O2 TOOL STEEL | 1.2842

4. O2 Steel Heat Treatment

Achieving O2 tool steel’s renowned hardness and wear resistance relies on a precise heat treatment process. As an oil-hardening cold-work steel, its exceptional properties are developed through controlled thermal cycling. 

4.1 The Annealing Process

O2 tool steel is typically supplied annealed. This initial heat treatment softens the steel, relieves stresses, and refines its microstructure, making it easier to machine or prepare for cold forming.

For severe cold-forming, spheroidize annealing is preferred:

• Heat the steel near or slightly below its lower critical temperature (Ac1).

• Hold at this temperature for a prolonged period.

• Cool slowly. This transforms carbides into a globular shape for maximum softness and ductility.

4.2 The Hardening Cycle

Hardening is the critical phase where O2 steel develops its characteristic high hardness. It involves heating to form austenite, then rapid cooling (quenching) to create a predominantly martensitic structure.

4.2.1 Preheating

While O2 is an oil-hardening grade, preheating is highly recommended, especially for larger sections or intricate parts, to minimize thermal shock and reduce distortion or cracking.

• Recommended Preheat Temperature: Around 650°C (1200°F).

• Tip: Placing the part atop the furnace before preheating can help gradually raise its temperature.

4.2.2 Austenitizing

Austenitizing involves heating the steel to a specific temperature to fully transform its structure into austenite, allowing carbides to dissolve.

• Recommended Austenitizing Temperature for O2 Steel: 790–815°C (1454–1472°F). Some sources suggest 800°C (1475°F).

• Soaking Time: Hold for 30–45 minutes per 25mm (1 inch) of thickness to ensure uniform heating and carbide dissolution.

• Caution: Proper furnace atmosphere control is important to prevent excessive decarburization or oxidation.

4.2.3 Quenching in Oil

After austenitizing, O2 steel is rapidly quenched in oil to transform austenite into hard martensite.

• Quenching Medium: Oil is specific to O2 steel, offering effective hardening with less distortion risk than water, especially for complex shapes.

• Target Quench Temperature: Quench until the steel reaches approximately 66–93°C (150–200°F).

4.2 Tempering

As-quenched martensite is very hard but brittle and stressed. Tempering is an indispensable post-quench treatment to improve toughness and ductility, reduce hardness to the desired level, relieve internal stresses, and enhance dimensional stability.

Critical Timing for Tempering:

Temper O2 steel parts as soon as they reach 52–65°C (125–150°F) after quenching. Delaying can lead to cracking.

• Tempering Temperature: Commonly around 175°C (350°F) for O2 steel (similar to O1). The typical range is 149–232°C (300–450°F), depending on desired final hardness. Lower temperatures yield higher hardness; higher temperatures increase toughness but reduce hardness.

• Soaking Time: Soak for at least 2 hours per 25mm (1 inch) of the thickest section.

Multiple Tempering Cycles:

Multiple tempering cycles (typically two) are often recommended for O2 tool steel. A second temper (after cooling to room temperature from the first) further refines the microstructure, relieves more stress, and can transform retained austenite. Air cool to room temperature between cycles.

4.3 Optional Advanced Treatments for O2 Steel

For specific needs, consider these treatments:

4.3.1 Stress Relieving for Enhanced Stability

Stress relieving minimizes residual stresses from manufacturing (machining, forming). Heat below Ac1, hold, then cool slowly.

• Timing: Before hardening, or after hardening and tempering.

• If Post-Hardening: Use a temperature ~25°C (50°F) below the final tempering temperature to avoid over-softening.

4.3.2 Sub-Zero Treatment (Cryogenic Treatment)

Sub-zero treatment can transform retained austenite (untransformed during quenching) into martensite by cooling to very low temperatures (e.g., -75°C / -103°F or lower). This may increase hardness and dimensional stability.

• Critical Post-Treatment: If used, O2 steel must be tempered immediately afterwards to relieve stresses from new martensite and improve toughness.

4.4 Summary of O2 Steel Heat Treatment Parameters

A quick reference for the typical O2 steel heat treatment process:

Adhering to these O2 steel heat treatment recommendations is essential for achieving target hardness (typically 60–62 HRC) and optimal performance.