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guide on how to heat treat cpm MagnaCut knife steel.

How to heat treat CPM MagnaCut

CPM MagnaCut was specially designed and developed for knives in 2021. Its composition was conceived by Larrin Thomas, a doctor in metallurgy (son of Devin Thomas), and is well known to knifemakers through his Knife Steel Nerds blog and book, Knife Engineering. Which has become the defacto reference for amateur and professional knifemakers.

CPM MagnaCut is a unique powder metallurgy stainless tool steel design that eliminates chromium carbide in the heat-treated microstructure. An excellent combination of toughness and wear resistance is achieved by having only small, high hardness, vanadium and niobium carbides, giving CPM MagnaCut properties similar to non-stainless steel CPM 4V.

CPM MagnaCut Chemical Composition (nominal) %:

CPM MagnaCut is a stainless steel manufactured by Crucible Industries, an American company renowned for its expertise in special steels. Being stainless, it is perfectly suited for wet, humid or coastal environments where you also desire excellent performance and toughness.

As you can see in the graph below [1] MagnaCut occupies a unique band that separates it from many other steels.

magnacut stainless toughness | Topham Knife Co
Overview of where MagnaCut compares against other Stainless Steels.

Here is a composition comparison graph of the steels usually compared to CPM-CruWear, CPM-4V, Vanadis 4 Extra, S110V and S45VN (visit zKnives.com for a graph).

  • Carbon 1.15%
  • Chromium 10.7%
  • Vanadium 4.00%
  • Molybdenum 2.00%
  • Niobium 2.00%
  • Nitrogen 0.20%

Heat Treatment Overview

Due to MagnaCut’s toughness, the recommended heat treatment for CPM MagnaCut is on the high end of hardness for knife steels which ranges between 60-64 HRC.

According to the Crucible steel datasheet [2], the optimal balanced heat treatment with a balance of toughness is 62.5 HRC. For some Japanese kitchen knives, where edge toughness is less important, Thomas indicates that MagnaCut can be heat treated to 62-64.

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.

Thermal cycling / Annealing (optional):

Normalizing: Heat the blade (780-840°C/1600°F) in the furnace and hold for 20 minutes. Allow the blade to air cool to room temp. The resultant microstructure ideally will be pearlite [1].

  1. Stress Relieving: Heat to 1100-1300°F (595-705°C), hold for 2 hours, then cool slowly in the furnace or cool in still air.
  2. Annealing: Heat to 1650°F (900°C), hold 2 hours, slow cool no faster than 25°F (15°C) per hour to 1100°F (595°C), then furnace cool or cool in still air to room temperature.
  3. Straightening: Best done warm 400 – 800°F (200 – 425°C)

Austenitizing / Hardening

  • Pre-heating (large complex parts): Heat to 1550-1600°F (845-870°C) and equalize.
  • Hardening Temp (Furnace): 1950 – 2200°F (1065 – 1205°C)
  • Sweet Spot: 2050°F / 1120°C for 20 minutes
  • Soak times: Please consult the summary table below.

The danger of an insufficient soak is much worse, leading to poor hardness, toughness, and corrosion resistance. While the chances of an overly long soak time are quite low. The changes to the steel in terms of transformation and carbides dissolving will “level off” after a certain amount of time and then changes are very slow after that. So it is recommended that the soak is “long enough” rather than trying to make it as short as possible.

Quenching media

There are no CTT or TTT curves available for CPM MagnaCut as yet (at the time of writing), but from what Larin says [1][4] and the measurable difference seen in the slower quench rates of the 2-bar vacuum quench data, means that a quick quench rate is preferable. Indeed large knives quench slower with a plate quench than small coupons. So hardness results may be somewhat lower than shown in the table depending on the size of the knife and the quenching speed. This is of course normal with any knife steel.

  • A medium-speed oil (warm to 125°F / 50°C) can be used until the part is black (1000°F (540°C), then plate/air cool.
  • Aluminium plates and compressed air to or below 125°F / 50°C
  • 2-bar gas/vacuum quench (larger setups)

Cryogenic Treatments

Cryogenic treatment or Deep cooling (-23°C / -196°C) is recommended to convert retained austenite, which should be done before the first temper cycle (limits the amount of stabilised austenite that is formed).

Place the blades in the freezer, bath or liquid nitrogen directly after quenching has been completed, to maximise the amount of retained austinite that can be converted to martensite.

You’ll note that I have included a graph for a typical single cryo step after quench as well as a double cyro, with an additional cryo treatment between the two tempering cycles. With the inherent toughness and a lack of retained autinite (RA) information at present, the double cryo step may well be worthwhile when using higher autinitising temperatures (2150°F – 2200°F / 1175°C – 1205°C). Be mindful that converting “all” the RA into martensite will reduce the toughness to a degree.

While liquid nitrogen is preferred, a chest freezer (-23°C), or a sub-zero bath (dry ice and kerosene/paraffin) will suffice for -100°F / -74°C. Submerge in sub-zero treatment for 2 to 6 hours or longer depending on thickness and number of blades. Clamping is recommended to avoid thermal shock-induced warp.

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

Tempering

cpm magnacut tempering vs toughness graph
1120°C / 2050°F had a superior toughness-hardness balance. [1]
  • Tempering Temperature Range: The recommended range is between 150°C – 230°C (300°F – 450°F)
  • Times: Two (2) times
  • Duration: Two (2) hours

It is not recommended to temper above 400°C / 750°F. Generally for knifemaking, secondary hardening / high-temperature tempers aren’t recommended and can insight embrittlement, leading to a reduction of toughness and corrosion resistance (should not be used for food handling applications).

*If using a small toaster oven or household kitchen oven for tempering, using a blade holding rack made from kiln furniture, a roasting tray lined with fine sand, or a similar large object will help retain thermal mass to reduce wide swinging temperatures as the device fluctuates trying to maintain temperature.

HRC Summary Table:

So there you have it, if you were looking to find out how to heat treat CPM MagnaCut, hopefully, I’ve answered most if not all of your questions.

CPM MagnaCut Heat treatment table

Links / References:

  1. CPM MagnaCut – The Next Breakthrough in Knife Steel
  2. CPM MagnaCut Datasheet (pdf)
  3. What is the Future of Stainless Knife Steel Design?
  4. What is the Best Hardness for MagnaCut Knives?
  5. Purchase: Knife Engineering: Steel, Heat Treating, and Geometry By Dr. Larrin Thomas
  6. Purchase: Magnacut Knife Steel
  7. Purchase: Knu-Foil Stainless Steel Tool Wrap
  8. Purchase: Condursal Z1100

Disclaimer:

Please note that your heat-treating kiln will also differ in its readings compared to mine. As such it’s best to do your own testing (coupons). All information above is from my readings of research papers, forum posts, and discussions with people. I am not a metallurgist; the above is presented here for the benefit of all knifemakers. You do not have to follow the recommendations and I’ll not be held responsible for any loss or damage you may experience.

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

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