Charles E. Bates, University of Alabama at Birmingham
Abstract from the DIS June, 2000 Meeting

EXECUTIVE SUMMARY

There is a growing demand in machining centers for consistent ductile iron castings that have a consistent machinability.  Higher speeds increase throughput and minimize the capital and labor costs per part.  However, machining at higher speeds requires parts with uniform microstructures, consistent properties, and a minimum volume fraction of abrasive inclusions.

Foundries that produce machinable castings occasionally encounter batches that are "hard-to-machine" or cause rapid tool wear.  When this occurs, there may be a loss of tolerances and surface quality, a loss in productivity, machine down time, and higher scrap rates.  Sometimes the only way to keep a machining center operating is to significantly reduce the cutting speed.

The Thin Wall Iron Casting Production and Machining project was started at the University of Alabama at Birmingham in 1995.  The goals of the program were to (1) develop benchmark data on the machinability of ductile iron, (2) compare benchmark data with data obtained from a variety of commercial castings, (3) define inclusions and other conditions that degrade machinability, (4) evaluate inoculants for their effectiveness in improving machinability, and (5) demonstrate approaches for mitigating factors that degrade machinability.  The general approach to the program consisted of correlating the machinability of non-commercial castings produced in participating foundries with the production conditions and casting compositions to build a data base.  Microstructures were examined and correlated with machining characteristics of each iron.

Tool life curves were developed using carefully produced and well-characterized High Speed Steel (HSS) drills.  "Acceptable" and "hard-to-machine" castings were also obtained from participating companies to determine differences in microstructure and composition that might explain reported differences in machinability.  The results obtained on 'hard-to-machine" castings were compared to results obtained in laboratory machining experiments.

Many factors can influence the machinability of iron.  One of the purposes of this study was to identify and rank by severity the phases and conditions that have undesirable effects.  Some of the factors that influence machinability include macro-inclusions, microcarbides, graphite distribution, strength, carbides and carbo-nitrides, and, to a lesser extent, the cleaning practices used on the casting.

An advancing tool must shear the metal microstructure to produce the desired cut, but in doing so, it encounters a variety of oxides, carbides, nitrides, sand, and other phases that may be present in the iron.  These particles may be abrasive and accelerate tool wear.

Several phases can be present in iron, and their volume fraction and distribution are thought to have significant effects on tool life.  Some of the phases that degrade machinability include (1) iron oxides and silicates formed during pouring; (2) carbides and ternary iron phosphides formed during eutectic solidification; (3) titanium, vanadium, and niobium carbides, nitrides, and carbonitrides formed by reactions in the iron; and (4) chromium and molybdenum carbides formed during cooling of the casting.

Finely distributed carbides that form during solidification of cast iron have a detrimental effect on machinability.  Carbon that remains in austenite grains after eutectic solidification must diffuse from the austenite and migrate to the graphite during cooling to the eutectoid temperature.  High cooling rates and the presence of elements that either inhibit carbon diffusion or form stable carbides (molybdenum for example) reduce the rate of carbon transfer and can result in austenite that is supersaturated with carbon.  (5) At or below the eutectoid temperature, the supersaturated austenite decomposes to produce abrasive micro-carbides distributed in the matrix.

Pouring conditions are important because these conditions determine the amount of oxygen and nitrogen that react with elements in the iron during pouring.  Maintaining a full sprue, having a properly designed sprue and runner system to minimize air entrainment, and the use of filters as flow control devices are all important in minimizing the formation of oxides, silicates, and nitrides that are thought to degrade machinability.

Inoculant additions and solidification rates are important because these factors control the graphite size and distribution.  The volume and distribution of the graphite probably affect the friction characteristics of the iron at the rake and flank faces of the cutting tool.  The friction characteristics affect the amount of heat produced, which in turn affects the tool temperature.  Higher tool temperatures generally cause faster tool degradation.

Most of the work done to date has been with high-speed steel tools.  However, the research is now shifting toward the use of higher performance tools including carbides and ceramics, and to other machining operations including turning and milling.

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