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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