Enhancing the transformation-induced plasticity effect in steel-based binders through deformation-induced martensitic transformation requires careful control of the austenite stability. This study investigated the effect of binder fraction on the thermal stability of austenite in the WC-(Fe,Ni) system with different binder contents within the WC + binder + graphite equilibrium region. Binder martensite fractions and carbon content across different binder fractions were compared using electron microscopy. Thermodynamic-based calculations, accounting for variation in binder carbon content, provided a baseline for distinguishing the roles of the binder fraction and chemical composition on the thermal stability of the austenitic binder. Samples with lower binder fractions and comparable binder carbon content revealed relatively similar martensite fractions, whereas the one with the highest binder fraction and the lowest carbon content exhibited the highest value. The predicted martensite fractions were comparable and in good agreement with the measured values for lower binder fractions.