Steel 4320
AISI/SAE 4320 · UNS G43200 · JIS SNCM420
4320 (UNS G43200; JIS SNCM420) is a Ni-Cr-Mo case-hardening steel that is carburized to give a hard, wear-resistant case over a tough, deeply hardenable core.
Values for the HR condition
4320 tempers and conditions
4320 is supplied in 7 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
Compare all 7 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
HRDefault Hot rolled | Standard stock form for carburizing-grade bar that will be machined and then heat treated. | 415 | 640 | 22 | 187 HB | — |
full anneal ~840–870 °C, furnace cool | Softest condition; pick it for heavy machining, deep hobbing or cold forming before carburizing. | 425 | 580 | 29 | 163 HB | — |
~895 °C, air cool | Refines grain and homogenizes forgings/bar before carburizing; best pre-heat-treat condition for dimensional stability. | 464 | 793 | 21 | 21 HRC | — |
Cold rolled | Thin strip/sheet form for stamped, carburized parts needing good surface and tight thickness. | 550 | 700 | 12 | 15 HRC | 7.8 % IACS |
Cold drawn | Choose for close-tolerance, bright-surface bar that needs minimal turning before carburizing. | 620 | 745 | 15 | 217 HB | — |
Quenched and tempered | Direct-hardened condition for shafts and non-carburized parts needing high core strength with good toughness. | 900 | 1,080 | 15 | 34 HRC | — |
case 58–62 HRC, core ~30–40 HRC | The intended service condition: hard wear/contact-fatigue-resistant case over a tough Ni-Mo core for gears and bearing races. | 1,000 | 1,200 | 12 | 60 HRC | — |
What is 4320?
4320 (UNS G43200; JIS SNCM420) is a Ni-Cr-Mo case-hardening steel that is carburized to give a hard, wear-resistant case over a tough, deeply hardenable core. Engineers specify it as annealed or normalized bar and plate where 4118 or 4620 lack core strength; before carburizing the default condition gives 415 MPa yield, 640 MPa tensile, 22 % elongation, 187 HB (90 HRB) and a machinability score of 50. Weldability is restricted, cold formability is limited (score 45) and there is no corrosion resistance, so plating or oiling is required. 8620 is the cheaper and more widely stocked alternative. 4320 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). 4320 has a yield strength of 415 MPa (60.2 ksi) and a tensile strength of 640 MPa (92.8 ksi) — weaker than 56% of steel grades. Elongation at break is 22% and the elastic modulus is 205 GPa (29.7 Msi).
What is 4320 used for?
- Gears
- Pins
- Shafts
4320 properties
Typical room-temperature values for 4320-HR; bars are the position among steel grades.
Physical3
4320 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
4320 has a yield strength of 415 MPa (60.2 ksi) and a tensile strength of 640 MPa (92.8 ksi) — weaker than 56% of steel grades. Elongation at break is 22% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
4320 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 11.7 µm/m·K (6.5 µin/in·°F).
Electrical2
4320 has an electrical resistivity of 2.2×10⁻⁷ Ω·m.
Chemical & Environmental3
Corrosion resistance is not recommended (11/100), worse than 73% of steel grades.
Sustainability4
Producing a kilogram of 4320 takes about 31 MJ of energy and emits 2.3 kg of CO₂ — more than 64% of steel grades. Typical recycled content is 30%.
Manufacturability7
4320's machinability is fair (50/100, worse than 53% of steel grades); weldability fair (40/100); formability fair (45/100).
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. 4320 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 4320
Is 4320 easy to machine?
Machine in the annealed or normalized condition with carbide tooling and rigid setups; leave grind stock because carburize-quench distorts, and finish-grind the case.
Can 4320 be welded?
Restricted weldability — preheat 200–300 °C, use low-hydrogen Ni-Mo filler and PWHT; never weld after carburizing.
Can 4320 be formed, bent or molded?
Cold-form only in the annealed state and with generous radii; hot forge 1150–1200 °C, finish above 900 °C and normalize afterwards.
Manufacturability
The process limits FabDigit applies to 4320. Every 4320 part is priced by an engineer; design inside these and the quote comes back without a redesign loop.
CNC machining
| Machinability | Better50 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 180 / 210 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 5× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Sheet metal
| Formability | Better45 / 100 |
|---|---|
| Weldability | Better40 / 100 |
4320 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 – 0.22 | 0.2 |
| Mn | 0.45 – 0.65 | 0.55 |
| Si | 0.15 – 0.35 | 0.25 |
| Ni | 1.65 – 2 | 1.82 |
| Cr | 0.4 – 0.6 | 0.5 |
| Mo | 0.2 – 0.3 | 0.25 |
| P | ≤ 0.04 | |
| Smax; 4320H/E-grades tighter | ≤ 0.04 | |
| Fe | balance |
What surface finishes work on 4320?
No anodizing; protect with oil, black oxide, manganese phosphate, or zinc/zinc-nickel plating with post-plate bake to avoid hydrogen embrittlement.
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).
Plating
CVD Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel 4320 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- G43200
- JIS
- SNCM420
How much does 4320 cost?
4320 is a premium-cost steel ($$$), cheaper than about 58% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
4320 — frequently asked
What is 4320 used for?
4320 is typically used for gears, pins and shafts. In short: carburizing Ni-Cr-Mo bar.
What is the yield strength of 4320?
4320 has a typical yield strength of 415 MPa (60.2 ksi) and a tensile strength of 640 MPa (92.8 ksi) — weaker than 56% of steel grades. Strength varies by condition: see the 7 listed tempers.
Is 4320 easy to machine?
Reasonably — machinability is rated fair (50/100, worse than 53% of steel grades). Machine in the annealed or normalized condition with carbide tooling and rigid setups; leave grind stock because carburize-quench distorts, and finish-grind the case.
Can 4320 be welded?
With care — weldability is rated fair (40/100). Restricted weldability — preheat 200–300 °C, use low-hydrogen Ni-Mo filler and PWHT; never weld after carburizing.
What surface finishes work on 4320?
No anodizing; protect with oil, black oxide, manganese phosphate, or zinc/zinc-nickel plating with post-plate bake to avoid hydrogen embrittlement.
Can 4320 be formed, bent or molded?
Cold-form only in the annealed state and with generous radii; hot forge 1150–1200 °C, finish above 900 °C and normalize afterwards.
What is the maximum service temperature of 4320?
4320 is rated for continuous use to about 350°C (662 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 4320-HR and 4320-CD?
HR is the default condition — standard stock form for carburizing-grade bar that will be machined and then heat treated. CD: choose for close-tolerance, bright-surface bar that needs minimal turning before carburizing. Yield strength is 415 MPa in HR versus 620 MPa in CD.
How much does 4320 cost?
4320 is a premium-cost steel ($$$), cheaper than about 58% 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.
Can FabDigit make parts in 4320?
Yes — 4320 is available for CNC machining and sheet metal; every part is priced by an engineer rather than instantly. Upload a STEP file to get a price and a DFM check.
Sources
- SAE J404 — Chemical Compositions of SAE Alloy Steels — SAE International (2021)
- ASTM A29/A29M — Steel Bars, Carbon and Alloy, Hot-Wrought — ASTM (2020)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4 — Heat Treating (carburizing of Ni-Cr-Mo steels) — ASM International (1991)
- GB/T 3077 (20CrNi2Mo) — SAC (2015)
- JIS G 4053 (SNCM420) — JSA (2016)
- Mysteel / SMM alloy bar and nickel price references — 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.