By adopting means

such as metallographic microscope

scanning electron microscope

and transmitting electron microscope etc.

the microstructure variation of high-temperature bolts made from 12%Cr steel has been investigated.Through comparison with that of the new bolts

the results show that the δferrite content in some localities of structure for the investigated bolts has been over 10%.After the bolt being in operation for 60-thousand hours

its metallographic structure has gradually aged

the carbides being separated out along the boundaries of δ ferrite

the alloy elements start migrating to the carbides

the material entered into initial period of aging;after being in operation for 350-thousand hours

the coarsening of carbide particles being more remarkable

chains of rough and large carbides have been formed along the boundaries of δ ferrite

the material has already entered into the final stage of aging.