A manufacturer's technical comparison of two JIS G4053 chromium-molybdenum carburizing steels — covering carbon content, case depth, core toughness, hardenability, and application selection guidance based on Jiangsu Liangyi's forging experience.
The JIS G4053 standard — published by the Japanese Industrial Standards Committee — defines a complete family of low-alloy Cr-Mo steels engineered for case-hardening. Case-hardening, also called carburizing and quenching, produces a hard, wear-resistant surface over a tough, ductile core. This dual-zone structure is the engineering foundation of virtually every gear, pinion shaft, and transmission component designed for both surface wear resistance and cyclic impact tolerance.
The naming convention is straightforward: SCM = Steel, Chromium, Molybdenum. The three-digit suffix encodes the nominal carbon content in hundredths of a percent. SCM415 nominally targets 0.15% carbon; SCM420 nominally targets 0.20% carbon. The chromium range (0.90–1.20%) and molybdenum range (0.15–0.35%) are chemically identical between the two grades.
Most engineers ask "which grade is harder?" The more precise engineering question is: "Which grade gives the right core hardness for my section thickness and load case?" Surface hardness after carburizing is nearly identical. The carbon difference only controls what happens at the core — and that is where structural failures initiate in over-loaded components.
| Element | SCM415 (wt%) | SCM420 (wt%) | Engineering Function |
|---|---|---|---|
| Carbon (C) | 0.13 – 0.18% | 0.18 – 0.23% | Determines core hardness and hardenability after quenching — the key differentiator |
| Silicon (Si) | 0.15 – 0.35% | 0.15 – 0.35% | Deoxidizer; minor contribution to solid-solution strengthening |
| Manganese (Mn) | 0.60 – 0.90% | 0.60 – 0.90% | Hardenability improvement; neutralizes sulfur embrittlement |
| Phosphorus (P) | ≤ 0.030% | ≤ 0.030% | Tightly controlled — grain boundary embrittlement if elevated |
| Sulfur (S) | ≤ 0.030% | ≤ 0.030% | Controlled — reduces transverse impact toughness if high |
| Chromium (Cr) | 0.90 – 1.20% | 0.90 – 1.20% | Surface hardness after carburizing; wear resistance; mild corrosion improvement |
| Molybdenum (Mo) | 0.15 – 0.35% | 0.15 – 0.35% | Hardenability depth (shifts Jominy curve right); prevents temper embrittlement |
Source: JIS G4053 Low-Alloy Steels for Machine Structural Use. Values are specified ranges — actual heat chemistry is confirmed in MTC EN 10204 3.1 documentation.
Gas carburizing exposes the forged component to a carbon-rich atmosphere (typically endothermic gas or nitrogen-methanol at 880–950°C) for 4–20 hours, diffusing carbon into the surface layer until the surface carbon concentration reaches 0.80–0.90%. Both SCM415 and SCM420 respond identically to this surface enrichment — the carburizing atmosphere sets the surface carbon regardless of base carbon content.
After carburizing and oil quenching, the surface of both grades converts to martensite with high carbon content, producing HRC 58–63. The interior, which remains at the base carbon level (0.13–0.18% for SCM415, 0.18–0.23% for SCM420), also transforms to martensite — but the martensite hardness and brittleness scale directly with carbon content.
SCM415 core after quench: Lower-carbon martensite at the core. Higher ductility. Charpy impact 80–120 J at 20°C. Better resistance to brittle fracture under sudden shock loading. The softer core acts as an energy-absorbing buffer between the hard case and the component center.
SCM420 core after quench: Slightly higher-carbon martensite. Core UTS reaches 950–1,200 MPa vs 850–1,050 MPa for SCM415. Yield strength 750–1,000 MPa vs 650–850 MPa. This additional core strength is the design margin that prevents tooth-root bending fatigue fracture in large-module gear sections.
SCM420's higher base carbon creates a more gradual carbon gradient from the surface (0.85% C) to the core (0.20% C) compared with SCM415 (0.85% → 0.15%). This shallower gradient reduces the stress concentration at the case-core interface under cyclic loading — a meaningful advantage in heavy-duty gearbox applications where fatigue cracks tend to initiate at this boundary.
Based on our forging production experience: for effective section thickness below 30 mm, SCM415 provides sufficient core strength while maximizing ductility. Above 30 mm, where through-hardenability governs core microstructure, evaluate SCM420. Above 40 mm, if SCM415 is specified on your drawing, request a DFM review — the grade may be under-specified for core load requirements at this section size.
