of martensite laths to subgrain is complete after 1000 hours.
Nevertheless the retained high dislocation density is
connected with a stable microstructure also in the more
aged structures.
In all the samples, the MX type carbide or carbo-nitride
rich in vanadium and niobium were observed primarily
precipitated on dislocations inside the subgrains,
although it was difficult detect them because of their
very small dimensions. They are rather stable in terms
of composition and size with increasing ageing time.
The M23C6 carbides detected in all the samples are
observed predominantly on subgrain boundaries. Both
steels show the coarsening of the M23C6 carbides, with
an increase of the size from an original value of 200
nm up to 400 nm after 10000 hours of ageing at 650°C.
The extensive quantitative microanalysis investigation
shows that the relative amount of Fe, Cr, Mo and W in
the M23C6 carbides varied as a function of the carbide
size. Particularly the Mo content increased with the
size, and a tendency to an enrichment in Cr appears
with increasing the ageing time. An EDX spectrum of
M23C6 carbides in P91 and E911 steels respectively is
shown in Fig.10.
The precipitation of the intermetallic Laves phase
(Fe,Cr)2(Mo,W) during ageing represents the most
relevant microstructure change. The particles of Laves
phase are localised at the subgrain boundaries and
often these precipitates are associated with M23C6 carbides. At 625°C the nucleation of the Laves phase is
observed just in the first 1000 hours in both steels.
The rate of coarsening of this phase was very high and
these particles grow up to dimensions of approximately
1 µm after an ageing time of 10000 hours. At 650°C
the Laves phase precipitation occurred during the first
1000 hours for E911, where in the P91 steel was
observed only after 3000 hours. After 10000 ageing
hours the Laves phase in both steels reached micron
sizes. In Fig 10 the typical EDX spectra of Laves phase
particles in P91 and E911 steels are shown.
3. DISCUSSION
The stress dependence of the minimum creep rate (Fig. 11) can be described by a well-known equation of the form:
![]() |
(1) |
where A and n are constants at given temperature, σ is the applied stress and Q represents the apparent activation energy. The value of the mean stress exponent n is related to the operating mechanism of creep deformation. This equation can be used for the estimation of Q and n separately for low stress and high stress domains and the values calculated for E911 are reported in Table III.
The stress exponent n, calculated by means of all the
data, decreases with increasing temperature. The
observed values are ranging between 15 at 575°C and 5
at 650°C.
The activation energy calculated by Eqn. (1) gives a Q value of 552 KJ/mol. The values of Q and n calculated
in the present work on E911 steel agree with found
values for P91 steel reported in (Slenicka et al., 1994).
Nevertheless two distinct domains of the stress
dependence of creep rate are observable in agreement
with the case of precipitation strengthened low alloy
and modified Cr steels (Slenicka et al., 1994;
Nishimura et al., 1994; Foldyna et al., 1996). This
behaviour is also confirmed in E911 steels and the
calculated n and Q values for both the domains are
reported in Table III. According to literature the value
of n = 19 for high stress domain and n = 5 for low stress
domain have been determined. With increasing
temperature the low stress domain becomes more
important and the difference between two domains is
smoother. The activation energies calculated in the low
and high stress domains are respectively of 458 and
912 KJ/mol. The differences in n and Q are thought to
result from the different dislocation process operating
at high and low stress. As reported in literature at high
stress regions the dislocations can overcome particles
of secondary phases by Orowan mechanism while at
low stress a climb mechanism is predominant (Foldyna et al., 1996).
The dependence of n and Q on temperature and stress
can be related to the microstructure evolution during
the creep that is principally the precipitation of Laves
phase and the particle coarsening. The precipitation of
Laves phase in the first period of creep exposure that
increases the strengthening due to the precipitation give
a reason of the high n and Q found values. The particle
coarsening, correlated to a long time exposure at high
temperature, becomes the predominant effect at low
stress. The resulting strengthening loss can explain the
reduction of n and Q.
The procedure used for the assessment of the creep
rupture data to extrapolate up to time to rupture of 105 hours is based on the Larson Miller parametric
equation:
![]() |
(2) |
where CLM is a constant, tr is time to rupture. The procedure used is based on the assumption that the stress dependence of PLM can be described by a polynomial stress dependence in the form:
![]() |
(3) |
By using all the experimental data, at high and low stress domains, the calculated CLM for the E911 and P91 steels are respectively 31,9 and 31,3. In Fig. 12 the results of creep tests in the form of stress to Larson- Miller parameter is shown.
By using the above values of the constant CLM a preliminary extrapolation of the base material creep results with the data at this moment available, shows at 600°C a stress of 105 MPa to have the rupture in 105 hours, instead of 94 MPa for the P91, with a possible increase of about 10 Mpa. In Fig. 13 the isothermal curves at 600°C are reported for the both P91 and E911 for comparison. As suggested by (Foldyna et al., 1996) the prediction of long-term stress rupture should be more correct if the constant CLM is calculated taking into account only the experimental results obtained at low stress domains. In effect the data for E911 steel are not yet enough to give a reasonable extrapolation by taking in account only the low domain stress data.
In Fig.14 a very good agreement is shown between calculated stresses and the experimental values to obtain similar time to rupture. The higher resistance of E911 steel is related to the addition of W, which is completely in solid solution in the ferrite only for short time tests. The presence of Laves phase, that produces a loss of Mo and W in the ferrite, is normally connected with a loss of creep strength due to the precipitation of this intermetallic phase. The creep results for E911 at 625°C and 650°C and after 1000 when the Laves phase is just precipitated show a higher creep resistance of this steelwith respect to the P91 steel. As suggested by Hald (1995, 1996), the highest creep resistance cannot be associated with the solution strengthening of W but it is due to the beneficial effect of the precipitation of Laves phase that increase the total volume fraction of secondary phases and can contribute to the precipitation strengthening.
The loss of ductility observed after time to rupture of 15000 hours particularly at 650°C can be associated to the increased size of these precipitates that grow up to dimensions at around 1µm..
4. CONCLUSIONS
The mechanical properties such as tensile, hardness
toughness and creep rupture strength have been
discussed for the new steel Grade E911, in comparison
with the Grade P91.
The tensile and creep rupture properties of E911
appears to be higher than the values measured on P91.
E911 presents however a limited loss of ductility for
creep test at 650°C after time to rupture of 15000 hours
that can be associated with the coarsening of Laves
phase, as suggested by the results of the microstructure
analysis. By considering that this steel has been
developed for application temperature of 600°C, where
the coarsening process is slower than observed at
650°C, the microstructure stability of E911 steel can be
guaranteed in terms of precipitation state.
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Received: Janury 15, 2001.
Accepted for publication: December 15, 2001.
Recommended by Subject Editor E. Dvorkin.