O1 Tool Steel Tempering Chart

O1 Tool Steel Tempering Guide (WordPress Embed)

Tempering Parameters for O1 Tool Steel
Key steps and considerations for achieving desired properties in O1 steel through tempering.
Parameter	Details & Recommendations
Purpose of Tempering	Relieves internal stresses from quenching, increases toughness, adjusts final hardness, and improves dimensional stability. Tool steel is rarely used in the as-quenched state due to low toughness.
Timing of Tempering	As soon as possible after quenching. Delaying is risky and can lead to cracking. If delay is unavoidable, a temporary soak in boiling water is recommended.
Pre-Tempering Cooling	Cool part to 125°F (52°C) – 200°F (93°C) before placing in tempering furnace.
Heating for Tempering	Heat slowly and uniformly to tempering temperature. Using a cold furnace and bringing it up to temperature is recommended to prevent cracking.
Typical Tempering Temperature Range	300-450°F (149-232°C) for high hardness. ~350°F (177°C) → ~62-63 HRC ~400°F (204°C) → ~62 HRC ~500°F (260°C) → ~60 HRC Higher temperatures (e.g., 550-600°F / 290-315°C) for increased toughness, but lower hardness. O1 is a Class 1 steel: hardness continuously decreases as tempering temperature increases.
Soak Time	Hold at tempering temperature for 2 hours per inch (25mm) of the thickest cross-section. Do not short-cycle the tempering operation.
Cooling After Tempering	Slow cool in still air to minimize residual stresses.
Number of Tempers	Normally, a single tempering cycle is sufficient for O1 steel. A second temper (optional, 25-50°F / 14-28°C lower than the first) may be used for further stress relief or refining grain structure in specific applications.
Dimensional Stability	O1 shows relatively small dimensional changes. Slight expansion up to ~400°F (205°C). Overall contraction at higher tempering temperatures.
Post-Heat Treatment Operations (Grinding, EDM, Welding)	These operations can introduce stresses. A stress-relief temper is strongly recommended after these operations, typically at a temperature 25-50°F (14-28°C) below the final tempering temperature.

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