Vacuum Heat Treating
Also: vacuum hardening, vacuum furnace heat treatment, bright hardening, high-pressure gas quench, HPGQ, vacuum tempering, vacuum annealing, vacuum aging
Vacuum heat treating hardens, tempers and ages parts in an evacuated furnace with a gas quench: bright, decarb-free tool steel and aerospace parts.
3,387 finishing and heat treating companies in the Noramark directory
A vacuum furnace pumps the chamber down before heating, so there is no oxygen to scale or decarburize the surface and no atmosphere to add carbon or hydrogen. Parts are heated by radiation from graphite or molybdenum elements, held, and then cooled by backfilling the chamber with nitrogen or argon (helium in some furnaces) and driving it through the load with a large fan. The same furnace tempers, anneals, stress relieves, ages and brazes, so many precision parts go through their whole thermal route without seeing air at temperature.
Cooling rate is set by gas pressure and velocity. Standard furnaces quench at about 2 bar; high-pressure gas quench (HPGQ) furnaces run 6, 10, 12 or 20 bar. Air-hardening tool steels (A2, D2, H13, M2, S7) and martensitic stainless harden fully at 2-6 bar in normal sections; heavy H13 die blocks need 10 bar or more to meet the cooling-rate requirements of die-casting specs such as NADCA #207. Oil-hardening grades such as O1 and 4140 need an oil quench in all but thin sections, which some vacuum furnaces provide in an integral oil tank. A gas quench is more even than oil, so parts move less and vary less from part to part.
Titanium must be heat treated in vacuum or argon, because it picks up oxygen in air and forms a brittle surface layer called alpha case; AMS 2801 governs aerospace titanium heat treatment. Superalloys such as Inconel 718, PH stainless and implant alloys are routinely vacuum treated to keep them clean, and AMS 2769 covers vacuum heat treatment in general. Metal 3D-printed parts are stress relieved in vacuum or argon furnaces, often still on the build plate. The trade-offs are cost per load, slower radiant heating, and a gentler quench than oil or water, which limits vacuum hardening to steels with enough hardenability for a gas quench.
At a glance
| Typical tolerances | Hardness to the same bands as atmosphere hardening: A2 and D2 typically 58-62 or 60-62 HRC, H13 44-48 or 46-50 HRC for dies, and 17-4 PH to the condition minimum (40 HRC min at H900, 35 HRC min at H1025). A 3-4 point HRC range is normal. Distortion is the lowest of any through-hardening route because the gas quench is even, but tool steels still change size by roughly 0.05-0.1 percent, and D2 and A2 can grow or shrink depending on the hardening and tempering temperatures. Leave about 0.1-0.25 mm (0.004-0.010") per side for grinding on precision tools; hardened blanks that will be wire EDM cut need a double or triple temper so stress does not open the cut. |
|---|---|
| Size limits | Common job-shop vacuum furnaces have work zones around 600 x 600 x 900 mm (24 x 24 x 36") with load limits of roughly 450-900 kg (1,000-2,000 lb); larger shops run 900 x 900 x 1,200 mm (36 x 36 x 48") furnaces taking about 1,400-2,300 kg (3,000-5,000 lb), and a few have furnaces several meters long for aerospace structures. Heavy die blocks are limited by quench rather than space: the gas pressure available decides the largest section that will fully harden. |
| Surface finish | Bright to light silver-grey, with no scale and no decarburization, so hardened tool steel can go straight to grinding or EDM with minimal stock. Chromium-bearing steels may show a faint straw tint if the vacuum or backfill gas was not clean. Titanium should come out silver to light straw; blue, purple or grey colour indicates oxygen pickup and calls for an alpha-case check. No cleaning is normally needed beyond removing fixture marks. |
| Lead time | Tool steel hardening and tempering in 3-7 business days, with 24-48 hour rush common for tool and die shops. Aerospace solution and age cycles with certification in 1-2 weeks. Long cycles, such as the 18-hour aging of Inconel 718 or cryogenic treatment plus a triple temper on tool steel, add days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Tool steels are the core work: A2, D2, H13, S7 and M2 harden in vacuum with a gas quench and come out bright, with no decarburized skin to grind off. Martensitic and PH stainless (410, 420, 440C, 17-4 PH, 15-5 PH) are hardened or aged in vacuum to keep them clean. Titanium (Ti-6Al-4V) is annealed, stress relieved, and solution treated and aged in vacuum or argon. Nickel superalloys such as Inconel 718 are solution treated and aged in vacuum. Low-alloy steels that need an oil quench (4140, 4340) can be hardened in vacuum furnaces with an oil tank and are otherwise done in atmosphere furnaces. Aluminum is almost never vacuum heat treated because it needs a fast water quench.
