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

Introduction

AISI/SAE 431 and 440C are two widely used martensitic stainless steels that commonly compete for applications requiring a balance between strength, wear resistance, and corrosion performance. Engineers, procurement managers, and manufacturing planners face the selection dilemma of prioritizing higher hardness and wear resistance versus a better balance of toughness and corrosion resistance at reasonable cost. Typical decision contexts include bearings, valve components, fasteners, shafts, and knife or tooling applications where heat treatment route, surface finish, and environment govern the optimal choice.

The principal distinction between these grades is their alloying strategy: one is formulated to deliver very high hardness and wear resistance through elevated carbon and chromium (440C), while the other sacrifices some potential peak hardness to achieve improved toughness and elevated corrosion resistance through additional alloying (431). That trade-off underlies differences in heat treatment response, machinability, weldability, and typical uses.

1. Standards and Designations

• Common international standards and designations:
• AISI/SAE: 431, 440C
• ASTM/ASME: Various ASTM specifications reference these alloys in bar, wire, or drawn products (consult specific ASTM product standards)
• EN: Closest equivalents sometimes mapped to EN martensitic stainless categories (check manufacturer datasheets)

• JIS/GB: Japanese and Chinese standards have similar martensitic stainless grades; consult conversion tables when exact equivalents are required.

• Classification:

• 431: Martensitic stainless steel (stainless alloy with medium-to-high chromium, with nickel and small Mo additions) — used where higher strength and corrosion resistance than plain carbon steels are required.
• 440C: High-carbon martensitic stainless steel / tool-grade stainless — optimized for hardness and wear resistance; considered a stainless tool steel.

2. Chemical Composition and Alloying Strategy

Below is a concise composition table showing typical nominal ranges encountered in supplier data and common specifications. These are approximate ranges — always verify against the specific mill certificate or standard referenced for the intended product form.

How alloying affects behavior: - Carbon: The defining difference — high carbon in 440C produces a higher volume fraction of martensite with carbides, enabling much higher hardness and wear resistance after quenching/tempering. Lower carbon in 431 moderates peak hardness to preserve toughness. - Chromium: Both grades are martensitic stainless with comparable chromium for passivity; combined with carbon, Cr influences carbide formation and hardenability. - Nickel and molybdenum: Present in 431 to enhance corrosion resistance and toughness; 440C typically omits significant Ni and Mo to favor carbide-forming Cr and high carbon for wear resistance. - Carbide formers (Cr, V): Promote hard carbides in 440C, improving wear resistance but reducing toughness.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• 431: Martensitic matrix with relatively fewer large carbides. When properly austenitized, quenched and tempered, 431 offers tempered martensite with good toughness and a moderate amount of fine carbides. It responds well to tempering to achieve a balance between strength and ductility.

• 440C: Martensitic matrix heavily populated with chromium-rich carbides (M23C6 and similar types) due to high carbon and chromium. After hardening, microstructure contains a high volume fraction of hard carbides embedded in martensite, producing high hardness and wear resistance but lower impact toughness.

• Heat treatment sensitivity:

• Austenitizing temperature and time control carbide dissolution. 440C requires careful austenitizing control to avoid excessive grain growth or retained austenite. Subsequent quenching to form hard martensite followed by tempering at low to medium temperatures achieves target hardness; however, over-tempering reduces hardness significantly.
• 431 tolerates a wider tempering window, enabling tempering to higher temperatures to trade strength for toughness as required.
• Thermo-mechanical processing (controlled rolling, controlled cooling) can refine prior austenite grain size and improve toughness for both grades, but 440C’s high carbon limits the degree of ductility improvement achievable.

4. Mechanical Properties

Properties depend strongly on heat treatment. Typical comparative behavior (qualitative and indicative ranges):

Interpretation: - 440C achieves superior peak hardness and wear resistance because of its high carbon and carbide population. This also yields higher tensile strength in hardened condition but at the expense of ductility and impact toughness. - 431 offers a better strength–toughness balance and improved resistance to cracking in dynamic or fatigue-loaded service when compared to fully hardened 440C.

5. Weldability

Weldability is influenced by carbon equivalent, hardenability, and microalloying.

