About 17-4|A precipitation-hardening martensitic stainless. In Condition A (annealed) it machines similarly to
304, but once aged to H900 it gets significantly harder and tougher on tools. Most shops machine it in Condition A then send it out for heat treat. It shrinks predictably during aging (roughly 0.0005 in/in), so leave allowance. Responds well to high-efficiency trochoidal toolpaths with low radial engagement and high axial depth. Use coated carbide, high-pressure coolant, and keep the chipload up to avoid work hardening. Used in aerospace, oil and gas valves, and anywhere you need 304-level corrosion resistance with much higher strength. Compare to 15-5 PH (similar but better transverse properties) and 304 (easier to machine but cannot be heat treated to anywhere near the same strength).
