SUS304 vs SUS316: Which Stainless Steel Should You Choose for CNC Machined Parts?

SUS304 and SUS316 are widely used austenitic stainless steels. Both offer useful corrosion resistance, formability and weldability, but they should not be treated as automatic substitutes. SUS316 normally contains molybdenum, which improves resistance to localized corrosion in many chloride-bearing environments. SUS304 is generally more economical and widely available for general service where chloride exposure is limited. The correct choice depends on more than the grade name. Exposure chemistry, chloride level, temperature, crevices, cleaning cycles, welding, surface finish, tolerances and the governing specification all matter. This guide focuses on the information that designers and buyers should review before requesting CNC machining.

SUS316 is commonly considered for components exposed to chlorides, stronger cleaning chemicals, coastal atmospheres, industrial water or more demanding hygienic conditions. Threads, covered areas, joints and alternating wet-dry zones deserve special attention because solution can concentrate locally. SUS316 is not a universal guarantee against corrosion. Warm seawater, high chloride concentration, acids or elevated temperatures may require a detailed materials review. Descriptions such as “outdoor use” or “water contact” are not sufficient inputs for material selection.

Both grades work-harden rapidly and conduct heat less effectively than common carbon steels. If the tool rubs rather than cuts, the surface hardens, cutting temperature rises and tool life falls. Long, tough chips can also wrap around the tool, scratch the part or block coolant.

Rough surfaces can retain deposits, moisture and cleaning chemicals. Deep scratches, dead zones, tight gaps and heat tint can reduce cleanability and corrosion performance. A requirement that states only “316 stainless steel” may therefore be incomplete. The RFQ should identify machined, ground, polished, passivated or electropolished surfaces; required Ra on functional areas; grain direction for cosmetic surfaces; and allowable clamping marks. When specialist passivation or electropolishing is required, VietTech can coordinate with qualified partners and inspect dimensions within the agreed scope.

Annealed SUS304 and SUS316 are generally nonmagnetic or only weakly magnetic, but cold work can create measurable magnetic response. A magnet is therefore not a reliable method for distinguishing these grades. Material traceability, certificates and agreed composition testing are more appropriate.

Include the following information so VietTech can review manufacturability and quote accurately: VietTech provides CNC milling, CNC turning, precision grinding, wire EDM and dimensional inspection within the agreed project scope. Specialist surface treatments are coordinated with suitable partners when required. Any material selection or substitution remains subject to the customer's design approval.

• Matching 2D drawing and 3D model with revision and data-authority rules.
• Full grade: SUS304, SUS304L, SUS316 or SUS316L; material standard and stock form.
• Material certificate, heat traceability or composition-verification requirements.
• Service environment: chemicals, chloride, temperature, cleaning cycle, crevices and liquid traps.
• Dimensional tolerances, GD&T, surface roughness and critical features.
• Welding, passivation, electropolishing, polishing and masking requirements.
• Prototype and production quantities, inspection reports and packaging needs.