Semi-Stainless
Semi-Stainless Steel
Semi-stainless steels sit between carbon and full stainless. They will rust if left wet or neglected, but under normal use they tend to develop a patina — a dark, protective layer — rather than forming aggressive surface rust. Rinse and wipe dry after use and you will rarely have issues. Much more forgiving than true carbon steel, while still offering excellent cutting performance. PM / Powder Steel Powder Metallurgy (PM)
PM stands for powder metallurgy. Fine steel powder is consolidated under heat and pressure. This can produce a fine, even carbide structure, but performance still depends on the steel and its heat treatment. The PM label alone does not guarantee a harder, tougher or more chip-resistant knife. Hitachi Metals (now Proterial), Japan 🇯🇵
Semi-stainless steels sit between carbon and full stainless. They will rust if left wet or neglected, but under normal use they tend to develop a patina — a dark, protective layer — rather than forming aggressive surface rust. Rinse and wipe dry after use and you will rarely have issues. Much more forgiving than true carbon steel, while still offering excellent cutting performance. PM / Powder Steel Powder Metallurgy (PM)
PM stands for powder metallurgy. Fine steel powder is consolidated under heat and pressure. This can produce a fine, even carbide structure, but performance still depends on the steel and its heat treatment. The PM label alone does not guarantee a harder, tougher or more chip-resistant knife. Hitachi Metals (now Proterial), Japan 🇯🇵
HAP-40 粉末ハイス鋼 HAP40
Hardness
Rockwell C Hardness(HRC) measures how resistant a steel is to permanent deformation. For kitchen knives:
55–58 HRC — Soft. Easy to sharpen and forgiving of rough use, but dulls faster. Typical of most German knives.
59–61 HRC — Balanced. Good edge retention with manageable sharpening. Common in mid-range Japanese knives.
62–64 HRC — Hard. Excellent edge retention, holds a very fine edge. Requires more care and a fine stone to sharpen.
65+ HRC — Very hard. Exceptional edge longevity but more brittle — more prone to chipping if used roughly.
Higher is not always better — it depends on how you use and maintain your knife.
55–58 HRC — Soft. Easy to sharpen and forgiving of rough use, but dulls faster. Typical of most German knives.
59–61 HRC — Balanced. Good edge retention with manageable sharpening. Common in mid-range Japanese knives.
62–64 HRC — Hard. Excellent edge retention, holds a very fine edge. Requires more care and a fine stone to sharpen.
65+ HRC — Very hard. Exceptional edge longevity but more brittle — more prone to chipping if used roughly.
Higher is not always better — it depends on how you use and maintain your knife.
64 to 68
HRC
555759616365+
HAP-40 is a powdered high-speed tool steel from Proterial, formerly Hitachi Metals. Proterial's YASUGI Specialty Steel page matches it to JIS SKH40, and the YSS list describes it as a powder metallurgy high-speed steel with high abrasion resistance and toughness for general use.
Proterial's table gives 3.70 to 4.70% chromium and 7.50 to 8.50% cobalt, and a hardening hardness of 66 to 68 HRC under one quench condition and 64 to 66 HRC under the other. Its chromium is under the 10.5% that the Japan Stainless Steel Association gives as the stainless minimum, so it can rust: dry the knife after use.
We list HAP-40 in Hatsukokoro Hayabusa Damascus (gyuto, nakiri, petty), Sentan (gyuto, petty) and Sukenari (a hairline slim K-tip gyuto and a mirror polished yanagiba).
Proterial's table gives 3.70 to 4.70% chromium and 7.50 to 8.50% cobalt, and a hardening hardness of 66 to 68 HRC under one quench condition and 64 to 66 HRC under the other. Its chromium is under the 10.5% that the Japan Stainless Steel Association gives as the stainless minimum, so it can rust: dry the knife after use.
We list HAP-40 in Hatsukokoro Hayabusa Damascus (gyuto, nakiri, petty), Sentan (gyuto, petty) and Sukenari (a hairline slim K-tip gyuto and a mirror polished yanagiba).
Composition
HAP-40 Element Composition
Compare with
HAP-40
—
8.0%
6.0%
4.0%
2.0%
0
C
1.3%
Cr
4.0%
V
3.0%
Mo
4.8%
W
6.0%
Co
8.0%
C — Carbon
Cr — Chromium
V — Vanadium
Mo — Molybdenum
W — Tungsten
Co — Cobalt
Typical values from third-party sources.
Hardness
64–68 HRC
555759616365+
HAP-40 Composition
What's in HAP-40
What each element does in your knife.
| Element | Composition | What it does in your knife |
|---|---|---|
| CoCobalt | 7.5–8.5% | Allows higher hardening temperatures for more hardness and wear resistance, and strengthens the steel matrix. |
| WTungsten | 5.6–6.5% | Forms hard carbides for wear resistance and edge stability, and keeps the steel hard even at elevated temperatures. |
| MoMolybdenum | 4.6–5.4% | Improves hardenability and toughness, boosts corrosion resistance, and helps the steel hold hardness under heat. |
| CrChromium | 3.7–4.7% | Raises corrosion resistance; at roughly 13%+ free chromium the steel qualifies as stainless. Also improves hardenability and wear. |
| VVanadium | 2.8–3.3% | Forms extremely hard vanadium carbides for superior edge retention, and refines the grain for a finer, tougher edge. |
| CCarbon | 1.27–1.37% | The primary hardening element. More carbon means a harder edge that holds longer — but past a point it trades away toughness and rust resistance. |
Composition from zknives (K&S steel dataset) reference material. These ranges come from the K&S steel composition dataset, which is derived from zknives; maker figures are cited separately only when they are used on the page.
HAP-40 Buying Guide
Should you buy HAP-40?
Where it shines
- Proterial describes HAP 40 as "Most standard grade with good balance of hardness, toughness and wear resistance".
- Proterial lists a hardening hardness of 66 to 68 HRC or 64 to 66 HRC, depending on the quench.
Frequently asked questions
Is HAP-40 stainless?
Reviewed by K&S Staff · Last updated 21 October 2026
How hard is HAP-40?
Published by K&S Education Desk; staff review pending
Knives in HAP-40
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Sukenari
Sukenari HAP40 Mirror Polished Yanagiba 330 mm, Ebony Brass Handle (Hatsukokoro Engraved))
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Sukenari
Sukenari HAP40 Mirror Polished Yanagiba 330 mm, Ebony Brass Handle
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Hatsukokoro
Hatsukokoro Hayabusa HAP-40 Damascus Petty 150mm with Western Handle
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Hatsukokoro
Hatsukokoro Hayabusa HAP-40 Damascus Nakiri 165 mm with Western Handle
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Hatsukokoro