Approximate guide to carbon content in iron.

Discussion in 'Sand Casting' started by Ironsides, Sep 26, 2021.

  1. Ironsides

    Ironsides Silver



    This is how I check the approximate carbon content in cast iron so two things are needed, 1. Carbon gouging rods 6.5 mm Dia. 310 mm long. These rods can be purchased from any welding shop. 2. Concrete re- enforcing steel rod 13 mm Dia.

    Cast iron has two states, it is either carbon hungry or steel hungry. In my video from 10 years ago it shows I use a 100% steel charge to make cast iron and before all the steel has been added a gouging rod is stirred in the melt for 20 seconds ( not shown in the video). An amazing thing happens to the rod, all the carbon dissolves below the molten cast iron to a sharp point like a spear. This only happens when the carbon content is very low. If a steel rod is stirred in the melt very little happens because there is little carbon for the steel rod to absorb. As the carbon content reaches 3.5% it can take a long time to absorb carbon.

    Nearly all cast irons that are melted and poured by foundries will have a carbon content at 3.5% or below because above 3.5% the cast iron will become too soft and brittle. So the next time you melt cast iron from scrap use a carbon gouging rod to stir in the melt for 20 seconds. It will only dissolve the sharp edges on the end of the rod. Very hot cast iron and lots of time will dissolve more carbon but can be difficult to achieve in a home foundry setting.

    An interesting read is in the foseco foundrymans handbook on page 272-273 and shows carbon and silicon content for all classes of iron. In those pages the carbon content rarely goes above 3.5%. The next interesting experiment to do is, just before ferrosilicon is added put the steel rod into the furnace but not into the molten cast iron and wait for 20 seconds and see how hot the end of the rod is. Pull the rod out and let it cool off so it is not glowing any more. Then add ferrosilicon and stir in with the steel rod for 20 seconds. You will find that the rod that is submerged in the molten iron will dissolve to a sharp point like the carbon rod did in the earlier experiment. This shows that cast iron is steel hungry. I have found that the small amount of steel that is dissolved when stirring in the ferrosilicon does not effect the hardness of cast iron.

    The temperature of my furnace just before adding ferrosilicon is in the range 1550-1570C and the metal temperature is 1350-1420C. So why did the steel rod not melt in 20 seconds but did in the melt when plunged into molten cast iron? When ferrosilicon has been added to cast iron there is the maximum amount of free carbon in cast iron for the steel to absorb. This is why I used 100% steel so carbon will be dissolved quicker and then add ferrosilicon at the last moment to make sure it will be machineable. Scrap cast iron has the right amount of silicon in cast iron it so if you try to dissolve more carbon it will take a long time to do it.

    In my video a 50% steel 50% ball bearing race mixture was used to make the cast iron harder wearing but ball bearing steel is a chromium steel. So what happens is chromium dissolves a lot more carbon than steel does but over a certain amount ferrosilicon will not release free carbon into cast iron. This is why only 50% ball bearing steel was used to bring the chromium level into a safe range. Most cast irons are kept very close to C.E. 4.3% range because it has the lowest melting point and is soft enough to machine so now you can see why carbon is harder to dissolve when it is close to C.E. 4.3%.
     

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