Study On The Microstructural Evolution And Strength Degradation Of 9% Chromium Ferritic/Martensitic Heat Resistant Steels

Microstructure and properties of 9Cr ferritic/martensitic heat resistant steels during aging and creep at 650℃and 700℃have been studied in the paper by the scanning electron microscope(SEM) equipped with energy dispersive spectrum analysis(EDS),transmission electron microscope(TEM),X-ray diffraction(XRD) and creep tests.The results show that:After normalized at 1100℃for 1h,cooled in air and then tempered at 700℃for 1h and at 750℃for 1h,followed by air cooling,the test steels all have a tempered lath martensitic microstructure.M₂₃C₆ carbide with size of 0.2μm and nanosized MX carbonitride precipitates distribute along grain and lath boundaries and within laths in the steel with 0.020wt.%C and 0.005wt.%N.The amount of nanosized MX precipitates increases with the decrease of carbon content and increase of nitrogen content.Two kinds of nanosized MX precipitates distribute densely and homogenously in the matrix of the ultra low carbon 9Cr ferritic/martensitic heat resistant steels.Negative creep phenomenon is found in the present steels during creep under conditions of 650℃below 250 MPa and 700℃/140MPa.The negative creep phenomenon is more obvious in the steel with higher dense nanosized MX precipitates.Its occurrence might be attributed to the precipitation of Fe₂W-type Laves phase during creep exposure.The precipitation of Fe₂W-type Laves phase along lath and grain boundaries decreases the specific capacity of the steels, resulting in volume shrinkage.And with the help of strengthening of nanosized MX,the dislocation slip deformation of the steels is too less to offset the volume shrinkage caused by the precipitation of Fe₂W-type Laves phase.The creep test results show that with the decrease of carbon content and increase of nitrogen content,creep ruptue life of the steels is prolonged,but the strength degradation is faster.The strength degradation is mainly attributed to the premature formation of Z phase,which consumed nanosized MX,leading to decrease of the strengthening effect of MX precipitates.According to the analysis of TEM,Z phase is found to nucleate on the larger nanosized MX particle.The Z-phase and MX phase have the following orientation relationship:[112]_Z phase//[001]_MX and(110) _{Z phase}//(200) _MX.