How and Why I heat treat Bohler K110 (D2) steel the way I do

Bohler K110 (AISI D2 / JIS SKD11, DIN 1.2379) is one of the main steels that I use for my kitchen knives. It’s classified as a hypereutectoid cold work tool steel that is typically used for blanking dies, slitting cutters, shear blades, forming dies, knurls, gages (plug and thread), punches, trimming dies, etc.

The performance of a tool (in this case a knife blade) depends on the design, the accuracy with which the tool is made, and the choice of the how-to properly heat treat k110 (D2) to bring the best qualities out.

Bohler K110 Chemical composition (average %)

C: 1.55, Mn: 0.30 , Si: 0.30, Cr: 11.30%. V: 0.75, and Mo: 0.75.

Why I like it

• The high carbon content for high hardness: 60+HRC
• High wear resistance. Approximately 8 times that of plain carbon steels due to the presence of carbides.
• Stainless. Well almost. Due to the high percentage of carbon in the steel, it does pull most of the 12% chrome out into chromium carbides, which leaves about 5.8% remaining for stain resistance.
• Stability during heat treatment.
• Extremely well researched with many decades of use and studies to reference.
• D2 has been used in many knives, famously by makers such as Bob Dozier.
• Good edge retention: Better than N690, ATS-34/154CM, and 440C.

Concerns

• High Autinising temperatures will increase / coarsen grain size. which isn’t desired in knife blades as it decreases toughness.
• Protection during heat treatment is vital as an open atmosphere gives up to .014″(.35mm) layer of decarbonization and a noticeable loss of alloying elements.
• Longer hold times will increase the dissolution of carbides resulting in increased as-quenched hardness. But decreased toughness.
• Keep carbides small and finely distributed.

The Heat Treat Recipe:

• The goal is to reduce grain size (luckily a high alloy steel like D2 sees grain growth at much higher temperatures [10]), distribute carbides uniformly throughout the matrix, and obtain 61+ HRC, whilst minimizing retained austenite. As kitchen knives are predominantly slicing tools, toughness is not my primarly goal.

Stress Relieving / Annealing:

Intended to relieve stresses set up by extensive machining and or forging. Make the steel machinable so that it can be easily cut, drilled, and ground. It’s important to slowly cool K110/D2 as it’s air-hardening and may form martensite rather than the softer, more machinable structures we’re after.

• Stress relief is typically performed at 650 – 700°C (1202 – 1292°F). Heat for 1 to 2 hours in a neutral atmosphere (SS foil packet, etc) and then slowly cool in furnace / vermiculite.
• Annealing (slow transformation anneal) is typically performed at 800 – 850°C (1472 – 1562°F). Heat for 15-30 minutes in a neutral atmosphere (SS foil packet, etc) and then slowly cool in furnace / vermiculite.

Austinizing:

Highly alloyed tool steels, like D2 are susceptible to distortion and cracking on heating if temperature gradients develop through a cross-section because of their high hardness and complex microstructures even in the annealed and stress-relieved state.

1. A high austenitizing temperature should be avoided.
2. Protective coating/foil should be used to avoid decarbonization as the higher carbon content core can expand and puts the outer layer under tension.
3. If you have the ability to ramp, it is recommended to do so.

• Austenitizing Temperature: 1025 (1875°F) -1038°C (1900°F) see summary table below for aditional options
• Soaking Time: 15-30min

Quench:

I use aluminum plates to quench the blade rather than air cooling.

This keeps the blades straight and true. A plate quench is a bit faster than cooling in the air (despite the steel being “air hardening”) but it’s still slow enough not to put undue stresses into the steel and risk cracking. Plus the plates keep the knife blade straight as an arrow. (Read How to plate quench stainless steel)

You can plate quench with the stainless foil pocket still covering the blade.

Cold / Cryogenic treatment.

In highly alloyed austenite, a considerable fraction of the austenite might be retained in the microstructure at room temperature (10% RA), resulting in a much lower hardness than expected for more completely transformed microstructures.

The cryogenic treatment promotes the transformation of retained austenite into martensite, refines the size of the secondary carbides, increase their amount and population density, and leads to their more uniform distribution in the microstructures, at cryogenic temperatures, which is attributed to improved hardness and wear resistance (up to 817%) of the steel [3].

Even a cold treatment (4% RA) can yield beneficial results but shallow or deep cryogenic temperatures (-196°C) are better (0% RA). D2 has a Mf point below room temperature -125,1°C, meaning a sub-zero treatment is needed.

Time in liquid nitrogen or dry ice needs to be no longer than for the knife to reach the cold temperatures. In other words, 30-60 minutes is sufficient. Prolonged holding time leads to no measurable improvements.

NB! To be effective, the cryogenic treatment should be performed as soon after quenching as possible and before tempering. This is because RA transforms into untempered martensite which is brittle and needs to be tempered. Ie: Don’t do a snap temper on K110 (D2) if you’re doing a cold treatment.

Tempering

Tempering reduces residual stresses, increases ductility, toughness, and ensures dimensional stability. Tempering as-quenched martensite will also precipitate fine carbides, which are named “transition” carbides. D2 can withstand the effects of tempering, hence a long hold time is needed.

• Tempering Temperature: 150 – 205°C (300 – 400°F)
• Times: 2 – 3 times* (preferably)
• Duration: 2 hours (120 minutes)

*Tempering 3 three times in comparison to the recommended 2 times, can increase toughness by up to 25% in D2. This is due to the optimum distribution of alloying elements between carbides and the matrix, finer distribution of carbides, and periodizing of carbide on interfacial martensite boundaries.

Soak for at least 2 hours (120 minutes), withdraw from the furnace, and allow to cool in air. The second and third tempering should be a repetition of the first. (Ie: keep the temperatures and duration constant)

Summary / Results:

• 1025°C (1875°F) and 150°C (300°F) temper should equate to 64 HRC (toughness 5.8 ft-lbs)
• 1025°C (1875°F) and 205°C (400°F) temper should equate to 61.5 HRC toughness 8.5 ft-lbs)
• 1038°C (1900°F) and 205°C (400°F) temper should equate to 62 HRC (toughness 7.8 ft-lbs)
• 1050°C (1925°F) and 205°C (400°F) temper should equate to 62.5 HRC (toughness 4.5 ft-lbs)

Links/References:

1. Bohler K110 Spec Sheet
2. Heat Treatment and Toughness Behavior of Tool Steels (D2 and H13) for Cutting Blades by Attaullah. (Ayooq) Arain 1999
3. Evaluation of the cryogenic treatment in the destabilization of austenite retained in AISI D2 steel by R. Minaya Huamán, Lauralice de Campos Franceschini Canale 2017
4. All About D2 Steel – Development, Use in Knives, and Properties
5. How Much More Chromium Does D2 Need to be Stainless?
6. Sub-zero treatments of AISI D2 steel: Part I. Microstructure and hardness 2010
7. Comparison of Effects of Cryogenic Treatment on Different Types of Steels: A Review
8. Knife Engineering: Steel, Heat Treating, and Geometry by Dr. Larrin Thomas
9. How to Heat Treat D2, PSF27, and CPM-D2
10. How to thermal cycle knife steel
11. Annealing Part 2 – Temper Annealing, Cycling, and Final Properties

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.