What is quench severity (Grossmann H-factor)?

The Grossmann H-factor (quench severity) is a dimensionless number quantifying the cooling power of a quench medium relative to an ideal (infinite speed) quench. Typical H-values: still water = 1.0, agitated water = 1.0–2.0, still oil = 0.25–0.35, agitated oil = 0.35–0.70, air (still) = 0.02, nitrogen at 10 bar = 0.10–0.20, water-polymer (10%) = 0.4–0.8. Higher H gives faster cooling, more martensite, higher hardness — but also higher distortion and cracking risk.

How do I choose between oil quench and water quench?

Water quenching is more severe (faster) and achieves full hardness in lower-hardenability (plain carbon) steels, but causes significantly more distortion and cracking risk — particularly in complex shapes and higher carbon steels. Oil quenching is used for alloy steels with sufficient hardenability to achieve the target hardness at a lower cooling rate, giving less distortion and cracking. As a rule of thumb: carbon steels up to 10 mm section can be water-quenched; alloy steels should be oil-quenched when the hardenability permits.

What is the cooling rate at the centre of a quenched bar?

The centre cooling rate depends on bar diameter, quench severity, and bar material's thermal diffusivity. For a 50 mm diameter steel bar oil-quenched (H = 0.35), the centre cooling rate at 700°C is approximately 5–8°C/s, compared to 15–25°C/s at the surface. For a 100 mm bar, the centre rate drops to 1–2°C/s — insufficient for martensite in low-hardenability steels. This is why hardenability (the ability of a steel to harden in large sections) is the key alloy selection criterion.

What is the risk of quench cracking and how do I reduce it?

Quench cracking occurs when thermal and transformation stresses exceed the material's fracture strength during rapid cooling. Risk factors include: sharp corners and abrupt section changes, high carbon content (>0.6% C), water quenching of alloy steels, quench into too-cold oil or water, delayed transfer from furnace to quench, and excessive soak temperatures causing coarse grains. Mitigation: use interrupted quenching (marquenching/martempering), reduce quench severity, add section-change radii, preheat quench oil to 50–80°C, and minimise transfer time.

Quench Rate Calculator for Steel Heat Treatment — Oil, Water & Gas Quench Severity

Quench Medium Severity Reference (Grossmann H-Factor)

Quench Medium	H-Factor Range	Agitation	Distortion Risk	Typical Use
Air (still)	0.02	None	Very low	Air-hardening tool steels
N₂ gas (5 bar)	0.05–0.10	Fan	Low	Vacuum furnace hardening
N₂ gas (10 bar)	0.10–0.20	High fan	Low	High-alloy vacuum hardening
Quench oil (still)	0.25–0.35	None	Moderate	Alloy steel hardening
Quench oil (agitated)	0.35–0.70	Pump	Moderate	Production alloy steel
Polymer (10%)	0.40–0.80	Agitated	Moderate	Alloy / medium-C steel
Water (still)	0.90–1.10	None	High	Plain carbon steel
Water (agitated)	1.10–2.00	Spray/pump	Very high	Carbon steel thin sections
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Calculator Outputs

Grossmann H-factor for your quench setup
Surface and centre cooling rates (°C/s) at 700°C
Predicted surface and core hardness (HRC)
Hardenability check — will the section through-harden?
Distortion and cracking risk rating
Interrupted quench (marquench) option analysis

Quench Methods Covered

Still and agitated oil quench
Water and brine quench
Polymer (PAG) quench — concentration effect
Gas quench — N₂, He, pressure effect
Salt bath (martempering) — interrupted quench
Austempering — isothermal bainite formation

Why Quench Rate Selection Is Critical

The quench rate determines how much martensite forms in the hardened part — too slow and you get mixed microstructures (bainite, pearlite) with lower and inconsistent hardness; too fast and the thermal and transformation stresses cause distortion or cracking. The optimal quench medium gives the minimum cooling rate needed to achieve your target hardness while minimising distortion.

Choosing between oil, gas, and polymer quenching in a vacuum furnace requires understanding both the steel's hardenability (from its composition) and the required hardness at section centre. The Bloor Engineering quench rate calculator combines Jominy data with heat transfer models to give reliable predictions without expensive trial-and-error testing.

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Enter steel grade, section size, and quench medium — get cooling rates, predicted hardness, and cracking risk assessment. Free with a registered account.

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Quench Rate Calculator for Steel Heat Treatment

Calculator Outputs

Quench Methods Covered

Why Quench Rate Selection Is Critical

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