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Factors That Affect the Machinability of Cast Irons

 

Many factors can influence tool life when machining iron. These include metallurgical conditions such as graphite size and distribution, composition, ferrite/pearlite ratio, cooling rate from the eutectic through the eutectoid temperatures, and the presence of either endogenous or exogenous inclusions.

Several factors that influence machinability are schematically illustrated in Figure 1. This figure represents a tool advancing through a metal part containing a variety of graphite flakes and abrasive macro-and micro-inclusions that might include oxides, carbides, nitrides, sand, and other materials. The advancing tool creates a compression zone below and ahead of the tool rake and flank faces. The flow characteristics of the material are a function of the metal modulus, strength, workhardening coefficient, chip-forming characteristics, and metal ductility.




Figure 1: Schematic of a Tool Advancing Through a Metal Part



Plastic deformation produced by the advancing tool in the workpiece generates heat that must be dissipated either through the chip, workpiece, or the tool. The metal being removed also impinges on the tool rake face of the tool and produces frictional heat. Under some circumstances, the heat and abrasion cause craters to develop on the tool rake face. Several phases can be present in iron, and their volume and distribution have significant effects on tool wear. Massive carbides formed during solidification are hard and can obviously degrade the machining characteristics by chipping or breaking tool tips.

Some of the carbon dissolved in austenite during eutectic solidification must diffuse from the austenite and migrate to graphite flakes or nodules as the metal cools to the eutectoid temperature. The presence of elements that inhibit carbon diffusion reduces the rate of carbon transfer and produces austenite supersaturated with carbon. High cooling rates from the eutectic to the eutectoid temperature may not provide enough time for the carbon to diffuse to the graphite. Supersaturated austenite then decomposes in the eutectoid range to produce higher volumes of abrasive (micro) carbides in the pearlite (Kovaks).

Inoculant additions and solidification rates are also important. These two factors have significant effects on both the eutectic cell structure and the graphite size and distribution, which in turn affect the carbon diffusion distance and the chip forming characteristics of the metal. The carbon must be able, in the time available as the iron cools from the eutectic to the eutectoid temperature, to diffuse from the austenite and attach itself to the graphite flakes or nodules. Larger distances between graphite flakes and nodules require more time for carbon diffusion to the graphite.

The graphite distribution also affects the mechanical strain that must be overcome at the tool tip and the chip forming characteristics of the metal. The volume and distribution of the graphite may also affect the friction characteristics of the iron in contact with the rake and flank faces of the cutting tool. The friction characteristics affect the amount of heat produced during the shearing ahead of the tool tip and that, in turn, affects the tool temperature. Higher tool temperatures generally cause faster tool wear.

Molding and metal handling practices can introduce oxides into the metal that abrade, wear, and chip cutting tools. Sand grains picked up from the mold and incorporated into the metal or adhering to the surface of castings degrade machinability because of their abrasiveness.

The Casting Engineering Laboratory at the University of Alabama at Birmingham, in conjunction with the American Foundry Society and a number of industrial participants, are determining the factors that affect the machinability of cast irons. Previously, the group has focused on determining the root causes of changes in machinability as evaluated by drilling experiments (Bates). These machinability evaluations were performed by measuring the wear rates of high-speed steel drills on test castings that were produced in commercial foundries. During the past two years, the group has expanded its capabilities to include turning experiments using more wear resistant tool materials. The machinability of many commercial castings can be evaluated with the new method. This paper presents the results of one such study on the machinability of continuous cast gray iron. A companion paper presents similar results for a series of continuous cast ductile irons. Continuous cast iron has the advantage of reduced porosity and inclusions so that catastrophic tool breakage can more easily be avoided.

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