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How to Heat Treat Bohler K460 (O1) Tool Steel

Bohler-Uddeholm K460 (1.2510) (AISI O1) is a popular carbon cold work tool steel that knife makers have used very effectively for decades. Famously O1 has been used by Randall Knives starting in 1938 or so and continuing to today.

K460 Chemical composition (nominal) %:

C 0.95, Mn 1.10, Cr 0.55, W 0.55, Si 0.25, V 0.1

K460 is an oil hardening carbon manganese tool steel, possessing excellent dimensional stability during heat treatment. Cutting tools (dies and punches), blanking and punching tools, threading tools, woodworking tools, machine knives for the timber, paper, and metal industries, measuring tools and gauges, molds for the plastics industry.

The vanadium addition is optional though that helps to maintain a fine grain size. The tungsten also helps with fine grain and wear resistance. The carbides are relatively small and well distributed. Fine carbides generally mean good toughness and edge stability.

The main benefit of O1 compared with simple carbon steel like 1095 is that it can be hardened in oil and better avoid cracking, distortion, and size changes.

Here is a composition comparison graph of the steel (visit zKnives.com).

Heat treatment

NB! It’s important to protect the steel from oxidation and decarburization during hardening. Cordusal, Turco, ATP-641, (anti-scale compounds), and/or stainless steel foil (during annealing) are probably the best choice unless you want to spend time removing the affected surface post-heat treat.

Annealing / Normalization / Stress Relieve:

An often overlooked step, but due to its susceptibility to warping during oil hardening, an annealing step or can save you countless headaches.

  • Normalizing: Heat to 857°C/1575°F for 10-15 minutes and then air cool to form pearlite. This will leave us with a consistent microstructure going in to our next steps.
  • DET (Divorced Eutectoid Transformation): Heat to 760°C/1400°F for 10-15 minutes and then slow cooling (placing in vermiculite) the steel transforms to ferrite while growing the carbides by diffusing carbon to the carbides.

Austenitizing / Hardening

  • Hardening Temp (Furnace): 780 – 820°C [1310°F – 1508°F]
  • Sweet spot: 801°C/1475°F @ 15min
  • Soak time: 10min (2.5mm thickness), 15min (5mm thickness), 30mm (20mm thickness)

Quenching media

  • A medium-speed oil is recommended (due to the high hardenability) until the part is black.
  • Aluminum plates can be used afterward to minimize any warping.

Cold / Cryogenic Treatment:

A cold treatment can be added after the initial quench for a small increase in hardness as well as a corresponding decrease in toughness.

NB! To be effective, the cryogenic treatment should be performed as soon after quenching as possible and before tempering. This is because RA transforms into untempered martensite which is brittle and needs to be tempered.

Tempering

Bohler K460 01 Tempering Graph | Topham Knife Co
  • Tempering Temp:
    • 65HRC: 150°C (300°F)
    • 62 HRC: 200°C (400°F)
    • 60HRC: 230°C (450°F)
  • Times: 1 times
  • Duration: 1 hour min

References:

  1. O1 Steel – History, Properties, and How to Heat Treat
  2. The Testing of Steel for Leather Paring Knives
  3. Reader Question – O1 vs 80CrV2
  4. O1 v. A2 (one more time)
  5. https://www.bohler-edelstahl.com/en/products/k460/
  6. How to Thermal Cycle Knife Steel

Disclaimer:

Please note that it’s best to do your own testing. 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 to add anything to the above.