SCM415 vs SCM420: Selecting Carburizing Steel for CNC Parts

SCM415 and SCM420 are chromium–molybdenum low-alloy steels for machine structural use under JIS G 4053. They are commonly considered for gears, shafts, pins, cams, bushings and other components that need a wear-resistant case with a load-bearing core. Parts may be machined in a suitable supplied condition and then carburized, quenched and tempered to an approved route. Daido Steel's published product data lists 0.13–0.18% carbon for SCM415 and 0.18–0.23% for SCM420. That difference can influence the heat-treated core response, but it is not a complete selection rule. Section size, hardenability, stock condition, required case depth, service load and distortion allowance must be reviewed together.

Carburizing enriches the surface with carbon before quenching, enabling a harder wear-resistant case than the core. A drawing should therefore define more than surface hardness. State the treatment, effective or total case-depth definition, test location, surface-hardness range, core requirement and areas to be protected from carburizing. An excessively shallow case may be lost during grinding; an unnecessarily deep case adds time and can increase distortion risk. Threads, narrow grooves, sharp corners and thin sections may respond differently from heavy sections. If a sectioned coupon is required, define its quantity, representative location and evaluation method. A surface HRC reading alone does not demonstrate case depth or core condition.

A possible route is stock preparation, rough machining, stress relief where needed, semi-finishing, carburizing–quenching–tempering, cleaning, controlled straightening and grinding or final machining of functional surfaces. This is only a framework. Before heat treatment, distribute allowance and avoid abrupt section changes where design permits. Post-heat-treatment datums need enough stock to restore geometry without removing the required effective case. Threads, keyways and no-case surfaces may require protection or later machining under the heat treater's qualified procedure. VietTech reviews datums, allowance and inspection from the drawing; specialist carburizing is coordinated with a suitable partner.

Bending, ovality, flatness and runout change depend on geometry, mass distribution, residual stress, furnace loading and quench conditions. A long shaft behaves differently from a thin gear or uneven bushing. Grinding stock must be sufficient to restore size and geometry while preserving case depth. For new or tight-tolerance parts, a pilot lot measured before and after treatment is the most defensible way to establish distortion and allowance. Drawings should identify post-treatment runout, cylindricity or flatness, the measurement datum, support condition and final surface state.

Before treatment, machinability depends on supplied hardness, microstructure, product form and surface condition. Cutting data must follow the actual tool, machine, material and setup rather than copied SCM440 values. After treatment, cylindrical, surface or internal grinding is selected by geometry and retained case requirements. Dimensional inspection should follow cleaning and thermal stabilization. Surface hardness, core hardness and case depth are different evaluations and may require separate equipment and specimens. A CMM supports positional and geometric checks but does not automatically replace roundness equipment, indicators, gauges or hardness testing.

Identify SCM415 or SCM420, JIS G 4053 edition, stock form, supply condition and certificate. Submit the controlled 2D PDF and 3D model, mark carburized and protected areas, define hardness, case depth and evaluation method, and state all final dimensions and GD&T after treatment. Identify grinding surfaces, restricted removal and no-indent zones. If an equivalent is allowed, define the comparison and approval route. Include pilot quantity, repeat volume, inspection report, heat-treatment certificate, lot traceability and rust-protection packaging.

• Exact grade, JIS G 4053 edition and supply condition
• Certificate and substitution policy
• Carburized/protected areas, hardness and case depth
• Post-treatment dimensions and GD&T
• Grinding surfaces, allowance and roughness
• Quantity, coupons, reports and lot traceability

This article was checked against JIS G 4053:2026 in the Japanese Standards Association catalogue, Aichi Steel's alloy-steel product range and Daido Steel's published SCM415/SCM420 composition data. The carbon ranges are manufacturer data, not a guarantee for every source or certificate. No universal carburizing cycle, hardness, temperature, time or case depth is prescribed here. VietTech performs CNC machining, grinding and inspection within confirmed capability; specialist heat treatment is coordinated with qualified partners. Grade or heat-treatment changes require design-owner approval.