304 vs 316 vs 17-4 PH: what actually changes in a stainless quote

Three buyers sent drawings in the same week: a bracket in 304, a valve body in 316L, a small splined shaft in 17-4 PH. Similar size, similar tolerance, and the quotes came back at prices that looked almost random.

They are not random. Stainless is not one material with three labels, and the difference shows up in three places: how the tool wears, how the part moves, and what has to happen after machining.

The starting point: machinability index

Published machinability ratings use AISI B1112 free-machining steel as 100. The numbers below are typical published values, and they shift with condition and supplier, but the gaps between them are the point:

The practical reading: a 316 part at the same geometry as a 304 part is not "a bit slower", it is a different tool life curve and a different finishing strategy.

304: the work hardening trap

304 hardens under the tool. Take a light pass with a dull insert and the surface hardness rises, which makes the next pass cut worse, which raises hardness again. The failure mode is a part that measures fine on the outside and has a 0.03 mm hardened skin that destroys the finish pass.

What shops do about it: keep the depth of cut above the work-hardened zone, never let the tool rub, use sharp geometry with a positive rake, and accept a higher cutting speed than instinct suggests. On a thin wall, 304 also deflects; the spring pass is not optional.

316: same family, more wear

316L adds 2-3% molybdenum for chloride resistance, and that molybdenum is not friendly to carbide. Tool life drops further, notch wear at the depth-of-cut line appears sooner, and surface finish on a deep bore becomes the limiting feature rather than the tolerance.

The commercial consequence is simple: if the drawing says 316L only because someone wrote "marine grade" out of habit, and the service is a dry enclosure, 304 will do the job for less. That is a design conversation worth having before the quote, not after.

17-4 PH: the one that moves after you measure it

17-4 is martensitic and precipitation hardening. You machine it in the annealed or overaged condition, then age it to H900, H1025, or H1150. Aging is not dimensionally neutral: the part generally shrinks by a few ten-thousandths of an inch per inch, which is enough to eat a tight tolerance if nobody planned for it.

Two consequences for a drawing:

• Features that must hold after aging have to be either machined with the shrink allowance or finished after heat treatment. Grinding after aging is common on bearing seats.
• If the drawing calls out H900 and a 0.01 mm tolerance on a bore, ask where in the sequence the bore gets its final cut. The answer changes the price more than the material cost does.

Also worth knowing: 17-4 is martensitic, so it is magnetic in both the annealed and the aged condition. Type 316 is essentially non-magnetic. If the assembly assumes non-magnetic stainless, 17-4 is the wrong pick, and no amount of machining skill fixes it.

Where this lands in the quote

Three line items move when the alloy changes, and none of them is the material price:

1. Cycle time. Tool life and feed rates, not the raw stock cost, dominate at low volume.
2. Inspection and rework. 316 bores and 17-4 seats are where parts get scrapped, because the finish pass is where the tolerance is either there or not.
3. Post-processing. Passivation for 300-series, heat treatment and sometimes grinding for 17-4, and a dimensional check after both.

A quote that lists material, machining, and finishing as three numbers but does not mention the sequence is a quote you should question. The sequence is where the risk lives.

CHX Precision Metal machines stainless, aluminium, titanium and engineering plastics in Shajing, Bao'an, Shenzhen, working to +/-0.005 mm where the drawing calls for it. 220 machines, MOQ 1 piece, prototypes 3-7 working days, batch production 7-15 days, EXW Shenzhen. chxmetal.com