Investigation of Ta-MX/Z-Phase and Laves Phase
as Precipitation Hardening Particles in a 12 Pct Cr
J.P. SANHUEZA, D. ROJAS, O. PRAT, J. GARCI
A, M.F. MELE
and S. SUAREZ
A 12 pct Cr martensitic/ferritic steel was designed and produced to study Laves and Z-phase as
precipitation hardening particles under creep conditions (650 °C). According to thermodynamic
calculations, W and Cu additions were selected to ensure the precipitation of Laves after
tempering. It is known that Z-phase formation does not follow the classical nucleation theory.
Indeed, MX particles are transformed into Z-phase by Cr diﬀusion from the matrix to the
precipitate. Therefore, to promote fast Z-phase formation, Ta, Co, and N additions were used
to produce Ta-MX, which will be transformed into Z-phase. The main result achieved was the
precipitation of Laves after tempering, with a particle size of 196 nm. As regards to Z-phase, the
transformation of Ta-MX into Z-phase after tempering was conﬁrmed by the formation of
hybrid nanoparticles of 30 nm. Although W and Ta have a low diﬀusion in the martensitic/fer-
ritic matrix, characterization of the precipitates after isothermal aging revealed that Laves and
Z-phase have fast growth kinetics, reaching 400 and 143 nm, respectively, at 8760 hours.
Consequently, creep test at 650 °C showed premature failures after few thousand hours.
Therefore, it is expected that future research in the ﬁeld of martensitic/ferritic steels will focus on
the growth and coarsening behavior of Laves and Z-phase.
Ó The Minerals, Metals & Materials Society and ASM International 2018
are widely used in new supercritical power plants
(600 °C/30 MPa) for key components such as steam
pipes, turbines, and boilers.
They combine high creep
strength, oxidation resistance, good weldability, thermal
fatigue resistance, and competitive production costs.
Nowadays, T/P91 and T/P92 grades are the most
common martensitic/ferritic steels used in fossil fuel
power plants due to their high microstructural stabil-
In general, precipitation hardening involving
carbides and MX particles is the main creep
strengthening mechanism in 9 to 12 pct Cr heat-resistant
A high volume fraction of M
heterogeneously nucleate on sub-boundaries and grain
boundaries avoiding the recovery of the martensitic/fer-
ritic matrix during long-term creep.
additional creep strength is obtained by the precipitation
of MX carbonitrides along sub-grain boundaries as
exhibited in T/P91 steels, which have a creep rupture
strength of 94 MPa at 600 °C/10
as shown in T/P92 steels, rupture strength can be
increased by 20 pct with the addition of W and B.
turn, W and B increase creep strength by solid solution
hardening and reduce the coarsening rate of M
carbides, avoiding the recovery of the martensitic/fer-
Nowadays, new environmental regulations for CO
emissions have motivated many researching groups to
focus on the development of new 9 to 12 pct Cr
creep-resistant steels under operating condition of
650 °C/30 MPa, which would result in fossil fuel power
plants with more eﬃcient steam cycles and lower CO
Several studies of conventional 9 pct Cr
steels (T/P91 and T/P92) showed that their oxidation
resistance was not enough to operate at 650 °C.
ﬁrst attempt to overcome this barrier was to increase the
Cr content to 12 pct, but no good results were obtained.
Although a superior oxidation resistance was achieved,
long-term creep resistance drastically declined due to the
transformation of MX particles into detrimental coarse
J.P. SANHUEZA, D. ROJAS, O. PRAT and M.F. MELE
are with the Universidad de Concepcio
n, Departamento de Ingenierı
de Materiales, Edmundo Larenas 270, Concepcio
n, Chile. Contact
e-mail: email@example.com J. GARCI
A is with the AB Sandvik
Coromant R&D, Lerkrogsvgen 19, 12680Stockholm,Sweden.S.
SUAREZ is with the Department of Materials Science, Saarland
University, 66123 Saarbrucken, Germany.
Manuscript submitted November 27, 2017.
Article published online May 11, 2018
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2951