Steel SCM421
JIS SCM421
SCM421 is a JIS chromium-molybdenum carburizing steel equivalent to AISI 4320, carrying more alloy than SCM415 and SCM418 for deeper hardenability in heavier-duty gears.
Reference datasheet — SCM421 is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the A condition
SCM421 tempers and conditions
SCM421 is supplied in 8 conditions, soft to hard. A is the default FabDigit quotes; the values on this page are for it.
Compare all 8 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
ADefault Solution annealed | Standard stocked machining condition — softest, most uniform structure before carburizing or cold forming. | 350 | 560 | 25 | 170 HB | — |
Isothermally annealed | Preferred pre-carburizing condition for gears: uniform ferrite-pearlite, low distortion and consistent hobbing behaviour. | 380 | 600 | 23 | 180 HB | 7.8 % IACS |
Hot rolled | Cheapest mill condition for parts that will be rough-machined then carburized; hardness scatter is highest. | 430 | 680 | 20 | 14 HRC | 7.8 % IACS |
Normalized | Used to refine grain and homogenize forgings before carburizing, or as a moderate-strength as-supplied core condition. | 460 | 720 | 20 | 16 HRC | 7.8 % IACS |
Cold drawn | Pick for small shafts and pins needing close size and good finish straight from bar, with minimal machining. | 590 | 700 | 12 | 16 HRC | 7.8 % IACS |
Q&T core condition | Direct-hardened (non-carburized) use for shafts and fasteners where a tough medium-strength body is enough. | 780 | 950 | 15 | 30 HRC | — |
Carburized, quenched & low tempered | The service condition for gears and splines: hard wear/pitting-resistant case (0.6–1.5 mm typical) over a tough alloyed core. | 930 | 1,180 | 12 | 60 HRC | — |
Carburized + hard finished | Carburized condition with tooth flanks ground/honed after heat treat for high-accuracy, low-noise, high-load gearing. | 950 | 1,250 | 11 | 60 HRC | 7.5 % IACS |
What is SCM421?
SCM421 is a JIS chromium-molybdenum carburizing steel equivalent to AISI 4320, carrying more alloy than SCM415 and SCM418 for deeper hardenability in heavier-duty gears. Supply conditions on file include hot-rolled, annealed, isothermally annealed, normalized and cold-drawn bar, quenched and tempered to 285 HB, and case-hardened to 58-62 HRC including coarse-grain practice. Default annealed properties are 350 MPa yield, 560 MPa tensile, 25% elongation and 170 HB (87 HRB). It has no corrosion resistance and needs grinding stock left for post-carburizing distortion; SCM418 is cheaper for lighter sections. SCM421 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,425°C (2,597 °F). SCM421 has a yield strength of 350 MPa (50.8 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 74% of steel grades. Elongation at break is 25% and the elastic modulus is 206 GPa (29.9 Msi).
What is SCM421 used for?
- Heavy-duty transmission gears
- Differential ring and pinion gears
- Planetary pinions
- Splined shafts
- Axle shafts
- Bearing races
- Construction machinery gearbox gears
Advantages
- Easy to machine — 62/100, better than 89% of steel grades
- Polishes to a fine finish — 65/100, better than 78% of steel grades
- Ductile — tolerates forming and impact — 25%, better than 77% of steel grades
Limitations
- Limited service temperature — 200°C (392 °F), worse than 93% of steel grades
SCM421 properties
Typical room-temperature values for SCM421-A; bars are the position among steel grades.
Physical3
SCM421 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,425°C (2,597 °F).
Mechanical13
SCM421 has a yield strength of 350 MPa (50.8 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 74% of steel grades. Elongation at break is 25% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
SCM421 is rated for continuous service to 200°C (392 °F). It conducts heat at 42 W/m·K (24.3 BTU/hr·ft·°F), worse than 56% of steel grades. Thermal expansion is 12.3 µm/m·K (6.83 µin/in·°F).
Electrical3
SCM421 conducts electricity at 7.8 % IACS, worse than 57% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (12/100), worse than 53% of steel grades.
Sustainability4
Producing a kilogram of SCM421 takes about 30 MJ of energy and emits 2.1 kg of CO₂ — more than 56% of steel grades. Typical recycled content is 30%.
