Stainless Steel 17-4 PH

Mostly before. The normal route is to machine in Condition A (solution-annealed, roughly 33–38 HRC), then age. Precipitation hardening is a low-temperature step with small, predictable dimensional change — published data put the contraction around 0.04–0.06% for H900 and roughly double that for H1150 — so most features can be finished in the soft condition and only the tightest fits ground after ageing. That is the whole attraction of the alloy compared with quench-and-temper steels. Tell the supplier which condition the drawing dimensions apply to; "after H900" and "before ageing" are different parts.

H900 (aged at 482 °C) gives the highest strength and hardness, about 40–44 HRC, with the lowest toughness and the least resistance to stress-corrosion cracking. H1025 (552 °C) lands around 35 HRC with noticeably better toughness and is the common general-purpose choice. H1150 (621 °C) is the softest, roughly 28–32 HRC, with the best toughness and SCC resistance; sour-service and pressure-retaining specifications typically call for H1150 variants. Specify the condition on the drawing rather than the hardness alone, and if the part sees chlorides under sustained stress, avoid H900.

In mild atmospheres and fresh water it behaves much like 304. It is below 316L in chlorides and in aggressive cleaning chemistry, so it is not a drop-in for wetted process parts. Two practical differences matter on the drawing: 17-4 PH is martensitic and therefore magnetic, unlike 304 and 316L; and it needs passivation after machining (ASTM A967 is the usual reference) to restore the passive layer that cutting disrupts. For product-contact surfaces, electropolishing on top of passivation is the normal route.

In Condition A it machines like a tough stainless: rigid setups, sharp positive-rake tooling and a steady feed so the tool does not rub and work-harden the surface. It is easier than 304 in one respect — the chips break better. In the aged conditions it is markedly harder and cutting speeds fall; finishing after ageing is usually confined to grinding critical diameters and faces. Call out surface finish only on the surfaces that need it, and expect the supplier to plan roughing, ageing and finish operations around those callouts.

Choose 17-4 PH when the part needs strength or wear resistance — its yield strength in H900 is roughly three times that of 316L — such as shafts, gearbox components, loaded instrument hinges and fittings. Choose 316L when chloride corrosion, cleaning chemistry or a non-magnetic requirement dominates: wetted process parts, product-contact surfaces, implants. In medical work both are common, for different jobs: 17-4 PH in loaded instrument mechanisms, 316L where the part touches tissue or product.