Root Cause of Degradation in the Creep Strength of Martensitic Steel

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. The activation energy (Q ), activation volume (V ), and Larson–Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q , V , and C  even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 °C compared to Fe2W in Grade 92 steel.

Macroscopic imprints of ductile deformation

The fundamentals of structural geology are presented, namely, folds, planar structures (cleavage or schistosity, foliation) and linear ones (lineations), regarded as emblematic for geologists. Ductile imprints of folds, affecting stratified formations, combined with brittle imprints, often remain modest in terms of strain intensity. Folding is essentially inhomogeneous and often results from the buckling (bending) of the layers (or stratification) as a consequence of layer parallel compression. Folded structures are frequently accompanied by fractures. Hence they may be classified as brittle–ductile. They are mostly encountered at low depths and constitute the upper structural level of the Earth’s crust. Ductile deformation sensu stricto appears at the lower structural level. The macroscopic aspects of ductile deformations and their implications will be examined. The principal operating mechanism, crystalline plasticity, represents the mechanical aspect of deformation, sometime assisted by chemical aspects (pressure-solution). While homogeneous deformation constitutes our principal concern, heterogeneous deformation is often present, particularly when examined at fine scales. At low shear strain (γ‎ < 0.7, or θ‎ ~35°, equivalent to ~30% shortening), plastic deformation generally leads to a planar and a linear anisotropy strengthening with increasing deformation. At higher shear strain, any pre-existing planar structure becomes so stretched that it cannot be recognized. The new structure may be purely planar, purely linear or plano-linear. Lattice fabrics, appearing in rocks subjected to plastic deformation and resulting from deformation mechanisms at the grain-scale, are examined in detail in Chapter 6.

Analysis on Degradation in Creep Strength of 9Cr-W Martensitic Steel

In order to clarify the creep mechanism of high Cr martensitic steel, creep curves of 9Cr-1W and 9Cr-4W steels were analyzed applying an exponential law to the temperature, stress, and time parameters. The activation energy, Q, the activation volume, V, and the Larson-Miller constant, C, are obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening by swept dislocations out of sub-grains occurs followed by strengthening due to the rearrangement of M23C6 and the precipitation of Laves phase. After Q&nbsp;reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of some weakest boundaries due to coarsening of the precipitates, which triggers the unexpected degradation in strength due to the accelerating coarsening of precipitates. Stabilizing not only M23C6 but also Laves phase is important to mitigate the degradation of rupture strength of martensitic steel. The above creep mechanism for martensitic steel can be applicable to the explanation for the degradation in long term rupture strength of high Cr martensitic steel, Grades 91 and 92.