4320 is a nickel chromium molybdenum carburizing steel used where a part needs a hard, wear resistant case over a tough, shock absorbing core. Carbon is kept low so the core stays ductile, and carbon is added back at the surface during carburizing.
It is a standard choice for aerospace and industrial gearing, pinions, bearing races and pins, where contact fatigue governs the surface and impact governs the core.
Stocked as bar and forging billet, normally supplied annealed or normalized for machining before the carburizing cycle.
Sizes and stock
- Round bar: through 10" DIA
- Custom shapes and forgings: per request
Processing capabilities
- Vacuum melted (VAR / VIM-VAR) stock with full melt traceability
- Normalize, anneal, quench and temper to the ordered condition
- Saw cutting, trepanning, gun drilling and rough machining
- Magnetic particle and ultrasonic inspection to AMS 2301 / AMS 2154 on request
Specifications
Material is released to the applicable specification with mill test reports and, on request, independent lab verification.
Why engineers specify 4320
- Hard, wear resistant carburized case with a tough core
- Good hardenability for medium and heavy sections
- Well established for gear and bearing applications
- Aircraft quality with melt traceability available
Nominal chemistry
Percent by weight, nominal composition per the governing specification. The mill test report for your heat is the controlling document.
Typical mechanical properties
| Condition | Tensile strength | Yield strength | Elongation | Hardness |
|---|---|---|---|---|
| Normalized | ~115 ksi | ~70 ksi | ~20% | - |
| Carburized and hardened | - | - | - | 58-62 HRC case |
Typical published values for reference only, not for design. Properties vary with section size, product form and processing. Order to the specification and condition your drawing requires.
Typical applications
- Gears and pinions
- Bearing races
- Pins, shafts and splines
- Transmission hardware
Fabrication notes
Machining
Best machined in the annealed or normalized-and-tempered condition, then heat treated to final strength. Rough machining before hardening, with a finish pass after temper, controls distortion on long or thin sections. We can supply rough turned, trepanned or gun drilled stock so your first operation starts closer to net shape.
Welding
Weldable with low hydrogen practice, preheat appropriate to carbon equivalent and section size, and a post weld stress relief or full re-heat treat. For high strength conditions the weld and heat affected zone should be re-treated to restore properties.
Forming
Hot work in the annealed condition. Cold forming is limited at high hardness, so bend and form before hardening and follow with stress relief where the geometry is tight.
4320 compared with similar grades
9310
Higher nickel for better core toughness and hardenability in aerospace gearing.
4620
Lower alloy carburizing grade for lighter sections and lower cost.
Designations and cross reference
- Material family
- Alloy Steels
Working from an obsolete, foreign or superseded designation? Send it over and we will cross reference it to a current grade and specification.
Quality, testing and traceability
Supplied aircraft quality with mill test reports, melt and heat traceability, and, on request, magnetic particle inspection to AMS 2301 or AMS 2300, ultrasonic inspection to AMS 2154, chemical assay, hardness and full mechanical testing.
We are AS9100D and ISO 9001:2015 registered. If our stock does not already carry the inspection your specification requires, we can quote the material with that work performed. More on our quality system
Available forms
Custom sizes converted from billet, plus saw cut, waterjet and near-net blanks. See custom conversions
4320 frequently asked questions
What is 4320?
4320 is a nickel chromium molybdenum carburizing steel used where a part needs a hard, wear resistant case over a tough, shock absorbing core. Carbon is kept low so the core stays ductile, and carbon is added back at the surface during carburizing.
What forms is 4320 available in?
We supply 4320 in bar, forging. Custom sizes are converted from billet, and we can provide saw cut, waterjet or near net blanks so material arrives closer to your finished part.
What specifications can 4320 be supplied to?
Commonly ordered specifications include ASTM A322. Material is released to the specification and condition on your purchase order, with mill test reports supplied as standard.
What conditions can 4320 be ordered in?
We can supply Normalized, Carburized and hardened. If our stock is not already in the condition you need, we can quote it with the heat treatment performed before shipment.
Can you cut 4320 to size?
Yes. Saw cutting, waterjet, laser and shearing are available, along with trepanning, gun drilling, lathe turning and CNC machining, so you can order blanks or rough machined pieces rather than full lengths.
Do you provide test reports and additional testing on 4320?
Every order ships with mill test reports and heat traceability. If your print calls for ultrasonic inspection, penetrant dye, magnetic particle, chemical assay, micrography, hardness or mechanical testing and our stock does not already carry it, we can quote the material with that work performed.
Can 4320 be supplied to DFARS domestic melt requirements?
Tell us at quote stage if the order is DFARS or domestic melt restricted and we will confirm availability against that requirement before you commit.
What do you need to quote 4320?
Grade, form, dimensions, quantity, specification and condition, plus any testing or certification requirements and the date you need it. Send a drawing if you have one. Quotes are typically ready within 24 hours.
How does 4320 compare with 9310?
Higher nickel for better core toughness and hardenability in aerospace gearing.