Hot compression tests at temperatures from 900 to 1100 8C and strain rates from 0.01 to 10 s À1 are performed on as-cast borated stainless steel in order to investigate the effect of boride particles on the workability and microstructure evolution during hot deformation. Microstructure examinations are carried out by applying optical microscope, scanning electron microscope, transmission electron microscope, and electron backscatter diffraction. True stress-true strain curves indicate that borides increase the stress level at low temperature (<1100 8C) while sacrificing the strength at 1100 8C. Processing map shows that flow instability mainly concentrated at temperatures lower than 990 8C and strain rates higher than 2 s À1 . Microstructure study results reveal that boride particles can be the particle-stimulated nucleation (PSN) sites, but only small fraction of new grains formed by PSN. The nucleation of cavities will occur by: (i) boride cracking; (ii) vacancy condensation at boride-matrix interface; and (iii) debonding of boride-matrix interface. Moreover, cracks almost originate from cavities around borides and propagate mainly via debonding at boride-matrix interface and original grain boundary.
The effect of long-term thermal aging on microstructure and high-temperature strength of a 10% Cr martensite ferritic steel is investigated. Comparison of the long-term aging behavior at 700 8C with that at 650 8C reveals that the low angle boundary fraction increases and the coarsening of the M 23 C 6 carbide accelerates at higher temperature, thereby the speed of subgrain growth increases. Additionally, elevated aging temperature suppressed nucleation of Laves phases and did not affect the K-S orientation relationship between M 23 C 6 carbides and ferrite matrix. The yield strength at 700 8C decreases, obviously, with increase of aging time at 700 8C, but its decline trend is not fully consistent with the change of the dislocation density, which confirms that loss of high-temperature strength is not only due to decreasing of free dislocations. On the other hand, the variation on increasing of the subgrain width shows a similar abrupt trend to that on decreasing of the yield strength. It is suggested that for aged steel, subgrain coarsening is also an important factor for the deterioration of yield strength.
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