AISI 440A vs 440B vs 440C: The Short Answer
AISI 440A, 440B, and 440C are all high-chromium (16–18% Cr) martensitic stainless steels. The only structural difference is carbon content, which determines hardness, corrosion resistance, and forgeability.
440A (0.60–0.75% C, 56 HRC max) — lowest carbon, highest corrosion resistance, best toughness. Ideal for surgical instruments, cutlery, and mild-service valve parts. Equivalents: UNS S44002, AMS 5631, SUS 440A, DIN 1.4109.
440B (0.75–0.95% C, 58 HRC max) — balanced grade. Near-440C hardness with better corrosion resistance and machinability. Best all-round grade for heavy industrial forgings. Equivalents: UNS S44003, SUS 440B.
440C (1.00–1.20% C, 60 HRC max) — highest hardness and wear resistance, lowest corrosion resistance. For precision bearings, gauge blocks, dies, abrasive valve seats. Equivalents: UNS S44004, AMS 5630, SUS 440C, X105CrMo17, DIN 1.4125.
Forging temperature for all three grades: 925–1,150°C; preheat to 760–820°C; never forge below 900°C. Post-forge anneal at 843–871°C is mandatory.
What Is the AISI 440 Series of Martensitic Stainless Steel?
When engineers specify a martensitic stainless steel for demanding forging applications — a valve seat that must resist both corrosion and abrasion, a surgical blade that must hold an edge through repeated sterilization cycles, or a bearing ring that must maintain dimensional accuracy under high contact stress — they almost always evaluate the AISI 440 family.
The AISI 440 series comprises three closely related grades: AISI 440A forged parts (UNS S44002), AISI 440B (UNS S44003), and AISI 440C (UNS S44004). All three share the same high-chromium martensitic backbone (16–18% Cr), the same heat-treatable microstructure, and the same class of industrial applications. What separates them is a single variable — carbon content — and that single variable drives every meaningful difference in performance, forgeability, and cost.
This guide is written by the engineering team at Jiangsu Liangyi Co., Limited, an ISO 9001:2015 certified open die forging manufacturer based in Jiangyin, Jiangsu Province, China, with over 25 years of experience producing custom forgings for customers across North America, Europe, the Middle East, Southeast Asia, and Oceania.
All three 440-series grades are high-chromium (16–18% Cr) martensitic stainless steels. The only structural difference is carbon content: AISI 440A (0.60–0.75% C), AISI 440B (0.75–0.95% C), AISI 440C (1.00–1.20% C). Every performance difference — hardness ceiling, corrosion resistance, forgeability, toughness — follows directly from this single variable.
Carbon Content & Chemical Composition
In a high-chromium martensitic steel, carbon performs two competing roles simultaneously:
- It increases hardness — dissolved carbon in the martensite lattice raises the hardness ceiling after quenching. More carbon = harder steel.
- It consumes chromium — carbon bonds with chromium to form chromium carbides (Cr₂₃C₆). Every carbide precipitate removes chromium from solid solution, reducing free chromium available for the passive oxide layer — directly lowering corrosion resistance.
AISI 440A: 0.60–0.75% C. AISI 440B: 0.75–0.95% C. AISI 440C: 1.00–1.20% C. All three contain 16–18% chromium and up to 0.75% molybdenum. The increasing carbon content is the sole cause of all hardness, corrosion resistance, and toughness differences between the three grades.
| Element | AISI 440A | AISI 440B | AISI 440C |
|---|---|---|---|
| Carbon (C) % | 0.60 – 0.75 | 0.75 – 0.95 | 1.00 – 1.20 |
| Chromium (Cr) % | 16.0 – 18.0 | 16.0 – 18.0 | 16.0 – 18.0 |
| Molybdenum (Mo) % | ≤ 0.75 | ≤ 0.75 | ≤ 0.75 |
| Manganese (Mn) % | ≤ 1.00 | ≤ 1.00 | ≤ 1.00 |
| Silicon (Si) % | ≤ 1.00 | ≤ 1.00 | ≤ 1.00 |
| Phosphorus (P) % | ≤ 0.040 | ≤ 0.040 | ≤ 0.040 |
| Sulfur (S) % | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
| Iron (Fe) % | Balance | Balance | Balance |
Hardness & Mechanical Properties
The hardness ceiling after quench and temper is the most cited specification difference, but ductility and impact toughness decrease significantly as carbon rises — critical for large-section forgings and shock-loaded components.
