JIS SUS316 Steel: AISI 316 Equivalent — Mo-Alloyed Stainless for Chloride Environments

SUS316 (JIS G4303) is the molybdenum-alloyed variant of the 18-8 austenitic stainless family — Japan’s direct equivalent of AISI 316 and EN 1.4401. The addition of 2–3% Mo raises the pitting resistance equivalent number (PREN) from ~20 to ~25, making SUS316 the minimum specification for marine, pharmaceutical, and chloride-process environments where SUS304 pits and stains within months. This guide covers international equivalents, full composition and property data, the PREN calculation that determines real corrosion limits, and the three situations where engineers pay the 30–50% price premium unnecessarily.

1. International Equivalent Grades

SUS316 maps to the globally recognized Type 316 austenitic stainless grade. The composition windows are nearly identical across standards, with minor differences in maximum carbon and nitrogen limits.

2. Chemical Composition

The molybdenum addition is the single defining difference from SUS304. Everything else — Cr, Ni, C limits — is nearly identical.

Source: JIS G4303:2012, ASTM A240/A240M-22, EN 10088-1:2014. Note the EN specification controls Mo max at 2.50% vs 3.00% in JIS/ASTM — confirm Mo content when ordering to EN for high-chloride applications requiring maximum PREN.

3. Mechanical Properties

All values for solution-annealed (1010–1150°C, water quench or rapid air cool) bar and plate. SUS316 cannot be hardened by heat treatment — only cold work raises hardness.

4. Physical Properties

5. Pitting Resistance: What Mo Actually Does

Pitting in stainless steel begins when chloride ions (Cl⁻) break down the passive chromium oxide film at microscopic defects. Once a pit initiates, the local chemistry inside accelerates further attack — a self-reinforcing process that can perforate thin sections in weeks.

Molybdenum combats this by two mechanisms: it enriches the passive film at the pit mouth, slowing breakdown; and it raises the critical pitting temperature (CPT) — the temperature above which pitting becomes inevitable in a given chloride concentration.

PREN: Putting a Number on Pitting Resistance

The Pitting Resistance Equivalent Number (PREN) is the standard screening tool:

As a field rule: PREN > 25 is generally required for continuous seawater immersion at ambient temperature. SUS316 sits right at this boundary — adequate for splash zones and intermittent contact, marginal for full immersion in warm seawater (>30°C / 86°F).

6. Machinability

SUS316 is moderately difficult to machine — rated at approximately 50% of AISI 1212 free-machining carbon steel. The austenitic matrix work-hardens rapidly under the cutting tool, and the low thermal conductivity concentrates heat at the tool tip.

• Turning: Cutting speed 80–120 m/min (262–394 ft/min) with coated carbide; use sharp edges and positive rake angles to minimize work hardening
• Drilling: Use low speeds with high feed rates — dwelling causes work hardening. Peck drilling recommended for deep holes
• Surface finish: Achieves Ra 0.4–0.8 μm without difficulty; Ra < 0.4 μm requires polishing or electropolishing (standard for pharmaceutical and food equipment)
• vs SUS304: SUS316 is marginally harder to machine due to higher nickel content — expect ~10–15% shorter tool life at equivalent speeds

7. Weldability & SUS316L

SUS316 welds readily with no preheat required. The austenitic structure eliminates hydrogen-induced cold cracking, and thermal conductivity is low enough that large heat-affected zones are rare.

The critical issue is sensitization: heating SUS316 in the 450–850°C (842–1562°F) range causes chromium carbide precipitation at grain boundaries, depleting Cr and creating a corrosion-susceptible zone. In light-gauge sheet this is usually not an issue — the weld zone passes quickly through the sensitization range. For heavy sections or multi-pass welds on equipment that will contact corrosive media, use SUS316L (C ≤ 0.030%) instead.

8. Common Mistakes

9. When to Choose SUS316 (vs SUS304 and SUS316L)

10. FAQ

Q: Is SUS316 the same as AISI 316?

Yes, for all practical engineering purposes. JIS G4303 SUS316 and ASTM A240/A276 Type 316 share identical composition limits for Cr, Ni, Mo, and C. Mill certificates from either standard are acceptable for most applications. Verify with your QA requirements for regulated industries.

Q: What is the difference between SUS316 and SUS316L?

Carbon content only: SUS316 allows up to 0.08% C; SUS316L limits carbon to 0.030%. The lower carbon in 316L prevents chromium carbide precipitation during welding (sensitization), which would reduce corrosion resistance in the heat-affected zone. For welded assemblies in corrosive service, always specify 316L. Corrosion resistance of both grades is otherwise identical.

Q: Can I substitute SUS316 for SUS304 without changing dimensions?

Yes — the two grades have the same density, elastic modulus, and nearly identical yield and tensile strength. A direct dimensional substitution is valid. The only consideration is thermal expansion: both have the same coefficient (~16 μm/m·K), so no change needed. Cost will increase 30–50%.

Q: Is SUS316 magnetic?

In the solution-annealed (soft) condition, SUS316 is essentially non-magnetic (relative permeability ≤ 1.02). Cold working — drawing, forming, straightening — can induce martensite and increase permeability to 1.5–3.0. If magnetic permeability is critical (MRI environments, sensitive instrumentation), specify annealed bar and avoid cold-formed shapes.

Q: Does SUS316 rust?

SUS316 does not rust in the conventional iron-oxide sense. However, it can stain (surface deposits of iron contamination), and it can pit in high-chloride environments that exceed its PREN (~25). The result — brown discoloration or small pits — is sometimes called “rusting” by end users but is a distinct corrosion mechanism. In environments within SUS316’s limits, the surface remains bright and passive indefinitely.