Evolution of microstructure and deformation resistance in creep of tempered martensitic 9–12%Cr–2%W–5%Co steels

doi:10.1016/j.actamat.2006.02.038

Acta Materialia

Volume 54, Issue 11, June 2006, Pages 3003-3014

https://doi.org/10.1016/j.actamat.2006.02.038 Get rights and content

Abstract

The microstructural evolution during creep at 923 K of four tempered martensite 9–12%Cr-steels modified with 2%W and 5%Co was quantified by electron microscopy. Coarsening of subgrains with strain towards the stress dependent steady state was confirmed. The evolution of the precipitate structure is similar with regard to M₂₃C₆ (M: metallic element, X: N, C) and Laves phase (Fe,Cr)₂(W,Mo) but differs strongly with regard to V-bearing precipitates. Low temper temperatures promote precipitation of V as M₂X instead of VX. This appears to be critical in causing anomalously fast breakdown of initially high creep resistance. Dissolution of small V-bearing precipitates and corresponding loss of precipitation hardening in the subgrain interior is proposed to be the reason for this breakdown. The dissolution is caused by fast coarsening of M₂X precipitates and precipitation of Z-phase at subgrain boundaries. Large M₂X precipitates at subgrain boundaries are proposed to be nucleation sites of Z-phase.

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 $\dot{ϵ}$ 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 $\dot{ϵ} – ϵ$ 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 $\dot{ϵ}$ has increased from the minimum ${\dot{ϵ}}_{\min}$ 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 d_aust, the $\dot{ϵ} – ϵ$ 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 C_MG dependent on the rate $(d \log \dot{ϵ} / d ϵ)_{III}$ 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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View full text

Evolution of microstructure and deformation resistance in creep of tempered martensitic 9–12%Cr–2%W–5%Co steels

Abstract

Introduction

Section snippets

Material

Tests at constant stress

Discussion of accelerated degradation of creep resistance

Conclusions

Acknowledgments

Mater Sci Eng A

Acta Metall

Mater Sci Eng A

Curr Opin Solid State Mater Sci

Acta Mater

Mater Sci Eng A

Comput Mater Sci

Mater Sci Eng A

Scripta Metall

Mater Sci Eng A

Steel Res

Steel Res

VGB Kraftwerkstechnik

ISIJ Int

Materialprüfung

Z Metallkd

Verformungsverhalten und Mikrostruktur warmfester martensitischer 12%-Chromstähle