AISI 440C is the hardest, reaching up to 60 HRC. AISI 440B achieves up to 58 HRC, and AISI 440A reaches up to 56 HRC after quenching and low-temperature tempering at 150°C.
| Property | AISI 440A | AISI 440B | AISI 440C |
|---|---|---|---|
| Max Hardness (Q&T) | 56 HRC | 58 HRC | 60 HRC |
| Tensile Strength (Q&T) | ~1,650 MPa | ~1,750 MPa | ~1,900 MPa |
| Yield Strength (Q&T) | ~1,480 MPa | ~1,590 MPa | ~1,860 MPa |
| Elongation (Q&T) | 5 – 8% | 2 – 5% | 1 – 2% |
| Impact Toughness | ✅ Best of three | Moderate | ⚠️ Lowest |
| Annealed Hardness (max) | ≤ 217 HB | ≤ 229 HB | ≤ 241 HB |
| Density | 7.75 g/cm³ | 7.75 g/cm³ | 7.75 g/cm³ |
| Elastic Modulus | 200 GPa | 200 GPa | 200 GPa |
While AISI 440C wins on hardness, it pays a significant price in ductility (1–2% elongation vs 5–8% for 440A) and impact toughness. For large-section forgings over 100 mm diameter, 440C's brittleness can be a serious manufacturing and service liability. Many engineers find that 440A or 440B is the safer, lower-risk specification for heavy-section work.
Corrosion Resistance: The Trade-Off Every Engineer Must Understand
When carbon content is high (as in AISI 440C), more chromium is locked into chromium carbide precipitates (Cr₂₃C₆) during heat treatment. These carbides do not contribute to corrosion resistance — the net result is less free chromium in the matrix and a weaker passive oxide layer.
AISI 440A has the best corrosion resistance of the 440 series. Its lower carbon content leaves more free chromium in solid solution to form the passive oxide layer. It performs well in mild atmospheric conditions, fresh water, and food-contact environments. None of the 440 grades are suitable for marine or strong acid service.
| Environment | 440A | 440B | 440C |
|---|---|---|---|
| Mild atmospheric exposure | ✅ Excellent | ✅ Good | ✅ Good |
| Fresh water immersion | ✅ Good | ✅ Good | ⚠️ Moderate |
| Food contact / weak acids | ✅ Good | ⚠️ Moderate | ❌ Not Recommended |
| Autoclave / steam sterilization | ✅ Good | ⚠️ Moderate | ❌ Not Recommended |
| Marine / seawater | ❌ Not Suitable | ❌ Not Suitable | ❌ Not Suitable |
| Strong acids / chlorides | ❌ Not Suitable | ❌ Not Suitable | ❌ Not Suitable |
Engineers sometimes over-specify AISI 440C for applications requiring moderate corrosion resistance — such as surgical instruments or food-processing cutlery. AISI 440A open die forgings achieve adequate hardness (56 HRC) with measurably better resistance to sterilization chemicals and biological fluids. Choosing 440C here adds brittleness risk with no real-world gain.
Forging Process Differences
All three AISI 440-series grades share the same forging temperature window, but 440C requires tighter process control due to its higher carbon content and greater volume of chromium carbides. The critical rule for all three grades — especially 440C — is: never forge below 900°C. Below this threshold, hard chromium carbides become abrasive within the matrix, causing surface cracking and internal tearing.
