420 Round Bars / SS420 Bar / AISI 420 Bar / SAE 420 Bars

Product Description

AISI 420 (UNS S42000) is a high-carbon modification of the baseline AISI 410 martensitic stainless steel. By increasing the carbon content, Type 420 delivers significantly higher hardness, superior wear resistance, and higher strength post-heat treatment compared to 410, while maintaining a similar level of basic corrosion resistance.

When sourced as a round bar or rod, AISI 420 is the industry standard for components that demand an ultra-sharp cutting edge, high rockwell hardness, or exceptional metal-to-metal wear resistance.

1. Chemical Composition (Weight %)

The elevated carbon content is the defining characteristic of 420 stainless steel, giving it a much stronger response to quenching than lower-carbon martensitic options.

2. Mechanical Properties by Condition

AISI 420 round bars are heavily dependent on their thermal history. They are usually purchased in the annealed state for ease of machining, then hardened by the end manufacturer.

Annealed Condition (As-Shipped Baseline)

Optimized for cold cutting, forming, and machining.

• Tensile Strength: $\approx 95,000 \text{ psi}$ ($655 \text{ MPa}$)

• Yield Strength: $\approx 50,000 \text{ psi}$ ($345 \text{ MPa}$)

• Hardness: $\le 241 \text{ HBW}$ ($\approx 22 \text{ HRC} / 99 \text{ HRB}$)

Hardened & Tempered Condition (Typical Final State)

Quenched in oil or air from $1800^\circ\text{F} - 1900^\circ\text{F}$ ($982^\circ\text{C} - 1038^\circ\text{C}$) and tempered. Depending on the tempering temperature, it achieves distinct property profiles:

• Maximum Hardness: Can reach 50 to 56 HRC when given a low-temperature stress-relief temper (around $300^\circ\text{F} - 400^\circ\text{F}$ / $149^\circ\text{C} - 204^\circ\text{C}$).

• High-Ductility Temper: If tempered at higher ranges ($1000^\circ\text{F} - 1100^\circ\text{F}$ / $538^\circ\text{C} - 593^\circ\text{C}$), hardness drops to 30 to 38 HRC, but impact toughness and elongation increase significantly.

⚠️ Metallurgical Warning: Like Type 410, Type 420 is highly susceptible to temper embrittlement and localized chromium-carbide precipitation if tempered between $800^\circ\text{F}$ and $1000^\circ\text{F}$ ($427^\circ\text{C}$ to $538^\circ\text{C}$). Avoid this thermal window to prevent premature, brittle structural failure and a drastic reduction in corrosion resistance.

3. Key Differences: AISI 410 vs. AISI 420 vs. AISI 422

Because these three martensitic grades are frequently cross-referenced on engineering prints, it helps to understand their specific application niches:

4. Fabrication & Machining Constraints

Machinability

• In the soft, annealed state, Type 420 has a machinability rating of roughly 45-50% of AISI 1212.

• It can be gummy and form continuous chips if uncut speed is too low; sharp tooling and solid chip-breakers are required.

• Once hardened past 45 HRC, standard machining is highly impractical; components should be machined to near-net shape in the annealed state, leaving only a minor grinding allowance for post-heat-treat finishing.

Welding Limitations

• Welding AISI 420 is not recommended for standard structural applications. The high carbon content makes the heat-affected zone (HAZ) incredibly brittle and highly prone to immediate underbead cracking.

• If welding is absolutely unavoidable, the bar must be preheated to a minimum of $450^\circ\text{F} - 600^\circ\text{F}$ ($232^\circ\text{C} - 316^\circ\text{C}$) and immediately given a post-weld anneal at $1350^\circ\text{F} - 1400^\circ\text{F}$ ($732^\circ\text{C} - 760^\circ\text{C}$) before the material cools below the preheat threshold.

5. Standard Sourcing & Compliance Specifications

When reviewing mill test certificates (MTRs) or drawing specifications for 420 rods, look for these universal standards:

Typical Applications

Due to its high attainable hardness and scratch resistance, AISI 420 round bars are heavily utilized for:

• Medical & Dental Instruments: Surgical scalpels, forceps, bone scrapers, and retractors.

• Industrial Blades & Cutting Tools: Shears, dicing blades, and textile cutters.

• Mechanical Moving Components: High-wear valve needles, hydraulic pistons, needle valves, and pump shafts handling abrasive slurries.

• Molds & Dies: Plastic injection molds where high mirror-polishing capability and surface hardness are required to prevent abrasive wear from filled polymers.