Manufacturability7
SCM421's machinability is good (62/100, better than 89% of steel grades); weldability fair (45/100); formability good (60/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. SCM421 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with SCM421
Is SCM421 easy to machine?
Machine in the annealed or isothermally annealed state (≈170–200 HB) with coated carbide; leave grinding stock for post-carburizing distortion, and expect only grinding/EDM once the case is hardened.
Can SCM421 be welded?
Weldable only before carburizing — preheat 200–300 °C, low-hydrogen consumables, and temper immediately after; never weld a carburized surface.
Can SCM421 be formed, bent or molded?
Hot forge 1150–1200 °C then normalize or isothermally anneal; cold heading and upsetting need a spheroidized structure.
SCM421 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| C | 0.17 – 0.23 | 0.2 |
| Si | 0.15 – 0.35 | 0.25 |
| Mnhigher than SCM415/418/420 — source of the deeper hardenability | 0.7 – 1 | 0.85 |
| P | ≤ 0.03 | |
| S | ≤ 0.03 | |
| Cr | 0.9 – 1.2 | 1.05 |
| Mo | 0.15 – 0.3 | 0.22 |
| Niresidual max | ≤ 0.25 | |
| Curesidual max | ≤ 0.3 | |
| Fe | balance |
What surface finishes work on SCM421?
No anodizing; protect uncoated parts with Mn/Zn phosphate, black oxide, oil or zinc plating (bake to avoid hydrogen embrittlement); shot peening after carburizing raises bending fatigue significantly.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Organic Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel SCM421 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- Similar grades
- 4320
How much does SCM421 cost?
SCM421 is a budget-cost steel ($), cheaper than about 90% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
SCM421 — frequently asked
What is SCM421 used for?
SCM421 is typically used for heavy-duty transmission gears, differential ring and pinion gears, planetary pinions, splined shafts, axle shafts and bearing races. In short: JIS 4320 equivalent; deeper case than SCM418; heavy-duty gear grade.
What is the yield strength of SCM421?
SCM421 has a typical yield strength of 350 MPa (50.8 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 74% of steel grades. Strength varies by condition: see the 8 listed tempers.
Is SCM421 easy to machine?
Reasonably — machinability is rated good (62/100, better than 89% of steel grades). Machine in the annealed or isothermally annealed state (≈170–200 HB) with coated carbide; leave grinding stock for post-carburizing distortion, and expect only grinding/EDM once the case is hardened.
Can SCM421 be welded?
With care — weldability is rated fair (45/100). Weldable only before carburizing — preheat 200–300 °C, low-hydrogen consumables, and temper immediately after; never weld a carburized surface.
What surface finishes work on SCM421?
No anodizing; protect uncoated parts with Mn/Zn phosphate, black oxide, oil or zinc plating (bake to avoid hydrogen embrittlement); shot peening after carburizing raises bending fatigue significantly.
Can SCM421 be formed, bent or molded?
Hot forge 1150–1200 °C then normalize or isothermally anneal; cold heading and upsetting need a spheroidized structure.
What is the maximum service temperature of SCM421?
SCM421 is rated for continuous use to about 200°C (392 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between SCM421-A and SCM421 CH-CG?
A is the default condition — standard stocked machining condition — softest, most uniform structure before carburizing or cold forming. CH-CG: carburized condition with tooth flanks ground/honed after heat treat for high-accuracy, low-noise, high-load gearing. Yield strength is 350 MPa in A versus 950 MPa in CH-CG.
How much does SCM421 cost?
SCM421 is a budget-cost steel ($), cheaper than about 90% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- JIS G 4053 — Low-alloyed steels for machine structural use — JSA (2016)
- GB/T 3077 — Alloy structural steels (20CrMnMo / 20CrMo comparison) — SAC (2015)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4 — Heat Treating (carburizing of Cr-Mo gear steels) — ASM International (1991)
- Machinery's Handbook (machinability ratings) — Industrial Press (2020)
- Gear steel bar datasheets (SCM420/SCM421, 20CrMnMo) — Daido Steel / Sandvik / Baosteel (2023)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.