Guide
8620 steel: how should a carburized part be specified and machined?
8620 steel is a low-carbon nickel-chromium-molybdenum alloy steel made for carburizing: it machines easily in the soft condition, then takes a hard, wear-resistant case over a tough core. Specify the grade (or 8620H when the hardening response must be guaranteed), the case depth and hardness after any grinding, and a machining sequence that finishes soft features before carburizing and grinds only what must be hard and precise afterward.
By Saola Manufacturing engineering team · Published · Updated
8620 shows up on drawings for gears, splined shafts, cam followers, pins and wear bushings. The material line is short; the trouble lies in what happens around the heat treatment. This guide covers why designers pick 8620, what the H and RH variants change, and how the part should be routed through machining, carburizing and grinding. How to write and verify the case depth callout itself is covered in the effective case depth guide, and is not repeated here.
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Why is 8620 the default carburizing steel?
Because its low carbon keeps the core tough and the bar easy to machine, while its nickel, chromium and molybdenum give enough hardenability for the carburized case and core to harden in common section sizes.
Carburizing diffuses carbon into the surface, so the base steel only needs enough carbon for the core. 8620 carries typically about 0.2% carbon, which is what the 20 in its European counterpart 20NiCrMo2-2 refers to. Ovako’s data sheet for its 20NiCrMoS2-2 variant, which lists SAE 8620 among similar designations, describes it as an alloyed case-hardening steel for very highly strained parts with good core toughness, used for bevel gears, crown wheels, gears, shafts and bolts, and suitable for direct hardening (Ovako, 2025). The same sheet gives a core tensile strength range of 700–900 MPa for that grade (Ovako).
The practical alternatives are plain-carbon carburizing grades such as 1018 or 1020, which are cheaper but harden less deeply, and higher-alloy grades such as 9310, chosen for heavily loaded aerospace and gear work. Through-hardening steels such as 4140 are a different route: hardness through the section rather than a hard skin on a tough core.
| Option | What it gives | When to choose it |
|---|---|---|
| 1018 / 1020, carburized | Hard case, softer core, lowest material cost | Thin sections, light loads, pins and wear surfaces |
| 8620, carburized | Hard case over a tough, stronger core in common section sizes | Gears, splines, shafts and pins with contact and bending load |
| 8620H | Same steel bought to a guaranteed end-quench hardenability band | Production gears or shafts where core hardness must repeat lot to lot |
| 8620RH | A narrower, restricted hardenability band | Distortion-sensitive parts where heat treat response must be tightly controlled |
| 4140, through-hardened | Uniform hardness through the section, no case | Moderate wear with higher core strength and no carburizing step |
What is the difference between 8620, 8620H and 8620RH?
The chemistry is nearly the same; the H and RH versions are bought to an end-quench (Jominy) hardenability band, which makes the core hardness after carburizing more predictable.
H-grade bars are purchased under ASTM A304, which covers alloy, carbon and carbon-boron steels that must reach a specified depth of hardening in the end-quench test, with the hardenability requirement on the purchase order. Restricted-hardenability RH steels fall under ASTM A914/A914M, whose scope states that they show a hardness range not greater than 5 HRC at the initial position on the end-quench bar.
For a one-off shaft, standard 8620 is normally enough. For a gear in production, where the heat treat cycle is set once and must give the same core hardness and distortion every lot, 8620H or RH removes the largest source of variation the heat treater cannot see in advance. Put the grade, the bar specification and the condition on the drawing, and ask for a mill certificate that shows the hardenability result.
What should be machined before carburizing, and what after?
Machine everything soft that can tolerate heat-treat distortion, leave grinding stock only on the hard, precise features, and protect anything that must stay soft.
A typical route for an 8620 part looks like this:
- Soft machining. Turn and mill the part from bar in the as-supplied condition, which for the Ovako grade is annealed below 212 HB on round bar (Ovako). Cut gear teeth, splines, keyways, cross holes and threads now, because they are hard to machine after hardening.
- Leave stock. On bearing journals, seal diameters and precision bores, leave grinding stock sized for the expected distortion and for the case removal the drawing allows.
