Comparison Of 420 (Medium-Carbon Martensitic) And 440 (High-Carbon Martensitic) Stainless Steel

420 (medium-carbon) and 440 (high-carbon) are both martensitic stainless steels, with the core difference being carbon content (420: 0.16-0.35%, 440: 0.60-1.20%). This difference leads to significant gaps in hardness, wear resistance and toughness, making them suitable for different wear resistance and strength requirements.

Core Parameter Comparison

Parameter	420 Stainless Steel	440 Stainless Steel
Chemical Composition (wt%)	C=0.16-0.35, Cr=12.00-14.00, Fe=Balance	C=0.60-1.20, Cr=16.00-18.00, Fe=Balance
Mechanical Properties (Quenched & Tempered)	Tensile Strength ≥725-820MPa, Yield Strength ≥520-620MPa, Elongation ≥12-15%, Hardness 48-57HRC	Tensile Strength ≥795-950MPa, Yield Strength ≥550-700MPa, Elongation ≥10-12%, Hardness 55-62HRC
Service Temperature	-20℃ to 300℃	-20℃ to 250℃
Equivalent Grades	SUS420J1/J2 (JIS), EN 1.4021/1.4028, UNS S42000/S42020	SUS440A/B/C (JIS), EN 1.4104/1.4109/1.4125, UNS S44002/S44003/S44004

Key Performance Differences: 1. Hardness & wear resistance: 440's high carbon content leads to higher hardness (55-62HRC) and excellent wear resistance, 1.5-2 times that of 420 (48-57HRC); 420's wear resistance is suitable for general scenarios. 2. Toughness: 420 has better toughness (impact toughness ≥15-25J) than 440 (≥10-20J), not easy to brittle fracture. 3. Corrosion resistance: 440's higher chromium content (16-18%) makes its corrosion resistance slightly better than 420 (12-14%); both are prone to rust in humid environments. 4. Machinability: 420 is easier to machine in annealed state, suitable for mass production; 440's high carbon content increases cutting resistance. 5. Cost: 440 is 20-30% more expensive than 420.

Applicable Scenario Distinction: 420 is suitable for general wear-resistant components, such as household kitchen knives, scissors, general plastic mold cores, valve stems, fasteners, and textile machine parts. 440 is suitable for high-wear components, such as high-end cutting tools, precision bearings, ultra-precision molds, surgical scalpels, measuring tools, and high-end hardware accessories.

Practical Q&A

Q1: Why is 420 widely used in mass-produced hardware tools? A1: It has good hardenability, can achieve 48-57HRC hardness after simple heat treatment; easy to machine and polish, suitable for mass production; low cost, cost-effective for general wear-resistant tools.

Q2: What is the wear resistance difference between 420 and 440 in practical applications? A2: In textile machine needle applications, 440's service life is 2-3 times that of 420; in kitchen knife applications, 440 can maintain sharpness for 6-12 months, while 420 needs sharpening every 2-3 months.

Q3: Can 420 be used for precision molds? A3: Yes, but only for general precision plastic molds; its dimensional stability after heat treatment is worse than 440; for ultra-precision molds (dimensional tolerance ≤0.001mm), 440C is recommended.

Q4: What welding precautions are there for 420 and 440? A4: Both need preheating before welding: 420 requires preheating to 200-300℃, and 440 requires preheating to 300-400℃ to avoid cold cracking. Use matching martensitic stainless steel welding wires (such as ER420 for 420, ER440 for 440), control low heat input to prevent grain coarsening. Post-weld tempering at 200-300℃ is recommended to eliminate residual stress and improve weld toughness.

Q5: Can 440 replace 420 in all wear-resistant scenarios? A5: No. Although 440 has better wear resistance, its poor toughness and higher cost limit its application. For components that bear slight impact while requiring wear resistance (such as ordinary scissors, valve stems), 420 is more cost-effective; 440 is only suitable for low-impact, high-wear scenarios.