Microstructure and deformation rate during long‐term cyclic creep of the martensitic steel X22CrMoV12‐1

Abstract

The microstructure of three specimens of the martensitic steel X22CrMoV12-1 which had been subjected to long-term cyclic creep at 873 K with intermittent phases of unloading (stress ratio R = 0) and compression (R = -1) was quantified by electron microscopy with regard to carbides, dislocations and pores. The laws of time dependent coarsening of carbides and strain controlled growth of subgrains found for monotonic creep hold also for cyclic creep. The longer time it takes cyclic creep to reach a given strain leads to a growth advantage of carbides compared to monotonic creep. The microstructural model of plastic deformation previously developed for monotonic creep on X20 (22)CrMoV12-1 allows to calculate the cyclic creep acceleration due to this advantage in carbide growth. Mikrostruktur und Verformungsgeschwindigkeit wah rend langzeitigem zyklischen Kriechen des martensitischen Stahles X22CrMoV12-1. Die Mikrostruktur dreier Proben des martensitischen Stahles X22CrMoV12-1, die langzeitigem zyklischen Kriechen bei 873 K mit Entlastungs-(Sparmunqsverhaltnrs R = 0) und Druckphasen (R = -1) unterworfen worden waren, wurde elektronenmikroskopisch auf Carbide, Versetzungen und Poren untersucht. Die fOr monotones Kriechen gefundenen Gesetze der zeitabhanqlqen Carbidverqroberunq und des dehnungskontrollierten Subkornwachstums bleiben auch fOr zyklisches Kriechen gOltig. Der srhohte Zeitbedarf, der bei zyklischem Kriechen bis zu gegebener Dehnung erforderlich ist, fOhrt zu einem Wachstumsvorsprung der Carbide im Vergleich zu monotonem Kriechen. Das fOr monotones Kriechen von X20(22)CrMoV12-1 entwickelte mikrostrukturelle Modell der plastischen Verformung erlaubt, die aus dem Wachstumsvorsprung der Carbide folgende zyklische Kriechbeschleunigung zu berechnen.

Microstructural evolution of the martensitic cast steel GX12CrMoVNbN9‐1 during long‐term annealing and creep

“…This strain-controlled softening cannot be due to formation and growth of pores, because even atively high stresses and deformation rates may be due to acceleration of oxidation by repeated cracking in the oxide layer with negligible spalling, figure 4c. Below the oxide layer depletion of Cr was found as was observed in [3]. The thickness ddepl of the Cr-depleted zone is distinctly smaller than d ox .…”

“…The thickness ddepl of the Cr-depleted zone is distinctly smaller than d ox . Under the same assumptions as in [3] the factor of increase in creep rate i: during the tests due to the increase in a associated with oxidation was estimated to vary between 1.42 (ao = 160 MPa) and 1.60 (ao = 105 MPa) from the power law t (X a l 2 found for constant phase structure [4]. This factor is small compared to the total s-increase during tertiary creep.…”

The Role of Dislocation Climb across Particles at Creep Conditions in 9 to 12 Pct Cr Steels

“…The total line is summarized as a Pythagorean addition in Eq. [10]. It can be observed how the larger M 23 C 6 contributes with lower, but nearly constant, particle strength at lower temperatures.…”

“…Particles in the Substructure In 9 to 12 pct Cr steels, M 23 C 6 carbides are present, where M stands for Cr, Fe, Mo, or W. The M 23 C 6 carbides nucleate mainly on subgrain boundaries during tempering. The distribution of M 23 C 6 carbides between subgrain boundaries (f h,M23 ) and interiors (f s,M23 ) can be described as [9,10] …”

Strain Enhanced Growth of Precipitates during Creep of T91

“…10) In the case of T91, two kinds of precipitates, M 23 C 6 and MX, contribute to the structural stability, and they also act as the origin of threshold stress for creep. [11][12][13] Two of the present authors (Spigarelli and Evangelista) have studied the coarsening law of T91 to understand the difference in creep behavior of some ferritic steels. 13) They obtained the empirical coarsening law exhibiting apparent stress dependence.…”