This article considers the problem of brittle fracture of steels due to stress raisers (SRs), including cracks, at low test temperatures. The authors consider the ductile-to-brittle transition at the critical temperature of fracture of specimens with SRs, $T_{C}$, as a mechanical instability of the ultimate strength, $\sigma_{NF}$, at fracture of above specimens below the yield stress of metal, $\sigma_{0.2}$. The mechanical strength stability of metal materials is provided by ductility, which is manifested within the strain-hardening range between the yield stress $\sigma_{0.2}$ and true fracture stress SK as the break resistance index $B_{r}$ = $S_{K}$/$\sigma_{0.2}$ at ductile break in specimen ‘neck’. The stress raiser localises the entire margin of mechanical stability of metal, $B_{r}$, in the ductility zone, which causes the macroscopic brittle behaviour of specimen at temperatures $T$ $\leq$ $T_{C}$, where strength of the specimen with SRs is $\sigma_{NF}$ $\leq$ $\sigma_{0.2}$. Dependence of the critical level, $B_{rс}$, at $T_{C}$ on both the margin of mechanical stability of strength, $B_{r}$, and the SR type is ascertained. Effectiveness of the embrittlement effect of various SR types (notches and cracks) is considered using the example of armco-iron ($\alpha$-Fe), as well as of structural steels over both a wide strength range ($\sigma_{0.2}$ = 150-1500 MPa) and ductility range ($\Psi_{K} \approx$ 10-83%). It is shown that the critical parameter of mechanical stability margin, $B_{rс}$, may become a key tool for ranking structural alloys in practical materials science, when certifying these alloys for their use in products containing structural stress raisers or fatigue cracks.