Hydrogen effects on directional solidification of Te-doped cast irons

In a recent study, directional solidification experiments were employed with increasing and decreasing velocities to evaluate the effect of small Te additions on the transition velocities between gray to white cast irons during acceleration, V{sub g-w}, and the reverse transition (white to gray) during deceleration, V{sub w-g}. The irons had a hypoeutectic composition of 3.4 wt pct C, 2 wt pct Si and were doped with 0.1 wt pct Te. In an extension of this work to lower Si levels, a hypoeutectic iron of 3.9 wt pct C, 1 wt pct Si + 0.05 wt pct Te, an important discovery was made involving the effect of H on the transition velocities. In conclusion, the authors have made the following discoveries: (1) small additions of H to the atmosphere present in directional solidification experiments of Te-doped cast irons have a large effect on the gray-white and white-gray transition velocities; (2) addition of 10% H to the atmosphere allows growth of white cast iron at rates down to 1 and 2.5 {micro}sm/s in irons containing 1 and 2 wt pct Si, respectively; (3) it appears that the action of the H is to reduce compounds of Te, most likely oxides, thereby allowing more Te to remain in liquid solution and to adsorb on the graphite/liquid iron growth front interfaces; this, in turn, reduces the graphite growth kinetics and permits white iron growth to dominate.