304 Stainless Steel CNC Machining
CNC machining of 304 stainless steel: collected property envelope with heat-treatment condition, corrosion-service selection points, and what to send for a quotation.
Last updated: 2026-10-26
304 is the general-purpose austenitic stainless steel most machine shops cut daily, and the one buyers usually mean when a drawing says "stainless steel" without a grade. It balances corrosion resistance, formability and weldability well enough to serve indoors and in most atmospheric service, which is why it dominates enclosures, food-contact hardware, fasteners and general process equipment.
Machining 304 is straightforward but not friction-free: the austenitic structure work-hardens, so tools must keep cutting rather than rubbing, and heavy passes beat light spring passes. Sharp tooling, adequate feed and flood coolant carry the standard countermeasures; parts that need extensive welded assembly should plan for annealing or passivation after welding to restore corrosion performance at the heat-affected zones.
Specifications
| Property | Value | Condition |
|---|---|---|
| Tensile strength | 515–620 MPa | annealed |
| Yield strength (0.2% offset) | ≈205–215 MPa | annealed |
| Hardness | ≈70–85 HRB | annealed |
| Density | 8.0 g/cm³ (0.29 lb/in³) | annealed |
Values above are rendered from the collected model-evidence layer, each carrying the heat-treatment condition it was reported under; fields without collected evidence are stated as not established rather than estimated. Requirements outside the table are confirmed for the specific geometry and batch at quotation.
Applications & Selection Points
General atmospheric and indoor service
When the working environment is ambient air, fresh water or food-grade washdown, 304 is usually the economical stainless choice. Housings, brackets, conveyor components and sanitary-style fittings machined from 304 hold up where plated carbon steel would pit. If the drawing does not state a chloride environment, 304 is the default grade worth quoting first.
Where 304 is the wrong call
Chloride exposure — marine spray, de-icing salt, brine processing — is the classic boundary. Pitting corrosion in those conditions is the reason drawings move to the molybdenum-bearing grade. Similarly, parts that need high strength from heat treatment cannot get it from 304: austenitic grades strengthen by cold work only, so a strength-critical machined fitting may belong to a precipitation-hardening grade instead.
Welded assemblies and post-machining
304 welds readily with standard austenitic practice. For machined-then-welded assemblies, sensitization near the weld is managed with low-carbon practice or post-weld annealing; the low-carbon variant exists for exactly this. Passivation after machining removes free iron picked up from tooling and restores the passive surface — worth specifying on parts with cosmetic or sanitary surfaces.
Machining-behavior selection points
Expect work hardening to shape the process plan: continuous chips, positive rake tooling and non-interrupted light-finish passes. Deep bores and thin walls are where 304 punishes a sloppy sequence. Cross-drilled features and thread cutting are routine, but the shop should clear chips aggressively — re-cutting work-hardened chips is the fastest path to edge failure.
Frequently Asked Questions
Is 304 the same as 18-8 stainless steel?
Essentially yes as a family nickname: "18-8" describes the approximate chromium and nickel balance that defines the austenitic family, and 304 is its most common member. But 18-8 is not a specification — procurement documents should name the actual grade and product form, because composition limits, product chemistry ranges and mechanical-property requirements differ between related designations within that family.
Should my part be 304 or a more corrosion-resistant grade?
Decide from the working environment, not the catalog. Ambient and washdown service usually keeps 304. Chlorides, marine spray or continuous salt exposure push the selection to the molybdenum-bearing grade, and strong acids or high-temperature reducing conditions can point elsewhere entirely. If the service environment is ambiguous, state it on the RFQ — an environment statement lets the quoting side propose the grade instead of guessing.
What should I send for a 304 machining quote?
The standard package: a STEP model plus a 2D drawing with tolerances and threads, the grade and product form called out (sheet, bar or forging matters to strength and finish), quantity and batch intent, finish and any passivation requirement, and inspection needs. If corrosion service defines the part, include the environment — it is the single input most likely to change the proposed grade.
Ordering & Documentation
Documents
Send a STEP or IGES model with a 2D drawing carrying tolerances, thread callouts and the finish specification. Name the grade and product form explicitly rather than writing "stainless" — several austenitic designations look interchangeable on a drawing and are not interchangeable in a supply chain.
Quotation
A quote is priced from the model and drawing, the named grade and form, quantity and batch intent, tolerance class on critical features, finish and passivation requirements, and destination country. Corrosion-service context belongs in the RFQ because it can change grade proposals, not just price.
Sample & Supply
Prototype quantities from one piece are normal for machined stainless, and prototype runs validate both the fixturing and the work-hardening behavior of the specific geometry before a production batch. First-article inspection with material certification is available on request — agree the scope at quotation.
Storage & Shipping
Machined 304 ships with the surface condition the finish callout defines — mill finish, passivated or polished. Stainless should be packed separated from carbon-steel handling contact to avoid free-iron pickup that later rusts. On receipt, store dry; the grade tolerates ordinary warehouse conditions well.