A2 TOO STEEL | 1.2363 | SKD12

5. Heat Treatment

Below, we outline the critical steps and considerations for a successful A2 tool steel heat treatment, based on industry best practices.

5.1 Initial Preparation

1. Assessing Initial Condition:

A2 tool steel is typically supplied in an annealed condition. This soft state is ideal for machining. Before commencing any A2 tool steel heat treatment, ensure all parts are thoroughly degreased.

2. Pre-Hardening Stress Relief (If Applicable):

For components that have undergone significant machining, particularly those with substantial material removal or intricate geometries, a stress-relief cycle is highly recommended prior to the main hardening phase. This step is vital for minimizing distortion during the subsequent A2 tool steel heat treatment.

* Procedure for Unhardened Material: Heat the steel slowly and uniformly to 1200–1250°F (649–677°C). Hold at this temperature for approximately 2 hours per inch of the thickest section. Follow this with a slow cool, preferably within the furnace.

5.2 Protecting A2 Tool Steel During Heat Treatment

Surface Integrity (Preventing Decarburization):

A2 tool steel is prone to decarburization—the loss of carbon from the surface—when heated to high temperatures. To counteract this and ensure the integrity of your A2 tool steel heat treatment, parts should be processed in:

* A controlled neutral atmosphere furnace

* A vacuum furnace

* Or, by wrapping them securely in stainless steel foil. Sealing parts in stainless steel foil is a widely adopted and effective method.

5.3 Key Stages of A2 Tool Steel Heat Treatment

The following stages are fundamental to achieving the desired properties in your A2 tool steel.

5.3.1 Preheating:

Preheating is an indispensable step in the A2 tool steel heat treatment process. It serves to:

* Condition the material’s molecular structure.

* Equalize temperature throughout the component, minimizing thermal shock.

* Dissipate internal stresses before reaching higher austenitizing temperatures.

* Recommended Preheat Temperature for A2: 1200°F (650°C).

* Hold Time: Generally 10-15 minutes. Ensure the part’s color matches the furnace interior before proceeding to the next stage.

5.3.2 Austenitizing (Hardening):

This is the critical heating stage where the steel transforms into austenite, and essential alloy carbides are dissolved into the matrix. This step dictates the final hardened properties.

* Standard Hardening Temperature for A2: Approximately 1775°F (968°C). Some protocols allow a range of 1775-1825°F (970-995°C).

* Soak Time: The holding time at the austenitizing temperature is crucial. For air-hardening steels like A2, a general rule is 1 hour per inch (25mm) of the component’s thickest cross-section. Avoid excessive soaking, as it can negatively affect the final microstructure.

5.3.3 Quenching:

Quenching is the rapid cooling phase that transforms the austenite into hard martensite.

* Method for A2: A2 is an air-hardening tool steel. This means it achieves its martensitic structure by cooling in still room air or via a slow gas quench from the austenitizing temperature. Air cooling is the gentlest common quenching method, significantly reducing thermal shock and internal stresses.

* Foil-Wrapped Parts: If using a stainless steel foil envelope, keep it sealed during air cooling until all visible red heat has dissipated. This prevents atmospheric contamination.

* Size Consideration: Air hardening has limitations. For A2 tool steel, achieving full hardness might be challenging if the cross-section exceeds approximately 5 inches (127mm) when air cooled.

5.3.4 Handling After Quenching:

Once the A2 tool steel part has air-cooled to around 150°F (65°C), it can be handled.

* Critical Next Step: Begin tempering immediately after parts cool to this 125-150°F (52-65°C) range. Delaying tempering can lead to an unstable structure (due to untempered martensite and retained austenite), increasing the risk of cracking or diminished tool life.

* Straightening (If Necessary): Minor straightening can be attempted when the part is above 400°F (205°C), before the martensitic transformation is substantially complete.

5.4 Tempering: Refining A2 Tool Steel Properties

Tempering is a vital post-quenching step in the A2 tool steel heat treatment cycle. It serves to:

* Reduce retained austenite (untransformed austenite that can lower hardness and cause dimensional instability).

* Transform retained austenite into more stable microstructures.

* Relieve internal stresses.

* Increase toughness and reduce brittleness.

5.4.1 First Temper:

* Typical Temperature for A2: 400°F (205°C).

* Process: Heat to temperature, soak, and then typically air cool.

* Soak Time: Approximately 2 hours per inch (25mm) of cross-section. This initial temper helps stabilize the newly formed martensite and transforms retained austenite.

5.4.2 Multi-Tempering (Double/Triple Tempering):

Double tempering is highly recommended for A2 tool steel, and a third temper can offer further benefits.

* Advantages: Significantly refines the grain structure, enhances wear resistance, and provides superior stress relief. This is particularly beneficial for components with complex geometries or sharp corners.

* Common A2 Double Tempering Cycle:

* First Temper: 400°F (205°C)

* Second Temper: 375°F (190°C)

* Important Note: Always allow the part to cool to room temperature between tempering cycles. The chosen tempering temperature directly influences the final hardness of the A2 tool steel.

5.4.3 Summary of Key A2 Tool Steel Heat Treatment Parameters:

5.5 Post-Hardening Stress Relief

After the complete hardening and tempering cycle of your A2 tool steel heat treatment, additional stress relief may be necessary. This is particularly true if the component undergoes significant grinding, welding, or Electrical Discharge Machining (EDM).

* Procedure: Temper the hardened material at a temperature 25-50°F (14-28°C) lower than the final tempering temperature used in the main heat treatment cycle.

By diligently following these detailed A2 tool steel heat treatment steps, you can ensure your A2 tooling achieves its designed mechanical properties and optimal microstructure. This commitment to proper thermal processing will maximize the service life and operational performance of your A2 components.