On the formation and growth of Mo-rich Laves phase particles during long-term creep of a 12% chromium tempered martensite ferritic steel

2009 ◽  
Vol 61 (11) ◽  
pp. 1068-1071 ◽  
Author(s):  
A. Aghajani ◽  
F. Richter ◽  
C. Somsen ◽  
S.G. Fries ◽  
I. Steinbach ◽  
...  
Keyword(s):  
Ferritic Steel ◽  
Laves Phase ◽  
Tempered Martensite ◽  
Term Creep

On the nature of internal interfaces in a tempered martensite ferritic steel and their evolution during long-term creep

2012 ◽  
Vol 66 (12) ◽  
pp. 1045-1048 ◽  
Author(s):  
E.J. Payton ◽  
A. Aghajani ◽  
F. Otto ◽  
G. Eggeler ◽  
V.A. Yardley
Keyword(s):  
Ferritic Steel ◽  
Tempered Martensite ◽  
Term Creep

Formation of the Fe2Hf Laves Phase through Eutectoid Type Reaction of δ→γ+Fe2Hf in Ferritic Heat Resistant Steels

2015 ◽  
Vol 1760 ◽  
Author(s):  
Satoru Kobayashi
Keyword(s):  
Phase Transformation ◽  
Ferritic Steel ◽  
Laves Phase ◽  
Ferritic Steels ◽  
Microstructural Observation ◽  
Interphase Precipitation ◽  
Δ Ferrite ◽  
Term Creep ◽  
Heat Resistant Steels

ABSTRACTPeriodically arrayed rows of fine Fe2Hf Laves phase particles were found to form in 9Cr ferritic steel. Microstructural observation demonstrates that the particles were formed on cooling through the interphase precipitation on the phase transformation from the δ ferrite to the γ austenite along the eutectoid transformation route of δ → γ+Fe2Hf and subsequently a phase transformation from the austenite to the α ferrite took place. This eutectoid route is expected to be effectively used for improving the long term creep strength of ferritic steels with Laves phase.

Novel Concept of Creep Strengthening Mechanism using Grain Boundary Fe2Nb Laves Phase in Austenitic Heat Resistant Steel

2011 ◽  
Vol 1295 ◽  
Author(s):  
Imanuel Tarigan ◽  
Keiichi Kurata ◽  
Naoki Takata ◽  
Takashi Matsuo ◽  
Masao Takeyama
Keyword(s):  
Grain Boundary ◽  
Strengthening Mechanism ◽  
Local Deformation ◽  
Creep Rupture ◽  
Laves Phase ◽  
Resistant Steel ◽  
Heat Resistant Steel ◽  
Heat Resistant ◽  
Term Creep

ABSTRACTThe creep behavior of a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%), strengthened by intermetallic Fe2Nb Laves phase, has been examined. Particular attention has been given to the role of grain boundary Laves phase in the strengthening mechanism during long-term creep. The creep resistance increases with increasing area fraction (ρ) of grain boundary Laves phase according to equation ε/ε = (1−ρ), where ε0 is the creep rate at ρ = 0. In addition, the creep rupture life is also extended with increasing ρ without ductility loss, which can yield up to 77% of elongation even at ρ = 89%. Microstructure analysis revealed local deformation and well-developed subgrains formation near the grain boundary free from precipitates, while dislocation pile-ups were observed near the grain boundary Laves phase. Thus, the grain boundary Laves phase is effective in suppressing the local deformation by preventing dislocation motion, and thereby increases the long-term creep rupture strength. This novel creep strengthening mechanism was proposed as “grain boundary precipitation strengthening mechanism” (GBPS).

TEM replica analysis of particle phases in a tempered martensite ferritic Cr steel after long term creep

2021 ◽  
pp. 111396
Author(s):  
H. Wang ◽  
A. Kostka ◽  
W.E. Goosen ◽  
G. Eggeler ◽  
J.E. Westraadt
Keyword(s):  
Tempered Martensite ◽  
Term Creep

Microstructural Evolution During Long Term Creep Tests of 9%Cr Steel Grades

2007 ◽  
Author(s):  
Leonardo Cipolla ◽  
Augusto Di Gianfrancesco ◽  
Dario Venditti ◽  
Giuseppe Cumino ◽  
Stefano Caminada
Keyword(s):  
Creep Strength ◽  
Creep Resistance ◽  
Power Plants ◽  
Volume Fraction ◽  
Laves Phase ◽  
Creep Tests ◽  
Creep Properties ◽  
Cr Steels ◽  
Term Creep

