SS 431 Round 1.4057 X17CrNi16-2 Bars

Product Description

AISI 431 (commonly specified by its European equivalents 1.4057 and X17CrNi16-2) is a high-chromium, low-nickel, hardenable martensitic stainless steel.

Among the standard 400-series martensitic grades (like 410 and 420), Type 431 stands out because its nickel addition ($1.50\%\text{ to }2.50\%$) drastically improves its impact toughness and torque strength. Furthermore, its higher chromium content ($15.00\%\text{ to }17.00\%$) gives it the highest corrosion resistance of any conventional, heat-treatable martensitic stainless steel—approaching the baseline performance of 304 austenitic steel in many mild environments.

1. Chemical Composition Cross-Reference (Weight %)

The strict coordination between the American (AISI) and European (EN/DIN) standards ensures a highly clean chemistry designed to minimize delta-ferrite retention, which can otherwise compromise toughness.

2. Mechanical Properties (Typical Condition QT 800 & QT 900)

European standards typically mandate delivery of 1.4057 round bars in a specific Quenched and Tempered (QT) condition. The two most common industrial heat-treat designations are QT 800 and QT 900 (where the number designates the minimum tensile strength in MPa):

3. Metallurgical Realities & Heat Treatment

The Dual-Phase Risk (Delta Ferrite)

Because Chromium is a strong ferrite-stabilizer and Nickel is an austenite-stabilizer, any chemical imbalance or improper heating can trap pockets of Delta Ferrite within the hardened martensite matrix. Delta ferrite aligns in stringers along the rolling direction of round bars, introducing directional weaknesses and reducing transverse impact toughness. Premium mills use tight chemical control to guarantee a virtually 100% martensitic matrix post-quench.

Heat Treatment Parameters

• Austenitizing: Heated slowly to $980^\circ\text{C} - 1030^\circ\text{C}$ ($1796^\circ\text{F} - 1886^\circ\text{F}$), followed by an oil or air quench.

• Tempering: Typically carried out in two windows:

  • For Maximum Hardness/Strength: Temper at $204^\circ\text{C} - 300^\circ\text{C}$ to achieve up to 44 HRC.

  • For Structural Toughness (QT Conditions): Temper at $600^\circ\text{C} - 650^\circ\text{C}$.

• Temper Embrittlement Warning: Just like 410 and 420, never temper 431 between $425^\circ\text{C}$ and $550^\circ\text{C}$. This causes severe interstitial precipitation that destroys the alloy's corrosion resistance and impact properties.

4. Fabrication Constraints

• Machinability: In the annealed state, it rates around 40% of AISI 1212. It is tough and highly prone to galling or tearing against the tool edge. It produces long, stringy chips; rigid tooling configurations, constant feeds, and chlorinated sulfurized lubricants are heavily recommended.

• Welding: High air-hardenability makes welding Type 431 exceptionally difficult. If structural welding cannot be avoided, preheating to $200^\circ\text{C} - 300^\circ\text{C}$ is mandatory, followed immediately by a post-weld sub-critical anneal at $650^\circ\text{C}$ before the component cools to room temperature.

5. Procurement & International Compliance Standards

When reviewing mill test certificates (MTRs) or drawing blocks for 1.4057 / 431 bars, compliance usually verifies against:

Typical Applications

Due to its unique positioning as a high-strength, highly corrosion-resistant martensitic steel, 1.4057 round bars are heavily utilized in marine and aggressive industrial environments:

• Marine Architecture: Propeller shafts, drive shafts, and high-load marine fasteners/bolts.

• Aerospace Engineering: Aircraft structural fasteners, bomb racks, and valve components.

• Chemical & Pump Industries: Heavy-duty pump rotors, mixer shafts, valve stems, and piston rods handling mildly corrosive chemical media.