304 vs 316 vs 17-4 PH: Choosing Stainless Steel for CNC Machined Parts

Stainless requests arrive as "304 or 316?" more often than any other material question. The right answer depends on where the part lives and what loads it sees. Here is how the three workhorse grades compare, from the cutting side of the business. For a deeper dive into just the 304 vs 316 decision, including the 316L question, see Stainless Steel 304 vs 316 for Machined Parts.

304: The Default Workhorse

Good corrosion resistance for most industrial and food-contact environments, weldable, and the cheapest of the three. The machining catch is work hardening: 304 hardens under the tool if feeds drop, which is why our programmers keep constant engagement and avoid dwelling. Thin-wall 304 parts (we have held 0.3 mm walls on semiconductor components) need staged stress relief between roughing and finishing or the walls move after unclamping.

316: When Chlorides Show Up

Add molybdenum and 316 resists chloride attack that pits 304: marine hardware, medical instruments exposed to sterilization chemistry, and wastewater processing parts. Machining behavior is close to 304, slightly gummier, slightly slower. Where pitting risk stays high after machining, passivation restores the chromium oxide layer the cutting process disturbed. If your part never sees chlorides, paying the 316 premium buys nothing.

17-4 PH: When Strength Leads

Precipitation-hardened 17-4 reaches roughly twice the strength of 304 after heat treatment (H900 condition), which is why aerospace brackets and high-load actuator parts use it. Two practical notes. First, machining is easiest in the solution-treated condition, so we rough before hardening and finish after. Second, 17-4's corrosion resistance is below 316, not above; it is a strength grade, not a marine grade. Standard properties are published in MatWeb's database if you need values for calculation.

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Tolerance Reality Check

All three hold our standard plus or minus 0.005 mm on stable features. The differences appear in thin walls, long unsupported parts, and post-machining distortion, all of which are process-planning problems we handle with sequencing and stress relief. More on that in our thin-wall stainless guide.

Send the drawing with the service environment described in plain words (temperature range, exposure, load) and we will recommend a grade with the quote. You can also compare other alloys in the stainless steel materials page.