Understanding the 440 Series Family

Quick answer: AISI 440A, 440B, and 440C are high-chromium (16–18% Cr) martensitic stainless steels that differ primarily in carbon content. 440C (1.0–1.2% C, 60 HRC) provides the highest hardness; 440B (0.75–1.0% C, 58 HRC) offers the best balance; 440A (0.6–0.75% C, 56 HRC) gives the best corrosion resistance.

AISI 440A, 440B, and 440C belong to the same family of high-chromium martensitic stainless steels, standardised under ASTM A276 and A473. All three share a chromium range of 16–18% and a molybdenum addition of up to 0.75%, giving the entire 440 series its characteristic combination of moderate corrosion resistance and high hardness — properties no austenitic grade can match in the hardened condition.

What separates these three grades is deceptively simple: carbon content. Moving from 440A to 440B to 440C, the carbon level rises progressively, pulling hardness upward with it. But higher carbon is not unconditionally better — it narrows the forging window, reduces toughness, complicates machining, and modestly reduces corrosion resistance after hardening. Understanding these trade-offs is the entire job of grade selection.

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Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified open-die forging manufacturer producing AISI 440C forged bars, seamless rolled rings, and shafts from 30 kg to 30 tons, with rings up to 6 m OD. This blog covers grade selection theory; for product specifications, datasheets, and custom quotations, visit the product page.

Chemical Composition — The Carbon Variable

Key fact: All three 440 grades share 16–18% Cr and ≤ 0.75% Mo. Only carbon differs: 440A has 0.60–0.75% C, 440B has 0.75–1.00% C, and 440C has 1.00–1.20% C. This single variable controls hardness, corrosion resistance, toughness, and machinability across the series.

The full chemical composition is specified in ASTM A276 and ASTM A473. The table below shows nominal ranges for all three grades. Notice how chromium remains nearly constant while carbon climbs — this single variable drives the entire performance divergence.

Chemical composition of AISI 440A, 440B, and 440C stainless steel per ASTM A276
Element 440A (S44002) 440B (S44003) 440C (S44004) Role in Steel
Carbon (C) 0.60 – 0.75% 0.75 – 1.00% 1.00 – 1.20% Highest Hardness via martensite distortion & carbide precipitation
Chromium (Cr) 16 – 18% 16 – 18% 16 – 18% Corrosion resistance; forms Cr₂₃C₆ carbides with carbon
Molybdenum (Mo) ≤ 0.75% ≤ 0.75% ≤ 0.75% Pitting resistance; secondary hardening
Manganese (Mn) ≤ 1.00%≤ 1.00%≤ 1.00% Austenite stabiliser; deoxidiser during melting
Silicon (Si) ≤ 1.00%≤ 1.00%≤ 1.00% Deoxidiser; oxidation resistance at elevated temperature
Phosphorus (P) ≤ 0.040%≤ 0.040%≤ 0.040% Controlled impurity; limits toughness degradation
Sulfur (S) ≤ 0.030%≤ 0.030%≤ 0.030% Controlled impurity; limits corrosion performance loss

Why Carbon Dominates the Grade Decision

In martensitic stainless steels, carbon serves a dual function. During austenitising, carbon dissolves into the austenite matrix. On rapid quenching, it is trapped in the distorted body-centred tetragonal (BCT) martensite structure — and this lattice distortion is the physical origin of hardness. The higher the dissolved carbon, the greater the distortion, and the higher the resulting HRC value.

However, carbon also reacts with chromium during slow cooling to form chromium carbide (Cr₂₃C₆) precipitates. These deplete the surrounding matrix of free chromium available for corrosion protection — explaining why 440C in the hardened condition has modestly lower corrosion resistance than 440A, despite identical nominal chromium content.

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Important: In the annealed condition, all three 440 grades have comparable corrosion resistance. The divergence only becomes significant after hardening and tempering, when chromium carbides deploy throughout the matrix.

