SUS440A Stainless Steel: Low-Carbon High-Carbon Martensitic Grade for Balanced Hardness & Toughness

SUS440A is the low-carbon variant in the SUS440 series (C=0.60-0.75%, Cr=16.0-18.0%), which balances hardness, toughness and wear resistance. After heat treatment, the hardness can reach 55-57HRC, which is lower than SUS440B/C but has better toughness. It is suitable for utility tools and components requiring both wear resistance and impact resistance.

Core Parameters

Chemical Composition (wt%): C=0.60-0.75, Si≤1.00, Mn≤1.00, P≤0.040, S≤0.030, Cr=16.00-18.00, Fe=Balance

Mechanical Properties (Quenched & Tempered): Tensile Strength ≥795MPa, Yield Strength ≥550MPa, Elongation ≥12%, Hardness 55-57HRC

Service Temperature: -20℃ to 250℃ (continuous service)

Equivalent Grades: EN 1.4104, UNS S44002, GB/T 7Cr17

Performance Advantages: Balanced hardness and toughness, not easy to chip or crack under small impact loads; good wear resistance and edge retention; easier heat treatment and machining than SUS440B/C; moderate corrosion resistance in dry environments; can be polished to a mirror finish.

Typical Applications: Household high-grade kitchen knives, scissors, utility knives, bearing rings and balls for low-speed and medium-load, valve cores for medium-pressure systems, precision mold cores for general wear requirements, textile machine needles, measuring tools.

Practical Q&A

Q1: What is the optimal heat treatment process for SUS440A? A1: Quench at 1010-1050℃, oil cool + temper at 180-220℃, air cool. This process can stably achieve hardness of 55-57HRC, and the impact toughness is ≥20J, which is the best balance state of wear resistance and toughness. If higher toughness is needed, temper at 300-350℃, but the hardness will decrease to 48-52HRC.

Q2: How does SUS440A compare with SUS420J2 in performance? A2: SUS440A has higher carbon and chromium content, so its hardness (55-57HRC) and wear resistance are better than SUS420J2 (52-57HRC); the corrosion resistance of SUS440A is also better due to higher chromium content. SUS440A is suitable for high-grade tools, while SUS420J2 is more cost-effective for general wear-resistant components.

Q3: Is SUS440A suitable for food contact tools? A3: Yes. After heat treatment and passivation, the surface oxide film is dense and stable, meeting food safety standards (such as FDA 21 CFR 175.300). It should be noted that it should not be soaked in acidic detergents for a long time to avoid damaging the passivation film.

Q4: What are the precautions for polishing SUS440A? A4: Adopt dry polishing process with abrasive belts of increasing fineness (120#→240#→400#→600#), then use polishing paste for fine polishing; during polishing, control the polishing pressure to avoid overheating and material hardening; after polishing, clean the surface thoroughly and perform passivation treatment.

Q5: What is the service life of SUS440A in practical applications? A5: In general wear environments (such as textile machine needles), its service life is 2-3 times that of SUS420J2; in high-load wear environments, its service life is shorter than SUS440B/C, and it is recommended to replace it with SUS440B/C.