Structural Criterion of Ductile-Brittle Transition Under Conditions of Weakened Grain Boundary Strength

Abstract

The effect of various factors (hydrogenation, segregation of phosphorus, arsenic and other elements), which reduce the cohesive strength of grain boundaries, on the ductile-brittle transition in structural steels is considered. It is shown that during brittle fracture accompanied by weakening of cohesive strength of grain boundaries, the free path length of brittle crack of Λb becomes a variable value, i.e., $$ {\Lambda}_b={d}_f+\xi {f}_i{d}_f^i, $$ where df is the size of brittle fracture cleavage facet, $$ {d}_{\mathrm{f}}^{\mathrm{i}} $$ is the size of brittle intercrystalline fracture; ƒi is the frequency of microcracks through grain boundaries, ξ is a geometric coefficient for interrelationship between df and $$ {d}_{\mathrm{f}}^{\mathrm{i}} $$ . For the case of mixed fracture, ductile-brittle transition is described by expression $$ {T}_c={T}_c^0+B\left(1-{f}_i\right)/{d}_f\left(1+\xi {f}_i\right). $$ In practice, by measuring the proportion of intergranular fracture in the brittle fracture zone it becomes possible to estimate the change in the Tb level. Results are confirmed by experiments for the phenomena of thermal brittleness and hydrogen corrosion.