The Key Difference: Carbon Content
If you've spent time specifying stainless steel for valves, bearings, cutting tools, or turbine components, you've encountered the AISI 440 series. Three grades, one family, near-identical chromium content (16–18%) — yet their behavior in service can be dramatically different. The single variable that separates them is carbon content.
Carbon drives hardness. Carbon also consumes chromium — every 0.1% of carbon dissolved during austenitizing pulls approximately 0.7% chromium into carbide precipitates, removing it from the passive film that provides corrosion resistance. This creates a fundamental trade-off that runs through every property comparison in this guide:
The harder the grade, the less corrosion-resistant it becomes. There is no way around this physics — only a choice of where to sit on the spectrum.
The choice between 440A, 440B, and 440C is therefore not about which hardness number looks best on a datasheet. It's about which grade survives longest in your specific operating environment, at your required section size, within your machining and heat treatment capabilities.
Performance Spectrum at a Glance
Full Technical Comparison Table
| Property / Characteristic | 440A · S44002 | 440B · S44003 | 440C · S44004 |
|---|---|---|---|
| Carbon Content (%) | 0.60 – 0.75 | 0.75 – 1.00 | 0.95 – 1.20 |
| Chromium (%) | 16.0 – 18.0 | 16.0 – 18.0 | 16.0 – 18.0 |
| Max Hardness (Rockwell C) | HRC 56 | HRC 58 | HRC 60 Hardest |
| Tensile Strength (MPa, hardened) | ~1,590 | ~1,750 | ~1,900 |
| Corrosion Resistance | Best ▲ Winner | Good | Lowest in 440 series |
| Wear Resistance | Good | Very Good | Excellent ▲ Winner |
| Impact Toughness (Charpy CVN) | 40–60 J ▲ Highest | 14–20 J | 8–15 J (lowest) |
| Machinability (vs AISI B1112) | ~45–50% | ~40–45% | ~30–40% |
| Forgeability >100 mm section | Good | Good ▲ Recommended | Difficult — quench crack risk |
| Weldability | Poor (preheat required) | Poor (preheat required) | Very poor |
| PRE (Pitting Resistance Equiv.) | ~18–21 | ~17–20.5 | ~14–16 |
| JIS Equivalent | SUS 440A | SUS 440B | SUS 440C |
| Closest DIN / EN Equivalent | 1.4109 | 1.4112 (approx.) | 1.4125 |
| Magnetic? | Yes (martensitic) | Yes (martensitic) | Yes (martensitic) |
AISI 440A — The Corrosion-First Choice
HRC 56
With the lowest carbon content in the series, AISI 440A sacrifices the top few hardness points in exchange for the best corrosion resistance. Because fewer chromium atoms are consumed by carbide precipitation during heat treatment, more chromium remains dissolved in the matrix to maintain the protective Cr₂O₃ passive film.
440A also delivers the highest impact toughness in the series — Charpy CVN values of 40–60 J versus 8–15 J for peak-hardness 440C. For components subjected to shock loading, vibration, or dynamic stress in moist environments, this combination is unmatched in the 440 family.
In environments with constant moisture, mild acids, or salt spray exposure, 440A's advantage becomes measurable in extended service life. Its Pitting Resistance Equivalent (PRE) of 18–21 puts it on par with AISI 304 austenitic — a useful benchmark when comparing stainless options.
AISI 440B — The Engineer's Recommendation
HRC 58
AISI 440B occupies the midpoint of the series and, in the vast majority of industrial forging applications, is the correct choice. Its carbon range delivers peak hardness of HRC 58 — within 2 Rockwell points of 440C — while retaining measurably better corrosion resistance because carbide precipitation is less aggressive.
The large-section forging advantage: For heavy forgings above 100 mm in cross-section, 440B is strongly preferred over 440C. The higher carbon of 440C generates greater volumetric expansion stresses during the austenite-to-martensite transformation on quenching. In practice, 440C forgings in large sections can crack at quench rate variations of as little as 10–15% from optimal. 440B's lower carbon eliminates most of this risk while delivering HRC 58 that is, in almost every real-world wear application, indistinguishable from HRC 60 in terms of service life.
For detailed specifications, dimensional range, heat treatment protocols, and application cases, visit the product page for our AISI 440B open die forgings and seamless rolled rings — including forged bars, valve components, and bearing rings from 30 kg to 30,000 kg per piece.
AISI 440C — When Only Maximum Hardness Will Do
HRC 60
AISI 440C is the grade engineers reach for when HRC 60 is a hard requirement — precision ball bearings rated ABEC-5 and above, surgical cutting instruments, and gage blocks where surface hardness directly governs contact stress resistance and dimensional stability.
The corrosion compromise is frequently underestimated. At 1.20% carbon (maximum), approximately 8–9% of the 16–18% chromium can be tied up in carbides. In the worst case, effective dissolved chromium falls to ~9%, barely maintaining passivation. This is why 440C components can rust noticeably faster than 440B equivalents in humid or wet environments — even at equivalent surface finish and passivation treatment levels.