The following typical mechanical properties are measured on forged, carburized, quenched and tempered test specimens at a 150–200°C temper temperature. Values represent the range observed across production orders at Jiangsu Liangyi. Actual values depend on section size, carburizing cycle parameters, quench severity, and tempering temperature. All final values are confirmed in the Mill Test Certificate (MTC EN 10204 3.1) provided with every shipment.
| Property | SCM415 (typical) | SCM420 (typical) | Unit | Winner for… |
|---|---|---|---|---|
| Surface Hardness (case) | HRC 58 – 62 | HRC 58 – 63 | Rockwell C | SCM420 (marginally) |
| Core Hardness | HRC 25 – 35 | HRC 30 – 42 | Rockwell C | SCM415 (softer = tougher) |
| Core Tensile Strength (UTS) | 850 – 1,050 | 950 – 1,200 | MPa | SCM420 (higher load capacity) |
| Core Yield Strength (0.2% proof) | 650 – 850 | 750 – 1,000 | MPa | SCM420 (higher static load) |
| Elongation (core, gauge length 5d) | 14 – 18% | 12 – 16% | % | SCM415 (more ductile core) |
| Charpy Impact (core, V-notch, 20°C) | 80 – 120 J | 60 – 100 J | Joules | SCM415 (shock absorption) |
| Effective Case Depth (HV 550 criterion) | 0.5 – 1.5 | 0.8 – 2.0 | mm | SCM420 (deeper case possible) |
| Pre-hardened Hardness (normalized) | HB 170 – 210 | HB 190 – 230 | Brinell HB | SCM415 (easier pre-carburize machining) |
| Carburizing Temperature Range | 880 – 940°C | 880 – 950°C | °C | Equivalent |
↑ Blue cells indicate SCM415 advantage. Green cells indicate SCM420 advantage. Neither grade is universally superior — the "winner" depends on which property controls your application's failure mode.
Relative performance of each grade across six key engineering criteria, normalized to a 0–100% scale. These bars show direction of advantage — not absolute values. Consult Section 4 tables for actual measured properties.
In our DFM review process, we regularly identify cases where SCM415 is specified on drawings where section thickness exceeds 40 mm, and we recommend upgrading to SCM420 to meet core load requirements. This review is included at no additional charge with every quotation request.
Submit your drawing for a free DFM review and grade confirmation within 48 business hours.
Both SCM415 and SCM420 are forged in the austenitic phase, with initial reduction between 1,100°C and 1,250°C and finish forging above 850°C to maintain refined grain size. Our standard forging ratio for Cr-Mo gear and shaft components is a minimum of 4:1 — sufficient to close porosity, refine the cast grain structure, and establish the directional fiber flow that improves fatigue strength versus machined bar stock. The same hydraulic presses (2,000 T to 6,300 T), the same tooling design workflow, and the same post-forge cooling programs apply to both grades without modification.
Post-forging, both grades receive identical normalizing (880–920°C, air cool) to relieve forging stress and homogenize grain structure. The carburizing cycle that follows is where the two grades require differentiated process control.
For SCM420 in thin-wall geometries (wall thickness under 20 mm), the carburizing atmosphere carbon potential must be actively managed to prevent case depth overrun — a condition where the carburized zone extends so deep that the ductile core layer disappears entirely. When the soft core is eliminated, the component becomes uniformly hard but brittle through the section, losing its shock-absorption capability. Our heat treatment team monitors carbon potential continuously with in-situ oxygen probes throughout the carburizing cycle for SCM420 thin-wall components.
Both grades are oil-quenched from carburizing temperature (820–870°C for direct quench, or re-austenitized at 820–850°C after sub-critical anneal in a double-quench sequence). SCM415's lower base carbon gives it a slightly lower hardenability index, meaning quench severity requirements are marginally less aggressive than SCM420 for equivalent core hardness targets. For dimensionally sensitive components — long shafts, thin-walled rings — we recommend marquenching (stepped quench into a bath at 150–200°C) for both grades to minimize distortion from thermal gradients.
Our standard NDT protocol for carburized Cr-Mo forgings is identical regardless of whether the material is SCM415 or SCM420: 100% ultrasonic testing per ASTM A388 or EN 10228-3 for internal integrity detection; magnetic particle testing per ASTM E1444 for surface and near-surface discontinuities; dimensional inspection per drawing; and hardness verification at case and core by destructive coupon. Third-party witness inspection is available on request — please specify your preferred inspection body when submitting your RFQ.