- Mask. Threads, areas to be welded or machined later, and anything that must stay soft are masked with copper plating or stop-off paint, or are left oversize and machined off after carburizing so the carbon-rich layer goes with the chips.
- Carburize, quench and temper per the drawing’s heat-treat specification, for example SAE AMS2759/7 for gas and vacuum carburizing.
- Finish. Grind the precise hard features; straighten if allowed; then inspect hardness, case depth on a sample, and dimensions.
The order matters most for threads. A thread carburized with the rest of the part becomes brittle and cannot be chased afterward. Either mask it, or leave extra stock over the thread area through carburizing, machine that stock off afterward and cut the thread in the soft material underneath. Say on the drawing which features are “soft after heat treat” so the routing is not left to guesswork.
How much grinding stock should a carburized 8620 part get?
Enough to clean up distortion and no more, because every bit of grinding stock removes case, and the case depth on the drawing should apply after grinding.
There is no universal figure; stock depends on part length, section changes, the quench and how the part is fixtured in the furnace. Long, slender shafts and thin rings distort most. The working method is to agree the stock with the heat treater from the first articles, measure the distortion, and then set the as-carburized case depth so that the minimum still remains after grinding at the worst point. A shallow case plus generous stock is the classic way to get a part that meets size and fails in service.
Two drawing notes prevent most disputes: “case depth applies after finish grinding” on the hard features, and a maximum stock or a minimum remaining case on the grinding operation. Grinding burn is the other risk on hardened 8620; the effective case depth guide covers the temper etch check used to catch it.
What should the drawing and RFQ say for an 8620 carburized part?
Grade and bar specification, condition as supplied, the heat-treat specification, case depth and hardness after grinding, core hardness, soft areas, and the inspection evidence you want.
- Material: “8620H per ASTM A304” (or 8620 to the bar specification you use), with condition, for example annealed or normalized.
- Heat treatment: carburize, harden and temper per the named specification; effective case depth range, limit hardness and stage per the case depth guide.
- Surface and core hardness, and where each is measured.
- Soft areas: threads and other features to be masked or left soft, shown on the view.
- Evidence: mill certificate with hardenability for H grades, heat-treat certificate, hardness results, and a case depth report on a sample or coupon per load.
How is an 8620 part handled on a Saola order?
The soft machining runs at our lead CNC plant; the 8620 bar and the carburizing step are confirmed at drawing review, because neither is on the plant’s published lists.
Parts are turned and milled at our lead CNC plant in Vietnam on CNC turning and CNC milling. 8620 is not on the turning page’s bar list, so the material grade and source are confirmed at drawing review. Carburizing is not among the plant’s listed processes either; how and where it is done is confirmed at drawing review and stated on the quote. The plant has grinding/polishing and a hardness tester, so a surface hardness check can be part of the inspection plan, and material certification is available on request. New parts get a first-article inspection report and every lot a dimensional inspection record. Send the drawing for a quote.
Frequently asked
Can 8620 be used without carburizing?
Yes, as a tough, weldable low-alloy steel in the normalized or quenched and tempered condition, but its low carbon limits the hardness it can reach. If no case is needed, a through-hardening grade is usually the better choice.
Is 8620 easy to machine?
In the annealed or normalized condition it machines well, though it can be gummy when very soft. Sulfur-modified variants exist for better chip control; check that the drawing and the fatigue requirements allow them.
Can a carburized 8620 part be welded?
Not in the hardened case. If a weld is needed, mask or machine away the case at the weld area and plan the weld before hardening, or design the weld zone to stay soft.
What is the European equivalent of 8620?
20NiCrMo2-2 (material number 1.6523) is the usual counterpart. Composition limits differ slightly between standards, so name the standard you buy to rather than the equivalent.
Sources
- Material data sheet, 20NiCrMoS2-2, Ovako, revised January 2025
- ASTM A304-20, Carbon and Alloy Steel Bars Subject to End-Quench Hardenability Requirements, ASTM International, 2020
- ASTM A914/A914M-19, Steel Bars Subject to Restricted End-Quench Hardenability Requirements, ASTM International, reapproved 2024
- SAE AMS2759/7, Gas and Vacuum Carburizing and Heat Treatment of Carburizing Grade Steel Parts, SAE International, 2026