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

Introduction

Engineers and procurement teams frequently face a trade-off between hardenability/wear resistance and corrosion resistance when selecting stainless steels for components such as fasteners, blades, valve parts, and appliance panels. The choice between grade 420 and grade 430 typically centers on whether the part requires heat-treatable hardness (and therefore higher strength/wear resistance) or improved general corrosion resistance with good formability at lower cost.

The fundamental practical distinction is that one grade is formulated to be heat‑treatable to high hardness and strength (through martensitic transformation when quenched), while the other is a ferritic alloy that remains essentially non‑hardening by standard quench treatments and offers better general corrosion resistance and formability. That difference drives most downstream decisions in design, fabrication, and procurement.

1. Standards and Designations

Both grades are commonly specified in international stainless-steel standards. Typical standards and designations you will encounter include:

• ASTM / ASME: common coverage in stainless bar, sheet, and fastener specifications (e.g., A276/A240 families — check each spec for grade listing).
• EN (European): commonly cited descriptors for ferritic and martensitic stainless steels.
• JIS (Japanese Industrial Standards): SUS420, SUS430 nomenclature is common in Japanese-sourced material.
• GB (Chinese standards): local equivalents and grade names are available for both stainless types.

Classification: - 420: martensitic stainless steel (heat-treatable stainless). - 430: ferritic stainless steel (non‑hardening, chromium ferritic stainless).

2. Chemical Composition and Alloying Strategy

Below is a concise comparison of typical composition ranges used to differentiate the two grades. Actual limits depend on the specific standard or product variant; always check the mill certificate.

Element	Typical range — 420 (wt%)	Typical range — 430 (wt%)
C	0.15 – 0.40	≤ 0.12
Mn	≤ 1.00	≤ 1.00
Si	≤ 1.00	≤ 1.00
P	≤ 0.04	≤ 0.04
S	≤ 0.03 – 0.04	≤ 0.03
Cr	12.0 – 14.0	16.0 – 18.0
Ni	≤ 1.00	≤ 0.75
Mo	Typically none	Typically none
V, Nb, Ti, B, N	Trace/low, alloy-dependent	Trace/low, alloy-dependent

How alloying affects behavior: - Carbon: the higher carbon in 420 enables martensitic hardening and high hardness after quench and temper; it increases strength but reduces corrosion resistance and weldability. - Chromium: 430 has higher chromium content, improving passive film stability and general corrosion resistance relative to 420 in many environments. - Low Ni: both grades are low in nickel (particularly 430), making them cost‑effective compared with austenitic stainless steels but limiting low‑temperature toughness and corrosion resistance compared with Ni‑bearing grades. - Other elements: low Mo and absence of nitrogen in standard grades mean neither alloy is optimized for aggressive pitting environments.

3. Microstructure and Heat Treatment Response

• 420: In the annealed condition 420 typically contains ferrite plus carbides. On quenching from the austenitizing temperature it transforms to martensite, allowing substantial increases in hardness and tensile strength. Tempering after quench reduces brittleness and tailors toughness. Typical processing routes: anneal (soft condition) for machining, then harden (austenitize, quench) and temper to required hardness/wear resistance.
• 430: Primarily ferritic (body‑centered cubic) microstructure at room temperature and through practical thermal cycles; it is essentially non‑transformable to martensite by conventional quenching. Solution annealing and stress relieving are used for softening and grain refinement, but ferritic steels do not gain hardenability through quench and temper. Excessive high‑temperature exposure can coarsen grains, reducing toughness and weld performance.

Effects of processing: - Normalizing/annealing: both can be annealed to relieve stress; 420 is often annealed before machining. - Quench & temper: effective for 420 to reach high hardness/wear resistance; not effective for 430. - Thermo‑mechanical routes: cold work increases strength in both grades (work hardening) but is more commonly used to tailor properties in ferritic 430 where heat hardening isn’t available.

4. Mechanical Properties

Mechanical properties vary with product form (sheet, bar, wire) and heat treatment. Rather than single values, the table below compares expected behavior and typical condition‑dependent expectations.

Property	420 (martensitic, heat‑treatable)	430 (ferritic, non‑hardening)
Tensile Strength	Can be raised substantially by quench & temper; moderate in annealed condition	Moderate and relatively stable across heat treatments; limited increase from heat treatment
Yield Strength	Increases with tempering/hardness; higher than 430 in hardened condition	Moderate yield strength; increases mainly by cold work
Elongation / Ductility	Lower in hardened condition; better ductility in annealed state	Generally better ductility/formability than hardened 420
Impact Toughness	Can be low if over‑quenched or insufficiently tempered	Better retained toughness at room temperature than highly hardened 420
Hardness	Can achieve high hardness (suitable for knives, wear parts) after quench & temper	Modest hardness in annealed or cold‑worked conditions; not suitable for high‑hardness wear parts

Note: Specific numeric values depend on the exact alloy variant and heat treatment. Consult mill datasheets and perform acceptance testing when strength or toughness criteria are critical.

