Your Cart
how to harden / heat treat 440c for knives

How to harden 440C stainless steel

AISI 440C (X105CrMo17) has the highest carbon stainless steel of the 400 series. It has one of the highest attainable hardnesses (58-60HRC) of the corrosion and heat-resisting grades. Grades 440A and 440B are identical except for slightly lower carbon contents (0.60-0.75% and 0.75-0.95% respectively); these have lower attainable hardnesses but slightly higher corrosion resistances.

While 440C is an older steel, it still is used today for many applications and while it has been “surpassed” by more modern, high alloyed steels it still offers good value for money. Jay Fisher has a love/hate relationship with it but if you take the time to read his article, it’s largely to dispel the misconceptions surrounding this steel.

The typical applications of 440C include ball bearings, bushings, valve parts and cutlery. Balls, rollers, needles and rings for corrosion-resistant bearings. The high percentage of chromium and carbon leads to a large number of carbides (10 microns) which aids wear resistance.

440C Chemical composition (nominal) %:

Composition: C 0.95-1.10%, Si <0.80%, Mn <0.80%, P <0.035%, S <0.030%, Mo 0.40-0.70%, Cr 16.00-18.00%

Here is a composition comparison graph of the steels usually compared to 440B, N690, and N695 (visit zKnives.com).

Heat treatment of 440C

NB! It’s important to protect the steel from oxidation and decarburization during hardening. Cordusal, Turco, and ATP-641, (anti-scale compounds), are probably the best choice unless you don’t want to spend time removing the affected surface scale post-heat treatment.

When doing the research for this steel, I found many studies that used far higher temperatures (>1100°C) for the austenitising temperatures. While this reportedly resulted in the hardest / highest HRC values many of those studies did also point out that it also resulted in large grain sizes. The large grain size reduces toughness and for knife proposes, is something we generally try to avoid.

Thermal cycling / Annealing (optional):

  • An often overlooked step, but due to its susceptibility to warping during oil hardening, an annealing step can save you countless headaches.
  1. Normalizing: Heat the blade (780-840°C/1600°F) in the furnace and hold for 20 minutes. Allow the blade to air cool to room temp. The resultant microstructure ideally will be pearlite [1].
  2. Annealing: 840-870°C (1550-1600°F) for 2 hours soak and cool very slowly in the furnace.

Austenitizing / Hardening

  • Pre-heating: 780°C (1440°F) – 10 minutes (large or complex parts)
  • Hardening Temp (Furnace): 1040°C-1066°C [1900°F – 1950°F]
  • Sweet Spot: 1040°C [1900°F]
  • Soak time: 5min (2.5mm) to 10min (5mm) up to 30 minutes (12mm) Max

Quenching media

440C is generally considered an air-hardening steel due to the addition of Mo. Clamping flat after quenching, during cryo or tempering is recommended to avoid thermal shock-induced warping.

  • a medium-speed oil (warm – 30°C) can be used until the part is black.
  • Aluminium plates and compressed air to or below 125°F / 50°C.

The CCT below shows the time needed (how fast a quench) need to avoid forming ferrite, bainite and or cementite.

440C CCT | Topham Knife Co

Cryogenic Processing

Cryogenic treatment is recommended to convert retained austenite, which should be done before the first temper cycle. The hardness of this material is increased by 7% when subjected to Deep Cryogenic Treatment (DCT); and by 4% for Shallow Cryogenic Treatment (SCT) [4]

While liquid nitrogen is preferred, a sub-zero bath with dry ice and kerosene will suffice for -100°F / -74°C. Submerge in sub-zero treatment for 4 to 6 hours depending on thickness and number of blades. As stated above, clamping is recommended to avoid thermal shock-induced warp. Any cryo treatment should always be followed by a tempering cycle.


Once the blade is quenched and near ambient temperature, blades should be tempered accordingly, the times suggested are to ensure an even, consistent temperature.

*If using a small toaster oven or household kitchen oven for tempering, using a blade holding rack made from kiln furniture, a roasting tray lined with fine sand, or a similar large object will help retain thermal mass to reduce wide swinging temperatures as the device fluctuates trying to maintain temperature.

Bohler N695 440C Tempering Chart | Topham Knife Co

Most manufacturers warn against tempering at 800°F / 425°C (secondary hardening curve) and above as sensitization will result in a reduction of toughness and corrosion resistance. Tempering above 400°C is not recommended.

  • Tempering Temp (Recommended range is between 100-200°C):
    • 61 HRC: 149°C (300°F) with Liquid Nitrogen DCT
    • 60HRC: 149°C (300°F)
    • 59HRC: 204°C (400°F) with shallow cryogenic treated SCT
    • 57HRC: 260°C (500°F)
  • Times: 2 times
  • Duration: Two (2) hours each time

Note (AMS 2759/5): retained austenite levels (typically <7% requirement) and dimensional stability requirements (typically <0.00010 in./in.) are not normally achieved with a single temper, but for knives, temper for at least one hour.

Recommended Hardness: 58 – 60HRC (after tempering) depending on the intended use. Lower HRC is recommended for tough “prybars” and higher hardness for sharp slicers that will see little chopping.


Please note that there are various manufacturers of 440C whose tolerances/chemical composition will differ quite a bit, making exact heat-treatment specifications difficult to pinpoint for everyone. Your own heat-treating kiln will also differ in its readings compared to mine. As such it’s best to do your own testing (coupons). All info above is from my readings of research papers, forum posts, and discussions with people. I am not a metallurgist and the above is presented here for the benefit of all knifemakers. You do not have to follow them and I’ll not be held responsible for any loss or damage you may experience.

Please comment below if you have anything to add to the above.

Links / References:

  1. How to Thermal Cycle Knife Steel
  2. Bohler N695 Extra Specification Sheet
  3. What is the Best Budget Knife Steel?
  4. Influence of Deep Cryogenic Treatment on the Mechanical Properties of AISI 440C Bearing Steel
  5. Effects of Varying Austenitizing Temperatures on Vacuum Hardening of Type 440C Stainless Steel
  6. Purchase: 440C Stainless Steel
  7. Purchase: Knu-Foil Stainless Steel Tool Wrap

2 thoughts on “How to harden 440C stainless steel

  1. How long and at what temp do you draw back 440c on the spring for a slip joint knife ?

    1. Hi John, Slip joints aren’ my speciality but luckily I can call on other’s experience here. Generally you’ll want the spring to be around 50-55 HRC. 50 HRC will have the most longevity. You can temper the spring at 300°C. Don’t go beyond this temperature as you’ll enter the secondary harding curve and introduce embrittlement, which is not desirable in a spring. However to hit the lower hardness ranges it’s “unavoidable”, 550°C – 600°C (1022 – 1100°F). One option (and I haven’t tested this myself) would be austempering, however the cooling rate required for stainless steels is so slow and long, its impractical (plus you need access to a molten salt bath).

Leave a Reply

Your email address will not be published. Required fields are marked *