Steel SMnB420H
SMnB420H is a Japanese manganese-boron carburizing steel to JIS G4053, supplied to a guaranteed hardenability band (H) so it can replace chromium grades at lower alloy cost.
Reference datasheet — SMnB420H 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
SMnB420H tempers and conditions
SMnB420H is supplied in 6 conditions, soft to hard. A is the default FabDigit quotes; the values on this page are for it.
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 | — |
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?
SMnB420H is a Japanese manganese-boron carburizing steel to JIS G4053, supplied to a guaranteed hardenability band (H) so it can replace chromium grades at lower alloy cost. Engineers specify it for carburized gears, shafts and pins; it is stocked as hot-rolled, annealed, normalized and cold-drawn bar, with default values of 355 MPa yield, 560 MPa tensile, 26% elongation and 170 HB (86 HRB). It has no inherent corrosion resistance and only moderate weldability (score 55) and machinability (score 62). For deeper case depth and higher core strength, step up to SMnB443H. SMnB420H 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). SMnB420H has a yield strength of 355 MPa (51.5 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 73% of steel grades. Elongation at break is 26% and the elastic modulus is 206 GPa (29.9 Msi).
What is SMnB420H used for?
- carburized gears
- shafts
- pins
Advantages
- Easy to machine — 62/100, better than 89% of steel grades
- Ductile — tolerates forming and impact — 26%, better than 82% of steel grades
- Forms and bends easily — 62/100, better than 78% of steel grades
SMnB420H properties
Typical room-temperature values for SMnB420H-A; bars are the position among steel grades.
Physical3
SMnB420H 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 has a yield strength of 355 MPa (51.5 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 73% of steel grades. Elongation at break is 26% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
SMnB420H is rated for continuous service to 350°C (662 °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 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 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's machinability is good (62/100, better than 89% of steel grades); weldability good (55/100); formability good (62/100).
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. SMnB420H is highlighted; hover a dot for its name, or open the interactive family chart.
Working with SMnB420H
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.
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 |
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.
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
How much does SMnB420H cost?
SMnB420H 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.
SMnB420H — frequently asked
What is SMnB420H used for?
SMnB420H 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?
SMnB420H has a typical yield strength of 355 MPa (51.5 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 73% of steel grades. Strength varies by condition: see the 6 listed tempers.
Is SMnB420H easy to machine?
Reasonably — machinability is rated good (62/100, better than 89% 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 be welded?
Yes — weldability is rated good (55/100). 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?
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 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?
SMnB420H is rated for continuous use to about 350°C (662 °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.
How much does SMnB420H cost?
SMnB420H 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 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; 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.