Why choose vacuum hardening over normal hardening?

The part comes out clean, without scale or a soft decarburised skin, and with less distortion. That saves grinding and suits finished tools, dies and precision parts.

Can any steel be vacuum hardened?

It works best on steels that harden in air or gas, such as D2, H13 and HSS. Plain carbon and low-alloy steels such as EN8 or EN9 need a faster quench, usually oil or water, so they suit other furnaces.

Is tempering needed after vacuum hardening?

Yes, always. Tool steels are usually tempered twice, and sometimes deep-frozen in between, to reach a stable hardness and size.

Vacuum hardening job work: tool steels, hardness, benefits

Vacuum hardening heats parts in a furnace with the air pumped out, then quenches them with high-pressure nitrogen gas. With no oxygen there is no scale and no decarburisation, so parts come out bright and close to size. It is the usual way to harden tool and die steels such as D2, H13 and M2, and precision parts that are already finish machined.

Quick facts

Furnace: Vacuum, then gas quench (nitrogen, about 2–10 bar)
Suits: Air- and oil-hardening tool steels, HSS, some stainless steels
Surface: Bright, no scale, no decarburisation
Distortion: Lower than oil or salt-bath quenching
Typical hardness: About 52–64 HRC, by steel
Usually priced: Per kg, with a minimum charge per load

How it is done

Parts are cleaned and loaded on fixtures so they do not touch
Air is pumped out and the load is heated in stages
Parts are held at the hardening temperature for the steel
Nitrogen gas at high pressure is blown through to quench
Parts are tempered once or more (often twice for tool steels) to the required hardness

Steels and typical hardness

Steel	Typical use	Typical hardness after tempering
D2 / D3	Blanking dies, punches	About 58–62 HRC
H13	Die-casting and forging dies	About 44–52 HRC
M2 HSS	Cutting tools, punches	About 60–65 HRC
SS420 / SS440C	Moulds, knives, bearings	About 48–58 HRC

Values are typical; the exact hardness depends on the tempering temperature chosen for the job.

What to tell the heat treater

Steel grade, ideally with the mill certificate
Hardness required, with a range (for example 58–60 HRC)
Weight, size and thinnest and thickest sections
Number of tempers, or deep-freezing (sub-zero) if needed
Faces that must stay flat or bores that must hold size
Whether a hardness test report is needed

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Questions

Why choose vacuum hardening over normal hardening?: The part comes out clean, without scale or a soft decarburised skin, and with less distortion. That saves grinding and suits finished tools, dies and precision parts.
Can any steel be vacuum hardened?: It works best on steels that harden in air or gas, such as D2, H13 and HSS. Plain carbon and low-alloy steels such as EN8 or EN9 need a faster quench, usually oil or water, so they suit other furnaces.
Is tempering needed after vacuum hardening?: Yes, always. Tool steels are usually tempered twice, and sometimes deep-frozen in between, to reach a stable hardness and size.

The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 18 October 2026.