What drives the cost
- Cycle: triple tempers, cryogenic treatment and long aging cycles each add furnace runs
- Quench pressure: high-pressure gas quench furnaces cost more per load than 2 bar furnaces
- Load weight and minimum charge: vacuum loads are priced by the furnace run, so small jobs ride with others or pay the minimum
- Fixturing and support for long or thin parts to control distortion
- Specification work: AMS 2769 or AMS 2801, Nadcap, test coupons, pyrometry and furnace survey records
- Inspection: hardness on every part, or coupons sectioned for alpha case on titanium
When to use it
- Tool steel dies, punches, molds and cutting tools that must come out bright and free of decarburization
- Titanium, nickel superalloys and PH stainless on aerospace and medical work
- Precision parts that are finish ground or cut by EDM after hardening and need minimum grind stock
- Parts with fine features, threads or polished surfaces that scale or decarburization would ruin
- Stress relief and solution treatment of metal 3D-printed parts
When not to
- Carbon and low-alloy steels that need an oil or water quench to harden, unless the shop has an integral oil quench
- Aluminum solution treatment, which needs a fast water quench
- Large, heavy parts that will be machined all over afterwards: an atmosphere or car-bottom furnace is cheaper per pound
- Case hardening that adds carbon or nitrogen, which is done by vacuum carburizing or ion nitriding rather than plain vacuum hardening
Design tips
- Specify the tool steel grade, the hardness range and, for die-casting dies, the quench and toughness requirement (for example NADCA #207 for H13).
- Ask for at least a double temper on tool steels, and a triple temper or cryogenic treatment on D2 and M2 parts that will be wire EDM cut or must stay dimensionally stable.
- Design uniform sections and avoid thin fins next to heavy blocks; a gas quench is gentle, but thin areas still cool faster and move.
- Leave 0.1-0.25 mm (0.004-0.010") per side on precision surfaces for grinding; bright hardening means no allowance for decarb is needed.
- For titanium, call out AMS 2801 and ask for an alpha-case check on a coupon if the part will not be machined after heat treatment.
- Tell the heat treater which surfaces are finished or polished so they can be fixtured and kept clean.
Frequently asked questions
- Why heat treat in a vacuum?
- With no oxygen in the furnace, parts do not scale or decarburize, so hardened tool steel keeps its full surface hardness and needs less grinding, and titanium does not form a brittle alpha case. The gas quench is also more even than oil, so parts distort less. The trade-off is a higher cost per load.
- What gas quench pressure do I need?
- Air-hardening tool steels such as A2 and D2 harden in normal sections at about 2 bar. Larger sections, H13 die blocks and high-speed steels benefit from 6-10 bar or more, and die-casting die specs set a minimum cooling rate that only a high-pressure furnace meets on big blocks. Oil-hardening steels such as O1 and 4140 generally need an oil quench, not gas.
- Is vacuum heat treating more expensive?
- Per load, yes, often noticeably more than an atmosphere furnace. On precision tool steel and aerospace parts it usually costs less overall, because there is no scale or decarb to grind off and distortion is lower. For carbon and alloy steel parts that will be machined all over afterwards, atmosphere hardening is cheaper.
- Does vacuum heat treating prevent decarburization?
- Yes, when the furnace is tight and the backfill gas is clean. A well-run vacuum furnace returns parts with no measurable decarburized layer, which is why tool steel can be hardened close to final size.
