What Are Hardening and Tempering Treatments?

Hardening and tempering are essential processes for engineering steels, providing components with the necessary mechanical properties for their intended service. During hardening, steel is heated to its appropriate temperature (usually 800-900°C), held there, and then quenched (rapidly cooled, often in oil or water). This is followed by tempering, a process involving heating at a lower temperature to relieve stresses and achieve the desired mechanical properties. The specific conditions depend on the steel composition, component size, and required properties.

These treatments can be performed in open furnaces or controlled environments (gaseous atmospheres, molten salts, or vacuum) for “clean hardening” to avoid scale and decarburisation.

Specialized Quenching Options:

• Martempering (Marquenching): Uses an elevated-temperature quench to reduce distortion. Suitable for specific alloy steels and section sizes.
• Austempering: Ideal for thin sections of medium- or high-carbon steels, producing low distortion and a tough structure without the need for tempering.

Benefits of Hardening and Tempering

• Develops an optimal combination of hardness, strength, and toughness.
• Enables weight and material savings in component design.
• Components can be machined or formed in a softer state and then hardened to achieve high mechanical properties.
• Neutral hardening ensures surface integrity, ideal for precision components like bearings, bolts, and automotive parts.

Steels That Can Be Treated

• Most engineering steels with over 0.3% carbon, including those listed in BS970 and BS EN 10083-1 and -2.

Limitations of Hardening and Tempering

• Hardenability: Steel’s ability to harden in depth depends on composition and size. Larger components may require higher-grade steels.
• Aluminum content: Excessive aluminum in low-alloy steels can negatively affect hardening response.
• Steel Condition: Decarburised layers must be removed before hardening to prevent distortion or cracking.
• Temper Embrittlement: Some alloy steels may suffer from embrittlement in certain temperature ranges (250-450°C).
• Component Size and Shape: The treatment depends on equipment capacity. Verify suitable facilities for large components early in the process.

Potential Problems

• Distortion or Cracking: Caused by factors like rapid cooling, poor design, or incorrect heat treatment.
• Scaling and Decarburisation: Open furnace treatments may cause surface scaling and carbon loss.
• Mixed Batches: Using material from mixed batches can lead to inconsistent treatment results.

How to Specify Hardening and Tempering Treatments Provide the following details:

• Treatment type: harden and temper, martemper, or austemper.
• Steel specification: include grade, composition, and standard.
• Desired mechanical properties: hardness or tensile strength range.
• Testing requirements: include type of testing and locations for samples.
• Special requirements: include additional services like cleaning, straightening, or non-destructive tests.

Ensure Successful Treatment

• Use quality steel from reputable suppliers.
• Design components to avoid sharp corners and abrupt changes in section.
• Consult with your heat treater early in the process.
• Ensure proper removal of decarburised layers and surface defects.

By carefully following these steps and specifications, you can ensure the successful hardening and tempering of your engineering steels.