Creep Behavior of Double Tempered 8%Cr-2%WVTa Martensitic Steel

Abstract

Creep testing was carried out at around 650 C for a martensitic 8Cr-2WVTa steel (F82H), which is a candidate alloy for the first wall of the fusion reactors of the Tokamak type. Rupture strength of the double tempered steel (F82HD) is lightly higher than that of simply tempered steel (F82HS). On the other hand, creep rate of F82HD is obviously smaller than that of F82HS in acceleration creep, though creep strain of F82HD in transition creep, where creep rate decreases with increasing strain, is larger than that of F82HS. Hardness of the crept F82HD decreases with increasing creep strain, which corresponded with the transmission electron microscopy (TEM) observation. On the contrary, X-ray diffraction and electron back-scattered diffraction pattern measurements show that fine sub-grains are created during transition creep. The creep curves were analyzed using an exponential type creep equation and the apparent activation energy, the activation volume and the pre-exponential factor were calculated as a function of creep strain. Then, these parameters were converted into two parameters, i.e. equivalent obstacle spacing (EOS) and mobile dislocation density parameter (MDDP). While EOS decreases with increasing creep strain, MDDP increases with increasing strain during transition creep. The decrease in EOS and the increase in either EOS or MDDP are rate-controlling factors in transition and acceleration creep, respectively. On the other hand, in case of F82HS, EOS increases and MDDP decreases during transition creep. In this case, the decrease in MDDP controls the creep rate during transition creep of F82HS. It is concluded that both EOS and MDDP are representative parameters of the change in substructure during creep.

Tempering Process and Precipitation Behavior of 8%Cr–2%WTa Steel

“…4), but the apparent lattice strain and the apparent crystalline particle size (Fig. 5) change discontinuously with crest and trough between about 3 to 9 h. Referring the EBSP measurements reported in the literature 15,27) it is found from the Figs. 4 and 5 that with the progress of tempering the re-distribution of the crystalline particles or the generation of subgrains may occur, which causes a local increase in internal stress and, as a consequence, the apparent lattice strain, and finally the rotation of the preferred orientation should take place.…”

“…5 that both the apparent lattice strain and the apparent crystalline particle size vary largely from 2 to 12 h. Similar drastic changes among specific intensity, apparent lattice strain and apparent crystalline particle size during tempering or creep deformation of some martensitic steels are reported elsewhere. 10,15,27) In the literatures, electron back scattered pattern (EBSP) measurements were also reported along with the isothermal hardness curves, and the discontinuous changes in X-ray peaks of bulk specimens as shown in Figs. 4 and 5 were explained as follows: in case of the generation and/or growth of new subgrains through the recovery process of lath martensite structure, discontinuous changes in the preferred orientation takes place and, as a consequence, the generation of ultra fine grains around the block of martensite or subgrains should occur, which leads to the discontinuous changes in the local lattice strain and the average crystalline particle size as shown in Figs.…”

Tempering Behavior of 9%Cr–1%Mo–0.2%V Steel

“…5 instead of the black regions. Tamura et al 22) recently confirmed the existence of the ultra fine grains in crept specimens of a similar steel using the same experimental conditions. They also confirmed that TEM contrast corresponding to the ultra fine grains was not observed.…”

“…5 are observed in a crept specimen of a similar martensitic steel. 22) The grain tolerance angle defines a grain by specifying the minimum misorientation value that was caused by a grain boundary. Measurements with misorientations below this value are grouped together to difine a grain.…”

Recovery and Precipitate Analysis of 9 Pct Cr-1 Pct MoVNb Steel during Creep

“…[35] Further, they had also used it to describe the creep mechanism with parameters such as activation energy, activation volume, and mobile dislocation density. [36,37] Using these parameters, the equivalent obstacle spacing for mobile dislocations was calculated based on some assumptions. [36] The equivalent obstacle spacing calculated that was using creep curves was confirmed to be comparable with the interparticle distance or subgrain size that was observed using TEM.…”