<p>The effects of the cooling rate during solid solution (SS) treatment on the hardness and microstructure of Ni–38Cr–3.8Al alloy were studied using a modified Jominy test. Hardness decreased with increasing cooling rate after SS treatment but exceeded 50 HRC after precipitation aging under all conditions. Faster cooling led to lower initial hardness but higher hardness after aging. Microscopic analysis revealed precipitation was more widespread at lower cooling rates after SS treatment but became uniform after aging. Rietveld analysis of X-ray diffraction indicated that after SS treatment, the weight fraction of Ni₃Al (<i>γ</i>′) increased as the cooling rate decreased, and its final fraction remained nearly constant regardless of cooling rate, although <i>γ</i>′ increased further during aging. Despite this, hardness did not exhibit a direct correlation with the <i>γ</i>′ weight fraction after aging, suggesting that lamellar spacing and other microstructural factors also influence hardness. A simple hardness model incorporating phase volume fractions and lamellar spacing was developed to predict hardness trends. The model exhibits good agreement with experimental data, confirming that lamellar spacing and microstructural factors significantly affect hardness. These findings provide insights into optimizing heat treatment to achieve desired mechanical properties—particularly for applications requiring high hardness and low magnetic permeability.</p>