Jominy
Hardenability.
Specifying Heavy Sections.

Hardness Is Not Hardenability

The two words get used interchangeably on drawings and they mean different things. Hardness is a surface property — how well a material resists being indented or abraded, measured in HRC, HB or HV. Hardenability is about depth: how far into a section the steel can transform from austenite into martensite when it is quenched.

A steel can be very hard at the surface and disappointing 40mm below it. That is not a heat-treatment failure; it is the alloy running out of hardenability. Nickel, chromium and molybdenum all slow the transformation of austenite into ferrite and pearlite, which is what gives martensite time to form at depths where the metal is cooling slowly. That is the whole reason alloy case-hardening grades exist alongside plain carbon steels — not for surface hardness, which carburising delivers either way, but for what is happening in the core.

How the End-Quench Test Works

The Jominy end-quench test, standardised in BS EN ISO 642 and ASTM A255, isolates cooling rate as the only variable. A single specimen is quenched from one end only, so every point along its length cools at a different, repeatable rate. Measure hardness along that length and you have mapped hardness against cooling rate for that heat of steel.

The result is a curve, not a number. At 1.5mm from the quenched face the structure is fully martensitic. By 50mm the bar is effectively air-cooling. How quickly hardness falls away between those two points is the grade's hardenability.

The Mass Effect: Reading Jominy Distance as Bar Diameter

Here is where the test earns its keep. The centre of a large round bar cools slowly during quenching, and that slow cooling rate is equivalent to a position some distance up a Jominy specimen. So a position on a 100mm test piece stands in for the core of a bar you could never test directly. The standard equivalences look like this:

Two things fall out of that table, and both of them matter more than people expect.

The quench medium is as significant as the diameter. At J = 10mm the equivalent section is ⌀38mm in water but only ⌀20mm in oil — almost half. Alloy case-hardening steels are normally oil quenched to control distortion and cracking, so the oil column is usually the one that applies. Quoting a diameter without saying how it will be quenched leaves the most important variable undefined.

Heavy sections run off the end of the published table. Standard equivalence data stops around ⌀70mm water or ⌀45mm oil, because beyond that the core cooling rate is slower than anything on a 100mm test piece. That is not a gap in the data so much as the point: once you are specifying 150mm, 200mm or 300mm rounds, you are extrapolating, and the only thing that settles core hardness is the Jominy curve for the heat you are actually buying.

The Three Grades, and What Separates Them

All three are carburising grades — they are specified for a hard case over a tough core, not for through-hardening. What differs is how well the core holds up as the section gets heavier. The analysis below is as published on our own data sheets.

*Enquire for availability on 832M13 black rounds.

Note what that table does and does not say. 655M13 and 832M13 sit in the same nickel band — roughly 3.25% nominal — so the nickel is not what separates them. The molybdenum is. 832M13 carries 0.10–0.25% Mo where 655M13 carries none, and that is the whole difference in how far each holds its core.

Typical curves show it plainly:

Three readings worth taking from that, and the middle one is the surprise.

At the quenched face, all three are much the same. 41–48, 38–43, 40–45 HRC — you cannot tell these grades apart from a surface hardness test. Whatever you paid for the alloy content, none of it shows up at J = 1.5mm.

8620 and 655M13 track each other almost exactly all the way out. At J = 10mm they are 23–30 against 22–28; at J = 15mm they are identical at 18–24. That is worth dwelling on, because it means the ~3.25% nickel in 655M13 is not buying core hardness over 8620 — it is buying core impact toughness, which is a different property and does not show on a hardness plot at all. If you switched to 655M13 expecting a harder core, you bought the wrong thing.

832M13 is the one that actually holds. 34–40 HRC at J = 10mm and still 28–34 HRC at J = 20mm, where 8620 has dropped under 18 and 655M13 is down to 16–22. It has the same nickel as 655M13, so the entire gap is the molybdenum. That is the clearest illustration on this page of why hardenability is not something you can read off a nickel figure.

Expressed as ruling section rather than Jominy distance: the high-nickel-plus-molybdenum grades hold a core tensile above roughly 850 N/mm² out to around 150mm ruling section, while a low-alloy grade such as 8620 begins to soften in the core somewhere between 25mm and 50mm — considerably earlier than most people assume.

