Typically low expansion
alloys machine similar to, but not as good as
a Type 316 austenitic stainless steel; or, around
25 percent as good as AISI B1112. They develop
somewhat gummier and stringy chips and higher
forces, but do not work harden as rapidly as Type
316. Work hardened bars can result in some improvement
of the inherent machinability in some instances.
Tool geometries normally used for austenitic stainless
steels are applicable to these alloys. All tools
should be kept sharp with a fine finish, be as
large as possible and rigidly supported.
Cutting fluid selection is an important consideration
in the machining of low expansion alloys. For
best machinability, a 1 to 1 blend of a sulfachlorinated
pertroleum oil containing 8 to 10 percent fatty
oil and a paraffin blending oil would be used,
or a water emulsifiable cutting fluid with polar
and extreme pressure additives. However, the presence
of any residual sulfur from the cutting fluid
on the machined part, followed by heat treating,
could result in serious degradation or magnetic
properties. Meticulous degreasing and cleaning
of machined parts can remove sulfur residues resulting
from the sulfur-bearing cutting compounds. Animal
lard is also used as a cutting lubricant. In any
event, the parts should be degreased and cleaned
as soon after machining as possible. Residual
sulfur can also cause grain boundary embrittlement. |
