How to Heat Treat Bohler N690 Stainless Steel

Bohler-Uddeholm’s N690 (1.4528) is a popular knife-specific stainless steel with knife makers in South Africa and Europe (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) %:

Steel Grade	C	Co	Cr	Mn	Mo	Ni	P	Si	V
N690	1.07	1.5	17	0.4	1.1	–	–	0.4	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 additions. 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. It 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.

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 can save you countless headaches. If you’re using quench plates then this step is generally not needed.

• Heat the blade in the furnace at 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 aluminium 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 and toughness, as you access the secondary hardening curve. Usually, secondary hardening (450°C) is not recommended.

• Tempering Temp: 150°C – 180°C (302°F – 356°F)
• Times: 2 times – 2X
• Duration: At least 2 hours each time (or one hour per 20mm material thickness)

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)

References / Resources:

1. https://www.bohler-edelstahl.com/en/products/n690/
2. https://knifesteelnerds.com/2018/05/14/why-there-is-cobalt-in-vg-10/
3. Purchase: N690 Knife Steel
4. Purchase: Knu-Foil Stainless Steel Tool Wrap

Disclaimer:

All the information 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.