8620 Steel:Key Specs, Heat Treatment & Uses
SAE 8620 is a low-carbon nickel-chromium-molybdenum alloy steel commonly used for carburized gears, pins, bushings and shafts. Carburizing adds carbon near the surface; hardening and tempering then create a wear-resistant case over a tougher core. Its strength and hardness depend on the supply condition, section size and final heat treatment.
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What does the 8620 designation tell you?
The final two digits indicate a nominal carbon content near 0.20%. The grade is also listed as UNS G86200. That identifies the alloy; it does not specify the tensile strength, final hardness or depth of a hardened layer.
The low base carbon content suits a part that needs different properties at its surface and center. Alloying with nickel, chromium and molybdenum helps the hardening response. An untreated bar, an annealed blank and a finished carburized gear are therefore different material states even if all are called 8620.
A purchase order also needs a product standard and delivery condition. For example, ASTM A322 covers hot-wrought alloy-steel bars; a tube or a special-quality forging needs the appropriate specification for that product.
Can you use 530 MPa and 26% elongation as a property set?
Only if the same qualified source gives both for the condition you are buying or analyzing. The grade name alone does not establish this pair. Elongation also needs its tensile-test basis: elongation at break is not the same as uniform elongation.
The examples below show why copying individual numbers is unreliable. They are supplier reference values, not interchangeable design allowables or guaranteed acceptance limits.
| Source and stated condition | Tensile strength | Yield / proof stress | Elongation | How to read it |
|---|---|---|---|---|
| Online Metals 8620 guide Exact heat treatment and specimen geometry are not stated in its property table. | 92 ksi ≈ 634 MPa | 52 ksi ≈ 359 MPa | 26% at break | Keep the complete row together. Do not pair its 26% with a tensile value from another dataset. |
| Vulcan 8620 guide, p. 20 Typical as-rolled condition. | Approx. 820 MPa | Approx. 590 MPa 0.2% proof stress | Approx. 22% Gauge length not stated. | These figures concern the supplied bar, not a finished carburized case. Confirm the applicable stock and test basis. |
| Hillfoot 8620 datasheet Untreated, as-rolled surface. | Not specified | Not specified | Not specified | The sheet says this material is not typically mechanically tested. A chemistry certificate need not contain tensile results. |
1 ksi = 6.894757 MPa; displayed conversions are rounded. On a phone, swipe the table; keyboard users can focus it and use the arrow keys.
Before using a quoted 530 MPa value, request the original material condition, section size, specimen location and test method. A numerical value can be valid for one specimen without being valid for your finished gear.
Vulcan lists carburized surface hardness up to 62 HRC. This is a supplier capability statement, not the default hardness of 8620 bar or a universal drawing target. Surface hardness, effective case depth and core properties need separate acceptance requirements.
8620 chemical composition
The following ranges are weight percent, balance iron, as listed in Laxcon’s SAE 8620 reference. They describe the base steel before carburizing. Use the governing material specification and heat analysis to accept an order.
Keeps the base steel relatively low carbon for core toughness and machinability before carburizing.
Supports toughness and hardenability, especially important to the load-carrying core.
Improves hardenability and wear response, working with the carbon-enriched case.
Supports hardenability and tempering response while helping resist softening.
Assists hardenability and deoxidation while contributing to strength response.
Acts mainly as a deoxidizer and contributes modestly to strength.
Usually restricted because excessive phosphorus can reduce toughness and increase segregation concerns.
Controlled because inclusions can affect fatigue, toughness and directional properties.
Its chromium content is far below stainless grades. Bare 8620 can corrode and may need oil, coating, plating or another approved protection system when the service environment requires it.
What does carburizing change?
Carbon diffuses into the surface at high temperature. Subsequent quenching can produce a hard, carbon-enriched case; tempering sets the usable condition. This is a change within the steel, not an added coating. Bodycote’s carburizing overview explains the enrichment, hardening and tempering stages.
The case transitions gradually into the lower-carbon core. The core can also harden during the thermal cycle: “tougher core” does not mean it remains in its original soft supply condition. Its response depends on alloy hardenability, section and cooling.
Surface hardness, effective depth and total depth differ
- Surface hardness: a reading near the outside, made with a suitable test for the part and layer.
- Effective case depth: distance from the surface to a specified hardness criterion, measured at the specified location.
- Total case depth: the extent of the altered case by the specified metallurgical or chemical method. It is not interchangeable with effective depth.
- Core hardness: a separate result at an agreed interior location. The same surface hardness can accompany different core responses.
Plan the heat-treatment route around the finished part
Carbon potential—the atmosphere’s tendency to supply carbon—works together with temperature and time. Furnace loading, atmosphere uniformity, prior microstructure, section, quench transfer, oil condition, agitation and tempering all change the final profile and distortion.
Machine and plan allowances
Rough-machine the part in a suitable soft condition. Provide stock for finishing and identify distortion-sensitive features, sharp transitions, thin walls and masked surfaces.
Carburize to a controlled profile
Control atmosphere, temperature and time to develop the required carbon gradient. Case depth cannot be guaranteed from hours alone without process capability evidence.
Quench and temper
Select the hardening route for case structure, core response and dimensional risk. Tempering relieves stress and sets the usable hardness/toughness balance.
Finish and inspect
Grind or finish critical features, then verify effective case depth, case/core hardness, microstructure where required, dimensions, cracks and surface integrity.
Allow for the surface removed during finishing
Illustrative allowance calculation: suppose effective case depth is measured at 0.90 mm before grinding. Removing 0.15 mm normal to that surface leaves about 0.75 mm to the same hardness crossing, provided grinding does not thermally alter the profile. For an outside diameter, a 0.30 mm diameter reduction removes 0.15 mm radially from each side.
