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by George D. Haley
Metallurgical Consultant
Coping with higher molybdenum prices requires using the minimum amount of alloy effectively. While molybdenum is added to cast iron for a variety of reasons, its most common foundry use is to increase the tensile strength. Its single function in this application is to refine the pearlite spacing. The difference in tensile strength between coarse and fine pearlite has been reported to be as much as 15,000 psi. This improvement in strength can only be achieved by adding
molybdenum to a 100% pearlitic matrix. As the amount of pearlite decreases so does the strengthening effect. Molybdenum has little to no effect on the ferrite present in the matrix. Adding molybdenum to a mixed ferrite and pearlite iron is wasting money.
This leads us to Rule No. 1: When using molybdenum make sure the structure is completely pearlitic before making the addition. Adjust the chemistry of the base alloy to achieve the desired structure. The first approach to increasing the pearlite content is to lower the silicon content. Do this in small steps. Lower silicons promote more pearlite and finer (stronger) pearlite.
At a certain silicon level chill carbides will begin to occur in the thinner sections. Small additions of copper and/or nickel will counter the chilling tendency while promoting more pearlite.
Rule No. 2: Proper addition of molybdenum is critical to achieving good recovery. The recovery of molybdenum depends on the addition practice. The melting temperature of FeMo is over 3000°F. Time and temperature are critical to achieving good recovery. Every effort should be made to alloy in the furnace. Add 2” x 1” or 1” x ½” into a clean molten iron bath in the furnace a minimum of 10 minutes before tapping. If ferromolybdenum must be added to the ladle, the recovery will not be as good as when adding it into the furnace.
Do not add more than 0.50% FeMo to the ladle. Mix the FeMo with any other addition such as copper or inoculant. Add fine ferromolybdenum sized 8 mesh x down to the stream as the ladle fills. The stirring is critical for it to go into solution. The tap temperature should be a minimum of 2700°F. Do not add FeMo to a small ladle as the metal will cool too fast to achieve good recovery.
ADDITIONAL COMMENTS
There is no substitute for proper inoculation of the iron. Type “A” graphite is required to achieve the maximum tensile properties. The base metal should have a high potential for nucleation prior to inoculation and alloying. If the base metal is overheated, the charge is full of rusted scrap and there is no graphite added to the charge then alloy additions will not be as effective and will be wasteful.
If the molybdenum containing returns are not segregated, half the alloy is lost and the addition cost is doubled!
SUMMARY
Efficient use of molybdenum as ferromolybdenum to increase tensile strength depends on Rule No. 1 (an all pearlitic structure) and Rule No. 2 (proper addition of FeMo to the base metal). Achieving anticipated results requires proper preparation of the base metal and good inoculation practice. Lastly, do not throw molybdenum away in the returns.
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