430 vs 446 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Type 430 and Type 446 are two ferritic stainless-steel grades commonly considered when a design must balance corrosion resistance, thermal stability, formability, and cost. Procurement and engineering teams often face a selection dilemma: choose the lower-cost, more formable grade with adequate corrosion resistance for mild environments, or pay a premium for higher chromium, improved high-temperature corrosion resistance, and longer life in aggressive environments.
The principal distinction between these grades is their alloying strategy within the ferritic family: one is a standard, economical 16–18% chromium ferritic stainless (Type 430), while the other is a high-chromium ferritic stainless (Type 446) optimized for elevated-temperature oxidation and increased resistance to high-chloride or higher-temperature environments. This difference drives their selection in sheet, plate, tubing, and fabricated components.
1. Standards and Designations
- UNS: UNS S43000 (Type 430); UNS S44600 (Type 446)
- ASTM/ASME: Commonly specified under ASTM A240 / ASME SA-240 for flat product stainless steels
- JIS: SUS430; SUS446 (common JIS/SUS designations used in Asia)
- EN/ISO: Both grades appear in the EN/ISO 10088 series and national equivalents (specific numeric EN designations vary by country and product form)
- GB: Chinese national standards list equivalents (product standards vary by form and application)
Classification: Both Type 430 and Type 446 are ferritic stainless steels (magnetic, body-centered cubic matrix). They are not austenitic, tool, or HSLA steels.
2. Chemical Composition and Alloying Strategy
| Element (wt%) | Type 430 (typical range) | Type 446 (typical range) |
|---|---|---|
| C | ≤ 0.12 | ≤ 0.20–0.25 |
| Mn | ≤ 1.0 | ≤ 1.0 |
| Si | ≤ 1.0 | ≤ 1.0 |
| P | ≤ 0.04 | ≤ 0.04 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | 16.0–18.0 | 23.0–27.0 |
| Ni | ≤ 0.75 | ≤ 0.6 |
| Mo | typically 0 | 0–1.0 (some grades include small Mo) |
| V, Nb, Ti | usually trace or none | usually trace or none |
| N | very low | very low |
Notes: - The ranges above represent typical compositions for commercial ASTM/UNS grades in the annealed condition. Exact limits depend on the specification and supplier. - Type 430 is a low-cost ferritic alloy with moderate chromium for general corrosion resistance and good formability. - Type 446 increases chromium substantially (and sometimes adds small molybdenum) to improve oxidation resistance, pitting and crevice resistance at elevated temperatures, and scaling resistance in carburizing or oxidizing atmospheres.
How alloying affects properties: - Chromium is the principal element for passivity (stainless behavior); increasing Cr to the 23–27% range yields improved high-temperature oxidation and better resistance to aggressive localized corrosion. - Carbon raises strength but can promote carbide precipitation at grain boundaries; in ferritics this can influence creep and ductility at high temperatures. - Molybdenum, when present, enhances pitting resistance and high-temperature corrosion resistance. - Low Ni and low N mean these are ferritic: limited hardenability through heat treatment, magnetic, and generally not age- or precipitation-hardening.
3. Microstructure and Heat Treatment Response
Microstructure: - Both grades exhibit a ferritic (body-centered cubic, BCC) microstructure in the annealed condition. - Type 430: ferritic matrix with relatively few precipitates; carbide precipitation can occur at grain boundaries if exposed to sensitizing cycles with sufficient carbon. - Type 446: ferritic matrix with higher Cr content; may contain chromium carbides or chromium-rich precipitates at elevated temperatures, but the higher Cr content favors a more protective passive film and improved scaling resistance.
Heat treatment response: - Ferritic stainless steels are not hardenable by quenching and tempering like martensitic steels. Hardness and strength are primarily controlled by cold work, grain size, and alloy content. - Annealing: Both are solution-annealed to restore ductility; typical anneal for ferritics is around 800–950 °C followed by controlled cooling to avoid embrittlement. - Stabilization: For higher carbon versions, stabilization treatments (e.g., Ti or Nb additions in other alloys) can be specified to tie up carbon and reduce chromium carbide precipitation; Type 430 and 446 typically rely on low carbon or controlled processing. - Thermo-mechanical processing (rolling, controlled cooling) can refine grain size and improve strength and toughness; Type 446 benefits from process control when used for high-temperature components to manage precipitates and creep resistance.
4. Mechanical Properties
| Property (annealed, typical ranges) | Type 430 | Type 446 |
|---|---|---|
| Tensile strength (MPa) | 400–600 | 450–700 |
| Yield strength (0.2% offset, MPa) | 200–350 | 250–450 |
| Elongation (%) | 20–30 | 10–25 |
| Impact toughness (Charpy, J) | moderate; improves with lower carbon and fine grain | typically lower at room temperature vs. 430 due to higher Cr and lower ductility; elevated-temperature toughness better retained |
| Hardness (HB) | 120–180 | 140–220 |
Interpretation: - Type 446 is generally stronger and has higher elevated-temperature strength and creep/oxidation resistance than Type 430 because of its higher chromium content and, in some variants, slightly higher carbon and optional Mo. - Type 430 is typically more ductile and easier to form; impact toughness at low temperature can be better in 430 depending on processing. - Exact values depend on product form (sheet, plate, tube), gauge, and processing history. Both grades derive most of their strength from work hardening and microstructure control rather than heat-treatment strengthening.
