Superior long-term creep resistance of a 10% Cr steel

Creep is a time dependent effect of external forces in relation to mechanical properties of materials. Creep-resistant martensitic steels with an average of about 10% chromium have been found to be suitable materials for making turbine components of fossil fuel plants. Specifically, their alloys are tailored to provide stability to the tempered martensite lath structure (TMLS) which comprises prior austenite grains, blocks, packets and laths with a high dislocation density in the lath interiors under creep conditions. Previous studies have shown that TMLS is the main factor in the superior creep strength of these steels during long-term creep. Regardless, researchers have acknowledged that the instability of TMLS under creep reduces the creep strength and yields to the creep strength breakdown. Two processes promote the onset of creep strength breakdown: a knitting reaction and sub-grain coarsening. Unfortunately, the role that the microstructural changes in the long-term creep behavior and superior creep resistance without creep strength breakdown up to an extremely long rupture time for these 10% chromium steels has not been investigated in sufficient detail.

To this note, a team of three researchers led by professor Rustam Kaibyshev from the Belgorod State University in Russia proposed a study whose main objective was to elucidate the origin of the elimination of creep strength breakdown up to 4·10⁴ h at 650 °C under long-term creep with an applied stress of 120 MPa in this steel. They purposed to pay specific attention to the effect of the dispersion of the secondary phases on the stability of the tempered martensite lath structure. Their work is currently published in the research journal, Materials Science & Engineering A.

The material scientists here commenced their research work by studying the creep behavior and evolution of lathe martensite structure and precipitates during creep. Next, they analyzed various microstructural factors that affect the superior creep resistance including: homogeneously distributed M(C,N) carbonitrides, alloying by (W+Mo) elements and particles of M₂₃C₆ and Laves phases.

The authors observed that the tempered martensite lath structure of the 10% chromium steel remained stable during long-term creep testing until rupture. Again, they noted that during transient creep, the Laves phase particles precipitated in an analogous manner as that during long-term aging. It was also revealed that nanoscale M₂₃C₆ carbides and M(C,N) carbonitrides could compensate for the negative effects of W depletion from the solid solution and extensive coarsening of the Laves phase particles; this would result in superior long-term creep resistance.

The Roman Mishnev and colleagues study has demonstrated the creep behavior and evolution of lath structure and precipitates under an applied stress of 120 MPa for the 10% Cr steel with Cobalt, increased Boron and decreased Nitrogen. It has been seen that the M₂₃C₆ carbides demonstrate a high coarsening resistance under creep conditions and exert a high Zener drag pressure before rupture because of the coherency of their interfaces. Altogether, the strain-induced transformation of a portion of the precipitated V-rich M(C,N) carbonitrides to the Z-phase does not affect the creep strength because the Z-phase particles are nanoscale and negligible in quantity.