Comparison Of 440 And 431 Stainless Steel: High-Carbon Martensitic Vs Nickel-Added Martensitic Grade

440 (general designation of 440A/B/C) and 431 are high-performance martensitic stainless steels, with the core difference being alloying elements and performance orientation. 440 is high-carbon (C=0.60-1.20%), focusing on hardness and wear resistance; 431 adds nickel (Ni=1.20-2.00%), focusing on high strength and toughness. Both are suitable for high-performance mechanical components, with clear differences in impact load and wear resistance requirements.

Core Parameter Comparison

Parameter	440 Stainless Steel	431 Stainless Steel
Chemical Composition (wt%)	C=0.60-1.20, Si≤1.00, Mn≤1.00, P≤0.040, S≤0.030, Cr=16.00-18.00, Fe=Balance	C=0.15-0.25, Si≤1.00, Mn≤1.00, P≤0.040, S≤0.030, Cr=15.00-17.00, Ni=1.20-2.00, Fe=Balance
Mechanical Properties (Quenched & Tempered)	Tensile Strength ≥795-950MPa, Yield Strength ≥550-700MPa, Elongation ≥10-12%, Hardness 55-62HRC	Tensile Strength ≥1080MPa, Yield Strength ≥880MPa, Elongation ≥12%, Hardness 38-43HRC
Service Temperature	-20℃ to 250℃ (continuous service)	-40℃ to 450℃ (continuous service)
Equivalent Grades	SUS440A/B/C (JIS), EN 1.4104/1.4109/1.4125, UNS S44002/S44003/S44004	SUS431 (JIS), EN 1.4057, UNS S43100

Key Performance Differences: 1. Hardness & wear resistance: 440's high carbon content leads to ultra-high hardness (55-62HRC) and excellent wear resistance; 431's medium carbon content and nickel addition result in lower hardness (38-43HRC) and poor wear resistance. 2. Strength & toughness: 431 has higher tensile strength (≥1080MPa) and excellent toughness (impact toughness ≥50J at -40℃), suitable for impact loads; 440's toughness is poor (impact toughness ≤15-20J), prone to brittle fracture. 3. Corrosion resistance: 431's nickel addition improves corrosion resistance, better than 440 in atmospheric and weak corrosion environments; 440 is only suitable for dry environments. 4. High-temperature performance: 431's service temperature (450℃) is 200℃ higher than 440 (250℃).

Applicable Scenario Distinction: 440 is suitable for components requiring ultra-high wear resistance and low impact load, such as high-end cutting tools, bearing balls and raceways, precision mold cores, surgical scalpels, and measuring tools. 431 is suitable for components requiring high strength, high toughness and corrosion resistance, such as automotive drive shafts, suspension components, high-pressure valve bodies, marine machinery parts, and aircraft engine accessories.

Practical Q&A

Q1: What is the role of nickel in 431? A1: Nickel is a strong austenite stabilizer, which refines the martensite structure during heat treatment and improves toughness; it enhances the stability of the surface passivation film and improves corrosion resistance; 431's toughness and corrosion resistance are significantly better than nickel-free martensitic stainless steels.

Q2: Why is 440 not suitable for impact load components? A2: Its ultra-high carbon content leads to low toughness; under impact load, it is prone to chipping or cracking; for example, 440C kitchen knives are sharp but easy to chip if dropped, while 431 components can withstand certain impact without damage.

Q3: Can 431 be used in marine environments? A3: Yes. Its corrosion resistance in marine atmospheric environments is better than 410, 420 and 440; it is suitable for marine mechanical components such as ship hull accessories and pump shafts; for components immersed in seawater for a long time, 316L is recommended.

Q4: What heat treatment process is suitable for 440 to achieve ultra-high hardness? A4: Quench at 1010-1090℃, oil cool + temper at 150-220℃, air cool; different sub-grades have slightly different temperatures: 440A (1010-1050℃ quenching), 440C (1050-1090℃ quenching); avoid tempering at 300-450℃ to prevent temper brittleness.

Q5: How to select between 440 and 431? A5: Choose 440 if ultra-high wear resistance is the core requirement and impact load is small; choose 431 if high strength, high toughness and corrosion resistance are required and impact load is present.