Hardness & Mechanical Properties Compared

Key fact: After quench-and-temper, AISI 440C achieves 58–60 HRC and tensile strength ≥ 1,965 MPa — the highest of any commercial stainless steel. 440B reaches 56–58 HRC / ≥ 1,750 MPa. 440A reaches 54–56 HRC / ≥ 1,550 MPa. Higher hardness trades against lower elongation and impact toughness.
440C ≥ 1,965 MPa Tensile Strength (Q&T)
440B ≥ 1,750 MPa Tensile Strength (Q&T)
440A ≥ 1,550 MPa Tensile Strength (Q&T)

Full Mechanical Properties Table

Mechanical properties of AISI 440A, 440B, and 440C stainless steel in the quenched and tempered condition
Property 440A 440B 440C Test Condition
Max Hardness 56 HRC 58 HRC 60 HRC Highest Quenched & Tempered
Tensile Strength ≥ 1,550 MPa ≥ 1,750 MPa ≥ 1,965 MPa Q&T, 150°C temper
Yield Strength 0.2% ~ 1,410 MPa ~ 1,600 MPa ~ 1,900 MPa Q&T
Elongation at Break ~ 8% Best ~ 6% ~ 2% Q&T, longitudinal
Impact Toughness (Charpy) Higher Best Moderate Lower V-notch, Q&T
Density 7.75 g/cm³7.78 g/cm³7.80 g/cm³ All conditions
Elastic Modulus 200 GPa200 GPa200 GPa All conditions
Hardness — Annealed ~ 217 HBW ~ 229 HBW ~ 269 HBW Fully annealed

Corrosion Resistance: Where Each Grade Stands

Key fact: All three 440 grades have the same 16–18% chromium but differ in corrosion resistance after hardening. 440A is the most corrosion-resistant (fewest chromium carbide precipitates), followed by 440B, then 440C. None match austenitic grades like 316L in aggressive chloride or acidic environments.
Corrosion resistance of AISI 440A, 440B, and 440C in various environments
Environment 440A 440B 440C Notes
Mild atmosphere ExcellentExcellentExcellent All three comparable in dry indoor air
Fresh water GoodGoodFair – Good 440C may show minor pitting at prolonged exposure
Dilute acids (pH > 4) FairFairFair All 440 grades are marginal; consider 316L for acid service
Salt spray / marine chloride Moderate Best Moderate Fair 440C most susceptible due to higher carbide density
Petroleum / hydrocarbons GoodGoodGood All grades suitable for oil & gas valve service (dry)
H₂S sour service (NACE MR0175) Not listedNot listed Qualified ≤ 22 HRC limit 440C qualified under NACE/ISO 15156 with hardness restriction

The Chromium Carbide Effect Explained

440C's relatively lower corrosion performance versus 440A is a direct consequence of high carbon. During hardening, chromium carbide (Cr₂₃C₆) precipitates form throughout the matrix. Each carbide particle ties up surrounding chromium, creating local chromium-depleted zones — these zones are more vulnerable to corrosion initiation, especially in chloride-containing media.

This does not make 440C unsuitable for corrosive service — it means tempering temperature must be specified correctly. Tempering above 550°C partially dissolves the carbide network, improving corrosion resistance at a cost of some hardness. Jiangsu Liangyi's heat treatment team manages this trade-off based on your specific service conditions.

Grade Selector: Choose Your Priority

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Machinability, Weldability & Forgeability

Key fact: Machinability (vs B1112 = 100%): 440A ≈ 45%, 440B ≈ 42%, 440C ≈ 40%. All three are considered difficult to machine due to high hardness even when annealed (217–269 HBW). Welding is not recommended for any 440-series grade due to high cold-cracking risk.

Machinability

Machinability comparison of AISI 440A, 440B, and 440C stainless steel
Grade Machinability Index Annealed Hardness Key Machining Challenge
440A ~ 45% Best ~ 217 HBW Work-hardening; requires sharp, positive-rake carbide tooling
440B ~ 42% ~ 229 HBW Abrasive Cr₂₃C₆ carbides accelerate tooling wear
440C ~ 40% Most difficult ~ 269 HBW Dense chromium carbides highly abrasive; use CBN or ceramic inserts

For all three grades, rough machining should be done in the annealed condition before hardening, then finish-machined with CBN or ceramic inserts after hardening if tight tolerances are required.