SAE 8620 (805M20) — the economical choice, within its section range

Roughly half a percent each of nickel and chromium with a little molybdenum. Carburises to a 58–62 HRC case over a core around 620–800 N/mm² tensile, and it is the cost-effective answer for gear teeth, bushes, camshafts and king pins where the section is light and the core duty moderate. Where it catches people out is the ruling section: core softening starts earlier than the diameters we routinely stock it in, so 8620 in a heavy round is a specification worth questioning rather than assuming. Full data on the SAE 8620 (805M20) page.

EN36 (655M13) — nickel for core toughness

Around 3.25% nickel with up to 1% chromium and no molybdenum — it is the standard non-moly grade of the EN36 family, and the grade the old EN36A designation referred to. The nickel buys genuinely high core impact toughness, which is what matters for anything shock-loaded, and it slows the ferrite and pearlite transformations enough to keep a useful core through intermediate sections. What it will not do is hold that core indefinitely: past J = 20mm the curve drops away. See EN36 (655M13).

EN36C (832M13) — the same nickel, plus molybdenum

This is the grade most often misdescribed, so it is worth stating plainly: 832M13 sits in the same ~3.25% nickel band as 655M13. It is not a higher-nickel steel. What it adds is 0.10–0.25% molybdenum, and historically that addition is exactly why the grade exists — EN36B was the 3% Ni-Cr steel without mandatory moly, and adding moly to improve hardenability and resist temper embrittlement is what made it EN36C. That is the failure mode which catches heavy sections cooling slowly through the critical range. Case 60–64 HRC over a core around 1050 N/mm². For a 250mm gear blank this is the grade to reach for. Full analysis on the EN36C (832M13) data sheet.

What to Specify, by Section Size

1. State the section and the quench, not just the grade. This is the one that costs money when it is missed. A grade name alone cannot tell a heat treater what core hardness to expect, and the oil-versus-water difference is nearly a factor of two on equivalent diameter. Specify all three.
2. Light sections with moderate core duty — 8620 earns its place. Good case, adequate core, least expensive of the three. Do not pay for nickel you will not use.
3. Where core toughness is the driver — EN36. The ~3.25% nickel is bought for impact toughness at the core as much as for hardness. Right for shock-loaded work through intermediate sections.
4. Heavy sections, or where temper embrittlement is a risk — EN36C. Same nickel as EN36; the molybdenum is what holds the core further out and resists embrittlement on a slow cool through the critical range.
5. Genuinely high nickel is a different grade. If the application needs ~4% Ni rather than ~3.25%, that is EN39B (835M15), not EN36C.
6. Ask for the Jominy curve if the core is critical. Hardenability varies between heats of the same grade, and heavy sections sit beyond the published equivalence tables. Where core hardness is a design requirement rather than a preference, the curve for the actual heat is the only thing that settles it.

Where Parkside Fits

We hold all three grades in black and bright rounds, which is the practical reason this article exists: customers ring up asking for a case-hardening grade at 250mm and the honest answer is sometimes a different grade than the one on the drawing. Every bar ships with EN 10204 material certification tying it to its cast and chemical analysis, we supply 3.1 inspection certificates as standard on these grades, and Jominy verification can be arranged on request where the core matters enough to prove it. Bar is cut to length from a single piece, so a trial length to prove a heat-treatment route does not mean buying a mill quantity. If a designation on an incoming certificate is unfamiliar, our grade equivalents table maps the BS, EN and international names against each other, and the hardness conversion table covers HRC, HB and HV.

Our View

Hardenability is one of the few metallurgical properties that changes the purchase order rather than just the heat-treatment paperwork. A designer who specifies 8620 for a 250mm shaft has not made a small error — they have specified a component whose core will not reach the hardness the drawing assumes, and no amount of care in the quench tank will fix it. The Jominy curve is what turns that from an argument into a number.

If you are working at the heavy end of the range and are not certain the grade on the drawing will deliver the core you need, call us before you order. We would rather have that conversation at the enquiry stage than after the bar has been cut.