| Standard (Country) | SCM415 Equivalent | SCM420 Equivalent | Key Note |
|---|---|---|---|
| JIS G4053 (Japan) | SCM415 | SCM420 | Reference standard; this article |
| AISI / SAE (USA) | SAE 5115 / AISI 4118 | SAE 4120 / AISI 4120 | Check Cr range — slight difference in upper limit |
| DIN (Germany) | 15CrMo5 | 20CrMo5 | Direct carbon-based naming; close match |
| EN / Material No. (Europe) | 1.7264 / 15CrMo5 | 1.7208 / 20CrMo4 | Verify Mn range vs JIS specification |
| GB (China) | 15CrMo | 20CrMo | Widely used in domestic Chinese procurement |
| BS (United Kingdom) | 805A15 / 805H15 | 805A20 / 805H20 | Legacy BS designations; largely superseded by EN |
When your engineering drawing references a DIN 20CrMo5 or SAE 4120 specification and manufacturing is to be conducted to JIS standards, our engineering team confirms the mapping during technical review and notes any composition differences in the MTC. For procurement where compliance to a specific standard is required, please specify the target standard explicitly in your RFQ so our team can confirm feasibility before accepting the order.
For non-critical, thin-section components, many manufacturers treat them as interchangeable without issue. For components where core strength under cyclic load governs fatigue life — particularly sections above 25 mm effective thickness — the distinction matters. Upgrading from SCM415 to SCM420 typically adds a small material cost increment while providing meaningful additional core load margin — your supplier can confirm the difference for your specific order.
At Jiangsu Liangyi, both grades are maintained as standard inventory raw materials. Production lead times are identical — both are available from the same ingot stock pool. For orders above 10,000 kg, we can procure specific certified heat numbers on request to support third-party witnessed incoming material inspection.
For light-to-medium industrial gear shafts in the module range 3–6, the typical effective case depth target is 0.6–1.2 mm at HV 550. This falls comfortably within SCM415's achievable range. For module 8 and above, where tooth root sections exceed 30 mm, we typically recommend case depths of 1.2–2.0 mm — this is where SCM420 becomes advantageous in carburizing cycle efficiency.
Both grades have low carbon equivalent (CE) values and are technically weldable in the non-carburized condition. In practice, however, carburized components are almost never welded — welding after case hardening would destroy the case microstructure locally. All joining features, cross-holes, and attachment points should be finalized before carburizing.
Our MOQ is one piece for standard material grades and sizes. For unusual geometries, large section sizes, or materials requiring special heat certification, MOQ is negotiable. Submit your drawing to our team for a specific MOQ and lead time quotation at no charge.
Jiangsu Liangyi operates in-house heat treatment facilities for gas carburizing, oil quenching, and tempering. Please confirm specific heat treatment scope and sub-process details directly with our engineering team when submitting your drawing for quotation.
The "H" suffix designates the H-steel specification — a narrow hardenability band guarantee verified by Jominy end-quench testing. SCM415H and SCM420H are used in volume production of automotive gears where batch-to-batch consistency of case depth and core hardness is critical. For custom open die forgings and ring rolling, the standard grade (without H suffix) is normally specified and hardenability is confirmed by the carburizing trial. Contact our engineering team if H-grade specification is required.
For wind turbine planetary ring gears, where section thickness typically exceeds 50 mm and the core must sustain high bending fatigue load cycles over 20-year design life, SCM420 is the standard industry choice. The higher core UTS (950–1,200 MPa) and deeper achievable case depth (0.8–2.0 mm) are compatible with typical design requirements for this application class. Our engineering team can provide application-specific grade confirmation and heat treatment parameter recommendations for wind energy gear components.
SCM415 and SCM420 are not competing grades — they are two calibrated positions on a single design axis. The axis is core carbon content, and the variable it controls is the balance between core ductility and core strength after quenching. Surface performance is essentially equivalent between the two grades once carburizing brings both surfaces to the same carbon concentration.
Choose SCM415 when section thickness is under 30 mm, when the primary failure risk is shock fracture or high-cycle fatigue from impact loading, and when post-heat-treatment dimensional stability is a priority. Elongation of 14–18% and Charpy impact of 80–120 J make SCM415 the standard choice for light-to-medium duty carburized components worldwide — automotive transmission gears, differential pinions, light industrial gearbox shafts.
Choose SCM420 when section thickness exceeds 30 mm, when the core must carry significant bending or contact stress under combined loading, when you need case depth above 1.5 mm without extended carburizing cycles, or when you are replacing a SAE 4120 or 20CrMo specification in a JIS-standard procurement. Core UTS of 950–1,200 MPa and deeper case depth make SCM420 the correct engineering choice for heavy-duty ring gears, large output shafts, and mining equipment drivetrain components.
If you are uncertain, the correct next step is not to guess — it is to submit your drawing for a DFM review. Our engineering team reviews every order for grade fit, forging sequence, and heat treatment strategy before any raw material is committed. For full product specifications, dimensional ranges, and application data, see our custom SCM415 forging parts page — DFM review and quotation are included at no charge.