In-situ small-angle neutron scattering (SANS) experiments, with and without an applied magnetic field of 1.5 T, were performed for two duplex stainless steels: 22Cr-5Ni and 25Cr-7Ni (wt.%) during isothermal heat treatment at 450 ∘C. The kinetics of phase separation was suppressed by the external magnetic field in both steels; however, the suppression was much more pronounced in 25Cr-7Ni, where phase separation was nearly eliminated. The difference in magnetic energy contributions from the external field in each steel explain their different degrees of phase separation. The findings are believed to have large technical implications for mitigating low-temperature embrittlement in Fe-Cr-Ni based alloys.