Effect of Long‐Term Thermal Aging on Microstructure and High‐Temperature Strength of a 10% Cr Martensite Ferritic Steel

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 degrees C with that at 650 degrees C reveals that the low angle boundary fraction increases and the coarsening of the M23C6 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 M23C6 carbides and ferrite matrix. The yield strength at 700 degrees C decreases, obviously, with increase of aging time at 700 degrees C, 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.