ASTM A738 is a quenched-and-tempered plate steel specification for pressure vessels that need more strength than the normalized C-Mn plates covered by A516 or A515 can deliver. Rather than relying on carbon content or heavy alloying to reach that strength, A738 plate is heat-treated — quenched and tempered — which refines the microstructure and pushes yield and tensile strength well above what the same base chemistry could achieve in the as-rolled or normalized condition.

The specification covers three grades — A, B, and C — that step up in alloy content and strength. This page covers Grade A, a carbon-manganese-silicon composition that's the base grade in the family, with a minimum yield strength of 60 ksi (415 MPa) and a minimum tensile strength of 80 ksi (550 MPa). Grades B and C add manganese-molybdenum and manganese-molybdenum-boron alloying respectively to reach higher strength levels for applications that need it.

A738 plate is specified for pressure vessel shells, heads, and other components where wall thickness or weight needs to come down without sacrificing pressure rating — the higher allowable design stress from the increased yield strength lets designers use thinner plate for the same service pressure compared to A516. Because the mechanical properties come from heat treatment rather than raw chemistry, welding and fabrication typically require closer attention to preheat, interpass temperature, and post-weld heat treatment than with normalized plate, to avoid degrading the quenched-and-tempered microstructure in the heat-affected zone.

A738 Grade A's quenched-and-tempered condition is the single biggest factor in how it machines — this plate arrives at the shop already heat-treated to its final strength, not annealed like A516 or A515. That means higher cutting forces, more heat generated at the tool-chip interface, and faster flank wear than the normalized pressure-vessel plates it's often specified alongside, even though the base chemistry isn't dramatically different.

Chip formation tends to be more controlled than on softer normalized plate — the higher hardness promotes cleaner shear and shorter chips with a properly matched chipbreaker, rather than the long stringy chips typical of low-carbon annealed steel. The tradeoff is that the same hardness abrades the cutting edge faster, so tool life becomes the practical limiting factor more often than surface finish.

Because this material is heat-treated to a specific hardness range rather than left soft for machining, coated carbide grades with good hot hardness are strongly preferred over uncoated tooling for anything beyond light finishing passes. Rigid setups, adequate coolant, and cutting speeds pulled back from what would be used on normalized plate all help manage tool wear and hold dimensional accuracy through a cut.