5. Weldability
Weldability considerations for ferritic stainless steels revolve around carbon content, alloying that influences hardenability, and susceptibility to grain growth and embrittlement.
Relevant indices: - The carbon-equivalent (IIW) equation is useful to judge cold-cracking/hardenability tendencies in welded ferritics: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The $P_{cm}$ parameter gives a measure of weldability and tendency for hardening or cracking: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Type 430: generally good weldability with common fusion processes when preheat and interpass temperatures are controlled; lower carbon and moderate Cr give manageable CE and Pcm values. Grain growth can reduce toughness in weld heat-affected zones (HAZ). - Type 446: weldability is more challenging than 430 due to higher Cr and often higher carbon; HAZ embrittlement and reduced toughness are concerns. Preheat, controlled heat input, and post-weld annealing or stress-relief practices may be required in critical applications. Filler metal selection (austenitic versus ferritic fillers) influences joint performance—matching and dilution must be considered. - In both cases, avoid excessive heat input and rapid cooling that can promote hard, brittle microstructures in the HAZ. Use qualified welding procedures for structural or pressure applications.
6. Corrosion and Surface Protection
Both grades are stainless (form a passive chromium oxide film), but their corrosion behavior differs:
- General corrosion: Type 430 provides good resistance in atmospheric and mildly corrosive environments (indoor, mildly humid, and non-marine conditions). It is commonly used where pitting and crevice corrosion risks are low.
- Localized corrosion and high-temperature oxidation: Type 446, with significantly higher chromium, shows superior resistance to scaling, oxidation, and some forms of localized corrosion at elevated temperatures and in aggressive chloride-containing atmospheres. When Mo is present in 446 variants, pitting resistance is further improved.
Pitting Resistance Equivalent Number (where applicable): $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - PREN is most applicable for assessing pitting resistance in alloys where Mo and N are significant. For standard 430 with low Mo and N, PREN is low and not a useful discriminator; for some 446 variants with Mo, PREN will be higher, reflecting better pitting resistance.
Surface protection for non-stainless comparisons: - Both are stainless—special surface protection (galvanizing) is unnecessary and uncommon. Surface finishing (passivation, pickling, mechanical polish) improves corrosion resistance and appearance. Where chloride attack is anticipated, consider higher alloy content or coatings.
7. Fabrication, Machinability, and Formability
- Machinability: Ferritic stainless steels typically machine easier than austenitic grades. Type 430 machines reasonably well with conventional tooling; work-hardening can occur if feeds are low. Type 446, being harder and with higher Cr, is tougher on cutting tools and may require more robust tooling and slower speeds.
- Formability: Type 430 has better cold-forming characteristics (deep drawing, bending) due to higher ductility. Type 446 is less ductile and harder to form without cracking, especially in thicker sections.
- Finishing: Both take polish and surface finishes well; 446 may show slightly higher resistance to discoloration during heat processes.
8. Typical Applications
| Type 430 — Typical Uses | Type 446 — Typical Uses |
|---|---|
| Domestic appliances (oven trim, control panels) | High-temperature furnace parts, burner liners |
| Decorative trim, architectural interior panels | Heat exchangers in high-temperature corrosive environments |
| Automotive trim and trim components (non-structural) | Industrial process equipment exposed to oxidizing atmospheres |
| Washers, screws, trim where moderate corrosion resistance required | Flue gas systems, boiler liners, chimney liners |
| Foodservice equipment (in mild environments) | High-temperature flue and exhaust components |
Selection rationale: - Choose Type 430 when cost, availability, and formability are priorities in mild to moderate corrosion environments. - Choose Type 446 when high-temperature oxidation resistance, scaling resistance, or prolonged life in more aggressive conditions is essential despite higher material cost and potential fabrication challenges.
9. Cost and Availability
- Cost: Type 430 is generally one of the lower-cost stainless steels due to modest chromium content and high availability. Type 446 commands a premium because of the much higher chromium content and, where present, additional alloying (Mo).
- Availability: Type 430 is widely stocked in sheet, plate, strip, and common fabricated forms. Type 446 is less commonly stocked and is often available through specialty suppliers in specific product forms (sheet, plate, tubing) and may have longer lead times for large volumes or unusual geometries.
- Procurement tip: Evaluate life-cycle cost—higher initial cost of 446 can be justified where downtime, replacement, or warranty exposure in high-temperature/oxidizing environments would be expensive.
10. Summary and Recommendation
| Performance Metric | Type 430 | Type 446 |
|---|---|---|
| Weldability | Good (with standard controls) | More challenging; needs controlled procedures |
| Strength–Toughness | Moderate strength, good ductility | Higher high-temperature strength, lower room-temp ductility |
| Corrosion (high-temp/pitting) | Adequate for mild environments | Superior for high-temperature and aggressive environments |
| Cost | Lower | Higher |
Recommendation: - Choose Type 430 if you need an economical, easily formed ferritic stainless steel for interior architectural work, appliance trim, or moderately corrosive environments where high-temperature oxidation and aggressive pitting are not primary concerns. - Choose Type 446 if the application involves elevated temperatures, oxidizing atmospheres, furnace components, flue gas or exhaust environments, or situations where superior scaling and long-term high-temperature corrosion resistance justify higher material and fabrication costs.
Final note: Both grades are ferritic stainless steels and are best specified with detailed product form, expected service temperature, welding requirements, and surface finish. For critical or high-temperature service, consult material data from suppliers and perform application-specific corrosion and mechanical assessments.