Adding Mg metal to cast iron can lead to Explosion !

Discussion in 'Castings, finishing/ repair/ and patina's' started by metallab, May 23, 2021.

  1. metallab

    metallab Silver

    This afternoon I experiemented the trick of making ductile cast iron by adding magnesium turnings to molten cast iron. I wrapped them, along with ferrosilicon, in a piece of paper and added it to the crucible.
    Then I kept a safe distance as I expected a violent reaction and that indeed happened. Mg has a boiling point (1112 C) way lower than the temperature of the iron (1400 C), so it is like putting crushed ice into molten aluminum.
    It appeared that almost all Mg has burned away (as the video below shows at 0:38).
    In another topic I read that pre-alloying with nickel would help. I tried that but to no avail.

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  2. JCSalomon

    JCSalomon Copper

    That looks to have been an exciting pour.
  3. metallab

    metallab Silver

    But the result was not satisfactory. It is a 2mm thick mold with a large sprue and two thin risers which were meant to be for air displacement, so I did not expect them to fill, but the metal was so fluid, it did fill, but not where I wanted to.....
    I had the mold tilted for about 30 degrees.

    On the detailed photos parts of the surface are rougher than other parts with a clear boundary between them.

    RX600508.JPG RX600507.JPG RX600506.JPG
  4. oldironfarmer

    oldironfarmer Silver

    You should try not venting, pour with the flask flat, and give yourself enough of a pouring basin you don't splash so much molten metal. Your vents sucked a lot of metal from the pour. If you need a vent, a wire poked into the sand is enough. I've never seen t he need to vent.
    John Homer likes this.
  5. Melterskelter

    Melterskelter Gold Banner Member

    A few comments:

    1) It is unclear how much time elapsed between the introduction of magnesium and pouring, but it looks like it may have been a few minutes. You did not report on the iron’s ductility. The time interval may have eliminated any ductility that the magnesium might have introduced.

    2) The risers (unneeded for this flat part) killed your pour. They provided a low resistance-to-flow channel for metal flow allowing cooling and freezing of the metal front. After they filled, the metal could not break through the slushy frozen front.

    3) Tilting the mold with the pouring basin high (should have been low) exacerbated and maybe fully caused “2.” The metal was able to first run around the mold edge and into the risers. With the flow into the flat area being of higher resistance. Thus, flow stagnated and froze in the flat area. Positioning the pouring basin down would have caused a uniformly-advancing molten metal edge with risers filling last and the mold probably would have filled.

    HT1, Tobho Mott and oldironfarmer like this.
  6. Melterskelter

    Melterskelter Gold Banner Member

    One more comment: Your pouring basin is much too small or maybe non-existent. You lost a fair bit of iron as your target was very small and hard to hit. I'd suggest carving a 2 X 3 inch by 1"(at least) deep basin with tumblehome sides. That way you are likely to get your iron into the mold without spilling and, becasue of the capacitance of the PB, you will be able to keep the sprue choked throughout the pour.

  7. metallab

    metallab Silver

    Denis Melterskelter, you are right.

    Next time I am not gonna use risers and keep the flask in normal horizontal position and a larger pouring basin.

    About the magnesium: After I introduced the Mg curls, I saw that the crucible failed (the edges got soft) so I was unable to do a stir and there were only a couple of minutes between the introduction of the Mg metal and the pour, so it did not mix at all.
    Next time I'll use a new crucible and try again with the Mg curls and make the mold like the sketch below.

  8. Melterskelter

    Melterskelter Gold Banner Member

    Actually, in your drawing the splash basin is labeled as a pouring basin and your sprue is undesirably conical. Please take a look at the link in my prior post.

    Flat, platter-like parts are best poured inclined slightly with sprue a bit low. This encourages a smooth, uniform, and steadily progressing front of metal. Pour water on the level floor and it goes every which way. Pour it into a inclined sheet cake pan and it rises smoothly away from the low side.

  9. oldironfarmer

    oldironfarmer Silver

    Denis is saying the pouring basin is at the top of the sprue. A basin at the bottom serves little purpose. At the top you can pour quickly and flood an almost straight sprue. Maybe a 1/2" diameter sprue at most.

    Have you poured again?

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