1992 Volume 78 Issue 12 Pages 1838-1845
A process of producing a grain size-gradient structure was proposed for low alloy steels containing nitride formers such as Ti, V, Al and Zr. Using a 1Cr-1Mo-3/4V steel, the grain size-gradient structure which combines a fine grain surface region and a coarse grain interior was obtained by the process, and an effect of the structure on the fatigue bebavior was examined.
The producing process is as follows:(a) precipitation of VN in the surface region through heating at 1000°Cin N2, (b) grain refining by recrystallization at 1000°C, (c) grain coarsening in the interior through solution treatment at 1150°C.In the surface region, the fine grain swere maintained by the grain growth inhibitor of VN after the solution treatment.The obtained grain size-gradient structure is composed of a fine grain (<10μm)surface region(01mm depth), a traRsient grain size(1080μm)region(12mm depth), and a comparatively coarse grain(80μm)interior.
The specimens with the grain size-gradient structure were tested at 550°C in arotating-bending fatigue machine and found to display better fatigue behavior than non-gradient homogeneous grain specimens. It was thought that the fine grain surface region suppresses the fatigue crack initiation and increases the fatigue life.