of the mold, slide or not stick at all. One of the biggest challenges we don’t have our heads around is predicting how an inclusion reacts to a mold surface.” According to Blair, the best way to


avoid inclusions is to maintain clean metal practices. “Many years ago, we did some work


in pouring with a shroud on a bottom pour ladle,” Blair said. “Using the shroud, the metalcaster reduced the number of inclusions by an order of magnitude, meaning they were 10% of what they were before. One thing we did not anticipate was the machine shop found the insert life doubled or even tripled once the inclusions were reduced.”


Avoid Carbides For iron castings, one of the larg-


est factors affecting machining is the presence of hard carbides in the metal. “Carbides are just tiny hard particles distributed in what would normally be a machinable matrix,” said Charles Bates, an industry veteran who was written numerous research papers on machining castings. “They are about as hard as the tools used to cut them.”


Group 1-Free-cutting


alloys C83800 C84400 C84800 C94300 C85200 C85400 C93700 C93800 C93200 C93500 C97300


Group 2-Moderately machinable


C92200 C92300 C90300 C90500 C95600 C95300 C86500


Group 3-Hard-to- machine alloys


C95300 C95400 C95500


Common Name Leaded red brass


Leaded red brass


Leaded semi-red brass Leaded semi-red brass High-leaded tin bronze Leaded yellow brass Leaded yellow brass High-leaded tin bronze High-leaded tin bronze High-leaded tin bronze High-leaded tin bronze Leaded nickel brass Leaded high-strength


manganese bronze Leaded tin bronze Leaded tin bronze Tin bronze Tin bronze


Silicon-aluminum bronze Aluminum bronze


High-strength manganese bronze High-strength


manganese bronze 9% aluminum bronze 11% aluminum bronze Nickel-aluminum bronze


Table 3. Machinability Ratings for Aluminum Sand Castings


Alloy 220.0


Temper T


Rating* Excellent


242.0 T2, T571, T77 Very Good 295.0 308.0 319.0


T4, T6, T62 Very Good F


Fair F, T5, T6 T61


T61 T6 F F F F F


T4 F F F


Good


355.0 T51, T6, T7, T71 Good C355.0


Good


356.0 T51, T6, T7, T71 Good A356.0 357.0 358.0 443.0 511.0 512.0 514.0 520.0 535.0 710.0 713.0


Good Good Good Poor


Excellent Very Good Excellent Excellent Excellent Excellent Excellent


*Machinability rating is based on a range of cut- ting operations


Source: AFS Aluminum Casting Technology Carbides most often form when trace


elements are picked up in the melting scrap—often from high strength, high


Table 4. Machinability Ratings of Copper Casting Alloys UNS Number


Machinability* 90


90 90 90 90 80 80 80 80 70 70 70


60 60 60 50 50 50 35 30


20 20 20 20


*Ratings are based on free-cutting brass (61.5% copper, 35.5% zinc and 3% lead) earning a 100 machinability rating. Source: AFS Casting Copper-Base Alloys


MODERN CASTING / September 2010 29


alloy steel. The trace elements, which also can be picked up when using the same furnace to melt batches of steel and iron, combine with the carbon to form carbides, nitrides and carbo-nitrides, Bates said, which kill the machine tool quickly. Bates recommends keeping a close


eye on the charging scrap and inocu- lating castings to minimize carbides in thin sections and corners. Good molding and gating practices


can improve the machinability of cast- ings from any type of casting facility, according to Bates. Clean molds free of sand and dust will mean a casting surface free of inclusions. Good dimen- sional control is important in making castings that will fi t properly in the machine tool for ease of machining. “The orientation of the casting in the


mold exerts a strong infl uence,” Fields said. “Particularly for dross-prone alloys where oxides can be trapped in the cope surfaces or under ‘shelves’ in the mold formed by cores.”


MC For More Information


“Machine Shop Survival,” S. Wetzel, MODERN CASTING, September 2006, p. 39.


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