Steel SCM421 CH-CG
Finish state
JIS SCM421
CH-CG = carburized + hard finished
Carburized condition with tooth flanks ground/honed after heat treat for high-accuracy, low-noise, high-load gearing.
Values for the CH-CG condition
Other SCM421 tempers and conditions
SCM421 is listed in 8 conditions; see the grade overview at SCM421.
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 | — |
CH-CGThis page 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 CH-CG?
SCM421 CH-CG is SCM421 supplied in the CH-CG condition — carburized condition with tooth flanks ground/honed after heat treat for high-accuracy, low-noise, high-load gearing. SCM421 CH-CG has a yield strength of 950 MPa (138 ksi) and a tensile strength of 1,250 MPa (181 ksi) — stronger than 91% of steel grades. Elongation at break is 11% and the elastic modulus is 206 GPa (29.9 Msi). SCM421 CH-CG 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). A is the default condition FabDigit quotes; CH-CG is available on request or by drawing note.
Advantages
- Polishes to a fine finish — 80/100, better than 97% of steel grades
- High strength — 950 MPa (138 ksi), better than 91% of steel grades
Limitations
- Difficult to machine — 10/100, worse than 99% of steel grades
- Difficult to weld — 8/100, worse than 99% of steel grades
- Limited formability — 3/100, worse than 99% of steel grades
- Limited service temperature — 180°C (356 °F), worse than 98% of steel grades
- Limited ductility — 11%, worse than 89% of steel grades
SCM421 CH-CG properties
Typical room-temperature values for SCM421 CH-CG; 22 are specific to this condition. Other listed values are SCM421 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
SCM421 CH-CG 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 CH-CG has a yield strength of 950 MPa (138 ksi) and a tensile strength of 1,250 MPa (181 ksi) — stronger than 91% of steel grades. Elongation at break is 11% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
SCM421 CH-CG is rated for continuous service to 180°C (356 °F). It conducts heat at 41 W/m·K (23.7 BTU/hr·ft·°F), worse than 62% of steel grades. Thermal expansion is 12.3 µm/m·K (6.83 µin/in·°F).
Electrical3
SCM421 CH-CG conducts electricity at 7.5 % IACS, worse than 67% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (12/100), worse than 53% of steel grades.
Sustainability4
Producing a kilogram of SCM421 CH-CG 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 CH-CG's machinability is not recommended (10/100, worse than 99% of steel grades); weldability not recommended (8/100); formability not recommended (3/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with SCM421 CH-CG
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.
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.
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 | — |
SCM421 CH-CG — frequently asked
What is SCM421 CH-CG used for?
SCM421 CH-CG 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 CH-CG?
SCM421 CH-CG has a typical yield strength of 950 MPa (138 ksi) and a tensile strength of 1,250 MPa (181 ksi) — stronger than 91% of steel grades. Strength varies by condition: see the 8 listed tempers.
Is SCM421 CH-CG easy to machine?
Not especially — machinability is rated not recommended (10/100, worse than 99% 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 CH-CG be welded?
Not readily — weldability is rated not recommended (8/100). 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 CH-CG?
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 CH-CG 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 CH-CG?
SCM421 CH-CG is rated for continuous use to about 180°C (356 °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.
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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.