SUS420J1 Stainless Steel: Medium-Carbon Martensitic Grade for Cutting Tools and Blades

SUS420J1: A Medium-Carbon Martensitic Stainless Steel

SUS420J1 is a martensitic stainless steel defined in JIS G4303, with equivalents EN 1.4021 (X20Cr13), UNS S42000 and GB 20Cr13. The martensitic family is characterised by a body-centred tetragonal structure formed by quenching from the austenite range; this structure is hard and strong but has limited ductility. SUS420J1 sits between the low-carbon 410 type and the higher-carbon 440 types: it offers a practical balance of hardness, edge retention and corrosion resistance for cutting tools, blades and general engineering wear parts.

Because martensitic grades harden by heat treatment, the final properties of SUS420J1 depend almost entirely on the quenching and tempering cycle applied. In the annealed condition the steel is soft enough for machining and forming; after hardening and tempering it reaches the hardness range used in knife blades, shears, valve components and pump shafts.

Chemical Composition per JIS G4303

JIS G4303 specifies the following composition limits for SUS420J1 bars. Single values are maximums unless shown as a range.

Element	SUS420J1
Carbon	0.16-0.25
Silicon	1.00 max
Manganese	1.00 max
Phosphorus	0.040 max
Sulfur	0.030 max
Chromium	12.00-14.00

The carbon content is the key difference from the low-carbon 410/410S types: more carbon means higher achievable hardness, but also lower weldability and ductility. The 12-14 % chromium provides corrosion resistance in mild environments, fresh water, steam and many food-contact situations, though it is far below the resistance of austenitic grades in chlorides.

Heat Treatment and Mechanical Properties

The typical hardening sequence is austenitising at about 950-1010 °C, oil or polymer quenching, then tempering. Tempering temperature controls the final hardness: low-temperature tempering around 150-300 °C gives maximum hardness with reduced toughness, while tempering at 600-750 °C gives lower hardness with improved toughness. In the hardened and tempered condition the grade typically delivers a tensile strength of about 725 MPa or more, a yield strength of about 520 MPa or more, elongation of at least 15 %, and hardness in the range of approximately 48-53 HRC when tempered at low temperature.

Condition	Typical value
Tensile strength	725 MPa min (hardened and tempered)
Yield strength	520 MPa min (hardened and tempered)
Elongation	15 % min
Hardness	Approx. 48-53 HRC (low-temperature temper)
Annealed hardness	Up to about 223 HBW

Recommended service temperature is roughly -20 °C to 300 °C. Above about 300-400 °C the hardness falls off rapidly as over-tempering occurs, so the grade is not intended for elevated-temperature strength applications.

Corrosion Resistance and Limitations

In the hardened and tempered condition SUS420J1 resists atmospheric corrosion, fresh water, food juices and mildly acidic or alkaline media. It is not suitable for seawater, strong acids or environments with significant chloride content, where pitting and rusting occur quickly. For decorative or high-polish applications the grade is rarely used; chromium-oxide passivation is less robust than in austenitic grades, and the surface can discolour. Regular cleaning and drying extend the life of blades and tools in service.

Applications: Cutting Tools and General Engineering

The main uses of SUS420J1 are general cutting tools, kitchen and pocket knife blades, scissors, shears, surgical and dental instruments, valve seats and spindles, pump shafts, fasteners and mould components that need moderate corrosion resistance combined with wear resistance. For applications requiring higher hardness, the higher-carbon 440-type grades are preferred; where weldability and toughness matter more than hardness, the low-carbon 410L or 410 types are better choices.

FAQ

Q1. What is the difference between SUS420J1 and SUS420J2? SUS420J2 (EN 1.4028) allows higher carbon, 0.26-0.40 %, giving higher achievable hardness at the cost of reduced ductility and weldability. J1 is chosen when a balance of hardness and toughness is needed.

Q2. Can SUS420J1 be welded? Welding is difficult because the martensitic transformation creates hard, crack-sensitive heat-affected zones. If welding is unavoidable, preheat and post-weld tempering are required, and the weld zone will not match the parent hardness.

Q3. Why does the grade need heat treatment before use? In the annealed condition the steel is soft and does not have useful edge-holding ability. Hardening and tempering develop the martensitic structure and the 48-53 HRC hardness that cutting tools rely on.

Q4. Is SUS420J1 food safe? It is widely used for food-contact blades and cutlery, and it complies with typical food-contact requirements when the surface is clean and passivated. It is not a substitute for austenitic grades in corrosive food-processing environments.

Q5. What are the equivalents of SUS420J1? The main equivalents are EN 1.4021 (X20Cr13), UNS S42000 and GB 20Cr13. Properties may differ slightly between specifications, so the governing standard should be confirmed for the product form and application.