430 vs 446 – Composition, Heat Treatment, Properties, and Applications

Introduction

Selecting between stainless steel grades 430 and 446 is a common decision point for engineers, procurement managers, and manufacturing planners who must balance corrosion resistance, high-temperature stability, manufacturability, and cost. Typical decision contexts include choosing material for appliance housings and indoor trim (where cost and formability are important) versus selecting alloys for high-temperature flue gas systems, furnace components, or exhaust systems (where oxidation resistance and long-term scaling resistance are critical).

The principal technical distinction is that both 430 and 446 are ferritic stainless steels, but 446 is a high-chromium ferritic alloy designed for superior high-temperature oxidation and corrosion resistance; 430 is a lower-chromium, general-purpose ferritic grade optimized for cost, formability, and moderate corrosion resistance. Because both are ferritic (body-centered cubic structure), they are compared frequently in design work where ferritic metallurgy, weldability, and cost-driven choices intersect.

1. Standards and Designations

• 430: UNS S43000 — commonly specified in ASTM/ASME (e.g., A240 for plate/sheet), EN (e.g., 1.4016), JIS (SUS430), and GB standards. Classification: ferritic stainless steel.
• 446: UNS S44600 — commonly found in ASTM/ASME (e.g., A240 variants for high-Cr ferritics), EN (e.g., 1.4762/1.4763 depending on subgrade), JIS (SUS446), and GB equivalents. Classification: high-chromium ferritic stainless steel (ferritic).

Both are stainless steels (not carbon, tool, or HSLA steels). Exact designation numbers and compositions can vary by regional standard and subgrade; always reference the specific standard called out on drawings or purchase orders.

2. Chemical Composition and Alloying Strategy

Table: Typical composition ranges (wt%) — consult the specific standard or mill certificate for exact limits.

Notes: - Values are indicative and depend on the precise standard and mill. 430 is a nominal 16–18% Cr ferritic stainless grade with very low Ni and essentially no strengthening alloy additions. 446 is a high-chromium ferritic stainless steel; its elevated Cr content (often 23–27%) is the key alloying strategy to maximize high-temperature oxidation resistance and reduce scaling. Some 446 specifications include small amounts of Mo or higher Si to further improve corrosion/oxidation resistance. - Alloying strategy: chromium provides passive film formation and high-temperature scale adhesion. Silicon can improve oxidation resistance at high temperature. Low carbon keeps ferrite stable and improves ductility; higher carbon (if present within limits) can increase strength but may compromise some corrosion resistance.

3. Microstructure and Heat Treatment Response

• Microstructure (as-manufactured, annealed): Both grades are ferritic with predominantly equiaxed or banded ferrite grains and, in some cold-worked products, fibrous morphology. Neither grade transforms to austenite at room temperature (unlike austenitic grades) and both are essentially non-heat-treatable for strength via hardening by phase change.
• Effect of alloying: 446’s higher Cr and possible Si content enhance high-temperature oxide scale formation and stability; grain boundary chemistry can influence scaling and carburization resistance at elevated temperatures.
• Heat treatment and processing:
• Annealing (stress relief) is commonly applied to restore ductility and reduce residual stresses. Typical solution anneal for ferritics is above $900^\circ\mathrm{C}$ followed by controlled cooling; exact temperatures follow the applicable standard.
• Normalizing or quench-and-temper cycles are not generally used to increase strength as in carbon steels; ferritic stainlesses do not harden by martensitic transformation. Thermo-mechanical processing (cold rolling + anneal) controls final mechanical properties (yield, tensile, ductility) through work hardening and recrystallization.
• Grain size control via controlled rolling and annealing is important for toughness and creep resistance at moderate temperatures. 446’s high-Cr microstructure is targeted for scale-forming stability rather than hardenability.

4. Mechanical Properties

Table: Typical mechanical property ranges (annealed, room temperature). Values are indicative; verify with mill test report.

Interpretation: - Strength: 446 may show modestly higher strength in some product forms due to chemistry and processing, but neither grade is used primarily for high-strength applications; their strengths are dominated by cold work level and product form. - Toughness and ductility: 430 tends to be more ductile and formable at room temperature, while 446 — with higher Cr (and sometimes higher C or Si) — often shows somewhat reduced ductility and lower impact toughness, especially at low temperatures. Both perform poorly relative to austenitic stainless steels in cryogenic impact toughness. - These mechanical properties are highly process-dependent (sheet vs. plate vs. tubing) and must be confirmed with supplier data.

5. Weldability

Weldability depends on carbon equivalent, alloying, and thermal expansion. Ferritic stainless steels generally weld easily using common processes but have specific pitfalls (grain growth, brittle delta-ferrite control not relevant here since both are ferritic, but sensitization and sigma phase formation at elevated temps can be concerns in service).