| Parameter | 440A | 440B | 440C |
|---|---|---|---|
| Forging Temperature Range | 925 – 1,150°C | 925 – 1,150°C | 925 – 1,150°C |
| Minimum Forging Temp. | 900°C | 900°C | 900°C (strict) |
| Preheat Temperature | 760 – 820°C | 760 – 820°C | 760 – 820°C |
| Post-Forge Cooling | Slow furnace cool | Slow furnace cool | Slow cool — mandatory |
| Post-Forge Anneal | 843 – 871°C | 843 – 871°C | 843 – 871°C (critical) |
| Forging Difficulty | Moderate | Moderate–High | High |
| Cracking Risk (sections >100 mm) | Low | Moderate | Higher |
Heat Treatment Guide
| Tempering Temperature | 440A (HRC) | 440B (HRC) | 440C (HRC) |
|---|---|---|---|
| 150°C (300°F) | 54 – 56 | 56 – 58 | 58 – 60 |
| 200°C (390°F) | 52 – 55 | 54 – 57 | 56 – 59 |
| 260°C (500°F) | 50 – 53 | 52 – 55 | 54 – 57 |
| 315°C (600°F) | 49 – 52 | 51 – 54 | 53 – 56 |
| 370°C (700°F) | 49 – 51 | 51 – 53 | 52 – 55 |
| 425–565°C ⚠️ AVOID | Temper embrittlement zone — reduces toughness AND corrosion resistance | ||
Tempering in the 425–565°C range significantly reduces impact toughness and corrosion resistance for all three grades without substantially reducing hardness. This is the most common heat treatment error and frequently causes premature failures in valve and bearing components. Always temper below 370°C or above 600°C.
Applications by Grade
AISI 440A — Corrosion Resistance & Toughness Priority
Scalpels, scissors, dental probes — 440A holds a sharp edge and resists autoclave sterilization chemicals better than 440C.
Kitchen knives — 440A handles food acids and dishwasher cycles better. The 4 HRC difference vs 440C rarely impacts real-world edge retention.
Small-section components needing moderate hardness and good atmospheric corrosion resistance over long service life.
Valve stems, seats, and bonnets in freshwater or mild chemical service where 440C's lower corrosion resistance is a liability.
Open die forgings, seamless rolled rings, bars, shafts, and custom shapes in AISI 440A (UNS S44002 / AMS 5631). ISO 9001:2015 certified · EN 10204 3.1 MTC standard · 30 KG to 30,000 KG · Global delivery. Request a free quote within 24 hours.
AISI 440B — Heavy-Duty Industrial Balance
Near-440C hardness (58 HRC) with better toughness — preferred for large-diameter bearing rings where section size makes 440C's brittleness a manufacturing risk.
Forged valve bodies and seats in petrochemical service requiring a balance of hardness, corrosion resistance, and machinability in large sections.
Impellers, shafts, and wear rings in pumps handling mildly abrasive and mildly corrosive fluids.
Cams, followers, and tooling components where both wear resistance and dimensional stability after heat treatment are needed.
For AISI 440B custom forgings and seamless rolled rings, see our AISI 440B forging parts page.
AISI 440C — Maximum Hardness & Wear Resistance
The original application — 440C is the industry-standard material for precision ball bearings requiring the highest contact stress resistance.
440C's excellent dimensional stability after hardening makes it the reference-grade for precision gauging instruments.
In applications with abrasive particulates (sand, grit, scale), 440C's hard carbide network provides wear resistance 440A and 440B cannot match.
Cold-work tooling where wear from the workpiece material is the dominant life-limiting mechanism.
For AISI 440C custom forgings, see our AISI 440C forged parts page.
Grade Selection Matrix
The correct grade satisfies all requirements with the minimum over-specification. Choosing a harder grade than needed introduces manufacturing cost, brittleness risk, and longer lead time without any performance benefit in service.
Corrosion resistance and toughness outweigh maximum hardness. Ideal for precision instruments, food contact, sterilization environments, and thin sections.
Near-440C hardness with better toughness, machinability, and corrosion resistance. Preferred grade for most heavy-duty industrial forgings requiring long service life.
Maximum hardness and wear resistance are the primary requirements. Corrosion is a secondary concern or manageable with coatings or controlled service environments.
