Steel SMnB420H CARB
Heat-treated state
CARB = carburized, quenched and low-temperature tempered
Final service condition for gears and splined shafts needing a hard wear surface with a tough core.
Values for the CARB condition
Other SMnB420H tempers and conditions
SMnB420H is listed in 6 conditions; see the grade overview at SMnB420H.
Compare all 6 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
ADefault Solution annealed | Standard stocked condition for machining or cold heading before carburizing. | 355 | 560 | 26 | 170 HB | — |
Hot rolled | Cheapest mill condition for forging stock and parts that will be normalized or carburized later. | 400 | 620 | 22 | 187 HB | — |
Normalized | Use to refine forging grain and stabilise distortion before carburizing or final machining. | 430 | 640 | 22 | 190 HB | — |
Cold drawn | Choose for close-tolerance bright bar for automatic screw-machine pins and shafts. | 580 | 700 | 12 | 210 HB | — |
Quenched and tempered | Reference condition for the JIS mechanical acceptance test and for parts used without a case. | 700 | 900 | 17 | 30 HRC | — |
CARBThis page Carburized, quenched and low-temperature tempered | Final service condition for gears and splined shafts needing a hard wear surface with a tough core. | 780 | 1,000 | 12 | 60 HRC | — |
What is SMnB420H CARB?
SMnB420H CARB is SMnB420H heat treated to CARB — final service condition for gears and splined shafts needing a hard wear surface with a tough core. SMnB420H CARB has a yield strength of 780 MPa (113 ksi) and a tensile strength of 1,000 MPa (145 ksi) — stronger than 86% of steel grades. Elongation at break is 12% and the elastic modulus is 206 GPa (29.9 Msi). SMnB420H CARB 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; CARB is available on request or by drawing note.
Advantages
- High strength — 780 MPa (113 ksi), better than 86% of steel grades
- Forms and bends easily — 62/100, better than 78% of steel grades
Limitations
- Limited service temperature — 160°C (320 °F), worse than 99% of steel grades
- Difficult to machine — 12/100, worse than 99% of steel grades
- Difficult to weld — 10/100, worse than 99% of steel grades
- Limited ductility — 12%, worse than 84% of steel grades
SMnB420H CARB properties
Typical room-temperature values for SMnB420H CARB; 12 are specific to this condition. Other listed values are SMnB420H reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
SMnB420H CARB 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).
Mechanical14
SMnB420H CARB has a yield strength of 780 MPa (113 ksi) and a tensile strength of 1,000 MPa (145 ksi) — stronger than 86% of steel grades. Elongation at break is 12% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
SMnB420H CARB is rated for continuous service to 160°C (320 °F). It conducts heat at 44 W/m·K (25.4 BTU/hr·ft·°F), better than 57% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
SMnB420H CARB conducts electricity at 8.2 % IACS, better than 61% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (10/100), worse than 86% of steel grades.
Sustainability4
Producing a kilogram of SMnB420H CARB takes about 27 MJ of energy and emits 2 kg of CO₂ — less than 59% of steel grades. Typical recycled content is 30%.
Manufacturability6
SMnB420H CARB's machinability is not recommended (12/100, worse than 99% of steel grades); weldability not recommended (10/100); formability good (62/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with SMnB420H CARB
Is SMnB420H easy to machine?
Machine in the annealed/normalized state with carbide at ~150–220 m/min; the Mn-rich matrix is gummy, so use positive rake and generous coolant, and finish-grind after carburizing.
Can SMnB420H be welded?
CE ≈0.45–0.50 — weldable with low-hydrogen consumables, 150–250 °C preheat and slow cooling; never weld after carburizing.
Can SMnB420H be formed, bent or molded?
Good hot forgeability at 1100–1200 °C and acceptable cold heading when spheroidize annealed; normalize forgings before case hardening to control grain size and distortion.
What surface finishes work on SMnB420H?
No inherent corrosion protection — use phosphating plus oil, black oxide, or zinc/zinc-nickel plating (bake to avoid hydrogen embrittlement on hardened parts); shot peening markedly raises gear-tooth bending fatigue.
SMnB420H 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 |
|---|---|---|
| CH-steel range slightly wider than SMnB420 | 0.17 – 0.23 | 0.2 |
| Si | 0.15 – 0.35 | 0.25 |
| Mn | 1.15 – 1.6 | 1.35 |
| P | ≤ 0.03 | — |
| S | ≤ 0.03 | — |
| Beffective (soluble) boron; Al/Ti added to fix nitrogen | 5×10⁻⁴ – 0.01 | 0 |
| Curesidual | ≤ 0.3 | — |
| Niresidual | ≤ 0.25 | — |
| Crresidual | ≤ 0.2 | — |
| Fe | balance | — |
SMnB420H CARB — frequently asked
What is SMnB420H CARB used for?
SMnB420H CARB is typically used for carburized gears, shafts and pins. In short: chromium-free hardenability; low-cost carburizing steel; guaranteed H hardenability band.
What is the yield strength of SMnB420H CARB?
SMnB420H CARB has a typical yield strength of 780 MPa (113 ksi) and a tensile strength of 1,000 MPa (145 ksi) — stronger than 86% of steel grades. Strength varies by condition: see the 6 listed tempers.
Is SMnB420H CARB easy to machine?
Not especially — machinability is rated not recommended (12/100, worse than 99% of steel grades). Machine in the annealed/normalized state with carbide at ~150–220 m/min; the Mn-rich matrix is gummy, so use positive rake and generous coolant, and finish-grind after carburizing.
Can SMnB420H CARB be welded?
Not readily — weldability is rated not recommended (10/100). CE ≈0.45–0.50 — weldable with low-hydrogen consumables, 150–250 °C preheat and slow cooling; never weld after carburizing.
What surface finishes work on SMnB420H CARB?
No inherent corrosion protection — use phosphating plus oil, black oxide, or zinc/zinc-nickel plating (bake to avoid hydrogen embrittlement on hardened parts); shot peening markedly raises gear-tooth bending fatigue.
Can SMnB420H CARB be formed, bent or molded?
Good hot forgeability at 1100–1200 °C and acceptable cold heading when spheroidize annealed; normalize forgings before case hardening to control grain size and distortion.
What is the maximum service temperature of SMnB420H CARB?
SMnB420H CARB is rated for continuous use to about 160°C (320 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between SMnB420H-A and SMnB420H CARB?
A is the default condition — standard stocked condition for machining or cold heading before carburizing. CARB: final service condition for gears and splined shafts needing a hard wear surface with a tough core. Yield strength is 355 MPa in A versus 780 MPa in CARB.
Sources
- JIS G 4052 — Structural steels with specified hardenability bands — JSA (2016)
- JIS G 4053 — Low-alloyed steels for machine structural use — JSA (2016)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4A — Steel Heat Treating Fundamentals (boron hardenability, carburizing) — ASM International (2013)
- GB/T 3077 / GB/T 5216 — / (20MnB comparison) — SAC (2015)
- Mysteel / SMM (2025)
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.