440A vs 440C – Composition, Heat Treatment, Properties, and Applications

Introduction

440A and 440C are two closely related martensitic stainless steels widely used where a combination of hardness, wear resistance, and moderate corrosion resistance is required. Engineers and procurement teams commonly weigh trade-offs between cost, edge retention or wear life, and fabrication complexity when selecting between them — for example, choosing between lower-cost, easier‑to‑process material and a higher‑carbon grade with superior hardness and wear resistance.

The primary performance distinction between these grades stems from their different carbon content and the way that carbon interacts with chromium and other alloying elements to control martensitic hardenability, carbide formation, and final mechanical attributes. Consequently, 440C typically attains higher hardness and wear resistance at the expense of reduced toughness and more challenging welding/machining compared with 440A.

1. Standards and Designations

These grades are specified and cross‑referenced in a number of national and industry standards. Common designation systems in which you will find these grades include:

• AISI / ASTM / ASME: Often referenced by the AISI/UNS nomenclature (martensitic stainless steels).
• EN (European Norms) / ISO equivalents: Found in EN lists under chromium-bearing martensitic stainless designations.
• JIS (Japanese Industrial Standards): Identified as SUS440A and SUS440C.
• GB (Chinese standards) and other national standards: Similar compositions appear under local grade names.

Material classification: both 440A and 440C are martensitic stainless steels (commonly used as bearing/tool/knife steels). They are not HSLA steels; they are heat‑treatable stainless/tool steels designed for hardness and wear resistance rather than exceptional toughness or formability.

2. Chemical Composition and Alloying Strategy

The following table lists typical composition ranges for the two grades in weight percent. These ranges are representative of common specifications (JIS/EN/AISI family) and are intended for comparative purposes.

Element	440A (typical range, wt%)	440C (typical range, wt%)
C	0.60 – 0.75	0.95 – 1.20
Mn	≤ 1.00	≤ 1.00
Si	≤ 1.00	≤ 1.00
P	≤ 0.04	≤ 0.04
S	≤ 0.03	≤ 0.03
Cr	16.0 – 18.0	16.0 – 18.0
Ni	≤ 0.75	≤ 0.75
Mo	≤ 0.75 (often low)	≤ 0.75 (often low)
V, Nb, Ti	Typically nil	Typically nil
B, N	Trace / not specified	Trace / not specified

Alloy strategy and metallurgical consequences: - Chromium at ~16–18% provides stainless characteristics through a passive oxide film while also promoting carbide formation (Cr‑carbides) that influence wear behaviour. - Carbon is the key differentiator: higher carbon in 440C forms more and harder carbides and increases martensitic hardness after quenching, improving wear resistance and edge retention. - Manganese and silicon are deoxidizers and minor alloying elements; molybdenum, if present, slightly improves hardenability and corrosion resistance. - Low levels of Ni, V, and other microalloying elements are generally absent or minimal; the design relies on the Cr–C interaction for properties.

3. Microstructure and Heat Treatment Response

Microstructure comparison: - Both grades develop a martensitic matrix after hardening, with chromium carbides distributed through the matrix. The carbide size, volume fraction, and distribution are strongly carbon‑dependent. - 440A (lower carbon): produces fewer carbides with smaller carbide volume fraction; martensite tends to be less saturated with carbon, yielding lower hardness and relatively better toughness. - 440C (higher carbon): gives a higher volume fraction of chromium carbides and a higher carbon content in martensite; the result is higher hardness and improved abrasive wear resistance but lower toughness and ductility.

Typical heat treatment response: - Annealing: both grades are annealed to relieve stress and soften prior to machining. Annealed microstructure is typically ferrite/pearlite plus undissolved carbides; hardness is low enough for machining. - Quenching: oil quench or air/oil depending on section size and desired properties. Austenitizing temperatures are selected to dissolve appropriate carbides without excessive grain growth. - Tempering: tempering reduces brittleness and adjusts hardness. Because of the higher carbon, 440C attains higher tempering hardness at a given tempering temperature but can be more susceptible to temper brittleness and require careful temper selection. - Thermo‑mechanical processing: controlled forging and solution treatment can refine carbide distribution, improving toughness and wear resistance; both grades respond to such routes but 440C demands tighter process control to avoid coarse carbides.

4. Mechanical Properties

Mechanical properties depend strongly on heat treatment. The following table summarizes comparative behaviour rather than single absolute numbers — it is framed to aid selection decisions in manufacturing and procurement.

Property	440A (typical behaviour)	440C (typical behaviour)
Tensile strength	Moderate to high (dependent on hardening)	Higher (when fully hardened)
Yield strength	Moderate	Higher
Elongation (ductility)	Higher (more ductile)	Lower (reduced ductility)
Impact toughness	Better (greater resistance to brittle fracture)	Lower (more brittle when hardened)
Hardness (HRC, typical hardened range)	~48 – 56 HRC	~56 – 62 HRC

Explanation: - 440C achieves higher peak hardness and tensile strength because its higher carbon enables a harder martensite and more chromium carbides. That also reduces ductility and impact toughness relative to 440A. - If toughness and resistance to catastrophic fracture are priorities, 440A will generally perform better after comparable heat treatments. If wear resistance and edge retention are critical, 440C is usually preferred.

5. Weldability

Welding martensitic stainless steels requires caution because of their propensity to form hard, brittle martensite and to crack in the heat‑affected zone (HAZ). Key influences include carbon content, hardenability (Cr and other alloying), and microalloying.

