O1 is one of the most commonly available tool steels and has been an industry standard for many decades. It possesses a high dimensional stability during heat treatment and is not particularly vulnerable to decarburization. O1 is a fairly deep hardening steel when quenched in oil and it is for this reason that it has too often been incorrectly recommended as a good beginner steel for knifemakers.  While it will harden very easily, it is a richer alloy that requires more involved heating and soak times to unlock its full potential in the hardening operation.  O1 can cost as much as three times the price as a simple high carbon blade steel which can easily reach their full potential in heat treatment with the simpler equipment most bladesmiths possess, thus making it the less economically prudent choice for many bladesmiths. If you have only a forge to heat treat with why pay extra for a steel that you may not get 100% out of?  If however you have the equipment to soak at precise temperature for extended periods, O1 will most likely outperform any of the simpler steels in cutting applications.    

Forging:  Start forging at 1800 to 1950 °F (980 to 1065 °C). Do not forge below 1550 °F (845 °C)

Normalizing: Heat to 1600 °F (870 °C). cool from temperature in still air.

Annealing: Heat to 1400 to 1450 °F (760 to 790 °C). Use lower temperature for small sections and higher tem­perature for large sections. Cool at a maximum rate of 40 °F (22 °C) per hour. The maximum rate is not critical after cooling to below 1000 °F (540 °C). Typical annealed hardness, 183 to 212 HB

Grinding or Machining

Stress Relieving (optional):   Heat to 1200 to 1250 °F (620 to 650 °C) Cool in air

Hardening: Aus­tenitize at 1450 to 1500 °F (790 to 815 °C) for 10 to 30 min, then quench in oil. Quenched hardness, 63 to 65 HRC

Tempering:  Temper at 350 to 500 °F (175 to 260 °C)

Polish

I-T / TTT diagram for O1:

The following tempering guidelines apply to O1 that has been soaked at temperature for a proper amount of time to achieve maximum hardness.  Knifemakers using other methods will have to adjust the resulting hardness numbers downward for the same temperatures.

*The above information is based upon personal experience and testing along with data from ASM International, Carpenter Steel and Crucible Service Centers.

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