How to Heat Treat Böhler M390 Steel

Böhler M390 is a third-generation martensitic chromium steel produced with powder metallurgy. Due to its alloying concept, this steel offers extremely high wear resistance, decent toughness, good corrosion resistance (food-grade [5]) and polishability (mirror).

M390 has become the preferred “super steel” choice for high-end folders and small fixed blades as of 2010. The longevity of M390 is relatively impressive given that it was developed in the late 1980s as a modified stainless version of Böhler’s own K190. The steel saw little use in knives until Latrobe steel copied M390 and sold it as 20CV in the USA, which became popular with several production knife companies. Carpenter began producing their own version CTS-204P, and all three have been steadily building in popularity with knives ever since.

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

M390 has since been modified for yet higher edge retention with the newer 7% vanadium M398.

M390 Chemical composition (average %)

C: 1.90, Mn: 0.30, Si: 0.70, Cr: 20. V: 4.0, W: 0.6 and Mo: 1.0.

Max working hardness is about 62/63HRC, although production knives are mostly in the 59-61HRC range.

NB! Recently Bohler has advised local knife makers, not to laser cut thick blanks of M390. Reportedly the high alloy content of M390 combined with the heat of laser cutting causes brittleness in thicker stock (4.56mm and above) that can’t be recovered with annealing. As such, water-jetting is recommended.

Heat Treatment of Bohler M390:

Elmax, M390, M398 & Vanax all have a thin (0.2mm /.007″) layer of 300 series stainless capsule material (HIP) on the surface to protect the PM steel during the manufacturing process. It’s recommended to remove this jacket before hardening as it will “insulate” the core steel during the quench and or give lower HRC readings. If you’re unsure, you can etch the steel to see if the jacket is still present. [1]

NB! It’s important to protect the steel from oxidation and decarburization during hardening. Condursal Z1100, Turco, and ATP-641, (anti-scale compounds) and stainless steel tool wrap (knu-foil), are probably the best choices.

Austenitising / Hardening

1. A lower austenitizing temperature will maximise the impact toughness. 1100-1150°C (2010-2100°F)
2. A higher austenitising temperature will maximise wear resistance at 1180°C (2155°F)

• Austenitizing Temperature: 1100 -1150°C (2010-2100°F) see the summary table below for additional options
• Soaking Time/s:
  • 20-30 minutes at 1100-1150°C (2010-2100°F)
  • 5-10 minutes at 1180°C (2155°F)
• Recommendation: 1130°C (2066°F)^*1 for 30 min

Quenching Media

M390 is very sensitive to quench rate and thus cooling should achieve at least a -100°C/s or faster cooling rate for best or consistent results. [7]

• Plate quench (water-cooled, forced air)
• Warm Oil, quench until black, and then cool in still air.

Sub-zero / Cryogenic Treatment

Deep freezing is recommended for both high corrosion and wear-resistance applications (more so when a higher austenitising temperature is used). Cool to room temperature immediately after hardening.

Cryogenic treatment after quenching is to reduce the amount of residual austenite. Another possibility to reduce the amount of residual austenite is by tempering. The time should be longer (>) than 2 hours.

• The subzero treatment leads to increased hardness values (64HRC) at austenitising temperatures = 1150°C (2100°F)
• Remove from cryo and allow the blade to warm to room temperature in ambient air, before tempering.

Note: Any cryo treatment (whether between tempers, or after the quench, should always be followed by a tempering cycle.

Tempering

Slow heating to tempering temperature. 2 x 2 hours

• Low-temperature tempering of 150°C-260°C (300°F-500°F) is recommended to ensure maximum corrosion resistance and increased toughness [6].
• High-temperature tempering 540-560°C (1000-1040°F) is recommended to give the highest wear resistance, although it will come at the expense of stain resistance (food grade rating) and toughness (embrittlement).

Summary / Results

• Temperature/Hardness:
  • 63HRC: 1150°C (2100°F) @ 200°C (400°F) [1]
  • 62HRC: 1130°C (2066°F) @ 170°C (400°F)
  • 60 HRC: 1150°C (2100°F) @ 525°C (970°F) [5]
  • 59HRC: 1150°C (2100°F) @ 250°C (450°F) [5]
• Times: 2 Times (Tripple temper if aiming for maximum wear resistance) [3]
• Duration: 2 hours (120 minutes) with intermediate cooling to room temperature (1 hour between tempering cycles).

When maximum dimensional stability is required further subzero treatments may be necessary between the two tempering operations. In this case, it is important to always end with a tempering, as the last operation.

References:

1. M390 Steel – History and Properties (and 20CV and 204P)
2. https://www.bohler-edelstahl.com/app/uploads/sites/92/2018/10/M390EN.pdf
3. Effect of Heat Treatment on the Microstructure and Mechanical Properties of Sintered Stainless Tool Steel
4. M390 vs 20CV vs 204P – 3rd Generation Powder Metallurgy Technology?
5. Materials Testing for food product processing
6. PM Plastic Mould Steels – Wear Resistant and Corrosion Resistant Martensitic Chromium Steels
7. Analysis of heat treatment parameters on the properties of selected tool steelsM390 and M398 produced with powder metallurgy
8. Purchase: Knife Engineering: Steel, Heat Treating, and Geometry By Dr. Larrin Thomas
9. Purchase: Bohler M390 Microclean Stainless Steel
10. 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. Your kiln may differ from mine, so there will be variances in results. Test and find what works for you.

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