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

Introduction

410 and 420 are two widely used martensitic stainless steels that are often compared when designers must balance hardness and wear resistance against toughness, weldability, and cost. Procurement managers, manufacturing planners, and engineers commonly face the choice between a lower‑carbon martensitic grade that is easier to form and weld and a higher‑carbon variant that can achieve significantly greater surface hardness and wear resistance after heat treatment.

The principal technical distinction is that 420 contains higher carbon (and therefore greater hardenability and potential hardness) than 410, while 410 is formulated for better toughness, ductility, and general‑purpose fabrication. That difference drives how each grade is heat treated, machined, protected, and applied in industry.

1. Standards and Designations

• Common international designations and standards:
• ASTM/ASME: ASTM A276 (bars), AISI/UNS numbers (UNS S41000 for 410, UNS S42000 or S42000 variants for 420).
• EN: EN equivalents are typically expressed as XxCrNi or XxCr13 etc., but direct one‑to‑one mapping varies by specific composition limits.
• JIS and GB: Japanese and Chinese standards have corresponding martensitic stainless grades with similar chemistries but different limits.
• Classification:
• 410: Martensitic stainless steel (stainless carbon/low‑alloy martensitic).
• 420: Martensitic stainless steel with higher carbon content (often called “high‑carbon martensitic stainless”).
• Neither 410 nor 420 are HSLA, austenitic, or tool steels in the formal sense, though 420 is commonly used where wear resistance approaching tool steel is desired.

2. Chemical Composition and Alloying Strategy

Table: Typical composition ranges (approximate; report actual spec limits from relevant standard when specifying material)

Notes: - These ranges are indicative and vary by product form and standard. Procurement specifications should reference the applicable standard or UNS number for acceptance limits. - The alloying strategy: both grades rely on chromium for corrosion resistance and martensite formation after quenching. 420’s elevated carbon content increases the volume fraction of martensite and carbides available for hardening; 410 keeps carbon lower to retain ductility and toughness after heat treatment.

3. Microstructure and Heat Treatment Response

Microstructure: - As‑quenched, both 410 and 420 form martensite (body‑centered tetragonal martensite) when cooled from the austenitizing temperature. Carbide precipitation (primarily chromium carbides) is more pronounced in the higher‑carbon 420, which can lead to a higher proportion of hard, brittle carbide phases distributed in the martensitic matrix. - In the annealed condition both are typically found as ferrite/pearlite or soft martensite depending on processing. Thermomechanical history (cold work, prior austenite grain size) also influences final properties.

Heat treatment behavior: - Typical solution anneal: heat to the austenitizing range (roughly 980–1050 °C, depending on specification and section size), followed by quench to form martensite. - Tempering: used to adjust hardness/toughness balance. Lower tempering temperatures (~150–300 °C) preserve higher hardness but lower toughness; higher tempering (~300–600 °C) reduces hardness and increases toughness. - 420 responds to hardening more strongly due to higher C — it can reach much higher Rockwell hardness after quench and low‑temperature tempering; 410 is limited by lower carbon and therefore cannot achieve the same peak hardness but retains better ductility and impact strength. - Normalizing or controlled cooling can be used to refine grain size and optimize machinability or toughness prior to final tempering.

4. Mechanical Properties

Table: Typical mechanical property ranges (heat‑treatment dependent; values indicative)

Interpretation: - 420 can be hardened to substantially higher hardness and tensile strengths because of its higher carbon; this makes it superior for wear‑resistant components and cutting edges. - 410 offers better toughness and elongation in comparable conditions and is generally the more ductile, impact‑resistant choice. - Exact properties depend strongly on the chosen heat‑treatment cycle and section thickness; quoting a design value requires specifying hardness or heat‑treatment state.

5. Weldability

Weldability is controlled by carbon content, other hardenability elements (Cr, Mo, V), and restraint/heat input. Higher carbon increases the risk of martensite formation and cold cracking in the heat‑affected zone.