International Equivalent Standards
The European equivalent of AISI 440C is X105CrMo17, material number 1.4125, per EN 10088-3. In Japan (JIS G4303) it is SUS 440C. In China (GB/T 1220) the nearest equivalents are 11Cr17 and 9Cr18Mo. The UNS designation is S44004 and the AMS specification is AMS 5630.
| Standard System | AISI 440A | AISI 440B | AISI 440C |
|---|---|---|---|
| USA — UNS | S44002 | S44003 | S44004 |
| USA — AMS | AMS 5631 | — | AMS 5630 |
| USA — ASTM | A276 / A473 | A276 / A479 / A580 | A276 / A473 |
| Japan — JIS | SUS 440A | SUS 440B | SUS 440C |
| Europe — EN / DIN | X65CrMo14 / 1.4109 | X90CrMoV18 / 1.4112 | X105CrMo17 / 1.4125 |
| Europe — EN Standard | EN 10088-3 | EN 10088-3 | EN 10088-3 |
| China — GB/T | 7Cr17 | 8Cr17 / 9Cr17 | 11Cr17 / 9Cr18Mo |
| Korea — KS | STS 440A | STS 440B | STS 440C |
| Russia — GOST | — | — | 95Kh18 |
Frequently Asked Questions
What is the difference between AISI 440A, 440B, and 440C?
All three are high-chromium (16–18% Cr) martensitic stainless steels that differ only in carbon content. 440A (0.60–0.75% C) = 56 HRC max, best corrosion resistance. 440B (0.75–0.95% C) = 58 HRC max, best balance. 440C (1.00–1.20% C) = 60 HRC max, highest wear resistance but lowest corrosion resistance. Higher carbon = harder but less corrosion-resistant.
Can AISI 440A, 440B, or 440C be welded?
Welding is generally not recommended for any 440-series grade because of the high carbon content, which makes the heat-affected zone highly susceptible to cracking. If welding is essential, preheat to 200–300°C, use matching filler metal, and post-weld anneal at 843–871°C immediately. For most forging applications, mechanical fastening or shrink fitting is the preferred assembly method.
Is 440A or 440C better for kitchen knives?
For professional kitchen applications, AISI 440A is generally the better choice. While 440C offers marginally greater hardness, 440A's improved corrosion resistance holds up significantly better against food acids, moisture, and repeated washing cycles. The 4 HRC hardness difference rarely determines real-world edge retention in cutlery applications.
What is the minimum order quantity for custom 440-series forgings?
Jiangsu Liangyi Co., Limited produces custom 440-series open die forgings and seamless rolled rings from 30 kg to 30,000 kg per piece. Minimum order quantities vary by grade, shape, heat treatment, and inspection requirements. Contact our sales team for a project-specific quotation.
What certificates does Jiangsu Liangyi Co., Limited supply with 440-series forgings?
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 certification. All 440-series forgings are supplied with EN 10204 3.1 mill test certificates as standard, covering chemical composition, mechanical properties (tensile, yield, elongation, hardness), heat treatment records, and NDE results. Third-party inspection witness (EN 10204 3.2) can be arranged on request. Products are manufactured to ASTM A276, A473, and other applicable international standards.
How does AISI 440A compare to AISI 420 stainless steel?
Both are martensitic stainless steels. 440A achieves higher hardness (56 HRC vs ~52 HRC for 420) due to higher carbon. AISI 420 offers slightly better corrosion resistance and is easier to weld and machine in the annealed condition. Where hardness is the primary requirement, 440A is preferred. Where weldability or maximum corrosion resistance matters, AISI 420 may be the better option.
What NDE inspection is applied to 440-series forgings?
Standard NDE at Jiangsu Liangyi Co., Limited includes 100% ultrasonic testing (UT) per ASTM A388 for internal integrity, and magnetic particle inspection (MT) per ASTM E1444 for surface and near-surface indications. Additional methods — dye penetrant (PT), hardness survey, CMM dimensional inspection — are available per drawing requirement. All results are documented in the EN 10204 3.1 MTC package.
What forging temperature is required for AISI 440C?
Forging temperature range for AISI 440C is 925–1,150°C. Preheat to 760–820°C before heating to forging temperature. The absolute minimum is 900°C — forging 440C below this causes hard chromium carbides to crack the matrix. After forging, cool slowly in a furnace and anneal immediately at 843–871°C. Never skip the post-forge anneal.