Weldability

Forgeability & Forging Temperature Ranges

Forging temperature ranges for AISI 440A, 440B, and 440C stainless steel
Grade Start Temp (Max) Finish Temp (Min) Critical Note
440A 1,150°C 950°C Widest hot-working window; most forgiving grade to forge
440B 1,120°C 925°C Intermediate; similar to 440A in practical forging operations
440C 1,150°C ≥ 900°C (strict) Never forge below 900°C — risk of cracking from undissolved carbides

Heat Treatment Parameters

Key fact: All three 440 grades are austenitised at 1,010–1,065°C, quenched in oil or salt bath, then tempered at 148–175°C for maximum hardness. Full annealing is at 840–900°C. The 425–565°C temper range must be avoided — it causes temper embrittlement in all 440-series grades.
Heat treatment parameters for AISI 440A, 440B, and 440C stainless steel
Heat Treatment Step 440A 440B 440C
Full Anneal 840–900°C, furnace cool 840–900°C, furnace cool 840–900°C, ≤14°C/hr to 600°C
Austenitising (Hardening) 1,010–1,065°C 1,010–1,065°C 1,010–1,065°C (precision critical)
Quench Medium Air, oil or salt bath Oil or salt bath Oil or salt bath preferred
Temper — Maximum Hardness 148°C / 1–2 hr 148°C / 1–2 hr 148–175°C / 1–2 hr
Temper — Better Toughness 315°C315°C315°C (significant hardness loss)
⚠️ Avoid Temper Range 425–565°C — causes temper embrittlement in all 440 grades
Typical Result (HRC) 54–56 HRC 56–58 HRC 58–60 HRC
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Temper embrittlement zone — 425 to 565°C: Tempering within this range causes a significant drop in impact toughness without proportional benefit in corrosion resistance. This temperature window must be skipped entirely in production heat treatment specifications for all three 440 grades. Post-quench cryogenic treatment (−73 to −80°C) is available for 440C to convert retained austenite and maximise dimensional stability in precision bearing rings.

Applications: Which Grade Goes Where

Key fact: 440C is the global standard for bearing rings, valve seats, and gage blocks (max hardness needed). 440B is preferred for shafts, impellers, and heavy structural forgings (balance of hardness and toughness). 440A is chosen for springs, cutlery, and environments where corrosion resistance is co-equal to hardness.
Recommended AISI 440 grade by industrial application
Application Recommended Grade Key Reason
Ball & roller bearing rings 440C Primary Maximum hardness for rolling contact fatigue life — global industry standard
Valve seats & balls (non-sour) 440C Highest wear resistance at sealing surfaces
Valve trim — H₂S sour service 440C NACE limit Must limit hardness to ≤ 22 HRC per NACE MR0175 / ISO 15156
Gage blocks & precision instruments 440C Dimensional stability at high hardness; low thermal expansion coefficient
Cutting tools & industrial knife blades 440C / 440B 440C for maximum edge retention; 440B where toughness matters more
Surgical & dental instruments 440C Fine cutting edge + adequate corrosion resistance for autoclave sterilisation
Pump impellers & wear rings 440B / 440C 440B for impact zones; 440C for highest-abrasion sliding surfaces
Springs & leaf springs 440A Best Best fatigue life, elongation, and corrosion resistance at moderate hardness
Table cutlery & food-contact blades 440A / 440B 440A for best food-safe corrosion; 440B for sharper edge retention
Shafts under combined loading 440B Preferred Best balance of strength, toughness, and corrosion resistance
Nozzles & abrasion-resistant bushings 440C Dense Cr₂₃C₆ carbide dispersion gives outstanding sliding wear resistance

Decision Framework: Grade Selection in Practice

Work through this five-question decision sequence to identify the correct 440 grade for your specific forging project:

