These two grades show up in the same conversation more often than they should. They are both stainless steel, both used in demanding environments, and both available in bar, rod, and machined form. But they solve completely different engineering problems. If you are trying to decide between them, you are probably looking at a component where corrosion exposure and mechanical load overlap, and that is exactly where material selection mistakes get expensive.
This article breaks down the actual differences in composition, corrosion behavior, mechanical properties, and application fit, so you can make a defensible specification decision.
What the Chemistry Actually Means for You
316 is an austenitic grade. Its defining feature is 2–3% molybdenum, which shifts the passive film’s breakdown potential upward and makes it significantly more resistant to chloride-induced pitting than 304. It is non-magnetic in the annealed state, has good weldability, and does not harden through heat treatment. You get roughly 170–220 HB out of it regardless of what you do post-machining.
440 is a martensitic grade and comes in three sub-variants: 440A, 440B, and 440C. The difference between them is carbon content, which ranges from 0.60% in 440A up to 1.20% in 440C. Higher carbon means higher achievable hardness after heat treatment. 440C hardened and tempered can reach 58–60 HRC, which puts it in the same range as tool steel. That is the whole point of the grade.
The tradeoff is that the high carbon content ties up chromium as chromium carbide during hardening. This reduces the amount of free chromium available to form the passive film, which is why hardened 440 stainless is noticeably less corrosion resistant than 316, and in some chloride environments, less resistant than even 304.
Corrosion Resistance: 316 Wins, But Context Matters
In continuous exposure to chloride environments (seawater, CIP cleaning solutions, marine atmospheres), 316 is the clear choice. The molybdenum stabilizes the passive layer against pitting and crevice corrosion at concentrations and temperatures where 304 and 440C both fail prematurely.
316L (low carbon variant) adds resistance to sensitization during welding. If your component requires welded joints and will operate in a wet or chemically aggressive environment, 316L over 316 is a straightforward call.
440 in the annealed (non-hardened) state has decent corrosion resistance, comparable to 304 in mild environments. But most applications that specify 440 require it to be hardened, and hardened 440C in a humid, chloride-present environment will show surface staining and localized pitting faster than you expect. If the application is a cutting edge in a dry kitchen knife, that is acceptable. If the component is a bearing race exposed to saltwater spray, it is not.
⚠️ Critical selection note: 440C is sometimes marketed as “stainless” in bearing catalogs without a clear hardness note. A hardened 440C bearing in continuous seawater exposure will corrode. If your application requires both high hardness and genuine marine corrosion resistance, look at ceramic-coated bearings, silicon nitride inserts, or duplex grades for the housing instead of relying on 440C as a drop-in.
Mechanical Properties: Where 440 Has No Competition
If your application requires surface hardness above 40 HRC, 316 cannot get there. Full stop. You would need to apply a surface treatment (nitriding, hard chrome plating, DLC coating) to achieve wear resistance on an austenitic substrate, which adds cost and process complexity.
440C hardened to 58–60 HRC is used in:
- Precision bearing balls and races
- Surgical instrument blades and cutting jaws
- High-load valve seats
- Pump components handling abrasive slurries in low-chloride conditions
- Gauge blocks and metrology tooling
316, at 170–220 HB, handles fatigue loads, impact, and dynamic loading better than hardened 440 because it retains ductility. Hardened 440C is brittle relative to austenitic grades. Under shock loading or misalignment, it can crack rather than deform.
For motion and linkage components where the failure mode is fatigue rather than surface wear, 316L or 17-4PH is almost always the better specification than 440.
Side-by-Side Comparison
Property | 316 / 316L | 440C (Hardened) |
Structure | Austenitic | Martensitic |
Hardness | ~170–220 HB | ~58–60 HRC |
Corrosion resistance (chloride) | Excellent | Moderate to poor |
Weldability | Excellent | Poor (requires pre/post-heat) |
Magnetic | No (annealed) | Yes |
Heat treatable | No | Yes |
Typical applications | Marine, food, chemical, linkage | Bearings, blades, valve seats |
Which Grade Fits Your Application
Choose 316 when:
Your part operates in wet, chloride-containing, or chemically active environments and the primary load is tension, fatigue, or dynamic cyclic stress. Food processing equipment, marine hardware, clevis rod ends on hydraulic cylinders in coastal installations, and CIP-cleaned conveyor linkages all belong here.
Choose 440 when:
You need surface hardness above 40 HRC for wear resistance or cutting edge retention, and the corrosion exposure is limited (light humidity, non-chloride cleaning, or dry conditions). Bearing races in enclosed gearboxes, cutlery, and surgical blades are the textbook use cases.
When neither is right:
If you need both high hardness and serious corrosion resistance, you are looking at a different material category. 17-4PH (H900 condition, ~40 HRC) offers a reasonable balance. Duplex 2205 gives superior corrosion resistance with higher strength than 316 but without the hardness of 440C. Ceramic or polymer composites cover the cases where metal reaches its limits.
When your application requires custom-machined 316L or 440C components with tight tolerances, Profab Machine provides CNC-machined stainless parts with full material certification, direct from the factory in Ningbo.
FAQ
- ASTM A276 / A276M: Standard Specification for Stainless Steel Bars and Shapes
- ASTM A580: Standard Specification for Stainless Steel Wire
- ASM Handbook Vol. 13A: Corrosion Fundamentals, Testing, and Protection
- Wikipedia: Stainless Steel
- NIST: Thermophysical Properties of Stainless Steels
