Grey Iron

[Richard Brekne]

List. This is taken from a "nett info" side. I bumped into the other
day. Some pretty basic but presumably accurate info about grey iron.
Thought it might be of use / helpfull in our ongoing quest to better
understand the differing qualities of cast iron plates.  No reference is
made here about the casting process itself. But I am hunting down info
on that as well.

Richard Brekne
I.C.P.T.G.  N.P.T.F.
Bergen, Norway

Cast Iron: Introduction

The wide spectrum of properties of cast iron is controlled by three main
factors:

(1) the chemical composition of the iron;

(2) the rate of cooling of the casting in the mould (which depends in
part on the section thicknesses in the casting);

(3) the type of graphite formed (if any).

Cast irons may often be used in place of steel at considerable cost
savings. The design and production advantages of cast iron include

-low tooling and production cost

-ready availability

-good machinability without burring

-readily cast into complex shapes

-excellent wear resistance and high hardness (particularly white irons)

-high inherent damping

*** Main types of cast irons; their advantages and disadvantages:

-Grey cast iron (Flake graphite cast iron): Most common type of cast
iron. Cheapest material for metal castings, especially for small
quantity production. Very easy to cast- much
narrower solidification temperature range than steel. Low shrinkage in
mould due to formation of graphite flakes. Good machinability, faster
material removal rates but poorer surface
finish with ferritic matrix and vice versa for pearlitic matrix.
Graphite acts as a chip breaker and a tool lubricant. Very high damping
capacity. No difference in notched and unnotched
fatigue strength. Good dry bearing qualities due to graphite. After
formation of protective scales, it resists corrosion in many common
engineering environments. Limitations are that it is
brittle (low impact strength) due to sharp ends of graphite flakes,
severely limits use for critical applications. Graphite acts as a void
and reduces strength. Maximum reccomended design
stress is 1/4 of ultimate tensile strength. Maximum fatigue loading
limit is 1/3 of fatigue strength. Changes in section size will cause
variations in machining characteristics (due to variation in
microstructure). Higher strength irons more expensive to produce.