Useful weldability indices (qualitative guidance): - Carbon equivalent formula commonly used: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pitting/corrosion-related welding index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 430: Low carbon and low alloy content produce generally good weldability with common fusion and resistance welding methods. Post-weld corrosion resistance is acceptable for many indoor/service environments. Distortion is low because coefficient of thermal expansion is lower than austenitics. - 446: Higher chromium and sometimes higher carbon or silicon can increase susceptibility to grain growth and local embrittlement in the heat-affected zone if improper procedures are used. Preheating is rarely required, but weld filler selection and controlled heat input are important to avoid local hardness increases and microstructural changes. 446 is less forgiving than 430 — more attention to filler metal chemistry and post-weld treatments is warranted. - Always plan welding procedure specifications (WPS) and filler selection with supplier input; conduct weld trials for critical assemblies.

6. Corrosion and Surface Protection

• Stainless behavior:
• 430: Moderate corrosion resistance in mildly corrosive environments (atmospheric, indoor, and mildly acidic). Not suitable for chloride-rich or marine environments. Offers good resistance to mild oxidizing environments at ambient temperatures.
• 446: High chromium content greatly improves resistance to high-temperature oxidation and scaling; it performs better than 430 in flue gases, furnace atmospheres, and in some sulfidizing or carburizing environments. It is also more resistant to pitting and crevice corrosion at elevated temperatures.
• PREN (pitting resistance) is normally applied to austenitic/duplex grades but can be referenced: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Interpretation: PREN is of limited direct utility for ferritic grades because their chloride pitting behavior is different; however, the higher Cr (and any Mo) in 446 will give it a higher effective pitting/oxidation resistance than 430.
• Surface protection for non-stainless steels: not applicable here since both are stainless; however, for improved aesthetics or extra protection, both may be painted, passivated, or finished with mechanical polishing. For aggressive environments, consider coatings or higher-alloy alternatives.

7. Fabrication, Machinability, and Formability

• Formability:
• 430: Good formability in annealed condition; suitable for deep drawing, bending, and stamping common in appliance and architectural applications.
• 446: Less ductile; formability is lower, especially in thicker sections. Bending radii and tooling compensation must account for higher work-hardening and springback.
• Machinability:
• Both ferritic grades machine reasonably well when using appropriate tooling and speeds. 430 is generally considered easier to machine than 446 due to lower Cr and more favorable chip formation.
• 446’s hard scale-forming tendency at elevated cutting temperatures and its higher alloying can increase tool wear; use carbide tooling and adequate coolant.
• Surface finishing: Both accept mechanical polishing, grinding, and brushing. 446 may require more aggressive finishing to remove tough oxide scale when hot-formed.
• Welding and forming sequence planning is important: forming before final anneal improves ductility and reduces cracking risk for 446.

8. Typical Applications

Selection rationale: - Choose 430 when cost, formability, and acceptable ambient corrosion resistance are priorities and service temperatures are low to moderate. - Choose 446 when service involves continuous or cyclic high temperatures, aggressive oxidizing atmospheres, or where long-term scale resistance is required despite higher material cost and reduced formability.

9. Cost and Availability

• Cost: 430 is a high-volume, widely produced ferritic stainless grade and is typically among the lower-cost stainless options. 446, with significantly higher chromium content and often more specialized processing, commands a premium per kilogram/metre and is less economical for large, low-stress parts.
• Availability: 430 is widely available across product forms (sheet, plate, coil, strip, tubular). 446 is available but in fewer mill product forms and gauges; lead times can be longer, and minimum order quantities or specialty producers may apply for specific product shapes.
• Procurement note: for budget-sensitive projects, confirm total life-cycle costs: 446 may reduce maintenance and replacement costs in high-temperature applications even if its upfront cost is higher.

10. Summary and Recommendation

Table: Quick comparison (qualitative)

Recommendations: - Choose 430 if you need a cost-effective, formable ferritic stainless steel for indoor or mild service conditions where high-temperature oxidation is not a primary concern (appliances, HVAC panels, decorative trim, general sheet-metal parts). - Choose 446 if the application exposes the material to sustained or cyclic high temperatures, flue gases, or oxidizing/carburizing atmospheres where long-term scale and oxidation resistance are required (furnace or boiler components, chimney liners, high-temperature exhaust and flue parts), and you can accept higher material cost and more restrictive fabrication requirements.

Final note: Both 430 and 446 are ferritic stainless steels; the right choice depends on balancing service temperature, environment (oxidizing/halogen/chloride exposure), formability, weld procedures, and life-cycle cost. Always confirm composition and mechanical data with mill test certificates and validate welding procedures and forming trials for critical components.