Useful indexes: - International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Popular Pcm formula for welding risk: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 440C: High carbon leads to high $CE_{IIW}$ and $P_{cm}$ values — elevated risk of martensite formation, cracking, and hydrogen-assisted cold cracking in the heat-affected zone. Preheating, post-weld heat treatment, and low-hydrogen procedures are typically required; welding is generally discouraged for parts requiring full hardness unless substantial post-weld heat treatment is applied. - 431: Lower carbon and presence of Ni/Mo moderate hardenability and reduce cracking susceptibility compared with 440C. Still not as weldable as low-carbon austenitic stainless steels; preheat and controlled cooling are recommended, and post-weld tempering may be needed depending on application.

6. Corrosion and Surface Protection

• Stainless behavior:
• Both grades are martensitic stainless and can form passive films due to chromium. However, corrosion resistance depends on microstructure, carbide precipitation, and alloy additions.
• 431’s Ni and modest Mo give it a modestly better corrosion resistance in many environments than 440C, particularly when 440C’s chromium is tied up in carbides.
• Use of PREN:
• PREN is commonly used for austenitic/ferritic stainless grades; it is less meaningful for martensitic, low-nitrogen alloys. Nevertheless, the formula is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• For these grades, nitrogen is typically low, and the PREN calculation will not capture martensitic-specific phenomena (e.g., carbide precipitation).
• Surface protection for non-ideal environments:
• 440C often requires additional surface protection or passivation, especially if used in chloride-containing or wet environments; plating, passivation, coatings, or engineered corrosion allowances should be considered.
• Where higher corrosion resistance is required, consider austenitic stainless steels or duplex families instead of martensitic grades.

7. Fabrication, Machinability, and Formability

• Machinability:
• 440C: Harder to machine in annealed condition (due to high carbide content) and becomes more difficult after hardening. Grinding is common for finishing; tool wear is greater. Use of carbide tooling and appropriate cutting speeds is standard.
• 431: Better machinability than 440C, particularly in annealed or softer tempered conditions. Tooling life and cutting parameters are more forgiving.
• Formability:
• 440C has limited cold formability; shaping is usually done in annealed condition or by machining.
• 431 is more formable in annealed condition; can be formed and then heat treated.
• Finishing:
• Both can accept polished finishes; 440C can achieve high polish for bearing/knife surfaces due to hard carbides aiding wear resistance, but polishing is more time-consuming.

8. Typical Applications

Selection rationale: - When cyclic loading, impact, or moderate corrosive exposure is present, 431 is often chosen for its toughness and corrosion balance. - Where wear resistance, edge retention, and high hardness dominate the design requirements, 440C is the preferred choice despite tougher processing and lower toughness.

9. Cost and Availability

• Cost:
• 440C tends to be more expensive in finished hardened/ground forms due to higher alloy content, tighter heat treatment and grinding requirements, and increased tool wear in machining.
• 431 is typically less costly to produce and machine, especially when production routes avoid full hardening and require only moderate tempering.
• Availability:
• Both grades are widely available in bar, plate, and wire forms from specialty stainless and tool steel suppliers. 440C in small-diameter bearing and knife blanks is very common; 431 is common for bar and forged components.
• Form considerations:
• Finished, hardened parts in 440C may be supplied ground and polished — these value-added forms increase cost and lead time.
• Large forgings or thick-walled components in 440C are less common due to challenges in achieving uniform properties; 431 is more adaptable to larger cross-sections.

10. Summary and Recommendation

Summary table (qualitative):

Conclusion — choose based on functional priorities: - Choose 431 if you need a balanced alloy that offers better toughness, improved resistance to corrosion in many service environments, and easier fabrication/welding — for rotating shafts, valve components, fasteners, and parts subject to impact or fatigue. - Choose 440C if maximum hardness, wear resistance, and edge retention are the primary drivers and you can manage plating, finishing, and more stringent heat treatment and welding constraints — for bearings, cutting edges, wear components, and precision tooling.

Final note: Both alloys require careful matching of heat treatment, post-processing, and surface preparation to achieve the intended combination of mechanical properties and corrosion performance. Always consult mill certificates, supplier datasheets, and perform application-specific testing (fatigue, wear, corrosion) before final selection.