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 Pmax 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 Pmax(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 Pmax(zinc) as did in low strength SS 41 steel. In SKD 6 steel with highest strength, on the other hand, Pmax(zinc) was only slightly dependent on x1 and T, and the unstable crack propagation ocurred at Pmax(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=Pmax(zinc)/Pmax(salt), was smaller in the steel with higher strength.
Under the constant load test, the crack growth initiation time in liquid zinc ti increased with a decrease in stress intensity factor K, and ti was shorter in high strength SKD 6 steel than in low strength SS 41 steel under the same K.