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Steel watch cases can gain scratch and wear resistance in three different ways: carbon or nitrogen can be diffused into the steel’s surface, a coating such as PVD or DLC can be deposited on it, or the watch can use steel hardened through much of its thickness. These are not interchangeable. A hardened surface can resist everyday wear better, but it does not make a case dent-proof.
What “case hardening” means for a steel watch
For common austenitic stainless steels used in watch exteriors, such as 316L and 904L, “case hardening” generally refers to treating the near-surface region rather than hardening the entire case. The underlying steel remains as support for that harder surface zone.
That is different from adding a separate coating, and from choosing a steel alloy that can be heat-treated to high hardness through the material. The distinction matters when comparing claims about scratch resistance, corrosion, appearance, and future refinishing.
How diffusion hardening works
Carbon or nitrogen enters the surface zone
In a thermochemical surface treatment, carbon, nitrogen, or both are introduced or diffused into the steel under controlled conditions. This changes the structure and properties of the near-surface material, creating a transition into the underlying steel rather than a separately deposited film. The exact result depends on the alloy and the treatment parameters; there is no single recipe that applies to every hardened watch case.
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- Premium 316L Stainless Steel Construction & Scratch-Resistant Sapphire Crystal This watch case is precision crafted from durable 316L stainless steel, offering excellent corrosion resistance and a polished, long-lasting finish for daily wear. The watch face is protected by high-quality sapphire crystal glass, which provides superior scratch resistance and clear visibility. With a slim overall thickness of 11.8mm, it maintains a sleek, comfortable profile for any wrist size.
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Conventional heat treatment does not ordinarily through-harden austenitic grades such as 316L and 904L. Surface treatments offer another route to improve the hardness of those steels without making the whole case from a different, heat-treatable alloy. The technical overview from worldstainless describes surface hardening as an option for watch cases.
Depth and hardness depend on the specific process
One worldstainless example reports nitrocarburizing grade 1.4057 (431) for 75 minutes, producing an approximately 20 μm case depth and surface hardness above 1800 HV. Those figures describe that treatment of that grade; they are not a general specification for watch cases.
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- Waterproof depth：100m Waterproof
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MTS SA describes its INORYS process as a low-temperature treatment that hardens the surface and immediate underlying structure up to 50 μm deep. The company says the treatment can retain a part’s existing polished, satin-finished, or blasted finish. Its page also reports a pin-on-disk comparison in which the friction coefficient fell from 1.3 for untreated material to 0.55 for hardened material. That is the supplier’s test result, not a universal measurement of how a watch wears on a wrist. See MTS SA’s INORYS process description.
How diffusion treatment differs from coatings and hard steel
| Approach | What changes | What to check |
|---|---|---|
| Diffusion hardening | Carbon and/or nitrogen modifies a near-surface zone of the steel. | Named process, alloy, treatment depth, hardness scale, corrosion behavior, and refinishing implications. |
| PVD or DLC coating | A separate material is deposited over the steel; it is not the same as diffusion into the case. | Coating material, finish or color, and how the manufacturer describes wear and repair. Longines explains PVD as coating stainless steel or titanium with another material in its PVD explainer. |
| Bulk-hardened steel | The case uses a heat-treatable steel hardened through the material rather than relying only on a shallow treated surface. | Steel type, hardness claim and its test context, plus the manufacturer’s guidance for wear or service. Damasko describes this approach for its cases on its Ice-Hardened Steel page. |
A black watch is not necessarily made from hardened steel: its appearance may come from a coating. Conversely, a diffusion-treated case need not look black or have a visibly different finish. Do not treat “hard coating,” “surface hardened,” and “hardened steel” as synonyms.
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What a hardened case can—and cannot—protect against
A harder surface can help reduce everyday scratching, wear, or galling. It cannot guarantee that the case will remain unmarked. A sufficiently forceful impact may deform the softer material beneath a shallow treated zone or dent the case itself. Hardness numbers alone do not establish impact resistance.
The British Stainless Steel Association says nitriding austenitic stainless steel can produce surface hardness above 1000 Hv, while also noting a risk of reduced corrosion resistance in the nitride layer. That is general guidance about the process, not a performance guarantee for a watch. Corrosion behavior is treatment-specific: a manufacturer’s claim about one process should not be assumed to apply to another.
Rank #4
- Stainless steel case: Solid stainless steel cases are heavier and more durable than other cases. As long as it is carefully maintained, it can be used permanently. Waterproof: The case design ensures excellent waterproof performance, and the watch's waterproof performance can reach up to 100 meters, making it easy to cope with most situations in daily life. Durability: Made of 316L stainless steel, it has strong corrosion resistance and a sturdy and durable case design.
What to compare before choosing a hardened steel watch
- Method: Find out whether the case is diffusion-treated, coated, or made from bulk-hardened steel. A general phrase such as “scratch-resistant” does not identify the method.
- Evidence behind the figures: For any hardness or depth number, look for the alloy, treatment, test scale, and conditions. Values from different methods are not a reliable brand ranking by themselves.
- Corrosion behavior: Ask whether the maker addresses corrosion resistance for the specific treatment and alloy. Nitriding can affect the treated layer differently from other processes.
- Impact expectations: Surface hardness is not the same as resistance to dents or deformation. Consider the underlying material and case construction as well as the treated surface.
- Finish and appearance: Check whether the process preserves the existing polish or brushing, or changes the case’s color or texture. MTS says INORYS retains the finish already on the part; that claim is specific to its process.
- Refinishing: Polishing or abrasion may remove or compromise a shallow hardened zone. Do not assume a treated case can be refinished just like untreated steel; check the manufacturer’s service guidance.
Manufacturers describe particular materials and processes, not a universal guarantee for every case. For example, Tudor’s article on steel discusses 316L cases but does not state that those cases are surface hardened. Its 2023 article on steel watch cases should not be read as evidence of a hardening treatment.
Quick Recap
Best Value
- product type：40mm Watch Cases Tools & Parts
- Suitable for dial size:28.5-30.5mm Dial
- Waterproof depth：100m Waterproof
- Glass material：Sapphire crystal glass
- Suitable for strap width：20mm
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