Step-by-Step Grade Selection Guide

Q1
Is maximum hardness (≥ 58 HRC) the single most important requirement — bearings, valve seats, gage blocks, cutting edges?
→ If Yes: Specify 440C. No other commercial stainless steel matches it here.
Q2
Will the part be exposed to chlorides or mildly acidic media, AND is hardness requirement ≥ 54 HRC but maximum hardness is not essential?
→ If Yes: Specify 440A — best corrosion resistance / hardness compromise in wet environments.
Q3
Is this a heavy-duty forging (shaft, disc, housing) requiring high strength AND reasonable impact toughness but maximum hardness is not critical?
→ If Yes: Specify 440B — the all-round balance grade; avoids 440C brittleness, far harder than 440A.
Q4
Will the part be heavily machined post-forge, and minimising machining cost matters more than the last 2 HRC?
→ If Yes: Specify 440A (≥ 54 HRC acceptable) or 440B (≥ 56 HRC needed). Both machine easier than 440C.
Q5
Is this a sour H₂S service application under NACE MR0175 / ISO 15156?
→ If Yes: 440C can be used only at ≤ 22 HRC (solution-annealed condition). At that hardness, evaluate whether a duplex or precipitation-hardening grade better meets your requirements.
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The short rule: Choose 440C for static wear applications (bearings, gage blocks, valve seats); choose 440B for dynamic or impact-loaded forgings (shafts, impellers, structural components); choose 440A when corrosion resistance is co-equal to hardness (springs, cutlery, wet environments). Once your grade is confirmed, you can request a custom AISI 440C forging quotation directly from our engineering team.

Frequently Asked Questions

These questions are compiled from actual technical and commercial enquiries received by Jiangsu Liangyi's engineering and sales teams.

AISI 440A, 440B, and 440C differ primarily in carbon content: 440A (0.60–0.75% C, max 56 HRC), 440B (0.75–1.00% C, max 58 HRC), and 440C (1.00–1.20% C, max 60 HRC). All three share 16–18% chromium. Higher carbon increases hardness and wear resistance but reduces toughness, corrosion resistance, and machinability. 440C is chosen for maximum hardness, 440B for balance, and 440A for best corrosion resistance.
For the vast majority of industrial bearing applications, yes. AISI 440C achieves 58–60 HRC after quench-and-temper — the highest hardness of any commercial stainless steel — and its dispersed chromium carbide (Cr₂₃C₆) network provides outstanding rolling contact fatigue life. It is the widely accepted industry choice for stainless bearing rings per ASTM A276. Alternatives are only considered for high-temperature service above 180°C, sour H₂S environments under NACE MR0175, or weight-critical aerospace applications using silicon nitride ceramic bearings.
AISI 440C is austenitised (hardened) at 1,010–1,065°C, quenched in oil or salt bath, then tempered at 148–175°C for maximum hardness of 58–60 HRC. Full annealing is at 840–900°C with a controlled cool of ≤14°C/hr down to 600°C. The 425–565°C temper range must be avoided as it causes temper embrittlement. For improved toughness or better corrosion resistance at reduced hardness, tempering at 315°C is used.
AISI 440C (UNS S44004) is equivalent to DIN/EN designation X105CrMo17, material number 1.4125 per EN 10088-3. The JIS (Japanese) equivalent is SUS 440C per JIS G4303. The Chinese GB standard equivalent is 9Cr18Mo. All designations describe the same high-carbon (1.0–1.2% C), high-chromium (16–18% Cr) martensitic stainless steel with identical composition limits.
Yes. AISI 440C is widely used for surgical and dental instruments — scalpels, scissors, bone rasps, and dental burs — because it achieves a very fine cutting edge and withstands steam autoclave sterilisation adequately. For food-contact cutlery and washing-exposed items, 440A or 440B are generally preferred due to better corrosion resistance in acidic food environments. 440C is used in food-processing wear components (slicing blades, conveyor wear strips) where maximum hardness takes priority.
Yes. Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified manufacturer of open-die forgings and seamless rolled rings in AISI 440A, 440B, and 440C. Single-piece capacity: 30 kg to 30 tons. Seamless rings: up to 6 m OD. Shafts: up to 15 m length. All parts supplied with full heat treatment and EN 10204 3.1 mill test certificates as standard; EN 10204 3.2 (third-party witnessed) and NDE (UT, MT) are available on request. Free quotation within 24 hours at sales@jnmtforgedparts.com.
AISI 440C should be forged between 925°C and 1,150°C. The minimum finish forging temperature is strictly ≥ 900°C — forging below 900°C risks cracking because undissolved chromium carbides embrittle the steel at lower temperatures. This is a narrower window than 440A (≥ 950°C) and 440B (≥ 925°C), requiring precise temperature monitoring throughout the open-die forging operation.