In the last two decades the service pressure and temperature of components for advanced power plants increased significantly and more severe requirements on strength, corrosion resistance and creep properties were imposed on high temperature steels. To comply with these requirements, several new 9–12%Cr martensitic steels were developed and some of them, such as ASTM Grades 91, 911 and 92 are currently used in new high efficiency Ultra Super Critical power plants. The initial evaluation of their creep strength above 550°C was defined with relatively short term tests, but the long experience in service and long term creep laboratory tests showed that the original estimation of creep strength values were not reliable and a reduction of the creep resistance occurred at long service time. Short creep tests (elaborated with time-temperature-parameter methods, i.e. Larson Miller equation) usually give an over-estimation of the long-term creep properties of 9%Cr steels. The results of the creep assessments of Grade 92 (Japanese NF616) are an example of the significant lowering of the creep properties: the creep resistance of this grade was initially evaluated in 600°C/160MPa/105h by means extrapolation of short creep tests, within 103 hours; recently the creep strength was reduced down to 113MPa (ECCC assessment, 2005). Moreover some premature failures of Japanese Grade PI 22 took place and similar problems appeared on other 12%Cr steels. The lowering of creep strength in 9–12%Cr steels at long times is a consequence of the evolution of their microstructure during high temperature service. The causes of this phenomenon in Grades 91, 911 and 92 are examined in this article, paying special attention to the metallurgical explanation. The most evident changes in the microstructure of 9%Cr steels occur with the nucleation of Laves-phase as well as the nucleation of Z-phase at longer times. The precipitation of Laves phase has two relevant aspects by the creep strength point of view. On one hand, high amounts of Mo and W contents are incorporated in this phase, causing a depletion of these elements from the solid solution and thus a reduction of their contribution to the overall creep resistance. On the other hand, the increased volume fraction of secondary phases leads to a higher precipitation strengthening during the first precipitation phase: at the beginning, the precipitation of fine Laves phase increases the creep resistance; however if the coarsening rate is not taken under control, the mean diameter of these particles reaches micrometric dimensions with a detrimental effect on creep behaviour within 103 hours in the range 600°C–650°C. The high coarsening rate of Laves phase is therefore the major cause of the lowering of creep properties of Grades 91, 911 and 92. Coarsening of Laves phase particles over a critical size triggers the cavity formation and the consequent brittle intergranular fracture. Transition from ductile fracture to brittle intergranular fracture often occurs in long-term creep at the onset of coarsening of Laves particles, which result to be the preferential site for cavities nucleation in the 9%Cr steels. Z-phase was recognized in 9%Cr steels after long term exposure, but in far smaller amount than on 12%Cr steels: no dramatic drop in volume fraction of MX was observed in association to the nucleation of this phase, therefore it is believed that the modified Z-phase does not affect significantly the long term creep properties of Grades 91, 911 and 92. The dimple pattern is typical of ductile fracture, which occurs for short service period (hence highest stress). At low stresses, cavities are formed at the triple grain junctions at which Laves particles are often found, causing wedge crack, otherwise isolated cavities can form independently at coarse Laves phase particles (Figure 13). The latter type is often observed after long-term creep in the interganular fracture region. In both cases, brittle fracture occurs at the onset of coarsening of Laves particles, which result to be the preferential site for cavities nucleation in the 9%Cr steels.

scholarly journals Laves-phase in the China Low Activation Martensitic steel after long-term creep exposure

2014 ◽  
Vol 63 ◽  
pp. 333-335 ◽  
Author(s):  
Lixin Huang ◽  
Xue Hu ◽  
Wei Yan ◽  
Wei Sha ◽  
Furen Xiao ◽  
...  
Keyword(s):  
Martensitic Steel ◽  
Laves Phase ◽  
Term Creep ◽  
Creep Exposure

Study on the nucleation and growth of Laves phase in a 10% Cr martensite ferritic steel after long-term aging

2015 ◽  
Vol 621 ◽  
pp. 93-98 ◽  
Author(s):  
Yuantao Xu ◽  
Mingjia Wang ◽  
Yan Wang ◽  
Tao Gu ◽  
Lei Chen ◽  
...  
Keyword(s):  
Ferritic Steel ◽  
Nucleation And Growth ◽  
Laves Phase

Creep Damage Evaluation by Hardness in Advanced High Cr Ferritic Steels

2007 ◽  
Vol 561-565 ◽  
pp. 2217-2220 ◽  
Author(s):  
Hassan Ghassemi Armaki ◽  
Kyosuke Yoshimi ◽  
Kouichi Maruyama ◽  
Mitsuru Yoshizawa ◽  
Masaaki Igarashi
Keyword(s):  
Activation Energy ◽  
Rupture Life ◽  
Creep Damage ◽  
Ferritic Steels ◽  
Tempered Martensite ◽  
Short Term ◽  
Term Creep ◽  
Martensitic Lath ◽  
Short Term Creep

The apparent activation energy for rupture life sometimes changes from a high value of short term creep to a low value of long term creep. This change results in overestimation of rupture life recognized recently in advanced high Cr ferritic steels. The present study examined how to detect the decrease of activation energy in 9-12 %Cr steels with tempered martensitic lath microstructure. During aging without stress hardness of the tempered martensite microstructures remains almost constant in short term, whereas it decreases with increasing time after long term exposure. The onset of hardness drop can be a good measure of the decrease of activation energy. Causes of the hardness drop and the decrease of activation energy are discussed.

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