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How to Heat Treat Bohler N690 Stainless Steel

Bohler-Uddeholm’s N690 (1.4528) is a very popular knife-specific stainless steel with knife makers in South Africa as well as many parts of Europe (where it’s referred to as N690co). While relatively high alloyed, it’s not powdered steel which no doubt hampers its adoption in the US.

N690 Chemical composition (nominal) %:

C 1.07, Co 1.5, Cr 17, Mn 0.4, Mo 1.1, Ni -, P -, Si 0.4, V 0.1

Voestalpine Bohler-Uddeholm states this is their best conventional knife steel and describes the steel as Martensitic chromium steel with cobalt, molybdenum, and vanadium addition. For tools and components which can be hardened to very high hardness levels (60HRC). A fine ground or polished surface finish is required for good corrosion resistance in keeping with its European food-grade rating.

That being said, this Austrian steel is very capable in its own right and has a reported wear resistance just below M390 and other premium steels and as such it has been compared to VG10 (Japan) and S30V (USA) respectfully. More often than not it is directly compared to 440C as the addition of cobalt is the only material difference.

Here is a reference to composition between the steels.

http://zknives.com/knives/steels/steelgraph.php?nm=154CM,440C,D2,N690,CPM S30V&ni=51,420,196,521,276

Heat treatment

NB! It’s important to protect the steel from oxidation and decarburization during hardening. Both Cordusal and stainless foil are good choices.

Annealing:

An often overlooked step, but if you’ve had issues with any warping in the past or simply want to set the steel up for the best HT you can, an annealing step or can save you countless headaches.

  • Heat the blade in the furnace and hold for 30 minutes. Allow the blade to cool slowly inside the furnace to room temp. The resultant microstructure is ferrite and carbide. 800-850°C (1470-1560°F).

Austenitizing / Hardening

I tend to always favor the lower end (1,030°C) of the temperature range for chef knives as I’m looking for high hardness, maximum stain resistance, and as much toughness or “small grain size” as I can get.

Some alloying elements affect thermal conductivity, such as high chromium additions, so preheating is more necessary for stainless steels or high chromium tool steels.

That all being said, looking online and reviewing various tests, many makers choose a middle ground of 1050°C and 1080°C for their purposes.

  • Hardening Temp (Furnace): 1,030°C – 1,080°C [1,886°F – 1,976°F]
  • Sweet spot: 1050°C
  • Soak time: 7min (2.5mm) to 15min (5mm)

Quenching media

  • Bohler recommends medium-speed oil until the part is black.
  • Aluminum plates with compressed air for thinner blades, is also an option to minimize any warping.

Sub-zero / Cryogenic Treatment

While the addition of Cobalt raises the Ms/Mf temperature, cold treatment is a nice value-added step you can perform to complete the transformation of RA to martensite. If you can try to reach -85°C to -185°C for at least 2 hours or until the piece has reached a stable core temperature.

Using the highest austenitizing temperature of 1,080°C with a Cryogenic treatment does seem to lead to some of the hardest HRC results (62 HRC with a 200°C temper) in some tests.

Tempering

Low-temperature tempering is recommended to ensure good corrosion resistance. Due to the addition of Mo, W, and V, which contribute to tempering resistance N690 can be tempered at higher temperatures at the expense of stain resistance as you access the secondary hardening curve.

Bohler N690 Temperingdiagram | Topham Knife Co
  • Tempering Temp: 150°C – 200°C (302°F – 572°F)
  • Times: 2 times
  • Duration: 1 hour each time

Recommended Hardness: 58 – 60 (after tempering)

Resources:

https://www.bohler-edelstahl.com/en/products/n690/

https://knifesteelnerds.com/2018/05/14/why-there-is-cobalt-in-vg-10/

Disclaimer:

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.

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