Simple Solutions That Work! Issue 10

However, most pressure pour channel furnaces holding treated ductile iron, will always maintain a minimum heel. The minimum heel left inside of the furnace at the end of the week, or during an unscheduled breakdown on a molding line, will require a magnesium “refreshment or rejuvenation”. There are several methods that can be utilized for increasing or boosting the magnesium content in faded ductile iron. The three most common methods are: 1.) treating faded DI with a high magnesium-containing ferrosilicon alloy such as a 9% Mg, 2.) treating DI with a nickel- magnesium (Ni-Mg) master alloy, or, 3.) treating DI with a proprietary 10-15% iron-magnesium (Fe-Mg) alloy briquette. The first method commonly used method for increasing magnesium levels is using an elevated magnesium level in a magnesium ferrosilicon (MgFeSi) master alloy. Often, the magnesium levels in these special alloys will be in the range of 9 to 10%. Care must be used with these high magnesium ferrosilicon master alloys as their density is considerably lower than the more commonly used 5% MgFeSi alloys. The lower density from the high magnesium content favors alloy floatation and poor magnesium recovery. Although using a high MgFeSi master alloy will increase magnesium levels compared to the common 5% alloy, its use will still result in an increase in un-wanted silicon. An example of how additional silicon affects the mechanical properties of a typical 60-40-20 as cast DI is shown in the table above. The 0.44% increase in the silicon content increases the tensile strength 12,700 psi (20%), while decreasing ductility or elongation by 5.4% (22.6%). Although not shown, as the silicon content increases, the brittle to ductile transition temperature increases. A second method that is used to boost magnesium levels in faded DI is the use of a nickel magnesium master alloy (NiMg). A major advantage using Ni-Mg is that the density is higher than of the base iron and the alloy will sink, optimizing magnesium recovery. While the sinking characteristics of a 5% Ni-Mg alloy is well know, the higher Mg grades (NiMg15%) of this master alloy, do not sink and care must be taken to insure that the alloy doesn't float. A disadvantage is in 51 HOW TO using Ni-Mg master alloys is two- fold: 1.) the additional presence of nickel that may or may not be a chemistry requirement for most DI grades, and 2.) the high cost of the nickel alloy. A more economical approach for boosting faded DI in pressure pour furnaces as well as unheated ladles is to use an iron-magnesium iron (Fe-Mg) master alloy. There are current two grades of iron-magnesium alloys available, a 10% Fe-Mg grade along with a 15% Fe-Mg grade. The biggest advantage with these alloys is the overall unit cost of the magnesium units when compare to the magnesium nickel. Also, these products are iron based which fits well with treated DI. There is no need to add an alloy that is not needed. The density of the Fe-Mg alloys is less than the Ni- Mg and requires a cover material (such as cover steel or other dense ferroalloy cover). Besides controlling silicon levels, using Continued on next page Sample 1 Sample 2 Sample 3 Total Carbon 3.37% 3.38% 3.42% Silicon 2.45% 2.60% 2.89% Carbon Equivalent 4.11 4.16 4.29 % Ferrite 95 95 80 % Pearlite 5 5 20 Tensile Strength (psi) 63,500 67,300 76,200 Yield Strength (psi) 44,800 48,900 59,600 Elongation, % 23.9 18.5 18.5 Brinell Hardness (BHN) 152 156 179