Evolution of microstructure and deformation resistance in creep of tempered martensitic 9–12%Cr–2%W–5%Co steels
Introduction
The 9–12%Cr-steels gain their creep resistance mainly from a combination of hardening by dislocations and precipitates [1], [2], [3], [4], [5], [6]. The dislocations form a subgrain structure with subgrain boundaries, consisting predominantly of dislocation networks constituting low-angle grain boundaries, and free dislocations arranged in the subgrain interior. Each of the hardening contributions is necessary to reach the highest possible creep resistance. At elevated temperatures neither the dislocation structure nor the precipitate structure are absolutely stable. Under normal conditions of creep in power stations the subgrains coarsen with creep strain ϵ towards their stress dependent steady state [3], [4], [7] and the precipitates coarsen with time t [8], [9], [10]. This microstructural coarsening leads to degradation of the creep resistance [3], [11], [12], [6], [13] causing increase of the creep rate in the tertiary stage and thereby limiting the lifetime of the steels under creep conditions.
The present work was carried out within a cooperative project of partners from research and industry aiming at developing new super heat-resistant tempered martensite 9–12%Cr-steels for components in power plants [14], [15]. Four model steels with 2%W, 5%Co and micro-alloyed with B, Al and N were investigated with regard to evolution of creep resistance and microstructure at 923 K. The short-term creep resistances of the model steels were better than that of the commercial alloy P92. However, this advantage gets lost with increasing duration of creep and for two of the model steels even turns into a disadvantage, with rupture times becoming distinctly lower than those of P92. We will show that the differences in evolution of the creep resistance are related to subtle differences in the evolution of V-containing precipitates and suggest means to enhance microstructural stability.
Section snippets
Material
Four steels with 2 mass% W were investigated. They were produced within the cooperative project by the Max-Planck-Institut für Eisenforschung, Düsseldorf, and the Saarschmiede GmbH, Völklingen. Table 1 lists details on designation, manufacturing and composition. Steel 6A is similar to the commercial 9%CrMoVW-steel P92, except for the Co content, while steels 5A, 5C and 5E have a higher Cr content close to 12% leading to better corrosion resistance. As seen from Table 2, steels 5A, 5E, and 6A
Tests at constant stress
Fig. 2, Fig. 3, Fig. 4 show curves for steels 5A, 5C and 5E at 923 K. The dash-dotted lines obtained from interpolation of the tensile creep tests (Fig. 1) end when has increased from the minimum by one decade; higher creep rates make only negligible contributions to creep life, rendering further extrapolation uncertain.
In spite of the significant difference in prior austenite grain size daust, the curves of 5E and 5E-I in Fig. 4 are similar. There is a trend towards lower
Discussion of accelerated degradation of creep resistance
Our results have shown that in two of the four investigated model steels, 5C and 5E, there is accelerated degradation of creep resistance at 923 K for creep times above 103 h. At these times the minimum creep rate ceases to decrease with decreasing stress (Fig. 7) and the rate of increase of creep rate with strain becomes anomalously high (Fig. 5) so that the Monkman–Grant law no longer holds (Fig. 8). Abe [41] proposed to modify (5) by making CMG dependent on the rate of softening
Conclusions
Accelerated softening, visible by the anomalously high rate of relative increase of creep rate with strain in the tertiary stage of creep, occurs during long-term creep of two out of the four investigated tempered martensite steels with 4%W at 923 K after 103 h. An essential microstructural cause for this loss appears to be the dissolution of small hardening V-containing precipitates in the subgrain interiors. This is related to fast coarsening of M2X and precipitation of Z-phase at the subgrain
Acknowledgments
Thanks are due to the cooperation partners from research and industry within the joint project funded by the Deutsche Forschungsgemeinschaft, in particular to Dr. Knezevic and Prof. G. Sauthoff, Max-Planck-Institut für Eisenforschung, Düsseldorf, for producing steels 5A, 5C and 6A, to Saarschmiede GmbH, Völklingen for producing steel 5E, to Dr. A. Scholz and Prof. C. Berger, Institut für Werkstoffkunde, TU Darmstadt, for supplying crept specimens and the corresponding long-term creep data, to
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