How to Heat Treat Bohler N690 Stainless Steel

N690 is Bohler-Uddeholm's best conventional stainless knife steel. High hardness and stain resistance make it a durable steel for every day use. Here's how to heat treat it.

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 the composition between the steels.

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

N690 Heat Treatment Guide

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. If you’re using quench plates then this step is generally not needed.

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

Austenitizing / Hardening

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 1060°C for their purposes.

  • Hardening Temp (Furnace): 1,030°C – 1,080°C [1,886°F – 1,976°F] see summary table below for aditional options
  • Sweet spot: 1050/1060°C
  • Soak time: 7min (2.5mm) to 15min (5mm)

Quenching media

  • Bohler recommends medium-speed oil until the part is black.
  • Plate quenching with aluminum plates to minimize any warping and compressed air for thinner blades or blades that have been pre-ground, to ensure the edge cools at a sufficient rate.

Sub-zero / Cryogenic Treatment

While the addition of Cobalt raises the Ms/Mf temperature, introducing a 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 (-70°C/-95°F ) does seem to lead to some of the hardest HRC results (62 HRC with a 200°C temper) in some tests. If using a household freezer (-20°C/-5°F ) stick with the autenitising temperatures of 1030-1060°C.

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 450°C (Austenitise at 1080°C) at the expense of stain resistance as you access the secondary hardening curve. Usually, secondary hardening is not recommended.

Bohler N690 Tempering graph
  • Tempering Temp: 100°C – 150°C (302°F – 572°F)
  • Times: 2 times
  • Duration: 1 hour each time

Bohler’s Recommended Hardness: 58 – 60 (after tempering)

Summary / Results:

  • 62HRC: 1080°C (1975°F), Deep Freeze to -70°C/-95°F, Tempering at 100°C (212°F)
  • 60HRC: 1060°C (1940°F), Cold Freeze to -20°C/-5°F, Tempering at 150°C (302°F)
  • 59HRC: 1050°C (1920°F) @150°C (345°F)
  • 57 HRC: 1050°C (1920°F) @ 200°C (435°F)

Resources:

  1. https://www.bohler-edelstahl.com/en/products/n690/
  2. 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.

2 Comments

  1. hello brother, I followed the steps you gave for the N690 above step by step.
    my furnace propane furnace
    steel 3.7 mm N690
    my oil heat treat oil
    I burn the oven after I throw the material in the oven
    I reach temperatures of 800-850 degrees in about 15 minutes, and then we brew at 1055-1066 degrees for 18 minutes, then I cool the oil with a temperature of 62 degrees for 8 seconds, I let it cool between 2 plates and the result is a knife that passes the file test. The result is very good in the tests, but 55-57 HRC hardness is found in 2 different hardness measuring instruments. I want to get your opinion too.
    I would appreciate if you correct me where I did wrong, thank you in advance, good work
    Hope Aydemir
    KoToKnife

    • Hi Aydemir,

      N690 being a stainless alloy, needs very tight temperature control when at austenitising temperature (soaking). As you have an 11 degrees temperature swing/window it’s going to not give the best possible results. With a propane forge, you can put a tube/pine into the furnace to create a more stable temperature zone for the knife to be in. Additionally, you could add a cold treatment to boost the HRC a point or two after quenching. Depending on your application, 55 may be a bit too soft, with 58-59HRC being a decent range for most knives.

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