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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.

Typical applications: moulds for the processing of chemically aggressive plastics containing abrasive fillers. Moulds for the processing of duroplasts and for the production of chips for the electronic industry. Screws for injection moulding machines; non-return valves and linings for injection moulding cylinders.

M390 has become the preferred “super steel” choice for high-end lock 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 then 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 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 62HRC, although production knives are mostly in the 59-61HRC range.

How to Heat Treat Bohler M390:

  • Elmax, M390, M398 & Vanax 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]

Austinizing / Hardening

  1. A lower austenitsing temperature will maxmise the impact toughness. 1100-1150°C (2010-2100°F)
  2. A higher austenitising temperature will maximise wear resistance 1180°C (2155°F)
  • Austenitizing Temperature: 1100 -1180°C (2010-2155°F)
  • Soaking Time/s:
    • 20 minutes at 1100-1150°C (2010-2100°F)
    • 5-10 minutes at 1180°C (2155°F)
  • Recommendation: 1150°C (2100°F)*1 for 20 min

Quenching Media

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 austenisiting temperature is used). Cool immediately after hardening. Cryogenic treatment transforms retained austenite which can increase hardness a small amount (+- 1-3 HRC), however, for that increase, a far greater amount of toughness (bending strength and impact strength) is lost.

  • The subzero treatment leads to increased hardness values (64HRC) at austenitising tempersatures >= 1150°C (2100°F)
  • Remove from cryo and allow blade to warm to room temperature in ambient air.
Bohler M390 temper hardness graph cryo | Topham Knife Co
Bohler M390 temper hardness graph | Topham Knife Co

Tempering

Slow heating to tempering temperature.

  • Low-temperature tempering 150-260°C (300-500°F) is recommended to ensure enhanced corrosion resistance and increased toughness [6].
  • High tempuature tempering 540-560°C (1000-1040°F) is recommended to give the highest wear resistance.
  • Temperature/Hardenss:
    • 63HRC: 1150°C (2100°F) @ 200°C (400°F) [1]
    • 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).

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 Matensitic Chromium Steels

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.