If the final drawing requires at least 0.80 mm at that location, this example is short even if a surface-hardness check passes. The arithmetic explains why case-depth requirements must say whether they apply before or after finishing. Final inspection must also address grinding damage and the actual profile.
Keep the case and core requirements linked
A thin pin and a thick gear hub may not cool at the same rate. A small coupon can help monitor a process, but it must be representative of the feature whose properties it is intended to prove. Surface hardness does not certify tooth-root fatigue strength, center toughness or freedom from quench cracks.
8620, 8620H or 4140: which question are you solving?
Choose 8620 for a carburized surface and supporting core
It is a common starting point for gears, splines, pins and bushings where surface wear matters. Selection still depends on the load, geometry, lubrication, layer depth, core response and required life. A component name is not an approval for every duty level.
Specify 8620H when hardenability control matters
The H suffix identifies a hardenability-controlled grade. It does not mean a supplied bar is already hardened or will reach a higher surface hardness. Allowed chemistry ranges can differ from plain 8620; acceptance includes the specified hardenability limits rather than chemistry alone. ASTM A304 describes these H-grade bar requirements.
The Jominy end-quench test measures hardness at increasing distances from a water-quenched end. It characterizes response to changing cooling conditions. This distance is not carburized case depth and is not automatically equal to hardening depth in your part. ASTM A255 explains the test and reporting basis.
Compare 4140 when strength through the section dominates
4140 has a higher nominal carbon content and is commonly quenched and tempered for bulk strength. It can also use a separate surface treatment where needed. For a shaft governed by through-section strength rather than a carburized wear layer, compare its qualified route with 8620. See 4140 properties and heat treatment for the detailed alternative.
Hardness describes resistance to indentation at a location. Hardenability describes the ability to develop a hardened structure under given cooling conditions. Neither property alone establishes component fatigue life.
Machining, welding and repair depend on the current condition
Machine the features before final case hardening
Turning, drilling, hobbing and broaching are usually planned in a suitable softer condition. Specify incoming hardness if machining consistency matters, retain agreed finishing stock and identify surfaces that must be protected from carburizing. A masked feature and an exposed tooth surface can have different treatment requirements.
After hardening, grinding, honing or suitable hard machining may be needed. Control stock removal, runout, finish and surface damage. A part can meet size while failing its case-depth or surface-integrity requirement.
Do not treat a carburized part like untreated bar when welding
Welding locally changes the material state. The carbon-enriched case and a rapidly cooled heat-affected zone can create cracking and property risks. Where welding is part of the design, plan its sequence before final case hardening and qualify the complete route. Repairing an already hardened gear requires specific engineering approval.
For an arc-welded assembly, the procedure should address hydrogen control, actual composition, thickness, restraint, heat input, preheat/interpass conditions and any subsequent treatment. TWI’s hydrogen-cracking guidance explains why these factors act together. A filler choice or preheat temperature copied from another job cannot qualify an 8620 repair.
Laser welding, cleaning and marking also need trials in the actual condition. A successful result on untreated bar does not prove that a finished carburized surface will retain its hardness profile, dimensions or fatigue-relevant surface quality.
What should the drawing and purchase order specify?
Define the material being purchased and the evidence needed after treatment. A material test report (MTR) for the original heat supports traceability and chemistry; it does not automatically certify the final case.
- Product and condition: grade, product standard, form, size, supply condition, quantity and heat/lot traceability.
- Hardenability if required: the H grade or agreed band, method and how it relates to the controlling section.
- Final surface and depth: hardness range, effective depth and tolerance, hardness criterion, test location, method and whether results apply after finishing.
- Core and metallurgy: core test location and property limits; microstructure, retained austenite or cleanliness limits where the design needs them.
- Dimensions and release: masking, grind stock, distortion allowance, final size/runout, surface finish, crack inspection and who may approve repair or deviation.
Retained austenite is austenite that remains after quenching; its amount can matter to dimensional stability and service behavior. Request a limit and measurement method only when justified by the component requirements, rather than treating a generic material checklist as a specification.
Are international “equivalents” interchangeable?
20NiCrMo2-2 / 1.6523 and SNCM220-family designations appear in commercial cross-reference tables. These are comparison starting points. Check the actual standard editions, composition, hardenability, delivery condition, product form and tests, then obtain the drawing authority’s approval before substitution.
Sources and data scope
- Laxcon Steels — SAE 8620: base composition and the distinction between plain and hardenability-controlled grades.
- Online Metals — 8620 product guide: the 92 ksi, 52 ksi and 26% reference row; its table does not specify a full heat-treatment/test condition.
- Vulcan — Case Hardening Steels, printed p. 20: typical as-rolled properties and carburized surface-hardness capability. Its composition table includes broader ranges; it is not merged with the plain-SAE composition above.
- Hillfoot — 8620, untreated/as rolled: supply-condition description and absence of a specified mechanical-property set. Its chemistry limits vary from other supplier tables.
- Bodycote — Atmospheric carburising: carbon enrichment followed by hardening and tempering.
- ASTM A322, ASTM A304 and ASTM A255: alloy-bar scope, H-grade acceptance and hardenability test methods.
- TWI — Hydrogen cracks in steels: welding-crack mechanisms and contributing variables.
Plan a trial on the real 8620 surface
For a laser process review with Oceanplayer Laser, send the material specification, current heat-treatment and case condition, part geometry, coating or contamination, and the required result. Identify any surface whose hardness, layer depth or dimensions must be preserved.