S7 tool steel | 1.2355

3. S7 Tool Steel Heat Treatment

Only the correct heat treatment process can achieve the impact toughness of S7 tool steel. 

3.1 Annealing

S7 tool steel is typically supplied in an annealed condition, ensuring maximum softness and ease of machining. Should re-annealing be necessary to restore this optimal state for fabrication, the process involves:

1. Heating: Uniformly heat the S7 steel to 1550°F (843°C).
2. Soaking: Hold at this temperature for approximately 1.5 hours per inch (or 3.5 minutes per mm) of the thickest section to ensure thorough and uniform heating.
3. Cooling: Implement a slow cool, reducing the furnace temperature at a controlled rate of 25°F (14°C) per hour until it reaches 900°F (482°C).
4. Final Cool: After reaching 900°F (482°C), the steel can then be air-cooled to room temperature.

The expected hardness after a proper annealing S7 process is a maximum of 230 HB, which is an ideal state for subsequent machining operations.

3.2 Hardening

The hardening phase of S7 tool steel heat treatment transforms the annealed microstructure into a significantly harder martensitic structure, interspersed with fine carbides, which is fundamental to its end-use performance and toughness. This involves several key stages:

3.2.1 Critical Preheating Steps

Preheating is a crucial initial step in the S7 steel hardening sequence. Its primary purposes are to minimize thermal shock, which can lead to distortion or cracking, and to relieve any internal stresses induced during prior machining operations.

• First Preheat: Heat the S7 component uniformly to 1200°F (650°C).
• Hold Time: Maintain this temperature for 10 to 15 minutes, ensuring the heat is uniformly distributed throughout the entire cross-section of the part before proceeding to the austenitizing temperature.

3.2.2 Austenitizing

Following preheating, the S7 tool steel is rapidly heated to its austenitizing (hardening) temperature. This stage is designed to effectively dissolve alloy carbides and transform the steel’s crystalline structure into austenite.

• Austenitizing Temperature: The recommended temperature for austenitizing S7 is 1725°F (940°C).

3.2.3 Soaking

Once at the austenitizing temperature, the S7 component must be soaked. This soak time is critical for allowing the entire structure to uniformly convert to austenite and for ensuring the adequate dissolution of carbides necessary for optimal hardening.

• Soak Time Calculation:

1. For parts with a cross-section over 1 inch (25mm): Soak for 1 hour per inch (25mm) of the smallest cross-section.
2. For smaller sections: Use specific guidelines, e.g., 30 minutes for 1/8″ (3.175mm) sections and 1 hour for 1″ (25mm) sections.

• Caution: Avoid excessively long soaking times, as this can negatively affect the steel’s final grain structure and, consequently, its mechanical properties.

3.2.4 Quenching

Quenching is the rapid cooling phase that transforms the austenitized S7 steel into a hard martensitic structure. S7 is an air-hardening tool steel, making air cooling the standard and safest quenching method, minimizing distortion risks.

• Standard Quench: Air cool from the austenitizing temperature.
• Large Sections: For components with sections exceeding approximately 2.5 inches (63mm), air cooling might not achieve full hardness throughout. In such cases, oil quenching S7 may be necessary to ensure through-hardening.
• Post-Quench Cooling: After quenching, allow the part to cool until it is just warm to the touch (around 150°F or 65°C) before proceeding immediately to tempering. This step is vital to minimize the risk of quench cracking.

3.3 Tempering

Tempering is an indispensable final step in the S7 tool steel heat treatment process. It serves to reduce internal stresses developed during quenching, significantly increase the steel’s toughness (shock resistance), and achieve the desired final working hardness.

• Timing: Tempering should commence immediately after the S7 component has been quenched and cooled to approximately 150°F (65°C). The as-quenched structure is highly stressed and inherently brittle, making it prone to cracking if tempering is delayed.
• Typical Tempering Range: For most applications, S7 is tempered between 204°C (400°F) and 232°C (450°F).
• Recommended for Best Performance: Tempering S7 at 450°F (230°C) yields its optimal working hardness of approximately 58 HRC, effectively balancing strength with its characteristic high impact resistance.
• Hot Work Applications: If the S7 tool steel is to be used in applications up to 1000°F (540°C), it should be tempered at a temperature slightly above its anticipated maximum operating temperature to ensure stability.
• Holding Time at Temper: Hold at the selected tempering temperature for at least 2 hours for each tempering cycle. Alternatively, a common guideline is 2 hours per inch (25mm) of cross-sectional thickness.
• Multiple Tempers for Optimal Results: Double or even triple tempering is highly recommended for S7 tool steel. This practice ensures maximum toughness, refines the microstructure, and promotes greater dimensional and microstructural stability. Allow the part to cool completely to room temperature between each tempering cycle.
• Final Cooling: Slow air cooling after the final temper is advisable to minimize the development of residual stresses.

3.4 Post-Treatment Considerations for S7 Steel

If hardened S7 components undergo subsequent finishing operations such as grinding, welding, or Electrical Discharge Machining (EDM), a stress-relief temper is strongly advised. This should be performed at a temperature 25-50°F (14-28°C) below the temperature of the last effective tempering cycle used, to mitigate risks of cracking or dimensional instability induced by these processes.

3.5 Achievable Properties with Proper S7 Tool Steel Heat Treatment

When the S7 tool steel heat treatment is performed correctly, users can expect the following typical properties, making S7 a reliable choice for demanding applications:

• Working Hardness: Typically 56-58 HRC.
• Shock Resistance: Very high, a primary and defining characteristic of S7.
• Wear Resistance: Good, though generally lower than high-carbon, high-chromium cold-worked steels like O6 or D2 tool steel.
• Dimensional Stability: Good stability throughout the correct heat treatment process.
• Air Hardening Benefit: The air-hardening capability of S7 provides a good margin of safety during the hardening process, reducing the risk of distortion and cracking compared to liquid quenching steels.
• Elevated Temperature Performance: S7 exhibits good resistance to softening at moderately elevated temperatures, making it suitable for medium-temperature hot work applications.
• Surface Hardening: Through surface treatment processes such as carbonitriding or nitriding, the surface hardness of S7 tool steel can be further improved to around 64 HRC. However, such surface treatments can affect the S7 steel’s inherent shock resistance and should be considered carefully based on application requirements.
• Dimensional Change: Expect an approximate dimensional growth of +0.001 inches per inch (0.001 mm per mm) when S7 is air quenched from the correct hardening temperature.

To provide a quick reference, the key parameters for S7 tool steel heat treatment are summarized below: