SUJ2 vs SUS440C: Which Steel for Wear-Resistant Precision Parts?

SUJ2 and SUS440C can both be hardened and used where wear matters, but they solve different design problems. SUJ2 is a high-carbon chromium bearing steel developed around rolling-contact performance and contact-fatigue resistance. SUS440C is a high-carbon martensitic stainless steel selected when hardness and wear resistance must be balanced with better corrosion resistance than conventional bearing steel. The choice should begin with the failure mode, service environment, geometry, heat-treatment route and verification plan. A machining supplier should not substitute one grade for the other without design approval.

SUJ2 is commonly considered for bearing rings, rolling elements, pins and machine parts under repeated contact in controlled, lubricated conditions. JIS G 4805:2025 is the current Japanese standard for high-carbon chromium bearing steels, but the purchaser must still define product form, size, delivery condition and applicable quality requirements. Steel cleanliness, inclusions, decarburization, heat-treated structure and surface finish may matter more to contact fatigue than a single hardness value. SUJ2 is not stainless; moisture, wash fluids and storage conditions require a suitable corrosion-control plan.

SUS440C is a hardenable martensitic stainless grade used for bearings, shafts, valves, tools and precision components needing better corrosion resistance than SUJ2. Stainless does not mean immune to attack: heat-treated condition, roughness, crevices, iron contamination, chlorides and cleaning practice all affect performance. Steelmaker hardness data are grade-specific reference values, not a guarantee for every form or section. State the required hardness or mechanical property, test method and test location in the RFQ.

A typical route may include stock preparation, rough turning or milling, holes and grooves, controlled allowance, partner heat treatment, cleaning, grinding or hard finishing, and final inspection. Allowance cannot be selected from one universal table; it depends on size, wall thickness, section changes, datums and expected distortion. Too little stock may not remove distortion or affected surface, while too much increases grinding heat and burn risk. Functional bores and concentric features require datum planning before quotation.

Wear life also depends on rolling or sliding stress, lubrication, debris, impact, temperature, edge geometry, concentricity and finish. In a wet environment, corrosion may govern reliability; in a clean lubricated system, SUS440C's corrosion advantage may not justify its process and cost. Conversely, familiar use of SUJ2 can create preservation risk. Rank the actual failure mechanisms before comparing grade, heat treatment, coating and maintenance.

Grade name does not define final microstructure. Austenitizing, quench medium, tempering, optional sub-zero treatment and section size affect hardness, retained austenite, stability and distortion. VietTech machines and inspects within confirmed scope; specialist heat treatment is coordinated with qualified partners. Final dimensions, roundness, cylindricity, runout and finish should be checked after the final treatment and finishing state. Hardness scale, location and surface preparation must be specified, and material, heat-treatment and dimensional records must retain lot linkage.

Provide matching 2D and 3D revisions, material standard, product form, delivery condition, loaded surfaces, rolling or sliding contact, environment, lubrication, hardness, heat-treatment records, final grinding surfaces, tolerances and inspection documentation. State quantity and traceability level. If equivalents are allowed, define the approval route rather than leaving substitution to the machine shop.

• Grade, standard, form and delivery condition
• Load, environment, lubrication and failure mode
• Hardness and heat-treatment certification
• Post-treatment tolerance, finish and inspection
• Lot traceability and substitution approval

This article references JIS G 4805:2025, JIS G 4303:2021 and official technical information from JFE, JTEKT/Koyo, Daido Steel and Aichi Steel. Standard scope varies by product form. Typical composition or property tables do not replace a batch certificate or contractual specification; final material approval remains a design responsibility.