Carbide forming elements such as chromium promote the decrease of free ferrite in cast iron and reduce the total possible strengthening effect.
For example, aging decreased cutting forces after aging ductile iron with spherical graphite and significant free ferrite.
The microstructure in this case was pearlitic with some steadite and free carbide but no free ferrite.
It can be concluded from these tests that all gray iron showing improved machinability in the aged condition contained some amount of free ferrite, while gray iron showing increased cutting forces after aging had no free ferrite but was entirely pearlitic with cementite/ steadite phases.
These irons had some free ferrite and no free cementite or steadite.
If this iron has negligible free ferrite, aging will increase cutting forces in this iron.
The resulted microstructure presented partially spheroidized carbides and a certain quantity of free ferrite
To obtain maximum strength from pearlite refinement, though, it's essential to first have pearlite, i.e., eliminate free ferrite
. The reason?
Recent work suggests this could be associated with microstructures containing free ferrite
The microstructure in both bores was predominantly ASTM Type A, size 4-5 graphite flakes with an almost fully pearlitic matrix and minimal amounts of free ferrite
. The casting hardness was approximately 207 HB externally and 179 187 HB internally.
The elimination of chill, carbides and free ferrite
is the function of cooling curve control.
The strength increase is attributable to reducing the free ferrite