Useful formulas for qualitative evaluation: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - More comprehensive Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 410, with lower carbon, will generally have a lower carbon equivalent than 420 and therefore better weldability (lower risk of HAZ hardening and cracking). - 420, especially high‑carbon variants, often requires special welding procedures: preheat, controlled interpass temperatures, low hydrogen welding consumables, and post‑weld tempering or stress‑relief to avoid HAZ cracking. - Use of matching consumables is important to avoid excessive hardness in the weld metal. Where weldability is a priority, specify lower carbon limits or choose filler metals designed for martensitic stainless welds.

6. Corrosion and Surface Protection

• Both 410 and 420 are martensitic stainless steels with moderate corrosion resistance due to their chromium content (≈12–14%). They are not as corrosion‑resistant as austenitic grades (304, 316) and are susceptible to pitting, crevice corrosion, and general corrosion in aggressive environments.
• For corrosive environments, surface protection strategies include passivation, painting, plating, or galvanizing (if base metallurgy and service permit). Note that galvanizing is generally applied for carbon steels and may not be appropriate where stainless surface integrity is required; consult coating compatibility.
• PREN (Pitting Resistance Equivalent Number) is mainly used for austenitic/ferritic stainlesses with Mo and N: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• PREN has limited relevance for 410/420 because their corrosion performance is dominated by chromium content and microstructural factors, and both typically have low Mo and N.
• Practical guidance: choose 410 or 420 only for mildly corrosive environments or when corrosion control by coatings/plating is acceptable. For chloride‑rich environments, select a higher‑alloy stainless.

7. Fabrication, Machinability, and Formability

• Machinability:
• In the annealed condition both grades machine reasonably well. Machinability declines sharply with increased hardness.
• 420 in hardened condition is abrasive to tooling and can be difficult to machine; tool materials and feeds must be selected accordingly.
• Formability and bending:
• 410 annealed has better formability and can be cold‑formed with appropriate allowances for springback.
• 420 (higher carbon) has reduced ductility and becomes prone to cracking when formed if not in the annealed condition.
• Surface finishing:
• Both grades take a good finish in the annealed state; polishing hardened 420 to a mirror finish is common for cutlery and medical instruments.
• Heat treatment considerations in fabrication planning: plan forming and welding in the annealed state where possible, then perform final hardening/tempering as a separate operation to achieve required properties.

8. Typical Applications

Table: Typical uses by grade

Selection rationale: - Choose 410 when fabrication, welding, impact resistance, and cost are primary concerns and only moderate corrosion resistance is required. - Choose 420 when high surface hardness, wear resistance, and edge retention are required and when post‑heat‑treatment processing is feasible.

9. Cost and Availability

• Cost:
• 410 is generally less expensive than 420 because of its lower carbon and simpler processing in many product forms.
• 420 commands a premium when supplied in high‑carbon, precision‑hardened, and finely finished forms (e.g., cutlery grades).
• Availability:
• Both grades are widely available as bar, plate, and forged product forms, but specific tempers (pre‑hardened, fine‑ground cutlery bars) may be more limited for certain carbon levels of 420.
• Lead times can vary by surface finish and hardness state — hardened and ground 420 components often incur longer lead times and higher processing costs.

10. Summary and Recommendation

Table: Quick comparison (qualitative)

Conclusions and recommendations: - Choose 410 if you need a balanced martensitic stainless steel that is easier to weld and fabricate, offers reasonable corrosion resistance for mild environments, and prioritizes toughness and lower cost. Typical use cases: shafts, fasteners, valves, and parts requiring routine fabrication and post‑weld service. - Choose 420 if your design requires high surface hardness, superior wear resistance, or sharp edges (cutting tools, blades, wear surfaces), and you can accommodate more restrictive welding and fabrication procedures plus a dedicated heat‑treatment route. 420 is the better selection where hardness after tempering and edge retention dominate the design criteria.

Final note: always specify the required heat‑treatment state, maximum hardness, and applicable standard in purchase documents. Mechanical and corrosion performance in service will be governed primarily by heat treatment and surface condition rather than nominal grade alone.