For forgings with cross-sections exceeding 100 mm, 440C carries significant quench-cracking risk. At Jiangsu Liangyi, our rejection rate for 440C in sections 100–200 mm is 3–5× higher than for equivalent 440B geometries. Unless HRC 60 is a firm contractual or design requirement, always specify 440B for sections above 100 mm.
Heat Treatment: Where the Grades Diverge in Practice
All three grades follow the same broad sequence, but practical handling differs — especially around quench rate management and tempering window tolerance.
Forging Considerations
All three 440-series grades are forgeable in the 1,065–1,175°C hot work range, but behavior during and after forging differs significantly. For open die forgings and seamless rolled rings, 440B is the preferred grade for sections above 75 mm diameter — this is not a supplier preference but a metallurgical reality backed by rejection rate data.
A minimum forging reduction ratio of 4:1 is required for any of these grades to fully break down the as-cast dendritic ingot structure. At Jiangsu Liangyi we document this ratio on every forging traveller card — for fatigue-critical or aerospace applications, we achieve 6:1 or greater upon request. Lower reduction ratios retain columnar grains that reduce transverse impact toughness by 20–35% and shorten component fatigue life measurably. Buyers requiring AISI 440B valve forgings, bearing rings, and custom forged components with full EN 10204 3.1 documentation can find the complete product range and dimensional specifications on the product page.
After forging, all three grades must be slow furnace-cooled — never air-cooled or directly quenched from the forging heat. The martensitic transformation during uncontrolled fast cooling generates internal stresses that will crack heavy sections without warning.
Application Decision Matrix
Cross-reference your application's primary constraint against the recommended grade. When two grades seem equally appropriate, the primary tie-breakers are: section size (favor 440B above 100 mm) and corrosive severity (favor 440A when prolonged moisture or chemical exposure is present).
Applications by Grade
- Marine valve trim
- Offshore actuator parts
- Corrosive pump internals
- Cutlery (corrosion focus)
- Chemical process fittings
- Springs in wet service
- API 6A wellhead valve balls
- Bearing rings >100 mm
- Rolling mill rolls
- Turbine seal rings
- Pump impellers & shafts
- Mining wear parts
- Aerospace hydraulic valves
- ABEC-5+ ball bearings
- Surgical cutting tools
- Precision gage blocks
- Fine knife blades
- Small valve balls <50 mm
- Dental instruments
Cost and Lead Time: The Full Picture
Raw material cost between the three grades is negligible — all three share the same base chromium content, and the small carbon difference has almost no impact on feedstock price. Where costs diverge is in heat treatment rejection rates and machining cycle time.
For heavy forgings in the 100–300 mm cross-section range, Jiangsu Liangyi's internal production data shows 440C quench-and-temper rejection rates running 3–5× higher than equivalent 440B geometries. Each rejected piece carries the full forging cost and must be scrapped or re-worked — elevating the effective cost-per-accepted-piece for 440C in large sections by 20–40% above nominal pricing.
Machining costs follow a similar pattern. In the annealed condition, 440B machines approximately 8–10% faster than 440C due to its lower hardness. For machining-intensive parts such as valve balls and pump impellers, this difference compounds across cycle time and tooling consumption into a meaningful total cost advantage.
For most industrial forging applications, 440B delivers equal or better total value than 440C — lower quench-cracking risk, faster machining, better corrosion margin, and a 2-HRC difference that rarely changes real-world service outcome.
Frequently Asked Questions
Related Product Pages and Material Guides
Explore individual grade pages and related material guides from Jiangsu Liangyi for full chemical compositions, mechanical property tables, heat treatment protocols, and manufacturing capabilities.
References and Technical Sources
- ASTM A276/A276M — Standard Specification for Stainless Steel Bars and Shapes, ASTM International
- ASTM A479/A479M — Standard Specification for Stainless Steel Bars and Shapes for Boilers and Pressure Vessels
- ASTM A580/A580M — Standard Specification for Stainless Steel Wire
- ASM International, Metals Handbook Vol. 1: Properties and Selection of Irons, Steels, and High-Performance Alloys, 10th Edition
- ASM International, Metals Handbook Vol. 4: Heat Treating, 10th Edition
- ASM International, Metals Handbook Vol. 16: Machining, 9th Edition
- NACE MR0175 / ISO 15156 — Materials for Use in H₂S-Containing Environments in Oil and Gas Production
- API Specification 6A — Specification for Wellhead and Christmas Tree Equipment, 21st Edition
- JIS G4303 — Stainless Steel Bars, Japanese Standards Association
- EN 10088-3 — Stainless Steels: Technical Delivery Conditions, CEN
- Jiangsu Liangyi Co., Limited — Internal production data and quality control records, 1997–2026