Useful compositional indices (interpret qualitatively): - Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ A higher $CE_{IIW}$ indicates greater risk of HAZ hardening and cracking; 440C will generally give a higher value than 440A because of its higher carbon.

• Pcm (weldability parameter): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ Higher $P_{cm}$ correlates with poorer weldability and higher preheat/postweld heat treatment demands.

Qualitative interpretation: - 440A (lower carbon) is easier to weld than 440C but still requires preheat, controlled interpass temperatures, and post‑weld tempering or stress relief to avoid HAZ cracking. - 440C (high carbon) is more difficult to weld. In many cases, welding is avoided; mechanical fastening or brazing may be preferred. If welding is necessary, strict preheat, weld parameters, and post‑weld heat treatment protocols are mandatory.

6. Corrosion and Surface Protection

• Both 440A and 440C are stainless by virtue of chromium content, but their corrosion resistance is only moderate compared with austenitic stainless steels (300 series). Chromium carbides can form and deplete chromium locally (sensitization) if held in the critical temperature range, reducing localized corrosion resistance.
• For aggressive environments, surface protection (passivation, coatings) or alternative alloys should be considered.

PREN (Pitting Resistance Equivalent Number) formula for stainless steels when Mo and N matter: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - Application: PREN is most meaningful for duplex and austenitic grades with appreciable Mo and N. For 440A/440C, with low Mo and N, PREN is low and not a useful discriminator. - Practical note: Where corrosion resistance is critical (e.g., marine, acidic), choose stainless grades with higher Mo/N content (or austenitic/duplex alloys) rather than relying on martensitics.

Surface protection for non‑stainless uses (if material is not sufficient): galvanizing, plating, conversion coatings, and paints are options for carbon/alloy steels, but for 440A/440C the usual approach is passivation (acid passivation) and controlled surface finishes to minimize crevice/pitting initiation.

7. Fabrication, Machinability, and Formability

• Machinability: 440A (lower hardness) is generally more machinable in the annealed condition than 440C. 440C’s higher carbon and carbide content increase tool wear and reduce cutting speeds unless the material is annealed and special tooling/coatings are used.
• Grinding and finishing: 440C responds well to precision grinding and polishing — hence its popularity for knife blades and bearing components. More abrasive and slower than 440A at equivalent feed rates.
• Formability and bending: Both grades have limited cold formability relative to austenitic stainless steels. 440A is somewhat more forgiving during forming due to its lower hardenability; 440C is typically formed only when soft-annealed and then heat treated.
• Heat treatment after forming is common practice; final machining is often performed after heat treatment and tempering or via grinding operations.

8. Typical Applications

440A — Typical uses	440C — Typical uses
Lower‑cost cutlery, inexpensive knife blades, surgical tools where less edge retention is acceptable	High-end knife blades, precision cutlery, razor blades with longer edge retention
Small bearings, valve components where moderate load/wear and corrosion resistance suffice	Ball bearings, thrust washers, valve seats requiring higher wear resistance
Springs and shafts where moderate hardness and toughness are balanced	Wear rings, seals, and hydraulic components subjected to high sliding wear
General-purpose parts where machining economy matters	Applications demanding high hardness, abrasion resistance, and fine surface finish

Selection rationale: - Choose 440A for applications prioritizing cost, easier fabrication, and higher toughness where the wear demand is moderate. - Choose 440C where edge retention, abrasive wear resistance, and peak hardness are decisive and where stricter fabrication controls (heat treatment, finish grinding) can be applied.

9. Cost and Availability

• Relative cost: 440A typically costs less than 440C because of lower carbon content and somewhat easier processing. However, market prices are driven more by chromium content and form (bar, strip) than by carbon alone; both are commodity stainless grades and are widely available.
• Availability by product form: Both grades are readily available in bar, strip, sheet/plate (limited thickness), wire, and spring temper forms. 440C is particularly common in hardened and ground bar for bearings, knife blanks, and precision components.
• Lead times: standard commercial forms usually have short lead times; specialty forms (e.g., big forgings, custom heat‑treat cycles) may add time.

10. Summary and Recommendation

Summary table — relative assessment:

Attribute	440A	440C
Weldability	Better (but still requires care)	Poorer (high risk of cracking)
Strength – Toughness balance	Moderate strength, better toughness	Higher strength/hardness, lower toughness
Cost	Lower / more economical	Higher (processing and machining costs)

Choose 440A if: - You need a martensitic stainless steel with reasonable corrosion resistance, moderate hardness, better toughness, and lower fabrication cost. - Machinability, ease of welding (with precautions), or dent/resilience under impact is more important than maximum wear life or edge retention. - The application is cost-sensitive and the wear environment is moderate.

Choose 440C if: - Maximum hardness, abrasive wear resistance, and edge retention are priority attributes (e.g., precision knives, bearings, seal faces). - You can accommodate tighter heat‑treatment control, post‑weld/post‑process tempering, and potentially increased machining or grinding costs. - The design calls for hardened components where wear life outweighs the need for high toughness or ease of welding.

Final note: Both grades can perform excellently when specified and processed correctly. The key decision drivers are the service environment (wear vs. impact), fabrication constraints (welding, machining), and life‑cycle cost. For critical components demand trials and consult metallurgists to define specific heat‑treat cycles and surface finishes tailored to the intended service.