The machinability of A36 is often estimated at about 72% of the machinability standard (which is 100% for AISI 1212 steel). This means it is somewhat more difficult to machine than free-machining steels. Several factors contribute to this:
Chemical Composition: The variable and relatively higher carbon content (up to 0.29%) and manganese content increase hardness and abrasive wear on tools compared to low-carbon steels like 1018.
Presence of Mill Scale: The hard, abrasive oxide layer on the surface of hot-rolled material can rapidly dull cutting tools if not removed prior to machining.
Inconsistent Hardness: The as-rolled condition can have slight variations in hardness through the cross-section, leading to inconsistent tool wear and machining performance.
Inclusion Content: While controlled, the sulfur and phosphorus inclusions are not as favorable for chip breaking as the deliberately added inclusions in free-machining steels (e.g., leaded or re-sulfurized grades).
For these reasons, for high-volume machining of precision parts, a cold-rolled steel like 1018 or a free-machining steel like 12L14 is typically preferred. A36 is machined successfully for structural fabrications (drilling, punching, tapping) where tool life and surface finish are less critical.