How to Heat Treat Bohler Elmax for best results

ELMAX has been called the "best all-around knife steel," because it will make a razor-sharp blade and deliver the best possible knife experience. Here's how to heat treat it.

Bohler Uddeholm Elmax is a “high chromium-vanadium-molybdenum-alloyed knife steel that is perfectly balanced with high hardness, good corrosion resistance, and excellent edge retention.

ELMAX has been called the “best all-around knife steel,” because it is simply that, a quote on quote, “perfectly balanced PM grade” steel with good corrosion resistance and excellent edge retention. If convenience and quality, versatile features are what you are looking to get out of a knife, then ELMAX is your steel of choice.

Elmax Chemical composition (average %)

C: 1.7, Mn: 0.30 , Si: 0.80, Cr: 18%. V: 3.0, and Mo: 1.0.

The 3rd generation powdered metal hat has a reduced powder size that gives it round and uniform carbides. This contributes to increased wear resistance, edge retention (with minimum risk for edge chipping), and toughness when hardened to over 60 HRC.

On a CATRA (Cutlery Allied Trades Research Association) Edge Retention Test, when tested for Rockwell C Hardness, Uddenholm’s ELMAX scored higher than the other tested stainless steels, including Aisi M4, Aisi 440C, Bohler M390 Superclean, and Uddenholm Vanadis 4, at an impressive 62 HRC. In its TCC (total cards cut) test, which measures how many silica impregnated cards that a knife with each steel type can cut through at a time, ELMAX scored an impressive 930.7, higher than most other steels, and second only to Bohler’s M390 Superclean.

Max working hardness is about 62HRC, although production knives are mostly in 60-61HRC range (63/64 is very high for whilst, 61/62 is more reasonable.). When used with a coarse edge about 400-800 grit range it has a durable, very aggressive edge, but Elmax can also support very fine, high polished edges as well.

Bohler Elmax CCT Diagram

The Heat Treat Recipe:

The goal is to maximize hardness (62 HRC) and edge retention as pertaining to chef/kitchen knives.

Elmax has a thin (0.2mm) layer of 300 series stainless capsule material 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.

Austinizing / Hardening

NB! It’s important to protect the steel from oxidation and decarburization during hardening and both Cordusal (other anti-scale compounds) and stainless foil are good choices for this.

  • Stress Relieve: 650°C for 2 hours (Optional)
  • Austenitizing Temperature: 1150°C (2100°F) for 15min [4]
  • Alternative: 1080°C (1976°F) for 30min [3]
    • Soaking Time: Note the soaking times, will change depending on your chosen autentitising temp (higher temps will have a shorter holding / soaking time).
  • See summary table below for aditional options

Quenching Media

The choice of a quenching medium also has a measurable impact on RA and hardness. Uddeholm recommends as fast a quenching speed as possible to achieve maximum hardness. Tests show that Oil delivers a marginally higher hardness (62) over a forced air quench (60). That being said, an air quench delivers better toughness scores, lower retained austenite and impact resistance levels. So an air/plate quenching is recommended. [3]

  • Oil, preheated to 65°C (150°F). Quench until the part is black, then clamp/plate quench [2]
  • Aluminum plates with compressed air (my preferred method) (read How to plate quench stainless steel)

Sub-zero / Cryogenic Treatment

Deep cool immediately after quenching, followed by tempering. If liquid nitrogen cannot be used, deep cool in dry ice to -95°F for similar results.

Cryro treatment increases hardness a small amount (+- 1-3 HRC), however for that increase, a far greater amount of toughness (bending strength and impact strength) is lost. Given this trade-off, if toughness is needed in the final blade, the cold treatment step should not be used and replaced with a snap temper.

  • Uddeholm Elmax is commonly sub-zero treated between -150°C and -196°C (-240°F and -320°F) although occasionally -40°C (-40°F) and lower temperatures are used due to constraints of the sub-zero medium and equipment available.
  • A treatment time of 1–3 hours at temperature, will give a hardness increase of 1–3 HRC.
  • Retained Austinite (RA) will be reduced to 3-4% (when air quenched)

Tempering

If forgoing the cold treatment, temper immediately the blade reaches 50–70°C (120–160°F).

As with other grades of steel, low-temperature tempering is recommended to ensure higher corrosion resistance. The preferred tempering temperature range is between 200°C (390°F) or 250°C (480°F). In exceptional cases, a minimum tempering temperature of 180°C (350°F) can be used for small simple inserts and parts where toughness is of less importance.

The secondary hardening curve (high-temperature tempering) 500°C (930°F) is possible but at the expense of corrosion resistance and is therefore not recommended for most applications. (8-5% RA)

  • Tempering Temperature: 180°C (356°F) – 250°C (480°F)
  • Times: 2 Times (2x)
  • Duration: 2 hours (120 minutes) with intermediate cooling to room temperature (1 hour hold at room temp).

When maximum dimensional stability is required further or secondary subzero treatment may be used between the two tempering operations. This is only affecting if tempering temperatures are above 240°C (390°F) In this case, it is important to always end with a tempering, as the last operation.

Summary / Hardness Results:

Based on the obtained results [3] it has been found that optimum mechanical properties for the Elmax steel were obtained after hardening from a temperature of 1080°C in compressed air (without a sub-zero treatment) and tempering at a temperature of 180°C. Such heat treatment ensures that the steel has high hardness (60.6 HRC), the highest impact strength (88.3 J/cm ) and high bending strength.

  • 58 HRC: 1040°C (1900°F) for 30min. Cryo -195°C (-320°F). Temper 2 x 2 hours at 250°C (480°F)
  • 60 HRC: 1050°C (1920°F) for 30min. Cryo -195°C (-320°F). Temper 2 x 2 hours at 200°C (390°F)
  • 60.6 HRC: 1080°C (1920°F) for 30min. Air Quenched (No Cryo). Temper 2 x 2 hours at 180°C (356°F) [3]
  • 61 HRC: 1080°C (1980°F) for 30min. Air Quenched. Cryo -78°C (-320°F). Temper 2 x 2 hours at 200°C (390°F)
  • 62 HRC: 1080°C (1980°F) for 30min. Oil Quenched. Cryo -78°C (-320°F). Temper 2 x 2 hours at 200°C (390°F)
  • 62 HRC: 1150°C (2100°F) for 15min. Cryo -195°C (-320°F). Temper 2 x 2 hours at 200°C (390°F) [4]

References:

  1. https://www.uddeholm.com/files/PB_Uddeholm_elmax_english.pdf
  2. https://www.uddeholm.com/app/uploads/sites/12/2017/05/Uddeholm_Elmax_Heat_Treatment_Recommendation.pdf
  3. Effect of Heat Treatment on the Microstructure and Mechanical Properties of Sintered Stainless Tool Steel
  4. Uddeholm PM Steel for Knives

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.

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