5. Weldability

Weldability is governed primarily by carbon equivalent, alloying elements, and microstructural response to thermal cycles. Two common indices used to assess risk of hardening or cracking are:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

and

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - 420: Higher carbon content increases $CE_{IIW}$ and $P_{cm}$ relative to 430, so preheating, controlled interpass temperature, and post‑weld tempering are often required to avoid hydrogen‑induced cold cracking and to reduce residual stresses. Choice of filler metal must balance corrosion and mechanical properties; austenitic filler wires are sometimes used to reduce cracking risk at the expense of local corrosion performance and mechanical mismatch. - 430: Lower carbon gives better intrinsic weldability than 420. Ferritic stainlesss, however, can be sensitive to grain growth in the heat‑affected zone, which can reduce toughness, and to embrittlement if exposed to certain thermal cycles. Typical practice uses matching ferritic or austenitic fillers depending on service and corrosion requirements.

Practical guidance: qualify weld procedures with relevant preheat/postheat and filler selection; perform hydrogen control and PWHT where required for martensitic 420 welds.

6. Corrosion and Surface Protection

• For parts used in mildly corrosive atmospheres, 430’s higher chromium content generally provides better uniform corrosion resistance than 420. However, neither grade is as corrosion resistant as common austenitic grades (e.g., 304/316).
• For environments prone to pitting or crevice corrosion, neither 420 nor 430 is ideal because both typically lack significant Mo and N content to boost local corrosion resistance. PREN (Pitting Resistance Equivalent Number) is often used to compare pitting resistance:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

Interpreting PREN: since Mo and N are negligible in standard 420/430, PREN will be driven almost entirely by Cr and remain low compared with Mo‑bearing stainlesss. Thus, PREN has limited utility for these grades except to highlight low pitting resistance. - Non‑stainless scenarios: both grades may be surface treated (passivation, electro‑plating, painting) to improve appearance or localized corrosion performance. For non‑stainless steels, galvanizing or polymer coatings are typical; for these stainless grades, cleaning and passivation (nitric or citric acid) are common.

7. Fabrication, Machinability, and Formability

• Machinability: 420 (annealed) machines reasonably well; in hardened condition machinability deteriorates. Sulfur‑bearing 420 variants may be optimized for free‑cutting. 430 generally has good machining characteristics in annealed form, though ferritics can work‑harden.
• Formability: 430 is typically superior for forming and deep drawing (appliance panels, trim) because it remains ductile and does not require post‑forming heat treatment. 420 must be annealed before significant forming, and final hardening can distort parts.
• Surface finishing: 420 can take high polish and is often specified for cutlery and blade applications due to its ability to be hardened and polished. 430 is used where decorative finishes and brushed surfaces are common.
• Heat treatments and dimensional control: quenching 420 can introduce distortion; designers must account for post‑hardening machining or stabilization.

8. Typical Applications

420 — Typical Uses	430 — Typical Uses
Cutlery, knives, surgical instruments (where hardness and edge retention are needed)	Decorative trim, appliance panels, oven interiors, architectural sheet
Valve components, shafts, pump parts requiring hardness and wear resistance	Automotive trim, decorative hardware, drainage channels
Bearing parts and wear elements in mild corrosive environments (when hardened)	Heat exchangers and fabricated sheet parts in mild corrosive atmospheres
Tools and dies where corrosion resistance is secondary to edge/wear performance	Low‑cost stainless alternatives for visible, formable components

Selection rationale: - Choose the martensitic, heat‑treatable option when dimensional stability after hardening, wear resistance, and edge retention are priorities and corrosion exposure is limited or can be mitigated. - Choose the ferritic option when formability, surface appearance, general corrosion resistance, and cost are primary concerns.

9. Cost and Availability

• Cost: 430 is generally less expensive than 420 in commodity sheet and coil forms because of composition and market use for appliances and architectural applications. 420 can be costlier when supplied as hardened bars or precision ground products due to additional heat treatments and finishing.
• Availability: Both grades are widely available worldwide in sheet, plate, bar, and wire forms. Specialty product forms (hardened, tempered bars, precision ground, or specific surface finishes) may have longer lead times for 420.

10. Summary and Recommendation

Criterion	420	430
Weldability	Fair to poor without controls (high C)	Good to fair (lower C; watch HAZ grain growth)
Strength–Toughness	High strength achievable; toughness trade‑offs if over‑hardened	Moderate strength; better ductility and toughness in annealed state
Cost	Moderate to higher (special heat treatments)	Generally lower (commodity ferritic)

Conclusion: - Choose 420 if you need components that can be heat‑treated to high hardness and wear resistance (e.g., blades, wear parts, hardened shafts), and you can accommodate pre/post‑weld heat treatment, careful welding procedures, and lower corrosion resistance. - Choose 430 if you need economical stainless material with good formability, decent general corrosion resistance for mild environments (appliances, architectural trim), and easier fabrication without the need for quench & temper processing.

Always validate final material selection against application‑specific requirements (mechanical loads, environment, manufacturability, and regulatory approvals) and review mill certificates and process qualifications before procurement.