Evaluation of ageing behavior in modified 9Cr-1Mo steel by magnetic barkhausen emission technique

The paper addresses the effect of ageing on the magnetic, mechanical and structural behavior of modified 9Cr-1Mo steels. Magnetic Barkhausen Emission (MBE) technique has been used to evaluate the magnetic properties. Initial magnetic softening due to stress relaxation effects was indicated by an increase in MBE rms voltage. The subsequent precipitation of fine carbides above 50hrs of ageing was indicated by magnetic hardening (rms decrease). However, such precipitation hardening was indicated by monotonic increase in mechanical hardness upto 500hrs of ageing. Scanning electron microscopy (SEM) indicated the formation of such fine carbides. Ageing beyond 1000hrs led to coarsening of carbides and also compositional changes in the matrix as well as the grain boundary as observed from Energy dispersive Analysis of X-ray (EDAX). EDAX revealed the alteration in Cr and Mo concentration in the matrix and grain boundary carbides with aging.