Liquid Zinc Embrittlement in Precracked Specimen of High Strength Steels

Abstract

The constant tensile speed and constant load tests were carried out in liquid zinc on the precracked specimens of Ni-Cr-Mo steel (SNCM 439) and Cr-Mo-W-V tool steel (SKD 6) quenched and tempered, and the embrittling behavior of both steels were compared with that in low strength steel (SS 41).
The maximum load P_max obtained from load P vs. deflection Δx₂ curve was much smaller in liquid zinc than in air or salt bath both in SNCM 439 and SKD 6 steels. In SNCM 439 steel with medium strength, the maximum load in liquid zinc P_max(zinc) decreased with a decrease in tensile speed x₁ and with an increase in testing temperature T, and the crack propagation was stable before and after P_max(zinc) as did in low strength SS 41 steel. In SKD 6 steel with highest strength, on the other hand, P_max(zinc) was only slightly dependent on x₁ and T, and the unstable crack propagation ocurred at P_max(zinc). The crack propagation in liquid zinc occurred along preaustenite grain boundary in both SNCM 439 and SKD 6 steels.
The embrittlement coefficient, the ratio of maximum load in liquid zinc to that in salt bath or air, φ=P_max(zinc)/P_max(salt), was smaller in the steel with higher strength.
Under the constant load test, the crack growth initiation time in liquid zinc t_i increased with a decrease in stress intensity factor K, and t_i was shorter in high strength SKD 6 steel than in low strength SS 41 steel under the same K.