O2 Tool Steel Heat Treatment Guide

O2 tool steel heat treatment is crucial to achieving optimal performance. O2 tool steel is an oil-quenched cold work tool steel with high hardness, high wear resistance, and minimal deformation during heat treatment. Compared to water-quenched steel, O2 steel exhibits better dimensional stability and higher toughness after complete quenching¹. The equivalent grades of O2 tool steel include:

1. Europe (EN/DIN): 1.2842 or 90MnCrV8.
2. Japan (JIS): SKS31
3. China (GB): 9Mn2V

A Quick Checklist of O2 Tool Steel Heat Treatment

Preheating

Almost all tool steels, including O2, require preheating. It helps reduce thermal shock and lowers the risk of deformation or cracking when cold tools are placed in a hot furnace. It can also relieve stress during the processing of O2 materials and improve furnace production efficiency. Preheat to 649-677°C (1200-1250°F) until the O2 material is evenly heated.

Austenitizing (Hardening)

The austenitizing temperature range for O2 steel is typically 760 to 800°C (1400 to 1472°F). At this temperature, various complex alloy carbides are dissolved, and the microstructure is transformed into austenite. The soaking time at this temperature should be carefully controlled to avoid adverse effects on the molecular structure.

Quenching

After austenitizing, the steel is rapidly cooled, typically in oil for O2 tool steel, to transform the austenite into martensite, which is the desired hard structure. Oil quenching has a slower cooling rate than water quenching, but it has a lower risk of deformation and cracking. The O2 material should be cooled to a temperature between 66 and 93 °C (150 and 200 °F) before proceeding to the tempering step. Unlike air-hardening steel, oil-quenched tool steel is not typically wrapped in stainless steel foil during quenching, as the foil would impede the oil quench.

Tempering

O2 tool steel is typically tempered at low temperatures, usually between 150 °C and 260 °C (300 °F and 500 °F), to maintain its high hardness. O2 tool steel commonly requires a single tempering cycle, but double tempering can sometimes be preferred. Each tempering cycle typically requires a soak time of at least 2 hours per inch (4.7 minutes per millimeter) of cross-section, with air cooling to room temperature between multiple tempers. The timing of tempering is critical and must begin immediately after the temperature from the previous quenching step drops to 52-65°C (125-150°F) to prevent adverse effects on tool life and to avoid stabilization of retained austenite. The higher the tempering temperature, the lower the hardness.

Common Issues and Solutions in O2 Heat Treatment

Deformation and dimensional changes

O2 steel exhibits good dimensional stability, but improper heat treatment can still cause deformation and dimensional changes, primarily due to excessive quenching and uneven heating.

To effectively control deformation, the following measures may be taken:

1. Martempering: Quench the workpiece in quenching oil or molten salt at a temperature approximately 14°C to 28°C (25°F to 50°F) above the martensite transformation start temperature (Ms). After the workpiece has cooled uniformly throughout, proceed with air cooling.
2. Preheating Treatment: It is recommended to preheat the steel to approximately 650°C (1200°F) prior to austenitizing to reduce the risk of deformation during subsequent hardening processes.
3. Machining allowance: Sufficient allowance must be retained during machining to correct any potential deformation that may occur after final heat treatment.

Quenching Crack

Compared to water-quenched steel, O2 steel offers higher quenching safety but still carries a risk of cracking. The causes include excessive heating rates, incorrect selection of quenching media, and design factors such as sharp corners or abrupt changes in cross-sectional thickness in O2 tools and dies.

To prevent cracking, the following measures can be taken:

1. O2 steel must be oil-quenched; the use of water or brine is strictly prohibited, as it may easily cause cracking. The optimal temperature of the quenching oil should be maintained between 50°C and 70°C (120°F to 160°F) to achieve the desired hardness while minimizing thermal shock.
2. Tempering must be performed immediately after quenching. It is recommended to carry out this process before the workpiece has thoroughly cooled to room temperature to eliminate high residual stresses and prevent cracking promptly.
3. For complex-shaped workpieces, it is recommended to perform stress-relief annealing at 650–675°C after rough machining and before final quenching to reduce the risk of subsequent cracking.

Surface Decarburization

O2 steel is highly susceptible to surface decarburization during heating. Decarburization causes carbon loss in O2 steel, forming a soft layer with insufficient hardness, significantly reducing its wear resistance.

To prevent decarbonation, the following process controls must be implemented:

1. The heating process should be conducted in a protective medium, such as an endothermic atmosphere, a molten salt bath, or a vacuum environment, to prevent oxidation effectively.
2. Before final heat treatment, the oxide scale on the O2 surface—commonly known as “black scale”—and the existing decarburized layer must be thoroughly removed to ensure the O2 workpiece achieves the required surface hardness after hardening.

Retained Austenite and Hardness Control

The microstructure and final hardness of O2 steel are susceptible to the austenitizing temperature. Excessively high temperatures lead to a significant increase in retained austenite content within the O2 steel matrix, resulting in insufficient hardness after quenching and dimensional instability.

To ensure stable performance, the following control measures are recommended:

1. Strictly adhere to the recommended austenitizing temperature range. For O2 steel, this is typically 790°C to 815°C, effectively suppressing the formation of excessive retained austenite at its source.
2. The cryogenic treatment process at -196°C (-321°F) effectively promotes the transformation of retained austenite into martensite. While it cannot eliminate it, it significantly reduces its content.
3. It is recommended to temper at 175°C-205°C (345°F-400°F) to achieve the desired working hardness, typically 58–62 HRC.

Low Red Hardness

O2 steel is a cold-work tool steel that lacks red hardness. It is suitable only for cold stamping, shearing, and forming applications in low-temperature environments. Its use in hot-work scenarios, such as die casting and hot forging, is strictly prohibited.

FAQ

1. Nee, J. G. (Chief Technical Reviewer & Managing Editor). (2010). Fundamentals of Tool Design (6th ed.). Society of Manufacturing Engineers. ↩︎