Review of Z phase precipitation in 9–12 wt-Cr steels

Abstract

ABSTRACT9-12wt%Cr steels for high temperature applications in fossil fired power plants rely upon precipitate strengthening from (V,Nb)N MX nitrides for long term creep strength. During prolonged exposure at service temperature another nitride precipitates: Cr(V,Nb)N Z-phase. The Z-phase slowly replaces MX, eventually causing a breakdown in creep strength. This paper reviews the Z-phase and its behaviour in 9-12wt%Cr steels including thermodynamic modelling, crystal structure, nucleation process and precipitation rate as a function of chemical composition. The influence of Z-phase precipitation upon long-term creep strength is assessed from several different 9-12wt%Cr steel grades and alloy design philosophies.

Creep behavior and microstructural evolution of a 9%Cr steel with high B and low N contents

“…The size of (V,Nb)(C,N) carbonitrides increases slightly with increasing the creep strain ( Table 2). In contrast to other studies on creep behavior of high-Cr steels [19,20], the development of Z-phase or enrichment of (V,Nb)(C,N) carbonitrides by Cr are not observed in the present study. Two-phase separation between Nb-rich and V-rich carbonitrides becomes more pronounced with increasing creep time ( Figs.…”

“…The following changes among the dispersed particles may occur during long-term aging and creep [4,[11][12][13][14]19,20]: (i) precipitation of Laves phase that leads to depletion of W and Mo from ferritic matrix down to thermodynamically equilibrium values; (ii) coarsening of M 23 C 6 carbides, Laves phase particles, and (V,Nb)(C,N) carbonitrides; (iii) replacement of nanoscale V-rich (V,Nb)(C,N) carbonitrides by coarse particles of Z-phase through (V,Nb)(C,N)→Cr(V,Nb)N transformation. Such changes in distribution of secondary phase particles decrease the efficiency of solid solution strengthening and dispersion strengthening.…”

Role of copper on Laves phase morphology in 9-12%Cr steels

“…The alloys investigated in this work were based on a very low carbon 12%Cr composition with Nb or Ta instead of vanadium, Z-phase strengthened steels [5,6], which are slightly different from most other 9-12%Cr steels. A high Co and Ni content ensured a martensitic structure with little or no delta ferrite.…”

Study on creep behavior and microstructure of a Fe–Cr–Al–Nb–W ferrtic heat-resistant steel based on C, N and Al element control

“…The efficiency improvement of coal-fired thermal power generation is an important technology for carbon dioxide emission reduction. To utilize this important technology, thermal power plants must be required to operate at higher steam pressures and temperatures [1][2][3][4][5]. One of the key points restricting the development of thermal power units is the development of high-temperature materials for high-temperature superheaters and reheaters.…”

“…The high-chromium (9-12Cr) ferritic heat-resistant steel would sustain a significant decrease in creep strength, due to 'structural degradation' during long-range service. The room temperature structure of high Cr ferritic heat resistant steel is martensite+a low amount of δ-ferrite [2]. High density dislocations and fine precipitates in martensitic laths are mainly utilized to ensure the stability of microstructure.…”