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.
Step 2--Control microstructure: In a matrix without free carbide/steadite having a small amount of free ferrite around flake graphite, age strengthening can improve casting machinability according to the second scenario (Table 1).